Too-frequent sootblowing can damage boiler components and place a big load on plant efficiency, but not enough of it can be just as big a problem. A sootblowing optimization system can help you find the “sweet spot.”
With the increasing demands placed on today’s coal-fired power plants worldwide, operators are continuously looking for the best options to increase their efficiency and maintain process optimization—all while watching the bottom line. A big part of this is complying with ever-tightening emissions regulations.
In the U.S., for example, the Environmental Protection Agency is in the process of developing and implementing new standards requiring the power sector to cut carbon emissions by 30% by 2030, and the industry is looking to comply by the most efficient means possible. Fortunately, for coal-fired plants looking to achieve optimization without costly modifications, there are economical alternative technologies that can be considered.
In coal-fired power plants, optimizing any process can be demanding. Operators at the Kansas City Power & Light (KCP&L) Hawthorn Generating Station recently accepted the challenge of optimizing their sootblowing process. Located in Jackson County, Mo., and situated on the south bank of the Missouri River, Hawthorn Unit 5 is a 594-MW, wall-fired, water-cooled boiler that fires 100% Powder River Basin subbituminous coal (Figure 1).
|1. Clean-up time. Kansas City Power & Light’s (KCP&L’s) Hawthorn Generating Station recently implemented a comprehensive improvement in its sootblowing processes. Courtesy: KCP&L|
Beyond Ad-Hoc Sootblowing
Today’s coal-fired plants are increasingly expected to operate at varying loads while simultaneously dealing with operational influences such as fuel-quality variations and air quality–control requirements. For the most part, these plants still utilize traditional sootblowing technology to remove soot deposits. However, these traditional patterns can present challenges to optimal plant operation. If there is insufficient sootblowing, the transfer of heat from flue gas to steam is impeded, resulting in decreased boiler efficiency. Large soot deposits can also restrict flue gas draft, requiring additional fan power and further reducing efficiency. On the other hand, too-frequent cleaning causes heating surface erosion and related unit outages, high steam and metal temperatures, as well as increased spray flows that will reduce efficiency on most units.
Typical sootblower equipment uses jets of steam, water, or air to remove deposits on heating surface tubes. Sootblowing increases power generation costs, both from expenses associated with the cleaning medium as well as the parasitic power required to drive compressors and pumps.
The sootblower activation cycle is usually based on the operator’s experience and recommendations from the boiler manufacturer. This generally results in a simplistic solution based on continuous sootblowing at fixed intervals or individual operator judgment. Sootblowing at fixed intervals cleans the boiler and avoids hard-to-remove buildup, but it can also mean that sootblowing occurs when it isn’t truly necessary and it can cause undesirable process parameter fluctuations during load variations and other dynamic operating conditions. Sootblowing on individual operator judgment leads to inconsistent sootblowing patterns and requires continuous operator intervention on a 24-hour basis, which burdens operators and usually results in suboptimal sootblowing activities.
In 2000, as part of rebuilding work after an accident in 1999, Hawthorn Unit 5’s sootblowing system was reconstructed, and engineers encountered the traditional problems detailed above. Substantial engineering hours were spent developing sequences over the years that would accommodate the varying operating conditions. The system was functional but certainly not optimized.
There were several specific operational challenges with the existing sequence-based sootblowing control system at Hawthorn.
Catalyst Temperature Limitations. Economizer exit gas temperature (EEGT) is constrained by selective catalytic reduction (SCR) catalyst temperature limitations. Hawthorn had been forced to reduce load on several occasions when this EEGT constraint was encountered. EEGT is dependent on fuel type, combustion parameters, and cleanliness of the boiler surfaces. Specific sootblowing activity is known to help EEGT in certain conditions, but it needs to be carefully executed and relies on manual blower intervention when using sequence-based controls.
Reheater. Managing reheater cleanliness is especially challenging. There is a delicate balance between a clean reheater, which causes excessively high reheat temperatures (and maximum attemperation spray flow), and a dirty reheater, which can cause plugging and excessive fan horsepower. Managing this balance with sequences and manual intervention at Hawthorn proved to be problematic.
Platen Superheater Slagging. Platens are the most prone to slagging, and platen sootblowers are the most frequently blown sootblowers. Slag rate is heavily influenced by coal type and combustion parameters in the furnace. Sequence controls are unable to respond to these changes, leading to over-blowing during low-slag conditions and under-blowing during high-slag conditions.
Over-Blowing at Low Load. Although Hawthorn was designed as a baseload unit and intended to cruise near full load, in 2012 there were more than 3,000 operating hours between 350 MW and 550 MW. At these reduced loads, sootblowing needs are significantly lower than at full load, but the sequence-based controls were programmed for full load—resulting in significant over-blowing during these periods.
Carbon Emissions Reduction. Another wrinkle was that in 2007, the Sierra Club and KCP&L agreed on a set of initiatives to offset the utility’s carbon dioxide (CO2) and other emissions by adding wind power and taking steps to conserve energy on a large scale. KCP&L employed an engineering consultant to conduct a study of the best technologies to employ fleetwide to meet a portion of its CO2 reduction goals via plant efficiency improvements. The first tier of proposed projects focused on improved performance monitoring and manual optimization. The second tier focused on advanced optimization using closed-loop products, such as combustion and sootblowing optimizers. After completing the first tier, KCP&L reviewed a fleetwide proposal for combustion and optimized sootblowing.
Finding the Best Solution
Generally, operators want a sootblowing optimizer to clean the boiler using minimal blowing media and without damaging tubes or cleaning certain sections too heavily relative to others. They want to reduce the total number of blowing operations while maintaining other key process parameters.
Sootblowing is also only a piece of the optimization process. It’s important to consider a comprehensive program that includes the ability to optimize other processes such as combustion. Taking a holistic approach ensures that all of the subparts work together to maximize the impact of each.
For KCP&L, the key process parameters to optimize/minimize were:
■ Reheat and superheat steam temperature variations and excursions
■ Reheat and superheat attemperation spray flows
■ Flue gas exit temperature
■ Load derates
■ Auxiliary power usage
■ Unplanned outages
Looking to improve those parameters, increase their process and boiler optimization at Hawthorn Unit 5, and meet their CO2 emissions goal, KCP&L chose the Siemens SPPA-P3000 sootblowing optimization solution.
An important differentiator for the team when choosing the SPPA-P3000 optimizer was its adaptive technology. KCP&L wanted a flexible platform that would support both combustion and sootblowing optimization and allow for future alterations. For sootblowing, individual blower activation was important. Sootblowing a group of sootblowers in a given section was known to upset boiler operation, and the team knew most plants had little operational margin for upsets when operating at full load.
The SPPA-P3000 offers a customized solution that determines the need for sootblowing based on dynamic plant operating conditions, equipment availability, and plant operational drivers. The system then generates individual sootblower activation signals for propagation to the existing sootblower control system in a closed-loop manner at optimal times. With sootblowing optimization, new equipment and sensors are not required. The SPPA-P3000 interfaces with an existing control system, takes a current set of plant parameters that are available, and works with those. Unit-specific customizations can be viewed and modified by the end user—making it flexible, reliable, and extensible—which results in sustained benefits.
The SPPA-P3000 is able to work alongside a foreign distributed control system. This technology allows plants to keep their current control system while gaining the benefits from the sootblowing optimizer (Figure 2).
|2. Easy implementation. The P3000 sootblowing optimization system can be installed alongside an existing distributed control system. Courtesy: Alistair Tutton Photography|
Putting Optimization to Work
With the new system in place, KCP&L and Siemens engineers could attack individual plant-specific problems by fine-tuning sootblowing individually for each blower. To achieve successful results, however, engineers found a balanced approach was necessary. The optimizer was configured to make blowing decisions based primarily on EEGT. The measured results of previous blowing operations allowed the optimizer to automatically select the best blower to improve EEGT. Because of this, some areas were blown more frequently.
There were also sensitive areas that needed extra controls to prevent temperature and spray flow excursions. The reheat section, in particular, was configured to prevent any consecutive or near-consecutive blowing operations.
By the end of the initial tuning process, it appeared that the sootblowing optimizer was going to be a success. The next step was to release to plant operations staff for continuous operation. Despite a few hurdles, operators generally embraced the optimizer after it proved to reduce sootblowing-related problems. Operators had the ability to enable and disable the optimizer at any time, but its in-service rate during the first year was over 95%.
When the system was up and running, plant operators helped identify an opportunity for additional sootblowing optimization. During soft market conditions, Hawthorn was operating in evening hours at minimum load and significant daytime hours at low load. Generally, there are very few sootblowing needs at low load and not much time spent there, so it was Hawthorn’s previous practice to suspend automatic blowing and blow manually only as needed. The optimizer was initially set up to match this protocol. Operators saw the opportunity for the optimizer to handle this low-load blowing, and the optimizer was easily updated with logic that allowed for minimal blowing at low loads.
After commissioning, another configuration change was related to process stability. There had always been certain occasions when multiple sootblowers (in different areas of the boiler) were blown nearly simultaneously. This was known to cause short-duration furnace draft pressure excursions and steam pressure decays. The furnace draft pressure caused a temporary reduction in airflow, which affected combustion, and the steam pressure decay caused megawatt setpoint control issues. After discussion with Siemens, the optimizer was configured to incorporate a short delay between any consecutive blower initiations.
Less Work, More Efficient
Several positive operational results were realized at Hawthorn Unit 5:
■ EEGT/SCR inlet flue gas temperatures were reduced by about 10F on average. This increased boiler efficiency and eliminated most load reductions due to catalyst temperature concerns.
■ Superheat and reheat steam temperatures were maintained closer to their desired setpoint, and large excursions were significantly reduced, increasing reliability.
■ Superheat attemperation spray flows were lower by about 20 klb/hr to 40 klb/hr on average, and reheat attemperation spray flows were lower by about 15 klb/hr on average—a significant efficiency gain.
■ Generally, calculated cleanliness of various boiler surface areas is the same or better.
■ There are about 5% to 10% fewer sootblowing operations on the whole, reducing media usage and auxiliary power consumption.
■ Plant heat rate has improved by an estimated 1%.
The payback of sootblowing optimization technology can typically be measured in months, with a strong likelihood of seeing immediate positive results. KCP&L’s implementation on Hawthorn 5 has increased boiler efficiency and reduced sootblowing-related load constraints. Blowing operations have been reduced while maintaining boiler cleanliness, so there is less damage to the tubes and less energy wasted on unnecessary sootblowing operations. Prior to this technology, the reheat spray valves would frequently run wide open, reducing cycle efficiency and allowing the reheater tube temperatures to increase—potentially resulting in long-term damage to the boiler surfaces.
Looking at the future of the optimizer from a performance and life-cycle cost standpoint, KCP&L is pleased with the results so far. Currently, the performance engineer at KCP&L is spending less time on sootblowing, with improved results. The optimizer continues to be used for over 95% of the plant operating hours since it was commissioned in 2012.
“When we’re talking about optimization, we look at it as the true future of all coal plants,” says Dominic Scardino, Hawthorn plant manager. “We’re not planning on building more coal units at this time, so it’s important to be proactive in maximizing the value of the existing units. We’re actively trying to implement operational excellence at our plants to maximize the value of these critical assets. This is good for our rate-payers and our community.”
Following a successful pilot of the sootblowing optimizer at KCP&L’s Hawthorn Unit 5, the team is now working on placing optimizers at other plants in the fleet, including two units at LaCygne. ■
— Neel Parikh is a principal engineer with Siemens Energy Inc. Peter Rogge is a plant performance and combustion engineer, and Kenneth Luebbert is a supervising engineer, both with KCP&L.