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Biochar reviewed

‘Contemporary interest in biochar is, first and foremost, driven by its potential role as a response to the problem of climate change, through the long-term storage of carbon in soils in a stable form.’ With that preamble in mind, and noting that it could also reduced the use of fertilisers, the Biochar Research Group at Edinburgh University was commissioned by DEFRA to review the potential of biochar. and in particular to look at some major uncertainties surrounding its impacts upon soils and crops, its overall performance and its costs compared to other carbon mitigation options. As the preamble put it ‘whilst biochar might improve productivity, is this effect really understood well enough that we can factor-in a long-term enhancement of the carbon sink in vegetation and soils?’

As I indicated in an earlier blog, there were also uncertainties as to whether it would be as effective in terms of carbon dioxide gas abatement as other ways of using biomass, including use simply as a carbon-neutral fuel, offsetting fossil-fuel emissions. There were also concerns that if it did prove effective, the result might be vast biomass plantation undermining biodiversity and competing with food production.

The final report on Pyrolysis-Biochar Systems (‘PBS’) from the project has now emerged. It says that it provides ‘preliminary evidence that PBS are an efficient way to abate carbon, and tend to out-compete alternative ways of using the same biomass (in terms of carbon abated per tonne of feedstock, or in terms of abatement per hectare of land).’ It suggests that you can get abatement of 1.0–1.4 t CO₂eq per oven dry tonne feedstock used in slow pyrolysis. ‘Expressed in terms of delivered energy PBS abates 1.5-2.0 kg of CO₂eq/ kWh, which compares with average carbon emission factor (CEF) of 0.5 kgCO₂eq/ kWh for the national electricity grid in 2008, and current CEF for many biomass feedstocks of 0.05–0.30 kgCO₂eq/kWh. Expressed in terms of land-use, PBS might abate approximately 7–30 t CO₂eq/ha/yr using dedicated feedstocks compared with typical biofuel abatement of between 1–7 t CO₂eq /ha/yr.’

And so it concludes, provided the Carbon Stability Factor (the proportion of total carbon in freshly produced biochar, that remains fixed as recalcitrant carbon over a defined time period), remains above 0.45, ‘PBS will out-perform direct combustion of biomass at 33% efficiency in terms of carbon abatement, even if there is no beneficial indirect impact of biochar on soil greenhouse-gas (GHG) fluxes, or accumulation of carbon in soil organic matter’. But it says there is also ‘an, in principle, credible case that biochar deployment in UK soil will produce agronomic gains (and possibly suppress GHG emissions)’ so it’s doubly blessed, though, perhaps inevitably, the report says that more research is needed to be sure.

There are also some other caveats (e.g. on costs, which it puts at maybe £42/t CO₂). It says that: ‘Biochar is, currently, an expensive way of abating carbon, although the costs would likely come down with investment’. It notes that: ‘There has been relatively little attention to the logistics of PBS, even though this is likely to be very important to the economic and practical viability. The issues raised include the need for (and cost of) storage, the acceptability of truck movements, and how economies of scale in producing and distributing biochar might be achieved. Biochar is currently expensive to produce due to feedstock, capital and operational costs. Extensive PBS implies an extensive infrastructure, involving pyrolysis units probably at a range of scales that will take some time to be built and operated, especially given the current lack of dominant design.’

Nevertheless it says ‘Biochar could, however, increase quite significantly the opportunities for carbon abatement in the agriculture and land-use sectors. In the UK the availability of land is unlikely to present an absolute barrier to biochar deployment, although the land potentially providing the highest returns from biochar addition (such as horticulture) is relatively small in extent. The supply and cost of biochar also depends upon the extent to which organic waste feedstocks could be utilised. There are some ‘niche’ areas where PBS could have particular advantages over alternative ways of dealing with organic residues, even within current economic conditions.’

The Centre for Alternative Technology came to similar conclusions in its ‘Zero Carbon Britain 2030’ study: see my earlier blog. They saw biochar playing a key role.

Although the Edinburgh study does highlight some potential problems and unknowns (e.g. on cost and how long carbon will stay trapped), it calls for more pilot projects and it does look like biochar, if sensibly managed, could be a winner. However, somewhat oddly, DECCs new ‘2050 Pathways’ report only sees biochar as playing a fairly limited role, as one possible geo-sequestration option – perhaps trapping 1Mt CO₂ p.a. in the UK by 2050.

By contrast a US study, ‘Sustainable biochar to mitigate global climate change’ is very positive. Biochar could it says offset 1.8 bn tonnes of carbon emissions annually, in its most successful scenario – around 12% of current global greenhouse-gas emissions – without endangering food security, habitat or soil conservation.

The DEFRA/Edinburgh Biochar report is at: http://randd.defra.gov.uk/Document.aspx?Document=SP0576_9141_FRP.pdf

Posted by Dave Elliott on September 24, 2010 9:42 PM | Permalink