|With GE’s Steam Cycle solution, users can significantly cut solution implementation time and avoid development delays—increasing uptime and profitability. Photo courtesy of General Electric.|
By Alan Hinchman, GE global marketing director-infrastructure & head of energy, oil & gas, water and commercial buildings offerings
Today in the power industry, there are many companies that are dealing with an increasing number of plant engineers that are approaching retirement. The loss of that institutional knowledge is a challenge since boiler controls require experience to install, replace and operate. Years of experience are often needed to understand the art associated with controls at a specific plant using specific auxiliary systems.
Retaining the institutional knowledge of retiring engineers eventually nets out in the cost of time – searching for the right candidates, training time and on-the-job experience can take years and thousands of dollars. The classic alternative is to use consultants to fill that void. However, there can also be a significant amount of time associated with training the consultant and getting the solution implemented and tested as well as the additional use of consultants required whenever there needs to be a change in the system. Neither training new engineers nor utilizing consultants seems appropriate in today’s world of increasing efficiency.
The potential loss of institutional knowledge can be overcome in part by the use of pre-engineered control strategies. The intent is to allow power plants to obtain sophisticated control strategies without having to reinvent the wheel when it comes to controlling all of the elements required to deliver consistent steam to the turbine. Leveraging proven strategies ensures not only that the boiler will operate efficiently, but that it will also operate safely. Pre-engineered strategies can cut the implementation time of a new Distributed Control System (DCS), thereby allowing installation during a short maintenance outage. This allows the time savings to be realized months before a traditional DCS could be installed.
This strategy also allows current operators to increase the operating efficiency of the plant while optimizing the labor resources required to perform specific tasks such as startup of heat recovery steam generators. Users can implement sophisticated strategies used at larger power plants in mid-sized and smaller plants with more confidence and more quickly than installing a traditional DCS with custom or less sophisticated strategies. Additionally, the ease of installation is also accompanied by ease in changing the parameters to allow the knowledge of existing employees to be captured in the control system.
Advantages of Using Advanced Controls
Significant improvements can be made to boiler operations through advanced controls to help the plant to operate closer to the maximum nameplate ratings. Specifically, the benefits to be gained are focused on delivering consistent steam to the turbine. However, these improvements in operations also focus on improving safety, decreasing emissions and decreasing consumption of excess fuel and water.
Other benefits of advanced controls include reduction in capital costs for inconsistent Btu values in fuels. Sensors for determining the Btu content of the fuel (such as municipal solid waste) are expensive, and the benefits provided rarely cover the expense. Compensation from the software can provide a significant advantage to operations and limit the capital outlay.
A DCS with sophisticated control strategies can increase the performance of mid- and small-size plants because they make sophisticated strategies, usually only found at large plants, available to all plants. The result is a plant that has higher efficiency and safety while providing consistent steam to the turbine. Additionally, utilizing an advanced DCS can reduce the time required to implement a new DCS. The reduction in implementation time can save on custom engineering hours and allow for faster achievement of the efficiencies of sophisticated control strategies.
Power plants are often faced with problems that challenge their operational efficiencies. Listed below are some of the common problems plants face, and how sophisticated control strategies can address these problems. Often these are not implemented due to either a lack of knowledge about a control strategy solution, or the perception that the engineered solution is beyond the financial scope of a project. But what if that solution was pre-engineered, and the knowledge of how to solve that problem was part of a productized solution?
Plant Master and Fuel Master
Problem: Plants with multiple boilers are faced with balancing their output such that the steam delivery matches the load demand from the turbine or turbines. When a boiler that supplies steam into a common steam header for multiple boilers is taken out of service, the other boilers will have to compensate to ensure smooth and consistent steam to the turbine. The varying load swings on the boilers may lead to unbalanced conditions which could cause varying steam delivery or in the worst case scenario, water carryover to the turbine.
Solution: Proper system design should accommodate both a plant and fuel master. The plant master functionality should allow for selecting which controller is the overall plant master to allow maintenance on one of the boilers or its controls. The fuel master controls the feeders or pulverizers. Input to the fuel master is from the plant master. The fuel master should provide a balancing and auto loop response correction to account for the number of burners in automatic mode versus manual mode or out-of-service for maintenance. Manual operation of a burner should not be required even during ramp-up or while using fuel with varying Btu content.
Value: Correct balancing of the boilers provides consistent steam delivery from the boilers. Consistent steam decreases fatigue on turbines, thus decreasing maintenance costs on turbines. Additionally, balancing the boilers provides safe operations of the boilers by not having one boiler providing for the load variation and increasing the temperatures in the boiler beyond the desired set point.
Analyzer Delay Compensation of O2 Trim
Problem: During load swings, the O2 set point is adjusted based on a load index. There is, however, a delay from the O2 analyzer. If this delay is not taken into account, the control will incorrectly adjust the O2 either higher or lower than optimal, leading to efficiency loss through excess hot air escaping up the stack or a high level of combustibles exiting the stack with the flue gas. Losses of 1 percent or more of efficiency can occur during transients or ramping.
Solution: The control strategies should provide O2 trim control with a minimum O2 set point (which is operator adjustable). The control strategies should account for analyzer response delays and incorporate these delays to improve response and thus efficiency.
Value: Fuel usage is optimized during load swings which saves on fuel costs and reduces unintended emissions such as CO.
Steam Flow Combustion Control in Stoker Boilers
Problem: Solid fuels fired on grates such as coal, biomass and municipal solid waste have varying Btu per measured weight, and constant heat cannot be assured as when using natural gas or oil. The available Btu must be inferred in order to control relative air flow to maintain suitable combustion. Unfortunately, O2 trim cannot be used exclusively to control combustion because of outside air impingement, uneven fuel bed and over-fire air stratification (O2 content should be more of an operator guide in this type of boiler).
Solution: A steam flow/air flow approach with the steam flow corrected for heat storage is utilized. This ensures that the combustion process is efficient and safe. The use of the adjusted steam flow/air flow system over a basic parallel system can increase efficiency by five percent or more. A minimum air set point and smother protection override also provides increased safety and boiler availability.
Value: Both safety and maintenance concerns are alleviated by properly controlling the combustion cycle. By allowing a minimum air set point, the combustibles in the stack are limited below hazardous levels. Maintenance concerns are minimized by providing more consistent steam flow, thereby decreasing fatigue on the turbine.
Compensation for Multiple Fans, Feeders and Stoker Response
Problem: When multiple devices like feeders, FD and ID fans, etc., are controlled from a single PID loop, it is desirable to compensate the loop for the number of devices in manual mode for activities such as manual loading in order to maximize loop response. Upsets due to poor response can lead to decreases in efficiency and mechanical stress.
Solution: Control strategies should automatically adjust the control loop response based on the status of the individual driven devices. For example, where multiple feeders are driven from a single master fuel controller, the loop automatically compensates the fuel control loop as a feeder is put in or out of service and compensates for the weighted capacity of the specific feeder to minimize fuel feed upset. A bumpless slow automatic balance is provided to restore the device to the demand signal value when it is put back in service and set to automatic.
Value: Minimizing upsets in the boiler loading process leads to more consistent steam delivery. Improving the PID loop response by understanding that devices come in and out of service will allow better system response from monitoring available devices only.
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