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Biochar in The Bamboo Garden

Close-up of bamboo charBlack is the new green, if the buzz about biochar is to be believed. There are some good reasons to be optimistic about the potential for using black carbon as a soil amendment to improve local agricultural fertility and to combat global climate change. But of course, as with all technologies and tactics, there are serious strategic issues to consider, before we are able to declare “bio”char as “green”.

Char: the what, why and how

Char is familiar to most people as the byproduct of a smouldering fire. It is in general the incompletely-burnt carbon structure of the wood that remains after the water, oils, resins and other volatile chemicals that were in it have been driven off by heat (volatilised). The process of heating wood to volatilise these ingredients is an endothermic reaction (requires heat) called pyrolysis, occurring at around 200-300 degrees Celsius, after all water has been evaporated from the fuel. This reaction yields hot gases which may then be combusted - an exothermic reaction (yielding heat) called oxidation - and the heat released used to do work such as heating water and cooking.  In a typical open fire these two processes are simultaneous. A third process (another endothermic reaction known as reduction) in open fires produces ash.

Char is a common fuel for barbecues and metal forges, but has also experienced a recent resurgence of interest in its properties as a soil amendment. Char incorporated into soil improves the soil’s structure and potential for biological activity, by increasing its aeration, water-holding capacity and CEC (cation exchange capacity - increases availability of nutrients for plants). It has also been suggested that improving the carbon content of arable soils around the world (both with char and other methods) could compensate for industrial atmospheric carbon emissions and return carbon levels to pre-industrial levels.

To produce the char without reducing it to ash, we must separate the reactions as much as possible, deconstructing fire into its component processes. By heating the woody feedstock in a chamber with a restricted oxygen environment, we can begin pyrolysis. If we then allow the hot fuel vapours now coming off the wood to be drafted into a second chamber with a ready supply of oxygen, we can burn them (oxidation) to produce useful heat, with char as a secondary product.

Proprietary solutions are available at both consumer and commercial levels, but many people are finding success with DIY pyrolysis stoves (usually small-scale). A popular design is known as Top-Lit Up-Draft (or TLUD), many small-scale applications of which may re-purpose consumer “waste” items such as steel cans. A wide variety of resources to assist DIY-ers developing and operating their own can be found by a web search using keywords such as “pyrolysis stove”, “char”/”biochar”, “TLUD” etc.

Cautions and tips

In my research, and limited experimentation with char production here, several issues are evident which would influence its triple bottom line (environmental, social, economic) impact, both on a global scale and for the average home producer: 

  • Feedstock type and provenance (what is to be pyrolysed and where is it from?).
    Char from invasive (i.e. quick-growing) woody plants might be considered “green”, depending on harvesting techniques, energy source (for initial pyrolysis heat) and how the produced heat is utilised. We burned homegrown dry bamboo splints, split and hand-cut to length with a crowbar on concrete, so our environmental impacts of producing the feedstock were relatively low. Impacts of carbonisation of waste destined for landfill is likely to be far less benign, a fact glossed over conveniently by recent propaganda from the corporate “waste-to-energy” crowd. Far better uses can be envisaged for the mountain of disposables produced daily by consumer society. Contamination by industrial pollutants of such feedstocks, of their emissions, and of the resulting char may also be a risk to environmental and human health downstream.
  • Incomplete combustion byproducts (e.g. smoke/soot).
    Our stove wasn’t very efficient (built of dry laid concrete blocks with limited air-supply control) so most of the produced heat was wasted except for the relatively small amount required to boil the kettle on top of it. Emissions likely included acetic acid, CO, CO2, SO2 and a variety of other common byproducts of combustion. Airborne emissions such as these are not just a global hazard but also a personal health issue for those living downstream from emission sources.  For these reasons, clean air acts in cities may understandably outlaw wood burning stoves, and char-production by extension. Investigate local regulations and be sensitive to the needs of neighbours and others. Most issues can be avoided by combustion-efficient design and thoughtful operation of the system.
  • Getting it into the soil.
    Dry char is no fun to incorporate into soil on a windy day, or to breathe in either. On a small-scale, application of crushed char to compost bins is one sensible and hassle-free route into the soil, and also helps prevent the compost becoming wet and sour. Larger-scale operations such as primary producers (farmers, foresters etc.) may need to look at subsurface application of slurries (e.g. pumped in behind cultivator or in-the-hole application when planting) unless they already have appropriately scaled composting systems in place.

For advice and design of biochar-related systems for your home or business, contact us here.


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