Coal has traditionally been lambasted as an environmental evil, but its use within the power generation sectors looks set to grow over the next decade.
Coal has been used as a fuel for more than 2,300 years, since the time of the ancient Greeks. From those humble beginnings it has grown until today in excess of seven billion tonnes are burnt to generate more than 40 per cent of the world's electricity. Despite its less than glowing environmental credentials the use of coal is set to soar over the next decade, driven by the economic growth of the emerging nations, particularly China and India. Seemingly flying in the face of global attempts to curb carbon, coal – already the single largest source for generation of electricity – is predicted to grow by 600,000t a day over the next five years.
"Global demand for coal will continue to expand aggressively over the next five years despite public calls in many countries for reducing reliance on the high-carbon fuel as a primary energy source," International Energy Agency (IEA) executive director Maria van der Hoeven says. "For all of the talk about removing carbon from the energy system, the IEA projects average coal demand to grow by 600,000t every day over the next five years."
There is no hiding from coal's importance in the energy mix. It is the world's most abundant and widely distributed fossil fuel, with global proven reserves totalling nearly 1,000 billion tonnes. At present coal is burnt to produce in excess of 40 per cent of the world's electricity, although this rises significantly in some countries, with South Africa and Poland both topping 90 per cent and China pushing 80 per cent. Even the highly economically advanced US uses coal for almost half of its power generation.
Despite climate change concerns and legislation, coal's role in the energy mix is unlikely to change soon given the continued economic growth in regions such as China and India that at present rely heavily on coal.
"Demand for coal has grown rapidly over the last decade, outstripping that for gas, oil, nuclear and renewable energy sources," van der Hoeven adds. "But this comes at a cost. Coal contributes to over 40 per cent of global anthropomorphic CO2 emissions and more than 70 per cent of CO2 emissions that arise from power generation are attributed to coal.
"To play its role in a sustainable energy future, its environmental footprint needs to be reduced; using coal more efficiently is an important first step. Beyond efficiency improvements, carbon capture and storage (CCS) must be deployed to make deep cuts in CO2 emissions."
In coal-fired power generation, efficiency is an important performance parameter. Clearly raising efficiency will reduce emissions of carbon dioxide (CO2), a 1 per cent improvement in overall efficiency can result in up to 3 per cent reduction in CO2 emissions. There are also side benefits such as a reduction in the emission of conventional pollutants and extending the life of coal resources.
The fossil-fuel industry has made considerable progress in increasing efficiency with supercritical (SC) and ultra supercritical (USC) pulverised coal-fired (PC) technology. While sub critical plants can achieve efficiencies of almost 40 per cent, state of the art USC plants produce a far superior performance. USC plants operating at high-steam conditions, with temperatures of 600°C to 620°C and pressures over 25MPa (250 bar), result in design efficiencies of 45-46 per cent on bituminous coals in some parts of the world. Unit capacity for such plants has reached up to 1100MWe.
If ongoing developments in materials are successful, the efficiency of the best PC plants may approach 50 per cent in the next 10 to 15 years. Progress is also being made in the development of circulating fluidised bed combustion (CFBC) plants. These can make use of low rank fuels and result in lower emissions of conventional pollutants.
The first SC CFBC plant of 460MWe size was commissioned at Lagisza, Poland, in 2009, and designs for larger units exist. For CFBC, there will continue to be a significant market, particularly for lower-grade coals, high sulphur coals, opportunity fuels and biomass. Successful operation of the supercritical unit at Lagisza should pave the way for the construction of such units in other countries, particularly China.
As far as integrated gasification combined-cycle (IGCC) power generation is concerned, there are only six coal-based units in the world. However, a small number of new projects have been initiated worldwide, each at some stage of planning or construction. There are challenges to be overcome, however, such as improving the cost and availability of IGCC, before the technology can be more widely deployed. An important factor is that IGCC is well placed technically to capture CO2. Though additional costs will be significant, some consider they may be lower than for pulverised coal systems.
Another challenge for increased efficiency is the poor quality of the coal. Around 45 per cent of available coal is either high moisture or high ash, which often results in inefficiencies in the power plants using these coals.
The traditional solution to this is drying the coal pre combustion, but this is a highly energy-intensive process that has its own technology challenges. There is a concerted drive to develop less energy intensive coal drying technologies, with research underway in Australia, Germany and the US, but to date there are no plans to accelerate these efforts into large scale commercial plants.
There is a similar requirement to develop less energy and water-intensive technologies for coal beneficiation to reduce its ash and sulphur content. Success in developing more efficient coal drying and beneficiation technologies will promote the wider use of low rank coals in both ultra supercritical pulverised coal and IGCC applications.
When it comes to selecting the technology to be used in future coal-fired power plants the method of selection is key. While there are several proposals for further commercial demonstrations of IGCC and some are being constructed, SC and USC pulverised coal plants continue to dominate new plant orders.
It is possible that IGCC, with or without CCS, may penetrate the market on a large scale only where the co-production of power and chemicals can be economically demonstrated. *