This is a paper presented at Woodfire ’89 held in Gulgong NSW

WOOD: THE MOST ECOLOGICALLY SOUND FUEL?
I intend to look at some of the effects that the use of wood as a fuel for kilns and heating will have on the environment.
This will be divided into two sections. Firstly the effect of harvesting and growing wood compared to the use of other fuels, and secondly the emissions of smoke and fumes from combustion.

1.WOOD AS A FUEL:
The leaves of trees convert sunlight into energy by photosynthesis. The ingredients are sunlight, carbon dioxide, water and minor amounts of nutrients such as nitrogen, phosphorus and potassium. This produces carbohydrate, some of which is immediately reused by the tree as energy to run itself. The remaining carbohydrate moves down to the branches and trunk where it is stored as wood.
As a properly managed resource wood is a renewable resource - I think of it as canned sunlight, and apart from methane produced  in a methane digester, wood is the only renewable fuel that I know of which can be used for firing kilns. This contrasts with fuel oil, coal and in particular, l.p.g. ,which are all fossil fuels with finite reserves.
A mature forest will have little net effect on the greenhouse effect, the amount of carbon dioxide absorbed in the production of wood will be balanced by the carbon released by the decay of fallen timber, according to Dr Trevor Booth of the CSIRO in Canberra.
Combustion of wood releases about the same amount of carbon as decay, only in a much shorter time-span.
Compared to wood (and methane produced in methane digesters) all the other fuels for kilns release carbon which has been removed from the atmospheric carbon cycle by the process of burial and conversion into coal or oil.
So, subject to harvested trees being replaced with new trees there is little effect on the greenhouse effect from the burning of timber, and only a small nutrient loss from the forest occurs. However, it can have considerable effect on wildlife. Generally the types of trees selected for fuelwood are the standing dead trees, trees which have reached maturity and died. These are the trees which have hollows which provide habitats for birds and wildlife. Large quantities of trees are presently cut for fuelwood in specific areas, and for instance Canberra consumes an estimated 80,000 tonnes of timber each year, and standing
dead yellow and red box trees are removed over areas of several hundred kilometers around Canberra. Australia-wide over one million people rely on wood for domestic heating and in Tasmania over 40% of all households use wood as their main heating fuel.
The use of wood in kilns cannot be considered separately from this use of fuel for domestic heating, and the effect on wildlife. It seems appropriate to use wood waste from industry to lessen the impact on the local ecology.

Table 1 Estimate of annual fuelwood use in Australia and some speculative figures on quantities of wood waste and recoverable logging residues
(Dr John Todd Tasmania 1983)
Million tonnes

From Table 1 it can be seen that large quantities of processing wastes are not used as fuel, these are either dumped in landfill or burned, and vast quantities of logging residues are left in the forest after harvesting.

It is perhaps relevant here to make a few comments about wood as a fuel. Half the weight of green timber may be water,  air dry wood will contain about 15% water. The density of wood will vary from species to species for instance dry yellow box will weight approximately 1 tonne per m3 whilst radiata pine will weight approximately 1/2 tonne per m3, however on a weight basis the calorific value of woods of different species is about the same. Yellow box has about 17 Mj/kg whilst radiata pine has about 10 Mj/kg.

Another benefit of the use of wood as a fuel is the fact that the supply industry is decentralised, small scale and labour intensive. Because of this it provides some of the cheapest fuel in Australia (see table 2), it keeps funds circulating within the local community, and does not use large quantities of petrochemicals in an extended delivery network.

Table 2
Some typical rural domestic energy
1989). Adapted from Dr John Todd

Heating oil

From this chart it can be seen that wood provides a cheap source of energy, although the relative efficiency of the different types of kilns used would have considerable bearing on the cost effectiveness of different fuels.

PART 2: EMISSIONS
When vegetable matter, (including wood) is burned complex combustion products are produced that include carcinogenic compounds including benzopyrenes and nitrosamines, and 'toxic gases such as carbon monoxide, lung and eye irritants, and compounds that tend to destroy the ability of lungs to cleanse themselves of larger smoke particles'.(Jay Sheldon).
These compounds are similar to those found in tobacco smoke and 'there is no reason to believe that the toxic effects are much different to tobacco smoke at similar concentrations' (R.J. Dunn).
When wood is burned in an open fire about 1% of the fuel would be released into the atmosphere as particles, so in the ACT where approximately 80,000 tonnes of wood is burned each year about 800 tonnes of particles are released. This compares with an estimated release of 900 to 1000 tonnes of particles from vehicles each year. Therefore in Canberra cars and wood-fires contribute fairly evenly to smoke pollution.

It follows that any kiln will also contribute to such pollution and that the effects of wood fuel use will depend on the concentration of gases, Jay Sheldon suggests the use of a catalytic system is possible to lower the combustion temperature of chimney smoke from over 550 degreesC to lower than 300 degreesC which would provide cleaner chimney exhausts.
On the whole the intermittent nature of kiln firing and their generally rural position would tend to minimize the pollution problems from kilns.
In summary I would argue that the use of wood as a fuel will have less impact on the general environment than the use of fossil fuels provided that several points are considered.
1,         Trees are planted to replace those harvested to provide fuel,
2,         Wherever possible waste wood from industry or harvesting for timber production is used.
3,         Mature trees that provide habitat for wildlife (especially in a rural landscape) are not cut.
Given these conditions I believe that wood provides a renewable resource which can be used, and will cause less damage than fossil fuels.
BIBLIOGRAPHY
Dunn, R,J., Wood Smoke in Canberra: Institute of Foresters Firewood Forum, 1983.
Shelton, Jay, Wood as Energy: Studio Potter, Volume 11 No,l
December 1982.
Todd, John, Some Comments on Australian Fuelwood use, Institute of Foresters Firewood Forum, 1983.

Ian Jones fires mostly domestic ware in a forty foot long lube/dragon/anagama. kiln.

Post script  August 22nd 2009:  Looking at this twenty years later, although many of the figures have changed I believe that the conclusions that I drew at the time are still valid, and it is obvious that the issue has become much more important.  I will make a few observations.
In the original paper I made the point that the green house effect of wood-firing was essentially neutral.  There is an obvious fossil fuel input into cutting and transporting waste wood, but it seems sensible that wood that is to be burned or allowed to rot on plantation forest floors (generating methane, a greenhouse gas at least twenty times as potent as CO2) should be used as a substitute for fossil carbon based sources of energy.  I did not mention electricity in any great detail, but it is now well known that Australia’s coal based electricity generation releases huge quantities of CO2 each year, and that even by 2020 renewable sources of electricity will still be only 20%.  Rises in the price of oil have made the cost of oil firing unacceptably high, removing that as a problem for potters to worry about, but the use of recycled cooking oil to fire kilns is something that should be considered, and would be greenhouse gas neutral.