Effects of fuel properties and operating conditions on combustion of peanut and tamarind shells in a conical fluidized-bed combustor using an alternative bed material

This paper presents a comprehensive study on combustion of peanut and tamarind shells in the conical fluidized-bed combustor using alumina sand as the bed material. Morphology and surface texture of these lignocellulosic biomasses were investigated. Thinner cell walls, greater cellulose content and porosity of peanut shells indicated their higher reactivity compared to tamarind shells. Hydrodynamic regimes and characteristics of conical alumina–biomass beds were compared between the fuels for variable static bed height and biomass percentage in the binary mixture at cold-state conditions. With selected particle size (300–500 μm) and bed height (30 cm), alumina sand ensured stable bubbling/turbulent fluidization regime of the conical bed at operating variables specified for burning these shells. Thermogravimetric characteristics were investigated to compare thermal and combustion reactivity of the biomasses. The ignition, peak and burnout temperatures were determined for both fuels in this thermogravimetric study. Experimental tests were performed on the combustor at 60 kg/h and 45 kg/h fuel feed rates, while ranging excess air from 20% to 80% at fixed load. In each test run, temperature and gas concentrations (O2, CO, CxHy as CH4, and NO) were measured along the axial direction in the reactor as well as at stack. The axial temperature and gas concentration profiles in the combustor were found to exhibit sensible effects of fuel properties and operating conditions on combustion and emission performance of the reactor. At the maximum load, excess air of 40% seems to be optimal when burning peanut shells, whereas 60% excess air is more appropriate for tamarind shells. High (~99%) combustion efficiency is achievable when firing these fuels at the maximum fuel feeding and optimal excess air, while CO, CxHy and NO emissions from the combustor can be controlled at acceptable levels. No evidence of bed agglomeration was found in the reactor during the tests for burning peanut/tamarind shells when using alumina sand as the bed material.