©2014 FIRECRACKER™GUW ALL RIGHTS RESERVED

What is a gasifier

(Click to enlarge photo)

In an open log fire the flames that can be seen are not the logs burning directly but the gas that is produced when the logs get hot. Because an open fire can draw in as much oxygen as it wants there is no real control over the combustion and eventually the logs are reduced to ash.

An enclosed wood-burning stove does the job in a much more controlled way where the operator can set how much air enters under the grate and therefore how much gas is produced, this is called 'primary air'. To allow the gas to burn properly more air must then be added above the grate and this is called 'secondary air', where the aim is to allow as little unburned gas (smoke) as possible to travel into the flue or chimney. A gasifier is similar to this but usually without the secondary air being added, the gas (called 'producer gas' or 'wood gas') can then be piped elsewhere to be burnt later on. Coal gas ('town gas') used to be made in a similar way by turning coal to coke.

There are various types of gasifiers made in many different sizes. Some are lit at the top and some at the bottom, some have air moving upwards and some downwards but the principle is still the same. In the photo you can see the top of a small gasifier. This is essentially just a tube full of fuel that has been lit at the top where a small quantity of air is fed in at the bottom. As the hot area (know as the 'pyrolysis zone') moves downwards, it leaves wood-chips that have been converted to charcoal above. Eventually all of the wood-chips are used up but the process doesn't then stop. What happens is that the temperature now increases and the charcoal itself is turned into gas until all the charcoal is gone and then the process stops. The residual ash represents a volume of less than 1% of the original wood-chips.

The gasifier in the photo is designed to burn the gas almost immediately it is produced and through the holes it is possible to see the gas alight. The gas burner here works in the opposite way to a gas ring on a cooking hob as air is drawn in through the holes (secondary air) to combust the gas.