Effect of freeboard deflectors in the fixed bed combustion of biomass

Abstract

Deflectors have been used in the freeboard section of industrial combustors to reduce radiant heat loss through flue gases and for particle emissions abatement. Freeboard deflectors can also reduce the draft force of flue gases and affect flow dynamics. There have been no systematic studies to investigate the effects of deflectors on the temperature profiles and emissions in laboratory scale fixed bed biomass combustors.This research includes experiments conducted over lean conditions (λtotal) on fixed bed combustor, with a freeboard deflector located at different axial locations. The aim is to characterize the effects on temperature distribution (near-wall and near-centreline) and gaseous emissions (NO, CO, CO2) over a range of primary and secondary air flow rates. Experimental results indicate that deflectors affect upstream near-wall temperatures, but their impact depends on relative (axial) position (H). The presence of a freeboard deflector however decreases near-wall temperatures in the downstream which may be due to aerodynamic effects. Deflectors do not appear to affect the centreline temperature profiles downstream of the secondary air. Furthermore, results reveal that deflectors do not have significant effects on the fuel consumption rate when expressed via the burning rate (kgm−2s−1). Results also showed that NO, CO and CO2 emissions are also affected by the presence of a deflector in the mid-range of combustion stoichiometry (λprimary=0.439–0.509). However, deflector effects were found to be most prominent for NO and CO emissions by reducing and rising their levels, respectively.