NZ626549B2 - Biomethane production method - Google Patents

Abstract

Disclosed is a method for the production of biomethane from hydrocarbon feedstock (200), comprising at least the following steps: gasification (201) of the feedstock in order to produce a synthesis gas in a gasification reactor; purification (204), in a purification unit, of the synthesis gas produced during the biomass gasification step; methanisation (206) of the purified synthesis gas in a methanisation reactor; and adjustment to specifications (207) of the gas mixture obtained during the synthesis gas methanisation step, in order to obtain biomethane. During the specifications adjustment step, a flow comprising at least the excess carbon monoxide is recirculated (208) in the gasification reactor. The invention also relates to a device for carrying out the method of the invention. ed during the biomass gasification step; methanisation (206) of the purified synthesis gas in a methanisation reactor; and adjustment to specifications (207) of the gas mixture obtained during the synthesis gas methanisation step, in order to obtain biomethane. During the specifications adjustment step, a flow comprising at least the excess carbon monoxide is recirculated (208) in the gasification reactor. The invention also relates to a device for carrying out the method of the invention.

Description

BIOMETHANE PRODUCTION METHOD The present invention relates to the field of biomethane production and more specifically to a method for producing biomethane by gasification of hydrocarbon feedstock.

Currently, Biomethane (or SNG: Substitute Natural Gas) production can be achieved by thermochemical biomass conversion.

The biomass may consist of wood residue (chips, sawdust, bark), plant residue (peel, seeds, stems, cereals, wheat, etc.), farming sector residues, agrifood waste (fats, slaughterhouse residues), animal waste (manure), waste related to human activity (slurry), marine seaweed, etc.

This conversion is realized by a method consisting of three main steps: - gasification of the biomass to produce synthesis gas (syngas) composed mostly of hydrogen (H ), carbon monoxide (CO), carbon dioxide (CO ) and methane (CH ), - catalytic methanation that consists of converting the hydrogen and carbon monoxide into methane and, - adjusting to specifications, which is aimed at eliminating residual hydrogen, residual carbon monoxide, water and carbon dioxide so as to produce biomethane that meets the specifications for injection into the natural gas network, especially in terms of heating value (HHV) and Wobbe Index.

Biomass gasification is carried out within a reactor in which biomass undergoes different reaction steps.

The biomass is first subjected to thermal degradation by drying and then to devolatilization of organic matter to produce a carbonaceous residue (char), a synthesis gas (H , CO, CO , CH , etc.), and condensable compounds contained 2 2 4 within the syngas (tars and more generally, whatever is condensable). The carbonaceous residue can then be oxidized by the gasification agent (water vapor, air, oxygen) to produce hydrogen, carbon monoxide, etc. Depending on its nature, this gasification agent can also react with the tars or the major constituent gases.

Thus, if it is water vapor (H O), a gas reaction to water (or WGS: Water Gas Shift) occurs in the gasification reactor according to the following equilibrium: CO + H O ⇔ H + CO 2 2 2 The reactor pressure has little effect on this reaction. In contrast, the equilibrium is strongly linked to the reactor temperature and to the initial concentrations of reagents.

For existing methods, the H /CO ratio never exceeds two at the end of the gasification step and, for example, it is of the order of 1.8 for the dual fluidized bed FICFB (Fast Internally Circulating Fluidized Bed) concept. This H /CO ratio is an important factor for biomethane production as the methanation reaction that makes the production of methane possible and on which the biomethane production method is based is as follows: CO + 3 H ⇔ CH +H O 2 4 2 To maximize methane CH production and minimize excess carbon monoxide, hydrogen and carbon monoxide should be in a 3:1 stoichiometric ratio.

However, even by respecting to this ratio, the reaction remains incomplete because of the chemical equilibrium. The 3:1 ratio maximizing the reaction is achieved by performing an additional WGS reaction. This can be done prior to methanation in a dedicated reactor, in the presence of specific catalysts. It is possible for it to be performed directly in the same reactor as the methanation, with a possible adjustment of the catalyst. In both cases, this reaction requires a significant injection of water vapor.

On output from the gasifier, the mass fraction of water in the synthesis gas is generally of the order of 30%. It comes partly from the biomass moisture and, in the case of certain methods, partly from direct injection of vapor into the gasifier.

However, the step of syngas purification to remove pollutants before methanation (in particular, removing tars, dust, inorganic compounds, etc.) requires a cooling operation in which the water present therein is largely condensed and eliminated, and is thus no longer available for an additional WGS step.

Indeed, after these purification steps, the mass fraction of water is reduced to about 5%, which concentration is insufficient to achieve the additional WGS reaction and achieve optimum operation of the chain of methods in its typical configuration. Thus, to make it possible to adjust the H O/CO ratio required for the methanation reaction, injection of water vapor becomes essential to avoid excess carbon monoxide before the step of adjusting to specifications. This water vapor is generally produced by recovering high temperature energy from the method; such energy would be easily marketable and constitutes a loss of direct profitability of the system. In addition, implementing the WGS translates into increased complexity of the method and an increase in capital expenditure and operating costs.

Some known methods from the family of fast internally circulating fluidized bed reactor (FICFB) gasification systems offer some solutions to these problems. In these methods, biomass gasification is implemented in a first reactor by contact with a hot heat transfer medium. This medium and a portion of the pyrolysis char formed in this reactor are extracted continuously at the base of the bed. They are then sent to a second reactor (combustor) where a fluidization medium is heated by the combustion of the char and a fuel booster before being reinjected into the gasifier. The heat transfer medium can consist of an inert solid (e.g. sand) or a mineral (olivine) with catalytic properties for cracking or reforming tars coming from the imperfect conversion of biomass. In this case, the H /CO ratio achieved is between 1.3 and 1.8.

International application WO 01/23302 describes a method belonging to this family of FICFB methods, the AER (Absorption Enhanced Reforming) method, which relates to syngas production that can be used directly in SNG-production methanation reactors. The method described in this application makes it possible to achieve the optimum specifications (H O/CO ratio greater than or equal to three) required for the methanation step.

In this AER method, the fluidization medium is no longer olivine but lime (CaO). In the gasifier, the lime absorbs the present carbon dioxide to form calcium carbonate CaCO , according to the reaction below for a temperature range of 650 to 750°C.

CaO (s) + CO (g) → CaCO (s) Because of the capture of the carbon dioxide, the WGS reaction, which is balanced, is switched to hydrogen production and carbon monoxide reduction, which leads to an increase in the H /CO ratio. Calcium carbonate is then converted into lime by the inverse reaction at a higher temperature in the combustor. H O/CO ratios of five to seven can be achieved in this way. Higher ratios, while making it possible to minimize the risk of coking on the methanation catalyst, can result in a decrease in the overall biomethane production yield and difficulties in separating the hydrogen and the syngas for adjusting the natural gas to specifications. The use of lime also leads to a slight decrease in the concentration of methane in the syngas. In addition, while lime is a good fluidization medium, its mechanical fragility makes it very prone to attrition phenomena and limits its lifespan.

A catalytic method, typically in fixed bed reactors, is also known from patent application US2010/0286292, which proposes to incorporate a WGS reactor into the conversion chain in order to adjust the H O/CO ratio close to the methanation's stoichiometry. A disadvantage of this method is that it is necessary to inject a significant quantity of water vapor, firstly to perform the additional WGS reaction and secondly, to limit the deposits of coke on the catalysts.

Another method described in documents EP1568674 and WO2009/007061 relates to the production of SNG from biomass gasification. This method consists of a purification of compounds such as hydrogen sulfide (H S) or Carbonyl sulfide (COS) by physical or chemical adsorption in fixed beds of activated carbon (AC), of metal oxides (e.g. zinc oxide ZnO), and methanation in a fluidized bed of catalytic particles with a particle size of 20 to 2,000 µm. Additional fluidization water vapor can be supplied to the reactor and recycling of hydrogen coming from the adjustment to specifications may occur. This method also has the disadvantage of requiring additional methanation and/or separation steps that increase the complexity of the method.

The purpose of the present invention is therefore to overcome one or more of the disadvantages of the prior art by proposing a method aiming to increase the conversion of biomass into biomethane and improve the overall energy efficiency thereof. The method does not require modifying the design of the gasification and/or methanation reactors and requires no additional steps, in particular the WGS step, before or in parallel with methanation.

To achieve this, the present invention proposes, according to a first aspect, a method for producing biomethane from hydrocarbon feedstock, comprising at least the following steps: - gasification of the feedstock in order to produce a synthesis gas in a gasification reactor; - purification, in a purification unit, of the synthesis gas produced during the hydrocarbon feedstock gasification step; - methanation of the purified synthesis gas in a methanation reactor; - adjustment to specifications of a gas mixture obtained during the synthesis gas methanation step, in order to obtain biomethane; during the step of adjustment to specifications, a flow comprising at least the excess carbon monoxide being recycled towards the gasification reactor.

This recycling leads to an increase in the amount of carbon monoxide in the gasifier, thus promoting the production of hydrogen according to the thermochemical equilibria described by the WGS reaction directly within this same reactor. The increase in this amount of hydrogen itself leads to increased methane production during the methanation step.

According to an embodiment of the invention, the hydrocarbon feedstock is biomass.

According to an embodiment of the invention, the recycled flow comprises residual hydrogen.

According to an embodiment of the invention, the recycled flow comprises residual methane.

According to an embodiment of the invention, on exit from the gasification reactor, the temperature of the synthesis gas is between 600 and 1000° C.

According an embodiment of the invention, the method comprises a heat exchange step after the gasification step to cool the synthesis gas to the ambient temperature.

According an embodiment of the invention, the heat exchange step is carried out before the purification step.

According to an embodiment of the invention, the method comprises a dehydration step carried out after the heat exchange step.

According to an embodiment of the invention, the method is performed at a pressure of 0.5 to 70 bar.

According to an embodiment of the invention, the temperature of the gas mixture obtained on output from the reactor is between 250 and 700°C.

According an embodiment of the invention, the flow obtained after the step of adjustment to specifications is at ambient temperature.

This invention envisages, according to a second aspect, a device for producing biomethane from hydrocarbon feedstock comprising at least: - a gasification reactor carrying out a gasification of the feedstock to produce a synthesis gas in a gasification reactor; - a purification unit performing a purification of the synthesis gas produced by the feedstock gasification reactor; - a methanation reactor realizing a methanation of the purified synthesis gas; - a means of adjustment to specifications of the gas mixture obtained during the synthesis gas methanation step, in order to obtain biomethane, the means of adjustment to specifications being designed to recycle a flow comprising at least excess carbon monoxide in the gasification reactor.

Other goals, features and advantages of the invention will be better understood in reading the following description, with reference to the drawings in an appendix given as an example: - figure 1 is a schematic representation of an embodiment of a device of the prior art; - figure 2 is a schematic representation of the implementation of a particular embodiment of the method according to the invention; - figure 3 is a schematic representation of a particular embodiment of the device that is the subject of the invention; and - figure 4 is a schematic representation of the implementation of a particular embodiment of the method according to the invention.

Although described using the example of biomass, the method according to the invention can be applied to all products likely to be gasified: biomass, coal, coke, waste, etc., and more generally to all hydrocarbon feedstocks.

The particular embodiment of the method envisaged by the present invention illustrated in figure 2 and implemented in the device illustrated in figure 3 requires at least the following steps, which are identical to those described in the prior state of the art: - gasification (201): conversion of the hydrocarbon feedstock and, for example, the biomass into synthesis gas (syngas); - purification (204) of the synthesis gas produced during the hydrocarbon feedstock gasification step to remove polluting components harmful to the lifespan of the catalyst; - methanation (206) by catalytic conversion of the syngas into biomethane; - adjustment to specifications (207) of the gas mixture obtained in the step of methanation of the synthesis gas to separate the biomethane from the other constituents, in particular hydrogen, residual carbon monoxide, water and carbon dioxide, and thus adjust the composition of the biomethane to specifications.

The biomass, being at ambient temperature and more specifically at a temperature equal to 20°C, circulating in a biomass supply line (100), supplies a gasification reactor (10). The biomass undergoes a thermochemical conversion in this gasification reactor (10), to form during the drying/pyrolysis and gasification step a syngas containing hydrogen, carbon monoxide, carbon dioxide, water, tars and pitches and/or compounds with the general formula C H , etc. This syngas flows in the syngas circulation line (1).

Before exiting the reactor, the composition of this gas changes under the action of the water vapor (injected at a temperature of between 160 and 500°C and preferably equal to 400°C) or oxidizing agent (oxygen, air, etc.) This change in composition occurs, firstly, with the thermochemical equilibria in homogeneous phase and, secondly, because of the production of compounds by heterogeneous phase gasification of part of the char.

On exit from the gasifier (10) the syngas is at a temperature of between 600 and 1000°C, preferably between 800 and 900°C and very preferably equal to 850°C.

The resulting syngas, thus purified of these pollutants (tars, COS, H S, etc.) before supplying a methanation reactor (20) then supplying, via a supply line (4), a unit (30) making the step of adjustment to specifications (also called separation of compounds) possible. At the end of these steps, the method makes it possible to obtain biomethane (31).

Under these conditions, the H /CO ratio before methanation is lower than the stoichiometric ratio and the methanation reaction can only be at its highest for the missing species, in this case hydrogen. On exit from the methanation reactor (20) the temperature of the biomethane is between 250 and 700°C, and preferably equal to 300°C. In addition to the biomethane produced, the gas output from the methanation reactor therefore contains excess carbon monoxide which has not reacted.

The gas mixture produced during the methanation step is split into four flows: − a first flow comprising methane which constitutes the biomethane itself, intended to be used; − a second flow comprising carbon dioxide; − a third flow comprising water vapor; and − a fourth flow comprising at least excess carbon monoxide, and possibly hydrogen which has not reacted because of the thermochemical equilibria.

In particular embodiments of the method according to the invention, this fourth flow comprising at least excess carbon monoxide is recycled (208) within the gasification reactor.

After the adjustment to specifications step, the various flows are at ambient temperature and preferably at a temperature equal to 20°C.

Since it is thus not necessary to utilize the WGS reaction upstream of or in parallel with the methanation reaction to effect an adjustment of the H /CO ratio, an energy and matter gain is achieved equivalent to the flow of this vapor which is ordinarily added.

According to embodiments of the invention, to remedy any deposit of coke on the surface of the catalyst, due to the operation of the methanation reactor in these sub-stoichiometric conditions and for limited H O concentrations, a specific treatment of the catalyst is used, such as for example doping with boron or with a suitable catalytic support, in particular by adjusting its acidity.

According to embodiments of the invention, in order to limit the coke deposit, a limited intake of water vapor is performed to adjust its concentration before methanation, either directly or by adjusting the temperature of the upstream wash.

In relation to the step of adjusting the biomethane to specifications, which is typically a separation step, the following methods, for example, are used: PSA (Pressure Swing Adsorption); TSA (Temperature Swing Adsorption); membrane separation; amine chemical absorption or physical absorption with triethylene glycol.

These various technologies can be used singly or in combination with each other.

According to embodiments illustrated in figure 4, the gasification step (201) is followed by a heat exchange step (202) to cool the syngas before passing into the purifier (16). In this way, the flow of syngas, which is at a temperature of between 600 and 1000°C, preferably between 800 and 900°C and very preferably equal to 850°C after passing through a heat exchanger (14) is at a temperature between 4 and 80°C, preferably between 25 and 35°C, and very preferably at ambient temperature (more precisely, equal to 20°C).

At the end of this heat exchange step (202), the water is partially removed from the syngas (circulating in the duct (2) positioned between the heat exchanger (14) and the dehydration unit (15)), which is cooled and dried (203) in a dehydration unit (15) from which the condensates are removed. This step of dehydration by cooling and condensation of the water vapor (203) is followed by a step of purification (204) in a purification unit (16) in which the flow is brought by the duct (3) positioned between the dehydration unit (15) and the purification unit (16).

The method according to the invention can be applied generally to all gasifiers.

Preferably, the method can be applied to methods producing a gas without dilution by the nitrogen of the air.

The method is not specific to any particular methanation process and can be applied to all these methods. In the case of fixed bed recycling type of methods, the method even makes a decrease in the rate of recycling possible.

The method according to the invention is, in embodiments, utilized at a pressure of between 0.5 and 70 bar, and preferably between 1 and 5 bar.

The present invention also envisages a device for producing biomethane from hydrocarbon feedstock, comprising at least: - a gasification reactor (10) carrying out a gasification of the feedstock to produce a synthesis gas in a gasification reactor; - a purification unit (16) performing a purification of the synthesis gas produced by the feedstock gasification reactor (10); - a methanation reactor (20) realizing a methanation of the purified synthesis gas; - a means of adjustment to specifications (30) of the gas mixture obtained during the synthesis gas methanation step, in order to obtain biomethane.

The adjustment to specifications means (30) of this device is designed to recycle a flow comprising at least excess carbon monoxide towards the gasification reactor.

The invention will now be illustrated with the following non-limiting examples.

Examples: In order to demonstrate the value of the invention, comparative simulations of the previous solutions and of the present invention were performed with a CAPE (Computer Aided Process Engineering) process simulation tool.

The simulations were conducted for one metric ton of dry biomass to be converted per hour to obtain biomethane.

In the following examples, the flows correspond to: - flow 100, the biomass; - flow 101, the vapor injected into the gasifier; - flow 1, the syngas output from gasifier; - flow 2, the cooled syngas; - flow 3, the dehydrated syngas; - flow 102, the condensates, - flow 103, the vapor for the additional WGS; - flow 4, the biomethane before adjustment to specifications, - flow 104, the CO/H ; - flow 105, the CO ; - flow 106, the H O; Comparative example according to the prior art: In this example according to the prior art (illustrated in figure 1), the biomass is introduced into a gasification reactor (10), in which it undergoes a thermochemical conversion; then the synthesis gas (or syngas) output from the gasifier after passing through a heat exchanger (14) passes into a dehydration unit (15). This step of dehydration by cooling and condensation of the water vapor is followed by a step of purification (204) in a purification unit (16) and a WGS step in a WGS reactor (17) with the addition of water vapor. The syngas then undergoes a compression step (205) in a compressor (18). The syngas thus produced and purified of these pollutants supplies a methanation reactor (20) then undergoes a separation step in a dedicated unit for adjustment to specifications (30).

Table 1 summarizes the conditions and results of the simulation for the method according to the prior art at different steps in the method and for the various effluents.

Table 1 Examples according to particular embodiments of the invention: In this example according to the invention (illustrated in figure 3), the biomass circulating in a biomass supply line (100) supplies a gasification reactor (10), in which it undergoes a thermochemical conversion; then the synthesis gas (or syngas) output from the gasifier after passing through a heat exchanger (14) passes into a dehydration unit (15). This step of dehydration by cooling and condensation of the water vapor (203) is followed by a step of purification in a purification unit (16).

The syngas then undergoes a compression step (205) in a compressor (18). The syngas thus produced and purified of these pollutants supplies a methanation reactor (20) then undergoes a separation step in a unit for adjustment to specifications (30).

Table 2 summarizes the conditions and results of the simulation for embodiments of the method according to the invention at different steps of the method and for the various effluents.

Table 2 Analysis of the results The energy yield of the method based on methane production is defined by the following relation: m PCI CH * CH m PCI biomass * biomass wherein: m represents the mass of methane. m represents the mass of biomass. biomass PCI represents the lower heating value of the methane.

PCI represents the lower heating value of the biomass. biomass The simulations carried out show that this yield is increased to 61.2% by utilizing the proposed solution in embodiments of the present invention, while the existing solutions only make it possible to achieve 54.2%, i.e. a 7 % increase.

In embodiments, the method according to the invention is based on recirculation of the CO/H flow to the gasifier. The recirculation flow rate associated therewith is intimately linked to the partial pressure of water in the gasifier (water introduced with the biomass and possibly as gasification agent). The consequence of too low a concentration in the gasifier is a very high recirculation flow rate without any consequent impact on the quantity of CH produced.

Embodiments of the method that is the subject of the present invention thus make it possible, in comparison with the methods of the prior art, to achieve: - an increased hydrogen flow produced by the gasification, from 23.75kmol/h to 27.27 kmol/h; - a shift vapor flow (40,00 kmol/h) removed; - a decrease in the water flow to be removed on output from methanation from 50.57 kmol/h to 11.85 kmol/h, resulting in a saving in the cooling energy consumed by the method; - an adjustment to specifications of the volume fraction of hydrogen, a complex operation and source of loss of efficiency of the method, which is no longer needed because of a H /CH ratio on output from the methanation reactor output that goes from 6.3% to 2.7%. This improvement is mainly due to the excess of carbon monoxide in the methanation reactor. - a flow of biomethane (SNG) on output from the system that goes from 11.81 kmol/h to 13.33 kmol/h, a 12.9% gain.

The scope of this invention is not limited to the details given above and allows embodiments in many other specific forms without moving away from the invention's field of application. Therefore, the present embodiments should be considered illustrative, and can be modified without however moving outside the scope defined by the claims.

Claims (12)

1. Method for the production of biomethane from hydrocarbon feedstock, comprising at least the following steps: 5 - gasification (201) of the feedstock in order to produce a synthesis gas in a gasification reactor (10); - purification (204), in a purification unit (16), of the synthesis gas produced during the biomass gasification step; - methanation (206) of the purified synthesis gas in a methanation 10 reactor (20); - adjustment to gas specifications (207) of the gas mixture obtained during the synthesis gas methanation step, in order to obtain biomethane; characterized in that, during the step of adjustment to specifications (207), a 15 flow comprising at least the excess carbon monoxide is recycled (208) towards the gasification reactor (10).

2. Method according to claim 1, characterized in that the hydrocarbon feedstock is biomass.

3. Method according to one of claims 1 or 2, characterized in that the 20 recycled flow comprises residual hydrogen.

4. Method according to one of claims 1 to 3, characterized in that the recycled flow comprises residual methane.

5. Method according to one of claims 1 to 4, characterized in that on exit from the gasification reactor (10), the temperature of the synthesis gas is 25 between 600 and 1000° C.

6. Method according to one of claims 1 to 5, characterized in that it includes a heat exchange step (202) after the gasification step (201) making it possible to cool the synthesis gas to between 4 and 80°C.

7. Method according to claim 6, characterized in that the heat exchange 30 step (202) is carried out before the purification step (204).

8. Method according to one of claims 1 to 7, characterized in that it comprises a dehydration step (203), performed after the heat exchange step (202).

9. Method according to one of claims 1 to 8, characterized in that it is performed at a pressure of 0.5 to 70 bar.

10. Method according to one of claims 1 to 9, characterized in that the 5 temperature of the gas mixture obtained on output from the reactor is between 250 and 700°C.

11. Method according to one of claims 1 to 10, characterized in that the flow obtained after the step of adjustment to specifications is at ambient temperature. 10

12. Device for the production of biomethane from hydrocarbon feedstock, for implementing the method according to claims 1 to 11, comprising at least: - a gasification reactor (10) carrying out a gasification of the feedstock to produce a synthesis gas in a gasification reactor; - a purification unit (16) performing a purification of the synthesis gas 15 produced by the feedstock gasification reactor (10); - a methanation reactor (20) realizing a methanation of the purified synthesis gas; - a means of adjustment to gas specifications (30) of the gas mixture obtained during the synthesis gas methanation step, in order to 20 obtain biomethane; characterized in that the means of adjustment to specifications (30) is designed to recycle a flow comprising at least excess carbon monoxide in the gasification reactor.