IntelliCLEANTM
Intelligent Software for Sootblowing Optimization
Introduction
Furnace and convection pass slagging and fouling have a detrimental effect on boiler performance and emissions and represent the primary cause of reduced operating efficiency in fossil-fired boilers and in an increase in flue gas temperature at the boiler exit. Sootblowing is used to control the level of ash and slag deposits on the boiler heat transfer sections.
Furnace cleaning increases radiation heat transfer to the waterwalls and reduces the Furnace Exit Gas Temperature (FEGT). This decreases the amount of heat that is available to the convection pass.
Sootblowing of the convection pass increases heat transfer in that region of the boiler, which increases steam temperatures and desuperheating sprays. For best boiler performance it is important to maintain an optimal balance between furnace and convection pass heat transfer.
Effects of Sootblowing on Unit Operation and Emissions
While ash and slag deposition is a gradual process, sootblowing results in an abrupt change in local heat transfer. This can result in unfavorable energy distribution among different heat transfer sections, loss of thermal performance and increased unit heat rate. Figure 1 shows the effect of sootblowing on furnace cleanliness (expressed as FEGT).
Sootblower cleaning effectiveness depends on their location relative to the surface that needs to be cleaned, pressure of the cleaning media (steam or compressed air), the nature of the deposit (slag or fly ash) on tube surface, and the slagging or fouling rate (i.e., intensity of slagging and fouling).
Some sootblowers are very effective in removing slag and fouling deposits, and their activation results in a large and long-lasting effect. Others have less effect in magnitude and duration. Some sootblowers have no detectable effect on boiler performance, emissions, or surface cleanliness.
Erosion damage to the tubes can also be caused by very effective sootblowers due to the high pressure of the sootblowing steam. Activation of ineffective sootblowers results in a waste of high quality steam or ineffective use of auxiliary power (for air-blown sootblowers). In addition, too frequent activation of sootblowers results in over-cleaning of heat transfer surfaces and could cause erosion and corrosion damage to the tubes or induce high stress levels in tube wall resulting in shortened tube life.
Sootblowing can also be used to control NOx emissions, which varies with furnace cleanliness (Figure 2). Some utility boilers are characterized by high volumetric heat release and high temperature flame zones. Heavy slag deposits in the waterwall regions of these boilers can reduce local radiation heat transfer by as much as 50 percent. As a result, flue gas temperatures are kept above the thermal NOx formation temperature for a longer time. Therefore, lower NOx emissions can typically be achieved by maintaining cleaner furnace waterwalls. However, in some applications, there is an optimal value of furnace cleanliness above which there are no additional benefits to sootblowing. This optimal value is highly dependent on site-specific conditions, such as fuel and ash characteristics, furnace design and volumetric heat release, and boiler operating conditions.
Sootblowing in the upper furnace sections and in the convection pass of the boiler can also affect stack opacity for units with marginally sized electrostatic precipitators (ESP). Ash, dislodged by sootblowing from the heat transfer surfaces, is carried by the flue gas into the ESP. A large ash cloud increases particle loading in the ESP which can result in opacity spikes, opacity violations and unit derates.
Optimal Sootblowing Schedule
The challenge in developing an optimal sootblowing strategy is to determine which portions of the boiler to clean and on what schedule, considering the trade-offs between NOx, opacity, steam temperatures, heat rate and other factors such as tube life, sootblower steam or air consumption and maintenance cost.
Figure 1 - Effect of Furnace Cleaning on FEGT and NOx
Plant operators are typically provided with little or no information on slag and ash deposition levels or guidance regarding appropriate sootblowing operations. Some plants still use the original boiler manufacturer’s recommendations, which could be more than 30 years old and not appropriate for current operations. Therefore, operators use a wide variety of strategies for sootblowing according to their personal level of understanding of the causal relationships between sootblowing and boiler performance, individual preferences, and experience level. The most common strategy is sootblowing according to fixed schedules with little regard for plant performance or emissions.
Sootblowing Optimization
Developed by Lehigh University’s Energy Research Center (ERC), the IntelliCLEAN sootblowing optimization approach balances furnace and convection pass heat transfer to improve boiler performance, reduce NOx emissions, and minimize disturbances caused by sootblower activation. The approach is based on minimal data (volume and quality) requirements that reduces the need for additional instrumentation. The IntelliCLEAN approach can be described by the following five steps:
Step 1: Setup Instrumentation and Calculations
This step includes the review of available plant instrumentation, installation of additional instrumentation by the customer (if needed) to measure FEGT and close the heat balance calculations for the convection pass sections. FEGT probes are typically the only additional instrumentation that is required.
Step 2: Sootblower Characterization
Sootblower characterization tests are performed to develop a database on the effects of individual sootblower groups on cleanliness of various boiler heat transfer sections, steam temperatures, desuperheating spray flow, emissions, and other parameters.
Step 3: Create the Data and Knowledge Bases
The boiler response to sootblowing is analyzed and the results are used to create a sootblower characterization database. The database includes information such as the effect of sootblower group activation on cleanliness of different boiler heat transfer sections, NOx emissions, steam temperatures, desuperheating sprays, opacity, flue gas temperature at boiler exit, etc. The knowledge base, containing explicit knowledge as rules or statements, is created from the sootblower characterization database. The knowledge base is used, along with boiler operating data and boiler section cleanliness data, to determine an optimal sootblower activation strategy.
Step 4: Develop an Optimal Sootblowing Strategy
The optimal sootblowing strategy, which satisfies optimization objectives and operating constraints, is developed using the sootblower characterization data and knowledge bases. The resulting strategy can be time or event-driven. Optimization goals can include: maintaining FEGT below a prescribed limit for slagging control, controlling main steam and reheat steam temperatures for best performance, controlling flue gas temperature (for SCR applications), or maintaining lowest NOx emissions and opacity levels. Typical constraints might include an FEGT limit (to control furnace slagging), steam temperature limits to protect equipment, opacity limits, etc.
Step 5: Implement and Evaluate the Optimal Sootblowing Strategy
The time-driven optimal sootblowing schedule is implemented, field-tested and adjusted (if necessary).
IntelliCLEAN Sootblowing Approach
The strategy for optimizing sootblowing can be implemented using the IntelliCLEAN computer program. IntelliCLEAN uses a knowledge based-expert system and the sootblower characterization database created in step 3 above, live process data, and information on cleanliness of boiler heat transfer sections to make decisions on optimal sootblower activation. IntelliCLEAN creates an optimal sootblowing sequence that is event-driven and goal-oriented and selects sootblower groups to be activated when and where needed to satisfy the optimization goals and operating constraints.
IntelliCLEAN is designed as a system of modules (Figure 2). Each module performs specific functions, such as: range check on input parameters, buffering and time-averaging of live process data, maintaining sootblower characterization database, determining and displaying cleanliness status of boiler heat transfer sections (furnace, convection pass sections), and determining an optimal sootblowing strategy. The sootblowing recommendations are presented in the advice window and indicate which sootblowing groups to activate. Separate advice is given for the controlled and scheduled sootblowing groups. Controlled groups are activated to achieve and maintain the optimization objective.
IntelliCLEAN adapts the optimal sootblowing strategy to changes in fuel quality, operating conditions, and sootblowing equipment maintenance status, which affect slagging and fouling intensity, and sootblowing effectiveness. Figure 3 illustrates changes in definitions of dirty and clean furnace conditions as fuel quality is changing. When a fuel that produces a hard-to-remove slag is fired, sootblower effectiveness is reduced, resulting in relatively high FEGT values. When a different fuel that produces a slag that is easier to remove is fired, sootblower effectiveness in removing slag from the waterwalls is increased, resulting in significantly lower FEGT values. Without the ability to adapt to process changes, such as fuel quality, section cleanliness status would be determined incorrectly and frequent system recalibrations would be necessary.
IntelliCLEAN includes a slagging/sootblowing simulator which can be used to test the software advice and resulting optimal sootblowing strategy or as a training tool for the operators. The simulator uses actual slagging, fouling and recuperation rates, and the sootblower characterization database. The user can control simulation speed, which makes it possible to simulate hours, days, and weeks of plant operation in a very short period of time.
Figure 2 - Examples of Modules in IntelliCLEAN
» Open Figure 3: Furnace Cleanliness Status Affected by Fuel Quality to a new window
Operating Benefits Of Sootblowing Optimization
The benefits of using IntelliCLEAN for sootblowing optimization and maintaining optimal boiler cleanliness include:
- Increased boiler efficiency
- Improved furnace heat transfer and lower FEGT
- Reduced NOx emissions
- Improved unit availability
- Increased power output
- Reduced furnace slagging and reduced fouling in convection pass of the boiler
- Reduced tube erosion and corrosion
- Reduced sootblower usage
- Reduced attemperation spray flow rates
Using IntelliCLEAN’s Artificial Intelligence (AI) approach for sootblowing also:
- Provides the means of handling changing optimization objectives. This opens up the possibility of employing different sootblowing strategies during the Ozone Season
- Adapts to changes in fuel quality and maintenance condition of the equipment
- Eliminates operator variability in selecting sootblowing schedules
IntelliCLEAN will also benefit utilities, which either through downsizing or retirement have lost some of their more experienced operators.
Economic Benefits Of Sootblowing Optimization
The economic benefits of sootblowing optimization can be substantial. For a typical 600 MW unit, a heat rate improvement of 20 BTU/kWh and NOx emission reduction of 0.020 lb/MBTU due to optimized sootblowing practice can translate into an annual savings of approximately $700,000 (assuming fuel cost of $1.50/MBTU, NOx credits at $4,000/ton, and a five month ozone season).
The power replacement costs, which are incurred due to unit outages to repair tube leaks caused by erosion damage, and the maintenance costs to repair boiler tube leaks can also be substantial. A sootblowing optimization program that includes IntelliCLEAN could save a utility several hundred thousand dollars per year in operation and maintenance (O&M) costs.
The actual value of these performance, emissions and O&M benefits are highly site-specific and depend on factors such as unit size, operating conditions, fuel type and cost, plant NOx emission levels and allowances and needs to be evaluated for each individual unit.
Computer Requirements
IntelliCLEAN is an on-line, real-time application that runs on a personal or plant computer supporting Microsoft Windows XP, 2000, 98, NT and 2000 applications.
User Training
The user interface built into IntelliCLEAN makes the software particularly easy to use. One to two days of training is usually sufficient.
