The effect of variations in time-temperature history on product distribution from coal pyrolysis

Abstract

An electrically-heated wire-mesh pyrolysis reactor is described. The apparatus is capable of heating rates ranging from virtually zero to 5000 K s −1 as well as multi-step heating with variable heating rates and holding times at intermediate temperatures and at the peak temperature. An alternating current power supply is used, allowing thermocouples to be attached directly to the wire mesh to give close tracking of temperature changes. The heating circuit is driven by a micro-computer for real-time logging and control. A continuous stream of carrier gas is forced through the sample holder, giving mean volatiles residence times of around 2ms in the heated zone. The gas sweep gives sample cooling rates of 500–1000 K s −1; slower cooling rates may be selected by pre-programming the heating circuit. Pyrolysis yields of tar and total volatiles from a low rank British bituminous coal (Linby) obtained with single-step heating, show clear changes in the product distribution with increasing heating rate. The effect of holding times, relatively unimportant during slow heating (1 K s −1) runs, is quite pronounced for fast heating rates (l000 K s −1) at peak temperatures below 650–700 °C. Because of this, unless pyrolysis reactions are allowed to run to completion, the effect of the heating rate on product distributions may be masked.