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Five heat transfer system types for pyrolysis

10 minutes for read

Heat transfer systems are vital for pyrolysis as they improve the process, product quality, and production costs. 

There are five main types of heat transfer systems for pyrolysis. This article compares these five systems and elaborates on why molten salts usage is the most effective and safe form of heat transfer for waste tire pyrolysis.

What are the different types of heat transfer?

Heat transfer occurs when energy transferred from a warmer source heats a cooler object. There are three different mechanisms by which this energy transfer can occur, but a difference in temperature between the source and object is essential for all of them. Conduction, convection, and radiation are the three mechanisms of heat transfer.

Conduction involves energy transfer between molecules in direct contact. Heat energy gets transferred when molecules hit each other. Conduction can occur in solids, liquids, and air. It’s most common in solids, as molecules are closely packed, but some solids are better heat conductors than others, for example, metal is a better conductor than wood.

Convection is the heat transfer through the movement of a fluid, which can be a gas or liquid. When a fluid is heated, its molecules will move faster and away from each other. So the bulk motion of heated fluid carries the heat energy it contains in currents. Fluid movements can be natural or forced. When you boil a pot of water, the water is heated by convection. The warm air movement from the hot pot is also due to convection.

Radiation refers to the transfer of heat energy by electromagnetic waves, usually infrared and visible wavelengths, and doesn’t require any medium. The surface of a hot object emits heat energy that a cold object absorbs. For example, people standing by fire get warm on the side facing the fire but not on the other side.

Industrial heat transfer systems can simultaneously use more than one of these mechanisms.

What is a heat transfer system – and when is it used?

Heat transfer systems, also called thermal transfer systems, provide indirect heat to processes using a thermal transfer fluid.

The system uses gas or liquid, to transfer heat away from a heat source. The heat transfer fluid remains circulating, carrying heat from the energy source to cold streams and returning to the heat source for reheating. The use of heat transfer systems can reduce the number of heat sources needed.

Heat transfer systems allow for both heating and cooling and act as a temperature control unit.

All heat transfer systems involve convection, but the source of energy that heats them will determine whether conduction, radiation, or both are involved.

Heat transfer systems are necessary when processes require high temperature, even heating, and good temperature control. A well-designed high-temperature heat transfer system should have thermal and chemical stability, heat transfer efficiency, and low environmental impact. Typical fluids are thermal oils, water, glycol, or water-glycol mixtures.

The choice and flow rate of the heat transfer fluids are essential features of a heat transfer system. A heat transfer fluid must have good viscosity, thermal stability, expansion rate, flash and fire points, and oxidation resistance.

The ability to maintain a narrow range of even temperatures makes heat transfer systems ideal for pyrolysis, a thermochemical process.

During pyrolysis of end-of-life tires (ELTs) for recycling, synthetic rubber and Carbon Black are broken down into smaller and simpler compounds. Temperature influences how the polymers react; therefore, temperature control is crucial.

Moreover, the quality and the fraction of pyrolysis products from waste tires—recovered Carbon Black, steel, oil, and gas—depend on the temperature and heating rate.

The five types of heat transfer systems

The heat transfer system and mode of operation are the two criteria used to classify waste tire pyrolysis processes. There are five high-temperature heat transfer systems for pyrolysis: electricity, flue gases, microwaves, molten metals, and molten salts.

Electricity

One of the most widely used heat transfer systems for pyrolysis is electricity. Electricity use is common in kiln rotary reactors, auger or screw pyrolysers, and batch-type stirred pyrolysers. However, electricity is an expensive energy source and makes the whole process uneconomical. 

Flue gases

These gases are obtained from combustion plants and contain fuel combustion products like carbon dioxide, water vapour, heavy metals, and residual compounds such as nitrogen oxides, sulphur oxides, carbon monoxide, and particulate matter.

With flue gas, it’s possible to get high outlet temperatures up to 1200°C. However, additional pollution control technologies are necessary to remove the residual compounds. This energy source is also not safe due to the risk of gas escape and fires. Fixed bed reactor and fluidised bed pyrolysis plants use flue gas. 

Microwaves

The use of microwaves for pyrolysis is still in the early stages of development. Microwave heating has several advantages, as it can provide temperatures as high as 800°C, even heating, and easy control.

It is well suited for continuous pyrolysis and fixed bed reactors that improve pyrolysis efficiency and economics. Efforts are on in research institutes and companies to scale-up microwave pyrolysis. Pilot projects show that it is a promising technology, but many technical challenges still need to be overcome. 

Molten metal 

Liquid metals have high thermal conductivity up to 1,000°C and low viscosity, fulfilling two main criteria for heat transfer systems.

Typical molten metals are alkali metals (lithium, sodium, potassium, and their alloys), heavy metals (lead, bismuth, and their alloys), and the so-called fusible alloys (gallium, cadmium, indium, tin, thallium, and their alloys). They’re primarily utilised in solar concentrating power plants and nuclear plants.

However, molten metals are toxic, flammable, and corrosive. The complex and unique engineering requirements to handle them increase capital investments, limiting their widespread use. For example, heavy piping is necessary to transport these molten metals.  

Molten salts

Phase-change salts that remain liquid when heated are called molten salts. These can be chlorides, carbonates, nitrates, nitrites, and fluorides that can reach 1,000°C. They have a high thermal capacity, low viscosity, and moderate density. Molten salts flow like water and are easy to pump, reducing operational costs. 

Molten salt usage is ideal for pyrolysis, as it has a high heat efficiency, taking and giving heat energy with minor losses. For tire pyrolysis, molten salts usage is found in continuous, fluidised, screw, rotary, and fixed bed reactors.

The salts are safe for people, posing no fire or pressure build-up risks. Molten salts are sustainable; they can be reused for many years and have the same composition as fertilisers.

Contec: molten salts usage in pyrolisis

Compared with the other available heat transfer systems, molten salts usage gives even heating, consumes less energy, is cost-effective, environmentally friendly, and has a solid safety record for people.

Choosing a heat transfer system depends on the temperature, heating rate, and product mix required. Contec pyrolysis relies on molten salts usage to produce consistent quality rCB.

Because of its circular and safe performance, molten salts also meet clean environmental standards set by the EU for ELT recycling processes. For more information about molten salts, subscribe to our LinkedIn newsletter to receive industry-related information about the circular economy in manufacturing.

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