Municipal solid waste (MSW) is one of the important environmental challenges in the world. Improper methods of MSW disposal are detrimental to the environment and human health. Researchers have recently examined torrefaction and other viable methods for MSW management. Torrefaction refers to the thermal process of converting biomass that requires moderate heating without oxygen. Char, the result of this process, has intriguing future applications in the energy industry due to its unique properties. It is essential to compare the properties of char produced by the torrefaction of MSW to the char produced by other thermal treatments, such as pyrolysis. Understanding the properties of char is crucial to understand how several types of MSW may be utilized for energy.
Torrefaction involves heating of biomass to temperatures between 200 and 300 0C in the absence of oxygen. During torrefaction, moisture and volatile substances are released, and the remaining solid material is converted into char. The torrefaction process provides a solid fuel that is simpler to handle and transport than raw biomass. The method also boosts the fuel’s energy density, making it more suitable for energy applications.
The decomposition of MSW involves several phases. First, separate non-combustible materials such as metal, glass, and ceramics. Then, shred the MSW to increase surface area and homogeneity. Next, dry the shreds to eliminate any remaining surface moisture. When drying, MSW is torrefied, resulting in the formation of char. The char is filtered and stored for future use when it has cooled.

Properties of Char Produced from Torrefaction of MSW

Char produced by the torrefaction of MSW has unique properties that make it suitable for various energy applications. These features consist of the following:

High Carbon Content:

The char produced by the torrefaction of MSW contains up to 85 % carbon. This helps it burn more effectively, making it an excellent fuel for energy applications.

Low moisture content and high energy density:

The torrefaction process removes moisture from MSW, and the resulting char will have very low moisture content. This enhances its energy density and makes it more suitable for energy applications.

High heating value:

Char produced by the torrefaction of MSW has a high heating value of 30 MJ/kg, which makes it suitable for use in power generation and heating.

Low ash content:

The torrefaction procedure generates char with a low ash percentage. The low ash content reduces the total waste output considerably.

Reduced Emissions:

The removal of volatile substances during torrefaction minimizes the emission of contaminants during combustion. Thus, the char formed by the torrefaction of MSW emits fewer greenhouse gases and other pollutants than MSW. This makes the whole process more sustainable.

Comparison of char properties from pyrolysis and torrefaction of MSW

Pyrolysis is another thermal process for converting biomass that involves heating biomass at substantially higher temperatures (400-900°C) without oxygen. The method generates char in addition to bio-oil and syngas as byproducts. The pyrolysis of MSW produces char with different properties than torrefaction. The following characteristics vary:

Carbon Content:

The pyrolysis of MSW creates char with a lower carbon content than torrefaction. Typically, the carbon content of pyrolysis char is between 60 and 70%.

Moisture Content:

The pyrolysis of MSW produces char with a higher moisture content than torrefaction. The increased moisture content of pyrolysis char reduces its energy density, making it less effective for energy applications.

Heating value:

Char formed by pyrolysis of MSW has a lower heating value than char produced by torrefaction. Typically, pyrolysis char has a heating value between 20 and 25 MJ/kg.

Ash content:

The pyrolysis of MSW produces char with a higher ash content than the torrefaction process. The greater the pyrolysis char’s ash content, the less suited it is for use in energy applications since it creates more waste.

Emissions:

The pyrolysis of MSW releases more greenhouse gases and other pollutants than the torrefaction process. The pyrolysis process produces bio-oil and syngas, which emit considerable quantities of greenhouse gases and other contaminants.

Char usage in different applications based on quality

The torrefaction of MSW produces char with unique properties that make it suitable for various applications. Char can be utilized as follows depending on its characteristics:

Electricity generation:

Char, with a high heating value and low ash content, is suitable for use in the generation of electricity.

Heat generation:

Char with a high heating value and low ash content may be employed in heating applications such as boilers and furnaces.

Soil Amendment/Fertilizer:

High-nitrogen-content char can be used as a soil amendment or fertilizer.

Carbon Sequestration:

Char may also minimize greenhouse gas emissions by sequestering and burying it in the ground.

Properties of char based on types of MSW

The characteristics of char produced by the torrefaction of MSW vary depending on the kind of MSW used. Many types of MSW have distinct chemical compositions and properties, which impact the properties of char produced from them. For example:

Food waste:

The char produced from food waste contains a high nitrogen concentration, which makes it suitable to be utilized as a soil amendment or fertilizer.

Paper and Cardboard:

Due to its high carbon content, char generated from paper and cardboard may be employed in energy applications.

Plastics:

The high heating value of plastic char makes it suitable for use in energy applications.

Textile waste:

The low heating value of textile char makes it unsuitable for energy applications.

The future of char produced from the torrefaction of MSW

The char produced by the torrefaction of MSW has a significant energy use potential. However, some problems must be resolved so that their full potential is reached. The production rate of char using the torrefaction process should be improved to satisfy the demand for char in energy applications. To compete with alternative energy sources, the cost of producing char through torrefaction must be reduced. There must be regulations to ensure that the char produced is safe for energy applications and has no adverse environmental effects. Despite the current problems, the future of char formed from the torrefaction of MSW is bright. With further studies and developments, it might become a valuable renewable energy source and a solution to waste management issues.