Coal

Back to black

The realisation that we need coal-fired power generation has helped revive a once neglected industry

Modern life is unimaginable without electricity. It lights houses, buildings, streets, provides domestic and industrial heat, and powers most equipment in homes, offices and factories. Improving access to electricity worldwide is a key factor in alleviating poverty.

It has been estimated that there are over 847 billion tonnes of proven coal reserves worldwide. This means that there is enough coal to last us over 130 years at current rates of production.

With climate change fears underpinning energy policy around the globe, it appeared that coal would be forced to take a back seat in the drive for a sustainable future. But that has turned out not to be the case. Nations such as China, the US and India have huge reserves and were determined to use them. Aided by cleaner technologies and the advent of carbon capture and storage (CCS), coal is enjoying a renaissance and will form a central pillar of energy policy this century.

"Tackling climate change is a long-term issue and requires a long-term solution. We are not going to solve the problem in the next five to ten years. What we can achieve in this timeframe is to set the foundations for an effective, long-term solution to climate change by investing in clean energy technologies, particularly carbon capture and storage," Milton Catelin, chief executive of the World Coal Institute says.

"Worldwide there are huge reserves of coal, and countries will continue to use these reserves to supply affordable energy. Carbon capture and storage (CCS) will be vital in regions where there really is no alternative to fossil fuels," he stresses

"While there is a lot happening on CCS, we are behind the curve. We need to ramp up efforts if we are going to achieve a clean energy revolution. Concerted effort between government and industry is essential to achieve climate change goals."

Future growth

Both the International Energy Agency (IEA) and US Energy Information Administration (EIA) predict that coal's share of the energy market will remain at similar levels to that of today - around 25-26 per cent - and grow at rates of between 2 per cent and 3 per cent annually, Catelin notes

"Predictions suggest a near-doubling of coal demand by 2030, implying that overall world coal demand will exceed ten billion tonnes annually, possibly by a significant margin," he says.

Figures from the IEA show that the Far East and India account for 45 per cent of the total increase in world energy demand up to 2030, and 82 per cent of the increase in coal demand.

"There is little doubt that global resources of coal are so massive that when the world needs more coal, and prices are at a level to justify proving-up the reserves, this will happen," Catelin says. The latest assessments of currently proven recoverable reserves of coal are 8 per cent of the total resource, compared with 47 per cent for gas and 67 per cent for oil.

"Debate as to whether the coal resource amounts to six trillion tonnes or ten trillion tonnes, or somewhere in between, will doubtless continue, but from this massive resource base there is little doubt that reserves can be proved and exploited for as long as the world needs coal," Catelin assures.

In the age when security of supply is a mere driver it is worth noting that coal is located worldwide - it can be found on every continent in over 70 countries, with the biggest reserves in the US, Russia, China and India. These coal reserves could be extended still further through the discovery of new reserves through ongoing and improved exploration activities and advances in mining techniques, which will allow previously inaccessible reserves to be reached.

Additionally, significant improvements continue to be made in how efficiently coal is used so that more energy can be generated from each tonne of coal produced.

Power generation

Steam coal, also known as thermal coal, is used in power stations to generate electricity.E F Coal is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. In these pulverised coal combustion (PCC) systems, the powdered coal is blown into the combustion chamber of a boiler where it is burnt at high temperature. The hot gases and heat energy produced converts water - in tubes lining the boiler - into steam.

The high pressure steam is passed into a turbine containing thousands of propeller-like blades. The steam pushes these blades causing the turbine shaft to rotate at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. Electricity is generated when these are rapidly rotated in a strong magnetic field. After passing through the turbine, the steam is condensed and returned to the boiler to be heated once again.

Improvements continue to be made in conventional PCC power station design, and new combustion technologies are being developed. These allow more electricity to be produced from less coal - known as improving the thermal efficiency of the power station.

Combustion technologies

A range of advanced coal combustion technologies have been developed to improve the efficiency of coal-fired power generation.

Improving efficiency levels increases the amount of energy that can be extracted from a single unit of coal. Increases in the efficiency of electricity generation are essential in tackling climate change. Not only do higher efficiency coal-fired power plants emit less CO2 per megawatt, they are also more suited to retrofitting with CO2 capture systems.

Efficiency improvements include the most cost-effective and shortest lead time actions for reducing emissions from coal-fired electricity. This is particularly the case in developing and transition countries where existing plant efficiencies are generally lower and coal use in electricity is increasing.

Improving the efficiency of pulverised coal-fired (PCF) plants has been the focus of considerable efforts by the coal industry. Significant efficiency improvements and CO2 reductions can be achieved as the existing fleet of power plants are replaced over the next 10-20 years with new, higher efficiency supercritical (SC) and ultra-supercritical (USC) plants.

A one percentage point improvement in the efficiency of a conventional PCC plant results in a 2-3 per cent reduction in CO2 emissions, depending on the level of efficiency prior to the change.

Fluidised bed combustion

Fluidised bed combustion (FBC) is a very flexible method of electricity production - most combustible material can be burnt including coal, biomass and general waste.

FBC systems improve the environmental impact of coal-based electricity, reducing SOx and NOx emissions by 90 per cent.

In FBC, coal is burned in a reactor comprised of a bed through which gas is fed to keep the fuel in a turbulent state. This improves combustion, heat transfer and recovery of waste products. The higher heat exchanger efficiencies and better mixing of FBC systems allows them to operate at lower temperatures than conventional PCC systems. By elevating pressures within a bed, a high-pressure gas stream can be used to drive a gas turbine, generating electricity.

FBC systems fit into two groups: non-pressurised systems (FBC) and pressurised systems (PFBC), and two subgroups, circulating or bubbling fluidised bed.

Non-pressurised FBC systems operate at atmospheric pressure and are the most widely applied type of FBC. They have efficiencies similar to PCC - 30-40 per cent. Pressurised FBC systems operate at elevated pressures and produce a high-pressure gas stream that can drive a gas turbine, creating a more efficient combined cycle system - over 40 per cent.

Bubbling uses a low fluidising velocity so that the particles are held mainly in a bed, and is generally used with small plants offering a non-pressurised efficiency of around 30 per cent. Circulating uses a higher fluidising velocity so the particles are constantly held in the flue gases, and are used for much larger plants offering efficiency of over 40 per cent.

Supercritical and ultra supercritical

New PCC systems - using SC and USC technology - operate at increasingly higher temperatures and pressures and therefore achieve higher efficiencies than conventional PCF units and significant CO2 reductions.

SC steam cycle technology has been used for decades and is becoming the system of choice for new commercial coal-fired plants in many countries. Recent plants built in Europe and Asia use SC boiler-turbine technology and China has made this standard on all new plants 600MWe and upwards.

Research and development is under way for USC units operating at even higher efficiencies, potentially up to around 50 per cent.

The introduction of USC technology has been driven over recent years in countries such as Denmark, Germany and Japan, in order to achieve improved plant efficiencies and reduce fuel costs. Research is focusing on the development of new steels for boiler tubes and on high alloy steels that minimise corrosion.

These developments are expected to result in a dramatic increase in the number of SC plants and USC units installed over coming years.

Gasification technology

An alternative to achieving efficiency improvements in conventional pulverised coal-fired power stations is through the use of gasification technology. IGCC plants use a gasifier to convert coal (or other carbon-based materials) to syngas, which drives a combined cycle turbine.

Coal is combined with oxygen and steam in the gasifier to produce the syngas, which is mainly H2 and carbon monoxide (CO). The gas is then cleaned to remove impurities, such as sulphur, and the syngas is used in a gas turbine to produce electricity. Waste heat from the gas turbine is recovered to create steam which drives a steam turbine, producing more electricity - hence a combined cycle system.

By adding a 'shift' reaction, additional hydrogen can be produced and the CO can be converted to CO2 which can then be captured and stored. IGCC efficiencies typically reach the mid-40s, although plant designs offering around 50 per cent efficiencies are achievable.

There are currently four commercial-scale, coal-based IGCC demonstration plants worldwide and a number of other IGCC projects have been proposed. IGCC plants operate at Schwarze Pumpe, Germany and Vresov, Czech Republic.

Gasification may be one of the best ways to produce clean-burning hydrogen for tomorrow's cars and power-generating fuel cells. Hydrogen and other coal gases can be used to fuel power-generating turbines, or as the chemical building blocks for a wide range of commercial products, including diesel and other transport fuels.

Reliability and availability have been challenges facing IGCC development and commercialisation. Cost has also been an issue for the wider uptake of IGCC as they have been significantly more expensive than conventional coal-fired plant.

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