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2.8 Health and environmental hazards associated with the use of producer gas

2.8.1 Toxic hazards
2.8.2 Fire hazards
2.8.3 Explosion hazards
2.8.4 Environmental hazards

A review of the different types of hazards and environmental impacts of producer gas operation has been published by Kjellström (23).

Toxic, fire and explosion hazards are the main categories.

2.8.1 Toxic hazards

An important constituent of producer gas is carbon monoxide, an extremely toxic and dangerous gas because of its tendency to combine with the haemoglobin of the blood and in this way prevent oxygen absorption and distribution. A summary of the effects caused by different concentrations of carbon monoxide in the air is given in Table 2.9.

Fortunately normal producer gas installations work under suction, so that even if a minor leak in the installation occurs, no dangerous gases will escape from the equipment during actual operation. The situation is different however during starting-up and closing down of the installation.

During starting-up the gas is generally vented, and it is necessary to ensure that the gases produced cannot be trapped in an enclosed room. As a rule a suitable chimney will provide sufficient safety.

During closing-down of the installation a pressure buildup in the gasifier will occur, caused by the still hot and pyrolysing fuel. As a result gases containing carbon monoxide will be released from the installation during a relatively short period. It is because of the danger from those gases that it is generally recommended that a gasified installation be located in the open air, if necessary covered by a roof.

Table 2.9 Toxic effects of different concentrations of carbon monoxide in the air

Percentage of CO in air





no significant effects



possibly headache, mild frontal in 2 to 3 hours



headache frontal and nausea after 1 to 2 hours, in the back of the head after 2.5 to 3.5 hours



headache, dizziness and nausea in 45 min. collapse and possibly unconsciousness in 2 hours



headache, dizziness and nausea in 20 minutes, collapse, unconsciousness and possibly death in 2 hours



headache and dizziness in 5 to 10 minutes, unconsciousness and danger of death in 30 minutes



headache and dizziness in 1 to 2 minutes, unconsciousness and danger of death in 10 to 15 minutes



immediate effect; unconsciousness and danger of death in 1 to 3 minutes

There has been some dispute, deriving from Swedish experience, whether chronic poisoning can occur as a result of prolonged inhalation of relatively small amounts of carbon monoxide which give no acute effects. It-seems that the issue now has been resolved: no chronic symptons can occur through carbon monoxide poisoning.

However this does not mean that the symptoms mentioned in Swedish literature (tiredness, irritability and touchiness, difficulty in sleeping) did not result from prolonged exposure to producer gas. There is a possibility that some other compound(s) in the gas are responsible for the symptoms.

The above stresses again the importance of placing stationary installations in an open environment as well as of taking care to avoid close contact with the gases during the starting-up and closing-down phases.

2.8.2 Fire hazards

Fire hazards can result from the following causes:

- high surface temperature of equipment;
- risks of sparks during refuelling;
- flames through gasifier air inlet on refuelling lid.

Risks can be considerably decreased by taking the following precautions:

- insulation of hot parts of the system;
- installation of double sluice filling device;
- installation of back-firing valve in gasifier inlet.

2.8.3 Explosion hazards

Explosions can occur if the gas is mixed with sufficient air to form an explosive mixture.

This could occur for several reasons:

- air leakage into the gas system;
- air penetration during refuelling;
- air leakage into a cold gasifier still containing gas which subsequently ignites;
- backfiring from the fan exhaust burner when the system is filled with a combustible mixture of air and gas during starting-up.

Air leakage into the gas system does not generally give rise to explosions. If a leakage occurs in the lower part of the gasifier (as is generally the case) this will result in partial combustion of the gas leading to higher gas outlet temperatures and a lower gas quality.

When the pyrolytic gases in the bunker section are mixed with air (as is bound to happen during refuelling) an explosive mixture can be formed. It is not unusual for this to result in small and relatively harmless explosions, especially when the fuel level in the bunker is relatively low.

Risk to the operator can be obviated if the gases in the bunker section are burnt off through the introduction of a piece of burning paper or the like, immediately after opening the fuel lid. Another possibility is to install a double sluice type filling system.

Air leakage into a cold gasifier and immediate ignition will lead to an explosion. Cold systems should always be carefully ventilated before igniting the fuel.

During the start-up of an installation, the gases are as a rule not passed through the entire filter section, in order to avoid blocking the filters with the tars produced during start-up. The filter may thus still contain air, and after an inflammable gas is produced and led through the sometimes quite voluminous - filter section an explosive mixture can result. If the gas is now ignited at the fan outlet a backfire can occur, leading to a violent explosion in the filter section. It is fox this reason that it is advisable to fit the fan outlet with a water lock.

2.8.4 Environmental hazards

During the gasification of wood and/or agricultural residues, ashes (from the gasifier and from the cleaning section) and condensate (mainly water) are produced. The latter can be polluted by phenolics and tar.

The ashes do not constitute an environmental hazard and can be disposed of in the normal way. For the tar-containing condensate the situation is different, and disposal of those from a large number of gasifiers can have undesirable environmental effects. No hard data are available on the bio-degradation of the phenolic and tarry constituents of the condensates, and the problem of disposal needs careful study.

The properties of exhaust emissions from engines run on producer gas are generally considered to be acceptable, comparable to those of diesel engines.

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