Professor: Amid rising energy and environmental concern, don’t count out coal
Until recently, millions of Americans probably had not thought much about coal in years. But in January of this year, coal was front page news, first with the tragic death of a dozen West Virginia miners and then as a major component of President Bush’s vow in the State of the Union to end the nation’s “addiction to oil.”
Mechanical engineering Associate Professor Reginald Mitchell, on the other hand, has been thinking about coal every day for more than 20 years. He is an expert on the science of making coal burn more efficiently and cleanly, a task necessary for it to continue powering a world already on a shaky environmental footing. He is one of dozens of Stanford engineering professors who research clean energy ideas – others focus on solar, hydrogen, wind, and energy conservation– to maintain economic prosperity while increasing environmental sustainability.
“For the most part, all of the research in coal combustion for the last 20-25 years has been on how to burn coal cleanly,” Mitchell says. “It’s all been environmentally driven. There can very well be a zero-emissions coal power plant.”
As a non-renewable fossil fuel, coal is controversial. Power plants, after all, have only paid to implement some of what engineers like Mitchell have discovered about how to burn it cleanly. But its significance in the economy cannot be discounted. Coal generates more than 50% of U.S. electricity today and the nation still has more than a 125 year supply at current consumption. Meanwhile, other nations such as China are dramatically increasing their coal production. Cleaner coal combustion could therefore make a major contribution to the environment, especially as some emerging energy technologies make their way from promising to pragmatic.
Clean coal chemistry
Coal, of course, is not inherently clean-burning. Its base is carbon and in addition to hydrogen and oxygen it also contains significant amounts of nitrogen, sulfur and mercury. Sulfur and mercury are especially problematic pollutants, and both carbon and nitrogen can become atmospheric pollutants if they combine with oxygen. Carbon dioxide (CO2) is a major greenhouse gas, and nitrogen oxides (NOx) contribute to acid rain.
But research has shown than none of the polluting gases created in coal combustion have to leave the power plant, Mitchell says. Engineers have found ways–some more expensive to implement than others, but all feasible–to remedy every pollutant that coal burning emits. “We know how pollutants are formed and how to stop some of them from being formed, and how to remove pollutants from the emissions,” Mitchell says.
Take for example, a focus of much of Professor Mitchell’s ongoing research: handling nitrogen during coal combustion. Coal is 1 to 3 percent nitrogen by mass. When coal is heated at the high rates of typical combustion, about half the nitrogen in the coal is released as tars and low molecular weight gases in the forms of amines and cyanides. Some of these tars and gases are trapped in pores in the coal but some are released when solid bonds (the easiest ones to break) are broken as the coal heats up.
The remainder of the nitrogen is in the “char,” the solid residue left after the tars and lighter gases escape the coal particles. The char is composed of a matrix of mostly carbon atoms with nitrogen and other atoms mixed in. The char is what “burns,” (technically speaking, it oxidizes). The remaining nitrogen is released as cyanides as the char burns to CO and then to CO2, which can safely be sequestered from the atmosphere. The amines and cyanides, meanwhile, burn as gases, producing either N2 or NO, depending upon the environment they enter when released from the particle.
They key then, is to provide an environment all along that encourages forming N2 rather than NO. To start with, as combustion begins in so-called “low-NOx burners,” oxygen is purposely made scarce. Because carbon oxidation reactions are faster than nitrogen oxidation reactions, carbon atoms use up the available oxygen before nitrogen-containing molecules can. These nitrogen containing molecules instead collide with other molecules, and with each other. If they collide with each other, they can react to form N2.
Nitrogen oxide formation needs not only available oxygen atoms, but also high temperatures. As combustion goes on and char particles and gases flow away from the low NOx burners, they reach areas where temperatures are too low for significant NOx formation. At this stage additional air can safely be added to covert CO to CO2. The N2 that was formed before is not oxidized to NO in these cooler regions of the combustor.
For the little NOx that does manage to form along the way, engineering again has answers. One example is a process called Selective Catalytic Reduction (SCR), the NOx is combined with ammonia (NH3) at just the right temperature to get the nitrogen atoms to again bond with each other and the hydrogen atoms and oxygen atoms to bond into water.
Generating powerful ideas
Much of Mitchell’s current research is focused on understanding the fundamental chemistry and physics of coal combustion in such a detailed and precise fashion that it can be the basis of a computer model. A model that accurately predicts what will happen as coal is burned at different temperatures, oxygen levels, and pressures could lead to new combustion strategies that improve the efficiency of the coal conversion process with minimized formation of atmospheric pollutants. There is still a lot of room to optimize the process, which has many steps because of the many environmental remediation techniques employed.
While he works to improve the traditional way of using coal, he also has some ideas that are anything but conventional. One is to find ways of taking the hydrogen out of coal for use in hydrogen fuel cells, should those become a major energy source in the future. The hydrogen for fuel cells has to come from somewhere and Mitchell thinks an economical source could be cheap, abundant coal.
But Mitchell, in collaboration with materials science and engineering Consulting Professor Turgut Gur, is also working to run the first ever tests of a novel fuel cell that wouldn't use hydrogen at all. The “direct coal fired fuel cell,” which is licensed from Stanford by Clean Coal Energy, LLC, uses oxygen from the air and carbon from solid coal particles to generate electricity. The cell will primarily emit carbon dioxide, and it will be relatively simple to capture and sequester the CO2 in order to prevent this greenhouse gas from entering the atmosphere. Since the coal fuel cell operates at high temperatures, it has the potential to have a significantly higher voltage and higher conversion efficiency than a fuel cell that employs hydrogen as fuel.
To Mitchell, coal is not the sooty stuff of the bygone days of the Industrial Revolution, but an environmentally sound energy source with a century of potential ahead. “Coal is the energy source for the future,” he says. “Even if we change to hydrogen [energy] we could get the hydrogen from coal.”
