Canada has abundant sources of fossil fuels such as coal and renewable sources such as switchgrass, sawdust and biosolids. Gasification is a process that utilizes fossil fuels and renewable fuels to produce synthesis gas. Adding species such as alkali and alkaline earth salts can catalyze the gasification process. However, the issues with catalytic gasification are the cost and recovery of catalyst. There is also the chance of synergy during the co-gasification of biomass with fossil fuels due to high concentration of catalytic species in biomass. In this thesis, the availability of the gasification catalysts, especially alkali and alkaline earth elements, naturally present on biomass feedstocks during the co-gasification with non-biomass feedstocks was identified.
The influence of pyrolysis temperature on the physical and chemical properties of the feedstocks during transformation to char at 1 atm pressure and temperatures of 750 ºC to 950 ºC were investigated. The biomass feedstocks were switchgrass (16.8 wt% potassium in ash), sawdust and biosolids and the non-biomass feedstocks were sub-bituminous coal (30.5 wt% ash) and fluid coke (2.0 wt% ash).
The kinetics of CO2 co-gasification of switchgrass (char and ash) with non-biomass feeds was determined using a thermogravimetric analyzer. High interparticle mobility of the potassium in switchgrass to non-biomass feed was observed. The potassium in the switchgrass/coal mixtures deactivated, likely due to formation of potassium aluminosilicate during the co-gasification. Presence of excess potassium in the mixture (K/Al molar ratio > 1) for catalytic activity was required to satisfy the stoichiometric requirements of the deactivation reactions.
The extended random pore model satisfactorily described the gasification profile of single and binary mixtures. The activation energy was influenced by the concentration of the catalytically active form of potassium. An increase in potassium concentration through the addition of switchgrass ash instead of char resulted in a decrease in activation energy in switchgrass/ash-free coal mixture, while the trend was reversed for coal, which had more interfering inorganic matter. Analyzing the co-gasification kinetics of the switchgrass and biosolids indicated increasing calcium concentration may allow more potassium to remain in a catalytically active form.