Fresh biomass is difficult to transport and store. It is often not immediately suitable as fuel for power stations. It is too wet, too inconsistent in shape, difficult to grind and varies too significantly in heating value. “Purchasers from power stations are willing to use biofuel, providing it does not cause problems,” states Jaap Kiel from the Biomass, Coal and Environmental Research unit. “Our response to that is torrefaction. Together with Vattenfall we will now start the basic engineering of a demonstration plant. This company sees countless possibilities for guaranteeing the international supply of biomass and facilitating the co-firing of biomass in coal-fired power stations.”

Biomass turned into biofuel with ECN technology

With torrefaction, ECN researchers wanted to create a technology that meets two important requirements: convert biomass into a standard biofuel of a constant, high quality, and minimal energy loss during the production process. Quality refers to transport, storage and utilisation as a fuel for the production of electricity and heat or liquid transport fuels. Kiel: “Managers of large coal-fired power stations desire clarity; every shipment of biofuel must be of a high quality that is known beforehand. The likelihood of outages is at its lowest in that case. Torrefaction also makes biomass similar to coal in many respects, which is something they are accustomed to. And the criterion of ‘minimal energy loss’ is logical of course: utilise as much energy as possible from scarce, sustainable and expensive biomass.”
What torrefaction does with biomass is somewhat similar to what a refinery does with oil. The idea is to create one or more useful products from raw material that have highly predictable and standardised properties. With biomass, this has everything to do with the fact that the raw material is obtained from a location – such as in the middle of the forest – other than where it is needed: the fuel depots of large power stations and production units for transport fuels around the world. Kiel: “We started thinking about this in April 2003. French researchers had already used torrefaction, originally developed for roasting coffee beans, on wood. We linked a number of essential preconditions to the basic idea and started with development. In 2008, we were able to commission a pilot installation. It produces BO2 pellets with a higher heating value than ‘ordinary’ wood pellets. They are also water-repellent and less vulnerable to heating and rotting. And the pellets are very easy to grind.”

Stable process
The group of researchers from ECN imposed three criteria: 1) The process must be simple and cheap; 2) It must have a high energy efficiency; 3) The environmental impact must be as low as possible. All three are logical criteria for researchers who work at an institute devoted to sustainable energy technology. “The pilot plant has a capacity of 50 kg of biomass per hour and it actually became clear at the outset that our basic idea was all right,” says Jaap Kiel.
In some projects you take ‘1 step forwards, 2 steps backwards’, but development during the torrefaction project was exceptionally successful. The research group soon started focusing on follow-up questions: does the installation run in a (long-term) stable manner and does the technology also work on a large scale? Long-duration tests were conducted in the pilot installation to answer the first question. Kiel: “The development of a new process is often based on an existing reactor type. However, we started from scratch and reasoned on the basis of the desired end product and the three criteria mentioned above. Our torrefaction reactor is therefore unique and we have applied for a patent. But we also wanted to extensively test this reactor ourselves given that it is so unique. Hence the long-duration tests, including a recently successfully completed 100-hour test.” A demonstration installation is currently being developed with a capacity of approximately 5 tonnes of biomass per hour.

Schematic overview of ECN's Torrefaction technology.

Long-duration test
Fred Verhoeff and Heiko Gerhauser were present when the 100-hour test was conducted with the pilot installation. Verhoeff: “We changed a number of details in 2009 in the hope that these would lead to improvements. So, yes, everyone was slightly tense.” On Sunday, the installation was preheated, on Monday morning the operators turned up the thermostat and production started. “Production was very stable from the start; a bit boring actually. By Friday afternoon, we called it a day as a double-shift system is quite tiring for the operators. But the test was a great success and we could celebrate,” says Verhoeff.
The pilot plant is full of measuring instruments of course. The operators monitor them closely as they use these instruments to control the process. Gerhauser: “The instruments produced a virtually straight line. That indicates a stable process and was the reward for the modifications we implemented.”
The only mistake made by the researchers was that they had not counted on everything proceeding so successfully. On Tuesday, it became clear that the biomass supply was insufficient. Fortunately, a new shipment could be ordered and delivered on time. The biomass did not meet the criteria completely because it contained a 25% moisture level while the torrefaction reactor had been optimised for 10-20%. Verhoeff: “You have to remove that additional moisture within your process as it disrupts the energy balance to some extent. The big question was whether the process could restore itself.”
The satisfied faces after this statement speak volumes. The torrefaction reactor did not falter or become off-balance, and continued producing obediently.

Biomass supply
Biomass must eventually supplement shortages in fossil fuels. The position taken by Vattenfall is clearly indicative of this. This company seeks security of supply for biomass. Physical and chemical conditions have been standardised with the torrefaction technology of ECN. The ecological side must also be guaranteed. Kiel: “Biomass must not compete with food crops. It must also satisfy a series of other sustainability criteria, so the solution is biomass residue streams in particular. Water usage must be included in the chain analysis, as well as (in)direct land usage change. This must be enforced by (international) politics. Since the onset of development, we have always attached great importance to sustainability. This is one of the reasons why Vattenfall wishes to support the upscaling of the technology. Vattenfall wants to operate as sustainably and ‘green’ as possible. ECN’s torrefaction technology ties in seamlessly with this desire.”

Contact
Jaap Kiel
ECN Biomass, Coal & Environmental Research
Tel.: +31 (0)224-564590
E-mail: Jaap Kiel

Information
Read or download the ECN-report BO2 technology for biomass upgrading into solid fuel – pilot-scale testing and market implementation

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