Industrial waste heat recovery: save energy and reduce costs

During manufacturing, a significant amount of waste heat is generated—thermal energy released into the environment without practical use. Recent technological advancements have made it more feasible to capture and reuse waste heat for generating thermal and electrical power. If you work in an industry where thermal processes are essential, you may have heard about waste heat recovery. Despite this, many organizations still do not utilize its full potential, particularly in low-grade heat recovery. In this article, we will explore how heat recovery can help you save energy, reduce greenhouse gas emissions, and lower operating costs.

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What is industrial waste heat?

Waste heat is the thermal energy from industrial processes released into the environment without any practical use. Depending on your industry, this heat may escape as warm water, steam, combustion products, or in other forms. It is estimated that between 20% to 50% of industrial energy input is lost as waste heat.

In the Netherlands, for instance, the industries generating the largest amounts of waste heat include:

• Chemical and petrochemical
• Iron and steel
• Food and textile
• Construction
• Non-metallic minerals

Why you should consider waste heat recovery

Waste heat recovery is widely used across industries—and for good reason. The fundamental principle is to capture heat created in one part of your facility and reuse it in other applications, making it one of the easiest and most cost-effective methods to enhance overall efficiency in industrial processes. This approach offers significant advantages:

You save energy and lower GHG emissions

Conserving energy and reducing greenhouse gas emissions are becoming increasingly crucial. Our world has limited resources, and we face a climate crisis. Therefore, optimizing resource use should be a business priority. This not only helps avoid public backlash but also attracts talented employees who are concerned about their impact on the world. Additionally, adherence to regional regulations on energy savings and emission reductions presents further justification for making these efforts a priority.

You reduce operating costs

Recovering waste heat enhances overall efficiency in industrial processes. By doing so, you can increase output with the same resources. In some instances, recovered heat can replace other heating sources entirely, reducing the need for additional waste heat treatment facilities. Furthermore, waste heat recovery could help you qualify for governmental incentives and subsidies, positively impacting your bottom line.

How waste heat is classified

Waste heat media are typically classified based on temperature. A large majority of waste heat media falls under Low-Grade Waste Heat (LGWH). This typically results from processes in the chemical, petrochemical, food, and textile industries. Recovering low-grade heat presents more challenges than higher-temperature waste heat, as heat transfer is contingent on temperature differences between the source and intended recipient. However, ongoing advancements in cutting-edge heat recovery techniques may change this situation.

The main sources of low-grade waste heat

Identifying sources of waste heat is a vital step toward a successful recovery initiative. The following list provides various sources of low-grade waste heat found across industries. If you encounter any of these in your industrial activities, consider exploring the potential for low-grade waste heat recovery.

Liquid medium

• Wastewater from heat exchangers (60°C)
• Condensed water from drying machines (80-90 °C)
• Hot oil waste from cooking processes (100-175 °C)
• Wastewater from cleaning processes (60°C)

Gaseous medium

• Exhaust steam from cooking with fryers or ovens (150-200 °C)
• Exhaust air from drying with spray/rotary dryers (110-160 °C)
• Hot air expelled from clinker coolers during cement production (100°C)

How waste heat can be recovered

The choice of a suitable waste heat recovery method depends on two main factors: the waste heat sources and the potential users. Heat in hot, moist air exiting an industrial dryer can be directly recovered through a heat exchanger. Subsequently, the recovered heat can be utilized in an absorption chiller to cool products post-drying.

This diagram illustrates the process of transferring heat from a source to a recipient:

Regarding temperature, one can assume that waste heat media:

• with a high temperature is directly usable or exchangeable
• with a medium temperature may require heat upgrading
• with a low temperature will always need heat upgrading

It's important to note that not all heat can be repurposed. Generally, high-grade waste heat yields more energy when reused than low-grade waste heat. Nevertheless, new technological advancements are enhancing the feasibility of low-grade heat recovery.

How to recover low-grade waste heat

Options for recovering low-grade waste heat are relatively new, but recent promising technologies have emerged. Some examples include:

Heat upgrade or storage using adsorption techniques

Adsorption refers to the adhesion of molecules (typically in gaseous or liquid states) onto the surface of an adsorber. This can be observed, for instance, in the adhesion of water molecules onto silica gel beads commonly used for moisture control in packaging. The adsorption process is generally exothermic, meaning heat is released to the surroundings as vapor molecules saturate the adsorber surface. Conversely, the adsorber can be regenerated to its original state when heated.

Use cases

• Simultaneous dehumidification and heat upgrading can be applied in drying processes.
• Dry adsorbers can serve as heat storage media.

Working materials

• Water (vapor) or ammonia, combined with adsorbers like zeolites, silica gel, or activated carbon.
• New adsorber materials are still under development.

Advantages

• Simple setup featuring minimal moving parts.
• Adaptable adsorber beds for long-term energy storage.
• Minimum maintenance required in a closed system.
• Lower energy loss due to irreversibility, as the adsorber does not need to circulate.
• High scalability potential.

Heat upgrade using absorption techniques

Absorption describes the process in which molecules of a compound dissolve within the volume of another substance, resulting in a concentration change. In certain absorbent-absorber pairs, this phenomenon also generates heat that can be harnessed for thermal processes. Recently, absorption processes are being utilized in heat pumps, replacing traditional mechanical compressors.

Use cases

• Absorption heat pumps can be used for recycling waste heat with minimal additional energy consumption.
• A significant temperature upgrade can be obtained using an absorption heat transformer.

Working materials

• Water mixed with soluble compounds (e.g., LiBr or phosphate salt).
• Hydrocarbon-based pairs are still under development.

Advantages

• Simple setup with few moving parts.
• Continuous operation of the system.
• Insignificant energy input required for the solvent pump compared to classical vapor-compression systems.
• Using suitable absorber-refrigerant pairs can yield significant temperature boosts.
• High scalability potential.

Heat conversion (Organic Rankine cycle)

The Rankine cycle is a thermodynamic process for generating mechanical or electrical energy. This cycle has been adapted to utilize organic compounds as refrigerants, enabling operation at lower temperatures and allowing the use of waste heat.

Use cases

• An Organic Rankine Cycle (ORC) generator can produce electricity from low-grade waste heat.
• The shaft power generated by an ORC can assist in operating the compressor of another heat pump for heat upgrading.

Working materials

Organic compounds, such as butane or heptane.

Advantages

• Compatible with low-grade waste heat, as the ORC typically doesn't require superheating.
• Organic compounds do not condense within the turbine or expander.
• Low turbine inlet temperatures enhance longevity.
• ORC generators can be produced in a compact form.

The best solution for your situation

To select the most suitable waste heat recovery system, it’s important to answer the following questions:

What kind of heat are you dealing with?

The following variables will impact the construction of your waste heat recovery system:

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• The temperature of the waste heat.
• The quantity of the waste heat.
• When and where the waste heat is available.

Sometimes the combination of these factors may suggest that you cannot recover your waste heat, but often there is great potential.

How will you use the recovered waste heat?

Before selecting the best solution for your needs, consider how you intend to use the recovered waste heat. Where should the heat be directed?

How do I know if it’s worth the investment?

All the information provided in this article may sound appealing, but you may still be wondering:
Is this worth the investment?

This is an important question, but it cannot be addressed in general terms here. Payback times depend on several factors, including:

• The lifecycle costs of the heat recovery system.
• The expected lifespan of the heat recovery system.
• The system’s efficiency.
• Potential savings in energy usage and required equipment.
• Available governmental incentives and subsidies.

Every situation is unique, and we would be delighted to analyze yours. Through measurements and modeling, we can determine the energy balance of your industrial processes and predict the potential (positive) impact of energy recovery. Our goal is to provide you with a clear understanding of the potential ROI before you invest in a waste heat recovery solution.

How we can help

1. Site Survey

We conduct measurements and utilize our proprietary air technical modeling software (developed in-house) to gain an accurate understanding of your current situation. This approach clarifies where improvements can be made.

2. Identify possibilities & predict impact

Using the site survey results, we assess the energy balance of your industrial processes and forecast the positive outcomes from energy recovery. How can the recovered energy be utilized? What impact will it have on your bottom line?

3. Integration of a customized waste heat recovery solution

We oversee everything: design, construction, and implementation of the solution.

Find out how much energy and money you could save using waste heat recovery

Is waste heat a significant concern for your operations? Get in touch with us, and we will help you discover how your business could benefit from waste heat recovery.