Posted byCHRISTOPH SCHILLING
Wood pellets became a commodity for heat and power application over the last decades. The current global trade for wood pellets exceeds 25 million tonnes annually mainly for combustion applications. Canada is a major producer and exporter of pellets at about 2.1 million tonnes per year, with a capacity of 3.4 million tonnes.
International standards (ISO 17225) define the properties of several grades of tradable pellets and most of the pellets produced today achieve this standard. Durability is a major quality factor in pellet production. Pellets produced in Canada need to withstand thousands of kilometers of traveling to reach European and Asian markets. Hydrophobicity is another important characteristic. Water resistant pellets would allow outside storage and therefore reduce problems with off-gassing hazards and decrease capital costs. Additionally, pellets could be shipped in open vessels and loaded at all conditions. Ash content is another important factor, especially for residential application where the ash is usually removed manually and represents a serious health risk.
Similar to other fuels, the most important parameter is the calorific value or more specifically, the energy density. As with electric cars having a hard time competing with gas powered vehicles, pellets can hardly compete with natural gas or other fuels with high energy density. It is therefore crucial to further improve the energy density of pellets to compete on the market.
In addition to combustion applications, wood pellets would be the feedstock of choice for future biorefineries such as pyrolytic operations where bio oil is the first stage of aviation fuel production processes. Fast pyrolysis reactions can take advantages of the wood pellets low moisture content (10%), low ash content (less than 1%), and small particle size (2 mm). Wood pellets can also be used for biochemical applications. Our lab experiments show that the yield of sugars from pelletized Douglas fir was comparable to wood chips. The difference was more a function of enzyme strength than the form of feedstock.
A first step in improving pellets
The parameters of the pelletization process itself provide a lot of opportunity to produce high quality pellets. Major parameters such as pressure, die temperature, feedstock moisture content, and particle size are correlated. When adjusted properly, one can produce a high quality pellet with low energy consumption. Increasing moisture content and temperature lowers the pelletization pressure and thus, the energy consumption. On the other hand, increased pelletization pressure can enhance the pellet quality in terms of durability and density. Particle size of biomass has an impact on both quality and energy requirements. The key is to adjust those parameters to every new feedstock.
Taking a step further
Process parameters make it possible to fine tune the pelletization process to a certain extent. For further improvement, new technologies can take wood pellets to the next level. Increased hydrophobicity and calorific value are the main characteristics to improve the energy density, allow cost-efficient handling, and essentially bringing pellet properties closer to the ones of coal. Three major technologies have been the focus of many studies in the last years:
- Steam treatment
- Coating of pellets
During torrefaction, the biomass is heated in an oxygen-free atmosphere of up to 300ºC. This results in a further decrease in moisture content and volatiles, which leads to an overall increase in carbon content. The challenge, however, is the pelletization of torrefied materials. With big portions of the lignin gone after torrefaction, pelletization becomes an even more energy intensive process. Research conducted at Biomass and Bioenergy Research Group at UBC shows that torrefaction after pelletization provides more durable and denser pellets compared to pelletization after torrefaction.
Steam conditioned, steam treated or steam exploded biomass materials show lower energy requirements for pelletization compared to pellets made from untreated wood which also indicates less wear on the dies. Pellets also show an increase in density as well as durability. Depending on the temperature, steam exploded pellets can also show a slightly increased calorific value.
Individual coating of pellets using a variety of different liquid solutions showed promising improvements in terms of hydrophobicity. The encapsulation with paraffin wax lowered the moisture uptake of pellets from 16 to 2.5% in 72 hours.
However, it is still unknown whether the additional processes of torrefaction, steam treatment or individual coating add more value to the product to offset the additional production costs. All three technologies require higher investments and operational costs.
The current standard for high quality pellets represents pellets which are now widely produced by manufactures around the world. Unfortunately, this decreases the effort of improving the pellet itself. With improved pelletization parameters and new technologies such as torrefaction and steam explosion, it is possible to produce pellets with higher calorific value, hydrophobicity and improved physical quality. However, the industry must be able to work in a cost efficient manner in order to keep its operation sustainable. With rising prices on the energy market, it is possible that these technologies will be implemented in the future to provide better pellets. Challenges also lie in the feedstock availability. With less suitable materials for pelletization it will be more challenging to produce good pellets. Additionally, future applications such as bio-fuel production, might call for different properties of pellets.
A first step towards the improvement of the current product is to define a new level of quality for a “better pellet”.