Experimental investigation of the transformation. and release to gas phaseof potassium and chlorine during straw pyrolysis

When straw undergoes thermal treatment the initial process is a pyrolysis a t which some K and Cl can be volatilized, and this may result in problems w ith deposit formation and corrosion of the reactor containment. A laborator y batch reactor was applied to study the release and transformation of K an d Cl as a function of temperature, at an initial heating rate of approximat ely 50 degreesC/s. To facilitate the interpretation of the batch reactor ex periments thermodynamic equilibrium calculations at reducing condition were conducted, and SEM (scanning electron microscopy) and leaching investigati ons were carried out on straw and char samples. The experiments showed that chlorine was released in two steps, about 60% was released when the temper ature increased from 200 to 400 degreesC and most of the residual chlorine was released between 700 and 900 degreesC. Below 700 degreesC no significan t potassium release was observe; above that temperature it increased progre ssively until about 25% potassium release at 1050 degreesC. During pyrolysi s most K was released from the original binding sites, and the part that wa s 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 degrees C was caused by evaporation of deposited KCl particles. The release of Cl t o the gas phase was strongly affected by heating rate and sample size.