BioCycle December 2009, Vol. 50, No. 12, p. 52

Sally Brown

RECENTLY I got an email from Matt in Michigan where a ban on landfilling yard trimmings is close to being overturned. The landfill consultants are arguing that 300 MW of power generation would be available if yard clippings are allowed back into landfills.

Matt, the recycling and composting guy at Michigan Department of Environmental Quality, wanted to know if it was even possible to get that amount of power generation from that amount of yard clippings. There are 1.3 million cubic yards (cy) currently managed at Michigan’s registered composting facilities. What should he say, what should he do? This was happening fast and he would really appreciate some information.

How do you answer that? You could do what I did initially (but I wouldn’t recommend it). I asked myself, “How much electricity would yard trimmings generate?” And I answered, “Now that is a very complicated question.” You see it depends on the type, which varies by season and by region. Yard trimmings will be much wetter in the summer with high potential methane generation. That is unless there is a ban on grass clippings.

In the fall and winter, some regions don’t have pick-ups but if they do, there is a higher potential for woody material. And I’d just read a study out of Australia where they harvested wood waste from a sanitary landfill where it had been pretty much sitting for 20 years or so. In Florida, yard trimmings are likely to be much wetter and less woody than material in Michigan in the winter. In dry areas like the Southwest…

But my laundry list of variables didn’t stop there. Characteristics of the specific landfill also influence the amount of methane generated. If it is a bioreactor landfill, it is likely that significantly more methane will be generated. If there is no immediate gas capture system, it is likely that all of the fresh wet materials will have significantly decomposed before the gas collection system gets turned on. If it is just a sanitary landfill, there is a good chance that the high cellulose and hemicellulose materials will just sit there. Another study out of Germany saw very, very slow decomposition of cellulose in sanitary landfills.

My first reaction for this poor man was to send him a very long report with a lot of literature referenced where he could get familiar with all of these intricacies. Walking the dog the next day, I realized that the very long report was likely to be completely useless for Matt in Michigan. And it was my responsibility to give him an answer that he could use. That meant no qualifications and short and succinct. How to do that?

**BACK OF THE ENVELOPE MATH**

The first step is to figure out how much electricity can potentially be generated from the yard trimmings. This is not a precise value, but a back of the envelope calculation:

1. 1.3 million tons wet is maybe 650,000 tons dry.

2. Assume 50 percent is woody material, leaving 325,000 tons to degrade.

3. A landfill can’t degrade all available carbon due to inappropriate moisture, pH, etc. They typically have a 50 percent efficiency, which brings it down to about 165,000 tons to degrade.

4. Assume the material is 50 percent carbon, leaving a total of 80,000 tons to get gas from.

5. Assume half of that goes to CO2 and the other half to CH4 (methane). (The standard proportion of landfill gas can go up to 60 percent CH4.) That leaves 40,000 tons to generate CH4.

6. To correct for the formula weight of methane (16/12), multiply 40,000 by 16 and divide by 12. That equals 53,333 tons.

7. Using the conversion factor found on-line for methane to electricity (see box), 3 MW is generated from 1.3 million wet tons of yard waste.

This response prompted Matt to do my calculations one better. “This is great,” he emailed back. “By my calculations below (with your help) it looks like the yard clippings could generate 4.5 MW of electricity capacity. By the way, I dropped the woody material down to 25 percent. Composting facilities in Michigan do not take 50 percent woody material.”

Here is Matt’s version:

1. Michigan composts 1.3 million cy of yard clippings/year (not tons).

2. If all of the 1.3 million cubic yards of yard clippings currently composted go to landfills that would equal 0.325 million tons (1 ton of yard clippings equals 4 cy).

3. 325,000 tons of wet yard clippings equal 162,500 tons dry.

4. If 25 percent is woody material, that leaves 121,875 tons to degrade.

5. Using the 50 percent efficiency value for landfills results in about 60,937 tons available to degrade.

6. If the yard clippings are 50 percent carbon, 30,468 tons are available to turn into gas.

7. Because typically half of that goes to CO2 and half to CH4, 15,234 tons of the carbon in the yard clippings will be converted to methane.

8. That means 15,234 tons multiplied by 16/12 (correcting for the formula weight of methane) equals 20,313 tons of methane from the yard clippings disposed in Michigan landfills.

9. There are 18,424,193 kilograms (kg) in 20,313 tons (1 ton = 907 kg)

10. Each cubic meter of methane weighs .714 kg. Therefore 18,424,193 kg of methane equals 25,804,192 cubic meters of methane/year.

11. Converting 25,804,192 cubic meters of methane/year to cubic feet per minute (scfm) equals 1,733 (scfm).

12. 1,733 cubic feet equals 4.5 MW.

This email brought a gigantic smile to my face. A total of 4.5 MW using conservative and very realistic estimates is a far cry from 300 MW. Matt has a very realistic, short and easy to follow argument that refutes the claims of the landfill companies. And by my putting this in the column, you can too.

**METHANE TO ELECTRICITY CONVERSION**

1 ton of methane = 907 kilograms (kg)

1 cubic meter of methane = 9.714 kg

1 cu. meter of methane/year = 0.00006714 cfm*

(www.convert-me.com/en/convert/flow_rate_volume)

1 cfm = 0.0025948 MW

(www.epa.gov/lmop/res/converter.htm)

*cu.ft./min.

*Sally Brown, Research Assoc. Prof. at the Univ. of Washington, authors this monthly column on the connections of composting, organics recycling and renewable energy to climate change. slb@u.washington.edu.
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December 15, 2009 | General