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.

Wood Pellets

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.

Torrefaction

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.

More Attention

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.

Proven Success

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.

Integrated Approach

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.