Influence of Oxygen Availability on off-Gassing Rates of Emissions from Stored Wood Pellets

Abstract

The phenomenon of off-gassing from wood pellets during storage has been the cause of several, in some cases fatal, accidents due to toxic atmospheres in storages. To optimize safety measures the nature of the responsible processes needs to be clarified. In this study the impact of O 2 availability, which is a decisive factor for the presumed oxidation of fatty acids, is pointed out. Off-gassing rates of CO, CO 2 , VOC, and CH 4 of pellets at relatively constant O 2 levels of approximately 35%, 20%, and <1% over a period of 20 d at approximately 295 K were investigated. For this purpose 7 kg of spruce pellets was stored under simulated ventilation of the atmosphere in a 31 L tank. Gas concentrations were determined every 24 h by GC-FID/TCD. Compared to the mean emission rates at 35% O 2 of CO (0.22 mg kg −1 pellets d.b. in 24 h) and CO 2 (0.76 mg kg −1 pellets d.b. in 24 h) the lowest O 2 concentration of <1% resulted in a significant reduction of off-gassing rates of 40% for both gases. In contrast the release rates of VOCs and also CH 4 decreased with the higher O 2 concentration (0.035 to 0.025 mg kg −1 pellets d.b. in 24 h; 0.0085 to 0.0061 mg kg −1 pellets d.b. in 24 h), presumably, because of increased onward reactions to CO and CO 2 . Since offgassing was not prevented by the lack of O 2 (<1% O 2 -trial) it is assumed that the O 2 required for the reactions originated from the biomass itself. During the storage of pellets at 20% O 2, emission rates of CO (0.18 mg kg −1 pellets d.b. in 24 h) and CO 2 (0.79 mg kg −1 pellets d.b. in 24 h) at the start decreased by more than 20% and those for VOCs (0.032 mg kg −1 pellets d.b. in 24 h) by almost 30% after 3 weeks. It can be assumed that in ventilated storages the reactivity and thus a potential risk from off-gases from wood pellets decreases considerably in only a few weeks. The effects of aging, in terms of declining reactivity at relatively constant tank conditions, on off-gassing rates could be clarified for the first time. A realistic development of the decline of reactivity of the material itself could be determined.

Reducing Volatile Organic Compound Off-Gassing during the Production of Pelletized Steam-Exploded Bark: Impact of Storage Time and Controlled Ventilation

“…This was possibly due to condensation onto the solid product because no increase with time was found for either new or already existing possible secondary reaction products. Also, Meier et al found a reducing trend in total VOCs in wood pellets during low-ventilation storage, which is in accordance with the results presented here. The concentrations of lower weight compounds were more stable over time, and off-gassing from pre-press samples changed less with storage time compared to direct and pellet samples.…”

Monitoring of carbon monoxide in residences with bulk wood pellet storage in the Northeast United States

“…Wallace et al found that for most homes air change rates, assuming windows closed, ranged from 0.30 to 1.9 (Wallace et al, 2002;Howard-Reed et al, 2002) per hour. Given the published offgassing rates of CO from pellets in the range of 10 −5 mg CO/kg pellets/min, a relatively small amount of ventilation can provide protective measures (Meier et al, 2016;Soto-Garcia et al, 2015b;Fan and Bi, 2013;Kuang et al, 2009). Hence, from these data, it appears that most homes had infiltration rates that may have helped maintain the low CO concentrations.…”

Fatal Accidents During Marine Transport of Wood Pellets Due to Off-Gassing: Experiences from Denmark

“…The topic has been investigated in several recent publications, e.g. [6,7] but the exact nature of the chemical processes at play do still not appear to be well-understood.…”