Frequently asked questions

What are the units for biomass as a fuel?

By weight: kilograms (kg) or tons (t)

By volume: The volume is measured in bulk cubic meter (BCM)

One bulk cubic meter corresponds to one cubic meter (m³].

One BCM of wood chips can be converted into:

  • 0.33 cubic meter of log wood
  • 0.47 piled meter of split logs
  • 0.40 stacked cubic meter of chip wood
  • 0.67 bulk cubic meter of poured chip wood

Pellets are measured in kilograms and their energy content is 4.9 kWh/kg. Pellets are distributed either loose via tank-truck or in bags of 15 kg or 700 kg.

How much fuel do I need?

That depends on the quality of the fuel, the size of the building and the degree of isolation to other buildings.

For every square meter heated you will need:

  • with very good Isolation (30 kWh/m²/Year) 0.04 BCM of wood chips, 6kg of pellets or 0.016 staked cubic meter of beech split logs
  • with medium isolation (70 kWh/m²/Year) 0.09 BCM of wood chips, 11kg of pellets or 0.04 staked cubic meter of beech split logs
  • with bad isolation (120 kWh/m²/Year) 0.15 BCM of wood chips, 24kg of pellets or 0.06 staked cubic meter of beech split logs

1000 kWh will be achieved with 1.2 to 1.5 BCM of wood chips.

1000 litres of heating oil can be replaced by:

  • 5 – 6 scm Hardwood
  • 7 – 8 scm Softwood
  • 10-15 scm wood chips
  • around 2000 kg of pellets

By energy content

The consumed energy is measured with a heat meter, similar to measuring electricity with the electric meter. For payment the basis is Kilowatt per hour (kWh) or in larger quantities in Megawatt per hour (Mwh).

The advantage of this method is that the quality of the fuel used is almost irrelevant. In any case the costumer just pays the energy supplied to him.

What are the costs of heating with biomass compared to other energy carrier?

The example of a building that used to need 5000 litres of heating oil a year is shown in the table below.
(As of 1.6.2010)

Source (unit) € / Unit kWh / Unit € / kWh Use cost for 50.000 kWh pa. in €
Heating oil (l)          
low-weight 0,70 10 0,07 5000 l 3.500
Split logs (rm)          
Hardwood 58 1.800 0,032 27,5 rm 1.600
Softwood 36 1.300 0,028 38 rm 1.370
Wood chips (Srm)          
good quality 22 800 0,0275 62 Srm 1.360
medium quality 12 600 0,02 83 1.000
Own wood, choped 3,60 700 0,005 72 Srm 260
Pellets (kg) 0,20 5 0,04 10.000 kg 2.000
Current-peak electricity (kWh) 0,20 1 0,20 50.000 kWh 10.000
Off-peak electricity (kWh) 0,10 1 0,10 50.000 kWh 5.000

How does humidity ratio and fuel value relate?

The energy content of 1 kg wood does not vary much for different types of wood. The following table is showing the fuel value in kg:

Humidity ratio in % Fuel value in kWh/kg
  Hardwood Softwood
5 4,90 5,20
10 4,62 4,91
15 4,06 4,32
20 3,78 4,02
25 3,51 3,73
30 3,23 3,44
35 2,95 3,14
40 4,34 4,61
15 2,67 2,85
45 2,39 2,55
50 2,11 2,26
55 1,83 1,97
60 1,55 1,67

1 litre of heating oil has a fuel value of 10 kWh.
1 bcm of wood chips out of spruce (WG 30%) is worth 74 litres of heating oil. Wood from beech is far better with 102 litres.

1 solid cubic meter = 1.4 pilled cubic meter/stacked cubic meter = 2,5 cubic meter pilled wood

When do I need to have a buffer?

A buffer is not necessary with a wood chips or pellets heating system, because the fuel supply is controlled automatically and continuously.

With a bulk wood heating system the buffer is important to ensure a comfortable process. Just because of the heat in the buffer a continuous combustion is not necessary any more. The energy overflow at full combustion of the logs has to be stored in the buffer. Therefore it is important to set the size of the buffer in respect to figures drawn from past experience.
(Buffer volume = Volume of the tank x 10 to 15)

Calculation as in EN 303/5:
VSp = 15 x TB x QN x (1 – 0,3 x QH / Qmin)

VSp = Buffer volume in litre
TB = Time for full combustion at nominal load in hours (fuel dependant)
QN = nominal power of the tank in kW
QH = heat load of the building in kW
Qmin = Minimal approved nominal power of the tank in kW

Are their many technical faults at operative mode?

Faults do not occur when the feeding with fuel is under good surveillance. The main reason is unequal, fibrous wood or some contamination with non-natural objects. With paying attention to good quality a trouble-free operation can be ensured.

The fuel feeding parts have to be sized according to the power of the installation and produced with high quality material. Not the diameter of the metering screw, but the spaces and the shape of the spirals are important for trouble-free course.

How much diesel and petrol is used in the production of wood chips?

To get the wood out of the forest for the production of 50 bulk cubic meter and to split it into the end product 45 litres of diesel and 3 litres of petrol is needed.

Or: To produce the equivalent amount of heating oil (1000 litres) 12 litres of petrol are needed.

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