9.6 What is important to know about pyrolysis?
Pyrolysis is the first stage of all biomass combustion. The thermal decomposition of organic materials in the absence of oxygen into a solid, liquid, and gas mixture requires an efficient dry matter content of the input between 70 and 85%. Due to the lack of oxygen, the process does not involve complete combustion. While it is not an entirely fireless process (as heat is applied to initiate the decomposition), it may appear like a smokeless fire compared to incineration, which involves abundant oxygen and visible flames.
The pyrolysis process can be differentiated between slow, intermediate, and fast pyrolysis. While the different process modes may have similarities, the products produced by pyrolysis (char, oil, and syngas) vary significantly.
Amongst others, the products and efficacies will vary according to the type of processing. It can be summarised as follows:
- A slower heating rate, a lower pyrolysis temperature, and a longer residence time maximise the yield of solid char.
- A higher heating rate, a higher pyrolysis temperature, and a shorter residence time maximise the gas yield (syngas).
- A higher heating rate, an intermediate pyrolysis temperature, and a short residence time maximise the liquid yield (oil).
In conclusion, slow pyrolysis, characterised by comparably low temperatures between 350 and 550°C on average and long residence times that may vary from 5 minutes to several days, is used for char production. Further, considering setup and implementation, it can be viewed as the most suitable pyrolysis mode for emergency settings compared to intermediate and fast pyrolysis.
| Technology | Typical Temperature (°C) | Heating Rate (°C/s) | Typical Residence Time | Products (%) | |||
| Solid (Biochar) | Liquid (Bio-oil) | Gas (Syngas) | Carbon Content of the Solid | ||||
| Slow Pyrolysis | 350 – 550 | 0.1 – 1.0 | Long (5 – 30 min) | 35 | 30 | 35 | 50 – 95 |
| Intermediate Pyrolysis | 300 – 700 | 1.0 – 10.0 | Moderate (10 – 20 seconds) | 25 | 50 | 25 | 66 – 74 |
| Fast Pyrolysis | »500 | >200 | Short (<2 seconds) | 12 – 25 | 50 – 75 | 13 – 25 | 64 – 90 |
In addition to process conditions that influence the pyrolysis results, the composition and composition changes of sludge/biosolids may lead to materially different biochar products. Increases or decreases in process temperature or residence time will change the percentage of varying product yields and the product itself.
Some sludges may not be suitable for reuse for agricultural purposes due to the presence of micropollutants, including organic pollutants (for example, pharmaceuticals, hormone-disrupting molecules, etc.), per- and poly-fluoroalkyl substances (PFAS), polycyclic aromatic hydrocarbons (PAH) and microplastics. Scientific research has focused more on these emerging concerns and proven that sufficiently slow pyrolysis with high pyrolysis temperatures (>500°C) and long residence times (>3 min) eliminate the contaminants.
Furthermore, while pyrolysis generally can destroy several types of micropollutants in sludge, it is again slow pyrolysis with sufficiently high temperatures (>500°C) and longer durations (>3 min) that can completely or nearly wholly degrade all organic contaminants and micropollutants.
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