Interrelation of Volatile Organic Compounds and Sensory Properties of Alternative and Torrefied Wood Pellets

Abstract

The increasing demand for wood pellets on the market, which is caused by their excellent combustion properties, inspires the production as well as the utilization of alternative biomass pellets as fuel. However, the emission of volatile organic compounds gives pellet materials a distinct odor or off-odor, which is directly perceived by the end user. Thus, there is an urgent need for knowledge about the emitted volatile organic compounds and their potential formation pathways as well as their contributions to odor properties of the pellets. In this study, pellets made of biomass energy crops (i.e., straw or miscanthus), byproducts from the food industry (i.e., rapeseed, grapevine, or DDGS (dried distillers grains with solubles from beer production)), or eucalyptus, as well as torrefied pinewood and torrefied sprucewood were investigated with respect to the emitted volatile compounds and their possible impact on the pellet odor. Headspace solid-phase microextraction in combination with gas chromatography−mass spectrometry was used to enrich, separate, and identify the compounds. Techniques used in sensory science were applied to obtain information about the odor properties of the samples. A total of 59 volatile compounds (acids, aldehydes and ketones, alcohols, terpenes, heterocyclic compounds, and phenolic compounds) were identified with different compound ratios in the investigated materials. The use of multivariate statistical data analysis provided deep insight into product−compound interrelation. For pellets produced from bioenergy crops, as well as from byproducts from the food industry, the sensory properties of the pellets reflected the odor properties of the raw material. With respect to the volatiles from torrefied pellets, those volatiles that are formed during the torrefaction procedure dominate the odor of the torrefied pellets covering the genuine odor of the utilized wood. The results of this work serve as a substantiated basis for future production of pellets from alternative raw materials.

Off-Gassing and Oxygen Depletion in Headspaces of Solid Biofuels Produced from Forest Residue Biomass

“…Safe storage and transportation of wood pellets is an important aspect of using solid biofuels as an RES [16]. Various gases, such as carbon dioxide (CO 2 ), toxic carbon monoxide (CO), methane (CH 4 ) and volatile organic compounds (VOC), including low-molecular-weight aldehydes (mainly hexanal), are found to be released during pellet storage [16][17][18][19]. Combined with poor ventilation, the off-gassing lowers the air quality inside and seriously threatens human health (and even life), especially with high concentrations of CO.…”

“…Off-gassing and self-heating studies focus mainly on pellets produced from coniferous biomass because of their widespread presence on the market and the potentially high content of lipophilic substances susceptible to autooxidation. Pellets made from pine and spruce have been the most widely studied to date [18,26,[30][31][32][33][34][35][36]; less frequently studied are other species, such as loblolly pine, European larch, Larix gmelini, Douglas fir, cedarwood and eucalyptus [21,27,[37][38][39], and very rarely, typical deciduous trees such as common ash and cork [21,37]. Therefore, there is a knowledge gap regarding off-gassing from pellets made from deciduous species compared with coniferous species.…”

Torrefaction of Wood in a Quiescent Layer of Talc

“…24 At the same time, it can lead to an increase in particulate matter emission. 25,26 In addition to the direct use of torrefied biomass as a fuel, it can be used in the process of pyrolysis 27,28 and gasification. 29 Despite significant development in torrefaction technologies, their wide industrial application is limited and sometimes is not economically feasible.…”

“…Torrefaction of biomass improves the grindability of raw biomass by the transformation from a fibrous structure to a more brittle, coal-like material. , Moreover, biomass torrefaction favors the increase in the energy density and the hydrophobization of the biofuel surface . At the same time, it can lead to an increase in particulate matter emission. , In addition to the direct use of torrefied biomass as a fuel, it can be used in the process of pyrolysis , and gasification . Despite significant development in torrefaction technologies, their wide industrial application is limited and sometimes is not economically feasible.…”