What is the size/scale of a wood fire that is producing 1kW?
Based on what my understandings re energy content of fuels and combustion processes,
about 0.5 to 1 cubic inch per minute of typically dry wood in an open fire.Based on an utterly superb 80 page Wood Fuels handbook which I discovered along the way - about the same, rather to my surprise. See at end for details.
- From typically published figures for energy in fuels.
Hydrocarbons typically provide around 10 kWh/kg for complete combustion.
Wood is a combination of hydrocarbons plus long chain molecules with less energetic bonds. In addition, water content makes a very significant difference to heat output, with well dried wood providing perhaps double the energy output per mass.
For well dried wood a reasonable estimate is perhaps 3 to 5 kWh/kg thermal output with good combustion. An open fire is liable to result in less than complete combustion (as evidenced amongst other things by yellow carbon rich flames and smoke). A closed combustion space with insulated walls, good air supply directed in such a way as to encourage circulation and higher combustion temperatures may approach the ideal. A "fire" may be well under half the ideal. A first trial of say 3 kWh/kg is liable to be if anything high.
To achieve 1 kW at 3 kWh/kg will take 1/3 kg or about 330 grams or about 11.5 ounces of wood per hour. Wood is seldom provided in ounce or even gram lots :-).
Wood densities vary widely but, as long as you are not burning lignum vitae, a density of around 0.6 (relative to water or 600 kg/m^3) is liable to be roughly correct.
So the requisite 330 grams of wood will have a volume of about 330/0.6 = 550 cc or about half a litre. A classic Coke or Pepsi tin has a volume of 375 cc so 550cc is about
550/375 = a 1.5 x softdrink-can-sized piece of wood per hour.
In reality that is liable to be on the low side with an open fire and less than perfectly seasoned wood and probably double that would be safe. Double that is about 1 litre or 3 softdrink cans.
That's 1000/60 = 17cc/minute or about 1 cubic inch per minute.
Thats a reasonably easily visualisable amount and, as above, with well seasoned wood and a reasonable burner it could be half that amount.
- From the Wood fuels handbook
Published by AIEL - Italian Agriforest Energy Association in 2008
This is a superb resource for anyone interested in this subject.
This 80+ page booklet is aimed at the commercial wood burning market and covers all aspects of assessing and processing harvested wood and the affect of species, water content processing methods and much more.
A look through it's many tables and graphs suggested that
The results vary immensely with a number of major factors being relevant, and
My empirical results (based on input from many sources over too many years) are 'within the range expected'.
The chart below from page 25 in the report provides one useful result.
The red line represent kWh/kg plotted against moisture content.
At high moisture contents (60%) about 1.6 kWh/kg is available.
For almost perfectly dry wood about 5 kWh/kg is available.
For intermediate states they identify
Wood chips - 3.4 kWh/kg
Log woods - 4.0 3.4 kWh/kg
Pellets - 4.6 kWh/kg
These need to be reduced by the density when converting kWh/kg to kWh/litre
or to kWh per can of coke sized piece of wood. They provide a wide range of density values for different wood types and situations, with my density = 0.6 being close enough to usual to be useful.
