460000 Inhibition Effect during Catalyzed Biomass Char Gasification in Steam and Carbon Dioxide and Its Reversal

Monday, November 14, 2016: 4:55 PM
Union Square 14 (Hilton San Francisco Union Square)
Mohmed Akil Syed1, Ildar Musin1, John D. Muzzy1, Derrick W Flick2, Carsten Sievers1 and Pradeep K. Agrawal1, (1)School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, (2)The Dow Chemical Company, Freeport, TX

Gasification is a chemical process in which a gasifying agent reacts with carbonaceous materials to produce gases including high value CO and H2. Gasification of renewable resources is a promising and environmentally friendly solution to the increasing global demand for energy and chemicals. The process takes place at high temperatures where biomass undergoes pyrolysis to rapidly form the biomass char. The char continuously reacts with the gasifying agent as the rate limiting step in the process. The gas-phase composition influences the gasification rate and is one of many variables that can be used for process improvement. Observations of gasification with low cost gases, steam (H2O) and carbon dioxide (CO2), have been reported extensively. The gasification reactivity behavior appears to vary among reports showing either the presence or lack of a synergistic effect for mixtures of these gases. The difficulty of predicting or explaining the optimum gasification rate stems from a lack of understanding the fundamental gasification mechanism in mixtures of steam and CO2.

This study focused on understanding the mechanism in the evolution of gasification reactivity during conversion of char with CO2, steam, and their mixtures. Bagasse char as starting material was prepared by pyrolysis of potassium-rich Brazilian bagasse in an inert gas pressurized entrained flow reactor. The char was then exposed to varying gasifying environments in a thermogravimetric analyzer to undergo full conversion. The initial gasification reactivity was higher in steam than in CO2. However, overall and final reactivities were higher in CO2 than in steam. Gasification inhibition was observed as a continuously decreasing reactivity with conversion with steam that was not observed with CO2. On the other hand, CO2 gasification rate showed a continuous increase with char conversion. The underlying mechanism was explored further by studying the transient behavior under exposure to step changes in model gas compositions of CO2, steam, H2, and O2. It is found that the active sites for both CO2 and steam gasification are initially the same. However, as the char gasification progresses in steam, the active sites are likely to be blocked by in-situ hydrogen product formation (“inhibition”), which does not happen during CO2 gasification. On the contrary, concentration of active sites is increased as the char gasification progresses in CO2 due to the increasing K/C ratio. In this study, several methods were demonstrated for the reversal of the inhibition effect. A generalized mechanism, consistent with experimental observations, is proposed. The findings provide a key insight into the source of the elusive synergistic effect during gasification with steam and CO2.

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