Some frequently asked questions about biochar
Compiled by Dr Rowena Ball
Senior Research Scientist, The Australian National University, Canberra 0200 Australia
1. What is biochar?
Biochar is charcoal made from biomass. It is thus distinguished from charcoal made from petroleum products, such as diesel soot and charred plastics.
2. How is biochar made?
Biochar has been made by nature for many millions of years, for as long as there have been fires in forests and grasslands in fact. When biomass (leaf litter on the ground, for example) is heated (perhaps by a lightning strike) chemical reactions set in which decompose it partly to charcoal and partly to hot volatile organic compounds. If these volatile organics become hot enough in air they burst into flames. So the flaming combustion you see in a forest or grass fire is actually the gaseous volatile decomposition products burning, not the original solid biomass. The charcoal decomposition product can burn too, but needs higher temperatures – this is the glowing combustion you see in a hot charred log or stump. Usually the volatiles are all burned off and most of the char is left and cools down. The char contains anything from a few % to 50% of the carbon that was originally in the biomass. It also contains all of the minerals and trace elements that were in the biomass, such as phosphorus, iron, and magnesium- this is the ash.
The natural biomass burning cycle is a very important shaper of our terrestrial environment and significantly influences global carbon balances. It is worth studying the diagrammatic representation below in order to consider how farming and land management practices may take advantage of these facts.
Biochar can also be manufactured by adapting and optimising this natural process. Typically, charcoal is made by heating wood or other biomass in pits or kilns and excluding the air so that the char does not burn and the amount of charcoal product is maximised at the expense of volatile products. For hundreds of years charcoal has been produced this way, and until the mining and use of fossil coal became widespread in the nineteenth century it was the only fuel available for reducing metals from their ores.
3. Why is biochar somehow special?
Biochar has many quite remarkable properties. To begin with, it is very resistant to chemical or biological degradation, or “refractory”. As indicated in the diagram biochar persists in soils and sediments for very long times. Scientists are now attempting to establish exactly how long, and all the evidence indicates that biochar has a lifetime of many thousands of years. This fact alone tells us that biochar is a significant natural carbon storehouse, or sink.
Looking at the diagram again, we see that carbon dioxide is drawn out of the atmosphere by plants, which convert it to cellulose and lignin and proteins and oils and all the other materials that make up biomass. When a fire comes along the volatile thermal decomposition products are combusted to carbon dioxide and water, thus putting carbon dioxide back into the atmosphere. But some fraction of the biomass always forms charcoal, so some of the carbon that was in the biomass is deposited in the biochar warehouse at each burn. Biochar is raw carbon. It does not have a defined structure like graphite and diamonds, but it is as un-reactive, as refractory, as those other kinds of carbon at normal temperatures-it is “sequestered” carbon, unavailable for further reactions.
4. So if I converted all the cleared and dead timber on my property to charcoal I’d actually be helping to reduce the amount of carbon dioxide in the atmosphere?
Exactly.
5. Why not just leave the dead timber to decay away naturally?
You could, but you would be adding to greenhouse gas emissions and here is why: Dead timber is broken down mainly by termites and certain wood-rotting micro-organisms. In this process carbon dioxide is produced as well as quite large amounts of methane, which is one of the potent greenhouse gases that is causing our climate problems. So all of the carbon that was in the wood is returned to the atmosphere. With biochar production much of the carbon from the wood is deposited into the biochar warehouse.
6. Ok, so I burn off the scrub more often and make felled and dead timber on my property into biochar. What do I do with all that biochar?
This takes us back to the answer I began to give to question 3, referring to the remarkable properties of biochar. For centuries it has been known to folk wisdom that the most fertile soils are those containing a high proportion of charcoal or black carbon. Now scientists are studying the fertility of such soils seriously and in detail. A great many trial plantings have now been carried out with a wide variety of crops. The results are consistently impressive: plantings in soils that have added biochar significantly outperform those in control plots. Furthermore, a single amendment with biochar may increase yields for decades. We do not know yet all the details of how and why biochar has such profound effects on soil quality. Intensive research on these questions is currently being carried out.
It has recently been reported in the scientific literature that biochar amended soils emit less nitrous oxide and methane to the atmosphere than control soils. Since these are two very potent problematic greenhouse gases it would seem that biochar has even more useful side effects than we anticipated.
7. Apart from dead timber, what other kinds of biomass can be use to make biochar?
Almost any biomass waste stream can be used to make biochar. Bagasse, municipal green waste, crop residues, chicken litter, sawdust, and weeds, to name just a few. Consider the municipal waste of a city such as Canberra. Why, enough biochar could be produced from the waste paper of Canberra offices and the coffee grounds discarded by Canberra’s caffeine-addicted chattering classes to sequester all of the hot air generated over Parliament House and more!
8. What are the problems associated with biochar production?
The most urgent problem to solve is that of emissions control. Pyrolysis of biomass to biochar also produces emissions of gases, mostly carbon monoxide with smaller amounts of methane and hydrogen. Since these gases are fuels in their own right we are working on a system of using the gases to fuel the initial heat-up of the biomass.
9. The diagram of the biomass burning cycle shows that biochar itself is also a fuel. Why not sell the biochar as a fuel rather than put it on the soil?
Some biochar can indeed be sold for barbecue fuel, but this is a minor market. It has a higher value as a soil amendment and in environmental remediation projects.
10. What’s the catch? Why aren’t everyone and their dog jumping on the black carbon bandwagon?
They are – now. In 2009 we will see an explosion in biochar research (see bar chart below) as people belatedly accept biochar is one of our most valuable commodities in terms of a) responding to the inevitability of climate change by adapting our land use and management practices, b) sequestering surplus CO2 that is already, and will continue to be, dispersed in the atmosphere, and c) reducing our dependence on fossil fuels.
The bar chart shows the number of times the terms “biochar” or “bio-char” or “black carbon” or “charcoal” occur in the TOPIC field of indexed scientific publications for each year from 1970 to Oct 2008. The 1991 jump is the year following the first Gulf war and associated oil crisis, the steady increase from 2003 parallels oil price rises, the jump in 2008 mirrors major climate change adaptation initiatives by governments.
