Can biochar help stop climate change?

March 28, 2009

Biochar production has been the object of considerable research and experimentation in Australia.

The most extensive work has been carried out in Western Australia, where the firm Verve Energy operated a pilot Integrated Wood Processing (IWP) plant for several years, using advanced pyrolysis technology developed by the CSIRO to process oil mallee biomass.

Located at Narrogin south-east of Perth, the plant had a one-megawatt generating capacity, and produced high-value activated charcoal for industrial filters — rather than biochar for incorporation in soils.

Process heat was used to distil eucalyptus oil from mallee leaves. As well as having a market in the pharmaceutical industry, this was found to have promise as an industrial solvent. The plant shut down in 2006 after the demonstration program was completed.

A joint statement announcing the trial from the Australian Greenhouse Office and WA electricity provider Western Power argued the case for "full-scale, fully economic plants" that "will be five times the size of the demonstration plant, requiring the planting of 20 million trees each.

"There is potential for at least ten IWP plants throughout the wheatbelt of Western Australia", they said.

Uses of biochar

The rise of a biochar industry, however, faces a "chicken and egg" dilemma. Farmers are reluctant to switch from familiar modes of soil fertilisation until the economics of biochar are widely tested. Private investors are unwilling to construct biochar plants until farm-sector markets are known to exist.

Adding to these obstacles is a lack of government support. Still regarded officially as an unproven technology, biochar will receive carbon credits under the Rudd government's Carbon Pollution Reduction Scheme.

Biochar is created through heating plant matter in an oxygen-poor environment, a process known as pyrolysis.

As the heat breaks down the biomass, biochar is left behind, and hydrogen and a range of gaseous carbon compounds are given off. If combusted, these gases can provide much more energy than is needed to keep the process going.

In modern pyrolysis plants, the temperature to which the biomass is heated can be changed to determine the end product. Lower temperatures produce more biochar, while higher temperatures produce more gases. Part of the gas stream can also be condensed to a liquid "bio-oil".

Various options exist for the gases not needed to maintain pyrolysis.

They can be burned in air for electricity production, or reacted with steam to produce further hydrogen. Together with carbon monoxide, which is also present, this hydrogen can be used to produce diesel fuel.

Also, using the Haber-Bosch process, hydrogen can be combined with nitrogen from the atmosphere to produce ammonia gas. This can then be reacted with carbon dioxide from the flue gases to produce the nitrogen fertiliser ammonium bicarbonate, which is readily absorbed by biochar.

This fertiliser can serve as a source of soil nitrogen, displacing fossil fuel-based nitrogen fertilisers.

The bio-oil that is condensed from the gas stream contains large amounts of water and is relatively acidic. Nevertheless, researchers at the University of Georgia in the US report success in processing it into a biofuel that can be mixed with diesel. The bio-oil also has promise for use in the chemical industry.

Carbon-negative energy

Even when configured to favour the output of electricity, biochar production is strongly carbon-negative, taking much more carbon from the atmosphere than it puts back.

This effect can be still more marked if the carbon dioxide in the exhaust gases is used to fertilise microalgae in adjacent ponds.

How much carbon could the use of biochar be expected to sequester in Australian soils, and what proportion of the country's emissions would this represent?

Mark Diesendorf, senior lecturer in The Institute of Environmental Studies at the University of New South Wales, estimates in his 2007 book Greenhouse Solutions with Sustainable Energy that "biomass from existing agricultural crop residues, supplemented by oil mallee from the wheat belt, could generate [power equal to] 39% of Australia's electricity generation in 2003-04".

To this might be added an additional 22% from short-rotation tree crops on an area of two million hectares, "similar to the current area of plantation forests", plus an indeterminate amount from existing plantation forest residues.

Electricity generation in Australia in 2004 was responsible for about 195 million tonnes of carbon dioxide emissions, or 35% of the total carbon output of 565 million tonnes.

If we accept Diesendorf's estimates, biomass might have supplied energy in excess of 117 million tonnes of carbon dioxide emissions, or 21% of the total.

Drawing down carbon

What if this biomass were pyrolysed rather than burnt? Pyrolysis plants can turn as much as 50% of biomass into biochar; if we take a more modest figure of 40%, that suggests that biochar in 2004 might have been used to sequester about 8% of Australia's total carbon emissions.

There are excellent reasons to think this figure could be much higher. Diesendorf proposes planting two million hectares of fast-rotation tree crops.

However, more than 30 million hectares of forest and woodland have been cleared since European settlement in NSW alone. Across Australia as a whole, the new plantations could cover many times two million hectares.

If grazing of sheep and cattle were sharply reduced, might enough biochar be produced each year to sequester an amount of carbon corresponding to 15-20% of current emissions, roughly offsetting the carbon now released by Australia's agriculture?

In this scenario, the production of biochar, rather than gas for electricity, would predominate. Alternative sources of renewable energy would need to be found to take account of this.

Nevertheless, biochar technology represents an enormous plus for the effort to sharply cut Australia's carbon emissions. To the extent that reforestation will restrict food production, biochar use should restore many of the losses through higher soil fertility.

The fact that this technology removes carbon already in the atmosphere confers an important flexibility. Emissions in areas such as agriculture can be offset at a modest cost instead of having to be ended through expensive or disagreeable sacrifices.

At the same time, it must be kept in mind that biochar is not eternal; the carbon it stores in the soil will return to the atmosphere in millennia to come. Biochar use must therefore be accompanied by continued, long-term emissions-cutting efforts.

Private capital, meanwhile, remains unwilling to take the risks involved in developing a large-scale biochar industry. The need is obvious for governments to step in.

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