Unit Operations for Processing Municipal Solid Waste

Evaluate the evolution of technologies related to solid waste management.

MEE 5901, Advanced Solid Waste Management 1

Course Learning Outcomes for Unit III Upon completion of this unit, students should be able to:

  1. Evaluate the evolution of technologies related to solid waste management.
  2. Describe best practices of solid waste management in an urban society.

Reading Assignment Chapter 4: Mechanical Processes

Unit Lesson As the world begins to approach the peak rate at which natural resources can be delivered into commerce, the role of segregating, recycling, and reusing materials from waste refuse becomes more important to the global economy. In a world that has adopted the key principles of sustainable waste management, waste is now seen as a valuable resource and is no longer viewed as being without value. A high-level of resource recovery is accomplished by applying different processing strategies that extract valuable materials from waste refuse (Department for Environmental Food & Rural Affairs, 2013). No single technology can meet all recovery objectives. Therefore, multi-unit operations that operate in series to each other will be needed if materials are to be extracted that still have or can be converted into products having commercial value. Each of the different unit operations will exert its own positive impacts on the environment. The more material that can be extracted for recycling and reuse, the less residue remains that will need to be either thermally oxidized for heat recovery or placed into a landfill. While practicing engineers in the field generally talk about separation technologies, there is also the need to include a discussion about conversion processes. Conversion processes can be classified into the categories of physical, thermal, or biological. The physical technologies that are primarily used when refuse is to be prepared for energy recovery are comprised of the following: screening, picking, shedding, grinding, wet separation, drying, pressing, baling, and pelletizing. The end result is that the processed refuse is compressed and densified into a refuse-derived fuel (rdf) for combustion in an incineration unit. Thermal technologies convert solid materials into gaseous products that are commonly referred to as synthetic gas or syngas. When oxygen is present, the process is referred to as gasification. In the absence of oxygen, the process is known as pyrolysis. Pyrolysis units are designed to operate at temperatures greater than 925 degrees Fahrenheit in the absence of oxygen. In addition to generating a pyrolysis char and oil, pyrolysis units produce syngas comprised of methane, carbon monoxide and dioxides, hydrogen, and complex organics. The syngas that is generated can be combusted in boilers, gas turbines, or internal combustion engines to make electricity that is placed onto the grid and sold to the local utilities. Syngas can also be transformed and made into other more complex organic chemical substances. Gasification units operate in the presence of reduced oxygen levels at temperatures greater than 1,400

degrees Fahrenheit. These units convert the organic materials in the municipal solid waste (MSW) to produce a syngas along with carbon monoxide, hydrogen, and slag. Slag is derived from inorganic materials that are converted into a solid, glassy residue at high temperatures. Plasma gasification units are also capable of generating syngas in units that operate at temperatures greater than 7,000 degrees Fahrenheit by making an electrically conducting gas called a plasma. The plasma is


Unit Operations for Processing Municipal Solid Waste

MEE 5901, Advanced Solid Waste Management 2

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