Question

Design a fluidized bed gasifier with a required syngas output of 2.5 m3/min (standard conditions) using a cylindrical cross section. The air-to-fuel ratio should be 70% of the stoichiometric requirements and sawdust will be used as fuel. The ultimate analysis is as follows: C=49.7%; H=6.2%; O=42.53%, N=0.7%; S=0.17%, and ash =0.7%. The superficial velocity should be 0.03 m/s and the air density at standard conditions is equal to 1.2 kg/m3. About 25 kg of biomass is used every hour. [MW: C = 12, H = 1, O = 16, N = 14, S = 32]

Answer 1

Principle of Steam gasification of Biomass
The general process description of the steam
gasification is shown in Figure 1-1. The principle is the
same as for a dual fluidized bed gasifier having a
combustion and a gasification reactor. Here, biomass is
devolatized in RYield (Reactor) where conversion of
non-conventional stream biomass into conventional
components (C, H2, O2, N2, S) takes place. The
gasification process is simulated using a Gibbs reactor
in which steam is supplied to the reactor and char
combustion takes place in reactor RStoic with the
supply of air. The heat that is produced during the
combustion is used in the gasifier for the gasification
reaction. Finally, flue gas (N2, CO2, and excess O2) and
solid (bed materials) is separated by using cyclone
separator.

Temperature

The gasification temperature highly influences on the
syngas composition. To determine the effect on the
composition, the temperature is varied from 600 to
1100°C.

Steam Temperature

The
steam temperature was varied from 450 to 1000°C to
check the volume percent of syngas composition.

Combustion

The air stream
‘AIR’ is fed to this block reactor with the composition
of 0.79 N2 and 0.21 O2 and temperature of 450°C is
specified.

Steam to Biomass Ratio (STBR)

STBR has a significant impact on the syngas
composition. Over the STBR range, H2 increases from
45.39 to 48.64 vol. %. There is no influence of STBR
on the composition of CH4. However, there is a gradual
increase in the CO2 content. From the STBR value of
about 0.7 there is a sharp decrease in the CO
composition.

Comparison of the result

Steam gasification is used to produce a high-quality
product gas (CO, H2, CO2, and CH4) and these gas are
further processed for production of synthesis fuel. For
this gas quality, gas composition and purity are the
main important parameter. The main objective is to
look at the generated product gas composition..