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
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
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.