to read the full report - Ecolateral by Peter Jones

Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
the temperature of the reactor and critically the rate at which heat is applied to the biomass fractions.
Lower temperatures and fast heating rates favour vapours, higher temperatures and high heating rates
gas, low temperature and slow heating rates favour char formation.
There are two main categories of pyrolysis process, but many practical variants within them:
Fast or flash pyrolysis – typically finely divided biomass waste is injected into a fluidised bed of inert
material operating at 500°C. The size of the fuel and the excellent heat transfer characteristics of the fluid
bed ensure a very fast heating rate which maximises the production of vapour. The vapour is
subsequently condensed as a liquid that contains approximately 70% of the energy value of the waste
feedstock. The by-product char and gas is used in part to provide heat to drive the process. The liquid
fuel has been successfully used to fire boilers and kilns. Trials have been undertaken in reciprocating
engines and gas turbines. Excess char can be sold as a product for activated carbon manufacture or
reducing agent in metal production. The char can also be used as fuel either on its own or as a slurry
with the pyrolysis liquids.
The main use for fast pyrolysis processes at present is the manufacture of speciality chemicals and food
additives although this is expected to change to energy use when the current demonstration plants in
Canada come fully on stream.
Slow Pyrolysis – finely diced waste is pyrolysed in either in a screw conveyor or reactor vessel that is
indirectly heated. The slower heating rate favours char and liquid production over gas.
Carbonisation – large pieces of waste or wood are heated in a retort. The heat is provided by burning a
proportion of the vapour and gas product.
A variation of slow pyrolysis using a heated screw has recently been proposed as a step in the production
of thermo chemical fuels in the form of a slurry of charcoal and pyrolysis liquid product. The concept is a
series of distributed fuel preparation processes on a local and regional scale that feed supply a very large
gasification plant at national scale. The advantages are reduced transport costs and having the fuel in a
form that it can easily be pumped into the high pressure (60 bar) processes that are used in this type of
facility.
Feedstocks for Pyrolysis
Currently fast pyrolysis processes are being designed for both clean wood and wood extracted from the
waste stream. In all cases the wood will need to be ground to less that 3mm particle size before use.
Slow pyrolysis can use a wider variety of solid shredded material including SRF.
Table 82 Risks Associated with Pyrolysis
Level of Risk Comment
Technical High Operating commercial plant in existence for oil
production little experience in energy generation.
Social & Planning High / Low High where process is used to produce electricity
at a regional scale where it may be perceived as
an untried and unsafe incinerator.
Low for syn gas production or thermo chemical
fuel production where it wuill be perceived as
another industrial process.
Financial High Few suppliers and uneven track record.
Products have no established energy market and
would rely on dedicated customer.
Regulatory Medium Receives enhanced subsidy via the Renewables
Obligation.
Status of products is unclear.
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