Tips for combustion analysis of commercial boilers
Boilers and burners require a combustion analysis on a regular basis for proper operation. The combustion analyzer measures the air-to-fuel ratio of the burner and calculates the efficiency of the boiler. The analyzer also may display the emissions output from the boiler. Each cubic foot of natural gas burned produces:
- 8 cubic ft. of nitrogen;
- 2 cubic ft. of water; and
- 1 cubic ft. of carbon dioxide.
Imagine we have a 100%-efficient boiler and to achieve that efficiency, our burner requires 10 parts of air to 1 part of gas. It is not realistic or safe to operate at that efficiency, so we increase the supply from 10 to 12 parts of air and now have 20% extra or “excess air.” Our burner efficiency is sacrificed but we have a system that heats without continuous monitoring (see Figure 1).
Most boilers are designed to supply between 12 and 15 parts of air for each part of gas. Some industrial facilities can reduce the air-to-fuel ratio even lower than 20% by using an oxygen or O2 trim system. This system uses a sensor in the flue to monitor oxygen content while the burner is running and constantly adjusts the air-to-fuel ratio.
When do you believe is the best time to perform a combustion analysis: fall, winter or spring? To get the proper readings, the boiler should operate for 15 min. prior to any adjustments to allow the flame to stabilize. When performing the combustion analysis in the fall and spring, the boiler may shut off on temperature or pressure before the adjustments can be completed. I believe the best time to perform a combustion analysis is when the building is under a heavy load, such as mid-winter.
A combustion analysis should also be performed if you clean or replace the blower wheel, replace the blower motor, gas valve or gas pressure regulator.
Oxygen. The air we breathe and use for burner combustion contains about 78% nitrogen, 21% oxygen, and about 1% argon and trace elements of other gases. It is adjusted to the least amount possible for best efficiency. If the burner has too much air or insufficient fuel, it is called lean and lowers the efficiency. If the burner has too little air or too much gas, it is called rich and could lead to sooting and high emissions, including carbon monoxide.
The reading on the combustion analyzer will display the oxygen percentage as a percent of the total flue gas volume. For instance, a reading of 4% O2 means oxygen comprises 4% of the volume of flue gases. A typical reading would be 2% to 6% (see Figure 2). I like adjusting the air for the middle of the acceptable range in case the blower wheel gets dirty and delivers less air for combustion.
- Carbon dioxide. This is a byproduct of combustion and also displayed as a percent of flue gas volume. If you measure only carbon dioxide without considering the oxygen, carbon monoxide could be present in the flue gases. You could have a good efficiency with acceptable CO2 levels and spewing carbon monoxide up the flue. Many burner manufacturers look for a reading of between 8 1/2% to 10% in the flue gas. Carbon dioxide, when mixed with water, forms carbonic acid, which could damage the standard efficiency boiler or chimney.
- Carbon monoxide. This is a dangerous byproduct of combustion generated by incomplete combustion. It is measured in parts per million rather than a percent of flue gas. To see how tiny a part per million is consider that a part per million is one day in 2,739 years or 1 in. in 16 miles. I like to get the carbon monoxide levels as close to zero as possible.
CO air free. When measuring carbon monoxide in flue gases, we use a term called CO air free. It means you subtract the excess air from the calculation to measure the actual CO amount in the flue gases. If the burner has elevated carbon monoxide levels in the flue gases, some techs are tempted to add more air to reduce the carbon monoxide level. For example, you and a friend each purchase a 16 oz. large coffee. You take your coffee black but your friend adds cream and sugar. You both start with the same amount of coffee but your friend’s coffee would be diluted with the cream and sugar.
That is what happens when we add more air to a burner with an elevated carbon monixide level. The CO air free calculation subtracts the cream and sugar from the calculations. The limit in flue gases is 400 ppm CO air free, according to ANSI Z 21.1. If you measure the carbon monoxide without factoring in the air free portion, this is referred to as “as measured” carbon monoxide. To calculate CO air free by using the “as measured” reading, use the following formula:
(See Formula 1 above)
- Excess air. This is the air that is not required for combustion and is added to the fire to assure safe operation. Think of it like the Secret Service protecting the president. Excess air is typically about 20% to 50%.
- Draft. This is a measurement of the flue gas velocity through the boiler. If the draft is excessive, it could pull the flue gases through the boiler too quickly, lowering the efficiency of the boiler. If the draft is too low, damage could occur to the burner. Draft is slightly negative on Category 1 appliances, about -0.05 in. w.c. and should be verified with the manufacturer. Most condensing boilers have a positive flue and require a sealed vent.
- Stack temperature. This temperature is an indication of the heat transfer ability of the boiler. If the stack temperature is too high, the burner is not transferring heat into the boiler. This may be caused by a combination of dirty heating surfaces, over firing, water-side scale buildup, excessive draft or excessive velocity of the water inside the boiler. If the stack temperature is too low, the flue gases will condense and damage the boiler, flue and chimney unless it is a boiler designed for condensing.
- Ambient temperature. This is the temperature of the combustion air for the burner. When calibrating your combustion analyzer, it should be done in the environment where the combustion air is drawn, as close to the burner as possible. If you calibrate the probe outside and the boiler uses air from the boiler room, it will give a false reading. If the boiler uses direct-vented combustion air, I like to calibrate my analyzer in the combustion air duct feeding the burner.
If you have two boilers in the room and one takes outside air and one takes room air, you will have to calibrate your analyzer twice; once for each type of burner.
It’s important to note that when completing your adjustments, document your findings to protect yourself and your company. This can be done by either using the analyzer printer (Figure 3) or a picture on your cell phone. I like leaving a copy of the combustion report in the boiler room for future comparison.