Effects of Pyrolysis Parameters on Products Yields

The percentage yields of pyrolysis products can be impacted by several parameters, including temperature, residence time, pressure, heating rate, feedstock composition, catalyst, and reactor type. In this article, we highlight the impact of the aforementioned parameters on the pyrolysis process of plastic waste.

1- Temperature:

Temperature is one of the determinant factors affecting the pyrolytic reactions. It significantly influences both the quality and quantity of the produced gas, solid residues, and liquid products from the pyrolysis of waste plastics. Additionally, the operating temperature required depends strongly on the product preference. If the gaseous product is preferred, a higher temperature (above approximately 500⁰C) is suggested. If the liquid product is preferred instead, a lower temperature in the range of 300-500⁰C is recommended and this condition is applicable for all types of plastics. It has to be noted that the yield of the liquid product reaches a maximum at the optimum temperature and then begins to decrease with a further increase in temperature.

Various studies demonstrate the effect of temperature on the liquid yield of different feedstocks. For instance, the results of a study performed on the effect of temperature (250–500⁰C) on polystyrene waste pyrolysis showed that low temperatures resulted in the formation of long-chain hydrocarbons. However, higher temperatures resulted in the formation of short-chain hydrocarbons due to the rapid cracking of C–C bonds. Similarly, aromatic compounds were formed at higher temperatures as a result of the triggering secondary process reactions.

2- Heating Rate

Another factor that affects the output significantly is the heating rate. Slow heating rates coupled with relatively low temperatures (e.g. 400 ⁰C) maximize the yield of char. Moderate heating rates of about 10 °C/min and maximum temperatures of 600 °C give a more even weight distribution of pyrolysis outputs. High heating rates of about 1000 °C/s at temperatures below 650 °C and with rapid quenching, favor the formation of liquid products and minimize char and gas formation; these process conditions are often referred to as ‘flash pyrolysis’. High heating rates to temperatures above 650 °C tend to favor the formation of gaseous products at the expense of liquids.

3- Residence Time and Pressure:

Residence time and pressure are both temperature-dependent factors that may affect the pyrolysis process and the product yields at lower temperatures. For example, the results of a study focused on the effect of temperature and residence time on the HDPE pyrolysis product distribution showed that high residence time resulted in a higher gas product when the temperature did not exceed 685°C. At temperatures above 685°C, however, the influence of residence time was less on the yield of the gaseous and liquid products. In another study, the effect of pressure on thermal degradation of HDPE was investigated. It was found that as the pressure increased from 0.1 to 0.8 MPa, the gaseous product yield increased dramatically from about 6 wt% to 13 wt% at 410°C, but only slightly at 440°C from 4 wt% to 6 wt%.

Based on the literature, most researchers focus more on the temperature parameter rather than on residence time while conducting plastic waste pyrolysis studies. This is because the residence time effect at higher temperatures becomes less evident. Moreover, if the pressure factor is deemed, additional units such as pressure transmitter and compressor must be augmented to the entire system, thus increasing the operating cost. However, it should be noted that these two factors should be put under consideration based on the product distribution preference especially when operating at temperatures below 450 ⁰C.

4- Use of Catalysts

Catalysts are widely used in the pyrolysis process to speed up the chemical reaction and improve the hydrocarbon distribution in order to obtain pyrolysis liquid that has similar properties to conventional fuels such as gasoline and diesel. Catalysts can be applied either directly in the pyrolysis reactor (in-situ) or through a second reactor for the actual catalytic process (ex-situ). Three common catalysts used in plastic pyrolysis are zeolites, fluid catalytic cracking (FCC) and silica-alumina catalysts. Recent studies showed that the use of zeolite catalysts in plastic pyrolysis only maximized the production of volatile hydrocarbons. As for higher efficiency and longer cycle time usage, HZSM zeolite was recommended since the deactivation rate of the catalyst was extremely low and thus, more efficient for regeneration. Moreover, it was found that FCC and silica-alumina catalysts maximized liquid oil production. These two catalysts were comparable in terms of liquid oil production but FCC had better catalytic performance.

5- Reactor Type

The configuration of the pyrolysis reactor is one of the essential parameters, as it influences the heat transfer, mass transfer, residence time, interaction of the volatiles, the escape of main products, etc. Up to now, there are various types of reactors including rotary-kiln, fixed-bed, batch and semi-batch, fluidized-bed, tubular, plasma, microwave, and conical spouted bed reactors (CSBR) that have been used in the pyrolysis of different waste.

Each reactor may have its own advantages and disadvantages depending on the application. Batch or semi-batch reactors are usually used in thermal pyrolysis since the operating parameters can be easily controlled. Nevertheless, these reactors might not be suitable for catalytic pyrolysis because of the potential of coke formation on the surface of the catalyst, which reduces the catalyst efficiency over time. The fluidized bed reactor is considered to be the best reactor to perform catalytic plastic pyrolysis since the catalyst can be reused many times without the need for discharging, especially worth considering if the catalyst is a very expensive substance. On the other hand, CSBR also provides good mixing with the ability to handle large particle size distribution and low bed segregation than the bubbling fluidized bed reactors. However, a variety of technical challenges during the operation of this reactor have been encountered, such as its complicated design requiring many pumps, catalyst feeding, catalyst entrainment and product (solid and liquid) collection that make it less favorable.

6- Feedstock Composition

The feedstock composition also affects the pyrolysis process. For instance, PE and PP types of plastics require higher temperatures for their complete degradation as compared to PS plastic due to their complex structures. Under similar pyrolysis conditions, PS plastic is decomposed more efficiently and yields more oil than PP and PE, as PS contains branched side chains that can break at low temperatures due to their low activation energy for bond-breaking.