(600a) Effect of Carbonaceous Susceptors on Selective Production of Phenolics and Syngas from Lignin Via Microwave Pyrolysis

At present, global energy supply and production of chemicals are largely dependent on fossil fuels such as coal, petroleum and natural gas. However, the utilization of these resources is limited owing to their non-renewable nature and its environmental adversity. Lignocellulosic biomass is projected as the only source of renewable carbon on earth. The primary structural building blocks of biomass are cellulose, hemicellulose, and lignin; of these components, contributes to 30% and 40% by weight and energy of the total biomass, respectively. Energy, syngas, fuels and chemicals can be obtained from lignin through thermochemical processing techniques like combustion, gasification and pyrolysis. Fast pyrolysis is a promising process to produce bio-oil and chemicals in high yields. High yields of aromatic hydrocarbons and organics can be obtained due to the aromatic functionality in the lignin structure. In order to extract resources from lignin on a large scale by fast pyrolysis technology, challenges such as high yield of condensable products, product selectivity and energy efficiency are to be improved. Achieving fast pyrolysis conditions with improved efficiency is possible by microwave assisted heating. Microwave assisted fast pyrolysis technology can be used to overcome the techno-economic barriers by reducing the processing time, improving the heat distribution, enhancing the product selectivity, and aiding in better heating efficiency.

The intention of the current study is to enhance the yield of bio-oil and its quality from lignin by microwave assisted pyrolysis. The effects of (i) lignin (L) to activated carbon (AC) ratio , (ii) particle size of activated carbon (0.45 mm to 3 mm), and (iii) carbonaceous susceptors (AC, graphite and charcoal) on heating rate, product yields (oil, gas and char fractions), product composition and process efficiency were investigated at a constant microwave power of 480 W. It was found that, owing to the high amount of physically bound moisture in the mesoporous capillaries of AC (1mm), high bio-oil yield of 66 wt. % at optimum conditions of 10: 90 g/g of L: AC was observed. In L:AC blend of 100 g, increase in mass of AC from 10 g to 90 g decreased the heating rate significantly from 128 ^oC/min to 32 ^oC/min, correspondingly a decrease in energy efficacy of the system from 95% to 34% was observed. From the oil composition analyses, it was found that bio-oil comprises of phenolics (simple phenols, guaiacols and syringols), aromatic hydrocarbons, naphthalenes, indene derivatives and furans. The highest yield of total phenolics (64.2%) was obtained at 10: 90 g/g of L: AC (1mm). Initial lignin sample mass of 9 to 40 wt. % was converted into simple phenolics and its composition increased with increase in L: AC ratio.

The carbonaceous materials, such as activated carbon, graphite, charcoal and bio-char are found to be the efficient microwave absorbers because of their high microwave coupling nature even at ambient temperatures [1,2]. For this reason, we have chosen activated carbon, graphite and charcoal as the susceptors to investigate their effect on lignin pyrolysis. The effect of susceptors on the heating rate, product mass and energy yields were investigated.  The yield (wt. %) of bio-oil is in line with the heating rates and varies in the following order: charcoal (36 wt. %) > graphite (34.5 wt. %) > AC (0.45mm) (29.2 wt. %). Phenolic derivatives are dominant in oil fraction, 97% of oil obtained with AC (0.45mm) contains phenolics followed by charcoal (79%) and graphite (72%). Selectivity (%) of simple phenolics is varying in the following order: 60.1 (AC (0.45mm)) > 55.8 (charcoal) > 40.9 (graphite). A significant enhancement in heating value was noticed both in char (28-30 MJ/kg) and oil (28-33 MJ/kg) fractions compared to raw lignin (25 MJ/kg). Tiny micro plasma spots generated during the pyrolysis process increased tiny cylindrical and spherical micro-pores within the residual char. Yield of CO₂ decreased and that of H₂ increased with increase in pyrolysis temperature and its evolution profiles were influenced by the nature of susceptor and lignin to susceptor mass ratio.

References:

1. T. Ohra-aho, J. Linnekoski. Catalytic pyrolysis of lignin using analytical pyrolysis-GC-MS. J. Anal. Appl. Pyrol. 113 (2015) 186-92.
2.  S. Adhikari, V. Srinivasan, O. Fasina. Catalytic pyrolysis of raw and thermally treated lignin using different acidic zeolites. Energy & Fuels 28 (2014) 4532-38.