What Is It, and How Does It Produce Electricity?
We are a throwaway society. The United States currently produces over 200 million tons of garbage each year. We have always viewed our mountains of garbage and sewer sludge as a problem, seeking hiding places to bury them and secret them away. New York City ships 10,500 tons of waste each day to landfills as far away as South Carolina and Ohio!
Oh, how wrong we have been! The truckloads of trash, mountains of refuse, and tons of sludge from sewer treatment plants that we call municipal solid waste (MSW) turn out to be a valuable energy resource. Most of these previously unwanted mountains of trash can either be directly burned in power plants or be processed to form syngas (synthetic gas—a methane-rich gas mixture much like natural gas). Because of its high water content, sewer sludge (along with kelp and animal waste) is typically processed into syngas that can directly replace natural gas as fuel for a power plant.
Solid wastes off-loaded from garbage trucks can be sorted, shredded, and directly burned in power plant boilers. Either way, garbage becomes a free fuel source to drive a power plant.
A 400 Mw power plant converted to run off municipal waste can handle the waste from over 500 garbage truck loads a day (over 450,000 cubic feet of garbage). This demand for fuel can be met either by daily pickups or by slowly emptying existing (and often stuffed-full) landfills. On the downside, the trash has to be sorted, dried, and shredded before it can feed into power plant burners. On the upside, burning garbage in a power plant avoids the cost of dumping it in increasingly scarce landfill sites within driving distance of large cities.
Who converts the most municipal solid waste into electricity? World leaders are Switzerland (80 percent), Denmark (60 percent), and France (40 percent). Denmark, with a population of only 5.5 million, is expanding to 40 MSW power plants. That’s one MSW plant per 140,000 people. By contrast, the United States has only one MSW plant per 3.5 million people. In Denmark, MSW power is considered a clean energy technology, and these plants are thought of as an asset—even in wealthy suburban communities!
The United States ranks way down on the list of countries using MSW as an energy resource, converting only 16 percent of our MSW into electricity. California, Florida, New York, and Pennsylvania (in that order) are the leading states for electrical production from MSW. The United States started using MSW later than other countries (we still had no MSW power plants in 1980), but our use of MSW is now steadily rising. By 2000 U. S. MSW capacity was up to over 5,000 Mw.
What’s Happening Now?
The biggest impediment to expanded use of MSW is the fear of not knowing exactly what arrives in the stream of garbage that we burn. Someone might have thrown out toxic wastes that the utility doesn’t catch before feeding that waste into the boilers. When those toxic pollutants belch out of the plant’s smoke stack, the utility could be held responsible for the resulting dangerous pollution. Since refuse companies can’t control what people throw into their garbage cans, the utility company can never be sure if harmful trace chemicals will be released during burning.
New techniques for sorting and analyzing the incoming flow of garbage, and new techniques for scrubbing (filtering and cleaning) the smoke that billows up the smokestack from the boiler fires, are slowly reducing that fear. As more plants are able to successfully use MSW in more states, it encourages other utilities to do the same—now that they’re reasonably sure that they won’t be fined for polluting the air.
Some 300 million rubber tires are tossed away in the United States each year. Engineers began using old tires as raw material for construction in 1992. Thirty percent of discarded tires are now recycled—mostly ground up and used in new roadbeds.
However, research has shown that the ultimate best use of old rubber tires is as fuel for power plant boilers. EPA figures show that a rubber tire delivers 25 percent more energy than the same weight of coal. A new burning technique now allows power companies to burn old tires in their boilers without creating the thick black smoke and stink that were so much a part of older burning processes. Currently, most of the tires that are burned are burned in cementmaking kilns. But utilities are beginning to eagerly eye the acres of old tires as a possible fuel for electric generating plants.
How Does It Measure Up?
(The Good, the Bad, and the Ugly)
On the plus side for MSW:
• MSW reduces the volume of garbage pouring into landfills by as much as 90 percent.
• MSW is a proven, reliable, existing technology.
• If this waste is not used as a fuel stock, someone must pay to dump it, cover it, and secure it, as well as for the land used to dump it.
• MSW is predictable and always available, with no disruptions.
On the negative side for MSW:
• MSW is typically high in CO2 and other global-warming gas emissions.
• MSW emissions can contain many toxins: cadmium, mercury, dioxins, etc.
• Competition exists with recycling programs, conservation efforts, and conversion into other fuels (ammonia, methanol, charcoal, methane, diesel fuel, and gasoline).
What’s the Bottom Line? (How Much Can It Help?)
• Potential: The potential is huge. MSW could provide one-third of all U. S. electrical demand if—//—all MSW were used to produce electricity.
• Key Factors: Competition will limit the dedication of MSW to electrical production. Uncertainly about toxic pollutants in the waste stream will curtail utilities’ willingness to invest in MSW power plants.
• Timeline: Many MSW plants already exist. Look for total MSW capacity to steadily and slowly grow, but never to become a major contributor to the national electrical grid.
Classroom Activities
Municipal solid waste is an engineering term for garbage. It’s the stuff you throw away in your garbage can and that the smelly garbage truck picks up during its weekly rounds.
Here are three questions to research and answer:
1. How much do you throw away?
This is a fun and revealing research activity for the whole class. Each participating student needs to have a weight scale (a bathroom weight scale is ideal). For one month, each student will volunteer to carry out the trash—all of it. To begin the process, weigh each empty trash basket or trash can in the house. Create a chart with one column for each trash basket. List the room it is in and its empty weight at the top of each column.
If your community has you separate lawn clippings and recyclables from garbage, measure only the garbage (whatever will be thrown into the outside garbage container). That is the only waste stream that would be fed into an MSW power plant.
Before the students carry any of the baskets out to dump into the outside garbage can, they should weigh each filled basket and write that weight in the appropriate column of their charts (using a different line for each day that they actually carry out any trash).
You can find the weight of the trash itself by subtracting the weight of the empty basket (listed at the top of the column) from the measured weight of the basket when full.
At the end of the month students not only can add up the total amount of stuff their family tossed into the garbage can, they also know how much of that total came from each room in the house.
Compare results for different students. Discuss what differences and similarities mean.
2. Collectively, how much MSW does your community (city or county) toss out each week?
You know how much each individual family in the class tossed out. But what about the whole community—including stores, restaurants, schools, and factories? You’ll have to contact your local waste disposal company to gather these total figures. Use the Internet to compare your community to those in other areas and to the nation as a whole.
3. What is done with it?
If you were thinking of building an MSW power plant, this would be the key question. Find out from your local trash company what it does with the stuff collected by the garbage trucks. Is it dumped in a landfill and buried? Does the company separate the recyclables and then bury the rest? Is it burned in an incinerator? Does the company separate out any part of that waste stream to be reused? How many tons of garbage per month could the trash company feed into an MSW power plant?
Use the library and the Internet to research existing MSW power plants around the country. How many tons of garbage do they get each month? What is the capacity of each MSW plant? Can you create a graph or a chart that shows how much garbage flows into each plant and how much electricity flows out? From this graph, what size MSW power plant could your community support?
For Further Reading
de la Garza, Amanda. Biomass: Energy from Plants and Animals. Chicago: Cengage Gale, 2006.
Fix, Alexandra. Energy. Portsmouth, NH: Heinemann Library, 2007.
Ollhoff, Jim. Geothermal, Biomass, and Hydrogen. Edina, MN: ABDO Publishing, 2010.
Povey, Karen. Biofuels. Chicago: Cengage Gale, 2006.
Thomas, Isabel. The Pros and Cons of Biomass Power. New York: Rosen Publishing Group, 2007.
Walker, Niki. Biomass: Fueling Change. New York: Crabtree Publishing, 2006.
Web Sites
www. adb. org/Clean…/INO-PFS-Municipal-Waste. pdf
A description of the technology.
www. energy. ca. gov/biomass/msw. html
A state government site describing California’s MSW programs.
www. epa. gov/cleanenergy/energy-and-you/…/municipal-sw. html
EPA’s site on MSW power plants.
www. eia. doe. gov Renewables and Alternate Fuels
The Energy Information Administration site on MSW power conversion.
ieeexplore. ieee. org/iel5/4272346/4272347/04272398.pdf
A good technical paper on the engineering aspects of MSW power plants.
