Table 13 Summaries of recent studies on catalytic/microwave-assisted pyrolysis

[110]

Prosopis juliflora (PJF)

The experiment was performed to examine how (i) microwave power, (ii) susceptor, (iii) particle size, and (iv) Feed:Susceptor ratio effects on the yields of bio-oil, gas, and char, composition of bio-oil. By altering 5 different industrial wastes as microwave susceptors, (i.e., graphite, char, aluminum, silicon carbide, and fly ash)

Maximum bio-oil was realized using fly ash of 40 wt % with a heating value of 26 MJ kg⁻¹ at a microwave power of 560 W, particle size of 2–4 mm, and Feed:Susceptor ratio of 100:1. The bio-oil analysis showed mixtures of phenolic, aromatic hydrocarbons, cyclopentanones, carboxylic acids, ketones, and furan derivatives. In addition, it demonstrates that the yield and quality of bio-oil are dependent on key parameters such as microwave power, biomass particle size/composition, and type of susceptor

[135]

Larch

Experiment was carried out using a fundamentally designed scalable microwave system maximize pyrolysis oil yield and quality.

The results suggested that with controllable sample size, the liquid product yield is comparable to conventional pyrolysis and can be achieved at an energy input around 600 kWh/t. Similarly, quality of the liquid is significantly improved compared to conventional pyrolysis. This is because of the advantage the very rapid heating and quenching that can be achieved with MAP.

[136]

Corn cob, corn stover, saw dust and rice straw

The experimental materials were used without any pre-treatment and at standard microwave experimental conditions; also, MgCl₂ as a catalyst

The pyrolysis of corn cob and corn cob plus catalyst gave the highest bio-oil yield up to 42.1 and 40% (wt), respectively.

Higher HHV of 22.38 MJ/kg of bio-oil from corn cob was due to the presence of ethyl ether and 2-bromo-butane with a relative proportion of 15.63 and 4.60%, respectively.

In addition, GC–MS analysis of corn cob-based bio-oil showed the presence of ethyl ether, phenol, aliphatic hydrocarbons, furfural, furan derivatives, and acids in major proportions.

[137]

Palm kernel shell

Catalytic fixed-bed and microwave pyrolysis of palm kernel shell using activated carbon (AC) and lignite char (LC) as catalysts and microwave receptors are investigated.

The authors report addition of catalyst increased the bio-oil yield, but decreased the selectivity of phenol in fixed-bed. The highest concentration of phenol in bio-oil of 64.58% (area) and total phenolics concentration of 71.24% were obtained at 500 °C using activated carbon.

[138]

Oil palm empty fruit bunch pellets

Experiment was carried out in a multimode microwave system with 2.45 GHz frequency with and without the MW absorber, activated carbon.

The ratio of feed to absorber did influence the temperature profiles of the EFB pellets and also pyrolysis products such as bio-oil, char, and gas. The highest bio-oil yield of about 21 wt.% was obtained with 25% MW absorber. The bio-oil consisted of phenolic compounds of about 60–70 area% as detected by GC-MS and confirmed by FT-IR analysis.

[139]

Chlorella sp. strain and Nannochloropsis strain

Experiment was done in the presence of a microwave absorbent SiC and catalyst HZSM-5

Results for Chlorella at temperature of 550 °C without catalyst were the optimal conditions that resulted in a maximum bio-oil yield of 57 wt%. On the other hand, optimal condition for Nannochloropsis at temperature of 500 °C with 0.5 of catalyst ratio was achieved. Resulting in a maximum bio-oil yield of 59 wt.%.

[140]

Process design––to do without susceptors

Presented a multidisciplinary design methodology for a microwave fluidized bed system that was based on processing raw biomass without the need for added microwave susceptors.

The authors found that a minimum power density of 54 MW m⁻³ was necessary to reach temperatures of 400 °C for particles with an average size of 600 μm at the minimum fluidization velocity (0.38 ms⁻¹). The microwave fluidized bed system was shown to be effective in enabling pyrolysis while limiting heterogeneity and thermal runaway effects.

[141]

Corn stalk biomass briquettes

It was carried out in a developed microwave reactor supplied with 2.45 GHz frequency using 3 kW power generators.

The highest bio-oil, biochar, and gas yield of 19.6, 41.1, and 54.0% was achieved at different process condition. Quality wise, the biochar exhibited good heating value (32 MJ/kg) than bio-oil (2.47 MJ/kg).

[142]

Rice straw, rice husk, corn stover, sugarcane bagasse, sugarcane peel, waste coffee grounds, and bamboo leaves

Use empirical equation to predict the various product yields. Also applied energy return on investment.

Solid, liquid, and gas yields were in the ranges of 18–22, 40–48, and 30–40 wt%, respectively. The primary components of the gas product were H₂ (18–25 vol%), CH₄ (6–8 vol%), CO (51–59 vol%), and CO₂ (10–14 vol%), and the rest undetermined part was only 3–5 vol%. The energy return on investment of microwave pyrolysis can be approximately 3.56, so the technique proofs can be economically feasible.

[143]

Used cooking oil

Pyrolysis was designed in order to create contact with a bed of microwave absorbents heated by microwave radiation. Different materials were used in the reaction bed, including particulate carbon, activated carbon, and mesoporous aluminosilicate.

The use of particulate and activated carbon as the reaction bed provided a fast heating rate and extensive cracking capacity to pyrolyze the used oil, thus showing favorable features that could lead to short process time and less energy usage.

Thus, it resulted in a production of a high yield of a biofuel product (up to 73 wt%) in 35 min. Additionally, the biofuel showed a composition dominated by light C₅–C₂₀ aliphatic hydrocarbons with low amounts of oxygenated compounds (≤11%). In particular, the oil product obtained from activated carbon bed showed low nitrogen content and was free of carboxylic acid and sulfur.

[144]

Malaysian oil palm shell, empty fruit bunch, rice husk, and coconut shell and wood sawdust

The dielectric properties were measured from room temperature to ~700 °C and at six different frequencies (397, 912, 1429, 1948, 2466, and 2986 MHz) using a cavity perturbation method.

The dielectric properties recorded a fallen in the drying region (24–200 °C) which is due to removal of moisture, and further, it decreased in the pyrolysis region (200–450 °C) that is due to decomposition or removal of volatile matter. Still, dielectric properties increased drastically beyond temperature 450 °C. Minimum MW absorption was attained in temperature range of 300–400 °C. Biochar which was formed after the pyrolysis process, showed high loss tangent as compared to the original biomass, signifying it as a suitable material for MW absorber and catalyst applications.

[145]

Tire powders

Experiment was carried out under standard microwave pyrolysis with special attention to the yields, and composition over time was studied

At the end of the pyrolysis, 43% of the solid residues, 45% of the oils, and 12% of the pyrolysis gases were obtained.

[146]

Softwood, hardwood, and herbaceous biomass

In this paper, these were processed by microwave-assisted acidolysis to produce high-quality lignin.

The lignin from the softwood was isolated largely intact in the solid residue after acidolysis. For example, a 10-min treatment, microwave-assisted acidolysis produced a lignin with a purity of 93% and yield of 82%, superior to other conventional separation methods reported in literature.

[147]

Corn stover and scum

Fast microwave-assisted catalytic co-pyrolysis for the production of bio-oil was carried out with CaO and HZSM-5 as the catalyst. The aim of the study was to carry out investigations on the effects of reaction temperature, CaO/HZSM-5 ratio, and corn stover/scum ratio on co-pyrolysis product fractional yields and selectivity. A constant ratio of corn stover: scum: CaO: HZSM-5 = 1:1:1:1 was used in carrying the experiment.

Overall, the results showed that co-pyrolysis temperature of 550 °C gave the maximum bio-oil and aromatic yields. Mixed CaO and HZSM-5 catalyst with the weight ratio of 1:4 increased the aromatic feedstock yield to 35.77 wt.% which was 17% higher than that with HZSM-5 alone. Scum as the hydrogen donor, showed a significant synergistic effect with corn stover to promote the production of bio-oil and aromatic hydrocarbons. The maximum yield of aromatic hydrocarbons (29.3 wt.%) were obtained when the optimal corn stover to scum ratio was 1:2.