DDGS chars gasification with CO2: a kinetic study using TG analysis

Abstract

Dry Distiller’s Grains with Solubles (DDGS) is a by-product during ethanol production from cereals which is currently mainly used as feedstock for cattle. With the growth of the ethanol industry, the increasing supply of DDGS may saturate the livestock feed market; thus, its potential applications need to be explored. DDGS gasification in a 100-kWth circulating fluidized bed (CFB) steam-O2 blown gasifier has been studied. However, the modeling of DDGS gasification process encounters difficulties due to the unavailable knowledge of DDGS char gasification kinetics. Therefore, in this paper, gasification kinetics of DDGS char with CO2 was investigated using thermogravimetric analysis (TGA). Two different types of char samples have been tested. Char type one (PYR-Char) was obtained after DDGS pyrolysis in a TGA at a final temperature of 750°C or 850°C for 20 min. Char type two (CFB-Char) was obtained after DDGS gasification in the 100-kWth CFB gasifier within the temperature range of 790°C to 820°C with a steam/biomass mass ratio of 0.81 and oxygen to biomass stoichiometric ratio of approximately 0.38. The influences of pyrolysis temperature (750°C, 850°C), heating rate (10°C/min, 30°C/min, 50°C/min, 70°C/min), CO2 concentration (10, 20, 30 vol.%), and gasification temperature (900°C, 1,000°C, 1,100°C) on the reaction rate of char-CO2 reaction were determined. Two representative gas–solid reaction models, the volumetric reaction model (VRM) and the shrinking core model (SCM) were applied in order to determine kinetic parameters. It was found that the calculated activation energy (E a) values using SCM were slightly lower than those using VRM. The calculated E a values for PYR-Char using both models were in the range of 100–165 kJ/mol, while the calculated E a values for CFB-Char were in the range of 55–100 kJ/mol. It was observed by scanning electron microscopy (SEM) that CFB-Char was more fragile and PYR-Char obtained at lower heating rate had a less porous structure. Generally, the predicted results using both models showed a fairly good agreement with experimental results, and SCM model suited slightly suitable better for char gasification at high temperature.