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BR#1859
Weld Overlay Materials Comparison For Recovery Boiler Service
To meet specific requirements for highly loaded recovery boilers, there has been substantial reliance on weld overlay tubing. This application uses a machine applied spiral bead of filler metal over 360 degrees of the tube circumference along the length of the tube. This paper presents the results of a laboratory study on a number of weld overlay samples using different filler metals selected for comparison based on their diluted as-welded composition. Results for materials with established experience including 304L, 825, and 625 were compared with the selected weld overlay candidates, including several which performed better than existing materials in simulated corrosion resistance and corrosion cracking environments during lab testing. The results identify filler metals that would provide both a cost and performance benefit over existing materials when used to produce a WOL360 tube for recovery boiler applications.
BR#1858
Integrating a Reheat Steam Cycle Power or Recovery Boiler into an Existing Pulp Mill
Existing low pressure pulp mills have the potential to greatly improve their electrical power generation by incorporating a reheat steam cycle into the existing plant. Reheat has been used by electrical generating utilities since the 1950s to improve thermal efficiency of the power generation cycle. In 2009, B&W PGG introduced the dual pressure reheat recovery boiler. Applying reheat steam cycles to the power boiler has the potential to substantially improve the power generation potential at a pulp mill. A reheat bubbling fluidized-bed boiler allows a pulp mill to burn biomass and achieve much higher electrical power production. The integration of a reheat steam cycle boiler into an existing mill presents challenges and requires an evaluation of the existing steam plant to determine the most advantageous way to accommodate reheat. This paper presents several case studies where reheat power or recovery boilers would greatly increase the electrical power generating capacity of existing pulp mills.
BR#1855
An Alternative Approach to Flue Gas Heat Recovery
Many solid fuel-fired boilers are equipped with an economizer and air heater to provide economical heat recovery and hot air for good combustion. There are practical limitations to how much energy a given economizer/air heater arrangement can absorb. This paper will explore an alternative approach to flue gas heat recovery. In this approach, B&W PGG integrates a watercoil air heater and the economizer surface to achieve final exit gas temperatures lower than what is economically possible with traditional economizer/tubular air heater technologies. Lower stack temperatures directly correlate to higher boiler efficiencies. This unique patent-pending technology increases the temperature differential between the feedwater and the flue gas thus improving overall heat transfer rates. Being a modular design, the strategic component arrangement promotes AQCS equipment integration. Additional benefits include lower fan parasitic power and the capability of controlling gas temperatures for varying fuel or boiler operating loads and conditions.
BR#1836
A Comparison of Superheat and Reheat Steam Cycles with Black Liquor Gasification for Pulp Mill Power Generation
Black liquor gasification is a process that replaces the Tomlinson recovery boiler by gasifying the liquor and delivering the gas to a boiler, combustion turbine, or some other process. To date, several projects have been built, however, the ultimate goal of burning the gas in a gas turbine has not been realized. B&W PGG recently introduced the dual pressure reheat recovery boiler. The reheat cycle has the potential to improve the power generating potential of a pulp mill steam plant. The power generated from a pulp mill reheat steam cycle approaches the theoretical generation potential of a black liquor gasification combined cycle process. This paper discusses the comparison.
BR#1830
Recovery Boiler Reheat Steam Cycle
High electricity costs and incentives for renewable energy are driving pulp companies to improve the power generating potential of their mills. The reheat cycle greatly improves thermal efficiency of the power generation cycle. Utility boiler experience applied to a pulp mill steam cycle would greatly improve the electrical power generation potential of a pulp mill steam cycle. B&W PGG has made developments in the recovery boiler design that will allow a recovery boiler to operate at utility pressures and with reheat steam cycles. The new reheat recovery technology will generate more electrical power from the pulp mill steam cycle.
BR#1767
Innovative Solutions for a Challenging Biomass Fuel and Boiler Upgrade Project
A biomass and non-condensable gas (NCG) firing addition, steam capacity increase and air system upgrade was successfully commissioned in a paper mill located in the southeastern U.S. This paper discusses the upgrade and the results. Steaming capacity was increased by 19% while maintaining the original steam temperature. The mill operates continuously on biomass but has retained the existing stoker coal capabilities for future fuel flexibility.
BR#1729
Enhancing the Circulation Analysis of a Recovery Boiler Through the Incorporation of 3-D Furnace Heat Transfer Results from COMO™
In the design of a highly loaded new or upgraded recovery boiler, the heat absorption profile istribution in the furnace becomes an important design parameter in evaluating the circulation adequacy of the unit. To enhance the current circulation analysis for recovery boilers, furnace heat transfer results from the computational three dimensional combustion analysis program, COMO™, have been incorporated into the company's circulation analysis program. The evaluation of circulation results for a highly loaded recovery boiler, which incorporated the heat transfer results from the computational combustion model, is compared to calculations generated using typical heat transfer analysis procedures in an effort to offer a less conservative and more accurate evaluation of the unit. (217K)
BR#1704
Coal Ash Liquor Cycle Chloride Control Restores Recovery Boiler Availability
Kraft recovery boilers operating today are experiencing higher fouling rates due to increased chloride content within the liquor cycle. It has been shown that excessive chloride content in recovery boiler ash results in increased fouling rates on the boiler's convective surfaces.(1, 2) Tighter liquor cycles and more efficient precipitator ash collection have resulted in higher chloride content in the liquor being sent to recovery boilers. By purging a small portion of the precipitator ash, the Alabama River Pulp and Alabama Pine Pulp Companies of Claiborne, Alabama have taken dramatic steps to restore their recovery boilers to design levels and to improve unit availability. Their liquor chloride control strategy has allowed both boilers to run between planned outages without unscheduled cleanings or loss of steam temperature. The following examination of this strategy will describe operation at the mills before and after changes were made in the liquor cycles, and will compare the impacts of ash dumping to operational cost savings. (204K)
BR#1696
Development and Evaluation of a Low-Temp Gasification Process for Chemical Recovery from Kraft Black Liquor
A bench-scale test program was conducted by McDermott Technology, Inc. (MTI, formerly the Research and Development Division of Babcock & Wilcox) to obtain proof-of-concept data for B&W's low-temperature black liquor gasifier design. The test program verified that black liquor gasification (BLG) was possible in a low-temperature, bubbling bed mode. To address incomplete carbon conversion, a novel gasification concept was developed wherein char and tar byproduct combustion is integrated with low-temperature BLG. This process could potentially achieve complete carbon conversion; however, there are significant challenges associated with thermal and physical integration of the processes. Results of a comparative economic analysis indicated that the low-temperature black liquor gasifier design was competitive with conventional technology for incremental capacity increases over a wide range of cost scenarios. Whether a stand alone gasifier or combined cycle system, the resulting energy output would be delivered as fuel gas or electricity instead of high pressure steam. This is expected to better address the energy needs of the future pulp and paper mill. Pressurized, oxygen-blown BLG technology is expected to offer more benefits as a recovery boiler replacement technology. A significant pilot-scale test program is required to address the technical concerns associated with oxygen-blown BLG and demonstrate commercial readiness of the technology. (335K)
BR#1695
Three-Dimensional Numerical Modeling of Stoker-Fired Power Boilers
A numerical model has been developed to simulate the various interacting physical processes that occur within any stoker-fired power boiler burning wood, refuse-derived fuel (RDF), coal, or other biomass fuel and operating at steady state. The processes modeled are three-dimensional turbulent gas flow, particle motion, chemical reactions and heat transfer. This paper provides a detailed description of the model and presents an example of its use.(1539K)
BR#1675
From Liquor to Sludge - Conversion of a Recovery Boiler to a Bubbling Fluid Bed Boiler
When a mill no longer needs an existing recovery boiler it is faced with the problem of what to do with the boiler in the future. The mill may have a new recovery of a larger size or may have changed from Kraft to recycle product. The sludge that once was landfilled is now a valuable fuel to generate steam and can displace other more expensive fossil fuels. A U.S. mill, having semi-retired a vintage 1964 recovery boiler with a larger recovery boiler, wanted to replace older power boilers with a power boiler capable of firing papermill sludge, tire derived fuel and wood waste. This paper discusses the modifications required to convert this recovery boiler to a power boiler and the considerations that paper mills need to understand when converting equipment to solid fuel firing. (755K)
BR#1674
Advanced Combustion and Flow Modeling: Study Improves Recovery Boiler Operation
A Kraft recovery boiler was experiencing steam temperature degradation due to excessive fouling of the convection surfaces, leading to plugging within the generating bank. Build-up of deposits on these surfaces required the unit be taken off line for water wash cleaning after eight to ten weeks of operation. Numerical combustion modeling was used to evaluate current boiler operation and assess the benefits of changes in operation and hardware required for improved operation. The Babcock & Wilcox Company's Combustion Model for Process Recovery boiler (COMO-PR™) was used to evaluate the boiler operation and design. This model is based on a fundamental description of interacting processes of turbulent flow, heat transfer and black liquor combustion. Modeling of the boiler, as it was operated, showed a strong side-to-side imbalance in the gas flow and temperatures within the furnace, and a large amount of mechanical carryover that was adversely affecting boiler operation. Boiler testing and operating experience qualitatively confirmed model predictions.
The model was used to evaluate different methods of introducing air and liquor to improve flow uniformity and reduce the amount of inorganic material entrained by the combustion gases as carryover and fume. A significant improvement was predicted and then achieved, with simple operational and hardware changes to the existing air and liquor systems. A combination of firing strategies and operation of the secondary air ports to better interlace secondary air improved flow uniformity and reduced the predicted amount of carryover by 60%. The amount of fume generated increased slightly, but chloride content in the ash was lower and was potentially easier to remove by sootblowing. The functional performance of the unit was modeled relative to the superheater surface requirements. Modeling based on the operational history indicated that additional surface would be required to attain design steam temperature throughout a six month operational span. (402K)
BR#1673
Bubbling Fluidized Bed Installation Capitalizes on Sludge
In recent years, the pulp and paper industry has encountered increasing cost and environmental issues associated with the disposal of byproduct sludge. Fraser Papers, a recycled fine paper mill located in West Carrollton, Ohio, envisioned an opportunity to capitalize on the utilization of sludge as a waste fuel. Studies were conducted and culminated in the selection of a BFB as the optimum choice in technology to burn high moisture, low grade (Btu) fuel. The principal mode of operation is to burn 100 percent of the sludge produced by the mill, in combination with coal to maintain combustion and to augment the mill's steam production needs. This paper focuses on the design and operation of the multi-fuel boiler, and the challenges associated with design and construction. This project has enabled the mill to improve air quality, lower energy costs, and reduce fossil fuel consumption. (271K)
BR#1668
Improving Recovery Boiler Furnace Reliability With Advanced Materials and Applications Methods
Since introduction of Type 304L composite tubing to the pulp and paper industry two decades ago, the incidence of tube cracking in recovery furnaces has dramatically increased. Advanced materials in the form of composite and weld overlay tubing have found increasing application in recovery boiler furnaces since the early 1990s. This paper summarizes tube repair and replacement recommendations as a function of crack characteristics, and the experience with a variety of materials and applications methods. (401K)
BR#1661
Bubbling Fluid Bed Boiler Emissions Firing Bark and Sludge
This paper explains the interplay between the different emissions control strategies for a BFB installation at a northwestern U.S. mill. The process is a chemical reaction with multi-variable control strategies that conflict with each other, necessitating a predictive controller to expand the operating window of the system. Also described are emissions from other bubbling bed sources and fuels.(346K)
BR#1649
Completing a Recovery Boiler Capacity Increase During a Maintenance Outage
An Eastern Canada pulp mill was faced with a recovery boiler floor maintenance issue that required immediate attention and a desire to increase black liquor processing (well above rated boiler capacity). Working with the client, Babcock & Wilcox modeled the air and liquor delivery systems while a new floor was being designed and manufactured using a different tube protective coating. This paper describes the project and how advanced modeling techniques led to an increase in black liquor solids processing. It also describes results of laboratory and field tests to identify superior tube material and arrangement for either sloped or decanting furnace floors.(2395K)
BR#1619
Recovery Furnace Floor Design and Alternative Materials
Black liquor is burned in the lower furnace of a kraft recovery boiler in a high temperature, oxygen deficient environment. The gaseous and molten inorganic combustion products are extremely corrosive. The furnace walls and floor are protected against corrosion by a number of methods. Considerable operating experience with corrosion protection is available today and provides information to the boiler designer and operator for future applications. Experience is reviewed with composite tubes and with carbon steel, pin-studded tubes used in the lower furnace of many recovery boilers. The few incidents of pin stud tube wastage and failure are discussed. Composite tubes constructed of TP304L/SA210A-1 and Sandvik 3R12 material have been installed in many recovery furnaces and all boiler manufacturers have experienced cracking of the stainless clad. Experience with this material is reviewed. Research and development programs to understand the cracking mechanism and establish a better material are summarized. (262K)
BR#1590
Fundamental Approach to Black Liquor Combustion Improves Boiler Operation
Combustion of black liquor in a recovery boiler has been optimized to change the ash characteristics and rate of deposition to substantially improve the superheater steam temperature performance over time. Modifications to the approach of spraying black liquor into the furnace and distributing air demonstrate the importance of depositing the liquor char on the bed and at the perimeter of the furnace. Optimum lower furnace operating conditions result in a stable bed and higher quality smelt. (102K)
