There is a movement in the United States called the ‘25×25’ whose goal is for our country to get 25% of its energy from renewable resources such as wind, solar, and biofuel by the year 2025. That sounds great, but how are we going to do that? In 2010, thirty-four million head of cattle went to market; and what do cows produce (besides milk)? Poo! That’s right, maybe we can use their waste to fuel our economy. Manure contains undigested carbohydrates and proteins that benefit plant growth when used as fertilizer, but when heated can produce 8,500 btu/lb. To generate energy from manure, methane is extracted from the waste in a process called anaerobic digestion. However, due to the variability of animal manure and thus its energy producing capability, it would be best to mix it with less variable fuels in a process called co-firing, which is the mixing of bio- and fossil fuels.
Studies at Texas A&M University have shown that co-firing manure and coal could reduce emissions of nitrogen oxide (NOx) into the environment. Fossil-fuels create large amounts of NOx – especially compared to manure –when they are combusted; and are responsible for over half of the global emissions of these gases. Nitrogen oxide emissions are of concern as they are associated with increased rain acidity (i.e. acid rain) which damages aquatic and terrestrial ecosystems as well as human health.
For the reduction of NOx to take place when coal is burned, manure must be ingested into the system at the correct point, typically into the secondary combustion chamber. This allows for the manure to serve as a supplemental fuel and an organic source of ammonia. Researchers envision a manure-to-energy conversion system that could be housed right at the site of production on the farm. This would take the form of a modular anaerobic digestion system that directs the manure to a covered lagoon. The lagoon acts as a methane recovery system which would be extracted and used to power an onsite generator. The sludge from the digestion process could be used to run a gasification system. Gasification is the process by which any carbonaceous fuel (fossil or bio derived) is converted to a useable gaseous product, thus further eliminating waste.
At the 2011 Manure-to-Energy Summit for the Chesapeake Bay region, officials outlined the “triple-benefits” of using manure as fuel: it produces useable energy; it creates sustainable and profitable farms; and it improves water quality. The Summit came together to address issues the region has had with algae die-off in its waterways. Aquatic systems need 1,000 times less phosphorous than do terrestrial systems, and when they receive these input levels above this threshold, it disrupts aquatic life by robbing the water of oxygen. State and local governments in this region are currently working to address a federal mandate that calls for significant reductions in nutrient loads to the Bay. The requirements call for a cumulative 60-million pound reduction in nitrogen and a 4-million pound reduction in phosphorus for the watershed states of Maryland, Virginia, Pennsylvania, Delaware, West Virginia and New York by 2025.
Manure from livestock is the largest source of excess phosphorus and one of the top contributors of excess nitrogen to the Chesapeake Bay. It is estimated that 36% of the phosphorus and 15% of the nitrogen comes from livestock. While wind, water and solar are great options for the future, their inconsistency energy flow and inability to scale during period of high demands make them energy options that still need to be perfected. Further, these forms of energy production only operate at their designed capacity 17 to 30% of the time. Co-firing, on the other hand, is estimated to be behind only nuclear energy in terms of its efficiency – at 85.5%.
While energy production from manure might not solve all of our future power generation needs, as an abundant resource with high efficiency ratings, it does offer an opportunity to decrease our dependence on relatively-inefficient and foreign fuel sources. So the next time you drive by a farm, and you get that familiar smell, think about the possibilities manure can provide for a cleaner, more sustainable future.