Flue Gas Analysis and Efficiency Testing

Flue Gas Analysis and Efficiency Testing

Flue gas analysis measures combustion products (O₂, CO₂, CO, NOx) and stack temperature to evaluate combustion efficiency, diagnose burner performance issues, and verify emissions compliance. Comprehensive combustion analysis enables calculating thermal efficiency via stack loss method (typical results 80-92%), optimizing air-fuel ratio for target oxygen levels (2-4% O₂ typical), identifying incomplete combustion through CO measurement (<50 ppm target), and trending performance to detect fouling or misadjustment. Modern portable combustion analyzers provide real-time measurements enabling field adjustment, while permanent continuous emissions monitoring systems (CEMS) track performance and document regulatory compliance for industrial installations.

Measured Parameters

Stack Temperature

Measurement location:

• Downstream of heat exchanger
• Minimum 8 duct diameters from outlet
• Centerline of stack
• Above condensation temperature (>250°F for gas, >300°F for oil)

Temperature measurement devices:

Type K thermocouple:

• Range: 0-2000°F
• Accuracy: ±0.75% or 2°F
• Response time: 1-5 seconds
• Most common for combustion testing

RTD (resistance temperature detector):

• Range: 0-800°F typically
• Accuracy: ±0.5°F
• Slower response than thermocouple
• Better long-term stability

Stack temperature significance:

Gross stack temperature: Absolute temperature measured in stack.

Net stack temperature: Temperature rise above combustion air:

$$\Delta T_{stack} = T_{stack} - T_{air}$$

This is the temperature relevant to efficiency calculations.

Typical stack temperatures:

Oxygen (O₂) Analysis

Measurement principle:

Zirconia sensor:

• Operating temperature: 600-1500°F
• Electrochemical reaction generates voltage proportional to O₂ partial pressure difference
• Fast response: 5-15 seconds
• Accuracy: ±0.1-0.3% O₂
• Most common for combustion control

Electrochemical sensor:

• Operating temperature: 300-600°F
• Galvanic cell generates current proportional to O₂
• Response time: 10-30 seconds
• Accuracy: ±0.2-0.5% O₂
• Used in portable analyzers

Paramagnetic analyzer:

• Laboratory/CEMS quality
• Accuracy: ±0.1% O₂
• Expensive, high maintenance
• Rarely used in field

Oxygen content interpretation:

Dry basis vs. wet basis:

• Dry: Water vapor removed before measurement
• Wet: Water vapor included
• Most analyzers measure dry O₂

Typical O₂ ranges:

• 0-1%: Too little excess air, incomplete combustion, CO formation risk
• 1.5-4%: Optimal range for most burners
• 4-7%: Acceptable but lower efficiency

• 7%: Excessive excess air, efficiency loss

Excess air from O₂:

$$EA% = \frac{O_2}{21 - O_2} \times 100$$

CO₂ measurement (alternative to O₂):

Natural gas stoichiometric CO₂: 11.8% (dry basis)

Actual CO₂:

$$CO_2% = \frac{CO_{2,max}}{1 + EA}$$

For natural gas:

$$CO_2% = \frac{11.8}{1 + EA}$$

Relationship between O₂ and CO₂ (natural gas):

$$O_2 + 0.5 \times CO_2 \approx 10.5% \text{ (for natural gas)}$$

Carbon Monoxide (CO) Analysis

Measurement principle:

Electrochemical sensor:

• CO oxidizes at electrode, generates current
• Linear response 0-2000 ppm typical
• Accuracy: ±5% of reading or ±10 ppm
• Response time: 30-60 seconds

Non-dispersive infrared (NDIR):

• IR absorption at 4.6 μm wavelength
• High accuracy: ±1% of reading
• Used in laboratory and CEMS
• Expensive

CO significance:

CO formation mechanisms:

1. Insufficient oxygen (inadequate combustion air)
2. Poor air-fuel mixing
3. Flame quenching (cold surfaces)
4. Incomplete residence time

Acceptable CO levels:

• <50 ppm: Excellent combustion
• 50-100 ppm: Acceptable for most applications
• 100-400 ppm: Needs adjustment

• 400 ppm: Poor combustion, immediate correction required

Air-free CO:

Some analyzers report “air-free CO” corrected to 0% O₂:

$$CO_{air-free} = CO_{measured} \times \frac{21}{21 - O_2}$$

This normalizes readings to account for dilution air.

Nitrogen Oxides (NOx)

Measurement principle:

Chemiluminescence:

• NO reacts with ozone, produces light
• Light intensity proportional to NO concentration
• NO₂ converted to NO upstream
• High accuracy, used for CEMS and testing
• Expensive, requires calibration gas

Electrochemical sensor:

• Portable analyzer option
• Lower accuracy than chemiluminescence
• Adequate for field verification

NOx reporting:

Concentration basis: Typically reported in ppm by volume (dry basis).

Corrected to standard O₂:

Regulations specify reference O₂ (3% for boilers):

$$NOx_{@3%O_2} = NOx_{measured} \times \frac{18}{21 - O_{2,measured}}$$

Example: 45 ppm NOx measured at 5% O₂

$$NOx_{@3%O_2} = 45 \times \frac{18}{16} = 50.6 \text{ ppm}$$

Emission rate (mass basis):

$$E_{NOx} = NOx_{ppm} \times \frac{Q_{input}}{K}$$

Where $K$ = Conversion factor (depends on fuel, approximately 10⁶ for natural gas in units of ppm·Btu/h per lb/h)

Combustion Efficiency Calculation

Stack Loss Method

Overall efficiency:

$$\eta = 100% - L_{stack} - L_{radiation} - L_{incomplete}$$

Stack loss (dominant term):

$$L_{stack} = \frac{(T_{stack} - T_{air}) \times C_p \times m_{flue}}{Q_{input}}$$

Simplified stack loss using flue gas CO₂:

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

Where $K$ = fuel constant:

• Natural gas: $K = 0.65$
• No. 2 oil: $K = 0.54$
• Propane: $K = 0.63$

Example calculation:

Natural gas burner:

• Stack temperature: 450°F
• Air temperature: 70°F
• CO₂: 9.0%

$$L_{stack} = 0.65 \times \frac{450 - 70}{9.0} = 27.4%$$

Alternative oxygen-based stack loss:

Siegert formula:

$$L_{stack} = A_1 \times \frac{T_{stack} - T_{air}}{CO_{2,max} - CO_2} + A_2 \times \frac{CO}{CO_2}$$

For natural gas: $A_1 = 0.66$, $CO_{2,max} = 11.8%$

Radiation and convection loss:

Function of boiler/furnace size and surface area:

• Small residential (<350,000 Btu/h): 2-4%
• Commercial (1-10 MMBtu/h): 0.5-2%
• Large industrial (>10 MMBtu/h): 0.2-1%

Measured by:

$$L_{rad} = \frac{Q_{input,on} - Q_{input,off}}{Q_{input,on}}$$

Requires comparing input with burner cycling.

Incomplete combustion loss:

$$L_{incomplete} \approx 10.2 \times \frac{CO_{ppm}}{CO_{2%}}$$

For CO = 100 ppm, CO₂ = 9%:

$$L_{incomplete} = 10.2 \times \frac{100}{9} = 0.11%$$

Usually negligible if CO <100 ppm.

Total efficiency example:

$$\eta_{total} = 100 - 27.4 - 1.0 - 0.1 = 81.5%$$

Efficiency Optimization

Target oxygen level:

Trade-off between stack loss and incomplete combustion:

• Lower O₂ → less excess air → lower stack loss
• But too low O₂ → CO formation → incomplete combustion loss

Optimal O₂ determination:

1. Measure efficiency at several O₂ levels (1%, 2%, 3%, 4%, 5%)
2. Plot efficiency vs. O₂
3. Select O₂ giving maximum efficiency with CO <50 ppm

Typical optimal O₂:

• Natural gas: 2.5-3.5%
• No. 2 oil: 3.0-4.0%
• No. 6 oil: 3.5-5.0%

Efficiency sensitivity:

Approximately 1% efficiency change per:

• 40°F stack temperature change
• 1% O₂ change (at typical stack temperatures 350-450°F)

Portable Combustion Analyzers

Analyzer Components

Typical portable analyzer includes:

1. Sampling probe:

   • Stainless steel construction
   • Thermocouple integrated
   • Sintered metal filter prevents particulate damage
   • Length: 12-48 inches

2. Sample conditioning:

   • Water trap condenses and removes moisture
   • Particulate filter
   • Sample pump draws flue gas

3. Sensors:

   • O₂: Electrochemical or zirconia
   • CO: Electrochemical
   • NO/NO₂: Electrochemical (if included)
   • Stack temperature: Thermocouple

4. Display and processor:

   • Digital display of measurements
   • Calculates efficiency automatically
   • Data logging capability
   • Printer output (some models)

Measurement Procedure

Pre-test preparation:

1. Verify analyzer calibration (fresh air O₂ should read 20.9%)
2. Check battery charge
3. Prepare sampling location (drill hole if needed)
4. Allow burner to reach steady-state operation (15-30 minutes)

Sampling procedure:

1. Insert probe: Position in stack centerline, downstream of heat exchanger
2. Purge analyzer: Allow 60-120 seconds for readings to stabilize
3. Record ambient temperature: For net temperature calculation
4. Record readings: O₂, CO, CO₂, stack temperature
5. Calculate efficiency: Analyzer performs calculation
6. Document results: Record firing rate, fuel type, date, time

Multiple point sampling:

For large stacks, traverse multiple points:

• Minimum 3 points: Center, 25% and 75% of diameter
• Average readings for accurate result

Diagnostic Interpretation

High O₂ (>5%):

• Excessive combustion air
• Air leak in furnace (post-combustion air infiltration)
• Fuel valve restricted (low fuel flow)
• Corrective action: Reduce air damper opening, check for air leaks

Low O₂ (<2%):

• Insufficient combustion air
• Air damper stuck or restricted
• Fuel valve leaking (excessive fuel)
• Corrective action: Increase air damper opening, check air supply

High CO (>100 ppm):

• Insufficient oxygen
• Poor air-fuel mixing
• Flame impingement on heat exchanger
• Burner needs cleaning
• Corrective action: Increase air, inspect burner, clean heat exchanger

High stack temperature (>500°F gas, >600°F oil):

• Excessive firing rate
• Heat exchanger fouled (soot, scale)
• Insufficient heat transfer surface
• Bypass damper open
• Corrective action: Clean heat exchanger, check dampers, verify proper sizing

Low stack temperature (<250°F gas, <300°F oil):

• Excessive excess air (dilution)
• Oversized boiler (cycling)
• Heat exchanger very clean (good condition)
• May indicate condensation risk if <180°F
• Corrective action: Reduce excess air if too high

Continuous Emissions Monitoring Systems (CEMS)

System Components

CEMS for combustion sources:

1. Sample extraction system:

   • Heated sample line (prevents condensation)
   • Sample probe with filter
   • Sample pump
   • Pressure and temperature compensation

2. Gas conditioning:

   • Refrigerated condenser removes water
   • Particulate filter
   • Sample pressure control
   • Flow meter

3. Gas analyzers:

   • O₂: Zirconia or paramagnetic
   • CO: NDIR
   • NOx: Chemiluminescence
   • SO₂: NDIR (if applicable)

4. Data acquisition system (DAS):

   • Records all measurements
   • Calculates emission rates
   • Stores data per regulatory requirements (often 5 years)
   • Provides reports

Regulatory Applications

EPA regulations requiring CEMS:

40 CFR Part 60 (NSPS - New Source Performance Standards):

• Large boilers >250 MMBtu/h
• Requires continuous monitoring of O₂, NOx, opacity

40 CFR Part 75 (Acid Rain Program):

• Electric utility boilers
• NOx, SO₂, CO₂ monitoring required
• Very stringent data quality requirements

State/local air quality districts:

• Vary by jurisdiction
• May require CEMS for sources >10-50 MMBtu/h in non-attainment areas

CEMS data reporting:

1-hour average: Rolling average reported Daily calibration: Automated zero and span gas checks Quarterly audits: Relative accuracy test audit (RATA) Annual reports: Submitted to regulatory agency

Quality Assurance

Calibration procedures:

Daily calibration drift (CD):

• Automated zero and upscale gas checks
• Performed every 24 hours
• Acceptance criteria: <5% of span

Linearity check:

• Quarterly
• Inject 3-5 calibration gases spanning range
• Verify linear response

Relative Accuracy Test Audit (RATA):

• Annual or quarterly (depends on source)
• Reference method testing concurrent with CEMS
• Calculate relative accuracy:

$$RA = \frac{|\text{CEMS}{avg} - \text{Reference}{avg}|}{\text{Reference}_{avg}} \times 100%$$

Acceptance: RA <10% typical (varies by pollutant)

Data validation:

• Invalid data flagged (out of range, failed calibration)
• Substitute data methods per regulation
• Downtime tracked and reported

Field Testing Procedure Summary

Standard Combustion Test

Objective: Measure combustion efficiency and verify proper burner operation.

Equipment needed:

• Portable combustion analyzer
• Drill and hole saw (if no test port exists)
• Manometer (verify draft)
• Thermometer (measure ambient air temperature)

Test procedure:

1. Pre-test verification:

   • Burner operating ≥15 minutes (steady state)
   • Verify normal operating conditions
   • Note firing rate or manifold pressure

2. Measurements:

   • Ambient air temperature
   • Stack temperature
   • O₂ or CO₂
   • CO
   • Draft (if applicable)

3. Calculations:

   • Net stack temperature
   • Excess air
   • Combustion efficiency
   • CO air-free

4. Documentation:

   • Record all readings
   • Calculate efficiency
   • Compare to baseline or specification
   • Recommend adjustments if needed

Acceptance criteria:

• Efficiency ≥80% (standard boiler), ≥90% (condensing)
• CO <100 ppm (preferably <50 ppm)
• O₂ in range 2-5% depending on burner type
• Stack temperature appropriate for equipment type

Frequency:

• Commissioning: Required
• Annual: Recommended minimum
• Quarterly: Best practice for critical equipment
• After service: Always verify combustion

Burner Tuning Optimization

Objective: Adjust air-fuel ratio for maximum efficiency with safe operation.

Procedure:

1. Measure baseline:

   • Record O₂, CO, stack temp, efficiency at current settings

2. Adjust air damper:

   • Reduce air slightly (close damper 5-10%)
   • Wait 2-3 minutes for stabilization
   • Record new readings

3. Find optimal point:

   • Continue adjusting air in small increments
   • Monitor CO: Must stay <100 ppm
   • Find lowest O₂ with acceptable CO
   • This gives maximum efficiency

4. Verify across firing range:

   • Check combustion at low, mid, high fire (modulating burners)
   • Adjust cam or control curve if needed
   • Ensure O₂ and CO acceptable across full range

5. Set oxygen trim (if equipped):

   • Enable O₂ trim control
   • Set target O₂ setpoint
   • Verify trim maintains setpoint ±0.3%

6. Final verification:

   • Record final settings
   • Calculate efficiency improvement
   • Document in maintenance log

Safety note: Never adjust to achieve O₂ <1.5% or CO >100 ppm for sake of efficiency. Safety and complete combustion always paramount.