Unlocking the carbon power of biochar

Biochar is the carbon-rich byproduct of biomass and it has great potential for many applications beyond fuel. Michelle Morrison examines the options

In helping our planet to face a raft of pressing issues from shrinking energy resources, climate change and over-population to food and fresh water shortages, the production of biochar using biomass feedstocks could be one of the key tools at our disposal. This is mainly because it has the capacity to combine no less than three major environmental benefits: soil improvement, carbon sequestration and the production of renewable energy.

Biochar is the carbon-rich byproduct that is produced when biomass – such as wood waste and agricultural crop residues – is pyrolysed in the absence of oxygen, causing its thermal decomposition into a syngas and a char. Like charcoal, it is a form of black carbon with the potential for many applications beyond fuel.

Flexible application

First, it can be used to help plants to grow by improving soil structure and productivity through better nutrient retention, water-holding capacity and cation exchange capacity – whether in poor quality tropical or desert soils, or brownfield land. Second, as a very stable form of carbon, burial of biochar can provide a reliable means of sequestering, for hundreds or even thousands of years, the carbon which was originally taken up by the plant biomass as CO2 when it was growing.

This is because biochar is largely resistant to mineralisation by microbes. The third potential benefit is the production of renewable energy. The pyrolysis of biomass creates a syngas that can be combusted to produce electricity and heat. Some can be used to maintain the pyrolysis process, while the rest is exported for external use. Low-temperature pyrolysis can yield bio-oils that can be used as fuels or processed to produce chemicals.

As the temperature increases, less oil and more biochar is generated. Eventually, at high temperatures with the introduction of a limited quantity of air, gasification takes place – creating mainly syngas with some ash and little or no organic biochar carbon.

Versatile technology

Incorporating biochar into agronomy in arid, semi-arid or tropical areas is a relatively simple and cheap answer to soil fertility and productivity problems. As always there are caveats. The diverse nature of biochar feedstocks, the conditions within the pyrolysis plant and the eventual application can all result in different characteristics and therefore different results. But in theory, the versatility of biochar technology means that it can be applied to many different organic products and/or wastes with some adjustment of the operating conditions.

Other applications could include using biochar as a soil forming material, combined with others for the restoration of brownfield land – acting as an ameliorant for contamination through its capacity to adsorb pollutants, as well as a soil improver. In the resource management sector, pure biomass feedstocks such as tree cuttings, forestry and crop residues, heather and even diseased plants should all be usable. Some contaminated wastes might make the biochar unsuitable as a soil improver or pollution ameliorant. But even this could be buried deep beneath the surface.

A great deal of research is currently under way into biochar, including any risks to the environment or human health, its longevity in the soil, physico-chemical properties, and potential agronomical and agricultural benefits. The UK Biochar Research Centre is at the forefront of this. Wardell Armstrong is working closely with the centre on a number of projects, providing consultancy on policy and legislative requirements for the implementation of biochar projects.

Harness the right climate

If its potential as one of the ways of alleviating climate change and food shortages is to be realised, there needs to be a concerted application of the technology at a number of scales. Its great appeal is that it can be applied across a broad range of projects. These projects include small-scale family, community or farming ventures where modest investments in low capital equipment could create biochar for soil improvement coupled with localised energy production, to large-scale, high specification initiatives.

Many different biomass feedstocks could be used – organic waste materials such as waste water treatment works biosolids, waste wood, crop residues and forestry residues or proprietary crops such as short rotation coppice. Depending on the project, the technology could be optimised for biochar, bio-oil or syngas production by altering the pyrolysis conditions. The biochar could be applied to land as a soil improver, for land restoration, or for contamination remediation.

But none of this must come at the expense of the environment or indigenous peoples. The spectre of unethical land grabbing and deforestation for biomass production should be fiercely guarded against to prevent the environmental and socio-economic impacts, which accompanied the first wave of biofuel production, from happening again. As long as these risks can be legislated against and avoided, biochar could be an extremely useful and timely tool in combination with other measures for more sustainable living.

Dr Michelle Morrison is principal environmental scientist at Wardell Armstrong

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