Engineers to burn manure as fuel to power an ethanol plant

To calculate the greenhouse gas balance of a biofuel, all the energy inputs of its entire production chain must be taken into account. If an ethanol plant uses coal or natural gas to power its operations (as is the case in the US and the EU), then the fuel will not be that clean. If however it uses renewable and climate friendly biomass (as is the case in Brazil where bagasse powers the refineries), then a much greener fuel can be obtained.

Engineers from the Texas Cooperative Extension have understood this and are now working with feedyard owners to help them look at the manure produced by their animals as a valuable biofuel that will be used to power an ethanol plant. Besides ethanol, the refinery will produce a byproduct known as distillers' dried grain, which is a prime feed for the cattle that produce the manure. If collected and treated well, the manure has a heating value almost similar to that of Texas lignite coal. The main difference is that the first fuel source has a low carbon dioxide footprint, whereas coal is extremely climate destructive.

Dr. Brent Auvermann, the expert developing the manure combustion process, recently hosted a seminar titled "Producing High-Value Manure for BioFuels and Fertilizer", in Hereford, where Panda Energy International will use the biomass in its ethanol facility. The meeting outlined work by Texas Agricultural Experiment Station researchers to determine best management practices for scraping manure from the feed pens.

"We're doing something that has never been done before," says Arles Graham, Panda Energy International's general manager for the Hereford plant, who spoke at the event. "We're using your manure as an energy source," he told feedyard owners. "It's a very complex process."

After starting up the plant with natural gas as the boiler fuel, Panda Energy will eventually use manure as a fuel source when producing ethanol for an E10 fuel blend, Graham said. The plant will initially process corn for ethanol, although the company is looking at alternative sources of starch to make the ethanol, and it will produce distiller's grains as a by-product. "But manure is our future," Graham said, estimating each plant will use 1,500 tons a day. Jim Adams, Panda Energy vice president-fuels, said the plant will begin asking yards in June to sign up for a percentage of their manure.

The past winter was a wake-up call, Adams said. Sometimes when the weather is too wet, manure can't be harvested from the pens. Manure will be used by this fall, so they have to start stockpiling now to ensure a steady supply. Adams said the plant will use manure on a six-day basis, requiring 70 to 80 truckloads per day. Panda's contractor will collect from the pens when they are dry enough, but will need to pull from stockpiles when pen surfaces are too wet.

Manure quality key
Quality is the biggest issue, Auvermann said. The manure needs to burn at a minimum rate of 2,758 British thermal units per pound of manure. That number changes according to the amount of pollutants – moisture and dirt – included when the pen is scraped:
bioenergy :: biofuels :: energy :: sustainability :: climate change :: fossil fuels :: biomass :: ethanol :: manure :: combustion ::

If all the water and contaminants were removed from the manure, the highest quality would be 8,500 Btu, "but we can't do that, because we can't take the ash out completely," he said.

Manure from soil-surfaced pens may not always meet the minimum heating value on an as-received basis, Auvermann said. Feedyard operators will have to take some steps to improve it. The timeliness of collection and depth of scraping will be key to keeping dirt content below 60 percent and moisture content below 20 percent, he said. "Paving the pens with a crushed ash or a fly-ash material (from coal-fired power plants) will end up returning to you in the form of heating value – big time," Auvermann said.

Partially composted manure from paved pens can have a heating value almost equivalent to that generated by burning Texas lignite coal, he said.

Feedyard owners should consider the process as "harvesting manure" rather than cleaning pens, Auvermann said. The ultimate goal is to have a hard, smooth, well-drained corral surface. Implementing good practices will pay at the bottom line, he said. Conscientious manure harvesting can result in higher fuel and fertilizer values, reduced feed requirements for cattle, improved pen drainage, and reduced odor, dust and flies.

Image 1: Dr. Brent Auvermann, Texas Cooperative Extension engineering specialist, advises feedyard operators to pay close attention to blade depth when harvesting manure from corral surfaces as a boiler fuel source. (Texas Cooperative Extension photo by Sharon Preece).

Image 2: Cleaning manure from feed pens is a common practice, but one that will have to be done more carefully in the future if the harvested product is to be used as a fuel source, according to Dr. Brent Auvermann, Texas Cooperative Extension engineering specialist. (Texas Cooperative Extension photo by Sharon Preece).

More information:
AgNews, Texas A&M University: Cleaner Manure Burns Hotter in Ethanol Processing - May 23, 2007.