Combustion optimization of a 150 MW (net) boiler utilizing air and fuel flow measurement and control

Abstract

Coal-fired electric utilities constantly struggle to achieve optimum combustion. While many techniques are currently employed, such as fuel and air control based on O 2 and CO measurement of the furnace exit, little has been done with regard to optimizing combustion at each burner. Windbox airflow and pulverized coal flow distribution is known to vary significantly burner to burner. The absence of effective methods to measure coal and air has left the utility industry with no alternative but to accept the resulting performance inadequacies. In this age of deregulation and concern over utility emissions, the industry continues to search for better methods of fuel and airflow measurement and control. This is especially true with the use of low NO x burners that require critical airflow and fuel balance for optimum reduction of NO x while simultaneously minimizing unburned carbon. In 1999, American Electric Power (AEP) – Pro Serv, Inc. installed a microwave based coal flow measuring device for on line measurement of individual burner fuel flow. This technology utilizes low frequency microwaves to accurately measure relative coal density and true coal velocity in individual coal pipes to determine the mass flow distribution between individual burners. A well-proven technology for measuring combustion airflow had already been in service for measuring and controlling the airflow to each burner. Together, these two measuring systems provide fuel and air massflow inputs to the plant’s DCS system, allowing for combustion optimization and a reduction in NO x emissions. Pro Serv, Inc. provides professional services in licensing, engineering, design, procurement, construction management, consulting, operations and maintenance worldwide in all areas of transmission, distribution, generation and industrial projects. INTRODUCTION In the United States, the Clean Air Act and increasingly competitive markets for electricity are proving to be complimentary drivers of electric utility plant improvements. One factor that contributes to overall boiler efficiency and reduction of power plant emissions is individual burner fuel/air ratios, a factor which has not previously been the focus of plant performance measurement and management. Not only is the fuel/air ratio generally important to optimizing combustion efficiency, it is also critical to the performance of low NO x burners and combustion modifications associated with low NO x programs (e.g., staged combustion). Poor fuel/air ratio can lead to problems such as slagging, and high unburned carbon in the flyash which adversely impacts ash sales. Currently, the method for measuring coal flow into a boiler consists of volumetric or gravimetric coal feeders that measure the bulk coal entering each coal pulverizer. From the pulverizer, the coal is delivered to multiple coal pipes, then via the coal pipes to the individual burners. Balancing of the coal flow distribution between burners has traditionally been attempted through clean or dirty air traversing and the installation of orifice plates. Since it is widely accepted that pipe-to-pipe coal distribution changes with load and time, manual traverses, whose accuracy has been challenged, can not be sufficient for balancing coal pipes to achieve optimum combustion over a range of operating conditions. Prior to implementing pulverized coal flow measuring equipment, AEP Philip Sporn Station had installed individual burner airflow measurement (IBAM) equipment supplied by Air Monitor Corporation as part of an AEP designed low NO x burner system. This allowed for accurate measurement and control of secondary airflow to the ten burners on this 150 MWN B&W roof-fired boiler (Fig. 1). In 1999, AEP looked to AMC Power to provide pulverized fuel flow measurement to the ten burners using their Pf-FLO system. This would allow the plant to match secondary airflow to coal flow at each burner, thus achieving optimum combustion. This paper focuses on the pulverized coal flow measuring product and the control methodology used to improve boiler performance.