MEMORANDUM From: The Office of Air Qu ality Planning and Standards U.S. Environmental Protection Agency To: Combustion Turbine NSPS Docket (EPA-HQ-OAR-2004-0490)
Date: June, 2012 Subject: NO X Emissions Data

Introduction The Utility Air Regulatory Group (UARG) reque sted reconsideration and submitted supporting materials questioning the achievability of the final NO X emissions standard for stationary combustion turbines with continuous emissions monitoring system s. One issued raised by UARG is whether the 4- hour continuous average is appropriate for simple cycle stationary combustion turbines. Nitrogen oxide (NO X ) is formed in combustion turbines by the oxidation of molecular nitrogen (N 2 ) in the combustion air (thermal NO X ) and any nitrogen compounds c ontained in the fuel (fuel NO X ). The formation of NO X 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 in stalled today in the United States routinely include water/steam injection or dry low NO X (DLN) technology. Post-combustion tec hnologies that are designed to reduce the amounts of NO X formed during combustion, such as selec tive 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 NO X formation by decreasing the peak flame temperature. The degree of reduction in NO X formation is proportional to the amount of water in jected into the co mbustion 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) a nd volatile organic compound (VOC) emissions are formed as a result of incomplete combustion. Dry Low-NOX (DLN) Burners

NO X can be limited by lowering combustion temperatur es and by staging combustion (i.e., creating a reducing atmosphere followed by an oxidizi ng atmosphere). The use of dry low-NO X (DLN) burners as a way to reduce flame temperature is a common NO X control method. The combustor designs are called DLN burners because injecting water into the combustion chamber is not necessary to achieve low NO X emissions. This method is primarily used when firing natural gas.

NO X Control Performance Using C ontinuous 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 fi rst is to download “Unit Level Emissions” monthly files with hourly data from speci fic facilities. The emissions data available through this approach contains substituted missing data and bias adjustment s to get the “adjusted” emissions that are used by the Clean Air Markets Division. Compliance with th e New Source Performance Standards is done using the “unadjusted” emissions data. The “adjusted” emi ssions are generally only slig htly 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 stati onary combustion turbine continuous emission monitoring (CEMS) data that were reviewed are as follows. First, the facility had to have a NO X 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 c ontrols and not SCR to control NO X emissions. Approach EPA reviewed “unadjusted emissions data” for speci fic sources. To determine the applicable hourly NO X standard that would apply, EPA us ed the following sorting criteria:

1. Startup and shutdown hours were id entified as any hour with less than a full hour of operation, the previous or following hour of operation ar e not consecutive, both natural gas and oil are burned in the same hour (fuel switching), the firs t 30 minutes of operation constitute a startup hour regardless of the average lo ad (i.e., any hour where the previous hour had 29 minutes or less operation is also considered a startup hour regard less of the average load), and a shutdown hour included hours when the following hour average lo ad was equal to or less than 30% and had 15 minutes or less of operation. These final 2 condit ions would also likely re sult 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 le ss 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 gi ven hour applies (it is not necessary that the average hour load be less than 75%). Since we did not have instantaneou s data, there might be additional hours where the part-l oad 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 a ny of the potential excess emissions were a result of either peri ods of turbine tuning, malfunction of the NO X 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

NO X emission rates were calculated by dividing the sum of the unadjus ted 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 Divisi on (CAMD) of EPA. The parts per million (ppm) emission rates were calculated as the average of th e hourly emission rates. Ou t-of-control hours were defined as when either the unadjusted applicable em issions 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 turb ines, only Manchief 2 has an abnor mally high incidence of apparent excess emissions. The turbines used at the Manchief facility have been di scontinued and can no longer be purchased, so EPA removed them from the analysis. Since startup and shutdown are partof the regular operating practices of stationary combustion turbines, and approximately one-third of NO X emissions occur during these periods for simple cycle turbines, we are proposing that the NO X emissions standard includes startu p and shutdown emissions. Throughanalysis 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-com pliance with the standard. We analyzed NO X continuous CEMS data from 140,000 hours of operation for 87 existing large turbines that did not have post-com bustion controls to reduce NO X emissions. Even though many of these turbines were built prior to the ap plicability date of subpart KKKK, the theoretical compliance rate with a 4-hour rolling average in cluding all periods of operation wa s 99.88% on a lb/MMBtu basis. Even though the theoretical compliance rate is high when the NO X 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 NO X CEMS data from approximately 105,000 hours of operation for 42 existing medium turbines without post-combustion controls to reduce NO X emissions. The theoretical compliance rate calculating the NO X emissions standard without impl ementing the startup and shutdown procedures is 99.85%. Implementing the startup a nd shutdown approach increases the theoretical compliance rate to 99.92%. Expanding the averagi ng period to a daily average would increase the compliance rate to 99.93%.