286639 Gas Emissions From the Storage of Western Red Cedar
The utilization of woody biomass as alternative fuel to generate heat and power has become an important element of sustainability in today’s world. Before direct combustion and/or processing in biorefineries, the biomass is usually stored for an extended time period. Gas emission is one of the major problems with biomass storage due to its impact on the surrounding environment and human health, as well as the dry matter losses which would reduce the heating value of the materials. Western Red Cedar (WRC) is a popular wood species in British Columbia, and its harvesting residues can be used as a sustainable biofuel to replace fossil fuels. Aside from the above-mentioned problems associated with biomass storage, WRC has a characteristics pungent smell which would cause concerns about odor nuisance problems. The objective of this paper is to investigate and quantify gaseous emissions from stored WRC woodchips. Fresh WRC chips with an initial moisture content of approximately 50% (wet basis) were used as raw materials in the study. Experiments were conducted in lab-scale reactors under both aerobic and non-aerobic (airtight) conditions at a range of temperatures (5oC, 20oC, 35oC, 45oC and 50oC), which represents cool to hot climate conditions in different geographic locations. Air was pumped into the aerobic reactors after the daily gas sampling event in order to replenish and maintain a high oxygen level in the reactors. The concentrations of gaseous emissions - carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4) along with oxygen (O2) were analyzed by GC, whereas volatile organic compounds (VOCs) were determined by GC/MS.
Results showed that under non-aerobic conditions, the concentrations of CO2 and CO increased gradually with time for all temperatures. At higher temperatures, higher CO concentrations were measured. In contrast, we observed lower concentrations of CO2 along with higher O2 concentrations at higher temperatures. At 20oC, the concentration of CO2 was the highest; after three weeks, CO2 concentration reached a plateau of 16% as oxygen content was depleted to 0%, indicating that the storage environment has turned anaerobic. By comparison, oxygen content was lowered to 10-15% for other temperatures. Results further showed that CO2 and CO emissions from the aerobic reactors exhibit similar trends as the non-aerobic reactors with respect to the effect of temperature. The concentrations of both CO2 and CO emitted at each temperature decreased slowly with time despite higher oxygen levels of close to 20% being maintained during the entire test period. Biological reaction could be dominant during the storage of wet biomass, leading to the highest level of microbial activity at 20oC rather than at higher temperatures. Microbiological analysis of the WRC woodchips was performed to verify this phenomenon and the extent of microbial activity in the materials. Results from the qualitative GC/MS analysis indicated that the major VOCs emitted include methanol, terpene, aldehydes, acid, alkane, benzene, indoles, acetone, ethers and esters. A TVOC meter was also available for determining the total concentration of VOCs; it was found to be higher at higher temperatures for both aerobic and non-aerobic storage conditions. Under non-aerobic conditions, the initial VOC concentration was approximately 53 ppm and 2 ppm at 50oC and 5oC, respectively; and it gradually decreased with time. Overall, the gas emission results from this study reaffirm the importance of ensuring safe storage conditions for wet woody biomass.
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