A combination of legislation and technology has helped clean up many of the world’s coal-burning plants. Both developed and developing countries have adopted increasingly stringent environmental regulations to govern emissions from coal-fired power plants. In the United States, all coal-fired power plants built after 1978 must be equipped with postcombustion cleanup devices to capture pollutants before they escape into the atmosphere. Cyclones, baghouses, and electrostatic precipitators filter out nearly 99% of the particulates. Flue gas scrubbers use a slurry of crushed limestone and water to absorb sulfur oxides from flue gas. The limestone reacts with the sulfur dioxide to form calcium sulfate, which may be used to produce wallboard. Staged combustion and low-NOx burners are used to burn coal to minimize NOx formation. Another strategy, selective catalytic reduction, reacts ammonia with NOx over a catalyst to produce nonpolluting nitrogen and water vapor.
Conventional coal-fired power plants capture pollutants from the flue gas after it leaves the boiler. Circulating fluidized bed (CFB) combustors capture most of the pollutants before they leave the furnace. Crushed coal particles and limestone circulate inside the CFB combustor, suspended by an upward flow of hot air. Sulfur oxides released during combustion are absorbed by the limestone, forming calcium sulfate, which drops to the bottom of the boiler. The CFB combustor operates at a lower temperature (14001F) compared to pulverized coal (PC) boilers (27001F), which also helps reduce the formation of NO x .
Precombustion coal cleaning is another strategy to reduce sulfur emissions by cleaning the coal before it arrives at the power plant. Sulfur in coal is present as pyrite (FeS2 ), which is physically bound to the coal as tiny mineral inclusions, and as ‘‘organic sulfur,’’ which is chemically bound to the carbon and other atoms in coal. Pyrite is removed in a coal preparation plant, where coal is crushed into particles less than 2 inches in size and is washed in a variety of devices that perform gravity-based separations. Clean coal floats to the surface, whereas pyrite and other mineral impurities sink. Additional cleaning may be performed with flotation cells, which separate coal dust from its impurities based on differences in surface properties. Precombustion removal of organic sulfur can be accomplished only by chemical cleaning. So far, coal combustion emissions and chemical cleaning has proved to be too costly, thus flue gas scrubbers are often required to achieve near-complete removal of sulfur pollutants.
The tightening of environmental regulations is likely to continue throughout the world. In the United States, for example, by December 2008, it is anticipated that coal-fired power plants will have to comply with maximum emission levels for mercury. Emissions of mercury and other trace metals, such as selenium, are under increasing scrutiny of coal combustion emissions because of suspected adverse effects on public health.
Coal is sometimes combusted with waste material as a combined waste reduction/electricity production strategy. The disposal of waste from agriculture and forestry (biomass), municipalities, and hospitals becomes costly when landfill space is limited. Some wastes, particularly biomass feedstock, are combustible, but their low energy density (compared with coal) limits their use as an electricity production fuel. Blending coal with these fuels provides an economical method to produce electric power, reduce waste, and decrease coal plant emissions. Most wood wastes, compared to coal, contain less fuel nitrogen and burn at lower temperatures. These characteristics lead to lower NO x formation. In addition, wood contains minimal sulfur ( o 0.1% by weight) and thus reduces the load on scrubbers and decreases scrubber waste biomass.
Numerous electric utilities have demonstrated that 1–8% of woody drying biomass can be blended with coal with no operational problems. Higher blends may also be used, but require burner and feed intake modifications as well as a separate feed system for the waste fuel. Cofiring in fluidized bed boilers may avoid some of these drawbacks, but the economics of co-firing are not yet sufficiently attractive to make it a widespread practice.
Coal use today is no longer evocative of dirty power plants with polluting black smoke billowing from their smokestacks. Many of these plants have been transformed through technology to operate more efficiently and with significantly lower emissions. Some fire coal with other waste materials and others produce both electric power and heat transmission. Cases of plant retrofits and their new performance statistics are documented by various institutions, including the Energy Information Administration (http:// www.eia.doe.gov) and the World Coal Institute (http://www.wci-coal.com). The following examples highlight clean coal use throughout the world: (more…)
At present, in the United States and worldwide, motor vehicles are fueled almost exclusively by petroleum based gasoline (or reformulated gasoline) and diesel fuels. Since the first oil price shock in 1973, efforts have been made to seek alternative fuels to displace gasoline and diesel fuels and achieve energy and environmental benefits. Some of the alternative fuels that have been researched and used are liquefied petroleum gas (LPG), compressed natural gas (CNG), liquefied natural gas (LNG), methanol (MeOH), dimethyl ether (DME), Fischer– Tropsch diesel (FTD), hydrogen (H 2 ), ethanol (EtOH), biodiesel, and electricity. Production processes associated with gasoline, diesel, and each of these alternative fuels differ. (more…)
Agricultural and forestry residues provide the largest proportion of biomass used for the production of biomas bioenergy. Some estimates suggest that globally available biomass role in the form of recoverable residues represents about 40 Ejyr -1, enough to meet 10% of the total present energy use of 406 Ejyr -1 . However, realizing this potential is limited by factors such as ease and cost of recovery and environmental concerns relating to sustainable land use practices. (more…)
Do we have the resources? Rudolf Diesel developed the diesel engine which ran on biodiesel vegetable oil in the late 1800s. At the time, he speculated that his discovery seemed insignificant, but later could prove to be as important as mankind’s uncovering of future energy uses for petroleum and coal tar. Given the U.S.’ rapid expansion into biofuels, it would appear his vision was correct due to the drawbacks of biofuels. But widespread adoption of biomass-based fuels is not a foregone conclusion. Two questions haunt its progress. First, will it truly reduce U.S. reliance on fossil fuels? And second, is there enough farmland to accommodate widespread production, without jeopardizing food supplies? (more…)
Estimation of the future technical potential of biomass as an energy source is dependent on assumptions with respect to land availability and productivity as well as conversion technologies. With the emergence of energy crops as the major source of biomass fuel, land use conflicts, especially in relation to food production, may arise. However, with efficient agricultural practices, plantations and crops could supply a large proportion of energy needs, with residues playing a smaller role without compromising food production or further intensifying agricultural practices. (more…)
The quantification of the actual reduction in green house gases sourcess emissions resulting from the substitution of fossil fuels with energy from waste biomass requires a complete lifecycle assessment (LCA). A systematic framework for estimating the net Green House Gases emissions from bioenergy systems and comparing them against the fossil fuel reference system that it would replace has been developed. The major considerations of the life cycle assessment approach to quantifying the greenhouse impacts of bioenergy are as follows: (more…)
The gradual change in the energy consumption pattern of the United States from 1860 to 1990. In the mid-1800s, biomass, principally woody biomass, supplied over 90% of U.S. energy and fuel needs, after which energy biomass consumption began to decrease as fossil fuels became the preferred energy resources. For many years, a safe illuminant had been sought as a less expensive substitute for whale oils. (more…)
Over the last decade, the number of countries researching the potential of biomass and bioenergy for energy services rose rapidly. This contributed to the biomass as solid and liquid fuels, an interesting and promising option among renewable energy sources available. This includes solid waste and biomass, which comprises of charcoal, firewood, energy crops and forestry and agricultural residues to produce heat and electricity, (more…)
During the US Presidential Election 2008 Campaign, the renewable portfolio standard was one of the main agendas for Presidential candidate, Mr. Barrack Obama. He suggested that in order to have an environment fit for living, one-fourth of the energy used in United States must come from renewable energy resources, such as wind, water, solar and so on. (more…)
