In 2013, the Energy Information Administration (EIA) reported that industry, power plants and commercial buildings consumed approximately 19 trillion cubic feet of natural gas. How much of this combusted energy was wasted? About 40 to 60 percent, according to estimates. Flue-gas heat recovery has been around for many years, but because combusted natural gas is something not visible — like leaving the lights on — it is mostly ignored.

While America is blessed to have an abundance of this reasonably cheap energy, increased energy efficiency is still considered profit, and every company wants to realize increased profit. Yet, still we waste this energy.

Today, we also have to think about the environmental aspects. Reducing waste heat helps reduce global warming and CO₂ emissions. Then, there is the water. In every 1 pound of combusted natural gas resides 2 pounds of recoverable, distilled water. So the question is, “Why is industry not looking to increase their natural gas energy efficiency, reduce their carbon footprint and conserve water?” Sadly, the answer is usually because it is easier to ignore the issue.

What Is Wasted Energy?

In our personal lives, the government, utilities and enlightened society are constantly educating us that wasted energy is leaving lights on when there is no one in the room and leaving computers on over the weekend. Energy Star has all sorts of certified, high-efficiency TVs and appliances. It is about electricity and “flexing your power” — which is good — but there is more. Let us increase natural gas energy efficiency and call it “stretching our power.”

Natural gas is used to produce a lot of our country’s electricity. It also is used to process almost everything that we use, from the food that is processed and consumed every day to our warm homes and workplaces and our heated water. The fuel for our vehicles, steel, plastics and glass were all “touched” by natural gas.

Every natural gas appliance has a chimney, and leaving all those chimneys across the country are the products of combustion that did not make it into the process. Can this natural gas be consumed more efficiently? Absolutely. There will always be some cases where not all of the recovered heat energy will be usable. Ninety percent efficiency is still a whole lot better than 60 or 80 percent. And, in industrial applications, increases in efficiency and reductions in waste are even more significant.

Natural gas is combusted to generate heat, and a lot of that heat is used to heat water and to make industrial process steam. The warmer the water that can be put into the boiler, the less natural gas will be required to bring it up to its desired temperature and pressure. A feedwater economizer should be standard equipment for any boiler that is used to produce steam. This equipment uses the hot exhaust gases to further heat the feedwater water. This will increase the boiler’s energy efficiency by 2 to 4 percent. Because the flue-gas temperature only can be reduced to the temperature of the heated feedwater, there is still 250 to 350°F (121 to 176°C) exhaust leaving the chimney. There is a lot of heat available in the exhaust that is yet be recovered.  

To recover the heat energy that is left in the exhaust requires a condensing economizer. Condensing economizers have been used in North America for more than 30 years. Typically, the energy efficiency gain produced with this technology is 8 to 15 percent. (Condensing boilers are available to the residential and small commercial market, and these appliances can operate to 98 percent efficiency.)

A condensing economizer installed in a large industrial or commercial application can bring these facilities’ natural gas energy efficiency to more than 90 percent. However, reaching this 90 percent-plus energy efficiency is not accomplished simply by installing a condensing economizer. The facility manager or the company’s Energy Team has to determine the most efficient application for utilizing all this recovered heat energy.

Some processes are easily found. Water that is coming in from the street that is to be heated with steam at a heat exchanger can first be routed through the condensing economizer and then be sent to the heat exchanger. The colder the water, and with a decent flow, will mean the exiting flue gas will leave the plant at a temperature within a few degrees of the incoming street water temperature. There will be times in the heat of summer when the temperature of the exiting flue gas will be cooler than the outside air temperature. Can this be called mass cooling?

Not all applications for utilizing this recovered waste heat energy are readily — or as easily — available. There are times when the thinking caps have to be put on to find that best application, and there are those times when applications for utilizing this recovered heat energy have to be created. Doing this must align with benefits for the company. Some projects will have a relatively short ROI, and some will take more time.

The materials used in a condensing economizer have to be suitable to stand up to decades of the slightly acidic condensate that is created when the heat energy has been removed from the combusted exhaust gases. The condensing economizer will typically be placed on the floor beside the natural gas appliance or alongside the chimney. The exhaust gases will be diverted into the condensing economizer, and the cooled exhaust will be vented directly to atmosphere. Because the cooled exhaust gases are moist, it is not advised to direct the exhaust back into the boiler’s carbon steel chimney. With the heat energy removed from the exhaust gases, a short stub chimney above the roofline is all that is needed unless a fan is added. However, this requires spending electrical energy to save natural gas energy.

What industries benefit from this technology? The list is long, but food and beverage processors, petrochemical plants, pharmaceutical producers, textile plants, feed mills, meat processors, oil and gas processors, power plants, automotive manufacturers and the mining industry are among the big winners of heat recovery systems.

 In general, if a lot of natural gas is being consumed, and there is hot wasted exhaust going up the chimney, increasing energy efficiency should be investigated.