Moisture sorption properties and fungal degradation of torrefied wood in storage

Abstract

Replacing fossil fuels with renewable energy sources is an increasingly important research subject in order to combat the global climate change. Wood is a well utilised source of energy that has some problematic characteristics common to all lignocellulosic biomass. Moisture affects the supply chain of wood fuels negatively by complicating logistics and combustion. Hygroscopicity of stored wood leads to fungal deterioration and consequent losses in heating value. The problem has been addressed by reducing the hygroscopicity through the thermal pre-treatment process of torrefaction. The torrefied material is said to be resistant to fungal degradation and subsequent dry matter losses. However, only few studies exist, and the material’s performance in storage has been pointed out as an important research area. This thesis aims to provide much needed answers related to the storage properties of torrefied wood and charcoal, most importantly the effect of moisture. This thesis is made up of four studies, in which the sorption properties and fungal degradation of torrefied spruce and birch, as well as charcoal produced from the same feedstock, were investigated. In one part study, torrefied and steam exploded pellets were compared with the undensified material. The material adsorbed only minor amounts of water vapour, and the hydroxyl group accessibility and particle size were reduced. Although the capillary absorption became slower, the capacity for water uptake increased. This led to high moisture contents during the storage trials. It was also shown that the material is degraded by fungi. The degradation was slow, but dry matter losses were recorded in laboratory conditions. Furthermore, the fungal activity increased the material’s moisture content. The torrefied material hosted abundant fungal flora following outside storage trials, and many of the identified genera were known allergens. It was shown that torrefied pellets do not tolerate contact with water and should be stored covered.