Fine-tuning a pulverizer
The classifier can only supply sufficient fuel to the burners when the grinding zone is optimized for maximum pulverizer capacity. Pulverizer “capacity” is the amount of coal a pulverizer can grind at a given raw feed rate, to a given fineness, at a certain Hardgrove Grindabilty Index (HGI), and at a specific raw fuel size—typically crushed to ¾-inch plus tramp metal separation before entering the fuel silos. All four of these variables must be properly managed to achieve good pulverizer performance.
Classifier changes must be made to optimize both fuel fineness and fuel distribution. That is, classifiers can create good fuel balance provided the fuel lines are flow-balanced on clean air (Figure 6).
6. Balancing burners. Balancing burners begins with the classifier on pressurized pulverizers. The classifier has two purposes: to return course particles for regrinding and to balance fuel streams to each burner line. Poor coal fineness often yields poor distribution, because large coal particles and air mix to form a two-phase mixture that will not homogenize (left). Good fineness creates a homogenous and balanced mixture that behaves more like a gas than a two-phase mixture of air and fuel (right). Source: Storm Technologies Inc.
A typical pulverizer capacity correction graph is shown in Figure 7. Note that the capacity of a pulverizer originally designed for 45 HGI and 70% passing 200 mesh when operated with 40 HGI fuel at 80% passing 200 mesh will have a true capacity of about 70% of the base design. Of course, the penalty is greater if the pulverizer was originally designed for a typical eastern bituminous coal 40 years ago with an HGI of 50 to 60.
Figure 7 also clearly shows that fineness and hardness have a much more profound effect on pulverizer performance than moisture or coal feed size. This observation leads to an important conclusion: The common link for good furnace combustion and good pulverizer performance is fuel fineness, and that’s why we believe a properly tuned pulverizer that is capable of delivering rated fuel flow with the fuel fineness essential for good combustion is critical for optimum plant performance.
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.
An optimum air/fuel ratio is also critical for achieving best fineness and best furnace performance. Pulverizer throats that are oversized are often compensated for by raising the primary airflow above the optimum 1.6 to 1.8 lb air/lb fuel range to minimize coal rejects. Although high primary airflow will reduce coal rejects, it does so at the detriment of coal fineness, fuel distribution, heat rate, upper furnace slagging, and furnace NO
x production (Figure 8).
8. Sampling coal fineness. A typical sampling methodology uses an isokinetic coal sampler to determine if the coal pipes are properly balanced and if the coal is ground to adequate fineness. Source: Storm Technologies Inc.
Switching from an eastern coal to PRB coal entails much more than just getting a good price per ton for lower-sulfur coal. PRB coal can cause problems from the pulverizer through the furnace if the plant’s pulverizers are not up to the task. Many plants have switched to PRB only to experience much higher furnace and backpass slagging and fouling, which then prompts them to add more sootblowers and water lances in an effort to keep furnace surfaces clean. Chances are their problems are not with the furnace or the new coal but rather are the result of a poor tradeoff between pulverizer capacity and fuel fineness that was made to minimize the capital cost of the fuel switch.
—Richard F. (Dick) Storm, PE (rfstormsea@aol.com) is senior consultant for Storm Technologies Inc. and a POWER contributing editor.
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