U.S. patent application number 13/260754 was filed with the patent office on 2012-04-26 for process for producing a purified synthesis gas stream.
Invention is credited to Cornelis Jacobus Smit, Isaac Cornelis Van Den Born, Gijsbert Jan Van Heeringen, Alex Frederik Woldhuis.
Application Number | 20120096770 13/260754 |
Document ID | / |
Family ID | 41227573 |
Filed Date | 2012-04-26 |
United States Patent
Application |
20120096770 |
Kind Code |
A1 |
Van Den Born; Isaac Cornelis ;
et al. |
April 26, 2012 |
PROCESS FOR PRODUCING A PURIFIED SYNTHESIS GAS STREAM
Abstract
A process for producing a purified synthesis gas stream from a
feed synthesis gas stream comprising besides the main constituents
carbon monoxide and hydrogen also hydrogen sulphide,HCN and/or COS,
the process comprising the steps of: (a) removing HCN and/or COS by
contacting the feed synthesis gas stream with a water gas shift
catalyst in a shift reactor in the presence of steam/water to react
at least part of the carbon monoxide to carbon dioxide, and to
obtain a synthesis gas stream depleted in HCN and/or in COS; (b)
removing hydrogen sulphide in the synthesis gas stream depleted in
HCN and/or in COS by contacting this gas stream in a
H.sub.2S-removal zone with an aqueous alkaline washing liquid to
obtain a H.sub.2S-depleted synthesis gas stream and a
sulphide-comprising aqueous stream; (c) contacting the
sulphide-comprising aqueous stream with ulphide-oxidizing bacteria
in the presence of oxygen in a bioreactor to obtain a sulphur
slurry and a regenerated aqueous alkaline washing liquid; (d)
removing carbon dioxide from the H.sub.2S-depleted synthesis gas
stream, to obtain the purified synthesis gas stream and a gas
stream enriched in CO2.
Inventors: |
Van Den Born; Isaac Cornelis;
(Amsterdam, NL) ; Van Heeringen; Gijsbert Jan;
(Amsterdam, NL) ; Smit; Cornelis Jacobus;
(Amsterdam, NL) ; Woldhuis; Alex Frederik;
(Amsterdam, NL) |
Family ID: |
41227573 |
Appl. No.: |
13/260754 |
Filed: |
March 30, 2010 |
PCT Filed: |
March 30, 2010 |
PCT NO: |
PCT/EP10/54188 |
371 Date: |
January 5, 2012 |
Current U.S.
Class: |
48/197R |
Current CPC
Class: |
C01B 3/16 20130101; C01B
3/58 20130101; C01B 2203/0485 20130101; Y02P 20/129 20151101; B01D
2257/304 20130101; B01D 2257/504 20130101; C01B 2203/0465 20130101;
B01D 53/1425 20130101; B01D 2251/306 20130101; B01D 2251/604
20130101; B01D 2257/308 20130101; B01D 53/22 20130101; Y02E 20/18
20130101; B01D 2251/304 20130101; Y02P 20/13 20151101; Y02P 20/151
20151101; B01D 53/1462 20130101; B01D 53/77 20130101; C01B 3/52
20130101; B01D 53/002 20130101; B01D 2251/95 20130101; Y02C 10/02
20130101; B01D 2257/408 20130101; C01B 2203/0415 20130101; Y02A
50/2358 20180101; B01D 2251/606 20130101; C01B 3/501 20130101; B01D
53/869 20130101; Y02C 10/06 20130101; Y02A 50/20 20180101; B01D
2251/80 20130101; C01B 3/506 20130101; C01B 2203/0435 20130101;
C01B 2203/0475 20130101; Y02C 10/10 20130101; Y02C 20/40 20200801;
Y02P 20/152 20151101; B01D 53/84 20130101 |
Class at
Publication: |
48/197.R |
International
Class: |
C10J 3/46 20060101
C10J003/46 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2009 |
EP |
09156580.4 |
Claims
1. A process for producing a purified synthesis gas stream from a
feed synthesis gas stream, comprising besides the main constituents
carbon monoxide and hydrogen also hydrogen, sulphide, HCN and/or
COS, the process comprising the steps of: (a) removing HCN and/or
COS from the feed synthesis gas stream by contacting the feed
synthesis gas stream with a water gas shift catalyst in a shift
reactor in the presence of steam/water to react at least part of
the carbon monoxide to carbon dioxide, and to obtain a synthesis
gas stream depleted in HCN and/or in COS; (b) removing hydrogen
sulphide in the synthesis gas stream depleted in HCN and/or in COS
by contacting it in a H.sub.2S-removal zone with an aqueous
alkaline washing liquid to obtain a H.sub.2S-depleted synthesis gas
stream and a sulphide-comprising aqueous stream; (c) contacting the
sulphide-comprising aqueous stream with sulphide-oxidizing bacteria
in the presence of oxygen in a bioreactor to obtain a sulphur
slurry and a regenerated aqueous alkaline washing liquid; and (d)
removing carbon dioxide from the H.sub.2S-depleted synthesis gas
stream, to obtain the purified synthesis gas stream and a gas
stream enriched in CO.sub.2.
2. A process according to claim 1, wherein the sulphide-oxidising
bacteria are selected from the group of autotropic aerobic cultures
consisting of the genera Thiobacillus and Thiomicrospira.
3. A process according to claim 1, wherein the steam/water to
carbon monoxide molar ratio in the feed synthesis gas stream as it
enters the shift reactor is is in the range of from 0.2:1 and 0.9:1
and wherein the temperature of the feed synthesis gas stream as it
enters the shift reactor is in the range of from 190 to 230.degree.
C. and wherein the feed synthesis gas stream comprises at least 50
volume % of carbon monoxide, on a dry basis.
4. A process according to claim 1, wherein in step (d) carbon
dioxide is removed by contacting the H.sub.2S-depleted synthesis
gas stream with an absorbing liquid at low temperature and at
elevated pressure, thereby transferring carbon dioxide from the
H.sub.2S-depleted gas streams to the absorbing liquid to obtain
absorbing liquid enriched in carbon dioxide and the purified gas
stream.
5. A process according to claim 1, wherein in step (d) carbon
dioxide is removed using a membrane.
6. A process according to claim 1, wherein in step (d) carbon
dioxide is removed by cooling the synthesis gas stream depleted in
hydrogen sulphide to a temperature at which carbon dioxide will
separate from the gas streams.
7. A process according to claim 1, wherein the purified synthesis
gas stream is used in a combustion turbine to produce
electricity.
8. A process according to claim 7, wherein hot exhaust gas is
emitted from the combustion turbine and is introduced into a heat
recovery steam generator unit to produce a steam, which steam is
used to produce additional electricity.
9. A process according to claim 1, wherein the further purified
synthesis gas is used in catalytic processes selected from the
group consisting of Fischer-Tropsch synthesis, methanol synthesis,
di-methyl ether synthesis, acetic acid synthesis, ammonia
synthesis, methanation to make substitute natural gas (SNG) and
processes involving carbonylation or hydroformylation
reactions.
10. A process according to claim 1, wherein the gas stream enriched
in CO.sub.2 is at a pressure in the range of from 5 to 50 bara.
11. A process according to claim 10, wherein the gas stream
enriched in CO2 is further pressurised and injected into a
subterranean formation for use in enhanced oil recovery or for
storage into an aquifer reservoir or for storage into an empty oil
reservoir.
Description
[0001] The present invention relates to a process for producing a
purified synthesis gas stream from a feed synthesis gas stream
comprising contaminants.
[0002] Synthesis gas streams are gaseous streams mainly comprising
carbon monoxide and hydrogen. Synthesis gas streams are generally
produced via partial oxidation or steam reforming of hydrocarbons
including natural gas, coal bed methane, distillate oils and
residual oil, and by gasification of solid fossil fuels such as
biomass or coal or coke.
[0003] There are many solid or very heavy (viscous) fossil fuels
which may be used as feedstock for generating synthesis gas,
including biomass, solid fuels such as anthracite, brown coal,
bitumous coal, sub-bitumous coal, lignite, petroleum coke, peat and
the like, and heavy residues, e.g. hydrocarbons extracted from tar
sands, residues from refineries such as residual oil fractions
boiling above 360.degree. C., directly derived from crude oil, or
from oil conversion processes such as thermal cracking, catalytic
cracking, hydrocracking etc. All such types of fuels have different
proportions of carbon and hydrogen, as well as different substances
regarded as contaminants.
[0004] Depending on the feedstock used to generate synthesis gas,
the synthesis gas will contain contaminants such as carbon dioxide,
hydrogen sulphide, carbonyl sulphide and carbonyl disulphide while
also nitrogen, nitrogen-containing components (e.g. HCN and
NH.sub.3), metals, metal carbonyls (especially nickel carbonyl and
iron carbonyl), and in some cases mercaptans.
[0005] Purified synthesis gas can be used in catalytical chemical
conversions or to generate power. A substantial portion of the
world's energy supply is provided by combustion of fuels,
especially natural gas or synthesis gas, in a power plant.
Synthesis gas is combusted with air in one or more gas turbines and
the resulting gas is used to produce steam. The steam is then used
to generate power.
[0006] An especially suitable system for using synthesis gas in
power generation is the Integrated Gasification Combined Cycle
(IGCC) system. IGCC systems were devised as a way to use coal as
the source of fuel in a gas turbine plant. IGCC is a combination of
two systems. The first system is coal gasification, which uses coal
to create synthesis gas. The syngas is then purified to remove
contaminants. The purified synthesis gas may be used in the
combustion turbine to produce electricity.
[0007] The second system in IGCC is a combined-cycle, or power
cycle, which is an efficient method of producing electricity
commercially. A combined cycle includes a combustion
turbine/generator, a heat recovery steam generator (HRSG), and a
steam turbine/generator. The exhaust heat from the combustion
turbine may be recovered in the HRSG to produce steam. This steam
then passes through a steam turbine to power another generator,
which produces more electricity. A combined cycle is generally more
efficient than conventional power generating systems because it
re-uses waste heat to produce more electricity. IGCC systems are
clean and generally more efficient than conventional coal
plants.
[0008] As set out hereinabove, when synthesis gas is used for power
production, removal of contaminants is often required to avoid
deposition of contaminants onto the gas turbine parts.
[0009] When synthesis gas is used in catalytical chemical
conversions, removal of contaminants to low levels is required to
prevent catalyst poisoning.
[0010] Processes for producing a purified synthesis gas stream
generally involve the use of expensive line-ups. For example, cold
methanol may be used to remove hydrogen sulphide and carbon dioxide
by physical absorption. The concentrations of these contaminants in
the purified synthesis gas will still be in the ppmv range. For
applications where the synthesis gas is to be catalytically
converted, contaminant concentrations in the ppmv range are still
too high. Purifying the synthesis gas streams to a higher degree
using a methanol-based process would be uneconomical due to the
disproportionately large amounts of energy required to regenerate
the methanol. In addition, the absorbed H.sub.2S needs to be
removed, usually by contacting the methanol comprising H.sub.2S
with a stripping gas at elevated temperatures, resulting in a
stripping gas comprising H.sub.2S. H.sub.2S in this stripping gas
is then converted to elemental sulphur, requiring a considerable
capital and operational expenditure.
[0011] In US 2007/0072949 a process is disclosed wherein sulphur
species are removed using low-temperature amine-based absorption
processes, followed by a solvent regeneration Claus/SCOT process
unit. The disadvantage of such a process is that it is limited to
the feed gas composition suitable for a Claus process, which is
rather high in hydrogen sulphide concentration.
[0012] It is an object of the present invention to provide an
optimised process for purification of a synthesis gas stream
derived from a range of carbonaceous fuels, such that the purified
synthesis gas is suitable for further use, especially for power
production.
[0013] It has now been found that by removing H.sub.2S directly
from the feed synthesis gas stream, this object can be
achieved.
[0014] Therefore, the invention provides a process for producing a
purified synthesis gas stream from a feed synthesis gas stream
comprising besides the main constituents carbon monoxide and
hydrogen also hydrogen sulphide, HCN and/or COS, the process
comprising the steps of: (a) removing HCN and/or COS by contacting
the feed synthesis gas stream with a water gas shift catalyst in a
shift reactor in the presence of steam/water to react at least part
of the carbon monoxide to carbon dioxide, and to obtain a synthesis
gas stream depleted in HCN and/or in COS; (b) removing hydrogen
sulphide in the synthesis gas stream depleted in HCN and/or in COS
by contacting this gas stream in a H.sub.2S-removal zone with an
aqueous alkaline washing liquid to obtain a H.sub.2S-depleted
synthesis gas stream and a sulphide-comprising aqueous stream; (c)
contacting the sulphide-comprising aqueous stream with
sulphide-oxidizing bacteria in the presence of oxygen in a
bioreactor to obtain a sulphur slurry and a regenerated aqueous
alkaline washing liquid; (d) removing carbon dioxide from the
H.sub.2S-depleted synthesis gas stream, to obtain the purified
synthesis gas stream and a gas stream enriched in CO.sub.2.
[0015] The process enables removal of hydrogen sulphide, carbonyl
sulphide and/or hydrogen cyanide to low levels.
[0016] The purified synthesis gas, because of its low level of
contaminants, is suitable for use as fuel, suitably in gas
turbines, or for use in catalytical chemical conversions. The
purified synthesis gas is especially suitable for use in an
Integrated Gasification Combined Cycle (IGCC).
[0017] In step (d), a carbon dioxide stream at elevated pressure,
suitably in the range of from 4 to 12 bara is obtained. This carbon
dioxide stream can be further pressurised and used for example for
enhanced oil recovery.
[0018] The process is economical because H.sub.2S is removed
directly from the synthesis gas stream via conversion into
elemental sulphur. The synthesis gas stream depleted in H.sub.2S
has very low concentrations of H.sub.2S, enabling the use of an
inexpensive non-selective acid gas removal unit to remove remaining
H.sub.2S as well as CO.sub.2.
[0019] It will be understood that the amount and type of
contaminants in the feed synthesis gas stream can vary and depends
on the amount of these contaminants present in the feedstock used
to generate the feed synthesis gas stream.
[0020] Generally, the feed synthesis gas stream is obtained by
gasification of feedstock.
[0021] When using solid fossil fuels such as biomass or coal as
feedstock, generally the amount of H.sub.2S and COS in the
synthesis gas stream leaving the gasifier is below 15 volume %,
typically below 5 volume % based on the synthesis gas stream.
[0022] When using an oil residue as feedstock, generally the amount
of H.sub.2S and COS in the synthesis gas stream leaving the
gasifier will be below 20 volume %, typically below 10 volume %
based on the synthesis gas stream.
[0023] The synthesis gas stream generated from a feedstock may
comprise particulate matter, for example fly-ash or soot particles.
Therefore, in a preferred embodiment synthesis gas exiting a
synthesis gas generation unit is contacted with scrubbing liquid in
a soot scrubber to remove particulate matter, in particular soot,
thereby obtaining the feed synthesis gas stream. The synthesis gas
stream exiting the synthesis gas generating unit is generally at
elevated temperature and/or elevated pressure. Especially in the
event that the synthesis gas is generated in a gasifier, the
synthesis gas stream exiting the gasifier will be at elevated
temperature and at elevated pressure. To avoid additional cooling
and/or depressurising steps, the scrubbing step in the soot
scrubber is preferably performed at elevated temperature and/or at
elevated pressure. Preferably, the temperature at which the
synthesis gas is contacted with scrubbing liquid is in the range of
from 40 to 160.degree. C., more preferably from 110 to 150.degree.
C. Preferably, the pressure at which the synthesis gas stream is
contacted with scrubbing liquid is in the range of from 20 to 80
bara, more preferably from 20 to 60 bara.
[0024] The amount of HCN and/or COS in the feed synthesis gas
stream depends on the composition of the feedstock from which the
synthesis gas is derived and the technology applied for the
production of synthesis gas. Generally, the amount of COS in a feed
synthesis gas stream derived from solid fossil fuel feedstocks,
especially coal, is from about 100 to 3000 ppmv, based on the feed
synthesis gas stream. For biomass, the amount of COS is generally
in the range of from 1 to 100 ppmv.
[0025] In step (a), HCN and/or COS is removed from the feed
synthesis gas stream via catalytic conversion.
[0026] Catalysts for the hydrolysis of HCN and/or COS are known to
those skilled in the art and include for example TiO.sub.2-based
catalysts or catalysts based on alumina and/or chromium-oxide.
Preferred catalysts are TiO.sub.2-based catalysts.
[0027] The amount of water/steam is preferably between 5 v/v % and
80 v/v %, more preferably between 10 v/v % and 70 v/v %, still more
preferably between 15 v/v % and 50 v/v %, based on steam.
[0028] In step (a), the feed synthesis gas stream is contacted with
a water gas shift catalyst in a shift reactor to remove HCN and/or
COS and to additionally react at least part of the carbon monoxide
with water to form carbon dioxide and hydrogen.
[0029] In an especially preferred embodiment of step (a), carbon
monoxide in the feed synthesis gas stream is converted with a low
amount of steam in the presence of a catalyst as present in one or
more fixed bed reactors. A series of shift reactors may be used
wherein in each reactor a water gas shift conversion step is
performed. The content of carbon monoxide, on a dry basis, in the
feed synthesis gas stream as supplied to the first or only water
gas shift reactor is preferably at least 50 vol. %, more preferably
between 55 and 70 vol. %. The feed synthesis gas stream preferably
contains hydrogen sulphide in order to keep the catalyst sulphided
and active. The minimum content of hydrogen sulphide will depend on
the operating temperature of the shift reactor, on the space
velocity (GHSV) and on the sulphur species present in the feed
synthesis gas stream. Preferably at least 300 ppm H.sub.2S is
present in the feed synthesis gas stream. There is no limitation on
the maximum amount of H.sub.2S from a catalyst activity point of
view.
[0030] In the preferred embodiment of step (a), the steam/water to
carbon monoxide molar ratio in the feed synthesis gas stream as it
enters the first or only water gas shift reactor is preferably
between 0.2:1 and 0.9:1. The temperature of the feed synthesis gas
stream as it enters the shift reactor is preferably between 190 and
230.degree. C. In addition it is preferred that the inlet
temperature is between 10 and 60.degree. C. above the dewpoint of
the feed to each water gas shift conversion step. The space
velocity in the reactor is preferably between 6000-9000 h.sup.-1.
The pressure is preferably between 2 and 5 MPa and more preferably
between 3 and 4.5 MPa.
[0031] The conversion of carbon monoxide may generally not be 100%
because of the sub-stoichiometric amount of steam present in the
feed of the reactor. In a preferred embodiment the content of
carbon monoxide in the shift reactor effluent, using a fixed bed
reactor, will be between 35 and 50 vol. % on a dry basis, when
starting from a feed synthesis gas stream comprising between 55 and
70 vol. % carbon monoxide, on a dry basis, and a steam/CO ratio of
0.2 to 0.3 molar. If a further conversion of carbon monoxide is
desired it is preferred to subject the shift reactor effluent to a
next water gas shift conversion step.
[0032] The preferred steam/water to carbon monoxide molar ratio,
inlet temperature and space velocity for such subsequent water gas
shift conversion steps is as described for the first water gas
shift conversion step. As described above the feed synthesis gas
stream is suitably obtained from a gasification process and is
suitably subjected to a water scrubbing step. In such a step water
will evaporate and end up in the syngas mixture. The resultant
steam to CO molar ratio in such a scrubbed syngas will suitably be
within the preferred ranges as described above. This will result in
that no steam or water needs to be added to the syngas as it is fed
to the first water gas shift conversion step. In order to achieve
the desired steam to CO molar ranges for the subsequent steps steam
or boiler feed water will have to be added to the effluent of each
previous step.
[0033] The water gas shift step may be repeated to stepwise lower
the carbon monoxide content in the shift reactor effluent of each
next shift reactor to a CO content, on a dry basis, of below 5 vol.
%. It has been found that in 4 to 5 steps, or said otherwise, in 4
to 5 reactors such a CO conversion can be achieved.
[0034] It has been found that it is important to control the
temperature rise in each shift reactor. It is preferred to operate
each shift reactor such that the maximum temperature in the
catalyst bed in a single reactor does not exceed 440.degree. C. and
more preferably does not exceed 400.degree. C. At higher
temperatures the exothermal methanation reaction can take place,
resulting in an uncontrolled temperature rise.
[0035] The catalyst used in the shift reactor is preferably a water
gas shift catalyst, which is active at the preferred low steam to
CO molar ratio and active at the relatively low inlet temperature
without favouring side reactions such as methanation. Suitably the
catalyst comprises a carrier and the oxides or sulphides of
molybdenum (Mo), more preferably a mixture of the oxides or
sulphides of molybdenum (Mo) and cobalt (Co) and even more
preferably also comprising copper (Cu) tungsten (W) and/or nickel
(Ni). The catalyst suitably also comprises one or more
promoters/inhibitors such as potassium (K), lanthanum (La),
manganese (Mn), cerium (Ce) and/or zirconium (Zr). The carrier may
be a refractory material such as for example alumina,
MgAl.sub.2O.sub.4 or MgO--Al.sub.2O.sub.3--TiO.sub.2.
[0036] An example of a suitable catalyst comprises an active
.gamma.-Al.sub.2O.sub.3 carrier and between 1-8 wt % CoO and
between 6-10 wt % MoO.sub.3. The catalyst is preferably present as
an extrudate.
[0037] In a preferred embodiment of step (a), the feed synthesis
gas stream comprises at least 50 vol. % of carbon monoxide, and the
steam to carbon monoxide molar ratio in the feed synthesis gas
stream as it enters the shift reactor or reactors is preferably
between 0.2:1 and 0.9:1 and the temperature of the feed synthesis
gas stream as it enters the shift reactor or reactors is between
190 and 230.degree. C.
[0038] In the event that step (a) involves the shift reaction as
described hereinabove, preferably, a portion of the "shifted"
synthesis gas stream, optionally after removal of contaminants, is
used for hydrogen manufacture, such as in a Pressure Swing
Adsorption (PSA) step. The proportion of the shifted synthesis gas
stream used for hydrogen manufacture will generally be less than
15% by volume, preferably approximately 1-10% by volume. The
hydrogen manufactured in this way can then be used as the hydrogen
source in hydrocracking of the products of the hydrocarbon
synthesis reaction. This arrangement reduces or even eliminates the
need for a separate source of hydrogen, e.g. from an external
supply, which is otherwise commonly used where available. Thus, the
carbonaceous fuel feedstock is able to provide a further reactant
required in the overall process of biomass or coal to liquid
products conversion, increasing the self-sufficiency of the overall
process.
[0039] In step (a), a synthesis gas stream depleted in hydrogen
cyanide and/or in COS is obtained.
[0040] In step (b), the synthesis gas stream depleted in hydrogen
cyanide and/or in COS is contacted with aqueous alkaline washing
liquid to transfer hydrogen sulphide from the synthesis gas stream
depleted in hydrogen cyanide and/or in COS to the aqueous alkaline
washing liquid.
[0041] Suitable aqueous alkaline washing liquids include aqueous
hydroxide solutions, e.g. sodium hydroxide or potassium hydroxide
solutions in water and aqueous (bi)carbonate solutions.
[0042] Suitably, step (b) is performed at a temperature in the
range of from 5 to 70.degree. C., more preferably from 10 to
50.degree. C. Preferably, step (c) is performed at a pressure in
the range of from 1 to 100 bar(g), more preferably from 1.5 to 80
bar(g).
[0043] Optionally, the washing liquid is buffered. Preferred
buffering compounds are carbonates, bicarbonates phosphates and
mixtures thereof, especially sodium carbonate and/or sodium
bicarbonate.
[0044] The concentration of the buffering compounds depends inter
alia on the composition of the gas flow and is generally adjusted
in such a way, that the washing liquid is kept within the preferred
pH range.
[0045] Preferably, the pH of the washing liquid is in the range of
from 4.5 to 10, more preferably from 5.5 to 9.0.
[0046] In step (c) hydrogen sulphide in the scrubbing medium is
converted to elemental sulphur using sulphide-oxidising bacteria in
the presence of oxygen in a bioreactor.
[0047] Reference herein to sulphide-oxidising bacteria is to
bacteria which can oxidise sulphide to elemental sulphur. Suitable
sulphide-oxidising bacteria can be selected for instance from the
known autotropic aerobic cultures of the genera Thiobacillus and
Thiomicrospira.
[0048] The main reactions that can take place in the bioreactor are
the microbiological formation of sulphur and sulphate:
HS.sup.-+1/2O.sub.2.fwdarw.1/8S.sub.8+OH.sup.- (1a) Sulfur
production
HS.sub.5.sup.-+1/2O.sub.2.fwdarw.5/8S.sub.8+OH.sup.- (1b) Sulfur
production
HS.sup.-+2O.sub.2+OH.sup.-.fwdarw.SO.sub.4.sup.2-+H.sub.2O (2)
Sulphate procuation
[0049] The sulphur slurry may comprise one or more products of the
main reactions, including elemental sulphur and sulphates.
[0050] The regenerated aqueous alkaline washing liquid may comprise
sulphur particles.
[0051] Reference herein to sulphide-oxidising bacteria is to
bacteria which can oxidise sulphide to elemental sulphur. Suitable
sulphide-oxidising bacteria can be selected for instance from the
known autotropic aerobic cultures of the genera Thiobacillus and
Thiomicrospira.
[0052] Preferably, the reaction medium in the bioreactor is
buffered. The buffering compounds are chosen in such a way that
they are tolerated by the bacteria present in the oxidation
reactor. Preferred buffering compounds are carbonates, bicarbonates
phosphates and mixtures thereof, especially sodium carbonate and/or
sodium bicarbonate. The concentration of the buffering compounds
depends inter alia on the composition of the gas flow and is
generally adjusted in such a way, that the pH of the reaction
medium in the oxidation reactor is between 6.0 and 12.0, preferably
between 7.0 and 11.0, more preferably between 8.0 and 10.0.
[0053] Typical pressures in the bioreactor are between 0.5 and 2
bar(g).
[0054] Preferably, at least part of the aqueous sulphur slurry
obtained in step (c) is separated from the regenerated aqueous
alkaline washing liquid. Suitably, the separating step takes place
in a solid/liquid separator. Suitable solid/liquid separators are
described in Perry's Chemical Engineers' Handbook, 7th edition,
section 22 (1997).
[0055] The sulphur content of the separated aqueous sulphur slurry
is suitably between 5 w/w % and 50 w/w %, based on the slurry.
Typically, the water of the sulphur slurry is removed to an extent
that a sulphur cake with a dry solids content of between 55 and 70%
is obtained. Suitably, the sulphur purity of the sulphur cake is
between 90 and 98 w/w %, based on the dry weight of the sulphur
cake. Optionally, the sulphur slurry can be re-slurried, filtered
and dried to obtain a sulphur paste with a purity of at least 95 wt
% sulphur, preferably at least 99 wt % sulphur. The sulphur paste
thus-obtained can optionally be dried to produce a powder with a
dry weight content of at least 85%, preferably at least 90%. This
powder can suitably be applied as a fungicide, a fertilizer or as a
miticide.
[0056] In step (d) carbon dioxide is removed from the synthesis gas
stream depleted in hydrogen sulphide.
[0057] In a first embodiment of step (d), carbon dioxide is removed
by contacting the synthesis gas stream depleted in H.sub.2S with
absorbing liquid to remove carbon dioxide and remaining hydrogen
sulphide.
[0058] Suitable absorbing liquids may comprise chemical solvents or
physical solvents or mixtures thereof.
[0059] A preferred absorbing liquid comprises a chemical solvent
and/or a physical solvent, suitably as an aqueous solution.
[0060] Suitable chemical solvents are primary, secondary and/or
tertiary amines, including sterically hindered amines.
[0061] A preferred chemical solvent comprises a secondary or
tertiary amine, preferably an amine compound derived from
ethanolamine, more especially DIPA, DEA, MMEA
(monomethyl-ethanolamine), MDEA (methyldiethanolamine) TEA
(triethanolamine), or DEMEA (diethyl-monoethanolamine), preferably
DIPA or MDEA. It is believed that these chemical solvents react
with acidic compounds such as H.sub.2S.
[0062] In a second embodiment of step (d), carbon dioxide is
removed using a membrane.
[0063] It is advantageous to use membranes with a high selectivity
for carbon dioxide. The selectivity is defined as the ratio of the
carbon dioxide permeability over the permeability of carbon
monoxide and hydrogen as measured in single gas experiments.
Preferably, the selectivity of the membrane is between 10 and 200,
preferably between 20 and 150.
[0064] Suitably the membrane material is chosen from the group of
polyethylene oxide based materials, preferably polyethylene oxide
based material comprising block-copolymers, especially PEO 600/5000
T6T6T or a cross linked PEO, polyimide or polyaramide based
materials, cellulose acetate based materials, zeolite based
materials, preferably silica-alumina phosphate based materials,
more preferably SAPO-34, micro-porous silica materials and carbon
molecular sieves materials.
[0065] In a third embodiment of step (d), carbon dioxide is removed
by cooling the gas stream to a temperature at which carbon dioxide
will separate from the gas stream. Suitably, the gas stream is
cooled to a temperature at which carbon dioxide becomes a liquid or
a solid so it can be separated from the gas stream.
[0066] The purified synthesis gas obtained in step (d) has low
levels of contaminants, suitably in the ppmv or even in the ppbv
range.
[0067] Suitably, the gas stream enriched in CO.sub.2 obtained in
step (d) is at a pressure in the range of from 3 to 15 bara,
preferably from 5 to 15 bara. This pressurised gas stream enriched
in CO.sub.2 can advantageously be used for enhanced oil recovery,
with less compression equipment needed.
[0068] In applications where the CO.sub.2-enriched gas stream needs
to be at a high pressure, for example when it will be used for
injection into a subterranean formation, it is an advantage that
the CO.sub.2-enriched gas stream is already at an elevated
pressure.
[0069] In one embodiment, the CO.sub.2-enriched gas stream is
further pressurised and used for enhanced oil recovery, suitably by
injecting it into an oil reservoir where it tends to dissolve into
the oil in place, thereby reducing its viscosity and thus making it
more mobile for movement towards the producing well.
[0070] In another embodiment, the CO.sub.2-enriched gas stream is
further pressurised and pumped into an aquifer reservoir for
storage.
[0071] In yet another embodiment, the pressurised CO.sub.2-enriched
gas stream is further pressurised and pumped into an empty oil
reservoir for storage.
[0072] For all the above options, a series of compressors is needed
to pressurise the CO.sub.2-enriched gas stream to the desired high
pressures. Pressurising the CO.sub.2-enriched gas stream from
atmospheric pressure to a pressure of about 10 bara generally
requires a large and expensive compressor. As the process results
in a CO.sub.2-enriched gas stream which is already at elevated
pressure, preferably above 10 bara, the most extensive compressor
is not needed.
[0073] In a preferred embodiment, the purified synthesis gas is
used for power generation, especially in an IGCC system.
[0074] In the IGCC system, typically, fuel and an oxygen-containing
gas are introduced into a combustion section of a gas turbine. In
the combustion section of the gas turbine, the fuel is combusted to
generate a hot combustion gas. The hot combustion gas is expanded
in the gas turbine, usually via a sequence of expander blades
arranged in rows, and used to generate power via a generator.
Suitable fuels to be combusted in the gas turbine include natural
gas and synthesis gas.
[0075] Hot exhaust gas exiting the gas turbine is introduced into
to a heat recovery steam generator unit, where heat contained in
the hot exhaust gas is used to produce a first amount of steam.
[0076] Suitably, the hot exhaust gas has a temperature in the range
of from 350 to 700.degree. C., more preferably from 400 to
650.degree. C. The composition of the hot exhaust gas can vary,
depending on the fuel gas combusted in the gas turbine and on the
conditions in the gas turbine.
[0077] The heat recovery steam generator unit is any unit providing
means for recovering heat from the hot exhaust gas and converting
this heat to steam. For example, the heat recovery steam generator
unit can comprise a plurality of tubes mounted stackwise. Water is
pumped and circulated through the tubes and can be held under high
pressure at high temperatures. The hot exhaust gas heats up the
tubes and is used to produce steam.
[0078] The heat recovery steam generator unit can be designed to
produce three types of steam: high pressure steam, intermediate
pressure steam and low pressure steam.
[0079] Preferably, the steam generator is designed to produce at
least a certain amount of high pressure steam, because high
pressure steam can be used to generate power. Suitably,
high-pressure steam has a pressure in the range of from 90 to 150
bara, preferably from 90 to 125 bara, more preferably from 100 to
115 bara. Suitably, low-pressure steam is also produced, the
low-pressure steam preferably having a pressure in the range of
from 2 to 10 bara, more preferably from to 8 bara, still more
preferably from 4 to 6 bara.
[0080] In the heat recovery steam generator unit preferably high
pressure steam is produced in a steam turbine, which high pressure
steam is converted to power, for example via a generator coupled to
the steam turbine.
[0081] The purified synthesis gas, because of its low level of
contaminants, is also suitable for use in catalytic processes,
preferably selected from the group of Fischer-Tropsch synthesis,
methanol synthesis, di-methyl ether synthesis, acetic acid
synthesis, ammonia synthesis, methanation to make substitute
natural gas (SNG) and processes involving carbonylation or
hydroformylation reactions.
[0082] Without wishing to be restricted to a particular embodiment,
the invention will now be described in further detail with
reference to the Figures. In FIG. 1, there is shown a process for
producing a purified synthesis gas stream. This starts with the
gasification of biomass or coal with oxygen in a gasification unit
1 to form a feed synthesis gas stream comprising hydrogen sulphide,
HCN and/or COS. Preferably, removal of solids such as slag, soot
and the like is done in a solids removal unit (not shown). The
resulting feed synthesis gas stream is led to a shift unit 2, where
it is contacted with a shift catalysis, converting CO to CO.sub.2
and hydrolysing HCN and COS. The resulting synthesis gas stream
depleted in HCN and COS emanating from unit 2 is led to
H.sub.2S-removal zone 3 where H.sub.2S is removed by contacting the
synthesis gas stream with an aqueous alkaline washing liquid. The
resulting synthesis gas stream depleted in H.sub.2S is led from
H.sub.2S-removal zone 3 to acid gas removal unit 4, where it is
contacted with absorbing liquid to remove CO.sub.2 and remaining
H.sub.2S. This results in a purified synthesis gas stream and a gas
stream enriched in CO.sub.2. Aqueous alkaline washing liquid
comprising H.sub.2S is led from the H.sub.2S removal zone to
bioreactor 5, where H.sub.2S is converted to elemental sulphur.
[0083] In FIG. 2, a preferred embodiment is depicted, wherein the
purified synthesis gas is used for power production. In FIG. 2, a
purified synthesis gas stream as produced in a process according to
FIG. 1 is led to a power plant comprising a gas turbine (1) and a
heat recovery steam generator unit (2). In the gas turbine, an
oxygen-containing gas is supplied via line 4 to compressor 5.
Purified synthesis gas as produced in a process described in FIG. 1
is supplied via line 6 to combuster 7 and combusted in the presence
of the compressed oxygen-containing gas. The resulting combustion
gas is expanded in expander 8 and used to generate power in
generator 9. Remaining exhaust gas comprising CO.sub.2 and oxygen
is led via line 10 to a heat recovery steam generator unit 2. In
the heat recovery steam generator unit, water is heated against the
hot exhaust gas in in heating section 11 to generate steam. The
steam is led via line 12 into a steam turbine 13 to produce
additional power in generator 14.
* * * * *