Laserfiche WebLink
<br />The energy fuels that involve a refining process <br />distinct from both extraction and subsequent conver- <br />sion or consumption are nuclear fuels and oil <br />(including shale oil and synthetic oil from coal), and <br />are described in the following section. <br />Water demands in the nuclear fuel cycle have been <br />calculated by the Atomic Energy Commission (1972) <br />on the basis of annual requirements of a typical <br />I,OOO-mw light-warer reactor steam-electric plant <br />operating 80 percent of the time. Of a total consump- <br />tion of 163 million gallons (617 thousand m'), 65 <br />million gallons (246 thousand m'), or about 40 per- <br />cent, is assigned to the uranium-ore milling stage, <br />almost entirely as evaporation from tailings ponds. The <br />remaining consumption of water occurs mainly in <br />evaporative cooling in the uranium enrichment plant, <br />which is normalized to 90 million gallons (341 thou- <br />sand m') annually for a I,OOO-mw plant. The <br />remaining 8 million gallons (30 thousand m') is <br />assigned in about equal proportions to the production <br />of uranium hexafluoride and reprocessing of used fuel <br />elements. Not included in the above water consump- <br />tion caJcu)ations is water consumed at power plants <br />supplying electricity for the enrichment process. This <br />annual power requirement is estimated at 310,000 <br />mwhr (megawatt hours) that, if produced in a fossil- <br />fuel plant, would indicate an evaporative requirement <br />of roughly 160 million gallons (604 thousand m'). To <br />keep this demand in proper perspective, it should be <br />remembered that the electrical power produced by the <br />model I,OOO-mw nuclear station annually (at 80 per. <br />cent load factor) amounts to about 22 times the <br />energy consumed to produce an annual fuel require- <br />ment for a 1,000.mw station (U.S. Atomic Energy <br />Commission, 1972, p. D5). <br />Water demand for petroleum refining is highly varia- <br />ble, depending upon such factors as process employed, <br />refinery design, and cost and availability of water. <br />A sampling of refineries producing 30 percent of the <br />petroleum products in the United States in 1955 (OilS, <br />1963, p. 299), indicated an average withdrawal demand <br />of 468 gallons (1.76 m') of water per barrel (42 <br />gallons or 0.159 m') of crude-oil input. Some 90 <br />percent of this water was used in cooling processes at <br />various stages of refining. A more meaningful measure <br />of water demand, however, is the consumptive use, <br />which averaged 39 gallons (0.14 m') of water per <br />barrel of crude-oil input, or roughly I volume of <br />water consumed per 1 volume of crude. Of this <br />consumption, 7J percent was accounted for in <br />evaporative cooling, 26 percent as boiler feed water, <br />and the remaining 3 percent for sanitary and other <br />in-plant uses. <br /> <br />CONVERSION <br /> <br />Conversion embraces the concept of changing an <br />energy raw material into a more usable form of energy. <br />Examples include burning of coal, gas, or oil to produce <br />electricity, or converting energy of nuclear fission to <br />electricity. Other examples include changing coal or oil <br />into gas, a cleaner, more convenient fuel for space heat- <br />ing, or even changing coal into a form of oil for further <br />refining. Much of the present emphasis on conversion <br />seeks to use fuels abundant in the United States, such as <br />coal and oil shale, to meet the present energy crisis with- <br />out sacrificing air quality objectives. Gene-rally. this <br />involves processing near the site of extraction to produce <br />a nonpolluting fuel which can be transported to a distant <br />market. Alternatively, the coal can be used near the <br />mine to produce electricity for transport to market. <br />The processes of particular interest in the present <br />energy shortage are coal gasification, coal liquefaction, <br />oil-shale retOTting, use of geothermal energy for electric <br />generation, and increased use of coal-burning plants and <br />nuclear reactors for power generation. fn each mode <br />considerable flexibility is possible in plant design, proc- <br />ess employed, and location of processing facilities with <br />respect to site of extraction, source and use of water, <br />and location of market. It is impractical, if not impossi. <br />ble, to assign rigid values of water use per unit of energy <br />produced to all processes because of economic trade- <br />offs, but ranges of water demand are useful for planning <br />purposes. Moreover, in electric-power generation the <br />need for high fuel efficiency generally dictates water <br />demand within close limits; accordingly, water demand <br />for electric generation can be estimated reasonably well. <br /> <br />STEAM-ELECTRIC GENERATION <br /> <br />The most efficient method of meeting large steady <br />electric demand (base load) is by use of a steam turbine <br />to drive a generator (Ilg. I). The steam may be produced <br />from geothermal wells, by burning coal, oil, or gas, or by <br />heat given off by nuclear fission, The power output of a <br />steam turbine is greatly increased by reducing the pres- <br />sure on the outlet side of rhe turbine_ This is done by <br />use of a condenser, which lowers the temperature of the <br />exhaust steam, causing condensation and thus signifi- <br />cantly reducing the pressure. The cooling capacity <br />needed for the condensation phase accounts for the <br />greatest consumption of water in the entire energy. <br />production process. <br />Various systems are used for condenser cooling- <br />once-through circulation. cooling ponds, sprayers, wet <br />cooling towers, dry cooling towers, and combinations of <br />the preceding systems. Once-through cooling commonly <br /> <br />3 <br /> <br />0459 <br />