Emissions from the Co-Generation of Biochar and Bioenergy with Agricultural By-Products

Abstract

The utilization of agricultural by-products for the co-production of biochar and bioenergy offers a viable way to sequester carbon effectively with added agricultural benefits. The operating pyrolysis temperature (or ‘volatile production temperature’), however, strongly influences the balance between biochar production (yield and quality) and energy production (composition and higher heating value (HHV) of the volatiles). In this thesis, the term ‘raw pyrolysis volatiles’ is used to refer to the mixture of pyrogas and bio-oil produced during pyrolysis. The composition (moisture, ash, volatile matter (VM), sulfur, and nitrogen contents) of the biomass can also influence the yield of pollutants. Using a laboratory-scale combined pyrolysis and combustion (pyrolysis-combustion) process, raw pyrolysis volatiles were produced at varying temperatures (400 – 800 °C) from agricultural by-products and combusted at 850 °C. The particulate matter (PM10 and PM2.1), gaseous (H2S, SO2, and NOx), and PM-bound polycyclic aromatic hydrocarbon (PAH) emissions were evaluated. These pollutants are responsible for severe short and long-term harmful health impacts and are therefore subject to strict environmental legislation. Utilizing rice husk in the pyrolysis-combustion process, it was found that the highest yields of both PM10 and PM2.1 occurred at lower volatile production temperatures (400 – 600 °C). This was attributed to the increased contribution of biooil to the raw pyrolysis volatiles HHV which resulted in elevated C/H ratios in the volatile mixture. This increased the tendency of the pyrolysis volatiles to soot during combustion. Linear dependence was observed between PM emissions and the biooil fraction in the raw pyrolysis volatiles. Combustion of the raw pyrolysis volatiles produced at elevated volatile production temperatures resulted in significantly increased PM-bound PAH concentrations. This was primarily due to the elevated PAH and oxy-aromatic content of the bio-oil fraction at higher temperatures. Elevated temperatures also resulted in increased average molecular weights of the PM-bound PAHs. Increased PM toxicity was observed at higher volatile production temperatures due to the elevated concentration of 4, 5, and 6 ring PAHs. It was demonstrated that increased volatilization of the fuel-bound sulfur and nitrogen contents during pyrolysis at elevated volatile production temperatures increased the energy-based yields of SO2, H2S, NO, and NO2. Utilization of grape pruning demonstrated that elevated biomass VM content resulted in increased energy-based yields of PM. The majority of the increase resided in the sub-micron size fraction due to the increased pyrogas fraction in the raw pyrolysis volatiles. The PM emissions were found to be independent of the feedstock ash content due to its retainment in the biochar. It was also found that utilization of the as received (AR) rice husk resulted in greater energy-based yields of PM10 (1.2 times at 400 °C and 1.6 times at 800 °C). The PM-bound PAH concentration was observed to be 2.1 and 2.8 times higher for the AR rice husk at 400 and 800 °C, respectively. Nevertheless, the majority of the PM-bound PAH species generated from the AR rice husk consisted of 2 and 3 ring PAHs (naphthalene, acenapthylene, and acenaphthene) with relatively low toxicity. This resulted in the toxicity of the PM generated from the AR rice husk being lower than the dried rice husk. This thesis demonstrated that operation of a pyrolysis-combustion process for the optimized generation of either biochar or bioenergy, at lower and elevated volatile production temperatures, respectively, resulted in significant differences in the emissions of harmful pollutants. It is hoped that the outcomes of this work will provide guidance for facilitating effective evidence based policies for agricultural byproduct utilization with beneficial environmental outcomes. It is clear, however, that despite the potential for high ash content agricultural by-products to be utilized in pyrolysis-combustion systems, doubts surrounding the current market demand for biochar need to be addressed before the large-scale co-generation of biochar and bioenergy can become a reality.