Heat recovery from exhaust gases – how does it work and when is it profitable?

How does heat recovery from exhaust gases work?

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

When recovering heat from waste gases, the most commonly used technologies are:

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

What exhaust gases can heat be recovered from?

Exhaust gases from which heat can be recovered are generated in many industrial and energy processes, where fuels are burned or materials are processed at high temperatures. Heat recovery from exhaust gases is not limited to furnaces and boilers. It can be used in various industrial and energy processes, where hot exhaust gases are generated. These include, for example:

• gas turbines and combustion engines,
• metallurgical and steelmaking furnaces,
• cement and glass installations – hot exhaust gases from rotary and melting furnaces,
• chemical and refinery industry – waste heat from reactor or cracking furnace exhaust gases,
• waste incinerators and biogas plants.

In each of these cases, it is crucial to adapt the recovery technology to the temperature and composition of the exhaust gases in order to ensure maximum energy efficiency.

Application of heat recovery depending on the exhaust gas temperature

The possibility of using heat recovery, and in particular the profitability of such an investment, depends on the exhaust gas temperature and the type of available technologies:

• Above 500°C – high-temperature exhaust gases are generated, for example, in steelworks, cement plants and combined heat and power plants. Heat can be used to produce process steam or convert it into electricity in ORC systems,
• 200–500°C – industrial boilers, metallurgical furnaces and chemical installations operate in this range. For example, economizers and recuperators can be used here to heat process water or combustion air,
• 100–200°C – condensing heat exchangers will prove useful here, allowing energy recovery from exhaust gases in the food, paper and HVAC industries,
• Below 100°C – heat recovery becomes less profitable, but in some cases it can be used to preheat air, ventilation systems or it allows the use of a high-temperature heat pump. 

Can heat recovery from flue gases be implemented without stopping production?

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

"The degree of interference depends on the type of installation and available space – in the case of industrial boilers, gas turbines or metallurgical furnaces, modernization may require technological breaks, while in systems where heat exchangers can be installed parallel to the existing system, the work can be carried out in stages, minimizing the impact on production continuity. Before implementing heat recovery, a technical analysis is always carried out to determine the optimal method of installation with the least possible disruption to the plant." – says MSc Eng. Przemysław Wojciechowski, Project Manager at DB Energy.

Advantages of heat recovery from exhaust gases

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

Additionally, reducing the combustion of fossil fuels translates into a reduction in carbon dioxide emissions and other pollutants, which is important both due to environmental regulations and the ESG policy of many companies. Heat recovery also improves energy efficiency, allowing for the optimal use of resources. In some cases, it is even possible to generate electricity through the use of technologies such as ORC, which translates into significant savings. An important advantage is also the versatility of the solution – heat recovery can be adapted to various industrial processes and a wide range of exhaust gas temperatures, which means that it can be used in many sectors of the economy.

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Disadvantages and limitations of heat recovery from exhaust gases

Despite the obvious benefits, heat recovery is not always cost-effective or possible to implement. Why?

• if the exhaust gas temperature is too low, the recovery efficiency is limited, which may result in no return on investment or a very long payback period,
• the exhaust gases may contain substances that cause soot deposits or corrosion of heat exchangers, which increases maintenance costs,
• in some industrial plants there may be a lack of space for the installation of heat exchangers or additional energy recovery systems,
• installing heat exchangers, ORC systems or recuperators may require large capital expenditures, which may be discouraging,
• if the installation does not operate continuously, the payback period may be longer.

In summary, heat recovery from exhaust gases is a solution that can bring significant savings and environmental benefits, especially in energy-intensive industries and in high-temperature processes. Although investment in heat recovery technologies may require initial outlays, in the long term it allows for lower operating costs, reduced CO₂ emissions and improved energy efficiency. It is worth remembering that the key to the profitability of the solution is proper analysis, which will help to adapt the recovery technology to the specifics of the process and exhaust gas temperature. Properly implemented heat recovery is a step towards more sustainable and profitable energy management in industrial plants, which is an important element in the pursuit of improving energy efficiency and compliance with environmental regulations.