The preliminary sizes and operating parameters of a fluidized bed gasifier may be estimated as follows:

Main flow rates

Using the desired power output (Q), the volumetric lower heating value (LHVv) of the producer gas and assumed gasifier efficiency, one can calculate the biomass flow rate (F) for production of 1 Nm3 of gas as:

і ■. ■■ [ p:. ‘.I;u I [ І6.35]

LHKng

One can find the required air flow rate, Mair for an air-gasification unit assuming a value of equivalence ratio, ER within the range of (0.2-0.3) (Basu, 2006)

Mair = mth. ER. F kg air/Nm3 product gas, [16.36]

where mth is the stoichiometric amount of air kg/kg and F is the fuel feed rate.

For steam gasification gasifiers, one can use published data on the steam/ biomass ratio for a similar fuel to find the amount of the gasification medium Mmed as the first guess.

As the gasifying agent will also be the fluidizing gas, it is necessary to check that it would be adequate for proper fluidization. The fluidizing velocity is chosen based on the bed particle characteristics taking into consideration the bed hydrodynamics and the process occurring in gasifier.

Reactor dimension

The volume flow rate of the gasifying medium (steam/air or oxygen) divided by the chosen fluidizing velocity will give the reactor cross-section area.

л. [16.37]

bed л r r

PmJJ

where pmed and U are the density and the fluidization velocity of the medium (air, oxygen, steam, or their mixture) at the operating bed temperature, respectively.

Bed height

The bed height should be chosen such that it provides the required residence time for better carbon conversion and would avoid slugging. Also, its selection is governed by operating cost considerations, as higher bed height means a higher pressure drop, taller reactor, and greater auxiliary power consumption.

Freeboard height and its diameter

Ideally, the freeboard height should exceed the transport disengaging height (TDH), so that the particle entrainment with upward flowing gases will be low, but in most cases, that is too expensive. A compromise between cost and performance is used.