The key to 'greener' energy - advanced plant01 February 2005

It is increasingly obvious that only a combination of engineering technologies is likely to be able to meet the world's growing energy needs in the near term without totally destroying its climate and environment.

The world's needs will not be met even if whole countries are covered with wind turbines. Wave and large-scale solar power are still very much at the R&D stage, and some of the more advanced concepts, which may well provide major amounts of energy in future, are presently little more than gleams in the eyes of engineers and inventors.

All that is required to meet immediate needs is, however, available now, and can be implemented on a large scale at acceptable cost within the next ten years. In our opinion, the list should start with improving the efficiency of existing coal-fired power stations. It should then include: renewable biomass; using methane from old coal mines and rubbish dumps; geothermal power, especially when it improves the efficiency of heat pumps; and nuclear, with UK safety standards and proper attention paid to the disposal of waste.

In the UK, there are still 16 coal-fired power stations generating 35% of the nation's electricity. Mitsui Babcock, however, has projected that it should be technically possible to cut the carbon dioxide emissions (and fuel costs) of these stations by as much as 60% by retrofitting more advanced equipment at a fraction of the cost of constructing entirely new plant based on alternative technologies.

Somewhat sceptical, we ask Dr Les King, director of technology and engineering at Mitsui Babcock, and Douglas Spalding, business development new products, how this goal might be achieved.

Dr King explains that the 60% figure is entirely achievable and is based on what Mitsui Babcock calls its 'green coal technology' based on three 'building blocks'. The first would be to run at higher temperatures and pressures, where steam and water coexist without latent heat of vaporisation to change from one to the other.

Such a condition, known as 'supercritical', requires working at above 221 bar main steam pressure. The existing UK power generating fleet runs with steam at typically 170 bar and 540-560°C, which gives an overall efficiency of 36-38%. Raising pressure to 300 bar and main steam temperature to 600°C in new-build plant would increase efficiency to 46%. If it were possible to raise the temperature further to 700°C, Dr King believes this would increase efficiency to 55%, comparable to a combined cycle gas-fired plant.

Retrofitting boiler parts and some turbine plants to permit supercritical operation would raise efficiency to 43.5%, a 20% improvement. A retrofit to a 600 MW plant would cost around £100m and also allow nitrogen oxide emissions to be reduced to 200mg/m3, the 2016 standard. Time required would be 24 months, including 12 months during which the plant would have to cease operation. A new 600MW plant built from scratch would cost around £500m and take 48 months to build.

The second 'building block' would be to increase the proportion of biomass used as fuel. The argument is that although burning biomass creates carbon dioxide, the biomass growing to replace it in a managed environment absorbs an equal amount from the atmosphere. Most UK power stations currently burn biomass along with the coal, but the milling plants limit this to about 5%. If the biomass were to be processed in a separate feed stream, this would allow up to 20% to be burned in an otherwise coal-fired boiler. More than 20% can apparently result in unacceptable ash deposits and fouling.

The third 'building block', according to Douglas Spalding, would be to use either the output from a gas turbine or biomass combustion to preheat boiler feed water, which currently consumes 15% to 20% of the energy input of a thermal power plant, giving a further 10-20% CO2 saving.

Looking further ahead, if coal were burned with oxygen instead of air, the flue gas becomes 90% carbon dioxide, making it amenable to sequestration, either by injecting it into oil wells to assist oil recovery, or by injecting it into aquifers.

Even if only a 20% improvement in efficiency were to be obtained by retrofitting the existing coal-fired power station fleet with supercritical equipment, this would be equivalent to constructing 7,000-10,000 wind turbines. Switching to natural gas is not really an alternative since North Sea gas production peaked in about 2000. If biomass were to make a really large contribution, a large proportion of it would have to be imported. UK coal reserves, on the other hand, are sufficient to last for hundreds of years.

As well as that which presently comes out of the North Sea as natural gas, substantial quantities of methane leak into the atmosphere from decaying rubbish and old coal mines. Since methane has a global warming potential about 23 times as great as carbon dioxide, and is also a fuel, it represents both a threat and an energy opportunity.

The German company Pro2 Alkane Energy, headquartered in Willich, but with its UK base at Alkane Energy's head office in Edwinstowe, Nottinghamshire, has specialist expertise in taking gas from landfill or old coal mines and using it as fuel for gas engines or microturbine-based CHP (combined heat and power) plants. UK area manager Jochen Niedree tells Plant Engineer that old coalmines represent a particular opportunity because "all old coal mines produce methane and you don't have to pay for it if you get the licence to extract it." He says that UK official bodies are still often "in denial" about methane from disused coalmines being a problem, but the gas is widely used as a fuel in the Ruhr, and there are now a few installations in the UK.

According to Niedree, it may be necessary to drill a borehole to extract the gas but once gas starts to be drawn out, its value as a fuel often improves. His company supplies containerised installations from 350kW up to 1.3MW, with one installation producing 11MW. Either MAN or Deutz reciprocating engines, or 100kW Turbec microturbines, are used. Working lifetimes of the mine gas are not known for certain, but thought likely to be 10 or 20 years: the Methamine project in the old French Nord Pas de Calais coalfield has been supplying gas sufficient for a town of about 45,000 people since 1989.

He adds that while CHP is the most efficient way to use the fuel, "on most sites, it is hard to sell the heat," although he does mention biogas sites supplying heat to digesters producing gas alongside greenhouses. Regarding contaminants in the gas, there is sometimes a need to use active carbon filters to remove siloxane group gases, which otherwise lead to damaging deposits resembling white sand. However, in most cases, Niedree believes it is better to simply change the engine oil more frequently and occasionally take the cylinder head off and clean it. If gas flow from an old coalmine decreases with time, Pro2 engineers have the flexibility to replace the engine for a smaller one.

Core technology

Another free source of energy, which can either be exploited directly or used to aid the efficiency of heat pumps, is that produced naturally in the earth's core.

Heat pumps work on the same principle as refrigerators, heating buildings and/or their hot water systems by moving heat from the ground. Much more popular in other European countries than in the UK - Sweden had 380,000 heat pumps in 2002 and they are installed in 40% of new house builds in Switzerland - they deliver an amount of heat energy four or five times greater than that consumed as electricity. They work anywhere, but especially well where substantial amounts of heat rise from beneath.

A German manufacturer is ERW Elektrotechnik in Beckum. Manager Ralf Hanswille tells Plant Engineer that his firm makes standard units producing 4.6kW, more than sufficient for the average small centrally- or underfloor-heated house, costing ?3,000, 15.5kW models for ?5,500 and special units rated at up to 100kW. The company claims: "Detailed studies have shown that electric-driven heat pumps would reduce emissions of CO2 and other pollutants associated with space heating in Europe by between 30% and 50% depending on the conditions of use."

In Germany, the state government of North Rhine Westphalia is encouraging the use of heat pumps. It estimates that by extracting heat from water and antifreeze circulating in plastic pipes in boreholes 50m to 100m deep, it is possible to heat a single family home for an electricity cost of ?250-300 per year. It points out that such systems can additionally be used for air conditioning in the summer months. Optimal design depends on knowledge of the types and thicknesses of subterranean rock layers and on the ground water situation. Digital geothermal maps and additional information on subterranean structures across the entire state are available on CD.

This information was originally limited to 100m depths to promote heat pumps, but has since been extended to 2,000m, for geothermal heating and other energy projects. This information has allowed the sustainable geothermal heating of new housing estates in Hamm and Iserlohn and is scheduled to be published in January 2005. The only part of the UK where we know of similar information being available is the city of Southampton.

Nuclear energy also produces no global warming gases. It is presently out of favour mainly because of the effects of gross human incompetence at Chernobyl in 1986 and the unhappy mixture of monitoring system breakdown, human incompetence and pressure to keep going regardless at Three Mile Island in 1979.

UK reactors have in the past been built with three emergency shutdown systems: borated steel rods, boron balls and the facility to inject boron dust. Hence, even if the reactor core is in such a state that the rods cannot be inserted, the balls will drop in and quench the reaction and the dust can be injected to close down the reactor permanently with no possibility of foolish humans trying to re-start it. To the best of our knowledge, ever since the Windscale accident in 1957, British operators, designers and regulating authorities have consistently been much more safety conscious than those in most other countries, including the USA and former USSR.

This leaves the terrorist risk, which we feel has probably been overestimated, and the problem of safely disposing of the nuclear waste, which has probably been underestimated.

Speaking at the Horizon seminar, 'Toward a Sustainable Earth', held at Cambridge University in December 2004, Dr Simon Redfern of the Department of Earth Sciences expressed the opinion that the idea of melting down nuclear waste with other substances, favoured in the USA, is not safe since the glasses formed are not sufficiently durable. He favours turning them into crystalline ceramic materials similar to those that have successfully contained radioactive substances since Planet Earth came into being. The only possible complaint that can be levelled at his approach is that the technology is still at an early stage and looks potentially expensive. Lower in cost and nearer to market is a technology being developed at Greenwich University, which mixes toxic waste with cement kiln dust and treats it with carbon dioxide to turn it into limestone.

The combination of more efficient steam-based thermal power stations, recovery of stray methane, heat pumps, geothermal heat and cleaner nuclear operation could provide more than enough energy over the next decade or two, while exceeding Kyoto's goals. There is no need yet to invent any new technologies. Large-scale solar and wave power generation, and mining the moon for helium-3 for nuclear fusion, may well be required in the future, but there is no immediate need for them.

How we can exceed Kyoto - without tears

In December, the Prime Minister announced that the UK was unlikely to meet its self-imposed target of a 20% reduction of 1990 greenhouse gas emissions, which exceeds the 12.5% demanded by the Kyoto Protocol. Emissions of carbon dioxide, the main greenhouse gas, were 164 million tons of carbon (Mtc) in 1990 and 152.5Mtc in 2002. Kyoto requires getting down to 144Mtc, a reduction of 8.5Mtc from 2002 levels; the government's target is 131Mtc, a reduction of 21.5Mtc.

Achieving this is not half as difficult as some would have us believe. Re-equipping the 16 power stations that produce 35% of the nation's electricity with supercritical boilers raises their efficiencies from 37% to 43.5%. This, we estimate, would save around 5Mtc. Adopting all the methods suggested in this area takes this figure to 11.7Mtc.

Combusting fugitive methane emissions might not have a dramatic effect on reducing carbon dioxide emissions, but would eliminate a lot of methane, which produces a greenhouse effect 23 times greater than that produced by carbon dioxide.

Switching all homes to use heat pumps would reduce carbon dioxide emissions relating to domestic consumption by around 40%. If only half the buildings were converted, this should be around 20% of the current figure, saving around 5Mtc. Doing the same for office buildings would probably save another 5Mtc.

The 14 nuclear power stations in the UK generate about 23% of our electricity. The newer ones generate around 1,200MW each. It can be argued that building another five would save either 7.5Mtc relative to gas firing or 15Mtc relative to coal firing.

Given the nature of statistics, the variations in those cited by the different interest groups are appreciable. However, even taking these into account, it is not hard to see that the goals set by Kyoto, and even those set by our own government, are quite reasonable. Fulfilment only requires that proper use be made of already existing technologies. Developing the newer energy technologies alongside these should allow significant further reductions.

Provided the human race gets its act together, it should therefore still be possible to avoid climate change disasters without any requirement to reduce living standards.

SOE

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