15 Types of Pyrolysis Reactors and their Features Explained
Types of pyrolysis reactors are; fluidized-bed, fixed-bed, vacuum, circulating, ablative, auger, rotary kiln, drum, tubular, Heinz retort, vortex, entrained-flow, wire mesh, batch, and semi-batch reactors.
This article discusses the types of pyrolysis reactors, as outlined below;
-An Overview of Pyrolysis Reactor Types
An Overview of Pyrolysis Reactor Types
The types of pyrolysis reactors are fluidized-bed, fixed-bed, vacuum, circulating, ablative, auger, rotary kiln, drum, tubular, heinz retort, vortex, entrained-flow, wire mesh, batch and semi-batch reactors.
Factors which are used to categorize pyrolysis reactors include geometry, mode of loading, and mode of heat application.
The following sections discuss each main type of pyrolysis reactor.
1). Fluidized-bed Reactor
The fluidized-bed reactor (FBR) is a vessel which contains a layer of bed material like sand at its bottom, as well as a flowing fluid that helps to prevent unwanted reactions of the substrate that is undergoing pyrolysis.
Heat transfer is carried out by the bed material, which efficiently transfers this heat to the substrate.
At the bottom of the reactor, a gas is introduced under pressure. This gas is usually pure nitrogen [7].
The role of this gas is to maintain an inert atmosphere within the reactor [1]. Such an atmosphere helps to prevent unwanted chemical reactions like combustion and hydrolysis.
It also helps to fluidize the particles of the substrate and bed material, which enables heat transfer to occur with more efficiency.
Fluidized-bed pyrolysis reactor increases the yield of byproducts like bio-oils and gases. It is effective for particulate substrates like woody biomass, although it is also used in the petroleum and chemical industries.
2). Fixed-bed Pyrolysis Reactor
A fixed-bed pyrolysis reactor is a vessel with a simple-design, where the substrate is introduced at the bottom, and heat is applied.
It works by heat transfer from the walls of the vessel to the substrate at a consistent rate, leading to thermal decomposition.
The sample (substrate) particles are usually stacked on the bed or bottom of the reactor, to a desired depth [13]. When pyrolysis begins, heat energy diffuses from the walls of the reactor, inward to the substrate.
This heat causes thermal decomposition of the substrate, which results in the desired products.
Unlike the fluidized-bed reactor, fixed-bed reactor does not require a fluid to be introduced at the bottom of the vessel. Also, pyrolysis in a fixed-bed reactor does not always require a catalyst, as catalysts may be used in some cases.
It must be noted that fixed-bed and fluidized-bed pyrolysis reactors are classified based on the working principle and internal design of the reactor vessel.
Therefore, these reactor types can be sub-classified based on other factors like geometry, so that there are reactors like tubular fixed-bed reactor.
3). Vacuum Pyrolysis Reactor
Vacuum pyrolysis reactor is simply a reactor which is equipped with a vacuum system that lowers the internal pressure of the reactor and hence the boiling point (or decomposition temperature) of the substrate [4], which leads to rapid and efficient thermal decomposition.
As the above description implies, any type of pyrolysis reactor (fixed-bed, ablative…) can be converted to a vacuum pyrolysis reactor, by simply installing a vacuum system in the reactor.
A vacuum pyrolysis reactor may work efficiently without the need for a carrier gas or catalyst. It may also be used with fast or slow heating techniques to transform a substrate into byproducts of thermal decomposition.
4). Circulating Reactor
Also known as the ‘circulating fluidized-bed reactor’, the circulating reactor was developed mainly for fast pyrolysis of biomass to produce bio-oil, char and gas [5].
It is also usable for flash pyrolysis operations [15], where the heating rate is ultra-fast.
The suitability of circulating pyrolysis reactor for these uses is due to its ability to distribute heat equally to all parts of the internal system.
A circulating pyrolysis reactor works by distributing heat from n external source, internally, through the continuous circulation of the heated substrate and fluidized-bed material.
Circulating pyrolysis reactor technology is used extensively in renewable energy production and electricity generation, because its heat distribution capabilities lead to effective conversion of the substrate into energy and/or usable fuels.
The main drawback of this type of pyrolysis reactor is its heat transfer capability.
While the circulating pyrolysis reactor can distribute internal heat equally and effectively, it is not efficient at heat transfer, because the circulating mechanism can cause heat losses. This drawback can lead to energy wastage (in the form of heat) when using the reactor.
5). Ablative Pyrolysis Reactor
Ablative pyrolysis reactor is a pressure-driven reactor which works by ‘melting’ the substrate against the heated walls of the reactor vessel.
In an ablative pyrolysis reactor, wood is pressed to the wall of the reactor using pressure from a centrifugal or mechanical source [11].
When this wood is pressed against the reactor walls, it absorbs heat effectively and melts.
As the melted wood moves away from the reactor wall, it leaves a film of residual oil that acts as a lubricant for other particles of biomass that will subsequently come into contact with the wall.
Because the rate and effectiveness of heating in a circulating pyrolysis reactor are not dependent on heat transfer between particles of the substrate, this type of reactor can work with large substrate particles [14].
Another advantage of the circulating pyrolysis reactor is that it only depends on the rate of heat supply from the external source. This eliminates the need for catalyst, inert atmosphere, and other additional requirements that may be considered in alternative pyrolysis reactors.
Ablative pyrolysis reactor also does not require a large amount of heat supply to perform fast pyrolysis. Because it is a pressure-driven system, it can perform fast pyrolysis of biomass using external heat supply of 450-600°C.
6). Auger Pyrolysis Reactor
Auger of screw pyrolysis reactor is a simple-design reactor which uses a screw mechanism, driven by a variable-speed motor, to move the substrate through the hot zone of the reactor [6].
Like the ablative reactor, this system depends on mechanical force and pressure to transfer heat effectively. It differs from the fluidized and circulatory reactors that depend on fluid dynamics.
An advantage of the auger pyrolyzer is its ability to carry out pyrolysis with moderate heat supply.
Auger pyrolysis reactor is particularly suitable for materials that are heterogeneous or difficult to handle.
It is not a good option for the production of bio-oils and volatiles, because the main product of pyrolysis in an auger reactor is biochar [16].
7). Rotary-kiln Pyrolysis Reactor
A rotary-kiln pyrolysis reactor consists of a chain-and-gear system, which is used to rotate a screw conveyor that is connected to a kiln.
The kiln is inclined at an angle and rotates at a slow pace that enables it to distribute heat effectively to the substrate within the system.
Rotary-kiln pyrolysis reactor is suitable for slow pyrolysis operations [8], and is particularly effective for the production of oil from biomass.
Factors that influence the efficiency of this type of reactor include heat supply and speed of rotation.
8). Drum Pyrolysis Reactor
The drum (or ‘rotating drum’) pyrolysis reactor is a simple type of pyrolysis reactor, whose design is similar to the rotary-kiln reactor. This reactor is classified based on its geometry.
It consists of a drum-shaped vessel to which heat is applied, and which circulates this heat internally through a continuous-rotation mechanism.
The rotating drum is usually housed in a furnace, that acts as a heat-supply unit.
Drum reactors are good for slow pyrolysis of biomass to yield gaseous byproducts like syngas, that can be used to produce heat and electricity. Other byproducts like biochar are also produced by this type of reactor.
9). Tubular Pyrolysis Reactor
The tubular pyrolysis reactor is very similar to the screw/auger pyrolyzer, and may depend on pressure or fluid dynamics to transfer heat.
This type of reactor has a simple design, and relatively low cost of operation and construction.
Factors that influence the efficiency of the tubular pyrolysis reactor include flow velocity of the substrate and the amount of heat supply.
It can be used to perform slow, fast and flash pyrolysis. However, it is best suited for the slow method.
Tubular pyrolysis reactor can work based on fixed-bed or fluidized bed mechanism, among others. The classification of this reactor is only based on its geometry.
10). Heinz Retort Pyrolysis Reactor
The heinz retort pyrolysis reactor is an airtight vessel to which heat is supplied from an external source, to heat a substance in the vessel.
It may be considered similar to an oven, and is one of the simplest types of pyrolysis reactor designs.
Heinz retort pyrolysis reactor works by simple heat transfer through the vessel walls. This heat causes thermal decomposition of the substrate.
11). Vortex Pyrolysis Reactor
Vortex pyrolysis reactor is a vessel which contains an inert gas and a rotating bed, and in which thermal decomposition of materials occurs.
It is also referred to as a Gas/Solid Vortex Reactor (GSVR) because it depends on a fluidized model of heat transfer [2).
The rotary motion of the bed is often produced by a centrifuge system. Vortex reactors are useful for biomass conversion to produce bio-oil and biochar.
12). Entrained-flow Reactor
The entrained-flow pyrolysis reactor is a vessel in which the substrate particles are heated in a fluid suspension. This suspension may be either in the form of a gas or slurry.
Entrained-flow reactor is used mainly for experimental purposes, to study the chemical and kinetic processes involved in pyrolysis of a substrate [3].
The system is also used to analyze the effect of various parameters like particle size, dynamics, and heat supply, on the pyrolytic process.
Flash (ultra-fast heating) pyrolysis is usually done in an effective manned using the entrained-flow reactor. The use of this reactor also results in more gasification than other conventional types, like the fixed-bed reactor.
13). Wire-Mesh Pyrolysis Reactor
Wire-mesh pyrolysis reactor comprises of metal grids (the mesh) in which a sample is placed and subjected to heating in the absence of oxygen.
Usually, the sample is clamped between two metal grids or meshes [12], which are exposed to heat.
Fast pyrolysis (fast heating) is common with the wire-mesh reactor.
Also, this type of pyrolysis reactor is often used for experimental purposes.
It is used to investigate the initial stages of pyrolysis, when devolatilization and gasification occur [9]. Such investigations also help to provide information regarding the vapor-phase reactions of a substrate.
Because of the simplicity of its design, the wire-mesh pyrolysis reactor minimizes secondary reactions and enables the collection of primary volatiles from the thermal decomposition of a sample.
14). Batch Reactor
Also known as ‘fixed-batch reactor’, the batch pyrolysis reactor is a simple, sealed vessel with apertures for introducing the substrate material.
It may come in various shapes and sizes as required, and is ideal for pyrolysis operations that require energy stability.
The reactor usually depends on external heat supply, and operates as a closed system, based on thermodynamic principles.
Batch pyrolysis reactor is often used for investigation of the energy stability of pyrolytic reactions.
15). Semi-Batch Pyrolysis Reactor
The semi-batch pyrolysis reactor is designed such that substrate or reactants, can be fed into the vessel in batches at intervals during the pyrolysis process.
Semi-batch reactors are mostly used when the substrate occurs in multiphase form (solid/ liquid) [10].
It is used for thermal conversion of biomass to produce bio-oil and syngas.
Factors affecting the effectiveness of pyrolysis in a semi-batch reactor include temperature, carrier gas (nitrogen) flow rate, rate of heating, residence time, and catalyst.
Conclusion
A pyrolysis reactor is a sealed vessel in which a substrate undergoes heating and thermal decomposition in the absence of oxygen.
Types of pyrolysis reactors are;
- Fluidized-bed Pyrolysis Reactor
- Fixed-bed Pyrolysis Reactor
- Vacuum Pyrolysis Reactor
- Circulating Pyrolysis Reactor
- Ablative Pyrolysis Reactor
- Auger Pyrolysis Reactor
- Rotary-kiln Pyrolysis Reactor
- Drum Pyrolysis Reactor
- Tubular Pyrolysis Reactor
- Heinz Retort Pyrolysis Reactor
- Vortex Pyrolysis Reactor
- Entrained-flow Pyrolysis Reactor
- Wire-Mesh Pyrolysis Reactor
- Batch Pyrolysis Reactor
- Semi-Batch Pyrolysis Reactor
These types are classified based on geometry/shape, mode of operation, and internal design.
A combination of reactor types may also occur; such as entrained-flow tubular, fixed-bed tubular, and circulated fluidized-bed pyrolysis reactors.
Different types of reactors are also suitable for different types of pyrolysis (based on heating conditions).
Drum, auger and rotary-kiln reactors are suitable for slow pyrolysis, while fluidized-bed reactors are suitable for fast pyrolysis.
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