A 15,000-metric-ton/yr demonstration plant now being built by Choren Industries, Freiberg, Germany, will be the first plant to convert biomass into syngas that then will be turned into high-quality diesel fuel. Once started up early next year, the unit will feed in 75,000 mt/yr of wood chips and straw to make so-called SunDiesel.
The fuel can be used without modification in any diesel engine without compromising performance, claims the company. Moreover, says Choren, it is virtually free of sulfur and aromatics, so it burns cleaner without particulate emissions. It can be used on its own or blended with conventional diesel fuel.
Choren has operated a 2,000-mt/yr pilot plant for SunDiesel since October 2003; Shell now holds a greater-than-25% stake in the firm; automakers Volkswagen and DaimlerChrysler also are cooperating in the development.
Choren is teaming its Carbo-V biomass gasification process to convert biomass into synthesis gas with Shell’s latest-generation Fisher-Tropsch gas-to-liquids (GTL) technology, called the Shell Middle Distillate Synthesis (SMDS) process, to turn the syngas into biodiesel. Shell now is operating the first commercial plant using the SMDS technology in Bintulu, Malaysia; output is used in the production of V-Power Diesel, which is already available in Europe and is proving itself in auto racing (see p. 66).
Unlike conventional processes for biofuels, which use grain and other food crops as feedstocks, the Carbo-V process starts with agricultural wastes, such as wood chips, rice husks and straw, or trees specifically grown for energy production.
Traditional gasification systems aren’t suitable for making syngas from biomass, says Choren, because their physical design limits operation to around atmospheric pressure and 1,000°C. This leads to significant tar and methane in raw gas, which can pose difficulties in subsequent Fisher-Tropsch processing. Also, the systems cannot cope with varying feedstock sizes and quality, the company contends, and so only can handle restricted ranges of biomass sources.
The Carbo-V process gets around these drawbacks by processing the biomass in three stages: pre-conditioning, partial oxidation and chemical quenching.
First, the biomass feeds are blended and then dried to a water content of 15% by weight. This feedstock goes to a low-temperature gasifier, in which it smolders in a mix of O2, CO2 and steam at 400°C to 500°C and 5 bar. The biomass is broken down into volatile gases and char. The gases pass to a combustion chamber in which tars and long-chain hydrocarbon molecules are decomposed at temperatures above 1,400°C. Meanwhile, the char is ground and milled into a powder that is blown into the hot gases exiting the combustion chamber. This chemical quenching reduces gas temperature to about 900°C and contributes to the process’s high cold gas efficiency of over 80%, says Choren.
The Carbo-V gasifier yields syngas as well as significant amounts of steam and heat, some of which is used to dry the incoming feed. Choren says that overall thermal efficiency exceeds 90%.
Because the syngas may contain traces of sulfur and other contaminants that could poison Fisher-Tropsch catalysts, it is sent through scrubbers before going to the GTL unit.
If all goes well with the demonstration plant, Choren plans to build a full-scale (200,00 mt of SunDiesel per year) plant at Lubmin, Germany, for 2009 start up, followed by similar-sized plants elsewhere in Germany.
The company also is interested in installing plants in China and the U.S.