By Zhanhua Ma, Technology Development Team Leader, RMT
The formation of ash deposits on heat transfer surfaces has been one of the main problems encountered in pulverized coal–fired boilers. Ash deposits not only lower the heat transfer rates, resulting in frequent maintenance and unscheduled shutdowns, but may also increase the risk of fireside corrosion of the metal surfaces. Therefore, the formation of ash deposits has a significant impact on emission control, boiler efficiency, unit availability and unit capacity factor, as well as on operation and maintenance costs.
The formation of ash deposits in coal-fired utility boilers is a perplexing process. It relates to the ash formation and transportation processes during combustion, as well as the flue gas aerodynamics inside the boilers, all of which involve many different physical and chemical phenomena.
Historically, power plant engineers and operators had limited tools available in order to minimize ash-related problems and respond to changes in coal quality. Many efforts have been made to understand the complex phenomena of ash deposition and to assist utilities in minimizing their ash deposition problems in coal-fired boilers. However, flue gas aerodynamics and radiant heat transfer inside of the boiler have not been integrated into the prediction of ash deposit formation and growth in local areas inside of the boiler, nor have they been used in analyzing the effect of ash deposit formation and growth on heat transfer—until now.
Using applied computational modeling (ACM), RMT’s SmartBurn group has applied mathematical modeling to these complex phenomena to better understand the boiler combustion process. By visualizing the flow field and the combustion details, an ACM model can both predict the performance of the unit with various modification designs and provide guidance for design optimization.
ACM is an advanced form of computational fluid dynamic (CFD) modeling. Rather than using CFD software to make static pictures of a boiler unit, the SmartBurn approach combines existing boiler data with proprietary sub-models, such as ash behavior to simulate the complex nature of the boiler combustion process. ACM couples the inner complexities of boiler combustion using sets of partial differential equations solved via approximation toward a converged numerical solution. Arriving at the converged numerical solution can involve millions of discrete partial differential equations—and that’s just in one run. With the numerical solution, one can visualize how the “good” and the “bad” happen together inside the boiler and work out a way to enhance the “good” while alleviating the “bad.”
A unique advantage of ACM is that it can be used to evaluate localized slagging and fouling problems related to actual operating conditions. ACM can be used to identify the major causes of ash deposition and can guide changes in boiler operation, such as what adjustments to make when using various coal feeds. ACM can also be used to help to minimize plant operating costs by providing strategies to maximize unit availability, minimize ash-related forced outages, and optimize sootblowing.
RMT applied ACM to two 512-MW T-fired boilers. The objectives of the project were to:
- Investigate the impact of operating conditions and unit configuration on furnace wall slagging, and
- Provide insight into the optimization of sootblowing strategies in different regions of the furnace walls.
With any model, there needs to be validation. The ash deposit patterns predicted by ACM were validated with pictures of slag on the furnace walls taken by plant personnel. Predicted deposit patterns on the super-heater division panels were consistent with those on the photo taken. Therefore, the ACM model provided an accurate prediction of deposit patterns as shown in Figure 1 on page 81.
Further modeling outputs showed how air and coal flows from each burner contribute to the ash impaction on specific regions of the furnace walls. Figure 2 (also on page 81) shows the three different operating conditions on Unit 2: 1) baseline conditions, 2) low primary air, and 3) high auxiliary air. High auxiliary air produces the least ash impaction compared to the other two operating conditions, especially in the upper furnace region. Low primary air results in higher ash impaction in the near burner region of corner #4 and on the upper furnace wall near side over-fire air (OFA) port at corner #2.
Using ACM, SmartBurn was also able to analyze the implications of sootblowing frequency and pressure on boiler cleanliness. Deposit thickness with varying steam pressures and sootblowing frequencies over a six-hour period were predicted and validated. Results indicated that more frequent and lower pressure sootblowing (two hours and low pressure) provided better cleanliness than less frequent and higher pressure sootblowing (three hours and high pressure). However, due to the differences in deposit strength in various parts of the furnace, a different sootblowing strategy was needed for the different groups of sootblowers to handle the various ash deposits while maintaining the sootblowing sequence from top to bottom of the boiler.
For example, the upper right furnace walls had higher deposit strength and were more difficult to clean, requiring more frequency or higher pressure to remove. Meanwhile, upper left furnace walls had lower deposit strength and were easier to clean, requiring less pressure to remove.
In addition to the sootblowing frequency and pressure optimization recommended from the slagging evaluation, plant engineers also conducted some unique optimizations to specific sootblowers based on pictures taken during a non-boiler-related outage. Two sootblowers located close to the OFA caused some instability in boiler flue gas when they were operated, disturbing the OFA flow and resulting in high CO levels. From these observations, it was determined that the degree of rotation of these two sootblowers could be optimized to minimize their impact on OFA operation during sootblowing.
Using ACM, RMT’s SmartBurn unit was able to satisfy the two objectives of the project: investigate the impact of operating conditions and unit configuration on furnace wall slagging, and provide insight into the optimization of sootblowing strategies in different regions of the furnace walls.
The results of this project indicated that operating conditions had a significant impact on furnace wall slagging. Lower primary air flow produced higher ash impaction on the furnace wall, which had the potential to increase furnace wall slagging. High auxiliary air reduced ash impaction on the furnace wall and provided more opportunity to reduce furnace wall slagging.
Differences in unit configuration require different operating procedures to manage the ash in the unit. With a different OFA design, the upper furnace wall slagging pattern will likely be different due to specific flue gas flow patterns.
The furnace wall slagging evaluation of the two 512 MW T-fired boilers also provided valuable insights that were used to develop optimization strategies for sootblowing in different furnace regions. The recommended sootblowing frequency and pressure for the sootblowers in different regions of the furnace wall developed using ACM were implemented in the plant to reduce sootblowing cost and/or the risk of waterwall tube erosion due to over–sootblowing. The plant implemented the recommended sootblowing practice by reducing the header pressure to 450 psig, and by tuning the nozzle pressure at each individual blower at 175 psig on the upper furnace and 125 psig on the lower furnace.
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Self-Contained Hydraulic System in New Solar Energy Field
By Woodie Francis, Product Marketing Manager-Hydraulic Actuators, Parker Hannifin Corp.
At the Nevada Solar One parabolic trough power plant, hydraulic actuator devices, designed and built by Parker Hannifin, rotate and tilt the solar collector assemblies (curved mirrored surfaces) as they track the sun’s motion each day through the desert sky. The actuators also control minor adjustments to the position of the arrays to compensate for the effects of wind pressure, as well as locking them for safe storage against high wind and dust storms.
Located in Boulder City, about 25 miles south of Las Vegas, Nevada Solar One is a 64 MW solar thermal plant and ranks as the world’s third largest solar energy field. Spanning 300 acres, this is the first large solar plant of this kind to be built in the United States in over 15 years. It will generate approximately 129 million kilowatt hours of solar electricity annually.
Acciona Solar Power (ASP), a unit of Spain’s Acciona Group, selected Parker to develop the solar panels’ motion control system. The amount of solar electricity Solar One will produce illustrates the potential for more parabolic trough systems in the southwestern United States.
Parabolic trough systems use a different technology than the photovoltaic solar panels common on rooftops. Parabolic trough power plants use concentrated sunlight, in place of fossil fuels, to provide the necessary thermal energy to drive a conventional power plant. Curved mirrored surfaces, which are parabolic shaped, concentrate the sun’s heat on a receiver tube, or pipe, located at the focus of the mirrors. A heat transfer fluid (usually a synthetic or mineral oil) passes through the receiver tube to achieve temperatures as high as 700 F. This fluid is used to produce steam/vapor that drives a conventional turbine/generator system to produce electricity.
To capture maximum sun exposure, the solar collector assemblies at Solar One track the sun throughout the day. Positioning the collectors is the job of Parker’s self-contained hydraulic system. This is Parker’s third solar power project.
At the Solar One energy field, 760 solar collector assemblies are aligned in parallel rows on a north-to-south axis, enabling the single-axis troughs to track the sun from east to west throughout the day ensuring that the sun is focused on the receiver tubes. A single actuator positions each of the 760 solar collectors, comprised of 12 mirrored surfaces about 8 meters in length. There are 9,120 collector modules in the Solar One field.
When ASP began talking with Parker about a motion control system to rotate the solar panels at Boulder, company engineers were adamant about the need for more power, simple operation and a maintenance-free system. The power and precise position control associated with hydraulic systems led ASP engineers to work with Parker on the hydraulic motion system. Parker had to ensure that the motion control would be powerful enough to accurately track the sun during the day and provide safe, secure stowing of the panels at night.
The self-contained hydraulic system that Parker designed uses the same fluid to both power the system and to lubricate it. This design will deliver nearly maintenance-free operation for over 20 years. The fluid within the system is self contained and there are no filters to change, reducing hydraulic maintenance.
The Parker system on each of the 760 solar collectors is comprised of a Parker HTR300 Series rack and pinion hydraulic actuator, control valves, solenoid valves, pumps, cylinders, seals and wear bands and pre-bent tubing/fitting assemblies. Parker provides all components in the hydraulic system. In addition to powering and positioning the solar collectors, the Parker actuators provide lockdown capabilities with wind load control up to 84 mph to protect the solar arrays while in stow.
Each hydraulic actuator drives 12 panels that weigh over 2,000 pounds, and each panel has 20 curved mirrors. An aluminum hubbing system enables the mirrors to be mounted directly to the structure. Six panels of mirrored surfaces are on each side of the actuator. New designs in the receiver tube have increased a tube’s active area to 96 percent.
To further promote the efficiency of the receiver tubes, the Parker hydraulic power and control system tracks the sun in small increments. Small, low-speed displacement pumps provide two short pulses from the reservoir to the unit’s motor three times a minute to track the sun at a precise angle. Power-plant efficiency depends on how well the parabolic mirror array concentrates and maintains the sun’s energy at the focal point of the tubes carrying the heat-transfer thermal fluid. The pinpoint control provided by the hydraulic system eliminates the “dead zone” in the parabolic mirror trough and increases the amount of direct sunlight on the receiver tube, increasing the trough’s efficiency for generating thermal energy to be converted into electricity.
As the solar collectors plot the sun’s daily westward movement across the sky, occasional bad weather can generate wind gusts with a force that exceeds the actuators’ torque limit. The system is designed to withstand this by allowing the solar trough to slip and rotate in a controlled fashion without damage to the motion control mechanics. This designed “clutching action” is inherent in hydraulic systems with the use of pressure relief valves and allows the solar collector to realign and begin tracking the sun again when the wind subsides. It also eliminates the need for additional hardware and programming necessary to compensate for wind gusts if using an electro-mechanical gearbox.
At the end of each day, as the sun sets and the solar panels are facing west, the actuator moves the arrays in large intervals by engaging its high-speed displacement pump to increase motor speed to return the solar panels to their eastern home position for the following day’s tracking of the sun. At this point the hydraulic system’s duty cycle, the mechanical locking mechanism, featuring Parker 2H cylinders, stows the parabolic troughs for the night.
Solar Power’s Future
Power generated from parabolic trough technology is still nearly twice as expensive as conventional fossil fueled power plants. Solar power is also slightly more expensive than that generated by wind—between 9 and 13 cents per kWh. A major advantage, however, is that except for the solar collector arrays, the rest of the power plant is the standard design widely used by existing fossil-fuel electric utilities. New investments in solar powered electric generation, such as Solar One, should support further development and technology improvements to improve the return on investment on utility-scale solar energy.
Designs by developers such as ASP and Parker Hannifin continually contribute to improvements in the efficiency of the solar arrays’ frame functionality and durability, the solar collecting capabilities of the trough receiver tubes, and the reduction in the operation and maintenance costs of the motion control systems, respectively.
Because trough technology relies on sunshine, future designs will include thermal energy storage methods to set aside heat transfer fluid, keep it heated until evening hours, and then use it during the night to spin a turbine to produce electricity. This advance in technology would enable solar energy to be used around the clock.
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