Comprehensive Study Notes on Boiler Combustion, Slag Dynamics, and Coal Chemistry

Visual Analysis of Geological and Industrial Samples

• Hand samples are physical specimens of rocks or slag that are analyzed based on physical characteristics.
• Common geological hand samples include pumice or limestone.
• Industrial hand samples often consist of slag, which can appear as porous, bubbly material.
• Specific physical traits of slag include being black on the inside and brown on the outside.
• The black color indicates the presence of magnetite ($Fe_3O_4$) or unburnt iron, suggesting reduced oxygen or high $CO$ environments.
• The brownish-red material on the surface is rust, indicating oxidation. This occurrs if the deposit was roasted in high oxygen at high temperatures during a shutdown or if it was knocked into water.
• The "pedigree" of a sample is vital for analysis. This includes its location of origin, orientation (labeling the gas side versus the tube side), and specific conditions of the environment where it was collected.

Combustion Dynamics and Slag Formation

• Bubbles in slag are a diagnostic clue signaling a combustion problem. They form when pieces of coal get mixed into wet slag and continue to release gases.
• Microscopic analysis (likened to the methods of Sherlock Holmes) allows for the identification of specific coal particles trapped within these slag bubbles.
• Pulverized coal plants are designed with burners that point toward walls rather than the sky.
• Slag behavior is often non-linear. While a system might behave linearly at low loads, small changes near maximum capacity (such as moving from $95\%$ to $100\%$ load) can lead to steep increases in slagging or corrosion.
• Gladdenization occurs when molten slag seals the gaps between boiler tubes, creating a permanent obstruction that is difficult to remove.

Sulfur and Mineral Forms in Coal

• Sulfur exists in coal in two primary forms:
  • Organic Sulfur: Burns to create sulfur dioxide ($SO_2$).
  • Pyritic Sulfur: A dense, heavy mineral composed of sulfur and iron ($FeS_2$).
• Coal typically has a ratio of one-third to two-thirds pyritic to organic sulfur, though $50/50$ is a common benchmark.
• Powder River Basin (PRB) coal tends to be light in pyritic sulfur and higher in organic sulfur.
• Ocean-influenced coal may have higher levels, such as $1.5\%$ pyritic and $1.5\%$ organic sulfur.
• Sulfates ($SO_4$) are the sulfuric acid component. Their presence in a sulfur analysis indicates old, oxidized coal that has weathered or been stored in water (pond finds).
• Pyrite is famously known as "fool's gold" due to its dense, heavy nature and gold coloring.
• Pyrite varies in size. Large particles can be washed out; tiny particles burn readily; "cleats" or cracks in the coal must be ground into dust but are malleable and difficult to pulverize.

Bituminous Coal and Coking Properties

• Bituminous coals have coking properties, meaning they melt into tar when heated in the absence of air.
• Coke ovens heat bituminous coal without air to drive out volatiles, creating coke, a hard porous material used in steel production.
• In a boiler, a bituminous coal particle puffs up like a sponge as it burns, eventually turning into $CO_2$.

The Splat Factor and Kinetic Energy

• Pyrite particles are approximately four times denser/heavier than coal particles of the same size.
• The "Splat Factor" describes the phenomenon where heavy particles hit the boiler wall at high velocity, similar to bugs splattering on a windshield.
• Momentum ($p$) is defined as mass ($m$) times velocity ($v$):  $p = m \times v$
• Kinetic Energy ($KE$) is the governing factor for the splat effect, defined as:  $KE = \frac{1}{2} m \times v^2$
• Because kinetic energy involves velocity squared, doubling the velocity quadruples the impact energy.
• High mass particles (pyrite and quartz) maintain their trajectory toward the wall due to inertia, while lighter coal dust and gases turn and flow up the furnace.
• To reduce the splat factor to zero, the $50$ mesh particle count must be reduced to zero by grinding the coal finer.
• The calculation for the splat factor involves:
  • The mass of pyrite and quartz particles.
  • Pipe velocity.
  • Concentration of $50$ mesh particles ($0.3\,mm$ size).
  • Loading levels (pounds per million BTUs).

Water Wall Corrosion and Infrastructure

• Researchers suggest that high chlorine ($Cl$) coal (over $0.2\%$ chlorine) causes water wall wastage within weeks, though real-world experience varies.
• Chlorine specifically attacks stainless steel, affecting division panels and superheat platens.
• High chlorine coal often sells at a substantial discount, such as $\$34$ per ton.
• Corrosion mechanisms in low-NOx environments involve staged combustion, high $CO$, and the presence of pyrite.
• The chemical reaction for corrosion involving pyrite:  $FeS_2 + CO + H_2O \rightarrow FeS + H_2S + CO_2$
• $FeS$ (partially burnt pyrite) is a molten solid that sticks to the wall and causes damage, rather than just the gaseous $H_2S$.
• Preventative measures for corrosion:
  • Avoiding flame impingement.
  • Improving coal fineness.
  • Proper air and coal mixing.
  • Using water wall tube coatings (e.g., chrome coatings), although these act as "band-aids" that can lead to corrosion at the interface or spalling.

Elemental Partitioning and Heavy Metal Hazards

• Elemental partitioning occurs when elements separate based on physical properties or volatilization.
• Iron concentrations are typically higher in bottom ash than in fly ash.
• Arsenic and selenium tend to volatilize and re-condense on fly ash and superheater tubes.
• Case Study: A boiler in Michigan produced white crystalline ash containing $3\%$ arsenic, which is as toxic as rat poison (typically $1\%$ arsenic).
• Exposure to heavy metals like arsenic can lead to heavy metal poisoning. Industrial hygiene practices include wearing monitors, full-face protection, and blood tests before and after working in high-risk environments (e.g., copper mines).

Ash Chemistry and Loading Calculations

• Ash content should be measured in pounds of ash per million BTUs rather than simple percentages to allow for accurate comparisons between different coal types.
• Formula for Ash Loading:  $\text{Pounds of Ash per Million BTU} = \frac{\text{Percent Ash}}{\text{BTU}/10,000}$
• PRB coal ($5\%$ ash, $8,500$ BTU) and Illinois coal ($8\%$ ash, $11,500$ BTU) both result in approximately $7\,lb$ of ash per million BTU.
• To find the loading of a specific element (like iron):  $\text{Pounds of Element} = \text{Pounds of Ash per Million BTU} \times \text{Decimal Percent of Element}$
• Higher ash content can lead to higher total iron loading even if the percentage of iron in the ash is lower. For example, a coal with $12\%$ iron but high ash contributes more iron than coal with $25\%$ iron but low ash.

Boiler Thermal Profiles and Maintenance

• Boilers are typically designed for a temperature of $2,150^{\circ}F$ in the area before the convection pass.
• Slagged walls act as insulation, driving temperatures higher and pushing the $2,100^{\circ}F$ zone past the reheater, causing superheater deposits and high reheat sprays.
• Soat Blower Types:
  • Wall Blowers: Preventative tools to keep the furnace walls clean.
  • Retracts: Reactive tools used to clean the convection pass once deposits have already formed.
• Soot blower erosion is a major damage mechanism where the steam from the blower wears down the tubes.
• "Sparklers" are visible streaks of unburned coal in the furnace, indicating poor combustion on that side, often linked to imbalances in oxygen or coal flow.

Slag Viscosity and Phase Behavior in Cyclone Furnaces

• The Temperature Critical Viscosity curve identifies the point where slag transitions from liquid to solid.
• Illinois high-sulfur coal has a gradual viscosity curve, giving operators time to respond to changes.
• PRB coal has a very sharp, steep viscosity curve; a small temperature change can cause the slag to freeze, plugging the cyclone.
• Devitrification: When molten minerals (glass) cool, crystals like spinels can come out of solution and lock together, creating a "slush" that refuses to flow.

Fouling and Active Alkalis

• Fouling is primarily associated with sodium ($Na$) and potassium ($K$), known as active alkalis.
• In PRB coal, calcium and sodium are often organically bound to the coal. This is an ion-exchange process similar to a water softener.
• High-calcium, low-sodium coal seams are usually found where water flows through the seam; dry seams produce high-sodium coal.
• Sodium can volatilize and condense on quartz particles, melting their surface and making them sticky (the ping-pong ball dipped in honey analogy). This causes them to adhere to the superheater.

SCR Catalyst and Ammonia Slip

• Selective Catalytic Reduction (SCR) converts Nitrogen Oxides ($NO_x$) into nitrogen and water using ammonia ($NH_3$):  $NO_x + NH_3 \rightarrow N_2 + H_2O$
• Ammonia slip occurs when excess ammonia is injected or when the $NO_x$ level is lower than anticipated.
• Excess ammonia reacts with sulfur gases to form compounds that plug the air heater, causing maintenance headaches.

Questions & Discussion

Question: Does high arsenic exposure cause heavy metal poisoning? Response: Yes, it can be fatal. Rat poison like Decon is about $1\%$ arsenic; industrial samples have been found with as much as $3\%$ arsenic.

Question: What happens if the $\text{NO}_2$ is kept lower than anticipated during ammonia injection? Response: It leads to ammonia slip. Operators often try to do a good job by keeping $NO_x$ extremely low, but if it is lower than the ammonia injection set point, the unreacted ammonia goes through the system and plugs the air heater.

Question: What does Prairie State look like on the slagging chart? Response: It is in the "severe" to "unattainable" range. It was designed for raw Illinois coal with $17\%-19\%$ ash, which is economically difficult to run efficiently due to slagging.

Question: Does washing coal help with slagging? Response: Yes. While washing coal might make the ash fusion temperatures or base-acid ratios look worse on paper, reducing the total amount of ash per million BTU significantly reduces actual slag formation in the boiler.**