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The Engineer: Field of dreams?

Date Published: August 01, 2007

Producing a new generation of biofuels from waste products is an exciting next step for green energy, but considerable technical obstacles remain before it becomes a reality. Stuart Nathan reports

From the earliest days of the coal-powered railways through the rise of the internal combustion engine and into the jet age, mechanised transport has always depended on fossil fuels.

But concerns over climate change, increasing scarcity of fossil fuel resources and worries over energy security are leading to new ideas across industry, and vehicle fuel is no exception.

Attention is now turning to fuels derived not from fossilised vegetable matter, but from freshly harvested plants. Biofuels are big business, and are gathering headlines.

Most biofuels are derived from plant oils — rapeseed, palm oil, soya oil and so on. These are rich in fatty acids, which can be converted by a simple chemical process to produce compounds called fatty acid methyl esters (Fames), which have similar properties to diesel. They can be blended with petrochemical diesel, at concentrations of 5-10 per cent, without compromising the performance of the diesel engine.

And because they are derived from plants, they are effectively carbon-neutral when they burn — the CO2 released from their combustion would have been released anyway when the plants decomposed.

Although biofuels are big business in South America and Malaysia, where sugar-cane derived ethanol has been used to fuel cars for decades, they have yet to make an impact in the UK.

This is set to change, thanks to legislation that will mandate a 2.5 per cent proportion of biodiesel (in diesel) or bioethanol (in petrol) by 2009, going up to five per cent by 2011. The European Union wants this to rise to 20 per cent by 2020.

And this is where some of the problems inherent in Fame production come to the fore. Fame is made from crops that are grown for food, which puts pressure on land use. It is not very efficient — according to Jeremy Tompkinson, the chief executive of the UK’s National Non-Food Crops Council (NNFCC), a hectare of land can produce about 1.2 tonnes of fuel. So as this first-generation biofuel is being developed, engineers are looking to develop a new suite of technologies, known as generation 2, which have a quite different basis.

While generation 1 technology converts the plant oils, which are chemically similar to the target fuel compounds, generation 2 uses the plant purely as a source of carbon. A series of processes ‘gasify’ the complex cellulose and other carbon-containing natural polymers in plant material, breaking them down into simple forms, to form a mixture of carbon monoxide and hydrogen known as synthesis gas or syngas. It then uses the Fischer-Tropsch process, a technology invented to fuel Germany’s World War I effort, to reassemble these simple molecules into long-chain hydrocarbons, suitable for burning in internal combustion engines.

This technology does not require crops to be grown specifically for biofuels. Rather than using the valuable and nutritious oils from crops, the waste material —wheat straw, stems and husks from corn, residues from sugar cane production – can be used instead. Waste wood could become a source of diesel. Even municipal waste and sewage could be pressed into service. Anything that contains carbon could be grist to the mill. At least, that’s the theory.

In practice, things are more complicated. The technologies needed for generation 2 biodiesel are available — gasifiers have been used for decades to convert coal into syngas in oil-poor regions such as South Africa, and the Fischer-Tropsch process is well-understood. But the versions on the market are not suitable for use with biomass. David Bown, technology manager for natural resources at engineering consultancy AMEC, said there is relatively little knowledge of how to adapt gasifiers for biomass-to-liquids (BTL) applications.

Diesel-powered VW Beatles are among the cars demonstrating the use if biofuel blends around Europe

Bown was recently commissioned by the NNFCC to produce a report on the technology and economics of second-generation biodiesel technologies. What he found was not encouraging for the short term: ‘If you look at the final step in the process, making the actual product, there is a lot of technology out there at the moment, in terms of coal-to-liquids and gas-to-liquids. But the gasification and the syngas treatment, the developments have mostly been for coal, and it’s driven by power production, to turn it into energy.’

Gasification is an even more venerable process than Fischer-Tropsch — it was developed in the 19th century to turn coal into gas for lighting and cooking. It involves heating the carbon-containing material to release volatile materials, then reacting the material with oxygen and steam. Modern gasifiers carry out this process at high temperatures and with pure oxygen. ‘But developmental gasifiers for biomass are rather different in terms of design and operating conditions,’ said Bown. ‘They are low pressure and air-blown, and not of high enough capacity to produce large amounts of biodiesel.’

According to Bown, this is not a case of slow development, more that the development of gasification is recent, in industrial terms. ‘In the 1990s, everything went very quiet in this sector, because of the very low oil price. It’s only since oil went above 40-$50 a barrel that it has all woken up. China has huge coal reserves and no oil, so that’s caused a rush in development in gasification, and the US is now starting to get serious. As long as oil is over $40 a barrel, these sorts of processes are economic; when it’s $20 a barrel, it can never compete.’

With biofuels, development is even slower, because biomass is more complicated to convert than coal. Biomass is variable. Wood is very different from straw, and even more different from palm oil waste, for example. The same material can differ radically from season to season and day to day in terms of its moisture content and so on. Gasifiers cannot cope with these variations: they need consistent feedstocks, where the size of the feed particles, their moisture content and composition are more or less the same over long periods.

Biomass also contains a much wider range of chemical compounds than coal, and these can behave in awkward ways during the gasification process, producing sticky tars and methane, which cannot be converted by Fischer-Tropsch processes. ‘Cheaper feedstocks, such as agricultural wastes, tend to produce dirty syngas, and that has to be cleaned before it can go into the final conversion process,’ Bown explained. ‘There’s not a lot of experience out there on the details of that — exactly how variable are these different feedstocks, what actually happens to them in the gasifier, and what cleaning needs to go on.’

The only company anywhere near a commercial generation 2 biofuel process, said Bown, is a German firm called Choren. Based in the former East German province of Saxony, Choren has designed and operated a complete generation 2 process and is now commissioning what it describes as a ‘beta-plant’ — a demonstration-scale unit producing 16.5 million litres a year of a biodiesel, which will be sold under the name ‘SunDiesel’, from 68,000 tpa of biomass feedstock. The company has combined a proprietary gasifier with a Fischer-Tropsch process licensed from Shell, and plans to bring a full-scale ‘sigma plant’, producing 220 million litres of SunFuel, on-stream by 2011.

So why is Choren so far ahead? ‘We started before anyone else,’ said Mattias Rudloff, business development manager.

Choren’s expertise in gasification dates back to just after the partition of Germany. ‘In this part of the country, we had to produce everything from brown coal,’ explained Rudloff. Brown coal, or lignite, is a low-quality fuel containing a large amount of sulphurous impurity. ‘There was a lot of effort put into improving brown coal quality, but after the end of the GDR in the late 1980s, it was obvious that it was the end for brown coal.’ The researchers began adapting the technology so it could gasify wood, which the region also had in abundance.

‘The first ideas on this BTL field were published in 1995, and everybody laughed at them,’ said Rudloff. Choren’s engineers were trying to gasify wood cleanly, without producing tar. This needs a temperature above 1200°C in the gasifier, while most gasifiers then worked at 900°C.

‘The result of their analysis was that you had to use a specific gasifier type, entrained-flow gasifiers,’ said Rudloff. These work at very high temperatures, around 1400°C. ‘Entrained flow gasifiers were developed for specific types of coal in a dust form, so the challenge was, how do you convert a piece of wood into dust?’

Choren’s gasifier works in three stages. First, the biomass, dried to about 15 per cent moisture content using waste heat from the plant and ground to a coarse powder, is fed into a low-temperature chamber, at 400°C, where it breaks down into tarry volatiles and a solid charcoal char. The gas containing the tar then passes into a high-temperature combustion chamber, at 1400°C, where it reacts with oxygen to form a pyrolysis gas. The high temperature not only forces the tar to react with oxygen, it also melts any ash in the feed, which runs down the inside walls of the chamber into a water bath where it solidifies into an easily disposable glassy material.

The charcoal from the first stage is ground to powder and blown into the hot gases below the combustion chamber, which reduces the temperature through a process known as chemical quenching. The pulverised char and the pyrolysis gas react with the oxygen to produce a crude syngas.

‘Then there are some conventional gas cleaning steps to remove sulphur, chlorides, and other impurities, and then what we call syngas conditioning, to give the right hydrogen to carbon monoxide ratio for the Fischer-Tropsch process. This reacts the compounds over a cobalt catalyst, which forces them to combine to make a range of paraffin liquids and waxes, which then undergo further treatment to convert them to biodiesel.’

The environmental profile of SunDiesel is impressive. ‘We did a life-cycle analysis from forest to exhaust gas, and we can show greenhouse gas reduction of 90 per cent compared with the fossil route,’ said Rudloff.

However, for marketing and distribution reasons, SunDiesel will be used as a blend with conventional diesel from Shell. ‘You can go up to 20 or 30 per cent SunDiesel with no changes to engines and without any change of fuel standards.’

Choren is constructing a demonstration-scale biofuel factory in Saxony producing 16.5 million litres of biodiesel a year from woodchip

The process was developed to work on a range of feedstocks, said Rudloff, but for the alpha and beta plant stages, Choren is working with a specific feedstock. ‘They’re only going to process wood, and that is going to be grown locally as an energy crop, as a dedicated supply,’ said Bown. ‘That gives them as much control as possible over what biomass it is, where it comes from, and what they have to do with it to put it into their gasifier.’

If this becomes a trend, it could damage part of the logic for second generation biofuels — their ability to use the waste material from food and other agriculture, once again putting pressure on land use and competing with food crops.

‘I would say there is a lot of waste biomass available, though this is particularly straw,’ said Rudloff. ‘And there is increasing competition for wood, which will lead to increasing the productivity from forests — in Germany, only two-thirds of the wood produced from managed forestry is currently used, and the third that stays in the forest could be drawn out without affecting the system. And those aren’t wood plantations — they’re forests, with growing periods of 100 years.’

However, he agrees with the NNFCC’s conclusion that there is potential for growing energy crops in Europe without pressuring food growing.

‘This is because of the amount of set-aside or fallow land, which will increase in coming years,’ said Rudloff. ‘At the moment, we have 10-12 per cent set-aside, and this will increase to 20 per cent in the next 20 years, because the population is decreasing and farmers’ efficiency is improving. From a soil perspective, the best crops to grow would be short-rotation wood, such as poplar or willow.’

Bown agreed. ‘You can grow these things on land which isn’t suitable for food, so it doesn’t compete directly. But long term, you’d want to look at different sources of biomass, such as algae, which could be grown offshore. People are even thinking of that as a way of capturing the carbon dioxide from flue gases.’

Bown is clear that without climate change as a driver, development of second generation biofuel would wither. ‘It’s a high capital cost,’ he said.

Market research organisation Nexant Chemical Systems, again working for the NNFCC, estimates that a world-scale biomass BTL plant would cost €500m (£336m).

‘It boils down to what incentives government is providing, and how effective they are,’ said Bown.

Source: The Engineer
Date Published: August 01, 2007

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