GB2339842A - Burning oils and fats in an oxygen-enriched atmosphere in an engine - Google Patents

Description

2339842 A Method of Burning Oils and Fats The present invention relates to

a method of burning oils and fats, particularly contaminated products, in engines and in particular, but not exclusively, in internal combustion engines such as compression ignition engines.

A compression ignition engine works by injecting fuel, under high pressure, into air which has been compressed by a piston travelling in a cylinder. The fuel and air mixture is further compressed until the fuel ignites, which leads to a rapid increase in temperature and pressure inside the combustion chamber.

Peak cylinder pressures can be over 140 bar and combustion temperatures can reach 25000C. Fuels, such as diesel oil, have characteristics which have been specifically designed to enable the fuel to burn efficiently inside the combustion chamber of the compression ignition engine and the ignition quality of the fuel has to be carefully controlled by the fuel manufacturers. A spark ignition engine operates in a similar manner except that a spark is used to initially ignite the fuel.

Potential alternative fuels, such as animal and vegetable based oils and fats, do not usually have the correct properties to burn efficiently in conventional internal combustion engines. These particular materials tend to quickly form carbonaceous deposits in the combustion chamber and around the valves and the fuel injectors. This has a detrimental effect on the engine operation, and its efficiency, as well as leading to an increased level of pollutants in the exhaust gas stream because the fuel is not burning completely and cleanly.

The effectiveness of the combustion process is very dependent on how quickly and how readily fuel and oxygen molecules mix and react together in the combustion chamber. To provide sufficient oxygen to combust the fuel, compression ignition engines have been designed to operate with an excess air to fuel ratio. In addition, modern engines have sophisticated fuel injection systems to provide a homogeneous mix of fuel and air in the combustion chamber so that the fuel is more likely to burn efficiently. Improvements to engine design and efficiency have, however, concentrated on the continuing use of conventional fuels, such as diesel oil, in the engine. Alternative fuels, such as animal and vegetable based oils and fats, have a more limited carbon chain structure than the conventional fuel blends which are manufactured from mineral oils. Their properties can also be very different, for example, the viscosity, cetane number, flash point and specific gravity of a natural oil can be completely different from a petrochemical based oil product which has been designed to run in a compression ignition engine. Most animal and vegetable based oils and fats are therefore generally not suitable for use as fuel in conventional compression or spark ignition engines.

The present invention seeks to provide an improved method of burning animal or vegetable based oils and fats or indeed unrefined, partially refined and waste oils in an engine.

In accordance with the invention there is provided a method of burning animal or vegetable based oils and fats, unrefined, partially refined and waste oils in an engine, wherein an enriched oxygen atmosphere is provided in the combustion chamber of the engine.

Increasing the concentration of oxygen in the air entering the cylinders of a compression engine has been studied before. However, such research has been limited in scope, and has tended to concentrate on the use of oxygen enrichment, in conjunction with conventional fuel oils, as a means of reducing the environmental pollution 3 associated with compression ignition engines.

The Applicant has found, totally unexpectedly, that oxygen enrichment can be used to enable alternative non7 fossil fuels, such as animal and vegetable based oils and fats, to be burnt in a conventional internal combustion engine. It has been found that a compression ignition engine is particularly suitable, since providing the material can be supplied to the fuel supply system of the engine in a suitable liquid or fluid state, there is generally no need to modify the combustion properties of the raw material. Hereinafter, the terms "liquid" and "fluid" are intended to encompass any material or mixture of materials which flows and can pass through the fuel supply system of an engine, including liquids and fluids containing solid particles or liquid droplets dispersed therein.

oxygen enrichment ensures that the material is burned as completely as possible without leaving substantial carbonaceous deposits in the engine, and that the emission of potentially hazardous pollutants, such as carbon monoxide, volatile organic compounds and particulate matter, is minimal.

Also the higher partial gas pressures produced inside the engine can be utilised to help to ignite low quality fuels.

In addition to virgin animal and vegetable based oils and fats, old, waste, oxidised or rancid oils and fats could also be disposed of in this manner, as could waste mineral oils such as used lubricating oil.

Furthermore, blends of different oils and fats may be combusted in enriched oxygen in accordance with the invention, as indeed can fluid mixtures and emulsions of solids and oils. Where it is of particular interest to combust materials comprising solids, semi-solids or viscous liquids, it may be convenient to suspend the solid particles or liquid droplets in a fluid so that a dispersion is formed. In order to do this any solids are crushed, ground and milled to particle sizes which are small enough to pass through the fuel supply system of the engine.

The degree of oxygen enrichment required to burn the various types of fuel discussed above will vary from fuel to fuel and from engine to engine. Generally, however, oxygen levels of up to 20% above normal (41% oxygen, 59% nitrogen) are contemplated. Although a very small increase in oxygen concentration (e.g. above 0.5% above normal oxygen levels) of the inlet air is sufficient to allow many previously incombustible fuels to burn, preferably oxygen levels of 1 to 5% above normal, more preferably 1 to 3% above normal and most preferably 2 to 3% (23% oxygen, 77% nitrogen; 24% oxygen, 76% nitrogen) above normal are contemplated.

Preferably the oxygen enrichment is by means of apparatus comprising hollow fibre gas separation membranes, as are known and commercially available.

Membrane type separators can easily produce the preferred oxygen levels discussed above quite economically. However, other methods of oxygen concentration can be used including compressed or liquid oxygen units, cryogenic air separation or pressure swing adsorption systems.

The combustion process of the invention may be controlled and thereafter stabilised by monitoring the composition of the exhaust gases and adjusting the degree of oxygen enrichment in the supply to optimise the combustion process.

It has also been realised that because of the high temperatures, up to 25000C, and pressures, up to 140 bar, generated inside the combustion chamber, contaminated oils and fats can be burned to destruction by a method of the invention. The method of the invention can effectively and advantageously deal with a variety of contaminants which are susceptible to destruction by high temperatures, including, for example, prion proteins, bacteria, viruses, fungi or organic chemicals such as herbicides and pesticides. These contaminated products are difficult to dispose of at present. The compression chamber of the engine, in an enriched oxygen atmosphere, acts as a high temperature, high pressure incinerator, which will break down, oxidise and burn the carbon and hydrogen present in both the fuel and the contaminants.

An additional advantage of the system is that the contaminated material would be injected into the engine in tiny, discrete doses, and therefore only a small, relatively narrow inlet pipe will be required to feed the liquid material into the engine. In the event of machine breakdown or maintenance, only a small amount of contaminated material would then have to be isolated from the fuel inlet pipe, which is a significant advantage in safety terms.

Although all contaminants should have been destroyed in the combustion chamber, it is preferable that after leaving the engine, the exhaust gases are held for a period of time at an elevated temperature to provide further sterilisation of the exhaust gas stream. Preferably the temperature is up to 8000C, most preferably between 500 and 8000C. This may be achieved by suitable thermal insulation of the exhaust.

The present invention is particularly useful in that it enables tallow to be burned efficiently as a fuel in the combustion chamber of a conventional diesel engine. Tallow is a clarified animal fat and being able to effectively combust tallow has a number of potential benefits. Tallow is a by-product of the cattle rendering process, and as a result of the BSE cattle culling program the UK stock of tallow has grown significantly. To be able to claim compensation from the European Community, it is a stipulation that this stock of tallow has to be incinerated. However, no effective way of incinerating this stock has yet been found. Although it is unlikely that the cull tallow is contaminated by the prion protein considered to be responsible for spreading BSE infection, it is obviously desirable that the tallow is burned as completely and efficiently as possible, with the minimum of emissions. The present invention allows for such combustion. From a further broad aspect, therefore, there is provided a method of disposing of tallow comprising burning said tallow in an oxygen enriched atmosphere.

It will be appreciated that the present invention not only allows for the efficient combustion of non conventional fuels, but also for such combustion to be used in electrical power generation. In particular, one or more engines operating in accordance with the invention may be suitably coupled to electrical power generation means.

From a further broad aspect, therefore, there is provided an electrical power generating system comprising a generator coupled to at least one engine said engine burning animal or vegetable based oils and fats, unrefined, partially refined and waste oils in an oxygen enriched atmosphere.

An advantage of using animal and vegetable based oils and fats, whether pure or contaminated, in power generation is that they are a renewable, sustainable, non-fossil source of energy. They usually have relatively high calorific values, for example, tallow and rape seed oil have calorific values comparable with conventional fossil fuels. Combusting these natural, renewable, high calorific materials in an enriched oxygen atmosphere in a compression ignition engine provides high thermal efficiency, which makes it an ideal method of producing the heat and power needed to generate electricity.

Natural oils and fats also have a limited carbon chain and a "clean" content profile, for example both tallow and rape seed oil contain very low levels of sulphur, chlorine and heavy metals. These products therefore burn cleanly without producing significant quantities of some of the pollutants associated with fossil fuels, such as sulphur dioxide and chlorine acid gases.

In addition, when these natural fuel products are burned, the carbon dioxide produced is not a net contributor towards the greenhouse gas effect. Firstly, the carbon dioxide released during the combustion of animal and vegetable based products is merely replacing the carbon dioxide originally sequestered by plants during the natural growing cycle. Secondly, the use of these products, instead of fossil fuels, directly reduces the amount of carbon dioxide which would have been emitted from a conventional power station producing an equivalent amount of electricity from a fossil fuel.

In a typical system in accordance with the invention, one or more diesel engines are connected to an oxygen enriched air supply system and to a supply of animal or vegetable based oils and fats, such as tallow or rape seed oil, either in a pure or a contaminated state. The output shaft of the engines would be coupled mechanically to one or more electrical power generating devices. Because of the improved thermal efficiency and increased energy density, which can be attained with oxygen enrichment of the combustion atmosphere, the exhaust gas stream is hotter than is usual in a compression ignition engine. Preferably, therefore, this hot exhaust gas is used produce steam to power a further electrical generator system. Furthermore, any excess steam is preferably used for local heating.

Before entering a flue, undesirable gaseous products of high temperature combustion, such as oxides of nitrogen, may be removed from the exhaust gas stream by means of either catalytic or non-catalytic reduction. In this way, the system can be used to cleanly generate 8 electricity whilst at the same time safely destroying any contaminated oil or fat products.

It will also be apparent, to those skilled in the art, that in addition to the embodiments described above, the oxygen enrichment technique of the invention could well be applied to other means of combustion, for example, furnaces, incinerators and kilns, so that these combustion methods may also be capable of using alternative renewable energy sources of fuel, such as animal and vegetable based oils and fats, or blends, admixtures and emulsions of said oils and fats, or mineral oils not usually suitable for such means of combustion. From a yet further broad aspect, therefore, the invention provides a method of incinerating animal or vegetable based oils and fats, unrefined, partially refined and waste oils in an oxygen enriched atmosphere.

Some preferred embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a system embodying the invention; and Figure 2 is a schematic illustration of the head of a diesel engine.

With reference to Figures 1 and 2, pre-filtered tallow is contained in liquid form in a heated, sealed tank 1. The temperature of the tank 1 is about 30-400C which is sufficient to keep the tallow liquid. A pump 2 pumps the tallow from the tank 1 via a control valve 3 to the fuel injection valve 4(Fig 2) of a diesel engine 5.

Air from the atmosphere is pumped into a gas separation module 6 by pump 20 and the filtered air is drawn out of the module 6 by vacuum pump 7. The module 6 is of a known construction of the type which contains a plurality of hollow fibre membranes 8 which can selectively separate oxygen and nitrogen to provide large volumes of oxygen enriched air. The oxygen rich air 9 leaves the module 6 via a control valve 10 which controls the concentration of oxygen in the air supplied to the engine 5 by selectively admitting atmospheric air to the oxygen enriched air 9. The control valve 10 operates to provide optimum engine operating conditions, which are sensed by sensors 11 which analyse the exhaust gas stream 12. Nitrogen rich residual air 13 leaving the separation module 6 is vented to the outside atmosphere.

The outlet of the control valve 10 is connected to the air intake manifold of the diesel engine 5 and the oxygen enriched air is introduced to the combustion chamber 14 of the cylinder 15 of the engine 5 via the air inlet valve 16.

is In operation, the oxygen enriched air supplied to the combustion chamber 14 through the air inlet valve 16 is compressed by the movement of the piston 17 in the cylinder 15. A small, discrete amount of tallow is then sprayed into the combustion chamber 15 by the fuel injection valve 4, where it meets a compressed and enriched oxygen atmosphere, so that on ignition, the tallow burns as completely as possible, under high temperature and pressure conditions, and with minimal emissions. The full ignition of the fuel causes the piston 17 to move back down the cylinder, and this movement can be transmitted in conventional means to a power take off shaft for use as appropriate, for example by connection to an electricity generator. Furthermore the combustion conditions should effectively destroy all contaminants in the tallow.

The exhaust gas 12 passes along an insulated tube 18, to ensure that the gas is held at an elevated temperature of up to 8000C, which further assists in destroying any contaminants left in the exhaust gas stream. The exhaust then enters a flue stack 19. if necessary, the exhaust gas 12 can be filtered to remove particulates and treated to reduce gaseous pollutants such as oxides of nitrogen.

To confirm that tallow could be effectively burned in a compression ignition engine by using an oxygen enriched atmosphere, practical trials were carried out in the laboratory using a Lister-Petter two cylinder four stroke diesel, with direct fuel injection and a nominal capacity of one litre.

The engine was run at its point of maximum thermal efficiency, that is when the maximum Brake Mean Effective Pressure was achieved throughout the engine revolutions range. The best operating BMEP was found to occur at a speed of about 2300 rpm. The engine was operated in a special test rig, where the mechanical load consisted of a high power direct current motor with a variable field voltage. The engine was operated at 1 kW load increments, from the minimum stable load to the maximum desirable load, based on exhaust temperature and carbon monoxide emission level. Pre-filtered, heated tallow was used to fuel the engine. The engine was first run at an enriched oxygen level which was 20% above normal (41% oxygen, 59% nitrogen) and then under decreasing oxygen concentrations, reduced in steps of 2%, until normally aspirated conditions were reached (21% oxygen, 79% nitrogen). The fuel consumption, exhaust emissions and exhaust temperature were recorded at each operating step. Under optimum oxygen enrichment conditions ie. oxygen concentration between 2% and 5% above normal (23% oxygen, 77% nitrogen; 26% oxygen, 74% nitrogen), more preferably between 2% and 3% (23% oxygen, 77% nitrogen; 24% oxygen, 76% nitrogen) the results showed that it was possible to burn tallow efficiently and cleanly with minimal exhaust emissions when compared to normally aspirated conditions.

In particular it was found that using the method of the invention carbon monoxide emissions were significantly reduced, even at the more extreme engine power outputs. Carbon monoxide in the exhaust gas is an indication of incomplete combustion, and the results showed that in enriched oxygen the tallow was burning efficiently, over a range of power outputs, as most of the carbon monoxide was being converted to carbon dioxide.

It was also found that emissions of hydrocarbons and particulate matter fell dramatically when oxygen enriched air was used because of the more complete fuel combustion. This was accompanied by a much 'cleaner, smoke profile compared to running the engine naturally aspirated.

Furthermore, samples collected on filters placed in the flue gas exhaust stream were analysed for amino acid content to establish whether or not proteins were present after combustion. The results showed that after burning in oxygen enriched air, the level of protein in the exhaust gas was undetectable.

Also, with oxygen enrichment there was a noticeable decrease in both engine noise and the characteristic knock associated with diesel engines. This confirmed that the fuel was being burned efficiently, and with a more favourable pressure and temperature profile, than when the same engine was run normally-aspirated with standard fuel.

Exhaust gas temperature measurements indicated that with added oxygen, the engine was running with improved thermal efficiency. This enables the engine to be run at increased energy density, which would be accompanied by higher exhaust temperatures, without imposing additional mechanical strain on the engine.

Similar experimental trials were carried out on a number of alternative oils and fats which are not normally suitable for use in compression ignition engines. By adjusting the oxygen concentration level to provide optimum combustion conditions, other animal, vegetable and mineral based oils and fats, in liquid or fluid form, including blends and admixtures, were all effectively burned as well as tallow. These alternative fuel materials included different vegetable oils, for example rape and soya, and various petrochemical oils, such as crude oil, heavy fuel oil, waste lubricating oil and mixtures of mineral oil products. In all cases, with the correct level of oxygen concentration, it was possible to burn the oils tested efficiently and cleanly.

It is believed that oxygen enrichment could also be used to burn these alternative oils and fats in a spark ignition engine, as the method of combustion is similar to that of a compression ignition engine, or even in a gas turbine, allowing for the thermal constraints within such an engine.

From the above,-it will be seen that the present invention allows for the combustion of a wide range of unconventional fuel products, including in particular animal and vegetable based oils and fats and more particularly tallow which may potentially be contaminated by prion proteins. Furthermore it allows such combustion to be harnessed efficiently for the generation of electricity.

Claims (22)

Claims

1. A method of burning animal or vegetable based oils or fats, unrefined, partially refined or waste oils in an engine, wherein an enriched oxygen atmosphere is provided in the combustion chamber of the engine.

2. A method as claimed in claim 1 wherein said engine is an internal combustion engine.

3. A method as claimed in claim 2 wherein the internal combustion engine is of the compression ignition type.

4. A method as claimed in claim 2 wherein the internal combustion engine is of the spark ignition type.

5. A method as claimed in claim 1 wherein said engine is a gas turbine.

6. A method as claimed in any preceding claim wherein the material to be combusted is a liquid or a fluid.

7. A method as claimed in any of claims 1 to 5 wherein the material to be combusted is a blend, mixture or emulsion of animal and/or vegetable based oils and fats, including admixtures containing finely ground or milled particulate matter.

8. A method as claimed in any preceding claim wherein the atmosphere is enriched up to 20% above normal (41% oxygen, 59% nitrogen).

9. A method as claimed in any preceding claim wherein the level of oxygen enrichment is controlled in dependence on an analysis of the exhaust gases.

10. A method as claimed in any preceding claim wherein the material to be combusted is a contaminated product.

11. A method as claimed in any preceding claim wherein the material to be combusted is tallow.

12. A method as claimed in any preceding claim wherein after leaving the engine, the exhaust gases are held for a period of time at an elevated temperature.

13. A method as claimed in claim 12 wherein said temperature is up to 8000C.

14. A method of generating electricity comprising burning a material in an engine by a method as claimed in any preceding claim and coupling the output shaft of the engine to an electrical generator.

15. A method as claimed in claim 14 wherein exhaust gas is used to produce steam to power a further electrical generator system.

16. An electrical power generating system comprising a generator coupled to at least one engine said engine burning animal or vegetable based oils or fats, unrefined, partially refined or waste oils in an oxygen enriched atmosphere.

17. A combustion system comprising an engine, oxygen enrichment apparatus for supplying an enriched oxygen atmosphere to the combustion chamber of the engine, and a supply of animal or vegetable based oils or fats, unrefined, partially refined and waste oils for supplying to the combustion chamber.

18. A system as claimed in claim 16 or 17 wherein said engine is a compression ignition engine.

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19. A system as claimed in claim 16, 17 or 18 comprising means for analysing the exhaust gas produced and means for controlling the concentration of oxygen in the supply in dependence on the analysis. 5

20. A system as claimed in any of claims 16 to 19 further comprising means for thermally insulating the exhaust of the engine.

21. A method of disposing of tallow comprising burning said tallow in an oxygen enriched atmosphere.

22. A method of incinerating animal or vegetable based oils or fats, unrefined, partially refined or waste oils in an oxygen enriched atmosphere.