Axial Concentration Profiles and NO Flue Gas in a Pilot-Scale Bubbling Fluidized Bed Coal Combustor

Abstract

Atmospheric bubbling fluidized bed coal combustion of a bituminous coal and anthracite with particle diameters in the range 500-4000 µm was investigated in a pilot-plant facility. The experiments were conducted at steady-state conditions using three excess air levels (10, 25, and 50%) and bed temperatures in the 750-900°C range. Combustion air was staged, with primary air accounting for 100, 80, and 60% of total combustion air. For both types of coal, high NO concentrations were found inside the bed. In general, the NO concentration decreased monotonically along the freeboard and toward the exit flue; however, during combustion with high air staging and low to moderate excess air, a significant additional NO formation occurred near the secondary air injection point. The results show that the bed temperature increase does not affect the NO flue gas concentration significantly. There is a positive correlation between excess air and the NO flue gas concentration. The air staging operation is very effective in lowering the NO flue gas, but there is a limit for the first stage stoichiometry below which the NO flue gas starts rising again. This effect could be related with the coal rank.

Progress of circulating fluidized bed combustion technology in China: a review

“…Moreover, the mass transfer resistance increases between the bubble phase and fuel particle surfaces [60], which leads to an oxygen-lean condition for the fuel combustion. The intensified reducing atmosphere will restrain the transformation of char-nitrogen and volatile nitrogen of coarse fuel particle into NO x [9,61]. As shown in Fig.…”

Experimental study on combustion characteristics and NOX emissions of pulverized anthracite preheated by circulating fluidized bed

“…The research results of Tarelho [17] and Li [18] indicated that, under the air-staged combustion condition, the decrease in α r normally led to the reduction of NO X emissions. However, when α r is too small, HCN, NH 3 and char-N in the reducing zone will increase quickly and will be oxidized in the burn out zone.…”

Predictions of NOx emissions in pulverized coal combustion.

“…and is for that conducive to NO x destruction and therefore produce less NO x emissions until the furnace end [154,162,176]. The major problem regarding the residence time is that the temperature profiles inside a furnace cannot be calculated.…”

“…which then are being oxidized into NO x in the downstream process when additional oxygen becomes available [176]. When the burner stoichiometric ratio is too high, oxygen is relative sufficient present in the primary combustion zone, and this weakens the reducing atmosphere and leads to form more …”

“…The existence of appropriate concentration of oxygen within the reduction zone is beneficial to NO x reduction in deep or middle airstaged combustion [185], because of the above mentioned relationship of too low burner air ratios which causes a bigger amount of reducing species (HCN, NH 3 , etc.) which enter the burnout zone and then being oxidized into NO x [176]. Concerning to the combustion behavior of different coals means that, that a high volatile coal need more partial available oxygen as low volatile coals.…”