By Douglas J. Smith, IEng Senior Editor
After converting its Newton, IL electric power plant to burn Powder River Basin (PRB) coal in 1999, Ameren Energy Generating Company realized it needed to improve the accuracy of airflow measurement to the pulverizers and boilers. Accurate airflow measurement is essential for efficient and safe operation of coal pulverizers and boilers.
In addition to achieving overall process efficiency, and assuring safe operations, accurate process airflow measurement is important for controlling emissions. When Newton plant converted to low-sulfur PRB it expected to reduce SO2 emissions by 45 percent and increase plant capacity by 15 percent.
When the coal arrives at the plant it is conveyed from the coal storage area to 12 coal pulverizers. After grinding, air is used to move the coal from the pulverizers to the burners. At full capacity, the plant burns 900 tons/hr of PRB coal.
During the pulverizing process, flow meters installed in the air ducts measure the airflow used in the pulverizers. The air is used for drying the coal in the pulverizers and for transporting it to the burners. If too little air is supplied to the pulverizers it can cause:
- Pulverizer coal spillage
- Coal pluggage in the piping from the pulverizers to the burners
- Low pulverizer outlet temperatures
- Incomplete combustion
- LOI (loss on ignition)/unburned carbon.
Similarly, too much airflow to the pulverizers causes problems with grinding of the coal (fineness), slagging and increased NOx emissions. Higher airflows also lead to erosion within the pulverizer and the coal piping, which increases maintenance costs.
Newton plant management looked at two types of airflow meters: Differential pressure (DP) with averaging pitot tubes (JAM tubes) and thermal mass flow sensors. JAM tube flow meters, which were originally installed, consist of orifice plates, venturis, and sonic nozzles. However, when plugged, these flow meters create a non-recoverable loss of pressure and accuracy. Because of the small orifice openings Ameren continually experienced problems with plugging of the flow meter orifices.
In coal-fired power plants, the air that the JAM tube flow meters measure, in addition to being hot, also contains fine coal and ash particulates. Another drawback at the Newton plant is the ducts only have short straight runs where the airflow can be measured, thus making accurate airflow measurements difficult. Because of the inaccuracy and non-repeatability of the measured airflow the plant was unable to control the unit automatically.
Thermal mass flow meters at Newton power plant. Photo courtesy of Ameren Energy Generating.
An evaluation of different options by Ameren Energy determined that thermal mass MT Series Multipoint flow meters, from Fluid Components, would be the best solution. With no-moving parts or orifices to constrict the flow of air, the multi-point flow meters are resistant to pluggage. In addition, the airflow measurements are accurate and repeatable.
To reduce unit downtime, the plant was able to fabricate and install the required mounting connections for the FCI multi-point sensors during a routine maintenance outage and prior to the delivery of the flow meters to the site. Accuracy of the new flow meters is ±2 percent of the reading with a repeatability of ±0.5 percent.
The new FCI MT Series Multipoint flow meters have now been fitted on all 12 pulverizers. Today the data generated by the flow meters is used as the primary input for the automatic control of the combustion process. Since installing the new thermal mass flow meters the LOI has been reduced from 0.75 percent to 0.15 percent and NOx emissions reduced from 0.37 lb/MMBtu to 0.15 lb/MMBtu.