Evidence of interactions in microwave-assisted co-pyrolysis of different varieties of coals

In this study, microwave-assisted co-pyrolysis of blends of Indonesian coal (IC) with Indian medium ash coal (MAC), Indian high ash coal (HAC) and coking coal (CC) was performed to evaluate the product yield and quality of the pyrolysis tar, and to understand the interactions during co-pyrolysis. The tar yields from pyrolysis of individual coals, viz., IC, HAC, MAC and CC were 26 ± 3 wt%, 14 ± 2 wt%, 12 ± 2 wt% and 11 ± 2 wt%, respectively. In co-pyrolysis, tar yields decreased with decrease in IC fraction in the mixture. Importantly, the experimental yields of products were similar to that calculated using the mixture rule. However, interactions were evident in the gas and char yields from pyrolysis of IC-rich mixtures, and the deviation from calculated values was in the range of 3–5 wt%. Pyrolysis tar from IC predominantly contained phenols, whereas tars from HAC, MAC, and CC contained aromatic hydrocarbons, phenols and aliphatic hydrocarbons. The interactions were evidenced in the form of enhanced selectivity to naphthalene derivatives and polyaromatic hydrocarbons with a concomitant reduction in the selectivity to simple phenols in the coal tar. Interactions were predominant in equal composition and IC-rich mixtures of IC:HAC, IC:MAC and IC:CC, which is due to the early formation of char via decomposition of IC. Hydrogen and methane constituted a major fraction of pyrolysis gases from MAC, HAC and CC and their blends, while the pyrolysis gas from IC was rich in H2 and CO2. Higher heating values of coal chars varied in the range of 21–27 MJ kg−1.