To optimize performance, begin at the pulverizers

Richard F. (Dick) Storm, PE, and Stephen K. Storm, Storm Technologies Inc

Pages: 1 2 3 4 5

Optimizing combustion in pulverized coal (PC)-fired boilers today is more important today than ever. It is well known that the average American PC plant is over 30 years old and that over its lifetime NO_x and SO₂ emissions limits have been steadily ratcheted down (see box). Today, operators no longer wonder whether permissible levels will continue to fall but, rather, when and by how much.

Change is in the air

Tighter pollution control has been a common thread in the major evolutionary development of coal-fired generation over the past 30 years:

Low-NO_x burners, overfire air systems, and other "furnace solutions" have enabled major reductions in NO_x emissions, from 0.5 to 1.5 lb/million Btu to less than 0.15 lb/million Btu.
Many boilers designed to fire eastern and midwestern bituminous coals have been adapted to burn lower-sulfur Powder River Basin coal at reduced production costs.
Pulverizers designed for coals with a Hardgrove Grindability Index (HGI) of 50 to 60 are today working with coals that have an HGI in the low 40s.
Additions of electrostatic precipitator fields and back-end retrofits of baghouses, selective catalytic and noncatalytic reduction systems, and scrubbers have become commonplace.
Distributed control systems and advanced electronic hardware and software have modernized and optimized boiler operations.
Public and regulatory pressures are leaning toward mandatory CO₂ emissions caps.

The newest fork in coal-fired generation's path forward is determining how to capture plant emissions of carbon dioxide (CO₂) when—not if—the gas is regulated as a pollutant. Some advocate widespread installations of unproven integrated gasification combined cycle (IGCC) technology ASAP, to prepare it as a long-term solution. Others say building fleets of super-efficient supercritical and ultrasupercritical-pressure and -temperature plants would be a timelier, more prudent, and more cost-effective alternative. But while regulators, Congress, and the courts wrestle with the question of what to do about greenhouse gases, one thing remains clear: CO₂ emissions could be lowered considerably by raising the efficiency of the existing U.S. fleet of 1,100+ coal plants.

Today's average U.S. PC-fired plant operates at a heat rate of about 10,500 Btu/kWh. Yet a subcritical (2,400 psi/1,000F/1,000F) unit is capable of operating at least 10% more efficiently, at a heat rate of 9,500 Btu/kWh (Figure 1). There are many proven ways to improve a boiler's performance by continuously optimizing its controllable variables (see box). This article explores opportunities for raising a unit's efficiency by improving the performance of its pulverizers.

13 ways to optimize combustion of boilers with low-NO_x burners

1. The excess-oxygen level at the furnace exit must be 2% minimum (and preferably 3%).

2. The fuel lines to the burners should be balanced by a "clean air" test accurate to ±2% or better.

3. The fuel lines should be balanced by a "dirty air" test (using a high-velocity probe) accurate to ±5% or better.

4. The flow rates of fuel lines should be within 10% of each other.

5. At least 75% of the fuel particles in the lines should be fine enough to pass through a 200-mesh screen. Less than 0.1% of the particles should be able to pass a 50-mesh screen.

6. Primary airflow should be measured and controlled with an accuracy of ±3%.

7. Overfire air should likewise be measured and controlled at ±3% accuracy.

8. Primary air/fuel ratio should be accurately controlled when above minimum.

9. The minimum velocity within fuel lines should be 3,300 feet per minute.

10. Mechanical tolerances of burners and dampers should be ±1/4 inch or better.

11. Secondary air distribution to burners should be controlled with an accuracy of between ±5% and ±10%.

12. The fuel feed to pulverizers should be smooth during load changes and should be measured and controlled as accurately as possible, preferably by gravimetric feeders equipped with load cells.

13. Fuel feed quality and size should be consistent.

1. Room for improvement. The heat rate of most older coal-fired steam plants could be lowered by improving their combustion air and fuel systems. Source: Storm Technologies

Storm Technology's experience has demonstrated that, of the 20 key O&M controllable variables with the greatest impact on unit heat rate (see box), most involve the furnace's "burner belt." Essentially (and most often), in a plant operating at its lowest possible heat rate, the combustion airflows, pulverizers, fuel line balancing, burners, and air heaters will all be optimized.

20 boiler variables that can be controlled by O&M practices to improve unit heat rate

1. Flyash loss-on-ignition (LOI), or unburned carbon in ash.

2. Bottom ash carbon content.

3. Boiler and ductwork air in-leakage.

4. Primary airflow. (Measure and control more precisely, to reduced tempering airflows.)

5. Pulverizer air in-leakage on suction-fired mills. (Reduce it.)

6. Pulverizer throat size and geometry. (When optimized, reduces coal rejects and complements operation at lower primary airflows, which reduces tempering airflow and total airflow bypassing the secondary air heater.)

7. Secondary airflow. (When measured and controlled more closely, it enables more precise control of furnace stoichiometry—essential to low-NO_x operation.)

8. Peak upper furnace exhaust gas temperatures. (When too high, they foster "popcorn ash" carryover into the selective catalytic reduction system and air preheater, excessive spray water flows, and boiler slagging and fouling.)

9. Desuperheating spray water flow to the superheater. (Reduce the level.)

10. Desuperheating spray water flow to the reheater. (Reduce the level.)

11. Air heater leakage. (Reduce it; the level for Ljungstrom regenerative air heaters should and can be less than 9%.)

12. Superheater outlet steam temperature.

13. Reheater outlet steam temperature.

14. Air heater exit gas temperature. (Correct it to a "no leakage" basis and optimize it.)

15. Burner "inputs" tuning. (For lowest possible excess oxygen at the boiler outlet and satisfactory NO_x and LOI levels.)

16. Boiler exit (economizer exit) gas temperatures ideally between 650F and 750F, with zero air in-leakage (no dilution).

17. Cycle losses due to valve leak-through. (For spray water valves, reheater drains to the condenser and superheater, reheater drains and vents, and—especially—any low-point drains to the condenser or hotwell.)

18. Sootblowing frequency. (Optimized for maximum cleaning effect and minimal impact on heat rate.)

19. Steam purity. (Turbine deposits negatively impact unit heat rate and capacity.)

20. Auxiliary power consumption. (Minimize it by optimizing fan clearances, duct leakage, fuel and primary air system performance.)

Despite all the changes in regulations, equipment, fuels, and combustion controls over the past few decades, one thing has not changed in evaluating pulverizer performance: You need to get the inputs right! Table 1 breaks down the potential heat rate improvements achievable from giving your pulverizer and related systems a good tune-up.