Researchers looking hard at cleaner technologies for coal

Scan the list of 21st-century energy sources, and you’re in for a jarring surprise. Solar. Wind. Nuclear. Hydrogen. Biomass. Coal.

Coal?The earthbound rock of our ancestors? Maker of mines and museums, steel and smog? Fuel of the Industrial Revolution? How’d that get on the list?

Pennsylvanians have a special feeling for coal. It’s in our bones—and in our lungs. We know the complex, defining role it has played in our state’s, our nation’s, history. But even we tend to think of coal in the past tense.

“ ‘Isn’t coal dead?’ I’ve been hearing that for a long time,” says Harold Schobert, professor of fuel science at Penn State. But in a world whose oil is running out, where renewable energy technologies are full of promise but not yet ripe, coal is back. In fact, it’s the new black: the other hydrocarbon.

In some respects, of course, coal has never been away. Coal-burning power plants produce fully half of American electricity. With the price of oil fluctuating wildly, however, coal has achieved a whole new level of attention, its cheapness and availability making it attractive as the choice for “energy independence.” The U.S. sits on vast reserves of the stuff—200 years’ worth, by most estimates, and coal is already fueling economic booms in China and India, who possess it in similar quantities. Coal, its proponents say, is a fuel for the present, and for at least the near-term future.

This kind of talk makes a lot of people anxious, and with good reason. For although government legislation has largely curbed smokestack pollutants and acid rain in the U.S. since the 1990s, coal-burning power plants remain the leading manmade source of the greenhouse gases implicated in global warming. According to EPA data, annual carbon dioxide emissions from these plants are greater than the emissions from all cars, trucks, planes, trains, and other forms of transportation combined. Per unit of energy, coal emits far more carbon dioxide than its fellow fossil fuels, oil and natural gas.

Yet most experts agree that coal will be used increasingly for at least the first half of the coming century, both in the U.S. and around the world. The question, then, is whether the environmental impacts of that use can be mitigated. Can new conversion and “carbon-capture” technologies and new government regulations render coal’s use “carbon-neutral”? Can coal clean up its act? Penn State scientists and engineers are at the forefront of those who are working to find out.

Models of Molecules

Harold Schobert
Harold Schobert

Coal research has a proud history at Penn State, beginning in the 1890s with establishment of one of the first formal schools of mining engineering in the United States. A lineage of prominent coal scientists has followed over the years, including Walter Maximilian Fuchs, a refugee from Nazi Germany, who developed one of the first molecular models of coal in 1942. The Coal Research Section was established in 1957, and ten years later director William Spackman initiated a Coal Sample Bank and Database that is now among the largest in existence, with some 1,100 samples of coal catalogued and stored for researchers around the world.

Capitalizing on this historical strength, in October of last year Penn State announced the launch of a major research alliance with Chevron, one of the world’s leading integrated energy companies, to investigate coal conversion technologies.

All of which makes it a little disconcerting to hear Schobert say, as he did in a recent lecture, that we really don’t know what coal is. But it’s true. “Unlike other complicated materials, like plastics, polymers, or biological substances,” he explains, “there is no single unique molecular structure for a given coal, let alone all coals.”

“Coals are as different as the peoples of the world,” is the way Jonathan Mathews puts it. “You can take two very similar coals, at least by their bulk elemental composition, and yet they behave quite differently. It’s the structural differences that really impact how a coal behaves. If all you’re doing is burning it, this doesn’t matter—you’re hitting it with a sledge hammer. But if you’re doing more subtle chemistry then it makes a huge difference.”

Structural representation of South African coal
Daniel van Niekirk / Jonathan Mathews

Structural representation of a South African intertinite-rich Highveld coal. Carbon atoms are green, oxygen atoms are red, and sulfur atoms yellow.

Structural representation of a South African intertinite-rich Highveld coal. Carbon atoms are green, oxygen atoms are red, and sulfur atoms yellow.

Like his illustrious predecessor Fuchs, Mathews, an assistant professor of energy & mineral engineering, attacks this complexity with modeling. But while Fuchs’ early hand-drawn construct was limited to 100 carbon atoms, Mathews, with the aid of powerful computers, now builds models that encompass tens of thousands of atoms. When he manipulates these giant clusters in three-dimensional animations on the large desktop display screen in his cluttered office, the effect is something like flying through space.

His models have to be large, Mathews explains, to encompass a realistic diversity of molecular weights: the correct ratio of carbon atoms to hydrogen, nitrogen, oxygen, and sulfur, as well as other, disparate elements. “The problem is that much of our data gives us averages.”

One tool that helps is the transmission electron microscope, which allows perusal of coal samples at near atomic levels. “We look down at the sharp edges of very small particles,” Mathews explains. “Coal particles are glassy, many of them, so they have these nice sharp edges. We look at these fringes and that gives us a distribution of molecular weights.” Recreating this distribution in a model, he says, can yield a better understanding of how a given coal behaves—and how its behavior can be manipulated. “The ideal is that we can be molecular surgeons, and slice up the structure in a manner that we would like.”

Structural representation of South African coal
Daniel van Niekirk / Jonathan Mathews

Structural representation of a South African Waterburg coal loaded with a pyridine solvent. (Pyridine is blue.)

The right scalpel for the job could well be an ultra-sophisticated contraption known as a femtosecond laser, which uses an extremely fast (one million billionth of a second) laser pulse to observe and break apart individual chemical bonds within molecules. (To put it in context, a femtosecond pulse lasts as long as the time it takes light to make it one-thousandth of the way across the period at the end of this sentence.) Schobert is collaborating with Christien Strydom, a physical chemist at North West University in South Africa, who is beginning to apply femtochemistry to the structure of coal, and its industrial processing.

Gasification, for example, is a crucial step in the conversion of coal into other forms of energy. The core process is relatively simple: Raw coal is reacted with steam at very high temperatures until carbon and water molecules break apart, their atoms realigning to form carbon monoxide and hydrogen. “But there’s lots of things on a molecular level that we don’t understand,” Strydom says. “We do not know where the bonds break, how they break, how they react eventually to give you these small molecules. Coal is a large polymeric structure, and in gasification it breaks up.”

With femtosecond lasers, Strydom says, “One can follow the movement of atoms. So you can literally follow a chemical reaction as it occurs. And if we can understand that better, maybe we can control it better.”

Liquid Coal

Instead of burning coal directly, gasification turns its energy potential into synthetic gas, separating out mineral impurities as slag. In a process known as integrated gasification combined cycle, or IGCC, the syngas is then cleaned of other impurities and burned as fuel in a combustion turbine, which produces electricity. In addition to producing more energy per unit of coal than straight burning does, IGCC produces concentrated streams of carbon dioxide, which can be relatively easily captured rather than escaping into the atmosphere. It has been touted as the “core technology” for a next generation of clean coal-fired electric power plants. But IGCC remains expensive to implement, and the approach suffered a setback in January when the federal government canceled FutureGen, its flagship demonstration project for this technology, due to cost overruns.

As oil prices continue to rise, meanwhile, so does interest in converting coal into liquid fuel. Schobert acknowledges that the idea of using coal to make gasoline seems counterproductive in an age of global warming. “The long-term vision of most transportation, I think, has to be electric,” he says. “But there’s a significant period of time yet when we’re going to be dependent on liquid transportation fuels, and we’re in a major mess visa vis oil. And as much as I am sympathetic to bio-based fuels, there’s not going to be enough to pick up the slack, in my view. So I do see a significant role for clean liquid fuels from coal, maybe starting in the 2020 timeframe and going out to around 2060, 2075. And I think we oughta plan for that.”

The technology is not new: the so-called Fischer-Tropsch process, which transforms the product of gasification into liquid hydrocarbons, was developed by German chemists Franz Fischer and Hans Tropsch in the 1920s, and helped fuel the Nazi war machine. (Walter Fuchs worked with Fischer before Fuchs was forced to leave Germany, eventually winding up at Penn State.)

In South Africa, SASOL, the national gas company, has been producing synthetic fuels since the 1960s by a method known as indirect liquefaction, which employs the Fischer-Tropsch process. Today, liquefied coal accounts for 40 percent of that country’s transportation fuels.

Chunshan Song
Chunshan Song

At Penn State, in the late 1980s, Schobert and colleague Chunshan Song, who is now director of Penn State’s Energy Institute, began work on a different approach to converting coal to liquid. Developed by the German Friedrich Bergius long before Fischer-Tropsch, so-called “direct” liquefaction is even simpler in concept. As Schobert explains it, a hydrocarbon in liquid form, as petroleum, contains roughly two hydrogen atoms for every carbon atom, while in the solid form, as coal, the ratio is closer to 1:1. “So the idea is hammer hydrogen into the coal until you convert it into a petroleum-like liquid,” he says. “The chemistry is easy. The engineering has proven to be an absolute bear.”

Shortly after Schobert arrived at Penn State in 1988, the Air Force issued a challenge to researchers to develop a jet fuel from coal. Last year, after almost twenty years of work, the fuel that he, Song, and senior research associate Caroline Clifford came up with in response to that challenge finally passed the pilot-plant demonstration stage. Dubbed JP-900, it meets or exceeds most of the specifications for military or commercial jet fuel, including flashpoint, thermal stability, and energy density. It has also been tested successfully in helicopters and fuel cells. “Most recently,” says Schobert, “we have driven a diesel truck 300 miles on this fuel with no problems.”

Indirect and direct liquefaction each have both virtues and shortcomings, Schobert says. “The indirect process is extremely versatile: It can work for any hydrocarbon source, from coal to methane—even biomass. It produces a clean diesel fuel. But it requires high heat, and produces lots of carbon dioxide.” Direct liquefaction, on the other hand, is less energy intensive and produces far less carbon dioxide, but it only works with coal—“a small range of coals, at that. And the syncrude it produces requires lots of further refining before it’s usable as fuel.”

The biggest shortcoming for both approaches, Schobert says, is the huge capital cost and the sheer length of time required to build a new plant from the ground up. “It would take eight or 10 years to construct a plant comparable to SASOL’s Secunda plant in the U.S.,” he estimates. “To build a plant that produces 50,000 barrels a day—a tea kettle, by modern refinery standards—would cost $5 billion,” he adds.

“Our concern at Penn State has been what are we gonna do for eight or 10 years if there’s a sudden dislocation of global oil supply? And where are we gonna get the $5 billion? Our thinking has been there’s got to be a third way.”

Caroline Clifford
Caroline Clifford

That way, he suggests, involves adapting existing oil refineries to allow for coal conversion. “It’s basically direct liquefaction, except done at lower temperature, and lower pressure,” explains Caroline Clifford. “You grind up the coal, mix it together with light-cycle oil, a distillate of petroleum that’s already being produced at the refinery.” The oil acts as a solvent, drawing the energy-rich compounds out of the coal, and the liquid is then distilled through standard refinery operations.

Another idea she and Schobert are exploring is to modify the standard refinery process known as coking, whereby petroleum molecules are “cracked” or broken down, leaving behind the solid carbon byproduct known as coke. This process could be adapted for coal, Schobert says, and “if you do it right the solid coke that results could be a very valuable material,” a high-quality carbon suitable for graphite and other specialty applications.

That’s music to the ears of Chunshan Song. “Harold and I have been promoting the idea that we should make more comprehensive use of coal, based on its unique structural advantages,” Song says. “The conventional thought has been to try to do that by gasification. Here at Penn State we’ve taken a different approach.” Direct liquefaction, he notes, unlike the indirect process, separates out the aromatic compounds present in coal, which may be useful for making specialty polymers that are “lighter than aluminum and stronger than steel.” Song has pioneered a technique known as shape-selective catalysis, which he says can overcome the traditional problem of purifying these compounds.

Capturing Carbon

SASOL’s Secunda plant is the largest synthetic fuels facility in the world. It processes 120,000 tons of coal a day. Its importance to the South African economy is reflected in the fact that a drawing of the plant has its place on the reverse of the country’s fifty-rand note. “It’s an engineering triumph,” Schobert says. “It is also the largest single source of carbon dioxide emissions on the planet.” According to the International Energy Agency, Secunda releases more carbon dioxide per year than Norway.

Finding environmentally safe, cost-efficient methods of capturing and storing carbon dioxide is the key to coal’s acceptance as a 21st-century fuel. “We don’t have to continue to build plants to 1960s technology,” Schobert insists.

Existing carbon capture and storage technology can reduce carbon dioxide emissions by up to 90 percent, according to a 2005 report by the Intergovernmental Panel on Climate Change. But the costs associated with this capability would push energy prices up anywhere from 30 to 60 percent in order to compete with non-capturing plants. In the absence of government-mandated limits on carbon emissions, the utility industry has not been quick to make that kind of investment.

In addition to gasification, current approaches to carbon capture include post-combustion flue-scrubbing of the sort used for sulfur and nitrogen oxides (chemical components of acid rain), and improved combustion technologies. For the former, Chunshan Song explains, the current process uses liquid amine solvents, which are both energy-intensive to produce and costly to use. At Penn State, he has led an effort to develop instead a solid adsorbent that consists of a nanoporous material impregnated with special polymers, chosen for their affinity for carbon dioxide. By loading these polymers into the tiny pores, the technique creates many infinitessimal containers, each with hundreds of sites for adsorbing carbon dioxide. “Basically, we put the carbon dioxide molecule into a basket,” he says.

Song’s “molecular-basket” approach has performed well in preliminary testing. But as he points out, capturing carbon is only half of the probem; the other half is safely storing it. The myriad approaches being investigated for carbon “sequestration” range from pumping it deep underground to burial at sea.

Jonathan Mathews
Jonathan Mathews

Schobert and others have investigated injecting the stuff into oilfield brine reservoirs. “Many oilfields have a certain amount of water in them that has accumulated over geologic timescales,” he explains. These reservoirs contains salts which, when mixed with carbon dioxide will form insoluble carbonates, effectively tying up carbon in a relatively stable form.

“We’re also taking a very hard look at mineral carbonation,” he says. In this process, carbon dioxide is reacted with naturally occurring metal oxides like magnesium and calcium to create limestone—basically accelerating the process of natural rock weathering. (“I think it’s workable,” Schobert says, “and it certainly has the potential for producing some saleable byproducts. But I’m having a hard time getting my mind around the practicality of it.”)

Another proposed solution would be to store carbon dioxide far underground, in abandoned oil or gas wells. Or, says Jonathan Mathews, in unusable coal seams. “On the atomic scale,” he explains, “you can consider coal to be a sponge—there’s quite a bit of open porosity within that space. It typically contains some methane, or water, or CO2.

“Depends on the coal, but CO2 is sort of an earthworm for coal,” he continues. “It can wiggle its way into places perhaps it shouldn’t be able to get, and the coal will relax and allow it to enter the system a little bit more than it will some of the other molecules.”

Mathews and colleagues are using x-ray tomography to get a better idea of how coal behaves when you load carbon dioxide into it. “We can take coal cores and put them under geological conditions of stress, and we can see what’s happening inside the coal,” he says. “We can follow where the CO2 goes, how the coal swells. It allows us to quantify how much CO2 a coal can hold under realistic conditions. A lot of stress on the coal actually reduces that capacity.”

Over the long term, Schobert says, the best storage solution may be some type of photocatalytic conversion. “The one energy source we’re never going to run out of is solar,” he reasons. “How about using the energy in sunlight to convert CO2? In a rough sense, this would approximate the process of photosynthesis in green plants.”

“We’re looking at potential catalysts for this,” he continues. “The idea would be to have a photovoltaic cell whose surface is coated with this catalyst, which will react with CO2 in the presence of sunlight to form some kind of carbonate-like species. We could put arrays of these things around powerplants or along highways, and the darn things ought to run every time the Sun is up.”

A Market Approach?

No sequestration model will be widely implemented until the cost of carbon emissions demands it. But what form should regulation take? Some economists advocate a straight tax. Many others favor a cap-and-trade system, in which the government would set a downward-sliding cap on allowable carbon dioxide levels and issue tradeable permits to individual power plants, in effect granting them a limited and dwindling permission to pollute while they clean up their acts. Once a plant’s permits are used up, further emissions would incur a stiff penalty. Under such a scheme, plants that are more efficient at cutting emissions can sell their extra permits to those that are less efficient.

Some environmentalists and others have criticized cap-and-trade. But Fan Zhang thinks this market-driven approach would work better than a tax, and at less cost to world economies. Zhang, an assistant professor of energy policy and economics at Penn State, has spent the last several years analyzing the American electricity market, specifically the effects of price deregulation on the environmental performance of coal-fired power plants.

artist's conception for FutureGen
U.S. Department of Energy

Artist's conception for FutureGen, a prototype zero-emissions coal-powered plant, to be built by a private consortium in conjunction in partnership with the U.S. Department of Energy. Despite cancellation of DOE funding in early 2008, a FutureGen Alliance made up of private partners continues to move forward with the project.

Zhang thinks the same principle could give electric companies incentive to build power plants that are carbon-capture-ready. “A lot of new coal-fired power plants are being built around the world,” she explains. “What I’m proposing is to estimate the option value of making these plants capture-ready, so that once the technology is commercially viable they can be easily retrofitted. Then, when the price of carbon dioxide goes high enough, they can exercise the option to retrofit, and sell the CO2 credits they no longer need.”

If such a market could be developed, Zhang believes, the lure of profit would entice the financial industry into investing in new power plants where the risk-averse utility industry might not. And “if there are a lot of capture-ready plants, the industry will have an incentive to promote a regulatory framework so that their options become more valuable.”

Best of all, she suggests, “It would also allow this capture-ready technology route to be possible, rather than locking us into a route where a lot of new plants are built without this capability, then forced into early retirement when CO2 regulation becomes reality.”

That’s music to the ears of Chunshan Song. “Harold and I have been promoting the idea that we should make more comprehensive use of coal, based on its unique structural advantages,” Song says. “The conventional thought has been to try to do that by gasification. Here at Penn State we’ve taken a different approach.” Direct liquefaction, he notes, unlike the indirect process, separates out the aromatic compounds present in coal, which may be useful for making specialty polymers that are “lighter than aluminum and stronger than steel.” Song has pioneered a technique known as shape-selective catalysis, which he says can overcome the traditional problem of purifying these compounds.

In 1990, she notes, as part of the Clean Air Act, the EPA established an emission-permits market as a means of lowering atmospheric levels of sulfur dioxide. In the years since, those levels have indeed been dramatically cut. “The sulfur dioxide market is the largest and most successful application of a cap-and-trade system in the world,” Zhang says, and she traces some of that success to deregulation. “Since 1995, when deregulation occurred, electricity has been sold in the auction market, so the price has become very volatile,” she explains. “It changes from season to season, day to day, hour to hour.” Soon after, Zhang noticed, individual power plants began overcomplying with the sliding sulfur dioxide cap, by as much as 30 percent. Many plants decided to hold on to their emissions permits instead of selling them.

The permit is like an option, Zhang explains. “An option is the right but not the obligation to use the permit. So if in the future the demand for electricity is very high, a lot of plants will increase generation. The demand for emission permits will also increase, and so will the value of those permits.” If demand drops off, on the other hand, the permit’s value does too. Just like in the financial market, the idea is to buy low and sell high, and an option allows flexibility to do this. “It’s a way to hedge against uncertainty.”

Zhang thinks the same principle could give electric companies incentive to build power plants that are carbon-capture-ready. “A lot of new coal-fired power plants are being built around the world,” she explains. “What I’m proposing is to estimate the option value of making these plants capture-ready, so that once the technology is commercially viable they can be easily retrofitted. Then, when the price of carbon dioxide goes high enough, they can exercise the option to retrofit, and sell the CO2 credits they no longer need.”

If such a market could be developed, Zhang believes, the lure of profit would entice the financial industry into investing in new power plants where the risk-averse utility industry might not. And “if there are a lot of capture-ready plants, the industry will have an incentive to promote a regulatory framework so that their options become more valuable.”

Best of all, she suggests, “It would also allow this capture-ready technology route to be possible, rather than locking us into a route where a lot of new plants are built without this capability, then forced into early retirement when CO2 regulation becomes reality.”

The Bigger Picture

In one sense, the “technology route” the U.S. takes with regard to coal seems hardly to matter. Regardless of what we choose to do, emerging giants China and India will remain heavily reliant on cheap, available coal for the foreseeable future, in amounts that will dwarf U.S. usage. “And they are not building the cleanest coal power plants in the world,” Jonathan Mathews notes. “Adding pollution control adds a lot of cost, and their economies are growing at a rate where other things are considered more important. These nations are following the same pathway we did.”

On the other hand, however, energy experts hope that U.S. policy and technology could have a ripple effect. If the U.S. can show that “clean coal” is a possibility, can demonstrate that zero-emissions plants can be made economically feasible—even advantageous—our best practices may be adopted around the world.

“Coal is the most abundant energy resource that we have,” Song says simply. “And for us, in the U.S., this is an indigenous resource. We have the potential to develop the technology that will make coal’s use more efficient, more environmentally friendly, and we really need to seriously pay attention to that.”

“We have to learn to look at the overall picture,” Schobert adds. “Unquestionably, coal, used as a transportation fuel, is going to emit carbon dioxide into the atmosphere. But can we offset these emissions in other places? Can we build up high-speed rail, say, so that we could eliminate in-state airplane flights? These are the kinds of questions we need to be asking.

“Long term,” he says, “we will have to rely on solar. But what are we going to do in the meantime—for the next decades, possibility a century? Coal is one of the answers to that.

“We have to get away from away from the idea of ‘either/or.’ Developing countries are aspiring to move up. As they do they will consume more energy. We’re gonna need everything we have. Solar, wind, nuclear, biomass…. And we’re gonna need coal.”

Harold H. Schobert, Ph.D., is professor of fuel science in the College of Earth Sciences, hxs3@psu.edu. Jonathan P. Mathews, Ph.D., is assistant professor of Energy and Mineral Engineering in the College of Earth and Mineral Sciences, jpm10@psu.edu. Chunshan Song, Ph.D., is professor of fuel science and director of the EMS Energy Institute, csong@psu.edu. Caroline Burgess Clifford, Ph.D., is senior research associate at the EMS Energy Institute, ceb7@psu.edu. Fan Zhang, Ph.D., is assistant professor of Energy and Mineral Engineering in the College of Earth and Mineral Sciences, fxz10@psu.edu. Christien Strydom, Ph.D., is director of the School of Physical and Chemical Sciences at North-West University in Potchefstroom, South Africa.

Last Updated October 13, 2008