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Energy sources and processes

The choice of fossil fuels for power generation is moving away from coal in favour of gas, and, to a lesser extent, oil. Gas and good quality crude oil would both be much better utilised as raw materials, but short-term economics dictate their use as fuel, and it will be some years yet before there is a return to interest in the processing of low quality fuel for power generation. The apparent problems of global warming may mean that this never happens.

Much has been said, written and planned recently about expansion in renewable energy systems, but they can only ever be a top-up to established sources, even if the visual intrusion of large wind-farms, for example, can be accepted (to replace a 2400MW generating station with 3MW wind turbines, properly spaced, would require a wind farm that is 3 km wide by 100 km long). Even in 2020, renewable sources (other than hydroelectric schemes) are not expected to exceed 2% of the total supply in the UK.

On the other hand, nuclear energy offers itself as a reasonable source of power, if the public attitude to it could be changed, and a satisfactory answer found to the waste disposal problem. Finland is constructing a new nuclear power plant, the first in Europe for over 10 years, while there are active movements in both the UK and the USA encouraging governments to invest in nuclear power again. It is still an important source, representing nearly 6% of the sources for energy on a world scale and nearly 14% of electricity production. When looked at in the context of fossil fuel futures and prevention of carbon dioxide emissions, nuclear power has to be considered as a major source.

The main area for development has to be the filtration of hot exhaust gases. High quality air intake filters for gas turbines and better water filtration for boiler feeds will also demand attention.

The basic stages in the traditional process for the generation of electricity are the rotation of the core of the generator set, preceded by the provision of energy by which to turn it. In the case of wind or water power, that energy comes directly from the wind or water turbine driven by the natural force. In the much more common fossil fuel powered systems, the energy is released by the burning of the fuel, to produce a hot gas, which either drives a gas turbine as the power source, or generates steam, for use in a steam turbine. The highest efficiencies come from combined cycle systems, which use gas in a gas turbine, the hot exhaust from which is passed to a water boiler, producing steam to generate more electricity in a steam turbine. Nuclear power systems work in much the same way, utilising the energy released by the fission reactions to heat an intermediate fluid circuit that in turn boils water for a steam turbine generator combination.

Equipment uses

Filtration and related separation equipment finds its applications in this sector in ancillary roles rather than directly in the generation step. These applications include:

. treatment of the boiler feed water and of the recycled condensate;

. filtration or sedimentation of recycling cooling water;

. air intake cleaning for gas turbine combustion;

. contaminant removal from gas in pipeline transport;

. boiler furnace flue gas treatment; and

. any processing of coal, before or after combustion, to increase its cleanness.

Nuclear power has most of these applications, characterised by the particular requirements of safety and long life in a highly radioactive environment.

Small scale and captive local system generating stations have all of these problems, plus those of the conditioning of diesel fuel oils. Quite different requirements apply to the small scale generation using fuel cells - gases and liquids will require thorough cleaning to ensure an adequately long life for the barrier membranes.

Boiler feed water treatment has the same needs as any steam generation system: fine filtration of the incoming makeup water, probably with some sort of de-ionisation. The feed water once prepared and fit for the boiler is quite an expensive liquid, and so will certainly not be discarded, but will be recycled after condensation, and some filtration will again be necessary to remove particles picked up in the steam system.

The cooling water used in the steam condensation stage must also be cleaned - of scale particles, to stop deposition on heat exchange surfaces, and especially of Legionella bacteria, which breed far too easily in cooling water circuits.

Atmospheric air usually contains too much dust to allow it to be used in contact with gas turbine blades, so large filter installation are mounted at the air intake point, usually equipped with panel or V-bank filters, with prefilters, to remove the dust to a safe level.

Exhaust gases from gas turbines and especially from boiler furnaces are not only dusty but very hot, and frequently contain sulphur and nitrogen oxides. Dedusting is obviously a task for a filter, but it must be a filter that can withstand the exhaust temperature, so that the filter medium will need to be of temperature resistant metal or ceramic. Sulphur removal is mainly undertaken by scrubbing with a lime slurry, yielding a liquid waste of calcium sulphate and other materials - a separation task well suited to the centrifuge.

The main area for development has to be the filtration of hot exhaust gases. High quality air intake filters for gas turbines and better water filtration for boiler feeds will also demand attention. A return to favour of nuclear power will add considerably to the sector's need for reliable high grade filters.

Contact:
Ken Sutherland
Email: ken.suth@ntlworld.com
Ken Sutherland has managed his process engineering and market research consultancy, Northdoe Limited, for nearly 30 years, a business largely concerned with filtration and other such separation technologies. He was a co-author of Elsevier's Decanter Centrifuge Handbook, and has also written the second edition of Elsevier's Handbook of Filter Media. More recently he has written Elsevier's A to Z of Filtration.

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