Introduction and Problem Statement

Process heating systems are some of the largest energy-intensive systems in most industrial facilities and account for approximately 36% of industrial energy consumption.  These systems may include equipment such as furnaces, ovens, heaters, kilns, and lehrs.  They are used in a variety of processes in an industrial energy system to transfer heat from energy sources to the products.  The energy source in these systems may be electricity, fuel (natural gas, fuel oil, etc.), steam, other gases, or a combination of two or more of these sources.  Based on the energy assessments performed at several industrial facilities, the typical energy efficiency of process heating equipment is 15% to 8%.¹ The general losses in a process heating system can be categorized as flue gas loss (exhaust stack), wall loss (through insulation), opening (used for access in the system) loss, fixture/conveyor (for movement of product in and out of equipment) loss, water cooling (for maintaining temperatures) loss, and atmospheric loss (through changes in the inside air/gas or other atmosphere).

This article provides a summary of case studies resulting from an energy assessment of process heating systems at seven large industrial facilities.  The average energy savings in those facilities were estimated to be 13% (range, 6% to 28%).  The top five energy-saving potentials were realized in the following areas (see Figure 1):

1. Recover waste heat from the exhaust stacks to preheat combustion air for the system.
2. Recover waste heat from the exhaust stacks to preheat incoming material, heat the plant in winter, or use it in a waste heat boiler.
3. Adjust air to fuel ratio of the system.
4. Improve insulation of the process heating equipment and distribution pipes. Replace old burners with energy-efficient burners.

Figure 1: Profile of Average Percentage Savings

For each recommendation made to the plants, the estimated implementation cost was used to calculate simple payback period (ratio between implementation cost and energy cost savings).  This cost may include equipment, labor for installing or maintaining the equipment, and/or labor for planning activities.  In most plants, the threshold for the simple payback period was two years. For the top five energy savings recommendations, the simple payback was found to be within this threshold (Figure 2). Several states offer financial incentives and/or low-interest loans for improvements in energy efficiency, but they were not considered in this analysis.  Including those incentives may reduce the simple payback period further.

Figure 2: Profile of Average Simple Payback on Investment (in Months)

Example System and Recommendations

The plant considered in this example had one of the furnaces with a gross fuel input rate of approximately 14.15 million Btu per hour.  The exhaust stack gases for this furnace were analyzed by using a portable combustion analyzer, and a thermography study was conducted with a thermal camera.  Other operating characteristics were analyzed with the appropriate equipment.  It also was found that the charge material for this furnace was incoming at room temperature and that the furnace door had a water cooling system.  The results of the stack analysis were as follows: flue gas temperature, over 1400°F; oxygen in flue gas, over 5.75; combustion air temperature, ~450°F.

On the basis of the information available during this assessment, the furnace was modeled in the Process Heating Assessment and Survey Tool (PHAST)² provided by the U.S. Department of Energy (DOE) (Figure 3). It may be seen that in the present scenario, approximately 6.07 million Btu/h is used as useful output and the remaining 8.07 million Btu/hr is lost through the system.

Figure 3: Profile of Energy Consumption and Losses in the Example

1. After an analysis of the PHAST model, the following recommendations were made to the plant:
2. Preheat combustion air and charge material by using waste heat available in the stack.
3. Use automatic system for air to fuel ratio adjustment to maintain oxygen in the stack at ~3%.
4. Replace the water cooling–based furnace door with an insulated door.
5. Improve insulation on the furnace walls. Minimize the open time of the variable openings used to charge material, etc.

The proposed changes also were modeled in PHAST; the results are shown in Figure 3. It can be seen that the gross fuel input requirement is reduced to approximately 8.67 million Btu/hr (approximately 39% energy savings). The profile of energy consumption in present and proposed scenarios is shown in Figure 4.

Figure 4: Profile of Present and Proposed Energy Consumption in the Example Furnace

Conclusion

Process heating systems are one of the most energy-intensive systems in industrial facilities. On the basis of the operating characteristics, energy savings may be realized by means of small modifications to the system. DOE’s software tool PHAST can be used to model process heating systems and perform sensitivity analysis to evaluate recommendations for energy savings. The assessment methodology can be applied to any industry with process heating equipment, and energy savings can be realized by implementing similar recommendations.