Performance Comparison of Four Different Low NOx Burners Located at One Facility

Jon B. Belcher (P.E.), Thermal Ventures, Inc.

INTRODUCTION:

In January of 1993, San Francisco Thermal, L.P. (SFTLP) received a letter from the Bay Area Air Quality Management District which stated the facility would be required to meet the emission limits of Regulation 9, Rule 7 by January 1, 1996. Regulation 9, Rule 7 requires NOx emissions not to exceed 30 ppm corrected to 3% O2 and CO emissions not to exceed 400 ppm corrected to 3% O2. Upon verification of the facility’s existing emission rates using AP-42 calculations, the decision 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. This description will explain how this process resulted in four different burner makes at a single facility and will review the compliance test results along with the daily operational performance of the units.

BACKGROUND:

San Francisco Thermal, L.P. is a subsidiary of Thermal Ventures, Inc. Thermal Ventures owns and 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, the acquisition of existing energy plants and the operations of energy plants. A brief description of each of the Thermal Venture’s systems is included in Table 1.

The San Francisco downtown district heating and cooling system was formally owned and operated 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, the purchase had to go through an approval process with the local regulatory commission. This approval process took approximately three years to complete and the purchase was not finalized until June 1993. During this lengthy approval process, PG&E was slowly reassigning its personnel to other locations. As this occurred, Thermal Ventures provided replacement personnel in order to maintain operations at the facility.

PITTSBURGH, PA. AKRON, OH. YOUNGSTOWN, OH. SAN FRANCISCO, CA.
STEAM CAPACITY, LB/HR 240,000 545,000


(operational)

360,000 490,000
CHILLED WATER CAPACITY, TONS 11,000 5,100 1000 none
FUEL Natural Gas, #2 Oil Coal, Wood Chips, Natural Gas Coal Natural Gas, #2 Oil

Table 1,  Thermal Ventures System Capacities

San Francisco Thermal, L.P. consists of two boiler plants, Station S and Station T. These plants supply steam to commercial businesses, office buildings, apartments and institutions in the downtown San Francisco area through approximately 12 miles of underground steam distribution piping . 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 the five boilers at Station T is included in Table 2.

Boile

3

4

5

6


Manufacture

Keeler

Keeler Union City Iron Works Keystone/Erie City Combustion Engineering
Type

D

D A O A
Year Buil

1958

1958 1957 1970 1975
Capacity, lb/hr

55,000

55,000 50,000 100,000 100,000

Table 2, Station T Boilers

BOILER #5:

Boiler #5 was selected first to retrofit because of problems which the plant had experienced with its operation. Even though the unit is rated at 50,000 lb/hr, the maximum capacity obtained from the boiler had been 35,000 lb/hr. This was primarily due to the forced draft (FD) fan being undersized. Other problems with the unit concerned the back wall. The boiler has a very short firebox and the rear wall had been replaced several times due to flame impingement and extreme heat 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 new Todd VariflameTM low NOx burner, a new windbox, gas and oil valve train, new FD fan, new burner throat, and controls. The Todd VariflameTM burner uses induced flue gas recirculation to meet the NOx limits and has a heat input rating of 64.2 mmBtu/hr. The retrofit was completed and compliance tests were conducted. The results of the compliance test are included in Table 3.

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.

Table 3, Boiler #5 Compliance Test Summary

The biggest problem experienced during the retrofit was with Contractor “A”. The schedule was not kept, the physical size of the components of the system were understated (resulting in more space 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 the contractor. As for the burner performance, at full boiler load the unit runs efficiently with oxygen levels in the 3-1/2% to 4% range. However, the unit does not operate efficiently at loads less 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 takes place. 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 of this paper.

BOILER #7:

The second unit to be retrofitted was Boiler #7, a 100,000 lb/hr Combustion Engineering type A boiler (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 bids were 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® low NOx burner rated at 130 mmBtu/hr, a new windbox, the ductwork from the windbox to the existing FD fan, a 25 HP flue gas recirculation fan (with ductwork and injection nozzle), a new burner throat and controls. The compliance test was conducted in late December of 1995 and Table 4 represents a summary of the results.

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

Table 4, Boiler #7 Compliance Test Summary

As can be seen by the test results, the unit barely met compliance. The primary problem was that it could not meet capacity. The contractor and CSI continued to work on the unit for approximately a year and eventually replaced the initial 36″ burner with a second, 38″ unit. This change helped with the capacity problem, but the full capacity of the boiler was still not being realized.

The performance of this unit is not only hindered by its capacity problem, but additionally, it does not follow load well. This is due to the control linkage between the gas valve and the FD fan damper (the jackshaft). The unit will operate smoothly at a given setting, but when the set point is changed, the unit may not respond initially. Then, after several adjustments, the unit will suddenly 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 readings in 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 of this paper.

EMISSION COMPLIANCE DEADLINE:

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

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 their location in the plant and the resultant ease of access. Because the units were identical, they were bid together as a turn-key job. Remember, during this process, SFTLP had not been entirely pleased 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 an agreement between CSI and Contractor B, the only burner which Contractor B could offer was a CSI unit; therefore, Contractor B did not bid on Boiler #3 and #4 work. Because of this, SFTLP was once again a bit apprehensive about the result of Boiler’s 3 and 4 turn-key bids. The result of 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 bid included new burners, new FD fans, a new 50 HP motor on Boiler #4’s FD fan (Boiler #3 used a steam turbine-driven FD fan and the turbine would be reused), new gas and oil valve trains and controls. The result of these units compliance tests are included in Table 5.

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

Table 5, Boiler #3 and #4 Compliance Test Summary

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 is included 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 last because its conversion was going to be the most difficult. First, the unit’s physical location within the plant was going to make the retrofit difficult. Additionally, the unit was experiencing several operational problems. The combustion in this unit was very violent and as a result the unit 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 the burner’s flame to impinge upon the rear wall. This wall, as with Boiler #5’s rear wall, had been replaced 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 was extremely difficult to maintain. Because of these problems, the operators avoided running this unit.

Because of the many problems with this unit, the contractors and burner manufacturers were not thrilled about this unit’s retrofit. Some contractors opted not to bid on this unit and others even withdrew their bids because of the difficulties anticipated. Meanwhile, Contractor B (who had handled the installation of the CSI burner in Boiler #7) had reached an agreement with a different burner 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 Alzeta Pyromat CSB burner rated at 125 mmBtu/hr, a furnace access doorway, a fuel pressure regulating valve and the burner controls.

As mentioned, the Alzeta burner is a bit unusual in its design and construction. It is made of five 38″ diameter cylindrical segments which when bolted together measure 12-1/2 feet in total length. The cylinder is mounted so that it projects into the boiler’s furnace. Instead of one large flame shooting into the furnace as with most burners, this unit incorporates several small flames emitting radially from the cylinder along the full length of the burner. The result of this is a very mild, tranquil, flame. The results of the compliance test for Boiler #7 is included in Table 6.

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

Table 6, Boiler #6 Compliance Test Summary

The unit runs very smoothly and is easy to operate in automatic. It responds well to load swings and the rumbling and shaking experienced prior to the retrofit was eliminated. The only complaints with the burner is not capable of burning oil and the oxygen levels are high (6% to 8%) throughout its operating range. A photograph of the completed Boiler #6 burner installation is included as Picture 5 at the end of this paper.


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

% CO2

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 O2 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 O2 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 not handle load swings

·  Below 95% capacity, O2 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 well;  3% to 3½% O2

·  Does not handle load swings

·  Operates OK @ given points, but surges when set point is changed

·  At low loads,  O2 increases to 5% – 6%

Table 7, Burner Comparison

PLANT OPERATIONS:

Prior to the burner retrofits at Station T and the finalized acquisition of SFTLP by Thermal Ventures, boiler operations were not streamlined or systematic. The only consistency was that the operation of Boiler #6 was avoided and one unit was kept on standby at all times. Since Thermal Ventures acquisition and the burner retrofit, the units have been operated based on efficiency and electrical consumption avoidance.

First, in viewing the load duration curve, it is evident that the peak steam production of Station T in 1997 was approximately 285,000 lb/hr. Additionally, the lowest load experienced was approximately 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 its FD fan. The second unit to be brought on-line is Boiler #3. Boiler #3 operates efficiently throughout its range and it also uses a steam turbine drive on its FD fan. Once Boilers #5 and #3 reach their capacity, Boiler #4 is brought on-line. This is because Boiler #4 also operates efficiently throughout its range. Once Boilers #5, #3 and #4 reach their capacity, Boiler #7 is brought on-line to replace the capacity of Boilers #3 and #4; once again, this is done because Boiler #7 will operate efficiently at its capacity. When Boiler #7 is brought on-line, Boiler #4 is taken off-line because of its electric FD fan and Boiler #3 is kept on hot stand-by. As plant load increases, Boiler #3 is brought up to capacity and then Boiler #4 is brought back on-line. As these units all reach their capacity, Boiler #6 is then brought on-line to meet peak capacity. Once again, this operational sequence is dictated by the respective boiler’s efficiency and the avoidance of electric power usage (primarily electric FD fans). The only method that the plant currently has of continuously monitoring the unit’s individual efficiencies is with the oxygen readings.

The results of this operational method are displayed in Figure1 which compares the steam sendout efficiency of Station T for calendar years 1994, 1995, 1996 and 1997 (comparison of pounds 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 1, Station T Steam Sendout Efficiency Comparison

CONCLUSIONS:

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

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