Flue gas analysis directly measures the chemical composition of combustion exhaust — providing the data needed to calculate combustion efficiency, identify tuning opportunities, verify emission limits, and extend burner and heat exchanger life in boilers, furnaces, and industrial combustion equipment. In Singapore, boiler operation is regulated by the Energy Market Authority (EMA) under the Energy Conservation Act, and air emissions are controlled by the National Environment Agency (NEA) under the Environmental Protection and Management Act (EPMA). Both regulatory frameworks create obligations that flue gas analysis helps facilities meet.
Why Flue Gas Analysis Matters
Combustion is never perfectly efficient. The three fundamental inefficiency mechanisms are:
- Stack losses — sensible heat carried away in hot flue gases instead of being transferred to the process. Reducing excess air (the main lever) lowers stack temperature and raises efficiency.
- Incomplete combustion losses — unburned fuel leaving the stack as CO, unburned hydrocarbons, or particulate. This occurs when excess air is too low or mixing is poor.
- Radiation and convection losses — heat lost through the boiler casing; less controllable through combustion tuning.
The optimal combustion condition balances these two competing effects — enough air to burn all fuel completely, but not so much excess air that heat is wasted carrying it through the stack. Flue gas analysis quantifies this balance precisely and enables data-driven burner adjustment.
Gases Measured in Flue Gas Analysis
Oxygen (O2)
The residual oxygen in the flue gas is the primary indicator of excess air. At stoichiometric combustion (exactly the right amount of air for complete combustion), O2 in the flue gas is theoretically zero. In practice, burners operate with some excess air to ensure complete combustion. The relationship between flue gas O2 percentage and excess air percentage depends on the fuel type. For natural gas, approximately 3% O2 in the flue gas corresponds to roughly 15% excess air — a commonly targeted operating condition for industrial boilers.
Carbon Dioxide (CO2)
CO2 concentration in the flue gas rises as combustion becomes more complete and excess air decreases. Maximum CO2 (the theoretical maximum for the fuel) indicates stoichiometric combustion. For natural gas, maximum CO2 is approximately 11.7%; for fuel oil, approximately 15–16%. Monitoring CO2 alongside O2 provides a cross-check on combustion completeness.
Carbon Monoxide (CO)
CO in the flue gas indicates incomplete combustion — fuel carbon is being oxidised only partially to CO rather than fully to CO2. CO rises sharply when excess air falls below the minimum needed for complete combustion, or when burner mixing is poor. The CO breakthrough point is the inflection where CO rises rapidly with decreasing excess air. Optimal combustion sits just above this point, balancing efficiency against incomplete combustion losses. CO measurement also has direct safety significance — high CO in the flue gas can indicate that CO is leaking into the occupied space if heat exchanger integrity is compromised.
Nitrogen Oxides (NOx)
NOx (primarily NO and NO2) is formed in the combustion zone at high temperatures. NOx emissions are regulated by Singapore's NEA under the EPMA and its subsidiary Environment Protection and Management (Air Impurities) Regulations. The maximum permissible emission limits for boilers and furnaces vary by fuel type, equipment age, and rated heat input — facilities must refer to NEA's licencing conditions. Reducing excess air and peak flame temperature (through staged combustion, flue gas recirculation, or low-NOx burners) reduces NOx formation.
Sulphur Dioxide (SO2)
SO2 is produced from the combustion of sulphur-containing fuels — fuel oil, coal, and some process gases. NEA limits SO2 emissions from industrial sources. Singapore's progressive shift to natural gas as the dominant industrial fuel has reduced SO2 as a widespread concern, but heavy oil users and facilities burning waste or biomass still need to monitor and report SO2 emissions. Fuel sulphur content directly determines the SO2 emission rate.
Flue Gas Analysers: Types and Operating Principles
Portable Combustion Analysers
Portable flue gas analysers are the standard tool for commissioning, maintenance, and energy auditing of boilers, furnaces, and industrial heaters. They draw a sample of flue gas through a probe inserted into the flue, condition the sample (cool, dry, and clean it), and analyse it using electrochemical sensors (for CO, NOx, SO2) and an electrochemical or zirconia cell sensor for O2.
Modern portable analysers calculate and display combustion efficiency, excess air percentage, CO2 concentration (calculated from O2 and fuel type), flue gas temperature, and energy loss as a percentage — all in real time. Leading instrument manufacturers in this space include CS Instruments, Testo, and Kane International. View CS Instruments products available from Unitest Instruments.
For boiler maintenance under Singapore's EMA Energy Conservation Act reporting obligations, a calibrated portable analyser with a printed or digital report is the standard evidence format.
In-Situ Zirconia Oxygen Analysers
In-situ zirconia probes measure O2 directly in the flue gas stream without sample extraction or conditioning. A zirconia cell at the probe tip generates a voltage proportional to the ratio of O2 in the flue gas to O2 in a reference air stream. They respond quickly (seconds rather than minutes), require minimal maintenance compared to extractive systems, and are well suited to continuous combustion control feedback loops in large boilers and process heaters.
Continuous Emission Monitoring Systems (CEMS)
Facilities with large combustion sources or operating under NEA licence conditions requiring continuous emission reporting must install CEMS — automated, continuous monitoring systems that measure stack emissions 24/7 and report data to NEA via the Compliance Monitoring and Surveillance (CMS) system. CEMS must meet the performance specifications in NEA's guidance and be calibrated regularly using certified reference gases.
Calculating Combustion Efficiency
Combustion efficiency (as opposed to thermal efficiency, which also accounts for blowdown and other losses) is calculated from the flue gas O2 or CO2 content and the flue gas temperature relative to inlet air temperature. Modern analysers perform this calculation automatically using the Siegert or ASME PTC 4 method. The Siegert method is most common in European-origin instruments; ASME PTC 4 is used in American-origin standards.
A simplified guide to flue gas O2 targets for natural gas-fired equipment:
| O2 in Flue Gas (%) | Approximate Excess Air (%) | Efficiency Implication |
|---|---|---|
| 0–1% | 0–5% | Risk of incomplete combustion; CO likely rising |
| 2–4% | 10–20% | Near-optimal — target for well-tuned equipment |
| 5–7% | 25–40% | Moderate excess air; efficiency loss of 1–3% |
| 8–12% | 40–70% | High excess air; significant stack losses |
| Above 15% | Above 100% | Very high excess air; serious efficiency problem |
Singapore Regulatory Requirements for Combustion Monitoring
Singapore's Energy Conservation Act (ECA), administered by EMA, requires mandatory energy management for large energy consumers (registered corporations with annual energy consumption above 54 TJ per year). Key obligations include:
- Appointing a certified energy manager
- Submitting annual energy use reports
- Developing and implementing energy efficiency improvement plans
Combustion equipment (boilers, process heaters, furnaces) typically represents a major share of industrial energy consumption. Demonstrating combustion efficiency through documented flue gas analysis supports ECA compliance and the Singapore Green Plan 2030 targets.
NEA's EPMA and the Environment Protection and Management (Air Impurities) Regulations set emission limits for NOx, SO2, CO, and particulate matter from combustion equipment. Facilities with licenced emission sources must conduct regular emission tests (stack tests) using approved methods and submit results to NEA. These tests use calibrated sampling trains and analysers whose calibration must be documented and traceable.
Calibration of Flue Gas Analysers
Electrochemical sensors in portable flue gas analysers have limited operating lives (typically 1–3 years per sensor type) and require calibration at intervals specified by the manufacturer — typically every 6 to 12 months for regulatory use. Calibration uses certified calibration gas mixtures with known concentrations of each target gas traceable to national standards.
For CEMS used in NEA-mandated continuous monitoring, calibration procedures and intervals are specified in the NEA performance specifications. Zero and span calibration checks are typically required daily or weekly using certified reference gases, with full multi-point calibration at longer intervals.
Unitest Instruments provides calibration services for gas analysers and associated instrumentation. Our SAC-SINGLAS accreditation (LA-2023-0845-C) covers the relevant measurement disciplines. Contact our team for calibration scheduling and instrument supply enquiries.
For related energy efficiency measurement context, see our article on compressed air leak detection and energy savings and our guide to power quality analysis.