The present disclosure relates to a system and a method for controlling a methanation reactor by monitoring gas concentrations and/or temperatures within the methanation reactor.

Background

An increasing acceptance for the urgency to reduce the pollution of anthropogenic greenhouse gases and alleviate the dependency for fossil fuels has enabled and accelerated the green transition towards decarbonization of both the industrial-, transport-, and energy-sector. In 2015, many countries therefore signed the Paris Agreement, which is committed to address the climate changes by approaching net zero of global anthropogenic CO2 emissions. Accordingly, the International Renewable Energy Agency reported that the global renewable power capacity was expanded by 10.3% in 2020, where the expansion was predominately based on intermittent power sources as wind and solar power. Hence, technologies that can facilitate carbon capture and utilization (CCU) for the abatement of CO2 based on the renewable power sources (Power-to-X or PtX) will be imperative to mitigate the current CO2 emissions and close the carbon cycle.

One of the leading options for PtX technologies are based on using renewable hydrogen (H2), which is created by consuming renewable power to split water into its elementary components of hydrogen and oxygen with water electrolysis. Downstream from the electrolysis, a wide variety of different PtX applications based on the hydrogen platform with integrated CCU have been developed to reduce the CO2 into various demanded compounds. Several of these compounds can be produced through different biotechnological pathways to products as bioethanol, acetate, methane, biobased plastics (polyhydroxybutyrate), proteins amongst many others. The significance of dosing and monitoring the hydrogen feed is a common denominator in most of these technologies, which will become extremely relevant for the operation and control of the dynamic biological systems in the fast-developing biotechnological industry based on hydrogen derived from renewable power.

An emerging PtX technology with potential for integrated CCU is ex-situ biological methanation (biomethanation). Biomethanation is a carbon capture method using microbes, e.g. in the form of a biofilm, for valorising CO2 to supply carbon-based energy carriers as methane (CH4), which are critically needed in the transport and chemical industry sectors. The main challenge in H2/CO2 biomethanation has been identified to be the H2 gas-liquid mass transfer, which are impeded by the low liquid partitioning of H2. Optimization of the operation, process and reactor designs have therefore been focused on limiting the H2 gas-liquid mass transfer barriers. Among the different reactor systems, the trickle bed reactor (TBR) system has shown promising conversion efficiencies and high operational robustness. The capital and operational expenditures of the TBR technology depends on its CH4 production capacity and most research in biomethanation therefore centre around the optimization of the reactor configuration and operational parameters to acquire the maximal production capacity of CH ₄ .

Summary

The performance of a reactor system, such as a TBR, may be based on input and output data of the system, i.e. how much of the input (reactants) is converted to the desired output (products). Thus, the performance may be monitored based on a black box approach describing the overall productivity. However, the variations and oscillations of the physical, chemical, and microbial properties throughout the reactor or TBR may result in substantial gradients in performance through the reactor. For example, the reactor may be compartmentalized i.e. divided into different compartments or sections with different local productivities due to differences in biofilm composition and activity through-out the reactor. These differences necessitates constant monitoring of the reactors performance to improve conversion efficiencies and enable control in case of any changes in activity through-out the reactor.

It was found that a reactor performance may be improved by monitoring and controlling the spatial scales of the microbial and chemical properties of the system. For example, a methanation reactor may be monitored for certain parameters and the distribution of these parameters within the reactor, e.g. selected compounds such as gas concentration of H2, CO2, CO, and/or CH4 at a specific position or at specific positions within the methanation reactor, and the gas concentration at the specific position(s) may be used to profile the gas concentration distribution within the reactor volume and control the distribution. The methanation reactor can also be monitored with temperature sensors, since the biomethanation process is an exothermic process. An increasing temperature is an indication that the biomethanation process rate is also increasing, whereas a subsequent decrease in temperature can signify a decreasing methanation activity, which could need operational measures to be taken.

A first aspect of the disclosure relates to a system for controlling a methanation reactor comprising a methanation reactor, which can comprise of a first fluid inlet and a first fluid outlet. The methanation reactor may be configured for at least one first fluid flow between the first fluid inlet and the first fluid outlet defining a first direction of the at least one first fluid flow, or the methanation reactor may be configured such that at least one first fluid flow is flowable between the first fluid inlet and the first fluid outlet defining a first direction of the at least one first fluid flow. The system may further comprise a first gas sensor configured to measure a first gas concentration at a first gas sensor location within the methanation reactor, and/or at least a first temperature sensor and a second temperature sensor configured to measure a first temperature and a second temperature, respectively, at a first and a second temperature sensor locations within the methanation reactor. The system may further comprise at least one controller configured to regulate the at least one first fluid flow into the methanation reactor based on the first gas concentration or on the first and second temperatures. At least one second controller may be further comprised to regulate at least one second fluid flow into the methanation reactor, also based on the first gas concentration or on the first and second temperatures. The first and the second temperature sensor locations may be in the first half of the methanation reactor and in the second half of the methanation reactor, respectively. With these locations, the first and the second temperature sensors will provide data so that a reliable spatial distribution of the biological activity within the methanation reactor can be determined.

The first gas sensor may be preferably sensitive to H2, CH4 and/or CO2, for e.g. measuring the concentration of volume of H2, CH4 and/or CO2.

Thus, the application of a first gas sensor and/or at least a first temperature sensor and a second temperature sensor provide an evaluation of the methanation reactor profile, which unravels the black box of methanation reactor operation, and facilitates controlling and improving the performance.

Further specifically, the application of a first gas sensor and/or a first temperature sensor and a second temperature sensor provides the option of an online in-situ monitoring of e.g. critical and controlling compounds or reactions in a methanation reactor. Advantageously the sensor can be a high precision sensor, such as a microsensor, facilitating in-situ and advantageously real-time monitoring.

The concentration gradients of the compounds, e.g. the gaseous substrate, within the reactor may be evaluated via a first sensor and the flow pattern of the reactor. By having a first temperature sensor and a second temperature sensor, the amount of generated heat can be evaluated, which constitutes a direct measure for biological activity within the methanation reactor. A more precise concentration distribution may be obtained by two or more sensors, such as between 3-10 sensors, and where the sensors are placed at different locations within the reactor, e.g. at a bottom section, middle section, and top section.

Another aspect of the disclosure relates to a method for controlling a methanation reactor comprising the following steps: providing at least one first fluid flow between a first fluid inlet and a first fluid outlet of a methanation reactor defining a first direction of the at least one first fluid flow; measuring a first gas concentration at a first gas sensor location within the methanation reactor, and/or a first temperature and a second temperature, respectively, at a first and a second temperature sensor locations within the methanation reactor; and regulating the at least one first fluid flow into the methanation reactor based on the first gas concentration and/or the first and the second temperature.

The method provides a more efficient methanation reactor operation. The measurement of a first gas concentration at a first selected position and/or a first temperature and a second temperature, respectively, at a first and a second temperature sensor locations within the methanation reactor facilitates the control of the at least one first fluid flow into the methanation reactor or other operational initiatives, which can be based on these measurements, and an improved operational strategy of the methanation reactor operation may be obtained.

By measuring the gas concentration close to the first fluid outlet of the methanation reactor, the amount of H2 and/or CH4 can be determined in the gas to be supplied to a gas grid. Normally, there are gas grid quality gas requirements that set lower limits for the purity of methane delivered by the reactor. In e.g. the Danish gas grid quality requirements, the methane purity must be >98%. If the CH4 concentration close to the first fluid outlet is below 98% or the H2 concentration close to the first fluid outlet is above 2%, then the operator will know that the gas supplied by the methanation reactor does not achieve the Danish gas grid quality requirements, and the operator can take immediate action.

By measuring the gas concentration close to the first fluid inlet or in the middle of the methanation reactor, the reaction in time can be studied, to determine the effectiveness of the methanation production. If the amount of CH4 decreases that could be an indication that the acidity in the methanation reactor is increasing, which reduces the effectivity of the microbes. The operator can take immediate action e.g. by flushing the reactor with water

By measuring the temperature at two different positions in the methanation reactor, then the temperature gradient from the metabolic heat production relating to gradients in biomethanation rates can also be extrapolated. Changes in single points can also be related to changes in metabolic activity when reactor heating primarily is done through metabolic reaction heat.

Description of the drawings

In the following embodiment and examples will be described in greater detail with reference to the accompanying drawings:

Fig. 1 shows an embodiment of the system according to the present disclosure, where the system is configured to regulate the gas flow (biogas and H2 supply) into the reactor and the liquid medium flow into and out of the reactor, in response to the sensor measurements,

Fig. 2 shows a schematic view of an embodiment of the process and instrumentation schematics of a methanation reactor and a monitoring setup,

Fig. 3 shows an embodiment of a plurality of hydrogen substrate concentration profiles at different vertical positions in a methanation reactor,

Fig. 4 shows time series data of a local CH4 production capacities at three different positions along the vertical axis for a period of 135 days,

Fig. 5 shows early warnings of acidification in a methanation reactor revealed by gas concentration sensors at day 88 followed by full acidification at day 93 leading to reduced product gas quality,

Fig. 6 shows an evolution of the production capacity compared to the size of a trickle-bed reactor, Fig. 7 shows a plurality of early-warning events, where the acidification in a 10L trickle-bed reactor is monitored with a first H2 sensor located in the middle of the reactor and a second H2 sensor located close to the outlet of the reactor.

Detailed description

The present disclosure relates to a system for controlling a methanation reactor comprising a first fluid inlet and a first fluid outlet, wherein the methanation reactor is configured for at least one first fluid flow between the first fluid inlet and the first fluid outlet; a first gas sensor configured to measure a first gas concentration within the methanation reactor, and/or at least a first temperature sensor and a second temperature sensor configured to measure a first temperature and a second temperature within the methanation reactor; and at least one controller configured to regulate the at least one first fluid flow into the methanation reactor.

Throughout this patent application, the terms gas sensor and gas concentration sensor are used interchangeably to refer to the same underlying sensor, which is a sensor used for measuring a gas concentration. For the sake of clarity and understanding, both terms are used synonymously and do not imply any difference in the technology or its implementation.

Throughout this patent application, the methanation reactor can be interpreted as a biomethanation reactor. Biomethanation can be defined as a biological methanation. Biomethanation reactors and chemical methanation reactors may both use CO2 and H2 as feedstock, but the conversion to CH4 can not occur without a catalyst to catalyze the methanation. A biomethanation reactor uses a biological catalyst of microorganisms to catalyze the process or reaction, whereas a chemical methanation reactor uses metalbased catalysts to perform the reaction. The different catalysts can require different conditions to properly function. Chemical methanation may require high temperatures (>200°C) and pressure, while biomethanation reactors can require lower temperatures (<150°C). In one embodiment, the methanation reactor may have an internal temperature that is below 200°C, preferably below 175°C, more preferably below 150°C. Especially below 100°C may be a suitable internal temperature. The term internal temperature may mean or indicate the overall temperature in the methanation reactor or the temperature of one or more active sites of the methanation reactor, where methanation reaction is occurring. The biomethanation reactor may comprise microorganisms configured to be used as a catalyst or biocatalyst to perform a methanation. The microorganisms facilitating the conversion to CH4 are hydrogenotrophic methanogens, which can be used as a defined pure culture of only hydrogenotrophic methanogens, a defined co-culture of several hydrogenotrophic methanogens and associated archaea and bacteria, or an undefined mixed culture containing hydrogenotrophic methanogens. Natural sources of mixed cultures include sludge from sewage wastewater plants, sludge from biogas plants, manure, wetlands, marine sediments, digestive tracts of animals, and/or other organic sources.

The methanation reactor may have an internal length, and the first gas concentration sensor location can be located at a first distance from the first fluid inlet of between 1% and 99% of the internal length, more preferably between 12% and 99% of the internal length, such as 15% of the internal length. Preferably, the first gas concentration sensor location can be located within the methanation reactor, where the first gas concentration sensor may measure the first gas concentration at the first gas sensor location within the methanation reactor. The first gas concentration sensor location may have a preferable location within the methanation reactor, which may depend on the current operation conditions, where especially the gas flow rate and current conversion efficiency may influence the preferable location of the first gas concentration sensor. The first gas concentration sensor may be located at a first distance from the first fluid inlet, which can provide a sufficient reaction time for the operators to take action when the internal gradient changes. A first distance from the first fluid inlet of 1 % of the internal length can be applicable, but preferably, the first gas concentration sensor may be located at a first distance of 12% of the internal length, to create a more distinct concentration gradient. The internal length may be defined as the internal length between the first fluid inlet and the first fluid outlet.

If the electric price is low, because e.g. it is windy and/or sunny, it may be advantageous to utilize the low price and produce more H2. The higher production of H2 will have to be transformed to methane in the methanation reactor at a higher rate so that the inlet flow of H2 as well as CO2 into the reactor or into each reactor will have to increase. The higher flow means that the first gas concentration sensor will preferably be positioned further away from the first fluid inlet to measure a reliable gas (like H2) concentration in the reactor, like after a first distance of 20% or later of the internal length to be able to detect changes in gas concentration due to the methanation reaction. The first gas sensor location may be after a first distance of 30% or later of the internal length if the flow is very large, or even after a first distance of 40% or later of the internal length if the flow is very large indeed.

Likewise, if the flow of H2 as well as of CO2 into the reactor is high, the time from methanation production starts going down anywhere in the reactor, e.g. due to acidification, until the gas at the first outlet does not fulfil the gas grid quality gas requirements (too low methane concentration and/or too high H2 concentration) will be short. If the flow of H2 as well as of CO2 into the reactor is high, the first gas concentration sensor may preferably be placed at a first distance of 90% or earlier or even 80% or earlier to give enough time to react to the change in the methanation production. The first gas sensor location may be at a first distance of 70% or earlier of the internal length if the flow is very large, or even at a first distance of 60% or earlier of the internal length if the flow is very large indeed.

The first temperature sensor location may be within 1% and 50% of the internal length of the methanation reactor and the second temperature sensor location may be within 30% and 80% of the internal length of the methanation reactor. With these locations, the first and the second temperature sensors will provide data so that a reliable spatial distribution of the biological activity within the methanation reactor can be determined even when the flow is high. The first temperature sensor location can be from 1 % of the internal length of the methanation reactor. This can be relevant if the first fluid flow is at a relatively high temperature at the first fluid inlet. A relatively high temperature would involve the methanation to start as close as possible to the first fluid inlet, thereby involving an increase of the internal temperature of the methanation reactor already from the first fluid inlet.

Preferably, the first temperature sensor location may be within 5% and 50% of the internal length of the methanation reactor and the second temperature sensor location may be within 30% and 80% of the internal length of the methanation reactor. With these locations, the first and the second temperature sensors will provide data so that a reliable spatial distribution of the biological activity within the methanation reactor can be determined even when the flow is very high.

More preferably, the first temperature sensor location may be within 5% and 40% of the internal length of the methanation reactor and the second temperature sensor location may be within 30% and 70% of the internal length of the methanation reactor. With these locations, the first and the second temperature sensors will provide data so that a reliable spatial distribution of the biological activity within the methanation reactor can be determined even when the flow is very high indeed.

In an embodiment, the first temperature sensor location and the second temperature sensor location may be more than 1% of the internal length of the methanation reactor apart, preferably more than 3% of the internal length of the methanation reactor apart, more preferably more than 5% of the internal length of the methanation reactor apart, even more preferably more than 10% of the internal length of the methanation reactor, apart, and more preferably more than 20% of the internal length of the methanation reactor apart. Such separations between the first temperature sensor and the second temperature sensor will enable the system to monitor the methanation efficiency of the microbes inside the biomethanation reactor.

In an embodiment, the system may comprise a third temperature sensor for providing an even temperature distribution that is even more true to the real temperature distribution and thus provide an even more true picture of the methanation reaction inside the methanation reactor. With three temperature sensors, a first temperature sensor location of the first temperature sensor may be within 5% and 40% of the internal length of the methanation reactor, a second temperature sensor location of the second temperature sensor may be within 20% and 50% of the internal length of the methanation reactor, and a third temperature sensor location of the third temperature sensor may be within 40% and 70% of the internal length of the methanation reactor.

In a preferred embodiment, the gas sensor is a gas concentration sensor. A gas sensor can be a device capable of responding to the presence of gas defined by certain parameters. A gas sensor can be a device that may be designed to detect the presence and/or concentration of specific gases in an environment where it can be placed or arranged. Gas concentration sensors can be selective or non-selective. Selective sensors can be designed to respond to specific gases, while non-selective sensors may react to a range of gases. Gas sensors may typically provide an electrical signal as their output, which can be analog or digital. This output can be processed to provide information about the gas concentration, or any other parameters that the gas sensors may be able to measure and/or quantify. A gas concentration sensor may be a type of gas sensor that is preferably designed to measure the concentration or amount of a specific gas in an environment where it can be placed or arranged. In addition to the first gas concentration sensor, the system may comprise a second gas concentration sensor for measuring a gas concentration at a position in the biomethanation reactor different compared to the first gas concentration sensor. Two gas concentration sensors located at different locations in the biomethanation reactor may provide a better understanding of the state of the biomethanation reaction, so that a failure in the methane production can be detected earlier, and so that biomethanation reaction can be adjusted by e.g. reducing the input flow of H2 and/or CO2 before e.g. a too high concentration of H2 enters the first fluid outlet.

The first gas concentration sensor may be located at a first half of the biomethanation reactor closest to the first fluid inlet, while the second gas concentration sensor may be located at a second half of the biomethanation reactor closest to the first fluid outlet. Those locations will provide an arrangement that is able to detect a failure in the methane production at an early state, which will provide a sufficient response time for the plant operators to take action on the impairment of the reactor performance. Additionally, those locations will provide an arrangement that provides a reliable gradient, where a clear impairment can be observed. Alternatively, the first gas concentration sensor may be located around the middle of the biomethanation reactor (e.g. between 30%-70%, preferably between 40%-60%, of the internal length of the biomethanation reactor), while the second gas concentration sensor may be located at or close to the first fluid outlet (e.g. between 70%-100%, preferably between 80%- 100%, of the internal length of the biomethanation reactor). Those locations will also provide an arrangement that is able to detect a failure in the methane production at an early state. If the second gas concentration sensor is located at the first fluid outlet, the second gas concentration sensor will be to tell the quality of the gas delivered to the grid and whether the gas delivered to the grid fulfils the necessary conditions.

The first and/or the second gas concentration sensor may be designed to at least be sensitive to hydrogen, and/or methane, and/or carbon dioxide, and/or carbon monoxide.

The system may further comprise at least one second fluid flow, where the fluid can be synthetic mediums of water with nutrients or organic-based liquids as e.g. sludge from biogas digesters, reject water from wastewater treatment plants, or other nutrient rich feedstock, with the objective of supplying nutrients and liquid for the microbes in the reactor. The methanation reactor may be configured for having the second fluid flow to flow between the first fluid inlet and the first fluid outlet. The methanation reactor may comprise a second fluid inlet and/or a second fluid outlet. The methanation reactor may be configured for having the second fluid flow to flow between the first fluid inlet and the first fluid inlet/the first fluid outlet. The methanation reactor may be configured for having the second fluid flow to flow between the first fluid outlet and the first fluid inlet/the first fluid outlet. The methanation reactor can be configured for the at least one second fluid flow between the second fluid inlet and the second fluid outlet defining a second direction or a second fluid line of the at least one second fluid flow. The second direction or the second fluid line may be along the second fluid flow or along a path of the fluid between the second fluid inlet and the second fluid outlet. The advantage of the second fluid inlet and the second fluid outlet is that the methanation reactor can be provided with a fluid comprising e.g. nutrients in a controlled way and at a controlled rate or amount. The nutrients and particularly the right amount or right flow of nutrients can provide a good environment for the microbes so that the microbes may consume CO2 and produce CH4 more efficiently. The nutrients may also be consumed by the microbes, so that the microbes will increase in numbers and so that the microbes may produce even more CH4. During methanation in the methanation reactor, water may be formed, which can be drained or removed from the methanation reactor through the first fluid inlet, the first fluid outlet, or the second fluid outlet.

The system may comprise a plurality of gas sensors configured to measure a plurality of gas concentrations at a plurality of gas sensor locations within the methanation reactor. The plurality of gas sensors may be arranged within the methanation reactor at a plurality of gas sensor locations. The gas sensor locations can be located at the first fluid inlet and/or the first fluid outlet of the methanation reactor, or be distributed along the methanation reactor, between the first fluid inlet and/or the first fluid outlet. The plurality of gas sensor locations can be evenly distributed along the methanation reactor but can also be at specific and/or random locations. Using the plurality of gas sensors the methanation process in the methanation reactor can be described in even more detail, since these sensors can measure reactants and products of reactions taking place in the biomethanation reactor.

The first gas sensor or the plurality of gas sensors may be positioned within the at least one first fluid flow. More generally, the first gas sensor or the plurality of gas sensors can be positioned within the methanation reactor, but the sensing part of the sensor(s) may be positioned within the at least one first fluid flow and/or the at least one second fluid flow and/or any other areas within the methanation reactor.

The plurality of gas sensors can be located at a first predefined distance and/or downstream to each other. They can also be located at a first predefined distance and/or upstream to each other. The direction of the location may depend on a position reference. More generally, the direction of the location may be defined according to the stream of the at least one first fluid flow or the at least one second fluid flow. It may be understood that the at least one first fluid flow or the at least one second fluid flow have their own flow direction within the methanation reactor. The first predefined distance can be characterized by the distance between the plurality of gas sensors. Preferably, the system may comprise a second and a third gas sensor that can be configured to measure a second and a third gas concentration at a second and a third gas sensor locations within the methanation reactor. More preferably, the first gas sensor may be located adjacent to the fluid inlet, the second gas sensor can advantageously be located centrally in the methanation reactor or can be located in the middle between the fluid inlet and the fluid outlet, and the third gas sensor may be located adjacent to the fluid outlet. By having three gas sensors located at three different gas sensor locations within the methanation reactor, a gas concentration profile of the methanation reactor can be obtained from the gas concentration measurements performed by the three gas sensors. The gas concentration measurements performed by the three gas sensors can enable warnings at an earlier state than measurements at the gas outlet alone and may be given by at least two of the three gas sensors. The early warnings may avoid disrupting the methanation process by taking correct actions with regulation of the at least one first fluid flow and/or the at least one second fluid flow.

The system may be configured to calculate a gas concentration distribution within the methanation reactor based on the first and the second gas concentration, preferably based on the first, the second and the third gas concentration, and more preferably based on the plurality of gas concentrations. By calculating the gas concentration distribution within the methanation reactor, the gaseous substrate concentration gradients throughout the different sections of the methanation reactor can be profiled. This establishment of reactor concentration profiles may enable an evaluation of the responses from different operating strategies and hereby enable fast detection of any changes in reactant or product formation hereby giving early warning signals to allow any correctional actions to be taken. The gas sensor(s) can be selected from the group of sensors monitoring the substrate concentrations as hydrogen sensors, CO2 sensors, CO sensors and/or combinations thereof and from the sensors monitoring the product gases as CH4 sensors. CO is an alternative carbon substrate source that may be used for monitoring instead of CO2. Preferably, the gas sensor(s) can be hydrogen sensors, such as hydrogen microsensors. More preferably, the gas sensor(s) can be selected from the group of any sensors that would be suitable for measuring gas concentrations of gas that can potentially be comprised within a methanation reactor. Even more preferably, the gas sensor(s) may be selected from the group of any sensors that would be suitable for measuring gas concentrations of gas that would give useful information for controlling a methanation reactor.

Fig. 1 shows an embodiment of the system according to the present disclosure, where the system is configured to regulate a first fluid flow 103, being a gas fluid flow in this embodiment (biogas and H2 supply) into and out of a reactor 101 by a first fluid inlet 105 and a first fluid outlet 116, respectively, and a second fluid flow 104, which can be a liquid medium flow in this embodiment, into and out of the reactor 101, by a second fluid inlet 106 and a second fluid outlet 115, respectively, As shown in Fig. 1, the system comprises a methanation reactor 101 , a first fluid flow 103, which can be controlled by a mass flow controller 108, a second fluid flow 104, and a first gas sensor 102, a second gas sensor 112, and a third gas sensor 113 arranged within the methanation reactor. The first gas sensor 102 is advantageously arranged close to the first fluid outlet 116 of the reactor 101 if multiple sensors are installed to allow for control of the methanation process and outlet gas concentration. The second gas sensor 112 and third gas sensor 113 are advantageously arranged at or close to the middle of the methanation reactor with a certain space to monitor the concentration gradient of reactants or products in the gas stream. By arranging the second gas sensor 112 and the third gas sensor 113 at or close to the middle of the methanation reactor, a sufficient response time to respond to change within the biomethanation conversion activity can be provided. A liquid medium pump 110 drives or circulates the second fluid flow 104. One of the gas sensors, like e.g. the third gas sensor 113 can control the liquid medium pump 110 to regulate the supply rate of nutrient to the reactor in case of deficiency. The second gas sensor 112 controls the recirculation and replacement of fresh medium in the second fluid flow to alleviate acid accumulations and provide replenished nutrients to the reactor. A first gas sensor 102 controls the first fluid flow 103 by controlling the mass flow controller 108 with object of changing the rate and ratio of the gas substrate load to retain the product quality and limit the gas substrate availability for acid producing microorganisms. The first gas sensor 102 can deliver data, i.e. data about gas concentrations, for controlling the mass flow controller 108, for example through a data-directed control. A fresh medium reservoir 107 is arranged in order to supply fresh medium or nutrients to the second fluid flow 104 with the purpose of both supplying fresh nutrients and exchanging acidified medium to the recirculating second fluid flow 104. The liquid medium pump 110 is configured to make the second fluid flow 104 circulate within the methanation reactor 101 with a predefined flow. The first fluid flow 103 is composed of biogas and H2 gas, which is supplied from a biogas and H2 supply 114, and then regulated by the mass flow controller 108. The regulated flow of the first fluid flow 103 is then injected into the methanation reactor 101 through the first fluid inlet 105. Biomethane of natural gas grid quality is produced within the methanation reactor 101 and collected at the first fluid outlet 116, where it can be injected to the natural gas grid 111. Some valves can be implemented to control the different flows and possibly redirect flows in different directions, e.g. in the second fluid flow, where a three-directional valve 109 is arranged to potentially extract a sample of the second fluid flow in the fresh medium reservoir. Fig. 1 is an example of an embodiment of the system according to the present disclosure and not limiting to the presently disclosed system. For instance, the methanation reactor 101 may comprise more than three gas sensors, as previously described, and the locations of the gas sensors may not necessarily be the same as shown in Fig. 1 .

The first, second, and third gas sensors can be positioned differently compared to in Fig. 1. The third gas sensors can e.g. be close to the first fluid inlet 105 of the methanation reactor 101. With reference to Fig. 1 , the third gas sensor 113 can control the liquid medium pump 110, the second gas sensor 112 can control the recirculation of the second fluid flow, and the first gas sensor 102 can control the first fluid flow 103 by controlling the mass flow controller 108. However, alternatively, the third gas sensor 113 can control the recirculation of the second fluid flow, or the first fluid flow 103 by controlling the mass flow controller 108; the second gas sensor 112 can control the liquid medium pump 110, or the first fluid flow 103 by controlling the mass flow controller 108; and/or the first gas sensor 102 can control the liquid medium pump 110 or the recirculation of the second fluid flow.

The sensors may also control different features and not necessarily the ones described in Fig. 1. The sensors can output information that may be used by a processing unit, that may control the liquid medium pump and/or the mass flow controller based on the information received by the sensors.

The second inlet 106 and/or the second outlet 115 may be omitted. The second fluid flow can be provided through the first inlet or the first outlet into the methanation reactor. In that case, the second fluid flow is not necessarily flowing but can be still standing.

In one embodiment, the system further comprises a heating system configured for heating a pre first fluid flow, which is the first fluid flow before the first fluid flow enters the methanation reactor through the first fluid inlet. By heating the pre first fluid flow before injecting the pre first fluid flow into the methanation reactor through the first fluid inlet, the first fluid flow will not provide a chilling effect inside the methanation reactor. The absence of the chilling effect creates an increase in the internal temperature of the methanation reactor close to the first fluid inlet and allows the methanation to start closer to the first fluid inlet,. Generally, the pre first fluid flow may comprise gas, that can be stored in a compressed form or in a liquefied form. The pre first fluid flow is then decompressed, which makes the pre first fluid flow cold. A cold first fluid flow would decrease the chance of having a methanation as close as possible to the first fluid inlet, since a cold first fluid flow would decrease the efficiency of the methanation within the methanation reactor. In addition, the a cold first fluid flow would have a chilling effect on a temperature sensor positioned close to the first fluid inlet, and temperature sensor will provide the true internal temperature even if the temperature sensor is positioned close to the first fluid inlet. The heating system can be a resistive heater or can be using combustion of a combustible fluids for heating the pre first fluid flow.

In an embodiment, the heating system is a heat exchanger, wherein the heat exchanger is configured such that heat generated by the methanation reactor is transferred to the pre first fluid flow. The heat exchanger may be a mechanical device or system used to transfer heat from one medium to another. A primary function of the heat exchanger can be to efficiently exchange thermal energy between two fluids or substances at different temperatures, without allowing them to mix. By using the heat produced by the methanation within the methanation reactor, no extra energy need to be provided to the system in order to heat the pre first fluid flow. The second fluid flow after having been heated inside the methanation reactor may provide the heat from the methanation reactor to the pre first fluid flow.

A cooling fluid may be comprised in the system disclosed, preferably in the methanation reactor or in close contact with the methanation reactor, in order to regulate an operating temperature of the methanation reactor. The cooling fluid can be any type of fluid that can regulate the operating temperature of the methanation reactor. The cooling fluid may be typically a liquid or a gas, which is used to generally reduce or regulate the temperature of a system. An ideal cooling fluid has high thermal capacity, low viscosity, preferably low-cost, non-toxic, chemically inert and neither causes not promotes corrosion of the cooling system. The cooling system may be a circuit where the cooling fluid is circulated with the help of a pump, and where the cooling fluid may be circulated in a heat exchanger. A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes, but in the present disclosure, this is preferably used as in cooling processes, since the methanation process is generally exothermic. The circuit may comprise the first fluid inlet and/or the first fluid outlet for letting the cooling fluid into and/or out of the methanation reactor. The circuit may circulate the cooling fluid through the methanation reactor in the same direction or in the opposite direction as the first fluid flow.

The methanation reactor may comprise a third fluid inlet and/or a third fluid outlet for the circuit, where the third fluid inlet and/or the third fluid outlet are designed for only letting cooling fluid into and/or out of the methanation reactor, so that the circuit has its own inlet and/or outlet. In this way, the flow of the cooling fluid can be controlled very well without interference of any other fluid and/or of the flow of any other fluid.

The cooling fluid of the circuit may also enter and/or leave the methanation reactor by the second fluid inlet and/or the second fluid outlet and hereby have direct contact with the microorganisms and packed bed material of the reactor to hereby allow direct heat transfer to the fluid. The cooling fluid may comprise nutrients, so that the cooling fluid is both cooling the methanation reactor and providing the microbes with nutrients.

A plurality of temperature sensors may be configured to measure a plurality of temperatures at a plurality of temperature sensor locations within the methanation reactor. Hydrogenothropic methanogenesis, i.e. biomethanation or more generally methanation, is a strongly exothermic process that generates heat for each CO2 and 4 H2 molecules that reacts, which is around -165 kJ/mol under standard conditions. The amount of generated heat is thus an indirect measure for biological activity and the biomethanation reaction rate of the biological catalyst. By measuring the relative temperature between the plurality of temperature sensors within the methanation reactor, the spatial distribution of the biological activity within the methanation reactor can be determined. The spatial distribution of the biological activity within the methanation reactor can be determined and can be used for monitoring any changes in the biomethanation reactor, like e.g. a decrease in the methanation production. This may be especially relevant in systems where reactor heat is supplied wholly or partly by the metabolic reaction heat originating from the biomethanation process. The system may comprise a first and a second temperature sensors that can be positioned within the at least one first fluid flow and/or the at least second fluid flow and/or within the cooling fluid. The first and the second temperature sensors can be located at a second predefined distance and/or downstream to each other. Downstream may mean that they are located one after another, following the flow stream of either the at least one first fluid flow or the at least one second fluid flow. The first and second temperature sensors can also be located upstream to each other.

Preferably, the system may comprise between three and ten temperature sensors that can be configured to measure the temperature at different temperature sensor locations within the methanation reactor. The different temperature sensor locations can be located at different locations within the methanation reactor. They can either be close to each other relative to the dimension of the methanation reactor or they can preferably be distributed along the methanation reactor with the same distance between each other, or distributed in any other way, preferably within the methanation reactor. More temperature sensors distributed within the methanation reactor will provide a detailed description of the methanation process, so that the operator of the methanation reactor can optimise the methanation process even better.

A temperature distribution within the methanation reactor can be calculated by the system based on the first and second temperatures, and preferably based on the plurality of temperatures. In methanation reactors with at least one fluid flow, the overall performance and spatial distribution of biological activity can be obtained by measuring at least two separated temperatures within the methanation reactor. Preferably between three to ten temperature sensors can be applied for a better calculation of the temperature distribution by the system. The absolute temperature difference between high and low performing biological activity may depend on specific methanation reactor design and management, but the relative temperature difference is an indirect measure between the temperature sensors arranged within the methanation reactor. It may be expected that the temperature would be the highest in the first fluid inlet, but should be almost uniform along the methanation reactor, in the first direction. The temperature may decline towards the first fluid outlet due to reduced biological activity because of substrate limitation, unless mass transfer of reactants to the methanogenic layer of microorganism (i.e. biofilm) is rate limiting. In a methanation reactor with a temperature at the first fluid inlet being around 45-55 °C, the methanation reactor temperature could increase to around 10-20 °C above the first fluid inlet, preferably because of a healthy and active biocatalyst. In the other hand, if the temperature is declining more than 1- 2 °C or 3-5 °C in direction with the first fluid flow already with the first 1/3 of the methanation reactor, or 1/4 or 1/5, it may be a sign of underutilized capacity. Other patterns of temperature differences could signify impaired biological activity due to other reasons than substrate limitation. The other reasons can be nutrient limitation or other kind of inhibition or biomass decay. The temperature measurements can also be used to prevent local overheating of the reactor by reducing substrate supply.

Temperature measurements or input from the temperature sensors may also be used to prevent local overheating of the methanation reactor by adjusting the first and second fluid flows.

The system may further comprise at least one processing unit configured to receive an input from the one or more gas sensor(s) and/or from the two or more temperature sensor(s), and further potentially configured to calculate a gas concentration distribution or conversion rate within the methanation reactor, based on the input from the one or more gas sensor(s) and/or a temperature distribution within the methanation reactor, based on the input from the two or more temperature sensor(s).

Fig. 2 shows a schematic view of an embodiment of the process and instrumentation schematics of a methanation reactor 101 and a monitoring setup. The monitoring setup comprises a processing unit 201 , where the inputs from a first gas concentrations sensor 207, a second gas concentrations sensor 208, and a third gas concentrations sensor 209, and/or a first pressure sensor 210 and a second pressure sensor 211 and/or a temperature sensor 212 are displayed and/or potentially processed. The temperature sensor 212 can give indications of increase or decrease in reaction/process rate. A decreasing temperature would indicate a reduced rate while an increasing temperature would indicate an increasing rate. A biogas digester 202 provides biogas as a part of the at least one first fluid flow 103, and pressured H2 bottles or H2 produced from water electrolysis using electricity from renewable sources 203 provide H2 as another part of the at least one first fluid flow. The biogas digester output is controlled by a first pump like a peristatic pump 204, which is controlling the flow of the biogas fed into the at least one first fluid flow 103. A mass flow controller 205 controls the H2 flow coming from the pressured H2 bottles, fed into the at least one first fluid flow. Two unidirectional valves 206 ensure that the biogas and/or the H2 is not fed back into either the biogas digester or the pressured H2 bottles. The first gas concentrations sensor 207, the second gas concentrations sensor 208, and the third gas concentrations sensor 209 are arranged within the methanation reactor 101. The first gas concentration sensor 207 is arranged close to the first fluid outlet 116 or the second fluid inlet 106, the second gas concentration sensor 209 is arranged close to the first fluid inlet 105 or the second fluid outlet 115 and the third gas concentration sensor 208 is arranged substantially at the middle of the methanation reactor 101 . The first pressure sensor 210 and the second pressure sensor 211 are arranged within the methanation reactor. The first pressure sensor 210 is arranged close to the first fluid outlet 116 or the second fluid inlet 106 and the second pressure sensor 211 is arranged close to the first fluid inlet 105 or the second fluid outlet 115. The pressure sensors are arranged to allow for calculating the resistance of the first fluid flow through the reactor. The temperature sensor 212 is arranged substantially at the middle of the methanation reactor 101. A liquid sampling port 213 is arranged in order to collect some sampling of the second fluid flow. A media solution 214 provides the second fluid flow, which is injected in the methanation reactor through the second fluid inlet 106, thanks to a second pump like a peristatic pump 215. The monitoring setup collects information or measurements from the pressure sensors and the gas concentration sensors. A product gas 216 from the methanation process of the biogas and the H2 gas in the methanation reactor is collected at the first fluid outlet 116.

Fig. 2 is an example of an embodiment of the system according to the present disclosure and not limiting to the presently disclosed system.

A control algorithm may be developed based on prior and current temperature distributions within a methanation reactor, preferably over days or weeks, in order to determine a method for the processing unit to act on controllable outputs depending on the inputs from the different sensors within the methanation reactor.

The at least one controller may also be operated by a local user interface. The local user interface may be any interface that a user can control and manage. With this local user interface, a user may be able to control the different features of the system, preferably the controllers for the at least one first fluid flow and/or the at least one second fluid flow, and/or the circuit. The manual control of the fluid flows can be an option in order to shut down, increase or reduce the at least one first fluid flow and/or the at least one second fluid flow, and/or the circuit, if something critical may happen in the methanation reactor.

The methanation reactor can be a gas-phase reactor, such as a co-current or countercurrent trickle-bed reactor. A trickle-bed reactor is a chemical reactor and/or a bioreactor that uses a continuous or periodic downward movement of a liquid and a downward or upward movement of a gas over a packed bed of carrier materials. Preferably, the surfaces of the carrier materials are covered by microorganisms which catalyze the conversion of H2 and CO2 as part of the biological reaction processed in the trickle-bed reactor. A counter-current trickle-bed reactor has a downward movement of liquid while the gas has an upward movement. On the contrary, a cocurrent trickle-bed reactor has a downward movement of gas while the liquid has also a downward movement.

The methanation reactor may have an elongated chamber that can be advantageously arranged horizontally or vertically. The elongated chamber may be arranged with any angles comprised between a horizontal and a vertical position. The elongated chamber will provide a relatively long and narrow channel, where the first fluid flow will be exposed to a long stretch of the microbes, which means that the amount of first fluid flow fed into the methanation reactor as well as the product gas delivered by the methanation reactor can be high, thus providing an effective methanation reactor. Furthermore, the elongated reactor column supports the monitoring of the gradual conversion efficiency that describes the spatial distribution of the biological activity.

The at least one first fluid flow may be either a gas flow and/or a liquid flow. The at least one first fluid flow into the methanation reactor may comprise CH4, CO2, H2S, H2, CO, biogas, nutrient medium and/or any combinations thereof. Biogas may be defined as a mixture of gases, primarily consisting of CH4 and CO2, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and/or food waste. Biogas is produced by anaerobic digestion with anaerobic organisms, including methanogens, inside an anaerobic digester, biodigester or a bioreactor. Nutrient medium can be defined as a fluid substance that may be used for growing bacteria, yeasts or microscopic fungi, as well as algae, protozoans, viruses, and cultures of plant or animal cells. Nutrient medium can also be more simply defined as a substance used for the cultivation, isolation, identification, or storage of microorganisms. The at least one first fluid flow may preferably be biogas, hydrogen, flue gas, syngas, purified CO2 or nutrient medium. Flue gas may be defined as the gas exiting to the atmosphere via a flue, which is a pipe or a channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. The flue gas may refer to the combustion exhaust gas produced at power plants. Its composition can depend on the fuel being burned, but it may usually consist of nitrogen derived from the combustion along with carbon dioxide, and water vapor as well as excess oxygen, which is also derived from the combustion air. It may further contain a small percentage of a number of pollutants, such as particulate matter, carbon monoxide, nitrogen oxides and/or sulfur oxides. A typical flue gas from the combustion of fossil fuels may contain very small amounts of nitrogen oxides, sulfur dioxide and particulate matter. The particulate matter is composed of very small particles of solid materials and very small liquid droplets.

The system may further comprise one or more pressure sensor(s) configured to measure the pressure(s) at one or more pressure sensor location(s) within the methanation reactor. Preferably, the system can comprise between two and ten pressure sensors configured to measure the pressure(s) at different pressure sensor location(s) within the methanation reactor. The system may comprise two pressure sensors mounted adjacent to the first fluid inlet or the second fluid outlet, and two pressure sensors are mounted adjacent to the first fluid outlet or the second fluid inlet. The pressure sensors give information on the resistance of the packed bed material to the passage of the first fluid flow through the reactor. The methanation reactor has an internal length, and the pressure sensors at the first fluid inlet and/or the pressure sensors at the first fluid outlet are located at a distance to each other of between 1 % and 50 % of the internal length, more preferably between 5 % and 30 % of the internal length, such as 15 % of the internal length. The at least one controller may be configured to regulate at least one first fluid flow into the methanation reactor based on the first gas concentration, the first and the second temperature, and/or the pressure(s), preferably on the plurality of gas concentrations and/or the plurality of temperatures. The system can also be configured to vary the at least one first fluid flow in time, also depending on the first gas concentration, the first and the second temperature, and/or the pressure(s), preferably on the plurality of gas concentrations and/or the plurality of temperatures.

By regulating the first fluid flow of the fluid to be methanated, the first fluid flow can always be regulated to the most optimal flow and highest production capacity, where the product gas still has the quality necessary according to national and international gas grid standards.

The at least one second fluid flow can be driven by a pump, and the pump and/or a valve may be configured to vary the at least one second fluid flow. As shown in Fig. 1 and/or in Fig. 2, a liquid medium pump can control the flow of the at least one second fluid flow. This at least one second fluid flow can be either manually controlled by a user through a local user interface, or it can be automatically controlled by a secondary system configured to receive inputs from the first gas concentration, the first and the second temperature, preferably on the plurality of gas concentrations and/or the plurality of temperatures in order to regulate the at least one second fluid flow accordingly.

If the sensor(s) tell(s) that the concentration of H2 and/or CO2 at the first outlet is too high and/or the concentration of CH4 at the first outlet is too low compared to the gas grid quality gas requirements, then the system can reduce the first fluid flow to the optimal flow or discontinue the first fluid flow altogether, so that the problem causing the non-optimal condition can be removed. If the sensor(s) tell(s) that the concentration of H2 and/or CO2 at the first outlet is below and/or the concentration of CH4 at the first outlet is above the gas grid quality gas requirements, the first fluid flow can be increased, then the system will increase the first fluid flow to the optimal flow.

The system can be configured to increase or decrease the at least one second fluid flow at times to optimize the methanation process. The at least one second fluid flow can be preferably a nutrient solution, that may be configured to preferably counteract the acidification of the methanation reactor, and therefore optimize the methanation process. The at least one second fluid flow may be a flow of nutrient solution, such as a synthetic or organic nutrient solution. The second fluid flow is designed to add nutrients to the methanogenic microorganisms in the reactor and also regulate the pH and concentration of microbial metabolites like organic acids in the reactor. One example of a synthetic nutrient solution can comprise NH4CI, EDTA, MgCh-6H2O, FeCh-6H2O, C0CI2, Na2MoO4'2H2O, NiCh and/or Na2S. This example of a synthetic nutrient solution is only one variant. Many other solutions are possible to use, and also more complex solution based on organic fractions from i.e. waste streams (Digestate, wastewater, etc).

The at least one second fluid flow can be used to supplement essential nutrients within the methanation reactor. The at least one second fluid flow may preferably be injected into the methanation reactor as droplets, which may allow a sufficient distribution of the nutrients on the packing material inside the methanation reactor. When operating the methanation reactor, a large daily volume of metabolic water can be produced. This metabolic water may be collected and removed from the system to avoid dilution of nutrients in the liquid medium, or more generally in the at least one second fluid flow. During normal operation, the at least one second fluid flow may preferably be used to remove the accumulation of acids in the methanation reactor. The at least one second fluid flow may not necessary be constant, and may be disrupted if needed.

The system may further comprise a pH sensor for measuring a pH value of the at least one second fluid flow. Preferably, the system may comprise a plurality of pH sensors for measuring a plurality of pH values of the at least one second fluid flow. The measurement of the pH of the at least one second fluid flow can give an indication of the amount of acids in the substrate depleted areas of the methanation reactor, therefore this would give an indication of a non-optimal operation of the methanation reactor. If the pH sensor(s) indicate(s) a too low pH value, the second fluid flow can be controlled to flow through the methanation reactor to flush away the acids.

In another aspect, a method for controlling a methanation reactor is disclosed. The method for controlling a methanation reactor may comprise the steps of providing at least one first fluid flow between a first fluid inlet and a first fluid outlet of a methanation reactor defining a first direction of the at least one first fluid flow; measuring a first gas concentration at a first gas sensor location within the methanation reactor, and/or a first temperature and a second temperature, respectively, at a first and a second temperature sensor locations within the methanation reactor; and regulating the at least one first fluid flow into the methanation reactor based on the first gas concentration and/or the first and the second temperature.

The method may comprise a step of providing at least one second fluid flow into the methanation reactor. The at least one second fluid flow may be a gas flow and/or a liquid flow. The at least one second flow can be preferably a nutrient solution, that may be configured to preferably counteract the acidification of the methanation reactor, and therefore optimize the methanation process.

The at least one second fluid flow can be increased if the first gas concentration is above a first gas concentration level. The at least one second fluid flow can also be disrupted. By increase, it may also be understood that the at least one second fluid flow can be at a constant fluid flow, but that the frequency of disruption can be reduced, such that an overall flow over a given period of the at least one second fluid flow is increased. The first gas concentration level can be determined based on empiric observations on a methanation reactor during a methanation process that would give an indication that some actions are required on the at least one first and/or second fluid flow to avoid critical events in the methanation reactor.

The method may comprise a step of reducing or shutting off the at least one first fluid flow, if the first gas concentration is above a second gas concentration level. The second gas concentration level can be determined based on empiric observations on a methanation reactor during a methanation process that would give an indication that some actions are required on the at least one first fluid flow to avoid critical events that would make the methanation reactor unusable for a relatively long period of time.

Example 1

Trickle bed reactor configuration

A methanation reactor system configured as a 10 L trickle bed reactor (TBR) system was set up to facilitate the ex-situ biomethanation process, which comprised of a custom-made column in stainless-steel with an inner diameter and height of 10.5 cm and 105 cm respectively of the active bed (Landia A/S, Denmark). The active bed of the TBR was filled with 2.08 kg of expanded clay pellets, Filtralite® Nature (Leca, Denmark), with a size range of 2 to 10 mm for packing material. The packing material had a vendor quoted density of 250 kg nr ³ and the active bed constituted a volume of 9.09 L. The system was operated with thermophilic conditions at 54°C, which was maintained by a 10 W rrr ¹ trace heating cable (RS Pro Denmark) coiled around the reactor. The regulation of the temperature was controlled by the input readings from a PT100 temperature sensor (Correge, France) located in the middle of the TBR, which was sent to an ON/OFF regulator. The at least one first fluid flow comprising gas substrates, biogas and H2, were supplied in the bottom of the TBR to establish a counterflow configuration of the at least one second fluid flow, being downwards trickling liquid, and the at least one first fluid flow, being upwards moving gases. A relief valve was mounted on the top of the reactor as a safety precaution, which would open at 1.4 bar. In addition to the inlets and outlets for liquid and gas in the top and bottom of the TBR, the column had three ports for sampling and sensors at each section of the TBR: At the bottom (12.5 cm into the bed), the middle (52 cm into the bed) and in the top (91.5 cm into the bed).

Two pressure sensors were installed with 80 cm spacing at the bottom and top position in the TBR to monitor the pressure profiles of the reactor continuously (31 IS, GEMS Sensors & Controls, USA). Additionally, three hydrogen or more generally gas concentration sensors were installed in the top, middle and bottom section of the reactor as further explained. The schematics of the process and instrumentation are presented in Fig. 2.

Inoculation

The clay-based carrier material occupying the bed of the reactor was irrigated at a rate of 17.6 L IT ¹ for 2 hours prior to inoculation to remove dust and loose particles. Subsequently, the liquid was drained from the trickle bed reactor and the carrier materials were left to dry for 7 days to ensure that the liquid with microbes from the inoculum could fill the pores of the carrier material during the inoculation. The sludge containing the microbial culture used to inoculate the trickle bed reactor was derived from a 1200 m ³ manure-based primary anaerobic digester (Foulum, Denmark) operated with thermophilic conditions. The digester sludge was screened to remove any particles and fibres with a diameter above 250 pm that could cause the orifice in the trickle bed reactor to clog and be used as carbon source for the microbes. The screened sludge used for inoculation was characterised by a dry matter concentration of 41.6 ± 0.5 g kg ^-1 , a volatile solid concentration of 28.4 ± 0.3 g kg ^-1 , a pH of 7, a total volatile fatty acid (VFA) of 73.7 mg L’ ¹ , where 44.7 mg L' ¹ constituted of acetate. The trickle bed reactor was then flushed with H2 and raw biogas to remove any oxygen present in the reactor system and create an anoxic environment. When an anoxic environment in the reactor had been established, a volume of 2L of the screened sludge was circulated through the column by the irrigation system for 5 hours at a rate of 17.6 L h' ¹ .

Liquid nutrient supply

At least one second fluid flow comprising a synthetic nutrient solution was used to supplement the most essential nutrients. The composition of the nutrient solution contains 2139.6 mg L' ¹ NH ₄ CI, 43.8 mg L' ¹ EDTA, 40.7 mg L' ¹ MgCI ₂ ■ 6 H ₂ O, 6.8 mg L’ ¹ FeCI ₂ ■ 6 H ₂ O, 0.13 mg L' ¹ CoCI ₂ , 0.24 mg L' ¹ Na ₂ MoO ₄ ■ 2 H ₂ O, 0.13 mg L' ¹ NiCI ₂ and 23.4 mg L' ¹ Na ₂ S. The liquid nutrient solution was supplied to the trickle bed reactor with a peristaltic pump (WT3000-1JA Micro Gear, Longerpump, China). A topmounted cone spray nozzle with a diameter of 1.0 mm was installed to introduce the liquid nutrients to the system as droplets, which allowed a sufficient distribution of the nutrients on the packing material. The nutrient medium was irrigated with a gradual liquid flow of 278 mL min ^-1 for 3 min followed by a flow of 578 mL min ^-1 for 1 min, which was repeated 3 times. The changes in liquid flow allowed non-uniform liquid distribution to avoid the extent of liquid channelling. Additionally, the flowrates of the nutrient medium allowed the nutrient medium in the 1 L reservoir to be circulated 4.2 times, which should allow a frequent distribution of the active methanogenic microbes from the inlet to the entire bed. During the initial reactor inoculation and operation (day 1 to 93), the nutrient medium was sprinkled over the bed 2 times per week with an interval of 3 and 4 days respectively between sprinkling to accommodate the reduction of mass transfer restraining liquid in the TBR. However, after 93 days of operation, the sprinkling strategy was changed to sprinkling once per day to remove the accumulation of acids in the substrate depleted areas of the TBR. Operating the TBR created a large daily volume of metabolic water, which were collected in the bottom of the reactor and removed from the system to avoid accumulations of acids and dilution of nutrients in the liquid medium.

Gaseous substrate feeding

Raw untreated biogas was supplied to the TBR system by a multichannel peristaltic pump (LabV1 -I I Schenken, China) supplied from the headspace gas from a 3400 m ³ secondary manure-based anaerobic digester (Foulum, Denmark). The raw untreated biogas injected in the TBR had an average composition of 54.5 ± 1.2% CH4, 45.5 ± 1.2% CO2 and 1208.4 ± 103.6 ppm H2S during the experimental period based on weekly measurements of the biogas composition. The H2 was supplied by H2 cylinders (Air Liquide Denmark) with a flow controlled by a mass flow controller based on thermal conductivity (SLA5850, Brooks Instruments, USA). The gaseous substrates of H2 and raw untreated biogas were initially aimed to be supplied to the trickle bed reactor in a 4:1 ratio between the bottled H2 and the CO2 from the raw untreated biogas. However, the raw biogas was supplied directly from the large anaerobic digester headspace, which distorted the stochiometric ratio of H2:CC>2 due to variations in the biogas composition. The feed ratio was therefore increased at day 4 to aim for 4.1 :1 to ensure full conversion of the biogenic CO2, which provided an average feeding ratio as depicted in Table 1.

Table 1 : Different periods of operation with varying process parameters during the experimental period.

Name Period I Pause period Period II Period III

Low stable No feeding Re- High stable feeding establishment feeding of stable conversion

Period duration [Day] 1-51 52-79 80-87 88-135

Gas feeding H ₂ : 14.48 H ₂ : 0 H ₂ : 14.48 H ₂ : 52.58

I

^L L L ^-1 d ^-1 l J Biogas: 7.73 Raw biogas: 0 Biogas: 7.73 Biogas: 27.80

Average feeding ratio 4.03:1 0 4.01 :1 4.15:1

[H ₂ :CO ₂ ]

Sprinkling frequency 2 times a week No sprinkling 2 times a week Daily sprinkling

Temperature [°C] 54 25 54 54

Analytical methods

Gas samples were collected 2-3 times per week from the gas feed and the product gases from both the outlet and sampling port 1 , 2 and 3. The gas composition was analysed using a gas chromatograph fitted with a thermal conductivity detector (Agilent Technologies 7890A, USA). The gas constituents were separated on a CTR1 double column (Alltech, USA), which utilised argon as carrier gas. Liquid samples were collected daily, where the pH was monitored with a pH-meter (Portavo® 902 PH, Knick), while the volatile fatty acid (VFA), dry matter (DM) and volatile solids (VS) concentrations were measured at a frequency of 3 times per week for VFA and pH, and once per week for DM and VS. The DM and VS were determined based on gravimetric methods according to the EPA standard method. The VFA concentrations (C1-C8) of the reactor liquids, inoculum and liquid in carrier materials were quantified with a gas chromatograph (7890A, Agilent Technologies, USA) equipped with a HP-INNOVAX column (Agilent Technologies, USA) and a flame ionization detector, where helium was used as carrier gas. The liquid samples were prepared by mixing 1 mL of liquid sample with 4 mL of 0.3 M oxalic acid containing an internal standard of pivalic acid for 10 min on a rotation mixer. Following the mixing, the solid fraction was removed by centrifugation for 10 min at 4500 RPM, where the supernatant was analysed with the GC. The same procedure was conducted for the VFA in clay pellets, Filtralite® Nature (LECA, Denmark), but with an extraction step. To extract the VFA from the carrier material, 1 mL of demineralised water was mixed with 1 g of carrier material for 10 min. The dilution of the acids was calculated based on the DM concentration.

Continuous H2 concentration monitoring with H2-sensors

Novel H2 microsensors was used to monitor the hydrogen concentration at locations in the top, middle and bottom of the reactor to shed light on the black box of the TBR to create concentration profiles of the reactor. Classical microsensors are typically marked by the presence of H2S, a biproduct from the anaerobic digestion, which was present in the raw biogas at a concentration of 1208.4 ± 103.6 ppm. Specially designed H2 microsensors from Unisense A/S with a H2S guard were therefore utilized to ensure that continuous measurements could be achieved in a H2S rich environment. The output from the H2 microsensors was continuously validated at with point measurements for gas chromatography in the different sections to ensure the precision of the response, as they were installed in the harsh environment with high concentrations of H2S.

Hydrogen concentration profiles were created continuously throughout the entire experimental period with data sampling every 1 min, which allowed the examination and monitoring of the different reactor responses based on specific operation strategies. The sensors provided a high precision, as they revealed that the dynamic system responded promptly, and for a prolonged period, on different operation events as for example sprinkling. This would be a shortcoming by the point measurements conducted with a GC, which could show distorted results depending on the time of measurement.

Results and discussion

Advancement in process control and operation by reactor profiling

The operation of the trickle bed reactor was examined in three separate periods of 51 days (Period I: day 1-51), 7 days (Period II: day 80-87) and 47 days (Period III: day 88- 135) with a shutdown period of 29 days (day 52-79) in between period I and II. An overview of the key parameters that was evaluated has been provided in Table 2.

Table 2: Overview of the key parameters in the different operational periods.

Period Interval Maximum CH4 purity Average CH4 Average pH

[days] productivity (at maximum productivity

[L L- ¹ d- ¹ ] productivity) [L L- ¹ d ¹ ]

Period I 1 -51 3.52 >99 3.18±0.71 6.18±1.01

Pause period 52-79 0 - 0 7.35

Period II 80-87 3.52 >98 2.87±1.11 6.87±0.56

Period III 88-135 12.64 (initially, >99 initially 10.85±1.31 6.70±0.47 day 88-91 ). (day 88-91 )

8.40 (During 53.5% during acidification, acidification day 97). (Day 97)

12.31 (After 96.6% after recovery, day recovery (day

120). 120)

For period I, the substrate load was kept at a constant rate to map the expansion of the methanogenic activities in the TBR and the associated substrate gradient of H2 that it would induce (Figure 2).

Fig. 3 shows an embodiment of a plurality of hydrogen substrate concentration profiles at different vertical positions in a methanation reactor 101 or TBR. A first graph 311 shows the hydrogen concentration measured over time by the first gas concentration sensor 301 or hydrogen concentration sensor substantially arranged close to the first fluid inlet for determining the gas concentration up to 12.5 cm from the fluid inlet 305. A second graph 312 shows the gas concentration measured over time by the second gas (hydrogen) concentration sensor 302 substantially arranged close to the middle of the methanation reactor for determining the gas concentration between 12.5 cm and 51.5 cm away from the fluid inlet 305. A third graph 313 shows the gas concentration measured over time by the third gas (hydrogen) concentration sensor 303 substantially arranged close to the fluid outlet 316 for determining the gas concentration between 51.5 cm and 91.5 cm away from the fluid inlet 305. A steep and steady decline in the H2 concentrations were observed in both the second graph 312 and the third graph 313 of the TBR in the first 4 days of operation after inoculation of the TBR. The observed gradual decrease in H2 concentration during the operation, also reported in Fig. 3, was expected to be a result of the biofilm formation and enrichment of the mixed microbial culture. On day 7, gas chromatography analyses of the gas concentration revealed that all of the CO2 was converted, while the H2 available for conversion at the third hydrogen concentration sensor 303 was diminished to a concentration of < 2% as shown by the third graph 313 reaching a purity of the product gas at the fluid outlet 316, which meets the natural gas grid quality requirements in Denmark. The subsequent variations were originating mainly from the fluctuations in the composition of the raw biogas from the biogas plant due to varying amounts of organic matter added to the anaerobic digester supplying the raw biogas to the methanation reactor, which thereby varied the substrate feeding ratio. The trend of the improving conversion continued, and on day 20, the concentration of available H2 for conversion decreased to < 2% at the second hydrogen concentration sensor 302 as the second graph 312 shows.

An operation strategy that can be applied for biological methanation in TBRs is to follow a stepwise increase of the substrate load. The loading rate would be increased to a level, where the product gas still maintained natural gas grid qualities, which would result in an extended period of operation at that substrate loading rate. However, as the experimental data suggest, the inherent dynamic of the biological system leads to the growth of biofilm, which consequently gradually displaces the conversion of H2 and CO2 to the product gas to occur closer to the substrate inlet supply. Monitoring the H2 at strategical positions in the vertical reactor would thereby allow a more controlled ramp-up period with a load increased initiated on an informed basis instead of the random and strictly periodic substrate load increases that generally are applied.

Overall and local production capacities in the trickle bed reactor

During period I, a maximum production capacity of 3.52 L L' ¹ d ^-1 with a corresponding methane purity of > 99% was achieved. Subsequent to the pause period, similar gas substrate feed rates were commenced, and after 2 days of operation, the previous overall methane productivity had been recovered with a purity of > 99%. With the knowledge provided by the on-line monitoring of the H2 concentration, the feed rate was increased and optimized at the onset of period III (day 88) to a rate, where all the compartments were supplied with the substrate while the gas quality was maintained. The increment of the gas substrate feed by a 3.6-fold displayed a rapid adaption of both the active and substrate depleted compartments. The methane purity in the outlet maintained the Danish gas grid quality requirements of a methane purity above >98% with an overall methane productivity of 12.64 L L' ¹ d ^-1 until day 93, where the conversion efficiency began to decline. The performance was compromised by acidification of the reactor, which impaired the performance until day 107, where an overall production capacity of 11.66 L L' ¹ d ^-1 was recovered. Biomethanation facilitated by a trickle bed reactor systems in the production of < 97% methane has previously been reported up to a conversion rate of 15.4 L L' ¹ d ^-1 based on measurements from the inlet and outlet gas streams.

However, the results acquired from monitoring the different sections in the TBR revealed that the biomethanation performance potential was much higher than anticipated, as shown in Fig. 4. Fig. 4 shows time series data of a local CH4 production capacities at three different positions along the vertical axis for a period of 135 days, and where the period is spitted into two periods, comprising Period I (402) as well as the combination of Period II (403) and Period III (404). Period I shows a first time series data (406), a second time series data (408), and a third time series data (410) of a local CH4 production capacities as measured by gas concentration sensors for measuring methane, where the gas concentration sensors are located at a first position, a second position, and a third position along the vertical axis. Period I is the measured period of local CH4 production capacities from day 0 to day 51. Periods II and III show a continued first time series data (412), a continued second time series data (414), and a continued third time series data (416) of a local CH4 production capacities at the first position, the second position, and the third position along the vertical axis for a period from day 80 to day 135. Periods I, II and III are the same as presented above regarding Fig. 2 as well as Tables I and II, and the data presented in Fig. 2 as well as in Tables I and II may be relevant regarding the data presented in Fig. 4 as well.

The first position is close to the fluid inlet of the methanation reactor or up to 12.5 cm from the fluid inlet of the methanation reactor. The second position is close to the middle of the methanation reactor or between 12.5 cm and 51.5 cm away from the fluid inlet of the methanation reactor. The third position is close to the fluid outlet of the methanation reactor or between 51.5 cm and 91.5 cm away from the fluid inlet.

A dotted line 418 at 88 days indicates an increased feeding rate of H2 and CO2, which forms the transition from Period II to Period III.

At the first position of the sensor near the first fluid inlet, a local maximum methane conversion rate was measured to 17.70 L L' ¹ d ^-1 at day 22 in Period I, while a local maximum productivity of 54.77 L L' ¹ d ^-1 was measured at day 95 in Period III. These local measurements were advantageous for the assessment of the true volume specific potential of the TBR, which indicated that the current TBR systems for biomethanation, therefore, possessed a largely untapped and potential capacity that could directly improve the CAPEX of the technology.

Currently, the approach used for describing the biomethanation process in the TBR system has been based on uniform black box models, which describes the process behaviours simply on the overall balance of inputs and outputs. However, the dynamic system in the TBR based on biofilm growth limits the advancement in creating a stably controlled reactor, and will therefore require more deterministic models, which allows the description of the internal functions and behaviours. It can be observed in Fig. 4 that the concentration of hydrogen substrate considerably decreases from the bulk concentration throughout the TBR. These concentration gradients indicate that the conversion follows a non-linear pattern throughout the TBR, and the first signs of the creation of a non-linear conversion gradient occur already at day 2 of operation, where the methane production capacity was 4.98 L L' ¹ d ^-1 in the bottom section, 2.45 L L' ¹ d ^-1 in the middle section and 1.00 L L' ¹ d ^-1 in the top section. This phenomenon of decreasing productivity along the reaction path vertically in the reactor has previously been observed in TBRs after longer operation periods. They experienced a similar gradient of CH4 productivity, where the ratio between the first fluid inlet and the first fluid outlet accounted for 6.43. Likewise, an average ratio of 6.64 between the productivity at the bottom section near the first fluid inlet and the top section near the first fluid outlet were found in this study for period III.

To prevent the process from developing severe substrate conversion gradients, it will be advantageous to characterise the threshold of the critical process variables that has a direct influence on the creation of gradients. As suggested in this example, the monitoring of the H2 substrate is crucial to examine the black box of TBR operation for biomethanation and create advanced control strategies that promote an optimal operation.

Early warnings and consequences of non-optimal operation

The biological dynamics of the TBR system for biomethanation also induced certain operational consequences that must be tackled to utilize the full potential of the system and maintain stable CH4 production. When operating of TBRs in a continuous mode with a stable gas substrate feed for a longer duration, as in period I and II, the sections in the vicinity to the first fluid outlet becomes depleted for H2 and CO2 substrate due to the dynamic growth inside the TBR. Basing the control and operation solely on the product gas quality will inevitably lead to shifts in the microbial culture in the different sections of the TBR resulting in acetogenic disturbances. The results from the reactor revealed that when ramping up the gaseous substrate feed to accommodate a higher conversion at day 88, the TBR performance deteriorated while the pH dropped and the VFA concentration increased, as shown in Fig. 5.

Fig. 5 shows a first H2 concentration 502 as measured by the first gas concentration sensor close to the first fluid inlet of the methanation reactor, a second H2 concentration 504 as measured by the second gas concentration sensor, and a third H2 concentration 506 as measured by the third gas concentration sensor close to the first fluid outlet of the methanation reactor during part of Period II and during part of Period III, from day 85 until day 110.

The increased feeding rate at 88 days immediately causes an increase in the second H2 concentration 504 in the middle of the methanation reactor. At day 92 and more clearly from day 93, an increase in the third H2 concentration 506 at the first fluid outlet of the middle of the methanation reactor

Fig. 5 shows early warnings of acidification in the trickle bed reactor revealed by the second gas (H2) concentration sensor at day 88 followed by the full acidification at day 93 revealed by the third gas (H2) concentration sensor leading to loss of product gas quality.

The extractions and analysis of carrier materials during the performance loss period revealed that the new abrupt supply of gaseous substrate at day 88 to the top section, near the first fluid outlet, had resulted in a higher fraction of homoacetogenesis, which acidified the previous substrate depleted sections of the reactor (Table 3). This production of acids in the top, near the outlet of the TBR, would be transported down through the reactor with the metabolic water, which would lead to increased acids concentrations in the stable and hydrogenotrophic methanogenic zones in the TBR.

Table 3: Acetate and total VFA concentrations of liquid embedded in clay pellets, Filtralite® Nature (LECA, Denmark) carrier materials at different positions of the reactor. The extracts from the analyses were collected during (day 101), at the end (day 106) and after (day 111) the performance drop.

Total VFA Bottom (Near first Middle Top (Near first fluid

[mg L' ¹ ] fluid inlet) outlet)

Day 101 582.7 ± 5.4 726.4 ± 19.1 2403.8 ± 26.9

Day 106 489.7 ± 2.2 555.8 ± 11.0 1017.2 ± 10.9

Day 111 320.3 ± 10.3 401.1 ± 28.6 1061.8 ± 6.2

The porous carrier material used in this example had the advantage of withholding high quantities of liquid, which created a unique opportunity for providing the required minerals and nutrients for the microbes without the necessity for reducing the mass transfer rate by sprinkling. Accordingly, extracted carrier materials from the reactor were found to have a dry matter concentration of 50.50 ± 4.44 % averagely throughout the reactor 24 hours after sprinkling. In this example, the frequency of sprinkling was increased from two times per week to five times per week to remove accumulated acids produced by the microbes in the TBR from the porous carrier materials. Additionally, the increased sprinkling also supports the transfer of methanogens from the first fluid inlet at the bottom of the reactor to the first fluid outlet at the top of the reactor.

Interestingly, the acidification disturbances in the TBR were initially observed at day 88, where the conversion efficiency in the middle of the reactor began to decline, whereas the initial sign of a decline in performance was first observed in the product gas at day 93. The loss of performance could therefore have been negated by starting the de acidification strategy earlier. Because of the substrate concentration gradient through the reactor column, the TBR systems will have a certain flexibility, where the loss of performance closer to the inlet will provide a higher concentration of the substrate to the zones with low substrate concentrations, which will be able to partly conduct the conversion. From a bioprocess developmental point of view, the control, based on endgas quality, may not be sufficient to identify certain drifts towards performance loss in a profound way. Hence, it was possible to detect a decline in the performance in section 2 of the TBR at day 88, which was 5 days before the drifting of performance was observed at sensor 3 near the first fluid outlet. Early warnings of the drifting in specific sections would provide a buffer time to manoeuvre the TBR back to full performance without compromising the quality of the product gas. Employing a continuous control system can provide a projection of certain events and reactor failures before they actually affect the product gas quality and allow the operator of the TBR to take the necessary actions to ensure full conversion all the time.

The vertical microbial gradient in trickle bed reactors

Carrier materials were extracted at day 135 from the three sampling positions in the bottom, middle and top of the TBR to create a profile of the microbial community along the vertical axis. This spatial microbial gradient should strengthen the understanding of how the microbial consortium develop to the gradient of varying conditions throughout the reactor bed. The extracted samples were analysed with high-throughput sequencing of 16S rRNA gene amplicons with two separate primer sets for the archaeal and bacterial community.

The most abundant archaeal genera identified (>0.1 %) were selected and visualised in a heatmap. A narrow distribution of thermophilic hydrogenotrophic methanogens were found to dominate the overall archaeal community in the trickle bed reactor indicating that a suitable enrichment of the original thermophilic biogas digester inoculum had occurred. The methanogenic archaea were distributed between 5 genera including Methanothermobacter, Methanobacterium, Methanoculleus, Methanosarcina and Methanomassillicoccus, where, in particular, Methanotermobacter and Methanobacterium dominated the TBR. Methanothermobacter was the most abundant archaea in the TBR accounting for 61.9% in the bottom near the first fluid inlet section of the reactor, 99.3% in the middle section and 90.8% in the top section near the first fluid outlet of the reactor. The species belonging to the Methanothermobacter genus can mediate the hydrogenotrophic methanogenesis under thermophilic conditions (55- 70°C) with a pH optimum range of 6.8-8.2 depending on the specific species.

Additionally, the Methanothermobacter has been found tolerant and able to thrive in the H2S rich environments, which the raw biogas introduces to the TBR.

Methanothermobacter have previously been selectively enriched in continuous ex-situ biomethanation systems, and the species affiliated to the Methanothermobacter genus have previously been demonstrated to be suitable candidates for TBRs, as they are able to reach high methane purities and biofilm formations when combined with packing materials. Accordingly, pure cultures of Methanothermobacter have been shown to facilitate an efficient biomethanation process in a pressurized TBR with a CH4 productivity up to 8.4 m ³ rrr ³ d ^-1 and in a CSTR up to 47.9 m ³ m' ³ _CU iture d’ ¹ . It was observed that the middle section had the most selective archaeal growth, whereas the bottom section of the TBR had an additional abundance of 32.6% of the Methanobacterium genus.

The mixotrophic methanogenic genus Methanosarcina was also identified in the TBR. The Methanosarcina genus are capable of different metabolic pathways of methanogenesis using various substrates as acetate, methanol and H2. The species affiliated to this genus was Methanosarcina Thermophila, which metabolism is based upon acetate substrate, and have previously been demonstrated to relieve high acetic acid loads by promoting the degradation in thermophilic anaerobic digesters. Despite the elevated VFA and acetate concentrations during day 93 to 107, the abundancy of Methanosarcina was very low (1.6% in the bottom, 0.1% in the middle and 2.0% in the top). Accordingly, elevated partial pressures of H2 have been uncovered to inhibit the growth and acetate consumption of Methanosarcina Thermophila, and the continuous supply of H2 to the TBR would therefore inhibit a substantial part of their growth. However, micro-scale studies of H2 concentration profiles showed limited H2 penetration into the biofilm, which suggested that the hydrogenotrophic methanogens could dominate the zone of biofilm in close proximity to the H2 supply zone, while the acetoclastic archaea could be located in bulk phase of the biofilm.

Most of these genera in the bacterial community were identified as chemoheterotrophs, which was affiliated in catabolizing peptides or carbohydrates. A fraction of the high flow rates of CO2 and H2 would assimilate into methanogenic cell material, which could provide a continuously replenished source of organic carbon for the fermentative bacteria concurrently with the decay of the methanogens.

Energy conservation

Oxidation of VFA’s as acetate is not thermodynamically favourable, and syntrophic associations have previously been identified between syntrophic acetate oxidizing bacteria belonging to the Coprothermobacter genus and hydrogenotrophic methanogens belonging to the Methanothermobacter genus in reactors exposed to elevated acid concentrations.

The heterogeneity vertical through the TBR accounts is suspected to occur based on the availability of H2 and CO2 substrate. Many planktonic cells are expected to be travelling through the reactor column as a result of the circulation of mineral medium. The microbial gradient may therefore be counteracted by reintroducing substrate to the depleted zones by thorough control of the substrate feed rate or by switching the first fluid inlet and first fluid outlet port frequently. Additionally, these solutions can be combined with frequent sprinkling procedures to ensure inoculation of the depleted zones with microbes from the substrate rich zones.

The diverse bacterial community, which metabolism relies on the degradation of organic materials, indicates the presence of a carbon source in the reactor. Accordingly, the pause period of 29 days was monitored by pressure sensors, which revealed a constant and continuous production of methane accounting for 7.9 mL L' ¹ d ^-1 . This constant and continuous production of methane indicated that an anaerobic digestion of organic carbonic material to methane had to be occurring simultaneously with the methanogenic conversion during full operation. The vast amounts of H2 and CO2 substrate provided to the TBR would also result in enhanced growth of the hydrogenotrophic methanogens, which in continuation of decay would provide as substrate for the fermenting bacteria. Advantages by tapping into the unexploited capacity of trickle bed reactor for biomethanation

The operation and control of the TBR should aim to minimize the zonation in the TBR by examining the different gradients that give rise to the zonation, so the maximum specific volume production capacity for the entire reactor can be achieved. Increased production capacities are sought because they are directly related to reductions in the required size of the TBR, which will induce declines in CAPEX costs. However, the size of the TBR is exponentially related to the production capacity, which means that the profit of increasing the production capacity is likewise reduced, as shown in Fig. 6. Fig. 6 shows the relationship between the production capacity compared to the digester size of a trickle-bed reactor. Applying the average production of 1.4 m ³ bio _g as m ^-3 AD volume d ^-1 combined with the average CO2 concentration in the biogas from this study of 45.5%, the size ratio between the anaerobic digester and the trickle bed reactor in relation to the production capacity. Tapping into the unexploited production capacity would allow a reduction of the active TBR volume from 6.12% of the total anaerobic digester volume to 1.17%.

The overall production capacity in this study of 12.64 L L' ¹ d ^-1 was in the middle of the curvature shown in Fig. 6, which indicate that much profit may be acquired from increasing the production capacity. Meanwhile, if the TBR can be optimized to maintain the local conversion of 54.77 L L' ¹ d ^-1 throughout the entire reactor column, then further pushing of the productivity would only reduce the TBR volume minimally.

The utilization of hydrogen substrate is a common denominator for many of the previously mentioned biotechnological PtX technologies. The aspects of applying an on-line method for monitoring of the hydrogen concentration in-vivo in the reactors may be an advantageous tool for the development of the process control to many PtX technologies that rely on the conversion of renewable H2 in the future.

Example 2

Fig. 7 shows a plurality of early-warning events, where the acidification in a 10L tricklebed reactor is monitored with a first gas concentration H2 sensor preferably located in the middle of the reactor and a second gas concentration H2 sensor located close to the outlet of the reactor.

The middle of the reactor may substantially be at a distance from the first inlet and/or the first outlet, which can be between 40% - 60% like e.g. around 50% of the internal length of the reactor.

Multiple early-warning events, as illustrated in Figs. 7A-I, of acidification in a 10L trickle bed reactor were monitored with a first gas concentration H2 sensor located in the middle compartment of the reactor bed. The middle compartment of the reactor bed can be defined as between 40% - 60% like e.g. around 50% of the internal length of the methanation reactor, from the first and/or the second inlet or the first and/or second outlet. The vertical dotted line indicates the specific time when the product gas quality deteriorated to contain >2% H2 at the outlet. The loss in performance in the presented events was a result of acidification, as the volatile fatty acid concentration increased simultaneously with the performance loss. The data was collected for an experimental period of a year, and the events demonstrate the selected periods where n39reventiveve actions were taken to reduce the acid accumulation such as liquid trickling, gas flow reduction, or temporal reactor shutdown.

The validation of the technology potential was demonstrated by the multiple early- warning events shown in Fig. 7A-I, where acidification in the trickle bed reactor was allowed to drift, which induced a gradual deterioration in the reactor performance. The trickle bed reactor operated for the experiments was a custom-built trickle bed reactor in stainless steel with a total volume of 10L and an active bed of 9.09L. The trickle bed reactor was packed with crushed expanded clay aggregates (Filtralite NC 2-10) and inoculated with decantered digestate from Foulum biogas plant. These experiments were conducted in continuation from the experiments in Fig. 3-5, and the trickle bed reactor had an overall operation period of 594 days in total. The trickle bed reactor was operated with trickling frequency of once per day at a rate of 70.1 L L' ¹ d ^-1 for 1 min followed by 40.2 L L' ¹ d ^-1 for 5 min, which was repeated three times. The gas load applied at the experimental period of Fig. 7 was 52.28 L L' ¹ d ^-1 of H2 and 27.80 L L' ¹ d ^-1 raw biogas (corresponding to 12.51 L L' ¹ d ^-1 of CO2) for Fig. 7A to 7H and 13.3 L L' ¹ d ^-1 of H2 and 7.1 L L' ¹ d ^-1 raw biogas (corresponding to 3.25 L L' ¹ d ^-1 of CO2) for Fig. 7I. These reactor specifications are listed in Table 4.

Table 4: Reactor specifications of the reactor used for the validation measurements presented in Fig. 7A-I.

It can be seen from Fig. 7A-I that deterioration of the reactor performance can be observed in the middle of the reactor more than 24 hours before it is observed in the product gas. In contrast, the gas break-through of H2 in the outlet product gas is only observable 4-8 hours before the product gas contained >2% H2. However, the time will depend on the reactor configuration, reactor dimensions, and sensor position, which are illustrated with a longer x-axis time period in Fig. 71, which had a 3 times lower feed gas load.

Further details of the invention 1 . A system for controlling a methanation reactor comprising a methanation reactor comprising a first fluid inlet and a first fluid outlet, wherein the methanation reactor is configured for at least one first fluid flow between the first fluid inlet and the first fluid outlet defining a first direction of the at least one first fluid flow; a first gas sensor configured to measure a first gas concentration at a first gas sensor location within the methanation reactor, and/or at least a first temperature sensor and a second temperature sensor configured to measure a first temperature at a first temperature sensor location within the methanation reactor and a second temperature at a second temperature sensor location within the methanation reactor; respectively, and at least one controller configured to regulate the at least one first fluid flow into the methanation reactor based on the first gas concentration or on the first and second temperatures. The system according to item 1, wherein the system further comprises at least one second fluid flow. The system according to any one of the preceding items, wherein the gas sensor is a gas concentration sensor. The system according to any one of the preceding items, wherein the methanation reactor comprises a second fluid inlet and a second fluid outlet, wherein the methanation reactor is configured for or such that the at least one second fluid flow, optionally is flowable, between the second fluid inlet and the second fluid outlet defining a second direction or a second fluid line of the at least one second fluid flow. The system according to any one of the preceding items, wherein the methanation reactor has an internal temperature, and wherein the internal temperature is below 200°C, preferably below 175°C, more preferably below 150°C. The system according to any one of the preceding items, wherein the methanation reactor is a biomethanation reactor. 7. The system according to any one of the preceding items, wherein the methanation reactor is configured to comprise or comprises microorganisms for performing a methanation.

8. The system according to any one of the preceding items, wherein the methanation reactor has an internal length, and the first gas concentration sensor location is located at a first distance from the first fluid inlet of between 1% and 100% of the internal length, more preferably between 12% and 100% of the internal length, such as 15% of the internal length.

9. The system according to any one of the preceding items, wherein the system comprises a plurality of gas sensors configured to measure a plurality of gas concentrations at a plurality of gas sensor locations within the methanation reactor.

10. The system according to any one of the preceding items, wherein the first gas sensor or the plurality of gas sensors is/are positioned within the at least one first fluid flow.

11. The system according to any one of items 9 to 10, wherein the plurality of gas sensors are located at a first predefined distance and/or downstream to each other.

12. The system according to any one of the preceding items, wherein the system comprises a second and a third gas sensor configured to measure a second gas concentration and a third gas concentration at a second gas sensor location and a third gas sensor location, respectively, within the methanation reactor.

13. The system according to item 12, wherein the first gas sensor is located adjacent to the first fluid inlet, the second gas sensor is advantageously located centrally in the methanation reactor or is located in the middle between the first fluid inlet and the first fluid outlet, and the third gas sensor is located adjacent to the first fluid outlet. The system according to any one of the preceding items, wherein the system is configured to calculate a gas concentration distribution within the methanation reactor based on the first and the second gas concentration, preferably based on the first, the second and the third gas concentration, and more preferably based on the plurality of gas concentrations. The system according to any one of the preceding items, wherein the gas sensor(s) are selected from the group of: hydrogen (H2) sensors, CO2 sensors, CH4 sensors, CO sensors, and combinations thereof, and preferably hydrogen sensors, such as hydrogen microsensors. The system according to any one of the preceding items, wherein the system further comprises a heating system configured for heating a pre first fluid flow, which is the first fluid flow before the first fluid flow enters into the methanation reactor through the first fluid inlet. The system according to item 16, wherein the heating system comprises a heat exchanger, and wherein the heat exchanger is configured such that heat generated by the methanation reactor is transferred to the pre first fluid flow. The system according to any one of the preceding items, wherein the system further comprises a cooling fluid configured to regulate an operating temperature of the methanation reactor. The system according to any one of the preceding items, wherein the system comprises a plurality of temperature sensors configured to measure a plurality of temperatures at a plurality of temperature sensor locations within the methanation reactor. The system according to any one of the preceding items, wherein the first temperature sensor and the second temperature sensor are positioned within the at least one first fluid flow and/or within the cooling fluid. 21 . The system according to any one of the preceding items, wherein the first and the second temperature sensors are located at a second predefined distance and/or downstream to each other.

22. The system according to any one of the preceding items, wherein the system comprises between three and ten temperature sensors configured to measure the temperature at different temperature sensor locations within the methanation reactor.

23. The system according to any one of the preceding items, wherein the system is configured to calculate a temperature distribution within the methanation reactor based on the first and second temperatures, and preferably based on the plurality of temperatures.

24. The system according to any one of the preceding items, wherein the system further comprises at least one processing unit configured to receive an input from the one or more gas sensor(s) and/or from the two or more temperature sensor(s), and further configured to calculate

- a gas concentration distribution within the methanation reactor, based on the input from the one or more gas sensor(s) and/or

- a temperature distribution within the methanation reactor, based on the input from the two or more temperature sensor(s).

25. The system according to any one of the preceding items, wherein the at least one controller is operated by a local user interface.

26. The system according to any one of the preceding items, wherein the methanation reactor is a gas phase reactor, such as a co-current or countercurrent trickle-bed reactor.

27. The system according to any one of the preceding items, wherein the methanation reactor has an elongated chamber that is preferably arranged advantageously horizontally or vertically. 28. The system according to any one of the preceding items, wherein the at least one first fluid flow into the methanation reactor is a gas flow and/or a liquid flow.

29. The system according to any one of the preceding items, wherein the at least one first fluid flow into the methanation reactor comprises biogas, CH4, CO2, H2S, H2, CO, nutrient medium, and combinations thereof, and preferably is biogas, hydrogen, flue gas, syngas, nutrient medium, or purified CO2.

30. The system according to any one of the preceding items, wherein the system further comprises one or more pressure sensor(s) configured to measure the pressure(s) at one or more pressure sensor locations within the methanation reactor.

31. The system according to item 30, wherein the system comprises between two and ten pressure sensors configured to measure the pressure(s) at different pressure sensor locations within the methanation reactor.

32. The system according to item 30, wherein the system comprises two pressure sensors mounted adjacent to the first fluid inlet, and two pressure sensors mounted adjacent to the first fluid outlet.

33. The system according to item 32, wherein the methanation reactor has an internal length, and the pressure sensors at the first fluid inlet and/or the pressure sensors at the first fluid outlet are located at a distance to each other of between 1% and 50% of the internal length, more preferably between 5% and 30% of the internal length, such as 15% of the internal length.

34. The system according to any one of the preceding items, wherein the at least one controller is configured to regulate at least the first fluid flow into the methanation reactor based on the first gas concentration, the first and the second temperature, and/or the pressure(s), preferably on the plurality of gas concentrations and/or the plurality of temperatures. The system according to any one of the preceding items, wherein the system is configured to control the at least one first fluid flow and/or the at least one second fluid flow. The system according to any one of the preceding items, wherein the at least one second fluid flow is driven by a pump, and the pump and/or a valve is/are configured to control the at least one second fluid flow. The system according to any one of the preceding items, wherein the system is configured to reduce or increase the at least one second fluid flow. The system according to any one of the preceding items, wherein the at least one second fluid flow is a flow of nutrient solution, such as a synthetic or organic nutrient solution. The system according to item 38, wherein the flow of synthetic nutrient solution comprises NH4CI, EDTA, MgCh etW, FeCh etW, C0CI2, Na2MoO4'2H2O, NiCh and/or Na2S or another defined or complex source of nutrient medium. The system according to any one of the preceding items, wherein the system further comprises a pH sensor for measuring a pH value of the at least one second fluid flow. A method for controlling a methanation reactor comprising the following steps: providing at least one first fluid flow between a first fluid inlet and a first fluid outlet of a methanation reactor defining a first direction of the at least one first fluid flow; measuring a first gas concentration at a first gas sensor location within the methanation reactor, and/or a first temperature and a second temperature, respectively, at a first and a second temperature sensor locations within the methanation reactor; and regulating the at least one first fluid flow into the methanation reactor based on the first gas concentration and/or the first and the second temperature.

42. The method according to item 41 , wherein the method comprises a step of providing at least one second fluid flow into the methanation reactor.

43. The method according to any one of items 41-42, wherein the at least one second fluid flow is increased, if the first gas concentration is above a first gas concentration level.

44. The method according to any one of items 41-43, wherein the method comprises a step of reducing or shutting off the at least one first fluid flow, if the first gas concentration is above a second gas concentration level.

45. The method according to any one of items 41-44, wherein the method is configured to be carried out by the system according to any one of items 1- 40.