Use of microwaves in efficient torrefaction of municipal solid waste to char

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August 8, 2023


Energy / PERSPECTIVE / Renewable Energy


Use of microwaves in efficient torrefaction of municipal solid waste to char

Municipal solid waste (MSW) generation has steadily grown due to rapid urbanization, industrialization, and economic growth. Thus, there is a need to find a practical approach that can get rid of MSW affordably and quickly. One of the viable solutions for the disposal of MSW is incineration. It offers several benefits over other methods, including a significant reduction in waste bulk and volume and the possibility of energy recovery. Nonetheless, MSW from underdeveloped nations typically has high moisture content. Burning MSW that is very wet may lead to high pollutant emissions. Thus, MSW must undergo pre-treatment to be acceptable for incineration.

To make MSW better suited, researchers can utilize burning, torrefaction, low-temperature pyrolysis, or thermal degradation of organic components in flammable materials without air. Furthermore, researchers have found that microwave heating, which offers quick and uniform heating, is a potential heating method for pre-treating biomass. However, microwave irradiation alone cannot effectively heat the biomass of Municipal Solid Waste. Researchers must also add a microwave absorber, such as carbon black or silicon carbide. Researchers can use agricultural leftovers to make char, which is physically and chemically comparable to carbon black. Microwave absorbers can utilize them to improve the absorption of microwave irradiation.

Advantages and Optimization of MW-Torrefaction with Biochar Absorbers

Torrefaction in the presence of a microwave (MW) is a thermo-chemical process that has evolved through time. Several kinds of research have conducted investigations on the benefits of conversion and good process outcomes for MW torrefaction of agricultural/forest residues/wastes and woody biomass. MW torrefaction offers many benefits compared to conventional electric heaters, including selective and regulated heating without direct contact with the heated biomass. A few researchers have concentrated on Municipal Solid Waste-torrefaction, although more effective investigations have been done on MW-pyrolysis. In addition to removing moisture from biomass, torrefaction also dehydrates and partially decomposes cellulose and lignin, as well as breaks down hemicellulose into volatiles. According to many research works, torrefied biomass has better qualities than raw biomass.
The operating conditions for product recovery determine the effectiveness of MW-torrefaction. MW power level, temperature, and MW absorber addition are some variables. When torrefying biomass, researchers may employ biochar, a byproduct thermo-chemically generated from biomass and forestry waste, as a low-MW energy absorber. Application and the use of biochar are crucial for environmental and agricultural advantages and for enhancing environmental sustainability in thermo-chemical transformations. The decrease in energy usage during MW torrefaction is the expected result of adding biochar. Several investigations have shown that MW absorbers could increase the torrefaction reaction’s temperature, even at very low MW power. Under various treatment settings, it can also boost process energy efficiency and product yields, including biogas, bio-oil and solid yield.

Municipal Solid Waste-Torrefaction and the Impact of MW Absorbers on Biomass Conversion

In MW-torrefaction, researchers typically grind the biomass first to a small size of around 5-6 millimeters to process the microwave-torrefaction on the biomass of Municipal Solid Waste. Researchers mix the grounded biomass with biochar from other thermo-chemically treated waste resources. The researchers keep the mixture inside quartz vessels so that microwaves can pass through the container and heat the mixed biomass. Commercial microwave ovens radiate the biomass with 2.45 Gigahertz frequencies for a certain time, depending on the quantity of the biomass.
After microwave irradiance, the process produces different fractions of torrefied biochar, liquids, and non-condensable gases. The power of the microwave can be varied to get better biomass conversion to desired types of thermo-chemically converted products. A typical microwave power of 520-650 W for 10-20 min is sufficient for 100 g of biomass.
According to a research study, an MW absorber may raise torrefaction temperatures and heat biomass and alter the quality of the solid product. In addition to the biochar made from waste and forest residue, researchers might also employ industrial bio-waste products such as palm kernel shell activated carbon, coconut activated carbon, and petroleum coke as MW absorbers. Consequently, the reduction in the drying process enhances the heating rate of the particles.

Enhancing Biomass Conversion with MW Absorbers and Biochar Addition

Another study emphasizes combining coal and torrefied biomass with biochar at 250 0C for 15 min to improve the fuel characteristics of wheat and barley straws. A few other studies emphasize various benefits of Municipal Solid Waste absorbers, including their ability to mix with biomass easily to create a more homogeneous mixture, their lack of any issues with disposal or after-use problems, and their capacity to be recycled, thus saving expenses in the MW torrefaction system. At varied pyrolysis settings, biomass with or without MW absorbers has different characteristics and qualities. The evaluation depends on the feedstock, reactor settings, and procedural details.
According to a few research studies, when the torrefaction temperature and residence time increase, the solid fraction drops, and the carbon content rises, generating pore widening and structural ordering on the chars. Researchers observed that adding biochar to the biomass of Municipal Solid Waste significantly reduces the microwave power requirement while achieving a similar yield from biomass conversion. According to a study, using a low-power microwave for biomass torrefaction to biochar, liquids, or condensable gases makes the effect more achievable. When researchers mixed 20% biochar with the biomass, they found that at least 35% less microwave energy was required to achieve the same yield from the biomass. The biochar can absorb the microwave and heat up the biomass efficiently. However, more microwave power for prolonged irradiance time can significantly reduce the biochar contents and enhance liquids and non-condensable gas formation, leading to mass loss.

Future of MW-torrefaction of MSW

Torrefaction improves several of the physicochemical characteristics of biomass used for bioenergy.

Conclusively, it can be remarked that when mixing microwave-absorbing materials like biochar with MSW, microwave-torrefaction is a higher energy-efficient process compared to the conventional torrefaction process. Future research on microwave-torrefaction could focus on improving the process further, for example by selecting better microwave absorbers. High-quality biochar obtained from the torrefaction of other biomasses can be highly economical. Researchers could also conduct research to optimize the MW power irradiance and treatment time to obtain high-grade biochar from the biomass of Municipal Solid Waste. Researchers can further explore the use of microwave-torrefaction for other MSW components, such as plastics, to recover high-grade oils.


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