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• Water does not "burn"* so every percent of water mass present in a given mass of moist timber results in a reduction in the percentage of combustible material.
  So, simplistically Energy Wet = Energy dry x (100-M)/100 where M is moisture percentage of mass.

• Due to the temperature of 'flu gases' being well above the boiling temperature of water at 100C, water leaving a fire will usually be in the form of 'steam' - water vapour at > 100 C. The combustion process has had to bth heat the water to boiling point and then to supply the latent heat of vaporisation to turn it to steam. For typical moisture contents the energy required to do this is significant but relatively small compared to the energy loss due to fuel replacement.

eg assume 6 kWh/kg for a sample of dry wood.
10% moisture would displace 10% x 6000 Wh = 600 Wh from a 1kg mass.
600 Wh = 600 x 3600 = 2.16 MJ
Energy to heat 100g water 10C say to 100C ~= 4.3 J/C/G x (100c-10c) x 100g =~ 40 kJ
Energy to vaporise 100g of water from 100C to steam = 2260 J/g x 100g = 226 kJ

Total energy loss due to vapourising 10% water = 266 kJ = 11% Loss of energy from water replacing wood = 2160 kJ = 89%. Total loss to water = 2.426 MJ.
Energy loss % in 1 kg wood due to 10% moisture = 2.426 MJ / 21.6 MJ
= 11.2%

• Not taken into account is the "watergas" process whereby passing water vapour over hot carbon results in breakdown of the water into Hydrogen and Oxygen and re-re action to form CO and CO2. The net energy effects of this process vary immensely with circumstance and are ignored here.

Page 5 - Wood - open fire
0.4–0.6 m3 /100 kWh delivered;
125–200 kg/100 kWh delivered.

• Water does not "burn"* so every percent of water mass present in a given mass of moist timber results in a reduction in the percentage of combustible material.
  So, simplistically Energy Wet = Energy dry x (100-M)/100 where M is moisture percentage of mass.

• Due to the temperature of 'flu gases' being well above the boiling temperature of water at $\rm100^\circ C$, water leaving a fire will usually be in the form of 'steam' - water vapour at > $\,100^\circ C$. The combustion process has had to both heat the water to boiling point and then to supply the latent heat of vaporisation to turn it to steam. For typical moisture contents the energy required to do this is significant but relatively small compared to the energy loss due to fuel replacement.

eg assume 6 kWh/kg for a sample of dry wood.
10% moisture would displace $\rm10\% \times 6000\,Wh = 600\,Wh$ from a 1kg mass.
$\rm600\,Wh = 600 \times 3600 = 2.16\,MJ$.
Energy to heat 100 g water 10$^\circ$C say to $\rm100\,^\circ C \simeq 4.3\,J/^\circ C/g \times (100\,^\circ C-10\,^\circ C) \times 100\,g \simeq 40\,kJ$.
Energy to vaporise 100g of water from $100\,^\circ\,\rm C$ to steam = $\rm2260\,J/g\times100\,g = 226\,kJ$

Total energy loss due to vapourising 10% water = $266\,\rm kJ$ = 11% Loss of energy from water replacing wood = $2160\,\rm kJ$ = 89%. Total loss to water = $2.426\,\rm MJ$.
Energy loss % in 1 kg wood due to 10% moisture = $\rm2.426\.MJ/21.6\,MJ$ = 11.2%

• Not taken into account is the "watergas" process whereby passing water vapour over hot carbon results in breakdown of the water into Hydrogen and Oxygen and re-re action to form CO and CO$_2$. The net energy effects of this process vary immensely with circumstance and are ignored here.

Page 5 - Wood - open fire
$\rm0.4–0.6\,m^3/100\,kWh$ delivered;
$\rm125–200\,kg/100\,kWh delivered$.

• 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.

• Importantly - Based on a number of sources listed in the 2005 NZ Government sponsored report
  Warm Homes Technical Report: Detailed Study of Heating Options in New Zealand - about the same as the results asbove for a well designed enclosed burner, but down to perhaps 5 times as big a fire (20% of the efficincy, 5 x more fuel use) for informally designed open fires.

2. From the Wood fuels handbook

3. Based on Warm Homes technical report.

• 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.

• Importantly - Based on a number of sources listed in the 2005 NZ Government sponsored report
  Warm Homes Technical Report: Detailed Study of Heating Options in New Zealand - about the same as the results asbove for a well designed enclosed burner, but down to perhaps 5 times as big a fire (20% of the efficincy, 5 x more fuel use) for informally designed open fires.

2. From the Wood fuels handbook

3. Based on Warm Homes technical report.

• 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.

• 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.

• Importantly - Based on a number of sources listed in the 2005 NZ Government sponsored report
  Warm Homes Technical Report: Detailed Study of Heating Options in New Zealand - about the same as the results asbove for a well designed enclosed burner, but down to perhaps 5 times as big a fire (20% of the efficincy, 5 x more fuel use) for informally designed open fires.

Affect of moisture in wood: Energy reduction from moisture has two main sources.

• Water does not "burn"* so every percent of water mass present in a given mass of moist timber results in a reduction in the percentage of combustible material.
  So, simplistically Energy Wet = Energy dry x (100-M)/100 where M is moisture percentage of mass.

• Due to the temperature of 'flu gases' being well above the boiling temperature of water at 100C, water leaving a fire will usually be in the form of 'steam' - water vapour at > 100 C. The combustion process has had to bth heat the water to boiling point and then to supply the latent heat of vaporisation to turn it to steam. For typical moisture contents the energy required to do this is significant but relatively small compared to the energy loss due to fuel replacement.

eg assume 6 kWh/kg for a sample of dry wood.
10% moisture would displace 10% x 6000 Wh = 600 Wh from a 1kg mass.
600 Wh = 600 x 3600 = 2.16 MJ
Energy to heat 100g water 10C say to 100C ~= 4.3 J/C/G x (100c-10c) x 100g =~ 40 kJ
Energy to vaporise 100g of water from 100C to steam = 2260 J/g x 100g = 226 kJ

Total energy loss due to vapourising 10% water = 266 kJ = 11% Loss of energy from water replacing wood = 2160 kJ = 89%. Total loss to water = 2.426 MJ.
Energy loss % in 1 kg wood due to 10% moisture = 2.426 MJ / 21.6 MJ
= 11.2%

• Not taken into account is the "watergas" process whereby passing water vapour over hot carbon results in breakdown of the water into Hydrogen and Oxygen and re-re action to form CO and CO2. The net energy effects of this process vary immensely with circumstance and are ignored here.

3. Based on Warm Homes technical report.

The calorific value for dry/seasoned wood is in accordance with the other sources, as would be expected.

However, because this report deals with domestic heating it includes a range of burning scenarios ranging from open fires to enclosed controlled high temperature burners. It therefore also provides energy efficiency estimates which vary very widely with installation and which are worth looking into to see what it takes to achieve the target 1 kW in a given case.

Page 5 - Wood - open fire
0.4–0.6 m3 /100 kWh delivered;
125–200 kg/100 kWh delivered.

Efficiency of conversion to heat: 5 - 15% !!!!!!!

They note:

• The low efficiency of open fires is the result of heat lost by convection through the chimney. Most room heating occurs through heat radiation, but the effectiveness of this may be reduced by the effect of draughts caused by the movement of air towards the fireplace. The maximum efficiency of open fires is generally assumed to be 15%. Operating costs presented in this work are based on efficiencies of 10 and 15%.

• Nature of the heat: Heating is mainly radiant, with limited convection into the room (most convection heat is lost up the chimney). However, if the fireplace includes a wetback then some of the convection heat will be captured by this to provide water heating.

• Ability to heat whole house: The form of heat output, mainly radiant rather than convection, combined with the limited heat output and the lack of means to circulate or distribute this heat make this form of heating unsuitable for heating a whole house. Some older homes have multiple fireplaces, which allow heating of multiple rooms, but will still suffer from the very low efficiencies inherent in open fires.

Additional relevant entries are on pages

8 - Multi-fuel burner 55-75%
10 - Enclosed wood burner 55 - 75%
12 - Pelletised wood 75 - 92%
35 - Wood pellets 90 - 95%