Residential areas throughout the world have problems with waste disposal with over 2 billion tonnes of Municipal Solid Waste (MSW) produced annually around the world. The anticipated population growth of 9.5 billion by 2050, along with industrialization and rapid development of urban and suburban areas, is expected to keep increasing the amount of MSW generated. Conventional methods used for the treatment and management of MSW are landfilling, incineration, and composting. Landfilling is becoming problematic as current landfill sites are nearing capacity with new ones difficult to implement because of the lack of available land. Furthermore, the organic component of MSW is problematic since decomposition releases gases such as carbon dioxide and methane into the atmosphere which contribute towards climate change and global warming.

Biomass is a sustainable, clean, and green resource that can be used to make fuels to meet energy demands. However, there are many drawbacks to using biomass directly as biofuels, including relatively low calorific value, unfavourable moisture content, abnormal composition, and other unfavourable properties. By applying other thermochemical techniques such as torrefaction to biomass from MSW, it is possible to create char, coke, and biochar, which have qualities similar to those of coal. To produce biochar with excellent fuel characteristics, torrefied solid biomass is mildly pyrolyzed at 200-300°C in the absence of oxygen or an inert atmosphere. This process results in solid biomass with high calorific value, low moisture content, and additional positive qualities like grindability and hydrophobicity.

Current Research into Torrefaction of Municipal Solid Waste

Researchers at the University of Saskatchewan, Canada, torrefied Construction Demolition Waste (CDW) and grass clippings from MSW in a blown glass reactor using microwave irradiation under nitrogen-activated inert conditions.The research demonstrated that higher microwave power levels contributed to increased Higher Heating Values (HHVs) and final temperatures. Microwave power level and torrefaction time had a negative correlation with the moisture content of CDW and grass clippings after the torrefaction process.

Generally, higher reaction temperatures and lower moisture content of the torrefied Municipal Solid Waste (MSW) are achieved through higher microwave power and torrefaction time. Increasing the microwave power level and torrefaction residence time resulted in a corresponding increase in the thermochemical properties of the materials.

South African Research: CDW and Grass Clippings Torrefaction for Biochar

As a result of torrefaction of CDW, elemental carbon increased by 1.3% to 39% over that of the non-torrefied CDW while the elemental carbon of torrefied grass clippings increased by 9% to 21% over that of the non-torrefied grass clippings.Researchers from the Department of Chemical Engineering at the University of Johannesburg, South Africa undertook a research programme investigating the production of high-quality biochar with properties close to bituminous coal from landfill Food Waste (FW) and Municipal Solid Waste (MSW). Researchers performed proximate and ultimate analyses to ascertain the fuel properties and elemental composition of FW and MSW before torrefaction.

The researchers gradually increased the temperature from 200 to 300°C at a rate of 10°C per minute and maintained it at the higher temperature for 40 minutes. Researchers assessed the quality of the produced biochar by calculating its calorific value, mass yield, energy yield, and energy density.  also used elemental analysis to assess the quality of raw food waste and Municipal Solid Waste. Researchers then used Thermogravimetric Analysis (TGA) and Fourier Transform Infrared (FTIR) spectrometry to study thermal evolution. They carried out torrefaction at 225°C, 275°C, and 300°C. The HHV of the biochar was comparable to that of bituminous coal from Anglo Mafube in South Africa. Elemental analysis of the biochar showed an increase in carbon content with temperature due to the loss of oxygen-containing volatiles

Municipal Solid Waste Torrefaction for Fuel Production: Process Parameters and Properties

Researchers at the University of Michigan in the USA investigated torrefaction of various Municipal Solid Waste. They compared the properties of the resulting product with coal from fossil fuels. Grinding the biochar revealed that the grinding characteristics and size distribution were comparable to those of traditional coal. Additionally, the higher heating value (HHV) increased as the mass decreased. Extrusion also significantly improved the product’s uniformity, durability, and water resistance. The investigation’s outcomes demonstrated that torrefied wastes could serve as a substitute fuel in coal power plants. During the torrefaction of Municipal Solid Waste, researchers discovered synergies between fiber and plastic wastes. This discovery led to a subsequent study undertaken to understand these interactions.

Wroclaw University of Environmental and Life Sciences in Poland conducted a study on Refuse Derived Fuel (RDF) produced from MSW. Research aimed to determine RDF’s torrefaction kinetics and study temperature and residence time effects on biochar’s fuel properties. The temperature range used for the torrefaction process was 200-300°C, with temperature intervals of 20°C. For each temperature, the residence lasted 20, 40, and 60 minutes, respectively.

Researchers analyzed moisture, organic matter, combustible and volatile content, ash, and HHV for RDF and CRDF. This analysis was conducted to compare and understand the fuel properties of both types of fuels. According to the thermogravimetric analysis, the majority of the lignocellulosic and hemicellulosic biomass present in RDF breaks down during torrefaction. Studies have demonstrated how residence time and temperature affect the CRDF’s fuel properties. The 260°C temperature and 20-minute residence time produced the CRDF’s highest HHV.

Future Trends

Researchers are currently investigating numerous areas where research and development could significantly improve the economic viability of current facilities that process MSW. These research and development opportunities focus on making capital improvements that enhance the overall financial health of these facilities by lowering operating costs and increasing or supplementing revenue sources. These possibilities include the creation of cutting-edge waste pre-processing methods, strategies for recovering valuable species from incinerated ash, techniques to boost anaerobic digestion’s biogas yields, separation techniques, and procedures for turning biogas into higher calorific value biofuels and bioproducts. These research areas offer opportunities to advance a wide range of current waste-to-energy facilities throughout the world, even though the stages of technological development among different countries differ significantly.

Businesses may find future research and development opportunities in the next-generation Municipal Solid Waste processing facilities, but these technologies need further development or risk reduction before adoption. These research and development opportunities include the creation of MSW gasification systems, strategies for lowering anaerobic digesters’ capital requirements, and procedures for the direct conversion of MSW into biofuels and other bioproducts. These MSW feedstock hold great promise for the production of biofuels and bioproducts, which are both at an early stage of technological development. Utilizing ongoing basic and applied research on the production of biofuels and bioproducts from cellulosic materials, algae, and other feedstocks can make MSW processes more affordable.