Design and operating parameters of a fluidized bed for the combustion of municipal solid waste using standpipes air distributors

Abstract

Hydrodynamic studies and combustion of simulated and actual municipal solid waste were carried out in a fluidized bed system. A wide range of parameters was investigated in hydrodynamic study after which the optimum parameters were implemented in the combustion studies. A newly fabricated standpipes air distributor (primary air inlet) was designed based on findings of the optimum orifice diameter, orifice distance and distance between pipes. Orifice diameter, orifice distance and distance between pipes of 3 mm, 10 mm and 70 mm were used in the hydrodynamic studies of circular and rectangular columns (CHS and RHS). The operating parameters investigated in the CHS and RHS included the effect of sand sizes and aspect ratios on the fluidization profile. Standpipes air distributors having the same orifice diameter and distance but with a wider pipe distance of 200 mm were used in the hydrodynamic studies of a bigger rectangular (big scale) column. Different air flow strategies were implemented to ensure good mixing between sand and samples and to investigate the penetration of the incombustibles into the sand bed. Parameters studied in the combustion of municipal solid waste included the effect of fluidizing velocity and air factor on the combustion profile in the bed as well as the freeboard region with standpipe air distributor design and dimension established from the hydrodynamic studies of a bigger scale rectangular column. Findings from the CHS and RHS showed that sand particles with mean size of 0.34 mm performed good fluidization profile compared to other coarser sand sizes. The ratio of the bed height over diameter of column (Dc) for good fluidization was determined at H = Dc for the circular column whereas the ratio of the bed height (H) over the length (L) of column was observed at H<L for the rectangular columns. A two side air flow was seen as the best air flow strategy for good mixing in a bigger rectangular column. The range of fluidization number and air factor for the combustion of simulated municipal solid waste in a rectangular fluidized bed combustor was 5 - 7 Umf in which 5 Umf was found to be the optimum with air factor of 0.8 (primary air). Air factor of 0.4 (secondary air) was observed to show good temperature profile in the freeboard region for the combustion of municipal solid waste. The optimum total combined air factor for the combustion of municipal solid waste was 1.2 in which inlet primary air factor and inlet secondary air factor were 0.8 and 0.4, respectively.