Higher efficiency means lower emissions with fired heaters
- Economizers and air preheat systems raise thermal efficiency
- Economizers capture heat from the flue gases to raise the process fluid temperature so that it takes less energy to heat it
- Air preheat systems push ambient air into an exchanger positioned in the exhaust stack where it gets heated by the hot flue gases, the hot air is forced into the burner to aid in the combustion process
A typical fired heater’s thermal efficiency is between 75% to 80% low heating value (LHV). Thermal efficiency is the ratio between the useful output of a device and its input. In other words, in the case of a fired heater, it’s the ratio between how much heat is produced by the burner and how much of that heat is used in the process. The unused heat is wasted and released into the atmosphere as flue gas. Flue gases contain gases and particles that are regulated by government agencies to manage the impact on the environment. According to the second law of thermodynamics, it’s impossible to achieve 100% efficiency, but there are ways to increase thermal efficiency and therefore reduce emissions.
Emissions are measured in two different ways; particles per million (PPM) and pounds per million BTU. PPM is dependent on combustion efficiency. The number of harmful emissions over time can be measured in tons. While choosing the right burner to meet PPM requirements is imperative, you can also increase the efficiency of the heater to lower the overall amount of harmful emissions released per year.
For example, if you have a 10 million BTU heater running at 80% efficiency eight hours a day five days a week, the heater would emit about 1 ton of NOx per year. If you increase the efficiency of that heater by just 5%, it would emit 6% less NOx per year. You would also save about 6% in fuel costs per year. Raising the efficiency of a heater is done with different technologies and good design.
Technologies Help Raise Thermal Efficiencies
A couple of technologies used to raise the thermal efficiency of fired heaters are economizers and air preheat systems. Economizers are heat exchangers that capture heat from the flue gases and put that energy back into the process. The process fluid, a heat transfer fluid or a gas/liquid heated directly, flows through the economizer before it reaches the main heating section. This process helps raise the process fluid temperature so that it takes less energy to heat it in the main heating section. More of the flue gas is used and therefore less waste escapes into the atmosphere. An added benefit of an economizer is it helps burn less fuel and saves on fuel costs. An economizer can raise the thermal efficiency of a fired heater significantly.
Air preheat systems work similarly to economizers, the difference is they are heating air instead of a process liquid. In air preheat systems a blower pushes ambient air into an exchanger positioned in the exhaust stack where it gets heated by the hot flue gases. After going through the exchanger, the hot air is forced into the burner to aid in the combustion process. Using hot air for combustion instead of ambient air uses less energy and uses less fuel. As with economizers, adding an air preheat system can significantly improve the efficiency of the system.
If you have a fired heater that doesn’t have an economizer or air preheat system it can be retrofitted to the heater. First, an evaluation of the system and process would need to take place to make sure there are not any compatibility issues and that it makes sense economically.
Heater Design Important for Thermal Efficiency
While technologies to aid in thermal efficiency are significant, the design of the heater is equally important. Standard designs and stock units might get the job done, but a custom-designed unit in most cases will be the most efficient. Undersized heaters won’t be able to achieve desired outputs. Oversized heaters can be quite efficient, but you would be paying for more heater than you need, and the turndown ratio may not work for your process.
There are a number of factors that go into designing and sizing a heater for both productivity and efficiency. Things like surface area, process fluid composition, flow rates, inlet temperatures, outlet temperatures, fuel composition, ambient conditions, space constraints, government regulations, and standards, etc. all drive the design process of the heater. A custom-designed heater will be sized and components will be chosen to maximize efficiency for the desired application.
Adding an economizer or air preheat system may cost a little more in upfront capital, but over time, having a more efficient system will save you money and help reduce your impact on the environment. Astec designs and builds fired heaters for a variety of industries.