Regardless of fuel prices, energy efficiency is a top-of-mind consideration for most manufacturers these days. One Fortune 500 company in the food packaging industry achieved notable energy-efficiency improvements with an uncommon green initiative that combines pollution-control systems with process heating equipment.
The company produces resealable aluminum bottles for beverages. Because these containers come in direct contact with the consumable liquids, they must be washed and internally coated to ensure safety and product quality. Once applied, the inside coating is heated in an oven for curing. The entire process is energy intensive, requiring both hot water for washing the cans and hot air for drying the coating. In addition, harmful emissions are generated from the coatings as they dry.
To reduce emissions and cut energy use and costs, the food packaging company integrated a system that recovers waste heat from the stack of their pollution-control equipment to generate hot water.
The combination of environmental control systems and process hot-water heating allows the food packaging company to achieve environmental compliance, generate heat for the process and reduce overall greenhouse gas emissions.
Aluminum Bottle Maker Seeks Way to Cut Energy Usage During Coating and Curing
Historically, companies use independent systems to generate hot wash water. The heat typically is provided by a stand-alone boiler or hot-water generator that burns fossil fuels to create hot water.
During bottle coating and curing, volatile organic compounds (VOCs) are created in the process ovens as the coatings dry. Environmental regulations require the company to meet a high rate of VOC removal efficiency. As is the case with many industrial air pollution-control applications, at the food packaging company, emissions are destroyed by thermal or catalytic oxidation. Time, temperature and turbulence are used to break apart organic compounds into carbon dioxide and water vapor.
When process emissions are oxidized, heat also is generated from the exothermic process. This heat typically is used within the pollution-control device to minimize auxiliary fuel consumption during operation. At the food packaging company, however, some of this excess heat was diverted permanently to create the hot water for bottle washing. In this way, auxiliary fuel is not consumed to provide the hot water. Instead, the hot water is generated by the company’s environmental compliance efforts.
Hot-water generation occurs by transferring heat from the abatement system stack over a series of finned coils, where water within the coils flows counter-current to the airflow direction. For the oxidizer used at the food packaging company, the coils can generate up to 3 million BTU/hr of energy transfer. This energy can be used to heat 100 gallons per minute of water to the required 200°F (93°C) temperature in the washing stage.
Pumps deliver the water through the coils based on the heating demand of the washing stage. As more hot water is requested, the pumps can deliver the water at a higher rate of flow based upon the variable speed of the pumps. Water temperature is controlled by adjusting the amount of hot gases that are passed over the coils. Dampers direct the gases either to flow across the coils or to bypass them based on specific temperature requirements.
Though air-to-water heat exchangers are common, another popular fluid used for heat recovery is a hot-oil heat transfer fluid. Operating in much the same way as the air-to-water exchangers, the air-to-oil heat-recovery coils capture waste heat from the oxidizer. This allows an operator to utilize the hot oil within their ovens or other process equipment.
Another benefit to a hot-oil system is that they allow for temperatures up to 550°F (288°C) without needing to deal with the extreme pressures that come hand in hand with high temperature steam. The hot-oil systems also can be decoupled from the owner’s process with a second heat exchanger (an oil-to-water or oil-to-air, for example). This provides more flexibility in how the waste heat from the oxidizer is utilized.
Besides trying to minimize auxiliary fuel combustion and the subsequent greenhouse gases from their heated process water needs, the aluminum bottle facility also was using abatement technology to minimize the overall carbon footprint.
By using a regenerative thermal oxidizer (RTO), this facility was already limiting its environmental impact because this technology is recognized as a fuel-efficient pollution-control device. What differentiates regenerative thermal oxidizers from the catalytic, recuperative and direct-fired oxidizer technologies is their ability to recover and reuse the thermal energy generated during operation. Even under low emission input, destruction often is achieved with little to no auxiliary fuel once the regenerative thermal oxidizer is brought up to temperature.
Even though all process emissions from the aluminum coating operation could have been effectively removed using just a regenerative thermal oxidizer, the food packaging company sought additional ways to reduce natural-gas consumption. As a result, the bottle-manufacturing facility also incorporated an emission concentrator system for the low temperature emission sources.
Concentrator technology is an energy- and cost-saving add-on to thermal and catalytic oxidizers that can reduce the amount of treated airflow by 95 percent. At the food packaging manufacturer, the high volume of low concentration process exhaust from the internal bottle coating is collected and passed across a zeolite adsorbent. As the emissions are passed over the zeolite material, VOCs are captured. The emissions are desorbed into a much smaller and concentrated airstream that is then sent to the regenerative thermal oxidizer for VOC destruction. Excess heat from the regenerative thermal oxidizer is used for desorption, so supplemental fuel is not needed for the concentrator.
At the food packaging company, the addition of a concentrator resulted in a smaller exhaust volume being sent to the regenerative thermal oxidizer. This reduced the capital cost of the control device. The concentrator also delivers a higher VOC concentration to the regenerative thermal oxidizer, which ensures the system is more fuel efficient. In this case, the higher VOC concentration correlates to a higher regenerative thermal oxidizer stack temperature. This enables even higher flow rates of hot water to be used within the process.
Results-Driven Initiative Cuts Energy Consumption for Aluminum Bottle Maker
Using the water coils for their process heating needs rather than stand-alone water heaters saved the food packaging manufacturer 3 million BTU/hr of auxiliary energy consumption. This is equivalent to about 1,500 tons per year of carbon emissions that were eliminated for a continually running process.
The regenerative thermal oxidizer was able to provide 99 percent destruction of the VOC emissions — often while operating without supplemental natural gas. This technology choice was able to reduce auxiliary fuel consumption up to 5.5 million BTU/hr over older technologies, saving up to 2,800 tons per year of carbon dioxide (CO2) emissions. The concentrator further enhanced the system efficiency and reduced auxiliary fuel consumption by an additional 5 million BTU/hr and saving another 2,500 tons per year of CO2 emissions.
As a result of these improvements, the aluminum bottle manufacturer is considering additional heat-recovery projects at this facility. Applications such as hot-oil generation, supplementing oven heat and ambient air heating are all potential green initiatives for this company in the future.