GB2395478A

GB2395478A - Disposal of sewage by combustion in an engine

Info

Publication number: GB2395478A
Application number: GB0227315A
Authority: GB
Inventors: John Mcneil
Original assignee: Finch Ltd; Otwoempower Corp
Current assignee: Otwoempower Corp
Priority date: 2002-11-22
Filing date: 2002-11-22
Publication date: 2004-05-26
Anticipated expiration: 2022-11-22
Also published as: GB2395478B; GB0227315D0

Abstract

A method and apparatus for disposing of sewage is disclosed. An aqueous sewage liquor 1 is separated from raw sewage and blended with a non-aqueous liquid carrier fuel 2. The blended fuel is supplied to the combustion chamber(s) of a compression ignition engine 7. Air is also supplied to the combustion chamber(s) so that the blended fuel, containing sewage, may be disposed of by combustion. The air may be enriched with oxygen. Preferably, means 10,11 are also provided to control and abate prescribed substances in the exhaust gas.

Description

Disposal of Sewage by Combustion in an Engine The present invention

relates to a method of safely disposing of sewage, from either human or animal sources, by means of combustion in a compression ignition engine. The heat and power produced by the engine is preferably used to generate electricity.

Because of environmental and human health concerns, the collection, treatment and disposal of sewage is becoming increasingly regulated. Traditional methods of disposing of sewage sludge, i.e. by dumping at sea, spreading on agricultural land or burying in landfill sites, are either now unacceptable or are becoming tightly restricted.

Although sewage sludge can be incinerated, sewage contains a large amount of water and the sludge has to be dried before it can be burned effectively. Because of the need to dry the sewage before combustion, incineration plants for sewage sludge tend to be costly and relatively inefficient.

Sewage can be processed by means of anaerobic digestion to produce a combustible gas.

Alternatively, sewage can be subjected to gasification and pyrolysis techniques that also produce combustible gases, which are capable of being burned in either a gas engine or a gas turbine. However, the sewage sludge would again need to be dried before it could be processed efficiently by either gasification or pyrolysis.

Raw sewage is a complex mixture of organic materials, inorganic solids, volatile substances and water. Sewage can also contain contaminants, such as hazardous organic compounds, biological pathogens and heavy metals, which can pose risks to human health, and sewage sludge therefore requires careful handling and treatment before disposal. Although sewage sludge has a reasonable calorific value, its high moisture content makes it unsuitable for direct use as a fuel in conventional incineration systems. The need to dry sewage sludge before it can be used as solid fuel makes conventional incineration an energy intensive disposal option.

The present invention seeks to provide an effective and energy efficient method of disposing of sewage whereby the sewage is combusted in a wet state instead of a dry state.

From a first broad aspect the invention provides a method of preparing sewage so that the sewage is in the form of a liquid fuel, which is capable of being combusted in a compression ignition engine. The high temperatures and pressures generated inside the combustion

chambers of a compression ignition engine ensure that the sewage and any hazardous organic based substances present in the sewage would be completely incinerated.

In a preferred embodiment, an enriched oxygen combustion atmosphere would normally be used inside the combustion chambers of the engine. Combustion in an enriched oxygen atmosphere not only improves the efficiency of the combustion process, but also helps to ensure that any thermally stable hazardous organic materials or biological pathogens that may be present in the sewage will be completely destroyed.

To achieve a liquid with a viscosity mobile enough to be delivered to the combustion chambers of a compression ignition engine, an aqueous based sewage liquor is initially prepared from raw sewage at the sewage treatment works. The aqueous sewage liquor is processed from raw sewage in a manner whereby as much combustible organic material as possible is retained in the liquor. For example, the aqueous liquor may typically contain between 50 and 70 % water and between 30 and 50 % organic based material.

The composition of a typical aqueous sewage liquor is given in Table 1.

Table I

Composition of Typical Aqueous Sewage Liquor Composition Values of Typical Aqueous Sewage Liquor Approximate water content mass % 50 Approximate dry solids content mass % 50 Carbon content of dry solid mass % 34.1 Hydrogen content of dry solid mass % 5.4 Sulphur content of dry solid mass % 0.6 Nitrogen content of dry solid mass % 5.7 Zinc content of dry solid ppm 266 Copper content of dry solid ppm 309 Total other heavy metals of dry solid ppm 95 Although the aqueous sewage liquor has a calories value, the sewage liquor contains an appreciable amount of water and is not in a form that would be acceptable as engine fuel. The aqueous sewage liquor therefore needs to be mixed or blended with a conventional non-

aqueous liquid engine fuel that would act as a carrier to provide lubricity and other essential fuel properties and characteristics.

The carrier fuel could be a fossil based petroleum fuel, such as diesel gas oil or petroleum fuel oil. More preferably, the carrier fuel would be a non-fossil fuel, such as waste or virgin vegetable oil, waste or virgin animal fat or oil, or an alcohol, such as ethanol, or blends thereof of vegetable and animal oils or fats and alcohols.

Combining a non-fossil carrier fuel with sewage, which is also a nonfossil, sustainable energy resource, would have the environmental advantage that the resulting fuel blend would be derived completely from renewable energy sources.

The aqueous sewage liquor and the carrier fuel would preferably be mixed and blended together to provide a fully homogeneous suspension of the sewage liquor in the carrier fuel, and in a manner whereby micro droplets of the aqueous sewage liquor would be completely encapsulated by carrier fuel. If necessary, a suitable emulsifying agent could be added to the mixture to provide a homogeneous micro-emulsion of the sewage liquor in the carrier fuel.

The composition of the blended fuel would tend to vary but would typically be between 20 and 40 % aqueous sewage constituent and between 60 and 80 % carrier fuel. The composition of the blended fuel will be dependent on the constituents of the original sewage liquor and the physical properties required to provide effective fuel and combustion characteristics, such as, for example, the viscosity, lubricity and calorific value of the final fuel blend.

For illustrative purposes, the approximate composition and properties of a typical blended fuel consisting of a mixture of aqueous sewage liquor and vegetable oil are given in Table 2.

Table 2

Properties of a Typical Sewage and Vegetable Oil Blended Fuel Properties Typical Values Approximate sewage liquor content mass % 40 Approximate vegetable oil content mass % 60 Approximate water content mass % 20 Approximate dry solids content mass % 20 Calorific value MI/kg 32 Viscosity cSt at 40 C 44 Specific Gravity 0.96 Approximate total heavy metals content ppm 18 As sewage is a potentially contaminated waste material, due care has to be taken at all times when disposing of sewage. For example, when sewage is incinerated the combustion process would have to meet the requirements of local and national environmental legislation, as well as the EU Waste Incineration Directive. The WID specifies the maximum amount of prescribed harmful substances that are permitted to be released into the atmosphere from the combustion process.

Exhaust gas contaminants prescribed in the WID include carbon monoxide, nitrogen oxides, acid gases, heavy metals, particulates and polycyclic organic compounds.

Enriched oxygen combustion in an engine provides efficient high temperature incineration, which ensures that any organic based materials in the fuel are completely destroyed. Under enriched oxygen combustion conditions, carbon monoxide and particulate emissions from the engine will also be much lower than under normal naturally aspirated combustion conditions.

Some emission releases, such as acid gases and heavy metals, are actually dependent on the concentration of sulphur, chlorine and heavy metals in the original sewage itself. Blending the sewage liquor with a renewable carrier fuel such as vegetable oil, which contains very little sulphur, chlorine and heavy metals, provides a means of reducing the level of these particular contaminants in the final blended engine fuel.

The potential emissions of acid gases and heavy metals can be predicted from an analysis of the blended fuel, and the releases of these particular prescribed substances can be controlled, to some extent, by altering the relative composition of the final fuel blend. Abatement equipment, such as a catalytic de-NOX system and a filtration system to capture particulates and heavy metal oxides, can be included in the engine exhaust to further control the release of prescribed emissions.

Viewed from a further broad aspect, the invention provides a disposal system for sewage comprising a compression ignition engine, means for separating an aqueous sewage liquor from raw sewage, means for blending the aqueous sewage liquor with a non-aqueous carrier fuel to provide a blended liquid fuel suitable for combustion in said engine, means to supply the blended fuel to the combustion chambers of said engine, oxygen enrichment apparatus to supply an enriched oxygen atmosphere to the combustion chambers of said engine, and means to control prescribed substances in the exhaust gas coming from the engine.

The peak cylinder pressure inside a diesel engine can be over 200 bar and the temperature inside the combustion chamber of the engine can reach over 2000 C. Increasing the oxygen concentration in the combustion chamber of a diesel engine also encourages early fuel ignition and a more complete burn of the fuel. Prior research in the laboratory by the applicant has also shown that oxygen enrichment helps to combust diffcult- to-burn fuels, such as vegetable oils and animal fats, in a standard compression ignition engine.

In a preferred embodiment of the invention no adjustment is made to the normal mechanical timing of the engine to compensate for the early ignition of the fuel when using an enriched oxygen combustion atmosphere, so that the blended fuel is subjected to a longer and more complete burn inside the engine.

In this context, 'normal' refers to the conditions or engine settings that would customarily be used to run the conventional petroleum based fuel, i.e. diesel gas oil, which would normally be specified for this type of engine.

Operating under normal engine timing and running conditions ensures that the blended fuel and any combustible organic contaminants present in the fuel, such as biological pathogens or hazardous organic compounds, will be completely incinerated by the engine.

When using a fuel blend derived totally from non-fossil sources, the power produced by the engine can be used to generate 'green' electricity, because the carbon dioxide produced during the combustion of renewable fuel does not contribute directly to global warming. Heat from the engine cooling system and from the hot exhaust gas can also be used for localised heating purpose or raise steam to drive a turbine to produce more 'green' electricity.

From a further aspect therefore, there is provided a method of generating 'green' electricity by combusting a blend of aqueous sewage liquor and a renewable fuel oil, such as vegetable oil, animal oils or fats, alcohol or blends thereof, in a compression ignition engine, which incorporates an enriched oxygen atmosphere in the combustion chambers of the engine, and coupling said engine to an electrical power generator. Heat from the process could also be utilised for either localised heating or to produce more electricity, and the combustion process could therefore have a high degree of efficiency.

If the aqueous sewage liquor is blended with diesel gas oil, the resulting fuel could well be combusted under normal naturally aspirated engine operating conditions, although a slightly enriched oxygen combustion atmosphere, containing perhaps as little as 1% extra oxygen (i. e,22% oxygen, 78% nitrogen), would improve the efficiency of the combustion process.

Non-fossil fuels, such as vegetable oil, are, however, more difficult to burn and require an increased concentration of oxygen in the combustion atmosphere of the engine to encourage effective combustion. To efficiently combust, for example, a sewage and vegetable oil fuel blend, and also ensure the effective destruction of any hazardous organic contaminants present in the sewage, the level of oxygen enrichment required is likely to be between 2 and 6 % above normal (i.e. between 23 and 27 % oxygen), and probably more preferably between 4 and 5 % above normal (i.e. between 25 and 26 % oxygen).

At these low levels of oxygen enrichment, the oxygen rich air is safe to handle, would not cause oxidation damage to engine components and would be cost effective to produce.

The enriched oxygen air could be supplied by either a gas separation membrane or by diluting pure oxygen, produced by commercially available air separation methods, with normal air.

I he combustion process will be controlled in part by the built-in engine management systems, but also by monitoring the input of blended fuel and oxygen rich air to the engine and the temperature and composition of the exhaust gas coming from the engine, particularly the concentration of carbon monoxide in the exhaust gas.

Engine combustion trials were carried out in the laboratory using a homogenous blend of aqueous sewage liquor and vegetable oil, similar to that described in Table 2. A Lister-Petter twin cylinder, four stroke diesel engine, with a nominal capacity of one litre, and equipped with direct fuel injection was used for the trials.

The engine was run at its optimum speed of 2300 rpm as recommended by the engine manufacturer for continuous running with diesel oil. The engine was run in a special test rig where the mechanical load consisted of a high power direct current motor with a variable field

voltage. The engine could be operated at different power increments, from a minimum power output of about 5-kWe to the maximum power output based on an acceptable exhaust smoke, which in the case of diesel oil is about 11-kWe.

To confirm the normal engine operating parameters, the engine was run initially naturally aspirated (i.e. 21% oxygen, 79% nitrogen) using diesel oil as fuel. The emissions of carbon monoxide and nitrogen oxides, as well as the exhaust temperature and smoke density, were recorded at each power increment.

The engine was then run at different power outputs using the blended mixture of aqueous sewage liquor and vegetable oil, under naturally aspirated and then enriched oxygen combustion conditions. Different levels of oxygen enrichment were supplied to the engine, ranging from 2% above normal (i.e. 23% oxygen, 77% nitrogen) to 6% above normal (i.e. 27% oxygen, 73% nitrogen).

Supplying an enriched oxygen combustion atmosphere to the engine immediately improved the combustion performance of the engine, as evidenced by the reduced amount of carbon monoxide present in the exhaust gas emitted from the engine when burning the blended fuel.

Efficient combustion of the sewage and vegetable oil fuel blend was achieved at an oxygen concentration about 5% above normal (i.e. 26% oxygen, 74% nitrogen), the improvement in efficiency being confirmed by a relatively low emission of carbon monoxide and a clear exhaust smoke.

Table 3 compares the combustion of the blended aqueous sewage liquor and vegetable oil, under enriched oxygen conditions that were 5% above normal (i.e. 26% oxygen), with naturally aspirated diesel oil, at an engine power output of 10-kWe.

For ease of comparison, the values given in Table 3 are expressed relative to the results of the engine running naturally aspirated on diesel oil.

Table 3

Results of Engine Combustion Trials Properties Diesel gas oil Sewage Fuel Blend 21% oxygen 26% oxygen Power output kWe 10.0 10.0 Relative power output 1.0 1.0 Carbon monoxide emission relative 1.0 0.35 Nitrogen oxides emission unabated relative 1.0 2.50 Exhaust temperature C 510 490 The engine trials confirmed that it was feasible to combust a fuel blend consisting of aqueous sewage liquor and vegetable oil in a compression ignition engine incorporating an enriched oxygen combustion atmosphere.

The relatively low carbon monoxide emission, which was also accompanied by a clear exhaust smoke, indicated that the sewage liquor and vegetable oil fuel blend was burning efficiently and completely under 5% enriched oxygen conditions. The high efficiency of the combustion process also confirmed that any hazardous organic contaminants present in the blended fuel would have been completely destroyed.

Some preferred embodiments of the present invention will now be described by reference to the following illustrations: Figure 1 is a schematic illustration of a sewage disposal system embodying the invention, including fuel preparation, combustion in a diesel engine, electricity generation and abatement of exhaust gas pollutants.

Figure 2 is a schematic illustration of the cylinder head of a compression ignition engine.

The method of combustion is dependent on being able to utilise an appropriate raw sewage slurry at the sewage treatment works. For example, the slurry which is available just before the sewage enters the digesters at the sewage works is probably of a suitable composition and consistency to convert into a blended engine fuel.

Certain large sized solids, such paper, silicates and other incombustible materials, would have to be removed from the sewage slurry before the sewage could be used as a constituent in a compression ignition engine fuel. The sewage preparation process therefore concentrates initially on the removal of these particular materials

For example, large sized solids would be removed from the sewage slurry by firstly grinding the slurry to a fine paste-like consistency, and then passing the slurry paste through a series of screens, sieves, presses, filters and rollers to gradually remove large sized solid materials from the sewage slurry.

The slurry would be then be rehydrated and reconstituted back to a mobile liquid consistency, before finally removing any remaining solid particulate material from the slurry, firstly by settlement and then by centrifuging the slurry.

The aqueous sewage liquor from the centrifuge process would, by now, be free of large particulate material that could have blocked the fuel injectors in an engine, i.e. particulates larger than 200 microns, however, the liquor would still contain a considerable amount of combustible organic material, including small sized solid particles.

The aqueous liquor phase from the centrifuge process would be stored in a storage tank 1.

Sludge and solid residues from the slurry conditioning process would be treated and disposed of in the same way as normal sewage sludge. For example, the sludge residues could be digested in a sewage digester, and the residues from the digesting process could either be used as soil conditioner or be buried in landfill.

A non-fossil carrier fuel, such as waste or virgin vegetable oil, is stored separately in tank 2.

(As well as vegetable oil, other renewable fuels, such as animal oils or fats, or alcohol, or blends thereof, could be used as a potential carrier fuel, or alternatively a fossil petroleum oil such as diesel oil could be used as the carrier fuel).

A blended engine fuel would be produced, on a batch basis, by mixing the aqueous sewage liquor from tank 1 with vegetable oil from tank 2 in a mixing tank 3, in a typical approximate ratio of 1 part sewage liquor to between I and 2 parts vegetable oil. Laboratory tests have established that this approximate ratio of sewage liquor to vegetable oil would provide a reasonably consistent and mobile blend of fuel that would be suitable for combustion in a compression ignition engine.

The mixture would be stirred continuously in tank 3 by stirrer 4 to provide a homogeneous fuel blend where micro droplets of the aqueous sewage would be encapsulated by the vegetable oil. If necessary an emulsifying agent could be added to the mix in tank 3 in order to provide a fully micro-emulsified fuel blend. The blending process would usually be carried out at ambient temperature conditions, however, the liquid mixture could be gently warmed to facilitate blending if necessary.

g A sample of the blended fuel would be analysed to check that the fuel had acceptable fuel characteristics and viscosity, and if necessary the viscosity of the blended fuel could easily be adjusted by adding either more aqueous sewage liquor or more vegetable oil, as appropriate, to the mixture in tank 3.

When the blended batch of fuel has acceptable fuel properties, the blended fuel would be transferred to a work-in-progress tank 5, situated adjacent to the compression ignition engine, The fuel blend would be continuously stirred by stirrer 6 in tank 5 and gently warmed if necessary. Simultaneously, a further batch of blended fuel could be prepared in mixing tank 3.

Blended fuel is pumped from tank 5 to a fuel injector 18 located in the cylinder head 19 of a compression ignition engine 7.

Naturally aspirated atmospheric air and enriched oxygen air is supplied to a valve 9 that controls the amount of oxygen in the air supplied to the engine 7, by selectively admitting atmospheric air to the oxygen rich air. The oxygen rich air is supplied to valve 9 by an appropriate oxygen production unit, such as a gas separation membrane, a pressure swing or vacuum swing adsorption system, or a cryogenic oxygen production system. (The oxygen production unit is not illustrated in Figure 1).

Valve 9 is also able to adjust the concentration of oxygen in the engine air supply in response to sensor 10, which measures the amount of carbon monoxide in the exhaust gas coming from engine 7 and provides an indication of the efficiency of the combustion process.

Valve 9 is connected to the air intake manifold of engine 7 and the oxygen rich air is introduced to the combustion chamber 13 of engine cylinder 15 by the air inlet valve 14. At this time the exhaust valve 17 from the combustion chamber 13 is closed.

A piston 16 moving up cylinder 15 compresses the oxygen rich air in the cylinder, and at the appropriate time a small amount of the blended fuel is sprayed into the combustion chamber 13 by the fuel injector 18.

On further compression by piston 16 the fuel ignites and the rapid increase in temperature and pressure in the combustion chamber forces piston 16 back down cylinder 15. This movement is transmitted to a power take off shaft that drives a generator 8 to produce electricity.

When the piston 16 returns back up the cylinder on its exhaust stroke the hot exhaust gas is expelled from the combustion chamber 13 through the exhaust valve 17.

The engine operation will be controlled in part by sensor 10, which monitors the concentration of carbon monoxide and nitrogen oxides in the exhaust gas, and the exhaust gas temperature, and in part by the engine management system.

to I: he exhaust gas passes through an ammonia catalytic reduction unit 11 to reduce the concentration of nitrogen oxides in the exhaust gas to below the limit specified by the Waste Incineration Directive.

Other abatement equipment, such as, for example, a particulate filtration system, could also be included in the engine exhaust, to ensure that the exhaust gas released to the atmosphere through flue stack 12 complied with the specified WID emission limits.

Although not illustrated, the heat from the engine cooling system and from the hot exhaust gas could also be utilised for local heating or to produce steam to power a steam turbine.

From the above, it will be seen that the present invention provides a method of safely and cost effectively disposing of some of the sewage that is collected at sewage treatment works, by combusting the sewage in an engine. As well as human sewage, the principles of the invention could also be applied to the disposal of animal sewage.

A proportion of sewage, in the form of an aqueous sewage liquor, is blended with a suitable non-aqueous liquid fuel carrier, which could either be a fossil fuel, such as diesel gas oil or petroleum fuel oil, or more preferably a non-fossil fuel, such as vegetable oil, animal oils or fats, alcohol or a suitable blend of such non-fossil fuels. The sewage liquor and the liquid fuel carrier are blended together to produce a homogeneous fuel, where micro droplets of sewage liquor are encapsulated by the carrier fuel. The mixture of the blended fuel is adjusted until the fuel has a suitable consistency and viscosity to enable the fuel to be supplied to the combustion chambers of a compression ignition engine.

The high temperatures and pressures generated inside the combustion chambers of an engine during the combustion process ensures that the sewage and any organic contaminants in the sewage will be completely destroyed. An enriched oxygen combustion atmosphere can be used in the engine to further improve the effectiveness and efficiency of the combustion process. The electricity and heat produced by the engine could be utilised at the sewage works for a variety of on-site processes. When a non-fossil liquid carrier fuel is used as the blending medium, any surplus electricity from process could be sold off-site as 'green' electricity because the engine fuel would have been totally derived from renewable energy sources.

Although the method of the invention, as described, has been based on using a high-speed compression ignition engine, it is a well-known fact that large, low-speed, wide bore diesel engines operate in a similar manner. The method of the invention would therefore be applicable to both small highffpeed and large low-speed compression ignition engines.

Claims

1) Claims

1. A method of disposing of sewage wherein an aqueous liquor containing sewage is prepared from raw sewage, the aqueous sewage liquor is blended with a non-aqueous liquid carrier fuel, and the blended fuel is supplied to the combustion chamber(s) of a compression ignition engine, and wherein a supply of air that can also be enriched with oxygen, if necessary, is supplied to the combustion chamber(s) of said engine, and wherein said blended fuel containing sewage is then disposed of by means of combustion in said engine.

2. A method as claimed in claim 1, wherein raw sewage slurry is treated to remove large sized solids by mechanical processing, the processed sewage slurry is then rehydrated and reconstituted back to a mobile liquid consistency, and solid particles larger than 200 microns that remain in the processed sewage slurry are removed, preferably by settlement and by centrifuging, so as to produce a mobile aqueous sewage liquor.

3. A method as claimed in claim 2, wherein large sized solids are removed from the raw sewage slurry by mechanical means such as rollers, presses, screens, sieves and filters.

4. A method as claimed in any preceding 1, wherein a homogeneous blend of the aqueous sewage liquor and a non-aqueous liquid fuel is prepared by stirring the sewage liquor and the non-aqueous fuel together in a mixing tank gently warming the mixture, if necessary, to assist the blending process, and adding a suitable emulsifying agent, if required.

5. A method as claimed in any preceding claim, wherein the non-aqueous liquid carrier fuel is a fossil petroleum based fuel, such as diesel gas oil or petroleum fuel oil.

6. A method as claimed in any of claims 1 to 4, wherein the non-aqueous liquid carrier fuel is a non-fossil based fuel, such as waste or virgin vegetable oil, waste or virgin animal fat or oil, an alcohol such as ethanol, or blends of animal and vegetable oils or fats and/or alcohols.

7. A method as claimed in any preceding claim, wherein the blend of aqueous sewage liquor and the non-aqueous carrier fuel is analysed and its composition adjusted if necessary, so that the blended fuel has suitable physical properties and characteristics to enable the blended fuel to be supplied to the combustion chambers of a compression ignition engine.

8. A method as claimed in any preceding claim, wherein means is provided to supply either normal naturally aspirated air or air enriched with oxygen to the combustion chamber(s) of a compression ignition engine.

9. A method as claimed in claim 8, wherein the means to supply enriched oxygen air to the combustion chambers of the compression ignition engine is a gas separation membrane, a pressure swing or vacuum swing adsorption unit, or a cryogenic air separation system.

10. A method as claimed in claim 8 or 9, wherein the combustion air can either be naturally aspirated or can be enriched with oxygen, and wherein the oxygen concentration in the enriched oxygen air supply may be as little as 1% above normal (i.e. 22% oxygen), or more preferably between 2% and 6% above normal (i.e. between 23% and 27% oxygen), and even more preferably between 4% and 5% above normal (i.e. between 25% and 26% oxygen).

11. A method as claimed in claim 8, 9 or 10, wherein the level of oxygen enrichment supplied to the engine is controlled in dependence of an analysis of the exhaust gas coming from the engine and in particular on the level of carbon monoxide in the exhaust gas.

12. A method as claimed in any preceding claim, wherein the compression ignition engine is a high-speed, narrow bore engine.

13. A method as claimed in any of claims 1 to 11, wherein the compression ignition engine is a slow-speed, wide bore engine.

14. A method as claimed in any preceding claim, wherein the compression ignition engine is operated at a constant optimum speed that gives maximum thermal efficiency and at a constant optimum power output that gives efficient fuel combustion.

15. A method as claimed in any preceding claim, wherein the engine is used to generate electricity by coupling the engine to an electrical generator.

16. A method as claimed in any preceding claim, wherein prescribed pollutants in the exhaust gas from the engine are abated by means of a catalytic de-NOx unit and a particulate filtration system, and any other abatement means that may be necessary.

17. A method as claimed in any preceding claim, wherein the heat from the engine coolant system and from the hot exhaust gas is used for either localised heating purposes or to produce steam to drive a steam turbine to generate electricity, or both.

18. A disposal system for sewage comprising a compression ignition engine, means for separating an aqueous sewage liquor from raw sewage, means for blending the aqueous sewage liquor with a non-aqueous carrier fuel to provide a blended liquid fuel suitable for combustion in said engine, means to supply the blended fuel to the combustion chamber(s) of said engine, means to supply either normal air or enriched oxygen air to the combustion chamber(s) of said engine, and, preferably, means to control and abate prescribed substances in the exhaust gas coming from the engine.

19. A combustion system comprising a compression ignition engine, oxygen enrichment apparatus to be able to supply an enriched oxygen atmosphere to the combustion chambers of said engine, and a supply of a suitably processed blend of an aqueous sewage liquor and a non-aqueous liquid carrier fuel for use as the fuel for said engine.

20. An electrical power generating system comprising a generator coupled to a compression ignition engine, said engine combusting a suitably processed blend of an aqueous sewage liquor and a non-aqueous liquid carrier fuel, and said combustion process being carried out in a combustion atmosphere that can, if required, be suitably enriched with an appropriate amount of oxygen.

1q

21. A combustion system comprising a compression ignition engine and a supply of a suitably processed blend of an aqueous sewage liquor and a non-aqueous liquid carrier fuel for use as the fuel for said engine.