Future shock
In fact, the industry can expect fireside wastage to become more prevalent as more 30+-year-old plants install SO2 scrubbers and then are converted to be capable of firing higher-sulfur coals. Why? The reason is because most low-sulfur compliance coals also have low iron content in their ash. These coals are "forgiving" from the perspective of contributing to aggressive fireside corrosion of water walls and slagging. Once a plant owner has spent the money on a scrubber, he will be tempted to buy a higher-sulfur fuel (with higher iron content in its ash) for economic reasons—primarily escalating Western coal rail costs and improved Eastern coal cost-competitiveness. This "reverse fuel switch" trend is likely to foster more slagging, fouling, and aggressive furnace tube corrosion.
Fortunately, there are options available to prevent increases in slagging and fouling that result from a reverse fuel switch. One for minimizing fireside wastage is to optimize pulverizer performance to fuel fineness that's acceptable in all fuel lines at all times. Storm Technologies' standard minimum recommended fineness is greater than 75% of particles passing through a 200-mesh screen and none through a 50-mesh screen. Lowering superheater and reheater metal temperatures, to reduce slagging and fouling in the convection passes, is another step than can be taken to improve unit reliability and burner belt combustion.
Finally, operating a unit at its maximum efficiency and capacity should be an overriding economic objective. Optimizing pulverizer performance and burner belt inputs can help reach that goal. So can diagnostic performance testing of fuel lines, combustion airflows, and key upper furnace combustion parameters. When fuel line fineness declines, and reducing environments are found in the furnace, corrective action should be taken immediately.
Cleaner coal-burner
To sum up, there are three significant ways in which optimizing pulverizer performance can contribute to a reduction in a coal-fired boiler's NOx emissions.
One. Release of fuel-bound nitrogen in the burner's devolatilization zone is enhanced by making coal particles smaller, in effect increasing the overall fuel surface area. Low-NOx burners are most effective when they're fed coal that has been finely ground. Poor fineness traps fuel-bound nitrogen within the carbon char particles, beyond the reach of even the best low-NOx burners.
Two. Fuel balance usually improves with better fuel fineness. A powdery mixture of fine coal particles passing through a pulverizer, classifier, and coal riffles (if the unit is so equipped) will flow more uniformly when it is more finely ground. Such fuels actually flow more like a gas when entrained at the proper ratio in the primary air stream. Fine coal particles mixed in the transport air become more uniformly distributed than coarsely ground coal particles at a similar air/fuel ratio.
Three. A more homogeneous mixture of coal and air entering the burners will naturally reduce required excess air levels. By reducing the total airflow and reducing the excess air, thermal NOx production is reduced. The better the mixing of the combustion products in the available residence time, the less "extra air" that has to be added to create oxidizing zones in portions of the furnace that are fuel-rich (Figure 6). When these "peaks and valleys" of free oxygen and high temperatures are made more uniform, then it becomes possible to reduce the total excess air that has to be added to make up for imprecise fuel and air inputs into the burner belt.
6. Walk the line. Economic plant operation requires operation in the narrow zone of optimum combustion efficiency. Source: Storm Technologies
—Dick Storm is president of Storm Technologies (www.stormeng.com); he can be reached at (704) 983-2040 or rfstormsea@aol.com. Stephen K. Storm is a vice president of the company and its manager of technical field services; he can be reached at 704-983-2040 or skstorm1@aol.com.
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