The 35th International Technical Conference on Clean Coal & Fuel Systems, Clearwater, Florida June 6-10, 2010

ROFA and Rotamix System Reduced NOx below 200 mg/Nm3 at Elektrownia Opole

Brian Higgins, Baiyun Gong, Ed Pozzobon, Guisu Liu,Edward Kinal¤, Jan Pilipionek¤, Mirosław Pietrzyk¤,

Robert Zmuda§, Peter Hajewski§, and Wlodzimierz Blasiak§

Nalco Mobotec ¤Elektrownia Opole §Nalco Mobotec Polska2540 Camino Diablo Polska Grupa Energetyczn 43-110 Tychy

Walnut Creek, CA 94597 USA Opole, Poland ul. Przemyslowa 55 Poland

ABSTRACT

Nalco Mobotec’s Rotating Opposed Fired Air(ROFA®) and Rotamix® systems and extensivewindbox modifications were installed on PGE’sElektrownia Opole Unit 3 in Poland to reduce NOxemission. The RAFAKO BP-1150 boiler is a 380-MWtangentially fired, tower boiler burning pulverized coal.The installation included three steps: (1) Anoperational, mechanical, and performance review ofthe existing firing system, leading to tangentialwindbox modification and upgrade, (2) the existingSOFA system was replaced by Nalco Mobotec’sROFA system, and (3) A Rotamix SNCR system wasinstalled, introducing urea into the upper furnace. Thesystem design and CFD modeling and field results arepresented in this paper.

The combined effect of the ROFA and Rotamixsystems, with the tangential firing system upgrades,resulted in NOx emission below 200 mg/Nm3 at 6% O2

(0.15 lb/MMBtu), averaging 168 mg/Nm3 over a ten-day reliability test. This is an overall reduction of 52%to 66% from the baseline with LNB and SOFA (350-500 mg/Nm3). The improved combustion due tomixing introduced by the ROFA system, maintainedloss-on-ignition (LOI) below 5% as required forcontinued fly-ash sales. Simultaneously, the COemissions were held to an average value of 41 mg/Nm3

over the ten days. Ammonia slip from the SNCRsystem averaged 3.1 ppm (5 ppm limit) at theeconomizer outlet, 35 mg/kg average (50 mg/kg limit)in the ash, and 8.6 mg/kg average (10 mg/kg limit) inthe gypsum produced from the WFGD solids.

This paper presents the system design, CFD modeling,burner modification, and the field results in detail.

INTRODUCTION

In 2016, Polish NOx emissions regulations are to dropto 200 mg/Nm3 (at 6% O2) under the large combustionplant directive (LCPD)1. It is difficult to reach this

1 Directive 2001/80/EC of the European Parliament and of theCouncil of 23 October 2001 on the limitation of emissions of certain

level of NOx emission with standard overfire air(OFA) and selective non-catalytic reduction (SNCR)systems without causing undue operational challenges.Selective catalytic reduction (SCR) systems areprohibitively expensive. Due to the large number ofboilers in Poland that must be below 200 mg/Nm3,many are installing equipment early. In 2008, PGEElektrownia Opole2 procured the Mobotec System withthree elements:

1) Windbox modification and upgrade2) ROFA System3,4 for NOx reduction and

combustion improvement3) SNCR Rotamix System5 for urea mixture

injection, in order to further reduce NOx

Elektrownia Opole mandated that the installation couldnot degrade plant performance with specifications onCO, fly ash LOI, slag ash LOI, ammonia slip, ammoniain ash, ammonia in gypsum, ambient noise,availability, boiler efficiency, wastage, steam flow,steam temperature, and consumption of urea, water,electricity, and compressed air. The work for thisproject was performed by a consortium that includedNalco Mobotec and Remak-Rozruch6.

FURNACE DESCRIPTION

This BP-1150 RAFAKO7 tower boiler normallyoperates between 180 and 380 MWe. The boiler is asub-critical REFAKO tower-design boiler rated at 1170t/h of steam at 540°C and 18.7 MPa. There are fivepulverizers that feed ten burner levels. Only four

pollutants into the air from large combustion plants [OJ L 309,27.11.2001]2 Polska Grupa Energetyczna (PGE: www.pgesa.pl) operatesElektrownia Opole (www.elopole.bot.pl)3 Coombs, Crilley, Shilling, and Higgins, “SCR Levels of NOxReduction with ROFA and Rotamix (SNCR) at Dynegy’s VermilionPower Station” 2004 Stack Emissions Symposium, ClearwaterBeach, FL, July 28-30, 20044 Haddad, Ralston, Green, and Castagnero, “Full-Scale Evaluation ofa Multi-Pollutant Technology: SO2, Hg, and NOx”, MEGASymposium, Paper No.117, 20035 Liu, Higgins, and Zarzar, “Performance Testing and Modeling of anAdvanced SNCR NOx Control System,” MEGA Symposium, #103,20066 www.remak-rozruch.com.pl7 RAFAKO SA (www.rafako.com.pl)

pulverizers are required to reach full load. When firinghigh calorific coal only, full load can even be met withonly three mills in service. Through pre-modernizationplant operational tests, two operational ceilings werefound: (1) the electrical output limit for the steamturbine-generator and (2) the induced draft (ID) fansflue gas capacity limit when furnace O2 exceeds 4.5%at full load. There are three ljungstrom air heaters; twoare used for secondary and overfire air, and one is usedfor primary air, supplied to the mills to dry andtransport the coal.

Figure 1. Elektrownia Opole.

The Elektrownia Opole boiler side view is shown inFigure 2. Unit 3 is one of four similarly-sized units atOpole. The boiler had close coupled overfire air(CCOFA) and separated overfire air (SOFA) installedprior to the ROFA project. The original SOFA systemcould deliver 20% of the total air flow (TAF), but oftenonly operated between 12% and 14%. The existingCCOFA system was retained during the installation ofthe ROFA system, but the CCOFA nozzles werereplaced with new redesigned tips capable of providinghigher velocities and adjustable horizontal directioncontrol. The existing SOFA system was completelyremoved and replaced with the ROFA system. Abovethe burners along the water wall, there are radiant wallsuperheater bands (yellow in Figure 2). These extendfrom above the SOFA ports to the SH platens (green inFigure 2). The radiant wall superheater bands fit tightto the evaporator tubes and only single tubes wereshortened during the installation of the ROFA ports.

Figure 2. Furnace side view.

Biomass is sometimes cofired by mixing the biomasswith the coal in the fuel yard. Up to 8% of the fuel (byweight) can be biomass before encounteringoperational problems with the pulverizers. Typicalbiomass is wood waste or agricultural waste,depending on availability. When cofiring biomass, themill fineness changes as shown in Table 1. There is asignificant increase in large particles when cofiringbiomass.

Table 1. Mill fineness by fuel type

Coal Cofiring

Pass 90-μm mesh 97-99% 93-97%

Pass 200-μm mesh 75-85% 68-77%

While biomass cofiring is beyond the scope of thispaper, the ROFA system has been shown to improvecarbon burnout when firing biomass on a nearby boilerfiring pulverized biomass.8

The typical coal fired at Elektrownia Opole comesfrom as many as 18 different mines and has the averagecontent shown in Table 2. 9

Table 2. Average fuel analysis from 18 mines (wt%,as received)

Design

Heating Value 20 – 21 kJ/kg

Moisture 7 – 12%

Ash 20 – 28%

Sulfur 0.6 – 1.0%

Chloride Up to 0.2%

There are 12 oil burners located on the front and rearwalls of the furnace, which are used for start up and arestopped once pulverized fuel combustion is stabilized.When taken out of service, quite a large cooling airrequirement remains.

DESIGN

There are four design components to the system:Operational Review, Main Windbox Modification,ROFA System Design, and Rotamix System Design.These are described here.

8 Brian Higgins, Yan, Gadalla, Meier, Fareid, Liu, Milewicz,Repczyński, Ryding, and Blasiak, “Biomass Cofiring Retrofit withROFA for NOx Reduction at EdF-Wrocław Kogeneracja,” A&WMEnergy Efficiency and Air Pollutant Control Conference, Wrocław,20099 As reported by Elektrownia Opole.

REVIEW AND MODIFICATION

Since the ROFA and Rotamix systems are integral tothe operation of the boiler (i.e., primary NOx controlmethod), the operation of the boiler must be reviewedand alterations made as required. The following is asummary of components that were reviewed duringdesign.

The pulverizers and coal feed system were reviewed tovalidate operational performance. The two componentsof interest are the fineness at full load, when thepulverizers are heavily fuel burdened, and the fueldistribution to individual burners. For both parameters,the Opole pulverizers were found to be operationallysound with exceptional fineness and providedacceptable coal distribution.

Figure 3. Three-dimensional image of the uppercorner burner modifications

The windbox and lower furnace combustion airinjection was thoroughly reviewed next. To maximizeSNCR efficiency, it is important to minimize CO in theupper furnace. Improved fuel/air mixing in the burnerzone facilitates CO burnout. The existing burners andsecondary air (SA) nozzles had compartmental tiltcontrol, but it was manual and was not often utilizedduring the day-to-day operation of the boiler. NalcoMobotec felt that having automated SA nozzle tiltcontrol for superheat (SH) and reheat (RH) temperaturealong with the existing attemperation system wasbeneficial for the project. Drives were added to controlthe tilts for all three independent vertical burnersgroups (i.e., bottom, middle, and top burners). TheCCOFA ports were replaced with new higher velocitynozzles and were also provided manual control for bothtilt (vertical) and yaw (horizontal), shown in Figure 3.Also, the lower secondary air nozzles in each cornerwere modified with a Lower Furnace CO Control(LFCC) system.

As part of the windbox modifications, the SA nozzleslocated between pairs of burners were replaced with

new nozzles to increase exit velocity and windboxpressure control. They were designed with manualhorizontal yaw control. Due to both tilt and yawposition requirements, these nozzles were speciallydesigned to provide film cooling air to protect thenozzles from radiant heat flux, as shown in Figure 4.The yaw control was added to both protect the waterwall from coal impingement (i.e., boundary or curtainair) and to allow fireball manipulation under stagedfiring conditions.

Figure 4. Modified SA and CCOFA nozzle

The windbox SA flow dampers were also modified toprovide operation with increased air biasing control.The air flow to the windbox is controlled by two sets ofdampers in series. The first damper controls the totalair to all burner pairs and the second damper controlsSA flow to each port individually. The downstreamdampers were modified and entry loss optimized byadding a venturi shaped entry. A portion of thesecondary flow control dampers (all but coal nozzlesecondary air feed) were equipped with modulatedactuators to improve control of flow and windboxpressure across load.

The operational review and tangential burner anddamper modifications were provided in collaborationwith John Grusha, Director of Combustion Systemsand Product Development, with RV Industries Inc.10

RV Industries designed and supplied the tangentialfiring system components, including installation andstartup support.

Water Wall Wastage

Water wall wastage due to attack by coal combustionintermediate compounds (i.e., FeS, H2S, Cl, and UBC)is well understood11 and occurs in every coal-firedboiler. In fact, water wall wastage is a pre-existingissue for Elektrownia Opole. While some low-NOxburner and OFA systems have been correlatedincreased water wall wastage, Nalco Mobotec has

10 RV Industries Inc., Honey Brook, PA, USA (www.rvii.com)11 Davis, Linjewile, Valentine, Swensen, Shino, Letcavits, Sheidler,Cox, and Carr, “On-line Monitoring of Waterwall Corrosion in a1300 MW Coal-fired Boiler with Low NOx Burners,” MEGASymposium, #115, 2004

never seen an increase in water wastageinstallation with more than 50 installationsyears. Conditions where water wall wastage becomesproblematic correlate with unburned (aburned) fuel impingement on the water wallROFA design, we have taken great care to reduce fuelimpingement on the water wall, despite there beinghigh CO concentration in burner zone. For example,we have installed smaller SA nozzles with yaw control.

ROFA SYSTEM DESIGN

The ROFA system was installed to lower NOx throughfurnace staging. Secondarily, the ROFA system hasbeen designed to increase mixing in the upper furnacefor CO and LOI burnout. A ROFA system includes aboosted-pressure ROFA fan (Figure 5),air ducting, and air injection nozzles. The ROFA air istaken from ducts at the outlet of the air preheaterboosted in pressure by the ROFA fan and deliveredthrough nozzles into the furnace. The air pressure at thenozzles is optimized as required to achievedetermined during the CFD modeling. Allthe ROFA nozzles is controlled based onto boiler steam flow to maintain tuned box pressuresload changes. Additionally, feed-forward and feedback control strategies can be implemented to reducesystem upsets during load fluctuations.

The ROFA system consists of a variable freqdrive (VFD) controlled centrifugal ROFA fan. TheROFA fan takes air through two individual ducts fromthe outlet of the two air preheaters (duct has a venturi flow meter to allow the boilercontrol system to account for air flow through theROFA system.

Figure 5. Installed ROFA and Rotamix fan

Extensive on-site investigations were carried out todetermine the most efficient and cost effective way toredirect secondary air to the upper furnace ROFAports. The ROFA air suction and discharge duct routingwas meticulously engineered as the existing boiler andbuilding structure contained significant duct pathinterferences particularly adjacent to the furnace walls(Figure 6).

never seen an increase in water wastage due to a ROFAinstallations over 15

Conditions where water wall wastage becomescorrelate with unburned (and partially

d) fuel impingement on the water wall. In ourROFA design, we have taken great care to reduce fuelimpingement on the water wall, despite there beinghigh CO concentration in burner zone. For example,

with yaw control.

ROFA system was installed to lower NOx throughfurnace staging. Secondarily, the ROFA system hasbeen designed to increase mixing in the upper furnace

A ROFA system includes a, interconnecting

, and air injection nozzles. The ROFA air isthe air preheaters. It is

boosted in pressure by the ROFA fan and deliveredthrough nozzles into the furnace. The air pressure at the

optimized as required to achieve mixing asdetermined during the CFD modeling. All air flow to

based on a relationshiptuned box pressures as

forward and feed-back control strategies can be implemented to reduce

The ROFA system consists of a variable frequencydrive (VFD) controlled centrifugal ROFA fan. TheROFA fan takes air through two individual ducts fromthe outlet of the two air preheaters (Figure 5). Each

has a venturi flow meter to allow the boilercontrol system to account for air flow through the

Rotamix fans

site investigations were carried out toficient and cost effective way to

redirect secondary air to the upper furnace ROFAThe ROFA air suction and discharge duct routing

as the existing boiler andbuilding structure contained significant duct pathinterferences particularly adjacent to the furnace walls

Figure 6. Three-dimensional representation of theROFA duct work

ROTAMIX SYSTEM DESIGN

The Rotamix system consists of an air delivery system(Figure 7) and a liquid (urea and water) deliverysystem. The air delivery system contains aambient-air fan and ductwork similar tosystem, but smaller in size. Since theutilizes ambient temperature airductwork is reduced and the need for thermalinsulation is removed.

Figure 7. Rotamix fan

There are several levels of Rotamix ports; including,ports between gaps in the radiant wall superheaterbands (Figure 8) and ports in the upper furnacebetween SH platens. Urea can also be injected throughthe ROFA nozzles. The upper furnace injhave injection angle adjustment capability.

Figure 8. Rotamix port with air, diluted urea, andhumidification water injection.

Granulated urea (i.e., solid urea) is delivered to site andloaded into a storage hopper

epresentation of theROFA duct work

The Rotamix system consists of an air delivery systemand a liquid (urea and water) delivery

system. The air delivery system contains a smallfan and ductwork similar to the ROFA

Since the Rotamix systemutilizes ambient temperature air, the size of theductwork is reduced and the need for thermal

Rotamix fan

There are several levels of Rotamix ports; including,radiant wall superheater

) and ports in the upper furnacebetween SH platens. Urea can also be injected throughthe ROFA nozzles. The upper furnace injection portshave injection angle adjustment capability.

Rotamix port with air, diluted urea, andhumidification water injection.

Granulated urea (i.e., solid urea) is delivered to site andthrough pneumatic

conveying. The diluted urea mixture is created bybatch-mixing pre-calculated doses of urea and water ina day tank. The diluted urea is then distributed amongthe multiple furnace injection ports via variable-speedpumps (Figure 9). The variable speed pump feed ratesare controlled individually as a function of boiler load.Any number of the injection sites can be individuallyactivated (through modulated valves). An air purge ofthe urea delivery line and spray nozzle is performedwhenever an injection device is taken out of service.

Figure 9. Rotamix urea supply mixing and pumpskids.

In addition to the diluted urea, Rotamix humidificationwater is delivered to any injection device that is inservice. A humidification water storage tank supplieswater to the humidification water pumping system. Thetank is supplied with water from the existing plantdomestic water supply system. The humidificationwater system adds additional control and tuningcapability to the Rotamix system as urea injectionconcentrations can be manipulated to compensate forfurnace flow anomalies.

CFD MODELING

CFD Modeling has been performed for this unit to: (1)understand the baseline performance of the unit, and(2) optimize the ROFA/Rotamix system design.

Combustion Model Overview

Nalco Mobotec utilizes FLUENT for CFD modelingsimulations, simultaneously solving for density,velocity, temperature, and chemical species (includingfuel volatiles) concentrations fields of the gas phaseand fuel particle properties and combustion within thefurnace to steady state. The gas phase conservationequations are solved using a variable density, quasi-incompressible formulation embedded in an Eulerianreference frame, while the fuel particles are solvedusing a Lagrangian reference frame. The governingequations are the gas phase continuity, momentum,turbulent kinetic energy, turbulent dissipation,enthalpy, and the species conservation equations foreach gas species in the turbulent combustion model.

These conservation laws have been described andformulated extensively in standard CFD textbooks. Ak-ε turbulence model was implemented in thesimulations. Standard Eddy-Breakup (EBU) turbulencecombustion model is used. The following two stepmechanism was utilized for fuel combustion:

Fuel + a O2 → b CO + c CO2 + d H2O + e SO2 (1)CO + 0.5 O2 → CO2 (2)

Where the stoichiometric coefficients (a, b, c, d, and e)are determined from the fuel proximate and ultimateanalyses. For lower temperatures found in the back-pass, a modification to the carbon monoxide reaction isalso included to more accurately predict COconcentration.

FLUENT NOx submodel involves sophisticated fuel-Nconversion pathways. After fuel devolatilization, fuel-N is partitioned into volatiles-N and char-N. HCN isthe dominant nitrogen species in volatile-N releasedfrom coal. Char-N is released into the gas phase at arate that is proportional to the carbon burnout rate.Because char-N conversion chemistry is complex, thesimulation assumes a fixed fraction of char-N directlyconverted to NO with the rest of N converted to N2.This assumption is often used in literature12. SNCRchemistry with urea injection is also used for Rotamixsystem design.

Figure 10. The CFD modeling mesh.

Geometry and Model Inputs

As shown in Figure 10, the furnace enclosure for theCFD model domain for the base, ROFA, and Rotamixcases is defined as beginning at the burners and ROFAports (inlet boundary conditions) and ending at thehorizontal plane downstream of the reheater M2 (outletboundary condition). The furnace volume extendsupstream to the bottom ash hopper. The super heatpendants are depicted in the model using the actual

12 Niksa and Liu, “Incorporating detailed reaction mechanisms intosimulations of coal-nitrogen conversion in p.f. flames,” Fuel 81(18),pp. 2371-2385 (2002).

number of tube bundles and dimensions to account forheat absorption and flow stratification. The furnacegeometry was represented in the computer model withapproximately 1,600,000 computational cells in anunstructured, hybrid (all hexahedral) grid. The largenumber of computational cells is sufficient to resolvethe most relevant features of the three-dimensionalcombustion process.

The firing rate input for the CFD base case simulationswas calculated by the coal flow and coal heating value.The total air flow (TAF) was calculated from thereported coal composition and measured economizerO2 concentration. Nalco Mobotec modeled the designcoal for the boiler, which well matched the coalanalyses from the HVT testing. The design coalanalysis was provided by the plant and is listed inTable 3.

Table 3. Fuel analysis used in the CFD model

Proximate Design

Moisture [wt.% ar] 9.6

Ash [wt.% ar] 22.9

Fixed Carbon [wt.% ar] 34.8

Volatile Matter [wt.% ar] 32.7

HHV [kJ/kg ar] 23422

Ultimate

Carbon [wt.% ar] 56.71

Hydrogen [wt.% ar] 3.69

Oxygen [wt.% ar] 5.73

Nitrogen [wt.% ar] 0.89

Sulfur [wt.% ar] 0.87

Figure 11. Comparison between HVT and CFDmodeling for Temperature and O2

HVT Testing

High velocity temperature (HVT) probe measurementswere taken during the design phase. The objective ofunit testing was to gather field data used to validate theCFD results, but the measurements only provideinformation at a few specific locations in the furnaceand are subject to inaccuracies as well.

During HVT testing, water-cooled probes were used.Measurements were taken at two elevations above theradiant wall superheater band; specifically through twoports on the sidewalls and through three ports on frontwall. Flue gas temperature and concentrations of NOx,CO, O2, and CO2 were measured at four operatingconditions:

• 350 MW with SOFA on• 350 MW with SOFA off• 250 MW with SOFA on• 250 MW with SOFA off

During HVT testing, flyash samples and coal sampleswere taken for analysis.

The HVT temperature measurements are compared tothe CFD modeling results in Figure 11. During the setup of the model, some modeling factors were changedto better match the measured data. For example, theCFD model wall conditions (temperature andemissivity) are adjusted to match the measuredtemperatures. This is required because the radiant heattransfer characteristics are not known a priori. Ingeneral, the qualitative agreement between themeasured data in Figure 11 and the CFD modelingpredictions was good for the base case.

CFD Modeling Results

Base case model (with SOFA) results were comparedwith ROFA modeling results; specifically, the contoursof calculated field variables (i.e., temperature, O2, CO,NOx, and turbulent kinetic energy). Rotamix modelingis discussed later. Biomass cofiring was not modeled.Since firing coal is the worst case for NOx production,the results are considered to be conservative by onlymodeling coal.

The base and ROFA case results are shown in Table 4.The ROFA case was performed with less O2 relative tothe base case (with SOFA). This corresponds to theway the boiler has been tuned with the ROFA systemin service (discussed in the Field Results section).

Table 4. Exit results from CFD modeling

Base ROFA

O2 [%] 3.9 3.0

CO [ppm] 20 27

FEGT [°C] 1261 1291

Outlet T [°C] 650 678

NOx [ppm] 274 165

NOx [mg/Nm3] 472 264

Reduction [%] - 44

The resulting NOx prediction for the ROFA case(without urea injection) is 44% reduction from the basecase (with SOFA). This is due to staging and lower O2.Even with the reduced O2, the CO emission is notsignificantly altered. This is due to the high degree ofmixing in the upper furnace.

The furnace exit gas temperature (FEGT) waspredicted to be higher in the ROFA case, but the massflow through the furnace is lower. Specifically, themass flow rate through the furnace is 5% lowerbecause the O2 is reduced from 3.9% to 3.0%. The neteffect is that even with a higher predicted FEGT, theoverall heat absorption to the water wall is notsignificantly changed.

Temperature Distribution

The temperature distribution for the base and ROFAcases is shown in Figure 12. The combustion zone isshown in red near the burners. In the right panel, thecombustion zone is drawn out over the larger volume(extending higher in the furnace) for the ROFA case.This is consistent with industry results on tangentialcoal fired boilers equipped with overfire air systems.

Figure 12. Gas temperature of (left) base case and(right) ROFA case

Changes in the burner tilts were not modeled. For thisgeometry, the SH heat absorption (for the water wallbands) is predicted to go up. In the field, this will causethe control system to tilt the burners down, which willresult in reduced overall NOx production (due tolonger residence time under staged conditions). For thisreason, we did not model the change in tilt as aconservative approach to the design; that is, the NOxbenefit from lowering the tilts is in our design marginand manifested itself during tuning.

Figure 13. Water wall heat flux for (left) base caseand (right) ROFA case

Water Wall Heat Flux

In Figure 13, the heat flux to the water wall is plottedfor the base case and the ROFA case. While the figuresare indeed similar, the peak heat flux for the ROFAcase is slightly elongated vertically in both directionsdue to the staged combustion. The heat flux is spreadout over a larger surface area of the water wall. Thishelps balance the heat flux to the water wall andreduces the formation of hotspots.

Figure 14. Oxygen contours of (left) base case and(right) ROFA case

O2 Distribution

Figure 14 shows the O2 distribution in the furnace. Forthe ROFA case, O2 is clearly lower in the lowerfurnace and at the furnace exit than base case. Due tobetter mixing, the exit O2 for the ROFA case is moreuniform relative to the base case. The mixing of theROFA jets is visible in the upper furnace. Likewise, the

lack of penetration by the existing SOFA ports in thebase case is visible as well.

Even though the excess air is low in the lower furnace,the O2 concentration near the walls in the burner zoneis not zero. This helps reduce the possibility ofslagging and water wall wastage even when the lowerfurnace is staged for NOx reduction. Note that the yawcontrol has not been modeled. With yaw control, moreO2 can be put near the walls for better NOx reductionand more protection from slagging and water wallwastage.

Figure 15. NOx contours of (left) base case and(right) ROFA case

NOx Distribution

Figure 15 shows the NOx concentration distribution inthe furnace. The NOx reduction prediction from theROFA case (without urea injection) is clear (greenversus red). The blue portions in the lower furnacerepresent locations where NOx is reduced to nearlyzero by HCN released from the fuel bound nitrogen involatiles under a reducing atmosphere to form N2.

The majority of the NOx production is in the lowerfurnace. Air addition in the upper furnace does notsignificantly affect NOx chemistry in the upper furnaceother than through dilution.

CO Distribution

Figure 16 shows the CO distribution in the boiler. Forboth cases, thousands of ppm of CO is formed in theburner zone as expected due to staging. More CO isformed in the ROFA case than the base case, and overa larger volume. However, the ROFA jets mix wellwith the flue gas and the CO reacts quickly with theoxygen in the ROFA zone. CO in both cases is reduced

to below 30 ppm (see Table 4), even with the ROFAsystem tuned with reduced exit O2.

Figure 16. CO contours of (left) base case and(right) ROFA case

Turbulent Kinetic Energy (TKE) Distribution

Figure 17 shows the Turbulent Kinetic Energy (TKE)distribution for the two cases. The ROFA jets providemore TKE in the upper furnace. This is due both toincreased velocity as well as increased mass flowthrough the ROFA system relative to SOFA. TheSOFA velocities are low and this is recognized in thefigure as a lack of TKE. Note also that the oil burnersair flow has been reduced in the ROFA case (much ofthe TKE seen in the base case is due to the oil burners,not just the secondary air).

Figure 17. Turbulent kinetic energy contours of(left) base case and (right) ROFA case

TKE directly relates to the ROFA jet turbulent mixing.Turbulence promotes the combustion of unburnedcombustibles from the lower furnace. Turbulent mixingalso increases the chemical utilization of the urea

injected through the Rotamix SNCR system. This isagain illustrated visually above and quantitatively inFigure 18. In Figure 18, the TKE is seen to increase byan order of magnitude at the ROFA elevation.Subsequently, the TKE takes many meters to dissipate.This turbulent dissipation occurs through mixing and isprolonged in the upper boiler.

Figure 18. Mass-weighted turbulent kinetic energyplotted versus height for the base and ROFA cases

The windbox modifications were not modeled. Thesmaller SA nozzles: (1) increase the exit velocity,which increases the turbulence and mixing in the lowerfurnace and (2) create higher windbox pressuredifferential, which helps balance the air flow amongthe different SA nozzles, homogenizing the air deliveryto the fireball and giving more controllability of the airflow to the fireball and CO production control.

Rotamix (SNCR) Modeling

The Rotamix SNCR system was also modeled to aiddesign. The focus of this model was on mixing andpenetration of the Rotamix jets. Sufficient designmargin was included to account for real temperaturevariability in the furnace. Examples of design margininclude: dilution of the urea, humidification waterinjection, manual tilt control on the upper Rotamixports, and Rotamix air flow control. Nozzles aregrouped by two or four injection nozzles, which haveflow control. This allows for the adjustment of ureainjection depending on local temperature fluctuations.These factors were not varied in the CFD modeling ofRotamix chemistry since only one set of conditionswere considered.

The Rotamix modeling below used a normalizedstoichiometric ratio of urea-to-ROFA-NOx of 1.5. Theresulting NH3 slip was modeled to be 3.6 ppm. Themodel predicted 20% NOx reduction without designiteration. Our experience has shown that in the field wecan outperform the model and so this was sufficient forour design purposes.

Figure 19 shows NOx concentrations at the CFD modelexit boundary to illustrate the depth of urea penetrationinto the furnace for effective NOx control. Ureainjection into the platen region results in less NOxreduction than injection into the open furnace due tolimitations on mixing, penetration, and a rapid quenchrate in the SH region and serves as the design limitingcase.

Figure 19. Predicted NOx distribution at thefurnace outlet for the Rotamix case

FIELD PERFORMANCE

Low-NOx Combustion System Operation

The baseline conditions with SOFA produced 350 to500 mg/Nm3 of NOx as shown in Figure 20 (opengreen symbols). The baseline data is from December13-19, 2009, and is considered to be indicative ofnormal operation of the boiler. In order to keep LOIand CO manageable, the amount of air through theSOFA system was marginal (only ~13% of TAF) andthis did not create a sufficient sub-stoichiometricburner zone for de-NOx, leading to higher thanexpected NOx emissions with SOFA.

During the project outage, the tangential windboxmodifications discussed previously were installed andthe NOx emissions dropped to between 260 and 420mg/Nm3 (averaging 350 mg/Nm3) before the start ofROFA tuning. This data is plotted in Figure 20 with theclosed blue symbols. Note that for this data, there issome staging because of the air required to keep theROFA boxes cool. In fact, for the shown data, theaverage ROFA cooling air flow was 11% of TAF,which is about the same as the SOFA flow rate in thebaseline.

0

2

4

6

8

10

12

14

16

18

20

10 15 20 25 30 35 40 45 50 55 60

TKE

[m2

/s2

]

Furnace Heights [m]

BaselineROFA

Lower Burner Upper Burner Lower ROFA Middle ROFA Entrance of P3

Figure 20. One-hour NOx data versus load forbaseline (green), burner mods (blue), and

ROFA/Rotamix (red) conditions

The ROFA/Rotamix NOx data are shown in Figure 20as red closed symbols from the reliability test fromFebruary 8 to 17, 2010. Only one data point is above200 mg/Nm3 at 6% O2 (i.e., 203 mg/Nm3). The averageNOx for all loads over the entire test period was168 mg/Nm3. Note that no data is available for theROFA system in service without urea injection sincethe data comes from the reliability test where bothsystems were required to be in service for the entireten-day test.

Table 5. Fuel analysis (coal only, during 10-day test)Min Ave Max Var

Moisture [%] 8.7 10.2 12.0 32%

Ash [%] 22.6 26.6 32.8 38%

Volatile [%] 20.1 22.3 23.8 17%

ΔHc MJ/kg 18.8 21.4 22.7 18%

C [%] 46.0 52.2 55.4 18%

H [%] 3.2 3.6 3.7 15%

O [%] 4.3 5.6 6.5 39%

N [%] 0.80 0.93 1.05 27%

S [%] 0.51 0.77 1.02 67%

Cl [%] 0.119 0.16 0.22 62%

FBN mg/kJ 0.41 0.43 0.46 12%

Coal and LOI Analysis

Elektrownia Opole fires coal from 18 different mines.In Table 5 the minimum, average, and maximumvalues for proximate and ultimate chemical analysesare tabulated. In the last column, the variability ([max-min]/ave) is tabulated as an indication of the truevariability for 10 days of operation. Over the 10-daytest there was no observed correlation between the fuel

analyses and variations in NOx, CO, or LOI. TheROFA system performs well with highly variable fuel.

Figure 21. Fly Ash LOI for all four Opole units(Unit 3 has a ROFA/Rotamix system)

Fly ash LOI data is plotted versus date for Unit 3 (withROFA/Rotamix systems) as well as the other threeunits at Opole in Figure 21. The goal is to keep the flyash LOI below 5% to maintain ash sales. For most daysthis is the case, with several samples slightly higherthan 5%. The average LOI value for fly ash over theten-day test period was 3.5%, meeting the project flyash LOI requirement. The good mixing in the upperfurnace from the ROFA system contributes toacceptably low fly ash LOI.

Figure 22. Slag Ash LOI for all four Opole units(Unit 3 has a ROFA/Rotamix system)

In Figure 22, slag ash (or bottom ash) LOI is plottedversus date for ROFA/Rotamix systems and comparedversus the other units at Opole. It is clear in this figurethat the slag ash LOI for ROFA/Rotamix system ishigher than the other units. For the ROFA/Rotamixdata, the average slag ash LOI was 7.3%, while for theother units it was 2.7%. The increased slag ash LOI inthe ROFA/Rotamix data is largely due to coal fall outin the lower furnace.

While low slag ash LOI is desirable, it is not as dire aproblem as fly ash LOI as the ash can still be sold and

the efficiency impact is not large (due to the higherquantities of fly ash).

Figure 23. Five-minute O2 data versus load forbaseline (green) and ROFA/Rotamix (red) data

One added complication is the methodology forsampling slag ash LOI. The soot blowing cycles canhave an important impact on slag ash LOI. Ash that isblown off the walls will have lower LOI due to thelength of time they have been resident in the furnace. Ifthe slag ash sampling is not correlated with the sootblowing cycles, erroneous slag ash LOI numbers canbe measured.

Nalco Mobotec is working to manage this effectthrough boiler tuning, specifically with the LFCCnozzles and lower furnace air management.

Figure 24: Five-minute CO data versus load forbaseline (green) and ROFA/Rotamix (red) data

Efficiency Impacts

In Figure 23, the measured O2 in the furnace is plottedversus load. Excess O2 at low load is required tomaintain SH and RH temperature. Excess O2 at highload is only required for combustion efficiency. Lowerexcess O2 typically leads to high LOI and CO. Due tothe mixing that ROFA jets provide excess O2 is

reduced in the furnace. This is beneficial for tworeasons. First, NOx is lower with lower furnace O2.Secondly, the boiler efficiency is improved.

As seen in Figure 23, O2 in the furnace has beenreduced by almost 1% (from 3.7% to 2.8%). Thisresults in 5% less TAF and improves the boilerefficiency (through reduced stack loss) as well asreduced FD fan amps, ID fan amps, and air heaterleakage.

In Figure 24, CO emissions are plotted versus load forthe baseline SOFA and ROFA cases. The trends aresimilar, peaking at full load where there is lessresidence time and less excess air to promote CO burnout. There are spikes of higher CO with the ROFAcase, but this will be managed through ongoing tuning.More or less the CO trends before and after the ROFAinstallation are consistent despite the much lower O2

with the ROFA case.

Rotamix SNCR Analysis

Because the Rotamix system was never out of serviceduring the ten-day test, it is not possible to verify theactual SNCR NOx reduction during this time period. InFigure 25, the urea consumption rate and NH3 slip areplotted against load. The average NH3 slip for theentire 10-day test period was 3.1 ppm.

Figure 25: Urea consumption rate and NH3 slipversus load for the Rotamix system

The urea flow rate is highest at low load. Fewerinjection ports are used at low load, and these portsspray urea directly into the open furnace.Proportionally, more urea can be used at low loadresulting in better NOx reduction without concern forNH3 slip. During tuning, it was observed at low loadthat the Rotamix system reduced NOx by more than50%. In Figure 25 it is noted that at low load, slip isusually below 1 ppm. The normalized stoichiometric

ratio (NSR) for urea is estimated from design data to bearound 2.0 for loads between 180 MW and 230 MW.

For loads between 330 MW and 380 MW, the primaryRotamix injection points are located in the SH platensregion and are subject to rapid cooling. In this region,less NOx reduction is possible (i.e., 20% to 30%) andslip is the primary limiting factor. When the one-houraverage load was over 330 MW, the average NH3 slipwas 4.8 ppm. The urea usage rate is lower at high loadand the NSR is estimated from design data to bebetween 0.8 and 1.0 when load is over 330 MW. Thiscan result in a significant chemical savings relative tonon-boosted SNCR systems.13

Ammonia in Ash and Gypsum

After urea is injected into the boiler, it breaks downand ammonia (NH3) slip is measured downstream.Ammonia slip must be limited to certain values inorder to continue selling ash and gypsum products. InTable 6 the ammonia measurements from each day oftesting are summarized.

Table 6. Ammonia in ash and gypsum summaryresults

Min Ave Max Var

NH3 slag ash mg/kg 1.7 2.3 3.2 65%

NH3 fly ash mg/kg 29 35 44 43%

NH3 in gypsum mg/kg 8.1 8.6 9.5 16%

The amount of ammonia in the slag ash is very low(2.3%). This is expected since the Rotamix SNCRsystem is located near the exit of the furnace and is notconsidered to have a large effect on the amount ofammonia in slag ash.

Downstream, the ammonia content in the fly ash washigher, averaging 35 mg/kg. There is a Polish limit of50 mg/kg, which was never exceeded throughout theten-day reliability test.

When gypsum is made in the wet flue gasdesulfurization (WFGD) system, it is washed toremove chlorides. This removes NH3 as well. Thesellable limit for NH3 in gypsum is 10 mg/kg, whichwas exceeded one day (10.7 mg/kg) during production,but during onsite gypsum processing the peak NH3 inthe gypsum was reduced to 9.5 mg/kg before the NH3

was sent offsite.

CONCLUSIONS

13 EPA, “EPA Air Pollution Control Cost Manual,” Office of AirQuality Planning and Standards, EPA 452/B-02-001, 4 (1), 2002,www.epa.gov/ttn/catc/dir1/cs4-2ch1.pdf

Nalco Mobotec’s ROFA and Rotamix systems havebeen installed on PGE’s Elektrownia Opole Unit 3 toreduce NOx below 200 mg/Nm3 (0.15 lb/MMBtu). Theinstallation included three steps:

1) Following an operational review, thetangential windbox was modified andupgraded,

2) The existing SOFA system was replaced byNalco Mobotec’s ROFA system, and

3) A Rotamix SNCR system was installed,introducing urea into the upper furnace.

The combined effect of burner modification and theROFA and Rotamix systems resulted in:

1) NOx emission below 200 mg/Nm3, averaging168 mg/Nm3 (52% to 66% reduction)

2) Demonstrated ability to handle significant fuelvariability

3) Improved combustion due to mixingintroduced by the ROFA system

4) LOI for fly ash averaged 3.5%, which isbelow the limit for continued fly-ash sales

5) LOI for slag ash averaged 7.3%, which ishigh, but will be managed through tuning

6) CO emissions were held to an average valueof only 41 mg/Nm3 over the ten days

7) Ammonia slip from the SNCR system washeld to 3.1 ppm on average and below 5 ppmfor full load conditions.

8) Ammonia in ash averaged 35 mg/kg (belowthe 50 mg/kg limit)

9) Ammonia in gypsum averaged 8.6 mg/kg(below the 10 mg/kg limit).

Extensive CFD modeling was used to design theROFA and Rotamix systems.14

NALCO and the logo are Registered Trademarks of Nalco CompanyROFA and Rotamix are Trademarks of Mobotec AB, used withpermission© 2010 Nalco Company