The influence of fluidized bed co-combustion of cassava rhizome and eucalyptus bark on the combustor performance and time-related physiochemical changes of the bed material

Abstract

Pelletized cassava rhizome (base fuel with elevated fuel-N and potassium contents) and high-moisture eucalyptus bark (secondary/reburn fuel) were co-fired in a 200 kW fluidized-bed combustor with a silica sand bed, using fuel staging and reburning techniques to mitigate NO emission. Detailed analyses of (co-)combustion and emission characteristics of the combustor were performed for variable operating parameters: 0–0.25 energy fraction of eucalyptus bark, 20–80% excess air (for each fuel option), and 0.1–0.4 secondary-to-total air ratio (in reburning tests at fixed excess air). High CO and CxHy in a secondary/reburn zone of the combustor led to enhanced decomposition of NO in this zone for both co-firing techniques. With optimal operating parameters, a noticeable NO emission reduction (25% when using fuel staging and 50% for reburning), at some increase in the CO and CxHy emissions, compared to burning the base fuel alone, can be achieved at high (∼99%) combustion efficiency. However, a small proportion of silica sand was subjected to agglomeration in both techniques within ∼8 h. Two alternative bed materials (alumina sand and alumina/silica sand mixture) were therefore employed to inhibit bed agglomeration during long-term co-firing tests for reburning under optimal operating conditions. No evidences of bed agglomeration were found in these 30 h reburning tests with the alternative bed materials. However, the bed materials showed the time-domain changes in their physiochemical characteristics, as found with SEM−EDS and XRF techniques. A carryover of fine Al-rich particles from the beds with alumina sand and alumina/silica sand mixture gradually diminished bed resistance to agglomeration.