Texas A&M University

Biomass and Clean Air By Gene Charleton

Cattle manure may be the key ingredient in a newly patented process that takes almost all of an important pollutant out of power plant smokestack emissions.

Kalyan Annamalai
Kalyan Annamalai, Paul Pepper Professor in the Department of Mechanical Engineering, says adding manure to coal can eliminate almost all nitric oxide pollution from power plants.

If you’ve ever passed within sniffing distance of Amarillo, Texas, you already know something about the raw material Kalyan Annamalai uses in his air pollution research.

It stinks.

Amarillo may be the feedlot capital of the world. More than 7 million cattle pass through feedlots within a 200-mile radius of the Texas Panhandle city every year. That means lots of manure. Millions of tons of it. And Annamalai, Paul Pepper Professor in the Department of Mechanical Engineering, thinks all that manure is wonderful.

For most people, cattle manure is just something that smells bad on a hot day. For Annamalai, an expert in combustion processes — how fuel burns — the stuff cattle leave behind on their way to becoming brisket and steaks is a key ingredient in a new way to reduce polluting nitric oxide, or NOx, from coal-fired power plants.

“In experiments in our Coal-Fired Boiler Burner Laboratory, we’ve been able to remove as much as 90 percent of the nitric oxide from the stack gases,” Annamalai says.

“We’ve been able to remove as much as 90 percent of the nitric oxide from the stack gases,” Annamalai says.

Similar experiments conducted with bigger pilot-scale coal burners at the U.S. Department of Energy’s National Energy Technology Laboratory in Pittsburgh reached the same conclusion. If experiments later this year at a large utility-type facility in California work out the way the laboratory work predicts they will, this process could be for real. Using this process could allow generating-plant operators to replace expensive natural gas with cheaper coal and still get lower NOx emissions.

DOE and the Texas Commission on Environmental Quality, or TCEQ, think so. They’ve funded the research for a total of more than $2.5 million so far. An advisory committee that includes utilities, feedlot and dairy operators is advising Annamalai and his colleagues on how the research can best address agriculture and power generation.

“Once it works at the plant in California, I will be excited,” Annamalai says.

Just because the process works in the laboratory and in pilot-plant studies doesn’t mean it will be practical for electric utilities to use it at their generating plants.

Coal, nitrogen and heat

pile of manure Researchers at a feedlot owned and operated by the Agricultural Experiment Station and USDA Agricultural Research Service in Amarillo/Bushland, Texas, prepare manure for composting. Courtesy of Kevin Heflin

Power-plant air pollution starts with coal. Or more accurately, the nitrogen that coal contains. (We’ll call it coal nitrogen so you don’t confuse it with nitrogen in the air.) Electric utility companies burn more than a billion tons of coal every year to power steam-generating plants. We use 20 percent more electricity now than we did 10 years ago, and there’s no sign that growth is going to stop. Coal-fired power plants produce more than half of that pollution.

That coal nitrogen is released when the coal burns in utility-plant boiler fireboxes, and the coal nitrogen combines with oxygen to form NOx.

NOx has been on the federal Environmental Protection Agency’s air-quality hit list for decades. It’s the villain behind a lot of pollution that worries air-quality experts. NOx combines with oxygen in the air to become nitric acid and nitrogen dioxide. Nitric acid is an important ingredient in acid rain; nitrogen dioxide attacks the protective high-altitude ozone layer. At lower altitudes, nitrogen dioxide contributes to smog and haze.

Enter manure

You wouldn’t expect manure to have much to do with getting rid of polluting nitric oxide. But it can. It’s chemistry in action. Manure contains a lot of ammonia; it’s one of the substances that make manure smell bad. As manure burns, it releases the ammonia, which latches onto nitric oxide released by the burning coal. The result? Harmless nitrogen and water — no NOx.

In Annamalai’s process, finely pulverized dried manure is injected into the gases produced in combustion chambers and fired with the pulverized coal that’s the power plant’s primary fuel. So far, the most efficient mix of coal and manure seems to be about nine to one, coal to manure. If you inject nine pounds of coal, you’d inject one pound of manure.

It’s more complicated than it sounds, of course. One of the basic truths of engineering is that processes that work well in bench- and pilot-scale experiments don’t always work as well when you scale them up to a full-sized industrial operation. Even success in larger pilot-plant experiments doesn’t guarantee it.

With manure, the biggest potential problem in scaling up the process is ash — what’s left behind after the manure is burned. Burning manure with coal leaves behind more ash than coal by itself. At bench- and pilot-plant scale, the additional ash hasn’t been a problem, but full-sized boilers are more complicated, and the additional ash could clog the tubes that carry heat through the water in the boiler, Annamalai says.

“It gets them to look at manure as a valuable resource instead of something that they need to get rid of,” Sweeten says.

“The source of the high ash turned out to be soil collected with the manure,” he says. “So John Sweeten [an expert in livestock waste management and resident director of the Texas A&M Agricultural Research and Extension Center in Amarillo] and I came up with a scheme to pave the feedlots with ash from the power plants and then collect the manure.

“We cut down the ash by half, a percentage almost the same as what Texas lignite coal produces. The only way we can find out how well this works is to try the low-ash manure in a real boiler.”

In larger boilers, the combustion gases also take longer to get from the combustion chamber to the smokestack. How this additional time will affect the efficiency of the ammonia–nitrogen reaction remains to be tested. Annamalai is confident it will work, but again, there’s no way to find out except to try it.

Manure power

Annamalai has been fascinated with manure and energy production for almost 25 years. It began in 1982, when he got an odd telephone call. Sweeten wanted to know if Annamalai could help him figure out how to use the millions of tons of manure left behind by cattle passing through Amarillo feedlots to produce energy. Annamalai thought the idea was worth looking into.

“We made a bit of a team,” Sweeten says. “I’m the manure guy; Kalyan is the combustion expert.”

The two have worked together on manure-based energy projects for almost 24 years since, ranging from manure-fired fluidized bed combustors and development of manure-based air pollution reduction technology to using manure to supplement coal for power generation or to heat planned Panhandle-region ethanol plants.

TCEQ recently funded Annamalai and Sweeten to investigate whether trace amounts of manure could reduce the amount of mercury produced in coal combustion systems. The idea is that small amounts of chlorine in manure would react with mercury, and the resulting compounds could be washed away with water. Experiments on this use of manure are under way at the mechanical engineering department’s Renewable Energy Laboratory.

Altogether, these research programs involve about eight faculty members and nine graduate students in Texas A&M’s engineering and agriculture programs.

Finding ways to use feedlot manure is getting urgent, Sweeten says. For the last 30 years, fertilizer-needy Panhandle farmers growing corn and other grain crops were a steady market for almost all the 7.2 million tons of manure left each year in Amarillo-area feedlots. Dwindling water supplies and the resulting changes in farmers’ crop planting mean they use much less manure than they used to, and it keeps piling up.

“We should be able to make recommendations to the cattle feedlot managers on how they can improve the fuel characteristics of their manure,” Sweeten says. “It gets them to look at manure as a valuable resource instead of something that they need to get rid of.” end of story