Combustion analysis and why CO readings are not always correct

Have you ever compared your CO readings to another analyzer?  You might have noticed that Testo CO reading is lower than expected.  Why such a difference?  The secret lies in the Testo’s addition of the NOx gas filter.

In residential and light commercial combustion analysis there are several important combustion gases that are measured or monitored. Carbon monoxide (CO), however, is arguably the one gas with the most liability associated with it. This is due to safety concerns and simply from the standpoint of good combustion performance. But NOx is also present and can affect the CO readings, it’s the carbon monoxide levels that you just tested that will keep you up at night.

While performing a combustion test, the O2 and CO are right in front of you on the analyzer display, but it’s easy to overlook all the other gases flowing through the flue pipe,  like CO2, nitrogen, NOx, SO2, water vapor, and so on. It’s these other gases that need to be accounted for when testing furnaces, boilers heaters and other combustion sources. Why, you ask are these other gases important? Some of them can change the output of sensors, and it’s important to know this when you have critical decisions or combustion adjustments to make. This way you are making them based on good information.

How Electrochemical Sensors Function
Electrochemical (EC) sensors rely on the chemical reaction between the combustion gases and the chemicals/materials inside the sensor. As the targeted gas, (i.e., CO) comes in contact with the sensor materials, a reaction takes place. Specifically, between the metal electrodes and chemicals which in-turn will create an electrical output. Technically speaking, this process is call a redox reaction (oxidation or reduction) depending upon the sensor. When the flue gas component (i.e., CO) is no longer present, the chemical reaction and electrical output stops.

The technology behind an EC sensor is well established and is designed to detect both low and high levels of a specific gas through different sensor designs and materials. The key to a quality EC sensor is its’ ability to isolate a single measurement parameter while remaining unaffected by the wide range of other components in the flue gases.

How NOx Affects CO Sensor Readings
If a sensor within a combustion analyzer responds to gases other than what it’s intended to measure, an incorrect measurement will be displayed. This will occur when a CO sensor - without a NOx scrubbing filter - is exposed to flue gas. All flue gas contains some concentration of NOx (for simplification, NOx refers to Nitrogen Oxide). The concentration of NOx tends to be higher with elevated temperatures and low O2 concentrations. When NOx enters the CO sensor, additional reactions take place that increase sensor output.  It is impossible to know how much of the output is due to CO and how much is the result of NOx. 

Many government weatherization programs and performance testing guidelines require corrective action when specific levels of CO are detected. This can prove to be time consuming and costly when these corrective actions are based on potentially false CO readings.

CO Sensor Design
The design of a CO sensor should include a way to eliminate the cross interference effect to other flue gases, namely Nitrous Oxide (NO).  If the CO sensor is designed incorrectly, the CO measurement will be wrong. For example, if the  CO sensor is exposed to flue gas containing 100ppm CO and 200ppm NO (remember NO may not be displayed or measured on the combustion analyzer), the CO sensor will react with a positive output for CO and an additional positive output for the NO. As such, the CO sensor will calculate 100ppm of CO and also calculate an additional 50-100ppm of NO, resulting in a displayed value of CO as much as 200ppm.  As a result, technicians will attempt to mechanically correct the burner to lower CO levels. The correction will be wrong and will negatively impact burner/system performance.

Testo’s Sensor Design
For decades, Testo has used CO sensors with integrated NOx filters as a standard design. These filters “scrub” and remove NOx from the gas stream. Ultimately, this prevents NOx from ever reaching the CO sensor. This NOx filter is designed to last longer than the life of the sensor so there is no maintenance required when used under normal operating conditions. In some of Testo’s  analyzers the filters can be replaced in the field. Go to http://www.testousa.com/combustion/ to see more of Testo’s line of combustion analyzers.

Confidence in Testo Sensors
Filtering NOx from the combustion gas stream before it goes through the sensor is fundamental to achieving accurate CO readings. Without a NOx filter the validity of the CO reading will always be in question.

If there is ever a concern in your CO readings, it’s easy to confirm proper function by using calibration gas. Connect and flow NO calibration gas to your analyzer. The CO sensor should show 0ppm. If a filter is not present, expect to see a 25%, to over 100%, value of the NOx gas in your CO reading. Although this procedure is easy to do, most contractors don’t carry calibration gas. So to eliminate the questions, use the proper sensor, and analyzer for accurate combustion testing.

NOx NOx "Who's There?" - A Blog Post by Ed Voytovich

"Ellie." "Ellie who?"  "The Elephant in the living room."

The elephant in our room is NOx, and elephants eat a lot.  Then they poop.  A lot. We need to watch where we step.

As technicians following the BPI Standards, we test for CO in the flue as well as in the ambient air as part of any audit where there are combustion appliances.  Imagine my surprise when my new Testo 327 combustion analyzer gave me flue gas content results that seemed impossibly low . . . in some cases zero. This was something I very rarely – if ever – saw with my trusty Bacharach equipment in the past.

Check out the complete blog post by Ed Voytovich on Home Energy Pros discussing NOx and how he uses his 327 oxygen analyzer.

Understanding Stoichiometric Combustion

Stoichiometric, or theoretical, combustion is the ideal combustion process where fuel is burned completely. To calculate the excess air or excess fuel for a combustion system, the stoichiometric air-fuel ratio is determined. The stoichiometric ratio is the ideal fuel ratio where the chemical mixing proportion is correct. When burned, all fuel and air is consumed without any left over.

Furnaces, boilers, and process heating equipment cannot run at the stoichiometric ratio. "On-ratio" combustion used in boilers and high temperature process furnaces usually incorporates a modest amount of excess air (about 10 to 20% more) than what is needed for the fuel to completely burn. If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon monoxide exhausts from the boiler will result in heat transfer surface fouling, pollution, lower combustion efficiency, flame instability, and a potential for explosion.

To avoid unsafe conditions, which are far worse than the lost efficiency associated with some excess air, boilers normally set to operate on the fuel-lean side of the combustion curve. This approach will also accommodate variations in the fuel-air control system.

What is NOx?

NOx is a term used to describe nitrogen oxides. NOx is a reactive gaseous compound that is one of the criteria air pollutants regulated by the USEPA Clean Air Act. The two primary oxides of nitrogen involved in air pollution are NO and NO2. Nitric oxide (NO) is colorless and essentially odorless. Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent smell. NOx (NO & NO2) can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections. NOx is also a precursor to the formation of ground level ozone (smog) and a contributor to acid rain.

NOx emissions are produced during the combustion of fuels. The primary man-made sources of atmospheric NOx include; area mobile sources (cars and trucks), off-road sources (construction and agricultural machines, trains, etc.) and stationary sources (power plants, industrial boilers & heaters, etc,).

In a combustion process, three types of NOx can be formed:

• Thermal NOx: From high temperature combustion in the presence of free oxygen.
• Fuel NOx: From the nitrogen bound in the fuel when it is oxidized during combustion.
• Prompt NOx: From the combustion flame and the ambient nitrogen in the ambient air.

To learn more about how testo products measure NOx, check out testo350.com.

What are Electrochemical sensors?

Electrochemical sensors are devices that measure flue gas constituents (O2, CO, NO, NO2, SO2, H2S) through the principle of ion selective potentiometry The sensor contain a electrolytic matrix that is designed for a specific gas to be detected. Two or three electrodes (again gas specific) are placed in this matrix and an electrical field is applied. Flue gas enters the sensor and chemically reacts (oxidation or reduction) on the electrode releasing electrically charged particles (ions). This reaction causes the potential of this electrode to rise or fall with respect to the counter electrode. With a resistor connected across the electrodes, a current is generated which is proportional to the concentration of gas present. The output is converted then displayed as a concentration (typically in ppm, percent, or as a mass unit (i.e. lbs/hr or mmbtu).

Standard electrochemical sensors are affected by various environmental factors including: temperature, pressure, and other combustion gases. However, testo sensors are designed to eliminate these effects. Mounted on each sensor is a circuit board that contains calibration data, linearity data, and other critical information. This technology enables the sensor to be pre-calibrated at testo and installed in the field as a simple plug-in device. No need to have calibration gases on site. Additional technological advancements in sensor and analyzer design include the use of:

• Continuous temperature compensation to provide accurate response regardless of ambient temperature fluctuations.
• Control of sample pressure and automatic flow rate monitoring to eliminate pressure related effects.
• Sensor temperature control by using heated thermoplastic sensor blankets.
• Integrated on-board interference filter media that eliminates the hassle of replacing NOx beads
• Automatic cross compensation for interference gases.

To learn more about testo's complete line of products, check out testo350.com.