A review on the role of susceptors in the recovery of valuable renewable carbon products from microwave-assisted pyrolysis of lignocellulosic and algal biomasses: Prospects and challenges

Conversion of waste polystyrene into valuable aromatic hydrocarbons via microwave-assisted pyrolysis

The prime objective of the current research work was to understand the role of microwave-assisted pyrolysis for the upgradation of expanded polystyrene (EPS) waste into valuable aromatic hydrocarbons. Ethyl acetate solvent was used to dissolve the EPS to enhance the homogeneous dispersion of EPS with susceptor particles. Biochar obtained from the pyrolysis was used as a susceptor. The design of experiments method was used to understand the role of microwave power (300 W, 450 W, and 600 W) and susceptor quantity (5 g, 10 g, and 15 g) in the pyrolysis process. The pyrolysis was conducted till the temperature reached up to 600 °C, and this temperature was achieved in the time interval of 14-38 min based on the experimental conditions. The obtained average heating rates varied in the range of 15 to 41 °C/min to attain the pyrolysis temperature. The EPS feed was converted into char (~ 2.5 wt.%), oil (51 to 60 wt.%), and gaseous (37 to 47 wt.%) products. The specific microwave energy (J/g) was calculated to know the energy requirement; it increased with an increase in susceptor quantity and microwave power, whereas specific microwave power (W/g) was a function of microwave power and increased from 15 to 30 W/g. The predicted values calculated using the model equations closely matched the actual values showing that the developed model equations via optimization had a good fit. The obtained pyrolysis oil physicochemical properties including viscosity (1 to 1.4 cP), density (990 to 1030 kg/m3), heating value (39 to 42 MJ/kg), and flash point (98 to 101 °C) were thoroughly analyzed. The pyrolysis oil was rich in aromatic hydrocarbons and it was predominantly composed of styrene, cyclopropyl methylbenzene, and alkylbenzene derivates.

Conventional and microwave-assisted acid pretreatment of tea waste powder: analysis of functional groups using FTIR

Tea waste powder (TWP) is one of the potential biomass waste to recover valuable chemicals and materials. The prime objective of this work is to investigate the role of acid pretreatment on TWP. Diluted acids (HCl, H3PO4, CH3COOH, and H2SO4) were used to soak the TWP to understand the role of acids on bond cleavage and chemicals formation. One gram of TWP was soaked in 100 mL of diluted acids for 24 h. The soaked samples were further subjected to a hot air oven (temperature: 80 °C, duration: 6 h), orbital shaking (shaking speed: 80-100 rpm, duration: 6 h), and microwave irradiation (microwave power: 100 W, duration: 10 min) to understand the synergistic effects of acids and mode of exposure. The pretreated solid samples and liquid samples were analyzed using FTIR to understand the presence of functional groups. The mass loss of TWP after treatment significantly varied with the type of acid and exposure mode used. In the orbital shaker, the mass loss was varied in the following order: H2SO4 (36%) > CH3COOH (32%) > H3PO4 (22%) > HCl (15%). In hot air oven, high mass loss was observed compared to orbital shaking [HCl (48%) > CH3COOH (37%) > H2SO4 (35%) > H3PO4 (33%)]. The mass loss in microwave irradiation is lower (19 to 25%) with all acids compared to orbital shaking. In the solid samples, O-H stretching, C-H stretching, C=O stretching, C=C stretching, -C-O-, and -C-OH- functional groups were noticed. Similarly, C=O and C=C peaks and C-O and -C-OH peaks were noticed in liquid samples. Interestingly, microwave irradiation showed promising results in 10 min of pretreatment, whereas orbital shaking and hot air oven pretreatments require 6 h to achieve the same result.

A critical review on the influence of operating parameters and feedstock characteristics on microwave pyrolysis of biomass

Biomass pyrolysis is the most effective process to convert abundant organic matter into value-added products that could be an alternative to depleting fossil fuels. A comprehensive understanding of the biomass pyrolysis is essential in designing the experiments. However, pyrolysis is a complex process dependent on multiple feedstock characteristics, such as biomass consisting of volatile matter, moisture content, fixed carbon, and ash content, all of which can influence yield formation. On top of that, product composition can also be affected by the particle size, shape, susceptors used, and pre-treatment conditions of the feedstock. Compared to conventional pyrolysis, microwave-assisted pyrolysis (MAP) is a novel thermochemical process that improves internal heat transfer. MAP experiments complicate the operation due to additional governing factors (i.e. operating parameters) such as heating rate, temperature, and microwave power. In most instances, a single parameter or the interaction of parameters, i.e. the influence of other parameter integration, plays a crucial role in pyrolysis. Although various studies on a few operating parameters or feedstock characteristics have been discussed in the literature, a comprehensive review still needs to be provided. Consequently, this review paper deconstructed biomass and its sources, including microwave-assisted pyrolysis, and discussed the impact of operating parameters and biomass properties on pyrolysis products. This paper addresses the challenge of handling multivariate problems in MAP and delivers solutions by application of the machine learning technique to minimise experimental effort. Techno-economic analysis of the biomass pyrolysis process and suggestions for future research are also discussed.

Energy and exergy performances of low-density polyethylene plastic particles assisted by microwave heating

Plastic waste can exist naturally for hundreds of thousands of years and harm humans, animals, and the environment. In this study, the energy and exergy performances (absorbed energy, energy efficiency, absorbed exergy, and exergy efficiency) of LDPE (low-density polyethylene) plastic particles assisted by microwave heating based on the experimental data as affected by microwave power, feeding load, and chamber volume were evaluated and analyzed. The results showed that as the microwave power raised from 500 to 900 W, the feeding load changed from 10 to 30 g, and the chamber volume decreased from 200 to 100 ml, (a) the absorbed energy at the heating time of 60 min increased from 19.73 kJ, 5.84 kJ, and 22.71 kJ to 37.69 kJ; (b) the energy efficiency for the whole heating process increased from 1.10%, 0.32%, and 1.26% to 2.09%; (c) the absorbed exergy at the heating time of 60 min increased from 0.308, 0.091, and 0.091 to 0.724 kJ; and (d) the exergy efficiency for the whole heating process increased from 0.017, 0.005, and 0.023 to 0.040%, respectively.

Promotion of bio-oil production from the microwave pyrolysis of cow dung using pretreated red mud as a bifunctional additive: Parameter optimization, energy efficiency evaluation, and mechanism analysis

To address the issues of high oxygen content and energy consumption in the microwave-assisted pyrolysis of biomass for biofuel production, this study used high-temperature pretreated red mud (RM) as an additive. The pretreated RM exhibited dual functionalities, namely microwave absorption and catalytic properties, during the microwave-assisted pyrolysis of cow dung (CD). This study also evaluated the optimization potential of energy recovery efficiency. The results showed that the addition of pretreated RM significantly increased the oil yield during the microwave-assisted pyrolysis of CD. The highest oil yield (59.63%) was obtained via the microwave-assisted pyrolysis of CD over catalysis with RM pretreated at 750 °C (RM750). Through the optimization of the RM750-to-CD mixing ratio, optimal oil quality and energy recovery efficiency were achieved. At a mixing ratio of 1:1, the pyrolysis oil featured the highest aromatic hydrocarbon content and lowest acid content. The high-temperature pretreatment of RM increased the Fe2O3 content, which enhanced the dielectric properties and magnetic loss ability of the reactants. This resulted in localized high temperatures and the formation of "hot spots," which can promote the deoxygenation and hydrogenation reactions of oil. Consequently, the lower heating rate of oil increased from 35.12 to 40.11 MJ kg-1. The released oxygen escaped in the form of CO. In addition, pyrolytic char was used as an in situ microwave absorbing material owing to its increased Fe2O3 content and graphitization degree, leading to an increase in energy recovery efficiency from 4.71% to 9.98%. This study provides valuable guidance for the efficient utilization of diversified solid wastes and demonstrates the potential application of microwave-assisted pyrolysis technology in the resource utilization of solid wastes.