Coal is the most abundant fossil fuel and also regarded, as the dirtiest not only is coal inherently impure by composition by the fact that it contains ash and sulfur; coal is also difficult to burn completely. Many techniques to combust coal have been developed since the late 1800's. "The first innovation were more concerned with achieving more complete combustion and reducing manual labor than they were with pollution and economic" (Ruzie 1)
The history of coal can be dated back around 400 million years ago.
"When coal was formed, the earth was covered with swamps. Plants died and the remains settled to the bottom of the swampy area creating layers of soggy dense material called peat. Sandstone and other sedimentary rocks formed creating pressure that squeezed water from peat. As more layers formed the weight and pressure turned the peat into coal" (Hertz 1) There are many types of coal, including anthracite, bituminous, sub-bituminous, and lignite. Anthracite has the highest carbon content, anywhere from 86 to 98 percent.
And produces nearly 15,000 Btu's per pound (British Thermal Unit, is the amount of heat needed to raise the temperature of the pound of water one degree Fahrenheit) (Bartow 1) The second type of coal is Bituminous or soft coal. It's the most plentiful type of coal in the States, and is mainly found in the eastern and middle part of the North American continent. Bituminous coal is primary used to generate electricity, and has a carbon content of 45 to 86 percent and a heat value of 10,500 to 15,000 Btu's.
Sub-bituminous coal rank just below bituminous coal with 35 to 45 percent carbon content. The heat value of Sub-bituminous coal is between 8,300 and 13,000 Btu's per pound. It can be found in the western parts of the states. Lignite the lowest carbon content then the others "it usually has a brownish-black in color and often shows a distinct woody structure"(Encarta 97) The heat value of lignite ranges between 4,000 and 8,300 Btu's. As with Bituminous, Lignite is also used to generate electricity (Bartow 1).
What are ? is a new way that allows coal to be burned more efficiently, with reduced emissions of sulfur and nitrogen oxides, while providing distinct advantages over conventional coal utilization systems. In new coal-fired plants, can be used to reduce emissions to acceptable environmental limits with the newer technologies providing greater generating efficiencies while reducing carbon dioxide emissions.
The conventional procedure for burning coal is to use pulverized fuel combustors, using the steam raised to drive turbogenerators. The combustion byproducts are sulfur dioxide (SO2), nitrogen oxides (NOx) and particulates, which if not controlled contribute to the production of acid rain as well as the greenhouse effect. can be grouped in four categories, the first three of which refer to the stages in the combustion process where installation can occur (the fourth bypassing the combustion process altogether). The first stage is Precombustion help improve coal condition by removing ash, sulfur and other form of contaminants. Precombustion also know as coal cleaning (physical, chemical, and biological) have great possibilities for decreasing the cost of sulfur emission reduction when also used with other cost effective technologies whether it's advanced combustion or postcombustion. The second stage is Advance technologies can be use on top of your present power plants through modifying equipments to include improvements in design and use repowering. Advanced combustion processes are the most efficient coal technologies available for new power plant construction. The third stage is Postcombustion. Because some air-pollution-control device that uses a spray of water to trap pollutants and to cool emissions are incapable of removing Nitrogen, postcombustion technologies are being developed for retrofit applications. Unlike combustion technologies, postcombustion technologies target existing power plants exceeding emissions restrictions. Designed to remove sulfur and nitrogen after the coal has been burned, they are typically installed on the stacks of power plants In most cases, these retrofit systems offer short-term, cost effective, remedies for severe environmental problems. ("Clean Coal" 1) The forth stage Conversion technologies. For more than half a century, scientists have tried a variety of approaches to convert coal to synthetic oil. Only in the last decade have such dramatic technological improvements been made.
The Department of Energy's Office of Fossil Energy is pursuing two major efforts to bring down the costs of liquids from coal. One method is Indirect Coal Liquefaction. Indirect liquefaction converts coal into liquid fuels and chemical in two steps. The first step is a gaseous state using a coal gasifies with the presents of oxygen and hydrogen (synthesis gas). In the second step, the synthesis gas after being clean of debris, the gas is converted into a variety of products, which includes hydrocarbon fuels, oxygenated compound and premium chemical. The Department of Energy says "Oxygenated compounds and hydrocarbon fuels are both environmental friendly and it complies with the future air quality standard mandated by the Clean Air Act Amendments of 1990" ("Indirect Coal" 1) Indirect liquefaction has several possible routes into the marketplace, today. Many plants can now produce synthetic liquids of substitution for crude oil. Indirect liquefaction in a few years will become important in power plants generating electricity. "With Indirect liquefaction you can store energy from plants during off-peak periods when demands for electricity is low where it can be converted to methanol and either stored for later use as a turbine fuel or sold commercially"("Indirect Coal" 2).
The other method Direct Coal Liquefaction converts coal directly to distillate liquids in a single step. "As a result, the process has a relatively high thermal efficiency of between 60 and 70 percent, and a high product yield. Direct liquefaction breaks down the large, complex molecules of coal and converts them to liquid fuels in a single process. Then Hydrogen is added to the coal during the conversion process to improve the liquid products. Making them similar compared to petroleum". ("Direct Coal" 1) Also in the early 1980s, researchers in the Federal coal liquefaction program found that by separating the coal-to-liquid process into multiple stages, conversion efficiencies could be increased. Higher efficiencies meant a greater yield of liquids, and in turn, that meant better economics. Moreover, the quality of the liquids also could be improved during the liquefaction process, reducing the need for expensive upgrading before or during the refining step. ("Direct Coal" 1)
Department of Energy states researchers today are improving the "two-stage" liquefaction process. New techniques for preparing the coal prior to its conversion show promise in boosting its reactivity, making the coal-to-liquid process even more efficient. Improvements are being made in ways to remove oxygen and minerals in coal that can create unwanted chemical reactions and impede the coal-to-liquid process. ("Direct Coal" 2)
Other technologies also offer the potential for creating alternative fuels from coal.
Finely ground coal can be slurried with water or various alcohols to form a combustible fuel with many of the same combustion characteristics as oil. The Office of Fossil Energy is carrying out an extensive series of analyses and combustion tests to determine fuel specifications for these slurries. This research is showing which fuels are satisfactory substitutes for oil, and which have to be reformulated to improve their handling and combustion characteristics.
Researchers in the Federal program are also examining alternative solid forms of coal. For example, coal can be crushed and cleaned, then reconstituted in powders, granules or pellets. In this form, the coal can be more easily handled, transported and used. Cleaning potential pollutants from coal to meet new Clean Air Act standards remains a major objective. Advanced physical, chemical and possibly biological cleaning could offer utilities new environmental control options.
The Office of Fossil Energy's advanced research program is studying future concepts that could offer further breakthroughs. It may be possible to use biological processes to convert coal to liquid fuels. Already, researchers have isolated a fungus that feeds on lignite and other coals and produces a liquid, oil-like compound. Sulfur-tolerant bacterial are also being developed to reduce the cost of sulfur removal from coal gases before they enter the liquefaction process. Other studies are examining an integrated two-stage synthesis gas fermentation process for producing a mixture of alcohols from coal.
One route toward a better understanding involves the study of biological organisms that convert methane to methanol in nature. These studies could reveal new insights into ways to create improved catalysts and processes that mimic natural processes but without having to resort to exotic and expensive materials and processes that add to costs.
Proper pretreatment of coal can improve the efficiency of coal-to-liquid processes. And coal cleaning can remove impurities in the coal that can form air toxic emissions.