Biomass pyrolysis processes: performance parameters and their influence on biochar system benefits

This study focuses on performance of biomass pyrolysis processes for use in biochar systems. Objectives are to understand the range of control of such processes and how this affects potential benefits of pyrolysis biochar systems, in particular for climate change mitigation. Slow, intermediate and fast pyrolysis processes are reviewed. Product yield distributions change depending on feedstock composition and preparation, control of temperature and material flows. These allow some control over distribution of main products – char, liquids and gases. Typical yield ranges for pyrolysis processes are defined. Variability associated with char yield is estimated at ±5% (relative). Char yield should be considered an underlying, but minor source of variability in pyrolysis biochar systems. A model study is used to compare effects on greenhouse gas balance and electricity generating capability of slow, intermediate and fast pyrolysis processes with direct combustion; there is a trade-off between these benefits following from the different product yield distributions. High char yields allow greater net CO2 benefits but lower electrical output from slow or intermediate pyrolysis. Higher liquid and/or gas yields allow greater electrical output from fast pyrolysis but less than from direct combustion. Fast pyrolysis and direct combustion have similar net CO2 effects when retained char is low. If the objectives of pyrolysis biochar systems are for climate change mitigation then processes with higher char yields should be preferred. The model is sensitive to the reference case chosen for fossil fuel substitution and to the stability of biochar-carbon in soils – a major uncertainty in the analysis of pyrolysis biochar systems. Financial analysis shows the trend in income value for pyrolysis and combustion processes is opposite to the trend in climate change mitigation benefits. Lifecycle CO2 analysis suggests dominant factors in pyrolysis biochar systems relate to carbon sequestration in biochar and fossil fuels substitution by renewable electricity