A superficial velocity, SV, is defined as a ratio of the syngas
production rate at normal conditions and the narrowest cross
sectional area of the gasifier. A number of authors have indicated
that SV influences the gas production rate, the gas energy content,
the fuel consumption rate, the power output and char and tar
production rates. It is independent of reactor dimensions, allowing
a direct comparison of gasifiers with different power outputs.
Yamazaki et al. [27] reported a case with a good performance of the
gasifier when a low tar content in producer gas and high efficiency
were obtained for SV values of about 0.4 Nm/s. Low values of SV
result in a relatively slow pyrolysis process with high yields of char
and significant quantities of unburned tars. On the contrary, high
values of SV cause a very fast pyrolysis process, formation of
a reduced amount of char and very hot gases in the flaming zone.
However, such high SV values may significantly decrease the gas
residence time in the gasifier, resulting in lower efficiencies in the
tar cracking processes.