Comprehensive Study Notes on Coal Combustion, Boiler Tuning, and Maintenance

Coal Specification and Sampling Game Theory

• Specifications for coal size can easily be manipulated if the buyer does not conduct rigorous testing.

• A common specification might include coal being $40\%$ less than a quarter inch in size, but validation of these specs is rare.

• Conducting a proper ASTM coal size test is a massive undertaking, requiring approximately $2,000\,\text{lbs}$ of coal and manual sieving. Mechanical samplers cannot be used for size testing because they crush the coal.

• Labs often avoid the full ASTM process because sieving a ton of coal to find a "two by zero" number is labor-intensive. If a buyer does not test, suppliers may not be honest with their size and quality specifications.

• Small coal operations have largely disappeared, and the current industry consists of larger companies, such as Peabody and Island Creek. However, the culture of "making up" coal analysis results persists among some personnel trained in older mining environments.

• Suppliers can gain an extra $50\,\text{BTUs}$ per day simply by letting the coal dry out slightly before measurement, an advantage they will take if the buyer is not paying close attention.

Mine Inspection and Sampling Strategies

• Personnel at LG&E and KU, including the head mining engineer Delbert and a team of four or five other engineers, actively inspect coal mining operations.

• Inspectors like Sam, a veteran mine inspector, look for specific signs of coal manipulation, such as front-end loader tracks going to rail lines where raw coal is dumped into the bottom of cars.

• Sampling typically occurs by taking material from the top of a loaded rail car. Consequently, suppliers may hide lower-quality, raw coal at the bottom of the car, beneath the sampled layer.

• Utility companies are often forced by regulators to purchase the "lowest cost" fuel that is "suitable." However, suitability is relative to the load situation. High-cycling units require coal that can handle rapid starts and stops, otherwise, they face "green slides" and other operational dilemmas.

Structural Damage Caused by Boiler Slag

• Slag, which is molten rock, builds up on burner walls and the "nose" of the boiler. If it is not removed, it grows until it eventually falls under its own weight.

• The height of a fall can exceed $200\,\text{feet}$. When massive clinkers—which can be the size of a "Volkswagen" or a "school bus"—fall from the walls, they destroy the sloped membrane at the bottom of the boiler.

• This impact can burst membrane tubes, gouge metal, break buckstays (large structural supports), and create "dead air space." Heavy slag falls can result in tubes being crushed by over $50\%$

The Triggers and Consequences of Boiler Tuning

• Boiler tuning typically centers on three objectives: limiting NOx, limiting CO/LOI (Loss on Ignition), and limiting slag.

• Almost all facilities tune for NOx, but the technicians involved may not understand the impact of carbon monoxide and low oxygen on coal ash and slag. Low NOx tuning is essentially an indicator of poor combustion; good, hot flames and high oxygen naturally produce more NOx.

• Approximately half of all boiler slagging issues find their origins in low NOx tuning performed by external partners who leave the boiler "cockeyed," with low oxygen on one side and high on the other (the oxygen split).

• The average oxygen reading in the control room might be $2\%$, but this can mask actual conditions of $0\%$ oxygen on one side and $4\%$ on the other. This imbalance causes localized poor combustion even when the average appears acceptable.

• Traditional boiler tuners adjust registers around the burners to control airflow. This is often a "dead end road" because changing the mill load can cause the heavy coal flow to switch to a different pipe, rendering previous tuning invalid.

Primary Airflow and Coal Distribution Math

• To achieve balanced combustion, coal should be ground fine with primary airflow kept within design ranges.

• Design air-to-fuel ratios for boilers in Kentucky are typically around $1.8$. However, reality reflects ratios closer to $2.6$, meaning coal is blown through the mill so fast that it comes out too large.

• When pipe velocity exceeds $7,000\,\text{ft/min}$, burner adjustments like adjustable vanes become ineffective. High velocities result from low BTU coal combinations and high primary airflow.

• Over-fire air (OFA) is a strategy used by manufacturers like Alstrom and B&W to starve the main fireball of oxygen to reduce NOx. This creates a massive zone of CO. While this reduces NOx, CO causes tube wastage and "fireside corrosion," particularly with sulfur in the coal which remains as a sulfide and eats the metal.

Safety and Maintenance Philosophies

• Spontaneous combustion is a major risk for Powder River Basin (PRB) coals, biofuels, and lignite. These coals contain enough internal oxygen to self-heat and ignite if left stagnant in a bunker or pile.

• In some extreme cases, boilers become unsafe to inspect due to moving walls caused by broken buckstays.

• The term "mill puff" is a euphemism used in the industry to describe a coal mill explosion to avoid negative news coverage.

• Maintenance remains a "choke point" for plant performance. For example, a plant in Hong Kong suffered massive slagging because $37\%$ of its wall blowers were inoperable and the mills were smoldering. Basic maintenance of air preheaters and soot blowers is essential but often neglected due to budget constraints.

Air Preheater Efficiency and Leakage Calculations

• Air preheaters recover thermal energy by dropping gas temperatures from approximately $750\,^{\circ}\text{F}$ to $300\,^{\circ}\text{F}$, recovering about $10\%$ of total energy.

• The general rule of thumb is that every $40\,^{\circ}\text{F}$ drop in temperature represents a $1\%$ increase in efficiency.

• Modern air markers often "cheat" by claiming design leakages of $6\%$, while realistic design numbers from the 1980s were closer to $10\%-12\%$.

• In practice, leakage can reach extreme levels. While maintenance should occur when leakage exceeds $20\%$, some units operate with up to $45\%-47\%$ leakage. High leakage steals air from the furnace, forcing ID (Induced Draft) fans to handle $30\%$ more flow than designed, which limits unit load and hurts the efficiency of pollution control devices like scrubbers and precipitators.

• Leakage and excess air can be calculated using the oxygen levels at the economizer outlet ($\text{O}_{2 \text{in}}$) and the preheater outlet ($\text{O}_{2 \text{out}}$).

• The calculation for excess air based on oxygen is:

    $\text{Excess Air} = \frac{\text{O}_2}{20.9 - \text{O}_2}$

• Theoretical air requirements for coal are roughly constant relative to BTU content, approximately $770\,\text{lbs}$ of air per million BTUs. Total air at $20\%$ excess air ($3.2\%$ oxygen) is approximately $925\,\text{lbs}$ per million BTUs.

Coal Abrasion and the Hardgrove Grindability Index (HGI)

• The HGI measures ease of grinding, not actual hardness. A soft coal like Pittsburgh coal might have an HGI of $55$, while harder coals fall into the $30-40$ range.

• If coal is like a "potato chip," it crunches easily ($100\,\text{HGI}$). If it is like a "rubber band," it is resilient and difficult to grind but not necessarily abrasive.

• Abrasion is caused by minerals within the coal, specifically quartz. Quartz ($\text{SiO}_2$) has a Mohs hardness of $7$, making it harder than steel. Alumina ($\text{Al}_2\text{O}_3$) in the ash chemistry is often mistaken for corundum (which is very hard), but it usually exists as soft aluminum silicates (clays).

• The Rask Quartz Equation is used to estimate the abrasive potential of coal ash:

    $\text{Estimated Quartz} = \text{Total Silica} - (1.5 \times \text{Alumina})$

• Multiplying the estimated quartz percentage in the ash by the pounds of ash per million BTUs provides a metric for erosion potential.

Questions & Discussion

• Question: Can the slag be dropped off the walls by cycling the unit load?

• Response: While the cyclone wants to be slagged, once slag is up against the wall, it generally will not just fall off unless it becomes heavy enough to self-destruct or is physically removed. In some cases, it can cause significant damage to the lower slopes and membrane.

• Question: Is it possible for a different representative to attend the training tomorrow in someone's stead?

• Response: The training is flexible. If a different person comes in the morning, a certificate of attendance can still be issued. Many people, including coal sales reps, attend just to network and may spend the rest of the day golfing.

• Question: Where does the ID fan sit relative to the air preheater?

• Response: The Induced Draft (ID) fan is located far in the back of the plant, typically near the stacks, after the gases have passed through the precipitator and the scrubber. The Force Draft (FD) fan pushes air into the boiler through the burners.

• Question: What is the difference between "mill puffs" and explosions?

• Response: A mill puff is a "polite term" for a coal dust explosion used to avoid alarming the public or news media, even when people are injured.

• Question: Is there a filter in the air preheater?

• Response: No, it consists of baskets filled with ridged metal plates that act as heat exchange elements. They do not filter but can become plugged by ash and "ammonia slip" from SCR systems, which creates a sticky residue similar to "tobacco gum."