Direct Thermochemical Liquefaction

Pyrolysis System Components

Pyrolysis System Components

A fast pyrolysis system consists of an integrated series of operations starting with delivery of a roughly prepared feedstock such as whole tree chips from short rotation coppice, wood waste from furniture manufacture, energy crops such as miscanthus or sorghum, or agricultural residues such as straw.  The main steps are illustrated and summarised below:

 

Overall fast pyrolysis process

Reception and storage Low capacity systems of up to around 3 t/h feed typically consist of a concrete pad for tipping delivered feed and a front end loader to move it between reception, storage and handling steps. Larger size plants will increasingly require more sophisticated systems analogous to those employed in pulp and paper mills. This will include weighbridge, tipping units, conveyors, bunker storage.
Feed drying This is usually essential unless a naturally dry material such as straw is available. A typical specification is 10% moisture in the dried product as not does only the feed moisture report to the liquid product but also the reaction water from pyrolysis, which typically gives 12-15% water in the product. Waste low grade process heat and would usually be employed from char combustion, for example in a rotary kiln. Biomass may also be burned for this purpose.
Comminution Particles have to be very small to fulfil the requirements of rapid heating and to achieve high liquid yields as explained earlier. This is costly and reactors that can use larger particles, such as ablative pyrolysers, have an advantage.
Reactor A wide variety of configurations have been tested as described above that show considerable diversity and innovation in meeting the basic requirements of fast pyrolysis. the “best” method is not yet established.
Char & ash seperation Some char is inevitably carried over from cyclones and collects in the liquid. Almost all of the ash in the biomass is retained in the char so successful char removal gives successful ash removal. The char may be separated and exported if there is a viable market, otherwise it would be used to provide process heat either directly as in circulating fluid bed reactors or indirectly as in fluid bed systems.
Liquids collection Larger scale processing would usually employ quenching with an immiscible liquid such as hydrocarbon or cooled liquid product. Both are effective. Although collections of aerosols is difficult there has been considerable success with electrostatic precipitators. This technology is widely used on very large scales, so at least in principle this should not be a problem. Careful design is needed to avoid blockage from differential condensation of heavy ends. Light ends collection is important in reducing liquid viscosity.
Storage and transport The bio-oil will require some local storage prior to local or remote use. A tank farm will provide storage and blending facilities. Both storage and transport are features unique to fast pyrolysis and permit economies of scale to be realised from building as large a conversion plant as possible as well as offering economic supplies of bio-oil for distributed or decentralised small scale power and heat applications.