SunDiesel: From Trash to Treasure
Woodchips, once slated for the trash heap, may be a future source of biodiesel.
When many people think of alternative diesel fuels, the most common example is soy-based biodiesel, thanks to a powerful agricultural lobby, and a rapidly developing, though still small, production and distribution infrastructure for the vegetable-based fuel product. But, traditional biodiesel is not the only source for plant-based diesel fuel. In a joint venture between DaimlerChrysler and Volkswagen with Choren Industries, a German energy company, a fuel production and distillation process has been developed, known as BTL (biomass-to-liquid) synthesis, to produce diesel fuel. This process allows fuel to be distilled from forest waste, using wood that would otherwise be too defective or diseased to be used for lumber or other forest products, and would otherwise be unusable waste. Though this may seem cutting-edge, the basic chemical process, known as the Fischer-Tropsch process, has been around for nearly 80 years.
A BTL diesel reactor
As the saying goes, necessity is the mother of invention. During World War II, the Germans had the foresight to realize that continued dependence on oil-based energy would be a precarious situation as the war continued. Germany had abundant coal but little petroleum. Research into the Fischer-Tropsch process was given top billing. The process allowed for the production of fuel from any carbon-based feedstock, including plant or coal sources. Though more expensive to process than traditional petroleum-based fuels with technology at the time, it gave them some measure of reassurance with their energy strategy. The Allies ultimately prevailed, but interest in synthetic fuels by the Allies continued, although at a small-scale level. Little-known legislation introduced by Wyoming Senator Joseph O'Mahoney in 1944 called for the construction of plants for research and development of synthetic fuels. As early as 1948, concerns arose that the U.S. was becoming increasingly dependent on imported oil.
Loren Beard, Senior Manager of Environmental and Energy Planning at DaimlerChrysler, gives a glimpse into the environmental potential the Fischer-Tropsch method holds. "If you use forest waste or some sort of crop, the process becomes carbon-dioxide neutral. All the carbon dioxide that you produce when you burn the fuel goes back into growing the plants that you got it from, that you would otherwise have to take to a landfill or dispose of elsewhere," he explained. And, unlike traditional biodiesel, which requires harvesting of a virgin crop for processing, Fischer-Tropsch doesn't distinguish between virgin or waste sources, opening up a variety of environmentally beneficial opportunities. "There's a disease that's killing a lot of the trees in the Pacific Northwest. They are a huge fire hazard and are really a nuisance right now. The government is looking at billions of dollars to dispose of them. A concept we've been looking at would be to build plants close to where the trees are, allowing the trees to be cut down and processed into diesel fuel right there," Beard said.
Although much attention has been paid to biodiesel lately, there are some issues in integrating it into the existing fuel infrastructure. According to Beard, BTL diesel is much more adaptable into the conventional diesel infrastructure than high-concentration biodiesel. "Biodiesel takes vegetable oil and processes it to a monoester of triglyceride oil, which doesn't look like conventional diesel, chemically, whereas BTL diesel is very similar to petrodiesel chemically but embodies the very best properties of it," he said. "It's sulfur-free and very low in aromatics, which is a precursor to soot or particulates."There are some minor issues with BTL diesel in terms of flow and lubricity, but Beard said these are easily overcome with additional processing. An ongoing issue with biodiesel has been its susceptibility to algae growth, which doesn't affect BTL diesel, but like biodiesel, the cold-flow properties of pure BTL diesel aren't ideal. "BTL, as it comes out of the plant, doesn't have any problem with algae growth, but it does have pretty poor cold-flow characteristics. But, this can be fixed with additives or a refining process called isomerization."
The convergence of BTL and biodiesel into a blended fuel is also a likely possibility, according to Beard. "If you add two percent biodiesel to BTL, it brings up the lubricity to a point comparable to conventional diesel. We're starting to see 2 to 5 percent biodiesel blends right now, and I imagine this is going to be the trend moving forward," he said.