Performance Comparison

of Four Different Low NOx Burners

Located at One Facility

by

Jon B. Belcher, PE

INTRODUCTION

In January of 1993, San Francisco Thermal, L.P. (SFTLP) received a letter from the Bay Area AirQuality Management District which stated the facility would be required to meet the emissionlimits of Regulation 9, Rule 7 by January 1, 1996. Regulation 9, Rule 7 requires NOx emissionsnot to exceed 30 ppm corrected to 3% O2 and CO emissions not to exceed 400 ppm corrected to3% O2. Upon verification of the facility’s existing emission rates using AP-42 calculations, thedecision was made to proceed with the replacement of the existing burners with low NOx units.

This paper will describe the replacement process of the burners at SFTLP’s Station T. Thisdescription will explain how this process resulted in four different burner makes at a singlefacility and will review the compliance test results along with the daily operational performanceof the units.

BACKGROUND

San Francisco Thermal, L.P. is a subsidiary of Thermal Ventures, Inc. Thermal Ventures ownsand operates district heating and cooling plants in Pittsburgh, PA., Akron, OH., Youngstown,OH. and San Francisco, CA. Thermal Venture’s expertise lies in two primary areas, theacquisition of existing energy plants and the operations of energy plants. A brief description ofeach of the Thermal Venture’s systems is included in Table 1.

The San Francisco downtown district heating and cooling system was formally owned andoperated by Pacific Gas and Electric (PG&E). In the fall of 1990, Thermal Ventures, Inc.reached an agreement with PG&E for the purchase of the system. In order to be finalized, thepurchase had to go through an approval process with the local regulatory commission. Thisapproval process took approximately three years to complete and the purchase was not finalizeduntil June 1993. During this lengthy approval process, PG&E was slowly reassigning itspersonnel to other locations. As this occurred, Thermal Ventures provided replacementpersonnel in order to maintain operations at the facility.

TABLE 1: Thermal Ventures System Capacities

PITTSBURGH, PA. AKRON, OH. YOUNGSTOWN,

OH .

SAN FRANCISCO,

CA.

STEAM

CAPACITY,

LB/HR

240,000 545,000

(operatio nal)

360,000 490,000

CHILLED

WATER

CAPACITY,

TONS

11,000 5,100 1000 none

FUEL Natural G as, #2 Oil Coal, Wood

Chips, Natural

Gas

Coal Natural G as, #2 Oil

San Francisco Thermal, L.P. consists of two boiler plants, Station S and Station T. These plantssupply steam to commercial businesses, office buildings, apartments and institutions in thedowntown San Francisco area through approximately 12 miles of underground steam distributionpiping . Station S consists of two B & W, 65,000 lb/hr boilers and is operated as a peaking plant. Station T has five boilers ranging in size from 50,000 lb/hr to 100,000 lb/hr. A description of thefive boilers at Station T is included in Table 2.

TABLE 2: Station T Boilers

Boiler 3 4 5 6 7

Manufacturer Keeler Keeler Union City Iron

Works

Keystone /Erie

City

Combustion

Engineering

Type D D A O A

Year B uilt 1958 1958 1957 1970 1975

Capacity, lb/hr 55,000 55,000 50,000 100,000 100,000

BOILER #5

Boiler #5 was selected first to retrofit because of problems which the plant had experienced withits operation. Even though the unit is rated at 50,000 lb/hr, the maximum capacity obtained fromthe boiler had been 35,000 lb/hr. This was primarily due to the forced draft (FD) fan beingundersized. Other problems with the unit concerned the back wall. The boiler has a very shortfirebox and the rear wall had been replaced several times due to flame impingement and extremeheat buildup.

The replacement of Boiler #5's burner was bid as a turn-key job including a new FD fan. Contractor “A” was the low bid on the job at approximately $152,000 which included a newTodd VariflameTM low NOx burner, a new windbox, gas and oil valve train, new FD fan, newburner throat, and controls. The Todd VariflameTM burner uses induced flue gas recirculation tomeet the NOx limits and has a heat input rating of 64.2 mmbtu/hr. The retrofit was completedand compliance tests were conducted. The results of the compliance test are included in Table 3.

TABLE 3: Boiler #5 Compliance Test Summary

Burner Todd VariflameTM

Date of Test 11/11/95

Heat Input; mmbtu/hr n.a.

Fuel Natural Gas

NOx ppm @ 3% O2 23.7

CO ppm @ 3% O2 49.0

DSCFM n.a.

%O2; Average of 3 Runs 3.1

% CO2; Average of 3 Runs n.a.

The biggest problem experienced during the retrofit was with Contractor “A”. The schedule wasnot kept, the physical size of the components of the system were understated (resulting in morespace being used by the unit than anticipated), and based upon the construction experience,SFTLP was not comfortable with the inadequate level of knowledge demonstrated by thecontractor. As for the burner performance, at full boiler load the unit runs efficiently withoxygen levels in the 3-1/2% to 4% range. However, the unit does not operate efficiently at loadsless than about 95%; the oxygen levels drift up to as high as 10% at these lower loads. Additionally, the flame from the Todd unit is long and impingement on the back wall still takesplace. It is yet to be seen whether or not repairs to the back wall will still be necessary.

A photograph of the completed Boiler #5 burner installation is included as Picture 1 at the end ofthis paper.

BOILER #7

The second unit to be retrofitted was Boiler #7, a 100,000 lb/hr Combustion Engineering type Aboiler (reference Table 2). This retrofit was also bid as a turn-key job. Due to Contractor A’s“track record”, SFTLP was a bit apprehensive in awarding them a second job. When the bidswere received, Contractor B teamed with Combustion Specialties, Inc. (CSI) was the low bid. The bid price for this unit was approximately $130,000 and included a CSI NOXMISER® lowNOx burner rated at 130 mmbtu/hr, a new windbox, the ductwork from the windbox to theexisting FD fan, a 25 HP flue gas recirculation fan (with ductwork and injection nozzle), a newburner throat and controls. The compliance test was conducted in late December of 1995 andTable 4 represents a summary of the results.

TABLE 4: Boiler #7 Compliance Test Summary

Burner Combustion Specialties, Inc. NOXMISER®

Date of Test 12/29/95

Heat Input; mmbtu/hr 113.4

Fuel Natural Gas

NOx ppm @ 3% O2 29.5

CO ppm @ 3% O2 110.7

DSCFM 18,294

%O2; Average of 3 Runs 2.0

% CO2; Average of 3 Runs 10.3

As can be seen by the test results, the unit barely met compliance. The primary problem was thatit could not meet capacity. The contractor and CSI continued to work on the unit forapproximately a year and eventually replaced the initial 36" burner with a second, 38" unit. Thischange helped with the capacity problem, but the full capacity of the boiler was still not beingrealized.

The performance of this unit is not only hindered by its capacity problem, but additionally, itdoes not follow load well. This is due to the control linkage between the gas valve and the FDfan damper (the jackshaft). The unit will operate smoothly at a given setting, but when the setpoint is changed, the unit may not respond initially. Then, after several adjustments, the unit willsuddenly surge, usually over-shooting or under-shooting the new set point. Also, at lower loads,the unit is not as efficient. The unit will operate efficiently at the burner’s full load (O2 readingsin the 3% to 3-1/2% range), but at lower loads the oxygen levels will increase to 5% or 6%.

A photograph of the completed Boiler #7 burner installation is included as Picture 2 at the end ofthis paper.

EMISSION COMPLIANCE DEADLINE

The Bay Area Air Quality Management District’s (BAAQMD) letter in January of 1993 statedthat all of the units at Station T were to meet compliance by 1/1/96. With the installation andtesting of Unit #7 complete in late December of 1995, three boilers still required retrofitting tomeet compliance. SFTLP approached the BAAQMD and requested additional time for thesethree remaining boilers to meet the emission requirements. The BAAQMD responded withpraise for SFTLP having two units already in compliance. SFTLP was told that they were wellahead of other facilities in the area and a variance could be granted to complete the remainingretrofits in 1996. After a public hearing process, the BAAQMD officially granted a variance toSFTLP.

BOILER #3 AND #4

Boilers 3 and 4 are identical, opposite-hand, Keeler type D units rated at 55,000 lb/hr each(reference Table 2). These units would be the easiest of the five boilers to retrofit due to theirlocation in the plant and the resultant ease of access. Because the units were identical, they werebid together as a turn-key job. Remember, during this process, SFTLP had not been entirelypleased or comfortable with the result of Boiler #5's retrofit. During the retrofit of Boiler #7,SFTLP was extremely pleased with Contractor B, but not pleased with the CSI burner. Due to anagreement between CSI and Contractor B, the only burner which Contractor B could offer was aCSI unit; therefore, Contractor B did not bid on Boiler #3 and #4 work. Because of this, SFTLPwas once again a bit apprehensive about the result of Boiler’s 3 and 4 turn-key bids. The resultof the bid process was that a different contractor (Contractor C) was low with a bid price of$240,000. Contractor C’s bid included Coen QLN burners rated at 71.4 mmbtu/hr each. The bidincluded new burners, new FD fans, a new 50 HP motor on Boiler #4's FD fan (Boiler #3 used asteam turbine-driven FD fan and the turbine would be reused), new gas and oil valve trains andcontrols. The result of these units compliance tests are included in Table 5.

TABLE 5: Boiler #3 and #4 Compliance Test Summary

Boiler #3 #4

Burner Coen QLN Coen QLN

Date of Test 6/18/96 10/4/96

Heat Input; mmbtu/hr 60.9 63.4

Fuel Natural Gas Natural Gas

NOx ppm @ 3% O2 25.1 21.9

CO ppm @ 3% O2 <1 35.1

DSCFM 11,880 11,447

%O2; Average of 3 Runs 5.3 4.1

% CO2; Average of 3 Runs 8.9 8.9

Both Boiler #3 and #4 run well throughout their capacity range and hold oxygen levels in the 3%to 4% range. A photograph of the completed Boiler #3 and Boiler #4 burner installation isincluded as Pictures 3 and 4 at the end of this paper.

BOILER #6

The last boiler to be retrofitted at SFTLP’s Station T was Boiler #6. This unit was retrofitted lastbecause its conversion was going to be the most difficult. First, the unit’s physical locationwithin the plant was going to make the retrofit difficult. Additionally, the unit was experiencingseveral operational problems. The combustion in this unit was very violent and as a result theunit vibrated and rumbled during operation. This was partly due to a 125 HP FD fan which

generated a high differential pressure combined with a very short firebox which allowed theburner’s flame to impinge upon the rear wall. This wall, as with Boiler #5's rear wall, had beenreplaced on a regular basis due to cracked and damaged refractory resulting in warped casing. Beyond this, the unit was made even more difficult to operate because the drum level wasextremely difficult to maintain. Because of these problems, the operators avoided running thisunit.

Because of the many problems with this unit, the contractors and burner manufacturers were notthrilled about this unit’s retrofit. Some contractors opted not to bid on this unit and others evenwithdrew their bids because of the difficulties anticipated. Meanwhile, Contractor B (who hadhandled the installation of the CSI burner in Boiler #7) had reached an agreement with a differentburner manufacturer, Alzeta Corporation. Alzeta’s burner design is unusual and because of this,Contractor B was able to bid the job re-using the existing FD fan, ductwork and burner throat. Because of this, Contractor B was the resulting low bid at $140,000. His bid included an AlzetaPyromat CSB burner rated at 125 mmbtu/hr, a furnace access doorway, a fuel pressure regulatingvalve and the burner controls.

As mentioned, the Alzeta burner is a bit unusual in its design and construction. It is made of five38" diameter cylindrical segments which when bolted together measure 12-1/2 feet in totallength. The cylinder is mounted so that it projects into the boiler’s furnace. Instead of one largeflame shooting into the furnace as with most burners, this unit incorporates several small flamesemitting radially from the cylinder along the full length of the burner. The result of this is a verymild, tranquil, flame. The results of the compliance test for Boiler #7 is included in Table 6.

TABLE 6: Boiler #6 Compliance Test Summary

Burner Alzeta Pyromat CSB

Date of Test 10/4/96

Heat Input; mmbtu/hr 100.7

Fuel Natural Gas

NOx ppm @ 3% O2 20.4

CO ppm @ 3% O2 8.0

DSCFM 22,703

%O2; Average of 3 Runs 7.4

% CO2; Average of 3 Runs 7.1

The unit runs very smoothly and is easy to operate in automatic. It responds well to load swingsand the rumbling and shaking experienced prior to the retrofit was eliminated. The onlycomplaints with the burner is not capable of burning oil and the oxygen levels are high (6% to8%) throughout its operating range.

A photograph of the completed Boiler #6 burner installation is included as Picture 5 at the end ofthis paper.

BURNER COMPARISON

TABLE 7: Burner Comparison

Boiler #3 Boiler #4 Boiler #5 Boiler #6 Boiler #7

Burner

Test Date

Heat

Input;

mmbtu/hr

Fuel

NOx ppm

@ 3%O2

CO ppm

@ 3%O2

DSCFM

% O2

% CO 2

Comments

Coen QLN

6/18/96

60.9

Natural Gas

25.1

< 1

11,880

5.3

8.9

• No FGR req’d.

• Runs well

throughout

range

• Holds O 2 well;

(3% to 4%)

• no problems

Coen QLN

10/4/96

63.4

Natural Gas

21.9

35.1

11,447

4.1

8.9

• No FGR req’d.

• Runs well

throughout

range

• Holds O 2 well;

(3% to 4%)

• no problems

Todd VariflameTM

11/11/95

n.a.

Natural Gas

23.7

49.0

n.a.

3.1

n.a.

• Induced FGR

req’d.

• Full load runs

well - 3½% to

4% O2

• Does no t handle

load swings

• Below 95%

capacity, O 2

increases; 5% -

10%

• Flame impinges

rear wall

Alzeta Pyromat

10/4/96

100.7

Natural Gas

20.4

8.0

22,703

7.4

7.1

• No FGR req’d.

• Runs very

smoothly

throughout

range

• Drum level

easy to

maintain

• Rumbling and

vibration

eliminated

• Cannot burn

oil

• High O2

throughout

range; 6% to

8%

CSI

NOXMISER®

12/29/95

113.4

Natural Gas

29.5

110.7

18,294

2.0

10.3

• Forced FGR

req’d.

• At full burner

load runs w ell;

3% to 3½% O2

• Does not

handle load

swings

• Operates OK

@ given

points, but

surges when

set point is

changed

• At low load s,

O2 increases to

5% - 6%

PLANT OPERATIONS

Prior to the burner retrofits at Station T and the finalized acquisition of SFTLP by ThermalVentures, boiler operations were not streamlined or systematic. The only consistency was thatthe operation of Boiler #6 was avoided and one unit was kept on standby at all times. SinceThermal Ventures acquisition and the burner retrofit, the units have been operated based onefficiency and electrical consumption avoidance. Figure 1 is the 1997 load duration curve forStation T which demonstrates the current plant operations.

FIGURE 1: Station T 1997 Load Duration Curve

First, in viewing the load duration curve, it is evident that the peak steam production of Station Tin 1997 was approximately 285,000 lb/hr. Additionally, the lowest load experienced wasapproximately 65,000 lb/hr and the plant produced in excess of 963,245,000 pounds of steam. Initially, the plant is base-loaded with Boiler #5 because it operates efficiently at full load(Station T never sees a load below Boiler #5's capacity) and it uses a steam turbine drive on itsFD fan. The second unit to be brought on-line is Boiler #3. Boiler #3 operates efficientlythroughout its range and it also uses a steam turbine drive on its FD fan. Once Boilers #5 and #3reach their capacity, Boiler #4 is brought on-line. This is because Boiler #4 also operatesefficiently throughout its range. Once Boilers #5, #3 and #4 reach their capacity, Boiler #7 isbrought on-line to replace the capacity of Boilers #3 and #4; once again, this is done becauseBoiler #7 will operate efficiently at its capacity. When Boiler #7 is brought on-line, Boiler #4 istaken off-line because of its electric FD fan and Boiler #3 is kept on hot stand-by. As plant loadincreases, Boiler #3 is brought up to capacity and then Boiler #4 is brought back on-line. Asthese units all reach their capacity, Boiler #6 is then brought on-line to meet peak capacity. Onceagain, this operational sequence is dictated by the respective boiler’s efficiency and the avoidanceof electric power usage (primarily electric FD fans). The only method that the plant currently hasof continuously monitoring the unit’s individual efficiencies is with the oxygen readings.

The results of this operational method are displayed in Figure 2 which compares the steamsendout efficiency of Station T for calendar years 1994, 1995, 1996 and 1997 (comparison ofpounds of steam sent to the distribution system vs. cubic feet of gas burned). In some months,sendout efficiencies have increased close to 7% with an overall average increase of 3.3%.

FIGURE 2: Station T Steam Sendout Efficiency Comparison

CONCLUSIONS

Reviewing the process of the five boilers’ retrofits, a primary issue is evident: performance testsshould have been conducted on the units prior to the installation of the new burners. This wouldhave been in the best interest of all parties involved: SFTLP, the contractors and the burnermanufacturers. A performance benchmark of the individual units would have been establishedprior to the installation of the new burners. This would have enabled a fair and concise methodof judging the performance of the new units. Unfortunately, the timing of the BAAQMD letterwas such that it was received about the same time that Thermal Ventures formally acquired thesystem. Future burner replacements will be accompanied by performance testing prior to newburner installations.

As for the four different types of burners at a single location, obviously from the perspective ofspare parts and ease of operation, it would be beneficial to have similar burners. However, withfour different boiler manufacturers, three different boiler types and four different boilercapacities, none of the units operate similarly anyway. However, thus far, it is evident that theCoen low NOx burners operate the most efficiently and smoothly throughout their range of thenew burners at San Francisco Thermal’s Station T.

PICTURE 1: Boiler #5 with new Todd Variflame low NOx burner installed

PICTURE 2: Boiler #7 with new CSI NOXMISER low NOx burner installed

PICTURE 3: Boiler #3 with new Coen QLN low NOx burner installed

PICTURE 4: Boiler #4 with new Coen QLN low NOx burner installed

PICTURE 5: Boiler #6 with new Alzeta Pyromat CSB low NOx burner installed