Release and transformation characteristics of K and Cl during straw torrefaction and mild pyrolysis

• Release of Cl was synchronous with devolatilization at 200–350 °C.
• Particle size influenced the release ratio of Cl obviously.
• Secondary reactions had a limited influence on speciation transformation of K.
• The reaction intensities of KCl with four macromolecules were different.

The release and transformation characteristics of K and Cl during straw torrefaction and mild pyrolysis under different conditions were investigated. The results show that release ratio of Cl increased continuously with temperature and holding time and the release of Cl was linearly proportional to devolatilization. The particle size was one of most important factors influencing Cl release. The release ratio for 74–124 μm straw particle was 60.78%, which was much higher than 27.25% for 250–420 μm at 350 °C. The influence of sample weight, particle size and heating rate was mainly due to secondary reactions of once-released Cl and pyrolysis char (the AAEMs and carbon active sites) during the diffusion process of volatiles. The influence of secondary reactions on K release and transformation behavior was not significant. The reaction intensities of KCl and macromolecules (cellulose, lignin, xylan and pectin) were different. The release of Cl was not generally equal to the fractions of K transformed into ion-exchanged species and released to the gas phase during KCl-loaded macromolecules torrefaction and mild pyrolysis, mainly due to different intensities of their secondary reactions.