Industrial furnaces are widely used to melt metals for casting or heat materials for change of shape (forging) or change of properties (heat treatment).
Mainly, industrial furnaces can be two types based on the method of heating: combustion (using fuels) and electric. Combustion-type furnaces are used for applications such as steel, ceramics, glass and more. They work depending on the type of combustion: oil-fired, coal-fired or gas-fired.
The combustion process carries out ignition with the help of air (21% oxygen) and fuels. Ideally, air and fuel should be mixed thoroughly for proper combustion. There is a defined air-to-fuel ratio (known as Stoichiometric ratio) taken into consideration by the industrial furnace designer. It may vary depending on application and load pattern.
Many furnace operators, however, are incapable of monitoring the process. They are losing significant amounts of energy because of too much air entering the furnace, which results in heat loss through flue gases. The excess air results in oxygen that is not consumed during combustion, and this oxygen absorbs otherwise usable heat and carries it out of the stack.
The chemically ideal amount of air entering into a furnace is just enough for all the oxygen in the air to be consumed. However, this ideal mixture is difficult to reach because fuel and air do not completely mix. Therefore, a certain amount of excess air will always be necessary for complete combustion. In fact, too little excess air results in inefficient burning of fuel, soot buildup and unnecessary greenhouse gas emissions.
The optimum level of excess air will vary based on the furnace and its applications. Generally, excess air of 10-15% is recommended to maintain either the current input temperature or production output level, whichever is desired.
Why Air/Fuel Ratio is Important
In combustion processes, air/fuel ratio is normally expressed on a mass basis. We get maximum useful heat energy if we provide air to the combustion zone at a mass flow rate (e.g., pound/minute, kg/hour) that is properly matched to the mass flow rate of fuel to the burner.
Consider equation 1 for fuel combustion chemistry.
Air is mainly composed of oxygen (21%) and nitrogen (79%). Oxygen in the air combines with the carbon in the fuel at an elevated temperature in the combustion chamber. When burning hydrocarbons, nature strongly prefers the carbon-oxygen double bonds of carbon dioxide and will yield significant heat energy in an exothermic reaction to achieve this CO2 form.
Thus, carbon dioxide is the common greenhouse gas produced from the complete combustion of hydrocarbon fuel. Water vapor (H2O) is also a normal product of hydrocarbon combustion.
Less Air Increases Fuel Waste and Pollution
If the air-to-fuel ratio is lower, enough oxygen is unavailable. This will lead to incomplete combustion with higher unburned fuel/carbon, which means direct fuel waste. As the availability of oxygen decreases, exhaust gases including carbon monoxide will form. As the air/fuel ratio decreases further, partially burned and unburned fuel starts emitting from the exhaust stack as smoke and soot, which creates air pollution.
More Air Loses Fuel and Heat Energy
If the air-to-fuel ratio is higher, too much air is fed into the combustion chamber. This will complete the combustion, and the excess air present, having nitrogen and unwanted oxygen, absorbs heat energy. This causes operating temperature in the chamber to drop, and the heating substrate is unable to extract heat energy. Also, excess hot air generated will escape from the stack, resulting in heat energy losses.
Theoretical (Stoichiometric) Air
Theoretical air-to-fuel ratio is defined as the minimum amount of air and fuel needed to convert hydrocarbon fuel into carbon dioxide and water vapor. Even if we maintain a theoretical air in the real combustion process due to less contact time in the combustion chamber, the air and fuel mix is improper, which ends up in incomplete combustion; hence energy loss and increased pollution levels. With the help of air and fuel online monitoring, however, we can derive a ratio that works with lowest heat loss.
Advantages of Maintaining Correct Air/Fuel Ratio in a Furnace
The main benefits of controlling the air/fuel ratio are:
- Improves combustion and optimizes thermal efficiency
- Saves cost by reducing fuel consumption
- Improves product quality by even temperature profile
- Helps increase productivity
- Reduces pollutants/emissions
To achieve these advantages, flow measurement of air and fuel is vitally important. Air-to-fuel ratio control plays a fundamental role in the efficient and safe operation of fired furnaces. This is because the air-to-fuel ratio in the combustion zone of these processes directly impacts fuel-combustion efficiency and environmental emissions. The amount of excess air within the system can be determined by analyzing the amount of oxygen in the flue gas. When the air/fuel ratio is optimized, the resulting energy savings usually ranges from 5% to 25%.
Important factors while selecting the best flow sensor for furnace applications include:
- Smart sensor with robust design
- Versatile in use for all gas furnace applications
- Minimal pressure drop
- Higher sensitivity and resolution
- Higher accuracy and repeatability
- No maintenance
- Easy cleaning if needed.
Latest Advancements in Furnace Air and Gas Flow Measurement
Conventional flow measurement uses an orifice flowmeter or a turbine flowmeter, which has limitations such as lower accuracy, lower turndown ratio and high pressure drop. Considering the limitations of old flow techniques in furnace applications, a new calorimetric (thermal dispersion) technique is gaining ground for industrial furnace flow measurement. It has a simple differential temperature measurement based on the constant temperature anemometry principle. It has evolved in the past 20 years and become highly sensitive, adjustable and versatile for use with both air and fuel gas.
In addition to flow measurement, other parameters advisable to measure include:
- Exhaust gas temperature at the furnace outlet
- Percentage of oxygen in the combustion gases
- Carbon monoxide
Conclusion
To achieve optimum thermal or combustion efficiency, air-to-fuel ratio control is vitally important. For this reason, excess air (O2%) in furnace exhaust gas (flue gas) should be part of the required control logic.
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