No one in this industry underestimates the difficulty of transforming an unwieldy and distinctly nonuniform substance like coal into a fuel whose physical and chemical characteristics are consistent enough to supply a workhorse power plant boiler. Designers of fuel-handling and pulverizer systems have wrestled with this problem since the first traveling-grate furnace was invented. The more predictable a fuel is, the easier it is for engineers to tune a boiler burning it for maximum performance and minimum emissions.
Selective service
From the grizzly to the burner tip, the pulverizer plays an essential role in maximizing the consistency of fuel delivered to the steam generator. Most existing pulverizers—both the vertical-shaft and ball-mill types—come with a static classifier. Its blades reject coarse particles to produce a stream of coal particles that are mostly about 0.0029 inches (74 microns) across.
Obviously, not all of the particles leaving the pulverizer are that size. The fineness of coal is measured using a U.S. standard sieve with a sleeve opening of 0.0030 inches. Particles smaller than 74 microns will pass through a 200-mesh screen, and those larger will not. Passing coal through several screens in series yields a profile of the particle size distribution (PSD) of a pulverizer/classifier. The industry standard for PSD has been 70% passing through a 200-mesh sieve. Other common U.S. standard sieves are 325 and 50 mesh, which stop particles larger than 0.0017 inches (45 microns) and 0.0117 inches (300 microns), respectively.
Coarse particles of coal don't burn as quickly, easily, or cleanly as finer particles. Because they take longer to burn, coarse coal particles raise a boiler's average NOx emissions. They also foster agglomeration and deposition of slag, making boilers and heat-recovery boilers more vulnerable to fouling. Coarse coal can even poison the catalyst of a selective catalytic reduction (SCR) system. If enough coarse coal passes through a boiler without being burned completely, its flyash may have too much unburned carbon (UBC) for commercial use.
Classifier 101
Upgrading a pulverizer's classifier from static to dynamic can improve two key pulverizer performance measures: its throughput and its coal fineness. As their name implies, dynamic classifiers (see box) use static as well as rotating vanes to sort coal particles more precisely by size. Retrofitting a pulverizer in this way:
- Increases the slope of its fineness distribution on a Rosin-Rammler PSD plot.
- Reduces the amount of superfine and overly coarse material.
- Reduces internal recirculation of partially pulverized particles.
- Reduces overgrinding, which in turn reduces the mill's wear, power consumption, and vibration.
- Reduces the differential pressure across the pulverizer.
The following summaries of dynamic classifier retrofit projects (classified by application) further illustrate the benefits of this technology.
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