THERMAL-TREATMENT OF HAZARDOUS WASTES - A COMPARISON OF FLUIDIZED-BEDAND ROTARY KILN INCINERATION

Large volumes of sludge are produced by a wide variety of industrial p rocesses and by municipal waste water treatment. Interest in incinerat ing these sludges, either alone or co-fired with other fuels, is incre asing. The issues surrounding sludge incineration in rotary kilns and fluidized beds were identified through a series of pilot-scale tests u sing two slightly different paper mill sludges. The specific issues ex amined include hydrocarbon emissions, NO., emissions, and bottom and f ly ash properties. A 61-cm i.d. x 61-cm long, 130-kW pilot-scale rotar y kiln simulator (RKS) and a 23-cm i.d., 300-kW circulating fluidized bed combustor (CFB) were maintained at a nominal temperature of 1100 K and a stoichiometric ratio of 1.5. The rotary kiln was fed in a batch mode in order to simulate the passage of solids through a kiln. The f luidized bed was fed in both batch and continuous modes. Samples were removed from the kiln (bottom ash) and transition section (fly ash). S amples of the fluidized bed materials were removed from the bed (botto m ash) and after the cyclone (fly ash). The exhaust gases were analyze d continuously for hydrocarbons, CO, O2, NO, and CO2. This paper prese nts data on these analyses as well as NO conversion and ash properties . The production of NO in the RKS was dependent on the supply of nitro gen (in the sludge) and oxygen (in the gas phase) in the reactor. The availability of oxygen to the sludge was affected by the particle diam eter of the sludge, the charge size, and whether a solids bed was pres ent at the time of the incineration. In the CFB, the nitrogen-containi ng compounds were oxidized primarily downstream of the feedboard regio n, resulting in elevated levels of NO in the transition and cyclone re gions. Carbon monoxide concentrations were high immediately above the bed which lead to the reduction of NO inside the freeboard zone. In bo th the CFB and the RKS tests little unburned hydrocarbons were present in the exhaust gas streams. Formation of fly ash particles was depend ent on types of incinerated material (sludge; mixture of sludge and si lica sand). Bottom ash material resembled randomly organized skeletons (or cenospheric skeletons), the structure of which was independent of the type of sludge or reactor. Smaller fly ash and bottom ash particl es were formed during CFB incineration experiments.