Drying of Woodchips

Moisture in wood fuels and
drying of wood chips
Mikko Helin
North Karelia Polytechnic
Joensuu 7.6.2005
Wood as a fuel
{ A high volatile content (≈ 80 – 85 %) and

relatively low carbon content (≈ 50 %)
{ At harvest the moisture content of wood is
around 50 % (total weight)
{ The moisture in the fuel lowers the net heating
value
{ Moisture content decreases conversion
efficiency into energy and increases gaseous
emissions (incomplete combustion)
Wood as a fuel…
{ Heating value of wood in average = 19 MJ/kg

{ To evaporate 1 kg of water takes about 2,5
MJ/kg H2O (0,8 kWh/kg H2O)
{ e.g. 2 kg of wood chips, moisture content 50 %
= 1 kg dry-matter of wood + 1 kg water
→ net heating value = (19 – 2,5) MJ = 16,5 MJ
→ 8,25 MJ/kg of fuel
{ Moisture content affects greatly on heating
value of wood fuel (see tables 1 & 2 )
Heating value of wood
{ Table 1. Net heating { Table 2. Net heating

value of wood in average value of 1 loose-m3 of
wood chips, dry matter
Moisture per kg of fuel weight 180 kg
content % MJ/kg kWh/kg
0 19,0 5,3 Moisture

10 16,9 4,7 content % kWh/loose-m3
20 14,7 4,1 10 940

30 12,6 3,5 20 920
40 10,4 2,9 30 890
50 8,2 2,3 40 850
60 6,1 1,7 50 800
Small-scale boilers vs. moisture content
{ In practice the heating effect of small-scale

boilers decreases even more
{ The total efficiency of the boiler (see figure 1)
decreases rapidly when the moisture content
rises (boiler tests are usually made with fuel
that contains 30 % or less moisture/water)
{ It should also be considered that the boiler
stays cleaner, malfunctions and need for
maintenance decreases → uptime increases
Moisture content vs. boiler efficiency
{ Figure 1
An average efficiency of one 50 kW boiler with
different moisture content of chips
90
Efficiency rate [%]
80
70
60
50
40
10 20 30 40 50 60
Moisture content of chips [%]
Example of a small-scale boiler
{ We can calculate the amount of heat we will get out
of 1 loose-m3 of wood chips with different moisture
content:
a) chips: moisture content 50 %, net heating value 800

kWh/loose-m3, boiler efficiency: η = 62 %
b) chips: moisture content 20 %, net heating value 920
kWh/loose-m3, boiler efficiency: η = 77 %
a) Eheat = 0,62 x 800 kWh = 496 kWh
b) Eheat = 0,77 x 920 kWh = 708 kWh
You get 43 % more heating energy!

Drying of wood chips
{ Traditional drying
z Whole tree or chip pile
z In the open or covered
{ Artificial/forced drying of wood (usually a fan
is been used to produce a high drying-air flow)
z Un-heated drying-air
z Heated drying-air e.g.
{ Solar energy
{ boiler water energy
{ Flue gas energy
Dryer models
Dryer models…
Approximate investment costs
{ small movable dryer using un-heated drying-air

(capacity 70 loose-m3)
z dryer building 2 000 €
z fan 700 €
{ medium size dryer using solar energy heated

drying-air (capacity 300 loose-m3)
z dryer building 15 000 €
z fan 2 000 €
General specification for artificial drying
{ Air flow should be steady in the dryer and there is

always a counterpressure when blowing trough a
chip layer.
{ Air flow rate with un-heated air is about 400 –
500 m3/h per loose-m3 of chips. Height of the chip
layer in the dryer is 0,8 – 1,5 m.
{ hole/opening density on the dryer floor should be
at least 5 – 10 % of the floor area
{ It is evenly important to make sure that exhaust
air flow rate is big enough to remove the moisture
from the dryer (otherwise the moisture
condensates in the dryer).
Example…
z The drying time can be calculated approximately:
- chips: 80 loose-m3; moisture content: 50 %; bulk density 350 kg/loose-m3
- drying-air fan: 5,5 kW; 400 m3/hair per loose-m3 of chips
- desired value of moisture: 20 %; amount of water to be evaporated: 130
kg/ loose-m3 of chips
- air: density = 1,2 kg/m3; temperature = 20 C; relative humidy = 60 %; 1 kg
of air binds about 2,0 g of water (2,40 g/m3 = 0,0024 kg/m3)
- drying efficiency: 90 %
It would take
- Consumption of electric power is 828 kWh (10,3 kWh/loose-m3)

- If drying-air is been heated (solar energy) +2 °C, water binding capacity
will increase to 2,6 g/ m3 and it would take now 116 hours. Consumption
of electric power would decrease 191 kWh (consumption of electricity:
637 kWh, 8,0 kWh/loose-m3).
Conclusion
{ Drying of wood chips might be

beneficial if
z Charged by the amount of wood chips
z Troubles with boiler when using moist
wood chips
z Existing building for the dryer (smaller
investment)
z ???
Thank you for your attention!

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Moisture in wood fuels and

drying of wood chips

{ A high volatile content (≈ 80 – 85 %) and

{ Heating value of wood in average = 19 MJ/kg

{ Table 1. Net heating { Table 2. Net heating

0 19,0 5,3 Moisture

20 14,7 4,1 10 940

{ In practice the heating effect of small-scale

a) chips: moisture content 50 %, net heating value 800

a) Eheat = 0,62 x 800 kWh = 496 kWh

b) Eheat = 0,77 x 920 kWh = 708 kWh

You get 43 % more heating energy!

{ small movable dryer using un-heated drying-air

{ medium size dryer using solar energy heated

{ Air flow should be steady in the dryer and there is

- Consumption of electric power is 828 kWh (10,3 kWh/loose-m3)

{ Drying of wood chips might be

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