Policy DAR-6: Nitrogen in Fuel
Policy DAR-6: Nitrogen in Fuel
Department ID: DAR-6
Program ID: Air Guide-32
Title: Nitrogen in Fuel
Issuing Authority: Robert Warland, Director, Division of Air Resources
Originating Unit: Bureau of Stationary Sources
Signature: Robert Warland
Date: 13 February 1998
Issuance Date: 13 February 1998
Printable version (PDF 38kb)
This document provides information required to develop emission factors for boilers.
All applicable rules, regulations and federal requirements are listed in DAR-6.
The policy was developed to provide emission factors for boilers with heat inputs ranging from 10mmBTU/hr. up to and including 100mmBTU/hr., for the purposes of capping out of NOx RACT, Title V or to determine the baseline emissions for a cost effectiveness analysis.
This policy explains 6 NYCRR 227-2, NOx RACT, which became effective on February 19, 1994.
This policy defines the method which may be used to estimate NOx emissions.
Emissions of NOx may be estimated by the boiler owners.
This policy was developed to provide emission factors for boilers with heat inputs ranging from 10 mmBtu/hr up to and including 100 mmBtu/hr, for the purposes of capping out of NOx RACT, Title V, or to determine the baseline emissions for a cost effectiveness analysis.
6 NYCRR 227-2, NOx RACT, became effective on February 19, 1994. A facility is a major stationary NOx source under this regulation if it has the potential to emit 25 tons or more per year of NOx in the New York City Metropolitan area and the lower Orange County Metropolitan area. For the remainder of New York State, those facilities with the potential to emit 100 tons or more per year of NOx are major stationary sources. Facilities which limit their potential to emit through enforceable permit conditions, so they do not exceed the above thresholds will no longer be major x sources.
The nitrogen content of residual oil greatly affects the NOx emissions from boilers with 10 to 100 mmBtu/hr maximum heat input. Because of the low Sulfur requirements in New York City (O.3% S) and Nassau, Rockland and Westchester Counties (0.37% S), NOx emissions will be the controlling pollutant for residual oil fired boilers in New York City when determining Title V and NOx RACT applicability. S02 emissions will determine the Title V applicability for the remainder of the residual oil fired boilers in the State. However, facilities may still cap out of the NOx RACT requirements by limiting emissions to below the major facility threshold.
Before 1993, NOx emission estimates from AP-42 were based upon the emission factor formula:
22 + 400 * N2 = lb NOx/1000 gal. of oil burned
(where N is the percent fuel nitrogen content)1
The estimates could range from 38 to 122 lb/l000 gallons of fuel burned (0.25 to 0.81 lb/mmBtu) for fuels with nitrogen contents of 0.2% to 0.5%, respectively. In 1993, the EPA changed the formula to:
20.54 + 104.39 * N = lb NOx/1000 gal. of oil burned
The new expected emissions were 41 to 73 lb/1000 gallons of fuel burned (0.28 and 0.48 lb/mmBtu) for nitrogen contents of 0.2% and 0.5%, respectively.2
The conversion rate of fuel bound nitrogen into NOx emissions decreases as the nitrogen content increases. One of the reasons for this may be that with low excess air combustion, there may be pockets where there is insufficient oxygen to combine with the nitrogen in the fuel and the reactive pathway may be for the nitrogen molecules to combine to form elemental nitrogen.3
The old AP-42 formula estimated that the conversion rate increased significantly as the nitrogen content increased. The new AP-42 formula provides estimates based upon a linear conversion rate. While the new formula is not entirely accurate, it is more accurate than its predecessor.
The Department has compiled nitrogen in fuel data from Consolidated Edison, Long Island Lighting, Massachusetts DEP, and Maine DEP. The data was from 1990 to 1994. The nitrogen content ranged from 0.02% to 1.21%, and the sulfur content ranged from 0.25% to 2.0%. The nitrogen content averaged 0.381% with a standard deviation of 0.117% for the 192 data points collected. The Department is continuing its data collection effort to monitor nitrogen in fuel trends.
Ability to control N in residual oil
Nitrogen is removed by the same process used to remove sulfur from fuel oils.4 Nitrogen appears to be more tightly bound in the oil molecules than sulfur, so the removal efficiencies are not equal. When distillate oil is desulfurized by 75-90%, about-25-30% of the fuel nitrogen is removed. Residual oil is not commonly desulfurized in the United States.
Much higher pressures and temperatures are needed to remove nitrogen from fuel compared to those needed for sulfur removal. This adds to the cost of the residual oil. At these higher temperatures and pressures, the fuel hydrocarbons also begin to crack. These two factors make it more advantageous for refiners to continue cracking the fuel and turn it into more valuable distillate fuel than to keep it as residual oil.
Therefore, nitrogen removal from residual oil is not a practical option to control NOx emissions, nor can a steady supply of low nitrogen oil be expected to be readily and reliably obtained.
According to a preliminary study on the denitrification of fuel oil, fuel nitrogen content appears to roughly correlate to the sulfur content.4 However, this is not firmly supported by the data collected by the Department. The percent nitrogen in fuel for low sulfur oil (= 0.5% S) ranged from 0.1% to 0.65% and the nitrogen content of 1% S fuel ranged from 0.02% to 1.21%. This may be caused by the oil coming from different fields of the world (Venezuela, East Texas, etc.). Each field has its own characteristics which could result in differing nitrogen-sulfur relationships. Therefore, sulfur content cannot serve as a surrogate for predicting the nitrogen content when looking at residual oil as a whole.
Northeast Oil Study
The Stationary Source Review Committee of the Northeast States for Coordinated Air Use Management (NESCAUM) commissioned a study of fuel oil nitrogen effects on NOx emissions. The study developed high and low uncontrolled fuel bound NOx emission estimates for utility and industrial boilers.3 To compare the study results with the new AP-42 emission estimates, the Department used a baseline of 0.15 lb/mmBtu of NOx which is derived from thermal NOx. This was based upon the estimate the new AP-42 formula would provide for a residual oil which has no fuel bound nitrogen.2 The 0.15 was added to the estimated emissions from the Northeast oil study. The estimates were higher than the new AP-42 emission factor formula.
Emission Rate Plots
Attached to this policy are three figures. Figure 1 is a plot of the nitrogen content of residual oil versus the following NOx emission estimates, expressed in lb/mmBtu:
- The new AP-42 formula. (New)
- The low NOx emission estimates from the Northeast oil study. (Low)
- The high NOx emission estimates from the Northeast oil study. (High)
- The overall average of the new AP-42 estimates, the High estimates from the oil study and the low estimates from the oil study. (Average)
The Department added 0.15 lb/mmBtu of NOx derived from thermal NOx to the estimated emissions from the Northeast oil study.
Figure 2 is a plot of the nitrogen content of residual oil versus the average of the three estimates expressed in lb/mmBtu. Figure 3 is Figure 2 converted from lb/mmBtu to lb/1000 gallons of fuel burned. This was converted by using a heat content of 0.15 mmBtu/gallon for residual oil.
For the purposes of capping a facility, the emission estimate should be based upon a nitrogen content of one standard deviation above the average or O.5%. This nitrogen content level will provide some certainty that the facility will not exceed the major facility threshold of 100 or 25 ton per year. The estimated emission rate for this nitrogen content is 0.5 lb/mmBtu or 75 lb/1000 gallons of fuel burned.
If a facility believes that the NOx emissions from its units are below 0.5 lb/mmBtu when burning 0.5% nitrogen oil, the facility may perform stack tests to confirm the emission rate. The oil being fired must be sampled for nitrogen content. If the nitrogen content of the oil is not 0.5%, then Figure 2 should be consulted to determine the emissions expected at the particular nitrogen content (e.g. if 0.4% nitrogen oil is being fired, the expected NOx emissions would be about 0.45 lb/mmBtu).
If the tested NNOx emissions are different than the line in Figure 2, then the O.5 lb/mmBtu may be changed by the percentage that the tested emissions differ. For example, if the boiler actually emits 0.40 lb/mmBtu instead of 0.45 lb/mmBtu when firing 0.4% nitrogen oil, then the boiler could be expected to provide similar performance (12% below 0.5 lb/mmBtu or 0.44 lb/mmBtu) when firing 0.5% nitrogen oil.
The Department shall continue to collect nitrogen in fuel data and will revise this policy as necessary.
1. Compilation of Air Pollution Factors (Fourth Edition), AP-42, U.S. EPA, Section 1.3 Fuel Oil Combustion, October, 1986.
2. Compilation of Air Pollution Factors (Fourth Edition), AP-42, U.S. EPA, Section 1.3 Fuel Oil Combustion, 1993.
3. Fuel Oil Nitrogen Effect on NOx Emissions and Controlled Levels, prepared for Northeast States for Coordinated Air Use Management by Acurex Environmental Corporation, 1994.
4. Denitrification of Fuel Oil (Preliminary Study), Bill Johnson Ozone/CO Programs Branch, January 1993.