Experimental Investigation of the Transformation and Release to Gas Phase of Potassium and Chlorine during Straw Pyrolysis

Abstract

When straw undergoes thermal treatment the initial process is a pyrolysis at which some K and Cl can be volatilized, and this may result in problems with deposit formation and corrosion of the reactor containment. A laboratory batch reactor was applied to study the release and transformation of K and Cl as a function of temperature, at an initial heating rate of approximately 50 °C/s. To facilitate the interpretation of the batch reactor experiments thermodynamic equilibrium calculations at reducing condition were conducted, and SEM (scanning electron microscopy) and leaching investigations were carried out on straw and char samples. The experiments showed that chlorine was released in two steps, about 60% was released when the temperature increased from 200 to 400 °C and most of the residual chlorine was released between 700 and 900 °C. Below 700 °C no significant potassium release was observe; above that temperature it increased progressively until about 25% potassium release at 1050 °C. During pyrolysis most K was released from the original binding sites, and the part that was not transformed to gas phase existed as redeposited discrete particles of KCl and K2CO3, as potassium silicates, or bound to the organic matrix. The initial release of potassium to the gas phase at approximately 700 °C was caused by evaporation of deposited KCl particles. The release of Cl to the gas phase was strongly affected by heating rate and sample size.