Influence of stove type and cooking pot temperature on particulate matter emissions from biomass cook stoves

Energy for Sustainable Development 16 (2012) 448–455

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Energy for Sustainable Development

Influence of stove type and cooking pot temperature on particulate matter emissions from biomass cook stoves Christian L'Orange a,⁎, John Volckens b, Morgan DeFoort a a b

Engines and Energy Conversion Laboratory, Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80524, USA Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, CO 80524, USA

a r t i c l e

i n f o

Article history: Received 7 November 2011 Revised 28 August 2012 Accepted 28 August 2012 Available online 18 September 2012 Keywords: Cook stoves Particulate matter Indoor air pollution Biomass Emissions

a b s t r a c t A large fraction of the world's population burns biomass fuels indoors using primitive cook stoves. The emissions from this form of inefficient combustion are a significant contributor to human morbidity and mortality worldwide. Although many studies have examined the mass of particulate matter released from such cook stoves, a few studies have characterized the size distribution of emitted aerosol. The objective of this research was to investigate the influence of stove design, stove temperature, and cooking pot temperature on the mass and size of particles emitted by biomass cook stoves. Results indicate that the temperature of the cooking pot has a substantial impact on emitted particles. Correlations were also found between particle production rate, combustion efficiency, and stove firepower. The concept of a lower bound on particle emissions due to inorganic materials present within biomass fuels is discussed. International Energy Initiative. Published by Elsevier Inc.

Introduction Biomass fuels such as wood are a potential source of renewable, carbon-neutral energy. Approximately 10% of global energy demand is supplied by wood and over 3 billion people burn some form of biomass fuel daily for energy (Bruce et al., 2000). Biomass combustion, however, is typically inefficient and incomplete, resulting in the release of toxic gaseous and particulate matter (PM) species. As a result, a significant portion of the global burden of disease is attributed to biomass combustion emissions, particularly for indoor combustion using cook stoves. Human exposure to biomass combustion by-products has been associated with adverse effects on the respiratory, cardiovascular, immune, and nervous systems. Chronic exposure to PM from biomass combustion has been linked to an increased risk of respiratory illness and infection, lung cancer, and permanent decreases in lung function (Khalequzzaman et al., 2007). Cardiovascular effects include increased risk of myocardial infarction, irregular heart rate, and increased blood viscosity (Lippmann et al., 2003). Inhaled PM can also suppress the immune system, hindering a person's ability to fight off infection Abbreviations: PM, Particulate Matter; HEPA, High Efficiency Particulate Air; CO, Carbon Monoxide; FT-IR, Fourier Transform Infrared; LOQ, Limit of Quantification; LOD, Limit of Detection; MCE, Modified Combustion Efficiency; EF, Emission Factor. ⁎ Corresponding author at: 430 N. College Ave, Engines and Energy Conversion Laboratory, Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80524, USA. Tel.: +1 720 810 2215; fax: +1 970 491 4799. E-mail addresses: [email protected] (C. L'Orange), [email protected] (J. Volckens), [email protected] (M. DeFoort). 0973-0826/$ – see front matter. International Energy Initiative. Published by Elsevier Inc. http://dx.doi.org/10.1016/j.esd.2012.08.008

(Lippmann et al., 2003; Naeher et al., 2007), with a commensurate reduction in thymocte and T cell production (Albright and Goldstein, 1996). Long-term PM exposure may also increase the risk of Alzheimer's disease, brain inflammation (Calderon-Garciduenas et al., 2007), and still birth (Smith, 1993). The health effects of biomass combustion aerosols likely depend on particle size and composition, as well as the duration and intensity of exposure (Davidson et al., 2005). Particle deposition in the human respiratory tract depends strongly on particle size. For example, larger particles (dp > 10 μm) tend to deposit in the nasopharyngeal airways, whereas smaller particles (dp b 1 μm) tend to penetrate into the tracheobronchial and alveolar regions of the lung (Hinds, 1999). Ultrafine PM (defined as particulate matter with a diameter of less than 0.1 μm) has been shown to penetrate the lung-blood barrier and may eventually reach other organ systems (Lippmann et al., 2003). Most combustion aerosols consist of a wide distribution of particle sizes and an important distinction to note is that particle volume (and mass) scales with particle diameter cubed. For example, the mass of one 3 μm particle is equal to one million 0.03 μm particles of equal shape and density. Consequently, two stoves may emit the same levels of PM mass but with substantially different particle size and number concentrations. Because regional lung particle deposition depends on particle size, the health implications of inhaled PM may change according to the size distribution and concentration of emissions. Particulate matter formation during fuel combustion depends on many factors, including flame temperature, composition and concentration of combustion reactants, and residence time within the reaction zone (Shurupov, 2000); each of these factors can vary according to the specific design of the stove. Although PM formation from combustion is

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not fully understood it is suspected that the process involves both nucleation and condensation mechanisms (Haynes and Wagner, 1981; Mansurov, 2005; Tesner, 1979). Nucleation is believed to begin with the formation of benzene rings through the rearrangement of fragmented chemical chains released during pyrolysis (Fitzpatrick et al., 2008). These rings grow into graphene-like sheets, ultimately forming stable nuclei. Cooling combustion gases then condense onto these nuclei, increasing their size. Combustion-related PM can also be destroyed through oxidation processes in and around the combustion reaction zone (Haynes and Wagner, 1981). For carbonaceous species like wood, oxidation ultimately results in the conversion of carbon-based PM into CO2. The rate of oxidation is dependent on ambient gas composition and temperature. As temperature decreases, the rate of oxidation slows, eventually halting this reaction (Stanmore et al., 2001). Ideally, nucleated particles would completely oxidize during combustion and no PM would leave the reaction zone, but this rarely occurs when burning biomass in a cook stove (Haynes and Wagner, 1981). The size of particles formed during combustion is dependent on the time spent in the formation and oxidation zones. The size of a biomass exhaust particle can span a range from less than 0.01 μm to greater than 100 μm. However, the majority of biomass combustion aerosol is typically smaller than 1 μm in diameter (Naeher et al., 2007) with the mass distribution centered on 0.2 μm (Dasch, 1982). Studies of biomass cook stoves have reported mass median diameters as high as 0.78 μm (Venkataraman and Rau, 2001) although 0.2–0.4 μm is more typical (Li et al., 2007; Saksena et al., 1992; Venkataraman and Rau, 2001). These particles consist of fractal, chain-like agglomerates that develop from rapid coagulation of many primary particles (Chakrabarty et al., 2006). Emitted size distributions are often characterized as log-normal, with geometric standard deviations ranging from 1.7 to 2.7 (Tesfaigzi et al., 2002; Weinert et al., 2003). Abrasive wear of a stove will also produce particles larger than 1 μm, but these particles contribute only a small fraction to the total emissions. Previous research has characterized mass emissions from biomass stoves (Armendariz-Arnez et al., 2010; Park and Lee, 2003; Reid et al., 1986; Roden and Bond, 2006; Saksena et al., 1992; Venkataraman and Rau, 2001) with little work investigating the factors affecting the size distribution of emitted PM (Li et al., 2007). Li et al. examined particle sizes emitted by cook stoves; however, they focused on the impact of using different biomass fuels instead of the effects of stove design and operating parameters. In this work, we hypothesized that both stove wall and cooking pot temperatures affect the concentration and size of particles emitted from biomass cooking stoves. To evaluate these hypotheses, we measured particle size distributions and mass emission rates from ceramic, metal, water cooled, and gasifier stoves operating under various conditions.

Particle measurements were taken with a cascade impactor and gravimetric filter samplers operating in parallel. The cascade impactor (NanoMOUDI 125-R, MSP Corp., Shoreview, MN, USA) segregated particles by inertial impaction into 13 size ranges from 0.01 to 10 μm in a aerodynamic diameter. A cyclone separator (URG-2000-30ENB, URG Corp., Chapel Hill, NC, USA) and filter pack (URG-2000-30FDT) were used to sample particles with 10 μm in diameter and smaller (PM10) for subsequent gravimetric analysis. Each system was equipped with a vacuum pump and flow controller. The PM10 filter sampler was operated at a rate of 28.3 L/min and controlled by a mass flow controller. This flow rate was chosen in order to achieve a cut point of 10 μm from the URG cyclone. The cascade impactor was operated at 10 L/min, as per manufacturer's instructions, with flow regulated by a critical orifice. A Fourier Transform Infrared Spectrometer (FT-IR) and a Testo electrochemical analyzer were used to monitor carbon monoxide (CO) emissions during each test as an indicator of stove combustion efficiency. Systematic error can result from the loss of particles within the sampling system. Large particles tend to impact in pipe bends, whereas, small particles may be lost to the walls via diffusion. Therefore, sampling and transport lines were kept short and straight whenever possible. The percentage of particles expected to be lost in the sampling line leading to the cascade impactor, shown in Fig. 2, was estimated using equations given by Hinds (1999). Both diffusion and impaction losses were modeled. The cyclone sampling line was short with no bends to minimize losses. The masses reported herein are measured values; corrections were not made based on predicted particle loss. Substrates for gravimetric analyses were weighed to the nearest microgram with an analytical microbalance (Mettler Toledo MX5 Columbus, OH, USA). Filter substrates were PTFE, 47 mm in diameter (Whatman #7592-104, Maidstone, Kent, UK); cascade impactor substrates were aluminum, 47 mm in diameter (MSP #0100-96-0573A-X, Shoreview, MN, USA). Analytical limits of quantification (LOQ) and detection (LOD) were determined from Eq. (1) by weighing multiple test blanks over multiple days (MacDougall and Crummett, 1980). Filter and impactor substrates with mass gains below the LOQ were excluded from analysis. The LOD and LOQ results are presented in Table 1.

Materials and methods

Tests were conducted to determine how emitted particle size distributions change with stove wall material, stove wall temperature, and stove type. Tests were also conducted to determine how particle size distributions change with the temperature of the cooking task (i.e., the temperature of the water pot resting on the stove surface). All tests were conducted on single-pot cook stoves burning 7% moisture content (dry basis) Douglas fir, with the exception of the gasifier stove, which burned commercially-produced pellets, as per manufacturer's recommendation. The experimental test matrix is shown in Table 2; pictures of the test stoves are shown in Fig. 3. Except for the gasifier, all of the stoves tested were natural draft. The geometry of a natural draft stove is designed so that the majority of air movement and mixing is a result of thermal buoyancy of hot exhaust gases. In a natural draft stove pyrolysis and combustion occur in the same region. Gasifier stoves can be either buoyancy-driven or forced convection (i.e., fan-driven) but in these stoves pyrolysis and combustion occur in different locations. A more detailed explanation of gasifier theory and operation has been compiled by the Biomass Energy Foundation (Reed and Larson, 1996).

The objectives of this research were to characterize particle size distributions emitted from several types of biomass cook stoves and to determine how stove operating conditions, such as stove wall and cooking pot temperature, affect the size and concentration of emitted particles. Furthermore, because combustion efficiency has been linked to PM mass emissions (McMeeking, 2008), we evaluated the correlation between combustion efficiency, firepower, and PM emission rates. Experimental system All tests were conducted in a 1.2 m × 1.2 m × 4.3 m (L× W× H) fume hood at the Engines and Energy Conversion Laboratory of Colorado State University. A constant-volume displacement pump moved air through the hood at 6 m 3/min with inlet air passing through high efficiency particulate air (HEPA) filters to remove background aerosol. A schematic of the fume hood and sampling system is shown in Fig. 1.

LOD ¼ 3  sðmb Þ; LOQ ¼ 10  sðmb Þ

ð1Þ

Where: s(mb) is the standard deviation of repeated analyses of test blanks. Stove type and cooking pot temperature tests

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Fig. 1. Schematic of the cook stove test hood.

Size distributions Stove wall material comparison Two stoves of similar design but constructed of different materials (metal, ceramic) were tested to develop reference emission levels. These stoves were selected due to their similarity to improved stoves

currently sold in the developing world. Therefore, emissions from these stoves will serve as baseline values for subsequent comparisons. The emissions from traditional stoves may be different than the baseline stoves tested here and will be compared to values available from previous studies. Stove wall temperatures The temperature of the stove wall likely affects particle emissions by changing the combustion region and/or inducing thermophoresis. Lowering the overall temperature of the stove is expected to inhibit PM oxidation resulting in increased PM mass emissions. Thermophoresis occurs due to thermal gradients between the stove wall and combustion zone, resulting in a net diffusion of particles towards the colder surface. Small particles are preferentially affected by thermophoresis (Hinds, 1999) shifting the emitted size distribution towards larger diameters. A double-walled metal stove was used to determine the effect of wall temperature on PM size distribution. The use of a double walled stove allowed for the circulation of cold water between the walls to maintain a constant surface temperature during each test. The double-walled stove was designed and built at the Engines and Energy Conversion Table 1 Limits of quantification for gravimetric filter and impactor substrates.

Fig. 2. Estimated particle losses in the cascade impactor sampling line.

Filter Impactor substrate

LOD (μg)

LOQ (μg)

15 17

51 55

C. L'Orange et al. / Energy for Sustainable Development 16 (2012) 448–455 Table 2 Experimental test matrix. Experimental variable

Level of variation

Replicates per condition

Stove wall materiala Stove wall temperature Stove type Cooking pot temperature

Metal, ceramic Insulated, water-cooled Natural draft, gasifier Ice cold pot, full boil pot

2 2 2 2

a

451

to those of the gas stream or stove wall, they may still exert an effect on the emitted particle size distribution. Tests were conducted with a cooking pot kept at 0 °C then repeated with the pot at a full boil (95.5 °C in Fort Collins, CO). The zero degree tests were conducted using a cook pot filled with ice water, which was periodically replaced as needed. All pot temperature tests were conducted using the baseline metal stove.

Metal and ceramic reference stoves, selected as typical “improved” stove models.

Laboratory at Colorado State University. The stove follows basic rocket-elbow dimensions but by controlling the rate and/or temperature of the water flowing through the stove, the interior wall temperature of the stove can be set. This design gives the ability to regulate an aspect of the stove which is typically transient throughout a test. Cooking pot temperature Tests were conducted to determine how cooking pot temperatures affect particle emissions. Pot temperatures change temporally and with cooking habits and may alter particle emissions. Although variations in the temperature of a cooking pot are small compared

Gasifier tests with wood pellets A gasifier stove was tested to examine how an alternative combustion process might affect particle emissions. The pyrolysis and ignition zones are separated in a gasifier, which may subsequently affect PM emissions. Slight modifications were made to the test procedure to accommodate the gasifier stove. These variations included a kerosene ignition step (instead of directly lighting the wood) and placing the cooking pot on the stove after ignition. The impact of the ignition step was not investigated but should be explored in the future work as the kerosene was estimated to burn for approximately 20% of the total test duration. However, the kerosene accounted for less than 5% of the fuel consumed on a mass basis. Fuel to the gasifier was batch-fed as

Fig. 3. Stoves tested for particle size distribution emissions: a) baseline ceramic stove, b) baseline metal stove, c) metal, double-walled, water-cooled stove, and d) gasifier stove.

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opposed to the continuous feeding possible with other stove types. Emissions from the Gasifier were compared to the metal-wall reference stove, described in Stove wall material comparison. Total PM emissions and combustion efficiency Modest correlations have been found between measures of biomass combustion efficiency and PM emission factor (PM emission rate/fuel consumption rate). Therefore, we sought to examine whether stoves can achieve similar levels of combustion efficiency while producing variable levels of PM, depending on the operating condition. For each test, modified combustion efficiency (MCE) was used as a measure of combustion completeness. The MCE for a given stove test is calculated using Eq. (2) where each term represents the molar concentration of a compound emitted during the combustion process. MCE ¼ ðΔCO2 Þ=½ðΔCO2 Þ þ ðΔCOÞ

ð2Þ

The delta accounts for the increase in concentration of each compound above that found in ambient, background air. Nine different stoves were tested for combustion efficiency: the four stoves described in Size distributions, three wood-burning concept stoves developed by Envirofit International (Fort Collins, CO, USA), a commercially-available stove (S2100, Envirofit International Fort Collins CO, USA), and the Yue-Xiang stove (Zhejiang Shengzhou Stove Works Shengzhou, China). A modified version of the variable water temperature procedure presented above was used. Tests were conducted at two levels of firepower: ‘high power’ and ‘low power’. The high power test heated 5 kg of water from 15 °C to 90 °C. The low power tests maintained 5 kg of water at 90 °C for 45 min. Both phases require the stove to go through phases of start-up and full active flaming. No data was taken when the stoves were smoldering or operating with only a charcoal bed, only active burning with well developed flames. Fuel consumption rates were recorded for each test to obtain a measure of stove emission factor. Results and discussion Stove comparisons Plots of log-normalized particle mass distributions emitted by the baseline ceramic and metal cook stoves are shown in Figs. 4a and b, respectively. The average mass of PM10 emitted by each stove during each test, MT, was determined from the sampled PM10 mass and the ratio of total volumetric flow of the exhaust (6000 L/min) to the volumetric flow of the sampler (28.3 L/min). Total PM10 emissions were lower for the metal stove, although not statistically significant (p>0.05). Emitted size distributions between stoves were also similar, indicating that stove wall material does not directly impact the size of particles emitted. The distributions had peaks and distributions similar to those found by Li et al. (2007). Particulate matter size distribution was not found for a three stone fire but the peaks were in a similar range to those found by Hays et al. (2002) from wood burning in an open fire. A table summarizing the results from each of the tests can be found in Table 3. Effect of stove wall temperature Lowering the stove wall temperatures only had a slight impact on particle size distribution or mass emission rate, Fig. 5. A metal stove wall maintained at 0 °C slightly increased the fraction of large particles emitted and produced slightly more PM10 compared to the baseline metal stove, although these differences were not statistically significant (p>0.05). Gasifier Gasifier stoves are often thought to produce less PM than traditional stoves (Ravindranath and Balachandra, 2009; Smith and Haigler,

Fig. 4. Particle size distributions, by mass, for Ceramic (a) and metal (b) biomass cook stoves heating water from 15 °C to 90 °C. Solid line represents a lognormal fit to the distribution. Error bars represent one standard deviation for each impactor stage. Size ranges for each impactor stage are available in URG Corp documentation. MT is the average mass of PM10 emitted per test±one standard deviation. MMD and σg are the mass median diameter and geometric standard deviation of the size distribution, respectively.

2008), but this was not evident with the gasifier stove tested here. Particles produced from the gasifier stove spanned a considerably wider size range, a geometric standard deviation of 3.6 as compared to 1.7–1.8, and had greater PM10 emissions when compared to baseline stoves. The gasifier stove was found to have a statistically significant (p b 0.05) difference in both median diameter and distribution from both baseline stoves. The gasifier stove also had larger variability in measured emission rates, as a function of particle size. However, the bi-modal distribution of PM emissions appears to be consistent between repetitions (Fig. 6). Wood pellets typically contain a large percentage of inorganic material due to wood bark, which may account for the greater PM10 emission rate. Following ASTM method D1102-84, the ash content of the Douglas fir used in the traditional stoves and the wood pellets used in the gasifier was determined to be 0.39% ± 0.13% and 1.53% ± 0.22% respectively (based on three samples of each fuel type). Hasler and Nussbaumer (1998) have found that wood and bark mixes produce bimodal distributions with the second maximum around 5 μm, similar to the results shown in Fig. 6. The distribution shown in Fig. 6 can be modeled as two distinct distributions. The first distribution has a MMD of 0.21 μm and σg of 1.8, which is very similar to the baseline ceramic, baseline metal, and water cooled stoves. The second, larger distribution has a MMD of 0.8 μm and σg of 1.9. Additional work burning Douglas fir wood chips in the gasifier stove would help clarify the impact of the wood pellets but was beyond the resources available for this study. Table 3 Summary of experimental results. Stove

Mass emitted [MT] (mg)

Mass median diameter [MMD] (μm)

Geometric standard deviation [σg]

Ceramic Metal Water-cooled Gasifier

1395 ± 251 933 ± 134 1586 ± 400 2319 ± 676

0.23 0.22 0.25 0.33

1.7 1.8 1.9 3.6

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Fig. 5. Particle size distribution, by mass, for a double-wall, water-cooled biomass cook stove heating water from 15 °C to 90 °C. Solid line represents a lognormal fit to the distribution. Error bars represent one standard deviation. MT is the average mass of PM10 emitted per test ± one standard deviation. MMD and σg are the mass median diameter and geometric standard deviation of the size distribution, respectively.

Cumulative percent emissions of PM10, as a function of particle size, for ceramic, metal, cooled, and gasifier stoves are shown in Fig. 7. Each of the stoves produced similar mass distributions with the gasifier stove having slightly more large particles. Examination of Fig. 7 indicates that stoves of varying design (with the exception of the gasifier) emit relatively similar PM size distributions, when operated under similar conditions.

453

Fig. 7. Cumulative mass percentage of particles from biomass combustion cook stoves of different stove designs. Points designate upper limits of cascade impactor measurement bins and corresponding cumulative mass fractions.

of Figs. 7 and 9 indicates that, for a given stove, a shift in the cooking task may be accompanied by a shift in the emitted particle size distribution, even if total mass emissions remain constant.

Total PM production Pot temperatures The cooking pot temperature had a significant impact on PM emissions. The hot pot (95.5 °C) and cold pot (0 °C) tests, shown in Figs. 8(a) and (b), respectively, resulted in different PM10 emission rates and size distributions relative to the baseline stove conditions, shown in Fig. 4. The hot pot condition resulted in smaller particle sizes and reduced PM10 emissions, compared to the baseline metal stove. Cold pot testing resulted in similar PM mass emissions but had a distribution shifted slightly towards large particles. Cumulative percent emissions of PM10, as a function of particle size, are shown for the hot and cold cooking pot conditions in Fig. 9. Distinct differences can be seen between the size distributions of particles released depending on the cooking pot temperature. A statistical difference (pb 0.05) exists for the mass median diameter of the two distributions but not for the geometric standard deviation. This implies that the spread of the data does not vary significantly with changing cook pot temperature but the median particle size does. Examination

Fig. 6. Particle size distribution, by mass, for gasifier biomass cook stove heating water from 15 °C to 90 °C. Solid line represents a lognormal fit to the distribution. Error bars represent one standard deviation. MT is the average mass of PM10 emitted per test ± one standard deviation. MMD and σg are the mass median diameter and geometric standard deviation of the entire distribution, respectively. The MMD and σg for each mode can be found in the text.

Tests were conducted to determine how different stove designs and operating conditions influence the size and mass of particulate matter emitted. Although it is not practical for a stove operator to control these parameters during cooking, they may be important predictors of stove performance, especially when determining the conditions under which a stove is to be evaluated.

Fig. 8. Particle size distributions, by mass, for metal biomass cook stove when operated with constant water-pot temperatures. Hot water pot maintained at 95.5 °C for 90 min (a), cold water pot maintained at 0 °C for 90 min (b). Solid line represents a lognormal fit to the distribution. Error bars represent one standard deviation. MT is the average mass of PM10 emitted per test ± one standard deviation. MMD and σg are the mass median diameter and geometric standard deviation of the size distribution, respectively.

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0.6 5

0.45

0.8 5

4.5

55 0.

0.7 5

0.9 5

454

4.0

0.6 5

0.4 5

0.5 5

25 0.

0.15

0.3 5 0. 25

0.5 5

0.4 5

2.0

15 0.

0.05

1.5 94

Stove emission factors (EF) were calculated to compare PM10 production between different stove types and operating conditions. Emission factors are calculated by normalizing the total PM10 emissions emitted by a stove by the mass of fuel consumed during a test. Correlations were found between MCE and EF under both high power and low power testing conditions, with similar trends of reduced emissions with improving combustion efficiency (Fig. 10). The ceramic and water cooled stoves are shown for reference. Both stoves exhibited reduced PM10 emissions with increasing MCE, but with high variability between replicate tests. The two stoves also perform differently between high and low power testing. A multiple regression analysis was conducted to evaluate the combined effects of MCE and average firepower on PM production rate (mg/s). Firepower is a measure of the rate of energy conversion occurring within a stove; it depends on fuel heating value and the rate at which that fuel is combusted. Firepower, as defined here, was calculated as the fuel consumption rate multiplied by the lower heating value of the fuel (as determined by a bomb calorimeter). Results of this analysis, shown graphically in Fig. 11, indicate that collectively modified combustion

0. 35

0.25

3.0

2.5

Fig. 9. Cumulative mass percentage of particles from biomass combustion cook stoves with different pot temperatures. Points designate upper limits of cascade impactor measurement bins and corresponding cumulative mass fractions.

5 0.3

0.7 5

3.5

0.6 5

Firepower (kW)

0.5 5

45 0.

95

96

97

98

99

Modified Combustion Efficiency Fig. 11. Contour plot predicting particulate matter production rate (mg/s) as a function of modified combustion efficiency (MCE) and average stove firepower. Particulate matter production rates are shown as contour lines from a multiple regression analysis with an adjusted R2 of 0.598. Modified combustion efficiency and stove firepower were used as independent predictor variables in the multiple regression to predict the dependent variable, particulate matter production rate.

efficiency and firepower account for 59.8% of the observed variability in PM emission rate. Two important results are evident from Figs. 10 and 11. First, some PM will remain in the exhaust even as MCE approaches 100% (Fig. 10). Biomass fuels contain elements, such as calcium and potassium, along with other minerals that are unlikely to oxidize into gas-phase compounds during wood combustion within a cook stove. Therefore, these compounds will remain as PM emissions, regardless of combustion efficiency. The concentration of inorganic material in wood reported previously was similar to published results for Douglas fir of 0.1% inorganic material by mass (Tillman et al., 1981). Indeed, an inorganic content of 0.1% translates to approximately 1 mg of PM emissions per gram of wood combusted, very close to the estimated PM emissions at a MCE of 100%. A limit likely exists for the PM reductions that can be achieved by improving combustion efficiencies. Second, as combustion efficiency increases, its relative impact on PM production decreases (Fig. 11). This indicates that an incremental improvement in combustion efficiency for poor performing stoves will result in a larger reduction in PM production then would be seen from the same incremental improvement for stoves which already have high combustion efficiencies. As an example, a larger reduction in PM production would occur from improving the combustion efficiency of a cook stove from 94% to 95% then improving a stove from 98% to 99% efficient. 4. Conclusions

Fig. 10. PM production vs modified combustion efficiency. Unfilled and filled boxes designate high and low power tests, respectively. Dotted line and solid line represent linear regression trends for high and low power tests, respectively.

This work evaluated the effects of cook stove type and cook pot conditions on PM10 emissions and size distributions from biomass combustion. Emitted particle size distributions do not depend strongly on stove construction material or stove wall temperature. Variations were found, however, in the size and total mass of particles released from natural draft cook stoves compared to those from gasifier stoves. Cooking pot temperatures had a substantial effect on emitted particle size distributions, which implies that the size of particles emitted from a cook stove will change with stove use and throughout the cooking process. While not all of the factors explored

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can be controlled in a practical manner for real stoves, they are important to understand when developing standards and tests to evaluate biomass cook stoves. Results indicate that for the WBT total mass emissions (as compared to size resolved emissions) is likely sufficient to compare cook stove designs, however, comparing health results between test protocols (where elements such as cook pot temperature may change) may not be appropriate due to changing particle size distributions. Ceramic, metal, and cooled stoves all emitted similar particle size distributions and PM10 levels, with only slight variations between each. Although stove body temperature and surface material have some influence on particle size distribution, these factors are less substantial than the effect of pot temperature. The larger influence of pot temperature may be explained by the amount of physical interaction between the emission plume and the pot. The exhaust flows parallel to the stove walls as compared to impinging directly onto the bottom surface of the pot. Ultrafine particle emissions, which are somewhat greater for the hot pot, may be influenced by thermophoresis at the pot surface, with enhanced collection at the cold pot surface and reduced collection at the hot pot surface, respectively. Changing the temperatures of the walls and cooking pot may alter the PM formation process through changes in the oxidation and formation regions of the stove. The high number of ultrafine particles emitted from a stove with a hot pot could have important implications to human health. A better understanding of the conditions that result in the release of these ultrafine particles represents an important area for future research. PM size distributions emitted by the gasifier stove were broader than those from natural draft stoves. The broader distribution emitted by the gasifier stove tested may be a result of either the high inorganic content found in gasifier wood pellets or to the separation of the reaction zones. Further investigation into gasifier stove tests should be to better understand the factors leading to this broad distribution. Modified combustion efficiency correlates with PM10 emission factors, but there is wide variability among stove types and replicate burns. Average firepower also correlates with PM10 emissions. However, the degree to which these factors can predict PM10 production in biomass cook stoves is probably not sufficient to replace direct particle measurements in the laboratory or the field. There is still significant uncertainty associated with emissions testing of biomass cook stoves. Variability of the user, which was not investigated here, also likely contributes to between-test uncertainties. One limitation of this study is that only two replicates were performed for the majority of tests, which limited the statistical power of the findings. Due to the limited statistical strength which could be achieved from only two test replicates, these results should be viewed as likely trends as compared to fully verified results. Additional test replicates are recommended for future studies to improve reduce the level of uncertainty. Further tests should also be conducted to better understand how the temperature of the cooking process affects the size of particles being released during combustion.

Acknowledgments This work was made possible through the support and funding of the Engines and Energy Conversion Laboratory at Colorado State University and Envirofit International.

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