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
