Pyrolysis occurs when fuel sources are heated in the absence of oxygen, with lower temperatures and longer heating times favouring the production of charcoal.
Higher temperatures drive increased biomass conversion to gas, while moderate temperatures and short vapour residence time are optimum for producing liquids. Three products – gas, liquid and solid - are always produced, but the proportions can be varied over a wide range by adjusting the different factors.
What is slow, intermediate and fast pyrolysis?
- Slow Pyrolysis
Slow pyrolysis or carbonisation has been practiced for thousands of years. It is widely used to produce high-quality charcoal for metallurgical applications such as in the production of high-grade silicon, as a leisure fuel in many developed countries, and in developing countries as an essential and storable commodity for cooking. Small-scale local production is still common and represents a cottage industry in many developed countries. Still, there are limited truly commercial operations (a notable exception is charcoal production in Brazil for iron and steel production).
Yields of up to 35 wt.% on dry feed are typically achieved, with the rest being gas and vapours. Some chemicals can be recovered from the steam condensation, but most of the by-products are burned for process heat and/or disposal.
This process is being studied in EBRI to produce charcoal for evaluation and comparison against products using intermediate and fast pyrolysis.
- Intermediate Pyrolysis
Intermediate pyrolysis offers an alternative that is ideal for challenging feedstocks. The material is physically pushed through the reactor, and the residence time and movement can be controlled.
Wood produces four products, with typical proportions falling in the following ranges: charcoal 33wt.%, aqueous liquid with dissolved organics 37wt.%, gas 20wt.% organic liquid, 10wt.%, all on a dry feed basis.
EBRI has successfully converted a wide range of biomass and waste types into gases, liquids, and solids for use in a variety of renewable energy applications for heat and power.
The main products are charcoal, which is employed as the heat carrier in the reactor, a liquid and a gas. Both of the latter can be used in engines for producing heat and power. The solid's residence time can be adjusted by rotating the screws on the reactor. The feedstock can be in any form and size, ranging from chips through to pellets and powder, and the products resulting from this process can be used for a range of purposes including in CHP engines.
Two scales of the process are available at EBRI: a nominal 20kg/h unit for process development and a nominal 100 kg/h pilot plant to demonstrate the technology.
- Fast Pyrolysis
Although many technologies have been researched and developed, only two basic processes have been commercialised successfully – fluid bed and transported, or circulating fluid bed. EBRI has four laboratory-scale continuous fast pyrolysis fluid bed reactors capable of up to 7 kg/h.
EBRI has a distinguished track record in fast pyrolysis technology, including in the fields of feedstock pre-treatment, liquid product collection, product analysis and characterisation, and most recently, in upgrading the vapours and liquids to refinery feedstock-quality hydrocarbons.
There are three main products: bio-oil, gas, and char. The main product, bio-oil, is obtained in yields of up to 75 wt.% on a dry-feed basis, together with by-product char and gas. Both of these can be used to provide heat for the pyrolysis process and/or drying the biomass feed.
Pyrolysis oil typically is a dark brown, free-flowing liquid which is similar to biomass in its composition. The liquid is formed by rapidly quenching and 'freezing' the intermediate products of hemicellulose, cellulose, and lignin as they break down. The liquid contains many reactive products, which contribute to its unusual attributes. A tiny proportion of solid char may be left over.
The typical maximum yield of bio-oil from woody materials is around 75 wt.%, which contains 70% of the biomass's energy. Bio-oil has a higher heating value of about 17 MJ/kg as produced with about 25% wt. water that cannot easily be separated. While the liquid is widely referred to as "bio-oil," it will not mix with any hydrocarbon liquids.
The charcoal by-product contains virtually all the ash and is a very useful vapour cracking catalyst, so rapid and effective separation from the pyrolysis product vapours is essential. Also, any residual char in bio-oil may separate and deposit on liquid phase upgrading catalysts. About 25% of the energy in the biomass is contained in the char.
In fluid bed processes, the char is separated, and commercially, the part would be burned to power the pyrolytic process. The excess can be exported for other uses. Transported bed or circulating fluid bed reactors recycle all the char with the sand to a second reactor where the char is burned in air to reheat the sand, which is recirculated to the pyrolyser to provide the heat for pyrolysis. As a result, there is no production of char for export from the process in these processes.
About 5% of the energy in the biomass is contained in the gas. This usually has a low heating value as it is often heavily diluted with recycled gas for fluidisation. Depending on how the reactor is configured, it can sometimes be recovered for energy use.
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Energy and Bioproducts Research institute (EBRI)