Installation and process for the production of synthesis gas, presenting a means of limiting CO2 emissions by means of steam

Abstract

Syngas production installation comprising: A digester for producing biogas, A mixer M for mixing biogas with steam, A biogas reforming unit for producing syngas from the biogas-steam mixture, and A device for controlling the amount of steam injected into mixer M to limit carbon formation in the reforming unit.

Description

Title of the invention: Installation and process for the production of synthesis gas, exhibiting a means of limiting CO2 emissions by means of steam

[0001] The present invention relates to a synthesis gas production installation with reduced CO2 emissions by direct reforming of biogas, i.e. reforming of biogas without prior separation of methane and CO2 and to a process implementing such an installation.

[0002] Biogas is the gas produced during the decomposition of organic matter in the absence of oxygen (anaerobic fermentation), also known as methanation. This can be a natural decomposition – as observed in marshes or municipal waste landfills – but biogas production can also result from the methanization of waste in a dedicated reactor, called a methanizer or digester.

[0003] Due to its main constituents - methane and carbon dioxide - biogas is a powerful greenhouse gas; it also constitutes, at the same time, an appreciable source of renewable energy in a context of scarcity of fossil fuels.

[0004] Biogas mainly contains methane (CH4) and carbon dioxide (CO2) in varying proportions depending on the method of production, but also, in smaller proportions, water, nitrogen, hydrogen sulfide, oxygen, as well as other organic compounds in trace amounts.

[0005] Depending on the organic matter degraded and the techniques used, the proportions of the components differ, but on average biogas contains, on a dry gas basis, 30 to 75% methane, 15 to 60% CO2, 0 to 15% nitrogen, 0 to 5% oxygen and trace compounds.

[0006] Biogas is used in various ways. In particular, the production of synthesis gas (a mixture of H2 and CO) from biogas, and therefore subsequently hydrogen, should be noted.

[0007] The risk of direct biogas reforming lies in the formation of carbon by methane cracking at the inlet of the reforming tubes. This is due to the presence of a high concentration of CO2 and a low water vapor content in the mixture entering the reforming tubes.

[0008] From this, a problem that arises is to provide an installation and a process for the production of synthesis gas which presents a means of limiting CO2 emissions.

[0009] One solution of the present invention is a syngas production plant comprising:

[0010] a. A digester for producing biogas, b. A mixer M for mixing biogas with steam, c. A biogas reforming unit for producing synthesis gas from the biogas-steam mixture, and d. A device for controlling the amount of steam injected into the mixer M to limit carbon formation in the reforming unit.

[0011] Note that the synthesis gas production facility may be included in a hydrogen production facility.

[0012] This hydrogen production facility will include:

[0013] - The synthesis gas production installation according to the invention, - A water gas reactor (WGS) to convert carbon monoxide from synthesis gas into hydrogen, and - A means of recovering hydrogen at the outlet of the gas-to-water reactor.

[0014] Depending on the case, the installation according to the invention may have one or more of the following characteristics:

[0015] - the control device includes a means for estimating the carbon affinity in the biogas-steam mixture entering the reforming unit.

[0016] - the carbon affinity estimation means comprises: a means for measuring the temperature of the biogas-steam mixture at the inlet of the reforming unit, a means for measuring the pressure of the biogas-steam mixture at the inlet of the reforming unit, a means for measuring the mole fraction of water in the biogas-steam mixture at the inlet of the reforming unit, a means for measuring the mole fraction of carbon dioxide in the biogas-steam mixture at the inlet of the reforming unit and a means for measuring the mole fraction of methane in the biogas-steam mixture at the inlet of the reforming unit. - the control device includes a means for developing a control signal based on the estimated carbon affinity. - the installation includes a means of transmitting the control signal to mixer M. - the installation includes upstream of mixer M a compressor and a biogas pretreatment unit to remove impurities present in the biogas. - the installation includes a system for recovering heat from the combustion fumes from the reforming unit, said heat being used to produce steam. - the installation includes downstream of the reforming unit a heat recovery boiler to cool the synthesis gas and produce steam. - At least part of the steam produced is introduced into mixer M. - the reforming unit includes a nickel or nickel-rhodium catalyst.

[0017] The present invention also relates to a process for producing synthesis gas using an installation according to the invention and comprising:

[0018] i. An anaerobic fermentation step in the digester to produce biogas, ii. A step of mixing biogas with steam in mixer M, iii. A biogas reforming step from the biogas-steam mixture, including a sub-stage of combustion of a combustible gas, so as to produce synthesis gas, and iv. A step controlling the amount of steam injected into the mixer M in order to limit carbon formation in the reforming unit.

[0019] The process according to the invention described herein produces, on the one hand, biogas by anaerobic fermentation from biomass in a digester and, on the other hand, hydrogen from this biogas stream and an oxidant stream (CO2, steam or both) in a tubular furnace by the following reactions:

[0020] (1) CH4 + H2O = CO + 3H2

[0021] (2) CH4 + CO2 = 2CO + 2H2

[0022] These reactions are highly endothermic, so the necessary heat is supplied by the combustion of the fuel in burners located in a combustion chamber. Several reforming tubes are placed in this combustion chamber. They are filled with a reforming catalyst or structured packing.

[0023] Preferably the fermentation step produces a biogas stream composed of about 45-60% CH4 and about 40-55% CO2 plus certain impurities such as H2S, VOCs, siloxanes.

[0024] During the reforming stage, the biogas-steam mixture is sent into the reforming tubes comprising a catalyst.

[0025] In the case where it is desired to produce hydrogen from synthesis gas, the following steps will be added to the process according to the invention:

[0026] - A gas-to-water reaction step in the WGS reactor so as to convert carbon monoxide from the synthesis gas into hydrogen, and - A step to recover hydrogen at the outlet of the WGS reactor.

[0027] Depending on the case, the method according to the invention may have one or more of the following characteristics:

[0028] - the control step is carried out in such a way that the carbon affinity of the biogas-steam mixture produced in step ii. is less than 1.

[0029] - the control step includes the following substeps: a substep for estimating the carbon affinity of the biogas-steam mixture produced in the step

[0030]

[0031]

[0032]

[0033]

[0034] ii., a substep of comparing the measured carbon affinity with the value of "1", a substep of adjusting the amount of steam introduced into the mixer M in step ii. - The carbon affinity estimation substep includes: measuring the temperature of the biogas-steam mixture at the inlet of the reforming unit, measuring the pressure of the biogas-steam mixture at the inlet of the reforming unit, measuring the mole fraction of water in the biogas-steam mixture at the inlet of the reforming unit, measuring the mole fraction of carbon dioxide in the biogas-steam mixture at the inlet of the reforming unit, and measuring the mole fraction of methane in the biogas-steam mixture at the inlet of the reforming unit. - the measured data are entered into a calculation model allowing estimation of the carbon affinity of the biogas-steam mixture produced in step ii. - the control step includes a sub-step of developing a control signal based on the comparison performed and a sub-step of transmitting this control signal to mixer M. - the process includes upstream of step ii. a biogas compression step and a biogas pretreatment step in order to eliminate impurities present in the biogas. - the process includes a step of recovering heat from the combustion fumes from reforming in order to produce steam. - the process includes a step of recovering heat from the synthesis gas produced by reforming in order to cool said synthesis gas and produce steam. - the process includes a step of introducing at least a portion of the steam produced into the mixer M. Carbon affinity is calculated according to the "real gas principle" along the reformer tube, considering the gas temperature for calculating the equilibrium constant [Rostrup-Nielsen 2011: JR Rostrup-Nielsen, L. Christiansen, Concept in Syngas Manufacture, Catalytic Science Series, Imperial College Press, pp. 233-293, 2011]. With the "real gas principle," the reaction quotient is calculated using the actual gas composition. This is the most conservative way to determine whether carbon can form. The carbon formation reactions considered are: 1. Decomposition of methane CH4 = C + 2H2 2. Boudouard Equilibrium 2CO = C + CO2 3. Reduction of CO₂: CO₂ + H₂ = C + H₂O Finally, the thermodynamics considered for calculating the equilibrium constant assumed that the carbon formed was graphite. This calculation is performed for a range values ​​for the biogas composition, the water vapor mole fraction, and the operating parameters of the reforming unit (inlet temperature, reforming temperature, pressure) are used. For each calculation, the maximum carbon affinity value along the tube is stored in a database. From this database, a correlation of the following form is derived:

[0035] Max (ac) = a*T + b*P + c*H + d*T2 + e*P2 +f*H2 + g*TP + h*TH + i*PH + j

[0036] with T the inlet temperature of the reforming tube, P the inlet pressure, H the mole fraction of water in the gas mixture entering the reforming unit.

[0037] [Tables 1] Preferable More preferable Even more preferable min max min max min max a 2.92E-04 1.75E-04 2.57E-04 2.10E-04 2.45E-04 2.22E-04 b -1.93E-02 -1.16E-02 -1.70E-02 -1.39E-02 -1.62E-02 -1.47E-02 c -1.22E+01 -7.31E+00 -1.07E+01 -8.77E+00 -1.02E+01 -9.25E+00 d -4.39E-07 -2.64E-07 -3.87E-07 -3.16E-07 -3.69E-07 -3.34E-07 e 2.61E-06 1.56E-06 2.29E-06 1.88E-06 2.19E-06 1.98E-06 f 7.64E-04 4.58E-04 6.72E-04 5.50E-04 6.42E-04 5.81E-04 g 1.24E-04 7.42E-05 1.09E-04 8.90E-05 1.04E-04 9.40E-05 h 1.15E-02 6.90E-03 1.01E-02 8.28E-03 9.66E-03 8.74E-03 i 6.65E+00 3.99E+00 5.85E+00 4.79E+00 5.58E+00 5.05E+00 j 5.83E+00 3.50E+00 5.13E+00 4.20E+00 4.90E+00 4.43E+00

[0038] Consequently, the process according to the invention allows careful control of the quantity of steam sent into the reforming tubes in order to avoid the formation of carbon and thus limit carbon dioxide emissions.

Claims

Demands

1. Syngas production installation comprising: a. A digester for producing biogas, b. A mixer M for mixing biogas with steam, c. A biogas reforming unit for producing syngas from the biogas-steam mixture, and d. A device for controlling the amount of steam injected into mixer M for limiting carbon formation in the reforming unit, the control device comprising a means for estimating the carbon affinity in the biogas-steam mixture entering the reforming unit.

2. Installation according to claim 1, characterized in that the carbon affinity estimation means comprises: - A means for measuring the temperature of the biogas-steam mixture at the inlet of the reforming unit, - A means for measuring the pressure of the biogas-steam mixture at the inlet of the reforming unit, - A means for measuring the mole fraction of water in the biogas-steam mixture at the inlet of the reforming unit, - A means for measuring the mole fraction of carbon dioxide in the biogas-steam mixture at the inlet of the reforming unit, and - A means for measuring the mole fraction of methane in the biogas-steam mixture at the inlet of the reforming unit.

3. Installation according to one of claims 1 or 2, characterized in that the control device includes a means for developing a control signal as a function of the estimated carbon affinity.

4. Installation according to claim 3, characterized in that it comprises a means for transmitting the control signal to the mixer M.

5. Installation according to any one of claims 1 to 4, characterized in that it comprises upstream of mixer M a compressor and a biogas pretreatment unit enabling the removal of impurities present in the biogas.

6. Installation according to any one of claims 1 to 5, characterized in that it comprises a system for recovering heat from the combustion fumes from the reforming unit, said heat being used to produce steam.

7. Installation according to any one of claims 1 to 6, characterized in that it comprises downstream of the reforming unit a heat recovery boiler for cooling the synthesis gas and producing steam.

8. Installation according to any one of claims 6 or 7, characterized in that at least a part of the steam produced is introduced into the mixer M.

9. A process for producing synthesis gas employing an installation as defined in any one of claims 1 to 8 and comprising: i. An anaerobic fermentation step in the digester to produce biogas, ii. A step of mixing the biogas with steam in the mixer M, iii. A step of reforming the biogas from the biogas-steam mixture, comprising a substep of combustion of a combustible gas, to produce synthesis gas, and iv. A step of controlling the quantity of steam injected into the mixer M to limit carbon formation in the reforming unit, the control step being carried out such that the carbon affinity of the biogas-steam mixture produced in step ii. is less than 1.

10. A process according to claim 9, characterized in that the control step comprises the following substeps: - A substep for estimating the carbon affinity of the biogas-steam mixture produced in step ii., - A substep for comparing the measured carbon affinity with the value of "1", - A substep for adjusting the quantity of steam introduced into the mixer M in step ii.

11. A method according to claim 10, characterized in that the carbon affinity estimation substep comprises: - measuring the temperature of the biogas-steam mixture at the inlet of the reforming unit, - measuring the pressure of the biogas-steam mixture at the inlet of the reforming unit, - measuring the mole fraction of water in the biogas-steam mixture at the inlet of the reforming unit, - measuring the mole fraction of carbon dioxide in the biogas-steam mixture at the inlet of the reforming unit, and - measuring the mole fraction of methane in the biogas-steam mixture at the inlet of the reforming unit.

12. A method according to claim 11, characterized in that the measured data are entered into a calculation model allowing estimation of the carbon affinity of the biogas-steam mixture produced in step ii.

13. A method according to any one of claims 10 to 12, characterized in that the control step includes a substep of developing a control signal based on the comparison performed and a substep of transmitting this control signal to the mixer M.

14. A process according to any one of claims 9 to 13, characterized in that it comprises upstream of step ii. a biogas compression step and a biogas pretreatment step so as to remove impurities present in the biogas.

15. A process according to any one of claims 9 to 14, characterized in that it comprises a step of recovering heat from the combustion fumes from reforming so as to produce steam.

16. A process according to any one of claims 9 to 15, characterized in that it comprises a step of recovering heat from the synthesis gas from reforming so as to cool said synthesis gas and produce steam.

17. A method according to any one of claims 15 or 16, characterized in that it comprises a step of introducing at least a portion of the steam produced into the mixer M.