Build on the basics
The typical primary air-fuel ratio for maintaining coal velocity exiting the mill relative to the secondary air velocity around the burner nozzle on most vertical spindle pulverizer systems is 1.8 pounds of air per pound of coal. Regardless of excess O2 setpoints, excess air requirements, or overfire air, the primary airflow ratio is the same at any given coal flow rate. Tuning the system to operate at this air-fuel ratio is the first step in building a primary airflow control ramp for the plant control system.
The second, and most restrictive, design criterion required control changes to ensure that a 3,000-ft/min minimum line velocity in each coal pipe is maintained. This is the minimum velocity necessary to entrain pulverized coal particles in an air stream, regardless of coal flow. Next, the primary airflow ramp on a vertical spindle mill, such as the MPS 89 mill, can be optimized for a nominal 1.8 air-fuel ratio, which determines the amount of primary air flow required at any load (Figure 2).
2. Select the right ramp rate. The horizontal portion of the line represents an operating regime under which the minimum line velocity is the controlling variable; the sloped line indicates the rate of fuel flow when the 1.8 air-fuel ratio controls the ramp rate of primary airflow. Source: Storm Technologies Inc.
Active tuning of the air-fuel ramp based on the fuel and mechanical condition of the coal pulverizer tolerances is rare today. However, this task is extremely important when comprehensively evaluating mill grinding performance and overall combustion performance. These days SEC actively adjusts air flow curves based on mill performance, coal quality, and pulverizer rejects to maintain optimal air-fuel ratios and unit response. SEC has found that unit response at lower loads may be reduced by maintaining the minimum primary airflow (following the curve, Figure 2), requiring an upward biasing of airflow at lower loads.
Breathe easy
Just as important as maintaining the correct air-fuel ratio in the coal pipe from the mill to the burner is proper balancing of the air-fuel ratio between each mill and its burner. This is especially important because the path from each mill to the burners has a different pipe length and elevation to each of the five burner banks (Figure 3).
3. Injecting solid fuels. A properly tuned pulverizer; instrumentation to measure pulverizer throat velocity and venturi and orifices airflow; and a microprocessor-based gravimetric load cell coal feeder are required to balance the coal pipes from each mill to an individual burner and between the five mills. Fuel lines are also monitored for fuel fineness and distribution by periodic manual measurements. Source: Storm Technologies Inc.
Average boiler stoichiometry may not be a useful indicator for balancing mill performance, because the unit might be air rich on one side and fuel rich on the other. To obtain optimum performance in the furnace, air and fuel flows need to be balanced within acceptable tolerances. Our experience has shown that primary and secondary air flows should generally be balanced to within ±5% of the average, while fuel flow should be balanced to no more than ±10% of the average on a pipe-to-pipe basis. Keep in mind that 7% air imbalance and 14% fuel imbalance are conservative values for typical coal-fired plants. Not surprisingly, localized high-temperature zones and/or localized slagging issues are not uncommon under these conditions.
One of the first steps taken by the team was to balance the air-fuel lines within ±2% of airflow as part of a comprehensive fuel line–balancing process that accounted for:
- Clean air balancing within +2%.
- Measured primary air accuracy within +3%.
- Dirty air velocity measurements at optimum primary air-fuel ratio with a balance of +5%.
- Fuel line fineness balancing through classifier changes or fuel line distribution modifications to achieve +10% variation.
- Pulverizer “blueprinting.”
Plant technicians also began conducting periodic (quarterly or semi-annually, based on fuel quality) mill performance tests to optimize coal fineness and air-fuel ratios to the burners; at the same time they inspect classifiers to confirm that they are set with optimum tolerances. The coal feeders were also calibrated or refurbished as needed, and each mill was renewed to factory specs.
Email
Comments
Print
Reuse
