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- 16 - <br /> <br />ash. It must be recognized that less total tonnage of <br />screened fluff RDF is made because of grit removal; hence, <br />while screening improves the fuel product, some recoverable <br />energy is sacrificed. <br /> <br />Dust RDF is the most advanced RDF form. Fluff RDF is blended <br />with an embrittling agent (a chemical that hardens cellulose <br />so that paper and cardboard will shatter upon impact) and <br />processed in a heated ball mill until the product will pass a <br />100 mesh screen (0.15 mm). The product is homogeneous and is <br />believed to exhibit the least variability of any RDF. Dust <br />RDF consists primarily of small paper fiber platelets. Con- <br />sequently, it behaves like a powder, can be stored in silos, <br />and can be handled with conventional flour and pulverized coal <br />handling equipment. It is important to recognize, however, <br />that an explosion potential exists when dust RDF, mixed with <br />air and comes into contact with a spark. This is not a major <br />drawback to the system, because explosion-proof kandling eauipi <br />ment and fixtures are routinely employed when handling coal <br />dust and flour. Of particular interest, du~t RDF can be <br />slurred with oil and burned in conventional oil-fired boilers. <br />Dust RDF has a higher heating value of 6,900 Btu/lb. and con- <br />tains 10 percent ash and two percent moisture. It apears to <br />have unlimited shelf life. <br /> <br />Finally, any of the three forms of RDF may be agglomerated or <br />densified to form densified RDF (d-RDF). Coarse and fluff RDF <br />agglomerate well in pelletizers, briquetters, and extruders, <br />but agglomeration of dust RDF requires a lignin binder and pro- <br />cessing in a briquette. <br /> <br />b. Waterwall Furnaces <br /> <br />Municipal refuse is delivered to the facility and deposited on <br />a tipping floor or in the storage pits from which it is <br />transferred to the furnace feed hopper. From the feed hopper, <br />wastes are continuously drawn into the furnace where they are <br />burned and heat is recovered. <br /> <br />Waterwall furnaces are enclosed by closely spaced water-filled <br />tubes. Water circulates through the tubes to recover heat <br />radiated from the burning fuel bed. Integrally constructed <br />(attached) waste heat recovery boilers generate steam while <br />reducing the temperature of the exhaust gases. Heat is trans- <br />ferred by convection from hot gases passing over boiler tubes <br />in the convection section of the boiler. Thus, a marketable <br />product is created while reducing the stack gas volume and, <br />consequently, permitting the use of smaller gas cleaning <br />equipment. <br /> <br />In the combustion process, oxygen is required to burn the fuel <br />and release heat. The combustion process can be improved <br />consiferably by agitating the fuel bed. Agitation results in <br />rapid ignition, effective mixing, and leveling of the fuel <br />bed. These factors combine to assure that all refuse par- <br /> <br /> <br />