(18h) Modeling and Simulation of Coal Gasification in a Fluidized Bed

A dynamic
mathematical model to simulate the continuous gasification of coal particles in
a bubbling fluidized bed reactor (FBR) is presented. The bubbling fluidized bed
has been modeled based on the two phase theory i.e. emulsion and the bubble phase.
The emulsion phase consists of solids and the amount of gases required for
fluidizingit, while the
bubble phase consist of all the extra gases. The freeboard of
the fluidized bed consists of gases released from the bed and entrained and
elutriated solids. Heterogeneous reactions have been assumed to occur in the
emulsion phase, while gas-gas reactions are assumed to occur in the emulsion
and bubble phase. The model incorporates fuel devolatilization, chemical
reaction kinetics, bed and the freeboard dynamics.

Mathematical models
based on the two phase theory have been presented before, Yan et. al (Heidenreichayb
& Zhangagbf, 1998)
have modeled the bubble phase and emulsion phase gases as plug flow reactor
while the emulsion phase solids as a CSTR , they have also proposed the concept
of ?net flow' of gases from the emulsion to bubble phase which consists of the
extra volume of gases generated due to devolatilization , homogenous reaction
and the heterogeneous reactions. The model was further updated with  energy
balance equations (Yan, Heidenreich,
& Zhang, 1999).
Hamel et. al (Hamel & Krumm,
2001)
had proposed a cell model and validated the same with experimental results from
laboratory scale to commercial scale gasifiers. Chejne et. al  (F Chejne &
Hernandez, 2002)
proposed a model along with Gaussian distribution for the solids particle size.
The same model was later on (Farid Chejne,
Lopera, & Londoño, 2011) adopted for a pressurized gasifier wherein the high
pressure effects in transport phenomena , bed-fluid dynamics , physical
properties , etc. were taken into account.

Later on Goyal et.
al (Goyal,
Pushpavanam, & Voolapalli, 2010) modeled co gasification that included coal
and pet coke as feed and concluded that increase in the amount of pet coke
decreased the carbon conversion and efficiency while increasing the amount of
syngas produced. Most of the models have assumed plug flow for gas phase in
both the emulsion and bubble phase while for the solids in emulsion phase CSTR
has been assumed.

In this work we try
to develop a framework to capture the non-ideality in the reactor, by assuming
a battery of mixing cells in series, the number of cells and hence the degree
of mixing can be controlled in the model according to the residence time
distribution, thereby the same framework can be used to simulate a wide range
of operating parameters (i.e. laboratory to commercial scale). Apart from
optimization of operating parameters the model can be used to study the effect
of feed point locations, catalyst and inert material in bed, different
geometric configurations and feed properties on the performance of the gasifier.
The
developed model is a step towards development of a multi zonal model coupled
with the computational fluid dynamics based model of a gasifier.

Figure
1 Schematic representation of Multi cell framework for a fluidized bed gasifier

To estimate the
hydrodynamic parameters the model includes set of well-known correlations to calculate
the fraction occupied by the bubbles in the bed , the porosity and velocity of
gas phase, the size and the velocity of  the bubbles , interphase mass transfer
and the heat transfer coefficients . The program of the model consists of
different modules i.e. kinetics, energy equation and mass balance equations. In
all there are 7 algebraic equations and 30 differential equation to be solved
in each cell, ode15s solver of MATLAB^TM has been used to solve the
set of equations with their default tolerances.

The model has been
first validated with the data available in literature of the commercial Winkler
fluidized bed gasifier (Goyal et al.,
2010).It
was then used to validate the experimental work carried out with high ash
Indian coal in a fluidized bed at CIMFR, Dhanbad (Chavan, Datta,
Saha, Sahu, & Sharma, 2012). 18 sets of experimental runs conducted with Rajamahal
coal has been validated with the model. Kinetics for the reactions have been
taken from data reported in the literature (Weimer, 1981).

Isothermal case was
solved first and later on a loss parameter in the energy balance equation for
the gas in the emulsion phase was adjusted for the non-isothermal case. It was
observed that the emulsion temperature and the solids temperature were
approximately same, while that of bubble phase temperature was lesser. The
vapor phase temperature was more or less the same as the bubble temperature.

Sensitivity
analysis with respect to the modeling parameters such as diameter of bubbles,
bubble voidage and diameter of bubble has been carried out. Effect of various
operating parameters such as air/coal ratio, steam/coal ratio and temperature
on the carbon conversion has been studied. Effect of carbon properties such as
the volatile matter, fixed carbon and the ash content on the carbon conversion
has also been studied.

The simulations
shows the same trend as the experimental results i.e. increase in the carbon
conversion with the increase in volatile matter, air, steam and temperature,
while carbon conversion decreases with increase in fixed carbon. Similarly the
higher heating value (HHV) of the product gases increases with the volatile
matter and fixed carbon and decreases with increase in mineral matter, air and
steam.

Figure
4. Comparison of reported carbon conversion with the simulated data

As shown in figure
4 comparison of reported carbon conversions from 18 experiments conducted on
Rajamahal coal with the simulation results of the model shows reasonable match,
the framework can now be used to study the effect of change in feed and product
location, number of solids and more reactions can be included in the model to
study the effect of co gasification, catalysts and inert solids on the
gasification process. The developed model thus can be used to study the
hydrodynamics of the reactor and to select the best operating conditions to
obtain maximum carbon conversion and efficiency for the gasification of high
ash Indian coal in a fluidized bed reactor.

References :

Chavan, P., Datta, S., Saha, S.,
Sahu, G., & Sharma, T. (2012). Influence of high ash Indian coals in
fluidized bed gasification under different operating conditions. Solid Fuel
Chemistry, 46(2), 108?113. doi:10.3103/S0361521912020048

Chejne, F, & Hernandez, J. P. (2002). Modelling and
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Chejne, Farid, Lopera, E., & Londoño, C. a. (2011).
Modelling and simulation of a coal gasification process in pressurized
fluidized bed. Fuel, 90(1), 399?411.
doi:10.1016/j.fuel.2010.06.042

Goyal, A., Pushpavanam, S., & Voolapalli, R. K.
(2010). Modeling and simulation of co-gasification of coal and petcoke in a
bubbling fluidized bed coal gasifier. Fuel Processing Technology, 91(10),
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Hamel, S., & Krumm, W. (2001). Mathematical
modelling and simulation of bubbling fluidised bed gasifiers. Powder
technology, 120, 105?112. Retrieved from
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Harshe, Y. M., Utikar, R. P., & Ranade, V. V.
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Heidenreichayb, C., & Zhangagbf, D. (1998).
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Weimer, A. W. (1981). Modeling a low pressure
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Yan, H.-M., Heidenreich, C., & Zhang, D. K. (1999).
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