subscribe to the RSS Feed

Friday, March 29, 2024

Theory of wood gas production

Posted by Krzysztof Lis on March 5, 2008

This article describes in theory all the important parts of wood gas production process.

Wood gas is produced during a process called gasification in couple consecutive termochemical reactions. If you plan to use it for fueling internal combustion engines, the gas needs to be conditioned to attain required quality and contents.

This article describes theoretical side of all those phases. In article Wood gas production in practice (to be written in future) you’ll read more about manufacturing the gas in specific devices / installations.

Wood gas is produced during a process called gasification in couple consecutive termochemical reactions. If you plan to use it for fueling internal combustion engines, the gas needs to be conditioned to attain required quality and contents.

This article describes theoretical side of all those phases. In article Wood gas production in practice you’ll read more about manufacturing the gas in specific devices / installations.

Drying

The feedstock is first dried, i.e. all the moisture in fuel is being evaporated in a temperature below 200°C (390F).

Pyrolysis

Next heat causes all the volatile tars in fuel to evaporate. At this stage the temperature is much higher, 200-600°C (390-1110F).

Oxidation

In the temperature above 700°C (1290F) the atoms of carbon and hydrogen are being oxidized to carbon oxides (monoxide CO and dioxide CO2) and water vapor (H2O).

This is the first place where carbon oxide (main combustible component of wood gas) is produced.

In this stage take place the following reactions:
C + 0,5 O2 <=> CO + heat
CO + 0,5 O2 <=> CO2 + heat
C + O2 <=> CO2 + heat
2 H + 0,5 O2 <=> H2O + heat

Reduction

In reduction zone you’ll find layer of very hot char. Gases from previous stages flow through that layer and are being reduced from CO2 and water vapor to CO and hydrogen.

This is the most important stage of wood gas production, since here are being produced all it’s flammable components.

In this stage take place the following reduction reactions:
C + CO2 + heat <=> 2 CO
C + H2O + heat <=> 2 CO + H2
C + 2 H2 + heat <=> CH4.

If the wood gas is to be burned for heat production, it’s ready for that purpose in this place. Fueling internal combustion engine with gas at this point is impossible, because it’s too hot and contains too much tars and particles. The impurities would cause the engine failure and the high temperature means the gas is very diluted and has low calorific value per unit volume.

Coarse filtering

In coarse filter dust is extracted from the gas in dust separator, cyclone, settling tank, etc. At this point some tars are also collected.

Gas cooling

After gasifier the producer gas has very high temperature (200-600°C, 400-1100F). At this temperature the gas cannot be fed to engine, because is to dilute and has very low calorific value per unit volume. An engine fueled with this gas would probably not work at all or (in best case) has significant power drop. Gas has to be cooled because the secondary filter may be not temperature resistant.

In this point water vapor and tars are being removed from the gas – they condense inside the cooler. Water vapor is not combustible and therefore lowers the wood gas heating value. It may also be corrosive to engine parts if it is given chance to condense inside the engine.

Fine filtering

This filter is made of fine mesh filtration material. It removes the remaining ash or dust from the gas. The material used to filter the gas was varying, from some cloth (bag filters), small wood chips or cork chips (as used during WWII). The dust particles simply stick to the material’s surface.
At this point the wood gas is ready to be fed to the engine. It is then mixed with air to be burnt inside the engine’s combustion chamber.

Leave a comment, and if you'd like your own picture to show up next to your comments, go get a gravatar!

home | top