Will high oil price see the return of coal as a mainstream fuel?
Nuclear power is currently the hot topic, but the continuing high oil price could bring clean coal into contention as a mainstream fuel. Eric Russell reports
The coal industry has been cleaning up its act in recent years and a number of technologies have been developed to reduce the output of greenhouse gases and to increase combustion efficiency.
Some, such as low NOx burners, are very cost effective and can be used with immediate benefits. Other technologies are not economically viable just yet and await a greater take-up to reduce unit costs.
Most efficient power yet
One emerging technology, known as Integrated Gasification Combined Cycle (IGCC), is the most efficient method of generating power yet. It uses gasification to break down coal into a gas stream of carbon monoxide and hydrogen, known as synthetic gas or syngas. The process removes sulphur and pollutant carriers, and it is possible to create marketable products such as methanol and fertilisers from the compounds that are also removed.
The syngas is cleaned by filtering and scrubbing, and fires a gas turbine that can drive an alternator, also generating steam. The steam drives a
secondary turbine to generate more electricity while the waste gas is treated to remove the remaining pollutants.
In existing clean-combustion systems, coal is graded in cyclones or centrifugal separators and washed, which, by itself, considerably reduces emissions of ash and sulphur dioxide when the coal is burned. Coal preparation is standard practice in developed countries but few use it in less developed regions. Ultra-clean coal can reduce ash below 0.25% and sulphur to very low levels. When pulverised, such coal could be fed directly into gas turbines.
Pulverised coal burns more efficiently the finer the particles. It can be carried into combustion chambers on a stream of air and ignited at a burner outlet or fired in a bed with air flowing through and lifting the coal, known as a fluidised bed. Variations are being developed such as pressurised and circulating fluidised beds.
There are many versions of burner available, all with highly improved air and coal flows and better flame temperature profiles. These burn off more oxides of nitrogen and are known as low-NOx burners because they help reduce stack emissions by up to 40%.
Design of combustion chambers is also well advanced with multi-stage chambers, overfire air, and recycling of exhaust gases, which breaks down more pollutants through the higher temperatures and longer exposure to the heat of combustion. Oxygen can be added to the recycling flue gases, known as the oxyfuel process, to increase CO2 concentration and make it easier to capture.
Precise control of combustion has also developed with advances in sensor technology giving more accurate readings of temperatures and gas composition. The small, eight-bit microprocessors from early computers are now low-cost enough for industrial automation and powerful enough for accurate control applications, including neural networks.
There are several ways to treat the gas that emerges after combustion. Electrostatic precipitators and fabric filters can remove 99% of the fly ash from flue gases. In a precipitator, the particles in the smokestack are given an electric charge as they pass electrified electrodes, they are then attracted out of the gas stream, on to an earthed screen and dislodged into a collecting facility.
Flue gas desulphurisation typically uses a sorbent such as a slurry of lime or limestone to take up to 97% of CO2 out of the gas stream. Gypsum is produced as a result.
Amine scrubbing involves bubbling the gases through a large column containing a basic amine solvent. This reacts with the acidic CO2 and the CO2-rich solvent can then be regenerated by heating, giving pure CO2.
Sequestration is the capture and storage of CO2 in underground chambers. It is also being used in Enhanced Oil Recovery (EOR) in the USA and the North Sea.
When oil is drawn from wells, the underground pressure drops until it is too low to force oil to the surface. But there is often a substantial volume of oil still in the well. Pumping in CO2 forces oil to the surface again. In the USA, 32 million tonnes of CO2 is used annually for enhanced oil recovery.
In the North Sea, the Sleipner EOR project repaid its investment within 18 months. About one million tonnes per year of compressed liquid CO2 is injected into the reservoir about 1km below the seabed and the saline aquifer could store 600 billion tonnes of CO2.
FutureGen, the Integrated Sequestration and Hydrogen Research Initiative, is a $1 billion (£580 million) partnership that will design and build a coal gasification-based electricity and hydrogen production plant in the USA. The 275MW prototype plant will serve as a laboratory for testing new technologies.
Greenhouse gas emissions can also be cut by changing the process of manufacture. The British Cement Association, for example, says the amount of cement used in concrete mixes can be cut by using substitutes.
This industry is a significant contributor to global warming and is the third ranking producer of CO2 in the world after transport and energy generation. It is responsible for 7-10% of the world’s total CO2 emission and the figure is increasing by some 5% a year.
Additives such as fly ash, which is removed from flue gases using electrostatic precipitators, can replace Portland cement by 50%. And granulated blastfurnace slag can be used at rates up to 90%.
Alstom is developing new technology that enables the capture of greenhouse gas emissions from fossil fuels. It burns coal or any fossil fuel in oxygen, rather than air. Oxygen combustion, also known as oxyfuel firing, concentrates the CO2 in flue gas, making it easier to capture.
Director of Technology at Mitsui Babcock, Les King says: “The combination of technologies in the company’s carbon abatement portfolio, called Green Coal Technology, could achieve a 50-60% reduction in CO2 emissions so they match levels for many existing gas-fired power plants.”
One aspect of the company’s work is to make plants ‘capture-ready’ for when CO2 sequestration becomes routine. It also points out that biomass, a CO2 neutral fuel, can be co-fired with coal in large boiler plants to help reduce CO2.
The heat and gas flows of combustion lend themselves to computer modelling with computational fluid dynamics (CFD) software. This uses a database of coal and gas attributes plus the rules of heat physics to calculate temperature profiles and heat flows.
The volume under analysis is divided into a mesh of cells and the calculation moves across the domain, from the starting point, which could be a coal burner or the space in the combustion chamber. The finer the mesh, the more accurate the prediction of flow or temperature gradient. The results are in colour. A numeric table at the side shows actual values against colour.
Fluent is a company that has specialised in this area for years and its software is used to create burner designs, to calculate flame temperatures, to study gas swirls and to predict the NOx levels that are output by different designs.
Transporting coal creates extra pollution and this can be avoided with coal-by-wire, popular in China, where mine-mouth generating plant takes coal directly from the mine and the electrical output is fed into the local grid. This is a relatively small plant, so it can be easily moved to a new mine.
In the UK, clean coal technologies are supported by organisations such as the International Energy Agency (IEA) Clean Coal Centre, a collaboration between member countries of the IEA, the World Coal Institute – a non-profit, non-governmental association of coal enterprises – and the Coal Research Forum – which brings together those with interests in coal research.
As the Government makes plans to construct new nuclear power plants in the UK, maybe coal shouldn’t be overlooked in the challenge to find energy provision that won’t affect the country’s CO2 targets.
· For more information visit www.coalresearchforum.org, www.iea-coal.org.uk, or www.worldcoal.org
· For more on clean coal visit www.power.alstom.com, www.fluent.com or
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