Pyrolysis: Difference between revisions

[[Category:Technologies & Solutions]]

[[Pyrolysis]] is the thermal degradation of waste in the absence of oxygen to produce gas ([[Syngas|syngas]]), liquid and solid char fractions. The [[Syngas|syngas]] is then generally burnt to raise steam and create electricity, but many plants are exploring the option of cleaning the [[Syngas|syngas]] for use in a gas engine or separating the gas into usable fractions such as hydrogen for use, as an example, of liquid fuels which in turn may be eligible under the [[RTFO]]. The solid residue (sometimes described as a char) is a combination of non-combustible materials and carbon.

The process generally requires an external heat source to maintain the temperature required and to avoid the introduction of air. There are a range of different types of approach, often aligned to the waste being processed, and include fixed bed reactors, batch or semi-batch reactors, rotary kilns, fluidized bed reactors, microwave assisted reactors and some innovative solutions like plasma or solar reactors <ref> Czajczyńska et al, 2017. Potential of pyrolysis processes in the waste management sector. Thermal Science and Engineering Progress, [online] 3, pp.171-197.  </ref>.

Typically, lower temperatures of between 300C to 850C are used during Pyrolysis of materials such as [[MSW]]. Pyrolysis processes tend to prefer consistent feedstocks and there is a limited track record of commercial scale Pyrolysis plant accepting [[MSW]] but a better track record of, for example, Pyrolysis of [[Biomass|biomass]] <ref> Defra, 2013. [https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/221035/pb13888-thermal-treatment-waste.pdf Advanced Thermal Treatment of Municipal Solid Waste.] London. </ref>.

The pyrolysis process can be divided into three subcategories depending on the operating conditions: temperature, heating rate, particle size and solid residence time. The different types of pyrolysis are slow/conventional, fast and flash.

Slow/conventional pyrolysis is a process where the heating rate is kept slow (around 0.1-1°C/s) which results in higher char yield than the liquid and gaseous products. This type of pyrolysis has been used for thousands of years for the production of charcoal. [[Biomass]] is heated to around 500°C in slow [[Wood|wood]] pyrolysis and the long vapour residence time in the reactor means the gas-phase products have ample opportunities to react with other products to form char<ref name="ref1">[http://www.eagri.org/eagri50/AENG352/lec10.pdf Principles of Pyrolysis]</ref>.

Fast pyrolysis provides conditions to maximise the production of the liquid fraction of the products that are bio-oils (e.g. pyrolysis oil and biocrude). The primary goal is producing a renewable fuel intermediate in the pathway to finished hydrocarbon fuels from [[Biomass|biomass]] feedstock<ref>[https://www.sciencedirect.com/science/article/pii/B9780128182130000011 Pyrolysis of Biomass for Fuels and Chemicals]</ref>. The typical fast pyrolysis conditions are: moderate pyrolysis treatment temperatures (600-1000°C), rapid heating rates of biomass particles (>100°C/min), combined with short residence times of the biomass particles and pyrolysis vapours (0.5-2s) at high temperatures. Combination of medium pyrolysis temperature with short vapour residence time ensures high yield of good quality pyrolysis liquids while keeping the char and gas yields to a minimum. Due to the low thermal conductivity of [[Biomass|biomass]], small particle sizes are required to achieve the high heating rate needed<ref>[https://www.sciencedirect.com/science/article/pii/B9780081004555000217 Handbook of Biofuels Production]</ref>. Fluidised bed reactors are best suited for this type of pyrolysis due to offering high heating rates, rapid devolatilization and are easy to operate<ref name="ref1" />.

Flash pyrolysis (sometimes referred to as very fast pyrolysis) is characterised by rapid heating rates (>1000°C/s) and high reaction temperatures (900-1300°C), producing high yields of bio-oil with low resulting water content and conversion efficiencies of up to 70%. The residence time is shorter than fast pyrolysis at less than 0.5 seconds. The [[Biomass|biomass]] feedstock must go through vigorous pre-treatment to reduce particle size as much as possible (around 105-250 μm) which is required to achieve such high heating and heat transfer rates<ref>[https://www.sciencedirect.com/science/article/pii/B9780444538789000096 New and Future Developments in Catalysis]</ref>. Entrained flow or fluidised bed reactors are considered the best pyrolyzers for this purpose<ref name="ref1" />.  

==References==

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