280617 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: firstname.lastname@example.org, email@example.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
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.