Microwave-assisted co-pyrolysis of high ash Indian coal and rice husk: Product characterization and evidence of interactions

In this study, microwave-assisted co-pyrolysis of high ash Indian coal and rice husk is investigated to evaluate the yield and quality of the pyrolysis oil, and to understand the interactions during co-pyrolysis. Microwave co-pyrolysis experiments were conducted in a bench scale unit, and the effects of various parameters such as microwave power (420 W and 560 W), coal:rice husk ratio, and feed:susceptor (graphite) ratio on product yields and oil composition were thoroughly evaluated. Oil yield was not significantly affected by microwave power and feed:susceptor ratio for a particular coal:rice husk ratio. The oil yields were 11 +/- 2 wt%, 15 +/- 2 wt%, 18 +/- 2 wt%, 21 +/- 2 wt% and 26 +/- 2 wt% for 100:0, 75:25, 50:50, 25:75 and 0:100 wt./wt. coal:rice husk, respectively, at both microwave powers, and followed an additive effect with increase in rice husk amount in the mixtures. Pyrolysis oil contained monoaromatic hydrocarbons, naphthalene derivatives, aliphatic hydrocarbons and phenolic derivatives. The interactions between coal and rice husk pyrolysis intermediates were evidenced in the form of enhanced yields of polyaromatic hydrocarbons with a concomitant reduction in yield of methoxy phenols. Pyrolysis oil from coal had highest heating value (36-38 MJ kg(-1)) due to the presence of aliphatic hydrocarbons, while that from rice husk had the lowest heating value (28-30 MJ kg(-1)) due to high oxygenate content. The heating values of co-pyrolysis oils were in the range of 30-36 MJ kg(-1). Hydrogen constituted a major fraction in the pyrolysis gases, and the addition of rice husk to coal promoted the evolution of CO.