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.
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 CO2 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 CO2 and optionally H2 (gas sweetening) before injection into the natural gas grid.
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.
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
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, CO2-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 FIG. 1b (U.S. Pat. No. 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, CO2-mixtures; secondly the pure H2, CO, CO2-mixtures result in higher thermal losses in the synthesis due to the exothermic enthalpy of the methanation reaction.
By means of the subject process, the unfortunate temperature level sequence and the number of operation units of the process shown in
The first step following the Low-Temperature-gasification is a multifunctional process unit featuring hydrodesulphurization/denitrogenation, methanation, WGS, tar 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. 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.
Number | Date | Country | Kind |
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070 13 482.0 | Jul 2007 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/005464 | 7/3/2008 | WO | 00 | 1/11/2010 |