“Gasification of Organic Waste Streams”

Introduction

The invention relates to gasification of feedstock material such as waste streams, for example organic waste streams including bio-oil.

Incineration of liquid waste streams is becoming less attractive due to strict environmental legislation and the impact of incineration on overall process efficiencies. While organic streams with a low water content can be considered for incineration, the cost of incineration drastically rises as the water content of the waste stream increases. Supercritical water gasification (SCWG) can be used for the destruction of waste streams containing toxic organic compounds, solvents, oils and other heavy hydrocarbons while also generating a valuable combustible gas product. In SCWG there is conversion of various organic waste streams and biomass feedstocks into a treated liquid effluent stream with a low chemical oxygen demand (COD).

When utilising traditional waste treatment methods, dewatering activities result in additional cost. With SCWG no dewatering is required as water is one of the reactants in the process.

Various SCWG processes are known such as those described in Casademont, et al. (2016) ([1]). As is known SCWG is achieved by operating at a pressure between 225 and 320 Bar and at temperatures between 450°C and 700°C. These conditions result in a considerable change in the physical properties of water in terms of density, thermal conductivity and the ability of water to act as a solvent. The solubility of the organic, non-polar substances increases, with a marked decrease in the solubility of inorganic salts. Large organic compounds are subjected to hydrolysis under supercritical water conditions. Organic compounds, having a relatively high volatility, are completely miscible with the supercritical water, and gasses display a similar characteristic at supercritical water conditions. In the SCWG process it is common for hydrogen production to be in excess of the hydrogen molecules contained within the organic portion of the waste stream, proof that water serves as an oxidant at supercritical water conditions (Hong and Spritzer, 2002, [2]). With water acting as the oxidant, hydrogen is produced from two main reactions namely (1) the carbon-steam gasification and (2) the water gas shift reaction.

Carbon-steam gasification: C + H2O => CO + ¾ (1)

Water gas shift reaction: CO + H2O => CO2 + ¾ (2) The CO content in the gaseous product stream is low due to the water gas shift reaction (2) converting CO to CO2 and ¾.

At reduced operating temperatures (420°C - 460°C) the methane concentration in the off-gas increases. This can be attributed to CO reacting with ¾ in the methanation reaction. This decreases Th yields and increases CH4 production.

Methanation reaction: CO + 3.¾ => CH ₄ + H ₂ O

The feed temperature ramp-up rate impacts the formation of coke and tar and reduces the yield of useful gas released from the gasification process. Coke and tar formation impacts continuous processing, resulting in settling of solids and subsequent reactor plugging.

The rapid temperature increase as a result of partial oxidation at supercritical water conditions has been proven effective for reducing the coke and tar formation. Introduction of sub-stoichiometric oxygen (O2) allows for reaching self-sustainability in terms of the heating requirement with a minimal impact to the heating value of the syngas produced.

The invention is directed towards more effective and efficient gasification.

References

1. Casademont, C., Garcia- Jarana, M., Sanchez-Oneto, J, Portela, J.R., Martinez de la Ossa, E.J., Supercritical water gasification: a patents review, Rev Chem Eng, 2016

2. Hong, G.T. and Spritzer, M.H., Supercritical water partial oxidation, Proceedings of the 2002 U.S. DOE Hydrogen Program Review, NREL/CP-610-23405, 2002

Summary of the Invention

The invention provides a process for supercritical gasification of feedstock, as set out in claim 1 and the dependent claims 2 to 27. IT also provides a supercritical treatment apparatus as set out in claim 28 and its dependent claims 29 to 34.

We describe a process of performing supercritical gasification of a waste stream, the process comprising the steps of:

(a) pre-heating the stream,

(b) feeding the stream through a supercritical tubular reactor, (c) cooling the stream from the reactor outlet to provide a cooled stream,

(d) reducing pressure of the cooled stream to provide a first reduced-pressure stream,

(e) separating said first reduced-pressure stream into the gas and liquid components, and

(f) cooling the liquid component and performing a further pressure reduction to provide a reduced pressure liquid stream.

Preferably, the process includes a further step (g) of performing separation of the reduced-pressure liquid component to provide a further gas component and liquid component.

Preferably, step (b) includes oxidant injection at a plurality of injection points. Preferably, the injection rates are chosen according to the nature of the stream material and the subsequent requirement for rapid heating.

Preferably, the temperature in step (b) reaches a value in the range of 450°C to 700 °C and a pressure in the range of 225Bar and 320Bar.

Preferably, the step (d) reduces the pressure to a level which is lower than upstream but still sufficiently high such that the gas component after the first separation step (e) may be stored or flowed to a point of use without specific gas compression.

Preferably, the step (d) reduces the stream pressure to a value in the range of 20Bar to 40Bar.

Preferably, the second pressure let-down step reduces the pressure to approximately atmospheric.

In one example, the temperature in the reactor reaches a value in the range of 550°C and 700°C and the pressure reaches a value in the range of 240 Bar and 260 Bar.

In one example, said step (a) is performed by heat exchange with hot flue gas from the reactor heater. Preferably, said step (a) is performed by heat exchange with hot treated effluent from the reactor.

Preferably, the first pressure let-down step (d) allows for extraction of the gas component from the separation step (e) at a pressure of between 10 Bar and 50 Bar. Preferably, said pressure is in the range of 20 Bar and 40 Bar. Preferably, a portion of the gas component from step (e) is directed to a fired heater, heating the reactor to the required operating temperature, and hot effluent gas from the fired heater is used to preheat the air fed to the fired heater before combustion.

In one example, a portion of the gas from step (e) is utilised as feed to a downstream process, one example being a gas engine or microturbine and converted to electricity and hot water.

Preferably, the reactions inside the reactor include one or more of hydrolysis, cleaving, water gas shift, and reforming.

Preferably, the pre-heating of step (a) is performed by an economiser heat exchanger recovering heat from the reactor effluent stream, and the economiser may be a double-pipe heat exchanger. Preferably, the reactor is a finned tubular reactor. Preferably, control of oxidant injection in the reactor during step (b) is performed to control formation of char and tar and to ensure that the process plant has flexibility of processing any of multiple different feed streams.

Preferably, the first cooling step (c) is performed to cool the stream to a temperature of between 50°C and 160°C. Preferably, near complete oxidation of residual COD is achieved by injecting hydrogen peroxide (H2O2) into the liquid stream downstream of the first gas liquid separation step.

Preferably, the pressure reduction steps are performed by introduction of choke water from a pump (P-1401A/B) and subsequently passing the effluent stream through a continuous arrangement of several capillary coils (X-1401). Preferably, the pressure let-down steps are performed by by-pass valves that allow for partial bypass of the capillary coils in order to achieve a reduced pressure drop.

Preferably, the by-pass valves are used when two-phase flow through a capillary system results in a pressure drop higher than the desired pressure drop. Preferably, the second cooling step is performed with chilled water at a temperature in the range of 4°C and 8°C to remove moisture from the gaseous product stream.

Preferably, gaseous product released from the second gas-liquid separator (S-1502) is removed using an inert gas blanketing system, directing the gas to a catalytic convertor (X-1501) for conversion of any residual carbon monoxide to carbon dioxide. Preferably, at least some gas is delivered to a gas storage tank, and is subsequently distributed either back to the reactor heater or as feed to any downstream process, in one example being a gas engine or microturbine for the generation of electricity. Preferably, the gas used for heating the reactor (a supply of natural gas used for start-up) is replaced with the produced gas having sufficient energy available to maintain the reactor effluent temperature.

We also describe a supercritical treatment apparatus comprising components including a controller with a digital data processor and a reactor, adapted to perform a process of any example described herein.

Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:

Fig. l is a flow diagram for a gasification plant of the invention.

Referring to Fig. 1 supercritical gasification of a waste stream uses a supercritical finned tubular reactor (R-1301) with optional oxidant injection at a plurality of injection points, the injection rates being chosen according to the nature of the stream material and downstream process control parameters. After the reactor there is heat recovery by an economiser E-1101 from the effluent stream, subsequent cooling (C-1101) followed by pressure reduction (X-1401) to provide a first reduced-pressure stream which is separated (S-1501) into gas and liquid components. The liquid component is cooled (C-1402) and there is a further pressure reduction (X-1402) to provide a reduced-pressure liquid stream. There is then separation (S-1502) of the reduced-pressure liquid component to provide a further gas component and liquid component. Gas components from the first gas and liquid separation step (S-1501) are distributed based on the requirement of the process or downstream processes. This may include gas supply: for heating of the reactor (R-1301), as feed for further downstream processing of the produced gas, to a gas engine for the generation of electricity and to a microturbine for generation of electricity and hot water. Control of the apparatus is achieved by a controller with digital data processors as are well known in the art.

In more detail, a supercritical water gasification (SCWG) process produces a valuable gas stream (syngas) from an organic waste stream. Organic waste streams with a chemical oxygen demand (COD) are converted to valuable syngas and clean water. Supercritical water serves as the solvent and the oxidant in the process, supplying the required oxygen for effective COD reduction and in the process releasing hydrogen (¾) as a product of the gasification reaction.

In various examples, a horizontal, finned, tubular reactor is operated at pressures between 225 Bar and 320 Bar, and more preferably between 240 bar and 260 bar. The reactor is heated by a heater that may or may not be a gas-fired heater and has an operating temperature of between 450°C and 700°C, preferably 550°C to 600°C.

The reactor is constructed of a material that can withstand these operating conditions, typically a high-nickel alloy. Nickel may also serve as catalyst for some of the desirable reactions in the gasification process. Depending on the feed being processed, the reactor may or may not have multiple oxygen injection points for the purpose of initiating a partial oxidation reaction, thus releasing thermal energy utilised to further advance the gasification and reforming reactions. A feed preheater recovers energy from the reactor effluent and a bypass allows for flexibility of controlling the preheat temperature and temperature ramp-up rate. A two-step pressure let-down sequence allows for extraction of the gas products produced and storage at a pressure of between 10 Bar and 50 Bar (more preferably 20 Bar and 40 Bar) without any additional pressurisation step. The gas product (typically consisting of ¾, CO2, CH4 and CO) is directed to a heater H-1301, heating the finned gasification reactor to the required operating temperature. The hot exhaust gas from the heater is used to preheat the air fed to the fired heater before combustion. Gas product, from the gasification process, not utilised for heating the reactor may be utilised as feed to a downstream processing unit or converted to electricity and hot water in a gas engine or microturbine. The process allows for a gasification efficiency of 75% to 95%.

Reactions inside the reactor may include hydrolysis, cleaving, water gas shift, and reforming.

The organic stream to be treated contains water that takes part in the SCWG reaction. The feed stream can either be directly from the source or from a feed preparation step where the optimum ratio of water to organic feed is controlled. The feed stream is pressurised to a pressure of 225Bar to 320 Bar (preferably 240 Bar to 260 Bar) using a high-pressure pump, P-1001, and the flow rate (measured by the flow transmitter - FT) is controlled by a variable speed drive on the pump motor. The waste stream is preheated in an economiser heat exchanger, E-l 101, recovering heat from the reactor effluent stream. The economiser may be a double-pipe heat exchanger. The preheated waste stream is routed to the finned, horizontal tubular reactor, R-1301, which is heated to a temperature in the range of 450°C to 700°C (preferably 550°C to 600°C) using a heater, FI-1301.

During start-up, the heater H-1301, in an example where a gas-fired heater is utilised, is fuelled by burning of natural gas. The flow rate of the natural gas and air fed to the fired heater H-1301 is controlled to ensure that the reactor R-1301 outlet temperature is achieved. Energy is recovered from the hot flue gas from the fired heater and this is passed through an air preheater, H-1302, preheating the air to the reactor operating temperature. Multiple oxygen injection points may be added to the process stream, allowing for rapid heating of the stream due to partial oxidation. The use of inline oxygen injection allows for effective partial oxidation and the subsequent temperature rise as the energy released from the oxidation reactions can be absorbed by the bulk of the liquid in the line. Oxygen injection points may be before and after the economiser heat exchanger (E- 1101). This allows for utilising partial oxidation to drastically increase temperature ramp-up rates when and if required.

The hot effluent stream from the reactor, R-1301 is sent to the economiser heat exchanger E-l 101, where energy recovery to the feed stream is maximised. A bypass around the economiser heat exchanger allows for fine feed temperature (measured by the temperature transmitter (TT) downstream of E-l 101) control as well as controlled temperature ramp-up rates depending on the feed. Having control over the rate at which the temperature is increased plays a vital role in the formation of char and tar and thus in the continuous operation of the SCWG plant. Good control over the temperature ramp-up rate also ensures that the plant has the flexibility of processing various feed streams as opposed to being set up to treat a single feed stream with specific char and tar formation characteristics.

From the economiser heat exchanger, the hot process stream is sent to a cooler, C-1101. This cooler can be utilised to recover a considerable amount of energy, depending on the extent of reactor feed preheating. The process stream is cooled to a temperature (measured by a temperature transmitter (TT) downstream of C-1101) of between 50°C and 160°C after which it is sent to the first pressure let-down step performed by X-1401 where the effluent pressure is reduced from a value in the range of approximately 240 Bar to 260 Bar (measured using a pressure transmitter WO 2021/204608 . g . PCT/EP2021/058358

(PT) upstream of X-1401) to a value in the range of 20 Bar to 40 Bar. The pressure drop is achieved by the introduction of choke water from a pump P-1401A/B and subsequently passing the effluent stream through an arrangement of several separate capillary coils (X-1401) with small internal dimensions. In one example, for a feed rate of 2.5 m ³ /h, the total length of the capillary coil is 80 meters, preferably constructed of 6mm diameter, Schedule 40, coiled pipe.

Two-phase flow in the capillary coils result in an increased pressure drop. The system is fitted with bypass valves, enabling partial bypass of the capillary coils and the subsequent decreased pressure drop. The combination of choke water flow and bypassing of the capillary coils allows for fine pressure control. The pressure let-down system ensures a gradual pressure let-down without the use of any hard restrictions (valves or orifice plates) in the process line. This vastly improves pressure control and let-down reliability and largely eliminates the need for maintenance on the pressure let-down step.

After the first pressure let-down step the process stream is sent to a first gas-liquid separator, S- 1501. In one example, for a feed rate of 2.5 m ³ /hr, the gas-liquid separator (S-1501) is a vertical pressure vessel with an internal diameter of 0.8 meters and a total height of 2.8 meters. The gas- liquid separator operates at a pressure in the range of 20 Bar to 40 Bar and achieves near complete separation of the gas produced from the gasification reaction. A level transmitter (LT) on S-1501 and a level control valve on the liquid effluent stream is used to control the liquid level inside the separation vessel. The gaseous stream, containing among others ¾, CTB, CO2, CO and moisture is passed to a cooler, C-1401 which utilises chilled water at a temperature in the range of 4°C and 8°C to remove moisture from the gaseous product stream. After moisture removal, the gas stream composition is determined using an inline gas analyser (AT-1401) and is subsequently sent to a gas storage tank, T-1401, at a pressure in the range of 15 Bar to 35 Bar.

Components contributing to the overall COD of the waste stream, that are not converted to syngas, have a vastly increased biodegradability due to the high reactor operating temperatures. Any residual COD retained within the treated liquid effluent stream, downstream of the first gas liquid separation step (S-1501), is oxidised by the injection of hydrogen peroxide (H2O2). The process stream temperature of 50°C - 160°C is favourable for near complete oxidation of any COD containing components still present in the liquid stream.

The liquid stream from the first gas-liquid separator S-1501 is sent to a second cooler C-1402, cooling the liquid effluent stream to a value in the range of 20°C to 40°C. The liquid process stream is then sent to a second pressure let-down stage X-1402 (similar to the let-down stage X-1401), reducing the pressure from a value in the range of 20 Bar to 40 Bar (measured using a pressure transmitter (PT) upstream of X-1402) to approximately atmospheric pressure. The pressure let down is achieved using choke water and subsequently passing the process stream through a second set of continuous arrangement of several capillary coils in the let-down stage X-1402. Similar to X-1401, bypass valves allow for partial bypass of the capillary coils, reducing the total process stream pressure drop, in case of two-phase flow. In principle, the advantages of the second pressure let down step are the same as those for the first pressure let down step.

The effluent stream, at now reduced-pressure, is sent to a second gas-liquid separator S-1502 from where the gas-free liquid effluent is either pumped or flows under gravity to disposal. A level transmitter (LT) on S-1502 and a level control valve on the liquid effluent stream is used to control the liquid level inside the separation vessel. Any gaseous product released from the second gas- liquid separator is removed using a nitrogen (or other inert gas) blanketing system, directing the gas to a catalytic convertor X- 1501. The catalytic convertor ensures the conversion of any residual carbon monoxide to carbon dioxide. The carbon dioxide is released to the atmosphere.

From the gas storage tank T-1401, gas is distributed either back to the fired heater H-1301, heating the reactor (R-1301), to a downstream unit for further processing or to a gas engine or microturbine for the generation of electricity and hot water. The gas used for heating the reactor supplements the natural gas used for start-up and sufficient energy is available from the produced gas to maintain the reactor effluent temperature of 450°C to 700°C. Should additional energy be required to maintain the reactor operating temperature, natural gas is used to provide the required trim.

An online gas analyser continuously monitors the quality of the gas produced, providing gas quality information to the processes where the gases are being utilised. After combustion of the produced gas, the combined effluent stream released to atmosphere is at a temperature in the range of 180°C and 280°C and consists mainly of CO2 and water. One of the benefits of the process is the absence of pollutant NO _x gasses in the off-gas stream released to atmosphere.

The following are some features and advantages of the invention.

- Continuous processing of the waste and/or organic feed stream to be gasified.

- Excellent plug flow in the reactor, eliminates back mixing and allows for short residence times. Harnessing the energy released from the exothermic partial oxidation reactions to effectively heat the bulk process stream.

High operating temperature and pressure, achieving supercritical water conditions and ensuring fast water gas shift reactions that yield an off gas containing hydrogen.

Small working volume, to the extent that the apparatus can be built on skids, and also allowing improved safety.

Effective heating in a fired heater.

Multiple oxygen injection points into the feed line and/or reactor enabling control over the temperature ramp-up rates by controlling the specific region where partial oxidation is employed. This in turn reduces the formation of coke and tar.

Choke water ensures safe and reliable pressure control.

Capillary coils in the pressure let down system eliminates requirement for pressure control valves, increases reliability and decreases the maintenance requirement.

By-pass valves on the pressure let-down system allow for bypassing sections of the capillary coil to reduce the pressure drop due to two phase flow. The portion of the process stream that is bypassed is closely related to the amount of gas produced from the gasification process.

The two-step pressure let-down sequence eliminates costs related to gas compression. High-pressure gas storage ensures reduced equipment size.

Fired heater fuelled by the product from the process may be used, thus autothermal conditions exist at steady state, eliminating the use of natural gas or electricity for additional heat input.

Economiser heat exchanger recovering energy from the reactor effluent to preheat the process feed to the reactor.

Excellent control of the formation of coke and tar. This is achieved by use of a bypass on the economiser heat exchanger, and in combination with multiple oxygen injection points for partial oxidation, ensures fine feed temperature control and temperature ramp-up rates. Effluent cooler to recover energy in the form of either hot water or steam.

Produced gaseous product is extracted and reused for providing the energy required to heat the reactor to the required operating temperature.

Hot exhaust stream from the fired heater is used for preheating the air fed to the burner - Sufficient hot exhaust gas is available to pre-heat air to the reactor operating temperature of 450°C - 700°C. - Gaseous product from the SCWG process that is not used for heating the reactor can be directed as feed to a downstream process.

- Excess gaseous product is routed to a device converting gas to electricity - typically a microturbine or a gas engine (whichever is preferred or more suitable for the specific gaseous product produced).

- Waste streams with a very high COD can be treated without the requirement for adding stoichiometric amounts of oxygen.

It will be appreciated that the oxygen injection is dependent on the specific feed that is being processed, there being flexibility for such an optimisation. The oxygen injection for partial oxidation would result in a rapid temperature increase in the bulk of the process fluid. The rate of temperature increase might be beneficial in some instances for limiting tar/char formation

The invention is not limited to the embodiments described but may be varied in construction and detail.