MEMORANDUM 

From: The Office of Air Quality Planning and Standards 

To: EGU NSPS Docket (EPA-HQ-OAR-2011-0044) 

Date: March, 2011 

Subject: Control of NOX Emissions 

Introduction 

Nitrogen oxide (NOX) is formed in a steam generating unit by the oxidation of molecular nitrogen 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 an excess of combustion air. 

Under these conditions, significant quantities of NOX are formed regardless of the fuel type burned. 

Fuel NOX is generated when nitrogen contained in the fuel itself is oxidized. The rate of formation of 

fuel NOX is primarily a function of fuel bound nitrogen content of the fuel but also is affected by fuelair mixing. New steam generating units being installed today in the United States routinely include 

burners and other features designed to reduce the amounts of NOX formed during combustion. 

Beyond the lower levels of NOX emissions achieved using combustion controls, additional NOX

emission control can be achieved for steam generating units by installing post-combustion control 

technologies. These technologies involve converting the NOX in the flue gas to molecular nitrogen 

(N2) and water. The existing EGU NOX NSPS is 130 ng/J (1.0 lb/MWh) gross energy output on a 30-

day rolling average basis. 

NOX Control Technologies 

Combustion Controls 

Low NOX Burners (LNB). Low NOX Burners are designed to limit NOX formation by controlling the 

stoichiometric and temperature profiles of the combustion process. This control is achieved by design 

features that regulate the aerodynamic distribution and mixing of the fuel and air, resulting in one or 

more of the following conditions: (a) reduced oxygen in the primary flame zone; (b) reduced flame 

temperature; or (c) reduced residence time at peak temperature. The most recent applications of LNB 

are called Ultra Low NOX Burners due to their improvements over previous LNB technology where 

advances in control systems; combustion process modifications and post-combustion carbon burnout 

technology have further limited NOX formation. LNB are a mature technology for the reduction of 

NOX formation during combustion. Due to their cost effectiveness, LNB are expected to be furnished 

with a new boiler regardless of other post-combustion NOX emission reduction technologies employed. 

Overfire Air (OFA). Overfire Air, also referred to as air staging, is a combustion control technology in 

which 5% to 20% of the total combustion air is diverted from the burners and injected through ports 

located above the top burner level. OFA is generally used in conjunction with operating the burners at 

stoichiometric or slightly sub-stoichiometric conditions, which reduces NOX formation. The OFA is 
then added to achieve complete combustion. OFA is a mature technology most often utilized 

concurrently with the application of LNB. OFA compliments the stoichiometric to sub-stoichiometric 

operation of LNB by providing the air required to complete fuel combustion and limit the formation of 

CO and VOC. OFA is expected to be furnished with a new boiler regardless of other post combustion 

NOX emission reduction technologies employed. 

Rotating Opposed Fire Air (ROFA). The Rotating Opposed Fire Air (ROFA) design injects air into 

the furnace first to break up the fireball and then to create a cyclonic gas flow to improve combustion. 

ROFA differs from OFA in that ROFA utilizes a booster fan to increase the velocity of air to promote 

better mixing and to increase the retention time in the furnace. A modification of the ROFA process is 

RotaMix in which urea or ammonia is injected with ROFA. 

Induced Flue Gas Recirculation or Flue Gas Recirculation. Induced Flue Gas Recirculation (IFGR) 

recirculates boiler flue gas from the boiler outlet to the furnace where it is reintroduced into the 

combustion process. Fuel/air mixing in the combustion region is intensified by the recirculated flue 

gas when introduced into the flame during the early stages of combustion. This intensified mixing 

offsets the decrease in flame temperature and results in NOX levels that are lower than those achieved 

without IFGR. The level of NOX reduction is dependent upon the burner and furnace design. 

Oxygen-Enhanced Combustion (OEC). Oxygen-Enhanced combustion is a NOX combustion control 

technology that reduces the formation of thermal NOX in the boiler. Praxair Technology, Inc. 

developed a method of using oxygen in the LNB instead of air to achieve additional NOX reduction. 

During testing, OEC was reported to reduce boiler NOX emissions by as much as 60% when compared 

to utilizing ambient air at the test facility. 

Natural Gas Reburning. (NGR) Natural Gas Reburning (NGR) is a combustion technology in which 

part of the main fuel heat input is diverted to locations above the min burners, thus creating a 

secondary combustion zone called the reburn zone. In NGR, the secondary (or reburn) fuel, natural 

gas, is injected to produce a slightly fuel rich reburn zone. Overfire air is added above the reburn zone 

to complete burnout of the reburn zone. As flue gas passes through the reburn zone part of the NOX

formed in the main combustion zone is reduced by hydrocarbon fragments (free radicals) and 

converted to molecular nitrogen (N2). NGR has been demonstrated to reduce NOX emissions by 39% 

to 67% on several existing coal-fired boilers in applications ranging in size from 33-600 MW in the 

United States and up to 800 MW overseas. 

Fuel Lean Gas Reburning (FLGR). Fuel Lean Gas Reburning, also known as controlled gas injection, 

is a process in which careful injection and controlled mixing of natural gas into the furnace exit region 

reduces NOX. The gas is normally injected into a lower temperature zone than in NGR. Whereas 

NGR requires 15% to 20% of furnace heat input from gas and requires burnout air, the FLGR 

technology achieves NOX control using less than 10% gas heat input and no burnout air. Less NOX

reduction is achieved with FLGR when compared with NGR. FLGR has been demonstrated to reduce 

NOX emissions by 33% to 45% on several existing coal-fired boilers. 

Advanced Gas Reburning (AGR). Advanced Gas Reburning adds a nitrogen rich compound (typically 

urea or ammonia) downstream of the reburning zone. The reburning system is adjusted for somewhat 

lower NOX reduction to produce free radicals that enhance the selective non-catalytic NOX reduction. 

AGR systems can be designed in two ways: (1) non-synergistic, which is essentially the sequential 

application of NGR and selective non-catalytic reduction (i.e., the nitrogen agent is injected 
downstream of the burnout air); and (2) synergistic, in which the nitrogen agent is injected with a 

second burnout air stream. To obtain maximum NOX reduction and minimum reagent slip in nonsynergistic systems, the nitrogen agent must be injected so that it is available for reaction with the 

furnace gases within a temperature zone around 1800oF. Demonstrated NOX emission reductions 

ranged from 50% to 76%, however, ammonia slip could be an issue. 

Amine Enhanced Gas Injection (AEGI). Amine Enhanced Gas Injection (AEGI) is similar to AGR, 

except that burn out air is not used, and the selective non-catalytic reduction reagent and reburn fuel 

are injected to create local, fuel-rich NOX reduction zones in an overall fuel-lean furnace. The fuelrich zone exists in local eddies, as in FLGR, with the overall furnace in an oxidizing condition; 

however the reduction reagent participates with natural gas (or other hydrocarbon fuel) in a NOX

reduction reaction. AEGI has been demonstrated to reduce NOX emissions by 30% to 73% during fullscale commercial applications. 

Post-Combustion Controls 

Selective Non-Catalytic Reduction (SNCR). In Selective Non-Catalytic Reduction systems, a reagent 

is injected into the flue gas in the furnace within an appropriate temperature window. Emissions of 

NOX can be reduced by 30% to 50%. The NOX and reagent (ammonia or urea) react to form nitrogen 

and water. Because of higher stoichiometric ratios, both ammonia and urea SNCR processes require 

three or four times as much reagent as SCR systems to achieve similar NOX reductions. The 

temperature window for efficient SNCR operation typically occurs between 1,650°F and 2,000°F 

depending on the reagent and condition of SNCR operation. When the reaction temperature increases 

over l800°F, the NOX removal rate decreases due to thermal decomposition of ammonia. On the other 

hand, the NOX reduction rate decreases below 1,800 °F and ammonia slip may increase. Temperatures 

in the range of 1,500 to 1,900°F, along with adequate reaction time at this temperature, are required to 

use this technology. 

Selective Catalytic Reduction (SCR). Selective Catalytic Reduction systems catalytically reduce NOX

in flue gas to nitrogen and water using ammonia (NH3) in the chemical reaction. SCR requires the safe 

transport, handling, and storage of NH3. The NH3 is injected into the flue gas and mixed prior to 

entering the catalyst. Within the catalyst bed, the NOX reacts with the NH3 and is dependent on 

temperature. Because the reactions normally proceed at temperatures between 1,600°F and 1,800°F, a 

catalyst is used to promote the reactions at lower temperatures. The use of base metal oxides, such as 

vanadium pentoxide, titanium dioxide, or noble metals, for both the active and support materials for 

the composition of the catalyst has been generally acknowledged. Newer, more sulfur-resistant 

ceramic catalysts have recently been used. A temperature range of 570°F of to 750°F is typical for the 

reduction process, which exists within the high-pressure boiler section of the HRSG. If the catalyst 

bed is not located in the proper temperature zone, the reaction efficiency will be reduced if either the 

temperature is too low, resulting in excessive ammonia slip, or the catalyst may be damaged if the 

temperature is too high. SCR systems have been designed to achieve NOX reductions of 80% and 

above, while maintaining ammonia slip at acceptable levels. Low SO2 to SO3 conversion and erosion 

resistance have also been achieved through modifications to chemistry and the manufacturing process. 

Catalytic Absorption/Oxidation (SCONOxTM). The SCONOxTM system uses a single catalyst to 

simultaneously oxidizing carbon monoxide to carbon dioxide, hydrocarbons to carbon dioxide and 

water, nitrogen oxides to nitrogen dioxide, and then absorbing nitrogen dioxide onto its surface 

through the use of a potassium carbonate absorber coating. The potassium carbonate coating reacts to 
form potassium nitrites and nitrates, which are then present on the surface of the catalyst. When all of 

the carbonate absorber coating on the surface of the catalyst has reacted to form nitrogen compounds, 

NOX will no longer be absorbed, and the catalyst must enter the regeneration cycle. The regeneration 

of the catalyst is accomplished by passing a dilute hydrogen reducing gas across the surface of the 

catalyst in the absence of oxygen. The hydrogen in this gas reacts with nitrites and nitrates to form 

water and elemental nitrogen. Carbon dioxide in the regeneration gas reacts with potassium nitrites 

and nitrates to form potassium carbonate, allowing the oxidation/absorption cycle to begin again. This 

cycle is referred to as the "regeneration cycle." Because the regeneration cycle must take place in an 

oxygen free environment, a section of catalyst undergoing regeneration must be isolated from exhaust 

gases. This is accomplished using a set of louvers, one upstream of the section being regenerated and 

one downstream. During the regeneration cycle, these louvers close and a valve opens allowing 

regeneration gas into the section. Stainless steel sealing strips on the isolation louvers provide a 

durable and elective barrier against leaks during operation. A SCONOxTM system typically will have 

five to fifteen sections of catalyst, depending on the size and any special requirements this number can 

vary from system to system. SCONOxTM technology has not been demonstrated on flue gas generated 

by coal combustion. 

Hybrid Selective Reduction (HSR). The Hybrid Selective Reduction is a combination of SNCR and 

SCR that is designed to provide the performance of full SCR with lower costs. In HSR, the SNCR 

system is used to achieve some NOX reduction and to produce a controlled amount of ammonia slip 

that is used in a downstream in-duct SCR reactor for additional NOX reduction 

THERMALONOx. The THERMALONOx technology has been developed by Themlal Engineering 

International. The technology is based on the oxidation of NO to NO2 and then dissolving the NO2 in 

water. The THERMALONOx technology is intended for use with a wet flue gas desulfurization 

(FGD) system used for SO2 emission control. The NO oxidation is accomplished by injection 

elemental phosphorous into the flue gas stream in a gas reactor installed upstream of the wet FGD 

absorber. The NO2 becomes dissolved in the wet FGD absorber and can be removed as elemental N2

or various phosphate compounds that may be used as fertilizer and/or animal food additives. 

NOX Control Costs 

Cost estimates were prepared for the regulatory baseline and the proposed regulatory amendment. We 

defined two separate baselines for the comparisons. The first baseline considers the existing NSPS 

requirements in the absence of other regulatory requirements. The alternate baseline takes the 

requirements of New Source Review and the proposed Toxics Rule into account. The alternate 

baseline is lower than the NSPS since these regulations require additional reductions beyond what the 

NSPS requires. 

To estimate impacts of the proposed NSPS amendments we estimated the incremental costs for two 

500 MW model plants. One of the model plants burns Illinois bituminous coal and the other 

subbituminous coal from Montana. Under the existing NSPS, owners/operators of a new EGU would 

already be required to install combustion controls and selective catalytic reduction (SCR). The 

proposed amendments would require that the owner/operator of the subbituminous increase the NOX

removal efficiency from 45 to 61 percent. The owner operators of the bituminous facility would have 

to improve the NOX removal efficiency from 63 to 74 percent. The overall annual NOX reductions 

would be 1,100 tons and the annualized costs would be $840,000. The incremental cost of $750 is 

considered cost effective.