Wood Torrefaction: The Future of Biomass Fuel?

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Most people know torrefaction today for its application in food processing industries. However, the process of torrefaction is also rising in rank in the energy sector, quickly becoming one of the most advanced options to efficiently substitute the burning of coal.

For this reason, Baltania OÜ, a fully owned Estonian subsidiary of the Dutch private equity investment firm Momentum Capital, decided to invest in a first industrial-scale torrefaction plant in Vägari, Estonia. Providing that all conditions of the investment decision are met, the construction of the unit will start in 2018.

Biomass torrefaction involves heating the biomass to temperatures between 250 and 300 degrees Celsius in a low-oxygen atmosphere. When biomass is heated at such temperatures, the moisture evaporates and various low-calorific components (volatiles) contained in the biomass are driven out. During this process, the hemi-cellulose in the biomass decomposes, which transforms the biomass from a fibrous, low-quality fuel into a product with excellent fuel characteristics. After torrefaction, the product is cooled down and pelletised, resulting in a black, torrefied pellets with a high calorific value—the perfect fuel to replace fossil coal in power stations. In fact, the energy density and grindability of torrefied pellets are closer to thermal steam coal and significantly higher than that of white wood pellets or wood chips. This is why torrefied pellets have been nicknamed “bio-coal”. The torrefaction process is not bound to woody biomass feedstocks alone, but may also include straw, coconut shells, cacao shells, Miscanthus, elephant grass, and even olive residues—opening the door for further innovations.

In this nascent market, Baltania OÜ announced its willingness to develop a first commercial plant in Estonia whose construction should begin in at the end of 2018. The investment of approximately EUR 45 million will be funded by Momentum Capital, together with other investors and financial institutions. The European Union has approved NER300 grant of EUR 25 million for the torrefaction project. The torrefaction project will be carried out in co-operation with the Estonian Ministry of the Environment. Baltania OÜ indicated its focus will be on production of 160.000 tonnes of torrefied bio-coal pellets per annum, and that its main customer base will consist of utility companies in the Nordic countries and central Europe currently running on fossil coal. Besides limiting boilers adaptations, according to Baltania, the biocoal pellets produced can be stored and handled using existing fossil coal infrastructure due to their hydrophobic nature.

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Solid Biomass Conversion Processes

Logging, grinding, screening and drying operations are mechanical processes for enhancing solid biomass fuels. These operations help transform biomass into homogeneous fuel that is easy to handle, with a higher energy recovery (eg. woodchips, wood logs, agrofuel). In general, moisture content in woodchips used for energy generation in municipal or industrial plants ranges between 20-30%.

Densification is another popular way to transform woody material into an advanced fuel with high calorific value. This process involves compressing biomass and lowering moisture levels to a range of 8-10%, allowing for an homogenous fuel – either in the form of pellets or briquettes. The heat during compression fuses the lignin in the biomass, naturally binding the biomass together in a new, enhanced shape. Thanks to densification, the homogenous biomass fuel is easier to transport and can be used in automated biomass installations, such as pellet stoves and boilers.

Thermo-chemical conversions are now used to produce fuel with even higher calorific value, utilising torrefaction or steam explosion technologies. During the torrefaction process, wood is subjected to temperatures of 230°C-300°C at atmospheric pressure without oxygen. Comparable to some extent to coffee torrefaction, this method creates a fuel with unique characteristics. As compared to conventional wood, torrefied wood has very low (>5%) moisture content, is easily grindable and is relatively hydrophobic.

Thanks to advanced technologies, woody biomass can also produce liquid and gaseous fuels. Pyrolysis, for instance, is a thermal-chemical conversion that requires a high temperature (>400°C) and little oxygen to convert solid biomass into gas, liquid fuels (pyrolysis oil) and/or biochar. Gasification is another thermo-chemical conversion that takes place at high temperatures (>800°C), with limited oxygen and/or steam, and converts solid biomass into synthesis gas, known as syngas – which contains carbon and hydrogen and can be used to produce liquid fuels such as biodiesel.

Wet Biomass Conversion Processes

Wet biomass is present in high volume across Europe (manure, agricultural waste and by-products). As it contains too much moisture to be converted efficiently into energy via a direct combustion process, other conversion pathways and energy outputs have been developed by the biogas (eg. anaerobic digestion) and biofuel sectors (eg. fermentation). Anaerobic digestion and fermentation are the two main conversion pathways used for converting wet biomass feedstock into advanced fuel.

Anaerobic digestion is the microbiological process of decomposition of organic matter, in the absence of oxygen, and is common to many natural environments; it is largely applied today in order to produce biogas in airproof reactor tanks, commonly referred to as digesters. A wide range of micro-organisms are involved in the anaerobic process which has two main end products: biogas and digestate. Biogas is a combustible gas consisting of methane, carbon dioxide and small amounts of other gases and trace elements. Biogas can be used in direct combustion to produce heat, but also power if converted within a cogeneration plant or in adapted gas motors. Biogas can also be upgraded via purification processes to a biomethane that can be injected into the existing natural gas networks, used as a chemical product and/or vehicle fuel. Digestate is the decomposed substrate, rich in macro- and micro-nutrients and therefore suitable for use as plant fertiliser.

Fermentation is a biochemical conversion whereby microorganisms, including yeast and bacteria, convert matter into products such as ethanol and liquid transport fuel (biodiesel). This process is done in several stages. First, sugary and starchy feedstock such as corn must be crushed and combined with water, allowing enzymes to convert this starch into sugar. This ferments along with the addition of yeast and is distilled into bioethanol. Bioethanol can also be produced from cellulosic biomass, such as grass, wood and stalks, via fermentation; however, this process is more complex and involves a mechanical pre-treatment and the addition of enzymes, or the breakdown of the lignocellulose into sugar through hydrolysis, followed by the same procedure.

Solid bioenergy feedstock

Renewables represent 8,0% of EU-27 total energy consumption (22.473 kilotonnes of oil equivalent) in transportation, of which 68% is provided by biofuels in 2018. It is rather challenging to foresee how biofuels (in particular first-generation biofuels) will continue to develop, since recent EU legislation established a quota for these biofuels. EU statistics on renewables in transportation can also be misleading regarding the actual level of production, because multiple counting rules are applied in accordance with the EU renewable energy directive. Looking at it in more detail, the biofuel market is driven by biodiesel and bioethanol. Biodiesel was the first biofuel developed and used in the EU in the transport sector in the 1990s. EU-27 remains the world’s largest biodiesel producer and it consumed 13,303 Ktoe in 2018. By comparison, bioethanol consumption in 2018 reached 2,620 Ktoe for an actual production of 2,365 Ktoe (Biofuels Report, 2020).

Bioenergy comes in all shapes and sizes

Within our close environment, a great source of organic materials such as plants, algae, or organic wastes, can be valuable fuels as soon as a technology makes it possible to efficiently extract all of its energy potential. Bioenergy players traditionally refer to biomass to describe those usable feedstocks. Biomass currently used in Europe includes wood from forests, agricultural crops and residues, by-products from the wood and agricultural industry, herbaceous and woody energy crops, municipal organic wastes and manure, and could potentially integrate algae and marine biomass in the future.

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