Life cycle perspective of bio-oil and biochar production from hardwood biomass; what is the optimum mix and what to do with it?

Abstract

Many studies identified the optimum temperature to maximise bio-oil/biochar yield using fast pyrolysis from woody biomass. However, the optimum mix of biochar and bio-oil production and their final utilisation to achieve optimal environmental and economic benefits are yet to be investigated. Hence, the aim of this study was to identify the optimum mix and utilisation using life cycle assessment and costing approach. Two utilisation scenarios were analysed: Scenario 1 considered terrestrial carbon sequestration through spreading biochar in corn fields while Scenario 2 assumed the co-combustion of biochar to displace coal in a coal-fired power station. In both scenarios, bio-oil was assumed to substitute heavy fuel oil in an industrial boiler. The functional unit used in the study was 1 Mg of green thinned logs from hardwood plantations. Scenario 1 showed outstanding greenhouse gas emissions offset (1680 kg-CO2-eq per function unit). However, this scenario lagged behind when considering other environmental impacts. Scenario 2 delivered more modest greenhouse gas offset, but it had better overall environmental and economic performance. The results indicated that the overall environmental performance of Scenario 2 decreased with increasing pyrolysis temperature due to the decline in biochar yield as well as the increased energy consumption during the pyrolysis process. Meanwhile, lifecycle cost reduced when the pyrolysis temperature increased because of the increased bio-oil production, which has higher economic value than biochar. Assuming equal weights for the environmental and economic functions, the optimal performance of Scenario 2 is likely to be achieved when the pyrolysis process is run at 500 °C with the bio-oil and biochar yields being 64% and 22%, respectively. Monte Carlo Analysis revealed that for balanced environmental and economic weights (30%–70%) the solution is robust. The Monte Carlo Analysis results suggested that under the optimal conditions, there is 97.5% likelihood that Scenario 2 would achieve 1050 kg-CO2-eq greenhouse gas emissions offset and realise $25.8 life cycle cost savings per functional unit. This study demonstrates an easy method to incorporate life cycle assessment and costing into an optimisation decision making framework. Thus, allowing a more comprehensive assessment of policies incorporating multiple environmental impacts (criteria). It contributes to the debate about the economic and environmental validity of using forestry by-products, as well as other low cost woody biomass as a renewable energy source.