Abstract
The paper investigated the torrefaction of cones from three tree species: Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.), and European larch (Larix decidua Mill.). The objective was to determine the effects of torrefaction temperature on the properties of cones with a view to their further use as a renewable energy source. Torrefaction was conducted at 200, 235, 275, and 320 °C for 60 min under an inert gas atmosphere. Elemental composition, ash content, and lower heating value (LHV) were measured for the original and torrefied samples. Torrefaction performance was evaluated using formulas for solid yield, higher heating value (HHV), HHV enhancement factor, as well as energy yield. Scanning electron microscopy (SEM) was used to assess elemental composition and structural changes at the surface of the torrefied material. For all the studied conifer species, the higher the torrefaction temperature, the greater the carbon and ash content and the higher the LHV (a maximum of 27.6 MJ·kg−1 was recorded for spruce and larch cones torrefied at 320 °C). SEM images showed that an increase in process temperature from 200 to 320 °C led to partial decomposition of the scale surface as a result of lignin degradation. Cone scales from all tree species revealed C, O, N, Mg, K, and Si at the surface (except for pine scales, which did not contain Si). Furthermore, the higher the temperature, the higher the enhancement factor and the lower the energy yield of the torrefied biomass. Under the experimental conditions, spruce cones were characterized by the lowest weight loss, the highest HHV, and the highest energy yield, and so they are deemed the best raw material for torrefaction among the studied species.
Highlights
Biomass is unique among renewable energy sources (RES) in that it can be applied both in the production of electrical energy and in the manufacture of chemical products [1,2]
The present study examined the effects of torrefaction temperature on the physical properties of cones to be used for energy purposes
Solid yield was used to evaluate the effects of torrefaction on sample weight loss, which corresponds to energy yield [44,45], while the enhancement factor reflected the power output and energy density of the obtained bio-coal [21]
Summary
Biomass is unique among renewable energy sources (RES) in that it can be applied both in the production of electrical energy and in the manufacture of chemical products [1,2]. The main objective of the torrefaction process is to obtain a higher energy density of biomass by its increasing carbon content while lowering the concentration of oxygen and hydrogen per unit of volume [9]. This goal is similar to charcoal production by carbonization, with the important difference being that the latter does not retain maximum energy in the product, and so leads to low energy yield. The present study examined the effects of torrefaction temperature on the physical properties of cones to be used for energy purposes
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