Technology

Micro Gasification

Microg-gasification in Brief

Micro-gasification for household cooking is a fast-evolving development. The principle was invented in 1985 and the first commercial micro-gasifier stove was available in 2003.

The solid biomass undergo changes determined by the presence of heat and limited oxygen as follows;

As biomass is heated, it evaporates excess moisture and its surface temperature increases
At elevated temperatures, biomass pyrolyses (‘decomposes by fire’) into combustible vapors and a solid, known as ‘char’
Red hot char can be converted to ash if sufficient oxygen is available
Mixed with oxygen the vapors and gases generated can be combusted when ignited

During the whole conversion process, temperatures increase from ambient temperature to well above 800°C, depending on local conditions. In each step vapors and gases are released and the solids reduce in mass and volume. If complete combustion is attained, emissions should be clean and only contain carbon dioxide and water vapor. If combustion is not complete, the smoke and vapors composed of unburned fuel and carbon monoxide will result, both of them being harmful to human beings and the environment.

Steps of Micro Gasification

Step 1

Drying

The first change happens during drying. The amount of water transformed into vapor depends on the moisture content of the raw fuel, which also determines the heat input needed to evaporate all the water and the loss in mass and volume to get to dry fuel.

Step 2

Pyrolysis (Carbonization)

Increased temperatures and absorbed heat eventually cause a complete decomposition of the biomass, which separates into volatile gases and vapors, as a solid char remains behind. The vapors contain various carbon compounds with fuel value, referred to by the term ‘wood gases. As the solid product of this stage is char, it is also referred to as carbonisation.

Step 3

Char Gasification

Once char is formed, solid carbon atoms are converted into gases and the non-carbon portion to ash. This only happens if oxygen is available and reaches the char while it is still hot enough to react. During char gasification, oxygen reacts with the char solids, yielding carbon monoxide, carbon dioxide and creating additional thermal energy. The non-burnable solid mineral content of the char remains as ash.

Step 4

Gas Combustion

The final stage of ‘gas combustion’ is where the gases are burnt (combusted) and the bulk of the heat is released that can be used for example for cooking.

The Gasification Process

A short 3D Animation by made by Blucomb. After ignition the Flaming-Pyrolysis front (red) take place and moving downward gasifies the fuel (yellow) which turns into biochar (black). The Wood Gas produced moves upward and burns with a stable flame at the top of the reactor separated from the gas generation zone.