Resulting gas volume per kilo wood

[Deleted]

rough average composition of wood

Cellulose 43%
Cellobiose C12H22O11 342 g/mol

Hemicellulose 32%
Pentose C5H10O5 = 150 g/mol

Lignin 25%
Cumarylalkohol C9H10O2 = 150 g/mol
Coniferylalkohol C10H12O3 = 180 g/mol
Sinapylalkohol C11H14O4 = 210 g/mol

On average, lignin can be seen as formed from Coniferylalkohol
C10H12O3 = 180 g / mol
Easy to see why lignin will give the most value.

Average wood reduced to:
Hexose 75% C6H12O6 = 180 g/mol
Alcohols 25% C10H12O3 = 180 g/mol

Average for calculation of resulting volume C4H7O3 = 103 g/mol.
Nine mol per kg wood.

C4 = 48 g/mol = (432g per kilo)
(H20)*3 = 18g/mol = 54g = (486g per kilo)
H= (2g/mol)*1/2 = 1g = (9g per kilo)

Moisture ~ 7.3%

Per kilo
Carbon = 432g need 1152g oxygen to build 1584g CO2 = 36 mol
Hydrogen= 8g need 64 g oxygen to build 72g H2O

1224 g oxygen will be accompanied by 4002 g nitrogen

Resulting per kilo
1584g carbon dioxide = 36 mol ~ 806 liter at 0°C .
4002 g nitrogen ~ 143 mol ~ 3203 liter at 0°C.
Water = 624g ~ 35 mol ~ 784 liter at 0°C.

Total volume per kilo wood
4793 liter at 0°C
6548 liter at 100°C
22350 liter at 1000°C
31128 liter at 1500°C

With excess air the volume could exceed 50 cubic meter
per kilo wood at 1000°C.

Calculating with table sugar instead would not give a much different result.
However wood with a significantly higher amount of carbon will also result in a
significantly higher gas volume.

Coniferous wood gives the most BTUs per kilo due to the highest lignin content
and high content of extractives like fatty acids, resin acids, waxes and terpenes.
However denser wood is usually preferable despide lower BTUs per kilo.
High content of extractives may cause unwanted effects and poisonous products.

[Deleted]

Countercheck of the calculation.

Since 23.2 mass-percent of air is actually oxygen,
the stoichiometric air-fuel ratio is 5.276 kg air per kg wood in this example.

Since 23.2 mass-percent of air is actually oxygen,
the stoichiometric air-fuel ratio is 5.276 kg air per kg wood in this example.

Specific heat capacity:
air, CO2, nitrogen ~ 1.0 kJ/kg.K
steam ~ 2.0 kJ/kg.K

"Theory of Wood Firing"
www.videncenter.dk/Groenne%20trae%20haefte/Groen_Engelsk/Kap_06.pdf
Excess-air factor 2.3 (130% excess air).

For 900°K (droping from 1000°C to 100°C)
4.0 kg nitrogen ~ 3600 kJ
6.9 kg air ~ 6210 kJ
1.6 kg CO2 ~ 1440 kJ
0.624 kg steam ~ 1123 kJ
Total ~ 12373 kJ

Latent heat of water evaporation/condensation - 2270 kJ/kg
Latent heat of 0.624 kg Water ~ 1416 kJ
Total energy ~ 13789 kJ

Hard wood 3500 (lb/cord) ~ 1600 (kg/cord)
Recoverable energy of 1600 kg wood ~ 22 millions Btu in this example.

Corresponds fairly well to the values given in tables

[pabloook]

Been looking for this info.
Guess when you say "2235 liter at 1000°C", you mean 22350 liters.

Could you please state the total volume per kilo of wood at 150 and 200ºC?
My interest is to obtain the gas speed in the ducts, and therefore obtain the head loss.

Thanks.

[Deleted]

Jun 26, 2014 15:47:09 GMT -8 pabloook said:

Guess when you say "2235 liter at 1000°C", you mean 22350 liters.

Thank you, corrected.

Could you please state the total volume per kilo of wood at 150 and 200ºC?

V0 = volume at 0°C
VT = volume at given temperature
T0 = temperature in Kelvin at 0°C = 273
T = given temperature in Kelvin


Law of Gay-Lussac

VT/V0 = T/T0

VT = V0*T/T0