## Thursday, February 12, 2009

### Heating Value of Wood Gas and Coal (Town) Gas.

Since I have seen various estimates for the heating values of the fuel gas derived from the gasification of wood and of coal, to put my own mind at rest, I have worked-out from first principles what they should be according to quoted typical compositions for "wood gas" and "coal gas" (otherwise known as "town gas"), formed by heating coal in closed retorts in the absence of air. The strategy is different in the latter case, since bituminous coal will yield around 13% of its mass of town gas by simple heating, leaving around 65% of the original coal mass in the form of coke and the remaining 22% distilling over as coal-tar. Most usually, wood (or other biomass) is gasified by heating it in a stream of air, drawn either up or down the "gasifier" depending on its design as either "draw-up" or "draw-down" in which the process takes place, and consequently almost half the resulting gas is nitrogen. Apart from a few percent ash all of the wood is thus converted to gas, in contrast to coal as it is normally converted to town gas (see below).

WOOD GAS.
A composition is: N2, 50.9%; CO, 27.0%; H2, 14.0%; CO2, 4.5%; CH4, 3.0%; O2, 0.6%.

Per cubic metre (m^3) of gas, this amounts to:

270,000/24450 (volume of one mole of an ideal gas at 25 deg C and atmospheric pressure) = 11.04 moles CO, x 28 = 309.12 g.
140,000/24450 = 5.73 moles H2 = 11.46 g.
30,000/24450 = 1.23 moles CH4 = 19.68 g.

Given the molar enthalpies of combustion for these gases, we get:

11.04 x -283 (kJ/mol) + 5.73 x -286 + 1.23 x -890 = 3.12 MJ + 1.64 MJ + 1.09 MJ = 5.85 MJ/m^3.

Remaining (incombustible) gases:
509,000/24450 = 20.82 moles N2 = 582.90 g.
45,000/24450 = 1.84 moles CO2 = 80.98 g
6,000/24450 = 0.25 moles O2 = 8.00 g

Total mass of 1 m^3 of gas = 1,012.14 g. Hence the calorific value of wood gas is 5.85 MJ/m^3 or 5.85 x 1000/1012.14 = 5.78 MJ.kg.

TOWN GAS.
A composition is: H2, 51%; CO, 15%; CH4, 21%; C2H4 (ethene), 3%; CO2 + N2 = 10%.

Per m^3 of gas, this amounts to:

510,000/24450 = 20.86 moles H2 = 41.72 g.
150,000/24450 = 6.13 moles CO = 171.78 g.
210,000/24450 = 8.59 moles CH4 = 137.42 g.
30,000/24450 = 1.23 moles C2H4 = 34.44 g.
100,000/24450 = 4.09 moles CO2 + N2 = 151.33 g (assuming a 50:50 mixture).
Making a grand total of 536.69 g.

Again, using the molar enthalpies of combustion we derive:

20.86 x -286 + 6.13 x -283 + 1.23 x -1423 + 8.59 x 890 = 5.97 + 1.73 + 1.92 + 7.65 = 17.27 MJ/m^3. This translates to: 17.27 x 1000/536.69 = 32.18 MJ/kg.