We live in a throwaway society that accumulates vast quantities of waste every day. While this comes with pressing challenges, there are also opportunities for professionals including electrical engineers to process at least some of the waste to produce much-needed renewable energy.
According to the U.S. Energy Information Administration (EIA), in 2018 a total of 68 U.S. power plants generated around 14 billion kilowatt-hours of electricity from 29.5 million tons of combustible municipal solid waste (MSW). Biomass, which comes from plants and animals and is a source of renewable energy, was responsible for more than half (about 51%) of the electricity generated from waste. It also accounted for about 64% of the weight of the MSW used. The rest of the waste used was from other combustible materials including synthetic materials made from petroleum and plastics. Glass and metal are generally not noncombustible.
Municipal Solid Waste in the U.S.
Burning MSW is not only a sustainable way to produce electricity, it also reduces the volume of waste that would inevitably end up in landfills. Instead, the EIA estimates that burning MSW effectively reduces waste volumes by about 87%.
But, while more than 268 million tons of MSW are generated in the United States every year, in 2017, only 12.7% of it was burned to recover energy. More than half (52.1%) went to landfill, about a quarter (25.1%) was recycled, and the rest (10.1%) was used to generate compost.
According to a U.S. Environmental Protection Agency (EPA) fact sheet on sustainable materials management published in November 2019, the total MSW generated in 2017 by material, comprised:
- Paper and paperboard, primarily containers and packaging 25%
- Food 15.2% (see below)
- Plastics 13.2% (19.2% of the total materials that ended up in landfill were plastics)
- Yard trimmings 13.1% (most of this type of waste is composted)
- Rubber, leather and textiles 9.7%
- Metals 9.4%
- Wood 6.7%
- Glass 4.2%
- Other 3.5%
Indicating tremendous human waste in its worst form, 22% of the material that ended up in landfill was classified as food. Trashed food was also the product category with the highest landfill rate, at an alarming 75.3%. Nearly a quarter (22%) of materials that were combusted with energy recovery were food, and overall, food was also the highest product category to recover energy, with a rate of 18.4%.
The total MSW combusted to generate energy was made up of the following materials:
- Food 22%
- Plastics 16.4%
- Rubber, leather, and textiles 16.1%
- Paper and paperboard 13.2%
- Wood 8.4%
- Metals 8.6%
- Yard trimmings 6.2%
- Glass 4.3%
- Other 4.3%
Generating Electricity from MSW
There are a variety of MSW-to-energy technologies, but in the U.S. the most common system involves mass burning of unprocessed MSW in a large incinerator that has a boiler that produces steam, and a generator that produces electricity. Another entails processing MSW into fuel pellets for use in smaller power plants.
Generating electricity in mass-burn WTE plants is remarkably straightforward and follows seven basic steps:
- The MSW is dumped out of garbage trucks into a large pit.
- A crane with a giant claw attachment is used to grab the waste and dump it into a combustion chamber.
- The waste, which now becomes the fuel, starts to burn, releasing heat.
- The heat that is released turns water in the boiler into high-pressure steam.
- The steam turns the turbine generator’s blades and produces electricity.
- The mass-burn plant incorporates an control system to prevent air pollution by removing pollutants from the combustion gas before it is released through a smoke-stack.
- Ash is inevitably produced in the boiler and the air pollution control system, and this has to be removed before another load of waste can be burned.
While the volumes burned as fuel in different plants vary, for every 100 pounds of MSW produced in the U.S., potentially, more than 85 pounds could be burned to generate electricity.
Of course, the U.S. isn’t the only country that uses waste-to-energy plants to generate electricity from MSW. And in fact, when compared to a lot of other countries, the percentage of MSW burned with energy recovery in the U.S. is minimal. At least nine countries are named by the EIA as bigger producers of electricity from municipal waste. In Japan and some European countries, for instance, there are fewer energy resources and not much open space available for landfills. So generating electricity from MSW is an obvious opportunity.
The four leading nations identified by the EIA as burning the most MSW with energy recovery are:
- Japan 68%
- Norway 54%
- Switzerland 48%
- France 35%
- The United Kingdom 34%
One thing’s for certain, the percentages are all set to continue increases globally as the move towards sustainability gains momentum. And U.S. percentages are going to increase too.