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.