Abstract
Biomass pellets are a source of renewable energy; although, the air pollution and exposure risks posed by the emissions from burning pellets in biomass boilers (BBs) are uncertain. The present study examines the organic species in fine particle matter (PM) emissions from an BB firing switchgrass (SwG) and hardwood (HW) biomass pellets using different test cycles. The organic and elemental carbon (OC and EC) content and select semivolatile organic compounds (SVOCs) in filter-collected PM were identified and quantified using thermal-optical analysis and gas chromatography–mass spectrometry (GC–MS), respectively. Fine PM emissions from the BB ranged from 0.4 g/kg to 2.91 g/kg of pellets burned of which 40% ± 17% w/w was carbon. The sum of GC–MS quantified SVOCs in the PM emissions varied from 0.13 to 0.41 g/g OC. Relatively high levels of oxygenated compounds were observed in the PM emissions, and the most predominant individual SVOC constituent was levoglucosan (12.5–320 mg/g OC). The effect of boiler test cycle on emissions was generally greater than the effect due to pellet fuel type. Organic matter emissions increased at lower loads, owing to less than optimal combustion performance. Compared with other types of residential wood combustion studies, pellet burning in the current BB lowered PM emissions by nearly an order of magnitude. PM emitted from burning pellets in boilers tested across multiple studies also contains comparatively less carbon; however, the toxic polycyclic aromatic hydrocarbons (PAH) in the PM tested across these pellet-burning studies varied substantially, and produced 2–10 times more benzo[k]fluoranthene, dibenz[a,h]anthracene and indeno[1,2,3-c,d]pyrene on average. These results suggest that further toxicological evaluation of biomass pellet burning emissions is required to properly understand the risks posed.
Highlights
Interest in alternative and renewable energy sources is growing worldwide due, partly, to the economic, security, and environmental concerns associated with burning fossil fuels
Organic matter (OM) in the particle matter (PM) was estimated by multiplying OC by a factor of 1.8 [17], in which case the OM and EC accounted for 60% ± 31% w/w of the PM
Despite the lower PM mass emissions typical of pellet burning, these results show that the polycyclic aromatic hydrocarbons (PAH) distribution in the PM from pellet burning differs substantially from wood burning in biomass boilers (BBs) and residential wood stove and fireplace appliances, and should undergo further toxicological evaluation
Summary
Interest in alternative and renewable energy sources is growing worldwide due, partly, to the economic, security, and environmental concerns associated with burning fossil fuels. Renewable energy accounts for ~13% of total U.S and world energy supplies currently [1]. Data for 2017 show that approximately 5% of the total renewable energy supply in the U.S (11.6 quadrillion kJ) is used in the residential sector, and that biomass in the form of wood or wood byproducts dominates renewable energy consumption for residences [2]. Compared with most residential biomass burning appliances, BBs have unique operating characteristics, large fire-box enclosures capable of burning multiple fuel types, and relatively low emissions stack heights [4]. These latter two characteristics of BBs create serious concern
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