For the past five years, there has been a significant increase of interest in torrefaction as a pre-treatment technology for biomass fuels. In 2012, there are at least 40-50 torrefaction initiatives that have been identified in Europe and North America (IEA, 2012a). Most of these installations aim to demonstrate the technical and feasibility of torrefaction as a feasible pre-treatment option. Some would require several thousand tonnes of fuel for large commercial scale tests, where only a few seemed promising. Even though there is still no winning technology identified, there will be several viable torrefaction technologies capturing
the market over time (IEA, 2012a). Due to confidentiality and the high commercial interest, it is not easy to obtain data that are up-to-date and reliable.
Reactor technologies were modified to perform torrefaction and the important ones, with the companies involved, with specific type of torrefaction reactor they use are listed in Table 1.4. According to Dhungana et al (2012), the reactors can be classified into two categories: directly heated and indirectly heated. Directly heated reactors allow heat to be directly in contact with the biomass. The heating media can be a hot gas, hot solids, superheated steam or electromagnetic radiation. Indirectly heated reactors do otherwise but one disadvantage is the inconsistency of heating the biomass in the reactors.
1.19.1 Directly heated reactors
Convective reactor is the most common reactor used for torrefaction. The hot gas that passes through the biomass may be completely inert or contains a low level of oxygen. In a fixed bed reactor, the particles are stationary while in a moving bed, the particles move either by gravity or force of a mechanical device like augur. These particles move with respect to the wall of the reactor that can be horizontal, vertical or inclined (Dhungana et al., 2012). The heat transfer is usually through solid-gas convection. Fluidized bed involves the flow of gas through a bed of granular heat carrier solids. These heated solids are able to heat up fresh biomass fuels that dropped amongst them. The dominant heat transfer is particle-to-particle and the transfer is higher than that in convective bed. Microwave is also another directly heated type of reactor. It uses microwave irradiation and a typical microwave oven/reactor usually works at 2.45 GHz. Microwave reactor is different from other directly heated reactors, where the former heats the biomass from within while the latter heats the biomass externally.
1.19.2 Indirectly heated reactors
Rotary drum involves heat transfer from the hot drum wall to the biomass. Dhungana et al (2012) described that this type of reactor does not need to be oxygen free and that the volatiles are not diluted by the gas passing through it. In screw or stationery shaft, the torrefaction reactor is stationery. The rotating screw moves the biomass through the reactor and along its length. The heat transfer is similar to that in the rotary drum where the hot outer wall of the reactor heats the biomass indirectly.
Table 1.4. Overview of reactor technologies and some of the associated companies (IEA,
2012a).
Reactor technologies Companies involved
Rotating drum CDS (IK), Torr-Coal (NL), BIO3D (FR), EBES AG (AT), 4Energy Invest (BE), BioEndev/ETPC (SWE), Atmosclear S.A. (CH), Andritz, EarthCare Products (USA)
Screw reactor BTG (NL), Biolake (NL), FoxCoal (NL), Agri-tech Producers (US)
Herreshoff oven / Multiple Hearth Furnace
CMI-NESA (BE), Wyssmont (USA)
Torbed reactor Topell (NL) Microwave reactor Rotawave (UK)
Compact moving bed Andritz/ECN (NL), Thermya (FR), Buhler (D) Belt dryer Stramproy (NL), Agri-tech Producers USA)
Fixed bed NewEarth Eco Technology (USA)
Dhungana et al (2012) provides a quantitative comparison of four different types of reactors (convective heating type reactor, fluidised bed reactor, rotary drum reactor and microwave reactor) and examined how each reactor could affect the quality of the biomass in terms of mass yield, energy yield and energy density. They found out that the rotary drum reactor yields torrefied biomass that has the highest energy density, however the lowest mass and energy yield in comparison to convective and fluidised bed reactors. Since microwave reactor heats the biomass internally, the authors discovered that the core of the biomass heated very fast but the surface remained cold. The biomass experienced a large non-uniform heating, where the core was over-torrefied and the exterior remain unaffected.
In the development of torrefaction technologies, the goal is to produce a torrefied biomass fuel that can be converted to pellets or briquettes that can be handled and are durable enough to be stored outside and withstand the weather conditions. For large scale handling, this is still remains to be proven. There are also challenges with the torrefied biomass fuel in terms of difficulty to compact and dust from the torrefied fuel is active and prone to explosion in high concentrations. With regards to outdoor storage and leaching, the concerns are yet to be dealt with. Environmental impact due to leaching from outdoor storage must be well
understood. Dhungana et al (2012) discussed that at this stage, there is no commercial market that is fully developed for torrefied biomass, so the price is still uncertain. Torrefaction projects are largely based on clean biomass resources such as clean waste wood. Seems like waste streams and residues have gain attention as feedstock for torrefaction due to their low prices and high availability. However, they have unfavourable chemical compositions, concerns like ash fouling, emissions and efficiency that need to be resolved. Regulators may have to discuss with the energy producers on how waste derived torrefaction fuels could be used in existing facilities.
“Product quality standards and specific test methodologies for torrefied materials are
currently under development by ISO Technical Committee 238, expected to be published during spring 2013 as part of the ISO 17225 Standard, and criteria for sustainability is under development by ISO / PC 248” (IEA 2012a). Furthermore, since torrefied biomass fuels have
similar characteristics as those of low rank coals, safety classification under International Maritime Organisation (IMO) is required especially if they are to be transported by ocean vessels.