San Juan Generating Station FGD Retrofit Project Update
The station is owned by nine different entities including investor owned utilities, municipal utilities and cooperative organizations. Owners are located in New Mexico, Arizona, Utah, Colorado and California. Power from the generating station is transmitted to all of those states. Public Service Company of New Mexico (PNM) is the operating agent.
Prior to the Limestone Conversion Project there were 575 full-time employees at the generating station. After the project, 460 full time employees will be budgeted. Two open pit mines operated by Broken Hills Properties (BHP) supply coal to the station. One mine is located next to the plant and a second is located about 20 miles from the plant. All coal is transported by truck to the plant.
In 1994, SJGS owners became concerned over the comparatively high operating and maintenance costs for the flue gas desulfurization (FGD) system. The 20-year-old regenerable W-L system was originally chosen in the late-sixties because it was competitive in operating and maintenance (O&M) costs and because there was concern about sulfur waste being a hazardous waste. Early lime and limestone systems were having many difficulties in those early years. In the past 20 years, the W-L system aged until it required comparatively high maintenance cost while limestone systems have become reliable and efficient. In early 1995, SJGS contracted a comparison study of costs for the W-L system with the most common modern systems. The ERPI FGD Cost Model was used to make the cost comparisons. Results of this comparison study confirmed the comparative high cost of the W-L system and indicated that, for the plant location, a limestone forced oxidation system (LSFO) was the best economic choice. Babcock & Wilcox (B&W) developed the possibility for converting the existing W-L system into a LSFO system. The EPRI FGD Cost Model estimated the cost of a LSFO system for SJGS at $110 million. Through the collaborative efforts of PNM and B&W, the budgeted cost of the conversion was reduced to $80 million. The major items that allowed this to be accomplished were design refinement, utilization of existing equipment, utilization of plant personnel and facilities, value engineering and risk sharing. The LSFO FGD system presented an opportunity for drastic reductions in O&M and energy costs. Economic justification based only on the FGD system, however, was not satisfactory. SJGS also has an extremely complicated waste water recovery system in order to meet zero discharge requirements including sodium waste from the W-L process. The W-L system is based on a sodium sulfite solution while a LSFO system is calcium carbonate to calcium sulfate. A conversion to LSFO would greatly simplify the waste water treatment system by using cooling tower and boiler blow down directly in the limestone slurry. Direct use of the blowdown water eliminates the need for more than one-half of the waste water treatment equipment. The project was justified and approved in 1996 on the basis of savings in the FGD system and waste water treatment. Originally, additional justification was thought possible with excess emission allowances but fluctuations in the allowance market have made income from this source uncertain. This conversion project, with the realized savings to date, is now seen as a $77 million project that will provide a savings in O&M fuel costs of $16 to $23 million a year depending on the way fuel and power are valued.
The project is unique in three ways. First, it is justified on the basis of economics with no regulatory requirements. The new system is, however, more efficient and removes more sulfur dioxide than the old system. Second, it is a true retrofit project. Much of the existing equipment was converted and the station remained in compliance with the air emission regulations while it was being converted. A third unique aspect of this project is that it was designed and constructed under a teaming or partnership arrangement with B&W. This simplifies organization, reduces cost and involves the operator more deeply in the project.
The W-L system was based on the chemical reaction of sodium sulfite with sulfur dioxide to form sodium bisulfite, Figure 1. The sodium bisulfite was thermally regenerated and the resultant sulfur dioxide was used to produce sulfuric acid. The original SJGS system was designed to produce sulfur but the process was extremely difficult to operate and was later replaced with a sulfuric acid plant. W-L absorbers are counter current multi trays with recirculating solution on each tray. They have prescrubbers to remove residual flyash and chlorides and to cool the fluegas. The scrubbing liquid is in the absorber sump and is separate from the absorbing solution which is drawn off from the lower tray. They have a set of demisters between the sump and the lower tray and at the top of the absorber. The product solution is sent to a regeneration plant which has four sets of two stage evaporators to produce the sulfur dioxide and regenerate the solution. The process also must remove any oxidized sulfate solution because it cannot be regenerated. This is done with chrystallizers and evaporators and sodium sulfate is produced for sale. None of the byproducts are economic to produce. Very high energy costs are involved because of the high absorber pressure drops and the use of boiler steam to regenerate solution. In addition there are more than 200 pumps used in the W-L process.
In contrast, the limestone forced oxidation system is simple and uses much less energy, Figure 2. Limestone slurry is prepared in Svedala ball mills and pumped to the absorbers where it is sprayed into the absorbers at two levels. A B&W patented tray is also in the tower to improve gas liquid contact. Two sets of demisters are located at the top of each absorber. Spent slurry in the sumps or reaction tanks is oxidized to gypsum and is drawn off of the absorber sumps and dewatered. Primary dewatering is accomplished with hydroclones and secondary dewatering is done with vacuum drum filters. Dewatered gypsum (85% solids) is conveyed to a pile where it is hauled back to the coal mine with fly ash and bottom ash for disposal. Reduced absorber pressure results in lower booster blower power and the simpler process drastically reduces the number of pumps required. No steam is required for the LSFO system.
In order to retrofit a LSFO system into the existing W-L system, all equipment had to be evaluated for possible reuse. The new design was based on the reuse of the existing absorber towers and all of the inlet and outlet ductwork. The absorbers were stripped inside but the exterior walls remained. The inlet and outlet ductwork was reused. Since less power was required in the LSFO system, new smaller motors were added to six of the absorbers and smaller fan rotors were added to the other six absorbers. Two new recirculation pumps were added to each absorber and the old scrubber pumps and tray pumps were removed. The LSFO solution is a less corrosive environment than the W-L solutions. This results in less expensive materials and should result in less maintenance problems. Some existing pipe racks were used to tie in the limestone preparation and dewatering area to the absorbers. Several tanks and a silo were converted to limestone system use. All of the electric power supply was reused and the switchgear and breakers were converted to limestone system use. Four control rooms were consolidated into one with this project and another control room was eliminated. Controls were consolidated and updated to a Distributed Control System (DCS).
This retrofit project was designed to keep the plant in compliance with State and Federal Regulations which means that only one absorber per unit could be converted at a time. The project was constructed in three phases, one absorber per unit in each phase. It also meant that construction was required to work around operating equipment without causing downtime. The design of piping and electrical around the operating system was extremely difficult.
PNM and B&W decided that the best approach for this project was for the two companies to enter into an agreement that allowed both companies to work in a team environment rather than the traditional supplier purchaser agreement. This arrangement allowed the companies to work closely together toward the common goal of providing SJGS with a high-quality FGD system that performs reliably to meet environmental requirements for the lowest possible cost. This type of arrangement requires a large amount of communication and cooperation. It also requires a great amount of trust on the part of both companies.
PNM, B&W and some subcontractors participated in formal team building sessions prior to the start of engineering design and also prior to the beginning of construction. The team building sessions were a great help in developing the trust required for this type of approach to a project.
PNM, B&W and the subcontractors held monthly meetings and weekly conference calls throughout the project to help communications and foster a team environment. These sessions were attended by individuals from all levels in each organization. SJGS engineers, mechanics, electricians, instrument journeymen and operators were encouraged to give input on design issues for cost savings early in the project. This approach greatly enhanced the "buy in" to the teaming approach. Several brainstorming sessions were conducted in order to solicit ideas for cost savings early in the project. Many of the ideas that came from these meetings were incorporated into the project and have contributed significantly to the cost savings that have been realized to date. A few cost saving ideas could not be incorporated into the project because of the "fast track" nature of the conversion. A very definite schedule had been established early in the project and completion dates had been written into the contracts. Additional savings justified acceleration of Phase II of the project by working two 50 hour per week shifts. Much information was transferred in the weekly conference calls to keep the project on track. Design drawings were reviewed and approved by SJGS personnel.
The budget for the project was mutually developed by both companies through a detailed estimate. Once this budget was established, both companies went to work on value engineering improvements to lower the cost. All design issues including the reuse of existing equipment and new equipment selections were mutually agreed upon. Suggestions for design improvements and cost savings have come from both parties. The major subcontractors were given incentives to generate ideas for design improvements and cost saving methods also. These subcontractors were invited to team building sessions as well as the monthly site meetings and conference calls in order to foster teaming.
The first phase of construction consisted of building the limestone preparation and gypsum dewatering areas in a new building. It also included the conversion of the first absorber on each unit from the old W-L type to the new LSFO design, Figure 3 (Click here to view Figure 3). Basically this means removing all absorber internals and installing B&W's patented absorber tray and two spray headers with recirculation pumps in each cell. Most of the power savings for the new LSFO system comes from reduced pressure drop. The old W- system had a prescrubber and five sets of internal trays. Only about 60% of the blower power is required for LSFO. Additional power savings will be realized because of the elimination of equipment in the W-L regeneration process and more than one half of the waste water treatment system. Overall the auxiliary power requirements are expected to be reduced by almost 30 MW that translates into increased net capacity of about 30 MW. Since all of the absorber structures are being reused, the inlet and outlet ductwork is also being reused. Some tanks, piping and piperacks also are being converted or used as is. It was found that the electrical power and MCC systems could be converted to the new usage by phases. This saved the significant cost of new transformers, breakers and switchgear.
B&W was released to begin engineering May of 1996. Babcock & Wilcox Construction Company (BWCC) mobilized on the site one year later in May of 1997. This phase was scheduled to last 10 months and was completed the end of March 1998. The first four LSFO absorbers were started up as a system and were operated in conjunction with two W-L absorbers per unit for Phase II. Phase II of construction was conversion of the second absorber on each unit to LSFO absorbers. The second phase schedule was accelerated and completed by April,1998. After Phase II LSFO completion, two LSFO absorbers were operated on each unit and the W-L system was shut down. Existing compliance was met with two limestone scrubbers per unit because it was possible to pass more flue gas through the limestone scrubbers than was possible through W-L absorbers and also dibasic acid (DBA) was added. The final construction phase was conversion of a third absorber on each unit to limestone. Final construction project completion was Oct. 15,1998 and New Mexico emission regulations now require approximately 75% SO2 removal or 0.46 lb/million Btu on an annual average. Existing state regulations also require that a station rolling average of 0.65 lb/million Btu be maintained and a unit two rolling average of 72% removal be maintained.
This phased approach to construction complicated the work considerably as the new equipment was installed throughout an operating plant that was required to remain in compliance. Also, many of the piping systems and electrical systems were installed around and with operating systems. The majority of new equipment was powered from existing power distribution equipment as it became available from abandoned equipment or absorbers being converted. This type of installation required a high degree of coordination, communication and teamwork. The teaming arrangement utilized for this retrofit was ideal for this type of project.
Seven PNM employees were integrated into the construction site organization. This was done to reduce cost and take advantage of plant personnel's knowledge of the plant. This structure improved the efficiency of the site construction team dramatically. PNM is performing some of the design and construction work with site personnel. This has also reduced cost and has enhanced SJGS personnel's sense of ownership in the project. PNM has the responsibility for design and installation of a new Distributed Control System (DCS) for the LSFO system. This control system also retrofitted the old controls for the waste water system. Combined DCS allowed for the consolidation of four control rooms into one and the elimination of a fifth. PNM controlled the start-up or commissioning of the system after each construction phase was complete. PNM in cooperation with B&W performed all of the training required for Operations and Maintenance people. This involvement accelerated the learning curve for operating a new system as well as reducing project cost.
This conversion project was justified on the basis of O&M and power/fuel savings. The reduction of personnel by 110 positions was a large percentage of the savings. The old W-L and WW systems required 67 people in operations and the new limestone/waste water system will require 26 operations people. W-L and WW for the old systems required 70 people in maintenance. The new systems will require about 17 people in all crafts. The high cost for maintenance materials and equipment replacement required for the 20 year-old W-L system will be greatly reduced with the newer and much less complicated limestone system. Fuel savings from reduced auxiliary power and elimination of regeneration steam is estimated at almost five million dollars per year. All of these savings are expected to add up to about $20 million per year. In addition, there could be income from excess allowance credits if the market allows for it.
The LSFO system is in full operation with three absorbers on each unit. Removal efficiencies have been measured and absorbers are performing according to design. The limestone preparation and gypsum dewatering equipment are operating at design capacities.
Phase I of the project was under budget and savings of $6,600,000 was declared. Part of the savings was shared with B&W and the subcontractors and this shared savings showed up as project cost. Budgeting has been unusual in applying the shared percentage of savings as a cost added to the project. Since the project was constructed in three phases, equipment for each phase was entered into the accounting books as it was placed in service. This, along with the nine owners of different units and different percentages of ownership made accounting extremely complicated. Additional savings was realized in phases II and II through labor efficiency. The teaming arrangement helps project success and results in lower overall project cost. The predicted savings are being compared to actual savings as data is collected on energy and fuel usage. Personnel reductions produce O&M savings and actual budget changes are being compiled. SJGS is much better prepared for the competitive marketplace.
If you have any comments or questions about the previous article, please contact Henry S. Taylor at hytaylor@mail.pnm.com