Article pubs.acs.org/EF
Co-gasification of Biomass and Non-biomass Feedstocks: Synergistic and Inhibition Effects of Switchgrass Mixed with Sub-bituminous Coal and Fluid Coke During CO2 Gasification Rozita Habibi,† Jan Kopyscinski,† Mohammad S. Masnadi,‡ Jill Lam,§ John R. Grace,‡ Charles A. Mims,§ and Josephine M. Hill*,† †
Department of Chemical & Petroleum Engineering, University of Calgary, 2500 University Dr. N.W., Calgary, AB, T2N 1N4, Canada ‡ Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, V6T 1Z3, Canada § Department of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College St., Toronto, ON, M5S 3E5, Canada ABSTRACT: Co-gasification of biomass, namely, switchgrass, with coal and fluid coke was performed to investigate the availability of the gasification catalysts to the mixed feedstock, especially alkali and alkaline earth elements, naturally present on switchgrass. Rates of CO2 gasification of the single and mixed materials were measured at temperatures between 750 and 950 °C and atmospheric pressure by thermogravimetry. High interparticle mobility of the catalysts is indicated by a prompt and lasting effect on the mixed feed gasification rate when compared with the separate rates. The switchgrass−coal mixtures show a deactivation (antagonism), attributed to sequestration of the mobile alkali elements by reaction with aluminosilicate minerals in coal to form inactive alkali aluminosilicates, such as KAlSi3O8 and KAlSiO4. Remaining catalytic activity is evident when excess alkali is present in the feed mixture to satisfy the stoichiometric requirements of these deactivation reactions. In co-gasification of switchgrass with fluid coke, which has little interfering inorganic matter, a synergism is noted in the gasification of the mixed feed. The results document the effects of fuel mixture, composition of the coal or coke ash, and the gasification temperature on this behavior.
1. INTRODUCTION Co-gasification of biomass and non-biomass is beneficial with respect to char reactivity, tar formation, and greenhouse gas emissions.1−3 Extensive studies on co-gasification of biomass and non-biomass fuels have been reported in the literature in which physical and chemical characteristics,3 biomass ratio in the mixture,4 ash content, and gasification temperature5 are shown to be the main factors influencing the process. It is well-documented that alkali and alkali earth metals present in biomass, such as switchgrass, promote gasification.6,7 The alkali species that have the highest catalytic effect are potassium, sodium, calcium, and magnesium. The interparticle mobility of potassium, in particular, allows transfer of the catalyst from a biomass feedstock to a second feedstock. Thus, biomass feedstocks may provide an inexpensive source of gasification catalysts to co-convert fossil fuels, such as petroleum coke and coal, at lower temperatures, thus lowering the energy requirements of the process. Thus, co-feeding potassium-rich switchgrass to a non-biomass fuel increases the gasification rate of the latter. Several studies in the 1970s and 1980s found that potassium, added as a catalyst (as K2CO3 or KOH) to a variety of coals, chars, and cokes, deactivated during gasification6−8 due to the formation of potassium aluminosilicate, such as kalophilite (KAlSiO4), as the potassium reacted with mineral matter (e.g., kaolinite) in the coal. The formation of this potassium aluminosilicate might also occur during the co-gasification of © 2012 American Chemical Society
switchgrass, especially with coal of a high ash content. For example, the catalytic effect shown by Brown et al.9 for cogasification of switchgrass and low-ash (9 wt %) coal might be hindered. Hernandez et al.10 observed a maximum in the H2/ CO ratio by co-gasifying 50% biomass waste with 50% coal− coke at 750−850 °C in an entrained flow gasifier. Further understanding of this effect during co-gasification of binary feedstocks is needed. From a Canadian perspective, not only coal but also petroleum coke will continue to have a role in the energy mix because of its low cost and abundance. In particular, petroleum coke, which is a byproduct of oil sands bitumen upgrading, has a high carbon content (>90 wt %), a very low ash content (
