Automatic Fuel-Burning Equipment

Automatic Fuel-Burning Equipment

Automatic fuel-burning equipment encompasses oil burners, gas burners, and dual-fuel systems that provide controlled, efficient combustion for boilers, furnaces, process heaters, and industrial heating applications. Modern burner technology focuses on achieving high thermal efficiency (80-95%), minimizing emissions (NOx < 9-30 ppm, CO < 50 ppm), ensuring safe operation through sophisticated flame safeguard systems, and maintaining optimal air-fuel ratios across varying firing rates. Burner selection requires comprehensive analysis of fuel characteristics, firing rate requirements, turndown ratio needs, efficiency targets, emission compliance, and control system integration.

Combustion Fundamentals

Stoichiometric Combustion

Natural gas (methane) combustion:

$$\text{CH}_4 + 2\text{O}_2 + 7.52\text{N}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + 7.52\text{N}_2$$

Stoichiometric air requirement:

$$A_s = 9.52 \text{ ft}^3 \text{ air/ft}^3 \text{ CH}_4 \text{ (at standard conditions)}$$

No. 2 fuel oil combustion:

$$\text{C}{12}\text{H}{23} + 17.375\text{O}_2 + 65.4\text{N}_2 \rightarrow 12\text{CO}_2 + 11.5\text{H}_2\text{O} + 65.4\text{N}_2$$

Stoichiometric air requirement:

$$A_s = 1368 \text{ ft}^3 \text{ air/gal oil} \approx 13.9 \text{ lb air/lb oil}$$

Excess Air Requirements

Actual combustion requires excess air beyond stoichiometric to ensure complete oxidation:

$$\text{Excess Air (%)} = \frac{A_{actual} - A_s}{A_s} \times 100$$

Typical excess air by burner type:

• Atmospheric gas burners: 40-50%
• Power gas burners: 10-20%
• Pressure atomizing oil burners: 15-25%
• Rotary oil burners: 10-15%
• Dual-fuel burners: 15-25%

Relationship to oxygen content:

$$\text{O}_2% = \frac{21 \times \text{Excess Air}}{100 + \text{Excess Air}}$$

For natural gas with 15% excess air:

$$\text{O}_2% = \frac{21 \times 15}{115} = 2.74%$$

Adiabatic Flame Temperature

Maximum theoretical combustion temperature with no heat loss:

$$T_{flame} = T_{air} + \frac{HHV}{C_p \times (1 + EA) \times A_s}$$

Where:

• $HHV$ = Higher heating value of fuel (Btu/unit)
• $C_p$ = Specific heat of combustion products (Btu/lb·°F)
• $EA$ = Excess air fraction
• $A_s$ = Stoichiometric air (lb/unit fuel)

Typical adiabatic flame temperatures:

• Natural gas: 3600°F (0% excess air) → 3100°F (15% excess air)
• No. 2 oil: 3750°F (0% excess air) → 3200°F (15% excess air)
• No. 6 oil: 3800°F (0% excess air) → 3250°F (15% excess air)

Combustion Efficiency

Thermal Efficiency Calculation

Overall burner/boiler efficiency:

$$\eta_{total} = 100 - L_{stack} - L_{radiation} - L_{incomplete}$$

Stack loss (dominant loss mechanism):

$$L_{stack} = \frac{(T_{stack} - T_{air}) \times C_p \times m_{gas}}{HHV \times m_{fuel}} \times 100$$

Simplified form using flue gas CO₂ or O₂:

$$L_{stack} = K \times \frac{T_{stack} - T_{air}}{CO_2%}$$

Where $K$ = Fuel constant (0.65 for natural gas, 0.54 for No. 2 oil)

Alternative oxygen-based stack loss:

$$L_{stack} = \frac{(T_{stack} - T_{air})}{T_{stack} - 450} \times \frac{% O_2}{21 - % O_2}$$

Radiation and convection loss:

• Small boilers (<10 MMBtu/h): 1-3%
• Medium boilers (10-100 MMBtu/h): 0.5-1.5%
• Large boilers (>100 MMBtu/h): 0.2-0.8%

Incomplete combustion loss: Indicated by CO and smoke. Should be <0.5% for properly operating burners.

Efficiency Example

Natural gas burner operating conditions:

• Stack temperature: 450°F
• Combustion air temperature: 70°F
• Flue gas O₂: 3.5%
• CO: 10 ppm (negligible)

Stack loss calculation:

$$L_{stack} = \frac{450 - 70}{450 - 450} \times \frac{3.5}{21 - 3.5} \approx 20% \text{ (using graphical method)}$$

Simplified: For natural gas, approximately 1% efficiency loss per 40°F stack temperature rise above ambient, per % O₂.

$$L_{stack} \approx \frac{380°F \times 3.5%}{40} \approx 19.5%$$

Total efficiency:

$$\eta_{total} = 100 - 19.5 - 1.0 - 0.2 = 79.3%$$

Burner Types Overview

Air-Fuel Ratio Control

Control Methods

On-off positioning:

• Fixed linkage between air damper and fuel valve
• Single air-fuel ratio setting
• Accuracy: ±15-20% of optimal
• Applications: Small burners (<1 MMBtu/h)

High-low-off positioning:

• Two discrete firing rates
• Two air-fuel ratio settings
• Low-fire typically 30-50% of high-fire
• Applications: Residential and light commercial

Fully modulating:

• Continuous firing rate adjustment
• Mechanical linkage or electronic positioning
• Jackshaft systems: Air damper and fuel valve mechanically linked
• Parallel positioning: Electronic actuators maintain ratio
• Accuracy: ±5-10% across firing range

Cross-limiting:

• Air leads on increasing fire
• Fuel leads on decreasing fire
• Prevents dangerous fuel-rich conditions
• Essential for large burners (>10 MMBtu/h)

Oxygen Trim Control

Closed-loop control system maintains optimal excess air:

Components:

• Oxygen sensor (zirconia or electrochemical)
• Control algorithm (PID)
• Air damper actuator or VFD fan control

Target setpoint:

• Natural gas: 2.0-3.5% O₂
• No. 2 oil: 2.5-4.0% O₂
• No. 6 oil: 2.5-4.5% O₂

Efficiency improvement:

$$\Delta \eta = L_{stack,high} - L_{stack,optimized}$$

Reducing excess air from 30% (5.5% O₂) to 15% (2.7% O₂) at 450°F stack temperature improves efficiency by approximately 2-3%.

Trim response:

• Response time: 10-30 seconds
• Update frequency: 1-10 seconds
• Stability: ±0.2-0.5% O₂

Flame Safeguard Systems

Modern flame safeguard controls provide safe burner startup, operation, and shutdown:

Startup sequence:

1. Pre-purge: 15-60 seconds (4 air changes minimum)
2. Pilot ignition: 3-10 seconds
3. Pilot flame establishment: 4-10 seconds
4. Main fuel valve opening
5. Main flame establishment: 4-10 seconds

Flame detection methods:

• Cadmium sulfide (CdS) cell: Detects visible light (residential/light commercial)
• Ultraviolet (UV) scanner: Detects UV radiation 1850-2900Å (commercial/industrial)
• Flame rod (ionization): Detects flame conductivity (gas only)
• Infrared (IR) scanner: Detects specific IR wavelengths (high-reliability applications)

Safety lockout conditions:

• Flame failure during operation
• Flame not established within trial-for-ignition period
• Loss of atomizing medium (steam/air atomizing burners)
• Low fuel pressure
• High/low combustion air pressure
• Unsafe flue draft conditions

Emissions Characteristics

NOx Formation Mechanisms

Thermal NOx: Formed at high temperatures (>2800°F) via Zeldovich mechanism:

$$\text{Rate} \propto e^{-E_a/RT} \times [\text{O}_2]^{0.5} \times [\text{N}_2]$$

Doubles approximately every 100°F above 2800°F.

Fuel NOx: Oxidation of nitrogen compounds in fuel (significant for residual oils, coal).

Prompt NOx: Formed in fuel-rich flame zones via hydrocarbon radical reactions (minor contributor).

Emission Reduction Techniques

Flue gas recirculation (FGR):

• Recirculates 10-30% of flue gas to combustion air
• Reduces flame temperature by 200-400°F
• Reduces O₂ concentration
• NOx reduction: 40-70%

Staged combustion:

• Fuel staging: Primary zone fuel-rich, secondary zone fuel-lean
• Air staging: Primary zone sub-stoichiometric, secondary air downstream
• NOx reduction: 50-80%

Low-NOx burner design:

• Internal FGR
• Delayed mixing
• Distributed combustion
• NOx reduction: 50-85% vs. conventional

Selective catalytic reduction (SCR):

• Post-combustion ammonia injection
• Catalyst bed at 600-800°F
• NOx reduction: 80-95%
• Expensive; used when <9 ppm required

Burner Sizing Methodology

Required Firing Rate

Heating load:

$$Q_{burner} = \frac{Q_{load}}{\eta_{system}}$$

Where:

• $Q_{load}$ = Building/process heating load (Btu/h)
• $\eta_{system}$ = System efficiency (burner × distribution × heat exchanger)

Warm-up capacity:

$$Q_{warmup} = \frac{M \times C_p \times \Delta T}{t_{warmup} \times 3600}$$

Where:

• $M$ = Mass of system water/structure (lb)
• $\Delta T$ = Temperature rise (°F)
• $t_{warmup}$ = Allowable warmup time (hours)

Burner capacity:

$$Q_{burner} = \max(Q_{load}, Q_{warmup}) \times SF$$

Safety factor $SF$ = 1.10-1.25 for boilers, 1.15-1.30 for process heaters.

Turndown Requirements

Minimum burner turndown ratio:

$$TD = \frac{Q_{max}}{Q_{min}} \geq \frac{Q_{design}}{Q_{summer}}$$

Typical requirements:

• Boilers: 4:1 to 10:1
• Process heaters: 3:1 to 8:1
• High-efficiency modulating systems: 10:1 to 25:1

Section Components

This section provides detailed analysis of:

1. Oil burner types: Gun-type pressure atomizing, rotary cup, air atomizing, and steam atomizing systems with atomization theory and sizing
2. Gas burner types: Atmospheric, power, premix, and low-NOx configurations with flame stability and mixing analysis
3. Dual-fuel systems: Combination burners, automatic changeover, and fuel interchangeability
4. Burner controls: Flame safeguard programming, positioning controls, and safety interlocks
5. Combustion air: Supply requirements, fan sizing, and air temperature effects
6. Flue gas analysis: Efficiency optimization, continuous emissions monitoring, and diagnostic techniques
7. NOx reduction: FGR systems, SCR catalysts, and ultra-low NOx burner design

Sections