UPGRADES AND ENHANCEMENTS FOR COMPETITIVE COAL-FIRED BOILER SYSTEMS

Transcription

1 UPGRADES AND ENHANCEMENTS FOR COMPETITIVE COAL-FIRED BOILER SYSTEMS J.B. Kitto, Jr. S.A. Bryk J.M. Piepho Babcock & Wilcox Barberton, Ohio ABSTRACT Deregulation of the electric utility industry is resulting in significant opportunities and challenges for U.S. power generators. Existing coal-fired capacity potentially offers the lowest variable cost power production option if these units are upgraded to optimize capacity, operating cost (including fuel), efficiency, and availability while also meeting today s stringent emissions control requirements. This paper highlights a variety of boiler system upgrades and enhancements which are being utilized to make aging coal-fired boilers low cost competitors in the 1990s. INTRODUCTION Seventy-five percent of today s operating coal-fired capacity was built in the era before 1980 (McGraw-Hill UDI, 1995) when the paradigm for new boiler power system designs included: Baseload capacity to maximize full load efficiency. Optimization around a single fuel to minimize capital cost. Service to integrated, regulated electric utilities with total cost based prices. The Energy Policy Act of 1992 and the pending Federal Energy Regulatory Commission (FERC) implementing rules are turning the power industry upside down. Some utilities are breaking into separate generation, transmission and distribution companies. Others are selling stranded assets. Still others are merging to be more competitive. While the final outcome is far from certain, the result has been a shift towards competitive electric power generation where the low price wins and overall performance counts. Surviving power generators must strive for minimum cost operation in order to prosper or go out of business. The new paradigm for coal-fired electric generators has become: Minimize operating cost (if it costs too much you can t sell it) through: Increasing equipment fuel flexibility for low cost coal use (fuel is the largest single operating cost at 70%+), Reducing other operating costs, and Increasing efficiency. Improve availability (if it doesn t run, you don t get paid) Maximize capacity (minimize large asset additions) Reduce maintenance and outage time (these are truly now variable operating costs) Optimize emissions control to minimize cost Coal remains the lowest cost fossil fuel resource with a price which is expected to remain stable or decline in the years ahead. As a result, the use of coal-fired capacity is expected to climb in the near term (Males, 1995). Increasing the capacity factor of these existing assets with low cost fuel will provide some of the lowest incremental cost power available. However, many of these older units (median age over 25 years) must be upgraded to make the available, reliable, low cost power required. The range of coal-fired boiler technology still in operation is quite broad, extending from small (~100 MW e ) 1950s vintage low pressure roof-fired units to large 1300 MW e modern high pressure units. Their design has become so well developed over the past 50 years that even modest changes in coal or revisions in operation may result in dramatic deterioration in unit performance, requiring major equipment changes to maintain capacity (Kitto, 1991).

2 Success in this endeavor requires a comprehensive plan to implement key technology advancements and design improvements which have occurred over the last few decades, while successfully accounting for the complex interaction of the various system elements. Recent experience with a number of unit upgrades also indicates that the greatest value can frequently be obtained by: 1) Component upgrade instead of replacement in-kind where availability, performance, or cost can be improved by the use of cutting edge technology to gain a cost advantage, 2) Earlier than planned component upgrade if the impact is reduced operating cost, instead of waiting to replace individual components at the end of their useful lives, and 3) Partnering between owners and suppliers to maximize value by sharing risks and rewards on such upgrades and enhancements to identify more innovative and cost effective solutions. Some of the more popular and effective upgrades and modifications that can improve unit flexibility, reduce operating and maintenance costs as well as improve coal-fired units are identified in Fig. 1. The balance of this paper focuses on some of the issues, options, benefits, and system interactions of these modifications. Not surprisingly, the modifications which affect direct cost the most are those which permit lower cost fuels to be burned. The available fuel options cover quite a broad range: individual coals, blended coals, co-firing with gas, co-firing with low cost local fuels such as petroleum coke, and most recently Orimulsion fuel. FUEL PREPARATION Fuel preparation systems, and more particularly coal pulverizers, are critical elements of successful efforts to reduce operating costs, meet emissions control requirements and provide boiler system flexibility, especially when fuel switching is involved. Installing low NO x burners to reduce emissions or fuel switching to a lower volatile coal will increase the amount of unburned carbon unless coal fineness is improved. Switching to a lower cost or lower sulfur coal can limit pulverizer capacity when the new coal is more difficult to grind (low grindability) or has a lower heating value. Lower heating values result in increases in coal mass flow through the pulverizer (a mass flow machine) for the same heat input to the boiler (a heat flow machine). New plant operating modes may require lower pulverizer loads where vibration and uneven pulverizer operation become important. Sootblower Upgrades Capacity Increase Superheater Outlet Header Improvements Circulation Upgrades Cycling Boiler Modifications Furnace Replacement and Upgrades Economizer Upgrades New Combustion Systems and Controls Air Heater Efficiency Improvements Fuel Preparation Equipment Upgrades FIGURE 1 OVERVIEW OF MODIFICATIONS AND UPGRADES TO IMPROVE OPERATING PERFORMANCE AND REDUCE COSTS.

3 A variety of upgrades have been developed to address these issues (Bryk, 1994). Pulverizers with rotating classifiers (Fig. 2) extend the range of performance including increased mill load, improved fineness and reduced pressure loss. The mechanical design uses two stage separation, reducing the amount of fines which are recirculated to the grinding zone thus increasing overall mill capacity. This feature permits a trade-off between the desired mill capacity and the fineness of the coal transported to the burner. Field tests indicate that a 10 to 15% increase in capacity is possible or an increase in fineness from 70% to 90% of the coal passing through a 200 mesh screen. Depending upon load, a 20% reduction in mill pressure loss can also be obtained. For example, a current retrofit study is looking at a boiler system which will experience a 30% reduction in capacity because of the effects of fuel switching on the pulverizer capacity. A rotating classifier retrofit for this application will permit mill capacity to increase 14-28%, recovering most of the lost capacity. Alternately, the rotating classifier system will provide higher coal fineness values without affecting capacity. Higher fineness mitigates the increases in unburned carbon losses often associated with low NO x burner retrofits. Another pulverizer design upgrade is to replace the stationary primary air throat (ring) with rotating elements. The design, first patented in the 1980s, reduces pressure drop, extends component wear life, and is easier to replace. Patented profile tire and wear segment designs can also reduce power consumption by more than 10%. Greater pulverizer turndown capability, frequently associated with fuel switching, can be achieved by installing a variable loading system for the grinding elements. This permits superior mill turndown without the excessive vibration that occurs when the coal load in the mill is insufficient to maintain a stable coal layer between the grinding elements. Maintenance can be reduced by installing ceramic wear-resistant components in the mill. Finally, inerting systems can be installed to reduce the fire and explosion potential of high dust, high volatile coals like those from the Powder River Basin. CIRCULATION UPGRADES/CAPACITY INCREASES A variety of pressure part and steam-water system operating problems may occur due to initial design deficiencies, a change Ceramic (Cera-VAM) wear parts extend life in high erosion areas (turret, classifier, housing and roll wheels). DSVS rotating classifier can reduce unburned carbon losses, increase capacity and reduce power consumption. Steam Outlet Side Wall Central Downcomer Rear Boiler Screen Rear Wall Boiler Tubes Low profile assymetric tires reduce power consumption. Low pressure drop rotating throats reduce power consumption and extend life. Variable loading systems accommodate low load turndown without vibration. FIGURE 2 PULVERIZER UPGRADES AND ENHANCEMENTS FIGURE 3 ADVANCED NUMERICAL STEAM/WATER CIRCULATION MODEL (THE ANTHEM CODE) DETAILED RESULTS SUPPLEMENT CONVENTIONAL METHODS FOR UNIQUE SITUATIONS.

4 from baseload to cycling service or a change in heat absorption from a fuel switch. These result in less flexible load-following operation, increased downtime, increased maintenance costs, and limited boiler capacity, and are frequently characterized by tube overheating (flow, stability, or heat transfer problems), excessive waterside deposition due to low flow, feedwater control fluctuations, drum level excursions, and header cracking and distortion. Overall steam flow may be limited by excessive water carryover in the steam to the superheater or circulation-limiting steam carryunder in the downcomer water flow. A solution to these problems is very complex because a seemingly small pressure drop or flow change in one area can result in a significant deterioration elsewhere in the system. The first step to solving these problems involves a detailed circulation analysis of the existing design. Alternatives are then evaluated using new circulation analyses as well as advanced numerical models (Fig. 3) to identify the most cost effective upgrade solutions. These generally include: changes in steam drum internals to reduce pressure drop, water carryover and steam carryunder; redesign of circuit flow paths; addition/deletion of heat transfer surfaces; the use of ribbed tubing; and possibly material upgrades. The complex interactions are illustrated by the problems with and solutions for a large western U.S. drum boiler where wing wall tubes were failing from overheat. The problems prevented the boiler from operating at full load. A detailed engineering evaluation and model study of the original design found that the overheating was not due to chemistry and deposition as originally thought but rather to low flow velocities and flow imbalances which resulted in (steam) film boiling inside the tubes. The solutions included: Low pressure drop drum internals to increase flow Redesign of the wing walls to increase the absorption and driving head in the circuits to minimize any chance of instability Increase the angle of the panel tubes, and Use of ribbed tubes to prevent film boiling (Fig. 4). Low Pressure Drop Drum Internals Added Wingwall redesign increased length and heat absorption. 17 deg (0.3 rad) Extended Downcoming and New Downcomer Bottles Use of Multi-Lead Ribbed Tubes Location of Tube Failures 40 deg (0.7 rad) Increased Slope Angle Original Modified FIGURE 4 CIRCULATION UPGRADES REDUCE TUBE FAILURES AND EXPAND CAPACITY BY INCREASING STEAM/WATER FLOWS AND CORRECTING IMBALANCES.

5 CYCLING DUTY AND LOW LOAD UPGRADES As older, smaller units are retired from service, large 400+ MW e boiler systems will need to cycle and operate effectively and efficiently at low loads to economically meet total load demands. This is a particular challenge for large units designed for baseload service since many boiler and auxiliary components are affected. Selected issues for the boiler pressure parts include: fatigue failures in the economizer and lower furnace tubes, structural damage to such areas as windbox supports, large transient temperature differences of 200 to 400F (110 to 220C), and low individual circuit flows when load is reduced to 15% of maximum continuous rating. In addition, turbine issues such as temperature cycling limits and slow response may require modification to the boiler systems because of the mismatch between steam and gas flow requirements. Some of the upgrades and modifications to address these issues include: An off-line recirculation system for drum boilers to provide a small amount of circulation through the furnace and economizer to prevent temperature stratification and potential thermal shock upon start-up. Revised circulation systems for once-through boilers where series and parallel panels can be changed by control valves to maintain sufficient flow rates in all tubes at all loads. Superheater bypass and dual pressure capacity to permit better matching of steam pressure and temperatures from the boiler to the steam turbine (Fig. 5). These systems can greatly enhance the start-up speed of the boiler from low load or an off condition. Spiral circuitry furnace replacement may provide an economical option where significant cycling service from 100% down to 15% MCR is desired for once-through boilers (Fig. 6). This design is more flexible in meeting the cycling/low load conditions than the older Universal Pressure designs used in virtually all U.S. baseload once-through boilers in operation. Extensive experience with this type of design has been obtained in Europe and Japan. HIGH TEMPERATURE HEADERS High temperature superheater and reheater outlet headers (>900F [>480C]) present perhaps some of the most challenging FIGURE 5 SUPERHEATER BYPASS SYSTEMS PERMIT FASTER START-UPS BY MATCHING STEAM AND TURBINE TEMPERATURES. FIGURE 6 SPIRAL CIRCUITRY FURNACE IMPROVES CYCLING AND LOW LOAD OPERATION OF ONCE- THROUGH BOILER SYSTEMS. design issues as well as some of the best opportunities for improvement. These headers experience high temperature creep under normal operation which ultimately limits component life. Also, cycling thermal and mechanical stresses, which are accentuated by boilers operating in cycling service, can combine with creep damage to form creep fatigue which can lead to failure much sooner than creep alone. Many original header designs also accelerate weld, ligament, and nozzle cracking due to cycling stresses (Nakoneczny, 1995). A combination of design and material changes can upgrade header and boiler performance: forged instead of welded outlet nozzles, increased ligament spacing by spreading the header penetrations around the header circumference instead of clustering them together (reducing high stress locations, Fig. 7), redesigned tube hole penetrations to reduce stress concentrations, and longer unrestricted tube legs to permit more flexibility to accommodate motion. The use of 9V alloy (9Cr-1Mo-V: SA ) can significantly improve header life (Fig. 8). This is a ferritic alloy with high strength and toughness. The use of 9V permits thinner header tube walls which are less susceptible to creep fatigue damage. It has creep properties comparable to austenitic stainless steel, and it also avoids dissimilar metal welds. SA 335-P11 headers may also be upgraded to SA 335-P22 as well as P91.

6 Small Ligament Large Ligament Present Upgrade Lower Ligament Stress FIGURE 7 SUPERHEATER OUTLET HEADER DESIGN CHANGES REDUCE STRESS AND INCREASE LIFE. OTHER PRESSURE PART UPGRADES Superheaters, reheaters, and economizers are sensitive to ash characteristics and boiler operation. Fuel switching to reduce costs or emissions may result in excessive surface slagging, fouling, and plugging due to ash chemistry changes, with certain heat transfer surface configurations being more sensitive than others. Increased furnace slagging tends to shift heat absorption higher in the furnace and result in higher furnace exit gas temperatures (FEGTs). These elevated temperatures can cause superheater overheating, aggravate fouling, and result in excessive attemperator flow rates. These high attemperator flow rates in turn reduce overall cycle efficiency and have, in some cases, limited boiler load because of insufficient spray capacity. The steam temperature control range is also reduced. Finally, older unit designs tend to be more prone to corrosion, erosion, and mechanical/structural problems. Upgrades are generally site-specific custom designs which incorporate several options or features. In the low temperature portion of the convection pass, economizer redesign options include tube spacing, tube arrangement, gas velocity adjustment, bare/fin tube heat exchange surface, support relocation, gas bypass baffles/distributors, erosion barriers, and sootblower shields. These types of changes have been used to reduce fouling, pressure loss and ash plugging; enhance cleanability and extend time between outages; and limit corrosion, erosion and mechanical damage. The engineering challenge is to make an upgraded design perform in a limited, preexisting space. In the high temperature portion of the convection pass, changes in the furnace arch design have been used to give a more uniform gas flow distribution to the superheater inlet, thereby reducing overheating and fouling. As with economizers, superheater and reheater geometries can be adapted to reduce fouling/slagging, erosion, and plugging or modify overall thermal performance. Where excessive temperatures are limiting load, material upgrades have provided the extra margin for uninterrupted operation. Surface coatings and treatments such as chromizing can reduce steam-side oxidation/exfoliation and potentially reduce fireside corrosion. FIGURE 8 USE OF 9V ALLOY IN THIS SUPERHEATER OUTLET HEADER REDUCES SUSCEPTIBILITY TO CREEP FATIGUE DAMAGE. COMBUSTION SYSTEM MODIFICATIONS As part of efforts to address the 1990 Clean Air Act Amendments, virtually all coal-fired boiler systems will undergo combustion system upgrades to limit NO x emissions. Extensive articles have been provided elsewhere on possible upgrades depending upon the original equipment and fuel (Piepho, 1992; Mellody, 1995; Costanzo, 1995). Most coal-fired units will use low NO x burners with or without overfire air (furnace staging) to achieve compliance (Fig. 9). Furnace staging and gas recirculation are more common on gasand oil-fired units. For extreme cases, selective catalytic reduction (SCR) deno x systems are added. Sliding Air Damper Drive Burner Elbow Primary Air/ Pulverized Coal Conical Diffuser Air Measurement Pitot Grid Sliding Air Damper Adjustable Inner Vanes Burner Nozzle Adjustable Outer Vanes Flame Stabilizing Ring FIGURE 9 COMBUSTION SYSTEM RETROFITS (DRB-XCL BURNER) REDUCE EMISSIONS BUT FURNACE STAGING WITH HIGH SULFUR COAL MAY INCREASE WALL CORROSION.

7 The robust mechanical design as well as the performance improvements of combustion system upgrades minimize burner maintenance and significantly improve unit reliability. Where old burner systems are being replaced, some synergistic benefits from improved air and fuel distribution may include less slagging and improved furnace exit gas temperature control. However, it is also important to address other potential side effects of burner retrofits. Low NO x burners typically increase unburned carbon losses. As noted above, this can frequently be corrected by installing rotating classifiers in pulverizers to increase coal fineness. More consistent coal supply to the pulverizer is also usually required and can be provided by gravimetric coal feeders. The full benefit of low NO x burners requires more accurate control of the combustion system, and control system upgrades may be required. Low NO x systems may increase local tube and membrane panel corrosion. Here, surface coatings can be applied to extend furnace panel life. Low NO x burners may also change the ash particle characteristics sufficiently to affect performance of the back-end environmental equipment, in particular electrostatic precipitators (ESPs). The ash may have a smaller particle distribution leading to lower collection efficiencies. The density and flow characteristics of the ash may change, leading to collection hopper flow problems. Changes in the carbon loss usually alter the overall resistivity of the ash, making it more difficult to collect. SOOTBLOWER UPGRADES/WATER BLOWING Switching fuels can dramatically change the fouling and cleaning needs of a boiler system. Some fuels (western U.S.) promote very tenacious reflective ash deposits which reduce furnace absorption with only modest buildup of slag layers. In other cases, modified ash chemistry can lead to significant increases in slag deposit thickness. Either condition can lead to increased furnace exit gas temperatures, reduced steam temperature control, and possible unit derate of 20 to 30%. A variety of cleaning system changes can address some of these issues. Adding sootblowers or sootblower re-spacing can improve cleaning coverage. Water blowing may be used alone or in combination with steam or air sootblowing to remove particularly tenacious deposits. New intelligent sootblowers can clean only selected areas to avoid tube damage or can index for a more concentrated blowing pattern. New sootblower nozzles provide greater jet penetration. In other cases, sootblower upgrades alone will not be enough and the heat transfer surfaces must simultaneously be redesigned to optimize jet penetration and cleaning. OTHER EFFICIENCY IMPROVEMENTS A variety of other upgrades and enhancements are available to increase overall plant efficiency. Air heater leakage remains a major efficiency loss with leakage rates as high as 35%. For regenerative units, new automatic seal designs can return and hold air leakage down to original design values. For tubular air heater designs, simple re-tubing may be attractive, although revised air heater configurations using computer aided tools may permit upgrades which have lower pressure drop and reduced cold end or cold corner corrosion. ENVIRONMENTAL EQUIPMENT Overall performance of the back-end environmental equipment may deteriorate when fuels or operating procedures change, and upgrades may be needed to meet emissions limits. Opportunities also exist to upgrade systems to either reduce operating costs or increase performance to generate sulfur dioxide allowances or nitrogen oxide credits. Electrostatic precipitator (ESP) performance will usually decline when fuel switching reduces sulfur content because the final ash chemistry will change (higher resistivity). This may be aggravated by other ash chemistry changes or an increase in ESP gas volume flow. The installation of low NO x burners may also result in finer ash particle size and increased unburned carbon which can reduce ESP performance. While solutions are site specific, they generally include increased collection plate area, increased collection efficiency through options such as pulsed energization, and gas conditioning such as adding small amounts of sulfur trioxide (Kitto, 1991). Advances in wet scrubber designs have resulted in significant reductions in the cost and size of these systems. Opportunities exist to incorporate these features into older units to either reduce operating costs by reducing pump power or gas-side pressure loss, or by improving SO 2 removal efficiency so that the excess SO 2 allowances may be sold, banked or used elsewhere in the power system. Typically there is a trade-off where improving SO 2 performance will increase operating costs. Feeney, (1994,1995), reviews a number of upgrade opportunities in detail, some of which include: 1) redesigning older units for low pressure drop, 2) adding a sieve tray to improve gas/liquid contact and removal efficiency, 3) using an additive to reduce power costs and increase SO 2 removal, and 4) converting operation to forced oxidation to take advantage of gypsum sale or easier waste disposal. GETTING THE JOB DONE RE-ENGINEERING THE PROCESSES THROUGH PARTNERING Traditional approaches to upgrading and enhancing boiler system operation usually include various combinations of: preliminary design or engineering studies, bid package preparation, project material bids, and separate erection bids. Frequently, several different but related items are bid separately. This traditional approach is proving to be less acceptable because it can lead to ineffective relationships between customer and supplier. Different goals and objectives cause limited communications and suboptimal performance. The result is a solution to a problem instead of the best solution. By their very nature, upgrade and enhancement projects are unique, often one of a kind efforts. New and creative solutions are required, and Partnering (or Teaming) is a more effective means of getting the job done. Partnering is a formal, long term, commitment between customer and supplier to achieve a specific common project or business objective and maximize the effectiveness of each participant s resources. Clear project objectives are established with measurable benchmarks, targets, and incentives that are identified and understood by each team member. Open communications are paramount. Risks and rewards are shared.

8 The key to success in such upgrade projects is unlocking the innovation and creativity of the customers, equipment suppliers, and erectors to obtain the very best solution. In essence, partnering removes barriers to permit broader, potentially more cost effective and technically superior solutions. Perhaps the most powerful phase of this relationship is during project evaluation and conceptual design. In a recent retrofit project, 13 variables were reviewed leading to 480 feasible combinations that were short-listed to 24 options for economic analysis. In one low NO x retrofit design effort, the supplier/customer team began by assuming that a combustion system upgrade plus SCR deno x system retrofit would be required. However, through extensive work it was found that a modified combustion system alone would be sufficient to meet today s requirement thus cutting the current project scope and cost by $13 million. In another situation involving an air heater replacement, a range of options was considered including use of the existing air heater, changing the air heater size, and changing the air heater surface profile. Each offered both capital and operating cost advantages. GETTING THE JOB DONE ADVANCING TECHNOLOGY Further technology and system advancements will be required to keep pace with customer needs for improved performance as well as increasingly stringent emissions regulations. Improved burners, advanced fuel preparation and combustion systems, enhancements for post-combustion systems, and upgraded boiler components will all be needed. A key to rapidly bringing these improvements to the market place will be large scale testing in order to reduce risk and address the impact of various fuels on overall performance. The new Babcock & Wilcox Clean Environment Development Facility (CEDF) is an integrated state-of-the-art combustion and emissions testing facility that offers a unique platform for developing these improvements. The 100 million Btu/hr (10 MW e equivalent) unit includes pulverized coal feed system, furnace, convection pass, air heater, both wet and dry scrubbers, baghouses and electrostatic precipitators - effectively a complete coal-fired boiler system. It has been designed to accommodate a wide crosssection of fuels from virtually any coal to #2 and #6 oil, natural gas, or combinations of fuels. Scaleup issues are minimized because of the facility s size. New approaches to reducing NO x and SO 2 emissions are well underway with advances already being provided commercially. The facility also offers the opportunity to test a variety of fuels under controlled conditions to evaluate the impact of fuel switching to meet Clean Air Act requirements. The CEDF flue gas time-temperature history closely matches commercial conditions so that test results are representative of what will happen under actual field conditions. The wide range of post-combustion equipment installed at the CEDF permits optimization of system upgrades to meet site- and fuelspecific requirements. TIMING As the U.S. utility industry begins its transition from a highly regulated structure to a free-market dominated system, the role of upgrades and enhancements is changing. Highest priority is given to projects which minimize operating cost, improve availability, maximize capacity and meet minimum emissions standards. However, timing is now more important than ever. In the past, retrofits and upgrades were spread out over several years to meet specified capital budgets. Today, if an upgrade will reduce costs and meet economic return criteria, power generators will need to do the upgrade sooner than later. In a deregulated environment, if the upgrade is delayed too long a competitor will reduce their costs first and the generator will lose market share. Electricity generators can not afford to be a high cost producer. CONCLUSION Interactions between fuel preparation, burner, boiler, and cleanup equipment can be critical to successful boiler system operation. A variety of coal-fired plant upgrades and enhancements have demonstrated improved coal-fired boiler performance and reduced costs. Key among these are changes to provide more fuel flexibility to reduce fuel cost which makes up 70%+ of the plant variable operating costs. REFERENCES Bryk, S. A., Dougan D. R., Moen, N. S., and Piepho, R. R., 1994, Rotating Classifier and Springload Auto-Adjustor Improvements Increase Pulverizer and Combustion Efficiency, CEA Utility Thermal Plant Life Management Conference, Edmonton, Alberta, October 2-5. Costanzo, M. A., Perry, D. M., and Sharman, J., 1995, Low NO x Retrofit of a NSPS Boiler Burning Sub-Bituminous Western Fuel, Power-Gen Americas 95, Anaheim, CA, December 5-7. Feeney, S., 1994, Environmental Upgrades: Selected Case Histories, Kentucky Coal Utilization Conference, Lexington, KY, January Feeney, S., 1995, Upgrading Scrubbers to Improve Performance, Power, pp 32-37, August. Kitto, J. B., Kulig, J. S., and Bryk, S. A., 1991, Technical Considerations in Using Low Sulfur Fuel Switching as a Clean Air Act Compliance Option, ASME Paper 91-JPGC-FACT-7. Males, R. H., et al., 1995, The Implication for Coal Markets of Utility Deregulation and Restructuring, National Coal Council, Washington D.C., November. McGraw-Hill UDI, 1995, Power Statistics Database. Mellody, J. G., et al., 1995, Operational Results of a Low NO x Burner Retrofit on a 780 Net MW e PC-Fired Utility Boiler, Power-Gen Americas 95, Anaheim, CA, December 5-7. Nakoneczny, G. J. and Schultz, C. C., 1995, Life Assessment of High Temperature Headers, American Power Conference, Chicago, April Piepho, J. M., et al., 1992, Seven Different Low NO x Strategies Move from Demonstration to Commercial Status, Power- Gen 92, Orlando, Florida, November.