Fuel 95 (2012) 662–664

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Fuel

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Short communication

Dry matter losses in combination with gaseous emissions during the storageof forest residues

Xiao He a, Anthony K. Lau a,⇑, Shahab Sokhansanj a,b, C. Jim Lim a, Xiaotao T. Bi a, Staffan Melin a,c

a Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3b Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USAc Delta Research Corporation, 501 Centennial Parkway, Delta, BC, Canada V4L 2L5

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 September 2011Received in revised form 10 December 2011Accepted 13 December 2011Available online 27 December 2011

Keywords:Forest residuesDry matter lossesGaseous emissionStorageTemperature effect

0016-2361/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.fuel.2011.12.027

⇑ Corresponding author. Tel.: +1 604 822 3476; faxE-mail address: aklau@chbe.ubc.ca (A.K. Lau).

Past published research on the storage of fresh woody biomass has rarely presented observations ofgaseous emissions in combination with the related dry matter losses. The objectives of this study areto determine dry matter losses and gaseous emissions from stored logging residues. Lab-scale vesselswere set up to study the concentration of off gases at 15 �C and 35 �C. Results showed that the maximumconcentrations of CO2, CO and CH4 were 13.8%, 0.16%, and 0.15%, respectively over a period of 35 days.The oxygen level decreased to 0% at the end of storage. Volatile organic compounds (VOCs) were quali-tatively detected by GC/MS technique. The major chemical compounds identified were alcohols, terpenes,aldehydes, acids, acetone, benzene, ethers and esters. The total VOC concentration reached 85 ppm at35 �C storage temperature at the end of the storage period. Dry matter loss ranged from 0.78% to 2.0%increasing with storage temperature.

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1. Introduction

In Canada, large amounts of lignocellulosic biomass in the formof forest and agricultural residues constitute renewable resourcesfor conversion into solid or liquid biofuels. High moisture forestresidues are usually stored for a period of time up to a year beforeutilization. Major problems with the storage of these materialsinclude gaseous emissions and dry matter losses. For large openpiles, the core part can become anaerobic; the resulting gaseousemissions are related to dry matter losses. Researchers havereported previous studies on dry matter changes for a variety ofwoody biomass [1–6]. A number of factors such as the physicalcharacteristics of biomass feedstocks, weather conditions, andthe method and duration of storage can influence dry matter lossesduring storage. Dry matter produces energy in thermal processessuch as combustion. Losses of dry matter are due to the decompo-sition of forest residues; this will reduce the calorific value andhence the potential revenue. Therefore it is important to minimizesuch losses during storage.

Gaseous emission from stored biomass is related to losses in drymatter. The main emissions from wood pellet storage are CO2, CO,CH4 and VOCs (volatile organic compounds). CH4 generation isusually due to anaerobic decomposition of biomass due to the actionof microorganisms, whereas CO2 can be generated from thermal

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oxidation, aerobic biodegradation or anaerobic biodegradation. Forinstance, the breakdown of carbohydrates with oxygen consump-tion may be represented by the reaction C6H12O6 + 6O2 ? 6CO2 +6H2O. Svedberg et al. [7] and Arshadi and Gref [8] postulated thatCO is formed from the auto-oxidative degradation of lipids and fattyacids present in wood, and storage temperature is one of the criticalfactors. Hellebrand and Schade [9] suggested that CO generation isindependent of microbial activity in the feedstock, but is promotedby increased temperatures and available oxygen. Moreover, CO pro-duced from plant litter was most likely caused by thermochemicaloxidation rather than a biological process. Kuang et al. [10] con-cluded that peak emissions of these gases were associated withhigher temperature and relative humidity in the containers withwood pellets. Biofuel material with a pile temperature of 60–70 �Cdue to degradation had been found to cause a substance loss of0.7–5.5% per month [6]. Wihersaari [4] found that the greenhousegas emissions were almost three times higher for fresh versus driedforest residues. More terpenes were released if wood chip pilesstored outdoor were mixed with bark during storage, especiallywhen the amount of precipitation increased [11].

Published studies on the storage of fresh woody biomass withregard to both dry matter losses and gaseous emission are scarce.The objectives of this research were to investigate the gaseousemissions from the fresh feedstocks in enclosed storage undertwo temperature conditions, and to measure the dry matterchanges during the storage period. Gaseous emissions wereobserved in previous studies on stored wood pellets that had been

X. He et al. / Fuel 95 (2012) 662–664 663

dried, suggesting that microbes were not eliminated by pelletizing.It is desirable to extend the study to forest residues with live mi-crobes. They have much higher moisture contents that would pro-mote greater microbial activities; besides, enclosed space wouldgenerate anaerobic environment and it can represent the core partof forest residue piles.

2. Materials and methods

2.1. Experimental setup and raw materials

Fresh Douglas fir (Pseudotsuga menziesii) branches with theirbark intact were obtained from the Pacific Spirit Park (Vancouver,Canada) and used as materials in this study. The branches with adiameter range of 22–25 mm were cut into 180–200 mm in length.Average moisture content of the branches was 38% (wet mass ba-sis). The moisture content was determined by grinding thebranches and drying the ground wood in a convection oven at103 �C for 72 h. Four high density polyethylene (HDPE) plastic con-tainers (two 10 L in volume and two 12.5 L in volume) were fittedwith valves and sampling ports and assembled as reactors for theexperiment. We call these containers ‘‘reactors’’ in this paper.The larger reactors held 2.5 kg biomass with 2.5 L headspace,whereas the smaller reactors held 2.0 kg biomass with 2.0 L head-space. The reactors were sealed and placed in room temperaturearound 15 �C. The other two reactors were sealed and placed inan oven with temperature setting at 35 �C. The materials had thesame bulk density at 200 kg/m3.

2.2. Analytical methods

The composition of gaseous emissions from the reactors wasanalyzed by gas chromatography (Model GC-14A, ShimadzuCorporation, Japan) for CO2, CO, CH4 and O2. The GC was calibratedregularly with the corresponding standard gases. The total concen-tration of VOCs was measured by a portable VOC monitor (ModelPGM-7240, RAE Systems, San Jose, CA). GC/MS (Model CP-3800,Varian Inc., CA) was used for qualitative analysis of the VOCs. Eachday, two sets of gas sampling were made. Firstly, 10-mL volumegas samples were drawn from each reactor to measure CO2, CO,CH4 and O2. Then, another set of 10-mL gas samples was drawnfrom each reactor to analyze the VOC composition.

Fig. 1. Gas concentration profiles at two storage temperatures: (a) CO2; (b) CO; (c)CH4; (d) O2.

3. Results and discussion

3.1. Gaseous emissions

Results of gaseous emissions are shown in Fig. 1a–c. Evidently,at the higher temperature of 35 �C, the gas concentrations in-creased significantly. At 35 �C, CO and CH4 concentrations in thelarger reactor are always higher whereas CO2 concentrations arehigher on some days. This is likely due to greater microbial activi-ties under higher temperature. By comparison, at 15 �C, the largerand the smaller reactors have almost the same CO2, CO and CH4

concentrations. The peak concentrations of CO2, CO and CH4 wereobserved to be 13.8% (138,000 ppm), 0.16% (1600 ppm) and0.15% (1500 ppm), respectively after 10 days. Fig. 1d depicts theoxygen concentration in the four reactors. The concentration was13.5–15.5% 1 day after the test started and then decreased steadilyto less than 1–2% within 10 days. Thereafter, oxygen was depletedto a level close to 0%. The concentrations of CO and CH4 increasedduring the storage period, with a faster rate at the earlier stage, andthen gradually leveled off during the remaining time. As of days10–15, depending on the temperature, the concentration of CO2

declined at different rates.

We compared our findings to gaseous emissions pertinent tostored wood pellets. Kuang et al. [10,12] studied gas emission fromstored wood pellets at temperatures of 10–55 �C and using varioussizes of the lab-scale containers. Their results indicated the peakconcentrations of CO2, CO and CH4 were within the ranges of1.87–4.5%, 0.55–1.52%, and 0.04–0.16%, respectively. They con-cluded that a chemical process for wood pellets could be the dom-inant mechanism for these emissions and pointed out thatbiological process may also contribute to the emissions for moistbiomass such as wood chips. The peak CO2 concentrations in ourmeasurements for non-pelletized material from Douglas firbranches were 4–7 times higher than those derived from woodpellets. The corresponding CO concentrations were 4–8 times low-er while CH4 concentrations were similar. The differences may beattributed to the higher moisture content of the fresh branches

Fig. 2. Trends of total dry matter losses versus total CO2, CO and CH4 emissions forthe four reactors.

664 X. He et al. / Fuel 95 (2012) 662–664

with live microbes at 38% moisture content (wet mass basis),whereas wood pellets had lower moisture content at 3.7–10% be-cause of the pre-pelletization drying process at a high temperature.When the initial O2 content is high in the reactors, it is possible forthe fresh forest residues to release a greater amount of CO2 thanthe wood pellets due to aerobic biodegradation in addition tochemical oxidation (breakdown of carbohydrates). At the laterstage when anaerobic condition would prevail at O2 content closeto zero, CO2 concentration started to decline from its peak; thiscould be due to adsorption to the surface of the materials, or dis-solving in water. A similar trend was not observed for the relativelydry wood pellets [10]. The CO2:CH4 ratio being 100:1 is muchgreater than that associated with typical anaerobic digestion forbiogas production at 40:60, suggesting that the reactor environ-ment in our study might not be favorable for methanogens(microbes which generate methane under anaerobic conditions).First-order Arrhenius-type kinetic equations were well fitted tothe gas emissions from stored wood pellet [10]; in our study, theArrhenius fit is applicable to the reaction rates of CO and CH4 emis-sions but not to the emission of CO2 because of the aforementionedbiological process.

3.2. Characteristics of VOCs

The total non-methane VOC concentrations of 85 ppm and60 ppm were observed in the reactors at 35 �C and 15 �C, respec-tively at the end of the storage period. This trend was similar tothe results reported for the release of terpenes, a specific class ofVOCs, from wood chip piles [11]. For the gas samples analyzedby GC/MS, a range of chemical compounds were found. The identi-fied VOC’s were alcohols, terpenes, aldehydes, acids, acetone, ben-zene, ethers, esters, sulfur and nitrogen compounds. Bycomparison, alkanes, hexanal, acetone and benzene were detectedfrom wood pellets storage [7,8].

3.3. Dry matter losses during storage

Dry matter losses in the four reactors amounted to 1.17%, 2.29%,0.78% and 1.33%, respectively, during the 35 days of storage. Thereactor with 35 �C had the largest dry matter losses. By compari-son, decomposition of Douglas fir residues and bark incubated insoil for 60 days led to 8.4% of wood carbon and 7.8% of bark carbonreleased as CO2 [13]. The much greater losses are probably due to a

larger population of active microorganisms in the soil. Based on lit-erature review, factors such as temperature, moisture content, bio-mass particle size and storage methods all have influences on drymatter losses [5]. At higher temperature, greater dry matter lossescan result from higher initial moisture content [1,3,4]. At the end ofthe tests, materials from the 35 �C reactor appeared brown versusthose that appeared green for the 15 �C reactor, which is anotherindication of greater degradation under higher temperature. Thetrends of dry matter losses, in (g) versus total CO2, CO and CH4

emission, in (ppm) over the storage period for the four reactorswere plotted in Fig. 2, illustrating significant increases in dry mat-ter losses and gas emissions due to an increase in temperature. Drymatter losses have a positive correlation with CO2, CO and CH4

emission.

4. Conclusion

The results show that storage temperature is a crucial factorduring the storage of fresh forest residues. Higher temperatureleads to higher gas concentrations. The maximum concentrationsof CO2, CO and CH4 were 13.8%, 0.16% and 0.15%, respectively overthe testing period. A wide range of VOC compounds were qualita-tively detected in the process. Higher total VOC concentration wasobserved in the reactor under higher temperature. Gas emissionswere found to have a positive correlation with dry matter losses,and greater dry matter losses occurred at higher temperature.Microbiological analysis is recommended in future work in orderto delineate the extent of chemical oxidation versus biological pro-cesses during storage.

Acknowledgements

This research is supported by Natural Sciences & EngineeringResearch Council of Canada and the US Department of EnergyOffice of Biomass Program.

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