MEMORANDUM 

From: The Office of Air Quality Planning and Standards 

 U.S. Environmental Protection Agency 

To: Combustion Turbine NSPS Docket (EPA-HQ-OAR-2004-0490) 

Date: June, 2012 

Subject: NOX Emissions Data 

Introduction

The Utility Air Regulatory Group (UARG) requested reconsideration and submitted supporting 

materials questioning the achievability of the final NOX emissions standard for stationary combustion 

turbines with continuous emissions monitoring systems. One issued raised by UARG is whether the 4-

hour continuous average is appropriate for simple cycle stationary combustion turbines. 

Nitrogen oxide (NOX) is formed in combustion turbines by the oxidation of molecular nitrogen (N2) in 

the combustion air (thermal NOX) and any nitrogen compounds contained in the fuel (fuel NOX). The 

formation of NOX from nitrogen in the combustion air is dependent on two conditions occurring 

simultaneously in the unit's combustion zone: high temperature and sufficient residence time. New 

stationary combustion turbines installed today in the United States routinely include water/steam 

injection or dry low NOX (DLN) technology. Post-combustion technologies that are designed to reduce 

the amounts of NOX formed during combustion, such as selective catalytic reduction (SCR), are also 

available. 

Water/Steam Injection 

This type of control injects water or steam into the primary combustion zone with the fuel. The water or 

steam serves to reduce NOX formation by decreasing the peak flame temperature. The degree of 

reduction in NOX formation is proportional to the amount of water injected into the combustion turbine. 

However, a limit exists on the amount of water that can be injected into the system before reliability of 

the combustion turbine is degraded and operational life is affected. This type of control can also be 

counterproductive because carbon monoxide (CO) and volatile organic compound (VOC) emissions are 

formed as a result of incomplete combustion. 

Dry Low-NOX (DLN) Burners 

NOX can be limited by lowering combustion temperatures and by staging combustion (i.e., creating a 

reducing atmosphere followed by an oxidizing atmosphere). The use of dry low-NOX (DLN) burners as 

a way to reduce flame temperature is a common NOX control method. The combustor designs are called 

DLN burners because injecting water into the combustion chamber is not necessary to achieve low NOX

emissions. This method is primarily used when firing natural gas. 

NOX Control Performance Using Continuous Emission Monitoring Data

EPA used hourly emissions data available from EPA’s Clean Air Markets division’s database. There are 

two methods to get hourly emissions data. The first is to download “Unit Level Emissions” monthly 

files with hourly data from specific facilities. The emissions data available through this approach 

contains substituted missing data and bias adjustments to get the “adjusted” emissions that are used by 

the Clean Air Markets Division. Compliance with the New Source Performance Standards is done using 

the “unadjusted” emissions data. The “adjusted” emissions are generally only slightly higher, but can be 

significantly higher for specific facilities. Therefore, use of the “adjusted” emissions data could result in 

higher emissions than would actually be reported. 
The criteria for selecting stationary combustion turbine continuous emission monitoring (CEMS) data 

that were reviewed are as follows. First, the facility had to have a NOX CEMS and the permit condition 

of the turbine had to be similar or more stringent than the requirements in Subpart KKKK. Second, the 

facility could only use combustion controls and not SCR to control NOX emissions. 

Approach

EPA reviewed “unadjusted emissions data” for specific sources. To determine the applicable hourly 

NOX standard that would apply, EPA used the following sorting criteria: 

1. Startup and shutdown hours were identified as any hour with less than a full hour of operation, 

the previous or following hour of operation are not consecutive, both natural gas and oil are 

burned in the same hour (fuel switching), the first 30 minutes of operation constitute a startup 

hour regardless of the average load (i.e., any hour where the previous hour had 29 minutes or less 

operation is also considered a startup hour regardless of the average load), and a shutdown hour 

included hours when the following hour average load was equal to or less than 30% and had 15 

minutes or less of operation. These final 2 conditions would also likely result in a part-load hour 

since the part-load standard applies to the entire hour if the turbine operates below 75% load at 

any point. 

2. Part-load hours were identified as the average heat input being less than 75% of the base load 

heat input. To determine the base load for each facility, EPA first performed an internet search 

for the applicable state permit. If the permit was not available, or did not list the base load heat 

input, the heat input data from the CAMD database was used. This approach is conservative 

since the highest emissions standard for any given hour applies (it is not necessary that the 

average hour load be less than 75%). Since we did not have instantaneous data, there might be 

additional hours where the part-load standard would apply. 

3. All other hours were considered full load hours. 

During startup, shutdown, and part-load hours the less stringent standard applies. During full load hours 

the natural gas standard applies when the facility is burning only natural gas. The other fuel standard 

applies when any oil is burned. Since we were unable to determine if any of the potential excess 

emissions were a result of either periods of turbine tuning, malfunction of the NOX CEMS or 

combustion control equipment, periods of operation where the ambient temperature is less than minus 

17oC, or identify all periods when the “part-load standard” would apply and the actual level of 

theoretical compliance would be higher. 

Results

NOX emission rates were calculated by dividing the sum of the unadjusted emissions for all non out-ofcontrol operating hours by the sum of the heat input (or gross electrical energy output) over the same 

period as reported to the Clean Air Markets Division (CAMD) of EPA. The parts per million (ppm) 

emission rates were calculated as the average of the hourly emission rates. Out-of-control hours were 

defined as when either the unadjusted applicable emissions or gross output could not be determined for 

that operating hour. 

EPA reviewed a total of approximately 250,000 4-hour averages using CEMS data from 129 stationary 

combustion turbines. Of those turbines, only Manchief 2 has an abnormally high incidence of apparent 

excess emissions. The turbines used at the Manchief facility have been discontinued and can no longer 

be purchased, so EPA removed them from the analysis. 

Since startup and shutdown are part of the regular operating practices of stationary combustion turbines, 

and approximately one-third of NOX emissions occur during these periods for simple cycle turbines, we 

are proposing that the NOX emissions standard includes startup and shutdown emissions. Through 
analysis of CEMS data, we have determined that including periods of startup and shutdown in the 

averaging period would not result in additional rates of potential non-compliance with the standard. We 

analyzed NOX continuous CEMS data from 140,000 hours of operation for 87 existing large turbines 

that did not have post-combustion controls to reduce NOX emissions. Even though many of these 

turbines were built prior to the applicability date of subpart KKKK, the theoretical compliance rate with 

a 4-hour rolling average including all periods of operation was 99.88% on a lb/MMBtu basis. Even 

though the theoretical compliance rate is high when the NOX emissions standard is determined directly, 

implementing the startup and shutdown approach as described earlier increases the theoretical 

compliance rate to 99.96%. Expanding the averaging period to a daily average has the same compliance 

rate of 99.96%. 

Similarly, we analyzed NOX CEMS data from approximately 105,000 hours of operation for 42 existing 

medium turbines without post-combustion controls to reduce NOX emissions. The theoretical 

compliance rate calculating the NOX emissions standard without implementing the startup and shutdown 

procedures is 99.85%. Implementing the startup and shutdown approach increases the theoretical 

compliance rate to 99.92%. Expanding the averaging period to a daily average would increase the 

compliance rate to 99.93%.