Technical Field

The present invention relates to a process for combined biological and thermo-chemical treatment of biological sludge, manure, food waste, slaughterhouse waste, lignocellulosic wastes and plastic-containing wastes to recovery energy, nutrients, and carbon storage material.

Background Art

Although biogas is widely seen as a key contributor to achieve the ambitious carbon emission goals, current production is simply too costly and not sustainable enough for a wider deployment. This is mainly due to poor volumetric yields and limited feedstock availability for current anaerobic digestion (AD) processes. The current AD feedstocks are mainly limited to manure, biological sludge, and food waste.

On one hand, lignocellulosic waste, as woody feedstock, has an enormous potential for renewable energy production. In 2019, the global demand for biomethane was 23 TWh according to the International Energy Agent. Considering an energy density of ~4.7 kWh/kg and 50% conversion rate, 1 ,000 million tons of woody waste would be enough to cover this demand, being only 5% of its available capacity (20 Gt/yr). However, this type of biomass is currently not considered as primary raw material for AD plants, due to high pre-treatment costs and poor conversion rates for biomethane with state-of-the-art technologies. On the other hand, the typical organic conversion rates in an AD are in the range of 40-50%, which means over 50% of the organic material passes through the AD and end up in the digestate. Digestate is rich in nitrogen and phosphorus and can be used as organic fertilizer. However, significant greenhouse gases (GHG) can still be produced and released from the digestate during storage, transportation, and application in the field ¹ . Also, pollutants in the digestate, such as microplastics and

¹ Czubaszek, R. and Wysocka-Czubaszek A. 2018. Emissions of carbon dioxide and methane from fields fertilized with digestate from an agricultural biogas plant. International Agrophysics. 32, 29-37. pharmaceutical residues will go into the soil and pose a risk of entering the food 2 chain . Digestate from sewage sludge has been banned for use in agriculture in many of the European countries, including Sweden, Germany and Italy. Applying digestate from other wastes in the agriculture may be allowed, but costs involved in transporting and storing diluted digestate (normally 95% water content) can be prohibitive.

Conventional mesophilic digestion (35-42 °C) is a well-established process for stabilizing sludge, manure slaughterhouse waste and food waste in AD. Low organic conversion rates (to biogas) and large volumes (high investment costs) associated with mesophilic digesters have limited its commercial applications and urged researchers to improve the AD processes. Thermophilic type digesters (SO- S °C) can increase biological reaction rates (doubling with every 10 °C rise) and further stabilize feedstocks by better pathogen destruction. This is important for the digestate categorization, qualifying Type A biosolids over Type B biosolids and

3 resulting in lower disposal costs . However, thermophilic temperatures promote stronger inhibition of ammonium to methanogens, which in many cases leads to more unstable performance than mesophilic digesters, especially when treating high nitrogen content wastes such as sewage sludge, fishery sludge and chicken manure ⁴ .

Temperature-phased anaerobic digestion combines a thermophilic phase followed by a mesophilic phase, which is an approach to incorporate advantages of thermophilic digestion and mitigate the disadvantages through the addition of a mesophilic phase that enhances stabilization. Although documented benefits of higher (but limited) organic conversion rates and improved stability have been recorded, the return of investment is still not significant enough to enable its wide application.

² Weithmann N. et al. 2018. Organic Fertilizer as a Vehicle for the Entry of Microplastic into the Environment. Science Advances, 4:eaap8060.

³ https://www.epa.gov/biosolids

⁴ Wastewater Engineering: Treatment and Resource Recovery. Metcalf & Eddy Inc. Fifth Edition. Thermal hydrolysis pre-treatment (THP) is a method that can achieve both higher organic conversion and higher pathogen destruction. THP applies high temperatures (130-165 °C) and high pressure (7-9 bar) to open cell walls and lower viscosities in feedstocks and consequently increase biogas production in AD. THP also helps to improve digestate dewatering performance and with less water in the dewatered digestate, lower disposal costs can be achieved. The high temperature and high pressure required in the THP process demand high amounts of steam: typically, 0.8-1 .5 ton 12-bar steam required to pre-treat 1 ton sludge (dry matter).

The properties of lignocellulosic materials render them resistant to biodegradation. A wide range of physical and chemical pretreatment methods of lignocellulosic materials have been studies to improve their biodegradability in AD. The pretreatment methods includes milling, irradiation, liquid hot water (100-230 °C and 1 -28 bar), chemicals (alkali or acid), steam explosion (160-260 °C and 7-48 bar), wet oxidation (180-220 °C and 1 -12 bar) and biological pretreatment with fungi and enzymes . Although various pretreatment techniques have been studied on numerous of lignocellulosic feedstocks, only a few of the reported techniques achieved high biogas yield with reasonable costs to be attractive.

In a typical pyrolysis process organic feedstock can be refined to solids (char), pyrolysis liquid and gas (non-condensable gas). The pyrolysis liquid can be separated into an oil phase (pyrolysis oil) and an aqueous phase (APL). Studies have shown that AD can convert aqueous pyrolysis liquid (APL) to biogas, but at high bio-oil dosages the AD was inhibited ⁶ . Thus, the challenge is how to feed the majority of the syngas, including non-condensable gas, bio-oil and APL from pyrolysis to AD and still obtain high and stable biogas production.

5 Zheng Yi et. al. 2014. Pretreatment of lignocellulosic biomass for enhanced biogas production. Process in Energy and Combustion Science, pp. 1 -19.

⁶ Torri, C. and Fabbri, D. 2014. Biochar Enables AD of Aqueous Phase from Intermediate Pyrolysis of Biomass. Bioresource Technology, 172, 335-341. DE102009020200A1 describes a method for operating a biogas plant where fermentation substrate (digestate) is first dried and then gasified, and syngas produced in the gasifier is scrubbed by the liquid fermentation substrate and the cleaned syngas is fed into a gas engine for electricity and heat production. The fermenters are filled with methane producing microorganisms.

WO2018234058A1 describes a method for using a membrane to feed syngas into a digester and improve syngas mass transfer from gas phase to liquid phase in the digester for biogas production. The syngas temperature must be below 55 °C, ideally at 35 °C. The membrane is not designed to handle high temperature syngas. The syngas pressure is not greater than 2.5 bar when it enters the membrane.

W02015003273A1 describes a method for converting organic feedstocks to syngas and biochar, and syngas is further separated into non-condensable gas and condensate and feed both non-condensable gas and condensate into digester for biogas production. WO2017197508A1 describes a similar process and includes adding metal ions in digestate to form precipitate and then pyrolyze the digestate cake to enhance phosphorus recovery in biochar.

EP1754771 A2 describes a method for producing gas from a biomass gasification process and the gas is fed into a biomass fermentation process where biogas is produced. The gas after passing the fermentation process is used in a gas engine for electricity and heat production.

A journal publication with a title of Integrated Processes of Anaerobic Digestion and Pyrolysis for Higher Bioenergy Recovery from Lignocellulosic biomass: A Brief Review (in Renewable and Sustainable Energy Reviews, volume 77, 1272-1287, 2017) reveals a review of integrated anaerobic digestion (AD) and pyrolysis (PY) process with the purpose of achieving a higher bioenergy recovery. The review looks at different pyrolysis AD-PY, PY-AD, and AD-PY-AD. In the AD-PY-AD process, pyrolysis gas is recycled back to the anaerobic digester and the publication also indicates that a two-step anaerobic digestion process would be beneficial and the present of biochar in an AD process is also beneficial due to the water binding and metal adsorption capacity associated with biochar. The biochar enhanced fermentation is also presented in another article (Title: Enhanced Ethanol Production from Syngas by Clostridium ragsdalei in Continuous Stirred Tank Reactor Using Medium with Poultry Litter Biochar, Applied Energy, Volume 236, 1269-1279, 2019).

Summary of invention

These and other objects and advantages of the invention are obtained by a process for treatment of organic waste, said process comprising the following step: i) supply a wet feedstock to a dryer and obtain a dried feedstock. ii) mix and pretreat the dried feedstock with a dry feedstock to obtain a mixed feedstock. iii) supply the mixed feedstock to a thermo-chemical process and produce a char fraction and a hot syngas. iv) discharge the char fraction and separate the hot syngas stream into warm syngas and tar in a tar separation unit. v) supply an AD feedstock and the warm syngas from the tar separation unit to a fermenter and expose the AD feedstock and the warm syngas for a series of biological reactions to obtain a headspace gas and a slurry in the fermenter; vi) supply the slurry from the fermenter to a digester and expose the slurry for a series of biological reactions to obtain biogas and digestate; vii) supply a portion of the headspace gas from the fermenter to the thermochemical process as carrier gas for syngas production and supply the rest of the gas phase from the fermenter to a second thermo-chemical reactor as fuel for energy production.

The wet feedstock and the dry feedstock can be any solid organic streams chosen from the group consisting of garden waste, demolition wood, impregnated wood, plastic-containing waste, straw, sawdust, bark, white pellet, black pellet or combinations thereof.

The AD feedstock is preferably chosen from the group consisting of pre- treated food waste, biological sludge, manure, slaughterhouse waste or combinations thereof.

Preferably, the headspace gas from the fermenter is supplied to the thermo-chemical process as carrier gas or is supplied to the second thermochemical process as fuel for production of hydrogen or heat. The hydrogen or heat production in the second thermo-chemical process can be combined with supply of the biogas from the digester.

The biological reactions in the fermenter are preferably chosen from the group consisting of fermentation, hydrolysis, acidogenesis, fermentative hydrogen production, acetogenesis, homo-acetogenesis and combinations thereof.

Preferably, reactions of methanogenesis are suppressed in the fermenter on purpose.

The biological reactions in the digester are preferably chosen from the group consisting of fermentation, hydrolysis, acidogenesis, fermentative hydrogen production, acetogenesis, homo-acetogenesis, methanogenesis and combinations thereof.

Preferably, the method can comprise the additional steps of supplying the digestate from the digester to a dewatering unit and obtain a reject water fraction and a dewatered digestate fraction, and supplying the dewater digestate fraction to the dryer.

Preferably, all or a part of the biogas from the digester is supplied to the second thermo-chemical process as additional fuel.

Preferably, the thermo-chemical process comprises the following steps: a. supply the mixed feedstock and a headspace gas from the fermenter to a feed chamber and mix the feedstock and headspace gas; b. supply the mixture from the feed chamber to a thermo-mechanical unit and heat the mixture: c. obtain a char fraction and a hot syngas; d. supply the hot syngas to a tar separation unit and obtain a warm syngas; and e. supply the warm syngas to the fermenter.

The warm syngas from the thermo-chemical process is preferably supplied to a recirculation pipe of the fermenter and is mixed with recirculating headspace gas and/or the warm syngas and the feedstock is supplied to an inline mixer.

Brief description of drawings

Figure 1 depicts a first embodiment of the process according to the invention

Figure 2 depicts a preferred embodiment of the fermenter according to the invention

Figure 3 depicts a detail of the thermo-chemical process according to the invention

Figure 4 depicts a second preferred embodiment of the process according to the invention for including digestate in the thermo-chemical process feedstocks

Figure 5 depicts a third preferred embodiment of the process according to the invention for production of hydrogen and biochar

Detailed description of the invention

Figure 1 is a flow diagram showing the first embodiment of the process according to present invention.

A wet feedstock (2), which can be either garden waste, sawdust, bark, or plastic-containing wastes, and is dried in a dryer (14). The dried feedstock (3) is then mixed with a dry feedstock (20), which can be either demolition wood, white pellet, or black pellet, and pretreated in a pretreatment unit (38). The pretreatment unit (38) is used to produce a homogenous, high bulky density feed to downstream processes. The mixed feedstock (7) is then sent to a thermo-chemical process (15).

Inside the thermo-chemical process (15) (see also Fig. 3 showing a more detailed view of the thermo-chemical process (15)), the mixed feedstock (7) is first mixed with a headspace gas (10) from a fermenter (11 ) in a feed chamber (36). The headspace gas (10) acts as carrier gas in the thermo-chemical process (15). After the feed chamber (36), the mixture enters a thermo-chemical unit (17), which is typically operated at a temperature between 250-850 °C, at atmospheric pressure and under a low oxygen environment. Examples of the thermo-chemical process (15) can include processes such as pyrolysis, gasification, and torrefaction. The thermo-chemical unit (17) can be heated by a high temperature gas (26) or by electricity or by heat released from the mixed feedstock (7) or combinations of thereof. The high temperature gas (26) can be flue gas by burning biogas, biomethane, natural gas, biomass, or other energy materials.

In this thermo chemical unit (17) the mixed feedstock (7) is converted into a char fraction (23) and a hot syngas (25).

The solid char (23) comes out at the bottom of the thermo-chemical unit (17) via an airlock (37) by gravity and the hot syngas (25) comes out on the top of the thermo-chemical unit (17). The solid char (23) itself has a moisture content less than 1% but can be damped with water to reduce dust production during char cooling and transportation. The solid char (23) has a fixed carbon content of 20-90 wt-%.

The hot syngas (25) is then separated in a tar separation unit (18) to produce a warm syngas (8) and a tar (24). Some examples of the composition of the warm syngas (8) are shown in table 1 below. The warm syngas (8) enters a fermenter (1 1 ). The tar (24) is collected in a tar collection tank (28) and then either recirculated back to the inlet of the thermo-chemical unit (17) for volume reduction or used as raw material in another process, such as for heat production, transport fuel production in a refinery, as binding agent, or chemical production. The tar (24) can have the same inlet with the mixed feedstock (7) or a different inlet to the thermo-chemical unit (17). A cooling medium (27), such as cooling gas or cooling fluid, goes through a heat exchanging unit (35) and is used to help control internal temperatures in the tar separation unit (18) and the temperatures in the tar separation unit (18) are controlled between 70-320 °C. The heat exchanging unit (35) can in one embodiment be placed externally of the tar separation unit (18) and in this embodiment, the heat exchanging unit (35) can be placed either upstream or downstream of the tar separation unit (18). The heat loss in the heat exchanger (35) can be partially recovered and used in the overall process, such as for heating an AD feedstock (1 ). The high temperature gas (26) can be used periodically to clean the tar separation unit.

The AD feedstock (1 ), which is typically pretreated food waste, biological sludge, manure, slaughterhouse waste or combined more than one of the four, is led into the fermenter (11 ) where the AD feedstock (1 ) is mixed with the warm syngas (8) from the thermo-chemical process (15). In the fermenter (11 ), biological reactions occur under a temperature between 45-70 °C, a pressure of 1 -7 bar and a pH of 4.0-6.0. Digestate (5) from a digester (12) can be added into the fermenter (11 ) during system start-ups for inoculation.

Organics in the AD feedstock (1 ) and the warm syngas (8) are converted to small molecular, water soluble compounds via a series of biological reactions including fermentation, hydrolysis, acidogenesis, fermentative hydrogen production, acetogenesis, homo-acetogenesis and methanogenesis. Basically, a large portion of the warm syngas (8) (mainly hydrogen, carbon monoxide, and carbon dioxide) can be converted to water soluble compounds in the fermenter (11 ) by microorganisms and become a part of the slurry.

The methanogenesis reactions are intentionally suppressed in the fermenter (11 ) by applying operating pressures including temperatures, pH and hydraulic retention times to methanogenesis microorganisms, which will facilitate the conversion of the warm syngas (8) into water soluble compounds.

The fermenter (11 ) is shown in more details in Fig. 2. After the tar separation, the warm syngas (8) enters the fermenter (11 ) in two ways. One way is to inject the warm syngas (8) in a recirculation pipe (39) where headspace gas (10) is circulated to stir the mixture in the fermenter (11 ). A pump (31 ) is used to regulate the headspace gas (10) recirculation rates. The other way is to mix the warm syngas (8) and the feedstock (1 ) in an in-line mixer (16) and then the mixture is pumped into the fermenter (11 ) via another pump (32). The solid char (23) can be added into the fermenter (11 ) at a percentage of 0.5-10% to the AD feedstock (1 ) volume to improve the fermenter (11 ) and the downstream digester (12) performance. The process can use either one of the two ways or both ways combined. In one embodiment, the warm syngas (8) can be mixed into the headspace gas (10) and in another embodiment, the warm syngas (8) can be injected via the in-line mixer (16). It is also conceivable to split the warm (8) syngas into two streams and combine these two embodiments. There are a gas distribution system (34) and a feed distribution system (30) in the fermenter (11 ) for evenly distributing streams coming into the fermenter (11 ). The gas distribution system also helps to create small gas bubbles to improve the gas-liquid mass transfer between the headspace gas

(10) and the slurry in the fermenter (11 ). A mechanical mixer (33) can be used (as an option) to obtain an ideal mixing condition in the fermenter (11). Internal structures, such as bundles of plastic carriers, can be added in the fermenter

(11 ) to help increase gas-liquid mass transfer by shearing gas bubbles and to provide high surface areas for biofilm formation. The fermenter (11 ) can have different shapes such as cylinder or rectangular. The fermenter (11 ) has typical height-to-diameter ratios of 1.5-10.

The liquid stream from the fermenter (11 ), a slurry (4), enters a digester

(12). Periodically, part of the deposit (9) in the fermenter (11 ) is removed from the fermenter (11 ). The digester (12) has a larger volume than the fermenter

(11 ) and it is operated under a lower or the same temperature of between 30-55 °C, at atmospheric pressure and a pH of 6.5-9.5. The digester (12) can be either wet digester (dry matter content of the digester content < 15 wt-%) or dry digester (dry matter content of the digester content > 15 wt-%). Biogas (21 ) and digestate (5) are produced in the digester (12) via a series of biological reactions including fermentation, hydrolysis, acidogenesis, fermentative hydrogen production, acetogenesis, homo-acetogenesis and methanogenesis. The methanogenesis reaction is intentionally encouraged in the digester (12).

The headspace gas (10) from the fermenter (11 ) first enters a gas buffer tank (29) before it is split into two streams. One stream enters the thermochemical process and functions as carrier gas. The other stream enters a second thermo-chemical process (19) and functions as fuel. The split ratio (the flow to the thermo-chemical process (15) versus the flow to the second thermochemical process (19)) varies between 1 to 20.

The second thermo-chemical process (19) can be a hydrogen production (via methane reforming reactions, water-gas-shift reactions, chemical looping reforming reactions, or combinations thereof) process or a heat production (combustion) process. The biogas (21 ) can be optionally introduced into the second thermo-chemical process as additional fuel.

As shown in Figure 4, the digestate (5) is first dewatered in a dewatering unit (13) to separate reject water (22) and dewatered digestate (6). Ammonia recovery from the reject water (22) is typically applied before it is treated in a wastewater treatment unit. In one embodiment, the dewatered digestate (6) is mixed with the wet feedstock (2) to be dried in the dryer (14). In this way, the organic residue in the dewatered digestate (6) can be further processed in the thermo-chemical process (15).

As shown in Figure 5, the slurry (4) from the fermenter (11 ) can be sent to the dewatering unit (13) directly. After dewatering, the dewatered slurry (6) is dried in the dryer (14) and then thermo-chemically treated in the thermochemical process (15). The headspace gas (10) from the fermenter (11 ) enters the second thermo-chemical process (19) for hydrogen or heat production.

Table 1. Composition of the warm syngas (8) from lignocellulosic wastes

Advantages of invention versus prior art

Compared to the prior art, the present invention has following advantages by combining the features presented above:

The design of the fermenter (11 ) serves a main function of converting the syngas (8) to water soluble compounds. The pH values, the pressures in the fermenter, the temperatures, the headspace circulation pipe (39), the gas distribution system (34), the feed distribution system (30), and the internal structure of the fermenter are all designed for optimizing the syngas conversion to water soluble compounds. The methanogenesis reactions in the fermenter (11 ) are intentionally suppressed, which helps to increase the syngas conversion efficiency.

As the syngas conversion occurs in the fermenter (11 ), the headspace gas (10) is enriched with nitrogen and methane, which cannot be converted to water soluble compounds, and traces of convertible gases, such as carbon dioxide, hydrogen, and carbon monoxide, which remains in the headspace gas. It is not a good practice to introduce the nitrogen-containing headspace gas to an anaerobic digester since nitrogen is a contaminant in biogas, which either reduces biogas calorific value or generates additional costs for nitrogen separation when purified biogas (biomethane) is the final product. In the present invention, the headspace gas is separated from the slurry (4) in the fermenter (11 ) and only the slurry (4) enters the digester (12). The headspace, due to its nitrogen content, is valuable for the thermo-chemical process (15) as carrier gas. In a typical pyrolysis, gasification or torrefaction process, an oxygen-free carrier gas, such as nitrogen, is needed to maintain an anaerobic environment. A commercial carrier gas can be costly to the thermo-chemical process operation. Using the headspace gas (10), which is rich in nitrogen and free of oxygen, as carrier gas improves the thermo-chemical process performance and meantime, reduces the operational costs.

Methane is an ingredient in the syngas (8) and cannot be converted to water soluble compounds in the fermenter (11 ) and will be enriched in the headspace gas (10). When the headspace gas is used as carrier gas in the thermo-chemical process, the enriched methane content will help to shift the reaction kinetics towards more hydrogen production in the process, which will eventually help to increase biogas production in the digester (12). The headspace gas can also exit the system by entering the second thermochemical process (19) and its calorific value in methane, hydrogen and carbon monoxide residues can be valorized into heat or into hydrogen.

Some condensable organics in the syngas (8) are also converted to water soluble compounds in the fermenter (11 ). These organics are in the gaseous forms at high temperatures (70-320 °C) and become a part of the slurry in the fermenter (temperatures 45-70 °C), which will be converted to biogas and digestate in the digester (12). The sensible heat in the syngas (8) is also used to raise the temperature of the fermenter (11 ) which also receives the low temperature AD feedstock (1 ). The sensible heat in the hot syngas (25) and from the second thermo-chemical process (19) can also be used to raise the temperature of the fermenter (11 ). The heat required by the dryer (14) can also be supplied by the heat produced in the second thermo-chemical process (19).

The tar separation unit (18) separates the tar (24) from the warm syngas (8) and the tar (18) is handled separately. The warm syngas, which contains non-condensable gas, aqueous pyrolysis liquid (in gaseous form at the presented temperatures), and a part of bio-oil (in gaseous form at the presented temperatures), can enter the fermenter (11 ) and then the digester (12) for biogas production. By controlling the operational temperature in the tar separation unit, the present invention can potentially minimize the tar (24) production and maximize the warm syngas production, which will lead to high biogas production.

Digestate, depending on feedstock compositions, may contain microplastics or pharmaceutical residues or both, which makes it less valuable. By introducing the dewatered digestate (6) to the presented process (shown in Fig. 4), biogas plants will be able to remove the microplastics and the pharmaceutical residues in the digestate and produce additional biogas and biochar, which have high values in circular economy.

The biochar (23), as an option, is added into the fermenter (11 ) and the digester (12) to improve system performance by raising slurry alkalinity and providing high surface area for biofilm formation.

By combining different types of feedstocks (1 , 2, and 20), the presented invention can solve a wide range of environmental issues and unlock an enormous amount of unconventional feedstocks for biogas production, produce different types of biochar for a wide range of biochar applications, and generate multiple forms of renewable energy: biogas, thermal, and hydrogen.

Recovering nitrogen and phosphorus in the feedstocks. More than 90% of phosphorus and approximately 40% in the feedstocks will be kept in the char (23), which makes the char (23) a promising alternative for fertilizer products. Depending on heavy metal concentrations and phosphorus plant availability in the char, further downstream processing for the char (23) may be necessary to produce higher value-adding products.

List of reference numerals

1 anaerobic digestion (AD) feedstock

2 wet feedstock 3 dried feedstock

4 slurry from fermenter

5 digestate

6 dewatered digestate

7 mixed feedstock

8 warm syngas

9 deposit in fermenter

10 headspace gas

11 fermenter

12 digester

13 dewatering unit

14 dryer

15 thermo-chemical process

16 in-line mixer

17 thermo-chemical unit

18 tar separation unit

19 second thermo-chemical process

20 dry feedstock

21 biogas

22 reject water

23 solid char

24 tar

25 hot syngas

26 high temperature gas

27 cooling medium

28 tar collection tank

29 gas buffer tank

30 feed distribution system

31 pump

32 pump

33 mechanical mixer

34 gas distribution system

35 heat exchanger unit

36 feed chamber

37 char airlock

38 pretreatment unit

39 recirculation pipe