This chapter describes the range of technologies available now and in the future to satisfy the world’s energy demand. The Energy [R]evolution scenario is focused on the potential for energy savings and renewable sources, primarily in the electricity and heat generating sectors.
fossil fuel technologies
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 burned at high temperature. The resulting heat is used to convert 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 ‘bolton’ 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:
• integrated gasification combined cycle: Coal is not burned directly but reacted with oxygen and steam to form a synthetic gas composed mainly of hydrogen and carbon monoxide. This is cleaned and then burned in a gas turbine to generate electricity and produce steam to drive a steam turbine. IGCC improves the efficiency of coal combustion from 38-40% up to 50%.
• supercritical and ultrasupercritical: These power plants operate at higher temperatures than conventional combustion, again increasing efficiency towards 50%.
• fluidised bed combustion: 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 the recovery of waste products. By elevating pressures within a bed, a high-pressure gas stream can be used to drive a gas turbine, generating electricity. Emissions of both sulphur dioxide and nitrogen oxide can be reduced substantially.
• pressurised pulverised coal combustion: Mainly being developed in Germany, this is based on the combustion of a finely ground cloud of coal particles creating high pressure, high temperature steam for power generation. The hot flue gases are used to generate electricity in a similar way to the combined cycle system.
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.
gas combustion technologies Natural gas can be used for electricity generation through the use of either gas 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 fuelled 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 produces electricity and the exhaust gases from the 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.
carbon reduction technologies Whenever a fossil fuel 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 hard coal power plant discharges roughly 720 grammes of carbon dioxide per kilowatt hour, a modern gas-fired plant about 370g CO2 /kWh. One method, currently under development, to mitigate the CO2 impact of fossil fuel combustion is called carbon capture and storage (CCS). It involves capturing CO2 from power plant smokestacks, compressing the captured gas for transport via pipeline or ship and pumping it into underground geological formations for permanent storage.
While frequently touted as the solution to the carbon problem inherent in fossil fuel combustion, CCS for coal-fired power stations is unlikely to be ready for at least another decade. Despite the ‘proof of concept’ experiments currently in progress, as a fully integrated process the technology remains unproven in relation to all of its operational components. Suitable and effective capture technology has not been developed and is unlikely to be commercially available any time soon; effective and safe long-term storage on the scale necessary has not been demonstrated; and serious concerns attach to the safety aspects of transport and injection of CO2 into designated formations, while long term retention cannot reliably be assured.
Read more in Chapter 8 of the usa energy [r]evolution report.