I hope to provide 'food for thought' as much as supporting firewood and condemning the consumption of fossil fuels.
One tonne of air-seasoned wood releases 16 gigajoules (GJ) of heat
when burned; see
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.
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 three great problems associated with burning fossil fuels:
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.
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.
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.
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.
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.
|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.|
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.
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.
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.
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.
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.
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