WO1996019642A1

WO1996019642A1 - Igcc/refinery utilities unit

Info

Publication number: WO1996019642A1
Application number: PCT/GB1995/002993
Authority: WO
Inventors: John Griffiths; Christopher Leslie Winter
Original assignee: Jacobs Engineering Limited
Priority date: 1994-12-21
Filing date: 1995-12-21
Publication date: 1996-06-27
Also published as: GB2296255A; AU4269996A; GB9425868D0

Abstract

This invention is a process for the co-production of electricity and hydrogen wherein a carbonaceous fuel ('F') is partially oxidised (1) at elevated pressure with oxygen or an oxygen containing gas to yield a gas stream containing hydrogen and carbon monoxide, quenching (2) the gas stream to add water vapour, passing the water vapour containing stream through a carbon monoxide shift reactor (5) to increase its hydrogen content, and, passing the stream through a carbon dioxide removal device (12), wherein at least a part of this carbon dioxide depleted stream then has its pressure reduced (13) and at least a part of the part of the stream which has had its pressure reduced is used first to recover at least some of the carbon dioxide removed earlier (15) and then at least some of the stream containing recovered carbon dioxide is used as a fuel for a gas turbine (17) used to drive an alternator to produce electricity, wherein at least part of one reduced carbon dioxide depleted stream is used as a source of hydrogen with or without further purification.

Description

IGCC/REFINERY UTILITIES UNIT

This invention relates to the production of electric power and hydrogen, along with other utilities such as steam, oxygen, nitrogen, and ethanol for a refinery or an integrated gasification combined cycle (IGCC) plant utilising, particularly but not exclusively, heavy ends (eg refinery residue) as the feedstock.

This invention is a process for the co-production of electricity and hydrogen wherein a carbonaceous fuel is partially oxidised at elevated pressure with oxygen or an oxygen containing gas to yield a gas stream containing hydrogen and carbon monoxide, cooling the gas stream and adding water vapour, passing the water vapour containing stream through a carbon monoxide shift reactor to increase its hydrogen content, and, passing the stream through a carbon dioxide removal device, wherein at least a part of this carbon dioxide depleted stream then has its pressure reduced and at least a part of the part of the stream which has had its pressure reduced is used first to recover at least some of the carbon dioxide removed earlier and then at least some of the stream containing recovered carbon dioxide is used as a fuel for a gas turbine used to drive an altenator to produce electricity, wherein at least part of one reduced carbon dioxide depleted stream is used as a source of hydrogen with or without further purification.

The amount of any stream split off and used for any purpose (including hydrogen source stream and gas turbine fuel gas use) depends upon the local demand for each. The water vapour can be added to the gas stream either by direct quench or by cooling the stream and then adding steam. Preferably the stream is either fully saturated by direct quenching, or at least 0.5 moles of steam per mole of carbon monoxide and more preferably above 1.0 moles are added to the cooled stream.

By utilising a partial oxidation process any sulphur compounds in the primary fuel (e.g. the refinery residue) are changed into gaseous compounds - mainly hydrogen sulphide together with some carbonyl sulphide - which compounds may be removed prior to the remaining gas being used as gas turbine fuel, hydrogen production, etc. Present commercialised technology invariably requires that the sulphur is removed by washing the gas with a liquid, e.g. Selexol (TM) , or Rectisol (TM) . Such liquid wash systems work at quite low temperatures - typically around ambient. Because of their temperature of operation most of the water vapour in the gas streams is condensed out prior to their entry into the liquid wash system and the gases leaving such system have a low water vapour concentration.

In GB-A-2196016 to T. Nurse, a process is described wherein a carbonaceous fuel is partially oxidised, the resulting stream is quenched in water, the stream is then used to provide heat to raise steam in a boiler, and the temperature is adjusted to a level suitable for feeding the stream to a carbon monoxide shift reactor. Downstream of the shift reactor the gas stream is expanded and then the stream is used as fuel for a gas turbine.

However, when a part of the gas stream is to be used as a feed stream for the production of hydrogen then in the Nurse invention the composition of the hydrogen production feed stream is the same as that of the fuel gas stream fed to the gas turbine. If, as is usual nowadays, a pressure swing absorber (PSA) unit is used to purify the stream to give a stream of pure hydrogen, then, in the case of Nurse, the PSA unit has to remove all the carbon dioxide present in the steam, and the (partial) pressure energy of the carbon dioxide in the feed stream is lost.

By means of the present invention carbon dioxide is first removed from the stream which is used for the production of hydrogen, thus increasing its purity and thereby reducing the cost of any e.g. PSA unit used to produce the hydrogen stream. Also the carbon dioxide is removed in a manner which retains a significant amount of the original partial pressure of the carbon dioxide after it is recovered into the stream fed to the gas turbine as fuel, and this partial pressure can be usefully used, for example, by expansion in the expander of the gas turbine.

Additionally advantageous is that by using the carbon dioxide in the fuel gas, or preferably by adding water by means of saturating the carbon dioxide-containing fuel gas stream produced by this invention, the gas turbine can be used a source of compressed air which can be fed to any air separation unit for the production of the oxygen used in the partial oxidation reaction. This arises because commercial gas turbines are balanced in the sense that they require certain flows in the compressor and in the expander. By introducing a higher flow of fuel gas because of the added carbon dioxide and, if added, water content of the fuel, some of the air in the compressor can be 'blown off .

There are a number of other benefits from adding carbon dioxide to the stream which is to be used for fuel gas in the gas turbine: the amount of water needed to be added to effect any given level of NOX control is much reduced: also the temperature of the water used in a gas saturator needed to add any given amount of water to the stream is lower than would otherwise be the case because the carbon dioxide itself picks up water. This increases the overall thermodynamic efficiency of power production in the plant because heat that otherwise would be used to gain the required level of gas turbine fuel gas 'saturation' can then for example be used to raise higher pressure steam or used to heat the fuel gas fed to the gas turbine to an even higher temperature.

Additionally the mass flow through the combined cycle gas turbine and its flue gas heat recovery section can be increased because carbon dioxide has a higher molecular weight than gas turbine exhaust gas.

Refineries produce high-sulphur residues, the disposal of which is becoming more difficult because of environmental legislation. As similar legislation also restricts the sulphur content of refinery products, refineries are needing more and more hydrogen for desulphurisation. Gasoline reformulation is also causing changes to refinery operations.

Refineries require several other utility services for refining crude oil. Naturally electricity is required, as is oxygen for the Claus unit, nitrogen for tank blanketing, steam for many heating duties, and sulphur free fuel gas.

One means of producing these utilities is through the partial oxidation (gasification) under elevated pressure of low quality waste hydrocarbons which may or may not be produced in the refinery.

The raw gasification product gas consists mainly of equal quantities of carbon monoxide and hydrogen together with about ten percent of carbon dioxide, see e.g. US-A-2 992 906. In order to maximise the production of the more desired co-product - hydrogen, the raw gasification gas of the present invention, after quenching with water or cooling and the addition of steam, is passed through a bed of catalyst wherein the reaction between carbon monoxide and steam occurs producing more hydrogen and carbon dioxide (the so called water gas 'shift' reaction) . This may be effected before or after the removal of sulphur compounds from the stream, as is known in the art.

The hydrogen required for eg hydrotreating and the fuel needed for electricity generation and other uses can then be prepared from this stream. The electricity alternator is driven by a gas turbine fuelled by the fuel gas stream. Such a turbine may form a part of a combined cycle system to enhance the efficiency of power generation, and to produce steam, preferably by use of a pass out turbine. In a combined cycle, steam is raised in boilers heated by the exhaust of the gas turbine.

There is, however, a problem in extracting the hydrogen from such a gas stream without significant loss of useful energy. There are a number commercial processes such as membrane technology and PSA using molecular sieves which will separate the hydrogen from the gas, or liquid wash systems which will preferentially remove the carbon dioxide leaving a high hydrogen concentration in the washed gas. However it is desirable to keep both the hydrogen and the fuel gas at pressure, and if some of the fuel gas is to be used as a gas turbine fuel it is also desirable for the carbon dioxide to be part of that fuel in order to have a lower flame temperature which has the effect of lowering the NOX formation, and, because it is a high molecular weight gas, to be expanded to atmospheric pressure to produce useful shaft energy.

The available commercial processes referred to significantly reduce the pressure of either the hydrogen or carbon dioxide, often making energy- consuming gas compression necessary if the low pressure gas is to be used as a fuel or processing material. This invention enables a substantial part of the carbon dioxide in the shifted gasification gas to be placed selectively in a fuel gas stream with minimum loss of its partial pressure for use as a gas turbine fuel whilst leaving the remaining hydrogen rich gas at substantially its original pressure. Typically the carbon dioxide may be recovered at more than 35% of its original partial pressure, often at more than 50%, sometime more than 60% and even more than 70%, or 75%.

The carbon dioxide may be removed from the source stream by use of a physical or physical/chemical system or a molecular sieve system. Whichever system is chosen it would be operated with a back-pressured ventilated system on the carbon dioxide recovery side, at least one ventilating stream being the stream that is to become the fuel gas stream.

Preferable the carbon dioxide is removed from the source stream by means of a physical solvent. The carbon dioxide-rich solution being then passed down a column maintained at a pressure with degassing aided by the use of a stripping gas - the stream into which the carbon dioxide is to be added.

The pressure in the stripping column (or cycle in the case of PSA bed) is preferably such as to allow the resulting stream to be used as a gas turbine fuel without recompression.

An enhancement of this process is the further stripping on the recovery side (after the first stripping by the pressure reduced stream) by the use of pure hydrogen produced in the PSA unit, or by the use of pressurised nitrogen from e.g. the air separation plant (taking care to ensure that any associated oxygen is at a safe low level) . By this means the amount of carbon dioxide removed may be increased because, for example, the lean solvent going back to the top of the absorber would contain less carbon dioxide - more having been removed by the second stage of stripping. An additional advantage of using this enhancement is that the gas leaving the top of the absorber can contain less carbon dioxide, this in turn means that the feed to the e.g. PSA unit would contain an even higher hydrogen concentration thereby the e.g. PSA's unit's cost being reduced.

The amount of carbon dioxide removed depends on a number of factors, the main ones being the hydrogen purity required, and whether or not additional hydrogen purification is to be effected. If no further hydrogen purification steps are to be used then, within the technology of known means, particularly known physical solvent means, substantially all of the carbon dioxide can be removed.

If a partial pressure swing (ventilated) molecular sieve adsorber is used then the amount of carbon dioxide removed will be calculated according the economics of such systems.

The source stream for the process of the present invention is a stream from or derived from partial oxidation of a carbonaceous material such as refinery residue, coal, Orimulsion (TM) and other carbonaceous wastes such as sewage sludge.

Preferably the stream from the partial oxidation reactor is quenched in the well known manner originally patented and licensed by Texaco Development Corporation, see, e.g. US-A-2 992 906. The quenched stream is, possibly after some temperature adjustment, passed through a carbon monoxide and steam (water gas shift) catalyst bed wherein some of the carbon monoxide contained in the stream is reacted with the steam to form carbon dioxide and additional hydrogen. By means of the shift reaction the amount of carbon dioxide in the source stream is increased thereby making it easier to remove, in that for a given pressure of the source stream, the partial pressure of the carbon dioxide in that stream is increased - thus increasing the equilibrium or load carrying capacity of the physical solvent. Likewise the increase in partial pressure would increase the loading attainable on any molecular sieve used for the purpose of removing the carbon dioxide. Naturally an increase in the partial pressure in the source stream increases the pressure at which the carbon dioxide can be recovered into the stream into which it is destined to be added/mixed.

The pressure at which the partial oxidation takes place is generally above 30, bara, preferably above 40 bara, more preferably above 45 bara, more preferably still above 50 bara, even more preferably above 55 bara, and most preferably above 60 bara.

The pressure drop through which the carbon dioxide- depleted stream passes is preferably effected in a turbine for the recovery of shaft power, which can then be used, e.g. for producing additional electricity.

It would be possible to expand this stream (preferably after drying it) from a comparatively cold temperature such that the stream falls below say 10 degrees celsius. The resulting cold stream could then be used as a source of "coolth", i.e. heat removal particularly in the carbon dioxide removal unit. Alternatively it would be possible to heat the stream to be expanded such that in an expander it gives out more power.

The pressure drop through which the carbon dioxide depleted stream passes is generally more than 5 bar, preferably above 10 bar, more preferably above 15 bar, still more preferably above 20 bar, and most preferably above 25 bar.

Regarding the pressure drop which could be effected on the hydrogen stream just after it is split from the fuel gas stream: it should be noted that notwithstanding the fact that the hydrogen may be wanted at a high pressure, because of the efficiency and cost of further hydrogen stream processing, notably in any PSA unit, it may be desirable to reduce the pressure of any feed to a PSA stream to the economic level inclusive of the cost of any recompression of the further purified hydrogen stream. PSA units are often most economic at about 30 bar.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:-

Figure 1 is a schematic flow sheet of an IGCC process according to the present invention, and

Figure 2 is a schematic flow sheet of the carbon dioxide adsorption and desorption section of the process illustrated in Figure l.

In Figure 1, an IGCC plant incorporating the process of the invention comprises a partial oxidation reactor (1) to which is fed a carbonaceous fuel ("F") and oxygen ("O") ; a quench (2) into which the very hot product gases are fed causing a substantial amount, but not all, of the water ("W") to evaporate; a trim boiler (3) (not to be confused with a very hot gas boiler that would be located directly on the outlet of the partial oxidation reactor) ; and a shift reactor preheater (4) followed by a shift reactor (5) . Next follow a number of gas coolers (items 6 to 9) to further cool the raw gas and heat streams and raise steam ("STM") and then a final cooling water ("CW") cooler(lθ) . ("BFW" stands for "boiler feed water".) The cooled gas then flows through a sulphur compounds removal unit (11) wherein sulphur compounds are removed from the stream and concentrated into a stream that would be fed to a Claus unit (not shown) for the conversion of the sulphur compounds into elemental sulphur. The substantially sulphur free stream could then have a side stream taken from it for eg the production of ethanol. Next the stream flows through a carbon dioxide removal column (12) wherein at least some of the carbon dioxide is removed into a physical solvent. The carbon dioxide depleted stream is split into two streams, the one destined to be the fuel gas stream being next passed through an expander (13) to produce shaft power. The other stream is passed to a pressure swing adsorption unit (14) to further purify the hydrogen, the off gases being used as a clean fuel gas stream. The stream from the expander (13) is then used as a stripping gas in a stripping column (15) to recover the carbon dioxide from the physical solvent used in column (12) . The combined recovered carbon dioxide/ fuel gas stream is then passed up a saturator column (16) to add the amount of steam required to keep the burning characteristics of the fuel gas such that the NOX produced when this gas is burnt is below that allowed by local regulations. The 'saturated' gas is then used a fuel for the gas turbine in the combined cycle unit (17) to produce electricity.

A stream (18) may be taken off and used as sulphur free fuel in a refinery. This stream could also be taken off before the recovery of carbon dioxide either upstream or downstream of the expander.

In a variation (not shown) it may be desirable to raise high pressure steam from the stream from the shift reactor (5) , and also to raise lower pressure steam immediately thereafter - preferably at the same pressure as the steam raised in the trim boiler (3) located downstream of the whole gasification unit (1) (following quench (2)).

Preferably in this variation, having shifted the gas from the partial oxidation reactor (l) , the stream may be passed through a sulphur removal unit. Such unit may employ either hot or cold known means of removal such as the use of Selexol (TM) or methanol (Rectisol (TM) ) as a solvent.

Following the removal of sulphur and at least some of the carbon dioxide, the stream may undergo a second stage of carbon monoxide water gas shift reaction - preferably employing a low temperature shift catalyst such as that currently supplied by ICI. If a second carbon monoxide shift stage is employed, the residual sulphur compounds may be removed from its feed by the use of a bed of zinc oxide or any other known means.

The carbon dioxide thereby produced may similarly be removed giving a substantially pure stream of hydrogen whose main impurities would be methane and residual carbon monoxide. If necessary these may be removed by the use of a bed of molecular sieves to give a highly pure stream of hydrogen.

After the sulphur removal section of this variation, a side stream may be taken for the production of chemicals requiring 'synthesis gas' as a feedstock - especially for methanol production.

The oxygen required for the partial oxidation may be supplied by an air separation unit (ASU) , the capacity of which can be increased so as to also supply oxygen to the Claus unit. Additionally the ASU produces nitrogen which can be used, e.g. as an inerting gas in a refinery.

In the present invention the gas stream from which the carbon dioxide is removed and the manner in which it is subsequently recovered may be produced by known art such as is described in WO 92/15775 (see stream 6 in Fig la thereof) .

The carbon dioxide absorption and desorption sections of the IGCC plant illustrated in Figure 1 will now be described.

Reference is made to Table 1, the properties of the relevant streams, and Table 2, the molar flows of the streams, and to Fig 2 hereof. The carbon dioxide containing feed gas stream (100) , produced as described above, is fed into the bottom of a packed or plated absorption column (101) (equivalent to column (12) in Figure 1) . Into the top of the same column is fed a stream of a physical solvent in this embodiment tri- ethylene glycol TEG (103) . As this stream is recycled from the bottom of the stripping column (108) (equivalent to column (15) in Figure 1) (together with makeup TEG (104) ) , this stream contains some unstripped dissolved gases (see composition of 105) . On passing down the column (101) , the physical solvent absorbs carbon dioxide from the feed gas, leaving the bottom of the column stream comparatively rich in carbon dioxide (106) . This rich solvent stream then passes through a pressure reduction device such as a valve (107), but could also be a turbine to recover shaft power, and is then passed to the top of the stripping column (108) . The gas leaving the top of absorption column (101) is contains comparatively little carbon dioxide (109) . It is heated to 70 C in the expander preheater (110) by using any heating medium (steam is shown) , and is then fed to the expander to recover shaft power during the lowering of the pressure of the stream. The stream is lowered from 6500 kPa to 2500 kPa, the former being a typical pressure for high pressure partial oxidation, whilst the later is a typical pressure at which feed gas is required for a gas turbine, and also is a good pressure at which to design a PSA unit in which pure hydrogen may be produced.

The stream (112) leaving the expander (111) is split into two streams (in 115) . One of these streams (113) is used as the feed to the PSA unit for the production of pure hydrogen, whilst the other (114) is used as the stripping gas fed to the bottom of the stripping column (108) . In this column, the carbon dioxide is stripped out of the rich solvent, and leaves the column with the stripping gas (116) and it is this stream which is used as the fuel for the gas turbine, either with or without saturation with water in a saturation column.

The stripped physical solvent in this embodiment is pumped back to the absorption column 101 . The solvent lost in the gas stream leaving the stripping column (108) is made up by the addition of fresh solvent - the stream (104) .

One variation (not shown) of this embodiment would be to take the liquid stream leaving the bottom of the stripping column (108) and, using pure hydrogen produced in the PSA unit, strip more carbon dioxide out of this stream. The resulting hydrogen plus carbon dioxide stream could then either be fed back to the feed gas stream going to the absorption column (101) , or it could be used as fuel gas. By this means the amount of carbon dioxide removed in the absorption column (101) could be increased, and the hydrogen content of the PSA unit feed could also be increased, thereby reducing the cost of the PSA unit.

Electricity made by the process of the present invention may be identified by one of two means. Firstly by the signature of the particular alternator used, or secondly by the fact that the consumer of such electricity wherever located will be paying for the supply of the same.

TABLE 1 - Properties of Streams

TABLE 2 - Stream Molar Flows (Kg Mols/HR)

Claims

CLAIMS :

1. A process for the co-production of electricity and hydrogen wherein a carbonaceous fuel is partially oxidised at elevated pressure with oxygen or an oxygen containing gas to yield a gas stream containing hydrogen and carbon monoxide, cooling the gas stream and adding water vapour, passing the water vapour containing stream through a carbon monoxide shift reactor to increase its hydrogen content, and, passing the stream through a carbon dioxide removal device, wherein at least a part of this carbon dioxide depleted stream then has its pressure reduced and at least a part of the part of the stream which has had its pressure reduced is used first to recover at least some of the carbon dioxide removed earlier and then at least some of the stream containing recovered carbon dioxide is used as a fuel for a gas turbine used to drive an altenator to produce electricity, wherein at least part of one reduced carbon dioxide depleted stream is used as a source of hydrogen with or without further purification.

2. A process as claimed in claim 1 in which the stream containing the recovered carbon dioxide is contacted with liquid water so as to add steam to the gaseous stream to be used as fuel.

3. A process as claimed in claim 1 or claim 2 wherein the partial oxidation is carried out at a pressure above 30 bara.

4. A process as claimed in any one of the preceding claims wherein the pressure reduction of the carbon dioxide depleted stream is greater than 5 bar.

5. A process as claimed in any one of the preceding claims wherein the pressure reduction is carried out through an expansion turbine to produce shaft power.

6. A process as claimed in claim 5 wherein the shaft power is converted into electricity.

7. A process as claimed in any one of the preceding claims when adapted to be operated in an IGCC plant.

8. A process as claimed in any one of the preceding claims when adapted to be operated in a refinery.