It is nearly impossible not to be shocked by the fact that presently, the Unites States has a 24-acre per capita ecological footprint. On a planet with 33 billion acres of land, it would take 5 Earths if everyone lived like an average American. The two most significant contributors to the ecological footprint are municipal solid waste (MSW) and energy. Currently, the most common ways to deal with MSW globally are: dumping, burning and recycling. When it comes to energy, petrocarbons provide the greatest shares of electric power and transportation fuels.

Fortunately, there are existing technologies which effectively address both these major issues simultaneously. Waste-to-energy (WTE) is the concept of transforming MSW into energy. MSW volumes and mass are greatly reduced while transforming the carbon-based portions into multiple sources of energy such as: synthetic gas (syngas), bio-oils, steam and heat. These sources can then be used to generate electricity and to produce renewable fuels.

Several types of WTE technology are commonly available commercially now. The simplest is incineration, often used to dispose of medical waste, where the heat produced can be used for energy. More advanced technologies include pyrolysis, conventional gasification and plasma arc gasification.
 

Various Options

With incineration, it is not necessary to presort the MSW, however, one can always remove any noncontaminated recyclable materials. The process starts by metering the MSW onto a grate or fluidized bed, supplied with plenty of air or oxygen, and burned at temperatures between 1000 to 2200 degrees Fahrenheit (F). Steam can be produced by a boiler system which includes a waterwall of pipes carrying water through the hot portions of the reactor. This high-pressure steam is passed through steam turbines to generate electricity. Exhaust gases are cleaned and filtered before sending them to the stack. With incineration, it is possible to generate 544 kilowatt-hours (kWh) per ton of MSW, enough to run an average American home for 17 days.

The pyrolysis process begins when MSW, after presorting and shredding, is metered into a reactor with little or no oxygen. The temperature in the reactor is increased to a range between 1,200 and 2,200 degrees F. Combustion does not occur in this process. When carbon-based materials are exposed to high temperatures, chemical bonds begin to break. Depending on the temperature, this process results in the generation of solid char, oily liquids, and gases such as hydrogen (H2), carbon monoxide (CO), and volatile hydrocarbons such as methane (CH4). Such a mixture of these flammable gases is known as syngas.

Syngas is further treated to remove harmful substances including mercury, hydrochloric acid, sulfur oxides and particulate matter. After this process, the syngas may be used to generate electricity using a gas turbine. The resultant solid and fluid residues can be further processed to produce solid fuels for power plants, a petroleum substitute (bio-oil) and concrete filler. Pyrolysis typically generates 571 kWh per ton of MSW.
 

Gasification Types

Similar to pyrolysis, conventional gasification often begins after removing recyclables and large items such as refrigerators and car bumpers. The remaining MSW is fed into a gasifier. The gasification reactor is heated to temperatures between 1450 to 3000 degrees F. In contrast to pyrolysis and incineration, gasification uses a substoichiometric volume of oxygen, often called a starved-air process. This creates a smoldering reaction which generates syngas mixed with combustion products such as carbon dioxide (CO₂) and water vapor. Often, steam is added into the process in order to enhance production of hydrogen and hydrocarbon gases.

The syngas is cleaned up to remove hazardous components and can then be used to generate electricity. The remaining solids are useful as concrete and asphalt aggregates. The conventional gasification process can generate 685 kilowatt kWh per ton of MSW.

Indiana University Purdue University Indianapolis (IUPUI) is collaborating with industrial partners in development of a gasification process which requires no presorting. Metals and glasses remain intact afterwards for ready recycling. This gasification-plus-oxidation (GPOX) process is a batch operation, processing several tons of MSW in 8 hour cycles. Trash volumes are reduced 90 percent or more, with the bottom ash being suitable as sterile roadway filler.

The plasma arc gasification process may be the most advanced and efficient technology available. Its name derives from the process of generating plasma (the so-called fourth state of matter) by ionizing a gas in the reactor. The plasma flames generated are essentially lightning bolts, created by high-voltage arcs. Temperatures in this process range from 7,200 to 12,600 degrees F, which is hotter than the surface of the sun.

MSW is generally shredded, then fed into the plasma reactor with substoichiometric volumes of oxygen or air. The syngas produced can be cleaned and used to make electricity. The solid residue from plasma arc gasification is unique. A glass-like byproduct, known as vitrified slag, extrudes from the bottom of the reactor. An attractive characteristic of this material is that its components do not leach out.

The vitrified slag consists of metals and silicate glasses which fuse together into an inert solid. Vitrified slag has more uses than ash from conventional gasification, including as insulation material, flooring tiles and garden blocks. Plasma arc technology can generate 816 kWh per ton of MSW.

Pyrolysis, conventional gasification, incineration and plasma arc gasification are existing technologies that can effectively reverse the negative results of two primary human activities: MSW disposal and energy production. On a planet with finite land available and with finite petrocarbon resources, WTE technologies provide a sustainable solution.
 

The Opposite ‘Problem’

Ironically, forward-thinking communities in Norway are currently facing a different “problem.” They are running out of MSW to power their WTE facilities. To solve this problem, the authorities are importing trash from surrounding countries. Sweden also is fighting to get a share of trash imports. These two countries are great examples of what WTE technologies can accomplish.

Some other countries, like Germany, Denmark and Austria, are becoming more interested in WTE. Expectantly, this rise of interest in WTE may create a domino effect in which these technologies become an essential component of the energy generation that powers our global economy.

The Puente Hills Landfill outside Los Angeles just closed forever. The second largest municipality in America just lost its largest trash dump, a man-made mountain 500 feet tall and covering 700 acres. The largest city in the U.S., New York City, ships its sewer sludge to Texas. Until recently, Americans have been able to throw out garbage indiscriminately.

Now, with disposal costs soaring, electricity prices rising and with demand for petroleum ever-increasing, it is time for the U.S. to become a leader in WTE technologies, and make these available to the rest of the world. There is just not enough Earth for all of us to live like Americans.

The article was submitted by the Richard G. Lugar Center for Renewable Energy at Indiana University Purdue University Indianapolis (IUPUI). More information is available at www.lugarenergycenter.org.