Article pubs.acs.org/EF
Real-Time Chemical Composition Analysis of Particulate Emissions from Woodchip Combustion Aki Kortelainen,*,† Jorma Joutsensaari,† Liqing Hao,† Jani Leskinen,‡ Petri Tiitta,‡ Antti Jaatinen,† Pasi Miettinen,† Olli Sippula,‡ Tiina Torvela,‡ Jarkko Tissari,‡ Jorma Jokiniemi,‡,§ Douglas R. Worsnop,†,∥,⊥ James N. Smith,†,# Ari Laaksonen,†,⊥ and Annele Virtanen† †
Department of Applied Physics, and ‡Department of Environmental Science, University of Eastern Finland, 70211 Kuopio, Finland Fine Particles, VTT Technical Research Centre of Finland, Post Office Box 1000, 02044 Espoo, Finland ∥ Aerodyne Research, Incorporated, Billerica, Massachusetts 01821, United States ⊥ Climate Change, Finnish Meteorological Institute, 00101 Helsinki, Finland # National Center for Atmospheric Research, Boulder, Colorado 80305, United States §
S Supporting Information *
ABSTRACT: Residential wood combustion is one of the major sources of fine particles. The chemical composition of the particles plays a key role in both adverse health and environmental effects. It is important to understand how chemical composition of particulate emissions varies during different combustion processes and conditions. In this work, combustion of wood chips was studied in a moving step-grate burner in different combustion conditions (efficient, intermediate, and smoldering) in the laboratory. The particulate emissions were measured with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-TOF-AMS). It was found that two phases were occurring frequently in the intermediate and smoldering combustion. Phase 1 took place when gaseous carbon monoxide (CO) was rapidly increasing after the new fuel addition. Phase 2 was a stable, burn-out period with low CO emissions until the new fuel addition and automatic removal of fuel leftovers from the grate. The analysis on the organic aerosol by positive matrix factorization (PMF) extracted out five factors: hydrocarbon-like organic aerosol (HOA), low-volatile-oxidized organic aerosol (LV-OOA), biomass burning organic aerosol (BBOA), and two additional factors of “polycyclic aromatic hydrocarbon (PAH) factor” and “aromatic factor”. PAH and LV-OOA were found to be forming mainly during phase 1. HOA showed similar behavior as a PAH factor and LV-OOA in a time series. BBOA was consistent with levoglucosan formation during the combustion and became higher during phase 2. The aromatic factor was mainly composed of fragment ions of n-butyl benzenesulfonamide compound, which was observed in both phases. To our knowledge, this is the first work to report the particulate organics of combustion aerosols and PAH distinguished by PMF. The results prove that the particulate organic emissions can be reduced efficiently when keeping combustion efficiency high. This may help in targeting the efforts on emission reduction better in the future.
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INTRODUCTION Fine particle emissions have both adverse health and environmental effects.1−3 Current levels of respirable suspended particles (PM10; Da < 10 μm, particulate matter of aerodynamic diameter of 10 μm or less) and fine particles (PM2.5; Da < 2.5 μm) in Europe are associated with about 350 000 premature annual deaths, increased hospital admission, and restricted activity to tens of millions of children and people with chronic cardiovascular and pulmonary diseases.4,5 In addition, ultrafine particles (PM0.1; Da < 0.1 μm) have their own specific effect on mortality and illness. 6 Ultrafine particles have greater inflammatory response than fine particles because of efficient deposition in the respiratory system and surface chemistry.7 Recent toxicological studies show that the toxic potential of PM depends upon particle size, concentration (number, mass, and surface area), chemistry, and morphology.8 Primary fine particulate matter (PM2.5) emissions from low-altitude sources, such as traffic and residential combustion, may cause immediate exposure near the source.9 Residential wood combustion (RWC) produces high amounts of gaseous and particulate emissions into the © XXXX American Chemical Society
atmosphere. RWC for heat production has been assessed to be one of the major sources of fine particle mass emissions, particulate polycyclic aromatic hydrocarbons (PAHs), and certain gaseous pollutants, such as volatile organic compounds (VOCs), throughout Europe.10 RWC emission influence on the PM2.5 level has been found to be comparable to the particulate level observed on a busy street.11 It is possible that, in the near future, the RWC emissions will further increase because the use of fossil fuels is predicted to be reduced.12 The main components of particulate emissions in wood combustion are ash (inorganic species), soot (elemental carbon), and organic matter.10 The particulate organic matter formed in wood combustion is a complex mixture, including compound groups, such as anhydrosugars (e.g., levoglucosan), methoxyphenols, PAHs, organic acids, sterols, and alkanes.13,14 Combustion conditions and fuel quality (composition and geometrical morphology of fuel) have an important effect on aerosol Received: September 2, 2014 Revised: November 28, 2014
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DOI: 10.1021/ef5019548 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels chemical composition, particle size, and concentration.10,15 The composition of particles is a key role when either the health or atmospheric effects of the emissions are considered. Therefore, it is important to understand how chemical composition of particulate emissions varies during different burning processes and conditions. In previous studies (e.g., refs 10 and 16−18), the chemical composition of particulate emissions from RWC has mainly been analyzed from impactor or filter samples. However, these methods have poor temporal information from aerosol composition in rapidly changing circumstances. In contrast, the chemical composition of particulate emissions can be analyzed with high time resolution (ca. less than 1 min) by an aerosol mass spectrometer (AMS).19,20 Furthermore, the highresolution time-of-flight AMS (HR-TOF-AMS)21 separates ions of different elemental compositions at each m/z in nearly real time, which allows for improved characterization of organic aerosol (OA) with statistical techniques. Previously, AMS has been used in several studies on biomass burning (e.g., refs 22 and 23), and only a few concentrated on wood combustion in the laboratory. Schneider et al.24 and Weimer et al.25 used a quadrupole aerosol mass spectrometer (Q-AMS) to study the chemical composition of burner emissions from different wood types. Heringa et al.26 studied the burning cycles and the changes in the chemical composition of particle emissions from log wood and wood pellet combustion with HR-TOF-AMS. It shows that the inorganic material and OA dominated the composition, black carbon (BC) had minor contribution in normal pellet combustion conditions, and BC was the dominating species during the smoldering combustion. While in a log wood burner, BC was dominate, excluding the periods after fuel addition.26 Elsasser et al.28 studied the aerosol chemical composition of the particle phase with HR-TOF-AMS from different burning phases (ignition, harsh combustion, stable combustion, burnout) from a logwood stove loaded with beech and spruce logs with different load weights, compositions, moisture contents, and combustion conditions. By the use of the positive matrix factorization (PMF) method for organics, it was found that three burning factors were linked into burning phases. A previous investigation by AMS has performed good work on the characterization of organic species and BC during different combustion conditions, while a recent study26 has shown that the ratio of oxygen (O) to carbon (C) (O/C ratio)27 varies with the combustion stage, suggesting the different chemical compositions in organic particulate species. However, the study on the detailed specification on the organic aerosol from the combustion process has rarely been reported. In this work, we studied the composition of the particles emitted from a grate-fired burner fueled with wood chips. Studied combustion conditions were classified into efficient, intermediate, and smoldering combustions according to the combustion efficiency of the burning process. We used HRTOF-AMS to quantify organic and inorganic species and employed the PMF method to analyze the HR-TOF-AMS data from the combustion source. The detailed information on temporal changes in chemical composition of OA and the effects of different burning phases and conditions on the composition of the emitted particles were also obtained.
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Characterization” at the University of Eastern Finland in Kuopio, Finland, during October and November in 2010. The focus of the project was on further understanding the physicochemical properties (e.g., size, shape, structure, and detailed chemical composition) of combustion-generated particulate matter and their associations to health-related toxicological responses. The schematic figures of the sampling and experimental setups are shown in Figures SI1 and SI2 of the Supporting Information. The experiments were performed by burning wood chips in a moving step-grate burner (MultiJet bioburner, 40 kW, Ariterm, Finland) under different “combustion conditions” (efficient, intermediate, and smoldering combustions) and measuring the emission products. The used fuels consist of a mixture of spruce and deciduous wood (dry matter content with a total mass of 75.8% and H2O with a total mass of 24.2%).18 The detailed descriptions of the fuel (see SI7 of the Supporting Information) and air/fuel ratios was recently published by Leskinen et al.18 The wood chips were added to the heater a few times in a minute automatically by logic control. In every 20 min, the grate elements were moved automatically to remove the ash from the grate to the container, which lay below the grate. Primary air in the grate and secondary air flows at the burner walls were adjusted by the user in situ. Experiments were divided into efficient, intermediate, and smoldering combustion conditions by the emission rates of carbon monoxide (CO) (low, elevated, and high, respectively) (see Table 1). According to the CO concentration level, efficient combustion represents the process of biomass burning in a modern pellet boiler and smoldering combustion represents conventional batch combustion conditions. In addition, intermediate combustion represents partly for both conditions or malfunction of the continuously working burner.17,25 The combustion conditions were controlled by adjusting fuel feeding (feed rates: efficient, 7 kg/h; intermediate, 5 kg/h; and smoldering, 7−8 kg/h) and air staging in a way that the efficient conditions were achieved with primary and secondary air flows of ca. 210−350 and 410−460 L/min, respectively. While in intermediate and smoldering combustion cases, only primary air flow was used (ca. 250−390 and 160−260 L/min, respectively), which resulted in poor mixing conditions of gasified fuel and combustion air and, consequently, high gaseous and particulate emissions. The detailed descriptions of the combustion situations and respective emissions with offline methods have been published by Leskinen et al.18 Prior to aerosol sampling, flue gas was diluted with a porous tube diluter and two ejector diluters in series. Dilution ratios used in the experiments varied from 500 to 2000 depending upon combustion conditions. The schematics of the measurement setup, used dilution ratios, and sampling locations for different combustion conditions are shown in Figures SI1 and SI2 of the Supporting Information. A Fourier transform infrared spectroscopy method (FTIR DX4000, Gasmet, Finland) was used to monitor concentrations of CO, carbon dioxide (CO2), nitrogen oxide (NOx), and hydrocarbons (CxHy). The oxygen (O2) concentration was measured by a separate paramagnetic sensor connected with FTIR. FTIR was sampled directly from the stack. The total number concentration was measured with a condensation particle counter (CPC model 3775, TSI, Shoreview, MN). The represented results are dilution-corrected, reduced to 10% O2, and later called the reduction correction factor.29 Size-dependent chemical composition of aerosol particles at a size range of 40−1000 nm was measured in real time by HR-TOF-AMS (Aerodyne Research, Inc., Billerica, MA) using a vaporizer temperature of 600 °C that flash vaporizes non-refractory aerosol particles.19−21 The vaporized and ionized ion fragments were measured using mass spectrometers in V and W modes, having better sensitivity and better mass resolution, respectively. The data processing was conducted at high resolution (HR) to determine the mass concentrations of organics, sulfate (SO42−), chloride (Cl−), nitrate (NO3−), and ammonium (NH4+) species in an ion fragment range of 12−198 amu in V mode. W-mode data were processed to verify the ion identification in V mode. In addition, the ions in the range of 198−453 amu from unit mass resolution (UMR) were analyzed to determine PAHs (see SI3 of the Supporting Information). The pTOF (particle time-of-flight) was used to determine the size-resolved mass
EXPERIMENTAL SECTION
The measurements were carried out as a part of the European project of “Clean Biomass Combustion in Residential Heating: Particulate Measurements, Sampling, and Physicochemical and Toxicology B
DOI: 10.1021/ef5019548 Energy Fuels XXXX, XXX, XXX−XXX
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