Most of the fuel that plants purchase is converted into heat in furnaces and boilers. So, the efficiency of these units has a big impact on a plant's energy cost. In this and the next column, we'll focus on how to improve the efficiency of process heaters; we'll look at boilers in future columns.
Furnaces either directly heat the process or media like heat transfer fluids and air. Unfortunately, minor details often are overlooked at the design stage.
For example, a chemical plant I visited in Alabama vented considerable amounts of surplus steam. Because the designers missed the excess steam availability, modifying the process heater to utilize that steam would be very expensive. Only a few limited options, like selling steam to a neighboring plant, are available now.
The biggest loss in any process heater is in heat conversion, which normally results in higher stack-gas losses. The two factors that contribute to the stack losses are the quantity and temperature of the gases leaving the heater to the stack or exhaust fan. If these factors are higher than required, stack losses also are higher and heater efficiency is reduced.
Controlling the quantity of stack gases is the most-talked-about savings opportunity for process heaters. However, at 80% of the process plants I visit no significant efforts have been undertaken.
Plants of course supply air instead of oxygen for combustion. Sadly, many forget they're providing four times greater volume of nitrogen than oxygen to the burner. The excess nitrogen gets into the furnace at ambient temperature, picks up heat directly at the flame and leaves the stack at an elevated temperature without contributing anything. Hence, the first step in process heater optimization is to control the quantity of excess air supplied to the burner.
Most burner manufacturers recommend about 10% excess air. Many plants add a further safety margin, increasing this to 20% or more. At some sites, either operators are unaware of excess air levels or burner control systems are too primitive to make changes. In any case, it's worthwhile to evaluate the opportunity to trim excess air.
Burners firing natural gas typically require about 1% excess oxygen (or about 5% excess air) to achieve complete combustion. So, if the level exceeds 2%, the first step is to reduce it. If a heater already is maintained at 2% oxygen in stack gas, it still may be possible to trim further. For some heaters, you can reduce excess air without significant capital investment.
Address the quantity of stack gas by setting up a small team or task force and having it follow a simple four-step action plan:
1. Organize an on-site meeting about the furnace to discuss and finalize its tuning program. If multiple departments have heaters, conduct meetings at each department because each unit may have different characteristics of operation.
2. Initiate a heater-tuning program. Use a portable stack-gas analyzer to validate the readings of any on-line analyzers.
3. Give all relevant operators on-site refresher training on fuel efficiency fundamentals.
4. Start to measure stack condition on a daily basis and create a monitoring database for each heater.
Have an experienced engineer clearly define task details in such a way that operators can follow them easily.
Field-tune each major heater to meet a target operating level — a combustion efficiency exceeding 80% with 1%–2.5% oxygen and near 0% combustibles in the flue gas. This target range is neither new nor unrealistic. The tough part of the task is convincing plant operators to shift from their comfort zone to the optimum operating zone. Seeking the assistance of an external expert may be worth considering in a few cases.
Implementing the above tasks doesn't require big capital investment but instead a strong commitment from the team and plant management. You'll surely notice positive results after a month. Give the team and operators credit by duly reporting to management and publicizing the results.
The results usually are measurable at the end of the first year — and can add up to significant savings if the annual purchased fuel bill exceeds $25 million. Typically, the first year payoff is five times more than the cost of the first year efforts. The program also provides spin-off benefits, particularly a motivated in-house team.
Ven V. Venkatesan is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@Putman.net.