ultra low-nox gas fired boiler burner design requiring no

Proceedings of the 2001 Joint International Combustion Symposium: AFRC/JFRC/IEAToward Efficient Zero Emission Combustion/Advances in Air-Fuel and Oxy-Fuel Technologies

September 9-12, 2001; Kauai, Hawaii, USA

Section 7B - Boilers - Paper 1

Ultra Low-NOx Gas Fired Boiler Burner Design Requiring No FGR Fan

Dr. Lev Tsirulnikov and Jason McAdams, PE

Contact Info:Dr. Lev Tsirulnikov

John Zink Company, LLCTodd Combustion Group

2 Armstrong Road, 3rd FloorShelton, Connecticut 06484

E-mail: [email protected]

Jason McAdams, PEJohn Zink Company, LLC

11920 East Apache, P.O. Box 21200Tulsa, Oklahoma 74116-1220

www.johnzink.comE-mail: [email protected]

2001 John Zink Company, LLC

ABSTRACT

The presented paper describes the test data obtained on a 40 MMBTU/hr heat input standard“Variflame” low-NOx burner firing natural gas with ambient temperature air in a test furnace atJohn Zink Company. The burner was tested under various combustion conditions which allowedus to establish the impact of different design and operational parameters and their combinationson performance data. The major focus of the test was to find the conditions that allow us toobtain, without using a Flue Gas Recirculation (FGR) fan, the same low NOx and CO emissionsthat are available with FGR. The obtained test results are presented as alternatives to using anFGR system and are compared with data obtained with regular FGR introduced into thecombustion air flow.

The obtained test results demonstrate a repeatable 66-75% (3-4 times) reduction in NOxemissions from the furnace/burner without an increase in CO. To get the same NOx reductionwith FGR only, it is usually required to introduce at least 15% flue gas recirculation.

The best tested combinations allowed us to bring NOx emissions down to 11-12 ppm. ThisNOx reduction can be achieved with minimal risk and at a low installation cost whilemaintaining the existing high reliability of the boilers in full operational turndown.

mailto:[email protected]

mailto:tsirulnl@kochind

Ultra Low-NOx Gas Fired Boiler Burner Design Requiring No FGR FanTsirulnikov and McAdams, 2001

1 2001 John Zink Company, LLC

PROJECT HISTORY

The standard Todd Combustion Variflame™ low-NOx gas/oil burner has been installed inhundreds of packaged industrial steam boilers and hot water generators mostly operating withambient combustion air. The heat input range for this burner design is approximately 30 to 400MMBTU/hr [1].

The Variflame burner design has historically provided high thermal efficiency and reliablelong-term operation throughout a wide turndown range. In order to meet the strict NOxemissions required by environmental permits at many locations, an external flue gas recirculation(“FGR”) fan, which supplies flue gas into the combustion air flow (or directly into the furnace),can also be installed with the burner to lower NOx emissions.

Because an FGR fan adds significantly to an installation and operational cost of the burner,especially on applications designed to fire only gas fuels, it was desired to reduce the NOxemissions from the Variflame burner itself so that the FGR fan could be eliminated whileretaining the burner’s proven advantages. For this investigation, the standard 40 MMBTU/hrVariflame burner (referred to as “option #1”) was tested using natural gas and ambienttemperature air to determine the baseline burner emissions performance in the test furnace and tomeasure the impact of traditional FGR on the unmodified burner emissions. Three burnerupgrades designed to lower NOx emissions without FGR (referred to as “option #2”, “option#3”, and “option #4”) were developed, installed on the standard Variflame burner, and testedunder the same firing conditions as the baseline to determine the NOx emissions reduction and tocompare against traditional FGR.

DESCRIPTION OF THE TESTED BURNER AND FURNACE

The standard “Variflame” burner consists of the following major parts and assemblies shownon the schematic (Fig. 1):

12

3

4

5

67

Figure 1. Schematic of the standard “Variflame” burner



1) An inner venturi tube channel supplying the swirled primary and the straight secondary air(or mixture of cold combustion air with hot flue gas recirculation) flows

2) An outer channel supplying the straight tertiary air flow3) A primary air swirler installed at the burner axis4) A number of the pokers with gas nozzles (distributing oriented jets of natural gas or other

gaseous fuels into the above combustion air flows), surrounding the mentioned swirler5) An oil atomizer installed at the burner axis (optional)6) An igniter (or pilot)7) A flame scanner

The structure and characteristics of the air flows in the standard Variflame burner (referred toas “option #1” in this paper) have been optimized over the years to provide the best burnerperformance. In addition, the ratio between natural gas and air flow velocities has beenoptimized to provide the lowest NOx emissions. All three burner upgrade designs (options #2 -#4) have kept the same structure and characteristics of the air flows and only differ from eachother in the way that the fuel is distributed and injected into the burner air flow.

For testing, the burner was installed inside a windbox and attached to the front of a testfurnace at Research and Development Test Facility at the John Zink Company in Tulsa,Oklahoma. Figure #2 shows a schematic of this testing arrangement which will be describedfurther below.

Test Furnace

FlueGas

Cooler

Steam InjectionPoint (MeasureFlow, Temp,

Press)

FI FI

TI

FGR

Line

FIR Line

VariflameBurner

Forced Draft Fan

TI

Recirc Gas Sample

Stack Sample

Steam InjectionPoint (Measure Flow,

Temp, Press)

Recirculation Line

CoolerBypassDamper

Air Inlet

FuelGas

FIR Eductor

TI

Windbox O2 Sample

Fuel/FIRMixture

O2 Sample,Temperature,

Pressure

AtomizedWater

InjectionPoint

TITI

Combustion Air

PI

PI

PI

Figure 2. Schematic of the Test Setup



The burner was installed to fire horizontally into the water-cooled firing chamber of the testfurnace, which was designed to imitate typical furnaces found in both gas and oil fired industrialboilers. The floor, front wall (where the burner is) and back wall of the firing chamber arecovered with refractory. The dimensions of this firing chamber are 7.0 feet wide by 7.75 feetheight, which is close to the dimensions of most middle-size packaged boilers. However the 40-foot length of the furnace is longer than most middle-size package boilers. The larger furnacevolume for this testing resulted in NOx emissions lower than those typically encountered in fieldinstallations of similar heat release Variflame burners, but the comparison between theeffectiveness of the various NOx reduction options will still be applicable to these fieldinstallations.

The test furnace is also equipped with twenty (20) sight-glass ports located every two feetalong the side wall and a sight glass on the rear wall, which allowed us to watch the visible flameand to evaluate its approximate dimensions based on numerous observations.

The combustion air was supplied to the burner using a Forced Draft (FD) fan, which was alsocapable of drawing a restricted FGR flow from the furnace stack. Air flow and FGR supplied bythe FD fan comes into the windbox which was modeled prior to testing to insure equal airdistribution radially within the burner.

The test furnace is also equipped with a system that allows the burner fuel to induce flue gasfrom the stack and to be mixed with it before combustion. This Fuel Induced Recirculation (or“FIR”) flow dilutes the fuel, lowering the heating value, and lowering the temperature of theflame. The use of flue gas mixed with the burner fuel has been studied previously in references[2-4]. This FIR system includes an eductor to induce the flue gas, a heat exchanger to cool andto regulate flue gas temperature, and ductwork to bring the flue gas to the burner. Based on thisJohn Zink Company patent pending method, a technology called COOLFuel™ was developedand has been successfully implemented on a few industrial and utility boilers [5]. The FIRsystem on the test furnace allowed us to use FIR flow and to obtain a NOx reductioneffectiveness comparable to regular FGR when testing the Variflame burner under the similarconditions. The two burner design options # 3 and #4 (of the four mentioned above) weredesigned for testing with FIR.

In addition to the FGR and FIR systems available, the test furnace also had the capability ofinjecting dry saturated steam and "cold" (70-80 F) water at various points. During the testingcovered in this paper we investigated the NOx reduction effectiveness of five different methodsfor introducing this moisture to the burner. The methods investigated include: steam into FIRflow, steam into FGR flow, steam into combustion air flow, steam into both the FIR/FGR flowand air flows simultaneously, and water into FIR flow.

Furnace emissions during testing were measured using a reliable Continuous EmissionsMonitoring (CEM) system to measure O2, NOx and CO concentrations in the stack. This sameCEM system could be connected to the flue gas recirculation flow, burner windbox, and FIRdiluted fuel lines in order to get the gas compositions readings required for calculation methodsof the FGR and FIR flows rates. Deviations of O2, NOx and CO concentration readings from thecalibration gas composition usually did not exceed ± 0.1% O2, ± 1 ppm NOx and ± 2 ppm CO.The furnace was also equipped with the measurements needed to determine the major



independent parameters and indexes of the combustion processes. These instruments were foundto have high accuracy and allowed us produce repeatable performance data in the testedoperational turndown.

During testing, the combustion parameters which were most closely monitored were theflame envelope (especially its length), operational turndown, air side pressure drop across theburner called “register draft loss” (or “RDL”), excess O2 %, NOx emissions, and CO emissions.

TEST RESULTS

Baseline Data and FGR Effectiveness

The data obtained during tests of the standard (original) Variflame burner gas unit (option#1) containing six standard Todd Combustion gas pokers described in [1], without anyintroduced media (FGR, FIR, steam, water or any combination) are assumed as the baseline datafor this analysis.

At full load and ~3% excess O2, the flame length and width did not exceed 18 and ~ 4.5 feet,respectively. Reducing excess O2 from ~3 to ~1.5% increased the flame length by ~1 feet.Conversely, increasing the excess O2 from 3 to ~4.6% resulted in a flame length that wasapproximately 1 foot shorter. At partial loads 80, 50 and 35%, the flame length wasapproximately 17, 12 and 10 feet, accordingly. It was found that the introduction of ~7.5% FGR(measured in % of total flue gas products) into the FD fan inlet did not affect these measuredflame dimensions. It should be noted that, due to the test furnace operating under negativepressure, these visualized lengths of the flame are significantly longer than those seen on thesame 40 MMBTU/hr burner installed in boilers in the field that operate under positive pressurewhere the flame length is less than 12 feet.

Since the existing FD fan damper allowed air to leak past when completely closed, it did notallow us to obtain test data with low (~3%) excess O2 when the burner heat input was less than~8 MMBTU/hr. Because of this, we only tested the burner to a turndown of 20% of its full rate.Within the tested range of 20% to 100% of full load, the burner provided completely reliableoperation.

At full load, ~3% O2 and ~100 °F combustion air temperature, the average RDL measuredduring testing of the baseline configuration (option #1) with no FGR, FIR or steam injection was5.6” WC, which matches the design RDL value of 5.5” WC closely. Increasing excess O2brought a corresponding RDL change (see Fig. 3, curve 1).



0

1

2

3

4

5

6

7

8

0 1 2 3 4 5

Excess O2, %

1

2

Figure 3. RDL of Option #1 vs Excess O2 at full load without FGR (curve 1) andwith FGR (curve 2)

Under the same combustion conditions and with ~7.5% FGR (~1.25 lb FGR/ lb fuel gas),RDL increased on average from 5.6 to ~7.0” WC (see Fig. 3, curve 2). As would be expected,higher temperatures of the combustion air and FGR mixture resulted in a higher RDL for theburner. Figure 4 shows the impact of FGR temperature on RDL. On average, a 100 °F increasein the FGR temperature increased the burner RDL by 0.09” WC, or ~1.5 %.



4

5

6

7

8

0 100 200 300 400 500 600 700 800

FGR temperature, F

O2=1.37-1.75%

O2=2.7-3.3%

Figure 4. RDL vs FGR temperature at full load, ~100 F combustion airtemperature and ~7.5% FGR flow rate

NOx data obtained with the testing of the baseline (option #1) gas unit are presented in Fig. 5(curve 1). This curve, which is based on three days worth of testing, shows only a slightdependence of NOx (corrected to 3% O2) on excess O2. In the 1.5 to 4.0% O2 range, correctedNOx increases from ~24 to 27 ppm. At ~3% O2 and (95-100°F) air temperature the averageNOx emission is ~ 26 ppm, and the largest reading does not exceed 27 ppm. Under the sameconditions, an increase in the combustion air temperature from 96 to 113 °F brings a visible ~2ppm (or ~7.5 %) increase in NOx (see Fig. 6). In other words, a 10 °F increase in the airtemperature brings an average ~ 1.2 ppm (or 4.4 %) increase in NOx concentration.



0

5

10

15

20

25

30

35

0 1 2 3 4

excess O2,%

NO

x @

O2=

3%, p

pm

option #1, no FGR option #1, ~7.5% FGR option #2, no FGR option #2, ~7.5 % FGR

3

41

2

Figure 5. NOx vs O2 at full load

25

26

27

28

90 95 100 105 110 115 120

air temperature, F

Figure 6. NOx vs combustion air temperature at full load and O2=2.7-3.24% (no FGR)



Burner design option #2 is a variation on the baseline option #1 which is exactly the sameexcept that three of the six gas pokers are blocked off, routing the entire gas flow through thethree remaining pokers and doubling the fuel gas exit velocity. This arrangement is described inmore detail in reference [6]. Under the same operating conditions (full firing rate, ~3% O2,~100°F combustion air temperature), implementing option #2 raised the NOx emissions of theburner from 26-28 to 31-33 ppm. Just like with option #1, excess O2% was found to have littleimpact on the corrected NOx emissions for option #2 (see Fig. 5, curve 3).

16

17

18

19

400 500 600 700 800 900

FGR temperature, F

NO

x @

O2=

3%, p

pm

Figure 7. NOx vs FGR temperature (full load, O2 = 1.85-3.13%,~7.5% FGR, Tair = 88-92F)

A 7.5 % flow rate of FGR (1.25 lb FGR/lb fuel gas) of an average ~600 °F temperatureinduced into combustion air reduced NOx from 26-28 to 17-18 ppm on the baseline (option #1)burner configuration (Fig. 5, curve 2). From this we can determine that each pound of “high”temperature FGR per pound of fuel gas provides an average 28% NOx reduction in the 0-1.25 lbFGR/lb fuel gas range.

The effectiveness of the FGR, however, depends on the recirculated flue gas temperature (seeFig. 7). An increase in FGR temperature from 550 to 710 °F decreases the NOx reductioneffectiveness of FGR from ~ 30 to ~26 % for each 1.0 lb FGR /lb fuel gas (a 13-14% reductionof the total value). Following this same trend, it can be predicted that decreasing the FGRtemperature from ~550 to 350 °F, which is the flue gas temperature available on most packageboilers, will improve the NOx reduction effectiveness of FGR from ~30 to ~35 % for each 1.0 lbFGR /lb fuel gas.

When testing option #2, introducing ~3% FGR (~0.5 lb/lb fuel gas) of an average 258°F intoFD fan inlet brought NOx down from 32 ± 1 ppm (curve 2) to 26 ± 1 ppm (curve 3). This



comparatively “cold” FGR provided a ~37 % NOx reduction for each 1.0 lb FGR /lb fuel gas,and is similar to what was predicted above for option #1.

Both options #1 and #2 (with and without FGR) produced a negligible amount (less than 10ppm) of CO in the load range from 100% down to 35% (where the excess O2 exceeded 6-8%).These low CO concentrations can be associated with the increased residential time in thefurnace, which is longer than typical package boilers due to its increased length, and the lack of asteam tube bank immediately after the flame zone. Only at loads under 35%, when excess O2exceeds 8%, were any significant CO concentrations measured during testing.

Comparison of FIR and FGR NOx Reduction Effectiveness

Burner design option #3 was a modification that allowed us to introduce approximately 4%FIR (measured in % of total flue gas products) into the burner fuel (or ~0.67 lb FIR/lb fuel). Justlike options #1 and #2, this configuration also provided stable operation throughout the fulloperational turndown range - from low firing (~10%) to 100% heat input. When operating withFIR, the visible flame with option #3 was 3-4 feet shorter in comparison the undiluted gas flamesof design options #1 and #2, but the width of the flame was approximately the same – around 4.5feet. With the burner operating at full load and ~ 3% O2, introducing 4% of 124-250 °Ftemperature FIR did not appear to increase burner RDL.

Modifying the burner design from option #1 to option #3 decreased the ratio of gas jetvelocity to the air velocity, compared to the optimized ratio used on option #1, and increased thebaseline NOx from 25-27 ppm to 32-40 ppm (Fig. 8, curve 1). Although it was believed that thebaseline NOx for option #3 could be reduced, it was not within the scope of this investigation todetermine the minimum baseline NOx for each burner design option. Increases in the excess O2from ~1.5 % to ~3.6, 4.0 and ~5.0 % increased NOx concentrations (corrected to 3% O2) from~32 ppm to ~35, ~37 and ~40 ppm, respectively. However, introducing comparatively “cold”FIR into the burner reduces NOx significantly, especially with excess O2 less than 3%.



0

10

20

30

40

50

0 1 2 3 4 5 6

O2, %

Baselineno FIR, FGR, steam, water

~4% FIR onlyTfir=124-250F

~4% FIR (124-250F) + ~7.5 % FGR (~600F)

~4% FIR + 7.5% FGR+ ~0.5 lb steam/lb Fuel Gas

1

2

3

4

Figure 8. FIR influence on NOx vs O2 dependencies with option #3 at full load

Curves 2, 3, and 4 on Fig. 8 all show that when ~4% FIR is introduced into the option #3 gasunit, changes in excess O2 do not provide a visible impact on NOx concentration. In the testedexcess O2 range, there are only negligible NOx deviations from each of the three averagingcurves.

With ~4% “cold” FIR (124-250 °F flue gas temperature), the burner NOx emissions arereduced from the baseline level of 32-40 ppm down to ~26 ppm (in the 1.2 to 3.3% O2 range, seecurve 2). From this reduction we calculate that each pound of “cold” FIR per pound of fuel gasreduces NOx by about 40% in the 0-0.67 lb FIR/lb fuel range. This NOx reduction effectivenessis greater than that achieved with “cold” FGR when testing with option #2.



22

23

24

25

26

27

28

100 150 200 250 300

FIR temperature, F

Figure 9. NOx vs FIR temperature with option #3 at full load,O2 = 2.0-3.5%, FIR = ~4%

As with FGR, increasing the temperature of the recirculated flue gas similarly reduces theNOx reduction effectiveness of FIR (see Fig. 9). Greater NOx reductions were achieved withlower flue gas recirculation temperatures. With our test setup, the lower FIR temperatures weredue to (1) cooling in the ductwork with comparatively small FIR flows, and (2) partial FIR flowcooling in the installed flue gas/water cooler. Because the flue gas recirculation cooler used forthis test has a fixed surface area, higher FIR flow rates (greater than 6%, or ~1.0 lb/lb fuel) couldnot be cooled to the same 125-250°F range experienced with only ~4% FIR and could not betested.

Burner design option #4 was a modification that increased the FIR capacity of the burnerfrom ~4% to ~12% (or from 0.67 to ~2.0 lb/lb fuel gas). Like option #3, this option reduced theratio between average gas and air velocities (without FIR introduction) to one third (1/3) of theoriginal burner design (option #1), and raised the baseline NOx from 26 to ~40 ppm. Again, webelieved that the baseline NOx for this option may have been able to be improved, but it was notwithin the scope of this investigation to do so. With approximately 12% FIR flow rate, theburner combustion appeared improved with the flame becoming shorter and more transparent.



10

14

18

22

26

30

0 1 2 3 4 5

excess O2,%

0

2

4

6

8

10

RDL

NOx

Figure 10. NOx and RDL on design option #4 vs excess O2 at full load, ~100 Fcombustion air temperature, 1.85-2.24 lb/lb fuel gas FIR flow rate and 328-505 deg F

FIR temperature

The modifications to the gas insert required to be able to introduce 12 % FIR to the burnerblocked some area in the burner throat and increased the RDL by ~50% - from the ~5.6” WC onbaseline option #1 up to an average of ~8.4” WC. Adding the ~12% FIR flow to the burner didnot appear to significantly increase the RDL beyond the increase seen from the modifications.Actual RDL numbers are shown in Fig. 10 where is it is shown that increases in excess O2 from1.0% to 3.0% and ~4.6% increase RDL from ~7.4” WC to ~8.4” WC and ~8.8” WC,respectively.

As seen from Fig.11, the ~12 % FIR flow rate available on option #4 cuts NOx emissionsalmost in half - from ~40 ppm to 18 - 25 ppm, depending on the excess O2 and flue gastemperature. At 3% O2 and 500 –550°F FIR temperature, NOx is approximately 20 ppm. Underthese same conditions, reducing the flue gas temperature from 500-550 °F to 200-250 °F reducesNOx from 20 ppm to ~18 ppm.



0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12 14

relative FIR/FGR flow rate, %

1

2

3

4

Figure 11. NOx vs FIR/FGR flow rate at full loadcurve 1 - option #1, FGR, curve 2 - option #3 FIR,curve 3 - option #3, FGR, curve 4 - option #4, FIR

Fig. 11 demonstrates the results obtained when using FGR and FIR on three burner designoptions (option #1, #3, and #4). From this figure it can be seen that the same ~18 ppm NOx levelwas obtained with 7.5 % FGR for the option #1 (with ~26 ppm original NOx) and ~12 % FIR forthe option #4 (with ~40 ppm original NOx).



0

10

20

30

40

50

60

0 2 4 6 8 10 12 14

FIR/FGR recirculation flow rate, %

FIR, Option #4

FIR, Option #3 FGR, Options #1 and #3

Figure 12. NOx reduction vs FIR/FGR flow rate at full load

Corresponding NOx reduction data, presented in terms of percent (%) reduction from thebaseline NOx (no FGR, FIR, or steam injection) for that burner configuration, are provided inFig. 12. Comparing the FGR and FIR curves obtained when testing option #3 shows that thesame 4% (~0.67 LB/LB) flue gas recirculation flow rate results in a ~24% NOx reduction whenused as FIR as opposed to an only ~16% NOx reduction when used as FGR. In other words, 1%FIR provides ~6% NOx reduction against ~4% with 1% FGR. Though the FGR temperature ishigher in this comparison (550 –600 °F against 200 –250 °F FIR temperature), increasing thetemperature of FIR would reduce FIR’s effectiveness by no more than 20%, resulting in a ~5%NOx reduction per 1% FIR. The same conclusion can be made from a comparison of the FIR andFGR curves at the 7.5 % flue gas recirculation flow rate. With this flow rate, FIR at 350-400°Fachieves a 42% NOx reduction (or 5.6% reduction per 1% FIR) while 550-600°F FGR onlyachieves a 31% NOx reduction (or 4.1% per 1% FGR).



0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12 14

FIR/FGR flow rate, % recirculation

FIR

FGR

Figure 13. Comparison of relative FIR and FGR NOx reduction effectiveness

A comparison of relative FIR and FGR effectiveness (NOx reduction percentage per 1 % fluegas recirculation at 500-600 °F) obtained with all of the burner design options tested is presentedin Fig. 13. From this figure, it is seen that 1% FIR provides an average of approximately 25%greater NOx reduction than 1% FGR (7.5% reduction versus 6%). Comparatively smalldeviations of the experimental data from the averaging curves confirm that, under the testedcombustion conditions, the major factor for the NOx reduction is FGR/FIR flow rate.

The downward slopes of the two curves presented in Fig. 13 indicate that the NOx reductioneffectiveness for each additional 1% of both FGR and FIR added to the burner decreases as fluegas recirculation flow rates increase. For this reason it makes sense for some applications tocombine restricted amounts of both FIR and FGR flows to achieve the highest NOx impact withthe least amount of recirculated flue gas. For example, a combination of 4 % “cold” (< 250 °F)FIR and 7.5 % “hot” (~600 °F) FGR (induced into FD fan inlet) may provide a similar or greaterNOx reduction than if all 11.5% recirculated flue gas were used as FIR. The results obtainedfrom testing this combination, are shown in Fig. 8 (curve 3). In the 1.0 -3.3% O2 range, thiscombination of FIR and FGR resulted in a ~50 % NOx reduction - from 32-34 to 16-18 ppm.Another combination of flue gases at the same temperatures (7.5 % FIR and 4.0% FGR) was ableto obtain a 55% NOx reduction. By comparison, the same 11.5% recirculation of FIR only at350-400°F is predicted to obtain only a ~50% NOx reduction (see Fig. 12).

The NOx reduction effectiveness of both FIR and FGR can be increased by reducing therecirculated flue gas temperature. This improved effectiveness is usually available on packagedboilers equipped with water economizers where the stack flue gas temperature does not exceed350 °F. As per the presented test data, decreasing the recirculated flue gas temperature from ~



600 to ~350 °F will improve the relative NOx reduction effectiveness (% reduction / %recirculation) of both FIR and FGR by at least 5%.

Comparison of Steam/Water injection and FGR

Flue gas formed by the complete combustion of natural gas at low excess O2 contains at least15% water vapor by weight. However, it is well known that introducing even a small additionalamount of moisture into a burner flame usually brings a significant NOx reduction, whichdepends not only on amount of moisture but also on the furnace/burner design, combustion airtemperature, and excess air. In addition to these factors, the location where moisture isintroduced into the furnace or flame is also very important. It has been established on a fewindustrial boilers that the NOx reduction effectiveness of moisture injection is changed whenFGR is used as well. Even in cases with the same combustion conditions, the same amount of themoisture introduced at the same point can provide different NOx reductions, depending onwhether the moisture is steam, water, or a mixture of both.

This portion of the paper presents the NOx emissions reduction obtained on the burner testconfigurations from five different methods of moisture introduction. These methods include:

(1) Steam injected into the FIR which is then mixed with the burner fuel gas,

(2) Steam injected into the FGR which is induced into the FD fan

(3) Steam injected directly into combustion air flow without FGR

(4) Steam injected into both the FIR/FGR and air flows simultaneously, and

(5) Atomized water injected into the FIR which is then mixed with the burner fuel gas

0

10

20

30

40

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

relative steam flow, lb/lb fuel gas

T fgr = 710 F

T fgr = 550 F

Figure 14. NOx reduction with steam injection in the ~7.5% FGR flow on option #1



The percentage NOx reductions (from baseline) achieved from injecting saturated steam (10-20 psig) into a constant ~7.5% FGR flow with the original burner design (option #1) arepresented in Fig. 14. During this testing the combustion air temperature was ~100°F. This testwas performed for two different average FGR temperatures, ~550 and ~710°F. Based on theobtained test data, it can be determined that the injection of saturated steam (in the 0-0.6 lb/lbrange) into the “higher” temperature FGR flow provides an average ~55% NOx reduction foreach lb steam/1 lb fuel gas. This result is roughly two times greater than the effectiveness of“hot” FGR which resulted in ~28% NOx reduction for each lb FGR / lb fuel gas.

Fig. 15 shows the actual NOx emissions measured on the original burner design (option #1)when testing with 550°F FGR and steam injection into the FGR. From this figure it can be seenthat option #1 is capable of operation down to ~12 ppm NOx when ~7.5% FGR and 0.5 lbsteam/lb fuel gas are used. Since the NOx emissions were about 18 ppm when operated with the~7.5% FGR, but without steam, then it can be calculated that the steam reduced NOx by about66% for each 1 lb steam / 1 lb fuel gas used (in the 0-0.6 lb steam / lb fuel range). This NOxreduction effectiveness is ~ 2.35 greater than that of FGR on a mass basis.

0

5

10

15

20

25

30

1 2 3 4

excess O2,%

Baseline (no FGR, no steam)

~7.5% FGR(~550F), no steam

~7.5% FGR (~550F),~0.4 lb steam / lb fuel gas

~7.5% FGR (~550F),~0.5 lb steam / lb fuel gas

Figure 15. NOx vs O2 at full load with FGR and steam introduction into designoption #1

The same ~12 ppm NOx level was achieved when testing option # 3 with simultaneous FGR,FIR and steam introduction (Fig. 16). In this case the same relative steam flow rate (~0.5 lb/lbfuel gas) introduced into any of three flows (FIR, combustion air, or in both FIR and air)



provided the same 33% NOx reduction found when injecting steam into the FGR flow of thebaseline burner configuration (option #1). Corresponding data are presented in Fig. 17.

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6

O2, %

Baseline(no FIR, FGR, steam)

~4% FIR onlyTfir=124-250F

~4% FIR + ~7.5 % FGR ~7.5% FGRTfgr~600F

~4% FIR + 7.5% FGR+ ~0.5 LB steam/LB NG

Figure 16. FIR, FGR, and steam influence on NOx vs O2 dependencies testing option#3 at full load

A comparison of all NOx emissions data obtained when testing with steam and waterinjection using all four burner design options (#1-#4) is presented in Fig. 18. From this figure, itcan be seen that introducing steam into the FIR flow appears to provide a little greater (5 to 8 %)NOx reduction effectiveness than the same steam flow rate introduced into the FGR orcombustion air flow. The NOx reduction effectiveness of water injection into the FIR appears tobe between the effectiveness of steam injected into FIR and into combustion air (see Fig. 17 forfurther detail).



0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

relative steam (water) flow rate, lb/lb fuel gas

12

3

Figure 17. NOx emissions vs steam/water injection with testing the option #4(~12% FIR, full load, O2=1.94-5.16 %)

(curve 1- steam into FIR, curve 2-steam into air, curve 3 - water into FIR)



0

10

20

30

40

50

60

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

steam and water flow rates, lb / lb fuel gas

option #1, "cold" 7.5 % FGR, steam into FGR option #1, hot 7.5 % FGR, steam into FGRoption #2, 3% FGR, steam into air option #3. 4% FIR, steam into airoption #3, 4% FIR, steam into FIR option #3, 4% FIR, steam into FIR and airoption #3, 4% FIR+7.5% FGR, steam into air option #3, 4% FIR+7.5% FGR, water into FIRoption #4, 12 % FIR, steam into air option #4, 12% FIR, steam into FIRoption #4, 12% FIR, water into FIR

Figure 18. NOx reduction vs relative injected steam/water flow rate

The general trend of the points shown in Fig. 18 indicates that the NOx reductioneffectiveness of steam is generally not affected by different FIR and FGR flow rates. It was alsofound that changes in excess O2 % over a large range did not influence changes in NOxreduction effectiveness of steam. Unlike FIR and FGR, each additional amount of steam injectedinto the burner appeared to keep the same NOx reduction effectiveness as the original amountintroduced (within the 0-0.75 lb steam / lb fuel gas range tested). This could be due to the factthat since the amount of steam that could be injected to the burner was much smaller than theamounts of FIR and FGR available, the flow rate never became high enough to see the samediminishing returns.



By comparing the performance of various options presented in Figure 18, it can be seen thatgreater NOx reduction effectiveness can be reached with the same steam/water flow rate andinjection method when the baseline NOx emissions are higher. On the other hand, moistureinjection still allows us to bring NOx down significantly even when the initial NOx level is under30 ppm as well. The NOx reduction effectiveness of steam injection for all injection methodswas between ~40% to ~80% for each lb steam / lb fuel gas, with an average of ~66%, which is~2.35 times greater than that found for FGR. Even for the worst case, when the initial NOxconcentration does not exceed 26 ppm, the injection of 0.75 lb steam/lb fuel gas is capable ofproviding a NOx reduction of 38%.

Minimizing NOx Emissions by Using Simultaneous Dilution Methods

Taking into consideration that increasing the flow rate of any recirculated flue gas (FIR orFGR) decreases NOx reduction effectiveness, combinations of the described methods are thefocus and the major interest for future improvements and developments associated with thetested standard Variflame Burner. The data presented above show that an optimization of thetested combinations with a restricted introduction of flue gas recirculation and moisture into thefurnace brings the greatest effectiveness in NOx reduction, with ultimate NOx emissions levelsdown to 12 ppm. The test results described above demonstrate NOx reductions up to 75%, withcorresponding test data presented in Figures 7, 10, 11, and 14-17. The same combinations (FIRwith FGR and steam) used in the field already have brought a 95% NOx reduction on one boilerapplication [5].

CONCLUSIONS

1. The tested 40 MMBTU/hr heat input standard Variflame burner (option #1) combustingnatural gas with ambient temperature air at low excess O2 using no FGR fan or other meansto control NOx, provides great performance data including:

- A stable and comparatively short flame envelope with no flame impingement on thefurnace surfaces for the existing packaged boiler designs;

- High thermal efficiency;

- Reliable long-term operation throughout a wide turndown range;

- Comparatively low NOx and CO concentrations.

The presented test results are confirmed with a collection of high efficiency performancedata obtained during start up and testing on hundreds of packaged boilers equipped withstandard Variflame burners firing natural gas, recalculated for the combustion conditions thatexisted on the test furnace.

For the tested conditions without FGR, at full load and 1.5% O2 the burner produced arepeatable NOx emission of 24 ppm and CO of less than 10 ppm. Under standard 3% excessO2, the NOx concentration did not exceed 26 ppm and CO was lower than 5 ppm. Theintroduction of “hot” FGR (~550 °F) in the amount of 1.25 lb FGR/ lb fuel gas (~7.5%recirculation) decreased NOx from ~ 26 to 17-18 ppm, a reduction of ~30%.



2. Fuel induced flue gas recirculation (FIR) does not require an external FGR fan and providesa 25% greater relative NOx reduction than regular FGR of the same temperature introducedinto combustion air flow. Using FIR on packaged boilers equipped with economizers wherethe stack flue gas temperatures are decreased from ~600 to ~350°F will bring an additional(around 5%) effectiveness in NOx reduction.

3. Comparative testing of five different methods to introduce steam and water into the burnershows that the method of introduction affects the NOx reduction effectiveness, withreductions of at least ~40% for each lb steam / lb fuel gas possible. NOx reductioneffectiveness is improved by a repeatable 5 to 8% when the steam is injected into the FIRflow rather than the FGR or combustion air. Overall, moisture injection achieved a NOxreduction effectiveness that was at least double that of regular FGR introduced into thecombustion air flow.

4. The presented test data confirm that the tested Variflame burner equipped with FIR andmoisture injection system installed and optimized as per John Zink Company/ToddCombustion Group know-how can provide ultra-low NOx natural gas combustion withoutusing an external FGR fan. Using option #4 with 12% FIR, 0.5 lb steam / lb fuel injected intothe FIR (or alternately 0.9 lb steam / lb fuel injected into the combustion air flow) and noFGR resulted in repeatable NOx and CO emissions in the 11-12 ppm and 3-6 ppm ranges,respectively, at 3% excess O2. To get the same NOx reduction using FGR only, it is usuallyrequired to have at least 20% flue gas recirculation.

5. These tests demonstrate that NOx emissions reductions equivalent to those possible withFGR are possible on standard Variflame burner installations without the use of an FGR fan,and show the opportunity to reduce the NOx emissions from other burner designs installed onany packaged boilers firing natural gas fuel.

REFERENCES

1. L. Tsirulnikov, J.Guarco and T. Webster, Boiler Burners, Chapter 19, The John ZinkCombustion Handbook, CRC Press, 2001, 547-588.

2. J.J. Feese and S.R. Turns, NOx reduction by fuel injection recirculation: insights fromlaminar flame studies, 1996 AFRC International Symposium, Baltimore, 1996.

3. T.Motegi and T. Nakamura, Lifted flames: a new concept of ultra-low NOx combustion,1996 AFRC International Symposium, Baltimore, 1996.

4. M. Matsumoto, T. Koizumi, T. Nagata, Low NOx combustion for high temperaturepreheated air, 1996 AFRC International Symposium, Baltimore, 1996.

5. T.Webster, J.Guarco and T. Eldridge, Gas conditioning technology for emissions reductionon boiler applications.

6. E. Shindler and J.Guarco, Poker Array, US Patent # 5,860,803. 1999.

ultra low-nox gas fired boiler burner design requiring no

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