Arnhem, Netherlands — Utilities are facing major challenges in the coming decades. Current policy envisions a transition to a sustainable energy supply, while ensuring security of supply. Therefore, current energy policy is spuring utilities to improve the sustainability of their coal-fired power plants.
Co-firing biomass is one of the major measures widely applied to reduce CO2 emissions. Since the mid-1990s, power plants designed to burn pulverised coal have additionally been firing organic materials, such as wood and agricultural waste. However, coal-fired power plants are not designed to process biomass, which limits the co-firing percentage to some 5%-10%. With investments in dedicated supply chains and biomass pre-treatment equipment co-firing percentages of 25%-50% (thermal) have already been achieved.
From the fuel perspective, the ideal situation is to process the biomass so that its properties resemble those of coal. The main form of processed biomass currently in use is wood pellets – pelletised dry sawdust – because it is a relatively clean fuel that is internationally available, easy to handle (free flowing capabilities, less dust emission) and has relatively low transport cost. Wood pellets work well in coal-fired plants and are now regarded as a well-proven technology.
Nevertheless, wood pellets do have their drawbacks. Wood pellets need dedicated silo storage to avoid degradation. Co-firing wood pellets has consequences for the milling and combustion of the wood pellets. At >5% co-firing, the pellets need to be hammer milled in separate hammer mills to a typical particle size of up to 1 mm, whereas the coal mills grind the coal to a pulverised particle size of about 50 microns on average. Co-firing furthermore may influence primary air requirements, combustion behaviour, heat transfer pattern in the boiler, boiler efficiency, by-products and emissions. The various problems mean that wood pellets aren’t really a commodity fuel that can be blended with coal in whatever proportions are desired.
In order to increase the co-firing percentage further, utilities are looking for innovations. To create a biomass product that has superior handling and co-firing capabilities than wood pellets, torrefaction is an option. Indeed, torrefaction is one of the technologies that promises to realise the dream of a true commodity fuel.
Torrefaction is essentially a biomass cracking technique. It’s an additional pre-treatment step that heats the biomass to 260-320°C for up to one hour in an atmosphere of no or low oxygen content. After torrefaction the biomass has become brittle, due to the disintegration of hemicelluloses and to a lesser extent lignin and celluloses, which are responsible for the tough fibre structure. In other words, the fibrous structure of the biomass is partially broken down. The weakened fibre structure improves the milling properties of the biomass and enables the biomass to be processed together with coal at the power plant.
Furthermore, the calorific value of the biomass increases typically from 12-16 MJ/kg to 20-24 MJ/kg, due to the loss of volatiles and moisture. The features of torrefied biomass enable co-firing rates of more than 50% of generating output, while keeping the investments needed to a minimum.
Depending on the distance from biomass source to the co-firing site, it is economically attractive to pelletise the torrefied biomass. Torrefaction pellets have a volumetric energy density of 14.5-17.5 GJ/m3 (bulk density of 800 kg/m3), which is about 70%-80% higher than conventional wood pellets (8.5-10 GJ/m3). In order to pelletise, the torrefaction temperature must stay below 300°C to keep a large part of the lignin intact, which serves as a natural binding agent for making pellets. Biomass that has been torrefied at higher temperatures might need additives to produce good quality pellets. Once the hydrophobic nature is proven, they can be stored in the open air – doing away with the need for silos. It is also considered feasibile to use particles with a size larger than the standard 8 mm pellets.
Torrefaction of biomass was already developed in the 1970s and 1980s. After a quiet period, the biomass market started to grow more rapidly at the beginning of this century. A number of small equipment suppliers with different technical processes started to torrefy biomass in pilot plants. The small quantities produced proved that it is possible to torrefy woody biomass. At this moment, torrefaction is attracting more and more attention. Biomass suppliers, investors, and end users are all starting up projects. There are about 30 projects currently running, mainly in Europe and North America. And, although most projects are pretty small scale, some larger ones are getting off the ground as well. The best known torrefaction unit is Topell in Duiven, the Netherlands, which is designed for 60,000 tonnes a year of product output.
A number of torrefaction reactors are being developed in parallel. It’s too soon to say which approach is going to prevail with the suppliers of torrefaction technology in different stages of developing a commercial-scale installation. An inventory of the existing concepts for torrefaction, evaluating them on their technological performance, has shown that roughly all suppliers have developed an integrated concept, in which the energy efficiency is optimised by combusting the volatile rich torrefaction gases and by using the heat of the flue gases to dry and torrefy the biomass. It isn’t the case that one technique is fundamentally superior to the others. Several techniques will ultimately prove successful. The idea is to have a process that can be managed easily. Cracking is an extremely complex business; it’s not just one step on from drying.
The essential thing is to have an integrated approach. It is important to think not only about the reactor itself, but also about the drying, the milling and the heat recovery. If the material isn’t pre-processed properly, that has implications for how the reactor works. For example, the pre-drying step is crucial for good torrefaction conditions. Higher moisture contents of the biomass will result in ‘wet’ torrefaction gas, which requires energy to combust and lowers the overall energy efficiency. Seasonal aspects also play a role.
It’s no good looking at everything from a purely technical viewpoint; it’s about finding the most economical solution as well. Where the biomass is coming from makes a big difference to the viability of a scheme, for example. As does whether one needs to create something from scratch, or if a torrefaction unit can be added to an existing plant.
Types of Torrefaction Reactor
Torrefaction concepts differ in reactor technology, torrefaction conditions and heat exchange methods. An overview of the major technologies is shown below.
Multiple Hearth Furnace
The Multiple Hearth Furnace (MHF) consists of six hearths, each approximately one metre in height. The biomass is fed at the top of the reactor, after which it moves down through the different levels. An ‘IN hearth’ passes the biomass to the next hearth by moving the biomass to a centralised passage. An ‘OUT hearth’ processes the biomass to the next hearth by moving it to drop holes located at the reactor’s periphery. To process the biomass through the different hearths, a centralised shaft drives rabble arms at each hearth. In case of torrefaction, the reactor is operated down draft, which means that the flue gas flow follows the same direction as the product flow.
The steam injections result in very good temperature control and a high product quality with minimal energy loss, giving the process a relatively high efficiency. But they also demand gas consumption to heat the relatively wet torrefaction gasses for combustion.
A critical factor of the MHF is fuel flexibility. The particle size is limited by the space between the teeth of the rabble arm, the space between the drop holes and the quality of the product; larger particles will take more time to be torrefied.
Rotary kiln reactor
The rotary kiln process resembles the successful concept for commercial pyrolysis units. When the rotary kiln reactor is applied to torrefaction, the biomass needs to be dried to preferably 10%-15%/wt moisture. In one concept available on the market the rotary kiln is indirectly heated by thermal oils; in another it is directly heated by superheated steam.
The rotational speed of a rotary kiln is a crucial process parameter for the product quality of torrefaction. When the rotational speed is too slow, the biomass will be carbonised instead of torrefied. When it is too high, the biomass is not fully torrefied and has low product quality. Moreover, rotational speed has a wearing effect on the biomass, leading to a reduction in particle size over the reactor’s length. Variations in particle size should be avoided in a rotary kiln. The basic reactor technology has no option to differentiate in particle size, which means that these variations are critical for product quality.
The reaction time of torrefaction takes 30 minutes and the total process time is around two hours. The residence time needed for optimal torrefaction conditions primarily determines the size of the rotary kiln, which limits the upscaling possibilities of this reactor.
The principle of a Torbed reactor is the toroidal flow of the bed, which is created by injecting air with high velocity (50-80 m/s) through stationary angled ‘blades’. The injection angle results in a flow with a horizontal and vertical velocity vector, which lifts and moves the fuel bed in a horizontal motion at the same time. This creates a shallow solid material bed, which circulates around a vertical axis in the centre of the reactor and around a horizontal axis in the freeboard of the reactor. The toroidal motion allows a higher air speed, which reduces the boundary layer between solid particles and gases. As a result, heat and mass transfer between gases and solids improve, which allows lower retention times and a more homogeneous product.
The commercial scale Torbed torrefaction reactor consists of a four-stage continuous updraft process. In the first stage, the biomass is completely dried and fluidised by superheated steam. The second stage increases the temperature further to 350°C and serves as a buffer for all biomass particles that have not been dried in the first stage. In the third stage, the biomass is torrefied by directly injecting hot flue gas from the combustion of torrefaction gas. The last stage functions as an additional control measure to ensure that all biomass particles have been torrefied.
The time needed to process the biomass through these four different stages is claimed to be less than five minutes, which justifies higher torrefaction temperatures than other concepts and enables higher biomass throughputs. However, excellent process control is needed to avoid a loss of chemical energy, resulting in a lower overall energy efficiency. Another disadvantage of higher torrefaction temperatures is the volatilisation of phenol, acetone and other contaminants, which makes flue gas cleaning more challenging.
Compact moving bed reactor
In a moving bed reactor the biomass is fed at the top and moves slowly down to the bottom where the product is discharged. The length of the reactor is, in large part, determined by the retention time needed to produce the desired product. When applied for torrefaction the retention time is 25-30 minutes. The biomass is directly heated to 250-300°C by a partial recycle of the torrefaction gases. From the remaining torrefaction gases the tar is separated and the cleaned gas combusted in an afterburner, where it is combined with the gas of a biomass gasification unit and the resulting flue gases directly fed into the torrefaction gas recycle stream. The recycle consists of repressurisation of the torrefaction gas to compensate for the pressure drop in the recycle loop, and of the heating of the recycle gas to deliver the required heat in the torrefaction reactor.
A typical phenomenon in moving bed reactors is the unequal heating of the fuel bed, due to limited mixing possibilities. This effect becomes more severe in larger moving bed reactors, which limits the upscaling potential of this reactor technology.
Screw conveyor reactor
The screw reactor is heated by the flue gases after combustion of the torrefaction gases, as in the other concepts. However, heat transfer in a screw reactor is less efficient than fluidisation technology and, due to the transport capabilities of the screw, the biomass feed is limited to particles with a size smaller than 10 mm. Moreover, biomass with a very low bulk density and high moisture content needs to be pre-treated before feeding it to the screw reactor. In order to have a good product quality, the screw diameter is limited, which limits the upscaling potential.
As can be seen, various torrefaction concepts exist. All concepts have been tested to at least a pilot-scale size. Some concepts are currently being implemented or have already been implemented in a torrefaction plant. The typical size of realised plants or plants under construction is on the order of 20-60 kt/year on product output. Apart from the upscaling challenges, all suppliers of torrefaction technology struggle to find feasible solutions for a number of issues, such as:
• Flue gas cleaning: In order to avoid permit problems, additional flue gas cleaning is needed after combustion of the torrefaction gas. An alternative would be to inject the torrefaction gas in a coal-fired boiler to completely oxidise all organic compounds;
• Process control: The challenge is to control the biomass feed, torrefaction temperature and retention time in such a way that all biomass is completely torrefied without being carbonised;
• Fuel flexibility: European and national legislation is restricting biomass available for co-firing. A different type of biomass will change the process conditions significantly and thereby also the choice of optimal reactor technology and integrated concept;
• Sustainability: Concepts with relatively low efficiencies and relatively high emissions will fall off.
The co-firing rate will still be limited by the chemical composition of the biomass because components like alkaline metals, phosphor and chlorine will still be present after torrefaction and affect boiler integrity (corrosion, fouling), byproducts and emissions. Site-specific bottlenecks will be present in most cases, and may include dust emissions, health and safety, operational limits of primary air fans, operational limits of the coal mills, and shifting of the heat balance in the boiler. Models can calculate and predict these bottlenecks.
Torrefaction’s performance is highly dependent on the pre-treatment of biomass. And a large part of its added value will be allocated before the power plant gate. Nonetheless, we foresee that torrefaction will play an important role in co-firing biomass at coal-fired power plants. At the moment, torrefaction technology is making its first careful steps towards commercialisation, while the technology and product quality are still surrounded by uncertainties.