Emission characteristics of a pyrolysis-combustion system for the co-production of biochar and bioenergy from agricultural wastes

The co-production of biochar and bioenergy using pyrolysis-combustion processes can potentially minimize the emission problems associated with conventional methods of agricultural by-product disposal. This approach also provides significant added-value potential through biochar application to soil. Despite these advantages, variations in biomass composition, including sulfur, nitrogen, ash, and volatile matter (VM) content, may significantly influence both the biochar quality and the emissions of harmful particulate matter (PM) and gaseous pollutants (SO2, H2S, NO2, NO). Using a laboratory-scale continuous pyrolysis-combustion facility, the influence of biomass composition (rice husk and grape pruning) and volatile production (pyrolysis) temperature (400-800 °C) on the biochar properties and emissions during combustion of the raw pyrolysis volatiles were evaluated. Utilization of grape pruning resulted in higher energy-based yields of PM10 than the rice husk, the majority of which consisted of the PM1.1 fraction due to the elevated pyrogas content of the volatiles. The PM emissions were found to be independent of the feedstock ash content due to its retainment in the biochar. Greater volatilization of biomass sulfur and nitrogen during pyrolysis at higher temperatures resulted in higher yields of sulfurous and nitrogenous gaseous pollutants. The energy-based yields of NO and NO2 were found to increase by 16% and 50% for rice husk and 21% and 189% for grape pruning respectively between 400 and 800 °C. The same trend was also observed for the emissions of H2S and SO2 for both feedstocks.