Heat recovery from flue gas – how it works and when it’s worth it

The heat recovery process consists of capturing the thermal energy contained in flue gases and transferring it to another medium, e.g. water, air, or thermal oil. Various types of heat exchangers are used for this purpose, adapted to the temperature and composition of exhaust gases.

For heat recovery from exhaust gases, the most commonly used technologies are:

• economizers – used in industrial boilers to heat feed water, increasing combustion efficiency,
• recuperators – heat exchangers transferring energy to air, reducing fuel consumption in industrial and heating processes,
• condensing heat exchangers – use latent heat contained in water vapor, increasing energy recovery in low-temperature systems,
• ORC systems (Organic Rankine Cycle) – allow conversion of heat into electricity, even at relatively low temperatures.

Flue gases from which heat can be recovered are produced in many industrial and energy processes involving fuel combustion or material processing at high temperatures. Heat recovery from flue gases is not limited to furnaces and boilers. It can be applied in various industrial and energy processes where hot exhaust gases are produced. Examples include:

• gas turbines and internal combustion engines,
• metallurgical and smelting furnaces,
• cement and glass plants – hot flue gases from rotary and melting furnaces,
• chemical and refinery industries – waste heat from reactor flue gases or cracking furnaces,
• waste incinerators and biogas plants.

In each case, it is crucial to adapt the recovery technology to the temperature and composition of the flue gases to ensure maximum energy efficiency.

The possibility of applying heat recovery, and particularly the profitability of such investment, depends on the flue gas temperature and the type of available technologies:

• Above 500°C – high-temperature flue gases occur for example in steelworks, cement plants, and CHP plants. The heat can be used for process steam production or conversion into electricity in ORC systems,
• 200–500°C – this range covers industrial boilers, metallurgical furnaces, and chemical installations. Economizers and recuperators can be used here to heat process water or combustion air,
• 100–200°C – condensing heat exchangers work well here, enabling energy recovery from exhaust gases in food, paper industries and HVAC systems,
• Below 100°C – heat recovery becomes less profitable, but in some cases it can be used for preheating air, ventilation systems, or enable the use of high-temperature heat pumps.

In many cases, heat recovery systems from flue gases can be installed without stopping production, especially if the plant infrastructure allows for modular installation of heat exchangers or in existing flue ducts. However, in more complex installations, where, for example, flue gas system redesign, new connections, or large equipment installation is required, a production shutdown may be necessary.

Heat recovery from flue gases brings benefits both economically and environmentally. Primarily, it allows for a significant reduction in fuel consumption because energy that would otherwise be lost can be reused to heat water, process air, or generate electricity. This leads to obvious financial savings – by using already generated heat, costs for purchasing utilities can be avoided.

Additionally, reducing fossil fuel combustion results in lower emissions of CO₂ and other pollutants, which is important both due to environmental regulations and the ESG policies of many companies. Heat recovery also improves energy efficiency, allowing optimal resource use. In some cases, it is even possible to generate electricity by applying technologies such as ORC, which translates into significant savings. Another important advantage is the solution’s versatility – heat recovery can be adapted to various industrial processes and a wide range of flue gas temperatures, making it applicable in many sectors of the economy.

Despite obvious benefits, heat recovery is not always the best or easiest option. The main limitation is the initial investment cost – installation of heat exchangers and related infrastructure may be expensive and technically challenging, especially in older plants not originally designed for such systems. The complexity of installation and potential need to temporarily stop production may also discourage some companies.

Another issue is flue gas composition – flue gases containing corrosive or fouling substances may require special materials and cleaning systems, which increases maintenance costs and complexity. In some processes, temperature of flue gases is too low to economically justify heat recovery technologies. Lastly, the return on investment depends on the scale of energy savings and fuel prices, so for small installations or where fuel costs are low, the payback period can be long.

Heat recovery becomes profitable when the recovered heat is efficiently used and when the investment and operating costs are outweighed by savings on fuel and energy consumption. Factors that increase profitability include:

• high and stable flue gas temperatures,
• continuous operation and sufficient volume of flue gases,
• possibility to use recovered heat in production processes, heating, or electricity generation,
• rising energy prices and strict emission regulations,
• access to financial support, subsidies, or green financing.

Before investment, it is advisable to perform a detailed technical and economic analysis to estimate the expected savings, payback time, and possible risks.

Heat recovery from flue gases is a proven method to improve energy efficiency and reduce costs in many industrial sectors. By using various technologies adapted to flue gas temperature and composition, it is possible to significantly lower fuel consumption and emissions. Despite some challenges related to investment costs and technical complexity, heat recovery remains a profitable solution in many cases, especially when combined with ongoing energy modernization and sustainability goals.