Changing Wood into Coal

Sally Kuzemchak
June 01, 1994

Ancient forests that once stood tall and dark are now buried, making their way back into daylight as coal. Researcher Patrick G. Hatcher wants to know how.

"One problem with studying how wood converts to coal is that the reactions take place over an extremely long period," said Hatcher, who is an associate professor of fuel science and geosciences at Penn State. When trying to duplicate those chemical reactions in the laboratories, he added, "We are faced with comparing something that occurs over a billion years with something modern."

Hatcher asserts that coal does not derive from simple chemistry but is a complex mixture of wood, roots, stems, leaves and other organic material.

cross-section of orange and yellow coal

A cross-section of coal reveals its complex organic makeup (red is woody matter, yellow is leaves and plant spores) and hints at the manner of coal's formation: by a selective, not a random, process.

"If we accepted the traditional view of coal formation, we wouldn't be able to see the tree rings in a lump of coal," he said. Traditionally, coal is thought to be formed by a random polycondensation process, a chemical reaction that would lead to the formation of a homogeneous compound. But, says, Hatcher, "Even in bituminous coal, we can still see the basic heterogeneous structure of plant cells. Random polycondensation does not occur, but rather selective preservation of the original structure."

Hatcher studies this selective preservation by examining the coalified remains of conifers and other gymnosperms, tracking the changes from peat, Australian brown coals, lignite, bituminous coal, and anthracite. While modern wood is made up of cellulose and lignin, the cellulose degrades out rapidly during the peat stage of coalification. The remaining lignin, the basis for most coals, is a large, carbon-based molecule that makes up 30 percent of vascular plants such as trees. Using the molecular structure of lignin as a model, Hatcher can compare subsequent changes in the emerging coal structure.

"By comparing the structure of a specific stage to the original lignin, we can determine the chemical reactions that had to take place to create that stage," Hatcher said.

However, lignin itself is presenting a problem for Hatcher. Recent studies have shown that its generally accepted structure may not be accurate.

For this reason, Hatcher, Jean Loup Faulon, at the fuel science department at Sandia National Laboratories, and Gary G. Carlson, also at Sandia National Laboratories are working on a computer model of lignin.

"Researchers used to believe that the lignin molecule synthesized in the laboratory by a random polymerization was the same as that occurring in nature, but this may not be true," said Hatcher. "If we use a different model for lignin, one that is more ordered in the form of a helical structure, we may have to modify how we think of the chemical composition of coal."

Patrick G. Hatcher, Ph.D., is an associate professor of fuel science and geosciences in the College of Earth and Mineral Sciences, 209 Academic Project Building, University Park, PA 16802: 814-865-7838. Jean Loup Faulon, Ph.D., works at the fuel science department at Sandia National Laboratories. Gary G. Carlson, Ph.D., works in the fuel science department at Sandia National Laboratories. Reported by Andrea Messer.

Last Updated June 01, 1994