Energy management has become a core challenge for manufacturers. With rising costs, stricter regulations, and growing pressure to reduce emissions, industries are being pushed to rethink how they use energy. For manufacturers, meeting production targets must be balanced with efficiency and sustainability.
The paper and pulp industry, with its high energy demands for heating and drying, faces these pressures acutely. Air-to-water (ATW) waste heat recovery presents a highly effective solution that not only curtails energy expenditure but also enhances environmental sustainability by capturing and reusing energy that would otherwise dissipate unused.
The global paper and pulp industry (PPI) is a vital component of the industrial manufacturing landscape. Valued at approximately $350 billion worldwide, this industry plays a major role in the global economy and directly employs over 40,000 workers in U.S. mills and countless more indirectly throughout the supply chain.
The production process starts with wood, the primary raw material, undergoing pulping, where it is either mechanically or chemically broken down into fibrous pulp. Mechanical pulping is energy-intensive but retains most of the wood mass, making it ideal for products like newsprint and some packaging materials. In contrast, chemical pulping, which uses energy and chemicals to break down lignin, produces higher-quality pulp often used for premium writing papers, hygiene products, and food-grade packaging.
After pulping, the process moves to bleaching and papermaking. Bleaching brightens the pulp, particularly for products requiring whiteness, such as office paper and tissues. Finally, the papermaking stage forms, presses, and dries the pulp into finished sheets. These processes, particularly in how they manage thermal energy, present an opportunity for energy optimization and sustainability within the industry.
Air-to-water waste heat recovery (ATW WHR) systems capture and repurpose heat from exhaust gases or steam, two common byproducts in paper and pulp manufacturing. The core component of these systems is the heat exchanger, which facilitates the transfer of heat from the waste stream (exhaust gases or steam) to water. This transfer occurs without direct contact between the fluids, typically through a conductive barrier for maximum heat transfer efficiency. Once the heat is transferred to the water, it can be repurposed in multiple ways within the facility.
This reuse of heat tackles some of the biggest challenges in the paper manufacturing industry:
Paper manufacturing presents several prime opportunities for waste heat recovery:
Steam boilers are used to generate steam required in both pulping and papermaking. However, they're not perfect and typically only manage to use about 70% to 85% of the energy they consume. The remainder of the energy manifests as waste heat through boiler flue gases and blowdown water (that's the extra hot water dumped to keep things clean and efficient inside the boiler).
In the drying phase of paper production, the paper transitions from a moist fiber mat to a dry, usable sheet. This stage is critical for stabilizing the paper for commercial use and is one of the most energy-intensive parts of the mill. The process relies on large, steam-heated cylinders such as Yankee dryers and drum dryers. These cylinders apply direct heat to the paper, effectively removing moisture as the paper moves through them. The surfaces of these dryers, although insulated, lose heat through radiation and convection to the surrounding environment. More substantially, the dryer vents and hood exhaust systems expel warm, moist air, which carries away a significant amount of thermal energy. These losses represent a key area for energy recovery initiatives.
In the paper and pulp industry, chemical recovery processes are essential because they help reduce environmental impact and lower operational costs. By recycling the chemicals used in pulping, mills can decrease waste and cut down on the expense of buying new chemicals. These recovery processes include:
The Kraft Process
Also known as the sulfate process, this is the most widely used method for converting wood into pulp. Clean wood chips are cooked in a solution of sodium hydroxide and sodium sulfide, known as white liquor. The result of the cooking process is a mixture of solid pulp and a liquid called black liquor.
The black liquor contains dissolved lignin and spent chemicals. It is concentrated via evaporation and then burned in a recovery boiler. The burning of black liquor recovers the cooking chemicals and generates heat, which can be recovered to produce steam for electricity generation or other processes within the mill.
Lime Reburning
Lime kilns are used to convert calcium carbonate back into calcium oxide. This process generates considerable amounts of waste heat that can be captured and reused. The high-temperature exhaust from lime kilns, which typically operates at several hundred degrees Celsius, is an excellent candidate for waste heat recovery systems.
Many paper and pulp mills produce their own power using cogeneration systems. These systems generate both electricity and steam by burning biomass or other waste materials from the pulping process. The exhaust gases from these power units release a lot of heat that can still be used. This waste heat is often captured to help heat up combustion air or the water that feeds into boilers.
The integration recovered heat from ATW WHR systems can be strategically redirected to optimize various processes within the paper and pulp industry:
One of the most straightforward applications of recovered heat is preheating the feedwater for boilers. This approach uses the waste heat to warm up the water before it enters the boiler. By doing so, less energy is needed to heat the water to the required temperature for producing steam, which makes the boiler operate more efficiently and reduces the amount of fuel needed.
The warm air released during paper drying can be captured and put to good use through a waste heat recovery system. Using an economizer, this heat is transferred to water, which is then fed into the boiler. The boiler uses the heated water to generate steam or more heat, which is cycled back into the drying process. This setup creates a loop where the same heat is reused, reducing the need for extra energy from outside sources.
Recovered heat can be used in many different ways across a paper mill, depending on the process. For example, it can heat water for cleaning or other tasks, preheat boiler feedwater to improve steam production or provide hot air directly for drying. Some processes, like bleaching or digestion, may use recovered heat to maintain the precise temperatures needed for chemical reactions without involving the boiler. Since each stage has specific heating requirements, recovered heat is applied differently to match the needs of the process:
Downtime doesn’t have to be a barrier to adopting waste heat recovery. ENERVEX’s RHX Roof-Top Heat Exchanger System is built to keep your mill running while capturing and repurposing valuable waste heat.
With fast installation – often under 30 minutes – and a bypass damper that ensures uninterrupted exhaust flow during maintenance, the RHX eliminates the risks of production delays. It’s a turnkey solution that cuts energy costs, reduces emissions, and delivers real savings without compromising uptime.
Contact ENERVEX today to learn how the RHX can work for you.