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<br />""... <br /><:n <br />N <br />W <br /> <br />In plants taking all the water as steam, all of the water must be treated <br />by ion exchange or reverse osmosis. The quantity required is usually more <br />3 <br />than 790 X 10 Ib/hr because excess steam is added to the gasifier to control <br />the temperature. This is most noticeable in a low temperature gasifier such <br />as the Lurgi process. In the standard Lurgi gasifiers, 45 to 50 percent of <br />the steam is decomposed. More steam is decomposed in the shift reactor, but <br />not all. When the Lurgi gasifier is run in a slagging mode (the ash melts) , <br />4 <br />all the steam added to the gasifier decomposes <br />Water in the coal does not enter into reaction. Far many gasification <br />processes the coal is dried, but even when it is not, as for the Lurgi process, <br />a moist coal results only in an increased quantity of wastewater, not in a <br />reduced steam requirement. <br />ReGuirements for boiler feed water in coal qasification plants have been <br />3,4,5 1 . 1 <br />collected on Table 2-1 . In medium-Btu or ow-Btu gas the chem~ca s are <br />carbon monoxide and hydrogen, ra~~er than methane, and as less hydrogen is <br />used, the water requirements <br /> <br />fall at the lower end of <br />5 <br />lb water/10 Btu product <br /> <br />the scale. If details <br />-5 6 <br />(= 3.7 x 10 acre-ft/cO <br /> <br />are lacking, a figure of 100 <br />5 <br />atu = 12 gal/iO Stu) can be used :or estLmating, but it will not necessarily <br /> <br />be accurate. The requirements are large. 9y way of reference a 1000 MWe steam <br />3 <br />electric generating station requires boiler makeup of 45 ~o 80 x 10 lb water/~r <br />(: 145 to 260 acre-ft/yr = 90 to 160 gal/min) . <br />2.3.2 Quantities, Quality and Disposal of Ion Exchance Reqenerant Waste. <br />The volume of wastewater resulting from the preparation of boiler feed <br />wa=er (which is deionized, degassed water) depends on the quality of the <br />source water and on the preparation me~~od used. We have studied three met~cds <br />of producing boiler feed water: (1) co-current regenerated ion exchange, (2) <br />counter-current regenerated ion exchange and (3) reverse osmosis6. In this <br />section we will discuss the two ion exchange procedures. Reverse osmosis will <br />be discussed in the next sub-section. <br />Co-current reqenerated ion exchange is ~'e usual treatment used in this <br />country. Wastewater volumes vary from 8 to 15~ of the product ~ater voluce <br />and the wastewate~ concentration is abou~ 2' total dissolved solids_ <br />Counter-current regeneration of ~~e ion exchange beds means that regener~'t <br />is pumped upwards through ~~e beds necessitating same mechanism to prevent t~e <br />beds from lifting. counter-cur:ent regeneration results in a lower use of <br /> <br />l8 <br />