(585bg) Role of Active Sites in the CO2 and Steam Gasification of Model Rdf Char

Authors: 
Aluri, S., Georgia Institute of Technology
Agrawal, P. K., Georgia Institute of Technology
Sievers, C., Georgia Institute of Technology
Muzzy, J. D., Georgia Institute of Technology
Flick, D. W., The Dow Chemical Company
Stears, B., The Dow Chemical Company
Refuse Derived Fuel (RDF) is a refined form of Municipal Solid Waste (MSW) derived by following certain pre-processing steps to remove fractions like glass and heavy metals from MSW. For this study, a model RDF composition was established based on the EPA data and pyrolysis results of individual components in different categories, e.g., plastics, paper and food waste. The chars were prepared in a thermogravimetric analyzer (TGA) by heating at a rate of 20 K/min up to 800 °C. Gasification of pyrolyzed feed was conducted at 800 °C using different mixtures of CO2 and steam as the gasification reagents.

Active sites in the char correspond to different entities depending on the char constitution and pore structure. For chars with negligible inorganics, the concentration of active sites is a function of the surface area. However, in the presence of certain inorganic elements, active sites correspond to the inorganic-bound-carbon. It is well established that alkali and alkaline earth metals are more catalytically active than the other inorganics. Potassium present in the orange peel char and calcium present in the paper components were the dominant alkali and alkaline earth metals in the model RDF char, respectively. Several studies were conducted to determine the role of potassium and calcium bound carbon active sites in CO2 and H2O gasification.

The chemical phases of potassium and calcium species present in the model RDF char were investigated by weight – change studies in different gas mediums. Change in the chemical form of each of these inorganics was studied at different temperatures during the various stages of gasification. The activity of these inorganic-rich active sites in CO2 and H2O gasification was compared and analyzed. Both the type and amount of inorganics had an effect on the gasification reactivity in the two gasifying mediums. Nevertheless, potassium had a considerably greater catalytic activity than calcium. Reaction mechanisms were established to explain char gasification in both CO2 and H2O. Co-gasification studies with CO2 and H2O were conducted at different partial pressures to analyze the affinity of the active sites towards the two gasification agents. In addition to the effect of inorganics, steam gasification was influenced by hydrogen inhibition. Gasification studies were conducted at higher temperatures of 900 °C and 1000 °C to counteract the inhibition caused by hydrogen. Essentially, the role of inorganic-rich active sites was found to have a great effect on the reactivity of the model RDF char in both CO2 and H2O gasification which can be extended to other potassium-rich or calcium-rich char gasification systems.