Sources of biochar
Researchers have concentrated on making biochar from diverse sources of biomass at different production parameters. Purpose-grown energy crops such as switchgrass, miscanthus, and corn have been evaluated for biochar production but most research trials have used end-of-life biomass (ELB) such as animal manure, forestry and agricultural residues, and sewage sludge.
Dedicated biomass plantations for the sole purpose of producing biochar are, under current prices, unlikely to be profitable. Biochar produced from indigenous forest clearing does not result in net emission reductions from a life cycle perspective and would also pose a risk to biodiversity conservation. There is also competition between biomass resources. Therefore, the existing biochar plants are dedicated to specific ELB streams that are financially attractive.
Crop residues and animal manure, when left in the field play an important role in carbon sequestration, conservation of soil and water, microbial activity, and agricultural productivity. Their use avoids the extra application of synthetic fertilisers and currently are commercialised as compost and soil improvers. Moreover, some crop residues are used to feed livestock. Therefore, crop residues and animal manure represent no waste in agriculture and removal of these ELB from the land to produce biochar should be done carefully, i.e. the optimal amount of feedstock removal should be analysed and the resulting biochar may be reincorporated into the same land where the feedstock originated.
Biochar from sewage sludge produced in the treatment of household, municipal, and industrial sewage may contain heavy metals and/or organic pollutants that could contaminate the soil rather than ameliorate it. Because of this possibility, further research is needed to characterise different types of sewage sludge under various conditions as their level of contamination is likely to vary at different places and times. Moreover, wet feedstocks such as sewage sludge and animal manures raise the question of which kind of carbonisation process would be best to use since they would require significant energy to dry before going through slow pyrolysis.