The most commonly used fossil fuels for power generation around the world are coal and gas. Oil is still used where other fuels are not readily available, for example islands or remote sites, or where there is an indigenous resource. Together, coal and gas currently account for over half of global electricity supply.
coal combustion technologies In a conventional coal-fired power station, pulverised or powdered coal is blown into a combustion chamber where it is burnt at high temperature. The hot gases and heat produced converts water flowing through pipes lining the boiler into steam. This drives a steam turbine and generates electricity. Over 90% of global coal-fired capacity uses this system. Coal power stations can vary in capacity from a few hundred megawatts up to several thousand.
A number of technologies have been introduced to improve the environmental performance of conventional coal combustion. These include coal cleaning (to reduce the ash content) and various ‘bolt-on’ or ‘end-of-pipe’ technologies to reduce emissions of particulates, sulphur dioxide and nitrogen oxide, the main pollutants resulting from coal firing apart from carbon dioxide. Flue gas desulphurisation (FGD), for example, most commonly involves ‘scrubbing’ the flue gases using an alkaline sorbent slurry, which is predominantly lime or limestone based.
More fundamental changes have been made to the way coal is burned to both improve its efficiency and further reduce emissions of pollutants.These include:
Other potential future technologies involve the increased use of coal gasification. Underground Coal Gasification, for example, involves converting deep underground unworked coal into a combustible gas which can be used for industrial heating, power generation or the manufacture of hydrogen, synthetic natural gas or other chemicals. The gas can be processed to remove CO2 before it is passed on to end users. Demonstration projects are underway in Australia, Europe, China and Japan.
Natural gas can be used for electricity generation through the use of either gas turbines or steam turbines. For the equivalent amount of heat, gas produces about 45% less carbon dioxide during its combustion than coal.
gas turbine plants use the heat from gases to directly operate the turbine. Natural gas fulled turbines can start rapidly, and are therefore often used to supply energy during periods of peak demand, although at higher cost than baseload plants. Particularly high efficiencies can be achieved through combining gas turbines with a steam turbine in combined cycle mode. In a combined cycle gas turbine (CCGT) plant, a gas turbine generator generates electricity and the exhaust gases from the gas turbine are then used to make steam to generate additional electricity.The efficiency of modern CCGT power stations can be more than 50%. Most new gas power plants built since the 1990s have been of this type.
At least until the recent increase in global gas prices, CCGT power stations have been the cheapest option for electricity generation in many countries. Capital costs have been substantially lower than for coal and nuclear plants and construction time shorter.
Whenever coal or gas is burned, carbon dioxide (CO2) is produced. Depending on the type of power plant, a large quantity of the gas will dissipate into the atmosphere and contribute to climate change. A coal power plant discharges roughly 720 grammes of carbon dioxide per kilowatt hour, a modern gas-fired plant releases about 370g CO2/kWh. Some coal advocates are proposing a new technique for reducing the carbon dioxide released by power plants. In this scheme the CO2 is separated, and then pumped underground. Both methods - capture and storage - have limitations. Even after employing proposed capture technologies, a residual amount of carbon dioxide – between 60 and 150g CO2/kWh - will continue to be emitted.
CO2 captured at the point of incineration has to be stored somewhere. Current thinking is that it can be trapped in the oceans or under the earth’s surface at a depth of over 3,000 feet. As with nuclear waste, however, the question is whether this will just displace the problem elsewhere.
Ocean storage could result in greatly accelerated acidification (reduction of pH) of large areas and would be detrimental to a great many organisms, if not entire ecosystems, in the vicinity of injection sites. CO2 disposed of in this way is likely to get back into the atmosphere in a relative short time.The oceans are both productive resources and a common natural endowment for this and future generations worthy of safekeeping.Given the diversity of other options available for dealing with CO2 emissions, direct disposal of CO2 to the ocean, sea floor, lakes and other open reservoir structures must be ruled out.
Empty oil and gas fields are riddled with holes drilled during their exploration and production phases.These holes have to be sealed over. Normally special cement is used, but carbon dioxide is relatively reactive with water and attacks metals or cement, so that even sealed drilling holes present a safety hazard.To many experts the question is not if but when leakages will occur.
Because of the lack of experience with CO2 storage, its safety is often compared to the storage of natural gas.This technology has been tried and tested for decades and is appraised by industry to be low risk. Greenpeace does not share this assessment. A number of serious leaks from gas storage installations have occurred around the world, sometimes requiring evacuation of nearby residents. Sudden leakage of CO2 can be fatal. Carbon dioxide is not itself poisonous, and is contained (approx. 0.04 per cent) in the air we breathe. But as concentrations increase it displaces the vital oxygen in the air. Air with concentrations of 7 to 8% CO2 by volume causes death by suffocation after 30 to 60 minutes.
There are also health hazards when large amounts of CO2 are explosively released. Although the gas normally disperses quickly after leaking, it can accumulate in depressions in the landscape or closed buildings, since carbon dioxide is heavier than air. It is equally dangerous when it escapes more slowly and without being noticed in residential areas, for example in cellars below houses. The dangers from such leaks are known from natural volcanic CO2 degassing. Gas escaping at the Lake Nyos crater lake in Cameroon, Africa in 1986 killed over 1,700 people. At least 10 people have died in the Lazio region of Italy in the last 20 years as a result of CO2 being released.
Can carbon storage contribute to climate change reduction targets? In order to avoid dangerous climate change, we need to reduce CO2 globally by 50% in 2050. Power plants that store CO2 are still being developed, however, and will not be widely available more than a decade.This means they will not make any substantial contribution towards protecting the climate until the year 2020 at the earliest.
Nor is CO2 storage of any great help in attaining the goal of an 80% reduction by 2050 in OECD countries. If it does become available in 2020, most of the world’s new power plants will have just finished being modernised. All that could then be done would be for existing power plants to be retrofitted and CO2 captured from the waste gas flow. As retrofitting existing power plants is highly expensive, a high carbon price would be needed.
Employing CO2 capture will also increase the price of electricity from fossil fuels. Although the costs of storage depend on a lot of factors, including the technology used for separation, transport and the kind of storage installation, experts from the UN Intergovernmental Panel on Climate Change calculate the additional costs at between 3.5 and 5.0 cents/kWh of power. Since modern wind turbines in good wind locations are already cost competitive with new build coal-fired power plants today, the costs will probably be at the top end. This means the technology would more than double the cost of electricity today.
Renewable energy sources are already available, in many cases cheaper, and without the negative environmental impacts that are associated with fossil fuel exploitation, transport and processing. It is renewable energy together with energy efficiency and energy conservation – and NOT carbon capture and storage – that has to increase world-wide so that the primary cause of climate change – the burning of fossil fuels like coal, oil and gas – is stopped. Greenpeace opposes any CCS efforts which lead to:
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