Is food waste-to-energy technology sustainable? There was a lot of publicity last year about ethanol requiring more energy to produce than you can get from the fuel. Is biogas from food waste a better deal for the environment? The answer is yes. It turns out that a tonne of food waste produces enough biogas to not only fuel the collection vehicle that picks it up, but also provide enough energy to power the biogas production process (one tonne of food waste = 1.1 tons). In other words, the food waste will collect itself and turn itself into clean energy. All we have to do is supply the right equipment and feedstock. Sounds sustainable to me!
Here are the numbers:
· Your average garbage truck is a real fuel hog. It gets only about 2.8 miles or 4.5 km per gallon of diesel fuel, which is the fuel that most garbage trucks use.
· On an energy content basis, one gallon of diesel fuel is equivalent to about 4 cubic meters of natural gas.
· Some biogas digesters will produce about 65 cubic meters of natural gas equivalent from each tonne of food waste. The actual biogas production is about 100 cubic meters per tonne of waste, but it’s lower in energy content than natural gas so I’ve converted it to natural gas equivalent based on its energy content. This biogas production estimate is based on data collected from European plants that have been using this technology for several years. A similar plant, to be located in Richmond, BC, near Vancouver, will be the first in North America.
· Doing the math, 65 divided by 4 times 4.5 equals 73 km per tonne of food waste, which means we can make the garbage truck go 73 km for each tonne of food waste we put into that same garbage truck.
· Since a typical planned round-trip distance for collecting food waste may be only 30 km, there is energy to spare.
You may now say, ha! You haven’t considered all of the energy needed to process the gas that comes out of that food waste and pressurize it for those high-pressure CNG tanks on that garbage truck. Well, the ever-vigilant California Air Resources Board (CARB) has considered this. In a report released last year, CARB analyzed the total energy requirements for treating and pressurizing the gas to CNG fuel-tank pressures. They found that the total energy required, including consumption of electricity and the biogas itself for gas processing and pressurization was 317,000 BTU per million BTU of CNG produced. In other words, processing and pressurizing consumed about 32% of the gas. Estimates of the energy consumption of the digester plant itself, including all heating, lighting, pumps, fans, and motors would consume another 18% (see note 1), bringing the total consumed to about half the gas produced. This means the typical garbage truck can go about 36 km for each tonne of food waste that is fed into the system. But we only need to drive it 30 km. There is still energy to spare (see note 2).
Of course, biogas also emits much less greenhouse gas than diesel fuel while powering those garbage trucks. The same CARB study cited above found that the processing and compression of biogas reduces emissions of greenhouse gases. According to CARB, each tonne of food waste processed would save 100 kg CO2e (carbon dioxide equivalent). In addition, the diesel emissions saved in that 30 km round trip will save another 67 kg of CO2e.
These calculations illustrate how biogas from food waste is renewable energy. The plant near Vancouver may produce electric power for the grid, rather than CNG. Future projects, perhaps in your community, may produce CNG for vehicle fuel, an exciting prospect for closed loop equations whereby trucks are powered by the food waste within.
NOTE 1: Most of this energy is electrical and would not actually be produced by the biogas if we are making CNG for vehicles, but it is included for comparison to other renewable energy sources. Biogas conversion efficiency assumed is typical for small gas-to-electricity projects.
NOTE 2: In fact, a typical garbage truck can carry more than our hypothetical one tonne of organic waste. Typical capacities are 4 tonnes or even more. But on most trips the typical truck is never filled to capacity. And the average load is always less than half of the full load, because the truck starts the collection trip empty. The average load probably is more than one tonne, but we’re being very careful here not to overstate our case, and using the one tonne simplifies the presentation. But some waste is likely riding for free in our example, meaning we have more energy to spare.
Article by Tom Kraemer, Vice President for Project Delivery at Harvest Power. The company develops, builds, owns and operates next-generation organics recycling facilities that harvest the renewable energy, nutrients, and organic matter from discarded organic materials using best-in-class technologies for composting, anaerobic digestion, and biomass gasification.