Firewood

The Firewood Resource | The Need for Firewood in a Sustainable Energy System | The Firewood Supply | Combustion Principals | The Future | Further Information | References

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The Firewood Resource

Firewood has been, and continues to be, a very important energy supply for Australians, especially in rural areas. Figure 1 shows the relative heating sources used in Australia since 1966, and estimates the trend in future use after 2002.

According to Driscoll, Milkovits, and Freudenberger, Australian households are burning between 4.5 and 5.5 million tonnes of wood every year, over half of which is consumed in New South Wales and Victoria. Although capital cities account for 63% of the households in Australia, only one third of firewood is burned in capital cities. For households using firewood, average consumption varied from 1.3 tonnes/year in Queensland to 5.8 tonnes/year in Tasmania, with a national average of 3 tonnes/year. Households in capital cities burned significantly less than households in the rest of each State (2.2 versus 3.7 tonnes/year). About 1,300,000 households use firewood as their main heating fuel and a further 300,000 use firewood as an occasional or secondary heating fuel. Thus, just over 20% of all households use firewood (regularly or occasionally) for heating (Driscoll et al, 2000).

 

Location	Respondents	Household use (t/yr)	Std. error	Lower 95%	Upper 95%	No. of households	% that use firewood	Firewood used (million t.)	Lower 95%	Upper 95%
Cap. city	168	2.25	0.23	1.83	2.75	4378700	18.6%	1.82	1.48	2.24
Rest state	250	3.67	0.22	3.28	4.14	2586300	34.7%	3.30	2.95	3.71
NSW	91	2.65	0.24	2.21	3.15	2402454	22.3%	1.42	1.18	1.68
VIC	103	2.69	0.30	2.13	3.28	1747638	25.8%	1.21	0.96	1.48
QLD	41	1.31	0.21	0.92	1.76	1338442	18.9%	0.33	0.23	0.45
SA	36	2.64	0.35	2.03	3.39	609769	25.5%	0.41	0.32	0.53
WA	72	2.70	0.29	2.16	3.29	718988	29.2%	0.57	0.45	0.69
TAS	63	5.81	0.52	4.87	6.84	186272	66.8%	0.72	0.61	0.85
ACT	10	1.88	0.34	1.26	2.57	117290	22.3%	0.05	0.03	0.07
NT	-	-	-	-	-	64687	-	0.03	0.00	0.00
Australia	418	3.00	0.15	2.71	3.32	7120853	23.4%	5.00	4.52	5.54

 Table 1 Average consumption of firewood per household and total firewood used in capital cities, the rest of each state (pooled over states), and for each state separately (source: Driscoll et al, 2000).

Obtaining information about firewood prices, amounts of wood used, and for what purpose, is difficult. Therefore, there is a lack of current statistical information about some aspects of wood heating and firewood use in Australia today. For example there is a lack of information regarding the mix of controlled combustion woodheaters versus open fireplaces. However, it has been estimated that approximately 800,000 woodheaters, and in the region of 700,000 open fireplaces are used as main or secondary heaters. In Perth, Western Australia, it is estimated that about 100,800 households (21% of households) use firewood as their main heating source (these numbers are based on 1995 Census data for wood and coal, as very little coal is used). A further 20,600 households use wood (or coal) for secondary or backup heating. In areas of Western Australia outside the Perth metropolitan area, a higher proportion of households use wood (or coal) as their main heat source (79,700 or 48% of households) or secondary heat source (7,300 households). Although now dated, these numbers indicate that firewood continues to be an important domestic heating fuel throughout the cooler Australian states and in Western Australia.

Firewood prices can vary from zero to over $500/t. Free firewood may be available from some building sites, from landscape gardeners (quite common in Sydney suburbs) and land clearing operations. Many firewood users (about 50%) collect their own supplies from private land at no cost other than their time and fuel.  A cost of around $20 per tonne is sometimes used to cover those who collect their own firewood; this does not include any allowance for the person’s time. See Table 2 for current commercial firewood costs around Australia.

 

 

However, more expensive firewood is available, usually in small delivered loads or in convenience packs in supermarkets.  For example, 5kg packs of extruded logs (manufactured in Malaysia) are widely sold in supermarkets and convenience stores across Australia for about $4 per pack, which is $800 per tonne.  The expensive wood is usually only used by households lighting a fire for aesthetic reasons where the fuel cost is not a big issue.  However, some households run out of wood, or are faced with delays in getting firewood from their normal supplier, and have to resort to the expensive convenience packs from petrol stations or supermarkets.  Basically, this is poor fuel management and it leads to very expensive heating.  For the purposes of cost comparisons it is sensible and justifiable to leave these expensive firewood supplies out of the calculations.  However, some of the woodheater industry’s competitors do occasionally use these convenience firewood costs to ridicule firewood use. Firewood prices also vary depending on the time of year (usually cheaper in summer), the size of the load delivered (cheaper for large loads, i.e. greater than several tonnes), the length of the wood (usually cheaper for longer lengths), and by the wood species, as the preferred species vary from region to region (Todd, 2004).

 

The Need for Firewood in a Sustainable Energy System

Firewood is the oldest fuel supply. The ability to control fire and use it for heating, cooking and processing of materials has been fundamental to human social development. In Australia, Aborigines have used fire for heating, cooking, hunting and land management. For early European settlers, firewood was their most important fuel and for the first one hundred years following settlement, firewood continued to be the most significant source of energy. The good supply of firewood and the mild Australian climate meant that the open fireplace became the main form of domestic heating. In Europe and North America, where larger populations (which put more pressure on firewood supplies) and colder climates meant that the poor efficiency of the open fireplace was inadequate, and more efficient controlled combustion heaters were developed. As firewood can be  harvested in a sustainable manner, by cutting a similar amount of firewood that naturally grows every year, the use of this resource can play a part in reducing greenhouse gasses by avoiding the use of fossil fuels.

Our indigenous oil supplies, initially sheltered Australia from oil price rises until 1978, but then oil prices in Australia rose quickly. From 1978 to about 1990, woodheaters experienced a rapid increase in popularity. In the early 1990s, however, concerns over wood-smoke and a preference for more convenient heating appliances triggered a gradual decline in the number of households choosing to heat with wood. This coincided with large marketing campaigns by gas and electricity heater manufacturers, and the reduced costs of heat pumps. Figure 2 shows the declining trend in popularity of firewood for space heating in Australia from the 1950s to 2000. The numbers shown prior to 1975 are uncertain and the projection from 2000 reflects the trends in 2000, and is a 'best guess' of future use.

  

The Firewood Supply

Wood, or any other plant material, is formed from the photosynthesis of carbon dioxide, water and trace materials into energy-rich organic molecules in the presence of sunlight.

In temperate climates, the incoming solar energy amounts to about 75 TJ/ha per year, but only 0.1 to 0.5 percent of this energy is captured in the form of biomass. World wide, the annual energy content of new biomass (i.e. growth) amounts to roughly four times the world's present consumption of primary energy. But while there is a lot of biomass energy available, environmental, social and economic constraints mean that only a tiny fraction of total biomass is used as a fuel.

In Australia, most firewood is supplied from land clearing operations on private land. Farmers seeking to clear land for pasture or crops will often allow wood cutters onto their land because this means land clearing costs are reduced and small royalties can be obtained for the firewood. Royalties vary from $0 to around $10 per tonne. Some 'firewood runs' are also managed for rough grazing and on-going firewood supply.

Firewood from land clearing is not a sustainable energy supply. Large quantities of firewood are also available from logging residues and improved integration of firewood and logging activities could increase use of this resource. It is also possible to grow timber specifically for use as firewood, although economic and social considerations mean that multiple use plantations are more desirable. Some of the possible multiple uses for firewood plantations include: salt control (through lowering of the water table), treatment of sewage or industrial waste-water streams; shelter belts for stock; wind-breaks; erosion control; recreational areas; and aesthetics. If sensibly management practices are adopted, Australia could easily supply projected firewood demand on a sustainable basis.

 

Combustion Principles

Wood combustion processes are quite complex due to the nature of the fuel and its non-uniformity. Wood consists of cellulose, hemi-cellulose and lignin. It also contains water, small amounts of sulphur and nitrogen and some inorganic compounds that remain as ash after combustion is complete. For dry wood (zero moisture) a combustion equation is:

The ultimate analysis of dry wood (that is, the ratio of carbon, hydrogen, oxygen and other elements) varies slightly from one species to another, but for most practical purposes the above equation is adequate. Table 3 shows the air requirements for the complete combustion of wood.

 

The significance of the numbers in Table 3 is illustrated by the following calculation:

• 3 kg of wood are burned in a woodheater in one hour (which is equal to about 3.5 kg of air dry wood burnt per hour). It requires a minimum of 15m3 of air per hour;

• In practice, well-designed woodheaters draw about twice as much air as the minimum required to ensure good combustion, i.e. about 30m3 per hour.

• A typical living area, say 7m x 5m, has a volume of 84m3;

• This means the woodheater draws roughly one third of the room's air into the heater each hour. This is acceptable because it is desirable to have all the air in a living room changed each hour or two, in order to keep the air fresh. By contrast, open fireplaces draw far greater amounts of air up the chimney (see estimates in the open fireplace section below).

 

The combustion process described above releases about 20 MJ per kg of dry wood burnt. This varies slightly from one wood species to another, but most lie in the range 19 to 21 MJ/kg.

The first step in the process of wood combustion (see Figure 5) is the evaporation of any water in the wood. For wood with 50% moisture content (i.e. half the weight of the wet piece of wood is moisture), about 13% of the energy in the wood is needed to heat and evaporate the moisture. For air-dry wood (about 15% moisture content) about 2% of the energy is used to evaporate the remaining moisture. This is the reason why it is important to store firewood where it can be kept dry.

The second step in the combustion process involves heating the wood itself to the point where the reaction becomes self-sustaining. Below about 200°C, the chemical reactions in the wood are endothermic (i.e. absorbing energy). Above 200°C, the reactions become exothermic (i.e. giving off heat). As the wood increases in temperature, the large organic molecules are broken down into smaller molecules and escape from the solid wood as a gas. This gas, which is a complex mix of volatile organic compounds, will burn if heated to its ignition temperature (around 600°C). The burning gas is what we see as flame.

The final stage in the process of wood combustion is the burning of charcoal. As the gases are given off from the wood, the residue becomes carbon-rich. The burning embers we see in a fire are almost pure carbon (charcoal) and instead of burning gas, the oxygen in the air reacts with the charcoal, reaching high temperatures (600 to 1000°C). When the charcoal is burnt, only a small quantity of ash remains. The weight of the ash is typically about 0.5% of the weight of the original wood.

 

The Future

Firewood serves an important role in Australia's domestic heating sector. It is a cheap heating option for many families; particularly those not connected to natural gas. It is a  renewable fuel source (provided firewood supplies are properly managed) that does not add to a net greenhouse gas release. Woodsmoke is a critical issue for the industry and cleaner burning appliances will need to be developed. International pressures to reduce greenhouse gas emissions suggest that firewood may have a bright future. If sustainably managed, firewood recycles carbon from the atmosphere. Firewood plantations can also serve other ecologically desirable functions such as wastewater treatment, salination reduction, erosion control, wildlife habitat and aesthetics. It is more energy efficient to use the firewood in the home than fuel an electric power station with biomass and then use the electricity to heat the home.

 

Further Information

Information regarding renewable energy resources, technologies, applications, systems designs and case studies.

 

Woodheater Technologies

Shelton JW, 1983; Solid Fuels Encyclopedia; Garden Way Publishing, Charlotte, Vermont USA.

This is the most comprehensive book available on firewood and woodheaters. It is written for an American audience and is now out of date as far as the latest designs of woodheater are concerned. But it provides accurate and detailed information about all aspects of wood heating.

There is also the Australian Home Heating Association site. It contains background information on the Australian woodheater industry, a discussion of wood-smoke pollution and generally promotes the use of woodheaters.

An informative on-line book about wood-burning prepared by Natural Resources, Canada

  

References

Driscoll, D., Milkovits, G., Freudenberger, D., 2000. “Impact and Use of Firewood in Australia” (Online) http://www.deh.gov.au/land/publications/firewood-impacts/firewood2.html (Accessed 22 July 2008).

Todd, JJ., 2004; Health Impacts of Woodsmoke in Tasmania, In-House Fuelwood Report 66, Eco-Energy Options Pty Ltd, Hobart Australia.