(328c) Detailed Kinetic Modeling and Simulation of Coal/Corn Stover Mixtures in a Moving Bed Gasifier

This paper
presents the process simulation and modeling of a moving bed gasifier using two
Montana coals (DECS-38 sub-bituminous coal and DECS-25 Lignite coal), one
biomass sample (corn stover) and their blends (10% and 30% by weight of corn
stover). All the processes occurring in a moving bed gasifier, drying,
devolatilization, gasification, and combustion, are included in this model.

The model,
developed in Aspen Plus using external FORTRAN subroutines, is used to predict
the effect of various operating parameters including pressure, oxygen to coal,
and steam to coal ratio on the product gas composition. Developing Aspen Plus
model requires knowledge of reaction stoichiometry, reaction rates, kinetics,
mass, and heat transfer. The model was used to predict the composition of
product gas, temperature profile, and effect of process variables on the syngas
composition.

Assumptions in the model are as
follows:

(1) The model is in steady state.

(2) Coal and gas flow as plug flow. Plug
flow assumes constant velocity across the cross section of the gasifier
with   no back-mixing.

(3) Residence time of the coal in drying
and pyrolysis section is assumed to be negligible as compared to
combustion-gasification section.

(4) The pressure drop in the
gasifier is neglected.

(5) Volumetric reaction represents all
gas-solid reactions except char combustion which is according to shrinking core
model.

(6) Gas temperature is the same
as solid temperature at every point in the gasifier.

The results
obtained from the simulation were compared with experimental data obtained for
the same feedstocks using a laboratory scale moving bed gasifier. The predicted
composition of the product gases was in general agreement with the established
results. Carbon conversion increased with increasing oxygen-coal ratio and
decreased with increasing steam-coal ratio. Steam to coal ratio and oxygen to
coal ratios impacted produced syngas composition, while pressure did not have a
large impact on the product syngas composition.

All the feedstock materials have been gasified in a moving bed
reactor running in a completely auto-thermal batch mode using several
air/oxygen/steam ratios. A laboratory-scale gasification system has been
designed and constructed. The main components are schematically represented in
Figure 1. Coal/biomass is fed at the top of the gasifier by means of a
quick-open flange while air/oxygen and steam are fed simultaneously upwards through
the bottom of the gasifier. The product gas from the gasifier contains mainly
hydrogen, carbon monoxide, carbon dioxide and small amounts of methane.

The gasifier is a cylindrical stainless steel modular flange assembly
having an internal diameter of 1.37 inches with quartz/stainless steel tubing
of 0.075 inches on the inside and a height of about 10 inches from the grate.
The internal tubing is fitted with a stainless steel grate with holes large
enough to let the ash pass through but small enough to hold the feed material.
The grate is connected to a mechanical rotary linear feed-through to
periodically remove ash. The bottom zone, under the grate, has another
cylindrical stainless steel flange with a height of about 5 inches to collect
and then discharge the ash produced in the process.

The energy content of the gas produced through gasification depends
on numerous factors, such as the oxidizing agent, reactor type, fuel type and
form, etc. The oxidizing agent can be chosen as air, oxygen, steam, or a
mixture of these. When air is used, the resulting gas has a low calorific
value. This can be increased by using oxygen or steam but in the latter case
sufficient heat should be provided because steam gasification is an endothermic
process.