March 1, 2009

"Blueprint" Your Pulverizer for Improved Performance

Richard F. (Dick) Storm, PE

We complete our two-part series on pulverizer O&M practices with a step-by-step review of important areas where close attention to dimensions and design features will increase coal processing capacity. The first part of this series, published in the October 2008 issue of POWER ("Finessing Fuel Fineness"), explored the importance of balancing the fuel and air entering and leaving the pulverizer to achieve good coal combustion in the furnace. In this article, we focus strictly on pulverizer O&M practices that every plant should be following.

Manufacturing tolerances were much looser a generation ago, which resulted in a good deal more variation in key dimensions than we find acceptable today. For some readers, the term "blueprinting" will evoke memories of hot-rodding a Detroit V8 engine after carefully machining and assembling it to the manufacturer’s specifications or "blueprints." The expected result was power output greater than that of a stock engine straight from the showroom floor, and we were seldom disappointed. We’ll use the same vernacular in this article to refer to the process of restoring a pulverizer or mill (the terms are used interchangeably) to better-than-"showroom" performance.

Balancing Act

Pulverized coal ground finely and distributed to the burners is burned in suspension, much like natural gas. To optimize combustion in the furnace, the air-fuel mixture can be readily divided equally between coal pipes leaving the mill and will burn much like natural gas when the coal is ground very fine to a mean particle size of about 50 microns. Keep in mind that the density difference between coal and air is about 1,000 to 1. Performing this balancing act efficiently is a good indicator of excellence in pulverizer performance.

Improved pulverizer performance often can be achieved by blueprinting the tolerances and settings. Most of the coal-fired power plants we visit use vertical-spindle pulverizers or pulverizers with a vertical shaft that drives a grinding ring or table. "Verticle-spindle" is used generically in the industry to include pulverizers such as MPS, MB, MBF, RP, Raymond Bowl (RP), and others. Interestingly, when you look closely at two pulverizers sized for a given weight per hour throughput, many design and performance features are very similar. Let’s take two different original equipment manufacturers’ pulverizers sized for about 120,000 pounds of coal pulverization per hour: one an RP mill design (Figure 1) and the other an MPS design (Figure 2).

1. A typical RB pressurized bowl mill, size 983. Courtesy: Storm Technologies, Inc.


2. A typical MPS 89 pulverizer. Courtesy: Storm Technologies Inc.

Both mills have vertical spindles and horizontal grinding tables or bowls. In part one of this series, pulverizer capacity or throughput was described as being not simply a static rating in tons per hour. Rather, as that article explained, the capacity of a coal pulverizer is a complex function of the fuel fineness desired, the coal’s Hargrove Grindablity Index (HGI), and the coal’s raw feed size and moisture content (see Figure 3 in "Finessing Fuel Fineness"). Plant operators can only improve throughput of a given mill with a particular coal (moisture and HGI are typically outside operator control) by sacrificing fineness — by producing a coarser product for the burners.

This situation is akin to being between the proverbial rock and a hard place. Changing fuels or suppliers may not be possible to improve mill performance, and in many plants, mill throughput problems are more than likely caused by a new fuel supplier providing lower-quality fuel. That’s the rock. The hard place is the fact that reduced fuel fineness will directly contribute to increased NO _x production and poorer furnace performance. The only option available to maintain mill throughput in many plants, beyond burner tune-ups, airflow optimizations, and coal pipe balancing or adding more sootblowers, is to optimize or blueprint the pulverizers.