Most of today’s operating coal plants began service at least a generation ago and were designed to burn eastern bituminous coal. A switch to Powder River Basin coal can stress those plants’ boiler systems, especially the pulverizers, beyond their design limits and cause no end of operational and maintenance problems. Many of those problems are caused by failing to maintain good fuel fineness when increasing fuel throughput.Many of our pulverized coal plants began life 30 or more years ago burning eastern bituminous coal. Utility practice at that 7. Predicting pulverizer performance. Four key parameters must be considered when assessing the performance of a pulverizer: fuel feedsize, fineness, moisture, and Hardgrove Grindabilty Index. Often pulverizers are called to perform far above their true capacity. Source: Storm Technologies Inc.time was to simply burn the coal particles until the flyash contained 3% carbon or less and then call it a day. That practice is antiquated by today's competitive operating standards, yet the fuel preparation equipment we operate today remains unchanged.
Without question, the heart of a pulverized coal–fueled plant is its pulverizers. Familiar problems such as boiler tube slagging and fouling, excessive furnace gas temperatures, poor furnace combustion, and waterwall wastage are often traced back to poorly performing pulverizers. In my experience, about 75% of the efficiency improvements in coal-fired boiler systems center on the pulverizers and closely related issues of coal fineness, fuel distribution, fuel line balance, and primary airflow.
Last year (see POWER, October 2007, “Managing air to improve combustion efficiency”) I wrote about the importance of managing air throughout the steam generator system. This month, I direct your attention to properly managing the fuel component of the combustion equation.
Balancing act
Injecting the correct mixture of air and suitably prepared coal into the furnace in the right amounts, in the right location, and at the right time is the key to optimizing boiler efficiency, reliability, fuels flexibility, maximum capacity, and low stack gas emissions. This sounds easy in theory, but it’s much more difficult in practice given all the seemingly uncontrollable variables in the process.
However, pulverizer and related problems lend themselves to a structured analysis approach. There’s an old proverb that asks, How do you eat an elephant? The answer: One bite at a time. The same wisdom applies to a pulverizer tune-up (see sidebar).
Figure 2 shows the burner belt and furnace of a typical boiler and illustrates the short residence time, typically less than two seconds, that a coal particle has to complete combustion before impinging, and perhaps sticking, on a superheater or waterwall tube. If that particle continues to combust in the backpass, boiler efficiency is reduced, slagging and fouling increase, upper furnace gas temperatures are elevated, and emissions may increase.
2. In the zone. A fuel particle has less than two seconds to complete combustion before entering the superheater section of the boiler. Source: Storm Technologies Inc.
For example, pulverizer mechanical tuning and airflow management must work hand in hand to exploit the NO
x-reduction potential of a boiler. Experts estimate that about 70% of the NO
x produced from a pulverized coal–fueled boiler originates with the fuel-bound nitrogen. Nitrogen, usually between 0.5% to 1.6%, is usually a very small component of the fuel’s ultimate analysis but is the source of most of the NO
x produced.
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