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The pyrolysis technology

Pyrolysis is the term used for an irreversibile chemical change brought about by the action of heat in an atmosphere devoid of oxygen. Synonymous term are thermal decomposition, destructive distillation and carbonisation. The pyrolysis of biomass yelds: · · · · Char Organic liquids fuel gas (pyrogas) Water (present in either liquid or gaseous state depending on final pyrolysis conditions).

The char contains any mineral ash or other non-combustible material present in the biomass plus what is termed the "fixed carbon", which represents the carbonaceous fraction of the biomass that did not volatilize on heating. The char also usually contains small quantities of hydrogen, oxygen and sometimes nitrogen. The organic liquids are a complex mixture of chemicals often called pyrolysis oil (tar). The pyrogas consists of a number of combustible gases, such as carbon monoxide, methane, hydrogen and minor quantities of other higher hydrocarbons. The pyrogas also contains carbon dioxide and may contain water vapour if it has not been previously condensed out of the stream. Most pyrolysis reactions are endothermic (require heat). The heat may be generated by introducing air or oxygen into the bed and burning a small portion of fixed carbon, as in gasification, or it may be supplied externally, as in pyrolysis. In all cases, the heating value of pyrolysis products is the sum of the heating value of the original material and the net energy added during pyrolysis. In biomass, the amount of energy required for pyrolysis of cellulosic material is very small compared with the heating value of original material.

As described earlier, pyrogas can be produced by either pyrolysis or gasification. However, on this discussion, we will select a pyrolysis reactor design that can take advantage of the high volatile fraction found in biomass and produce a gas without the fuel dilution that occurs when either air or oxygen are introduced into the fuel bed. Generally, as in our case, an indirectly heated rotary kiln is used to produce pyrogas from biomass. The cylindrical retort section of this kiln is slightly inclined and slowly rotated, causing the biomass to move from the front end to the discharge end of the retort. The retort is enclosed in a refractorylined firebox. A portion of the char is burned in a postcombustor under the reactor, and the exhaust gases of this combustion flow in the annular space between the outside well of the cylindrical retort and the interior of the firebox. Heat is transferred through the alloy metal wall of the retort to

pyrolyze the biomass. Biomass moisture has a very pronounced effect on any thermal processing system, so the thermal efficiency of the system could be increased by operation at lower temperature and by using some of the heat in the flue gas stream to evaporate a portion of biomass moisture.

In actually pyrolysis process, such as the indirectly heated rotary kiln, the process doesn't reach chemical equilibrium, which makes it difficult, if not impossible, to predict the quantity and composition of the char, tar and pyrogas. The pyrogas composition is a function of pyrolysis temperature for a tipical biomass. Pyrogas can have many uses: it can be used directly as a boiler fuel, serve as a synthesis gas for the production of methanol or ammonia. Another interesting possibility is the direct use of pyrogas as a fuel an internal combustion engine or turbogas, as in this case.

Pyrolysis application

Pyrolysis process can be applied for different final uses. 1) The possibility to destroy, without burning, different kinds of plastic materials, tyres, paper, etc., provides application in the recovery of valuable metals, like copper or aluminium, through the pyrolysis of electrical cables or shredded pieces of coated aluminium sheets or profiles. In this case, the emerging pyrogas is useful to maintain the temperature of the rotary kiln, by direct burning in conventional burners. The absence of oxygen in the rotary kiln, allows the perfect recovery of the metals, without superficial oxidation. 2) The possibility to convert a specific waste, or a mixture of wastes, into pyrogas and carbon has been used to eliminate dangerous wastes, like tunnery residues, providing the thermal energy to the rotary kiln through the direct burning of the pyrogas and carbon, and the additional steam generation.

Electric energy production

The possibility to provide, from the pyrolysis of treated wastes, biomass, plastic materials, others, a pyrogas with sufficient LHV to be directly utilised to feed turbogas turbines or internal combustion engines is completely innovative. Till now, most companies tried to apply gasification process for the "thermovalorization" of residues or biomass, without interesting results, because the syngas exhibits a too low heat value ( presence of oxygen and nitrogen) . In addition, in case of failure or

maintenance of gasifier, the production plant will be completely out of operation : from the other hand all the residues can be converted into syngas and addressed to combined cycle( TG/TV). The process applied by Tecnofin Group to the Printer Plant, feeded by biomass, provide the gasification at low temperatures (about 500 °C) of the feeding materials, up to 70% in weight, and the convertion into carbon powders of the remaining part ( only a low percentage -5%- of tar are produced and recovered, for direct burning into boiler). As a consequence, a big part of the feeding material, converted into pyrogas, can be addressed to combined cycle, with high energy efficiency, while the carbon powder are used to energize the pyrolyser and to burn into the boiler. The plant arrangement allows the direct burning of the pyrogas into the boiler and the possibility to burn the feeding materials into the boiler, so overcoming the stop of the plant in case of turbogas or pyrolyser failure or maintenance.

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