Biochar production: Tackling the problems of waste management, energy generation and carbon sequestration

Crombie, K., Brownsort, P., Cross, A., Masek, O. and Sohi, S., UK Biochar Research Centre, University of Edinburgh, UK



Biochar is the porous, carbonaceous substance produced by pyrolysis of biomass for the purpose of safe, long-term storage of carbon in soils with renewable energy production from co-products. The properties of biochar in some cases, lead to improved fertility and crop growth following incorporation in soils. At UKBRC we are currently investigating the potential use of virgin (e.g. willow, pine, rice husk, etc) and non-virgin biomass feedstock (e.g. anaerobic digestate) for biochar application. Samples produced at four different temperatures (350oC – 650oC) have been analysed for physical, chemical and soil functional properties, such as stability and labile carbon. The yield of biochar produced was found to decrease as the temperature rose from 350-650oC resulting in elevated yields for liquid and gas fractions. Increased pyrolysis temperatures also resulted in higher fractions of stable C and reduced labile C. Pyrolysis of Anaerobic Digestate (AD) feed eradicated AD odour potentially due to large release of volatile matter as well as significantly reducing sample mass. AD generated very high biochar yields of up to 60%, due to the high ash content of the feed and biochar produced. Important areas of biochar remain under-researched such as the use of non virgin feedstock for biochar and energy production as well as biochar’s response in soil.

Key words
Biochar, pyrolysis, waste management, carbon sequestration, soil amendment

Global climate change and the inevitable depletion of fossil fuel reserves are major challenges facing the 21st century which has lead to a boom in research related to alternative energy sources and reducing greenhouse-gas (GHG) emissions. The current atmospheric carbon level is estimated at 800 Gt with roughly 8Gt being added annually from industry and the burning of fossil fuels [Mathews, 2008]. Reducing emissions would only slow rises in atmospheric CO2 levels, so any response strategy needs the incorporation of withdrawal of carbon dioxide from the atmosphere. Removal of CO2 from the atmosphere must have the potential to keep the CO2 stored in huge concentrations safely over a long time scale as return of carbon as CO2 into the atmosphere on too short a timescale could intensify climate change rather than alleviate it [Woolf, 2008; Shackley et al., 2009]. Biochar is a relatively new field of research which can be utilised to tackle the problems of both energy production and carbon sequestration. Biochar is a carbon-rich solid produced by the thermo-chemical conversion of biomass, such as wood, leaves and manure in an oxygen depleted atmosphere at relatively low temperatures (<700oC). Plants take in CO2 during photosynthesis and this CO2 is released back into the atmosphere via natural decomposition and burning [Renner, 2007]. Biochar is a way to counteract this process by transforming the biomass into a recalcitrant stable carbon form which decomposes at much slower rates than its parent feedstock and storing it in soils for hundreds to thousands of years [Lehmann et al., 2009].

The use of biochar as a soil amendment has the potential to not only play a role in the carbon cycle but the nitrogen cycle as well. Nitrous Oxide (N2O) is a green house gas which has a global warming potential several hundred times that of CO2. Rondon et al (2007) presented reductions between 50-80% in emissions of N2O and complete suppression of CH4¬ emissions after the incorporation of biochar into soybean and grass system soils however the results will differ with varying location and type of biochar. Through interactions with SOM, micro-organisms and minerals it is also possible for biochar to affect the aggregation of soil thus improving its capacity for water and nutrient retention [Verheijen et al., 2010]. During pyrolysis most of the Ca, K, P, Mg and plant micronutrients as well as half of the N and S in the feedstock are retained in biochar. The use of biochar as a soil amendment returns the majority of these nutrients to the soil from which they came [Laird, 2010]. In addition to altering the water holding capacity and nutrient concentration in soil, the typically alkaline nature of biochar can also increase the activity of micro-organisms in acidic soil by raising the pH. This can cause a source of ‘priming’ for the decay of pre-existing organic matter while also increasing plant productivity [Van Zwieten et al., 2010]. Further benefits of pyrolysis biochar systems consist of the collection and utilisation of volatile co-products (pyrolysis oil and gas) as renewable fuels as well as establishing sustainable disposal routes for organic waste residues.

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