Attachment 3 

Industry Position Paper: 

API/NPRA/WSPA Process Heater Turndown Position 11-19-09 
Page 1 

API/NPRA/WSPA Process Heater Turndown Position 11-19-09

API/NPRA/WSPA believe that some refinery process heaters that operate at high turndowns for 

extended periods cannot achieve the NOx emission limits that are included in the December 28, 

2008 proposed NSPS Ja amendments. Thus, the proposed NOx limits do not represent Best 

Demonstrated Technology (BDT) for those process heaters and EPA should supplement the 

proposed standard by establishing a standard that applies to these process heaters. 

Providing such a mass limit for turndown operations does not increase emissions, but only 

recognizes the characteristics of burner operation at turndown rates. Under a mass limit, NOx 

mass emissions during turndown are still well below the mass emitted at high rates at the 

concentration of the emission standard and in most cases will be well below the mass emitted by 

unregulated process heaters (e.g., those <40 MMBTU/hr). 

We would also point out that a shorter averaging time than the proposed 365 days would increase 

the number of process heaters that can not comply with the proposed emission limit, because 

process, production and maintenance related cutbacks of process heater duty can go on for weeks 

or even a few months. For instance, if a process heater were operated at 10% of capacity for one 

month and 90% the rest of the year, the annual average firing would be 83% and the average 

NOx concentration for the year would not be significantly impacted by the higher emissions 

concentration during the one month period. Yet, if the emission limit were a 30 day average, the 

process heater could be out of compliance at least 30 times. 

Basis for Position

1. Process heaters that operate at turndown rates for extended periods were not 

fully considered in developing the proposed process heater NOx emission limit. 

Docket Document EPA-HQ-OAR-2007-0011-0290 (Evaluation of Nitrogen Oxides Emissions 

Data for Process Heaters, December 19, 2008) reports EPA’s analysis of NOx emission data 

collected from industry and vendors that served as the basis for the proposed emission limits. 

This report summarizes data obtained from CEMS monitoring, stack tests (mostly from burner 

test cells) and vendors, but does not provide adequate detail to determine how the Agency used it 

to develop the proposed limits. It is clear from the data tables and discussion however, that 

process heater turndown operation was not well represented in the data. The little data that is 

included for turndown operation (four test cell sets of data reported for the latest generation of 

ultra low NOx burners on different fuels where emissions were determined at 20-30% of normal 

firing as well as at normal levels) showed in three of the four cases that the NSPS Ja NOx 

concentration limit would not be met at or just below these firing levels, even under test cell 

conditions. In general, the data referenced in this document and otherwise available does not 

seem to include any process heaters that normally operate at high turndown. This is reasonable, 

since CEMS are not justified for process heaters with small NOx emissions and performance 

testing is done at or near design rates. 

 
API/NPRA/WSPA Process Heater Turndown Position 11-19-09 Page 2 

None-the-less, the Agency appears to have accepted that the industry concern is valid, because 

there is an attempt to provide relief in the December 22 proposal. Specifically, EPA proposes 

applying a 365-day averaging time to the NOx limit, in order to address periods of turndown. It 

is stated in the proposal preamble: 

Turn downs, therefore, result in less efficient combustion, which tends to increase 

NOX concentrations in the heater exhaust. Even though the concentration of NOX 

increases during turn downs, the mass of NOX emitted does not because there is 

less exhaust gas produced. Turn downs typically occur in hydrotreater or 

hydrogen units that have varying operational rates. Some process heaters may be 

in turn down for months (e.g., when a hydrotreater is using a new catalyst). As 

Industry Petitioners point out, one way to allow for the variations in emissions 

that are due to process fluctuations, turn downs, and variations in fuel gas 

composition is to extend the averaging time over which compliance is determined. 

Based on the above information, we are proposing changes to the NOX limit to 

address these issues. [73 FR 78526, Dec 22, 2008] 

API/NPRA/WSPA agree that providing an annual averaging time along with the Part 60 General 

Provisions SS&M exemption, handles the problem for most turndown situations, since short and 

even intermediate-length periods of turndown would not drastically affect an annual average. 

However, the annual average approach still does not address the emissions from those process 

heaters that operate at turndown rates for all or a significant part of a year, where they are turned 

down to the point that their NOx emissions concentration corrected to 0% oxygen is above the 

proposed limits. We believe there are enough of these situations in the industry that further relief 

is required. 

Given that these low emitters are not typically equipped with CEMS, direct demonstrations of 

this problem are not available. Thus, we rely on two elements to support our argument. First, 

that NOx emission concentrations increase as process heaters are turned down, though the point 

at which the design emission concentration is exceeded varies with the heater. Second, there are 

a reasonable number of process heaters that operate at high turndowns for long periods of time. 

We also point out in Item 4 that there is precedent for the relief we seek. 

2. Most process heaters cannot meet the proposed standards at some turndown 

rate. 

NOx emission rates, in ppmv or lb/MMBTU, from process heaters increase at low firing rates 

when emissions are calculated on a dry, 0% excess air basis. Therefore, many Part 60 Subpart Ja 

process heaters will be unable to maintain their design NOx emission rate below the applicable 

40 or 60 ppmv (dry, 0% O2) emission limitation under turndown conditions, though the point at 

which these emission rates would be exceeded is process heater specific. This is amply 

demonstrated in the data available to the EPA. 

 
API/NPRA/WSPA Process Heater Turndown Position 11-19-09 Page 3 

As EPA says in the NSPS Ja Amendments Final BID (EPA-HQ-OAR-2007-0011-0228), page 

31, “CEMS are not cost-effective means of ensuring compliance for the smaller process heaters.” 

Thus, it is not surprising that CEM data for process heaters that normally operate under turndown 

conditions is lacking. Installing CEMs on such process heaters does not make economic sense 

and, because of the low emissions, usually doesn’t make environmental sense. There is however, 

quite a bit of CEMS data that has been provided to EPA for process heaters that normally operate 

at higher firing rates. This data shows clearly that when process heaters fire at low rates, the 

stack excess air increases and the NOx concentrations, corrected to 0% excess air, increase. 

This issue was explained by Dr. Waibel of John Zinc in his paper (Docket Item EPA-HQ-OAR2007-0011-0262.2) as follows. 

Modern ultra low NOx burners reduce NOx by staging fuel and entraining furnace 

gases into the fuel gas jets as the fuel is injected into the flame zone. The 

entrained furnace gases help moderate peak temperatures within the flame. 

Typically the more furnace gas entrained into the flame zone the lower the NOx 

emissions. At the same time, the higher the furnace operating temperature the 

higher the level of entrainment must be to get the desired emission reduction. 

For any given fuel composition and furnace gas temperature there is a limit to 

how much furnace gas can be entrained into the flame zone before the flame is 

cooled to the point where the combustion reaction is quenched resulting in flame 

instability and ultimately loss of flame. The maximum amount of furnace gas that 

can be mixed with the fuel depends on the temperature of the furnace gas and the 

oxygen content of the furnace gas. High levels of entrainment of cool, low oxygen 

furnace gases will result in flame quenching. 

The heater operating temperature is highest at maximum firing rate and burners 

must be designed with high levels of fuel staging and furnace gas entrainment to 

reduce NOx for this high temperature operation. Unfortunately, under turndown 

conditions when the furnace operating temperature drops the level of staging and 

entrainment that was necessary and acceptable under high temperature conditions 

will quench the flame unless the oxygen level in the furnace gases is elevated. 

Although the elevated oxygen level is good for flame stability it results in an 

elevated NOx concentration in the stack gases. However, the firing rate is low, so 

even though the pounds of NOx per million Btu is generally higher, the pounds of 

NOx per hour produced by the heater under turndown is generally less than the 

emissions under high fire operation. 

Dr. Waibel’s paper includes two examples of process heaters equipped with ultra-low NOx 

burners. In both cases, the proposed emission limits could not be met if these heaters had to 

operate under turned down conditions for extended periods. 

 
API/NPRA/WSPA Process Heater Turndown Position 11-19-09 Page 4 

Furthermore, Dr. Waibel states that “NOx guarantees provided by burner designers are generated 

by evaluating the worst case conditions specified for the maximum and normal firing rates for 

the heater. A maximum predicted emission level is determined and then a safety factor is applied 

to accommodate uncertainty and help insure that the guarantee level will be met. Guarantees are 

not made for operation below 50% of design since the operating conditions become uncertain 

and excess air levels must be set for safe operation rather than for NOx control.” 

3. Some process heaters operate at high turndown conditions for extended periods. 

For the reasons stated in item 2, it would be expected that some of these process 

heaters will not be able to meet the proposed emission limits. 

There are some situations where process heaters’ normal operations involve extended turndown 

operation. A common example is hydrofiner heaters, where minimal heater duty is required 

when the catalyst is fresh, which can result in years of operation at high turn down rates. The 

population of such process heaters is rising as the result of the recent rash of low sulfur diesel 

projects installed in response to the EPA Ultra Low Sulfur Diesel requirements. Extended 

turndown operation can also occur as a result of economic conditions. On September 9, 2009, in 

a letter to Mr. K. C. Hustvedt, we provided an analysis of the process heater operating data 

contained in the refinery industry consent decrees. A total of 800 process heaters with design 

firing over 40 MMBTU/hr were identified in the Consent Decrees reviewed. The review 

determined that approximately 30% (231 of 776) of the process heaters for which operating data 

was available operated at <50% of design firing for the year identified, 56 of those process 

heaters operated at <25% of design firing for the year and 16 operated at <10% of design firing 

for the year. The Consent Decrees generally contained operating data for two years and, in 

general, operating rates appear to track from year to year. 

Examples of process heaters operating at high turndown for extended periods provided by our 

members include: 

 One API member reports sizing a process heater to handle cold imported material in 

addition to warm site-generated material and having the import project cancelled for 

economic reasons after the process heater was installed, leaving the process heater to 

operate at much less the 50% of capacity. 

 Another member reports a process heater that changed service from a cat feed 

hydrotreater to a diesel hydrotreater in 2006 in response to EPA's Ultra Low Sulfur 

Diesel requirements, which resulted in an initial annual heat duty of about 9 MMBTU/hr 

versus a design duty of 51 MMBTI/hr. 

 A regeneration heater at a Powerformer (reformer) is used to heat nitrogen to 

initiate burns when reactor catalyst is regenerated and to heat up the reactor prior to 

putting it back on line. The capacity of the heater is 66 MMBtu/hr and it operated for 

7672 hours last year below 33 MMBtu/hr (50%). The average heat duty for the year was 

19.7 MMBtu/hr. 

 
API/NPRA/WSPA Process Heater Turndown Position 11-19-09 Page 5 

 A FCCU preheater is reported to typically fire about 50 MMBTU/hr, though it was 

originally designed for 350 MMBTU/hr. This particular preheater is equipped with a 

CEM that shows NOx concentrations increase by 50-100% at turndown rates versus 

design rates. 

In several of the above examples, the process heater is sized for the highest duty needed even 

though the heater operates at high turndown part of the time. In other examples, the process 

heater was sized for a different service than it is currently serving. In these later cases, operating 

in turndown mode is appropriate because the demand on the heater may increase in the future or 

because spacing issues, timing or cost make installing a replacement unjustified by the very 

small increment of NOx reductions that might be obtained. 

We believe these situations are representative of a subset of process heaters that may find they 

cannot meet the Subpart Ja NOx requirements on becoming subject to Subpart Ja and that relief 

is therefore justified. 

4. There is precedent for applying a mass emission limit for turndown operation. 

This is not a new issue and there is precedent for addressing turndown process heater NOx 

emissions by applying a mass limit. 

In the South Coast Air Quality Management District NOx rule for refineries (SCAQMD Rule 

1109), section (b)(1)(B)(iii) provides for a mass emission limit equal to the NOx emissions that 

would occur at maximum rates for boilers and process heaters that operate at “less than the 

maximum rated capacity.” 

As we indicated in our September 9, 2009 letter, the Bay Area Air Quality Management District 

has approved NOx mass emission limits equivalent to the NOx emissions that would occur at 

maximum rates for two process heaters at Valero’s Benicia refinery. 

More generally, in the early days of Title V in the Bay Area (2003-2004), the 5 Bay Area 

refineries negotiated the “NOx Box” permit condition which was for small heaters without NOx 

CEMs. Basically, the x-y coordinates are fired duty and O2 concentration, and source testing 

defined the “NOX Box” on those axes so that operation within the Box was deemed compliant 

and operation outside was a deviation requiring additional source testing to determine 

compliance. The NOX Box permit condition also recognized periods of curtailed operation 

<20% of rated fired duty for up to 5 days (refractory dry out, etc.). The key to the permit 

condition is the “either / or” language so that you have to meet a certain concentration averaged 

over X hours, OR a mass limit averaged over X hours.