|Fig. 1: Picture of garbage. (Source: Wikimedia Commons)|
The United States is one of the countries that generate the most amount of municipal solid waste. Where waste eventually ends up at is quite interesting to know as well: 7 percent is combusted through waste-to-energy systems, 24 percent is recycled and composted, and 69 percent is landfilled.  It is quite unfortunate to know that only 7% of waste is converted to energy. Waste-to-energy technology is so important not only because we want to get rid of all this waste but also because we need more energy. The worlds demand for energy will increase by 56% between 2010 and 2040.  (See Fig. 1.)
Currently, there are many different types of conversion technologies, including gasification, plasma gasification, and pyrolysis. Gasification, specifically, is a process that changes a carbon-based material such as biomass or MSW into other forms of energy without actually burning it. It converts waste materials into a gas via a chemical reaction. The reaction combines carbon-based materials with little amounts of oxygen and break them down into simple molecules. The end result is that we will have a mixture of carbon monoxide and hydrogen.  Gasification converts about 80% of the chemical energy in the waste fuel into chemical energy in the gas phase, so it is a pretty effective method. Air gasification is more commonly seen than oxygen gasification. Air gasification has a heating value of 4-6MJ/m3 while oxygen gasification has heating value of 10-18 MJ/m3. However, oxygen gasification requires oxygen supply which can be expensive. 
|Fig. 2: Picture of pyrolysis machine (Source: Wikimedia Commons)|
It perhaps is helpful to also briefly learn about what other conversion technologies are out there. There are two that are also commonly used: plasma gasification and pyrolysis. First, plasma gasification uses very high temperature in an environment that barely has any oxygen. This environment is required in order to decompose organic waste materials into basic molecules. On the other hand, pyrolysis is a thermal conversion process where waste is heated in the absence of oxygen. Pyrolysis happens in a low temperature, air-free environment.  Fig. 2 shows a pyrolysis machine.
Waste to Energy market has increased significantly in the past few years and is expected to grow. In 2012, waste-to-energy technologies market was valued at $24 billion, which is an annual increase of 5% from 2008.  As such, we are expecting a lot for this technology and hopefully it can continue developing so that we can convert more waste into energy. This market could change our entire future.
© Diana Kim. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 Gasification: the Waste-to-Energy Solution," Gasification Technologies Council, 2011.
 K. Whiting, M. Fanning, and S. Wood, Waste Technologies: Waste to Energy Facilities," WSP Environmental Limited, May 2013,
 "Waste to Energy," World Energy Council, 2013.