[Gasification] hearth load concept

[Mariodg77]

Dear forum,

I've got a question regarding the concept of the hearth load of a downdraft
gasifier.
in fact i must calculate the quantity of air necessary to a small scale
gasifier (70kWth) and I think that this parameter could be useful.
on the fao publication
(http://www.fao.org/documents/show_cdr.asp?url_file=//docrep/t0512e/T0512e0c.htm)
i've found that this value is defined in this way:

The hearth load B is defined as the amount of producer gas reduced to normal
(p, T) conditions, divided by the surface area of the "throat" at the
smallest circumference and is usually expressed in m³/cm²/h. Alternatively
the hearth load can be expressed as the amount of dry fuel consumed, divided
by the surface area of the narrowest constriction (Bs), in which case hearth
load is expressed in Kg/cm²/h. Because one kilogramme of dry fuel under
normal circumstances produces about 2.5 m³ of producer gas, the relation
between Bg and Bs is given by:

    Bg = 2.5 Bs

According to the information provided in (8) Bg may reach a maximum value of
about 0.9 (Bs = 0.36) in continuous operation in good "Imbert" type
gasifiers. Higher values of Bg give rise to extreme pressure drop over the
reduction zone of the equipment.

Normal "Imbert" type generators show minimum values of Bg in the range of
0.30 - 0.35, resulting in power turn-down ratios of a factor 2.5 - 3. Modern
gas producers are better insulated, and can operate tar-free at Bg values of
0.15 - 0.18. 

How am I supposed to calculate the narrowest diameter of a downdraft
gasifier of about 70kWth? which one can be a good value for the surface area
of the throath? 

thanks, 

mario

 
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