Managing Coal Quality Variations: Impacts on Boiler Combustion & Effective Solutions

In recent years, due to the diversification of coal types, coal quality has undergone significant changes. The prevalence of diverse coal types, inferior coal quality, and coal quality severely deviating from the design coal specifications has led to a substantial increase in labor intensity for workers, a significant rise in the failure rate of auxiliary boiler equipment, a deterioration in the working environment, heightened difficulties in environmental protection work, and operational challenges in boiler combustion, making it difficult to meet boiler outlet temperature standards.

The Coal Combustion Process:
From entry into the furnace until complete combustion, coal generally undergoes four stages:

1. Moisture Evaporation Stage: When the temperature reaches approximately 150°C, all moisture is evaporated.
2. Volatile Matter Ignition Stage: As coal continuously absorbs heat and the temperature rises, volatile matter is released. When the ignition point temperature is reached, the volatile matter ignites. Volatile matter combustion is fast, typically accounting for only about 1/10th of the total coal combustion time.
3. Coke (Char) Combustion Stage: After the volatile matter burns away, the remaining solid material composed of carbon and ash is coke (char). At this point, the coke temperature rises rapidly due to intense combustion of the fixed carbon, releasing a large amount of heat. The combustion speed and burnout degree of coal mainly depend on this stage. The goal is to completely burn the carbon in the ash residue to minimize incomplete combustion heat loss.

Conditions for Good Combustion:

1. Temperature: Higher temperatures accelerate chemical reactions and combustion. Furnace temperatures are typically between 900°C and 950°C.
2. Air: Faster air velocity scouring the carbon surface improves contact between carbon and oxygen, speeding up combustion.
3. Time: Coal must have sufficient residence time within the furnace to burn completely.

During carbon combustion, an ash envelope forms around the particle. Carbon monoxide (CO) and carbon dioxide (CO₂) generated during combustion diffuse outward through this envelope. The CO can encounter oxygen and continue burning to form CO₂. This means that around a burning carbon particle, enveloped by ash, there exists a layer of CO and CO₂ gases. Airborne oxygen must penetrate this outer layer to reach the carbon. Therefore, increasing the air supply and the velocity at which air scours the carbon particle helps carry away this outer gas layer. Simultaneously, enhancing turbulence disrupts the ash envelope, promoting direct contact between oxygen and carbon and accelerating combustion. Insufficient oxygen or inadequate agitation leads to incomplete combustion, resulting in unburned carbon particles in the slag and the emission of unburned CO with the flue gas.

Larger fuel particles require longer combustion times; insufficient residence time leads to incomplete combustion. Therefore, in actual operation, measures such as supplying ample oxygen, using secondary air for agitation, maintaining high combustion temperatures, and avoiding excessively low air volumes are generally adopted to ensure thorough fuel combustion.

1. Requirements for Coal Types:
Coal types have certain selectivity. Coal with a volatile matter content above 15%, an ash fusion temperature above 1250°C, weak caking properties, moderate particle size, and a calorific value above 3500-4500 kcal/kg is most suitable.

• Coal Ash Content: Should be controlled between 10-30%.
• Coal Particle Size: Particles 0-15mm should not exceed 50-55%; particles 0-10mm should be controlled below 30%. The maximum coal particle size should not exceed 20mm.
• Coal Moisture Recommended Values:
  • When particles <3mm constitute 20-40% of the total, moisture should be controlled at 8%.
  • When particles <3mm constitute 80% of the total, moisture should be controlled below 7%.
  • When particles <3mm constitute 100% of the total, moisture should be controlled below 6%.

Common Coal Quality Issues: High ash content; non-uniform particle size; presence of significant impurities; low calorific value; difficulty in ignition; variable moisture content; poor combustibility; high carbon content in slag.

Generally, boilers perform best using the design coal or coal closely resembling it to ensure stable combustion. Significant differences in coal quality greatly impact stable boiler combustion and normal heating operation.

2. Impact of Coal Quality on Stable Boiler Combustion:

• Calorific Value: A key indicator of coal quality. When the calorific value drops below a certain level, it not only causes unstable and incomplete combustion but can also lead to boiler flameout, making it very difficult to meet outlet temperature standards and affecting normal operation.
• Volatile Matter: Volatiles release and burn at relatively low temperatures. The heat released rapidly increases the temperature of the char particles, providing highly favorable conditions for their ignition and combustion. Furthermore, the release of volatiles increases the internal porosity and external reactive surface area of the char, enhancing its combustion rate. Therefore, higher volatile matter content means less hard-to-burn fixed carbon, easier burnout, more pores from volatile release, increased reactive surface area, and faster combustion reactions. Low volatile content significantly raises the ignition temperature, increases the heat required for ignition, prolongs the time needed to reach combustion conditions, reduces combustion stability, shifts the flame core upwards, decreases heat absorption by radiant heating surfaces, increases heat absorption by convective heating surfaces, raises the exhaust gas temperature, and increases exhaust heat loss.
• Ash Content: Ash absorbs heat during combustion without releasing any. Higher ash content means lower calorific value, leading to difficulties and delays in ignition, lower furnace temperatures, reduced coal burnout, increased unburned carbon in fly ash, and higher carbon content in the ash. Carbon particles may become encapsulated by ash, reducing their surface combustion rate and flame propagation speed, resulting in poor combustion. Additionally, increased fly ash concentration accelerates wear on boiler heating surfaces, increases dust removal load, raises the physical heat loss of fly ash and slag, and reduces boiler efficiency.
• Particle Size: Significantly impacts boiler combustion. Oversized particles have insufficient residence time; the char within doesn’t burn completely, increasing the carbon content in slag and the physical loss from slag. Excessively fine particles are easily entrained by air, preventing good contact between carbon and oxygen for reaction. These fine particles are also quickly carried away by flue gas, increasing fly ash loss. Therefore, air volume must be reasonably adjusted based on coal particle size.
• Moisture Content: Within certain limits, moisture has a similar impact on ignition characteristics as volatile matter. A small amount of moisture is beneficial for ignition. From a combustion kinetics perspective, in high-temperature flames, water vapor acts as a catalyst, enhancing char combustion, raising flame temperature, and strengthening radiant heat transfer to water-cooled walls. Hydrogen molecules and hydroxyl radicals generated from water vapor dissociation also increase flame conductivity. However, excessive moisture content increases the heat required for ignition. Furthermore, part of the combustion heat is used to heat and vaporize the water, lowering the flue gas temperature within the furnace. This reduces the temperature of the entraining flue gas and the radiant heat from the flame to the char, which is detrimental to ignition.
• Impurities: Not only absorb heat from combustion, hinder contact between coal and oxygen, affect combustion, reduce boiler efficiency, and increase slag/ash removal volumes, but also pose significant hazards to safe boiler operation.

3. Impact of Coal Quality on Boiler and Auxiliary Equipment Operation:
When coal entering the furnace deviates significantly from the design or reference coal specifications, it adversely affects boiler combustion and auxiliary equipment:

• Poor coal quality makes boiler ignition and operational adjustments difficult, causing unstable combustion, easy flameout, and seriously affecting the boiler’s ability to meet outlet temperature standards.
• Furnaces are prone to forming clinkers (slagging), causing severe wear on heating surfaces like water walls and superheater tubes. Ash buildup increases air resistance, reducing boiler thermal efficiency.
• Burning coal with high ash content not only affects dust collector efficiency but also increases the operational load on ash removal and disposal systems, leading to operational failures and adversely affecting both the working and external environments.
• Coal with high carbon content can lead to high-temperature corrosion on water walls and low-temperature corrosion in the rear flue, economizer, and air preheater, causing boiler tube bursts and jeopardizing safe operation.

4. Recommended Countermeasures:
Faced with the reality of coal quality changes causing boiler operational difficulties, proactive testing and exploration during actual operation are essential. Formulate corresponding, highly operational countermeasures to optimize stable boiler combustion, ensure outlet temperature standards are met, reduce auxiliary equipment failures, and guarantee the safety, stability, and economical operation of the entire system.

Specific Measures:

1. Enhance Operator Skills: Improve the technical operation level of boiler operators. Ensure they are promptly informed of the analysis results of coal entering the furnace, especially calorific value, volatile matter, ash content, and particle size, to make corresponding combustion adjustments for different coal qualities.
2. Optimize Coal Blending: Strengthen techniques for blending, co-firing, and proportioning different coal types. Through continuous combustion adjustments and trials, explore optimal operating parameters (like air distribution) for burning different coals. Experiment with blending, co-firing, and proportioning techniques to improve the utilization of various coals, especially inferior coal, reducing heating operational costs.
3. Refine Combustion Control: Ensure proper coal-to-air ratio and thorough mixing. Maintain high furnace temperatures to facilitate combustion. Adjust boiler load according to regulations. Monitor furnace draft (pressure), exhaust gas temperature, oxygen (O₂) content, carbon dioxide (CO₂) content, etc., to keep boiler operating parameters at optimal levels. Adjust the boiler for issues like incomplete or uneven combustion caused by non-uniform particle size.
4. Improve Coal Handling Management: Store different coal types separately whenever possible. Burn specific coal types or specific blends as needed during different periods. Coal handlers should communicate timely with boiler operators on duty. For coal with low moisture or extreme particle sizes (too fine/too coarse), methods like appropriate stirring or adjusting crusher settings should be employed.
5. Strengthen Inspection and Maintenance: Enhance patrol inspections and maintenance of boiler combustion equipment and auxiliary devices. Promptly troubleshoot faults in auxiliary equipment, especially boiler sealing, coal feeders, cyclones/separators, superheaters, economizers, air preheaters, etc.
6. Control Air Supply and Furnace Temperature: Maintain higher furnace temperatures, which favor coal ignition and burnout. Lower furnace temperatures are detrimental to combustion.
7. Control Main Steam Pressure and Temperature: Operate at high pressure and temperature close to design parameters whenever possible, favoring turbine power generation. High pressure and temperature at certain loads, combined with reduced main steam flow, contribute to energy savings.
8. Strengthen Coal Quality Testing Management: Implement effective measures to ensure timely and accurate coal test results are provided to relevant departments. This enables management to select operational adjustments best suited to the specific plant conditions, minimizing the adverse impact of coal quality changes on boiler combustion. Achieving optimized boiler operation not only improves overall economic efficiency but, most importantly, ensures the entire system operates normally, safely, stably, and economically.