In situ and ex situ catalytic microwave pyrolysis of biomass pellets using Ni/Al2O3 for hydrogen and bio-oil production

Four different biomass feedstocks, viz., rice straw (RS), sugarcane bagasse (BG), pine wood (PW), and Prosopis juliflora (PJ), were pyrolyzed in a microwave reactor at 800 W- 800 °C, and the hydrogen generation potential was assessed under different conditions using Ni/α-Al₂O₃ catalyst. Uniquely, four different configurations were evaluated in this study, which include non-catalytic pyrolysis of biomass in powder and pellet forms, and catalytic pyrolysis of biomass pellets in in-situ and ex-situ modes. The hydrogen yield was high in the case of biomass pellets and in the presence of catalyst. The general trend was: (biomass pellets) _In-situ > (biomass pellets) _Ex-situ > biomass pellets > powder biomass. The heating rates also followed the above trend for all biomass feedstocks. Gas yield increased by 2–10 % with pelletization of biomass, and 18–32 % with the use of catalyst due to efficient cracking of the pyrolysis vapours. The H₂:CO (mol/mol) ratio was higher in the case of in-situ pyrolysis mode, and the trend was: BG (1.60) > PW (1.52) > RS (1.32) ≈ PJ (1.31). In-situ catalytic pyrolysis of PW pellets yielded high amount of hydrogen (39 vol%, 27.7 g/kg_biomass). In-situ catalytic pyrolysis mode was effective for higher hydrogen yield due to better contact of the pyrolysates with the Ni catalyst that effectively promoted water gas shift reaction. Ex-situ catalytic pyrolysis of biomass pellets produced high yield of CO+CO₂. Detailed bio-oil composition analysis revealed that the selectivity to phenolics in the bio-oil obtained from biomass pellets (64–71 %) was more than that from biomass in powder form (51–61 %). Notably, in-situ catalytic pyrolysis of pellets resulted in a sharp decline in phenolic selectivity (22–45 %) accompanied by increased production of aliphatic oxygenates (38–52 %), and mild increase in aliphatic hydrocarbons. This study proves that microwave-assisted pyrolysis of biomass pellets is a promising strategy for hydrogen generation at high yields with good potential to reform the gases for further enhancement of hydrogen.