Comprehensive Study Guide for Power Plant Engineering and Coal Combustion

Power Plant Operations and Operator Dynamics

• Practical operations knowledge at power plants like NIPSCO is often gained through a combination of structured learning (e.g., a two-week operator training class) and hands-on experience by union workers.

• Control room simulators are used to train operators on recognizing critical levels, such as decreasing water/trouble levels, and executing corrective actions.

• Interpersonal relationships between management (engineers) and unionized operators are critical for problem-solving, particularly when coming off prolonged industrial disputes like an eight-month strike.

• Successful engineering diagnostics often require field verification rather than relying solely on control room gauges, as seen in cases where oxygen ($O_2$) averages displayed as $3\%$ actually masked a split of $0\%$ and $6\%$.

• Wind box dampers, which proportion airflow to sides of a boiler, can be set incorrectly during plant startup and remain unadjusted for years, leading to significant combustion imbalances.

• Operational proof of combustion mastery is demonstrated by the ability to intentionally manipulate Loss on Ignition (LOI) levels (taking them up and down) to prove that the crew, rather than coal variation, controls the process.

Pulverizer Mechanics and Classification Systems

• Coal pulverization involves a vertical pipe delivery system where the coal is crushed by rotating wheels on a table.

• Primary air is blown upward from the bottom of the mill and through a vane wheel to lift the ground coal particles.

• The classifier section consists of blades that cause a change in particle direction. Larger particles lose momentum and drop back down into the grinding zone for further processing.

• The pulverizer acts as a mechanical separator similar to a steam drum, separating coal dust from heavier particles rather than water from steam.

• The Primary Air (PA) fan provides the necessary airflow to the mill. This air must be preheated (often through a specific sector of the air heater) to assist in drying the coal during the grinding process.

• Old exhauster-style Combustion Engineering (CE) mills had airflow rates fixed by the mill speed, making them less variable for load changes compared to modern designs.

• Babcock & Wilcox (B&W) EL mills utilize large metal balls in a ring to grind coal. This specific design served as the basis for the standard Hardgrove grindability test machine.

Hardgrove Grindability Index (HGI) and Material Properties

• The Hardgrove Grindability Index (HGI) is a measurement of the ease of grindability, not a direct measurement of hardness or abrasion.

• The HGI test involves 200 revolutions of a sample of "coffee ground" sized coal in a standardized machine (the Hargrove grinder).

• High HGI numbers (e.g., $70$) indicate coal that is easy to grind (like potato chips), while low HGI numbers (e.g., $40$) indicate coal that is resilient and difficult to grind (like rubber bands).

• Operational reality of low HGI: Mill motors will hit their maximum amperage (amps) limit before reaching full coal tonnage capacity.

• Coal itself acts as a lubricant (graphite-like carbon layers); it is the rocks and minerals (quartz, pyrites) within the coal that cause mill wear and erosion.

Coal Washing and Mineral Composition

• Washed coal is processed via gravity floatation in a mixture of water and finely ground magnetite ($Fe_3O_4$).

• Specific gravity thresholds are set to separate coal (SG approximately $1.3$) from rock (SG set higher at $1.4$ or $1.5$ to ensure high BTU recovery).

• The goal of washing is typically to recover $70\%$ of material weight while retaining $98\%$ of the BTU content.

• Sulphur and ash are inherent in coal; pyrites found in the "cleat" (cracks in the coal) are intimately mixed and cannot be fully washed out.

• Coal ash analysis reports elements as oxides, which can be misleading:

  • $Al_2O_3$ (Corundum/Aluminum Oxide): Harder than steel ($9$ on Mohs scale), but does not exist in raw coal; it represents aluminum silicates (clays/shales).

  • $SiO_2$ (Silica/Quartz): The hardest mineral in coal ($7$ on Mohs scale).

  • $Fe_2O_3$ (Hematite/Iron Oxide): Reports pyrite ($FeS_2$) content after it has been fully oxidized in a lab setting.

• The Rask equation is used to estimate quartz content by subtracting silica associated with aluminum silicates from the total silica: $\text{Estimated Quartz} = \text{Total Silica} - (1.5 \times \text{Aluminum Oxide})$.

Pulverizer Performance and Slag Management

• Primary factors impacting pulverizer capacity include:

  • High BTU content (requires less tonnage for the same heat output).

  • Inlet coal size (standard yard crushers reduce coal from 2-3 inches to 0.75 inches, adding $3-5\%$ to mill capacity).

  • Moisture content.

  • Hardgrove Grindability Index.

• A common operational error is "jacking up" primary airflow beyond design limits (typically $1.6$ to $1.8$ air-to-fuel ratio). High velocity air carries large coal particles out of the mill before they are properly ground, resulting in poor fineness.

• Poor fineness leads to large particles impacting boiler walls, causing "wall slags".

• Mechanical maintenance, such as ensuring all classifier blades are symmetrical (within a quarter-inch), is vital for balancing coal distribution between burner pipes.

• Water cannons and heat flux sensors are reactive solutions to slagging that can cause "alligatoring" or stress fractures on boiler tubes.

• Slag removal techniques have historically included the use of 12-gauge or 8-gauge shotguns (kiln guns), though improper aiming can cause starburst cracking on the inner diameter (ID) of stainless steel superheater tubes.

Thermodynamic Efficiency and Calculations

• One British Thermal Unit (BTU) is the energy required to raise the temperature of one pound of water by $1^\circ F$.

• Boiling water into steam requires approximately $1,000\,BTU$ per pound (latent heat of vaporization).

• Bomb calorimeter tests measure high calorific value by cooling combustion byproducts back to room temperature. Practical boiler efficiency must account for the Net Calorific Value (NCV).

• NCV accounts for the heat lost to moisture in the coal and moisture formed from hydrogen combustion, both of which exit the stack as steam at $1,000\,BTU/lb$.

• Hydrogen ($H_2$) in coal converts to water weight at a ratio of 9:1. A coal with $4\%$ hydrogen produces $36\%$ water weight upon combustion.

• A $10\%$ increase in moisture results in approximately a $1\%$ penalty in boiler efficiency.

• Reheat sprays used to control steam temperature significantly penalize heat rates. Every $10,000\,lb$ of reheat spray can cost $10-15\,MW$ of efficiency because the energy is lost to the condenser rather than recovered in feedwater heaters.

Combustion Chemistry and Emissions

• Low NOx firing strategies (overfire air) intentionally create fuel-rich zones with high Carbon Monoxide ($CO$) levels.

• High $CO$ effectively "eats" NOx by scavenging oxygen, but it creates a reducing atmosphere that prevents iron from oxidizing to $Fe_2O_3$.

• In reducing atmospheres, unburned pyrites ($FeS_2$) can be sprayed onto furnace walls, leading to rapid tube wastage and sulfide corrosion.

• Loss on Ignition (LOI) levels indicate unburned carbon in fly ash. High LOI in economizer hoppers can lead to smoldering fires and "clinkering" when oxygen is introduced during low-load periods.

Pulverizer Testing and Sampling

• Isokinetic sampling ensures that the velocity of the sample entering the probe matches the velocity of the gas in the pipe to avoid biasing the particle size distribution.

• Testing ports should ideally be located 10 to 12 pipe diameters away from elbows; if located closer, additional sampling points are required to account for "roping" (centrifugal separation of solids and gases).

• Clean air tests and dirty air tests are performed to measure pipe velocities with and without coal flow to ensure pipe-to-pipe balance.

Questions & Discussion

• Question regarding the pulverizer process: Does the coal come down a pipe, get crushed by rotating wheels, and then get blown up by air coming from the bottom through a vane wheel?

  • Response: Yes, the process uses mechanical grinding and upward airflow. The classifier blades then cause turbulence, throwing larger droplets (or particles) against the wall to fall back down, similar to a mechanical separator in a steam drum.

• Question regarding primary air fans: Is the PA fan the only thing feeding air to the mill?

  • Response: Generally yes, but the air must be preheated. In some designs, the air is pulled from the main wind box, but in most modern systems, it passes through a dedicated sector of the air heater.

• Question regarding Tennessee Valley Authority (TVA) operations: Does TVA blend different types of coal?

  • Response: Yes, at plants like Kingston, they blend Powder River Basin (PRB) coal with higher BTU Illinois or Pennsylvania coal (e.g., $11,500$ or $13,000\,BTU$) because they cannot maintain full load on PRB alone. They are also constrained by strict chlorine limits.

• Question regarding boiler slopes: Why did the TVA representative ask what "slopes" were?

  • Response: This indicated the individual was relatively new to physical boiler hardware. Slag falls (the size of Volkswagens or school buses) can severely damage these slopes, sometimes knocking out the bottom of the boiler entirely.