Microwave-assisted pyrolysis and co-pyrolysis of coal and oil palm shell with coconut shell activated carbon as microwave absorber

Abstract

The increased energy insecurity and carbon dioxide (CO2) emissions from fossil fuel utilization demands sustainable and cleaner fuel resources. Bio-fuel and chemicals from biomass have been recognized as renewable energy resource. Coal has the potential to become an important source for liquid and gas fuels. Co-processing of coal with biomass is considered a step towards sustainable and clean coal utilization. In this research work, oil palm shell (OPS) and coal were subjected to microwave (MW) pyrolysis and co-pyrolysis conditions to produce liquid fuels. Coconut activated carbon (CAC) used as a MW absorber was distributed uniformly over pyrolysis material to reduce hotspots. Three process parameters; CAC loading, MW power and nitrogen (N2) flow rate were studied on pyrolysis performance. Initially, pyrolysis performance with 1, 2 and 3-Layer of carbons over isolated fuels were studied. Later, 3-Layer of carbon over isolated fuels was carried out with 35, 55 and 75 wt% CAC loading, increasing MW power and N2 flow rate. The MW co-pyrolysis of coal and OPS in segregation and blend were investigated to observe vapor-phase synergy. The effects of process parameters on the efficiency of co-pyrolysis of blend were tested to identify the optimal processing conditions. The highest bio-oil and coal-tar of 36.26 wt% and 18.59 wt% were obtained with 75 wt% CAC loading using 450 W and 600 W with 4 liters per minute (LPM) of N2 flow rate, respectively. This improved oil recovery is mainly due to the fact that higher MW power and CAC loading produced sustained pyrolysis conditions for longer duration for the complete conversion of fuel solids. The bio-oil was enriched in phenol with highest detected 71.77% gas chromatography-mass spectrometer (GC-MS) area with 3-Layer method at 75 wt% CAC loading, 300 W and 4 LPM of N2 flow rate. This higher phenol formation can be attributed to the slow and uniform process heating conditions, and in-situ upgrading of pyrolysis vapors over successive carbon surfaces. The coal-tar is composed mainly of aromatics (naphthalenes, benzenes and xylene) and saturated aliphatics (alkanes and alkenes) hydrocarbons. The gas produced from pyrolysis of OPS and coal is H2 with composition of 27.94–50.46 vol% and 40.23–65.22 vol%, respectively. The co-pyrolysis oil is composed of polars (phenol, phenolics and guaiacols) consisting of more than 50% GC-MS area. The MW co-pyrolysis in segregation of upper-bed-coal/bottom-bed-OPS produced higher polars of 71.62–76.33% GC-MS area with much limited aromatics and saturated aliphatics of 2.41–8.43% and 0.37–0.80% GC-MS area, respectively. Conversely, upper-bed-OPS/bottom-bed-coal segregated fuels produced lower polars of 50.92–61.82% GC-MS area with much higher aromatics and saturated aliphatics of 19.72–28.29% and 8.22–21.36% GC-MS area, respectively. The difference in polar, aromatics and saturated aliphatics in co-pyrolysis oil shows positive vapor-phase synergy. MW co-pyrolysis of blend optimum process conditions for 33.17 wt% oil were found to be at 71.38 wt% CAC loading, 582 W and 3.5 LPM of N2 flow rate.