Wood gas - Producer gas

Wood gas

Wood gas - partial burning of wood

Wood, or any hydrocarbon, can be partially burned with a little oxygen and water, and the result is synthesis gas which is a mixture of CO and H2. A typical analysis is:

Nitrogen N2: 50%
Carbon monoxide CO: 20-27%
Hydrogen H2: 14 - 18%
Carbon dioxide CO2: 4-8%
Methane CH4: 3-4%
Oxygen O2: 1%.

It is sometimes called "producer gas", but to be accurate that should refer to CO made by partially oxidising C.

Wood gas in combustion engines

Wood gasifiers can power spark ignition engines, with little change to the carburation. In diesel engines it is necessary to add little bit of injected fuel to ignite the mixture.. 

During the second world war when liquid fuel was in short supply, it was normal to run vehicles on gas from an on board gasifier.

Efficiency

Efficiency of the gasifier system is relatively high. The gasification stage converts about 75% of fuel energy content into a combustible gas.

The energy consumption is 1.5 times higher compared to the energy demand of the same car on petrol.

This means that 1000 kg of wood combustible matter has been found to substitute 365 litres of petrol

In power generation 1.1 kg wood produces 1 kWh of electricity.

Exhaust gas emission from an internal combustion engine is significantly lower on wood gas than on petrol. There are no particulates, and the gas produces very little carbon black in the motor oil.

Wood gasifiers

A gasifier converts solid fuel or waste to gas by partially burning them. 

The wood gas is filtered for tars and soot/ash particles, cooled and directed to an engine, furnace, or fuel cell.

Wood gas has a heating value of 5.7 MJ/kg versus 55.9 MJ/kg for natural gas and 44.1 MJ/kg for petrol / gasoline. The heating value of wood is about 15-18 MJ/kg.

Australia had 72,000 vehicles running on woodgas. Altogether, more than one million wood gas vehicles were used during World War Two.

Stoves for cooking

Using wood for cooking causes health problems due to the smoke. This can be solved by designing a stove to gasify the wood, then burn the gas.

Certain stove designs are in effect a gasifier working on the updraft principle—the air passes up through the fuel, which can be a column of rice husks, and is combusted, then reduced to carbon monoxide by the residual char on the surface. The resulting gas is then burnt by heated secondary air coming up a concentric tube. Such a device behaves very much like a gas stove. This arrangement is also known as a Chinese burner.

Coaxial downdraft gasification stove (below)

An alternative stove based on the downdraft principle and typically built with nested cylinders also provides high efficiency. Combustion from the top creates a gasification zone with the gas escaping downwards through ports located at the base of the burner chamber. The gas mixes with additional incoming air to provide a secondary burn. Most of the CO produced by gasification is oxidized to CO2 in the secondary combustion cycle; therefore, gasification stoves carry lower health risks than conventional cooking fires

Ref: Wikipedia: wood gas.

Google search for wood gas stoves

“inverted downdraft gasifier” stove

 

Chemical composition of wood smoke

Chemical g/kg Wood
carbon monoxide 80-370
methane 14-25
VOCs* (C2-C7) 7-27
aldehydes 0.6-5.4
substituted furans 0.15-1.7
benzene 0.6-4.0
alkyl benzenes 1-6
acetic acid 1.8-2.4
formic acid 0.06-0.08
nitrogen oxides 0.2-0.9
sulfur dioxide 0.16-0.24
methyl chloride 0.01-0.04
napthalene 0.24-1.6
substituted napthalenes 0.3-2.1
oxygenated monoaromatics 1-7
total particle mass 7-30
particulate organic carbon 2-20

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