Microchannel reactors in fuel production
A demonstration plant aims to confirm the potential for microchannel and other technologies in the distributed production of biofuels
Derek Atkinson Oxford Catalysts
Jeff McDaniel Velocys
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Biofuels produced entirely from waste such as agricultural by-products and municipal solid waste have attracted attention as a substitute for petroleum-based transport fuels. Since they do not contain aromatics or sulphur-containing contaminants, the liquid fuels produced via biomass to liquids (BTL) are typically of a higher quality, and they burn more cleanly than petroleum-based diesel and jet fuels. They could also prove to be valuable in the effort to reduce carbon emissions. A study carried out by the Southern Research Institute Carbon to Liquids Development Center
in the US used the GREET (Greenhouse gases, Regulated Emissions & Energy use in Transportation) model to show that biomass-based FT diesel (biodiesel) production and use results in net greenhouse gas (GHG) emissions savings of 135% compared to petroleum-based diesel, and GHG savings of 129% compared to
petroleum-based gasoline. This is largely because bio-derived synthetic diesel production relies on biomass, rather than fossil fuels, as a feedstock.
Despite their potential advantages, economic, environmental and technical obstacles remain to be overcome before biofuels produced from waste can achieve wider application. A major problem is that it takes roughly one tonne of biomass to produce one barrel of liquid fuel. As a result, to avoid the economic and environmental costs of transporting feedstock to central processing plants, BTL production facilities need to be relatively small and located near the source of the feedstock. Establishing small-scale distributed production of biofuels as a practical and economically feasible option requires, in turn, the development of relatively small facilities that can produce typically 500–2000 bpd of liquid fuels, efficiently and cost-effectively.
The Fischer-Tropsch (FT) process, in which synthesis gas (syngas), a mixture of carbon monoxide (CO) and hydrogen (H2), is converted into various liquid hydrocarbons using a catalyst at elevated temperatures, is a key process in BTL. However, fixed-bed or slurry-bed reactors, the two conventional reactor types currently used for FT processes, are designed to work at minimum capacities of 5000 bpd. They only function well and economically at capacities of 30 000 bpd or higher, and the technology does not scale down efficiently.
However, new reactor designs, such as microchannel reactors, combined with more efficient FT catalysts optimised for use in them, offer a practical way forward. Microchannel reactors are compact reactors that have channels with diameters in the millimetre range. They are well suited to the job because they greatly intensify chemical reactions, enabling them to occur at rates 10 to 1000 times faster than in conventional systems. For example, microchannel FT reactors developed by Velocys and using a new highly active FT catalyst developed by Oxford Catalysts accelerate FT reactions by 10–15-fold compared to conventional reactors, and exhibit conversion efficiencies in the range of 70% per pass. This is a significant improvement over the 50% conversion (or less) per pass achieved in conventional FT plants. Their efficient conversion rates, combined with their modular construction, makes microchannel FT reactors, in theory, an excellent tool for small-scale distributed production of biofuels from a wide variety of sources.
Developing the technology is one thing. Establishing it as a practical and commercially viable solution is another. A demonstration plant now being commissioned in the town of Güssing, Austria, by a coalition that includes project developer and lead engineering integrator SGC Energia (SGCE), the Oxford Catalysts Group, developers of the microchannel FT technology, along with the engineering firm, Repotec, the Technical University of Vienna (TUW), and gasification facility owners Biomass CHP Güssing aims to operate an FT microchannel reactor and effectively integrate it with other key steps in the BTL process, including biomass gasification and syngas cleaning.
In the late 1980s, the town of Güssing, located in southern Austria near the borders of Hungary and Slovenia, was the administrative centre of the poorest region in Austria. Then, in the 1990s, the city developed a model to replace energy dependence on fossil fuels with renewable sources. By 2001, Güssing had achieved energy self-sufficiency through the installation of a biomass plant that takes advantage of steam gasification technology. The developments in Güssing led to the establishment of the Renewable Energy Network Austria (RENET). As a result, Güssing has become a magnet for companies and researchers keen to develop renewable energy technologies.
Other factors determined the choice of Güssing as the site for a demonstration of FT microchannel biofuels production technology. These included the enthusiasm expressed by the local technology community as well as the availability of a new test facility and R&D building with the utilities in place for SGCE to install the FT and gas conditioning skids necessary for its trial. Güssing is also home to a gasification plant that has been operating in a stable manner for seven years (Figure 1). The syngas resulting from this gasification process has the necessary characteristics and high H2/CO ratio required for FT.
Microchannel process technology is a developing field of chemical processing that enables rapid reaction rates by minimising heat and mass transport limitations, particularly in highly exothermic or endothermic reactions. This is achieved by reducing the dimensions of the reactor systems. In microchannel reactors, the key process steps are carried out in parallel arrays of microchannels, each with typical dimensions in the range 0.1–5mm (see Figure 2). This modular structure enables reduction in the size and cost of the chemical processing hardware.
When microchannel technology is employed, plant size is small. Conventional FT reactors are up to 60m tall. In contrast, microchannel reactor assemblies are roughly 1.5m in diameter, have a low profile and sit horizontally. Their modularity and productivity make them convenient for use in small-scale biofuels production plants, and also opens up the possibility for their use on offshore platforms to produce liquid fuel via gas to liquids (GTL) processes.
Microchannel FT reactor design is also flexible. For example, where increasing the size of conventional reactors normally requires plant designers to increase the size of each reactor unit, which alters flow and reaction dynamics in the reactor, the modular structure of microchannel reactors means that increasing plant size to build demonstration or even commercial-sized plants can be done by “numbering up”. This involves simply adding more reactors with the same dimensions.
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