Dynamics of decomposition gases and release of volatile organic substances in long-term storage stockpiles of pine bark: Focus on mono- and sesquiterpenes

Abstract

Bark is one of the key sidestreams produced in sawmills and currently is typically used for energy generation. Storage of bark is required to ensure supply at times of high heating demand, i.e. in cold winter months. In addition to energy generation, mono-, sesqui-, and diterpenes are components of resin in coniferous trees that can be used in various applications. The main aim of the present work was to investigate emissions of mono- and sesquiterpenes and common greenhouse gases, their generation, and flux dynamics within stockpiles of Scots pine (Pinus sylvestris L.) bark occurring during long-term storage in outdoor conditions. The focus was therefore on conversions of organic matter by decomposition into various gaseous substances. The work adds to the understanding how the decomposition products and volatile organic compounds (VOCs) are emitted from a stockpile and their stockpile pore space gas dynamics. Furthermore, it demonstrates how ventilation of a stockpile can affect the dynamics of decomposition product release and timing of VOC emissions during the storage. The storage time lasted from mid-April until the end of September 2021, during which seven samplings were carried out. CO2 was the main greenhouse gas liberated, with some emission of CH4, CO, and methanol. The major content of monoterpenes was released in the first 11 days after the stockpile establishment, and approximately half the sesquiterpenes in a 50-day storage time. Ventilation showed an effect on the release in the analysed cases, as the compounds decreased more rapidly over time due to ventilation. While the main share of carbon (C) losses originated from the release through CO2 in the non-ventilated stockpile, approximately 39 % more C was released in the ventilated stockpile. Gases other than CO2 were responsible for about 1 % of the total gaseous C losses from the non-ventilated, and 2 % from the ventilated, stockpile.