Flue Gas Monitoring
Force of Gravity
October 8, 2022Environmental & Combustion Controls
October 8, 2022
Flue Gas Monitoring
Monitoring the flue gases on a boiler or burner makes sense ecologically as well as economically.
Reduced fuel cost and higher efficiency makes the return-on-investment on flue gas analysis very short.
Instruments for monitoring flue gases range from simple handheld devices, to installed Oxygen sensors to complete racks of equipment!
Handheld instruments are used for both system maintenance and for commissioning.
Why measure flue gas?
Combustion flue gas analysis has been used by Plant Operators for decades as a method of optimizing fuel/air ratios. By measuring the amount of excess oxygen (O2) and/or carbon monoxide (CO) in the flue gases resulting from combustion, plant operators can operate at the best heat rate efficiency, lowest NOX, and also generate the least amount of greenhouse gas. The theoretical ideal, or the stoichiometric point, is where all fuel is reacted with available oxygen in the combustion air, and no fuel or O2 is left over.
To ensure complete combustion of the fuel used, combustion chambers are supplied with excess air. Excess air increases the amount of oxygen and the probability of combustion of all fuel.
The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than the heat provided by more efficient combustion.
Typical excess air to achieve highest efficiency for different fuels are:
- 5 - 10% for natural gas
- 5 - 20% for fuel oil
- 15 - 60% for coal
Carbon dioxide - CO2 - is a product of the combustion and the content in the flue gas is an important indication of the combustion efficiency.
An optimal content of carbon dioxide - CO2 - after combustion is approximately 10% for natural gas and approximately 13% for lighter oils.
Toxic Gases
Some combustion processes give off toxic gases such as Sulphur Dioxide (SO2) and NOX. It is ideal to test for the presence of these toxic gases so we can avoid using contaminated fuels or we can adopt procedures to clean the flue gas prior to release into the atmosphere.
NOX is a generic term for the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). They are produced from the reaction of nitrogen and oxygen gases in the air during combustion, especially at high temperatures.
Emissions of SO2 are proportional to the sulphur content of the fuel being used. Coal and other fossil fuels may have various levels of SO2. SO2 emissions can be generated during combustion.
Toxic gases are more complicated to calibrate and test as the reference gas used to certify these sensors is very expensive and has a short shelf life – your instrument may need to be returned to the manufacturers for annual calibration of SO2 and only some laboratories in NZ can calibrate NO2 and NO sensors.
Selecting Product for Flue gas Analysis
Hand held Flue Gas Analyser (FGA)
A handheld FGA will typically measure O2 and/or CO2 and sometimes other toxic gases such as Nitric Oxide (NO) and Carbon Monoxide (CO).
Various technologies are used for the gas measurement e.g. Catalytic, Infrared and others. It’s worth giving some thought to these as the cost of replacement of these varies, as does the serviceable life and these will impact the total cost of ownership.
A handheld FGA includes a hand-held probe which is inserted into the flue stack at an appropriate point. A sampling pump in the instrument draws the flue gas and measures it
Some instruments include a manometer for checking draft or leaks at commissioning.
When selecting a FGA some manufacturers do not use oxygen sensors in their meters. They install CO and CO2 sensors and calculate oxygen from these two parameters. This is smart technology because the oxygen sensor in handheld flue gas analysers typically only last 2-3 years and are expensive to replace so cost of ownership can be dramatically reduced using a FGA that calculates oxygen.
Installed permanently in the flue, a ZrO2 Oxygen sensor (Zirconium and Platinum) converts oxygen levels to (typically) a 4 to 20 mA signal which is read on a display or into a SCADA system.
This is a very common method for a permanent installation.
Installed complete systems that can measure up to 5 flue stack gases such as NOX, SO2, CO, CO2 and O2 are usually specified & quoted to system requirements.