EVC8: Intermediate bioenergy carriers for power and heat
Energy carriers which are treated either mechanically and/or chemically before they are used as final energy carrier for heat and/or power production are called intermediate bioenergy carriers or biogenic secondary energy carriers. The main objectives for the specific treatment are to improve either the physical or chemical fuel properties for the specific conversion technology or to optimize the fuel supply chain (e.g. homogeneous fuel, high density and suitable for storage, automatic fuel supply). Important intermediate bioenergy carriers are wood pellets, also known as white pellets, and wood briquettes.
Wood pellets are bulk material and they are suitable for automatically fuelled combustion systems. Wood pellets commonly consist of compressed saw dust or wood shavings which derive as residues from the production process of sawmills. The shape of pellets is cylindrical and the diameter is ≤ 25mm. In Europe the diameter of high-quality wood pellets is typically 6 mmm. Wood pellet production is a specific process which has to respect the properties of the raw material as well as later quality criteria of the produced pellets. Typically, the feedstock is dried and/or grinded in advance of pelletizing in order to reach an optimal moisture content as well as size distribution for the compression process. Due to the high pressure and increased temperature during the pelletizing process, specific agglomeration mechanisms take place which lead to a positive connection of the feedstock. Frequently, a small number of additives, e.g. < 2 w-% of maize starch, is supplied to the feedstock directly before the pellet press in order to improve the agglomeration mechanisms and to increase the stability of pellets. Downstream the pellet press, the pellets are cooled in a counterflow cooler, e.g. using cool air. Typically, pellets are provided to the end-consumers as bulk goods or as bagged cargo on pallets. For wood pellets several standards as well as certification schemes are available which guarantee a specific product quality. Important parameters are type of used feedstock, the stability of wood pellets represented by the durability, moisture and ash content, lower heating value, share of fines and bulk density.
Wood briquettes are stable commodities or cargo. Similar to pellets they also consist of compressed wood feedstock. However, the size distribution of the used feedstock can be larger compared to pellet feedstock since the diameter of briquettes are bigger compared to pellets (>25 mm). Briquettes, which are commonly used in stoves or manually fuelled boilers, have a diameter in the range of 40 mm – 100 mm. Compared to firewood an important advantage of briquettes is the reduced demand of specific storage volume. Also, for briquette production, drying or grinding of feedstock is frequently necessary. The compression process could be either carried out continuously (extrusion process) or discontinuously (compartment process). Briquettes produced by the extrusion process are commonly round shaped, whereas briquettes produced by the compartment process are commonly rectangular shaped. Typically, briquettes are provided to the end-consumers as bagged cargo on pallets. There are also standards as well as certification schemes for briquettes available which guarantee a specific product quality. Important parameters for briquettes are type of used feedstock, moisture and ash content, as well as lower heating value.