There’s been an awful lot of activity these days about a proposed trash-fueled electricity generating plant out at the Ada County landfill. While it would be inappropriate for me to comment directly on the particulars of the situation, since it’s being considered by the Idaho Department of Environmental Quality, I would like address two consistent misconceptions that tend to arise around these types of facilities. Namely, that they convert trash to energy and that the energy they produce is renewable.
The phrase “trash to energy,” which is often used to describe these facilities, is quite misleading. While it’s true that energy is extracted from the stuff we throw away, the total amount of matter (measured by its mass) remains the same. For example, if a given plant takes in 400 tons of trash each day (as the proposed facility does), there will still be 400 tons of matter after the energy is extracted. The process of extracting the energy separates the solid wastes (consisting of everything from old tires to half-eaten hamburgers) into a mix of gases and a residual slag. While the actual ratio of those two outputs may vary, the leftover slag is usually less than 10 percent of the original waste. The gaseous mix (often called syngas) is a combustible fuel which is then used in a conventional engine or turbine to run a generator. Therefore, of the original 400 tons, some 40 tons remain as slag and 360 tons will go up the stack from the engine and into the atmosphere.
On one hand, it’s not a bad deal. The volume of trash disposed in the landfill is greatly reduced (by a factor of 10 or more) and we get electricity in the process. On the other hand, we add a lot of combustion products into an already fragile airshed and unlike similar plants that use conventional fuels (like natural gas), this fuel can have a very different makeup from one load of trash to the next, making it difficult to control the toxic combustion products that result from the engine or turbine.
Also, physics aside, it is interesting to consider whether the energy produced by such a facility could be truly called renewable. At first glance, we don’t seem to be running out of garbage anytime soon, so it would be easy to dismiss this point, but I would argue that it’s an important distinction. For example, can our garbage be considered truly renewable if it’s the byproduct of consuming nonrenewable resources? In 2007, the Energy Information Administration carried out a study of the energy content of MSW (municipal solid waste). This study, which looked at trends in MSW from 1989 through 2005, attempted to answer this question by looking at how much of our trash came from materials that most would consider renewable (biologically based materials like wood and paper) and how much came from other sources, a category which is dominated by plastics (which are petroleum-based and clearly non-renewable).
The results were astonishing and worth considering. In 1989, about two-thirds of the energy in our trash could be attributable to materials considered renewable, one-third non-renewable. But those percentages steadily changed through the decades to 2005 when the split was nearly even (56 percent renewable, 44 percent non-renewable) and the non-renewable energy was almost exclusively from plastics. To my knowledge, that study hasn’t been repeated recently, but is there any reason to assume that this trend hasn’t continued in the past seven years? On those trend lines, the mix would now be over half non-renewable sources. In addition, those materials that provide that energy are the target of recycling programs, since they are valuable for other reasons than just their energy content.
Like so much of the energy picture, the answers are not easy, but framing the problem correctly is key to finding the best path forward. Given that we throw away stuff that has valuable energy content, I’m all in favor of aggressively harvesting that energy, as long as it is done in an environmentally safe manner. For that, we rely on our elected officials and governmental agencies to protect the public interest.
But the most important question remains not just unanswered, but unasked. Why is there so much energy in our trash in the first place? We would be so much better off as a community, and as a society, if we were much more mindful of what we throw away. In other words, if valuable fuel is finding its way into the garbage trucks, we’re making a mistake.
John Gardner is director of the CAES Energy Efficiency Research Institute and professor of mechanical and biomedical engineering at Boise State University.