Staged combustion of rice straw in a fluidized bed

Abstract

Staged combustion of rice straw has been investigated using an atmospheric bubbling fluidized bed combustor. The combustor has a 300 mm ID and a 3300 mm height. Secondary air was introduced in the freeboard at 1500 mm above the primary air distributor. Rice straw was fed as cylindrical pellets of a 12 mm diameter and 10–15 mm lengths. The obtained results indicate that staged combustion appears an effective technique to reduce NO x emissions, in particular, at higher operating temperatures. Typically, at 850 °C bed temperature, NO x concentration is reduced by about 50% when 30% of fed air is introduced as secondary air. Staged operation has a slight, non-monotonic effect on SO 2 emission. Combustion efficiency improves with increasing secondary air ratio reaching a maximum value that is mainly attributed to a reduction in fixed carbon loss. With further increase in secondary air ratio, combustion efficiency; however, decreases again since entrained fixed carbon and exhausted carbon monoxide tend to increase. The range of secondary air ratio, over which combustion efficiency improves, expands at higher operating temperatures.