GASIFICATION PROCESS EMPLOYING ACID GAS RECYCLE

Abstract

A method for converting a feedstock comprising solid hydrocarbons to a sweet synthesis gas, involving the steps a. gasifying said feedstock in the presence of steam, an oxygen rich gas and an amount of sour process gas to form a raw synthesis gas optionally comprising tar, b. optionally conditioning said raw synthesis gas to a sour shift feed gas, c. contacting said sour shift feed gas with a sulfided material catalytically active in the water gas shift process for providing a sour hydrogen enriched synthesis gas, d. separating H.sub.2S and CO.sub.2 from said sour hydrogen enriched synthesis gas, for providing said sour recycle gas and a sweet hydrogen enriched synthesis gas.

Claims

1) A method for converting a feedstock comprising solid hydrocarbons to a sweet synthesis gas, involving the steps a. gasifying said feedstock in the presence of steam, an oxygen rich gas and an amount of sour process gas to form a raw synthesis gas optionally comprising tar, b. optionally conditioning said raw synthesis gas to a sour shift feed gas, c. contacting said sour shift feed gas with a sulfided material catalytically active in the water gas shift process for providing a sour hydrogen enriched synthesis gas, d. separating H.sub.2S and CO.sub.2 from said sour hydrogen enriched synthesis gas, for providing said sour recycle gas and a sweet hydrogen enriched synthesis gas.

2) A method according to claim 1 where said sulfided material catalytically active in the water gas shift process comprises 1-5% cobalt, 5-15% molybdenum or tungsten and a support comprising one or more metal oxides, such as alumina, magnesia, titanium or magnesium-alumina spinel.

3) A method according to claim 1 in which said sour process gas comprises at least 200 ppmv sulfur.

4) A method according to claim 1, in which said step (a) comprises the step of directing the tar to contact a material catalytically active in converting hydrocarbons to CO and H.sub.2.

5) A method according to claim 1, in which said step (b) comprises at least one of the following steps b.i heat recovery by transfer of thermal energy to a heat exchange medium, b.ii removal of tar, b.iii removal of particulate matter, b.iv compression.

6) A method according to claim 1, in which said oxygen rich gas is either atmospheric air or atmospheric air having undergone an oxygen enrichment procedure.

7) A method according to claim 1, in which said sweet hydrogen enriched gas is directed to contact a material having a sulfur absorption capacity prior to contacting said material catalytically active in methanation.

8) A method according to claim 1, in which said feedstock comprises an amount of sulfur resulting in from 80 ppmv to 500 ppmv H.sub.2S and COS in the synthesis gas.

9) A method according to claim 1, in which said feedstock comprises material taken from the group of plant material, animal material, biological waste, industrial waste and household waste.

10) A method according to claim 1, in which said feedstock comprises a sulfur dopant, taken from the group of sulfur rich biological material, sulfur rich waste or sulfur containing chemicals.

11) A method for production of methane involving production of a sweet synthesis gas according to claim 1, involving the further step of directing said sweet hydrogen enriched synthesis gas to contact a material catalytically active in methanation, for providing a gas rich in methane.

12) A method according to claim 11 in which said material catalytically active in methanation is cooled by thermal contact with a heat exchange medium in step e, and optionally transfer said heat exchange medium to step b.i if present.

13) A method for production of ammonia involving production of a sweet synthesis gas according to claim 1, involving the further step of directing said sweet hydrogen enriched synthesis gas to contact a material catalytically active in formation of ammonia, for providing a gas rich in ammonia.

14) A method for production of methanol or dimethyl-ether involving production of a sweet synthesis gas according to claim 1, involving the further step of directing said sweet hydrogen enriched synthesis gas to contact a material catalytically active in formation of methanol or dimethyl-ether, for providing a gas rich in methanol or dimethyl-ether.

15) A method for production of a hydrocarbon involving production of a sweet synthesis gas according to claim 1, involving the further step of directing said sweet hydrogen enriched synthesis gas to contact a material catalytically active in the Fischer Tropsch process, for providing a product rich in hydrocarbons.

Description

FIGURES

[0066] FIG. 1 shows a process for gasification of a feedstock according to an embodiment of the present disclosure.

[0067] FIG. 2 shows a process for gasification of a feedstock according to an embodiment of the prior art.

[0068] The following elements are referred to in the drawings. For ease of understanding the numbering is reused for elements having a similar function, but it does not imply identical function of the elements having similar numbers. [0069] Feedstock 2 [0070] Steam 4 [0071] O.sub.2 rich gas 12 [0072] CO.sub.2 rich gas 8 [0073] Gasifier 10 [0074] Raw synthesis gas 16 [0075] Atmospheric air 6 [0076] Air separation unit 14 [0077] Tar reformer 20 [0078] Tar-free synthesis gas 22 [0079] Heat exchange 24 [0080] Filter 28 [0081] Gas wash 32 [0082] Compressor 36 [0083] Sour shift feed gas 38 [0084] Sour shift reactor 40 [0085] Shifted synthesis gas 44 [0086] By-passed synthesis gas 42 [0087] Sweet synthesis gas 52 [0088] Acid gas removal (AGR) process 46 [0089] Acid gas 48 [0090] Acid gas enrichment 50 [0091] Sulfur guard 54 [0092] Synthesis section 56 [0093] Water 60 [0094] Saturated steam 62 [0095] Superheated steam 64 [0096] Tar cooling/filtering unit 68 [0097] Tar removal unit 70 [0098] Tar 72 [0099] Initial AGR unit 74 [0100] Waste gas 76 [0101] Pre-methanation unit 78 [0102] Methanation unit 80 [0103] SNG 82

[0104] FIG. 1 shows a process for gasification of a feedstock to form a sweet synthesis gas according to an embodiment of the present disclosure. In FIG. 1, a feedstock 2, steam 4, an O.sub.2 rich gas 12 and a CO.sub.2 rich sour recycle gas 8 are directed to a gasifier 10, which typically operates at 700° C. to 1000° C., and converts the feedstock 2 to a raw synthesis gas 16 comprising CO and H.sub.2. The O.sub.2 rich gas 12 may be substantially pure O.sub.2, obtained from atmospheric air 6, via an air separation unit 14, or in alternative embodiments atmospheric air or atmospheric air enriched in O.sub.2 depending on the desired product of synthesis. An amount of tar may also be present in the raw synthesis gas 16. The raw synthesis gas 16 is in the embodiment of FIG. 1 directed to an optional tar reformer 20, which also receives an O.sub.2 rich gas 12. In the tar reformer 20 the raw synthesis gas 16 contacts a material catalytically active in conversion of hydrocarbons such as anthracene or naphtalene to H.sub.2 and CO, providing a tar-free synthesis gas. The material may comprise nickel as the catalytically active material, which is not deactivated in the presence of moderate amounts of sulfur such as below 500 ppmv, e.g. partially sulfided nickel. Optionally the tar reformer may also be replaced by a tar removal unit or even omitted if the amount of tar if very low. The sour recycle gas 8 is added to the tar reformer to promote conversion of char to CO, to control the temperature development by dilution and if the catalyst in the tar reformer is sulfided, sulfur present in the sour recycle gas 8 will also assist in maintaining the catalytically active material sulfided. The tar-free synthesis gas 22 is cooled by heat exchange 24, and subsequently filtered 28 to remove alkali metal residue and other particles. The filtered synthesis gas is directed to a gas wash 32, where soluble impurities, such as chloride and ammonia are removed, providing a cleaned gas, having a temperature around 40° C. The cleaned gas is compressed 36, typically to 30 bar, and directed as sour shift feed gas 38 to a sour shift reactor 40 containing a material active in WGS in the presence of sulfur, providing a shifted gas. The composition of the sour shifted synthesis gas 44 is controlled by the conditions in the sour shift reactor 40, the amount of steam added upstream reactor 40 and the amount of by-passed synthesis gas 42. The sour shifted gas 44 will contain “acid gas” in the form of some CO.sub.2 from the WGS process and sulfur, typically in the form of H.sub.2S. The acid gas 48 is separated from a sweet synthesis gas 52, by an acid gas removal (AGR) process 46. The sulfur content of the acid gas 48 is optionally concentrated by acid gas enrichment 50, removing CO.sub.2 from a sour recycle gas 8 to be added to one or both of the gasifier 10 and the tar reformer 20. Alternatively, the acid gas enrichment may be omitted and the acid gas 48 may be used directly as sour recycle gas 8. The sweet synthesis gas 52 may then be directed to a synthesis section which may have an optional sulfur guard 54, to capture any remaining sulfur prior to the synthesis section 56, which may be designed for production of chemicals such as methane, methanol, dimethyl-ether, hydrocarbons or ammonia. The synthesis of these is exothermic, and therefore the synthesis section 56 is typically cooled by a cold heat exchange media 60. In the embodiment shown in FIG. 1, the cold heat exchange media 60 may be water, which is heated to saturated steam 62 in a boiler or a boiling water reactor and subsequently further heated to form superheated steam 64 downstream the gasifier 10.

[0105] In a further, more specific, embodiment of the process according to FIG. 1, biomass in form of wood pellets was fed to a fluidized bed gasifier together with steam and pure O.sub.2 from an air separation unit. The gasifier is filled with fluidized material (typically sand and/or olivine) and is run at 10 barg and 850° C. CO.sub.2 is typically introduced in the upper part of the bed or in the free-board.

[0106] The gas is, optionally after one or two hot cyclones, fed to a catalytic dusty tar reformer. Oxygen together with steam or CO.sub.2, from the AGR is injected to increase the temperature in between catalytic beds. More than 90% of tars are converted to CO+H.sub.2, contributing to make more synthesis gas and thus final product.

[0107] The gas leaves the tar reformer at about 780° C. and enters a cooling section. Saturated steam produced e.g. in a downstream boiling water reactor which typically may be too cold for use in a turbine can be superheated there.

[0108] A bag filter operating at moderate temperatures (below 250° C.) removes the particles and the ashes that went through the tar reformer monoliths as well as through the heat exchangers. The syngas is further cooled and fed to a water scrubber to remove the last traces of particles as well as chlorine and ammonia. Activated carbon beds are then installed to remove the last traces of tars and benzene. These are run at low temperature (about 40° C.).

[0109] The clean syngas is then compressed to about 30 bar g and passed through a hydrogenator, a chlorine guard and COS hydrolyzer step before being fed to the sour shift reactor.

[0110] The syngas at this point has a lack of hydrogen, therefore some CO is shifted to CO.sub.2 (when H.sub.2O is reduced to H.sub.2) in the presence of sulfur. A by-pass ensures a good control of the shift so that the syngas is qualified to being fed to the methanation section.

[0111] CO.sub.2 in excess is removed in an Acid Gas Removal section (amine wash or cold methanol wash or glycol wash) together with H2S. The effluent is recycled back (potentially with the help of a recycle) to the gasification section to enrich the gas in sulfur. The off-gas excess is routed to a sulfur recovery unit such as a WSA unit, a caustic scrubber or a SOLVE™ unit.

[0112] The syngas leaving the AGR has a module (M=H.sub.2—CO.sub.2/CO+CO.sub.2) of 3 ready for methanation. A sulfur guard ensures that no sulfur breakthrough, contributing to longer methanation catalyst lifetime. A boiling water reactor consisting of one or two passes produces on-spec bio-SNG and recovers the reaction heat as saturated steam (pressure from 80 to 120 bars).

[0113] The bio-SNG could be further dried (molecular sieves) and compressed to meet local requirements.

[0114] FIG. 2 shows a process for gasification of a feedstock to form a sweet synthesis gas according to the prior art. A feedstock 2, steam 4, an O.sub.2 rich gas 12 and a CO.sub.2 rich gas 8 are directed to a gasifier 10, which typically operates at 700° C. to 1000° C., and converts the feedstock 2 to a raw synthesis gas 16 comprising CO and H.sub.2. The O.sub.2 rich gas 12 may be substantially pure O.sub.2, obtained from atmospheric air 6, via an air separation unit 14, or in alternative embodiments atmospheric air or atmospheric air enriched in O.sub.2 depending on the desired product of synthesis. An amount of tar may also be present in the raw synthesis gas 16. The raw synthesis gas 16 is in the embodiment of FIG. 1 directed to a tar cooling/filtering unit 68, followed by a tar removal unit 70, from which tar 73 is removed.

[0115] The tar-free synthesis gas is compressed 36, typically to 30 bar, and directed to a gas wash 32, where soluble impurities, such as chloride are removed, providing a cleaned gas, having a temperature around 40° C.-60° C. The low amount of sulfur (less than 80 ppmv) in the cleaned gas is insufficient for operation of a sour WGS process, and therefore the remaining sulfur and CO.sub.2 must be removed in an initial AGR unit 74, providing a waste gas 76 comprising CO.sub.2 and a small amount of sulfur. The sweet synthesis gas is directed to a sulfur guard 54 providing a sweet WGS feed gas 38 directed to a sweet WGS reactor 40, where the ratio of H.sub.2 to CO in the synthesis gas is adjusted to the ratio required by the synthesis process, by the conditions in the sour shift reactor 40 and the amount of by-passed synthesis gas 42. In the embodiment shown in FIG. 1, the desired product is SNG, and accordingly the shifted gas 44 a is directed to a pre-methanation unit 78, which also forms an amount of CO.sub.2. The CO.sub.2 is separated from the intermediate methane rich gas in an AGR unit 46, providing a waste gas 58 comprising CO.sub.2. Finally, methanation 80 is completed forming SNG 82.

EXAMPLES

[0116] In the following 3 examples of processes for conversion of biomass to synthesis gas are given. Example 1 relates to a process without recycle of sour gas according to the prior art, as illustrated in FIG. 2. Example 2 and 3 relates to processes with recycle of sour gas, as illustrated in FIG. 1.

[0117] All examples assume the same biomass feed and a gasification process with presence of CO.sub.2.

Example 1

[0118] Example 1 relates to a process for conversion of wood pellets to synthesis gas. The hot syngas downstream the gasification chamber, with particles and ashes has a typical volumetric composition as that shown in Table 1, Table 2 and Table 3.

[0119] As Example 1 operates without recycle of sour gas, the concentration of H.sub.2S at the outlet of the gas wash will be 80 ppmv, which is too low for operation of a sour shift process, and too high for operation of a sweet shift process. Example 1 is therefore calculated for the operation of a sweet shift process in accordance with FIG. 2. In accordance with normal operation of such gasifiers, pure CO.sub.2 is added to the gasifier to support the conversion of carbonaceous char.

Example 2

[0120] The same feed as in Example 1, was treated in a process according to the present disclosure, similar to the process shown FIG. 1, but omitting the acid gas enhancement The sour gas recycled will thus contain around 1000 ppmv H.sub.2S. With a recycle ratio of 5% the 80 ppmv will be increased to 100 ppmv, which is sufficient for operation of sour shift. Table 2 shows the composition of selected streams in the process of FIG. 1.

Example 3

[0121] The same feed as in Examples 1 and 2, was treated in a process according to the present disclosure, similar to the process shown FIG. 1, including the acid gas enhancement, in which an amount of CO.sub.2 has been removed from the recycled sour gas. The sour gas recycled will thus contain around 10% H.sub.2S. With a recycle ratio of 5% the 80 ppmv will be increased to 300 ppmv, which is sufficient for operation of sour shift. Table 3 shows the composition of selected streams in the process of FIG. 1.

[0122] When comparing Examples 1, 2 and 3 it is clear that the product gas is highly similar, and therefore the three processes are identical from an input/output perspective. The extra cost of using two AGR units in Example 1 compared to the recycle configuration of Examples 2 and 3 is however problematic, and will almost always be beneficial to Examples 2 and 3. The choice between Examples 2 and 3, relates to the balance between the reduced recycle volume and the cost of an acid gas enhancement unit.

TABLE-US-00001 TABLE 1 No recycle 16 38 44 H2 (g) [%] 25.11 25.24 36.49 CH4 (g) [%] 6.20 6.23 6.23 CO (g) [%] 16.20 16.28 5.03 CO2( g) [%] 23.58 23.70 34.95 N2 (g) [%] 0.14 0.14 0.14 H2O (g) [%] 27.93 28.07 16.82 H2S (g) ppmv 70 0 0

TABLE-US-00002 TABLE 2 AGR to gasifier 16 48/8 38 44 H2 (g) [%] 24.61 0.04 24.73 35.71 CH4 (g) [%] 6.08 0.02 6.11 6.11 CO (g) [%] 15.88 0.02 15.95 4.97 CO2 (g) [%] 25.02 95.55 25.15 36.13 N2 (g) [%] 0.14 0.00 0.14 0.14 H2O (g) [%] 27.46 4.28 27.59 16.61 H2S (g) ppmv 89 1000 99 99

TABLE-US-00003 TABLE 3 AGR/AGE/Gasifier 16 8 38 44 H2 (g) [%] 25.18 0.00 25.18 36.40 CH4 (g) [%] 6.22 0.00 6.22 6.22 CO (g) [%] 16.24 0.00 16.24 5.02 CO2 (g) [%] 23.83 80.00 23.83 35.05 N2 (g) [%] 0.14 0.00 0.14 0.14 H2O (g) [%] 28.03 10.00 28.03 16.81 H2S (g) ppmv 300 100000 300 300