Possible off-farm resources include municipal green waste from gardens and parks,
composted or compostable urban waste, digested sewage sludge and mixed municipal
waste. In addition, in the future, by-products of other bioenergy or bio-fuel systems may be
available.
Utilisation of off-farm wastes for biochar production holds the attraction of potential cost
savings from avoiding landfill or other disposal charges. In addition, compared to typical or
existing disposal methods, there may be a lower emission of CH4 and N2O greenhouse
gases than that emanating from direct placement in soil, enhancing the net gain in carbon
equivalents through avoided emissions of high GWP gases. However, many such wastes
have a high water content which will incur increased emissions (and cost) associated with
higher requirement for process energy in pyrolysis.
In a ‘closed loop’ scenario, biochar is incorporated into the same land, or at least the same
enterprise or groups of enterprises, from which the pyrolysis feedstock originates. A typical
scenario would involve utilisation of cereal crop straw that in intensive arable areas is often,
effectively, a waste product. Although there is no published laboratory work to support the
use of biochar produced from wheat straw, there is limited existing information on the relative
stability of biochar from rice husk, sugarcane bagasse and straw from maize. In industrial
agriculture crop straw may constitute 2 t C ha-1. Putting this scenario in the context of the UK
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example, with approximately 4 M ha of cereal crops grown, annual total fossil fuel CO2
emissions amount to 170 M t.
Theoretical comparisons have been made for the carbon-equivalent gain offered by the
pyrolysis of maize straw versus use of a dedicated biomass crop (Gaunt et al., 2008).
Utilising biomass crops for energy on a large scale has a potential impact on the land
available for food production and may exert new pressure on non-agricultural land use.
Biochar produced from the pyrolysis of biomass crops might be incorporated into different
agricultural land from where the biomass feedstock was grown. This could be for greater
agronomic gain, to apply the product in rotation over a wider area to maximise benefits, or to
deal with the cumulative quantity of product. In the combined energy and bio-oil coproduction
case study considered by Ogawa (2006), the biochar by-product was also
returned to adjacent arable land.
Most scenarios considered to date have focused on conventionally managed arable land,
where biochar could be added to soil as part of an existing tillage regime. Biochar could be
incorporated during conversion of land to no-till, but a strategy of integration into no-till and
grazed grassland systems has not yet been considered.