12.2 Steam Generators, Boilers & Combustion Engineering

A steam generator or boiler is a closed pressure vessel designed to transfer heat from combustion products or nuclear reactions into water, generating saturated or superheated steam under controlled pressure. Boiler design directly impacts steam quality, plant safety, fuel consumption, and environmental emissions.

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1. Boiler Classifications & Structural Types

Boilers are categorized by relative fluid placement within internal heating surfaces, operating pressure ratings, firing arrangements, and mobility.

A. Fire-Tube Boilers (Shell Boilers)

In fire-tube boilers, hot combustion flue gases pass inside metal tubes submerged within a water-filled shell.

• Common Configurations: Horizontal Return Tubular (HRT), Scotch Marine (wet-back or dry-back), and Vertical Fire-Tube boilers.
• Operating Parameters: Limited to relatively low pressures (maximum $1.8\text{ MPa}$ or $250\text{ psig}$) and steam generation capacities below $15\text{ tons/hr}$ ($33,000\text{ lb/hr}$). Pressure capability is restricted because large-diameter outer shells require excessively thick walls under high internal pressure ($t = P D / 2 S$).
• Advantages: Large water holding capacity provides thermal storage, making them resistant to sudden load fluctuations.

B. Water-Tube Boilers

In water-tube boilers, water and steam flow inside small-diameter tubes while hot flue gases circulate around external tube surfaces.

• Common Configurations: Straight-tube (Babcock & Wilcox), bent-tube (Stirling, D-type, O-type, A-type package boilers), and once-through supercritical boilers.
• Operating Parameters: Suitable for high-pressure utility applications ($2.0\text{ MPa}$ to $> 30\text{ MPa}$) and capacities exceeding $2000\text{ tons/hr}$.
• Advantages: Rapid steam generation, enhanced fluid circulation, smaller tube diameters capable of safely containing high pressures, and flexible structural arrangements.

C. Package Boilers vs. Field-Erected Boilers

• Package Boilers: Factory-assembled units shipped complete with burner, controls, and auxiliaries mounted on a single skid for immediate industrial installation.
• Field-Erected Boilers: Large utility steam generators assembled on site, featuring multi-story furnace cavities and complex ducting.

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2. Essential Steam Generator Auxiliaries & Internal Components

Modern steam generators incorporate heat recovery systems and safety devices to maximize overall plant efficiency and maintain safe operating envelopes.

Feedwater --> Economizer --> Steam Drum --> Downcomers --> Mud Drum
                                 |                           |
                                 v                           v
                           Superheater <--- Waterwalls <-----+ 
                                 |
                                 v
                          Main Steam Out

• Steam Drum & Mud Drum: The steam drum separates saturated steam from boiling water via internal cyclone separators and chevron mist eliminators. The mud drum at the lowest loop elevation accumulates settled sludge and scale, which is periodically cleared via bottom blowdown.
• Continuous & Intermittent Blowdown: Continuous blowdown removes concentrated dissolved solids (TDS) from near the water level in the steam drum. Intermittent bottom blowdown discharges accumulated heavy sediment from the mud drum.
• Economizer: A tube-bundle heat exchanger located in the exit flue gas duct that preheats incoming boiler feedwater. Preheating feedwater by $5.5^\circ\text{C}$ to $6.0^\circ\text{C}$ increases overall boiler efficiency by approximately $1%$.
• Superheater: Elevates saturated steam temperature above saturation at constant pressure. Radiant superheaters absorb direct furnace radiation; convective superheaters extract heat from high-velocity flue gas streams.
• Air Preheater (APH): Transfers residual exhaust flue gas heat to incoming combustion air. Tubular and rotary regenerative (Ljungström) air preheaters elevate furnace combustion temperatures and enhance flame stability.
• Safety Valves: Direct spring-loaded safety valves required by ASME Boiler Code. Must pop fully open at set point pressure and close within a specified blowdown pressure differential (typically $2%$ to $4%$ below popping pressure).

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3. Boiler Performance Metrics & Calculations

Evaluating boiler thermal output requires standardizing steam parameters to allow performance comparison across varying operating pressures and temperatures.

A. Boiler Thermal Efficiency ($\eta_b$)

$\eta_b = \frac{\text{Useful Heat Absorbed by Steam}}{\text{Heat Released by Fuel}} = \frac{\dot{m}_s (h_s - h_f)}{\dot{m}_f \times HV}$ where $\dot{m}_s$ is steam generation rate (kg/h), $h_s$ is steam enthalpy (kJ/kg), $h_f$ is entering feedwater enthalpy (kJ/kg), $\dot{m}_f$ is fuel consumption rate (kg/h), and $HV$ is fuel heating value (kJ/kg).

B. Factor of Evaporation ($FE$)

The Factor of Evaporation compares the actual heat required to produce 1 kg of steam to the latent heat of vaporization of water at standard atmospheric pressure ($101.325\text{ kPa}$ at $100^\circ\text{C}$, where $h_{fg} = 2257.0\text{ kJ/kg}$ or $970.3\text{ Btu/lb}$): $FE = \frac{h_s - h_f}{2257.0\text{ kJ/kg}} = \frac{h_s - h_f}{970.3\text{ Btu/lb}}$

C. Equivalent Evaporation ($E_e$)

Equivalent Evaporation represents the mass of water that would be evaporated from and at $100^\circ\text{C}$ per hour under standard atmospheric conditions: $E_e = \dot{m}_s \times FE = \frac{\dot{m}_s (h_s - h_f)}{2257.0}\text{ (kg/h)}$

D. Developed Boiler Horsepower ($BHP$)

Historically, one Boiler Horsepower ($BHP$) is defined as the evaporation of $34.5\text{ lb/h}$ of water from and at $212^\circ\text{F}$ ($100^\circ\text{C}$), equivalent to $15.65\text{ kg/h}$ equivalent evaporation. $1\text{ BHP} = 33,475\text{ Btu/h} = 35,316\text{ kJ/h} = 9.8095\text{ kW}$ $BHP = \frac{\dot{m}_s (h_s - h_f)}{35,316\text{ kJ/h}} = \frac{E_e (\text{kg/h})}{15.65}$

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4. Combustion Engineering & Fuel Analysis

Combustion is the rapid high-temperature exothermic chemical reaction between fuel combustibles (Carbon, Hydrogen, Sulfur) and oxygen.

Theoretical (Stoichiometric) Air Requirement

Based on elemental fuel weight fractions ($C, H, O, S$), the theoretical minimum mass of air required to achieve complete combustion per kg of fuel is: $\left( \frac{A}{F} \right)_{stoich} = 11.53 C + 34.34 \left( H - \frac{O}{8} \right) + 4.29 S\text{ (kg air / kg fuel)}$ where $(H - O/8)$ represents net available (uncombined) hydrogen.

Excess Air & Orsat Flue Gas Analysis

To ensure complete combustion, industrial boilers operate with excess air ($10%$ to $25%$ for oil/gas; $20%$ to $45%$ for coal). The Orsat apparatus measures dry flue gas volumetric composition ($CO_2, O_2, CO$). Percent excess air is computed via: $\% EA = \frac{O_2 - 0.5 CO}{0.264 N_2 - (O_2 - 0.5 CO)} \times 100\%$

Higher vs. Lower Heating Value

• Higher Heating Value (HHV): Measured when water vapor in combustion products condenses into liquid at $25^\circ\text{C}$, releasing its latent heat of vaporization.
• Lower Heating Value (LHV): Measured when combustion product moisture remains in the gaseous vapor phase. $LHV = HHV - 9 \cdot H \cdot h_{fg,water} = HHV - 9 H (2442\text{ kJ/kg})$

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5. Worked Thermal Engineering Calculation

Problem Statement: An industrial water-tube boiler generates $15,000\text{ kg/h}$ of superheated steam at $3.0\text{ MPa}$ and $350^\circ\text{C}$ ($h_s = 3115.3\text{ kJ/kg}$) from feedwater supplied at $90^\circ\text{C}$ ($h_f = 376.9\text{ kJ/kg}$). The boiler burns $1,600\text{ kg/h}$ of coal having a Higher Heating Value of $30,000\text{ kJ/kg}$. Calculate:

1. Total heat energy absorbed by steam per hour
2. Boiler thermal efficiency $\eta_b$
3. Factor of Evaporation $FE$
4. Equivalent Evaporation $E_e$ in kg/h
5. Developed Boiler Horsepower $BHP$

Step-by-Step Solution

Step 1: Calculate Useful Heat Absorbed by Steam $\dot{Q}_{absorbed} = \dot{m}_s (h_s - h_f) = 15,000\text{ kg/h} \times (3115.3 - 376.9)\text{ kJ/kg}$ $\dot{Q}_{absorbed} = 15,000 \times 2738.4 = 41,076,000\text{ kJ/h} = 11,410.0\text{ kW}$

Step 2: Calculate Fuel Thermal Heat Input $\dot{Q}_{input} = \dot{m}_f \times HV = 1,600\text{ kg/h} \times 30,000\text{ kJ/kg} = 48,000,000\text{ kJ/h}$

Step 3: Calculate Boiler Thermal Efficiency $\eta_b = \frac{\dot{Q}_{absorbed}}{\dot{Q}_{input}} = \frac{41,076,000}{48,000,000} = 0.85575 = 85.58\%$

Step 4: Calculate Factor of Evaporation (FE) $FE = \frac{h_s - h_f}{2257.0} = \frac{3115.3 - 376.9}{2257.0} = \frac{2738.4}{2257.0} = 1.2133$

Step 5: Calculate Equivalent Evaporation (E_e) $E_e = \dot{m}_s \times FE = 15,000\text{ kg/h} \times 1.2133 = 18,199.5\text{ kg/h}$

Step 6: Calculate Developed Boiler Horsepower (BHP) $BHP = \frac{\dot{Q}_{absorbed}}{35,316\text{ kJ/h}} = \frac{41,076,000}{35,316} = 1163.1\text{ BHP}$ $\text{Alternatively: } BHP = \frac{E_e}{15.65} = \frac{18,199.5}{15.65} = 1162.9\text{ BHP}$