MEMORANDUM

From: The Office of Air Qu ality Planning and Standards

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

Date: March, 2011

Subject: Control of NO X Emissions

Introduction

Nitrogen oxide (NO X ) is formed in a steam generating unit by the oxidation of mo lecular nitrogen 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 an excess of combustion air. Under these conditions, si gnificant quantities of NO X are formed regardless of the fuel type burned.

Fuel NO X is generated when nitrogen contained in the fuel itself is oxidized. The rate of formation of

fuel NO X is primarily a function of fuel bound nitrogen cont ent of the fuel but also is affected by fuelair mixing. New steam generating un its being installed today in the United States routinely include

burners and other features desi gned to reduce the amounts of NO X formed during combustion.

Beyond the lower levels of NO X emissions achieved using com bustion controls, additional NO X emission control can be achieved for steam genera ting units by installing post-combustion control technologies. These technologi es involve converting the NO X in the flue gas to molecular nitrogen (N 2 ) and water. The existing EGU NO X NSPS is 130 ng/J (1.0 lb/MWh) gross energy output on a 30- day rolling average basis.

NO X Control Technologies

Combustion Controls

Low NO X Burners (LNB). Low NO X Burners are designed to limit NO X formation by controlling the stoichiometric and temperature prof iles 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 primar y flame zone; (b) reduced flame temperature; or (c) reduced residence time at peak temperature. The most recent applications of LNB

are called Ultra Low NO X Burners due to their improvement s over previous LNB technology where advances in control systems; combustion process modifications and post-combustion carbon burnout technology have further limited NO X formation. LNB are a mature technology for the reduction of

NO X formation during combusti on. Due to their cost effectiveness, LNB are expected to be furnished with a new boiler regardless of other post-combustion NO X 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 di verted from the burners and injected through ports located above the top burner level. OFA is genera lly used in conjunction with operating the burners at stoichiometric or slightly sub-stoi chiometric conditions , which reduces NO X formation. The OFA isthen added to achieve complete combustion. OFA is a mature technology most often utilized concurrently with the application of LNB. OFA compliments the stoi chiometric to sub-stoichiometric operation of LNB by providing the ai r required to complete fuel com bustion and limit the formation of CO and VOC. OFA is expected to be furnished with a new boiler regardless of other post combustion

NO X 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 fire ball and then to create a cyclonic gas flow to improve combustion.

ROFA differs from OFA in that ROFA utilizes a booste r fan to increase the velocity of air to promote better mixing and to increase the retention time in th e furnace. A modification of the ROFA process is

RotaMix in which urea or ammoni a is injected with ROFA.

Induced Flue Gas Recirculati on 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 combus tion region is intensified by the recirculated flue gas when introduced into the flam e during the early stages of com bustion. This intensified mixing offsets the decrease in flame temperature and results in NO X levels that are lower than those achieved

without IFGR. The level of NO X reduction is dependent upon th e burner and furnace design.

Oxygen-Enhanced Combustion (OEC). Oxygen-Enhanced combustion is a NO X combustion control technology that reduces the formation of thermal NO X in the boiler. Pr axair Technology, Inc. developed a method of using oxygen in the LN B instead of air to achieve additional NO X reduction. During testing, OEC was reported to reduce boiler NO X 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 mi n burners, thus creating a secondary combustion zone called th e reburn zone. In NGR, the sec ondary (or reburn) fuel, natural gas, is injected to produce a slightly fuel rich rebu rn 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 NO X formed in the main combustion zone is reduced by hydrocarbon fragments (free radicals) and

converted to mol ecular nitrogen (N 2 ). NGR has been demonstrated to reduce NO X 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 controlle d mixing of natural gas into the furnace exit region

reduces NO X . 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 NO X control using less than 10% gas heat input and no burnout air. Less NO X reduction is achieved with FLGR when compared w ith NGR. FLGR has been demonstrated to reduce

NO X emissions by 33% to 45% on seve ral existing coal-fired boilers.

Advanced Gas Reburning (AGR). Advanced Gas Reburning adds a nitrogen rich compound (typically urea or ammonia) downstream of th e reburning zone. The reburning sy stem is adjusted for somewhat

lower NO X reduction to produce free radicals that enhance the selective non-catalytic NO X reduction. AGR systems can be designed in two ways: (1) non-s ynergistic, which is e ssentially the sequential application of NGR and selectiv e non-catalytic reduction (i.e., th e nitrogen agent is injecteddownstream of the burnout air); and (2) synergistic, in which the nitr ogen agent is injected with a

second burnout air stream . To obtain maximum NO X reduction and minimum reagent slip in nonsynergistic systems, the nitrogen agent must be inj ected so that it is available for reaction with the furnace gases within a te mperature zone around 1800 o F. Demonstrated NO X emission reductions ranged from 50% to 76%, however, a mmonia slip could be an issue.

Amine Enhanced Gas Injection (AEGI). Amine Enhanced Gas Injec tion (AEGI) is similar to AGR, except that burn out air is not us ed, and the selective non-catalytic reduction reagent and reburn fuel are injected to create local, fuel-rich NO X 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 NO X reduction reaction. AEGI has b een demonstrated to reduce NO X 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

NO X can be reduced by 30% to 50%. The NO X and reagent (ammonia or ur ea) react to form nitrogen and water. Because of higher stoichiometric ra tios, both ammonia and ur ea SNCR processes require three or four times as much reagent as SCR systems to achieve similar NO X reductions. The temperature window for efficient SNCR operatio n typically occurs between 1,650°F and 2,000°F depending on the reagent and condition of SNCR ope ration. When the reacti on temperature increases over l800°F, the NO X removal rate decreases due to thermal decomposition of ammonia. On the other hand, the NO X 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 r eaction time at this temperature, are required to use this technology.

Selective Catalytic Reduction (SCR). Selective Catalytic Reduction systems catalytically reduce NO X in flue gas to nitrogen and water using ammonia (NH 3 ) in the chemical reaction. SCR requires the safe transport, handling, and storage of NH 3 . The NH 3 is injected into the fl ue gas and mixed prior to entering the catalyst. Within the catalyst bed, the NO X reacts with the NH 3 and is dependent on temperature. Because the reactions normally pr oceed at temperatures between 1,600°F and 1,800°F, a catalyst is used to promote the reactions at lower te mperatures. The use of base metal oxides, such as vanadium pentoxide, titanium di oxide, or noble metals, for both the active and support materials for the composition of the catalyst has been genera lly acknowledged. Newer, more sulfur-resistant ceramic catalysts have recently been used. A temper ature range of 570°F of to 750°F is typical for the reduction process, which exists within the high-pre ssure boiler section of the HRSG. If the catalyst bed is not located in the proper temperature zone, th e 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 system s have been designed to achieve NO X reductions of 80% and above, while maintaining ammonia s lip at acceptable levels. Low SO 2 to SO 3 conversion and erosion

resistance have also been achieve d 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, a nd then absorbing nitrogen dioxide onto its surface through the use of a potassium carbona te absorber coating. The potassi um carbonate coating reacts toform potassium nitrites and nitrates , which are then present on the surf ace of the catalyst. When all of the carbonate absorber coating on the surface of the catalyst has reacted to form nitrogen compounds,

NO X will no longer be absorbed, and the catalyst must enter the rege neration cycle. The regeneration of the catalyst is accomplished by passing a dilute hydrogen reducing gas acro ss 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 th e regeneration gas reacts with potassium nitrites and nitrates to form potassium carbonate, allowing th e oxidation/absorption cycle to begin again. This cycle is referred to as the "regener ation cycle." Because the regenerati on cycle must take place in an oxygen free environment, a section of catalyst undergoi ng 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 regene ration cycle, these louvers cl ose and a valve opens allowing regeneration gas into the section. Stainless steel seali ng strips on the isolat ion louvers provide a

durable and elective barrie r 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 Reducti on is a combination of SNCR and SCR that is designed to provide th e performance of full SCR with lo wer costs. In HSR, the SNCR system is used to achieve some NO X reduction and to produce a contro lled amount of ammonia slip

that is used in a downstream in -duct SCR reactor for additional NO X reduction

THERMALONOx. The THERMALONOx technology has been developed by Themlal Engineering International. The technology is based on the oxidation of NO to NO 2 and then dissolving the NO 2 in water. The THERMALONOx technology is intended for use with a wet flue gas desulfurization (FGD) system used for SO 2 emission control. The NO oxida tion is accomplished by injection elemental phosphorous into the flue gas stream in a gas reactor in stalled upstream of the wet FGD

absorber. The NO 2 becomes dissolved in the wet FGD absorber and can be removed as elemental N 2 or various phosphate compounds that may be used as fertilizer and/or animal food additives.

NO X Control Costs

Cost estimates were prepared for the regulatory ba seline and the proposed re gulatory 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 alte rnate baseline takes the requirements of New Source Review and the propos ed Toxics Rule into account. The alternate baseline is lower than the NSPS since these regul ations require additional reductions beyond what the NSPS requires.

To estimate impacts of the proposed NSPS amendmen ts 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 exis ting NSPS, owners/operato rs of a new EGU would already be required to install combustion contro ls and selective catalyt ic reduction (SCR). The proposed amendments would require that the ow ner/operator of the subbituminous increase the NO X removal efficiency from 45 to 61 percent. The owne r operators of the bituminous facility would have to improve the NO X removal efficiency from 63 to 74 percent. The overall annual NO X reductions would be 1,100 tons and the annualized costs woul d be $840,000. The incremental cost of $750 is

considered cost effective.