The wood for burning in the boilers adapted for that should have less than 20% moisture content. Therefore, the correct wood storage and drying is very important.

Wood needs minimum two years of drying. The diagram shows the content of water in the beech wood when stored outdoors. It can be seen that the wood absorbs moisture again after eight months of storage.
From 14th to 21st month its moisture content is below 20%.
Wood should be stored in places permeable to air, sunny and protected against rain – preferably from the south side.
The rows of wood should be spaced a hand-breadth apart from each other to enable the air a free circulation between them and moisture removal.
Storage of fresh wood in the boiler room is not allowed. It does not dry in such conditions, but its decay occurs caused by fungi.
Only well dried wood can be stored in the correctly ventilated boiler room.

Depending on drying time and conditions, the wood contains:

• Wood dried in the drying room up to 10% of moisture
• Well dried wood outdoors up to 15% of moisture
• Dried wood outdoors, protected against rain in autumn and winter: 20% of moisture
• Fresh wood from the forest: more than 50% of moisture

Wood moisture and its calorific value

The wood calorific value depends, to a large extent, on its moisture.
The higher wood moisture, the lower its calorific value.
Water contained in the wood must be evaporated during combustion. The more water in wood, the more energy contained in the wood is lost for evaporation.

The table below shows relation between the wood moisture in percentages and calorific value KWh/kg.

It can be seen that the fresh wood with moisture of 50% has only half calorific value of the wood with moisture of 10%. It results from the fact that combustion of the fresh wood is not only uneconomic, but harmful to the environment. High content of water in wood results in combustion temperature decrease, and thus excessive smoke, sooting, water condensation in the chimney and its damage. Increased emission of harmful substances, that was not burnt, in the flue gas.

The wood dried outdoors reaches the moisture of 20% during the following time:

Combustion process

The combustion process can be divided into three phases.

1. Drying phase
In this phase water contained in the wood is changed into water vapour at temperature of approx. 100°C.
As a result of fast drying, wood shrinks and breaks, which accelerates the process, but the combustion temperature in this phase is low, smoke is emitted, energy is lost for drying. Time of the drying phase depends on water content in wood.

2. Phase of wood carbonizing
After drying phase, temperature increases from 100°C to approx. 300C and carbonization process starts. Wood mass releases a very rich in energy wood gas with different chemical composition, whose main components are carbon, oxygen and hydrogen.
Although wood belongs to solid fuels, it is composed in 84% from components that are changed into wood gas during the combustion process. When wood gas is burnt in the suitable conditions, it produces the long blue and yellow flame at temperature of 800°C to 1000°C.

3. Final phase
When all volatile matters contained in wood are burnt, charcoal combustion occurs. The incandescent charcoal is combusted at temperature of 500-800°C. In that time you can see short blue flame and no smoke.

Combustion products

The main chemical components in wood are carbon and water. Joints of those components in the correct combustion process cause that wood is environment-friendly fuel. The substances harmful to the environment such as sulphur, chlorine and heavy metals are practically not present in wood. During combustion, firstly water H2O changes into water vapour and carbon joins oxygen to form CO2.

Experience teaches that wood combustion in inappropriate conditions results in emission of thick smoke and unpleasant odour. Long-lasting and not complete combustion of gas wood, may result in:

• Too low combustion temperature, caused by e.g. high moisture of wood
• Incomplete combustion may be caused by too low intake of the air needed for combustion.
• In the wood gasification boilers where combustion takes place in the lower part of the boiler, there is too low temperature of heat water return.
• Too low chimney cross-section or its bends hinder flue gas discharge, which makes correct air intake to the boiler impossible

The incorrect combustion produces the following harmful components: carbon oxide, acetic acid, phenol, methanol, formaldehyde and further hydrocarbon substances. It also associated by excessive smoke, tar, wood tar.

Wood gasification boilers

The boilers constructionally prepared for very efficient combustion of wood are more and more often used for heating our homes. They pass positively laboratory tests and test runs. The correct combustion processes are supported in his boiler  by the fan. In the top part of the boiler there is the loading chamber, where up to 50 cm long logs are put onto the incandescent charcoal.
The fan blows air into the loading chamber, which results in releasing the wood gas. The wood gas under influence of pressure generated in the loading chamber is directed through the burner nozzle to the lower combustion chamber. The secondary air, supplied to the nozzle, is mixed with the wood gas and cause its burning at very high temperature levels of 800-1000°C. The high quality combustion is recognized by the correct blue and yellow flame and minimum ash quantity.
All tests carried out on the wood gasification boilers show that it reaches the correct combustion parameters if it works with power of minimum 50% and boiler temperature from 65 to 90°C. However, the heating system of our house needs regulation in the range from 15 to 100%. One of the ways to achieve that range is operation of the wood gasification boiler with the accumulator tank.

Accumulator tank 

The accumulator tank is the equipment where the stored boiler water takes the excessive heat produced by the boiler, and that cannot be taken by the building heated by radiators and hot water heater.
When the accumulator tank is fully loaded up to 90°C, the boiler shut-down should occur and since then the building starts taking heat from the accumulator tank through the controlled mixing valve system depending on the needs.
In the accumulator tank, hot water in its top part flows into installation, while cool water returning from the installation flows into its lower part.
As a result of this circulation, the thermal layering is formed in the tank – in the top part of the tank we have water temperature which the boiler was heated up i.e. 80-90°C, while in the lower part water temperature is 30-40°C – between the top zone  and bottom zone there is the 10-15cm thick boundary layer. To achieve the correct layering , the tank should be always loaded above 80°C.
Planning the heating with the use of the accumulator tanks it should be noted that it is assumed 50 – 70 litres of water in the accumulator tanks per one kW of the boiler power.
Heating with wood, heat pumps, solar collectors, electric heating (second rate) are the systems that must necessarily use the heat accumulation technology and therefore we connect them easily e.g. heating with wood + solar collectors.

Mistakes made when heating with wood

The elementary mistake made when heating with wood is burning the wood with too high moisture content i.e. above 20%. Burning wet wood produces excessive smoke for a long time after ignition. Evaporation of water from wood absorbs a lot of energy, it is difficult to achieve the optimum combustion temperature, below which wood gas is not burnt, and   released into the atmosphere through the chimney. Low flue gas temperature results in the condensation of water vapour in the chimney and consequently damages its construction. Wood should be correctly stored and dried outdoors for two years as minimum. After delivery from the forest, it should be cut to the appropriate length and chopped.

A mistake is not to keep the boiler heat exchanger clean. The heat exchanger should be cleaned regularly at least once a month to remove soot and ash.

In the boilers using the wood gasification process, the flue gas temperature should be above 150°C and the boiler temperature above the dew point i.e. 60°C. The boiler operation below those temperatures results in a quick corrosion of the body and chimney damage.

During the periods of transition temperatures i.e. autumn – spring, the boiler should be loaded with wood only partially so that it could be combusted fully.

What is the calorific value of wood?

Moisture of 15% is assumed.