ARTICLE pubs.acs.org/EF
Influence of Feedstock: Air Pollution and Climate-Related Emissions from a Diesel Generator Operating on Soybean, Canola, and Yellow Grease Biodiesel Leila G. Lackey†,‡ and Suzanne E. Paulson*,† †
Department of Atmospheric and Oceanic Sciences, and ‡Environmental Science and Engineering Program, University of California at Los Angeles (UCLA), Los Angeles, California 90095, United States ABSTRACT: Global use of biodiesel is increasing rapidly. Combustion of biodiesel changes the emissions profile of diesel engines, altering their impact on both urban air pollution and climate. Here, we characterize exhaust emissions from conventional petroleum diesel and three neat biodiesels manufactured from soybean, canola, and yellow grease feedstocks. Exhaust was sampled from a fixedspeed 4.8 kW diesel generator at idle and full loads, and mass emission rates were determined for nitrogen oxides (NO, NO2, and NOx), particulate matter (PM), and elemental, organic, and black carbon (EC, OC, and BC). Additionally, particle size distributions were characterized. Largely consistent with a growing body of data on emissions from biodiesel, biodiesel emissions were cleaner by most metrics than those for conventional diesel. Emissions from the two primary-oil fuels, synthesized from soy and canola feedstocks, were cleaner by most metrics than emissions from diesel, producing approximately 55, 65, and 60% less PM, EC, and OC at engine idle and 40, 20, and 15% less at engine load. In addition, while primary-oil NOx emissions were 5% higher than diesel emissions at engine idle, they were more than 30% lower at engine load. Yellow grease emissions of PM, EC, and OC were reduced in comparison to diesel at engine idle by 60, 30, and 20%. However, at engine load, most yellow grease emissions were increased in comparison to diesel, resulting in approximately 5, 60, and 70% more PM, EC, and OC. Organic vapor emissions from primary-oil biodiesels were also lower, and aromatic emissions were much lower compared to diesel. Yellow grease NOx emissions were increased in comparison to diesel by approximately 5% at engine idle and 10% at engine load. Notably, NO2 accounted for a smaller fraction of NOx for all three biodiesels compared to diesel, a difference that may be more important than the somewhat higher NOx emissions in determining the impact of biodiesel on urban ozone formation. Taken together, our results suggest that widespread implementation of primary-oil biodiesels could result in improvements in urban ozone and PM pollution. In addition, by reducing both the mass and the EC content of those particles, primary-oil biodiesels may reduce anthropogenic climate forcing.
’ INTRODUCTION Biodiesel is currently under active development as an alternative to conventional petroleum diesel (hereafter referred to simply as diesel), spurred by its potential to improve energy security and independence, lower criteria pollutant and greenhouse gas emissions, and increase economic activity.1,2 Biodiesel/ diesel blends, particularly those containing 20% or less biodiesel, can be used in existing diesel engines without modification, potentially allowing for rapid and widespread implementation of biodiesel within the existing infrastructure. Soy and canola are the most widely used biodiesel feedstocks in North America. While yellow grease (produced from used cooking oil and other waste oils) is also used, the economics of collection and production and the quantity of feedstocks are less favorable. Yellow grease biodiesel has been predicted to attain a market share of approximately 0.3% of diesel fuel in the U.S. by 2030, as compared to 9% for all types of biodiesel combined.3 The effect of biodiesel on emissions is an active area of research. Studies of particulate matter (PM) and nitrogen oxides (NOx) emissions have been conducted for some time, while investigations of other exhaust components, such as black carbon, are beginning to emerge. In general, in comparison to diesel, engines operating on biodiesel emit decreased PM and hydrocarbons and register lower smoke opacity; however, several studies have reported increases in emissions of nitrogen oxides for biodiesel compared to diesel.412 r 2011 American Chemical Society
Pollutant emissions vary widely depending upon the specific engine, its maintenance history, and its operating condition.12,13 As a result, emissions collected from a variety of test engines taken together likely give a more accurate picture of real-world impacts than can be provided by any single study or even engine class. Biodiesel tests are frequently run on dieselbiodiesel blends ranging between 5 and 100% biodiesel (i.e., B5B100).14,15 The fraction of biodiesel in a blend can strongly influence the emissions profile. Table 1 provides a summary of studies that are directly comparable to our study. Included are B100 emissions studies using at least one of the biodiesels considered in this study (soy, canola, or yellow grease). These studies also reported a 100% diesel control and used units for which it was possible to compare to this study. Emissions are generally reported as mass of pollutant per volume of fuel, unit of energy (e.g., g kWh1), or unit distance traveled. It is not possible to compare our results to per distance reports; however, we have attempted to convert energy output to fuel volume for comparison purposes. Because our study reported mass to volume, literature results for mass to energy were converted using the brake-specific fuel consumption Received: August 3, 2011 Revised: November 10, 2011 Published: November 11, 2011 686
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Table 1. Emission Rates for Soy, Canola, and Yellow Grease Biodiesel and Average Biodiesel/Diesel Emissions Ratios (Bold) and Diesel Control Averages (Bold) diesel NOx (g L1)a
NO2/NOx
particle mass (g L1)a
mean mobility diameter (nm)
soy
4.88 14.37 17.410 18.95 19.04 22.8 ( 3.4b
19.728 25.49 31.711 35.527 41.212
0.0630 0.1433 0.4332 0.24 ( 0.06b
0.2631 0.3127
0.1019 0.105 0.324 0.56 ( 0.17b
0.897 0.9542 0.9828
11.839 1551 2649 415 427 4319 48 ( 7b
4748 4828 6227 6832 8850 8842
number (L1)a
1.9 10155 1.9 1015b
total hydrocarbons (g L1)a
0.419 1.035 1.1112 1.434 2.16108 8.67 ( 3.00b
18.110 19.95
1.05 ( 0.08b
1.08 ( 0.03c
0.86 ( 0.16c
0.1533
0.1030
0.3331 0.3527
1.07b
1.67c
1.15 ( 0.07c
0.164
0.627
951 4648
21.74
0.065 0.1019
0.234
0.77 ( 0.12c 567
0.97 ( 0.21b
305 3619
18.128 19.727
0.1019 0.204
21.04
0.4532
0.5342 1.0828
0.82 ( 0.13c 10550
0.93 ( 0.14c
2339 2949 3819 4028
4727 4732 6542
1.00 ( 0.17c
6.6 10155 3.47c 6.348 10.477 15.8619 22.5228 25.3827
0.169 0.4912 1.054
1.25108 3.288 12.157
0.64 ( 0.12b
1.919 1.9b
NOx (g L1)a
9.59 11.17 14.04 14.38 15.928 22.212 24.6 ( 4.3b
25.411 26.05 37.427 40.610 53.96
0.0232 0.0330 0.0727 0.04 ( 0.01b
0.0431 0.0533
0.055 0.367
0.564 0.7942
0.3828
1.0241
particle mass (g L1)a
yellow
Engine Idle 3.38 26.39 7 14.8 36.111 4 23.0 49.312
0.60 ( 0.01b
aromatics (mg L1)a
NO2/NOx
canola
Engine Load 6.69 26.312 7 11.5 42.711 4 18.4 82.16 8 24.6
0.615 0.724
11.4919
0.494 9.8519
0.61 ( 0.06c
0.54 ( 0.07c
1.119 0.58c
3.819 2c
19.44 29.95
43.310
15.828 20.427
1.31 ( 0.14c
1.20 ( 0.10c
1.02 ( 0.28c
0.0433
0.0230
0.0232 0.0327
0.80c
0.67c
0.89 ( 0.24c
0.134 0.447
0.4941
0.035 0.134
0.1519
0.104 0.1628
12.4828 16.4227
21.34
0.0531
0.2019 0.3942
19
0.43
0.51 ( 0.12b
0.64 ( 0.30c 687
0.39 ( 0.11c
0.39 ( 0.07c
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Table 1. Continued diesel mean mobility diameter (nm)
number (L1)a total hydrocarbons (g L1)a
aromatics (mg L1)a
49
30 4050 485 5751 577 7041 7228 82 ( 20b 9.8 10145 9.8 1014b 0.134 0.145 0.1628 0.1719 0.326 0.489 0.94 ( 0.35b 0.641 1.6 ( 1.0b
soy 19
74 7442 7827 7832 8239 31248
41
canola 7
34 4151
46 28648
0.73 ( 0.09c
0.5741 0.7912 1.277 3.1727 3.178
2.519
0.104 0.209 0.206 0.338
5
yellow 50
33 4519
79
1.09 ( 0.44c
0.8212 1.317 2.3041
2.2 10155 2.24c 0.065 0.0719
49
30 5028 5419 5532
5827 5939 6042
0.77 ( 0.04c
0.074
0.074 0.0819
1.15 ( 0.50c
0.46 ( 0.04c
0.67 ( 0.13c
0.241 0.33c
0.219 0.08c
1.519 0.60c
0.1028 3.2827
Results were originally reported as per kilowatt hour (kWh1); the authors converted kWh to L using the rated power output of the engine (268 g of fuel kWh1) and typical diesel (8500 g of fuel L1) and biodiesel (8800 g of fuel L1) densities.16 b Mean diesel emission rate ((standard error if multiple observations). c Mean normalized biodiesel emission rate ((standard error if multiple observations); each biodiesel rate was divided by the diesel control from the same reference, and the mean normalized value is reported. a
of our engine (268 g kWh1; supplied by the manufacturer) and the typical density of each type of fuel (8500 g L1 for diesel and 8800 g L1 for biodiesel16). Some studies suggest that the chemical profile, such as chain length and degree of saturation (properties that are determined largely by the feedstock), may impact fuel properties and engine emissions.17,18 Of the studies summarized in Table 1, only two report results for more than one biodiesel feedstock and each has significant differences between emissions for the two biodiesels tested.4,19 Nitrogen Oxides. Atmospheric nitrogen oxides contribute to tropospheric ozone formation, acid precipitation, nitrogen cycling, and fine PM formation.20 Ozone and PM impact respiratory and cardiovascular health, particularly for individuals with chronic conditions, such as asthma or heart disease.21,22 Deposition of atmospheric nitrogen can significantly affect nitrogen cycling in terrestrial and aquatic ecosystems.23 NOx is defined as the sum of NO and NO2; however, the ratio of NO/NO2 in primary emissions is a key determinant of photochemical smog formation. Because the conversion of NO to NO2 in the urban atmosphere consumes oxidants, which can be in short supply in the early morning, a higher NO/NO2 ratio slows the initial smogforming chemistry and results in lower final levels of pollutants, such as O3. Several modeling studies have shown that increasing the ratio of NO2/NOx in primary emissions increases the formation of ozone.24,25 Diesel is a major contributor to anthropogenic NOx emissions; for example, a bottom-up emissions inventory by Dallmann and Harley26 estimated 74% of U.S. fossil-fuel-associated NOx emissions were from diesel fuel combustion in 2006. NOx emission rates for biodiesel have been reported relatively widely and fall between 45% lower and 72% higher than diesel (Table 1).412,27,28 Of the reviewed studies, average NOx emissions rates from biodiesel were 86108 and 102131% of the
diesel control at idle and load, respectively. This is consistent with a recent review by Xue et al.,29 which found that 45 of 69 reviewed studies (many of which were B100) reported an increase in NOx for biodiesel compared to diesel.9 NO2/NOx ratios for biodiesel exhaust has been reported by three research groups.27,3033 They reported higher NO2/NOx ratios for biodiesel compared to diesel at engine idle: 7% higher than diesel for soy,33 67% higher than diesel for canola,30 and an average of 15 ( 7% higher than diesel for yellow grease.27,31,32. At load, the trend was reversed: 20% lower than diesel for soy,33 33% lower than diesel for canola,30 and an average of 11 ( 24% lower than diesel for yellow grease.27,31,32 Particulate Mass. Impacts of PM on health and climate depend upon specific particle characteristics, including size distribution, number, and chemical and optical properties.3438 Most studies to date have focused on PM mass, but a handful have characterized particles more thoroughly.39,40 PM mass emission rates reported for biodiesel (Table 1) averaged 1840% lower than diesel at idle and 3661% lower than diesel at load,4,5,7,19,28,41,42 also consistent with the Xue et al.29 review, reporting that 64 of 73 reviewed studies had reduced PM emissions for biodiesel compared to diesel. Particle Size and Number. The particle size is hypothesized to be a factor in particle toxicity. Ultrafine particles (mobility dp < 0.1 μm) may be easily transported into cells and have higher surface area/ volume ratios than larger particles. While the evidence is mixed, these differences may translate to increased cellular toxicity.4346 Because of the small mass of individual ultrafine particles and the potential importance of surface area in toxicity, number concentrations may be critical to estimating their health impacts.38,47 For individual literature reports (Table 1), particle mobility diameters for biodiesel range between 42% lower and 98% higher than the diesel control.5,7,19,27,28,32,39,41,42,4851 However, the 688
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literature average difference between fuel types is much smaller: 07% smaller at idle and 27% smaller to 9% larger at load. Particle number emission rates have not been frequently reported. Heikkila et al.5 reported 347 and 224% higher number emission rates for canola biodiesel compared to diesel, at idle and load, respectively. It remains to be seen how representative this study will be. The general literature trends are toward both a significant reduction (approximately 50%) in particle mass and a moderate reduction (approximately 15%) in particle diameter (Table 1). This implies either somewhat more modest increases in particle numbers or decreases, but as with other emissions metrics, wide variability is expected. Light-Absorbing Carbon (LAC). LAC, also known as elemental carbon (EC) or black carbon (BC), contributes substantially to anthropogenic climate warming. LAC warms the climate directly, and estimates of its radiative forcing fall between 0.3 and 0.8 W m2, in the same range as methane.5254 LAC also deposits on snow, where it can decrease albedo, increase melt rates, and alter the hydrologic cycle.52,55,56 Additionally, EC and BC are widely used to trace diesel emissions for health studies.5760 Both diesel exhaust and LAC have been shown to have pulmonary toxicities, including inflammatory, mutagenic, and carcinogenic responses.6164 “Black” and “elemental” carbon are frequently used to refer to the same black, sooty material, but are not completely interchangeable; each has a more limited operational definition. “BC” refers specifically to measurements made optically with instruments, such as the aethalometer used here,65 while “EC” refers specifically to measurements made by thermal and thermal optical analysis of particles collected on filters.66 In the 1996 global inventory of total BC and organic carbon (OC) emissions developed by Bond et al.,67 emission rates were estimated at 7.95 Tg year1, of which 18% were attributed to diesel combustion, with the remainder primarily from biomass burning. Approximately 6% of the total BC emissions were attributed to North America (0.5 Tg year1); the fraction of North American BC attributed to diesel was not given. A limited number of studies have measured EC in biodiesel exhaust, usually reporting lower EC emissions from biodiesel compared to diesel. Chung et al.39 measured EC/OC ratios of 0.1 and 3.0 for yellow grease biodiesel at idle and load, respectively, compared to 0.5 and 3.8 for diesel. Bugarski et al.33 measured 38% lower EC mass concentrations in underground mine air when operating a generator with soybean biodiesel compared to diesel. Wallen et al.40 measured EC from canola biodiesel and found it to have 188% more and 69% less EC than diesel at idle and load, respectively. Finally, Zhang et al.68 measured 5080% less EC for soybean and waste oil biodiesel compared to conventional diesel. Organic Carbon. In contrast to strongly warming LAC, the organic and inorganic fractions of aerosols enhance scattering, produce negative radiative forcing, and cool the climate. Chung and Seinfeld69 estimated the radiative forcing of anthropogenic OC between 0.09 and 0.18 W m2. In addition, some studies have linked the health effects of diesel PM to the organic fraction,64,70 possibly because of carcinogens, such as polycyclic aromatic hydrocarbons (PAHs) in this fraction.71,72 Bond et al.67 estimated 1996 global OC emission rates of 34 Tg year1 but attributed only 2% to diesel combustion.67 Reports of gas-phase total hydrocarbon emission rates from biodiesel (Table 1) span a wide range (90% lower to 300% higher than diesel);49,12,19,28,31,41 however, mean relative emission rates for biodiesel compared to the diesel control fell in a narrow range at idle; 3646% lower (Table 1). At load, mean relative
Table 2. Results of ASTM D6751 Testing Supplied by Biodiesel Producers ASTM D6751 test parameter73
a
soybean
canola
yellow grease
Ca + Mg (ppm)
