simulation of biomass gasification in fluidized bed using aspen … · 2017-10-24 · fluidized...
TRANSCRIPT
Abstract— Gasification is a thermo-chemical process to convert
carbon-based compounds such as biomass and coal into a gaseous
fuel. Among the various types of gasification methods, fluidized bed
gasification is one which is considered as more efficient method than
others as biomass is fluidized in a mixture of air/oxygen and steam.
In the present study, a comprehensive steady state process model has
been developed for biomass gasification in atmospheric fluidized
beds using Advanced System for Process Engineering (ASPEN
PLUS) simulator. Three ASPEN PLUS reactor models namely,
RYIELD for drying and decomposition of biomass into its elements,
RGIBBS for achieving both chemical and phase equilibrium of
volatile matter available in biomass and RCSTR for carrying out char
gasification have been used. The fluidized bed reactor has been
divided into finite small elements because there exists change in
number of moles, change in partial pressures of all the components
along the height of the reactor as the reactions take place inside the
reactor. To consider the change in superficial velocity which results
from change in number of moles along the height of the reactor, each
of these elements has been simulated by one RCSTR. Finally, the
effects of steam to biomass ratio, equivalence ratio and temperature
on the composition and lower heating value of product gas have been
studied. Using optimization tool available in Aspen Plus, the
optimum values of steam to biomass ratio and equivalence ratio have
been found out.
Keywords— Biomass, gasification, Aspen Plus, gas composition,
heating value, optimization, steam to biomass ratio and equivalence
ratio.
I. INTRODUCTION
IOMASS refers to plant based materials such as dead
trees, branches, wood chips and even municipal solid
waste. The process of conversion of biomass (or any other
carbonaceous material) to syngas is called as gasification.
Gasification occurs when oxygen and/or steam is reacted at
high temperatures with available carbon in biomass within a
gasifier. Air gasification produces a poor quality gas with
regard to the heating value, around 4-7 MJ m-3
, while O2 and
steam blown processes result in a syngas with a heating value
in the range of 10-18 MJ m-3
Lv et al.,. However, gasification
with pure O2 is not practical for biomass gasification due to
prohibitively high costs for O2 production using current
commercial technology (cryogenic air separation). The syngas
thus produced can be combusted in gas turbine or in an engine
to generate electricity and heat.
K. Anand Kishore1 is with the National Institute of Technology Warangal-
506004, Telangana, India (e-mail: [email protected]).
K. A. V. Ramanjaneyulu was with the National Institute of Technology
Warangal-506004, Telangana, India (e-mail:[email protected]).
II. SIMULATION DETAILS
In a typical atmospheric fluidized bed gasifier, feed together
with bed material are fluidized by the gasifying agents, such as
air and/or steam, entering at the bottom of the bed. The
objective of this study is to develop simulation capable of
predicting the steady-state performance of an atmospheric
fluidized bed gasifier by considering the hydrodynamic and
reaction rate kinetics simultaneously. The products of
homogeneous reactions are determined in RYIELD reactor
and RGIBBS reactor, and reaction kinetics are used to
determine the products of char gasification. A drawback in
using ASPEN PLUS is the lack of a library model to simulate
fluidized bed unit operation. However, it is possible for users
to input their own models, using FORTRAN codes nested
within the ASPEN PLUS input file, to simulate operation of a
fluidized bed. The work presents the simulation of fluidized
beds for biomass gasification in Aspen Plus. Fig. 1 shows a
simple schematic diagram fluidized bed gasifier. A preheated
mixture of oxygen and steam is introduced at the bottom of the
gasifier and flows upward with biomass to react with.
Fig.1 Schematic illustrative diagram of fluidized bed gasifier.
III. RESULTS AND DISCUSSION
Effect of Temperature on Syngas composition:
The effect of gasifier temperature on produced syngas
composition is shown in Fig.3. The temperature considered
varies from 700 °C to 1000 °C. The composition of syngas
varying with a small range with increasing gasifier
temperature. It can be seen that H2 composition and CH4
composition are almost independent of temperature. According
to Le Chatelier’s principle, higher temperatures favour the
Simulation of Biomass Gasification in
Fluidized Bed Using Aspen Plus
K. Anand Kishore1 and K. A. V. Ramanjaneyulu
2
B
6th International Conference on Chemical, Biological and Environment Sciences (ICCEBS'2015) Sept. 13-14, 2015 Dubai (UAE)
28
reactants in exothermic reactions and favour the products in
endothermic reactions. Therefore the endothermic reaction that
is steam gasification was strengthened with increasing
temperature, which resulted in an increase of CO composition.
Fig.3 Effect of temperature on Syngas composition
Effect of Steam to biomass Ratio on syngas composition:
Over the S/B range from 0 to 4, carbon conversion efficiency
exhibited decreasing trend, which can be explained by that
excessive quantity of low temperature steam lowered reaction
temperature and then caused gas quality to degrade. As shown
in Fig.4, in the S/B range from 0 to 4, the composition of H2
increases, this can be explained by more steam gasification
reaction is taking place because of increased steam quantity.
The change in CH4 composition is very small with increasing
steam-fuel ratio.
Fig.4 Effect of Steam to Biomass Ration on Syngas
Composition
Effect of Air to Biomass Ratio on syngas composition:
The effect of Air to Biomass ratio on the Syngas
composition is shown in Fig. 5. The effect of this ratio range
from o.5 to 1.8 was observed. In this range the mole fraction
of H2 decreases because increasing of Air to Biomass ratio
indirectly means that it will reduce the reaction temperature so
that the endothermic reaction that is steam gasification reaction
is weakened. In Fig.5 increasing the Air to Biomass ratio
increases the mole fraction of CO2 because of increasing the
flow rate of oxygen favours the oxidation reaction to occur.
Fig.5 Effect of Air to Biomass Ratio on Gas Composition.
IV. CONCLUSION
A model was developed for the gasification of biomass in an
atmospheric fluidized bed gasifier using the ASPEN PLUS
simulator. To provide the model, several ASPEN PLUS unit
operation blocks were combined with number of separator
blocks and a mixer block. The simulation analysis for the
product gas composition versus temperature, air to biomass
ratio and steam to biomass ratio was done.
Higher temperature improves the quality of syngas by
increasing the production of carbon monoxide and decreasing
the mole fraction of carbon dioxide. Increasing steam to
biomass ratio increases the production of hydrogen and carbon
dioxide and decreases the carbon monoxide. Increasing air to
biomass ratio decreases the production of hydrogen and
increases the carbon dioxide production.
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