Pyrolysis
Written by Katarzyna Jagodzinska Last updated on 14 Mar 2023
Ever had a barbecue? You must then have heard about BBQ charcoal, which is produced through pyrolysis. But what actually is pyrolysis?
Pyrolysis is a thermochemical process that utilises heat to promote the chemical transformation of the feedstock. It occurs in stages – first, when heating the feedstock, water (moisture) evaporates, then the breaking down of organic molecules occurs, resulting in releasing of volatile products (pyrovapours), which, eventually, react with each other (secondary reactions of volatiles). To make it happen, the absence of oxygen is required; otherwise, the feedstock would get incinerated. This means that pyrolysis occurs in an inert atmosphere. For example, in lab-scale test installations, such an atmosphere is ensured by flushing the pyrolysis reactor with nitrogen or other inert gas (e.g., argon). In contrast, in larger installations, due to an extensive amount of feedstock, the inert (or, to be more accurate, close to inert) atmosphere is ensured by the feedstock itself as it decomposes into pyrovapours (with low oxygen content). When pyrovapours reach a considerable amount, they cause a pressure increase in the reactor, thus preventing the air (and hence oxygen) from leaking in.
Another parameter crucial for pyrolysis is temperature. As its temperature range can be wide (200°C-1000°C), to communicate more precisely, the term low-temperature pyrolysis, or torrefaction, is used for processes occurring up to 300°C, whereas for processes occurring at temperatures higher than 700°C the term high-temperature pyrolysis is frequently used. The choice of process temperature depends on the desirable process product – here, the general rule is that lower temperatures favour solid process product formation, so the higher the temperature, the more pyrovapours produced. Also, increasing the temperature results in a more intense forming of non-condensable pyrovapours (often called permanent gases). Therefore, picking the correct process temperature is crucial as it significantly impacts the product characteristics.
The process parameter closely related to temperature is the heating rate, which also strongly determines the type of product obtained. Considering the heating rate, three main pyrolysis types are distinguished – slow, fast, and flash. There is no standard definition of these types, but the rule of thumb is that when speaking about slow pyrolysis, the heating rates are typically below 10°C/s, whereas fast pyrolysis can achieve rates exceeding 100°C/s. Last but not least, in some cases, the heating rate can exceed 1000°C/s – then the process is called flash pyrolysis. Similarly to the temperature, the relationship between heating rate and process product characteristics can be expressed as a general tendency – the slower the process, the higher the solid product yield.
Last but not least – the process temperature and heating rate are interwoven with the so-called residence time, so the time feedstock is subjected to pyrolysis. Here, the general rule is that the longer the time, the higher the solid residue yield, but also more intensive secondary or even tertiary reactions of pyrovapours. These reactions are often unfavourable as they are uncontrollable (due to their complexity and unpredictability) and can significantly change the process product characteristics. This is why properly optimising and closely monitoring pyrolysis parameters are crucial for reaching the desired process product.
Pyrolysis applications
As briefly mentioned before, pyrolysis can be divided into various types regarding its temperature or heating rate. However, the process can also be classified considering the desired product; hence often in discussions about pyrolysis, the terms ‘conventional pyrolysis’ (commonly called just pyrolysis), ‘catalytic pyrolysis’ or already mentioned ‘torrefaction’ can also be heard.
When the primary focus of the process is solid yield (often called biochar if the feedstock is of biomass origin), we are talking about torrefaction. In our daily lives, we might know torrefaction from coffee roasting. However, among industrial professionals, usually, torrefaction is mentioned in three contexts. First is the energy applications of biomass raw materials/waste, or, more precisely, the increased efficiency of such applications when replacing raw feedstocks with torrefied ones (biochar) characterised by higher energy density. Also, in this context, biochar as a carbon source in various industrial applications is promoted as a part of phasing out fossil fuels. Besides that, biochar as a fertiliser in agriculture is a second broadly discussed application. Thirdly, biochar gained much attention as an adsorbent and immobiliser of heavy metals, phenols, and other harmful compounds from soil and water. Several of these applications still need multifaceted research and development; yet, torrefaction broad commercialisation seems inevitable. Having said that, it is worth mentioning, as a curiosity, that torrefaction can also be seen as a waste pretreatment method before further processing, for instance, in the case of fine residues after shredding end-of-life vehicles, which broadens its utilisation potential.
In the public discourse, biofuels are present more and more frequently, and when discussing their production, the so-called conventional pyrolysis is often mentioned. In such cases, usually, the bio-based feedstocks (as in torrefaction) are subjected to medium temperatures (450-600°C) and fast pyrolysis (as it favours the formation of pyrovapours which condense to the so-called pyrolysis oil). However, in light of the supply risk of crude oil and various fuel types and the simultaneous global waste crisis, plastic waste, as the source of valuable chemicals and fuels, began to receive attention along with biomass. In most cases, though, for both biomass and plastic feedstocks, an upgrading of pyrolysis oil is necessary to obtain a high-quality end product. For that reason, increasing emphasis is put on catalytic pyrolysis, in which the catalyst is used to upgrade the quality of the obtained pyrovapours. The catalyst is individually selected to fit the feedstock properties, process parameters, and desired product. It can enable, among others, the stabilisation of pyrolysis oil composition (together with reducing its complexity) and decreasing oxygen content (considerable especially for biomass feedstocks). Thanks to that, in the case of plastic waste, the range of its recycling/recovery paths broadens to include the production of diesel or petrol fuels, monomers or food-grade plastics from their pyrolysis oils.
One of the emerging paths of applying pyrolysis in waste management systems, gaining more and more attention, is its use in energy and material (fibres) recovery from decommissioned wind turbine blades. Their pyrolysis is not commercialised yet, as there is still a need for research and development in this area; however, researchers claim that it has considerable economic potential after upscaling. Similarly, more and more attention is paid to the production of higher-value products from waste plastics, such as hydrogen and carbon nanotubes (CNTs), which can be used in several industrial applications (e.g., composite materials, which could be used in space applications or energy storage). Finally, what has to be said is that certainly, there are more potential and promising pyrolysis applications not mentioned here.
Pyrolysis reactors
The type of pyrolysis reactor depends on the feedstock properties and desired product characteristics determining the process parameters. In general, pyrolysis reactors are categorised by their scale, the required size of feedstock particles, operation mode (i.e., batch or continuous), and heat transfer mechanism (i.e., mixing with a preheated heat carrier or contact with a heated surface).
A concise yet thorough summary of pyrolysis reactors with their advantages and disadvantages can be found here.
Examples of existing pyrolysis installations: