Winter in many Australian country towns is accompanied by a pall of smoke from wood-fired heaters that lasts from late afternoon to the following morning. In larger towns and cities burning wood has been limited by laws to reduce the impact of smoke on respiratory health. This is not a significant concession, given that other sources of heat, with much lower emissions, are available.
The majority of heat and electrical energy we use is recovered from oxidizing compounds of mainly carbon or carbon hydrogen and oxygen, usually by combustion of fuel in a furnace. When fuel is burned, a variety of compounds are released with the energy. These include organic molecules of varying molecular weights, carbon, carbon dioxide, carbon monoxide, hydrogen, water, oxides of nitrogen and sulphur, ammonia, and others. Of course the products of combustion are dependent on the fuel composition.
Burning wood produces all the compounds listed above with the exclusion of sulphur oxides. These are more commonly a product of coal or, to a lesser extent, gas combustion, and are noted to produce acids in the atmosphere that fall as rain. Natural gas is a primarily a mix of gaseous alkanes (CnH2n+1) and some minor compounds such as hydrogen sulphide. Burning natural gas produces larger amounts of lower molecular weight compounds compared to wood, and because these can quickly distribute with the same compounds in the atmosphere, burning gas is considered to be cleaner. In addition, burning wood produces heavy compounds as liquids and solids. Smoke, as an example, is particulate carbon and minerals.
In Australia, particulate emissions are broadly limited to particles 10 micrometers in size and at a concentration of no more than 50 micrograms per cubic meter. This air appears clear to the naked eye. Particles of this size are very small and are drawn deep into the lungs when breathing. They are considered to be responsible for respiratory and cardiovascular disease in some cases. Liquid products (tar and organic solvents) can also be drawn into the lungs if suitably exposed, and additional health impacts can occur. Any (educated) smoker will agree.
The products of wood combustion can also be varied with the temperature of combustion. For example, more wood is converted to relatively harmless gasses and water if the temperature of combustion is high (>1000°C). Particulate emissions are much reduced due to more complete oxidation of carbon compounds. Lower temperature combustion produces less energy because some compounds remain incompletely burned and pass up the chimney. Open fires and slow-combustion heaters are less clean than burning natural gas. However, new wood-burning technologies, including secondary higher-temperature burning and catalytic conversion of primary products, are substantially cleaner and their use has been legislated in parts of the US and Europe.
Any discussion like this should consider the issue of atmospheric carbon emissions, and of course any biomass combustion represents no net increase in greenhouse gas concentration. Fossil fuels on the other hand are primary contributors. It appears that completely clean energy production will be achieved only where biomass is converted to fuel and/or energy with sufficient scale to cost efficiently incorporate technology for the control of emissions.
The economics of energy production and price is unfortunately a maze of scale, fuel, markets, technology, legislation, regional economic circumstances, and the list goes on. There is no single global solution. One easy observation is that if an individual collects wood with little or no monetary cost, and burns this to produce heat for warmth and cooking, then this is as economically efficient as an individual might become. This is probably the least clean of all options, but to argue against such freedoms of activity is difficult on cultural or economic grounds. To then impose an increasing cost on the alternatives is even more difficult to justify. Fortunately there are still places where individuals can grow and use wood for heating and cooking, and we are all free to choose to live in these places. These places should also be conserved in Australia.
Another easy observation is that with good building design, the need for heating energy and spending money is significantly reduced. Many households that seek the open fire tradition or comfort have a fire place and are usually older dwellings with no insulation in the walls or ceiling. Is it reasonable that this tradition and comfort be limited to the camp site or barbecue from time to time?
With rising fuel and energy costs, the use of wood obtained at no or low cost could save many households money and in rural households where incomes are lower, this saving is important. Where population density is high however, more people risk more concentrated exposure to the products of burning wood fuel over sustained periods. The population density is defined indirectly by the acceptable limits on particulate emissions. There are, and should be, controls to minimise the risk to people in populated areas who are susceptible to health impacts of particulate pollution. Burning wood is controlled in these places because there are just too many air-breathing people over small spaces to maintain a healthy environment. It is noteworthy that the cost of wood-fuel in urban areas, or the rising cost of gathering and transporting it to town, soon diminishes any economic advantage.
Infrequent and short term exposure to wood combustion products, for most people, does not threaten health. In rural Australia after 60,000 years, fire is still used in land management, and fires are started naturally all the time, without worrying consequences for rural people. With larger distance between households in the rural landscape, wood is still used cost efficiently for heating and cooking. At the same time there is nothing healthy about countless wood fires in the suburbs or towns, and there are good alternatives such as natural gas.
The argument that the romance of open fires should risk community health is weak. However, the discussion raises an important issue. There is an engineering challenge to be faced here, to provide clean biofuel combustion at any scale to consumers. In this circumstance all combustible industrial and domestic waste becomes a source of energy that can replace costlier imported fuels. A history of exploiting cheap fossil fuels has discouraged development, however.
Comments welcome below.