Feb-2025

Unlocking the potential of waste heat recovery

Boosting efficiency and sustainability with waste heat recovery systems in the chemical industry.

Sara Milanesi
Exergy International

Article Summary

Decarbonising chemical processes, much like other energy-intensive processes, is essential to achieve net zero by 2050 (IEA, 2023). In fact, the chemical sector accounts for approximately 14.5% of all industrial CO₂ emissions (1,342 Mt from a total of 9,316 Mt) and ranks as the largest industrial energy consumer (IEA, 2022).

Global demand for chemical products is expected to grow by around 2.5 times by 2050, leading to a projected increase in both energy and non-energy uses of raw materials, heat, and electricity from 47.6 EJ to 88 EJ per year (Perego & Ricci, 2023).

This scenario presents a unique challenge with respect to decarbonisation of the chemical industry as it is heavily reliant on fossil feedstocks (coal, crude oil, and natural gas) both as a source of energy and as raw materials.

Various technologies and measures can be deployed to reduce energy intensity and mitigate carbon emissions in the sector. These include the production and use of green hydrogen, carbon capture utilisation and storage (CCUS) solutions, circular reuse of plastic waste, replacement of fossil fuel raw materials with biomass, and improvements in energy efficiency for process heat.

Waste heat potential in the chemical industry
Among energy efficiency measures, the recovery of waste heat represents a viable and rapidly deployable solution that can optimise energy use and enhance the overall sustainability of chemical processes.

In one study, the Knowledge Center on Organic Rankine Cycle Technology (KCORC) identifies the waste recovery potential for 1,175 energy-intensive industrial sites across seven EU countries (see Figure 1) (KCORC, 2022).

Estimates show that more than 50 MW of thermal energy is produced at each site, with 11.7% represented by the chemical industry. For waste heat sources above 250°C, considered more economically attractive, Organic Rankine Cycle (ORC) waste heat recovery plants could allow the installation of around 3.6 GW of electrical power in these industrial sites (see Table 1).

Organic Rankine Cycle technology for waste heat recovery in the chemical sector
Exhaust heat is generated during various stages of chemical processes such as distillation, reaction processes, heat exchange and cooling systems, exhaust gases from combustion processes, and ventilation equipment.

This waste heat can be recovered and reused by employing different technologies depending on its characteristics and application.
One of the most efficient and economically viable technologies for waste heat recovery is the ORC, which operates effectively within a temperature range of 90°C to 400°C.

The ORC system is similar to the traditional Clausius-Rankine cycle, commonly used for electricity generation, but employs organic substances as the working fluid instead of water (steam). These substances have a lower boiling point and higher vapour pressure, making them more suitable for generating electricity from low-temperature heat sources. The organic fluid, which can be a hydrocarbon or a refrigerant, is selected based on its thermodynamic properties that best suit the available heat source. This allows for higher cycle and turbine efficiencies to be achieved.

The ORC operates as a closed thermodynamic cycle. Heat from a primary source warms and vaporises the organic fluid, which then expands through a turbine, producing mechanical energy that is converted into electricity. The fluid is then condensed and pressurised to restart the cycle.

Using an ORC cycle for waste heat recovery at low and medium-high temperatures or for small applications gives some advantages over a steam cycle, including:
• Higher efficiency and flexibility in operations, even at partial loads.
• Automated operations, which avoid dedicated trained personnel to run them.
• Low, easy maintenance and a long plant lifecycle.
• Enabling operation without water consumption by choosing an air-cooled condenser.

These characteristics make ORC technology a flexible, customisable solution suitable for retrofitting existing sites or greenfield installations.

In the chemical sector, ORC systems can recover waste heat from distillation processes by harnessing available heat from the overhead vapour of the distillation column and the non-condensable gas (NCG) stream from stripping columns. In these cases, the ORC system serves a dual function, providing significant benefits: it replaces the conventional condensers at the top of the column while simultaneously generating electrical energy (see Figures 2 and 3).

The process steam, NCG flow, and organic working fluid never come into contact, avoiding the potential issue of process steam and condensate contamination.

ORC waste heat recovery system with radial outflow turbine
Exergy International is a global provider of clean energy technologies and an expert in the design and supply of ORC systems. In 2009, the company introduced the Radial Outflow Turbine (ROT) for ORC systems (see Figure 4).