Internal combustion engines and gas turbines suffer from deterioration in performance and corrosion of their internal parts if the gaseous fuel supplied to them contains impurities. This occurs, in particular, where the fuel is, or contains as a principal component, methane. Such fuels are produced, for example, by the decomposition of waste matter on landfill sites, the decomposition of sewage in water treatment plants, the decomposition of manure or from one of several biogas technologies being developed. When the use of such fuels gives rise to a build up of deposits or matrices of carbon, silica and other contaminants within the engine the efficiency of the engine is compromised and the output of the engine deteriorates rapidly until the engine has to be dismantled and the contaminated parts either cleaned or replaced.

The build up of significant deposits on the combustion surfaces of an engine, besides causing corrosion, reduces the power output of an engine. For instance, with an engine generating electricity the generating power can be reduced by up to 10% typically from 1 Mw to 900Kw. Cleaning the incoming gas can eliminate these losses and allow the engine to run at full power for an extended period of time.

Whilst the build up of contaminants is often quite slow, on certain sites where the levels of silica are high, severe levels of deposits can build up rapidly. One particular example is where diesel engines are used for generating electricity on landfill and sewage sites, fuelled by the methane gas generated by the decomposition of the landfill waste or sewage waste.

The engine operating efficiency deteriorates rapidly accompanied by erosion of the engine parts by chemical contaminants derived from the waste.

Typically in such circumstances, chlorides, fluorides, silicates and carbons tend to form matrices with atmospheric contaminants and corrode engine parts over short periods of time. In addition the presence of hydrogen sulphide leads to the formation of unwanted acid derivatives that can lead to severe corrosion within the engine, especially where engine parts are in contact with lubricating oil.

It has been appreciated for some time that if the contaminants could be eliminated from the gas fuel feed, the formation of such matrices and acidic deposits can be avoided and thus corrosion of engine parts from these sources eliminated. Up until now, this has been achieved passing the gas through either an activated carbon filter, e. g. as described in US-A-5451249 or through a compounded liquid, usually water-based, e. g. as described in US-A-5059405. Known carbon filters are reasonably efficient but their expense is such that payback times for the installation and the carbon cartridges'are long, making the use of such systems only viable where severe problems exist. The use of compounded liquid systems is quite common, but the amount of contamination removed by such systems is very limited and they are considered to be largely inefficient.

We have now found that effective filtration of impurity-containing feed gases used to fuel combustion engines can be achieved simply and in a far more cost-effective fashion by the use of an activated earth filter between the gas supply and the engine itself.

Activated earth is a standard well-know commodity. It generally consists of a clay-type mineral, for example bentonite, which has been treated, e. g. by exposure to one or more commercially available acids, then dried and processed into small particles with a large total surface area per unit volume

of finished product. The treatment creates electrostatic points within the mineral structure. It is believed that in use in gas filtration, these electrostatic points have the ability to attract various and different elements from a feed gas stream. Because these electrostatic points have different properties, a wide range of contaminants can be eliminated from the gas and held captive by the activated earth.

Clearly for gas filtration, the gas needs to be passed through some sort of fitter structure during which it comes into contact with the activated earth.

The type of structure may vary widely, but simple arrangements tend to be inexpensive and easy to maintain. For example, a filter structure for use in the present invention may consist simply of a container filled with activated earth through which the incoming contaminated gas fuel stream is passed, i. e. the container is positioned between the engine and the incoming gas supply. As the gas passes through the activated earth. the contaminants within the gas are largely or wholly removed, leaving a purer gas for fuelling the. engine. Such a filter structure also removes liquid contaminants and water from the contaminated gas feed and so leaves a more combustible material to fuel the engine Most conveniently, the activated earth filter is presented in the form of a replaceable cartridge.

The activated earth in such a filter structure is preferably in the form of compounded granules rather than the powder material form in which activated earth is generally supplied, as this tends to give a filter through which the fuel gas can only pass with an unacceptable pressure drop, at least for a reasonable size filter. The fuel gas flow can be substantial for waste gas combustion applications, e. g. 700 m3 per hour.

The invention preferably uses various activated earths derived from a range of mineral deposits. After mining, the appropriate mineral, for example a clay such as bentonite, is treated by acidification with a strong acid, e. g.

sulphuric, which modifies the material to allow certain elements to be held captive within the structure of the material. The activated earths used in this invention preferably are types which have particular affinity to carbon or silicon-based contaminants in the feed fuel gas, particularly siloxane. These impurities are captured as the gas passes through the activated earth filter and held captive by the activated earth, so they do not reach, and thus harm, the engine. The cleaned gas then is fed into the engine as a purer, more combustible material that does not contaminate the engine with corrosive or power-reducing deposits.

After a period, the length of which depends on the levels of contaminants, usually one week to a month, the filter, e. g. a cartridge containing the activated earth, will become saturated and require changing for a new one.

This may be done, for example, simply by unbolting an old cartridge from the gas fuel line and inserting the new cartridge. The cartridge is usually fitted into a bypass line or two filters are arranged in parallel so that the gas can be redirected during cartridge replacement without the engine being turned off.

Preferably the cartridge or like container construction is one which enables the activated earth granules to be removed and disposed of so that the container can then be refilled with fresh uncontaminated activated earth granules. Alternatively, the cartridge may be regenerated if the nature of the granules and contaminants is appropriate. For example, if the contaminants are siloxane, the granules can be revived once saturated simply by heating them to boil off the siloxane, usually to around 200°C or slightly higher.

Such heating can, if desired, take place with the filter in situ, using heat from the gas-fired engine itself, or its exhaust, or by incorporating a microwave or RF heating system into the container.

Suitable activated earth granules are available widely in commerce. For

example the products made by Rockwell Industries of Widnes in Cheshire under the names Bleached Earth and Bentonite may be successfully used in practising the present invention.

The following example will serve to illustrate the invention: EXAMPLE A landfill site was identified, on which was located a 1 MW capacity generator powered by a Jenbacher 320 engine, the engine running on methane produced by the decomposing household waste at the site.

Analysis of the infeed gas showed it to contain around 10 mg/m3 of siloxane.

Inspection of combustion surfaces internally of the engine showed substantial deposits of siliceous material, mostly silicon dioxide.

Measurement of the power output revealed that despite a rated power output of 1 MW, the actual operating output was only 860 KW.

A filter was then inserted into the feed line to the engine, following a stripdown and cleaning of its combustion surfaces. The filter consisted of a cylindrical housing of diameter 800 mm and length 400 mm, which was filled with around 200 Kg of Fulcat 435 or Fulcat 230 (Ex Rockwell Industries). Analysis of the siloxane level in the output stream of fuel gas which constituted the infeed to the engine and which had a volumetric flow rate of around 700 m3/hour showed a siloxane content of less than 1 mg/m3. The resultant clean running of the engine enabled a measured power output of 1 MW to be achieved and maintained for several weeks essentially unchanged.