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Mobile plants drive biofuel production

A mobile biomass refinery is set to take biofuels further than ever before.

Mobile biofuel plants, the size of large lorries, will soon be roaming the US ready to convert any kind of biomass into liquid fuel, claim researchers from Purdue University, Indiana. Professor Agrawal and colleagues have already developed a laboratory-scale prototype and hope to have a full-scale version up and running within the next five years.

'The plant will process any biomass, wood chips, switch grass, rice husks, wheat straw and so on,' explains Agrawal. 'The actual reaction is quick, taking only minutes, and once the reactor is running you can feed the biomass rapidly through the system.'

As Agrawal also points out, his team's approach avoids the huge cost of transporting large quantities of heavy, bulky plant matter to central refineries. 'Liquid fuel from biomass is far more economical to transport,' he says. 'For low energy-density material like wood chip, it makes more sense to process this biomass into liquid fuel with a mobile platform and then move the fuel to a central refinery for final processing, as takes place [with crude oil] at a petroleum refinery.'

How H2Bioil works

Agrawal's team has pioneered a biomass-to-liquid fuel conversion technique called fast-hydropyrolysis-hydrodeoxygeneation, or 'H2Bioil' for short.

Here, solid biomass is fed into a high-pressure, hydro-pyrolysis reactor and rapidly heated to around 500C in the presence of hydrogen and a catalyst, breaking down the complex biomass molecules into smaller molecules. The oxygen in these smaller molecules is removed via a further reaction yielding high energy-density oil molecules. These are condensed into liquid oil for eventual use as fuel.

In order to make the process feasible in the short term the team has integrated a natural gas reformer to the plant. In doing this, the uncondensed gases that remain after the biomass to liquid fuel conversion can be fed into this steam methane reformer, providing a steady supply of hot hydrogen to the hydro-pyrolysis reactor.

'My goal with this process has been to keep it simple and do it on a small scale,' explains Agrawal. 'We can directly feed the biomass while heat is supplied by the hot gases... so [compared to a conventional fast-pyrolysis process] we have removed a lot of the equipment, which should enable the construction of small-scale mobile plants.'

Agrawal also asserts H2Bioil yields more liquid fuel per gram of biomass compared to conventional methods such as the Fischer-Tropsch process. Recent calculations indicate his process produces about twice as much fuel as current technologies, assuming its hydrogen is derived from methane.

Future work

After extensive modelling Agrawal and fellow researchers are busy perfecting their laboratory prototypes. Having built two reactors, calibrations are underway.

'For example, we wanted a biomass feeder that was very small in size. We couldn't buy one anywhere and everybody told us it couldn't be done,' explains Agrawal. 'It's taken us a couple of years and we had to develop three prototypes but now we can feed the biomass very slowly, of the order of one gramme a minute, at high pressure.'

In doing this, Agrawal and colleagues will be able to assimilate as much data as possible to help them build the full-scale version. 'If you and I were to have this conversation in six months time we would have quite a collection of data,' he says. 'We've got the equipment and we're adding a few fixtures so that we can induce hydrogen under high pressure.'

Reactor work aside, the team is also spending a lot of time assessing different catalysts that could be used in the process. According to Agrawal, biomass typically contains around 35 per cent oxygen whereas gasoline or diesel hardly has any, which is one of the reasons biomass has a relatively low energy density.

'The trick is to remove all that oxygen without 'hurting' the molecules and that's why the catalyst is so important,' he says. 'We really don't have a particular catalyst yet, although my colleagues are currently going through a list of different catalysts [with different properties].'

In the long term Agrawal would like to use solar power to produce the hydrogen, by, for example, splitting water. In such a set-up the roaming biofuel plant could make use of a solar-powered electrolyser instead of the methane steam reformer, making the process entirely renewable.

'My research is spent closing my eyes and imagining what the world will look like when we don't want to use fossil fuels,' he laughs. 'For using solar power, the pieces will come together much sooner than you think - I would say even in the next few years as the costs have come down so much. This is happening.'

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