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