Principles Of Wood Combustion For; Principles Of Log Combustion For Generating Heat; Units Of Measurement For Wood Fuel; Calorific And Emission Values - Viessmann PYROMAT ECO Technical Manual

Principles of wood combustion for generating heat

1.1 Principles of log combustion for generating heat

Units of measurement for wood fuel

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The units of measurement commonly used in forestry and the timber
industry for wood fuel are solid measures of timber (smt) and stacked
cubic metres (stcm). The solid measure of timber (smt) describes
3
1 m of solid timber mass in the form of assorted round timbers.
Conversion table for assorted common wood fuel
Unit of measurement
Assorted
1 smt round timber
1 stcm split logs
1 m long, stacked
1 stcm logs
split, stacked
1 tcm logs
split, tipped
1 tcm (forest) - woodchips
G 50 "medium"

Calorific and emission values

Timber is a renewable fuel. During combustion, on average approx.
4.0 kWh/kg units of energy are released.
The table shows the net calorific value of various types of timber with
a water content of 20 %.
Type of timber Density
kg/m
Conifers
Spruce 430
Fir 420
Pine 510
Larch 545
Deciduous
Birch 580
Elm 620
Beech 650
Ash 650
Oak 630
Hornbeam 720

Influence of moisture on the net calorific value

The water content of timber substantially influences its net calorific
value. The more water timber contains, the lower its net calorific value,
since the water evaporates during combustion and consumes energy
in the process.
Two measures are used to specify the water content.
■ Water content
The water content of timber is its water mass as a percentage of the
total timber mass.
■ Timber moisture level
The timber moisture level (hereafter referred to as moisture or mois-
ture level) is the water mass as a percentage of the total timber mass
excluding water.
The graph shows the relationship between the water content and the
moisture level, as well as the dependency of the net calorific value.
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Solid measure
of timber (smt)
Round timbers
Net calorific value (approx. at 20 %
water content)
kWh/smt kWh/stcm
3
2100 1500
2200 1550
2600 1800
2700 1900
2900 2000
3000 2100
3100 2200
3100 2200
3100 2200
3300 2300
The stacked cubic metre (stcm) is a measure for stacked or tipped
wood, measuring a total volume of 1 m
age, one solid measure of timber equals 1.4 stacked cubic metres.
Stacked cubic Stacked cubic
metre (stcm) metre (stcm)
Split logs
1 1.40
0.70 1.00
0.85 1.20
0.50 0.70
0.33 (0.50)
1 litre of fuel oil can be replaced by 3 kg of timber, considering the usual
efficiencies. A stacked cubic metre (stcm) of beech corresponds to the
energy of approx. 200 litres of fuel oil or 200 m
fore burning wood contributes to the preservation of the finite reserves
of oil and gas.
Timber has a generally neutral CO
combustion is immediately reabsorbed into the photosynthesis cycle
kWh/kg
and therefore contributes to the formation of new biomass. Another
environmental aspect is the fact that timber contains hardly any sul-
4.0 phur, therefore almost no sulphur dioxide is created during combus-
4.2 tion.
4.1
4.0
4.1
3.9
3.8
3.8
4.0
3.7
5
4
3
2
0 10
Water content in %
0 11
Moisture level in %
Freshly cut timber has a moisture level of 100 %. During storage over
one summer, this moisture level reduces to approx. 40 %. During stor-
age over several years, this moisture level reduces to approx. 25 %.
3 , including air gaps. On aver-
Tipped cubic Tipped cubic me-
metre (tcm)
Logs
Stacked Tipped
1.20 2.00
080 1.40
1.00 1.70
0.60 1.00
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of natural gas. There-
balance, as the CO
2
20 30 40 50
25 50 100
PYROMAT ECO
tre (tcm)
Woodchips
G 50 "medium"
3.00
(2.10)
1.00
created during
2