Physical and chemical properties of pellets from energy crops and cereal straws

The physical and chemical properties of biomass pellets based on wood, willow, miscanthus, wheat, barley and rape straws were each tested using European Committee for Standardisation (CEN) standard methods to determine their calorific, ash, chlorine, sulphur, nitrogen, carbon and hydrogen contents as well as durability, bulk density and moisture. In terms of chemical composition, it was found that when compared to wood, willow or miscanthus the straws (i.e. wheat, barley and rape) had higher levels of chlorine, sulphur and nitrogen which could lead to problems in terms of boiler degradation and gaseous emissions. Energy contents of all pellets were very similar with no significant differences between biomass types. The moisture content of wood pellets was significantly lower than the other types of biomass indicating that these raw materials need a higher level of moisture present for binding. While all pellet types had durability levels above 90%, wood was the only pellet to achieve a value above the standard 97.5% level. It was concluded that, the majority of pellet boilers currently on the market are designed specifically for wood pellets and thus, are not capable of adapting to the differing properties of these pellets. The new European standard (EN 14961) fuel specifications give a broad spectrum of quality levels. With improvements in pellet quality through addition of binders and mixtures, developments in boiler design, and with the introduction of new standards, alternative biomass pellets will, in the future, compete favourably with wood pellets for both heat and electricity generation. ► Biomass pellets are compared in terms of their physical and chemical properties. ► The new EN 14961 fuel specifications give a much broader spectrum of quality levels. ► Straws chemical content makes them unsuitable for the majority of pellet boilers. ► Developments in boiler design necessary for future alternative biomass utilisation.