Once pulverized coal flows have been measured, they can be balanced and optimized. Until then, tuning is simply guesswork. The right way to balance furnace fuel flows is to establish solid baseline performance by proper measurement of fuel flow, fineness, and velocity. Only then can all the coal pipes be accurately balanced and followed by a tune-up of the boiler controls.
Operating a coal-fired boiler efficiently is all about carefully and constantly managing air and fuel flow. To use a familiar football metaphor, success in efficient boiler operation means consistency in the fundamentals — blocking and tackling — not hurling an end zone pass at the buzzer. The most successful plants make steady progress by perfecting one portion of their combustion efficiency program at a time. Make a game plan, stick to it, and good results (unlike in football) are guaranteed.
Over the past several years we have discussed in depth how to efficiently combust coal by breaking the system down into manageable parts. This series of articles on the fundamentals discussed:
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Managing air flow ("Managing Air to Improve Combustion Efficiency," October 2007).
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Optimizing boiler efficiency ("Boiler Optimization Increases Fuel Flexibility," June 2008).
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Improving pulverizer performance (" ‘Blueprint’ Your Pulverizer for Improved Performance," March 2009).
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Measuring coal flow rate through coal pipes (this issue).
Each of the articles is available in the POWER archives at www.powermag.com.
A Six-Step Program
Efficient furnace performance is absolutely mandatory, regardless of what kind of coal you are firing. The source of the coal matters less than optimizing the fuel system (starting with the pulverizers) and the air system (starting with the primary air fans), which meet at the burner system. Each boiler system must be properly tuned to work with suitably prepared coal, and the process begins with putting the right amount of fuel with the right amount of air in the burner at the right time (Figure 1).
1. Balancing act. Combustion tuning of a pulverized coal boiler begins with balancing the air and fuel flows in the coal pipes from the pulverizers to the burners. Note that each of the coal pipes is a different length. On most boilers, burners are also present on the opposite side of the furnace, adding to the air/fuel-balancing challenges. Courtesy: Storm Technologies Inc.
Tail pipe air quality systems add to the complexity of achieving efficient combustion. For example, many coal-fired plants chose to continue using higher-sulfur coals after they added a flue gas desulfurization, or scrubber, system. Lower-cost coal may save on purchased fuel costs, but the higher-sulfur fuel option poses more of a challenge to plant operators, who must wrestle with the effects of nonhomogeneous mixing of fuel and air, which include reducing atmosphere furnace zones that increase slagging and tube corrosion. The difficulty with properly tuning a boiler experiencing poor combustion is compounded by having only 1 or 2 seconds of furnace residence time to completely combust the fuel.
Balancing fuel flows in a pressurized classifier is best achieved with optimum coal fineness, blueprinted mills, synchronized classifier blade angles/lengths, and a properly calibrated and repeatable optimum air/fuel ratio. Achieving the optimum fuel balance to the burner belt requires very good fuel fineness, determined by current standards, blueprinted pulverizers, and optimized classifier geometry.
The main goal of testing and tuning the coal pipe flows is to tune the entire combustion system to efficiently use every millisecond of the limited furnace residence time available. Here is our six-step program for achieving a satisfactory fuel balance in your burner belt.
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