(70e) Modeling of Pyrolysis and Boudouard Reactions of Various Coals, Biomasses and Coal-Biomass Blends Using Thermogravimetric Analysis, Experimental Moving Bed Gasification System and Stable Carbon Isotope Ratio Mass Spectroscopy

Modeling
of pyrolysis and Boudouard reactions of various coals, biomasses and
coal-biomass blends using thermogravimetric analysis, experimental moving bed
gasification system and stable carbon isotope ratio mass spectroscopy

Abhijit
Bhagavatula¹, Naresh Shah², Gerald Huffman² Christopher
Romanek³

1.       Corresponding author: Department of Chemical and Materials Engineering,
University of Kentucky, Lexington, KY, 40506-0043, USA email: abh222@uky.edu, abhijitbv@gmail.com

2.      
Department
of Chemical and Materials Engineering, University of Kentucky, 533 S.Limestone
St, Suite 107, Whalen Building, Lexington, KY, 40506-0043, USA

3.      
Department
of Earth and Environmental Sciences, University of Kentucky, 216 Slone
Building, Lexington, KY, 40506-0053, USA

Abstract

Non-isothermal
thermogravimetric analysis has been performed at different heating rates of 5,
10, 20 and 40 °C/min to investigate the thermal decomposition kinetics of two
coals: lignite and sub-bituminous; four biomass materials: pinewood, poplar,
corn stover and switchgrass; and blends of each biomass type with both coals at
10 % and 30 % by weight. The pyrolytic behavior of the samples was analyzed
under these conditions using an inert nitrogen atmosphere while a carbon
dioxide atmosphere was employed for studying the gasification kinetics, or the
Boudouard reaction kinetics. A distributed activation energy model was utilized
to estimate the kinetic parameters (order, activation energy and
pre-exponential factor) for both sets of experimental data. Predicted results
from the optimum kinetic parameters have been compared with the experimental
data. The DAEM equation predicts the experimental data very well for different
heating rates.

Apart from this, 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. The core of the system is an updraft
gasifier, where pressure and temperature profiles are measured by a pressure
transducer and a set of thermocouples, respectively. Coal/biomass is fed at the
top of the gasifier by means of a quick-open flange. Air/oxygen and steam is
fed at the bottom of the gasifier and its rate is measured by a rotameter. A
safety valve is located on the gas exit tube. Here, the gas stream enters two
condensers, where steam and tars are condensed and collected at the bottom
before sampling for gas chromatographic (GC) analysis. The product gas from the
gasifier contains mainly hydrogen, carbon monoxide, carbon dioxide and small
amounts of methane. Finally, the exit gas flow rate is again measured by a wet
test meter.

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 feedthrough
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 grate at the bottom of
the gasifier is used not only for holding the solid particles together but also
as an oxidant distributor.  The oxidant, fed at the bottom of the reactor,
flows along the channels and exits through the small holes along the grate, so
it is distributed across the whole section of the gasifier. At the bottom, a
small tube allows the use of pre-heated steam to enter the bed. Temperature
profiles along the gasifier axis are measured by a set of K-type thermocouples
placed within a steel protective tube.

Figure
1: Schematic of the laboratory scale moving bed gasification system.

Also,
a stable carbon isotope analysis was performed on both the feedstocks as well
as the effluent gases from the moving bed gasifier in order to determine their ¹³C/¹²C
ratios. The difference in the ¹³C/¹²C ratios of the
feedstocks acts as a natural isotopic tracer for determining the amount of coal
and biomass converted to each component in the product gases. This novel
technique can be utilized for quantitatively determining the individual contribution
or source apportionment of coal and biomass to the product gases by calculating
the isotopic carbon mass balance for each product gas.