Article pubs.acs.org/EF
Characterization of Gaseous- and Particle-Phase Emissions from the Combustion of Biomass-Residue-Derived Fuels in a Small Residential Boiler Edvinas Krugly,† Dainius Martuzevicius,*,† Egidijus Puida,‡ Kestutis Buinevicius,‡ Inga Stasiulaitiene,† Inga Radziuniene,† Algirdas Minikauskas,† and Linas Kliucininkas† †
Department of Environmental Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania Department of Heat and Nuclear Energy, Kaunas University of Technology, K. Donelaicio g. 20, LT-44239 Kaunas, Lithuania
‡
S Supporting Information *
ABSTRACT: Biomass is considered as one of the most promising fuels worldwide, mostly because of its renewability and almost-neutral carbon balance. At the same time, numerous studies have shown that the combustion of biomass fuels results in emissions of multiple gaseous and particle phase pollutants. The aim of this study was to fill the gap in the data of emissions from the combustion of agricultural biomass fuels. Five agricultural residue-derived fuels were tested: sunflower stalk pellets, straw pellets, buckwheat shells, corn stalk pellets, and wheat grain screenings. In addition, wood and sewage sludge pellets were investigated as reference fuels. Experiments were performed in a commercially available domestic 13 kW pellet burner during optimal and stable combustion conditions. The characterization of the emissions of gaseous basic pollutants (CO, CO2, SO2, NOx), as well as combustion specific pollutants (size-segregated particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), as well as BTEX (benzene, toluene, ethylbenzene, xylenes) was conducted. The emissions of PM were mostly represented by PM1 fraction (PM1/TSP > 0.8) in the case of all fuels. Total PM emissions ranged from 0.28 g/kg to 5.23 g/kg. Total emissions of PAHs ranged from 469.4 μg/kg to 7212.2 μg/kg. Size-segregated PAH analysis revealed that the most of PAHs were detected in fine aerosol fraction (0.056−0.18 μm). Sewage sludge pellets were determined as the most polluting fuel, including PAH emissions. Several fuels, including sunflower stalk pellets, buckwheat shells, and sewage sludge pellets, were found to be the least favorable fuels for combustion in a small-scale pellet-type burner, because of increased emissions of CO and PAHs. have a major effect on the formation of pollutants.7,15 Some researchers found that emissions from the combustion of biomass fuels were higher than emissions from the combustion of coal.16 Concentrations of PAHs and PCBs may increase several times because of inefficient combustion.17 While the latter is aimed to be controlled by the constant tuning of combustion devices, new biomass/fuel products constantly appear in the market and their fuel properties are not wellresearched. These fuels have a different morphology (logs/ pellets/chips/grains), composition (various wood or other biomass species), elemental content, moisture, ash content, and calorific value. Achieving optimal combustion conditions with such an unknown/new type of biomass fuel sometimes is a difficult task. Not all burners on the market are capable in efficiently burning these new types of fuel, because some of them are characterized by a high ash, chlorine, sulfur, nitrogen, or moisture content, which leads to inefficient combustion and high emissions. This issue has gained substantial attention from researchers worldwide. For example, Sippula et al. investigated emissions from the combustion of Finnish wood species (birch, spruce, pine, alder, and willow) in a small-scale burner, as well as emissions from mixtures of the heavy fuel oil and wood.18
1. INTRODUCTION Europe is turning its energy production toward renewable energy.1 Biomass is considered as one of the most important and promising fuels worldwide, mostly because of its renewability and almost-neutral balance of CO2 emissions.2,3 Because of the increased consumption, fuel producers introduce various types of new biomass fuels.4 At the same time, numerous studies have shown emissions of various gasphase and particle-phase pollutants such as fine and coarse particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), monocyclic aromatic compounds, such as benzene, toluene, ethylbenzene, xylenes (collectively referred to as BTEX), dioxins, and furans during combustion of biomass fuels.5−9 Up to 80% of total PM emission during the winter in Europe is carbonaceous and mostly comes from the combustion of biomass for heating purposes.10 Emissions from the biomass combustion have a critical effect on an ambient air quality in settlements utilizing biomass as the primary fuel for the energy production.11 Moreover, these pollutants penetrate indoor and affect indoor air quality.12,13 The decreased indoor air quality causes adverse health effects to human beings.14 The production of the biomass fuel will continue to grow, thus the pollution arising during the combustion process must be assessed and managed. The processes of the formation of pollutants during the biomass combustion have been well-researched. The type of biomass, burner characteristics, and combustion conditions © 2014 American Chemical Society
Received: February 18, 2014 Revised: July 15, 2014 Published: July 15, 2014 5057
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Figure 1. Images of the tested biomass fuels.
Table 1. Elemental Composition and Main Characteristics of the Researched Fuels biomass fuel type
C, %w/w
O, %w/w
H, %w/w
N, %w/w
S, %w/w
Cl, %w/w
humidity content, %w/w
ash content, %w/w
calorific value, MJ/kg
fuel feed rate, kg/h
wood pellets sunflower stalk pellets straw pellets buckwheat shells corn stalk pellets grain screenings sewage sludge pellets
46.15 42.38
40.97 38.66
5.30 4.94
0.046 0.665
0.011 0.117
0.005 0.041
7.10 10.10
0.42 3.10
16.9 17.0
1.68 ± 0.28 1.38 ± 0.32
39.15 49.05
36.40 38.10
5.17 5.69
0.798 0.560
0.053 0.025
0.025 0.022
12.30 5.10
6.10 1.45
14.1 15.9
1.58 ± 0.23 1.02 ± 0.08
45.26
38.35
5.92
0.194
0.028
0.010
7.54
2.70
15.6
1.80 ± 0.26
42.07
36.21
4.37
2.450
0.155
0.056
10.90
3.80
14.8
1.20 ± 0.06
33.21
12.85
3.06
2.575
0.363
0.049
12.70
35.20
11.9
1.08 ± 0.17
analytical method
EN 15104
EN 15104
EN 15104
ASTM D516-41
EN 14774-1
EN 14775
EN 14918
content, calorific value) of the above listed fuels are presented in Table 1. 2.2. Combustion Setup and Conditions. The laboratory setup for the experimental modeling of the combustion of solid fuels is schematically presented in Figure 2. A commercial 13 kW pellet boiler (KSM-175-13-U, UAB Kalvis, Lithuania) was utilized for the experiment. Such type of burners is widely spread in central and northern Europe. The pelletized fuel was continuously supplied to the combustion chamber from a storage tank by an auger transporter. The ash was removed from the combustion chamber by means of a mechanized rake, followed by an auger transporter. Both the supply rate of fuels and the frequency of ash removal were regulated for each tested type of fuels. The air was supplied to the combustion chamber by a controlled blower. All of the above parameters were controlled by a user interface. The produced heat energy was extracted to the circulating water in a heat exchanger. The temperatures of heat transfer medium and exhaust gases were recorded by temperature probes. Exhaust gases were directed to a thermally insulated exhaust duct (a diameter of 150 mm). The exhaust gas velocity was monitored by an anemometer (Portable Thermal Anemometer Series 2440, Kurz Instruments, Inc., USA), and the flow rate was calculated accordingly.
Several reports were published on the emissions from the combustion of Portuguese wood and agriculture residues.15,19−21 The objective of this study was to characterize emissions from the combustion of several fuels derived from agricultural residues in a small-scale, modern pellet burner. Emissions of fine particles, PAHs, BTEXs, and gaseous pollutants were investigated during an optimal combustion regime.
2. METHODOLOGY 2.1. Fuels. The following seven fuels were evaluated in the experiments: wood pellets, sunflower stalk pellets, straw pellets, buckwheat shells, corn stalk pellets, wheat grain screening residues, and sewage sludge pellets. These agricultural-residuederived fuels represent some of local ones produced by Lithuanian agricultural/food processing and environmental companies, but they may represent similar fuels for a vast area of central−eastern Europe. Municipal sewage sludge was researched as an alternative to biomass fuels, although its application for energy-generating purposes must be controlled. Pictures of biomass fuel samples are presented in Figure 1, while the morphology and fuel feed rate are described in Table 1. The characteristics (elemental composition, moisture and ash 5058
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was diluted using the dilution system of two-stage heated ejector diluters (DI-1000, Dekati Ltd., Finland). The dilution factor (∼50 times) was determined by measuring the CO2 concentration before and after the dilution. The dilution air was treated with silica gel, activated carbon and HEPA filter in order to remove moisture and pollutants (PM, hydrocarbons). Aerosol samples (duplicate for each fuel) were collected on aluminum foil substrates (25 mm). In addition, MOUDI 110 equipment (MSP Corp., USA), at a flow rate of 30 L/min, and PM10 impactors (Dekati, Inc., Finland), at a flow rate of 10 L/ min, were utilized to collect size-segregated aerosol samples for gravimetrical and chemical analyses. The total suspended particulate matter (TSP) fraction of the aerosol was collected on quartz fiber filters (Pall Corp., USA), by using filter cassettes, at an air flow rate of 2 L/min. The gaseous-phase PAHs were sampled using a XAD-2 sorbent (SKC, Inc., USA), at an air flow rate of 2 L/min. BTEX samples were taken using an Anasorb CSC sorbent (SKC, Inc., USA), at a same flow rate. The ELPI+ device was sampling continuously during the experiment, while the PM10 impactor, MOUDI, TSP, gaseous PAH, and BTEX samples were taken at a steady state for a shorter period of time. All latter devices (except of the ELPI+) were sampling from the first stage of the diluter, thus utilizing a dilution factor of 7. It must be noted that the particle size distribution of the aerosol sampled after the first heated stage of the diluter might have been affected by the condensation of water vapor on particles in the impactor. Most of the particles larger than 10 μm (fly ash) were lost during the process of the ejector dilution; thus, our results represent only the fine fraction of the aerosol. Before the sampling, the aluminum foils were heated at 300 °C for 6 h and left to cool in the thermostated area with an adjustable relative humidity (50%) and a temperature of 20 ± 1 °C for 24 h. Before and after sampling, the substrates were gravimetrically analyzed using a microbalance (Model MXA-5, Radwag Wagi Elektroniczne, Poland). After the gravimetrical analysis, the substrates were stored in a freezer at −20 °C until the chemical analysis, together with the samples of gaseous PAHs and BTEXs. The gaseous pollutants in the combustion emissions were measured by a real-time instrument (Model IMR 2000, IMR Environmental Equipment, Inc., Germany). The following flue gases were measured: O2, CO, CO2, NOx, and SO2. In addition, the parameters of temperature and λ also were measured. The data were recorded every 5 s during the 2-h experiment and stored in a computer. The hydrogen chloride (HCl) concentration was measured using sorbent tubes (Drägerwerk AG & Co, Germany), operating based on the change of the sorbent color. The analysis of HCl has been carried out only for the combustion of biomass containing chlorides (Table 1). 2.4. Analytical Methods. Concentrations of solid-phase PAHs were analyzed in PM10 (particulate matter, having and aerodynamic diameter smaller than 10 μm) samples and in various fractions of ELPI+ and MOUDI. In order to obtain a sufficient sample mass for chemical analyses, PM fractions of ELPI+ were polled to three groups, generally representing nucleation, accumulation, and coarse modes of the aerosol: Fraction 1 (0.017−0.4 μm); Fraction 2 (0.4−1 μm); Fraction 3 (1−10 μm). MOUDI samples were grouped as Fraction 1 (0.056−0.18 μm), Fraction 2 (0.18−1 μm), and Fraction 3 (1− 18 μm). The extraction and chemical analysis was conducted following methods similar to our earlier studies.13,22 PM
Figure 2. Experimental setup for testing the emissions from the combustion of biomass fuels.
Before starting each experiment, the heat transfer medium (water) was preheated to 70 °C by an additional natural gas boiler, with the objective to reach optimal combustion conditions quicker and avoid condensation on the surfaces of the boiler during the startup phase. The combustion of pellets was initiated by using a propane gas torch. After reaching the intended temperature, the supply of fuel and air was initiated. The stable combustion regime was reached by adjusting the supply of fuels and air, and the removal of ash. The optimal combustion conditions were set by measuring the temperature and the CO concentration in the exhaust gas. The conditions of combustion (temperature of water in the boiler, flue gas temperature, and a coefficient of an excess air (λ)) were kept stable during the measurements. Every measurement cycle was repeated two times with the same combustion conditions. After each measurement cycle, the boiler was cleaned in order to remove all residuals and particles. Ash from the combustion zone was removed by an automatic rake every 60 min. During the removal, pollutant concentrations in the exhaust gas increased several times. This period of sampling was not taken into account, and all of the data was eliminated. The temperature of water in the boiler during optimal combustion was 65 °C. Results include data from 14 measurement cycles using the seven biomass fuels. Each measurement cycle took ∼2 h. 2.3. Sampling of Pollutants. The gaseous pollutant measurement probe was installed to the exhaust duct 0.3 m above the boiler, while particle sampling probes were positioned ∼1 m downstream. The real time size-segregated particle concentration and samples were taken using the Electrical Low Pressure Impactor (ELPI+, Dekati, Inc., Finland), at a flow rate of 10 L/min. The combustion aerosol 5059
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Table 2. Emission Factors for TSP, PM10, PM2.5, and PM1 Fractions PM Emissions, g/kg PM fraction TSPa >PM10 + PM10b PM10 PM2.5 PM1 morphology of biomass fuelsc
wood pellets 0.33 0.29 0.28 0.28 0.28
± ± ± ± ±
0.07 0.05 0.05 0.05 0.06
sunflower stalk pellets 4.01 3.42 3.41 3.41 3.34
± ± ± ± ±
1.05 0.89 0.81 0.85 0.72
straw pellets 1.15 0.96 0.96 0.95 0.93
± ± ± ± ±
0.24 0.21 0.24 0.25 0,22
buckwheat shells 1.95 1.91 1.91 1.90 1.74
± ± ± ± ±
0.21 0.21 0.24 0.26 0.30
cylinder; D = 6 mm; cylinder; D = 8 mm; cylinder; D = 8 mm; shell; D = 5 L = 10−15 mm L = 10−20 mm L = 10−20 mm mm; L = 4 mm
corn stalk pellets 0.88 0.80 0.80 0.80 0.77
± ± ± ± ±
0.31 0.19 0.19 0.22 0.23
grain screenings 5.16 4.91 4.90 4.87 4.39
± ± ± ± ±
1.40 0.98 1.11 0.80 1.15
cylinder; D = 6 mm; irregular; D = L = 10−15 mm 1−2 mm
sewage sludge pellets 5.45 5.33 5.31 5.23 5.03
± ± ± ± ±
2.01 1.80 1.56 1.93 1.2
irregular; D = 5 mm; L = 8−10 mm
a
Represents TSP fraction sampled by a cassette. bRepresents TSP fraction sampled by the PM10 impactor. cD = diameter of pellet; L = length of pellet.
analyzed for every 10 samples. The amount of any given compound in the analyzed blank samples did not exceed the lowest point of the calibration curve. The target PAHs standard stock solution was used for the linear response measurement. The calibration curve of target PAHs compounds consisted of 10, 5, 2.5, 1.25, 0.65, 0.31, 0.16 ng/μL standard solutions. Solutions of deuterated PAH compounds were used as the internal standards. A standard solution of octachloronaphthalene was used as a recovery standard for the measurement of recovery coefficients of internal standard solution compounds in real samples. The extraction procedures of PAHs were validated in a series of recovery experiments. Recoveries of individual PAHs during analytical method validation procedure were from 56.4% to 98.7%. The linearity of the BTEX concentration response was checked using the calibration curve prepared from 10, 5, 2.5, 1.25, and 0.625 μg/mL standard solutions. As an internal standard for BTEX, 2-fluorotoluene was used. Laboratory blanks, field blanks, and solvent samples did not show contamination by target compounds. The detection limits of each target compound were estimated as three standard deviations of a base (noise) line. The detection limit range was 1.5−21 ng per PAH sample and 1.2−2.6 μg per BTEX sample. 2.6. Data Analysis. Results of analyses were statistically processed using SPSS 12 (IBM Corp., USA) and Origin 9 (OriginLab Corp., USA) software. Descriptive statistics were used for the representation of PM, PAH, VOC gaseous pollutant levels. Considering a small sample of fuels (n = 7), the Spearman ranking correlation coefficient (rs) was used to determine the relationship between fuel properties and emissions. Emission factors were presented as mass of pollutant per one kilogram of dry fuel (g, mg, or μg/kg) and were normalized to an oxygen concentration of 6%. Particle number emissions were represented by the number of particles per one kilogram of a fuel (#/kg). The amount of pollutant per mass of a fuel was selected as a unit of representation because, in a small-scale energy production, the amount of fuel is a more tangible unit, compared to the amount of energy.
substrates from ELPI+, MOUDI, and PM10 impactor equipment were extracted in an ultrasonic bath for 10 min with 4 mL of dichloromethane. An internal standard (deuterated PAHs, 50 ng per sample) was added to the samples before the extraction. Samples were concentrated to 0.5 mL by a nitrogen flow concentrator. The samples were cleaned in a column with silica gel and anhydrous sodium sulfate, utilizing a dichloromethane and hexane mixture (50/50 %w) as an eluent to 4 mL of volume. After an additional concentration by a nitrogen flow concentrator to 0.2 mL, the liquid was mixed with octachloronaphthalene (50 ng per sample) and moved to a chromatographic vial with a micro insert. The GC/MS system (Model GCMS-QP2010 Ultra, Shimadzu Corp., Japan) with a capillary column (Model Rxi-5 ms, Restek, Inc., USA) operating according to a single ion monitoring method was used for the quantitative analysis of the extracted PAHs. The following conditions were used: injection volume, 2 μL; splitless mode; injector temperature, 250 °C; and carrier gas (helium) flow, 0.58 mL/min. The temperature program was set to start at 50 °C for 3 min, then increase at a rate of 10 °C/min to 300 °C and remain at that temperature for 10 min. The MS scanning was performed from 30 m/z to 450 m/z. BTEX samples were extracted by a solvent extraction method. Vials with samples were placed in the water bath with ice; then, 2 mL of CS2 (GC low benzene content, Sigma− Aldrich Co., USA) and 10 μL of internal standard solution (1 ng/μL of 2-fluorotoluene) were added into the sorbent. Samples were shaken in a shaker for 30 min. After the extraction, samples from a vial were drawn by a Luer Lock glass syringe with a fiberglass filter and placed into a 1.5-mL chromatography vial. The GC/MS system (Model GCMSQP2010 Ultra, Shimadzu Corp., Japan) with a capillary column (Model Rxi-5 ms, Restek, Inc., USA) was used for the quantitative analysis of BTEX. The following conditions were used: injection volume, 2 μL; splitless mode; injector temperature, 250 °C; and helium flow, 0.58 mL/min. The temperature program was set to start at 30 °C and remain at that temperature for 1 min, then increase at a rate of 7 °C/min to 205 °C, remain at that temperature for 2 min, then increase at a rate of 10 °C/min to 305 °C and remain at that temperature for 10 min. The MS scanning was performed from 30 m/z to 500 m/z. 2.5. Quality Control. The sampling, sample preparation, and analysis were performed according to good laboratory management practices.23 The gas analyzer was calibrated by a manufacturer and checked by the metrological center. One sampling blank sample and one laboratory blank sample were
3. RESULTS AND DISCUSSION 3.1. Characterization of Fuels. The calorific value of the tested biomass fuels ranged from 14.1 MJ/kg to 17.0 MJ/kg. The sewage sludge pellets had a lower calorific value than other fuels (11.9 MJ/kg), but it was sufficient for self-supporting combustion. The moisture content in the researched fuels ranged from 5.1% to 12.7% (i.e., within the range where fuels 5060
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Figure 3. Particle size distributions (PSDs) based on the number and mass concentrations of the combustion aerosol.
may burn efficiently).2 The straw and sewage sludge pellets had the lowest calorific values (associated with a relatively high moisture content, low carbon content, and relatively high ash content). Characteristics of sewage sludge were similar to these found by Pettersson et al.24 Sunflower stalk pellets, together with wood pellets, had the highest calorific values. The latter appeared to be the “cleanest” fuel, with a low ash content (0.42%), low sulfur content (0.011%), and low chlorine content (0.005%). Among biomass fuels, wheat grain screening residues contained the highest concentrations of nitrogen, sulfur, and chlorine. As expected, the sewage sludge was the lowest-quality fuel, characterized by a low calorific value (11.9 MJ/kg) and a high ash content (35.20%, which was almost 10 times higher than that of the biomass fuels). The properties of biomass fuels were similar to those reviewed by Vassilev et al.25 3.2. Emission Factors and Size Distribution of PM. The emissions of size-segregated particulate matter (TSP, PM10, PM2.5, and PM1), as measured during the combustion of tested biomass fuels, are presented in Table 2. Most of the measured PM mass in the exhaust gases was accumulated in the submicrometer particle size (PM1/TSP > 0.8 in the case of all fuels). This ratio is comparable to those obtained by Fernandes et al.21 and McDonald et al.26 Note that fly ash particles (>10 μm) were not adequately included to the TSP fraction, because of the loss of larger particles in the ejector diluter. More-detailed analysis of the particle size distributions is presented later in this section. The TSP emissions ranged from 0.33 g/kg (wood) to 5.45 g/kg (sludge), and correspondingly 0.28−5.31 g/kg for PM10,
0.28−5.23 g/kg for PM2.5, and 0.28−5.03 g/kg for PM1. The variation in PM emissions was relatively high among the fuels. Based on PM emission factors, the tested fuels may be classified into two groups. The first group (sunflower stalk pellets, grain screenings, and sewage sludge pellets) were characterized by relatively high emissions (>3.00 g/kg) of PM in all fractions. This group of fuels was characterized by a high sulfur, chlorine, and ash content. The correlation analysis supports the relationship between PM1 concentration and sulfur (rs = 0.89), chlorine (rs = 0.93), and ash content (rs = 0.71). The second group (wood pellets, straw pellets, buckwheat shells, and corn stalk pellets) were characterized by a relatively low PM emission (
