In Part 1, we discussed how the quantity of air supply to a burner contributes to the stack loss of a heater. The second factor that causes such losses is the stack temperature. A few steps may reduce the impact of this and help save energy at the process heaters.
It's very common to see stack temperatures in excess of 400°F in process heaters that were designed prior to 1980. Some process heaters that are recently designed operate with stack temperatures of around 250°F. Approximately 1% additional fuel is consumed for every 40°F rise in the stack temperature at the same process heating load.
Step 1: Always keep the process heater's heat recovery surfaces clean. Many operators think that their heater is not designed with sufficient heat recovery surface to reduce the stack temperature. It's true that more heat recovery surface reduces the stack temperature, but few recognize that the heat recovery surface should be cleaned. Adding soot blowers or better maintaining and utilizing the existing soot blowers are simple actions that operating personnel can take.
If steam soot blowers are very difficult to maintain, consider options like sonic soot blowers. It is very common to see about 5°F-to-8°F rise in stack temperature after each 8-hr. shift in a process heater that fires liquid fuel. In such heaters, efforts to keep the heat recovery surfaces clean pays back nicely.
At a petroleum refinery in the Caribbean, the simple addition of soot blowers saved more than 3% of the heater's fuel use. The biggest benefit was the absence of smoky flue gas leaks that were experienced when no soot blowers were at the air-preheaters and convection section.
Step 2: Do a pinch analysis and look for heat sinks in the processes nearby. In the past, the inlet process temperature to a heater set the limit of the lowest stack temperature reachable. Many times the process inlet temperature itself used to be higher than 350°F and hence the stack temperatures exceeded 400°F. However, with developments like pinch technology, where multiple heat sources and heat sinks are integrated, it is possible to recover more heat from the existing process heaters. Most established engineering consulting firms presently offer pinch analysis software programs. Though software programs help, certain obvious opportunities need not wait for a full pinch analysis.
In one of the heaters at a client site, we replaced its low pressure (LP) steam generating coil with an economizer coil to preheat the fresh makeup water from ambient to moderate temperatures before sending it to the deaerator. The stack temperature of the heater dropped from 400°F to about 310°F. Also the deaerator's reduced LP steam use more than compensated the LP steam generation loss from the old coil. Hence, looking for better heat sink options or considering an additional heat sink may reduce the stack loss further.
Step 3: Explore the options for alternative use of low level heat. In process plants, finding suitable heat sink to recover additional heat from heater stacks is always a challenge. Sometimes process plants may need additional cooling. In such plants, it's possible to improve process cooling from the excess heat recovered from the stacks of process heaters by installing an absorption chiller. The chilled water from that chiller may improve process cooling.
In a Wyoming refinery, we came across a situation where process condensers cooled by tower water were insufficient to recover all the light ends from the refinery fuel gas stream — especially in the summer. Our recommendation to install an absorption chiller to complement the condenser cooling was well received. In that refinery, LP steam is generated by additional heat recovery from the process heaters, and the excess LP steam is utilized to run the absorption chiller. Absence of critical non-moving parts in absorption chillers keeps their maintenance costs low.
Many more steps may be possible, but these three would definitely help improve the heat recovery from stack gases in process heaters. The stack loss reduction through additional heat recovery is usually more significant than the excess air control.
Ven V. Venkatesan is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@Putman.net.