Giant foods processor General Mills joined the DOE’s Better Plants program, committing to reducing its energy intensity by 20 percent by 2022. It is well on its way to achieving that goal via heat recovery and other energy-efficiency initiatives. Photo courtesy of General Mills.
General Mills, maker of hundreds of food products sold into more than 100 countries, including its best-selling “Big G, little o” Cheerios cereal, set a goal in 2014 to reduce its energy intensity by 20 percent, with 2012 as its baseline year. The energy reduction is part of its overall sustainability goal to reduce natural resource consumption and sustainably source raw materials. The company is well on its way to energetically achieving its goal, in large part because of its heat recovery initiatives.
“In 2012, we ramped up our energy conservation efforts to meet our sustainability goals and also financial savings,” said Graham Thorsteinson, General Mills energy platform leader. “So by 2022, we will reduce energy intensity by 20 percent. The scope is large. The other goal that we’re excited about is, from 2010 to 2025, we are going to reduce our greenhouse gas impact by 28 percent. That’s from farm to fork to landfill.”
The manufacturer piloted several energy projects at its Covington, Ga., plant. “We started by hiring a plant-based energy engineer at our largest plant,” Thorsteinson said. “We developed some tools around optimizing compressed air, chillers, steam, and all of our major process energy users. We saw some really large benefits financially by focusing on energy and heat recovery.”
The energy engineer uncovered more than $1 million a year in cost savings, Thorsteinson said. “This person was delivering multiple times his salary.” Another energy engineer was hired at another General Mills plant, and additional, significant energy savings were recovered as a result.
The success of the pilot programs provided the substantiation to hire additional energy engineers for all the manufacturer’s major plants. The program grew from one engineer in one plant in 2011 to dozens working in 20 plants today. “We’ve found that focusing resources on energy initiatives delivers savings multiple times what they cost. We’ve gotten a great ROI,” Thorsteinson said.
Joins DOE’s Better Plants
To continue its momentum, the manufacturer joined the U.S. Department of Energy’s (DOE) Better Plants program in 2014 as a Challenge partner. The President Obama-initiated Better Buildings, Better Plants Program is part of the broader Better Buildings Initiative, launched in 2011 (see Figure 1). The program’s goal is to help make industrial, commercial, public, and residential buildings 20 percent more energy efficient over 10 years from when partners join the program. It offers technical assistance and national recognition and facilitates sharing of successful best practices.
Figure 1: General Mills is a partner in the President Obama-initiated DOE Better Plants program, which helps partner manufacturing companies achieve a 20 percent energy intensity goal over 10 years. Better Plants partners now represent more than 11 percent of the manufacturing sector’s total energy footprint, with more than 2,500 facilities across the U.S. So far, partners have reported cumulative energy savings of 600 trillion BTUs and nearly 35 million metric tons of avoided climate-changing carbon emissions, the agency reports.
“We were really interested in joining the Better Plants Challenge because of the free technical expertise and assistance we could get from the DOE to help solve problems, and gain energy management expertise, receive in-plant training, attend conferences such as DOE’s annual Better Buildings Summit and information sharing with other large, respected companies,” he said. “The DOE has been very helpful.
“We are very committed to achieving our sustainability goals, so signing up for a program that offers assistance with that made sense. It’s a really good program. We’re excited about it,” he added.
How Heat Recovery Saved a Cool $4.3 Million
The heat recovery initiatives comprised three components: reusing waste heat generated from one area of the plant to heat ingredient water in another area, using flash steam to regenerate a desiccant wheel for packaging dehumidification; and installing coil run-around loops to reduce dehumidification costs.
“In cereal plants, there is no reason we should be using energy to make hot water or to heat the building. There are some parts of the process that are crazy hot. Even in the winter, a room might be 110 degrees F,” Thorsteinson remarked.
1. Waste Heat Reuse
Figure 2: Using a heat exchanger and ordinary plumbing, General Mills created a heat recovery loop that transfers waste heat from three different sources to other operations needing low-grade heat.
Thorsteinson and his team turned their attention to heat recovery efforts at General Mills’ Cedar Rapids, Iowa, plant. They identified multiple waste heat sources from which they could generate “free hot water.”
“First, we looked at all the areas in which we were expending energy to generate heat for the process—we call those heat sinks—and which waste streams they had—we call those heat sources. The team at the plant compiled an exhaustive list of them and looked at what the most efficient and economical system is of lining up heat sources with heat sinks,” he said. “It’s just a matter of looking at your waste heat and lining it up with areas that need heat,” he said.
Thorsteinson said they started piping recovered, heated water from the heat source operations across the facility to those that needed low-grade heat (see Figure 2). Many of the connections are simply plumbed, and each of the heat sources has a heat exchanger connected to it, Thorsteinson said. “So we kind of created a heat recovery loop, if you will. This first loop that’s getting waste heat from three different sources became almost a new utility for us.” The three sources are:
Desuperheater. The first amount of waste heat was recaptured from a desuperheater, which is a large heat exchanger that has cold water flowing through it as part of the refrigeration cycle. Previously in the cycle, hot gas from the desuperheater was piped to a cooling tower outside, similar to a residential air conditioner condenser. Then that gas was piped to the chiller, he explained. “So previously that hot gas was piped to these condensers to cool it. Now the hot gas leaves the heat exchanger as warm gas and the cold water leaves as warm water, so with a heat exchanger and a bunch of piping, we send the heat where it’s needed.
“By taking the heat from the refrigerant there, we get free heat. Plus, we significantly reduce the load on our outside condenser because the heat is redirected to this heat recovery water system. It’s a win-win,” Thorsteinson said.
Air Compressors. The second waste heat reuse was from the air compressors. “Air compressors lose almost all of their energy as heat, so that’s another great place to pick up hot water,” Thorsteinson said. “Hot water from the compressors used to go to outside condensers to cool it down so the air compressors wouldn’t overheat. Now we put in a heat exchanger and steal some of that heat,” he said.
“Like the superheater, by picking up heat from the air compressor, we no longer have to cool the air compressor as much, so there’s double impact.”
Condensing Economizer. A condensing economizer is an air-to-water heat exchanger. “It’s a stainless steel box, about 15 ft. by 8 ft. and 8 ft. tall, so it’s a good-sized box with a lot of coils in it,” Thorsteinson said. A condensing economizer can handle very low temperatures—those lower than standard economizers can. “It’s kind of a harsh environment. So it takes a specialized, hardy piece of stainless steel to be able to do that.”
Hot air exhausted from the boiler goes through a regular economizer where it drops from 450 degrees F down to 270 F, he explained. That air runs through the condensing economizer, which drops its temperature from 270 degrees to 130 degrees. Because the process extracts a lot of heat out of the air, the water gets extremely hot. That hot water is then piped to where heat is needed in other areas of the plant. Additional heat exchangers are installed where needed.
“We use the hot water to heat the ingredient feed water, or for cleaning, or to preheat boiler combustion air. Anywhere heat is needed, we pipe this recovered heated water to it,” Thorsteinson said.
2. Heat Regeneration Via Flash Steam
Another area ripe for reuse was flash steam (see Figure 3). Water boils at 212 degrees F. When high-pressure condensate is collected in a tank, water can remain as water at 350 degrees under pressure. But during its transition to atmospheric pressure, flash steam is emitted, Thorsteinson explained. “A lot of times, when you go to manufacturing plants, you see a tremendous amount of flash steam coming out of the room, depending on whether they are collecting that condensate,” he said.
Figure 3: Before implementing heat regeneration from its flash steam source, all of the heat and energy General Mills generated in the steam was vented and lost. The foods manufacturer since captured the heat and energy from steam and has realized significant energy and costs savings as a result. Photo courtesy of General Mills.
So instead of venting that flash steam, the General Mills engineers piped it to a heat regeneration station that is used for dehumidification. “One way you can dehumidify air is by using a dessicant dryer, which absorbs the moisture,” Thorsteinson explained. “But then you have to get the moisture out of the dessicant, or it won’t be able to absorb any more moisture.” The dessicant rotates slowly, and a hot air source dries the dessicant so it can continue to absorb more moisture from the air.
“We just used waste heat to be able to better generate heat to that wheel, and that saved a lot of money.”
3. Coil Run-around Loops
Figure 4: A third heat recovery project uses a heat exchanger to exchange thermal energy between cool glycol and hot outside air.
The third heat recovery project, called a runaround loop concept, is also HVAC-related. “Again, this is just coils and piping. It’s pretty ingenious,” Thorsteinson said, explaining how the system works. The first coil of the unit, or the precoil, connects to hot outside air. It cools the glycol, which is the fluid being pumped through the runaround system (see Figure 4). The glycol in the coil is cooler than the outside air, so the outside air becomes cooler and the glycol in that coil becomes warmer. This glycol then is used to condition the air after the cooling coil, but upstream of the reheat coil, so it reheats the air and recools the glycol.
“Now the air is being heated for free because it was heated by the outside air. So then it just pumps around and around, but it’s constantly getting heated by the outside air and is cooled for free because it comes after the cooling coil,” he said.
Thorsteinson said that the system only is applicable with a typical mechanical dehumidification-type unit that drops moisture out as temperatures lower and reheat to maintain relative humidity.
Results: Three-year ROI, Water Savings, 5,500 Tons CO2 Prevented
All together, the three heat recovery projects at the Cedar Rapids plant and the projects that began in 2012 netted a 23 percent energy intensity savings, and has prevented the release of approximately 5,500 tons of CO2 emissions annually, Thorsteinson relayed.
After the energy efficiency projects were implemented, the energy costs at the Cedar Rapids plant–$12.8 million in 2011–were chopped to $8.5 million in 2015, Thorsteinson reported. The $4.3 million cost savings on a $10 million investment point to a three-year return on investment (ROI).
Additional cost savings resulted from the reduction of 2.2 million gallons of water per year because water was reused.
General Mills subsequently replicated the heat recovery practices at several other locations.
“The cost savings definitely got people’s attention. One is the complexity and size of it. It’s one of the largest energy projects that we have ever executed. The other aspect is that the Cedar Rapids plant was already one of our most efficient plants. A ton of efficiency work had already been done, so to be able to make this impact kind of just showed people that continuous improvement really never stops.”
Better Plants Helps Corporate-wide Expansion
General Mills is expanding the heat recovery project to 14 other sites and is continuing to get assistance from the DOE’s Better Plants program. “We have a lot of plants in a lot of different areas, and it’s kind of a complex environment,” Thorsteinson said.
As part of the Better Plants program, the DOE awarded General Mills an in-plant training event in March 2015 at the company’s Cedar Rapids, Iowa plant on compressed-air systems. “That plant was, by far, our biggest compressed-air user. We used thousands of horsepower of energy on air compressors.
“They came out and taught us how to optimize the energy use on compressed air and then walked us through how to use the controls and how to find energy losses in the facility,” he continued.
The program is set up as a hands-on, learn-to-teach model, he said. Twelve engineers from various General Mills plants were present at the in-plant training. They took the training concepts back to their plants to replicate the best practices that were shared with them.
The company signed up for the DOE’s combined heat and power (CHP) technical assistance (CHP-TAP) program. CHP sources natural gas to generate power on-site and reuses the waste heat to produce steam for the site.
Thorsteinson said the company especially appreciates the opportunity to confer with other large manufacturers like 3M and Alcoa.
The General Mills plant was recognized by the DOE in a showcase document on the agency’s site, and DOE Undersecretary for Science and Energy, Lynn Orr, toured the plant in recognition of its achievements.
“We were looking to partner a bit more outside of our own four walls and benchmark with other people, and Better Plants enables us to do that.”
Graham Thorsteinson is corporate energy platform leader for General Mills, 404-375-0785, gthorst@gmail.com, www.generalmills.com.
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