From the article:
The “superficial velocity” (hearth load) of a gasifier is the most important measure of its performance, controlling gas production rate, gas energy content, fuel consumption rate, power output, and char and tar production rate.
The superficial velocity, SV, of a gasifier is defined as: SV = Gas Production Rate/Cross Sectional Area = (m3/s)/(m2/s) = m/s It is easily estimated or measured by measuring gas production rate or fuel throughput and gasifier dimensions. It controls the rate at which air, then gas, passes down through a gasifier. This in turn exercises a primary effect on heat transfer around each particle during flaming pyrolysis of the volatiles, combustion of the tars and gasification of the charcoal.
A low SV causes relatively slow pyrolysis conditions at around 600°C, and produces high yields of charcoal - 20-30%, large quantities of unburned tars, and a gas with high hydrocarbon content and high tar (volatile) content. A high SV causes very fast pyrolysis, producing less than 10% char-ash at 1050 C and hot gases at 1200-1400 C in the flaming pyrolysis zone. These gases then react with the remaining char-ash to yield tars typically less than 1000 ppm, 5-7% char-ash and a producer gas with less energy.
These relationships have been investigated in a velocity controlled inverted downdraft gasifier with a 7.5 cm diameter. As the superficial velocity was varied from 0.05 m/s to 0.26m/s , the gas production rate increased from 102 to 679 cm3 /s, charcoal production decreased from 13.0% to 4.7% and tar in the gas decreased from 8330 to 300 mg/kg (ppm).
At low Superficial Velocities (and low Biot numbers), the particles are heated slowly to pyrolysis temperature and remain essentially isothermal. At high superficial velocities the outside of the particle can be incandescent (> 800°C) while the center is still at room temperature. This permits the escaping gases to react with the charcoal, thus reducing the charcoal yield and increasing the gas yield. We call this phenomenon “simultaneous pyrolysis and gasification”, SPG and believe that it is the fundamental reason why the Superficial Velocity controls all other aspects of gasification.
In producing heat the “tars” in producer gas are a useful fuel, providing no cold surfaces intervene. The “inverted downdraft” stoves provide potentially simple, clean cooking for developing countries. At low SVs they also produce charcoal. In producing electric power, tars are detrimental to engine operation, and so high SVs must be maintained to minimize tar and char production.