Control Initiatives to Improve Coal Pulverizer and Unit Dynamic Performance under Adverse Conditions

摘要

Changing electricity market conditions, particularly from the growth of uncontrolled renewables, have demanded that fossil-fuel generators seek to increase capacity margins and become more flexible in operation. Consequently utilities are often seeking higher ramp rates, wider load ranges and the ability to maintain bid output even under adverse conditions. Pressure to "push the margins" of performance can increase reliability risks, both from plant and operational limitations. The performance of coal pulverizers has become increasingly important in this environment, as they significantly influence a power plant's ramp rate, stable operating range, frequency response and operational security. Yet generators may extend pulverizer service hours, increase working range and continue operation with running faults or poor coal conditions to meet economic imperatives. In this paper the author presents a range of control initiatives that have each been introduced on several power projects to reduce the impact on performance under such adverse conditions. The control enhancements help to detect and circumvent choking, provide 'smart' capability calculations to optimise fuel runbacks, and introduce pulverizer air flow demand shaping to compensate for worn pulverizers and so maintain consistency in the pulverized coal flow dynamic response during ramps. The paper also describes milling and classification process dynamics and the mechanisms that cause variations in the pulverized fuel output response. Simulation results from a pulverizer model are presented to demonstrate the effects of air flow and fuel input changes on several key process parameters, and thus help explain the reasoning behind some of the control enhancements.