Green comes in many shades. Remember when something is being billed as “green” in terms of the environment, that green is also the color associated with money, and since release of the movie The Green Hornet (based on a Marvel Comic), with superheroes. These days you are likely being forced to navigate a wide range of end use/disposal options for organic residuals. Even though many of these promise that they are beneficial for the environment, it is critical to make sure that the green being offered is the kind of green you are seeking. Otherwise, not only may your decisions be bad for the planet, they may be really bad for your company’s or municipality’s bottom line. When you see red associated with green on a balance sheet, you generally don’t get into the holiday spirit.
My goal when I started to write this column was to take the mystery out of green energy. I wanted to provide a table filled with facts that could be used to quell poorly informed renewable energy decisions that lead to poor and irrational residuals management, and replace them with reasoned thinking. The table was supposed to clearly show net electricity produced per ton of different residuals under different conversion technologies, the cost per unit of electricity produced, and the associated CO2 emissions. The table was to be titled “Shades of Green.” To do this, I started looking around for papers and on the web, figuring that somewhere this information would already exist.
I found a great website and lots of information at www.eia.gov — the website for the U.S. Department of Energy Information Administration. All the information I was looking for was readily available for coal, petroleum and natural gas. When I searched the site for information on renewables, it was a different story. There was information on combustion of energy crops, some data on availability of urban wood waste at different price points, and some information on the few gasification facilities currently operating.
It was at that point I realized that another part of the allure of the green energy term is the high level of uncertainty and variability associated with it. You may really want to believe some of these claims as they would make things a whole lot easier. I have to tell you though, facts in the long run are generally a better option. Same way that boring cardiovascular stuff like running, swimming or walking is generally cheaper and more effective than those miracle machines you see on late night TV.

Distinguishing The Hues

The table I was looking for doesn’t yet exist. Instead, I found a range of articles. I did my best to summarize these in Table 1. The values for coal and natural gas came from the eia.gov website I mentioned earlier. References for the food waste values and the landfill are given below. What should be noticed first off is that natural gas is a much cleaner fuel than coal. You can calculate the “cleanness” in terms of CO2 emissions for each energy source by dividing the kWh by the CO2 emissions. A higher number means a cleaner source of energy. The ratio of energy to CO2 emissions for natural gas is about 8 times greater than for coal.
The next thing to notice is that controlled digestion of food waste packs a lot of energy, about half as much energy as coal. In other words, something that we still pretty much consider garbage has half the energy potential as something that we destroy mountains to get. You should also notice the blank space for the associated CO2 emissions with anaerobic digestion of food waste. While there are likely some, these are miniscule in comparison to coal and natural gas. Next we come to the landfills. Based on the data from the study used for the table, you can see that energy from landfill gas is often much cleaner than coal (except for one case where gas was being collected and the other where it wasn’t). In fact, it ‘s comparable to natural gas. Note though, that while natural gas is recognized as cleaner than coal, it is not typically categorized as “green.” To me this makes “green” energy from landfills a very pale green, verging on grey.
The most important thing to notice about the landfill example is the variability. These results were from direct measures at three landfills in France and were used by the French government as a basis for default values for energy recovery. Each of the landfills measured in the study were young with waste entrained and emitting methane for less than 6 years. There was a greater than 10-fold difference in energy production and close to a 100-fold difference in methane emissions. While the highest emissions were from a cell without gas collection, it did have a clay cover in place.
In fact, if you want unpredictable behavior and volatility, landfills are the place to go. Another paper I read (Faoar et al., 2007) used data from different landfills to determine the value of the variables used to determine annual gas emission rates. The variables are part of an equation:
n
Q= ∑ 2kL0Mie-kti
i=1
This equation says that the total landfill gas emission rate (m3/year) is based on the number of years of waste placement, the gas generation constant (how fast gas is produced), how much pent up methane is in the feedstocks, the quantity of feedstocks and how long they’ve been there. The authors focused on bioreactor landfills where leachate recirculation is supposed to accelerate decomposition and make conditions within the landfill more uniform. They found that the time it takes for methane production to really get going (lag time in years) varied from 1 to 6.6. Once it starts to produce methane, the rate at which the gas is produced or the decay rate constant (k) varied from 0.23 to 2.2. The methane generation potential (L0 measured in m3/Mg) ranges from 9 to 88. Differences close to a factor of 10 are not indicative of reliability.
There are many things that the table does not show, including capital, operational and maintenance costs. These are all critical to consider, however this type of information for alternative energy technologies may not be as robust as for well- established technologies. Before being seduced by the term “green energy” I would encourage you to make your own tables, however tedious an exercise it may be. Green energy is a terrific thing — if you are talking about a vibrant green that is backed by solid data. If you are referring to one of these shaded types of green, I would recommend that you do your research and keep a clear head as you ask questions. That may take the powers of a superhero!
Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at slb@u.washington.edu.

References

Faour, A.A., D.R. Reinhart and H. You. 2007. First-order kinetic gas generation model parameters for wet landfills. Waste Management, 27: 946-953.
Gray, D., P. Suto and C. Peck. 2008 Anaerobic digestion of food waste. US EPA Region 9 Funding opportunity No. EPA-R9-WST-06-004 www.epa.gov/region9/organics/ad/EBMUDFinalReport.pdf.
Spokas, K., J. Bogner, J.P. Chanton, M. Morcet, C. Aran, C. Graff, Y. Moreau-Le Golvan and I. Hebe. 2006. Methane mass balance at three landfill sites: What is the efficiency of capture by gas collection systems. Waste Management, 26: 516-525.