Maybe it sounds simple, but actually when you check literature you will see what is reported as superficial velocity is not (u=airflow (m3/se)/Area of reator (m2)) because if you consider this method then the air flow would be quite different compared with the one which is calculated based on ER value.
Any idea?
You are correct that the calculated superficial velocity differs from any actual velocity or estimate of an actual velocity, but that is the intention. Since you generally do not know the particle volume fraction at any point in the fluid bed, the superficial velocity ignores it and is the velocity the gas would have if there were no particles at all. Similar definitions for superficial velocity exist for flow through porous media (oil reservoirs, filters, fixed beds, etc.) and for other multi-phase flows. It is a useful characterization in the sense that you can anticipate consequences if it increases or decreases, but it does not represent any actual velocity of anything in most applications.
So in that case if you have the air flow and biomass flow, how do you calculate superficial velocity? What I have checked is that the value would not be the same as the one calculated by that simple equation I wrote in the question
The air superficial velocity is its volumetric flow rate divided by the cross sectional area of the empty container. The biomass superficial velocity (which would be an uncommon number to calculate) is the same. The ER (I assume equivalence ratio) number is nearly always represented in standard cubic feet whereas superficial velocities are typically done at the operating temperature and pressure, so this may be part of the confusion.
The superficial velocity (hearth load) of a gasifier is one of the most important measures 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) = m/s
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.
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