(77b) Think Beyond Efficiency Gain When Adding an Air-Preheater to a Process Fired Heater

Installing an Air-preheater (APH) for a fired heater is a common practice to increase the efficiency of fired heater, thereby reducing the fuel consumption. The fuel saving is primarily due to increase in sensible heat of combustion air which recycles heat from low temperature flue gas to the radiant section of fired heater. Fuel saving also comes from improvement in combustion efficiency. The extent of fuel saving depends on many factors including flue gas temperature, excess air, fuel composition etc.

In many cases of new fired heater installation and revamps, energy efficiency improvement scheme has been looked into isolation rather than analyzing entire fired heater system performance. Air-preheater is recovering the residual low quality heat from cold flue gas and transferring that to combustion air. The heat from air-preheater gets recycled in radiant section, which results into an increase of radiant heat flux and higher NOx emission. This increase in radiant heat flux translates into reduction in fired heater run length, must be carefully evaluated for new fired heaters as well as fired heater revamp. 

For optimum performance of a fired heater, an appropriate APH size & duty selection along with precise fired heater design is very important. Evaluation of every fired heater design parameter is critical therefore any oversight may adversely affect fired heater performance resulting in decreased fired heater capacity, run length, efficiency etc. There are several critical parameters, some of these are listed below, which should be considered and evaluated for a well-designed fired heater with air preheater for a successful run length:

• Evaluate the effect of Air preheater size and duty on radiant heat flux and hence on fired heater run length

• Evaluate the effect of preheated air temperature on adiabatic flame temperature, firebox temperature, NOx emissions and consequently on fired heater run length

• Evaluate the effect of convection section size and heat recovery on heater run length

An integrated analysis of air preheater and fired heater system is essential for a successful operation of fired heater with an air-preheat system. An effort has been made in this paper to summarize and highlight the major parameters related to APH size / duty which will affect the overall fired heater performance. The recommendation and discussion in the paper shall also be emphasized using a case study.