What Is Pyrolysis / Gasification?
Pyrolysis involves the thermal decomposition of organic material in an oxygen-free or low oxygen environment. Pyrolysis generates three main products; char, oil ('pyrolysis oil') and gas - in varying ratios depending upon the composition of the waste materials fed into the pyrolysis reactor and process conditions. Gasification is the process whereby most of the char, tar and volatile gas resulting from pyrolysis are converted into a more useful combustible gas (syngas) by a reaction with steam, with or without air (P.Toms, pers.comm.; S. Joseph, pers.comm.). The two terms are often used in tandem, since there are many Pyrolysis/Gasification technologies which apply the two processes in series.
While the ratios of products vary, most Pyrolysis/Gasification processes produce a combination of the following four products:
- a gas (often referred to as syngas) for use in energy recovery;
- a liquid alcohol fuel to be used as chemical feedstock or energy recovery;
- a residual char with potential uses such as activated carbon; and
- a solid slag (when inorganic components are found in the feedstock) which may be disposed of or refined.
Waste Streams Handled
Gasification technologies can process material from the following waste streams (Waste Inquiry 2000):
- dewatered sewage sludge;
- shredded plastics;
- agricultural wastes;
- sorted mixed organic waste including:
- food waste;
- green waste; and
- paper pulp.
Gasification technology operations generally include a series of PRR processes such as dewatering, shredding, separating, sorting and drying. This ensures that the material going into the gasifier is in a relatively homogeneous form (S. Joseph, pers.comm.). The thermal reactor is usually where the waste is heated and the gas is captured. Residual energy and gas can be used for process heating or drying, for steam or hot water production or for building heat (S. Joseph, pers.comm; P.Toms, pers.comm).
These are well established technologies but have not traditionally been applied to the MSW stream (Waste Inquiry 2000). There are a number of different processes based on Pyrolysis/Gasification which have emerged in last few decades and some systems have reached commercial status (Waste Inquiry, 2000). However, the commercial viability and public acceptability of these technologies has yet to be proven. For example, a pilot gasification facility for green waste opened in Woollongong, NSW in 2001 but subsequently closed due to financial difficulties. In Western Australia a similar facility was proposed but never realised - reflecting concerns about a community backlash. Nevertheless, several such facilities can be found in Europe and North America.
- Joseph, S., BioEnergy Systems and Technology. pers. comm. 15 November, 2000
- Toms, P. Business Development Manager, Primergy. pers comm. 14 November, 2000.
- Waste Inquiry (2000) Report of the Alternative Waste Management Technologies and Practices Inquiry.
Pyrolysis and Gasification, 2002, Friends of the Earth (FOE)