How does a tree trunk become a coal lump? A chemist’s answer suggests a new view of how a tree is built in the first place.
You probably think you know how coal formed: from trees and ferns and other plants that died and were buried in the muck of primeval swamps-- buried deeper and deeper, until the heat and pressure reduced them to a black, energy-rich essence. This picture is true enough as far as it goes. But it doesn’t go very far for Patrick Hatcher, a fuel scientist at Penn State. Fuel scientists want the details--what chemical reactions happened when during the hundreds of millions of years it took for living plants to become black rock. It’s almost an intractable problem, says Hatcher, unless you can simplify it by narrowing in on a specific part of a specific type of plant, and then follow the evolution of that part.
For the past decade, Hatcher has focused on lignin, the molecule that strengthens the soft, sugary cellulose walls of wood cells the way reinforcing rods strengthen concrete. Lignin keeps trees from falling over while they’re alive, and when they die it becomes the primary component of coal. In a series of laboratory experiments, Hatcher has analyzed samples of conifer lignin and coal--and of all the stages in between--to see how that transformation takes place. In the process he has turned up some controversial evidence that lignin, and thus trees, may be put together differently from the way researchers had imagined.
The first thing that happens when a tree trunk crashes into a swamp is that microbes eat away the cellulose. Within a thousand years or so, only the lignin skeleton of the wood cells is left. (The log remains virtually the same size, however, because lignin is so rigid.) The lignin itself begins to be transformed once the log has been buried under further sediments.
Lignin is a polymer molecule--a long chain of identical units. Each unit consists of ten carbon and four oxygen atoms: a six-carbon ring with a three-carbon chain attached to one side and a single carbon attached to another, and two oxygens each on the ring and side chain. The oxygens play a key role in the coalification of lignin.
Hatcher analyzed samples of successive stages of coalification: peat, brown coal, lignite, subbituminous coal, and bituminous coal. When peat becomes brown coal, he found, the lignin skeleton is simply rearranged into a more compact, stable configuration. As the heat and pressure increase, though, they begin plucking oxygen atoms from the lignin units. In lignite, one oxygen has been pulled from the side chain; only three oxygens remain. Subbituminous coal has lost the second side-chain oxygen and one from the ring, leaving only one oxygen per ring. Finally, bituminous coal loses the last oxygen from every other ring on the lignin strand. While the coal is losing oxygen it is also having water squeezed out of it, which makes it more compact. But it is the progressive loss of oxygen that makes the coal more energy-rich: the less oxygen the coal contains, the more it can be oxidized and the more energy it releases when burned.
This process could never unfold in such an organized way, Hatcher thinks, with specific oxygen atoms being lost at predictable times, if strands of lignin in wood cells were arranged in the way fuel scientists have always thought--each long strand randomly clumped and knotted like yarn. That’s what synthetic lignin, made in a test tube, looks like; but real lignin, according to Hatcher, must be more orderly. The problem is that it’s hard to look at real lignin while it is embedded in cellulose.
Instead, Hatcher and his colleague Jean-Loup Faulon used a computer to calculate the forces among the atoms of a lignin strand, and thus how the strand is most likely to arrange itself in three-dimensional space. The picture they got was orderly indeed: it was a helix, without clumps or knots. Not only did the computer-generated lignin helix nestle neatly into the spaces between computer-generated cellulose fibers, but also it required less energy to form than a knotted strand of lignin. Both observations support the view, says Hatcher, that real plants probably make lignin helices.
But so far he has no direct evidence of that, and his computer model has been greeted with skepticism. We’re running up against a lot of fixed minds on this concept, he complains. It’s almost dogma. Hatcher is still looking for a way to prove the dogma wrong.