GB2365492A

GB2365492A - Method of combusting fuel in a compression-ignition engine

Info

Publication number: GB2365492A
Application number: GB0019360A
Authority: GB
Inventors: John Mcneil
Original assignee: Finch Ltd; FINCH INTERNAT Ltd; Finch International Ltd
Current assignee: Finch Ltd; FINCH INTERNAT Ltd; Finch International Ltd
Priority date: 2000-08-07
Filing date: 2000-08-07
Publication date: 2002-02-20
Anticipated expiration: 2020-08-07
Also published as: GB2365492B; GB0019360D0

Abstract

A combustion unit 1 comprises means to recirculate exhaust gas from the combustion unit back into the combustion chamber of said combustion unit. Surplus carbon dioxide from the exhaust gas stream is liquified in a liquifying unit 19 during recirculation, and the recirculated exhaust gas is enriched with oxygen from an air separator 29 prior to entering the combustion chamber of the combustion unit. Accordingly the combustion atmosphere is essentially oxygen and carbon dioxide.

Description

<Desc/Clms Page number 1> METHOD OF COMBUSTING FUEL The present invention relates to a method of combusting fuels in a compression ignition engine, and a combustion system for carrying out the method. The invention is particularly suitable for combusting non- fossil fuels, although fossil fuels may also be used. The invention is particularly suitable for reducing C02, NO,, and other pollutant emissions from the exhaust of combustion processes.

A compression ignition engine operates by injecting fuel, in either gaseous or liquid form, under pressure, into air that has been compressed by a piston travelling up a cylinder. The fuel and air mixture is further compressed until it is hot enough to ignite the fuel. Combustion of the fuel leads to a rapid increase in temperature and pressure inside the combustion chamber, which forces the piston back down the cylinder. The temperature inside the combustion chamber can reach over 20000C.

At these high temperatures various combustion reactions can take place inside the combustion chamber. Carbon in the fuel reacts with oxygen to form carbon dioxide, although incomplete combustion also produces carbon monoxide; hydrogen in the fuel reacts with oxygen to produce water vapour; inter-reactions between carbon, hydrogen and oxygen can lead to the formation of various organic compounds.

There is increasing concern that the release of greenhouse gases into the atmosphere during the combustion of fossil fuels is a major contributory factor towards global warming. The greenhouse gas of most concern is carbon dioxide.

Other undesirable combustion pollutants that are present in the exhaust gas include carbon monoxide, volatile organic compounds and particulates. Many

fossil fuels also contain sulphur or chlorine and produce sulphur dioxide or hydrogen chloride acidic gases, when they are burned.

It is therefore of great interest to reduce carbon dioxide and other pollutant emissions into the atmosphere, and it would be valuable in particular if a method could be found for allowing combustion of fuels in diesel engines and so forth without releasing carbon dioxide and other pollutants into the atmosphere.

Diesel engines tend to be fuel specific and operate best on the petrochemical fossil fuels that have been especially formulated for this type of engine.

Non-fossil fuels, such as animal and vegetable oils and fats, are generally difficult to combust in standard diesel engines because they burn inefficiently and carbonaceous deposits are formed in the combustion chamber of the engine.

other non-fossil fuels, such as those that are synthesised by the pyrolysis or gasification of biomass material, tend to have poor ignition qualities and low calorific values. These fuels are more difficult to ignite than petrochemical fuels and normally they can only be combusted in modified diesel engines that include either a pilot or spark ignition system to initiate ignition of the fuel.

It has been proposed, for example in WO 00/05492 of Finch International Limited, that some non-fossil fuels., such as animal and vegetable oils and fats, can be efficiently combusted in a standard diesel engine by increasing the oxygen concentration in the combustion chamber of the engine to a level about 2% to 6% above naturally aspirated conditions (i.e. 23 to 27% oxygen in the combustion atmosphere).

However, at the high temperatures that exist in the combustion chamber of a diesel engine, the oxygen and nitrogen in the air inside the chamber can react together to form nitrogen oxides (Nox). It is therefore

a characteristic of diesel engines that they produce relatively high NOx emissions when compared to other combustion systems. Nitrogen oxides are indirect greenhouse gases that contribute towards global warming by assisting in the formation of direct greenhouse gases such as tropospheric ozone. Nitrogen oxides also have an acidification impact in the atmosphere. Further increasing the oxygen concentration in the combustion chamber of an engine, to enable non-fossil fuels to be effectively combusted, results in an increased level of NOx in the exhaust, for example up to or even over three times the levels produced when diesel fuel oil is combusted under naturally aspirated conditions.

To reduce the level of NOx emitted from the exhaust of diesel engines, attempts have been made to recycle a small part of the engine exhaust gas back into the air inlet on the engine. This has the effect of reducing the oxygen content of the atmosphere in the combustion chamber of the engine, to below the naturally aspirated level, and therefore there is less oxygen available to react with the nitrogen in the air. Although this tends to reduce the formation of NOx, lowering the oxygen content produces less efficient combustion, and the formation of other combustion pollutants, such as carbon monoxide, tends to increase.

In addition to NOx, the exhaust gas from a diesel engine contains other pollutants that can have a detrimental effect on both the environment and on human health.

The present invention seeks to decrease the problem of NOx emissions in the combustion of fossil and non- fossil fuels. It is a further object of the invention to reduce or prevent the emission of carbon dioxide and the release of other pollutants from combustion processes into the atmosphere.

The present invention proposes replacing the normal air in the combustion chamber of an engine with a gas

mixture consisting substantially of only oxygen and carbon dioxide. The benefit of this is that there is no air, and hence no nitrogen, present in the combustion chamber of the engine, and therefore NOx is not produced during the combustion process. Also, the carbon dioxide in the exhaust gas may be present in sufficient concentration to facilitate its removal from the exhaust gas and prevent its release it into the atmosphere.

Thus viewed from one aspect, the invention provides a method of combusting fuel in a compression ignition engine wherein an atmosphere consisting essentially of a mixture of oxygen and carbon dioxide is provided in the combustion chamber of said engine, and wherein the oxygen constitutes at least 21% of said gas mixture.

The invention also provides a combustion system comprising a combustion unit, preferably a compression ignition engine, and means to provide a combustion atmosphere consisting essentially of oxygen and carbon dioxide, said oxygen constituting at least 21% of the combustion atmosphere.

Preferably, the amount of oxygen in the gas mixture for combustion is enriched. By "enriched" is meant that the concentration of oxygen in the combustion atmosphere is greater than that found in atmospheric air, which is normally considered to contain about 21% oxygen.

In the case of oxygen, the amount of enrichment needed will depend on the nature of the fuel to be combusted. In general, high quality fuels such as natural gas and diesel oil are easy to ignite and therefore oxygen enrichment in accordance with the invention may be very low e.g. no more than 1% above normal (i.e. approximately 22% oxygen in the combustion atmosphere) .

On the other hand, some other fuels which are more difficult to burn and ignite (e.g. pyrolysis oil) may require higher amounts of oxygen in the combustion atmosphere to achieve ignition and/or complete

combustion of the fuel intake. It is readily within the routine skill of the reader to ascertain a suitable degree of oxygen enrichment according to the type of fuel to be combusted.

Preferably, however, the oxygen concentration in the inlet gas mixture would be up to about 30%, more preferably about 22 to 29%, more preferably at least about 230-., or even more preferably between about 25 to 27% oxygen (4%-6% above the oxygen level in normal air) As mentioned above, these amounts are particularly preferred for non-fossil fuels.

Hence, it is preferred that the combustion atmosphere comprise at least about 70% carbon dioxide, more preferably about 71 to 78% and most preferably about 73 to 75% carbon dioxide. It is therefore preferred that the combustion atmosphere comprises from about 22 to 29%, more preferably 25 to 27% oxygen and from 71 to 78%, more preferably 73 to 75% carbon dioxide.

Preferably, the degree of oxygen enrichment is controlled in dependence on an analysis of the engine operating conditions and the exhaust gas composition, and in particular the level of carbon monoxide in the exhaust gas when it leaves the engine, as the presence of carbon monoxide is an indication of incomplete combustion. Small amounts e.g. up to 1% of other substances, such as organic compounds, carbon monoxide, water vapour and so forth, which may have formed during the combustion reactions, could be present in the engine combustion atmosphere, without having a detrimental effect on the combustion process.

Fuels to be combusted according to the invention include fossil and non-fossil fuels and these may be in liquid or gaseous form, in the form of mixtures and emulsions, and in some cases solid fuel may also be used, e.g. in the form of finely dispersed slurries. Examples of typical fossil fuels for combustion in

compression ignition engines include fossil based mineral oils or hydrocarbon gases.

In a particularly preferred aspect of the invention, the fuel to be combusted is a non-fossil fuel. Examples of non-fossil fuels include, but are not limited to, animal and vegetable oils and fats, pyrolysis oil, landfill gas and synthetic gas mixtures produced by the gasification of biomass material. Examples of animal and vegetable oils and fats include tallow and cooking oil, especially waste cooking oil. Mixtures of these fuels, including mixtures of fossil and non-fossil fuels are also encompassed.

Non-fossil, renewable fuels, such as vegetable oils and pyrolysis oil, have the advantage that they do not contribute directly towards the greenhouse effect when burned, because the carbon dioxide released to the atmosphere during combustion can be absorbed by new crops planted to replace those originally used to produce the non-fossil fuel.

A further advantage of most non-fossil fuels is that they generally have very low sulphur and chlorine contents and when these fuels are burned the amount of sulphur dioxide or hydrogen chloride in the exhaust is usually negligible.

Replacing fossil fuels with non-fossil fuels in an engine therefore provides a number of environmental advantages, particularly when the non-fossil fuels are combusted in an enriched oxygen atmosphere. Using, an enriched oxygen combustion atmosphere improves the efficiency of the combustion process, which results in much lower emissions of carbon monoxide, organic compounds and particulates.

In a preferred embodiment, the carbon dioxide in the combustion gas mixture is supplied by recirculating the engine exhaust gas stream, which is highly concentrated in carbon dioxide, back to the engine. The recirculated exhaust gas will also contain oxygen that

was not used up during the initial combustion of the fuel. The amount of oxygen remaining in the exhaust gas will be dependent on the original concentration of oxygen in the carbon dioxide and oxygen gas mixture supplied to the combustion chamber of the engine.

For example for a combustion atmosphere containing 260-. oxygen, 740-. carbon dioxide, the exhaust gas stream leaving the engine may consist of about 16-06 oxygen, 790-o carbon dioxide, 5-*o water vapour depending on the type of fuel combusted. The exhaust gas would also contain very small amounts of combustion pollutants. For example carbon monoxide may account for about 0.020-. of the exhaust gas composition.

Hence it is evident that although the exhaust gas is depleted in oxygen, it still contains a significant amount of oxygen, as well as an increased amount of carbon dioxide, and both of these could be usefully employed to make up the oxygen/carbon dioxide gas mixture required for the combustion atmosphere in the engine.

A further advantage of the invention is that the carbon dioxide in the exhaust gas is highly concentrated and so facilitates the removal of the surplus carbon dioxide, which has been produced during the combustion reactions, from the exhaust gas stream.

The exhaust gas that remains after the surplus carbon dioxide has been removed can then be topped up with fresh oxygen, so that the recirculated exhaust gas stream is brought back to the required oxygen/carbon dioxide composition (i.e. in the example given above 260-. oxygen, 74% carbon dioxide) before entering the combustion chamber of the engine.

The surplus carbon dioxide can be removed from the exhaust gas stream by utilising specific properties of carbon dioxide that are very different from the other

gases, mainly oxygen and carbon monoxide, present in the exhaust gas. For example, carbon dioxide has a smaller molecular diameter, a higher boiling point and is denser than either oxygen or carbon monoxide. Various methods of separating out the surplus carbon dioxide from the exhaust gas could be considered, including the use of gas separation membranes.

Liquefaction, however, is a preferred method. A portion of the exhaust gas stream, equivalent to the volume of exhaust gas that contains the increase in carbon dioxide that has been formed by the combustion reactions, can be diverted through a liquefying unit. Within the liquefying unit the carbon dioxide is liquefied out of the exhaust gas stream, whilst the oxygen and carbon monoxide remain in gaseous form.

The carbon dioxide separated out from the exhaust gas is in a suitable state to be readily disposed of in a manner that would prevent it from entering the atmosphere. For example, it could be used in other industrial purposes, or he used to encourage plant growth in greenhouse horticulture, or be used to pump out nearly depleted oil wells and then be entrapped in the ground.

When combusting fossil fuels, being able to sequester the carbon dioxide from the atmosphere, by the means described, is a major environmental benefit, as it helps to reduce the greenhouse gas effect. It is even advantageous when burning renewable non-fossil fuels, since it results in a negative greenhouse gas impact instead of the neutral effect usually associated with these fuels. The combustion process would then become a carbon dioxide 'sink,, because the crops planted to replace those originally used to produce the non-fossil fuel would be able to absorb some of the carbon dioxide that is being released into the atmosphere by conventional fossil fuelled power stations.

Although oxygen enrichment reduces the emission

levels of most exhaust gas pollutants, it is preferred that the exhaust gas is treated and cleaned, whilst being recirculated back to the engine, to remove water vapour and particulate matter, and also sulphur dioxide and hydrogen chloride if the fuel being combusted contained either sulphur or chlorine.

It is not necessary to remove combustible pollutants, such as carbon monoxide and organic compounds, from the recirculated exhaust gas, as they can be returned to the combustion chamber of the engine where they will be combusted during subsequent combustion cycles. Consequently the exhaust gas recirculation system prevents the release of these particular pollutants into the atmosphere.

Viewed from a further aspect therefore, the invention provides a method of recovering carbon dioxide produced from the combustion of fuel wherein the combustion atmosphere consists essentially of oxygen and carbon dioxide.

Viewed from yet a further aspect, the invention also provides a carbon dioxide recovery system for a combustion unit comprising means to recirculate exhaust gas from a combustion unit back into the combustion chamber of said combustion unit, means to liquefy surplus carbon dioxide from the exhaust gas stream during recirculation and means to enrich the recirculated exhaust gas with oxygen prior to entering. the combustion chamber of the combustion unit.

It is also preferred that the exhaust gas is treated and cleaned, whilst being recirculated back to the engine, to remove water vapour, particulate matter, and sulphur dioxide and hydrogen chloride (if the fuel combusted contains either sulphur or chlorine).

The present invention is particularly suited to static compression ignition engine installations that are used for the commercial generation of electricity.

Accordingly, a further aspect of the invention

includes a method of generating electricity and an electricity generation system wherein an engine or similar combustion unit (e.g. a turbine) operating in accordance with the invention is coupled to an electrical power generating device.

Furthermore, the heat from the exhaust gas and the engine cooling systems can also be used to produce steam to drive a steam turbine to generate more electricity. Thus, preferably the hot exhaust gases from the engine are used to raise steam in a steam boiler, which may then optionally be used to drive a steam turbine.

In addition to heat, steam and electricity, the process of the invention provides a number of other valuable by-products, including oxygen, nitrogen and carbon dioxide, in either liquefied or gaseous form. These by-products could be used in other industrial applications.

Conveniently, at least a portion of the oxygen in the combustion atmosphere is supplied in purified form from an air separation unit. Hence the nitrogen by- product from the air separation unit can be used to manufacture pyrolysis oil in a pyrolysis oil production process, whilst excess oxygen from the air separation unit can be used in a partial oxidation gasification process.

In a further preferred aspect of the invention, products from the combustion system may be utilised in manufacturing processes that actually produce non-fossil fuels for combustion in the engine. For example, the combustion method of the invention could be combined with either a pyrolysis or a partial oxidation gasification process that manufacture non-fossil fuels from biomass material, as these processes require supplies of either an inert gas or oxygen as well as heat and steam.

The invention will now be described by way of example only with reference to the following Examples

and the accompanying drawings, in which: Figure 1 shows a schematic illustration of a combustion and exhaust gas recirculating system that embodies the invention, where the engine exhaust gas is recirculated back to the engine and is topped up with pure oxygen provided by a cryogenic unit; and Figure 2 shows a schematic illustration of a pyrolysis oil manufacturing plant that could utilise the by-product streams available from the process described in Figure 1.

With reference now to Figure 1, fuel is pumped from a storage tank 2 along the fuel line 3 to a fuel injector located in each cylinder head of the engine 1.

Oxygen is produced from a cryogenic unit 29 by feeding air by an air blower 28 into the cryogenic system. The pure oxygen is fed from the cryogenic unit 29 to a storage tank 31 by pump 30.

Liquid nitrogen from the cryogenic unit 29 is stored in tank 33 and gaseous nitrogen can be fed from tank 33 by a pump to a storage tank 34 for use in either associated processes or other industrial applications.

Carbon dioxide gas is stored at a pressure slightly above normal atmospheric pressure in tank 21. Carbon dioxide is fed from tank 21, by pump 22, through a gas valve 26 into the combustion gas supply line leading to the engine.

The pressure in the gas supply line is controlled by a pressure regulating valve 27, which controls the feed of carbon dioxide to a gas mixing valve 5, where the carbon dioxide is mixed with oxygen.

Pure oxygen is fed from storage tank 31, by pump 32, to the mixing valve S where the required amount of oxygen is added to the carbon dioxide, in dependence on a gas analysis that is carried out by an analyser/valve 23 positioned in the gas supply. For example, assuming that an enriched oxygen level 5% above normal was required, the gases would be mixed by valve 5 in the

ratio of 26% oxygen to 74% carbon dioxide.

The mixed gas would be at a pressure slightly above atmospheric pressure, typically about one tenth above normal atmospheric pressure. The gas supply line 4 would feed the homogeneous gas mixture to a turbo charger on the engine, which would present the gas at the correct pressure to each air inlet valve in the cylinder head of the engine 1.

On engine start up, the residual normal air in the engine cylinders and in the exhaust circulation system would be vented by analyser/valve 23 to the atmosphere through flue 25, until such time that the exhaust gas circulation system was filled with exhaust gas only and air had been purged from the system.

The composition of the exhaust gas after combustion is significantly different from that of the inlet gas because of the combustion reactions. Assuming that the inlet gas was enriched with oxygen to 5% above normal, the exhaust gas may typically consist of about 16% oxygen, 79% nitrogen, 5% water vapour and small amounts of carbon monoxide, organic compounds and particulates.

A sensor/gas analyser 6, positioned in the exhaust as it leaves the engine, monitors the temperature and pressure of the exhaust gas and the levels of the main combustion pollutants. In particular the level of carbon monoxide is continually monitored as this provides a good indication of the efficiency of the combustion process. Sensor 6 is linked to the gas mixing valve 5, so that the concentration of oxygen in the inlet gas supply can be adjusted electronically, if necessary, to provide optimum combustion conditions.

The exhaust gas leaving the engine is at a much higher temperature and also higher pressure than the inlet gas supply because of expansion due to the high combustion temperature and the increased volume of gases in the exhaust due to combustion reactions. The exhaust gas enters an expansion tube to relieve the increased

pressure, however, a positive pressure will be maintained within the system to drive the exhaust gas around the recirculating system. As the exhaust gas circulates back to the engine, the bore of the exhaust tube is gradually restricted to compensate for the progressive drop in pressure as the gas cools and as gas constituents are removed. Eventually the tube reduces down to the same diameter as the gas supply tube 4.

The engine 1 is connected to a generator 7 to produce electricity. The engine cooling system is connected to a heat exchange unit 12 so that heat from the engine can be used for heating purposes in heat loops 35.

The hot exhaust gas heats a boiler 8 to produce steam 9 that is supplied to a steam turbine 10, which in turn drives a further generator 11 to produce more electricity. Either liquid nitrogen or liquid carbon dioxide, which are available from the combustion/exhaust circulation process, can be used as coolant for the steam turbine 10. Excess steam in the steam loop 9 can be used for other heating purposes. The exhaust gas is partially cooled by passing the exhaust gas through a heat exchanger 12 so that heat can be used in heat loops 35. Liquid carbon dioxide can be used to further cool the exhaust gas.

On leaving the heat exchanger 12, the exhaust gas enters a water-cooled condenser 13, which cools the gas down to near ambient temperature and condenses out the water vapour present in the exhaust gas stream. The water supply to the condenser is circulated through a power cooling system 14 to maintain a constant flow of cold water in the condenser. Either liquid nitrogen or liquid carbon dioxide, available from the process, can he used to aid the cooling of the water supplied to the condenser 13 by the power cooling system 14. Condensed waste water 15 from the condenser 13 is treated, if necessary, before being sent to drains.

The exhaust gas from the condenser 13 is filtered in a bag filter 16 or alternatively a cyclone unit to remove particulate matter. On leaving the filtration unit, the exhaust gas primarily consists of oxygen, carbon dioxide and small amounts of carbon monoxide and organic compounds.

A by-pass valve 17 diverts a proportion of the exhaust gas, so that the main circulating exhaust gas stream contains a volume of carbon dioxide equivalent to the original carbon dioxide level in the gas mixture supplied to the engine. The diverted portion of exhaust gas passes to a liquefying unit 19, which liquefies the carbon dioxide out of the exhaust gas. The volume of the liquefied carbon dioxide is equivalent to the amount of additional gaseous carbon dioxide that was produced during the combustion reactions.

Gases in the diverted portion of the exhaust gas that have lower boiling points than carbon dioxide, i.e. oxygen and carbon monoxide, are fed back into the main exhaust gas stream from the liquefying unit 19.

A non-return valve 18 prevents the exhaust gas stream returning to by-pass valve 17. Liquid carbon dioxide is transferred to storage tanks 20 from the liquefying unit 19, and a supply of carbon dioxide gas is maintained in storage tanks 21, supplied from the liquid carbon dioxide tanks 20.

A gas analyser and regulating valve 23 analyses the remaining recirculated exhaust gas stream. At this stage, the volume of carbon dioxide in the exhaust gas should be back to its original level before combustion. However, gas analyser/valve 23 can electronically trigger fine adjustments of the carbon dioxide concentration by valves 26 and 27. Valve 26 can add carbon dioxide from tank 21 if the carbon dioxide level is too low, whilst valve 27 can divert some of the exhaust gas to the flue 25 if either the carbon dioxide level or the pressure in the exhaust gas is too high.

If the gas analyser 23 senses that the composition of the exhaust gas stream has gone completely out of balance, for whatever reason, valve 23 can divert the whole exhaust gas stream to the flue 25, where air blower 24 would dilute the exhaust gas with air before venting it to the atmosphere. If this happened, the combustion/recirculating exhaust gas system would automatically start again using fresh supplies of carbon dioxide and oxygen from storage tanks 21 and 31 respectively.

The exhaust gas passes through regulating valve 27 to the gas mixing valve 5, which adds oxygen from tank 31, in accordance with the oxygen level sensed by the gas analyser 23 linked to valve 5, until the oxygen level in the recirculated exhaust gas is back to the specified operating concentration. The combustion operation is then ready to start again.

If the fuel being combusted is known to contain appreciable levels of sulphur or chlorine, a dry scrubbing unit, to remove sulphur dioxide and hydrogen chloride from the exhaust gas, can be included in the exhaust gas recirculation system, between condenser 13 and bag filter 16.

In practice, of course, the combustion/ recirculating exhaust process would operate on a continual basis. The small amounts of carbon monoxide and organic compounds that remain in the recirculated exhaust gas will be burned off during subsequent combustion cycles. For clarity, Figure 1 is a much simplified representation of the recirculating exhaust gas process. In practice, the recirculating system would have a plurality of sensors, gas analysers, pressure regulators, vents, control valves, pumps, and gas cooling systems all electronically linked and controlled, to ensure the integrity of the system.

The heat, power and steam available from the combustion process, along with byproducts from the

system, such as oxygen, nitrogen and carbon dioxide in liquid or gaseous form, could be used in associated processes that manufacture fuel for the engine, such as, for example, pyrolysis or gasification systems that produce non-fossil fuels from biomass material.

This is illustrated with reference to Figure 2, which schematically illustrates a pyrolysis oil production plant, in a simplified form. Pyrolysis oil, a complex mixture of various chemicals including lignin, aldehydes, carboxylic acids, carbohydrates, ketones, phenols and alcohols, is manufactured by the controlled combustion of biomass material in an inert atmosphere. By combining the combustion and pyrolysis processes, full use can be made of the product streams available from the combustion/exhaust gas recirculation system.

A partial oxidation biomass gasification process is similar to the pyrolysis oil process, except that the combustion conditions are varied and steam and oxygen are used as the reactant medium instead of an inert gas, so that a predominantly gaseous product is produced rather than liquid pyrolysis oil.

In Figure 2, biomass matter 36, such as grass, straw or wood chips, is fed to a processing plant 37, where it is chopped to size and dried by heat 35 supplied from the combustion process. The dried material 38 is fed to a hopper 39.

By-product nitrogen from the combustion process, is fed by pump 40 from tank 34 into the dried material 38 from hopper 39 and the biomass material and nitrogen are blown together onto the bed of the pyrolysis reactor 41. Nitrogen from tank 34, pre-heated by steam 9 and heat 35 from the combustion process, is introduced to a chamber 42, where it fluidises the bed of the reactor 41.

The heat for the pyrolysis reaction, typically about 500'C, is supplied by heat 35 and steam 9 from the combustion process. Char product from the pyrolysis reaction is removed at a cyclone unit 43. The char is

dried in a drying unit 44, using heat 35 and steam 9 from the combustion process. The dry char 45 has commercial value as a solid fuel.

The hot pyrolysis gas, which includes water vapour, is condensed at a powered water cooled condenser 46, and liquid nitrogen or liquid carbon dioxide from the combustion process can be used to help cool the condenser.

The liquid pyrolysis oil residues are pumped, by pump 47, to a centrifugal separator 48, which removes particulate matter and excess water remaining in the pyrolysis oil. The oil is then stored in tank 2 for use as fuel 3 in the compression ignition engine.

Residual pyrolysis gas from the condenser 46, which mainly consists of nitrogen, carbon dioxide, carbon monoxide and methane is filtered through a bag filter 49 and can then be concentrated for use as fuel in say a gas engine.

From the above it will be seen that the present invention enables liquid and gaseous non-fossil fuels to be efficiently combusted in a compression ignition engine, with the purpose of generating electricity, and in such a manner that the release of exhaust pollutants to the atmosphere are at nil or negligible levels and the surplus carbon dioxide, produced during the combustion reactions, can be removed and sequestered from the atmosphere. As well as electricity, the process of the invention also provides a number of potentially valuable product streams, including heat, steam, oxygen, nitrogen and carbon dioxide.

The combustion method of the invention is applicable to various non-fossil fuels, including animal and vegetable oils and fats, pyrolysis oil, landfill gas and syn-gas mixtures obtained from the gasification of biomass material. Waste non-fossil fuels, such as waste cooking oils, could also be combusted in the manner described.

The method of the invention could also have applications in the combustion of fossil based liquid and gaseous fuels. The system would provide a means to sequester the carbon dioxide from the combustion process, which in turn would help to reduce the impact that fossil fuel combustion is having on global warming.

The combustion system of the invention is effectively self-supporting once it is in operation, since carbon dioxide and oxygen in the exhaust gas from the engine can be recirculated back to the combustion chamber. In other words, the combustion system is self- supporting since it is not necessary or desirable to allow any atmospheric air into the combustion system once it is in operation. Hence it is necessary only to top up the amount of oxygen in the inlet gas mixture in order to provide a suitable combustion atmosphere comprising oxygen and carbon dioxide.

Claims

Claims 1. A method of combusting fuel in a compression ignition engine wherein the atmosphere in the combustion chamber of the engine consists essentially of a mixture of carbon dioxide and oxygen, and wherein the oxygen constitutes at least 21% of said gas mixture.
2. A method as claimed in claim 1 wherein said atmosphere comprises between about 23 and 29% oxygen and 71 to 77% carbon dioxide.
3. A method as claimed in claim 2 wherein said atmosphere comprises about 25 to 27% oxygen and about 73 to 75% carbon dioxide.
4. A method as claimed in any one of claims I to 3 wherein at least a portion of the exhaust gas from said engine is recirculated back into the combustion atmosphere of said engine.
5. A method as claimed in claim 4 wherein carbon dioxide and oxygen in the exhaust gas are recirculated back into the combustion atmosphere.
6. A method as claimed in any preceding claim wherein at least a portion of the carbon dioxide in the exhaust gas is removed preferably by means of liquefaction.
7. A method as claimed in any preceding claim, wherein the degree of oxygen enrichment is controlled in dependence on an analysis of the engine operating conditions and the exhaust gas composition, and preferably on the level of carbon monoxide in the exhaust gas when it leaves the engine.
8. A method as claimed in claim 6 wherein the amount

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of oxygen added to the recirculated exhaust gas stream and the amount of carbon dioxide present in the gas mixture is dependent on an analysis of the exhaust gas after surplus carbon dioxide, from the combustion reactions, has been removed from the exhaust gas.
9. A method as claimed in any preceding claim wherein the fuel is a non-fossil fuel.
10. A method as claimed in claim 9 wherein the non- fossil fuel is pyrolysis oil, animal or vegetable oils or fats or landfill gas, or synthetic gas produced by the gasification of biomass.
11. A method as claimed in claim 10 wherein said non- fossil fuel is tallow or waste cooking oil.
12. A method as claimed in any one of claims 1 to 8, wherein the fuel is a fossil based mineral oil or hydrocarbon gas.
13. A method as claimed in any preceding claim wherein the engine is used to generate electricity by coupling to an electrical generator.
14. A method as claimed in any preceding claim wherein the hot exhaust gases from the engine are used to raise steam in a steam boiler.
15. A method as claimed in any preceding claim wherein at least a portion of the oxygen in the combustion atmosphere is supplied in purified form from an air separation unit.
16. A method as claimed in claim 15 wherein the nitrogen by-product from said air separation unit is used in the manufacture of pyrolysis oil in a pyrolysis

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oil production process.
17. A method as claimed in claim 15 or 16 wherein excess oxygen from the air separation unit is used in a partial oxidation gasification process.
18. A method as claimed in any preceding claim, where the exhaust gas is treated and cleaned, whilst being recirculated back to the engine, to remove water vapour, particulate matter, and, if necessary, sulphur dioxide and hydrogen chloride.
19. A method as claimed in any preceding claim, where combustible pollutants in the exhaust gas stream, such as carbon monoxide and organic compounds, are recirculated in the exhaust gas stream back into the combustion chamber of the engine to be combusted during subsequent combustion cycles.
20. A method of recovering carbon dioxide produced from the combustion of fuel wherein the combustion atmosphere consists essentially of a mixture of oxygen and carbon dioxide.
21. A method as claimed in claim 20 wherein the excess carbon dioxide produced during combustion is removed from the mixture of the products of combustion and recovered in a form suitable for storage.
22. A method as claimed in claim 21 wherein the excess carbon dioxide is removed by liquefaction.
23. A combustion system comprising a combustion unit, preferably a compression ignition engine, and means to provide a combustion atmosphere consisting essentially of oxygen and carbon dioxide.

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24. A combustion system as claimed in claim 23 further comprising means to recycle at least a portion of the exhaust gas back into the combustion atmosphere.
25. An electricity generation system comprising a combustion unit as defined in claim 24 coupled to an electrical power generating device.
26. A carbon dioxide recovery system for a combustion unit comprising means to recirculate exhaust gas from a combustion unit back into the combustion chamber of said combustion unit, means to liquefy surplus carbon dioxide from the exhaust gas stream during recirculation and means to enrich the recirculated exhaust gas with oxygen prior to entering the combustion chamber of the combustion unit.