Currently unwanted forest residues such as stumps, bark and twigs are proving of great benefit in a new Swedish test facility producing high-quality synthesis gas intended for transportation fuels.
The Swedish Gasification Center (SGC) at Lulea University of Technology (LTU), Lulea, Sweden, was formed in May last year as the first step in a ten-year, SEK 540-million ($78-million) project to investigate new biomass-to-fuel technologies. The ultimate aim is to develop full-scale manufacturing plants capable of extracting environmentally friendly fuels.
The SGC's first move was to bring together gasification technologies developed by three different Swedish organizations: the Energy Technology Center (ETC) in Pitea, an R&D center that focuses on combustion, gasification and biorefinery processes; renewable energy specialist KTH Royal Institute of Technology, Stockholm; and sustainable development specialist Chalmers University of Technology, Gothenburg.
Engineers at Lulea say they have now obtained a valuable synthesis gas from the forest waste materials.
"Primarily, we make use of low-quality forest residues which the wood and paper industry cannot use. People often talk of the need to pre-treat these kinds of raw materials or to use it with charcoal to produce synthesis gas effectively. What we have done is show how to use forest residues directly — and this is an important aspect of our success [at the SGC]," explains ETC CEO Magnus Marklund.
At the heart of the process is an eight-meter-high gasifier manufactured by IVAB, Pitea, a commercial partner in the project.
"The gasifier has succeeded with that which many scientists have tried to achieve for several years: to produce high-quality synthesis gas from forest residues. The gasification project at ETC is based on simplicity, with the direct input of untreated pulverized forest residues, but with intricate technical challenges, which scientists and engineers at ETC, LTU and IVAB had already been working on for three years," says Marklund.
"The actual input of the raw materials in the gasifying apparatus is a challenge. It is a pressurized process and the powder that is fed into the gasifier is composed of fibers and particles, which vary in characteristics depending on the origin of the material, for example whether it comes from birch or pine forests. It places great demands on the design in order to achieve a smooth and stable feed into the gasifier," notes Fredrik Weiland, research engineer at ETC and PhD student in energy engineering at LTU.
The fuel feeding system is based on a combined mechanical and pneumatic concept, but further details haven't been revealed at this stage.
To minimize unwanted nitrogen produced from synthesis gas, pure oxygen and carbon dioxide are used when the raw material is transformed in the gasifier.
"Our synthesis gas has very low levels of hydrocarbons, which is good when you want to produce fuels from gas. A possible final product could be methanol, hydrogen and even synthetic benzene," adds Marklund.
In trials, the gasifier has mainly operated at 2 bar abs. and between 1,200°C and 1,300°C with an oxygen concentration around 80%. The syngas has been cooled, separated and partly cleaned in a direct-water-cooled bubbling quench.
"No further handling of the produced syngas has been done so far. However, the syngas has been characterized by composition and particulate matter. Further processing and synthesis of the syngas is planned … at increased operating pressures," notes Marklund.
Stockholm-based Sveaskog, Sweden's largest forest owner and leading supplier of timber, pulpwood and biofuel, and Dublin, Ireland-based Smurfit Kappa, a global manufacturer of paper-based packaging materials, have provided additional funding for the project.
Meanwhile, other scientists at ETC have found that extremely small holes (half a nanometer in diameter) in its proprietary "zeolitemembranes" can purify synthesis gas from carbon dioxide — giving the potential to develop affordable biofuels such as methanol and dimethyl ether.
"We have come a long way and have discovered that when we have high pressure on synthesis gas, as in an industrial process, our zeolitemembranes function very well, says LTU professor Jonas Hedlund, who heads the project.
Initial tests successfully separated carbon dioxide and synthesis gas. However, mathematical modeling suggests that even better results are possible at much higher pressures. Pilot plant tests should begin later this year.
SEÁN OTTEWELL is Chemical Processing's Editor at Large. You can e-mail him at email@example.com.