New market potential: torrefaction of woody biomass
According to researchers at Idaho National Laboratory’s Bioenergy Program, torrefaction of woody biomass could reduce variability in biomass feedstock and create a commodity-type product for green energy generation and usage. Faculty members Jaya Shankar Tumuluru and Richard Hess explain.
Biomass was the primary source of energy worldwide until a few generations ago, when the energy density, storability and transportability of fossil fuels enabled one of the most rapid cultural transformations in the history of humankind — the Industrial Revolution. In just a few hundred years, coal, oil and natural gas have prompted the development of highly efficient, high-volume manufacturing and transportation systems that have become the foundation of the world economy. But over-reliance on fossil resources has led to environmental and energy security concerns. One of the greatest advantages of using biomass to replace fossil fuels is reduced greenhouse gas emissions and carbon footprint.
Europe is leading the way in offsetting its growing fossil fuel demand with renewable bioenergy, and through policy it has created markets for the wood pellet trade to support residential and industrial heating and cofiring with coal to produce electricity. However, mobilising sustainable, cost-effective and reliable biomass feedstock supplies is one of the greatest constraints limiting expansion of bioenergy production globally. To compete effectively with fossil fuels and garner investor confidence in bioenergy, supplies can be made more appealing by becoming more like the resources they are intended to replace. Researchers in the Bioenergy Program at Idaho National Laboratory (INL) are focused on developing feedstocks and supply systems that do just that – making woody biomass more like the fuels that have changed the world. One approach they are exploring is torrefaction of wood to give it important ‘coal-like’ handling and heating characteristics.
Biomass has limitations as fuel, in comparison with fossil fuel resources in general, because of its high moisture content, lack of bulk density, tendency to degrade and low calorific value (CV). All of these factors are interdependent and result in increased costs for energy output. For example, high-moisture-content biomass requires more energy to grind, which is required for feeding it into heating and power-generation appliances. This also leads to decomposition during storage and results in costly dry matter losses. Grinding this type of wood chips results in irregular shapes, creating yet another handling issue, especially during feeding in cofiring or gasification systems. In terms of chemical composition, high-moisture-content biomass has a higher oxygen content than carbon and hydrogen, making it less suitable for thermochemical and cofiring applications.
The torrefaction makeover
There is much interest in torrefaction as a method of transforming biomass into a product more like coal. Torrefaction is not a new technology — it has been used industrially for hundreds of years to roast coffee beans, but its application to biomass for bioenergy energy production is new. Torrefaction addresses woody and other biomass limitations that are restricting biofuels industry expansion. It makes both woody and herbaceous biomass brittle (easy to grind), improves the fuel properties (increases the carbon and reduces the oxygen and hydrogen) and makes it more suitable for combustion and gasification applications. It produces a low-moisture, hydrophobic product with very low biological activity, making it very stable in different storage environments. Torrefaction combined with densification increases the energy density by 25% compared with densified biomass.
Torrefaction occurs as biomass is slowly heated to a temperature range of 200–300°C in an environment without oxygen. This is hot enough to almost completely dry the material and produce chemical changes without causing combustion. At thermal treatment temperatures of 50–150°C, biomass loses free water, or the water that flows throughout the plant tissues, which reduces the material’s overall bulk density. At temperatures of 150–200°C, hydrogen and carbon bonds begin to break, which allows elimination of bound water, or water that is held tightly in the plant’s micropores, and causes the biomass to lose its fibrous nature, making it even easier to grind.
Finally, at temperatures of 200–300°C, or the torrefaction range, not only has the material given up most of its moisture while retaining most of its energy value (70% of initial wood weight and 90% of initial energy content), it actually undergoes chemical changes that greatly improve its coal-like qualities. Carbonisation and devolatilisation occur, resulting in the emission of off-gases that can be recycled to help power the torrefaction process. This makes transportation of biomass over long distances and combustion of biomass safer for humans and the environment, because volatiles are not emitted into the atmosphere, as they are in high-volume wood chip and pellet transport. Destruction of the plant’s cell structure makes it even more brittle, further improving its grindability and making the material more uniform and consistent. The combination of these changes reduces the material’s ability to rehydrate, so it sheds rather than absorbs external moisture and is less prone to rot. Torrefied biomass has similar combustion characteristics to coal, and the blackened material appears more like it, too.
Torrefaction of ground wood results in a product with ~22 million British Thermal Units (mmBTU per tonne), which is comparable with 24mmBTU of coal. However, the difference in bulk density makes it difficult for biomass to be cost-competitive with coal, which has a bulk density of 850kg/m3, compared with torrefied wood at ~230kg/m3.
Combined torrefaction and pelletisation was proposed by the Energy Research Centre of the Netherlands (ECN) for production of high mass density biomass for energy applications. Densifying torrefied wood by pelleting or briquetting increases the bulk density to 750–850kg/m3, significantly reducing transportation and logistics limitations because larger volumes of material can be handled in one load for a similar cost.
In comparison with conventional wood pellets, the ECN estimates a 30% logistics cost saving when transporting torrefied, pelleted biomass in the existing pellet infrastructure. Torrefied densified wood products have great near-term market potential because they are compatible with existing high-volume transportation infrastructures, and existing conversion facilities and appliances can use the material or be readily adapted to cofire higher percentages with coal.
Torrefaction also makes biomass easier to trade alongside other energy commodities, while reducing risks posed to feedstock producers and biorefiners by fluctuating demand and supply balances. These attributes should enable cost-competitive, reliable feedstock supplies and consistent, uniform physical, chemical and storage properties to meet biorefinery specification requirements.
This process is an effective way to make biomass more like conventional energy commodities, but one of the greatest advantages of torrefied densified biomass is that it offers environmental and energy security advantages over fossil fuels. This makes use of woody biomass attractive for a variety of heating and energy production applications. Improved physical, chemical and storage properties make torrefied wood pellets a good replacement for regular wood pellets in cofiring and gasification plants. High-energy-content torrefied wood pellets increase the thermal energies of combustion and gasification systems. The variety of product outlets for torrefied and densified biomass gives feedstock producers and biorefiners greater flexibility in buying and selling biomass products to meet future energy demand.
Improved storability – hydrophobicity studies (temperature zone D) conducted by INL and Oak Ridge National Laboratory (ORNL) found that torrefaction dramatically reduces the need for expensive covered storage and helps to retain energy value during storage and long-distance transportation.
Improved flowability – grinding studies (temperature zones D and E) at INL have found that torrefied biomass has better particle size and shape sphericity after grinding, which makes it easier to handle in existing high-volume transportation systems and more suitable for thermochemical applications, such as gasification, cofiring and pyrolysis.
Improved energy value – torrefaction studies (temperature zones D and E) conducted by INL indicated that biomass retains most of its energy content while giving up moisture and low-energy-content volatiles, which increases
heating value and improves combustion efficiency.
Reduced logistics costs – the Energy Research Centre of the Netherlands (ECN) reported that torrefaction and densification increase bulk density nearly four-fold and can reduce logistics costs by 30%.
Reduced preprocessing costs – INL grinding studies on deep dried (temperature zone C) and torrefied biomass (temperature zones D and E) found significant reduction in grinding energy required (50–70% less) compared with raw biomass.
Reduced variability – the ECN has found that torrefied biomass (temperature zones D and E) has more consistent moisture content (less than 3%) and pulverises more evenly than untreated biomass, resulting in better blending of varying plant fractions.
Reduced Storage off-gases – INL and University of British Columbia studies indicated torrefied woody biomass (temperature zone D) emits lower CO, CO2 and CH4 off-gases compared with non-torrefied wood chips and commercial wood pellets.
The authors acknowledge the contributions of Leslie Park Ovard and Christopher Wright. The work at INL is supported by the US Department of Energy.