The co-combustion of pulverized-biomass in a natural gas-fired large-scale laboratory furnace is investigated experimentally. The biomass fuels used included pine sawdust, pine shells, pine branches, and olive stones. For comparison purposes, the co-combustion tests have also included the study of natural gas/pulverized-coal flames. For each solid-fuel, flue-gas data were obtained for pollutant emissions and particle burnout as a function of the solid-fuel cofiring ratio. Ratios up to 20% (energy basis) were used in the experiments. Subsequently, in order to improve the understanding of the processes that occur during the co-combustion of natural gas and solid fuels, the study focused on detailed in-flame measurements of major gas species, including NO x , and gas temperatures for three representative furnaceoperating conditions. The results show that (1) cofiring of biomass fuels, compared with pure natural gas firing, has no significant effect on NO x emissions provided that the fuel-bound nitrogen is low because of the positive impact of the reduced in-flame temperatures of the cofiring in the thermal NO formation that serves to attenuate, to some extent, the fuel NO formation associated with biomass fuels; (2) cofiring of biomass fuels, compared with pure natural gas firing, may have an important impact on CO and hydrocarbon emissions if particle sizes are too large because this extends the volatilization process downstream into regions with relatively low temperatures; and (3) cofiring of biomass fuels, compared with cofiring of coal, allows for much larger particle sizes without a deleterious effect on particle burnout performance due to its higher content of volatile matter.

The aim of the present work was to evaluate the effectiveness of the reburning process using biomass (rice husk) as reburn fuel in a large-scale laboratory furnace. For comparison purposes, tests were also conducted using natural gas and ethylene as reburn fuels. The paper reports new data for flue-gas emissions for a wide range of experimental conditions which quantify the effects of the reburn fuel fraction (energy basis), residence time in the reburning zone, and initial NO _x concentration for the three secondary fuels on NO _x reduction from the present combustor. The results show that at reburn zone residence times of about 0.7 s the reburning performance of the rice husk (1) is comparable to that of the natural gas reburning at high reburn fuel fractions, with almost 60% NO _x reduction achievable at reburn fuel fractions of 25 and 30%, and (2) approaches those of the natural gas and ethylene at high initial NO _x concentrations, with nearly 60% NO _x reduction attainable at initial NO _x concentrations between around 500 and 970 ppm. The results also reveal that there is a correlation between the extent of NO _x reduction and particle burnout: the higher the reduction, the lower the burnout. However, the present results combined with previous data from the same furnace using coal as reburn fuel suggest that the use of rice husk as reburn fuel in full-scale equipment would have a lower impact in the carbon burnout as compared with coal.