Decarbonisation Technology August 2026 Issue

Heat recovery below the acid dew point Corrosion-resistant heat recovery offers a practical pathway for industrial decarbonisation, increasing the efficiency of existing energy systems

Nicolas Schiffer Fiedler Technoform

E very day, energy-intensive industries this heat is not lost because it has no value, but because recovering it would require operating in one of the most corrosive environments of the plant. For sectors such as waste-to-energy, cement, refining, and chemical processing, this represents a largely overlooked opportunity to reduce emissions while improving efficiency, and reliability. A large proportion of industrial waste heat is released at low temperature levels, typically below 150°C. At these temperatures, technical and economic barriers often limit recovery, leading to substantial energy losses to the atmosphere. This is particularly evident in flue release significant amounts of recoverable heat through their stacks. In many cases, gas streams, where conventional systems avoid operation below the acid dew point due to corrosion risks. As a result, valuable heat in the range of approximately 40-150°C remains largely unused ( Feodorova, et al., 2019 ). The relevance of this operating window is increasing. District heating systems are moving towards lower supply temperatures, industrial sites are seeking efficiency gains without fundamental process changes, and carbon capture applications require defined flue gas conditions for stable operation. Low- temperature flue gas heat is therefore becoming a strategic energy resource rather than a neglected byproduct. The main barrier is not the availability of heat, but the ability to recover it reliably under corrosive conditions. Cooling flue gases below the acid and water dew point leads to the formation of aggressive condensates

that rapidly degrade conventional materials. Overcoming this challenge is key to unlocking a largely untapped resource for industrial decarbonisation. Why heat recovery stops at the acid dew point In industrial flue gas systems, heat recovery is limited not only by thermodynamics but also by the chemical environment created during cooling. As hot exhaust gases cool down, their capacity to hold water vapour decreases. At the same time, acid-forming components, such as sulphur oxides (SOx), hydrogen chloride (HCl) and other trace compounds, can react with water vapour to form highly corrosive acids. The problem is therefore not condensation alone, but the combination of condensation, acid formation, and material attack. The temperature at which acidic components begin to condense is referred to as the acid dew point. It typically lies between 110°C and 150°C, depending on the flue gas composition, moisture content, and concentration of acid- forming species ( HeatMatrix Group B.V., 2021 ). Below this threshold, corrosion rates can increase sharply. Further cooling towards the water dew point, often in the range of 40-60°C, depending on operating conditions, leads to larger amounts of liquid condensation and therefore more intensive exposure of heat exchanger surfaces to acidic media ( Feodorova, et al., 2019 ). This creates a highly aggressive operating environment. Acidic condensates can attack metallic surfaces, penetrate or destabilise protective oxide layers, and initiate rapid