NOx Reduction from Biodiesel Fuels - Energy & Fuels (ACS

376

Energy & Fuels 2006, 20, 376-382

NOx Reduction from Biodiesel Fuels Sandun Fernando,* Chris Hall, and Saroj Jha Department of Agricultural and Biological Engineering, Mississippi State UniVersity, Mississippi State, Mississippi 39762 ReceiVed July 6, 2005. ReVised Manuscript ReceiVed October 21, 2005

The demand for energy around the world is increasing, specifically the demand for petroleum-based energy. Appeasing this growing energy demand without irreparably damaging the environment is of primary concern. With rising fuel prices and environmental concerns, alternative fuels could satisfy the need for renewable energy with low environmental impact. Some of the more popular alternative fuels for new vehicles are ethanol, hydrogen, and biodiesel. Although gasoline engines are expected to be replaced by hydrogen-powered fuel cells, compression-ignition engines, the diesel engines, are expected to remain in use for high-power applications because of limitations of hydrogen-storage densities. The viable environmental friendly alternative fuel for compression-ignition engines is methyl esters (commonly known as biodiesel), which is derived from vegetable oils or animal fats. Using biodiesel instead of conventional diesel fuel reduces emissions such as the overall life cycle of carbon dioxide (CO2), particulate matter, carbon monoxide, sulfur oxides (SOx), volatile organic compounds (VOCs), and unburned hydrocarbons significantly. However, biodiesel increases nitrogen oxides (NOx) emissions, mostly NO and NO2, which are considered as zone A hazardous compounds. This paper reviews the kinetics of NOx formation in relation to thermal, prompt, and fuel NOx formation processes and critically reviews the techniques that have been attempted to reduce NOx emissions from mechanisms to effectively reduce the NOx formation with biodiesel fuel.

1. Introduction The demand for energy around the world is increasing, specifically the demand for petroleum-based energy. World energy consumption is expected to increase by 50% to 180 000 GWh/year by 2020. Appeasing this growing energy demand without irreparably damaging the environment is a considerable challenge. In the United States, California has set rigid regulatory goals for the emissions of hydrocarbons, carbon dioxides, and nitrous oxides from new vehicles. On December 21, 2000, the EPA signed emission standards for model year 2007 and later heavy-duty highway engines (the California ARB adopted virtually identical 2007 heavy-duty engine standards in October 2001). The rule includes two components: (1) emission standards and (2) diesel fuel regulation. The first component of the regulation introduces new, very stringent emission standards, as follows: ‚ PM, 0.01 g/(bhp h) ‚ NOx, 0.20 g/(bhp h) ‚ NMHC, 0.14 g/(bhp h) The PM emission standard will take full effect in the 2007 heavy-duty engine model year. The NOx and NMHC standards will be phased in for diesel engines between 2007 and 2010. The phase-in would be on a percent-of-sales basis: 50% from 2007 to 2009 and 100% in 2010 (gasoline engines are subject to these standards based on a phase-in requiring 50% compliance in 2008 and 100% compliance in 2009).2 These goals are nearly * To whom correspondence should be addressed. Telephone: +1-662325-3282. Fax: +1-662-325-3853. E-mail: [email protected]. (1) Environmental Protection Agency. A ComprehensiVe Analysis of Biodiesel Impacts on Exhaust Emissions. Draft technical report. EPA 420P-02-001 2002 (cited May 27, 2005); http://www.epa.gov/otaq/models/ analysis/biodsl/p02001.pdf.

unreachable with current technologies, and as discussed later in this paper, revolutionary rather than evolutionary technologies are required to meet effectively the stringent emissions standards. Experts suggest that current oil and gas reserves would suffice to last only a few more decades. To cope up with the rising energy demand and dwindling petroleum reserves, fuels such as hydrogen, ethanol, and biodiesel are in the forefront of the alternative technologies. Ethanol has been successfully commercialized and is a mature technology. However, ethanol is used in the spark ignition engine, which is thermodynamically less efficient. In addition, ethanol could only be used as a 85% blend because of low ignition qualities in a gasoline engine. The more efficient alternative to gasoline engines is hydrogenbased fuel cells. However, for effective commercialization of hydrogen, there are many technical challenges to overcome including hydrogen production, storage, and high fuel cell production costs. Although hydrogen-powered fuel cells could replace spark-ignition engines, the compression-ignition engines, the diesel engines, are expected to remain in use for high-power applications such as railroad locomotives, ships, and overland transport trucks. This is mainly due to the challenge associated with hydrogen storage density. Accordingly, the viable alternative for compression-ignition engines is biodiesel. At present, distillate fuel oil consumption in the United States is approximately 60 billion gallons per year. According to EIA’s International Energy Outlook 2005, the world demand for crude oil grows from 78 million barrels/day in 2002 to 103 million barrels/day in 2015 and to just over 119 million barrels/day in (2) California Air Resources Board. Exhaust Emission Standards and Test Proceduress1985 and Subsequent Model HeaVy Duty Urban Bus Engines and Vehicles; (cited May 27, 2005); http://www.arb.ca.gov/regs/ components/pubtransit_buses.pdf.

10.1021/ef050202m CCC: $33.50 © 2006 American Chemical Society Published on Web 11/19/2005

NOx Reduction from Biodiesel Fuels

Figure 1. World, OPEC, and non-OPEC oil production life cycles. Oil production curves for years 1960-2040 are graphed. Years 19601997 are historic data. Years 1998-2040 are forecast by the use of the World Oil Forecasting Program. World production peaks in 2006.4

2025. Under these growth assumptions, approximately half of the world’s total oil resources would be exhausted by 2025.3 Also, with many studies suggesting that the world oil production would peak in the next 10 years (Figure 1),4 alternatives to crude oil derivatives is warranted. On the basis of U.S. Department of Energy projections, a national goal of 1.2% renewable fuel use in 2002 increasing to 4% by 2016 (these goals were based on legislation requiring a percentage of U.S. motor fuels to contain biodiesel or ethanol) is realized. The study concluded that a 4% level would displace the annual equivalent of 302 million barrels of crude oil by 2016 or nearly 2.9 billion barrels of crude oil between 2002 and 2016. In light of this, biodiesel is a viable alternative fuel to diesel to be used in compressionignition engines. Pure biodiesel is a fuel composed of “mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats which conform to ASTM D6751 specifications for use in diesel engines. Biodiesel refers to the pure fuel before blending with diesel fuel. Biodiesel blends are denoted as, “BXX” with “XX” representing the percentage of biodiesel contained in the blend (i.e., B20 is 20% biodiesel, 80% petroleum diesel)”.5 Use of biodiesel can greatly benefit the environment by reducing emissions. It can also reduce the dependency on foreign oil, because biodiesel is a renewable resource. Biodiesel can be used in standard diesel engines with little or no engine or fuel system modifications.6 When compared to standard diesel fuel, biodiesel fuel provides comparable fuel efficiency, torque, and horsepower.7 Using biodiesel instead of conventional diesel reduces emissions such as the overall life cycle of carbon dioxide (CO2) emissions, particulate matter, carbon monoxide, sulfur oxides (SOx), volatile organic compounds (VOCs), and unburned hydrocarbons. However, while reducing the aforementioned types of emissions, nitrogen oxides (NOx) emissions, mostly NO and NO2, are increased.8 Figure 2 shows the impact that biodiesel-diesel blends have on emissions. (3) Energy Information Administration. Forcast and Analysis of Energy Data; International Energy Outlook 2005; Report: DOE/EIA-0484(2005) (cited October 14, 2005); http://www.eia.doe.gov/oiaf/ieo/oil.html. (4) Duncan, R. C.; Youngquist, W. OPEC oil pricing and independent oil producers, in Presented at the PTTC Workshop “OPEC Oil Pricing and Independent Oil Producers”. 1998. Petroleum Technology Transfer Council Petroleum Engineering Program. University of Southern California, Los Angeles, CA. (5) National Biodiesel Board. Biodiesel 101. 2005 (cited March 27, 2005); http://www.biodiesel.org/resources/biodiesel_basics/default.shtm. (6) National Biodiesel Board. FAQ. 2005 (cited March 27, 2005); http:// www.biodiesel.org/resources/faqs/default.shtm. (7) Pacific Biodiesel. Why Biodiesel. 2005 (cited March 27, 2005); http:// www.biodiesel.com/why_biodiesel.htm.

Energy & Fuels, Vol. 20, No. 1, 2006 377

Figure 2. Average emission impacts of biodiesel for heavy-duty highway engines.1

2. NOx Problem Nitrogen gas (N2) makes up approximately 78% of the Earth’s atmosphere. The partial pressure of N2 ranges from 0.55 atm at 10 000 feet to 0.78 atm at sea level, while the volume fraction is 0.78 at both altitudes. The reactivity of N2 is low because it possesses a strong triple bond that requires 941 kJ/mol to break. Because of this strong triple bond, only combustion, lightening, and nitrogen fixing organisms can remove N2 from the atmosphere. The atmospheric lifetime of nitrogen gas is approximately 107 years, while the atmospheric lifetime of NOx gases range from 1 to 8 days.9 A complete overview of atmospheric nitrogen can be found elsewhere.9 NOx are produced during the combustion of many fuels. The two most common NOx are nitric oxide (NO) and nitrogen dioxide (NO2). NOx emissions pose serious problems for both the public health and the environment. NOx is a major contributor to ground-level ozone. While atmospheric ozone is beneficial in reducing harmful solar radiation, ironically, groundlevel ozone can cause respiratory irritation, a reduction of lung function, asthma attacks, and permanent lung damage.10 Ozone can also damage vegetation and reduce crop yields. Ozone is produced from NOx through the following reactions:11 hν(λ < 410 nm)

NO2 98 NO + O ∆H ) 57 kJ, ∆G ) -51.3 kJ O + O2 f O3 ∆H ) 142.7 kJ, ∆G ) 163.2 kJ NOx, in the form of nitric acid, is a contributor to acid rain, which causes damage to man-made structures and can increase the acidity of waterways, making them unsuitable for aquatic life. Inhalation of particles derived from NOx emissions can cause lung damage, aggravation of pre-existing conditions, and aggravation of heart disease.12 NOx emissions also contribute (8) British Association for Bio Fuels and Oils. Submission for Biodiesel and Bioethanol. 2001 (cited April 26, 2005); http://www.biodiesel.co.uk/ press_release/submission_for_biofuell_1.htm. (9) Trogler, W. C., et al. OVerView of Atmospheric Nitrogen Compounds. 1997 (cited April 28, 2005); http://chem-faculty.ucsd.edu/trogler/CurrentNitroWeb/Section2/Section3.shtm. (10) Environmental Protection Agency. AIRNowsOzone and Your Health. 2004 (cited April 26, 2005); http://www.epa.gov/airnow/ozone2.html. (11) Trogler, W. C., et al. Urban Smog and Reaction Mechanisms. 1997 (cited April 26, 2005); http://chem-faculty.ucsd.edu/trogler/CurrentNitroWeb/Section4/Section5.shtm. (12) EnviroTools. Nitrogen Oxides. September 2002 (cited April 26, 2005); http://www.envirotools.org/factsheets/contaminants/nitrogenoxides.shtml.

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excess nitrogen to water bodies, which can lead to eutrophication of the water bodies and can adversely affect the flora and fauna of these ecosystems. NOx emissions, specifically N2O, a greenhouse gas, can contribute to global warming. N2O has 310 times the global warming potential of CO2.13 The NOx emissions can decrease visibility by increasing the particulate matter in the air as well as by increasing smog.12 NOx emissions are increased anywhere up to 15% with a 10% average above regular diesel fuel when using biodiesel.14 3. NOx Formation NOx formation can be described generally by three methods, i.e., thermal NOx, fuel NOx, and prompt NOx. Each of these three pathways contributes to the overall NOx emissions from a fuel. Thermal NOx formation is the main contributor to NOx emissions from a diesel engine. NOx is generally formed through the following reaction:

N2 + O2 T 2NO ∆G° ) 86.596 kJ/mol ∆H° ) 90.291 kJ/mol This reaction shifts to the right at higher temperature, and rapid cooling of the products, such as occurs in an automotive exhaust, traps the gases in their high-temperature equilibrium distribution.15 In attempting to reduce NOx emissions from biodiesel, understanding the kinetics behind the NOx-forming reactions is important. At the molecular level, the NOx-forming reactions are complex. Therefore, a theoretical approach combined with experimental data, while coupling computational fluid dynamic modeling, experimental data, and graphic simulations, is proposed to be effective in understanding the elevated NOx problem with biodiesel. 3.1. Thermal NOx Formation. Thermal NOx is formed at high temperatures in the combustion chamber when oxygen and nitrogen from the air combines. This type of NOx is generally formed during fuel combustion, such as gas or diesel.17 Figure 3 shows the increase in NOx formation with an increase in the temperature. The basic kinetic equations for thermal NOx formation are described as the extended Zeldovich mechanism and are k1

O + N2 798 N + NO k2

N + O2 798 O + NO k3

N + OH 798 H + NO

(1.1) (1.2) (1.3)

where k1, k2, and k3 are the rate constants for the forward reactions as given in the Table 1. The rate constants for the (13) International Carbon Bank and Exchange. Calculating Greenhouse Gases Emissions, 2000 (cited April 26, 2005); http://www.icbe.com/ emissions/calculate.asp. (14) National Biodiesel Board. Biodiesel Emissions. 2005 (cited April 26, 2005); Available from: http://www.biodiesel.org/pdf_files/fuelfactsheets/ emissions.pdf. (15) Trogler, W. C., et al. Kinetics of the Reaction Between Nitric Oxide and Oxygen. 1997 (cited April 28, 2005); http://chem-faculty.ucsd.edu/ trogler/CurrentNitroWeb/Section3/section4.shtm. (16) Allied Environmental Technologies Inc. The Formation of NOx. (cited April 26, 2005); http://www.alentecinc.com/ papers/NOx/The%20formation%20of%20NOx_files/The%20formation% 20of%20NOx.htm. (17) Bacharach, Inc. Oxides of Nitrogen. 2005 (cited April 26, 2005); http://www.bacharach-training.com/combustionzone/nox1.htm.

Figure 3. NOx formation with respect to flame temperature and excess oxygen.16 Table 1. Rate Constants for Thermal NOx Reactions (1.8 × 108)e-38370/T [m3/(gmol s)] (3.8 × 107)e-425/T [m3/(gmol s)] (1.8 × 104)Te-4680/T [m3/(gmol s)] (3.8 × 103)Te-20820/T [m3/(gmol s)] (7.1 × 107)e-450/T [m3/(gmol s)] (1.7 × 108)e-24560/T [m3/(gmol s)]

k1 k-1 k2 k-2 k3 k-3

reverse reactions are k-1, k-2, and k-3 and are also given in Table 1.18 The net rate of NOx formation through the above reactions is given by the following formula:

(

1-

dNO ) 2k1[O][N2] dt

(

1+

)

k-1k-2[NO]2 k1[N2]k2[O2] k-1[NO]

k2[O2] + k3[OH]

)

gmol/(m3 s) (1.4)

where all concentrations are in gmol/m3. To solve this equation, both the concentration of O atoms and the concentration of OH radicals are needed in addition to the concentrations of both N2 and O2. The concentration of O radicals can be calculated from either of the following equations:

[O] ) 3.97 × 105T-1/2[O2]1/2e-31 090/T gmol/m3 [O] ) 36.64T1/2[O2]1/2e-27 123/T gmol/m3

(1.5) (1.6)

The first equation is the equilibrium approach, and the second equation is the partial-equilibrium approach. The concentration of OH molecules can be calculated with the following equation:

[OH] ) 2.129 × 102T-0.57e-4595/T[O]1/2[H2O]1/2 gmol/m3 (1.7) For most lean fuel cases, such as a diesel engine, eq 1.3 can be neglected and [OH] is no longer needed. This is due to the following observation:

k2[O2]eq . k3[OH]eq

(1.8)

Reducing the rate of the reaction (given in eq 1.4) will reduce the overall production of NOx. This is because combustion only takes place during a limited time in the engine. To reduce the rate of thermal NOx formation, the temperature inside the (18) Fluent, Inc. Thermal NOx Formation. November 29, 2001 (cited April 26, 2005); http://www.imp.cnrs.fr/intranet/fluent6.0/help/html/ug/ node581.htm.



combustion chamber must be reduced. This will reduce the rate of NOx formation as well as the overall NOx formation. 3.2. Prompt NOx Formation. Prompt NOx is produced when hydrocarbon fragments react with nitrogen in the combustion chamber to form fixed nitrogen species such as HCN. These nitrogen-containing fragments then react with atmospheric nitrogen.19 The path now acknowledged for prompt NOx creation is

CH + N2 T HCN + N

(2.1)

CH2 + N2 T HCN + NH

(2.2)

N + O2 T NO + O

(2.3)

HCN + OH T CN + H2O

(2.4)

CN + O2 T NO + CO

(2.5)

The two largest contributors to prompt NOx formation are CH and CH2 (eqs 3.1 and 3.2). The formation of prompt NOx is proportional to the number of carbon atoms in each unit volume. Prompt NOx is independent of the parent hydrocarbon. The amount of HCN created increases as the concentration of hydrocarbon radicals increases. The amount of hydrocarbon radicals increases as the equivalence ratio increases. Prompt NOx formation increases with an increasing equivalence ratio and then reaches a peak and decreases because of a shortage of oxygen. According to Fluent, Inc.,20 for most hydrocarbon fuels, the prompt NOx formation rate is given by the following equation:

dNO ) fk′pr[O2]a[N2][FUEL]e-E′a/RT dt

a)

{

(2.6)

f ) 4.75 + 0.0819n - 23.2φ + 32φ 2 - 12.2φ 3 (2.7)

XO2 e 4.1 × 10-3 1.0, -3.95 - 0.9 ln XO2, 4.1 × 10-3 e XO e 1.11 × 10-2 2 -0.35 - 0.1 ln XO2, 1.11 × 10-2 < XO < 0.03 2 XO2 g 0.03 0,

Therefore, the contribution of prompt NOx formation for a biodiesel-fueled engine can be ignored. 3.3. Fuel NOx Formation. Fuel NOx is created when nitrogen that is chemically bound in the fuel combines with excess oxygen during the combustion process. This type of NOx formation is only a problem with fuels containing chemically bound nitrogen.17 The main pathway for this type of NOx creation involves the creation of intermediate nitrogen containing species such as HCN, NH3, NH, or CN. These molecules can then be oxidized to form NOx.23 This type of NOx formation is not very prevalent with biodiesel because biodiesel has an average nitrogen concentration of only 0.02%.24 Biodiesel produced from vegetable oil inherently does not contain any nitrogen. The amount of biodiesel currently produced by animal sources is comparatively very small. Because of this reason, fuel NOx formation from a biodieselfueled engine can be considered negligible. 4. Previous Research and Methods for NOx Reduction Significant work has been done on reducing NOx emissions in both gasoline and diesel engines. One of the most effective ways to reduce NOx is to reduce the temperature inside the engine cylinder. NOx are formed in significant quantities starting above 1500 °C, and formation increases rapidly as the temperature increases.18 Diesel fuel has a flame temperature of over 2000 °C,25 almost 500 °C above the threshold. The most common method of reducing engine temperature is by using an exhaust gas recirculation (EGR) system. This system takes a portion (about 10-25%) of the exhaust gas and circulates it back into the intake. In this way, the heat of combustion from the fuel is used to heat the exhaust gas. The exhaust gas is essentially inert and therefore does not react in the combustion chamber but only absorbs heat.26 This however, results in a small loss of power along with a decrease of up to 80% of NOx emissions. The NOx emission level can be reduced up to 90% using air preheat systems.27 Another way of reducing NOx emissions is postcombustion control of the exhaust gas to remove NOx. Emissions of fuel NOx as well as thermal NOx can be successfully controlled using these methods. One such method is reducing NOx emissions using the three-point catalytic converter. The catalytic converter changes NOx to N2, CO to CO2, and unburned hydrocarbons into H2O with the following reactions:

(2.8)

2NOx ) N2 + xO2

where f is the correction factor (given in eq 3.7) from a curve fit of experimental data valid for aliphatic alkane hydrocarbons (CnH2n+2) and equivalence ratios between 0.6 and 1.6, a is the oxygen reaction order (given in eq 2.8), k′pr and E′a should be selected in accordance with reference 21, R is the universal gas constant, T is the temperature, n is the number of carbon atoms per fuel molecule, and Φ is the equivalence ratio. Prompt NOx formation is only prevalent in fuel-rich combustion.22 Diesel engines run lean; therefore, this type of NOx formation only contributes a small amount of NOx to the overall NOx emissions.

2CO + O2 ) 2CO2

(19) Hutton, J. Literature Research into NOx Formation. 2005 (cited April 26, 2005); http://www.chemeng.ed.ac.uk/people/jack/Projects/proj2000/ degussa/degussa.html. (20) Fluent, Inc. Prompt NOx Formation. November 29, 2001 (cited April 26, 2005); http://www.imp.cnrs.fr/intranet/fluent6.0/help/html/ug/node582.htm. (21) Dupont, V., et al. The reduction of NOx formation in natural gas burner flames. Fuel 1993, 497-593. (22) National Energy Technology Laboratory. Combustion NOx Formation. January 3, 2005 (cited April 27, 2005); http://www.netl.doe.gov/coal/ turbines/background/noxform.html.

CxHy + (2x + y)O2 ) xCO2 +

(2y)H O 2

However, the materials used in catalysts include platinum, palladium, and rhodium, which are expensive. In addition, the (23) Fluent, Inc. Fuel NOx Formation. November 29, 2001 (cited April 26, 2005); http://www.imp.cnrs.fr/intranet/fluent6.0/help/html/ug/node583.htm. (24) Gardner, J. M. Biodiesel Fuel Quality Study; BRABMIJ, Inc.: Pittsburgh, MO, 1995. (25) The Natural Gas Vehicle Coalition. How Safe Are Natural Gas Vehicles? September 28, 1999 (cited April 26, 2005); http://www.ngvc.org/ ngv/ngvc.nsf/bytitle/techbull2.html. (26) Chevron. Emission Reduction Technologies. 1998 (cited April 26, 2005); http://www.chevron.com/prodserv/fuels/bulletin/diesel/L2_6_9_rf.htm. (27) Agrawal, R. K.; Wood, S. C. InnoVatiVe Solutions for CostEffectiVe NOx Control. 2002 (cited June 15, 2005); http://www. etecinc.net/article_images/Innovative%20Solutions%20for%20CostEffective%20NOx%20Control.pdf.


Fernando et al.

Figure 4. Selective catalytic reduction process.29

Figure 5. Selective noncatalytic reduction process.30

catalytic converters work best in a stoichiometric air-fuel ratio of about 14.7:1. Diesel engines tend to run lean, which makes the catalytic converter less effective at reducing NOx. Running lean also produces more overall NOx because of increased engine temperatures, and the effect of running lean on NOx emissions can be seen in Figure 3.28 In addition, catalytic converters work best at high temperatures. It is found that diesel exhausts are at a lower temperature than gasoline exhaust, making conventional catalytic converters less effective on diesel engines.29 A separate method used in reducing NOx is the selective catalytic reduction (SCR). It involves the injection of ammonia in the presence of a catalyst. Selective catalytic reduction can be utilized where exhaust gases are between 500 °F and 1200 °F, depending upon the catalyst used. Selective catalytic reduction can result in NOx reductions up to 90% (Figure 4).30 Selective noncatalytic reduction is another method used to reduce NOx emissions. This involves the injection of a NOxreducing agent, such as ammonia or urea, into the boiler exhaust gases at a temperature of approximately 1400-1600 °F in the absence of any catalyst. The ammonia or urea breaks down the NOx in the exhaust gases into water and atmospheric nitrogen. Selective noncatalytic reduction can reduce NOx emissions up to 70% (Figure 5).31 Direct water injection has been effective in reducing NOx emissions. Research done at Kyushu University indicates that water injection significantly reduces the engine temperature with leads to a reduction of NOx formation, as seen in Figure 6.32 (28) Chevron. Diesel Engines and Emissions. 1998 (cited April 27, 2005); http://www.chevron.com/prodserv/fuels/bulletin/diesel/L2_6_8_rf.htm. (29) Pelley, J. NASA InnoVation Helps Clean Diesel Exhaust. Environmental Science and Technology Online 2003 (cited April 26, 2005); http:// pubs.acs.org/subscribe/journals/esthag-w/2003/feb/tech/jp_nasa.html. (30) Martin, B. Emissions Control Techniques Applied to Industrial Vehicles. 2005 (cited June 15, 2005); http://www.ifp.fr/IFP/en/files/cinfo/ IFP-Panorama05_11-DepollutionVA.pdf. (31) SNCR Committee of the Institute of Clean Air Companies, Inc. SelectiVe Non-Catalytic Reduction (SNCR) for Controlling NOx Emissions. 1997(citedJune15,2005);http://www.ammoniapro.com/Ammonia%20Library/ NOx%20Reduction/Institute%20of%20Clean%20Air%20Co_SNCR.pdf.

Emulsified fuel or fuel-water emulsions (FWE) has resulted in a clear reduction of NOx emissions at a low cost with no significant design changes and with no adverse effect on the reliability of the engine. However, some observers claim that fuel-water emulsions in a conventional injection system can cause considerable problems associated with low lubricity.33 There are some more methods in which water is used to reduce the temperature of the combustion zone such as combustion air saturation system (CASS), humid air motor, etc. CASS uses a very fine mist of water that is injected after the turbocharger using special nozzles to humidify the combustion air. In the humid air motor, hot compressed air from the turbocharger is led to a humidification tower, exposed to a large surface area, and flushed with hot water. The water can be heated by a heat exchanger connected to the jacket cooling system or using exhaust gas. Thus, the humidified air leads to the reduction of the combustion air temperature and causing a reduction in NOx emissions.34 Some more methods involving a change in the combustion process to cause the reduction of NOx levels are reducing the amount of scavenge air and common rail control. Reducing the amount of scavenge air reduces the quantity of excess O2 available for conversion to NOx. Common rail fuel injection has proven to be a very effective way in combating smoke problems as well as a NOx reduction technique. It gives the freedom to choose the injection pressure and timing completely independent of the engine load. Also, adding an element of computerized control makes it possible to consider several engine parameters and then automatically optimize the injection and therefore the combustion in each load situation.35 As of yet, not much research has been done in reducing NOx emissions specifically from biodiesel fuel. In one study,36 Nabi et al. found that retarding ignition timing reduced NOx emissions from diesel-biodiesel blends, while advancing ignition timing increased NOx emissions. Szybist et al.37 found a linear relationship between injection timing and NOx emissions. This (32) Australian Maritime College. Marine Engine Research Projects. 2005 (cited April 27, 2005); http://www.amc.edu.au/research/areas/ marine.engines/projects/. (33) Environmental Protection Agency. Impacts of Lubrizol’s PuriNOx Water/Diesel Emulsion on Exhaust Emissions from HeaVy-Duty Engines. 2002 (cited June 15, 2005); http://www.epa.gov/otaq/models/p02007.pdf. (34) Lloyd’s Register. Emissions of Nitrogen Oxides from Marine Diesel Engines. 2002 (cited June 15, 2005); http://www.cdlive.lr.org/ information/Documents/Approvals/NOX/NOx%20Questions%20and%20 Answers%20July%2002.pdf. (35) Busch, R. AdVanced Diesel Common Rail Injection System for Future Emission Legislation; 10th Diesel Emission Reduction Conference, 2004 (cited June 14, 2005); http://www.eere.energy.gov/vehiclesandfuels/ pdfs/deer_2004/session8/2004_deer_busch.pdf. (36) Nabi, M. N., et al. Combustion and exhaust emission with diesel fuel and diesel-biodiesel blends. AlternatiVe and Oxygenated Fuels (special publication) 2004, 139-146. (37) Szybist, J.; Boehman, A. Biodiesel injection timing effects on NOx emissions. Chem. Phys. Proc. Combust. 2003, 201-204.



Figure 6. Simulated effect of water injection in parallel with fuel injection, at 12° after TDC.31

relationship was independent of the fuel blend used. In this study, conventional diesel, pure biodiesel, and a 20% blend of biodiesel with diesel were studied. Biodiesel has a 12% lower energy content than diesel fuel by weight, and when a greater volume of fuel is injected in correction for this, some fuel injection pumps will advance the start of injection timing, causing an additional increase in the NOx emission. In addition, the faster propagation of pressure waves caused by biodiesel’s higher speed of sound and the more rapid pressure rise that results from biodiesel’s greater bulk modulus may shift the injection timing settings from their optimized factory settings, leading to earlier combustion.42-44 The combustion rate also has a significant effect on the NO production. More premixed combustion means a high initial rate of combustion, which causes the fuel to burn earlier, resulting in higher gas temperatures and increased NO production.46 The cetane number and fuel volatility are the two most important fuel properties that determine the amount of premixed combustion and thus the combustion rate.42,45 The lower volatility of biodiesel decreases the amount of fuel vaporized during the ignition delay and, therefore, also decreases premixed combustion. A study completed by Monyem et al.38 used a John Deere diesel engine to observe the effects of timing and oxidation on emissions from

biodiesel fuel. No significant reduction of NOx was found when comparing unoxidized and oxidized biodiesel. Yoshimoto found that emulsified blends of water and diesel fuel reduced NOx and smoke. When biodiesel and water emulsions were studied, a mixture of 30% water with biodiesel greatly reduced NOx emissions while still achieving the minimum fuel consumption desired.39 In a study by McCormick, it was found that biodiesel fuel emissions were substantially affected by its molecular structure. NOx emissions increased with an increasing fuel density and a decreasing cetane number. With fully saturated fatty acid chains, NOx emissions increased as the carbon chain lengths decreased from 18 to 12 molecules.40 Peterson et al. showed that, when the iodine number was increased from 8 to 129.5, the NOx increased by 29.3%.41-47 They reported no significant difference in NOx and PM emission (38) Monyem, A.; van Gerpen, J. H. The effect of biodiesel oxidation on engine performance and emissions. Biomass Bioenergy 2001, 20, 317325. (39) Yoshimoto, Y.; Onodera, M.; Tamaki, H. Reduction of NOx, smoke, and BSFC in a diesel engine fueled by biodiesel emulsion with used frying oil. AlternatiVe Fuels (special publication) 1999, 167-174. (40) McCormick, R. L., et al. Impact of biodiesel source material and chemical structure on emissions of criteria pollutants from a heavy-duty engine. EnViron. Sci. Technol. 2001, 35, 1742-1747.


of methyl and ethyl esters of biodiesel fuels with identical fatty acid distributions. For biodiesel blends with diesel, the blend aromatic content is lower than that of the base diesel fuel (oxygenates contain no aromatics).48,51 This dilution of the aromatics should lower both PM and NOx emissions. Typical biodiesel fuels consist of mixtures of saturated and unsaturated methyl esters containing carbon chains with 12 or more atoms in length.50 NOx emissions vary significantly with fuel composition but are well-correlated with biodiesel density. NOx emissions appear to be different for biodiesels from different feedstocks. All neat biodiesels increased NOx emissions relative to the certification fuel. In particular, feedstocks containing unsaturated fatty acid chains (soy, canola, and soapstock) produce significantly higher NOx emissions than more saturated materials. For the suite of biodiesels prepared from nearly pure fatty acids, all biodiesel fuels produced higher NOx than certification diesel with the following exceptions: methyl palmitate, methyl laurate, ethyl stearate, and the ethyl ester of hydrogenated soybean oil.49,52 One of the factors influencing NOx formation is the excess air levels. High excess air levels (>45%) may result in an increased NOx formation because the excess nitrogen and oxygen in the combustion air entering the flame will combine to form thermal NOx. The amount of excess air that is entering (41) Cleaver-Brooks. Emissions. 1997 (cited June 15, 2005); http:// www.cleaver-brooks.com/Emissions1.html. (42) Tat, M. E.; van Gerpen, J. H. The kinematic viscosity of biodiesel and its blends with diesel fuel. J. Am. Oil Chem. Soc. 1999, 76, 15111513. (43) Tat, M. E., et al. The speed of sound and isentropic bulk modulus of biodiesel at 21 °C from atmospheric pressure to 35 MPa. J. Am. Oil Chem. Soc. 2000, 77, 285-289. (44) Tat, M. E.; van Gerpen, J. H. Speed of sound and isentropic bulk modulus of alkyl monoesters at elevated temperatures and pressures. J. Am. Oil Chem. Soc. 2003, 80, 1-8. (45) Tat, M. E.; van Gerpen, J. H. The specific gravity of biodiesel and its blends with diesel fuel. J. Am. Oil Chem. Soc. 2000, 77, 115-119. (46) Heywood, J. B. Internal Combustion Engine Fundamentals; McGrawHill: New York, 1988. (47) Peterson, C. L.; Taberski, J. S.; Thompson, J. C. The effect of biodiesel feedstock on regulated emissins in chasis dynamometer tests of a pickup truck; 1999 ASAE/CSAE-SGCR Annual Internatinal Meeting Paper No. 996135. (48) Ullman, T.; Mason, R.; Montalvo, D. Study of Cetane Number and Aromatic Content Effects on Regulated Emissions from a HeaVy-Duty Engine; Coordinating Research Council Contract VE-1; Southwest Research Institute: San Antonio, TX, 1990. (49) Graboski, M. S.; Ross, J. D.; McCormick, R. L. Transient emissions from no. 2 diesel and biodiesel blends in a DDC series 60 engine, SAE 961166, 1996. (50) Graboski, M. S.; McCormick, R. L. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24, 125. (51) Ullman, T. L.; Spreen, K. B.; Mason, R. L. Effects of cetane number, cetane improver, aromatics, and oxygenates on 1994 heavy-duty diesel engine emissions, SAE 941020, 1994. (52) Graboski, M. S.; McCormick, R. L.; Alleman, T. L.; Herring, A. M. Effect of biodiesel composition on NOx and PM emissions from a DDC series 60 engine; Draft Final Report 2003; National Renewable Energy Laboratory.

Fernando et al. Table 2. Some Examples of NOx Control Technologies NOx control technologies

NOx reduction (%)

stoichiometric-based combustion modifications low excess air (LEA) reduced air preheat (RAPH) dilution-based combustion modifications water/steam injection (WSI) recirculation (EGR) postcombustion flue gas cleanup selective noncatalytic reduction (SNCR) selective catalytic reduction (SCR)

0-15 25-50 40-60 50-80 25-50 70-90

the combustion process can be limited to limit the amount of extra nitrogen and oxygen that enters the flame. This can be accomplished through burner design and can be optimized with oxygen trim controls. This type of control is known as low excess air (LEA) (see Table 2).41 5. Conclusion and Final Remarks The world energy demand continues to increase. The most feasible way to meet this growing demand is by utilizing alternative fuels. One such fuel that exhibits great potential is biodiesel. However, there is elevated NOx emission problems associated with biodiesel. There are several physical and chemical properties of biodiesel that are closely interrelated to the emission of NOx. From three main types of NOx formation: (i) thermal NOx, (ii) prompt NOx, and (iii) fuel NOx, the main contributor to NOx emissions from biodiesel is thermal NOx. This type of NOx formation is highly dependent upon temperature. For biodiesel, there are two effective ways of reducing this temperature. The first is by using water injection or an emulsion of biodiesel and water. The second method is by retarding the ignition timing in a biodiesel-powered engine. When ignition timing is retarded, the injection of the diesel fuel is moved from before the top of the compression stroke to very close to the top or after the top of the compression stroke. This would reduce the residence time and maximum temperature developed in the combustion chamber. The most effective way to reduce NOx emissions through combustion modification from a biodiesel-powered engine is by using a combination of water injection and retarded injection timings. However, some problems have been found affecting the performance of the engine with such measures. Postcombustion catalytic and noncatalytic reductions of NOx are also the effective means. In addition, the technologies to reduce the NOx emissions using biodiesel as a fuel in the diesel engine require insights in the comprehension of the physical and chemical phenomena occurring inside the combustion chamber. This will increase the viability of biodiesel to be used as a fuel in the old engines, and simultaneously, new development in the engine design and fuel properties should be assessed. EF050202M

NOx Reduction from Biodiesel Fuels - Energy & Fuels (ACS

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