CA2693459A1

CA2693459A1 - Process to produce a methane rich gas mixture from gasification derived sulphur containing synthesis gases

Info

Publication number: CA2693459A1
Application number: CA2693459A
Authority: CA
Inventors: Serge Biollaz; Tilman J. Schildhauer; Martin Seemann
Original assignee: Individual
Current assignee: Scherrer Paul Institut
Priority date: 2007-07-10
Filing date: 2008-07-03
Publication date: 2009-01-15
Also published as: WO2009007061A1; US20100205863A1; EP2167617A1; CN101802146A

Abstract

The present invention discloses a method for converting a raw gas into a methane-rich and/or hydrogen-rich gas, comprising the steps of : a) providing the raw gas stemming from a coal and/or biomass gasification process, thereby the raw gas comprising beside a methane and hydrogen content carbon-monoxide, carbon-dioxide, alkanes, alkenes, alkynes, tar, especially benzole and naphthalene, COS, hydrogen sulfide and organic sulfur compounds, especially thiophenes; thereby the ratio of hydrogen to carbon monoxide ranges form 0.3 to 4; b) bringing this raw gas into contact with a catalyst arranged as a fluidized bed reactor at temperatures above 200°C and at pressures equal or larger than 1 bar in order to convert the raw gas into a first product gas, thereby simultaneously convert organic sulfur components into hydrogen sulfide, reform tars, generate water/gas shift reaction and generate methane from the hydrogen/carbonmonoxide content; c) bringing the first product gas into a sulfur absorption process to generate a second product gas, thereby reducing the content of hydrogen sulfur and COS
from 100 to 1000 ppm down to 1000 ppb or less; d) optionally bringing the second product gas into a carbon diodide removal process to generate a third product gas at least almost free of carbon dioxide; e) bringing the third product gas into a 2nd methanation process to generate a forth product gas having a methane content above 5 vol%; f)) optionally bringing the fourth product gas into a carbon dioxide removal process to generate a fifth product gas at least almost free of carbon dioxide g) bringing the fifth product gas into an hydrogen separation process in order to separate a hydrogen rich gas from a remaining methane-rich gas, called substitute natural gas.

Description

Process to produce a methane rich gas mixture from gasification derived sulphur containing synthesis gases The present invention relates to a method for converting coal or biomass to at least almost sulfur-free substitute natural gas. Further, the invention relates to a process to produce a methane rich gas mixture from gasification derived sulphur containing synthesis gases.

In particular, the present invention relates to a continuous production process of synthetic natural gas (SNG) from biomass, coal or naphta. More specifically, the present invention relates to the production of clean gaseous heating fuels from these less valuable sulphur containing hydrocarbonaceous materials.
Description of the prior art The production of SNG from biomass is the conversion of a "dirty/difficult" fuel into a clean burning well known commodity. The costumer has the freedom to use the SNG for power generation, heating or mobility. A big plus is the already existing infrastructure such as pipelines and compressed natural gas (NG) cars. To insert the product gas of the methanation into the grid it has to be cleaned of COZ.
and compressed to 5 to 70 bars to meet the standards of average natural gas.

The conversion of biomass to SNG is a complex process, which can be structured roughly into four main units; gasification, raw gas cleaning, fuel synthesis and gas sweetening. A solid feed is thermally converted to a raw gas and subsequently cleaned of particles, tars and sulphur. In the fuel synthesis, the raw gas is converted into raw SNG (a CH4/CO2 mixture) that is cleaned from COz and optionally H2 (gas sweetening) before injection into the natural gas grid.
SUBSTITUTE SHEET (RULE 26) The presence of sulphur in the feedstocks leads to the formation of H2S, COS or organic sulphur species, depending on the temperature of the gasification. Low temperature (LT) gasification promotes the formation of organic sulphur species such like thiophenes, mercaptanes and thio-ethers, whereas high temperature (HT) gasification leads to the formation of exclusively H2S and COS.
The typical raw gas composition of HT and LT gasification is shown in table 1.
Table 1 Low Temperature gasification High Temperature (600-1000 C) gasification (1000-2000 C) H2, CO, C02, H2O H2, CO, C02, H20 CH4 a few ppm alkanes, alkenes, alkynes nil (especially ethylene) Tars (Naphtalene,...) nil H2S, COS H2S, COS
Org. S-species (mercaptanes, thio- nil ethers, thiophenes HCN, NH3 HCN, NH3 Org. N-species (e.g. pyridine) nil For the synthesis of SNG, LT-gasification is advantageous (higher overall cold-gas efficiency), as the raw gas contains already substantial amounts of CH4. Drawbacks of this kind of raw gas are the high amount of poisonous components, such as alkenes, alkynes, H2S, COS, organic S-species, HCN, NH3, organic N-species.

For that reason, an efficient gas cleaning is required to protect the catalysts applied in the fuel synthesis. An example of a state of the art to produce synthesis gas for applications such as Fischer-Tropsch-Synthesis or production of Methanol, DME, and SNG is shown in figure la.

SUBSTITUTE SHEET (RULE 26) A scrubber at low temperatures is used to remove the tars and the organic S-species and N-species. H2S, COS are absorbed on solid absorbers available for this duty (active carbon, ZnO
or other metal oxides...). In general, the gas cleaning is followed by a Water-Gas-Shift reactor, C02-seperation and multiple methanation units. To increase the calorific value of the gas to the quality limits of the gas grid, CO2 and H2 is removed. The order of units 4-9 can be different.

Disadvantage of this process scheme is the high number of operation units and the different temperature levels of the units (especially cooling down to the scrubber temperature).
To avoid this kind of temperature gradients in the process, the use of raw gas from a HT-gasification is common, an example of such process is shown in figure lb (US 3'928'000, EP 0'120'590). The different gas composition enables S-resistant Water-Gas-Shift (WGS) and S-resistant Methanation and lowers the amount of operation units. However, the raw gas composition is less favorable for the SNG synthesis as the SNG composition results in higher energetic losses.
First, the energetic effort in the gasification unit is higher for the production of pure H2, CO, C02-mixtures;
secondly the pure H2, CO, C02-mixtures result in higher thermal losses in the synthesis due to the exothermic enthalpy of the methanation reaction.
Description of the invention By means of the subject process, the unfortunate temperature level sequence and the number of operation units of the process shown in figure la as well as the energetic losses of the process shown in figure lb can be avoided. A methane-rich stream can be produced from sulphur containing feedstocks containing 10 to 95 mol% of methane.

The first step following the Low-Temperature-gasification is a multifunctional process unit featuring hydrodesulphurization/denitrogenation, methanation, WGS, tar SUBSTITUTE SHEET (RULE 26) reforming and cracking and the hydrogenation/reforming of alkenes and alkynes simultaneously. The H2S produced from the organic sulphur species by hydrolysis and the COS are removed by absorption on common absorber materials such like ZnO, CuO. CO2 can be removed before or after the 2nd methanation step. For the adjustment of the calorific value excess H2 is separated and may be recycled to unit 2.

The hydrodesulphurization unit (HDS) is a common process step for the desulphurisation of feedstocks in the petrochemical industry or of natural gas before steam reforming. The applied catalysts for these units tend to catalyse both methanation and watergas shift reaction which is unwanted as these exothermic reactions may lead to a thermal runaway of the reactor. In the subject process, however, the methanation and WGS-reactions are desired.

To control the temperature rise due to exothermic reactions, a fluidised bed reactor equipped with means for heat removal can be applied. The catalyst fluidisation offers additionally the potential for internal regeneration of the catalyst from carbon deposits caused by compounds like ethylene or tars in the LT-gasifier producer gas. Such an internal regeneration can be found for fluidised bed methanation and can be enhanced by staged addition of recycle H2 and /or steam in the upper part of the fluidised bed.

Moreover, the raw gas stream leaving the unit can be tailored to the requirements of a 2nd methanation unit to minimise the total number of process units by the addition of steam, H2 from the recycle and the proper choice of temperature and pressure.

SUBSTITUTE SHEET (RULE 26) alkenes and alkynes simultaneously. The H2S produced from the organic sulphur species by hydrolysis and the COS are removed by absorption on common absorber materials such like ZnO, CuO. CO2 can be removed before or after the 2nd methanation 5 step. For the adjustment of the calorific value excess H2 is separated and may be recycled to unit 2.

The hydrodesulphurization unit (HDS) is a common process step for the desulphurisation of feedstocks in the petrochemical industry or of natural gas before steam reforming. The applied catalysts for these units tend to catalyse both methanation and watergas shift reaction which is unwanted as these exothermic reactions may lead to a thermal runaway of the reactor. In the subject process, however, the methanation and WGS-reactions are desired.

To control the temperature rise due to exothermic reactions, a fluidised bed reactor equipped with means for heat removal can be applied. The catalyst fluidisation offers additionally the potential for internal regeneration of the catalyst from carbon deposits caused by compounds like ethylene or tars in the LT-gasifier producer gas. Such an internal regeneration can be found for fluidised bed methanation and can be enhanced by staged addition of recycle H2 and /or steam in the upper part of the fluidised bed.

Moreover, the raw gas stream leaving the unit can be tailored to the requirements of a 2nd methanation unit to minimise the total number of process units by the addition of steam, H2 from the recycle and the proper choice of temperature and pressure.

Claims

1. A process to produce a methane rich gas mixture for further application in high temperature fuel cells or for manufacturing of synthetic natural gas (SNG) from gasification derived synthesis gas mixtures that contain at least some compounds problematic to conventional methanation units such as organic sulphur or nitrogen compounds, alkanes, alkenes, alkynes, aromatic hydrocarbons like naphthalene, toluene, benzene, phenols etc. or other non-aromatic hydrocarbons, the process includes:
a) at least a unit that allows for methanation, water gas shift reaction and for converting most or parts of the above mentioned group of problematic compounds to less problematic compounds, e.g.
- by hydrodesulphurisation of organic sulphur species, - by hydrodenitrogenation of organic nitrogen species, - hydrogenation or reforming of alkanes, alkenes, alkynes, - hydrogenation, reforming or cracking of hydrocarbons, whereas the unit is operated at temperatures between 200°C
and 900°C, pressures between 0.8 bara and 70 bara and comprises a catalyst that contains metals, e.g. molybdenum, cobalt, ruthenium, nickel, wolfram or their sulfides as active phase and is supported on materials containing e.g.
aluminum, silicon, titanium, zirconium, cer, gadolinium, manganese, vanadium, lanthanum, chromium or their oxides.

2. The process as described in claim 1, but carried out in a fluidised bed reactor with catalyst particles in the size range of 20 - 2000 µm.

3. The process as described in claims 1 and 2, but equipped with heat transfer means to control the temperature, preferably in the fluidized bed reactor.

4. The process as described in any of the claims 1 to 3, but with an additional hydrogen feeding (e.g. from a recycle stream) at the top, the bottom of the reactor and/or in between, e.g. as secondary injection into a fluidised bed.

5. The process as described in any of the claims 1 to 4, but with an additional steam feeding at the top, the bottom of the reactor and/or in between, e.g. as secondary injection into a fluidised bed.

6. The process as described in any of the claims 1 to 5, but with an additional bed of active carbon, ZnO or other metal oxides to remove species like H2S and/or COS.

7. The process as described in claim 6, but with an additional water removal before the bed of active carbon, ZnO
or other metal oxides to enhance the separation efficiency, e.g. by means of a membrane.

8. The process as described in any of the claims 1 to 7, but with an additional removal of carbon dioxide.

9. The process as described in any of the claims 1 to 8, but with an additional feeding of steam, followed by a second methanation step.

10. The process as described in any of the claims 1 to 8, but with an additional feeding of steam, followed by a second methanation step that is carried out in a fluidised bed reactor.

11. The process as described in claim 9 or 10, but with an additional removal of carbon dioxide.

12. The process as described in any of the claims 1 to 11, but with an additional removal of water.

13. The process as described in any of the claims 1 to 12, but with an additional removal of hydrogen.

14. The process as described in claims 1 to 12, but with an additional removal of hydrogen that is used as a recycle stream fed into the first methanation unit.

15. A method for converting a raw gas into a methane-rich and/or hydrogen-rich gas, comprising the steps of:
a) providing the raw gas stemming from a coal and/or biomass gasification process, thereby the raw gas comprising beside a methane and hydrogen content carbon-monoxide, carbon-dioxide, alkanes, alkenes, alkynes, tar, especially benzole and naphthalene, COS, hydrogen sulfide and organic sulfur compounds, especially thiophenes; thereby the ratio of hydrogen to carbon monoxide ranges form 0.2 to 5;
b) bringing this raw gas into contact with a catalyst arranged as a fluidized bed reactor at temperatures above 200°C and at pressures equal or larger than 1 bar in order to convert the raw gas into a first product gas, thereby simultaneously convert organic sulfur components into hydrogen sulfide, reform tars, generate water/gas shift reaction and generate methane from the hydrogen/carbonmonoxide content;
c) bringing the first product gas into a sulfur absorption process to generate a second product gas, thereby reducing the content of hydrogen sulfur and COS from 100 to 1000 ppm down to 1000 ppb or less;
d) optionally bringing the second product gas into a carbon dioxide removal process to generate a third product gas at least almost free of carbon dioxide;
e) bringing the third product gas into a 2nd methanation process to generate a forth product gas having a methane content above 5 vol%;
f) optionally bringing the fourth product gas into a carbon dioxide removal process to generate a fifth product gas at least almost free of carbon dioxide g)bringing the fifth product gas into an hydrogen separation process in order to separate a hydrogen rich gas from a remaining methane-rich gas, called substitute natural gas.