Experimental Studies on Co-pyrolysis of Biomass and Plastic Waste

Abstract

The depletion rate of non-renewable resources and their utility, which mostly ends up in polluting the environment, are the major reasons for which biomass usage has come into the limelight. Energy production from biomass is mostly done by thermo-chemical and biological conversion routes among which pyrolysis is considered as the most appropriate and efficient thermo-chemical method for biomass conversion. The present work is mostly based on the co-pyrolysis of Mahua seed (Madhuca indica) with plastic. Mahua seed is a widely available biomass, whose pyrolysis at different conditions of heating rate, temperature and residence time yields around 49% of bio-oil. But this bio-oil is highly viscous, unstable, and has high water content, which limit its application. Therefore, co-pyrolysis of Mahua seed (MS) with plastic (Polystyrene) has been done in a semi-batch reactor in the presence of inert atmosphere in different blending ratios (9:1, 3:1, 4:1 and 1:1) at a constant heating rate of 20 ºC/min and temperature ranging from 400-600 ºC. A maximum of 74.25% bio-oil has been obtained at 1:1 blend which is higher by about 25.25% than the oil yielded from the pyrolysis of Mahua seed alone.

Also, this bio-oil, obtained at 525 oC with 1:1 blend, possesses better quality and quantity in comparison to the bio-oil from Mahua seed. It had lower oxygen, higher carbon and higher hydrogen contents, having higher calorific value than Mahua bio-oil, which has been characterized through elemental analysis. Due to the addition of plastic in biomass, the physical properties such as viscosity, water content, flash point, pH, distillation temperature, and carbon residue are decreased near to petroleum based fuel. The FTIR, GCMS and 1H˗NMR analyses show that there is a significant decrease in phenolic, acidic compound; however most of the functional groups present in co-pyrolysis oil are aromatic compounds. The FTIR spectrum of the oil obtained from the co-pyrolysis closely resembles to that of Polystyrene (PS) pyrolysis oil rather than that of Mahua bio-oil. Further GC-MS analysis shows that most of the compounds present in co-pyrolysis oil are similar to those of Polystyrene pyrolysis oil. The aliphatic compound present in co-pyrolysis oil reduced as compared to the Mahua bio-oil. The co-pyrolysis oil could be ranked as carbon chain range of C6–C18, which is the mixture of gasoline and diesel.

In addition to that, the by˗product (bio-char) obtained from Mahua seed pyrolysis and co-pyrolysis at an optimum temperature of 525 ºC was also characterized and it was found that the calorific value of co-pyrolysis bio-char is more than that of Mahua seed bio-char; however both are more than that of Indian standard coal. The pH of Mahua seed and co-pyrolysis bio-char were 11.9 and 12.5 respectively, which is probably good for acidic soils. From the SEM images of Mahua seed and co-pyrolysis bio-chars it can be concluded that co-pyrolysis bio-char is more porous than that of Mahua seed bio-char. The obtained surface area of co-pyrolysis bio-char is more than that of Mahua seed bio-char.

The results of the thermal kinetic study of Mahua seed, Polystyrene and co-pyrolysis kinetics of Mahua seed:Polystyrene 1:1 blend shows that the behavior of the blends are quite different to the combination of the individual materials of biomass and Polystyrene. The Mahua seed and Polystyrene 1:1 blend exists good interaction and significant synergic effect between the plastic and biomass co-pyrolysis. The values of Activation energy (EA) and pre-exponential factor (A) are higher for mixtures than for individual components in the Kissinger method, whereas the activation energy and pre-exponential factors obtained for FWO and KAS methods of mixture were lower than those of individual one. The obtained kinetic parameters from Kissinger, KAS and FWO methods are good in agreement, but KAS and FWO methods are more efficient in the description of the degradation mechanism of solid-state reactions.

To further evaluate the efficiency of this upgraded bio-oil, engine performance study was carried out where the oil has performed well up to 60% blend whereas bio-oil from Mahua seed oil ran up to 30% with diesel blend. This analysis further bolsters the potentiality of the obtained bio-oil from co-pyrolysis to be used as an alternative fuel in combustion devices after proper treatments.