Carbo-V is the patented biomass gasification process of CHOREN Industries that is used to produce SunDiesel. Shell Deutschland Oil Gmbh recently purchased an equity interest in CHOREN. The combination of Shell’s SMDS (Shell Middle Distillate Synthesis) Fischer-Tropsch (FT) synthesis process with the Carbo-V gasification process may be a winning combination in BTL processes. CHOREN is planning to build a commercial 15,000 tonnes per year (4,620,000 US gallons/yr) plant using these technologies.
The Carbo-V’s three step biomass gasification process produces syngas without the tars and other impurities usually associated with biomass gasification. The Shell Middle Distillate Synthesis (SMDS) technology is the low-temperature, cobalt catalyst based FT process that they use for production of synthetic oil liquids. FT liquids are very clean, sulfur free and aromatic free; that meet the most stringent environmental standards.
The following are properties of SunDiesel taken from sample batches currently being manufactured by CHOREN:
- cetane number > 75
- aromatic hydrocarbons < 0,3 %
- polycyclical aromatic hydrocarbons < 0,01 %
- sulfur content: not detectable
- very good oxidation stability
- very good compatibility with elastomers
Fuels of this type are becoming increasingly important as environmental standards increase and supplies of conventional oil become deplete and become more expensive.
The CHOREN gasification process consists of the following steps:
- Process heat is first used to dry the biomass to ensure that it only contains 15 – 20 % water.
- In the first stage of gasification, a low temperature gasifier breaks down the biomass into biocoke (a type of charcoal) and a low-temperature carbonization gas containing tar. This involves partial combustion (carbonization) with air or oxygen at temperatures between 400 °C and 500 °C.
- In the second stage a high temperature gasifier takes the carbonization gas from the first stage and feeds it into a combustion chamber where it is partially oxidized with oxygen. The heat, which is released as a result of the oxidation process, warms up the carbonization gas to temperatures that exceed the ash melting point of the fuels that have been used, i.e. 1,300 °C – 1,500 °C. At these temperatures any unwanted longer-chain hydrocarbons, e.g. tar and even methane, are broken down. The gas that is produced primarily consists of carbon monoxide, hydrogen, carbon dioxide and steam.
- In the third stage of gasification, endothermic entrained bed gasification, the biocoke discharged from the first stage is cooled, ground down to pulverized fuel and is then blown into the stream of hot gas exiting the combustion chamber of the second stage. A huge amount of heat is absorbed when gasifying the biocoke and this lowers the temperature of the gas to 800 °C – 900 °C in a matter of seconds. This “chemical quenching” process produces a tar-free gas with a low methane content and with high proportions of combustible carbon monoxide and hydrogen.
- This hot raw gas is then cooled, and dust particles and any unwanted substances (e.g. chlorides, sulfides etc) are removed using multi-stage gas washing process. The solids, e.g. residual coke and fuel ash are fed back into the hot combustion chamber pneumatically. The ash elements melt and flow down the inside wall of the combustion chamber into a water bath at the foot of the reactor. The vitrified, solid ash can then be used as a slag granulate, e.g. for road building purposes.
- The gas is then transmitted to the synthesis unit.
In essence, the Shell MDS technology is normally a three-stage process.
- In the first stage synthesis gas is obtained by partial oxidation of natural gas with pure oxygen in the Shell Gasification Process. Presumably this step is replaced by the CHOREN process.
- In the second stage, Heavy Paraffin Synthesis (HPS) the synthesis gas is converted into liquid hydrocarbons via a Fischer-Tropsch (FT) reaction. The multi-tubular fixed bed reactor is filled with a Shell proprietary Fischer-Tropsch catalyst. The Fischer-Tropsch synthesis reaction can be modelled as a chain growth reaction of CO and hydrogen on the surface of a heterogeneous catalyst.
- In the third and final stage, the waxy syncrude is fractionated into high-quality products, a part of which is converted into middle distillates by means of the Heavy Paraffin Conversion (HPC). The waxy part of the raw synthesis product is selectively hydrocracked to the desired middle distillate products. Simultaneously the product is isomerised to improve the cold flow properties. Crucial for the performance is the use of the proprietary hydrocracking catalyst. The HPC product is subsequently fractionated in a conventional distillation column.
Construction of their commercial plant, to be located at CHOREN's Freiberg facility, would probably begin in 2007-2008 with production to fuel starting in 2010-2012. CHOREN already has a smaller plant which has been used to develop their process and produce fuel samples to car makers such as Volkswagen and Daimler Chrysler.
This investment by Shell in CHOREN complements their $46 million investment in Iogen. maker of a proprietary enzyme for ethanol production, for them to complete a bigger facility that will produce 200,000 tons of ethanol a year—at an estimated cost of $1.30 per gallon—once it goes online in 2008.
Acknowledgement must be given to Green Car Congress for bringing Shell's inventsment in CHOREN to my attention.
"Shell Takes Minority Stake in Biomass-to-Liquids Producer CHOREN", Green Car Congress, 8/17/05
"Shell Buys Stake in CHOREN to Build Biomass-to-Fuel Factory", Bloomberg, 8/17/05
CHOREN Industries, Freiberg, Germany
"BTL Made by CHOREN Process, Environmental Impact and Latest Developments", Dipl.-lng Matthia Rudolf, EAEC Congress, Belgrade, 5/05
SMDS Technology and Process Overview, Shell website
About Shell - Biofuels, Shell website, Article discussing biofuels and Shell's interest in biofuels and their investments in Iogen and CHOREN