746 NITROGEN OXIDES REDUCTION INTRODUCTION Nitrogen oxides are one of the most persistent categories of globally emitted air pollutants because they are combustion products of both stationary and mobile sources. By far the highest concentration of the oxides formed during combus- tion are NO and NO 2 . Given time, and in the presence of O 2 either in the flue gases or in the atmosphere most of the NO is converted to NO 2 . There are six commonly encountered oxides of nitrogen: (1) nitric oxide (NO); (2) nitrous oxide (N 2 O); (3) nitrogen dioxide (NO 2 ); (4) nitrogen trioxide (N 2 O 3 ); (5) nitrogen tetroxide (N 2 O 4 ); (6) nitrogen pentoxide (N 2 O 5 ). Typically, nitrogen oxides, or NO x , refers to a collective name for nitric oxide, NO, and nitrogen dioxide NO 2 , both of which are formed as a by-product during combustion. Nitric oxide is a colorless, odorless, toxic, nonflammable gas which is slightly soluble in water. Nitrogen dioxide, however, is a reddish-brown gas that is toxic and highly corrosive with a pungent odor. This gas can contribute to highly visible plumes. Emissions of nitrogen oxides are of concern due to their potential role in ozone formation, acid rain deposition, health effects, and formation of toxic air pollutants. EFFECTS OF NO X Human and Animal Health Effects Of the two compounds, nitrogen dioxide is the most toxic and dangerous to humans. A variety of studies has been performed to observe the effects of NO 2 on humans and animals. Most of these studies have been performed using pure NO 2 . Effects of acute NO 2 exposure have been reported as nose and eye irri- tation, obliterative bronchiolitis, pulmonary congestion and edema, pneumonitis, and death. Most of these reactions, such as pulmonary edema and obliterative bronchiolitis, can occur at extremely high concentrations (150–500 ppm) for short periods of time minutes to an hour. It appears, however, that mixtures of oxides tend to lessen the discomfort and the poten- tial to contract severe disorders. It has also been shown that chronic, intermittent exposure to NO 2 (10–40 ppm) can result in chronic pulmonary fibrosis and emphysema. 1 In animal studies, continuous exposure to NO 2 for 90 days at 5 ppm has resulted in the deaths of 18% of the rats, 13% of the mice and 66% of the rabbits. 1 On the other hand, intermittent exposure for 18 months at the same level (5 ppm) did not result in any deaths. In addition to the chronic and acute effects, it appears that daily exposure to NO 2 concentrations of 5 ppm can lead to slightly accelerated lung tumor formation, but not at a level of any statistical significance. On the other hand, exposure strictly to nitric oxide has not been reported to result in human poisoning probably due to its relatively low toxicity and its conversion to NO 2 . The relative toxicity can be seen in the exposure standards set by NIOSH (National Institute for Occupational Safety and Health) and OSHA (Occupational Safety and Health Administration). For nitric oxide, the threshold limit value (TLV) for an 8-hour time weighted average (TWA) expo- sure is 25 ppm. 2 For nitrogen dioxide, the TLV is 1 ppm for a short-term exposure limit (STEL). The short-term expo- sure limit is a 15-minute TWA exposure that should not be exceeded at any time during a workday. Likewise, the imme- diately dangerous to life or health (IDLH) concentration for nitric oxide is 100 ppm, whereas for nitrogen dioxide, the concentration is 50 ppm. 2 Toxicologists have reported that nitric oxide can be a mild nose, eye and throat irritant. At high concentrations, nitric oxide can lead to a progressive depression of the central nervous system. 1 Environmental Effects Not only does NO x affect the human population, but it also has an adverse effect on the environment, in particular vege- tation. Gaseous pollutants damage plants by entering through the stomata during the respiration cycle. 3 The pollutants can disrupt the photosynthesis process and can destroy plant chlorophyll. Experiments have shown that concentrations of NO 2 as low as 0.5 ppm can result in reduced plant growth by as much as 35%. In particular, it appears that plants are more susceptible to nitrogen dioxide effects at night than during the day. Scientists feel that NO x has played a significant role in the deforestation of central Europe. Not only does direct exposure to nitrogen oxides result in plant deterioration, but nitrogen oxides combine with certain hydrocarbons to form ozone and peroxyacyl nitrates (PAN’s), two compounds that have been found be more toxic towards plants than nitrogen oxides alone. Exposure to these compounds has been shown to result in plant growth suppression, bleaching, glazing and silvering on the lower surface of the leaves. 3 C014_002_r03.indd 746C014_002_r03.indd 746 11/18/2005 1:26:52 PM11/18/2005 1:26:52 PM © 2006 by Taylor & Francis Group, LLC NITROGEN OXIDES REDUCTION 747 In addition to vegetative kill, nitrogen dioxide is an extremely corrosive gas that can have deleterious effects on a wide range of materials such as plastics, fabrics, rubber and metals. Studies on Nylon-6 and Kevlar have revealed that the ultimate tensile strengths of these materials are reduced when exposed to a NO x environment in the range of 0.5–0.8% volume concentration. Global Warming/Greenhouse Effect The greenhouse effect occurs due to the buildup of gases which can absorb heat. The earth maintains a constant aver- age temperature by radiating heat to space; thus, greenhouse gases absorb a portion of this heat and radiate it to the lower atmosphere. Scientists believe that the heat is trapped in the lower atmosphere, resulting in global warming, rising sea level and global climatological alterations. There is consid- erable debate over the global warming issue when viewed from the geologic time scale. In any case, there has been considerable research done in this field and it will continue to be a topic of concern for years to come. Principally, carbon dioxide is the most important green- house gas, contributing to roughly half of the global warming that has been reported. The other half of the global warming has been attributed to approximately 20 other gases, most notably methane, chlorofluorocarbons (CFC-11 and CFC-12), ozone, nitrous oxide, and nitrogen dioxides. Some scientists have estimated that a 50% increase in the current concentra- tion of N 2 O will result in a mean global temperature increase of 0.2–0.5ЊC. 4 Of the nitrous oxide emissions, 25% has been attributed to fossil fuel combustion. If these predictions are accurate, then it becomes even more important to control the emissions of nitrogen oxides from combustion related processes. Acid Rain Deposition Acid rain forms when sulfur dioxide and nitrogen oxides mix with water vapor to form sulfuric and nitric acid. In par- ticular, nitrogen oxides are transformed into nitric acid by the following reactions: 5 2NO O 2NO NO O NO O 2NO 2H O O 4HNO 22 322 22 2 3 +→ +→ + ++→ . On an annual basis, nitric acid is responsible for approximately 30% of the acidity of rainfall. This percentage increases to around 50% during the winter. Acid rain is one of the most damaging effects of NO x emissions. It leads to the destruc- tion of ecosystems in lakes, deforestation, and the stripping of organic material in soils, creating erosion and potentially metal laden soils. In addition, NO x emissions contribute substantially to the acid pulse in snowmelt, which in turn severely impacts the freshwater ecosystem. Therefore, acid rain is yet another reason for controlling and limiting NO x emissions to the atmosphere. Ozone Formation In addition to being a cause of acid rain, nitrogen oxides are also considered one of two precursors to the formation of pri- mary ozone, O 3 . Ozone is thought to be formed from the com- plex reaction of certain hydrocarbons and nitrogen oxides. The role of nitrogen oxides in ozone formation is significant because of the health effects associated with ele- vated levels of ozone. Ozone exposure can lead to coughing and chest discomfort, headaches, upper respiratory illness, reduced pulmonary function, eye irritation, and increased asthma attacks. For ozone, the NIOSH ceiling exposure limit is 0.1 ppm; the OSHA threshold limit value (TLV) for an 8-hour time weighted average (TWA) exposure is 0.1 ppm. The OSHA short-term exposure limit is 0.3 ppm. 2 The immediately dangerous to life or health (IDLH) concentra- tion for ozone is 10 ppm, which is five times less than the IDLH for nitrogen dioxide and nitric oxide, respectively. In addition to human health effects, studies have shown that ozone damages agricultural crops and forest ecosystems. NO x REGULATIONS Stationary Source Regulations Because of the harmful effects associated with nitrogen oxides, governments around the world have established increasingly stricter regulations over the past few decades to curb and reduce the amount of NO x emissions. In par- ticular, the United States federal government enacted the Clean Air Act (CAA) to protect the nation’s air quality. The CAA was first passed into legislation in 1970, with substan- tial amendments being added in 1977 and most recently in 1990. The amendments in 1990 greatly increased the scope of the existing CAA. The CAA is divided into seven primary Titles, I–VII. Some of he new major points in these Titles are as follows: 6, 7 • Title I —A new system was established to deter- mine if an area is classified as either an ozone attainment or an ozone nonattainment area. The CAA effectively divides the United States into attainment and nonattainment areas, based on the level of criteria pollutants in the area’s ambient air. Because of the formation of ozone from vola- tile organic compounds (VOC’s) and NO x , Title I requires more stringent requirements for these two classes for compounds. • Title II —Clean fuel requirements and increased restrictions on motor vehicle emissions were introduced in Title II. Under Title II, new vehicle tailpipe emissions of NO x were to be reduced by 60%, starting with 40% of all new vehicles in the 1994 model year and increasing to 100% by the 1998 model year. • Title III —Emission limitations for 189 hazard- ous air pollutants (HAP’s) are to be set by the C014_002_r03.indd 747C014_002_r03.indd 747 11/18/2005 1:26:52 PM11/18/2005 1:26:52 PM © 2006 by Taylor & Francis Group, LLC 748 NITROGEN OXIDES REDUCTION United States Environmental Protection Agency (USEPA). The National Emissions Standards for Hazardous Air Pollutants (NESHAP’s) set in ear- lier versions of the CAA will remain intact, but the USEPA will now be required to use the best dem- onstrated emissions control practices in a particu- lar industry to regulate sources for that industry. Under standards set by the USEPA, major sources will be required to apply Maximum Available Control Technology (MACT). A major source is defined as one that emits 10 tons/year of a HAP or 25 tons/year of any combination of HAPs. • Title IV —Under Title IV, the USEPA is required to establish a program to reduce the occurrence of acid rain. Because SO 2 and NO x are the two main contributors to acid rain, facilities that fall under control of Title IV will be forced to meet certain standards and will be required to obtain an acid rain permit. • Title V —A comprehensive operating permit pro- gram was established for all significant air emission sources. • Title VI —Under this Title, a new national program was developed to phase out the use of chlorofluoro- carbons (CFCs) and similar compounds to protect the stratospheric ozone layer. • Title VII —The USEPA’s enforcement ability was greatly enhanced with more criminal and civil powers. Included in the Title I requirements are the National Ambient Air Quality Standards (NAAQS), which specify the maximum allowable concentrations for six criteria pollutants. These pollutants are: (1) carbon monoxide; (2) lead; (3) nitrogen oxides; (4) ozone; (5) particulates (Յ 10 microns diameter); and (6) sulfur oxides. There are two types of NAAQS, which are defined in the USEPA 40 CFR Part 50 Regulations: • primary—standards are designed to protect the public health. • secondary—standards are designed to protect the public from a pollutant’s effects on visibility, per- sonal comfort, properly, etc. If an airshed area exceeds the ambient air concentrations of one of these pollutants, then that area is considered to be in nonattainment. For nitrogen oxides, the primary and sec- ondary National Ambient Air Quality Standards expressed as annual arithmetic mean concentration are 0.053 parts per million (100 m g/m 3 ). 8 This limit should not be exceeded, during any 12 consecutive month period, for the annual aver- age of the 24-hour concentrations. Figure 1 9 shows the con- trol technologies required by facilities to meet the NAAQS. The three main categories are BACT, LAER, and RACT. BACT or Best Available Control Technology, is defined by the CAA as “ … an emission limitation based on the maxi- mum degree of reduction of each pollutant … which the per- mitting authority … ” 9 considers appropriate. BACT applies to new or modified sources of emissions in attainment areas. LAER or Lowest Achievable Emission Rate applies to new or modified sources in nonattainment areas. It refers to the most stringent emission limitation achieved by a similar facility or a particular source category. 9 RACT or Reasonably NAAQS Sources in attainment areas New or modified sources Best Available Control Technology Lowest Achievable Emission Rate Reasonably Available Control Technology Maximum Achievable Control Technology Generally Available Control Technology New or modified sources Areas sources NESHAP Major sources Sources in non- attainment areas New or modified sources Existing sources Case by case Fewer than 30 sources More than or equal to 30 sources FIGURE 1 Relationships between control technologies and standards. 9 C014_002_r03.indd 748C014_002_r03.indd 748 11/18/2005 1:26:52 PM11/18/2005 1:26:52 PM © 2006 by Taylor & Francis Group, LLC NITROGEN OXIDES REDUCTION 749 Available Control Technology applies to existing sources in nonattainment areas. RACT has been defined by the USEPA as “the lowest emission limitation that a particular source is capable of meeting by the application of control technology that is reasonably available considering technological and economic feasibility”. 9 One of the ways the CAA provides for regulation of emissions is through the New Source Performance Standards (NSPS), which is also a component of Title I. The USEPA continually promulgates new standards, under the 40 CFR Part 60’s, to regulate the emissions of criteria pollutants from new or substantially modified stationary sources. These new sources may also have to undergo a New Source Review (NSR) permitting process. Under this process, the regulating agency determines whether the facility can begin operation and under what conditions. The following is a list of new source performance stan- dards that pertain to facilities that are regulated for NO x emissions: 10 • Subpart D-Fossil Fuel Fired Steam Generators (facilities that began construction after 8/17/71) (Ͼ250 MBtu/hr) 10 • The NO x emission limits are found in Table 1. • For combinations of fuels, the following proration applies: E wx y z wxyz n = ++ + +++ 260 86 130 300 where E n ϭ NO x limit (ng/J heat input) w ϭ % of total heat input derived from lignite x ϭ % of total heat input derived from gaseous fossil fuel y ϭ % of total heat input derived from liquid fossil fuel z ϭ % of total heat input derived from solid fossil fuel. • Subpart Da-Electric Utility Steam Generating Units (facilities that began construction after 9/18/78) (Ͼ250 MBtu/hr) 10 • x • For combinations of fuels, the following proration applies: E 86w 130x 210y 260z 340v 100 n = ++++ where E n ϭ NO x limit (ng/J heat input) w ϭ % of total heat input derived from combustion of fuels subject to the 86 ng/J heat input standard x ϭ % of total heat input derived from combustion of fuels subject to the 130 ng/J heat input standard y ϭ % of total heat input derived from combustion of fuels subject to the 210 ng/J heat input standard z ϭ % of total heat input derived from combustion of fuels subject to the 260 ng/J heat input standard v ϭ % of total heat input derived from combustion of fuels subject to the 340 ng/J heat input standard. • Subpart Db-Industrial-Institutional Steam Gene- rating Units (facilities that began construction after 6/19/84) (Ͼ 100 MBtu/hr) 10 • x • Subpart Ea-Municipal Waste Combustors (facili- ties that began construction after 12/2/89) (Ͼ250 tons/day) 10 • 180 ppm by volume, corrected to 7% O 2 (dry basis). • Subpart G-Nitric Acid Plants (facilities that began construction after 8/17/71) 10 • 3.0 lb NO x (as NO 2 )/ton of acid produced expressed as 100% nitric acid). • Subpart GG-Stationary Gas Turbines (facilities that began construction after 10/3/77) 10 • The NO x emission limits for stationary gas tur- bines are given by two equations. For units with a heat input at peak load of Ͼ107.2 GJ (100 MBtu) per hour, the allowable NO x emission at 15% O 2 , dry is given by the following equation: E Y F n =+0 0075 14 4 . (.) For units with a heat input peak load of Ͼ10.7 GJ (10 MBtu) per hour but Ͻ107.2 GJ (100 MBtu) per hour and for units with a base load at ISO conditions of 30 MW or less, the TABLE 1 USEPA 40 CFR Part 60 Subpart D-Fossil-fuel fired steam generators NO x emission limits 10 Fuel type NO x emission limits, ng/J(lb/ MBtu) (expressed asNO 2 ) for heat input (1) Gaseous fossil fuel 86 (0.20) (2) Liquid fossil fuel 129 (0.30) (3) Liquid fossil fuel ϩ wood residue 129 (0.30) (4) Gaseous fossil fuel ϩ wood residue 129 (0.30) (5) Solid fossil fuel 1 300 (0.70) (6) Solid fossil fuel ϩ wood residue 1 300 (0.70) (7) Lignite, except (9) 260 (0.60) (8) Lignite ϩ wood residue, except (9) 260 (0.60) (9) Lignite mined in North Dakota, South Dakota, or Montana, and which is burned in a cyclone-fired unit 340 (0.80) 1 Except liginite or a solid fossil fuel containing 25%, by weight, or more coal refuse. C014_002_r03.indd 749C014_002_r03.indd 749 11/18/2005 1:26:53 PM11/18/2005 1:26:53 PM © 2006 by Taylor & Francis Group, LLC The NO emission limits are found in Table 3. The NO emission limits are found in Table 2. 750 NITROGEN OXIDES REDUCTION allowable NO x emission at 15% O 2 , dry is given by the following equations: E Y F. n =+0 0150 14 4 . (.) where E n ϭ NO x limit (% by volume, dry) Y ϭ mfg.’s rated heat rate at mfg.’s rated peak load or the actual measured heat rate based on lower heating value of fuel as measured at actual peak load for the facility. The value of Y shall not exceed 14.4 kJ per watt hour. F ϭ defined according to the nitrogen content of the fuel as follows: Fuel-bound nitrogen (% by weight) F (NOx % by volume) N Ͻ 0.015 0 0.015 Ͻ N Ͻ 0.1 0.04 (N) 0.1 Ͻ N Ͻ 0.25 0.004 ϩ 0.0067 (N-0.1) N > 0.25 0.005 In addition to NSPS and NAAQS, the USEPA is reduc- ing NO x emissions through Title IV, the Acid Rain Program. The regulations under this program were published as final in the April 13, 1995 Federal Register and became effective on May 23, 1995. The regulations are aimed directly at coal fired utility plants in which the combustion of coal on a BTU basis exceeds 50% of its annual heat input. The type of boil- ers used at these plants has been subdivided into Group 1 and Group 2 boilers. Group 1 boilers include tangentially fired boilers or dry bottom wall-fired boilers. Group 2 boilers include wet bottom wall-fired boilers, cyclone boil- ers, vertically fired boilers, arch-fired boilers, or utility boilers (i.e. fluidized bed or stoker boilers). In addition, the CAA has set standards based on Phase I and Phase II. In the acid rain regulations, NO x emission limits have been established for Phase I coal fired utility units with tangentially fired boilers (95 units) or with dry bottom wall-fired boilers (84 units), which were to be effective January 1, 1996. For tan- gentially fired boilers, the limit is 0.45 lb/MBtu of heat input on an annual average basis. For dry bottom wall fired boilers, the NO x limit is 0.50 lb/MBtu of heat input expressed on an annual average basis. The facilities that cannot meet the requirements will be allowed to apply for a less restrictive emission standard or to join an “averaging pool,” through which the overall emis- sions limit average is attained. Phase I standards for Group 2 boilers are scheduled to be set by January 1, 1997 with imple- mentation by January 1, 2000. Phase II standards for Group 1 and 2 boilers are required to be established by 1997. 11 Similar to the federal government under the Clean Air Act, State governments have the authorization to enact regulations that maintain ambient air quality and to set limits for sources of air pollution. In New York State, the agency charged with TABLE 2 USEPA 40 CFR Part 60 Subpart Da-electric utility steam generating units NO x emission limits 10 Emission limit for heat input Fuel type ng/J (lb NO x /MBtu) Gaseous fuels Coal-derived fuels 210 0.50 All other fuels 86 0.20 Liquid fuels Coal-derived fuels 210 0.50 Shale oil 210 0.50 All other fuels 130 0.30 Solid fuels Coal-derived fuels 210 0.50 Any fuel containing more than 25%, by weight, coal refuse 1 ( 1 )( 1 ) Any fuel containing more than 25%, by weight, lignite if the lignite is mined in North Dakota, South Dakota, or Montana, and is combusted in a slag tap furnace 2 340 0.80 Any fuel containing more than 25%, by weight, lignite not subject to the 340 ng/J heat input emissions limit 2 260 0.60 Subbituminous coal 210 0.50 Bituminous coal 260 0.60 Anthracite coal 260 0.60 All other fuels 260 0.60 1 Exempt from NO x standards and NO x monitoring requirements. 2 Any fuel containing less than 25%, by weight, lignite is not prorated but its % is added to the % of the predominant fuel. C014_002_r03.indd 750C014_002_r03.indd 750 11/18/2005 1:26:53 PM11/18/2005 1:26:53 PM © 2006 by Taylor & Francis Group, LLC NITROGEN OXIDES REDUCTION 751 enforcing these regulations is the New York State Department of Environmental Conservation (NYSDEC). The USEPA has delegated authority to the NYSDEC to issue permits to con- struct to sources for the modification or construction of any stationary source subject to federal NSPS, NESHAPS, or PSD (Prevention of Significant Deterioration) requirements, and to implement and enforce the federal standards and PSD requirements where they apply. 12 The NYSDEC requires that any new or modified air emission source acquire a permit to construct and a certificate to operate under the requirements of 6 NYCRR Part 201. Any air contamination source subject to the NSPS, NESHAPS, or PSD requirements must be in com- pliance with the federal standards as well as any applicable State standards, such as New York State Ambient Air Quality Standards (NYSAAQS) or New York State performance stan- dards. The NYSAAQS for NO 2 is 0.053 ppm (100 m g/m 3 ) 13 with the same averaging period as that of the USEPA NAAQS. Recently, the NYSDEC made revisions to the 6 NYCRR Part 200, 201, and 227 air pollution regulations. In particular, these revisions included Subpart 227–2 entitled “Reasonably Available Control Technology (RACT) for Oxides of Nitrogen (NO x ).” 14 This subpart requires operators of existing major sta- tionary sources of NO x to use RACT to limit NO x emissions. A source is classified as a major source if it has the potential to emit 100 tons NO x per year or if it has the potential to emit 25 tons NO x per year in a “severe” ozone nonattainment area. The major stationary sources as defined in Subpart 227–2 are: 14 • Very large boilers—Q MAX Ͼ 250 MBtu/hr • Large boilers—100 MBtu/hr Ͻ Q MAX Յ 250 MBtu/hr • Mid-size boilers—50 MBtu/hg Ͻ Q MAX Յ 100 MBtu/hr • Small boilers—20 MBtu/hr Ͻ Q MAX Յ 50 MBtu/hr • Combustion turbines—Q MAX Ͼ 10 MBtu/hr • Internal combustion engines— ≥ 225 HP in severe nonattainment areas; Ͼ400 HP in all other areas • Other combustion sources where, Q MAX is equal to the maximum heat input capacity or rate. 4–8 14 contain information regarding the NO x emission limits for the above sources. These limits were to be met by the existing sources by May 31, 1995. Small boiler operators are only required to perform annual tune-ups and to maintain a log book containing process information. Other major combustion sources of NO x not covered under one of the specific sources above were required to submit a RACT proposal detailing the technology and potential emissions to the NYSDEC by May 31, 1995. Outside of state and federal regulations, certain cities promulgate their own air pollution control regulations. The New York City Department of Environmental Protection (NYC DEP) revised their air pollution code in March, 1992. This document contains guidelines for obtain- ing local permits and certificates as well as standards for air pollutant emissions. In regards to NO x emissions, the NYC DEP set a limit for boilers with a capacity of Ͼ500 MBtu/hr depending on whether the boiler was completed before (150 ppmv NO x ) or after (100 ppmv NO x ) August 20, 1971. 15 The petrochemical and refinery sectors have been subjected to stringent regulatory requirements in recent years by state TABLE 3 USEPA 40 CFR Part 60 Subpart Db-industrial/commercial/institutional steam generating units NO x emission limits 10 Fuel/steam generating unit type NO x emission limits, ng/J(lb/MBtu) (expressed as NO 2 ) for heat input (1) Natural Gas and distillate oil, except (4) (i) Low heat release rate 43 (0.10) (ii) High heat release rate 86 (0.20) (2) Residual oil (i) Low heat release rate 130 (0.30) (ii) High heat release rate 170 (0.40) (3) Coal (i) Mass-feed stoker 210 (0.50) (ii) Spreader stoker and fluidized bed combustion 260 (0.60) (iii) Pulverized coal 300 (0.70) (iv) Lignite, except (v) 260 (0.60) (v) Lignite mined in North Dakota, South Dakota, or Montana, and is combusted in a slag tap furnace 340 (0.80) (vi) Coal-derived synthetic fuels 210 (0.50) (4) Duct burner used in a combined cycle system (i) Natural gas and distillate oil 86 (0.20) (ii) Residual oil 170 (0.40) C014_002_r03.indd 751C014_002_r03.indd 751 11/18/2005 1:26:53 PM11/18/2005 1:26:53 PM © 2006 by Taylor & Francis Group, LLC Tables 752 NITROGEN OXIDES REDUCTION and federal governments. 33,34,35 NO x control options have been applied to fired process heaters 34 whose uncontrolled emis- sions are in the range of 0.10 to 0.53 lb/10 6 Btu. The percent- age reductions attained for key control technologies include, (1) Low NOx burners (25–65%) (2) Selective Catalytic Reductions (SCR) (65–90%) and (3) Combined Low NOx and SCR (70–90%) Additional factors involved in choosing a desired technology include capital, operating and maintenance costs. 35 Mobile Source Regulations In addition to stationary combustion, mobile sources, such as passenger cars and trucks, contribute almost 50% of the NO x produced in the United States. To reduce the emissions from these sources, federal and State governments have imple- mented standards on tailpipe emissions of NO x . Typically, vehicles are divided into three main weight categories: (1) light duty, (2) medium duty, and (3) heavy duty. Vehicles are further subdivided in each main category; emission limits are applied to these subcategories. Emission limits are expressed two different units: (1) g/mi, grams per mile, or (2) g/bhp-hr, grams per brake horse-power-hour. In the federal and California regulations, standards are expressed in g/mi for light and medium duty vehicles, whereas g/bhp-hr is used for heavy duty vehicles. As a comparison of some of the emission 16,17 standards for passenger and light duty vehicles. To further reduce air pollutant emissions from vehi- cles, the federal and State governments have implemented a number of other programs. One such program is the Clean-Fuel Vehicles Fleet Program which is designed to reduce emissions, in highly polluted regions, from vehicles belonging to a fleet. Tailpipe emissions generally account for 60% of the total vehicle emissions. 16 Thus, governments are beginning to focus on the other emission forms asso- ciated with vehicles. These include evaporative emissions, CO emissions at cold temperatures, air toxics emissions, emissions testing and procedures, and emissions control diagnostics systems. Regulating emissions is not the only method to curb NO production. The composition of the fuel has also been a target of regulations. Requirements have been promulgated to establish the use of reformulated gaso- line, oxygenated gasoline, reduced volatility gasoline, and cleaner diesel fuel. Thus, the federal and state governments have taken active role in the limiting of NO x emissions. Depending on the political climate in the United States, these gulations may become less or more stringent over the next few years. TABLE 4 NYS DEC NO x RACT emission limits for very larger boilers (lbs NO x /MBtu) 14 Boiler configuration Primary fuel type Tangential Wall Cyclone Stokers Gas only 0.20 0.20 NA NA Gas/oil 0.25 0.25 0.43 NA Coal wet bottom 1.00 1.00 0.60 NA Coal dry bottom 0.42 0.45 NA 0.30 1 1 The limit is 0.33 lbs NOx/MBtu when 25% of other solid fuels, on a Btu basis, are utilized. TABLE 5 NYS DEC NO x RACT emission limits for large boilers (lbs NO x /MBtu) 14 Primary fuel type Emission limit Gas only 0.20 Gas/oil 0.30 Pulverized coal 0.50 Coal (overfeed stoker) 0.30 1 1 The limit is 0.33 lbs NO x /MBtu when 25% of other solid fuels, on a Btu basis, are utilized. TABLE 7 NYS DEC NO x RACT emission limits for combustion turbines (ppm, corrected to 15% O 2 ) 14 Combustion turbine and fuel types Emission limit Simple cycle and regenerative Gas only 50 Multiple fuels 100 Combined cycle Gas only 42 Oil only 65 TABLE 6 NYS DEC NO x RACT emission limits for mid-size boilers (lbs NO x /MBtu) 14 Primary fuel type Emission limit Gas only 0.10 Distillate oil 0.12 Residual oil 0.30 TABLE 8 NYS DEC NO x RACT emission limits for internal combustion engines (gms NO x /brake HP-hr) 14 Engine and fuel types Emission limit Rich burn 2.0 Lean burn Gas only 3.0 Other fuels 9.0 1 Rich burn engine—O 2 by volume is less that 1.0% in the exhaust. 2 Lean burn engine—O 2 by volume is greater than or equal to 1.0% in the exhaust. C014_002_r03.indd 752C014_002_r03.indd 752 11/18/2005 1:26:53 PM11/18/2005 1:26:53 PM © 2006 by Taylor & Francis Group, LLC limits, Table 9 shows the federal, California and New York NITROGEN OXIDES REDUCTION 753 Conversion of Emission Standards As can be seen from the above regulations, NO x standards come in various units. Some regulations state NO x limits in ppm, on a dry volume basis, whereas other regulations may x been developed for converting emission units at different oxygen percents in the flue gas for natural gas, oil, and coal fired units, respectively. Figure 2 is based on a typical gaseous 18,19 based on a number of different liquid and solid fuels (see 18,20 and 12 18,21 , respectively). Three Lotus spread- sheets were utilized to develop Figure 2. First, a spreadsheet on a percent volume basis, to the elemental component mass fractions. In addition, the spreadsheet also calculates the lower and higher heating values of the natural gas based on the components in the natural gas. The lower heating value is compared for accuracy to experimental results. Next, the component mass fractions were read into the second spread- the pounds of flue gas produced per pound of fuel on a dry weight basis. Using the quantity of flue gas and the higher heating value of the natural gas, a “K” factor was developed for all three figures via: K HHV V FG MW M = × × where HHV ϭ heating value of the natural gas, Btu/lb FG ϭ volume of flue gas per lb of fuel, dscf/lb MW ϭ molecular weight of NO 2 , lb/lbmol V M ϭ molar volume, dscf/lbmol In this equation, V M is equal to 385.1 dscf/lbmol at 68ЊF and 29.92 in. Hg. The “K” factors for a new selected fuels are “K” factors do not vary widely for each type of fuel, which implies that Figures 2–4 could be readily applied to a par- ticular fuel type. The exception to the rule are gaseous fuels that have a moderately high carbon dioxide content, such as the natural gas from Germany. Because the carbon diox- ide does not play a role in the combustion process, a high carbon dioxide content gas will produce significantly lower flue gas per pound of fuel, thereby increasing the “K” factor. The “K” factor was then read into the third spreadsheet to be used in the following equation to develop the data points for each conversion graph: X K E (20.9 %O ) 20.9 2 ϭ ϫϫ Ϫ where X ϭ NO x concentration, ppmvd E ϭ lb NO x /Mbtu %O 2 ϭ % oxygen in the flue gas. TABLE 9 NO x emission standards for passenger cars and light duty trucks (g/mi) 16,17 New York California Federal PC LDTI 2 LDT2 3 PC LDTI 2 LDT2 3 PC LDTI 2 LDT2 3 5 yr/ 0.4 0.4 1.0 0.4 0.4 1.0 1.0 1.2 1.7 50,000 mi (0.7) 1 (0.7) 1 1993 10 yr/————————— 100,000 mi 5 yr/ 0.4 0.4 1.0 0.4 0.4 0.7–1.0 0.4–1.0 0.4–1.2 0.7–1.7 50,000 mi (0.7) 1 (0.7) 1 1994 10 yr/ — — — 0.6 0.6 0.9 0.6 0.6 0.97 100,000 mi 5 yr/ 0.4 0.4 0.7 0.4 0.4 0.7 0.4–1.0 0.4–1.2 0.7–1.7 50,000 mi 1995 10 yr/ — — — 0.6 0.6 0.9 0.6 0.6 0.97 100,000 mi 1 The standard for in-use compliance for passenger cars and light duty trucks certifying to the 0.4 g/mi NO x standard shall be 0.55 g/mi NO x for 50,000 miles. 2 LDTI refers to light duty trucks from 0–3, 750 pounds loaded vehicle weight. 3 LDT2 refers to light duty trucks from 3,751–5750 pounds loaded vehicle weight, but less than 6,000 pounds gross vehicle weight. C014_002_r03.indd 753C014_002_r03.indd 753 11/18/2005 1:26:53 PM11/18/2005 1:26:53 PM © 2006 by Taylor & Francis Group, LLC fuel, a US/Texas gas (see Table 10 was developed, Table 13, that converts the gas components, sheet, Table 14, to calculate the flue gas constituents and Tables 11 found in Tables 10–12. As can be seen from the tables, the require the emissions in lbs NO /MBtu. Figures 2–4 have ). Figures 3 and 4 were 754 NITROGEN OXIDES REDUCTION Oxygen in Fluegas (Excess Air),% 2 (9.5) 3 (15) 5 (28) 7 (45) 9 (68) 11 (100) 13 (148) 15 (229) 1000 100 10 0.01 0.1 1 E (PPMV, DRY) X (LB/MBtu, HHV) NYC DEP 8 NYS DEC 2 NYC DEP 7 NYS DEC 3 US EPA 1 , NYS DEC 5 NYS DEC 6 US EPA 4 (0.2) (150) (100) (0.1) (42) (50) (75-145) TURBINES BOILERS - New Source Performance Standards (NSPS) - > 100 MBtu/hr in size -50 - 100 MBtu/hr in size - Depends on heat rate (NSPS) - Simple cycle and regenerative combustion turbines - Combined cycle combustion turbines 1 2 3 4 5 6 - > 500 MBtu/hr in size, after 8/20/71 - > 500 MBtu/hr in size, before 8/20/71 7 8 FIGURE 2 Conversion of emission units and comparison of various standards for NO x natural gas units. C014_002_r03.indd 754C014_002_r03.indd 754 11/18/2005 1:26:54 PM11/18/2005 1:26:54 PM © 2006 by Taylor & Francis Group, LLC NITROGEN OXIDES REDUCTION 755 Oxygen in Fluegas (Excess Air),% 2 (9.5) 3 (15) 5 (28) 7 (45) 9 (68) 11 (100) 13 (148) 15 (229) 1000 100 10 0.01 E (PPMV, DRY) 0.1 1 X (LB/MBtu, HHV) NYS DEC 2 , NJ DEP US EPA 1 NJ DEP 3 NYS DEC 4 NJ DEP 5 (0.20) (0.25) (0.28) (0.3) (0.43) 1 2 - NSPS-Subpart D- >250 MBtu/hr - Dry bottom cyclone boiler - Dry bottom wall utility boilers - Dry bottom tangential & wall boilers-> 250 MBtu/hr 4 3 NJ DEP - utility boiler NYS DEC - >250 MBtu/hr 5 - Dry bottom tangential utility boilers FIGURE 3 Conversion of emission units and comparison of various standards for NO x oil-fired units. C014_002_r03.indd 755C014_002_r03.indd 755 11/18/2005 1:26:54 PM11/18/2005 1:26:54 PM © 2006 by Taylor & Francis Group, LLC [...]... 2 - NSPS - Subpart Da - > 250 MBtu/hr 4 - NSPS - Subpart Da - > 250 MBtu/hr 6 - Dry bottom tangential boilers (bituminous & anthracite coal) (subbituminous coal) 7 - Dry bottom tangential utility boilers FIGURE 4 Conversion of emission units and comparison of various standards for NOx coal-fired units © 2006 by Taylor & Francis Group, LLC C014_002_r03.indd 756 11/18/2005 1:26:54 PM 757 NITROGEN OXIDES. .. result in substantial NOx reductions Off stoichiometric combustion (OS) In off stoichiometric combustion techniques, NOx reduction is achieved by altering the fuel/air ratio in the primary combustion zone Burners-out -of- Service (BOOS) One such technique is known as burners-out -of- service As the name implies, this operational control method involves taking one or more burners out of service, in a multiburner... of 8.31439 Joules/mol-deg.K or 1545.4 ft-lb/lb mol deg.R and thus 385.1 scf/lbmol The specific volume for nitrogen is based on composition of dry air (20.9% O2, 79.1% N2) where N2 is the “atmospheric” nitrogen containing about 1% Ar, 0.03% CO2, and trace amounts of Kr, Ne, Xe; molecular weight of atmospheric nitrogen is 28.08 ϳ Humidity at 60% R.H and 80 deg.F often used as standard © 2006 by Taylor... C014_002_r03.indd 760 11/18/2005 1:26:54 PM NITROGEN OXIDES REDUCTION TABLE 15 NOx Control methods and reduction2 2,23 NOx reduction (%) Control method Low excess air Off-stoichiometric combustion Low-NOx burner Staged-air burner Staged-fuel burner Low excess air burner Burner w/external FGR Burner w/internal FGR Air or fuel-gas staging w/internal FGR Air or fuel-gas staging w/external FGR Flue gas recirculation... combustion, Sox and NOx,” The Chemical Engineer, 32–38 (1989) 33 NJ Department of Environmental Protection—State implementation plan for the attainment and maintenance of the ozone and carbon monoxide national ambient air quality standards (1999) 34 Baukal, C.F., Hayes, R., Grant, M., Singh P., and Foote, D., Nitrogen Oxides Emissions Reduction Technologies in the Petrochemical and Refining Industries” Environmental. .. control of the temperature and does not provide the same reductions in NOx as SCR The advantages of SNCR over SCR include lower capital and operating costs and safer handling of chemicals when urea based reagents are used A comparative study of the two processes on a 200 ton/hr oil and gas fired boiler revealed that the SNCR would require a capital cost of $0.4 million as compared to a capital cost of $2... Quality Standards,” NYCRR, Title 6, Part 256 and 257 (1989) © 2006 by Taylor & Francis Group, LLC C014_002_r03.indd 767 11/18/2005 1:26:56 PM 768 NITROGEN OXIDES REDUCTION 14 NYSDEC “Reasonably Available Control Technology for Oxides of Nitrogen (NOxRACT),” NYCRR, Title 6, Parts 200, 201 and 227 (1994) 15 NYCDEP “Air Pollution Code” (1992) 16 Renna, Steven P and Rubin, Laurence M., Present and Future... controlled portion of the combustion air, normally 10–20%, is redirected above; flames to the OFA ports.25 Effective implementation this control method relies on a number of parameters, most notably adequate mixing of the overfire air with the primary combustion production In addition, OFA is a function of the location and number of ports, ports spacing and geometry, and on the fan capacity and furnace dimensions... in the reactor oxygen and water vapor concentration in the flue gas flue gas temperature effective mixing and distribution of ammonia and air in the flue gas stream © 2006 by Taylor & Francis Group, LLC C014_002_r03.indd 764 11/18/2005 1:26:55 PM 765 NITROGEN OXIDES REDUCTION • • • • amount of ammonia ammonia slip catalyst surface area dust loading Ammonia slip and sulfur content of the fuel are two... combustion control technologies of varying types on the market today: selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) These methods have been used extensively on an international scale and have become a common feature on gas-turbine cogeneration and combined cycle systems in the United States These systems can provide NOx reductions of up to 90% One of the most popular post . precursors to the formation of pri- mary ozone, O 3 . Ozone is thought to be formed from the com- plex reaction of certain hydrocarbons and nitrogen oxides. The role of nitrogen oxides in ozone formation. DEP (0.7) (0.6) (0.55) (0.42) (0.38) (0.5) (0.45) 1 - NSPS - Subpart D - > 250 MBtu/hr 2 - NSPS - Subpart Da - > 250 MBtu/hr (bituminous & anthracite coal) 3 - Dry bottom cyclone utility boilers 4 - NSPS - Subpart Da - > 250. 4 (0.2) (150) (100) (0.1) (42) (50) (7 5-1 45) TURBINES BOILERS - New Source Performance Standards (NSPS) - > 100 MBtu/hr in size -5 0 - 100 MBtu/hr in size - Depends on heat rate (NSPS) - Simple cycle and regenerative