WO2003080503A1

WO2003080503A1 - Method for producing syngas with recycling of water

Info

Publication number: WO2003080503A1
Application number: PCT/GB2003/001197
Authority: WO
Inventors: Stephen Bruce John Scott
Original assignee: Jacobs Engineering U.K. Limited
Priority date: 2002-03-21
Filing date: 2003-03-21
Publication date: 2003-10-02
Also published as: AU2003216847A1; AU2003216847A8; WO2003080503A8; GB0206701D0

Abstract

Disclosed is a production process for synthesis gas ('syngas') which can be used to generate power or as a chemical feedstock for, e.g. an ammonia plant, which process includes the steps of: partial oxidation of a carbonaceous feedstock to produce very hot raw syngas, quenching the very hot raw syngas with liquid water, reacting the carbon monoxide in the raw syngas with water to form carbon dioxide and hydrogen, condensing water out of the shifted raw syngas by cooling to heat up process make-up water, and recycling the condensed water and at least some of the heat of the heated make-up water to the quench.

Description

METHOD FOR PRODUCING SYNGAS WITH RECEC ING OP WATER

This invention relates to the production of synthesis gas (hereinafter referred to as "syngas") which is a mixture of gases containing at least hydrogen and an oxide of carbon from a carbonaceous feedstock.

Syngas is a term well nown to those versed_. in the art of the production of gas from fossil fuels such as natural gas, coal, oil, or other hydrocarbons such as petcoke. Syngas is used mainly for the synthesis of chemicals such as ammonia, methanol, hydrogen, and Fischer Tropsche hydrocarbons, but it can be used as a thermal fuel such as is used for feeding to gas turbines to generate power. Such a gas turbine may be part of an integrated gasification combined cycle (IGCC) arrangement for the generation of electric power. IGCC is a widely practised process for converting by partial oxidation fossil fuels into raw syngas which is cleansed and treated to become suitable for use as gas turbine fuel .

A major design consideration in particularly an IGCC plant is the recovery of the heat generated during partial oxidation. Typically up to 20% of the chemical energy in the original fuel is released as heat in the partial oxidation step. That is, only 80-90% of the feedstock chemical energy is converted to syngas, the remainder being released as gasification waste heat . The cost and effectiveness of the recovery system for this waste heat has a major effect on a plant's commercial viability.

This invention seeks to provide a process in which there is an improvement in the cost and effectiveness of the recovery system for this waste heat. Specifically, this invention relates to the production of syngas by a process comprising at least the steps of -.

A. Partial oxidation of a carbonaceous feedstock to produce very hot raw syngas ("POX") ,

B. Quenching the very hot raw syngas with liquid water ("Quench"),

C. Reacting the carbon monoxide in the raw syngas with water to form carbon dioxide and hydrogen

("Shift") ,

D. Condensing water out of the shifted raw syngas by cooling ("Desaturation") to heat up process make-up water,

E. Recycling the condensed water and at least some of the heat of heated make-up water to the quench ("Recycle"),

Figure 1 is a schematic flowsheet showing the interrelationship between these unit operations for a preferred embodiment of the present invention.

Preferably other steps such as the use of cold makeup water in the desaturation step, steam raising, sulphur compounds removal, heat interchanging, heat transfer by means of hot water distribution, syngas expansion, and syngas re-saturation may be used in the present process depending on the particular feedstock and desired end product .

All of these unit operations are well known to those skilled in the art, but the following summaries are provided as teaching regarding their use in the present invention. POX: This is commercially practised in many plants in a unit often referred to as a "gasifier" . Fossil fuel is oxidised with an oxygen-containing gas, such as oxygen produced in an air-separation unit, at high temperature to produce what might be termed *raw syngas' . This raw syngas contains a number of components that have to be removed before the syngas can be used for one of the above uses .

Preferably the pressure in the POX section is above

10 bar, more preferably above 20 bar, more preferably above 30 bar, more preferably above 40 bar, and most preferably above 60 bar.

Quench; In this step, the very hot temperature (often well over 1,100°C) of the partially oxidised stream is reduced using liquid water. This is preferably effected by directly quenching the gases by contact with sufficient liquid water in a manner such that liquid water remains when the mixture of hot gases and liquid water reach thermal equilibrium. This direct contact method has the advantage of removing much of the solid material present in the raw syngas. These solid particles are generally removed from the system through a liquid water purge stream. This is known as "blowdown" .

In commercial plants, following the quench as described above, finer particulates carried forward with the gas stream from the quenched stream can be scrubbed out with water in a counter current scrubber. In the present invention, this water preferably comes from the Desaturator, as will be described later. Shift : This exothermic equilibrium reaction is also commercially practised in many plants e.g. those for ammonia production, and its characteristics are well known. By means of the use of Recycle as will be described later the performance and margin of operation of Shift in the present invention is improved.

Desaturation: In the context of this invention this step is downstream of the shift reaction and is distinct from the quench and any associated scrubber. This step is usually known as desaturation because, in reducing the temperature of the raw syngas, the majority of the water remaining in the syngas (as steam) after the shift reaction is condensed, thus leaving a relatively dry gas. Although tubular heat exchangers can be used, in the present invention it is preferred to use direct contact cooling. The device normally used to effect this step is referred to as a "Desaturator" and consists of a tower containing trays or packing up which flows the syngas and down which flows a relatively large volume of liquid water. On entering the tower this water is relatively cold. Because of the close temperature approaches achievable with direct contact cooling (often only a few degrees centigrade) , by controlling the flow of water to the top of the tower, the water leaving the tower can have a temperature close to that of the incoming syngas, and the syngas leaving the tower can have a close temperature approach to that of the incoming water.

The advantage of the close temperature approach of the syngas leaving the tower is that known sulphur compound removal processes, such as Selexol™ work better at low temperatures . The low water content of the syngas is also advantageous within the Selexol™ unit in that the water balance over the unit is such that water evaporates into the purified syngas - giving a helpful evaporative cooling effect. It is an advantage of a preferred embodiment of this invention that the amount of water reacted in the shift reaction together with any that is required to make up for blowdown losses, plus any water required as make-up when a coal slurry feedstock is used is about equal to the economical amount required to effect the final cooling of the syngas. Thus advantageously the present invention can provide a low water consumption syngas production process. The temperature of the water flowing to the top of the Desaturator is below 140°C, preferably below 100°C more preferably below 80°C, more preferably below 60°C, more preferably below 0°C, more preferably below 30°C, and most preferably below 25°C.

Recycle: A major advantage of this invention arises from the recycling of heat in the form of hot water from the Desaturator to the quench, particularly when there is a close temperature approach between the incoming syngas from the shift reactor and the outgoing water. This recycled heat significantly increases the water content of the resulting quenched syngas, which in turn means that the following equilibrium shift reaction converts more of the carbon monoxide to carbon dioxide. This then means that low levels of NO_x in flue gases from IGCC plants utilising the syngas as fuel can be achieved without the need to add additional inert diluent such as water vapour or nitrogen to the fuel . In this invention the temperature of the recycled water is generally above 80°C, preferably above 100°C, more preferably above 120°C, more preferably above 140°C, more preferably above 160°C, more preferably above 180°C, and most preferably above 200°C.

Not all of the hot water from the bottom of the Desaturator need be recycled to the quench. In some embodiments it is preferred that the majority is used for other process heating duties, with any balance being used to raise low pressure steam. The high temperature achieved by the close approach at the bottom of the Desaturator gives added advantages in that the exergy of the heat (i.e its higher temperature) allows significantly more, higher pressure, steam to be generated from the heat recovered in the Desaturator. If some of the flow of hot water from the bottom of the Desaturator is used in the preferred way i.e. to generate steam, its temperature on leaving the boiler is above 140°C, more preferably above 150°C, more preferably above 160°C, more preferably above 170°C, and most preferably above 180°C. This has the very significant advantage of allowing higher pressure steam to be generated with the waste heat .

A further important advantage of this invention is that it facilitates a rise in the concentration of steam present in the quenched syngas from a level whereat the shift catalyst is only just operating within its required conditions - for example at a steam to dry gas ratio of below 1 - to a level whereat the shift catalyst is comfortably within its required conditions - for example a steam to dry gas ratio of above 1. By means of this rise, any operating perturbations of the plant do not then pose a risk to the operation of the shift catalyst, such as it starting to catalyse unwanted, highly exothermic, Fischer Tropsch reactions or it degrading. This invention can raise the steam to dry gas ratio to above 1, more preferably above 1.2, more preferably above 1.4, more preferably above 1.6, and most preferably above 1.8, by volume .

In IGCC designs heretofore, if the desulphurised syngas is to be used for electric power production by utilising it as a fuel for a gas turbine, which is usually followed by a steam turbine driven by steam at least partly raised from the heat in the exhaust of the gas turbine, then, depending on the desired (required) NO_x levels in the flue gas, water vapour/heat may be added to the syngas by passing it through a saturator. In this invention, depending on the locally permitted emission concentration of NO_X/ it may not be necessary to carry out this step. This is because, in this invention, the carbon dioxide formed by the shift reaction is very effective in reducing the amount of NO_x formed during the combustion of the syngas in the gas turbine. However, the use of a saturation step may become necessary if carbon dioxide is removed from the syngas because of, for example, environmental regulations.

It is well known that a gasifier followed by a shift provides an opportunity to remove carbon dioxide from a fuel gas prior to its combustion. This procedure may also be combined with and is enhanced by this invention. That is, it is possible to capture carbon dioxide from the fuel gas before combustion for export and possible sequestering. In such a case, it would be necessary to replace the captured carbon dioxide with either nitrogen or water vapour, or a mixture of the two, in order to keep down the flame temperature and to maintain plant output. The quantity of carbon dioxide removed may be varied for fiscal reasons and therefore the provision for nitrogen and/or water vapour addition can be incorporated into the design of some embodiments of the present invention.

Another important advantage of this invention is the fact that the carbon dioxide in the syngas is generally present at a relatively high partial pressure in the syngas stream, which is itself preferably at a high pressure. This means that should it be desired to remove the carbon dioxide, for example to reduce carbon emissions to the atmosphere for environmental reasons, the cost of its removal is very much less than the cost of removing the same quantity of carbon dioxide from e.g. flue gases at atmospheric pressure. In these embodiments of the present invention the partial pressure advantage factor is of the order of one hundred.

Generally expressed, this invention provides a process for the production of syngas from a carbonaceous feedstock, which comprises the steps of: partially oxidising the fuel with an oxygen-containing gas to yield a gas stream containing carbon monoxide generally at supra-atmospheric pressure, usually above 10 atmospheres, quenching the said gas stream with liquid water thus increasing the steam content of the gas stream, and subjecting the gas stream to a carbon monoxide shift reaction whereby carbon monoxide reacts with steam to form carbon dioxide and hydrogen, wherein the process includes the steps of condensing out at least some of the water from the shifted gas stream, preferably by means of a direct contact cooling, to cool the stream and to heat up process make-up water, and recycling at least some of the condensed water and at least some of the heat of the heated make-up water back to the quench step.

Preferably some of the evolved shift reaction heat is used to raise steam, and, preferably after further cooling the syngas stream is passed through a sulphur compounds depleting step.

In some embodiments of the present invention one means to utilise the heat from cooling the wet syngas stream is to use it to saturate the syngas used as fuel for the gas turbine of an IGCC plant thereby increasing its mass flow as taught in European Patent 0 384 781. This also reduces the gas turbine fuel flame temperature, and hence NOx formation

In the process of the present invention the shift reaction raises the temperature of the reactants substantially and thereby increases the number of possible applications of the waste heat produced.

Obviously, the consumption of water used as a chemical reactant in the shift reaction must be balanced by an equal quantity of make-up water to the syngas generation plant. Additional water may also be required to make up for losses incurred in blowdown from the quench and as make-up when a coal slurry feedstock is used. In the present invention, the extra amount of water present in the quenched syngas has advantage in that it both 'pushes' the equilibrium in the shift step to cause more carbon monoxide to react, and its extra mass results in a lower temperature rise from the heat of the shift reaction which in turn causes more carbon monoxide to react because of the higher shift equillibrium constant at the lower temperature .

European Patent EP 0 575 406 relates to an IGCC process, in which it is described as preferable to feed the gasification waste heat into the gas turbine rather than to a steam cycle. It also teaches the use of shift heat to preheat the fuel gas and to generate and/or preheat any inert diluents that need to be added to the fuel before it is fed to the gas turbine. This is usually beneficial to overall IGCC efficiency, however there are practical limits to the amount of waste heat that can be used in this way, such as the choice of materials of construction for the associated equipment.

Following this prior art, the final result is that a significant proportion of the gasification waste heat cannot be effectively recovered and recycled to the plant but has to be rejected to a heat sink through an air or water cooling system. This reduces the thermal efficiency of an IGCC plant.

By contrast, in accorance with the present invention, the incorporation of a carbon monoxide shift stage in an IGCC power-generating plant in combination with a heat recycle to the quench confers several advantages : (i) The release of more shift heat means that the syngas is more superheated and so higher pressure waste heat steam can be raised.

(ii) The greater dry gas volume produces generates more power, not only when the syngas is to be expanded before use as gas turbine fuel, but also when it is used as a uel. This is because a higher mass flow is expanded in the back end of the gas turbine without having to be compressed by the front end compressor in the gas turbine .

(iii)The increase of shifted syngas water content increases the dew point temperature of the wet syngas stream entering the Desaturator

(iv) The shift catalyst also hydrolyses COS and HCN in the syngas which would otherwise have to be separately processed.

(v) The higher carbon dioxide content of the syngas reduces flame temperature and hence NOx formation.

(vi) On a mol-for-mol basis, carbon dioxide suppresses NO_x formation better than water.

The use of the present invention unexpectedly can obviates the need for cooling the syngas other than that necessry to heat up the make-up water needed, which means that a greater proportion of the gasification waste heat plus that remaining from the shift reaction is recovered and recycled to the plant to improve its overall thermal efficiency. The use of the make-up water for gas scrubbing in the Desaturator and the reduction/removal of the need for any further diluents to be added to the fuel gas also reduces the water requirements of an IGCC plant based on the invention. Brief Description of the Drawings

Figure 1 is a schematic flowsheet showing the principal elements of the invention.

Figure 2 is a preferred embodiment of the present invention using the partial oxidation of coal to produce a non-condensable fuel gas consisting of a mixture of combustible, non-combustible gases, and vapours.

Description of the Preferred Embodiment

The primary fuel consisting of a coal/water slurry containing some 66% by weight of coal is reacted with 95% by volume pure oxygen at a pressure of over 65 bar in a partial oxidation unit (1) . The exiting gas stream is Stream 1 in Table 1. The resulting mixture of gases known generally as "syngas" is quenched (2) using an excess of liquid water, i.e. not all of the water evaporates, down to the saturation condition at a pressure of 63 bar and at about 245° C. The syngas produced after quenching passes to a scrubber (3) where it is scrubbed with makeup water pumped from the bottom of the desaturator (9) to remove particulates, and then passed to the gas processing section. This is Stream 2 in Table 1. The scrubber water bottoms is fed to the quench together with more process water pumped from the bottom of the desaturator as a balanced make-up to the quench system.

After scrubbing, the syngas passes through a heat interchanger (4) before entering the shift catalyst reactor (5) . The interchanger heat exchanger (4) is used to preheat the inlet gas/steam mixture to above the temperature required to initiate the catalytic shift reaction and to prevent steam condensing on the shift catalyst. The scrubbed syngas and water vapour is superheated to about 300°C in the interchanger heat exchanger (4) and passed to the shift reactor (5) where most of the CO is catalytically converted to C0₂, with the evolution of heat. The amount of carbon monoxide shifted to carbon dioxide is such that the approach to shift equilibrium is about 10 to 30 degrees centigrade. Concurrently, sufficient COS in the syngas is converted to H₂S and HCN to NH₃ to negate the need for further treatment of these compounds. The exit temperature from the shift converter is above 447° C and the liberated heat is used to raise HP steam (135 bar) in a boiler (6) and then to raise the temperature of the feed water to that boiler in a heat exchanger (7) . The syngas is then cooled against the inlet stream to the shift reactor in the interchanger heat exchanger (4) - Stream 3 in Table 1 - and further cooled against BF in a heat exchanger (8) . Final syngas cooling and condensation of surplus water vapour is carried out in a desaturator (9) .

Desaturation is effected by counter-current direct contact cooling with water in a packed column. The bulk of the cooling is by a high capacity water circulating system using a pump (10) to transfer the heat as hot water to an LP steam boiler (11) . The cold process water make-up to the gasification/gas treatment systems at 15° C finally cools the exit syngas to 23° C. The thus cooled syngas contains typically 0.05 vol% of water - Stream 4 in Table 1.

The hot condensate and process water make-up exits from the bottom of the desaturator (9) at 215° C. After pumping (10), this is split into two steams. The gasification unit water make-up is pumped to the quench/scrubber (2) and (3) as an effective heat recycle. The bulk of the desaturator bottoms is used for process heating purposes as follows:

(i) To raise LP steam at 8.3 bar in the LP steam boiler (11)

(ii) To provide reboil duty for the Acid Gas Removal (AGR) system (14) in heat exchanger (12)

(iii)For BFW heating in heat exchanger (13)

A small stream from the cooled condensate is fed to the coal preparation unit (not shown) for coal slurrying, and the remainder used as cold water recycle in the desaturator (9) .

In this way, the syngas is cooled below 25°C before being fed to the AGR (14) for reduction of sulphur compounds to below the limit permitted for release to atmosphere.

The sulphur reduced syngas - Stream 5 in Table 1 together with the saturated HP and LP steam is sent to the Combined Cycle Unit (CCU) (15) for the generation of electric power. Optionally the whole or a proportion of that cleansed syngas can be taken off for export, e.g. as a chemical feedstock for the synthesis of, say, ammonia.

Table 1

Claims

CLAIMS :

1. A process for the production of syngas from a carbonaceous feedstock, which comprises the steps of partially oxidising the feedstock with an oxygen- containing gas to yield a gas stream containing carbon monoxide, quenching the said gas stream with liquid water thus increasing the steam content of the gas stream, and subjecting the gas stream to a carbon monoxide shift reaction whereby carbon monoxide reacts with steam to form carbon dioxide and hydrogen, wherein the process includes the steps of condensing out water from the shifted gas stream to cool the stream and to heat up process make-up water, and recycling at least some of the condensed water and at least some of the heat of the heated make-up water back to the quench step.

2. A process as claimed in claim 1 wherein the condensing step is effected by direct contact cooling with liquid water.

3. A process as claimed in claim 1 or claim 2 where the recycled water is at a temperature of above 80°C, preferably above 140°C, and more preferably above 200°C.

4. A process as claimed in any one of the preceding claims wherein the shifted gas stream has a steam/dry gas ratio of above 1.2, preferably above 1.6, and more preferably above 1.8, by volume.

5. A process as claimed in any one of the preceding claims wherein sulphur compounds and/or carbon dioxide is removed from the shifted gas stream after the water condensing step.

6. A process as claimed in any one of the preceding claims wherein the shifted gas steam is fed to an IGCC process .

7. A process as claimed in any one of the preceding claims including expanding the shifted gas stream to produce shaft power .

8. A process as claimed in any one of the preceding claims wherein nitrogen and/or water vapour is added to the shifted gas stream after the water condensing step.

9. A process as claimed in any one of the preceding claims including the step of passing the shifted gas steam through a steam-raising boiler before the water condensing step.

10. A process as claimed in any one of the preceding claims wherein the partial oxidisation is effected as a pressure of between 30 and 160 atmospheres.

11. A process as claimed in any one of the preceding claims including the step of scrubbing the quenched gas stream using some of the recycled water.

12. Syngas when produced by a process as claimed in any one of the preceding claims.

13. An IGCC plant when operating using syngas as claimed in claim 12.

14. Power in the form of electricity when produced by a process or in a plant as claimed in any of claims 1 to 11 or claim 13.