Energy & Fuels 1993, 7, 273-278
273
Fluidized-Bed Incineration of Refuse-Derived Fuel Pellets S.C. Saxena' and N. S.Rao Department of Chemical Engineering, The University of Illinois at Chicago, P.O.Box 4348, Chicago, Illinois 60680 Received August 19, 1992. Revised Manuscript Received November 24, 1992
Combustion characteristics of refuse-derived fuel (RDF) pellets were investigated in a fluidizedbed incinerator in cocombustion mode with propane gas in an inert bed of fused alumina (about 800 bm average particle diameter). Effects of operating variables such as fluidizing gas velocity, bed temperature, fractional excess air, and fuel pellet feed rate on combustion efficiencywere investigated. The incinerator was characterized by measuring the gas distributor pressure drop, minimum fluidization velocity, bed voidage, and temperature distribution. Fuel pellet combustion efficiency was found to increase linearly with increase in superficial gas velocity at a constant fuel feed rate and incinerator temperature. At a fixed temperature, the influence of feed rate was nominal provided the excess air level was held approximately constant. At a fixed temperature, the emission of carbon monoxide increased as the fuel pellet feed rate was increased a t a constant superficial gas velocity. The concentration of carbon monoxide in the flue gas a t a constant fuel feed rate and incinerator temperature decreased with increase in superficial gas velocity.
Introduction
Thermal destruction of a variety of waste materials is effectively achieved in a fluidized-bed incinerator from the viewpoint of combustion efficiency as well as environmental pollution. This is because such reactors provide good solid-solid and gas-solids contact, good solids mixing, and uniform temperature distribution. Such reactors have been used successfully for over halfa century in petroleum industry, more recently for combustion and gasification of coal, and are currently being evaluated and employed for wastes of diverse nature varying from wood chips to radioactive materials.14 Fluidized-bed incinerators are also used for specialized wastes as industrial sludge waste,7 hazardousspent potliner waste byproduct from aluminum smelters? and solid wastes contaminated with metah9J0 McFee et al." demonstrated successfully the use of fluidized-bed incinerators for the effective destruction of ~~~
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(1) Bailie, R. C.; Ishida, M. Gasification of Solid Waste Materials in Fluidized Beds. AIChE Symp. Ser. 1972,68, No. 122, 73.
(Z)Copeland, G. G. Industrial Waste Disposal by Fluidized Bed Oxidation. AIChE Symp. Ser. 1972, 68, No. 122, 63. (3) Becker, K. P.; Wall, 3.C. Fluid Bed Incineration of Wastes. Chem. Eng. hog. 1976, 72, 61. (4) Ravindram, M.; Kalvinskas,J. J. CoalDesulfurizationin aFluidized Bed Reactor. Enuiron. h o g . 1986,5, 264. (5) Corry, R. G.; Rasmussen, G. P. Design Considerations and Metals Disposition in Fluidized-Bed Incineration of Refinery Wastes. Enuiron. h o g . 1990, 9, 57. (6) Bailie, R. C.; Burton, R. S. Fluid Bed Reactors in Solid Waste Treatment. AIChE Svm. Ser. 1972.68. No. 122,140. (7) Brunner, C. R. industrial Sludge' Waste Incineration. Enuiron. hog. 1989,8, 163. (8) Tabery, R. S.;Dangtran, K. FluidizedBed Combustionof Aluminum Smelting Waste. Enuiron. B o g . 1990, 9, 61. (9) Ho, T. C.; Tan, L.; Chen, C.; Hopper, J. R. Characteristics of Metal Capture During Fluidized Bed Incineration. AIChE Sym. Ser. 1991,87, No. 281, 118. (10) Ho,T. C.; Chen, C.;Hooper, J. R.;Oberacker, D. A. Characteristics
of Metal Capture in Fluidized Bed Incinerators and Waste Heat Boilers. Fluidization VII, Processings of the Seuenth Engineering Foundation Conferenceon Fluidization; Potter, 0.E., Nicklin,D. J.,Eds.; Engineering Foundation: New York, 1992, pp 463-470. (11) McFee, J. N.; Rasmussen, G. P.; Young, C. M. The Design and Demonstration of a Fluidized Bed Incinerator for the Destruction of Hazardous Organic Materials in Soils. J. Hazardous Mater. 1985, 12, 129.
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hazardous organic materials in soils. Cofiring of refuse derived fuel with coal and its combustion above the grate in a fireball created by the combustion of pulverized coal has also been reported.12 We have been examiningthe fluidized-bed incineration and coincineration mode of operation in a pilot-plant facility13originally designed and used for coal combustion. In this context, we14have investigated the characteristics and appropriateness of a specially designed propane ring burner and an Engelhardt-Eclipse tube-fired burner for preheating the combustion air and the use of propane in the incineration process as an auxiliary fuel. Coincineration of wood15and peanut-hull16pellets has been studied to establish the favorable combustion conditions for optimum incineration and emission control. We report the results obtained for refuse-derived fuel (RDF) pellets using propane gas as an auxiliary fuel in an inert alumina bed. Experimental Section The fluidized-bed pilot-plant facility consists of a 1.52 m tall air preheating section which is connected to the main 2.8 m tall calming, test bed and freeboard sections through two elbows arranged in U-shape. The entire assembly was built from 0.254 m diameter, schedule 40, type 304 stainless steel pipe sections and was provided with an internal 50 mm thick Purolite-30 insulating material lining. Before the present series of experiments were initiated, the Purolite-30 lining which showed signs of wear and tear, was repaired by reinforcing it with another refractory material, Descon A60, produced by M/SHarbison(12) Gershman, Brickner and Bratton, Inc., Small-scale Municipal Solid Waste Energy Recouery Systems; Van Nostrand Reinhold Co.: New York, 1986; Chapter 5. (13) Saxena, S. C.; Rao, N. S.; Zhou, S. J. Fluidization Characteristics of Gas-FluidizedBeds at Elevated Temperatures. Energy 1990,15,1001. (14) Saxena, S. C.; Rao, N. S.; Thomas, L. A. Combustion of Propane and Fluidized-Bed Cocombustion. Energy, to be published. (15)Saxena,S.C.;Rao,N. S.;Thomas, L. A. Fluidized-Bedlncineration of Wood Pellets, Fuel, to be published. (16) Saxena, S. C.; Rao, N. S.; Kasi, A. N. Fluidized-Bed Incineration of Peanut-Hull Pellets. Presented at the 12th International Conference on Fluidized Bed Combustion, May 8-13, 1993, San Diego, California.
0887-0624/93/2507-0273$04.00/0 0 1993 American Chemical Society
274 Energy & Fuels, Vol. 7, No. 2, 1993 Table I. Particle Size Distribution of Inert Fused Alumina Bed mass fraction of solids retained av size U.S.sieve no. (um) initial final -14 + 16 1290 0.215 1095 0.198 -16 + 18 0.262 0.405 -18 + 20 925 0.228 0.224 -20 + 30 723 0.061 0.299 -30 + 35 543 0.019 0.024 -35 + 40 463 0.014 0.035 -40 + 45 390 0.004 0.013 -45 + 50 328 av particle dia (pm) 873 677
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Walker Refractories Dressor Inc. This refractory is capable of sustaining temperatures up to 1977 K. Two 18.65-kW compressors supplied the combustion air and the incinerator flue gas was cleaned by two cyclones and a microfilter before it was exited into the atmosphere. The air and the inert alumina bed were heated by an Englehardt-Eclipse tube-fired burner14 using air and propane mixtures. This burner was housed in the preheater section. Another propane ring burner with 27 orifices was installed in the calming section to provide additional thermal energy. The calming section has a perforated plate gas distributor with an open area of 0.84% while the test bed was provided with a multiorificenozzle distributor plate (details are given by Saxena et The engineering design details of the incinerator have been reported by Saxena et al.l8 Six thermocouples and six pressure taps were located on the vertical section of the incinerator to monitor the temperature distribution and pressure drops across different sections. The flue gas was continuously analyzed for concentrations of oxygen, carbon monoxide, and carbon dioxide by Cole-Parmer and Teledyne gas analysers. The output signals from the thermocouple and pressure probes and gas analysis instruments were logged on an automated computerized data acquisition system consisting of a dedicated personal computer, monitor, printer, and a plotter. The inert material used in the bed was fused alumina particles with the size distribution given in Table I. It was supplied by the Chicago Fire Brick Co. The initial average particle size of 873 pm was reduced to 677 Km due to thermal decrepitation during the combustion runs. The California Pellet Mill Co. supplied the RDF pellets used in these experiments. The pellet diameter was about 12.7 mm and their lengthvaried between 1.1 and 2.9 cm. A commercially available Acrison solids feeder after several modificationswas used to feed the RDF pellets into the incinerator at a point 76 mm above the distributor plate. The pellets from the hopper were injected into a vertical downspout, 50.8 mm in internal diameter and about 1m long, by an auger whose rotational speed could be varied over a range to alter the feed rate to the incinerator. The pellets dropped into a horizontal pipe and were pushed into the bed by a specially designed auger driven by a gear motor at a constant speed of 30 rpm. This auger was made from a 50.8 mm diameter stainless steel rod by cutting 12.7 mm deep spiral threads at a pitch of 25.4 mm. A metered amount of air was also introduced at the top end of the vertical downspout to assist in pushing the pellets into the incinerator and also to cool the horizontal pipe and auger assembly. In order to establish the appropriate range for U, values in incineration experiments and to establish the quality of fluidization, it is essential to know the minimum fluidization gas velocity, Umf,for the fused alumina bed. It was determined in
Saxena and Rao Table 11. Ultimate Analysis in Percentage of RDF Solid Pellets. carbon 47.98 hydrogen 6.88 sulfur 0.16 chlorine 0.088 phosphorus 0.071 nitrogen (ppm)
