Understanding Combustion Efficiency

Understanding Combustion Efficiency

Combustion efficiency is a measurement of how well the fuel being burned is being utilized in the combustion process. This is different from the efficiency number produced on the analyzer, which is reflective of the total amount of heat available from the fuel minus the losses from the gasses going up the stack. Stack loss is a measure of the heat carried away by dry flue gases and the moisture loss. It is a good indicator of appliance efficiency. The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the appliance, and reflects the energy that did not transfer from the fuel to the heat exchanger. The lower the stack temperature, the more effective the heat exchanger design or heat transfer and the higher the fuel-to-air/water/steam efficiency is. The combustion efficiency calculation considers both the stack temperature and the net heat and moisture losses.  This would include losses from dry gas plus losses from the moisture and losses from the production of CO.  
 
Combustion converts the carbon in the fuel to CO2. For each type of fuel there is a maximum CO2 that can be converted. When you select the fuel in the analyzer, the CO2 is calculated from the fuel type by the percentage of O2 left in the flue gas. Typically for natural gas the ultimate CO2 is 11.7%. This would be achieved when the O2 in the flue gasses was at 0% Some analyzers also allow for the max CO2 to be input by the user if the heat content of the fuel is known.
 
Again, the ultimate CO2 would be derived during stoichiometric combustion in which there is no excess air and no excess fuel present during the combustion process. In reality, in the HVAC industry we are striving not for stoichiometric combustion, but complete combustion in which all hydrogen and carbon in the fuel are oxidized to H2O and CO2. For complete combustion to occur, we have to have excess air, or air supplied in excess of what is needed typically because of poor mixing of the fuel and air during the combustion process. If excess air is not provided we will not have the complete conversion of carbon to CO2, and will end up with the formation of partially oxidized compounds, such as carbon monoxide and aldehydes. While ideal operating range for burners is not as efficient as stoichiometric combustion, it does provide us with an additional factor of safety.
 
The percentage of excess air required is based upon several things including
 

1. Equipment application (commercial, residential, industrial)
2. Expected variations in fuel properties (wobble numbers)
3. Combustion air supply rates and air density
4. Degree of operator supervision required or available (summer winter adjustments)
5. Control requirements such as O2 trim

 

For maximum combustion efficiency low excess air is desirable. For residential furnaces it is typically 50% however additional air may be required for dilution to prevent condensation of the flue gasses. This could be introduced into the appliance after the point of combustion through a draft diverter or as excess air which goes completely through the combustion process.
 
Each type of fuel has specific measurable heat content.  The maximum amount of heat that can be derived from a fuel is based on using pure oxygen as the oxidizer in the chemical reaction and maximizing the fuel gas mixture. In field practice, the oxygen is derived from the air which is 20.9% oxygen, 78% nitrogen and 1% other gasses.  Because the oxygen is not separated from the air prior to combustion, there is a negative effect on the chemical reaction. Air is primarily nitrogen.  While nitrogen is inert, and plays no role in the combustion process, it cools the chemical reaction (burning temperature) and lowers the maximum heat content deliverable by the fuel.  Therefore, it is impossible to achieve combustion efficiencies above 95% for most fuels, including natural gas, when air is used as the oxidizer in the combustion process. 
 
The combustion efficiency or maximum heat content of the fuel is then based upon the quality of the mixture of fuel and air, and the amount of air supplied to the burner in excess of what is required to produce complete combustion.  The efficiency calculated by the combustion analyzer is a modified equation that considers combustion efficiency and stack losses.  It is a part thermal, part combustion efficiency calculation. The equation is a reasonable estimation of the steady state operating efficiency of the appliance. This is true of all analyzers currently manufactured.[1]
 
The entire system (furnace/boiler, ducting, and piping) must be evaluated to determine the true efficiency of the system.  Combustion efficiency is a valuable part of the system evaluation, but it is only one part of the evaluation process and cannot be used as the sole reason or justification for keeping or replacing existing equipment. If the excess air is carefully controlled, most furnaces are capable of performing at higher levels than their rated Annualized Fuel Utilization Efficiency or AFUE level, AFUE levels typically range from 80% to 97% [2]
 
The ultimate thermal efficiency of the appliance is determined by dividing the heat output rate of the appliance by the rate of fuel input.  During the combustion process, all furnaces that operate with the same combustion efficiency will produce the same amount of heat with the same fuel input.  The combustion efficiency has no bearing on how well the appliance utilizes the heat produced after the combustion process has taken place.   Heat exchanger design and its ability to transfer the sensible [3] and possibly the latent [4] heat to the room air determine how well the heat produced by the combustion process is utilized.
 
Net combustion efficiency calculations assume that the energy contained in the water vapor (which is formed as a product of combustion) is recovered and is not exhausted from the flue or stack. For example: a combustion analyzer user will see a net efficiency reading of say 95-97% in a 90 plus furnace, as the secondary heat exchanger is "wringing out" the latent heat of vaporization in the water vapor by condensing it from vapor to liquid. Gross combustion efficiency calculations assume that the energy contained in the water vapor is not recovered. In the example above the gross efficiency (only from burning the fuel) might be 86-88%. Typically the difference between the value of net combustion efficiency and the value of gross combustion efficiency for a natural gas fueled system is around 7-9% with the net value being higher than the gross value.

During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust gasses. Most of the exhaust gas comes from chemical combinations of the fuel and oxygen. When a hydrocarbon-based fuel (Natural Gas) burns, the exhaust gasses include water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust gasses can also include chemical combinations from the oxidizer alone. If the natural gas is burned with air, which contains 21% oxygen, 78% nitrogen and 1% trace gasses, the exhaust can also include carbon monoxide (CO), oxides of nitrogen (NOX, nitrogen + oxygen) and if sulfur is present in the fuel, sulfur dioxide, SO2 (Sulfur + oxygen).
 
The temperature of the exhaust will be high because of the heat that is transferred to the exhaust during combustion. Because of its high temperature, exhaust usually occurs as a gas, but there can be liquid or solid exhaust products as well. Water (H2O) is always present in natural gas and oil combustion in residential furnaces. Soot, which is incompletely burned fuel, is a form of solid exhaust that occurs in some combustion processes.
 
During the combustion process, as the fuel and oxidizer are turned into exhaust products, heat is generated. Interestingly, some source of heat is also necessary to start combustion. Gasoline and air are both present in your automobile fuel tank; but combustion does not occur because there is no source of heat. Since heat is both required to start combustion, and is itself a product of combustion, we can see why combustion takes place very rapidly. Also, once combustion gets started, we don't have to continue to provide the heat source, because the heat produced by the combustion process will keep things going. We don't have to keep lighting a campfire, it just keeps burning.
 
Flue gasses are the gasses produces by burning fuel. These gasses are hot, but have not given up all their heat in the combustion process. Depending on the type of furnace, a certain amount of heat must go out of the flue to prevent the gasses from condensing. With high efficiency furnaces, condensing is desirable because of the additional heat extracted from the flue gasses.
 
A digital combustion analyzer performs all of the mathematical calculations and measurements necessary to determine efficiency, safety, dew point, and the amount of pollution the appliance is producing.  For most technicians, the safety (CO) and efficiency (EFF.) readings will be the most important and most frequently referenced numbers.  When safety or efficiency is compromised, other portions of the chemical reaction (CO2, O2) will be referenced, along with calculated values like excess air, to determine the cause of the problem in the combustion process. Other variables like NOx and SO2 are referenced and controlled to keep them at levels that are safe for the environment and acceptable to the local authority having jurisdiction over these matters.  Some areas do not currently regulate levels of NOx and SO2 and where they are not controlled they are also not typically measured. Usually, larger exhaust sources (higher BTU systems) are targets of NOx and SO2 regulations. (NOTE: Several manufacturers have a full lines of affordable emissions products to measure regulated emissions.)
 
As a service technician, unless a component has failed, there are only three things can be adjusted on a gas/oil appliance that will affect the combustion process. 

• Fuel pressure
• Primary air (on newer furnaces this is not adjustable)
• Draft, which will impact secondary air

Other factors can affect the combustion process.  These include impingement for example from an improperly placed pilot, excess air from a cracked heat exchanger, insufficient combustion air due to tight construction or improper ventilation, an improperly installed venting system, or incorrect orifices.  These are considered defects or installation problems, and require mechanical correction rather than adjustment.  It is the service technician’s responsibility to determine if combustion problems are caused by improper adjustment, incorrect installation, component failure, or equipment defect.  Therefore, it is important that the technician completely understands how each of the subsystems affects the chemical reaction called combustion.
 
It should be noted that there is not a national industry standard for calculating measured efficiency with a combustion analyzer. Manufacturers of analyzers use differing calculations to derive efficiency values.  Oftentimes this discrepancy is due to values that have been extrapolated into the condensing range.
 
Heat removed from the flue gasses on a condensing furnace is latent or hidden heat.  A combustion analyzer that measures only temperature and not volume of condensate cannot measure the quantity of heat removed from the flue gas during the condensing process.  Although terms of thermal and combustion efficiency are often used interchangeably on non-condensing units, they cannot be used in the same manor on condensing appliances.
 
The thermal efficiency of a condensing appliance and combustion efficiency will be different.  The only way to calculate the actual thermal efficiency of an appliance is to measure the exact airflow across the heat exchanger and the change in air temperature across the heat exchanger and input the measured values into the sensible heat formula to calculate the heat energy input into the conditioned air. There will be some minimal loss to the furnace cabinet by radiation and conduction. Depending on how much of the heat energy is extrapolated from the water in the flue gas, an average of 970 BTU per pound, the efficiency readings can differ by as much as 10%.  This assumes that either all latent heat energy was extracted from the flue gasses after they reached the dew point or none of the latent heat energy was extracted. 
 
This extrapolation of values is distorted, and has led manufacturers of appliances to inadvertently post higher than actual thermal efficiency numbers. Due to the readings achieved on their analyzer. (NOTE: This calculation does not affect the AFUE numbers, which are derived by a different means.)  By not taking this discrepancy into account, some in the industry have suggested that fuels are being delivered with low BTU levels.  This leads them to suggest that fuel pressures be raised to provide the net heat output that the manufacturer has published.  For this reason, we recommend that the fuel pressure be set per the manufacturer’s instructions.  The combustion efficiency will then be a function of the actual dry flue gas and not of the thermal efficiency of a condensing appliance.  This avoids use of a calculated rather than a measured parameter.  Some manufacturers have chosen to use a combustion calculation that does not extrapolate the thermal efficiency values of flue gasses below the dew point, as those values are not representative of the heat that is removed from the flue gasses during the condensing process. Although this may result in the appearance of a lower thermal efficiency of the appliance, the science used for measuring combustion efficiency is not artificially high.  Once differences in combustion and appliance thermal efficiency are understood, the methodology of scientific measurement verses extrapolation of measured values can be appreciated and applied, allowing manufacturers to publish combustion and thermal efficiencies that are representative of the actual efficiency of their appliance, thereby creating a standard that is based upon actual measurement rather than an extrapolation.

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[1]Manufacturers use differing forms of combustion equations.  This modified equation is often referred to as combustion efficiency, even though as a matter of pure science it is not.