Wood Energy and Log Burning
To fully understand how wood (log) heating works, it is important to have a little bit of knowledge about the theory of wood heating.
What is biomass (wood)?
Biomass (wood) is produced by photosynthesis. This takes place in the leaves of plants where chlorophyll is involved in the process of converting carbon dioxide and water into sugars and starches. These are also called carbohydrates. The process of conversion requires energy. This energy comes from sunlight. The energy contained in wood originates as solar energy.
Photosynthesis combines CO2 and H2O to produce carbohydrates and oxygen
What is wood?
Wood is hard, fibrous, lignified structural tissue produced in trees. The "raw material" for the lignin are the carbohydrates produced by photosynthesis.
Types of Wood
Wood is commonly classified as either softwood or hardwood. The wood from conifers (e.g. pine) is called softwood, and the wood from broad-leaved trees (e.g. oak) is called hardwood. These names are a bit misleading, as hardwoods are not necessarily hard, and softwoods are not necessarily soft. The well-known balsa (a hardwood) is actually softer than any commercial softwood. Conversely, some softwoods (e.g. yew) are harder than most hardwoods.
Examples of European hardwoods from evergreen trees include holly.Common deciduous European and North American hardwood species include the oaks (Quercus species), beech (Fagusspecies), ash (Fraxinus species), maple (Acer species) and cherry (Prunus species).
Important tropical hardwoods include teak (Tectona), mahogany (Swietenia), iroko (Chlorophora excelsa), ebony (Diospyros ebenum) and rubberwood (Hevea brasiliensis).
Wood fuels can take a number of forms. These include Firewood, Wood Chip, and densified wood such as Pellets or briquettes
Energy Contained in Wood
Typically, freshly cut wood is as much 60% water. If you burn freshly cut wood, it will use nearly all the energy it contains to evaporate the water.
As a result, it is important to burn dried wood only. A log which is naturally air-dried will have about 12-18% moisture.
A wood pellet should always contain less than 10% as it is artificially dried.
How does wood burn?
There are several stages in wood combustion. These are heating, drying, gasification and charcoal burnout.
The wood is first heated and water evaporates from the surface. Heat is absorbed during this stage. The wetter the wood, the more heat is wasted in drying it. This is followed by gasification. The flames of a wood fire are due to combustion of gases released by gasification, not combustion of the wood itself.
There are two stages of gasification;pyrolysis and gasification.
Pyrolysis begins at about 150C. It absorbs some heat and does not require any oxygen. It produces tar compounds and gases such as CO and hydrocarbons.
Gasification under the influence of oxygen starts at about 230C. Approximately 80% of the weight of the dry wood is converted to gas.
The remaining part becomes charcoal which is burned out between 500C and 700C. The incombustible impurities become the ash. The ash in wood is mainly due to soil and sand absorbed in bark and a smaller proportion comes from salts absorbed during the growth of the tree.
Burning wood for central heating
Dried logs can be used for central heating purposes. This offers the lowest possible heating costs as unprocessed wood is very cheap.
A boiler known as a "Wood Gasification" boiler is specially designed to provide the best combustion of logs.
The image above is a rendering of how a wood gasification boiler works.
Split wood is loaded into the combustion chamber (#1) and is lit with a gas torch. Due to the design of the boiler, the flame from the logs is downwards. The flame then passes up through the back of the boiler and out through the flue at the top. As it does so, it heats the water which is circulated through the boiler.
Because the wood burns using the gasification process above, very little ash and impurities are left behind in the boiler.
The design of the boiler is such that it will typically only need to be loaded once per day and once lit needs no more attention.
Read on about wood gasification boilers here: Homeowners:Wood Gasification Boilers