Operation of modern spark ignition or compression ignition stationary engines with gasoline or diesel fuel is generally characterized by high reliability and minor efforts from the operator. Under normal circumstances the operator's role is limited to refuelling and maintenance. There is little need for action and virtually no risk of getting dirty. Start and operation can in fact be made fully automatic.
Anybody expecting something similar for wood gas operation of engines will be disappointed. Preparation of the system for starting can require half an hour or more. The fuel is bulky and difficult to handle. Frequent feeding of fuel is often required and this limits the time the engine can run unattended. Taking care of residues such as ashes, soot and tarry condensates is time-consuming and dirty.
It is a common mistake to assume that any type of biomass which fits into the opening of the refuelling lid can be used as fuel. Many of the operational difficulties which face inexperienced users of gasifiers are caused by the use of unsuitable fuels. In order to avoid bridging in the fuel bunker, reduced power output because of large pressure losses, or "weak" gas, slag cakes, tar in the engine and damage to the gasifier caused by overheating, it is necessary for most designs that the fuel properties are kept within fairly narrow ranges. This is not necessarily a more serious limitation than the need to use gasoline of super grade for high compression spark ignition engines rather than regular gasoline or diesel fuel. But in the case of gasifier operation, more of the responsibility for quality control of the fuel rests with the operator. The need for strict fuel specifications is well documented in the experiences reported from the Second World War (43). It is unfortunate that some commercial companies, with little practical experience, but trying to profit from the renewed interest in gasification, have advertised the possibility of using almost any kind of biomass even in gasifiers which will work well only with fuels meeting fairly strict standards. This has in some cases created unrealistic expectations and has led to disappointments with the technology.
Operation of wood gas engines can also be dangerous if the operator violates the safety rules or neglects the maintenance of the system. Poisoning accidents, explosions and fires have been caused by unsafe designs or careless handling of the equipment. It may be assumed that modern systems are designed according to the best safety standards, but it is still necessary to handle the equipment in a responsible manner.
Finally, it must be realised that the current technology is generally based on the designs of the mid-1940's. Only a few persons have retained detailed practical knowledge of design, material selection and operation and maintenance procedures. Many of the currently active manufacturers have no access to the experience of such persons and base their designs on information available in the literature, and on recent and comparatively limited experience. There has been some improvement of the technology, for instance of filter designs based on new materials, but the practical operating experience with these improved systems is limited. A consequence of this is that equipment failures caused by design mistakes, choice of the wrong materials, or incomplete instructions to the user on operation and maintenance, must be expected in the first period of reintroduction of wood gasifiers.
The reports on operational difficulties presented in this publication and elsewhere must be evaluated with this in mind. It can safely be assumed that second generation systems will show improved performance.
Those interested in the technology must accept that it demands hard work and tolerance of soiled hands by a responsible operator, and that it is not yet perfect. But as will be shown, it is both serviceable and economic in many applications in spite of its inconveniences.
