Characteristics of flue gas emissions during large wood pellet combustion

Abstract: Due to the influence of the mass and heat transfer, some obvious differences in flue gas emissions may occur during the combustion of wood powders and pellet. However, many studies report on the characteristics of flue gas emissions during wood powder combustion, and researches on the single wood pellet combustion and its flue gas emission are limited. The aim of this work was to obtain dynamic data on the release of CO, NO, and SO2 during single large wood pellet combustion by performing experiments in a tube furnace under different temperatures (800, 900, 1 000, 1 100, and 1 200℃) and air flow rates (3, 4, and 5 L/min). The results from the experiment can serve as an important reference for efficient and clean combustion of biomass. The total emissions and their conversions from biomass combustion can be quantified with these data. The experimental results showed that CO emission curve present a double-peak shape at 800℃, mainly because of its slow devolatilization and combustion in low temperature. With increasing temperature, CO emission time decreased, indicating that it needed shorter time during devolatilization and char burnout. Because of sufficient burning, the peak CO concentrations and total CO emissions almost decreased as the temperature grown in 4, 5 L/min air flow rate. NO was released to the gas phase in significant amounts at 900℃, while decreased with increasing temperature or reducing air flow rate. A part of NO was transformed to N2 with the reductive gas of CO as the temperature increased. At temperatures of 1 100 and 1 200℃, NO forming through HCN and NH3 declined because of inadequate oxygen supply in 3 L/min air flow rate. The change tendencies of NO emissions in 4 L/min air flow rate were similar to the change tendencies in 5 L/min air flow rate, probably associated with CO emissions in 4, 5 L/min air flow rate. Fuel-N conversion rate reached the minimum and maximum values (12.32%, 41.79%) in 3 L/min air flow rate under the combustion temperature of 1 200℃ and 5 L/min air flow rate under the combustion temperature of 900℃,respectively. During wood pellet sufficient burning, no SO2 was released to the gas phase, since sulfur mainly converted to sulfate stored in ashes or discharged with the flue gas at high temperature. Above 1 100℃, SO2 emission was observed presumably by organic sulfur oxidation or sulfate dissociation in 3 L/min air flow rate, mainly because of the low conversion of SO2 transformed to sulfates under the condition of lean oxygen. Calculations indicated that 33.78% and 25.99% of the fuel-S was released to the gas phase as SO2 at 1 100 and 1 200 ℃,respectively. On the contrary, most sulfur in the wood pellet discharged with the forms of H2S, CaS, etc.