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Operation factors of pyrolysis process

Pyrolysis processThe Speed. On the basis of laboratory trials one can conclude that the speed of process significantly influence mainly the stage of decay in the temperature range 260-290 oC. Intensive heat supply during this time by the grinded wood leads to the growth of resin output and decrease of charcoal output when the output of low-molecular products is almost not changeable: acids, methanol, aldehyds. We can suppose that during the slow heating some part of resin is coked on the surface of the coal increasing its output and simultaneously the oxygen situated in the wood assimilates with hydrogen giving the water and with carbon – CO2.

During the fast distillation the output of gases increases significantly and the output of charcoal and liquid distillates decreases.

High pressure generated under the intensive heating inside the device has a great influence on the output.

The speed of product withdrawal from the sphere of reaction has more influence in this case. The faster the resin is withdrawn the bigger its output is.  

The pressure in the equipment. It has a great influence on the process of thermal decomposition. For example when the pressure is changed from 200 atmosphere to 5 mm of mercury column the outputs (for absolutely dry hardwood) of charcoal and methanol fall from 34 to 20 % and from 3,1 to 1,2 % correspondingly but the output of acetic acid and resin increases.

Simultaneously in vacuum the composition of condensate is sharply changed: a great number of reducing matters appear in it and sedimentary resin is completely absent.

These relations in the industrial type equipment are not used because the economic effect from their existence won’t compensate the technical complications arising when the high-temperature process in the metal devices in pressure or vacuum conditions is organized.

Final temperature of the process. The products emitted during the process of wood pyrolysis are produced in the wide range of temperatures. Each product has definite temperature of early formation, its maximum and end of formation. The maximums of the overwhelming majority of products of wood thermal decomposition relate to the temperature interval corresponding to the exothermic reaction (release of excess heat). There are important temperature points:

  • 260 °C – when the wood is getting brown having released constitutional and some part of reactionary humidity, CO2, formic and acetic acids.
  • 400 °C – when the separation of the main mass of liquid products is already finished.

Further increase of the process temperature (tempering) leads to the separation of small amount of hard resins (1,5-2%), significantly increases the output of noncondencing gases and gives the charcoal with higher carbon content – up to 90-95%.

In CBK-5 type installations (prevailing among the small entrepreneurship) the wood is heated by the combustion gases in which the oxygen content is significantly reduced. The gases go through the wood mass and give the warmth before the beginning of exothermal reaction. After that the important criterion is nonadmission of the Oxygen into the pyrolysis chamber to avoid the overheating and distruction of the charcoal kiln.

Combustion of charcoal briquettes

The pyrolysis processes running in the area of overheated water vapor, water and high boiling neutral liquids are slightly detached. Wood decomposition in the stream of overheated water vapor leads to the detaching of methoxy groups, sharp rise of output of volatile acids and aldehydes, appearance of matters of carbohydrate nature in the distillate, decrease of charcoal output and complete absence of sedimentary resin. Such kind of technologies is mostly used to study chemical processes.



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