EN 326 Combustion: Comprehensive Notes on Combustion Equipment and Power Plant Systems

Layout and Components of a Typical Thermal Power Plant

• System Identification and Overview:

  • The structure of a modern thermal power plant involves a complex interaction of fuel processing, combustion, heat transfer, and energy conversion systems.
  • Primary Components (14 Key Stages):
    1. Coal conveyer
    2. Stoker
    3. Pulverizer
    4. Boiler
    5. Coal Ash handling
    6. Air preheater
    7. Electrostatic precipitator
    8. Smokestack (Chimney)
    9. Turbine
    10. Condenser
    11. Transformers
    12. Cooling towers
    13. Generator
    14. High-voltage power lines

• Detailed Layout Components:

  • The steam power plant layout includes specific circuits for air, steam, fuel, and water:
    • Economiser: Utilizes flue gases to preheat feed water before entering the boiler.
    • Superheater: Located within the boiler assembly to increase the temperature of steam beyond its saturation point.
    • Feed water pump: Circulates water through the economiser into the boiler tubes.
    • Condenser circulating water pump: Moves water between the condenser and the cooling tower.

Functions of Modern Power Plant Components

• 1. Boiler: The location where water is converted into wet steam through heat provided by fuel combustion.

• 2. Superheater: Receives wet steam from the boiler and converts it into superheated steam to improve thermodynamic efficiency.

• 3. Reheater, Air Heater, and Economizer: Supplementary heat exchange components designed to recover energy from flue gases and optimize the steam cycle.

• 4. Turbine: A machine where high-pressure steam expands to drive mechanical rotation, which in turn drives the generator.

• 5. Condenser: Converts used steam from the turbine exhaust back into liquid water (condensate), which is then recycled as feed water.

• 6. Cooling Tower: Dissipates the heat absorbed by the condenser circulating water into the atmosphere.

• 7. Condenser Circulating Water Pump: Responsible for the continuous loop of water between the condenser and the cooling tower.

• 8. Feed Water Pump: Functions to pump water into the high-pressure environment of the boiler tubes against the existing steam pressure.

• 9. Air Preheater: Uses residual heat in the flue gases (products of combustion) to heat the incoming combustion air, improving thermal efficiency.

• 10. Ancillary Support Facilities:

  • Crusherhouse: For initial coal processing.
  • Coal Mill: For further refinement of fuel.
  • Induced Draught (ID) Fan: Draws flue gases out of the boiler.
  • Ash Precipitators (Electrostatic): Removes particulate matter from flue gases.
  • Boiler Chimney: Vents processed gases to the atmosphere.
  • Forced Draught (FD) Fan: Forces air into the furnace for combustion.
  • Water Treatment Plant: Ensures purity of the water used in the steam cycle to prevent scaling and corrosion.

Thermodynamic Subsystems and Calculations

• Subsystem A (Turbine, Condenser, Pump, Boiler):

  • This core system manages the power generation cycle (Rankine Cycle).

• Energy Balance and Performance Equations:

  • Work done by the Turbine ($\dot{W}_t$):$\dot{W}_t = \dot{m}(h_1 - h_2)$
  • Heat rejected by the Condenser ($\dot{Q}_{out}$):$\dot{Q}_{out} = \dot{m}(h_2 - h_3)$
  • Work required by the Pump ($\dot{W}_p$):$\dot{W}_p = \dot{m}(h_4 - h_3)$
  • Heat input to the Boiler ($\dot{Q}_{in}$):$\dot{Q}_{in} = \dot{m}(h_1 - h_4)$
  • Cooling Water Heat Transfer:$\dot{Q}_{out} \approx \dot{m}_w (h_5 - h_6)$$\dot{Q}_{out} \approx \dot{m}_w c_w (T_5 - T_6)$

Coal Combustion Systems and Equipment

• General Categories of Combustion Applications:

  • 1. Grate Fired Furnaces: Used primarily for solid fuels with a stationary or movable fuel bed called a stoker.
  • 2. Chamber Fired: Includes systems for pulverized coal, liquids, and gases.

• Categories Based on Particle Size:

  • Large particles are burned on fuel beds.
  • Small particles are burned in pulverized form.

• Types of Coal Burning Equipment:

  1. Over Feed Stokers
  2. Under Feed or Retort Stokers
  3. Traveling-grate or Chain-grate Stoker
  4. Pulverized Coal Burners
  5. Cyclone Furnace

Over Feed Stokers and Fuel Bed Zoning

• Operating Principle: Fresh coal is continuously dropped onto the top of the fuel bed.

• Fuel Bed Structure: The thickness varies between $10\,cm$ and $30\,cm$. The bed is organized into four distinct functional zones:

  • Distillation Zone (Top): Raw coal loses volatile matter (VM) through heating.
  • Reduction Zone: Characterized by endothermic reactions where coke reduces $CO_2$ back to $CO$.         $C + CO_2 \rightarrow 2CO$
  • Oxidation Zone: Characterized by exothermic reactions where oxygen reacts with carbon.         $C + O_2 \rightarrow CO_2$
  • Ash Zone (Bottom): Spent fuel remains.

• Air Flow and Chemistry:

  • Primary air enters from below the grate, cooling the ash/grate and absorbing heat.
  • In fuel beds less than $10\,cm$ deep, all oxygen is typically depleted in the glowing (oxidation) region.
  • Gases leaving the glowing region include $N_2$, $CO_2$, $CO$, $H_2$, and $H_2O$.
  • Secondary Air: Supplied at high speed above the fuel bed to create turbulence and ensure rapid combustion of volatile matter and $CO$.

Under Feed (Retort) Stokers

• Operating Principle: Coal and air move in the same direction, forced into the fire from below by a screw feeder or power ram.
• Combustion Process:
  • Air is supplied through holes in the grate and passes through layers of incandescent coke.
  • As coal burns, it moves outward to the sides.
  • Combustion gases meet secondary air for complete burning.
  • Ash is collected in an ash pit, though some unburnt coal may occasionally pass through.

Traveling-Grate and Chain-Grate Stokers

• Design: A mechanical firing device where coal is fed from a hopper onto a moving grate composed of interlocking cast iron bars. Traveling-grate bars are more closely interlocked than chain-grate bars.
• Process: Coal enters at one end; by the time it reaches the opposite end, combustion is complete and ash falls into the pit.
• Advantages:
  • Simple, low-cost construction.
  • Self-cleaning and reliable.
  • Low maintenance.
  • Heat release rate controlled by stoker speed.
  • High heat release rate per unit volume of furnace.
• Disadvantages:
  • Ash fusion or clinkering can cause uneven air distribution.
  • Preheated air temperature is limited to $180^{\circ}C$.
  • Loss of fine coal through the grate.
  • Capacity limit: Not suitable for high-capacity boilers (generally those $> 200\,tonnes/hr$).

Spreader Stokers

• Operation: A feeder drops coal onto a spreader (rotor) that throws coal uniformly onto the grate.
• Air Management: Primary air comes from the bottom. Secondary air is injected through high-velocity nozzles over the fuel bed to create turbulence.
• Advantages:
  • Burns a wide variety of coal types.
  • Reduces clinker and coking difficulties.
  • Allows for high-temperature preheated air.
  • Quick response to load changes due to small fuel bed depth.
  • Efficient combustion of volatile matter.
• Disadvantages:
  • Difficult to operate with variable coal sizes and moisture (effective only for sizes $\leq 40\,mm$).
  • High production of fly ash, necessitating a dust collector.

Pulverized Coal Burners

• Technical Specifications: Over $85\%$ of coal particles must have a diameter less than $0.063\,mm$.
• Operation: Finely ground coal is blown into the chamber by hot primary air, burning similarly to a cloud of liquid droplets.
• Pros and Cons:
  • Advantages: Higher efficiency, great operational flexibility, and capability to use various coal qualities.
  • Disadvantages: High cost associated with pulverizing and the fact that most ash is carried away in the exhaust (fly ash).

Cyclone Furnaces

• Mechanism: Small coal particles ($< 6\,mm$) are burned in suspension.
• Swirl Action: Fuel swirls into the chamber, meeting tangential secondary air and axial tertiary air.
• Ash Handling: High combustion temperatures cause ash to melt. Centrifugal force carries molten ash (slag) to the furnace walls. Since the furnace is inclined, molten slag flows down to a tap hole.

Fuel Oil Burners

• Vaporization and Atomization: Combustion of heavy oils requires breaking the liquid into small droplets (atomization) to ensure rapid evaporation and mixing.

• 1. Rotating Cup Burner:

  • Oil is fed into a cup rotating at $3,500$ to $10,000\,rpm$.
  • Centrifugal force creates a thin film; primary air ($10-15\%$ of theoretical requirements) blasts the film into a fine mist.
  • Flame shape is controlled by the cup geometry and air nozzle position.

• 2. Mechanical or Oil-Pressure Atomizing Burner:

  • Uses high fluid pressure to force oil through an orifice.
  • Oil is preheated to a viscosity of $10$ to $30\,centistokes$.
  • Rotational energy is generated in a conical swirl chamber; half of the pressure is used for this rotation.

• 3. Steam or High-Pressure Air Atomizing Burner:

  • Operates on the "scent spray" principle.
  • Steam preheats the heavy oil, reducing viscosity.
  • Air/steam pressure ranges from $1\,kg/cm^2$ to $7\,kg/cm^2$.

• 4. Low-Pressure Air Atomizing Burner:

  • Operates at air pressures between $0.035$ and $0.15\,kg/cm^2$.
  • Best suited for light, less viscous oils like kerosene.
  • Requires higher primary air amounts ($20\%$ or more).

Gas Burners

• Pressure Type Aerated Burners:
  • Utilize concentric primary air and gas jets under pressure.
  • Large furnaces use multiple jets in a common head.
  • Premixed stoichiometric proportions of gas and air can be burned in tubes or tunnels.
  • Flames can be laminar or turbulent; turbulence is preferred for better mixing in high-heat release applications.

Questions & Discussion

• Question/Prompt: Give the layout of a modern gas fired conventional steam power plant and explain its various circuits.
• Context: The student is directed to visualize the thermal power plant system focus (Subsystem A) and specifically how fuel/air/stack gas processes integrate with the steam cycle components (Turbine, Condenser, Boiler, Pump).
• Question/Prompt: Watch "UNDERFEED STOKER | PRINCIPLE | HOW COMBUSTION TAKES PLACE IN | COAL FIRING | GTU" on YouTube.
• Question/Prompt: Watch "SPREADER STOKER | TYPE OF OVERFEED STOKER | THERMAL POWER PLANT | GTU" on YouTube.