Firewood: an environmentally responsible fuel

Contents of this page

Firewood: Environmental disadvantages
Firewood and smoke
Firewood and destruction of live trees
Firewood and destruction of habitat
Firewood and greenhouse
Firewood and overpopulation
Firewood-overpopulation-greenhouse-air pollution
Firewood and exercise
Water heater: wood-fired
Water heater: wood-fired_drum
Water and home heating: wood-fired
Growing trees_Land required
Firewood into liquid fuel
Index

Created as a separate page 2005/06/09, modified 2008/11/08
Comments welcome, email daveclarkecb@yahoo.com

Introduction

This page aims to explain some of the advantages and disadvantages of firewood, mainly in relationship to the environment.

I hope to provide 'food for thought' more than supporting firewood and condemning the consumption of fossil fuels.

One tonne of air-seasoned wood releases 16 gigajoules (GJ) of heat when burned; see Wikipedia. At the time of writing a tonne of firewood sold for around Aud$160 in Adelaide, South Australia. For comparitive energy costs see Energy cost calculator.

Background

Until the Industrial Revolution firewood was mankind's main source of heat. It was also used for the production of charcoal which then was used to smelt metallic ores and to work iron and steel.

Fossil fuels can be obtained in greater quantities than firewood, and petroleum, being a liquid, is more convenient to use as a fuel. However, there are two great problems associated with burning fossil fuels:

They are nonrenewable,
They produce an increase in the world's greenhouse carbon-dioxide.

Firewood is becoming scarcer. See also, native trees in the Clare Valley. They are suitable for firewood production and may be a viable crop as such.

Environmental disadvantages resulting from firewood use

Smoke

Burning wood produces smoke. The amount of smoke varies depending mainly on two factors: how dry the wood is and how efficient the burning is. If the wood is burned in people's homes then these factors are very much in the hands of individual householders.

Modern stoves are designed to produce little smoke if used properly. Some stoves have a calalytic 'after-burner', a ceramic honeycomb in which smoke and other particles are consumed.

It is at least theoretically possible to efficiently burn wood in large facilities that could then provide heat to whole communities via piped hot water; so minimizing smoke and avoiding the consumption of fossil fuels and the net production of greenhouse carbon-dioxide.

Destruction of live trees

Wood comes from trees. A great many of the world's trees have been destroyed to obtain either timber, wood chips (for paper pulp), or firewood. The loss of trees is resulting in environmental problems such as: increased erosion, increased flooding, soil salinisation, and eventual shortage of timber, pulp and firewood.

Destruction of habitat

Dead wood and hollows in old trees provides habitats for various animals and fungi. Rotting wood provides ideal sites for the establishment of some other plants. If dead wood is collected as firewood, habitat is lost.

Wood contains minerals such as phosphorus that are valuable plant nutrients. Phosphorus is already being removed from rural land and dumped in city sewers via cropping; this removal of nutrients from soil is unsustainable.
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Firewood and greenhouse

Other forms of home heating, oil, gas and electricity, consume fossil fuels. (Most electricity in Australia is produced by burning fossil fuels). Consumption of fossil fuels is unsustainable because the reserves of petroleum (in particular) are finite, and carbon-dioxide, a greenhouse gas, is produced when fossil fuels are burned.

While carbon-dioxide is also produced by burning wood, it is reabsorbed by growing the trees that produce replacement wood. So long as the net amount of wood in the world remains constant, no net carbon-dioxide is released into the atmosphere by burning firewood.

Firewood and overpopulation

As discussed above, firewood can provide home heating sustainably. The energy stored in the wood was absorbed from sunlight while the trees grew. However, there are too many people in the world for enough firewood to be grown to supply the home heating demand. And there are so many people in densely populated cities that the smoke produced by wood fires becomes a health problem.

A significant factor is that where home heating is most needed - at high latitudes - people are wealthier and are generally able to use petroleum for heating. Where people are forced to gather and use firewood by poverty - in the third world - there is less need for home heating because most third world nations are in the tropics.

To change from petroleum heating to wood heating in rich nations would require a huge consumption of firewood. poverty

Firewood/overpopulation/greenhouse/air pollution

There is an inter-relationship between firewood, overpopulation, and production of greenhouse gas and smoke.

Overpopulation results in excessive consumption and leads to a shortage of firewood.
Burning of firewood in areas that have high population densities results in air pollution within those areas.
Overpopulation makes firewood less desirable as a source of heating in the short term, and make fossil fuels more desirable in the short term, while increasing the consumption rate of fossil fuels and making their use impossible in the longer term.
Overpopulation combined with increased consumption of fossil fuels drives the man-made greenhouse component.
Greenhouse is another reason why population levels should be minimized.
There is some trade-off between local air pollution (smoke) and pollution of the whole atmosphere (greenhouse carbon-dioxide) in the choice of burning wood or fossil fuels.

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Firewood and exercise

Another imbalance is that the use of firewood generally involves exercise, and those parts of the world where firewood is most used as a fuel are where people are in least need of additional exercise; ie. the Third World. In the West, where heating is mostly achieved by burning fossil fuels, there is a major problem with insufficient exercise and obesity.

Both the collection of firewood, and reducing it to a convenient size for consumption in the home, involve considerable exercise.

Wood-fired water heaters

The photo shows my 'rocket' water heater (the reason for the name being obvious), part of the poly pipe solar water heater (lower left), and the 'drum' water heater (to the right of the rocket. The poly pipe has since been replaced with a much more efficient solar water heating panel, see About Me

My only complaint with the Rocket is that the water smells of (I think it is) ethyl mercaptan, the substance that is used to give liquefied natural gas its distinctive smell. I have found the smell remarkably long-lasting; it was sickeningly strong for the first few showers. The best solution seems to be to remove the top and bottom pipe connections and allow the heater to air out for a week or two; so far I've been needing it too much to do this for more than a very few days. Why the manufactures could not leave the tanks sitting in the Alice Springs sun for a couple of weeks after cutting the large holes for the flue, I don't know.

Rocket

Few people in Australia (and, I suspect, the developed world generally) use firewood for water heating. A good wood-fired water-heater is available from 'The Center for Appropriate Technology' in Alice Springs, Australia. Loosely called 'The Rocket' for its appearance, it is made out of a recycled 80kg liquefied gas cylinder. It heats enough water for two generous showers in 30 to 40 minutes (starting from water at about 5 degrees Celsius).

If you were to get the water temperature up to around boiling point I would think there would be enough for four generous showers in winter. Of course if you keep the fire going you could have a shower about every 15 minutes. About 7kg of firewood is sufficient to heat the water to showering temperature in winter.

The cost, as of July 2002, is about $550 plus whatever it costs you to arrange transport.

Very cheap drum water heater

A cheaper wood-fired water heater can be made by placing a fireplace under a 200L drum lying on its side. Inlet and outlep pipes can be connected to the two bungs of the drum. See photo above.

Of course a 200L drum is not built to take much internal pressure, so care must be taken to not burst it. A drum bursting due to the water boiling inside it would be very dangerous.

This type of water heater has been used in the station (outback) country of Australia for many years. The fire must be kept going for about two hours to heat the water.
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Wood-fired water and home heating

Commercial 'combustion' wood heaters or wood stoves can be combined with water heating. Water heating combined with wood-fired space heating is not widely available in Australia, although there is at least one manufacturer of a water jacketted section of flue that can replace a normal section. The flue section is double-walled with the water between the outer and inner walls. As the water heats it must be permitted to convectively circulate with a hot-water storage tank at a higher level.

Wood cooking stoves do often seem to be combined with water heating.

How much land is needed to grow the trees?

In South Australia trials have shown that, in a 500mm rainfall area, five tonnes of firewood can be grown per hectare each year.

I have read, and can believe from my own experience, that about 0.2ha of land, 0.4ha at a maximum, is sufficient to grow enough trees to supply a household with firewood. This assumes that firewood is the main source of space and water heating. It is based on a 500mm annual rainfall and 25 years experience with growing native trees in Australia and the use of firewood.

Turning firewood into liquid fuel

Forestry Insights (NZ) describe how methanol can be produced from wood by a process known as gasification and how ethanol can be produced from wood by acid hydrolysis and fermentation.

A litre of methanol, when burned, yields about 45% as much energy as a litre of petroleum, and ethanol yields about 68% as much energy as petroleum; see Energy units

Tore Högnäs wrote a page on the efficient production of liquid fuel from wood, it is available from www.confor.org.uk/timber_transport/.

Below is a quote from this page:

The production of wood liquid fuel is based on rapid pyrolysis. Crushed wood is heated in an airborne bed reactor to 500-600 ºC for some seconds, which leads to a separation of gas, liquid and solid material. The gas is liquefied by rapid cooling. The process is efficient: 60-70 % of the dry weight of the wood is transformed into liquid. The product is a liquid and not tar since it contents 20 % water. The energy content for wood liquid fuel is about half of that for normal mineral oil. The product cannot be mixed with mineral oil.

The liquid can be burned in heating plants and boilers now heated by mineral oil. It can even be used in diesel engines. However, the replacement of mineral oil with wood liquid fuel requires some technical changes in the burning facilities or engines. Wood liquid fuel is sour and requires for instance rust-resistant pipes and tanks. Burning of wood liquid fuel is as clean as mineral oil and there are no net emissions of carbine dioxide. Compared with traditional wood fuels wood liquid fuel is cleaner to use.