Catalytic steam generation
Exploration of an unconventional fuel mixture has resulted in a novel â€¨catalyst-based technology
Franck Letellier and Dave Wardle
Oxford Catalyst Group
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A spin-off from the Department of Chemistry at the University of Oxford in 2004, Oxford Catalysts works on the development of metal carbide catalysts for the generation of clean fuels from both conventional fossil fuels and sustainable, renewable sources such as biomass waste. These include improved hydrodesulphurisation (HDS), chemisorption, steam methane reforming (SMR) and Fischer-Tropsch (FT) catalysts.
The group, as a whole, focuses on developing new and improved catalysts and catalyst-based technologies to create cleaner fuels for the future. Within the group, Velocys concentrates on the development of microchannel reactor technology, while catalyst development is carried out by Oxford Catalysts.
Microchannel reactors are com-pact reactors that have channels with millimetre-sized diameters. Small-diameter channels dissipate heat more quickly than conventional reactors with larger-diameter channels in the range of 2.5–10cm (1–4 inch), so more active catalysts can be used. Mass and heat transfer limitations reduce the efficiency of large, conventional high-pressure reactors used for hydroprocessing. Microchannel processing enables chemical reactions to occur 10 to 1000 times faster than in conventional systems.
Microchannel FT reactors, developed by Velocys and using a new, highly active metal carbide FT catalyst developed by Oxford Catalysts, will soon be trialled for the small-scale distributed production of biofuels from waste. These reactors exhibit conversion efficiencies in the range of 70% per pass and are designed for economical production on a small scale. A single microchannel reactor block might produce up to 40 barrels (bbls) of liquid fuel per day. In contrast, conventional FT plants are designed to work at minimum capacities of 5000 bbl/day, and function well and economically at capacities of â€¨30 000 bbl/day or higher. They typically exhibit conversion efficiencies of 50% or less per pass.
Oxford Catalysts has also developed and patented a catalyst preparation method, known as organic matrix combustion (OMX), which makes it possible to achieve high metal loadings while reducing the need for precious metal promoters and enabling precise control of crystallite sizes. OMX played a key role in the development of these improved HDS catalysts, which are designed to enable refineries to meet the requirements for cleaner fuels from less than ideal feedstocks. OMX also lies behind the development of the new, highly active metal carbide FT catalyst. This catalyst has been optimised for use in FT microchannel reactors, to enable the small-scale distributed production of next-generation biofuels from a wide range of waste feedstocks. Demonstrations of FT microchannel reactor technology, to produce liquid fuels from biomass, are due to take place in 2010. The technology is also being adapted to create small-scale gas-to-liquid (GTL) facilities for use offshore to capture the energy from flare gas.
Although the group’s main focus is on technology for cleaner and greener liquid fuels, it also has a novel catalyst and fuel combination that produces instant steam. This development is based partly on work originally carried out by Dr Tiancun Xiao, one of the founders and Senior Scientific Advisor at Oxford Catalysts, as part of his search for a catalyst-based reaction to create a portable hydrogen generator for powering fuel cells. Xiao’s starting point was an unconventional fuel mixture of an organic compound (methanol), with hydrogen peroxide passed over a platinum catalyst to produce hydrogen at room temperature via an exothermic reaction:
H2O2 (hydrogen peroxide) + CH3OH (methanol) ==> 2 H2 + CO2 + H2O
The large amounts of heat and steam produced during this reaction also made it possible to reformulate the fuel to produce high-temperature steam at room temperature and pressure via the reaction:
3 H2O2 + CH3OH ==> CO2 + 5 H2O
This work attracted the â€¨seed funding from Oxford University that led to the establishment of Oxford Catalysts. It also formed the basis for the company’s first patents.
Although initially the idea was to develop the reaction for hydrogen production, in the short term the commercial uses for high-temperature steam were more obvious. As a result, the focus soon shifted to developing the reaction as a way of generating portable steam. The fact that the reaction starts at room temperature in just a few seconds sug-â€¨gests a range of possible applications.
The prototype demonstrations of portable, instant steam were impressive. In one demonstration, an ordinary plastic spray bottle was used to hold the fuel, and the nozzle was adapted to hold the catalyst and serve as the reactor. When the trigger was squeezed, high-temperature steam emerged from the nozzle (see Figure 1). Another prototype reactor, the size of a sugar cube, was able to pump steam at a rate of 7 l/min at up to 800°C from room temperature in just 1–2 minutes, while a third prototype, just 2cm high, proved capable of producing 70 l/min of 650°C steam. The technology has been developed so that it can now produce steam at 100–800°C.
The steam team had to develop a new platinum-based catalyst especially for the reaction. Moving towards commercialisation of this new technology presented additional challenges. The overall goals were to reformulate the fuel to enhance safety and to reduce â€¨costs, and to enhance catalyst performance.
Literature searches offered some clues as to the best way to tackle outstanding problems. These ranged from increasing the life and activity of the catalyst while reducing catalyst cost, to formulating safe fuel mixtures to produce steam of different temperatures, as well as identifying a catalyst support material capable of surviving the hot oxidising and water-wet atmosphere produced by the reaction. Since water produced by the reaction also inhibits the reaction, the steam team had to discover the right balance between fuel composition and catalyst activity to enable the reaction to proceed efficiently.
Developing new methods to test variables, including catalyst lifetime, catalyst activity, steam temperatures, flow rates, space velocity through the reactor and the composition of effluents, presented further difficulties. Since steam is generated by passing the fuel mixture over the catalyst, an â€¨additional challenge involved designing a suitable reactor — a capsule to contain the catalyst — to ensure the fuel was presented and distributed over the catalyst in the most effective way. It also required determination of the right particle size and diluent for the catalyst; finding a way to manage the heat to ensure the catalyst bed did not overheat or become too cold; and finding ways for the reactor to tolerate changes in the flow of fuel.
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