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<br />,~ <br />-<:"' <br />t ..~_ <br />...~ <br />'..) of conventional coal-fired steam generating systems. The <br />~_' bed's lower operating temperature and controlled combustion <br />substantially reduce generation of nitrous oxides. The mix <br />is in constant contact with the coiled metal tubes of the <br />heat exchanger, and heat is transferred more efficiently than <br />in present systems. The limestone or dolomite, in addition <br />to transferring heat, absorbs sulfur and can eliminate sulfur <br />dioxide emissions. The resulting sulfurous ash has been recycled <br />successfully as a soil conditioner. <br /> <br />All coal contains sulfur which, when burned, produces sulfur <br />dioxide (SO?) in stack gases. These emissions can be detrimental <br />to health, especially in cases of atmospheric inversions where <br />polluted air is trapped near the ground. It is also thought <br />that such emissions may contribute to production of acid rain. <br />The Clean Air Act specifies SO? emissions must be reduced, <br />and dry flue gas desulfurizati6h is an emerging technology <br />for achieving that goal (Figure 7). The technique would remove <br />S~ by treating stack gases with natural minerals or compounds <br />su~h as nahcolite, trona, or calcium carbonate (substances <br />similar in consistency to baking powder). Two possible approaches <br />have been identified: <br /> <br />1. The substance could be mixed with coal before burning. <br />Then, when SO? is emitted, the nahcolite, trona, or <br />calcium carbonate would catch and hold the SO . <br />2. When coal is burned and gases travel up the s~ack, <br />a slurry of the substance could be sprayed into the <br />gases to capture S02' <br /> <br />It may be possible in some cases to recover the sulfur for <br />further marketing and use. <br /> <br />Another emerging technology involves use of coal refuse as <br />a fuel for powerp1ants. The refuse, obtained after coal cleaning, <br />might be burned using fluidized-bed combustion techniques <br />to provide a more economical energy source while reducing accumu- <br />lations of waste material. (About 25 percent of all mechanically <br />cleaned coal is rejected and available as refuse.) Two factors <br />are creating the incentive for use of coal refuse: (1) rising <br />fuel demands and costs, and (2) the technology of f1uidized- <br />bed combustion which is not sensitive to the characteristics <br />or quality of fuel being burned. As a byproduct of energy <br />that could be produced from refuse, CO2 in stack gases might <br />be captured for use in the AQUATRAIN pTpe1ine system. <br /> <br />Full development of these new technologies will be spurred <br />by the ready availability of saline water and coal and/or <br />coal products, economically transported by AQUATRAIN. <br /> <br />36 <br />