In response to these energy-conservation endeavors, it has been suggested that equipping gas-fired furnaces with sensor and control systems that optimize the fuel-to-air ratio could lead to dramatic energy savings. Such control systems are said to save substantial fuel over uncontrolled furnaces.

This article will demonstrate that, although combustion optimization does save fuel, the cost of deploying such controls is substantially higher than the resulting fuel savings on a heat-treating furnace. Alternative ways of achieving similar cost savings at a substantially lower investment cost will be presented.

Fig. 1. Combustion is most efficient between 0.75% and 2% excess oxygen.

An Overview of Combustion

The process we refer to as combustion is nothing more than the chemical combination of fuel and oxygen to produce heat and by-products (see sidebar). Air – the primary source for oxygen used in combustion processes – is comprised of oxygen (20.95%), nitrogen (78%) and traces of other gases. This is a critical point because nitrogen adds nothing to the combustion process and, in fact, it absorbs a significant amount of the heat generated by the fuel and air. In addition, nitrogen leads to the production of nitrous-oxide compounds that are regulated by the EPA.

In perfect combustion, the oxygen and fuel combine in an ideal ratio so that neither oxygen nor fuel remains after combustion, and the by-products are primarily carbon dioxide and water. This point of perfect combustion is commonly referred to as stoichiometry. It ensures that no unnecessary nitrogen is being heated, and it is the point where heat generation is highest and pollution emissions the lowest. Figure 1 shows a chart of combustion efficiency versus excess oxygen.

No real combustion system, however, can ever achieve perfect combustion because the fuel BTU content and fuel and airflow rates vary. Additionally, the uniformity and mixing of the gases is not perfect. In reality, combustion is either oxidizing (too much oxygen) or reducing (too much fuel).

Oxidizing combustion (too much oxygen) causes the heat loss and excess pollution production mentioned previously, but it also results in a shorter life for alloy components such as radiant tubes and – in the case of open-fired furnaces – refractory brick.

Reducing combustion (too much fuel) produces a sooty emission as unburned fuel goes up the stacks (wasting fuel and dollars). What’s worse is that reducing combustion shortens the life of tubes even more quickly than oxidizing and can also lead to a greatly reduced burner life span.

Optimized combustion within the heat treat is vital to achieving the longest possible life for tubes, burners and brick and can result in substantial fuel savings.

Fig. 2. Temperature dependence of combustion optimization: for every 1% reduction in excess oxygen there is a corresponding 1-3% reduction in fuel use.

The Economics of Combustion Control - Potential Savings

As a general rule of thumb, for every 1% you can reduce your excess oxygen in your firing system, you can reduce the fuel needed to produce the same heat between 1-3% (Fig. 2). For example, if your furnace is currently firing with 10% oxygen in the exhaust flue and you reduce that to 1% oxygen, you can expect a savings of at least 9% on the fuel used for that furnace. If you’re firing at higher temperatures you might see savings as high as 27%.

Fig. 3. Combustion control systems require multiple, high-dollar components, resulting in a high per-point investment.

The Economics of Combustion Control - The Cost of Control

In order to obtain a net savings on your combustion optimization, you must consider the start-up costs of any such system and the ongoing costs. This is where combustion control for heat treating breaks down as a real solution.

First, let’s look at a traditional application for combustion-control upgrades: steel billet reheating. In a real-world example, a single producer was using over $120 million per year in heating fuel. After their upgrades they saved 7% per year on fuel use. That’s $8.4 million per year. The cost of their initial upgrade was about $750,000 with an annual cost of about $60,000. That means they got complete payback within a month and realized a 14,000% return on investment every year thereafter.

Unfortunately, heat-treating furnaces are very different from steel reheat furnaces:
  • The temperatures are much lower (1600°F vs. 2400°F)
  • The amount of fuel used is much lower
  • Most importantly, the number of burners is much lower in steel reheat while the size much larger
Let’s take a look at the savings potential for combustion control in a “typical” heat treat. Our typical heat treat uses $540,000 per year in natural gas for heating. They can save 7% on this after upgrading to a combustion-control system – that’s $37,800 per year. Figure 3 shows how such a system is configured.

Counting PID control loops, motorized actuators for the burners and oxygen sensors – all at one per burner – they can expect an initial investment of about $118,000 and an annual cost of $45,000. In other words, this is a very bad investment!

Fig. 4. Combustion imbalance monitoring requires substantially fewer components, resulting in lower overall investment.

Alternate Approach: Imbalance Monitoring

So, are there options somewhere between constant manual checks of burners and full-blown combustion control? Consider an automated monitoring and alarm/notification system, or “combustion imbalance monitoring.”

In an imbalance monitoring system (Fig. 4) the PID control loop is replaced with a low-cost “smart transmitter,” and the motorized actuator is eliminated completely. The monitoring system acts as a watchdog to make sure the excess oxygen stays within a predefined “band” where efficiency is high. If something, such as fuel BTU content, causes a deviation from high efficiency, the smart transmitter sounds an alarm. What’s more, the system sends a continuous digital record of temperature and excess oxygen to the plant’s data-logging system so that trends can be recorded. When a deviation occurs, plant personnel manually adjust the burner until it comes into compliance with the high-efficiency band.

Looking back to that “typical” heat-treat shop that can save about 7% on fuel, we can now reduce the initial investment from $118,000 to about $56,000 and the annual cost from $45,000 to about $14,000. There is still no payback in fuel savings until year two, but remember that tubes will now last significantly longer and the ongoing payback is now almost $24,000 per year.

Conclusions

In spite of high gas prices and voices pushing for combustion controls on heat-treating furnaces, the economics for control simply aren’t there. Combustion, however, still represents an area where most heat treats can find savings in the form of less fuel use, lower emissions and longer radiant-tube and refractory life. The way to do it economically is either manually or using an automated system to notify operators when burners are operating inefficiently. The savings won’t double profits or put a new shine on an old furnace, but they’re real and achievable. IH

For more information: E.S. Boltz, Y.H. Boltz, B. Knight and P.J. Barker, Marathon Sensors Inc., a member of United Process Controls, 8904 Beckett Rd., West Chester, Ohio 45069; tel: 513-772-1000; fax: 513-326-7090; e-mail: yboltz@marathonsensors.com; web: www.marathonsensors.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: oxidizing combustion, reducing combustion, stoichiometry, oxygen sensor, efficiency

SIDEBAR: Natural Gas Combustion

Natural-gas-fired combustion combines natural gas and oxygen to produce heat and by-products. Air, which is 78% nitrogen, is used as the oxygen source. All of that nitrogen contributes nothing and actually absorbs the heat generated, leading to wasted dollars. By optimizing the ratio of fuel and oxygen, one can save fuel while reducing pollution and extending alloy (tube) and burner life.

SIDEBAR: A Word About Automotive Lambda Sensors

It’s tempting to think that those inexpensive oxygen sensors found in cars might be part of the solution for combustion optimization in heat treat. Automotive oxygen sensors, however, are called Lambda sensors for good reason. “Lambda” is a term for fuel-to-air ratio and Lambda sensors do not actually sense oxygen percentage. They instead detect the point where combustion goes from oxidizing to reducing. Since they’re designed for gasoline, these sensors flip-flop their output when the fuel-to-air ratio is equal to 14.7:1 – a ratio that makes no sense for natural gas.

What’s more, in your car the engine control unit doesn’t pick an efficient excess oxygen point. Since Lambda sensors are so inaccurate, it only makes sure that the output continuously flip-flops between oxidizing and reducing (a minimum of 60 times per minute) so that the average is roughly near the point of good combustion.

If you ever find a furnace fired with gasoline and a control system that can flip-flop fuel output hundreds of times per minute, then you might consider Lambda sensors. Until then, a good oxygen sensor is a must.