Venselaar (46) compares the design characteristics of a number of gasifiers that were commercially available during the Second World War. A distinction is made between "no throat" "single throat" and "double throat" (Imbert type) designs (see Figure 2.15). He comes to the conclusion that the three types differ mainly in maximum allowable hearth load, giving values of B max. of 0.03, 0.11 and 0.4, respectively for "no throat", "single throat" and "double throat" gasifiers.
Further conclusions from this comparison are that:
- nozzle air inlet velocities should be around 30 - 35 m/s;
- throat inclination should be around 45° - 60°;
- hearth diameter at air inlet height should be 10 cm larger than "throat" diameter in case of a "single" throat design, and about 20 cm larger than the diameter of the narrowest constriction in case of "double" throat design;
- height of the reduction zone should be more than 20 cm (the average height of reduction zone for the gasifiers reviewed was 32 cm);
- height of the air inlet nozzle plane should be 10 cm above the narrowest constriction.
As far as "double throat" or "Imbert type" gasifiers are concerned, there is good agreement between design rules presented by the authors of (43) and (46).
It must be emphasized that the above empirical design rules are based on experiences with gas producers fuelled by wood blocks varying in size between 3 to 5 x 6 to 8 cm.
A more theoretical approach is given by Groeneveld (17) who points out the importance of the fuel size. He proposes that the determining factor of the maximum load of any given gasifier is the residence time of the fuel in the pyrolysis zone. If this residence time is too short, heat penetration into a given fuel particle is insufficient to cause complete