Product characteristics of rice straw pyrolysis at different temperature: Role of inherent alkali and alkaline earth metals with different occurrence forms

The inherent alkali and alkaline earth metals (AAEMs) in biomass have a significant impact on biomass pyrolysis. However, the catalytic effects of water-soluble (WAM), ion-exchanged (IAM) and acid-soluble alkali metals (AAM) on properties of products are still unclear. In this study, pyrolysis characteristics of biomass containing different intrinsic AAEMs were performed in a fixed-bed reactor at different temperatures. The results showed that the effects of AAEMs on char characteristic and on bio-oil composition were influenced by temperature, but their effects on product yield were independent of temperature. Almost all the Na released in water-soluble form below 400 °C, but part of K element still remained in the chars even at 750 °C, which played a catalytic role in the release of volatiles at high temperature. IAM promoted the formation of C–O groups and reduced the production of CO groups in chars prepared at low temperature. At the range of 400–750 °C, WAM and IAM suppressed the formation of esters, while AAM promoted the production of ketones and aldehydes. WAM had a positive effect on the formation of phenols, while IAM and AAM showed negative influence on phenol production at 750 °C. In addition, WAM, IAM and AAM had different effects on gas composition at different temperatures.