Pyrolysis: Difference between revisions

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 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 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, 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 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" />.