(Also known as producer gas generation and destructive distillation.)
Wood Gasification is the production of flammable gas products from the heating of wood. This is done by burning wood in a burner which restricts combustion air intake so that the complete burning of the fuel cannot occur. A related process is the heating of wood in a closed vessel using an outside heat source. Each process produces different products.
Given all the oxygen it needs to burn cleanly the by-products of wood combustion would be carbon dioxide, water, a small amount of ash, ( from the inorganic components of wood) and heat. This would be the ideal in a wood stove.
In full combustion of a hydrocarbon such as wood, oxygen will combine with the carbon in the ratio of two atoms to each carbon atom.
It also combines with the hydrogen in the ratio of two atoms of hydrogen to one of oxygen. This produces CO2 (carbon dioxide) and H2O (water).
The heat from combustion breaks down the chemical bonds between the complex hydrocarbons in the wood while the combination of the resultant carbon and hydrogen with oxygen, combustion, produces heat, therefore the process drives itself.
Producer Gas Generation
If the air is restricted, and there is enough heat, combustion will continue, but imperfectly. With this restricted combustion one atom of oxygen will combine with one atom of carbon, while the hydrogen will sometimes combine with oxygen and sometimes not combine with anything.
This produces carbon monoxide, (CO) (the same gas as car exhaust and for the same reason) water (H2O), and hydrogen gas (H). It will also produce a lot of other compounds and elements such as carbon, (C) which is smoke.
The heat will be enough to break down the the wood but the products of this inhibited combustion will be carbon monoxide and hydrogen, fuel gases which have the potential to continue the combustion reaction and release heat since they are not completely burned yet.
(The other products, predominately carbon dioxide and water, are those of complete combustion and can be carried no further.)
It is simple technology to produce a gaseous fuel from solid wood. A properly designed burner combining wood and air is one relatively safe way of doing so.
Where wood is bulky to handle, wood gas is convenient and can be burned in various existing devices, not the least of which is the internal combustion engine.
Destructive distillation, is heating wood in a closed container until it is hot enough for the chemical bonds between hydrogen and carbon break. Quite different process from that of combustion since no outside oxygen is introduced.
All wood contains some water and this can be anywhere from about 7% to 50% or higher, so this water is available to play a part in the destructive distillation process.
Wood also contains a wide variety of other chemicals, From alkaloid poisons to minerals. These also become part of the process, be they can be assets or indeed great liabilities.
The products of this process will depend on the make up of the wood and the temperature it is heated to, but are generally, methane gas, methyl gas, hydrogen, carbon dioxide, carbon monoxide, wood alcohol, carbon, water, and small quantities other things. Methane gas may make up as much as 75% of the mixture.
Methane is a simple hydrocarbon gas which occurs in natural gas and can also be obtained from anaerobic bacterial decomposition as "bio-gas" or "swamp gas". It has high heat value and is simple to handle.
Methyl gas is very closely related to methyl alcohol (wood alcohol) and can be burned directly or converted into methyl alcohol (methanol), a high quality liquid fuel suitable for use in internal combustion engines with very small modification.
Both of these wood gas production methods, incomplete combustion or destructive distillation, will produce an easily handled fuel that can be used as a direct replacement for fossil fuel gases (natural gas or liquified petroleum gases such as propane or butane).
It can be handled by the same devices that regulate natural gas and it will work in burners or as a fuel for internal combustion engines with some very important cautions.
It's important to cover some of the safety considerations of fuel gas:
Producer gas generators run under a modest vacuum, while destructive distillation proceeds under relatively low pressure. Obviously all the gases are flammable and the usual precautions taken with more common gaseous fuels, such as natural gas and liquid petroleum gases, should be observed.
Carbon monoxide is a large component of producer gas and it is deadly.
It's not sufficient for anyone overcome by carbon monoxide to just reach fresh air because the monoxide combines with the hemoglobin in blood to render it permanently inert.
Blood so affected can no longer carry oxygen to the body and brain. Thus extensive emergency measures must be taken to treat a victim of carbon monoxide poisoning.
Wood gas generated by a burner is NOT AT ALL suitable for use in stoves, water heaters, gas refrigerators, or any enclosed area where concentrations of the gas might collect if a flame went out. This gas must only be used in VERY well ventilated areas, preferably outside or under a roof with no walls.
Methanol (wood alcohol) is also quite deadly, including the fumes which might be breathed.
Methyl alcohol interferes with the function of the nervous system and will cause blindness and death in small quantities if consumed.
The process which produces either a gaseous product from destructive distillation, or which is the first step toward production of wood alcohol, can contain methanol as a hot vapor.
Therefore it is not actually necessary to consume wood alcohol to be poisoned by it.
Breathing methanol vapor from a poorly sealed generator system will have the same deadly effects.
Having similar or even greater dangers the above cautions also apply to many of the gas and liquid fuels that are used in everyday life and similar precautions apply, no gas appliance should be run without a automatic safety cut off.