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Is Chasing Grid Demand Killing Your Large Coal Boiler? Here Are Four Concerns to Address

02 Dec 2019

Operating pulverized coal-fired boilers under low-load conditions presents several challenges. This article offers tips to keep units in the best possible operating condition.In today’s energy market with the increased penetration of renewable energy and high-efficiency combined cycle power plants, many of the once-baseloaded pulverized coal (PC) power plants must cycle and/or operate at low load. Many PC plants are operating at 25% to 50% of their maximum continuous rating during periods when renewable energy supply peaks. This is exposing new operating challenges that impact not only the efficiency of the plant at these loads but also the reliability of the boiler.Grid stability and reliability depends on the coal fleet for rapid load changes as renewables are ramped up or down. In the summer and winter during extreme weather, coal plants are depended on for major electric generation. Storm Technologies’ experience with low-load operation and the challenges it can present date back to before the recent operating characteristics of today’s plants. Most professionals in the power industry are aware of the additional stresses cycling or low-load operation can have on the boiler materials, dissimilar metal welds, and water chemistry to name a few. Conversely, Storm has found that there are a number of improvements in operations and maintenance controllable parameters that are of comparable low cost and often overlooked. These changes when addressed and monitored can improve the operational flexibility and the reliability of the overall plant.
 
Pulverizer Operation
 
While cycling or operating at low load, the pulverizers at some point are operating on the lower end of the primary air ramp or what is referred to as the minimum air setpoint (Figure 1). At this point, the pulverizer is most susceptible to coal rejects. Coal rejects are considered a “stealth” heat rate penalty by Storm Technologies; it is also a safety concern due to the raw fuel that is spilling through the throat and is exposed to the hot primary airflow in a very air-rich environment.If coal rejects appear when operating at lower throughputs, it is often directly related to the lower primary airflow, resulting in a lower free jet velocity through the pulverizer throat. Addressing how many pulverizers are needed for operating across the entire load range of your boiler should be an easy enough task and something operations likely has a procedure for. However, Storm often finds plants could be operating with less pulverizers in service, but operators simply choose not to cycle pulverizers out of service. This reduces the throughput through the pulverizers and in many cases results in pulverizers operating on the minimum air portion of the primary air curve.Figure 1 illustrates a simplified example that by removing two pulverizers from service and operating with only two pulverizers, you effectively increase the primary airflow through the operating pulverizers by moving to the sloped portion of the primary air ramp. As a result of removing the pulverizers from service, you increase the velocity through the throat of the pulverizers in service, which in most cases eliminates the coal rejects that can plague pulverizers at the minimum air setpoint.Barring a mechanical problem within the pulverizer, coal rejects are almost exclusively related to the geometry of the pulverizer throat for a given set of inlet conditions. If the open area around the throat is sized such that the primary air velocity drops below 7,000 feet per minute (fpm), then raw coal will begin spilling out of the pulverizer. Again, this in most cases results in wasted fuel and increases the potential for an excursion.For many years, Storm has been applying engineered solutions to increase the throat velocities so that at the minimum air setpoint the pulverizer throat velocity is 7,000 fpm and there are no coal rejects by installing a Storm-designed rotating throat and deflector assembly (Figure 2). Through the years, there has been one concern that plant engineers have had with this proven design, that is, by reducing the open area immediately surrounding the throat, the pressure drop across the throat increases as the primary airflow increases. As a result, Storm engineers designed and patented an adjustable throat that relieves the increased pressure drop at higher primary airflow rates, while maintaining the required 7,000-fpm throat velocities at the minimum primary airflow setpoint. This is effectively done by providing a second primary air path to the pulverizer and accurately controlling the primary airflow to each zone by a calibrated venturi. Overall, both the original and new patented Storm throat designs will eliminate coal rejects associated with operating a pulverizer at low throughputs, which can improve the reliability of your power plant.