Energy costs represent a large fraction of the operating cost of an industrial furnace, most of which use natural gas as their source of heat. Companies that operate thermal-processing systems can maintain a competitive edge with efficient fuel use while minimizing the emission of CO₂ and NOx. An Excel burner model can calculate the fuel savings attainable by using thermal-capture technology to recover and return process heat.

Preheating the combustion air by using flue- or stack-gas heat requires some sort of heat-recovery device. Commonly, these devices are incorporated into the burner, which makes them easier to retrofit into an existing furnace without hot-air piping. Irrespective of the method, the idea is to capture thermal energy from the stack gas and transfer it to the combustion air. How does preheating the air improve operation and save fuel? Four ways:

1. Less fuel is required to bring the products of combustion (POC) to the furnace temperature.

2. The stack (waste) gas temperature and volume are lower, thus decreasing CO₂ emissions.

3. The flame structure can be controlled to minimize NOx emissions.

4. The flame has a higher velocity, thus improving in-furnace POC circulation.

We’ll look at the first two points in this article and the other two in a follow-up article in October.

Potential for Saving

Companies with energy-management programs have adopted best practices to improve the thermal efficiency of their system. These practices are familiar to readers of Industrial Heating – careful control of air-fuel ratio, thermal capture, fire rate, furnace pressure, better insulation and others.^[1] Along with making sure that burners and other combustion equipment are operating at peak efficiency, recovering and reusing waste heat is likely the most beneficial improvement you can make.^[2]

Fuel Savings

Let’s look at the fuel-saving possibilities for an 1800°F (982°C) furnace fired at 500,000 BTU/hour at 15% excess air. The combustion heat input is the lower heating value (LHV) of the fuel per scf, multiplied by the flow rate. A heat balance shows the heat to load is 212,120 BTU/hour. Figure 1 shows the results of using unheated air, while Figure 2 shows the results for using 1000°F (538°C) air. With no preheat, nearly 49% of the combustion heat leaves in the stack gas. In contrast, only 27% of the combustion heat is carried out of the stack when using preheated air.

Without air preheat, the load heat is 42% of the combustion heat. With air preheat, 75,160 BTU/hour is transferred from the stack gas back to the furnace as heat in the combustion air, bringing the load heat to 60% of the combustion heat. The fuel savings is 29%, while the stack heat is only 40% of what it was using unheated air. Clearly, using cold air for process heating is a costly waste of energy.^[3]

Stack-Gas Savings

Preheating the air makes a big difference in the stack-gas properties. Let’s take a look at the same 1800°F furnace for five different air preheat temperatures. The firing rate is adjusted to provide a constant net-available heat (NAH) of 257,120 BTU/hour to deliver 212,120 BTU/hour to the load. Figure 3 shows that the temperature and flow rate of stack gas bothdecrease with increasing air preheat temperature. Over the same range, the fuel savings increase from 24 to 34%, and the stack-gas heat content drops from 32 to 22% of the combustion heat.

Air preheating has a multiplier effect on fuel savings because both the stack-gas temperature and flow rate decrease with air preheat. The results show that about 1.9 BTU less combustion heat is required for every BTU transferred from the stack gas to the burner by means of preheated air. In addition, about 3% less CO₂ is emitted for every 100°F increase in air preheat temperature.

Figuring all this out involves a lot of heat-balance arithmetic. You can avoid this by using an Excel burner model (workbook AirPreheatCalc.xlsx), which you can download here (IndustrialHeating.com/APC). Templates with examples show how to calculate the thermal efficiency derived from air preheating. The results can replace the multipage charts and tables found in typical combustion handbooks.^[4] As an example, Figure 4 shows workbook results on the fuel savings over a range of air preheat temperatures for four different POC temperatures.

Classification of Combustion Air Heating Systems

Air preheating requires the installation of a heat exchanger capable of transferring heat from the hot POC to the combustion air and a burner capable of using the heated air. Moving heat from one fluid (the POC) to another (air) is accomplished in two different ways. First, direct transfer across a conductive membrane: recuperative heat exchange. Second, indirect transfer by heat storage and removal: regenerative heat exchange. These devices are most commonly made as part of the burner itself. A recuperator operates continuously, with counter-current gas flow through the heat exchanger. A regenerator involves alternate storage and extraction of heat from a ceramic bed. Figure 5 depicts the two types of heat exchangers.

Table 1 shows the general features of the two types of heat exchangers.^[5] We’ll give more details in a follow-up article in the October issue and show how to make thermal-efficiency calculations for each type. We’ll also show how burner technology has evolved to take best advantage of the potential energy savings and environmental improvements obtainable by using heated combustion air.

Conclusions

The results from an Excel burner model show that combustion-air preheat can provide fuel savings of up to 40% for recuperative burners and 60% for regenerative burners as compared to using unheated air. The choice of recuperators or regenerators depends on several factors, as indicated in Table 1. Benefits include:

•  Faster heat-up times (higher flame temperatures and heat-transfer rate using high-velocity burners)

•  Greater efficiency (more available heat per unit of fuel)

•  Reduced pollution (minimum exhaust volumes, lower NOx)

•  Cost savings (less fuel used and greater productivity)

Use of one type versus the other is application-dependent, with selection based on the costs of the device, installation and maintenance. However, either choice will provide beneficial results.

The author gratefully acknowledges the help of Dave Toocheck of Bloom Engineering, Martin Schönfelder of WS Thermal Process Technology, Dennis Quinn of Fives North American Combustion and Jake Mattern of Hauck Manufacturing in the preparation of this article, as well as Chet Allen of Eclipse for providing the Eclipse Engineering Guide for the Excel workbook.