Quality reaction

A small-scale biomass reactor that utilises the Fischer-Tropsch process is said to produce superior-quality diesel and jet fuel. Siobhan Wagner reports

A new biomass reactor with a diameter of only 1.5m has the potential to produce ultra-clean biofuels from waste such as agricultural by-products and construction debris.

The reactor, which uses a catalyst developed by the UK’s Oxford Catalysts, was designed by microchannel reactor specialist Velocys, based in the US.

It will be commercialised by Velocys in 2010 and is now in demonstration form. When in full use, Velocys claims the reactor has the potential to churn out 300-500 barrels of biofuels a day.

The reactor uses a process known as Fischer-Tropsch (FT), which was first developed by Franz Fischer and Hans Tropsch in Germany in the 1920s and 1930s to produce liquid fuel from coal.

The process takes synthesis gas, composed of a mixture of carbon monoxide and hydrogen, and converts it into various forms of liquid hydrocarbons using a catalyst at elevated temperatures and pressures.

The FT reaction process can work with any source of carbon. There are several FT plants worldwide that rely on coal or natural gas as a source for the large-scale production of liquid fuels, lubricant feedstocks and industrial waxes.

Up until now, however, there has been no economically viable way of producing biofuels from non-foodstock biomass through the FT process.

Derek Atkinson, business development director at Oxford Catalysts, said this is because of the many logistical and technical challenges that biomass presents. Biomass is available in much more limited quantities compared to sources such as coal or natural gas.

Atkinson said FT plants that process natural gas can be as tall as 60m and have the ability to produce 140,000 barrels of fuel a day. This would be impossible with biomass, he said, because one ton of biomass only produces a barrel of biofuel.

‘That means you would need to process 140,000 tons a day of biomass,’ he added. ‘You can’t imagine how you would collect that biomass and transport it to the facility 365 days a year.’

Therefore, biomass is only an attractive source for FT reactors if it can be processed on a much smaller scale. The challenge for engineers at Velocys was to develop a smaller-scale reactor that could churn out fuels economically.

The team came up with a design based on arrays of microchannels with diameters in the 0.1-5.0mm range. Thousands of these channels are arranged in 10 individual reactor blocks housed inside a pressure vessel that is approximately 1.5m wide and 8m long. An individual reactor block has the capacity to produce 30-50 barrels of biofuel a day.

Each reactor block has a cross-flow structure of microchannels. The channels that run vertically contain Oxford Catalysts’ specially prepared catalyst. The channels running horizontally are designed to remove heat from the process. Coolant water enters on one side and leaves as steam on the other.

Synthesis gas flows vertically downward through the channels containing the catalyst.

The catalyst, which is based on cobalt metal, was created using Oxford Catalysts’ patented preparation method, known as organic matrix preparation.

The method isolates cobalt in an organic component inside a calcification device. ‘It then catches fire and combusts,’ said Atkinson. ‘The temperature rises from about 100°C to about 1,000°C almost instantaneously.’

With this, small metal crystallites are formed. Atkinson said the crystallites, which are approximately 8-15nm, have a particle-size distribution that is much narrower than what is possible with existing technology.

He added that this is ideal because lots of small crystallites increase the catalyst’s surface area and activity level.

The crystallites also exhibit a terraced surface, which is ideal for promoting reactions. ‘A flat surface of atoms is not a good surface for reactions to occur on,’ said Atkinson. ‘The reacting atoms have to sit on the catalyst somehow. A terraced surface with edges tends to be the catalytic site. The more of those edges you’ve got, the more activity you have got per surface area of catalyst.’

As synthetic gas passes over these catalysts in the reactor, up to 70 per cent is converted into FT products, which include diesel, jet fuel and parrafinic waxes.

‘The diesel produced is far superior to any other diesel,’ he claimed, adding that it has zero sulphur and a high cetane number.

The cetane number measures the combustion quality of diesel fuel during compression ignition. ‘It is more efficient as a fuel, but it also produces less nitrogen oxide and particulates,’ said Atkinson.

The only problem is FT diesel does not meet the government’s density specs for diesel, so it cannot be used alone. ‘These specs were written years ago before FT diesel was invented,’ said Atkinson. ‘There are ongoing discussions to rewrite this spec.’

Apart from better diesel, the process also produces better jet fuel. ‘It has much lower smoke than existing jet fuel,’ he said.

There is, however, one drawback. The seals for jet engines have been designed for existing kerosene products, which have a large amount of aromatics in them. These aromatics cause the seals to swell, which stops them leaking. Therefore, pure FT kerosene would leak out of the engine.

Atkinson said that using a 50-50 mix of FT kerosene and regular kerosene does not cause this problem. He said the US Air Force has a programme that will convert all its jet fuel into such a 50-50 blend by 2016.

‘It may not sound significant that the US Air Force is doing that, but the US Air Force is the world’s largest user of aviation fuel,’ he said. ‘Those jets do not get many miles to the gallon.’

Velocys is beginning a demonstration of its reactor at Wright Patterson Air Force Base in Dayton, Ohio, in the US. The reactor will run for six months before the company commercialises the reactor for non-foodstock biomass to liquid fuel conversion next year.