Dry matter loss and heat release due to oxygen depletion in stored wood pellets

https://doi.org/10.1016/j.biombioe.2023.106848Get rights and content

Highlights

  • The dry matter loss (dml) in wood pellets was calculated from oxygen consumption data.

  • The dml and heat release (HR) rates increased exponentially with temperature.

  • The dml and HR rates were parabolically related to moisture content.

  • Pellets at a low moisture content of 10% show more oxidative reactions than wood chips do.

Abstract

When stored in closed environments, wood pellets consume and react with atmospheric oxygen in oxidative reactions. These reactions result in the loss of dry matter and the release of oxidative heat. To measure the oxygen consumption, pellets with moisture content (m.c.) between 4 and 50% (wb) were stored in sealed jars for 10 days at 25, 40, and 60 °C. Air was drawn from the jar using a syringe. The oxygen concentration of the extracted air was determined using gas chromatography. Assuming a complete oxidative reaction, 43.2 mmol or 1.057 L of oxygen gas is required to convert 1 g of wood under room conditions. Pellets stored at 25 °C had a rate of dry matter loss between 0.002 and 0.005%/day. The rate of dry matter loss more than doubled when pellets were stored at 40 °C. Pellets with 35% m.c. stored at 60 °C had the highest rate of dry matter loss of 0.036%/day. The oxidative heat rate ranged from 1 μW/g at 10 °C to 84 μW/g at 60 °C for 35% m.c. (wet mass basis) sample. When compared to published dml data of wood chips, wood pellets had higher dml rates at moisture contents lower than 10% but were more stable at higher moisture contents. Our results may be applied to estimate dry matter loss and oxidative heat in stored pellets without extensive analytical devices.

Introduction

Wood pellets, as a solid biofuel, react with atmospheric oxygen to produce off-gases such as carbon monoxide, carbon dioxide, and methane [1,2]. These reactions, which are initiated primarily in wood resins, terpene and fatty acids, cause off-gas emissions and dry matter losses in the pellets and release oxidative heats, which contribute to self-heating phenomena in stored pellets [3]. Dry matter loss in pellets causes losses in merchantable and useable material. Dry matter losses may be caused by abiotic chemical reactions and aerobic biotic reactions which both involved atmospheric oxygen consumption [4]. The primary abiotic chemical reactions are oxidation reactions. The main aerobic biotic reactions are cellular respiration which occurred in fungi and bacteria. Under controlled indoor storage conditions, the rates of off-gas emissions and dry matter loss of wood pellets usually peaked within a few days and then gradually decreased thereafter [2].

Besides their use as solid biofuel, wood biomass has been proposed as a material for long-term carbon storage and sequestration [5,6]. Most recently, Zeng and Hausmann [5] proposed the “wood vault” concept where uncomminuted wood logs are stored in an enclosure under controlled atmospheric conditions to ensure minimal losses from natural degradation. Although producing pellets requires more energy and results in more greenhouse gases emissions than wood chips or logs, wood pellets may be an ideal candidate to be used for carbon sequestration as they have three times the bulk density of wood chips and logs, have low moisture, and are easy to handle and transport [7]. One unanswered question is whether dry matter losses in wood pellets due to natural degradation is comparable to other forms of woody biomass over a long storage period.

He et al. [4] demonstrated that the amount of dry matter loss in wood chips stored in an aerobic environment is correlated to the amount of carbon dioxide (CO2) emission. When storage temperature increased from 5 to 50 °C, the CO2 emission factor for stored wood chips increased from 3 g/kg dry matter (DM) to 18 g/kg DM while the dry matter loss increased from 0.97% to 2.36% for wood pellets stored for 9 weeks. The oxidative reactions in biomass consume atmospheric oxygen and result in CO2 emissions and dry matter losses. Therefore, oxygen consumption from the atmosphere is an indication of dry matter losses.

For wood pellets, Yazdanpanah et al. [8] showed that the peak CO2 emission factor increased from 0.007 to 0.415 g/kg DM as storage temperature increased from 25 to 60 °C and the moisture contents increased from 4% to 50% (wb). The results showed that the CO2 emission factors were positively correlated to storage temperature and the pellets’ moisture content. Meier et al. [9] reported that the CO2 emission from off-gassing from wood pellets increased from 0.45 to 0.76 mg/kg pellets as oxygen concentrations increased from <1% to 35%. The authors speculated that part of the oxygen required for the off-gassing reactions originated from the biomass itself.

Although numerous studies were conducted on gas emissions of stored wood pellets as summarized by Alakoski et al. [10], the authors were unable to locate any work done on dry matter loss in stored wood pellets. It may be tempting to assume that the dry matter loss in wood pellets is similar to that of dried wood chips. However, Afzal et al. [11] and Hofman et al. [12] established that the simple acts of bundling and covering wood chips can change the amount of dry matter loss in wood chips significantly. Previous research into dry matter loss in wood chips showed that high moisture content, high availability of nitrogen through high bark and foliage contents, higher microbial activity, and finer particle sizes will promote greater dry matter losses through fungi and bacterial growth [4,13,14]. These fuel parameters are usually very different in wood chips and wood pellets. Consequently, the concentrations of carbon monoxide emissions from wood chips were reported as one order of magnitude smaller compared to that of wood pellets [10]. The densified nature and low bark content of wood pellets, in addition to the usually lower moisture contents, may result in lower dry matter loss relative to wood chips.

The objective of this study is to present data on oxygen depletion and then develop equations to predict the dry matter loss and oxidative heat in stored wood pellets from oxygen concentration data.

Section snippets

Materials

Wood pellets were obtained from a commercial pellet plant in the BC interior. The pellets were made from the sawdust from a sawmill processing lodgepole pine (Pinus contorta). The samples had a moisture content (m.c.) of ∼4% as received. All moisture contents are expressed on a percent wet mass basis. The samples were conditioned to nominal 9%, 15%, 35%, and 50% m.c., respectively, by spraying set amounts of distilled water on the pellets in a slow-rotating container for up to 10 min.

Results and discussion

The off-gas results from this work were published in Yazdanpanah et al. [8]. Data from Kuang et al. [21] was also used. Both studies used pine pellets. We expect pellets made from different tree species will have different dry matter loss behaviors due to the difference in resin and fatty acid contents [22]. In this paper, only oxygen (O2) concentration data between 1 and 8 days are used and presented to represent the maximum rate of dry matter loss.

Conclusion

Although the dry matter losses of stored wood chips are studied in many published literatures, there is a lack of published dry matter loss data for stored wood pellets, a commercially relevant solid biofuel. This study attempted to estimate the dry matter losses in wood pellets using the published oxygen concentration data. The rate of dry matter loss (dml) was fitted as a function of temperature and moisture content. The rate of dml increased exponentially with temperature from 0.002%/day at

Acknowledgments

The Natural Sciences and Engineering Research Council (NSERC) of Canada supported this research under the Discovery Grant program.

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