Each category of pyrolysis process is characterised by a contrasting balance between
biochar, bio-oil and syngas (Table 1). The precise ratio in these products may vary between
plants, and may be optimised at a particular installation; however, it is critical that maximising
the production of biochar relative to mass of initial feedstock (Demirbas, 2006), is always at
the expense of usable energy in the liquid or gaseous form. Although a greenhouse gas
mitigation strategy may favour maximising the biochar product (Gaunt et al., 2008), the
balance that is realised is a function of market and engineering constraints.
In a generalised analysis, the economic cost of maximising the retention of carbon in biochar
using slow pyrolysis has been assessed against the possible net gain in CO2 –equivalent
emissions from using the product in soil, after accounting for the additional fossil-carbon
offset that could be obtained through complete combustion of the feedstock (Gaunt et al.,
2008). The net carbon gain over fossil fuel scenarios was 2–19 t CO2 ha-1 y-1, encompassing
figures 2–5 times higher than those for strategies based on biomass combustion. The eligible
portion of this added saving would have to attract CO2 -offset at a value sufficient to cover
the USD 47 t-1 value of residual energy in biochar. A more detailed description of this
analysis and discussion of the competing processes associated with energy and char
production is discussed in detail in Chapter 3.
Each category of pyrolysis process is characterised by a contrasting balance between
biochar, bio-oil and syngas (Table 1). The precise ratio in these products may vary between
plants, and may be optimised at a particular installation; however, it is critical that maximising
the production of biochar relative to mass of initial feedstock (Demirbas, 2006), is always at
the expense of usable energy in the liquid or gaseous form. Although a greenhouse gas
mitigation strategy may favour maximising the biochar product (Gaunt et al., 2008), the
balance that is realised is a function of market and engineering constraints.
In a generalised analysis, the economic cost of maximising the retention of carbon in biochar
using slow pyrolysis has been assessed against the possible net gain in CO2 –equivalent
emissions from using the product in soil, after accounting for the additional fossil-carbon
offset that could be obtained through complete combustion of the feedstock (Gaunt et al.,
2008). The net carbon gain over fossil fuel scenarios was 2–19 t CO2 ha-1 y-1, encompassing
figures 2–5 times higher than those for strategies based on biomass combustion. The eligible
portion of this added saving would have to attract CO2 -offset at a value sufficient to cover
the USD 47 t-1 value of residual energy in biochar. A more detailed description of this
analysis and discussion of the competing processes associated with energy and char
production is discussed in detail in Chapter 3.
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