The effect of alkali and alkaline earth metals in biomass ash on the bio-oil components derived from biomass fast pyrolysis

The alkali and alkaline earth metals (AAEMs) in biomass ash have a significant impact on the yield and component distribution of rapid plytic biooil. In this paper, corn straw is selected as the raw material. First, the effect of cascade deash removal pretreatment (distillation water, ammonium acetate and hydrochloric acid) on the selective removal of AAEMs and its biological oil components is studied, and then the effect of the type of AAEMs (K, Ca, Na and Mg), the concentration of chloride salt (0.5%, 2.5% and 5%), and the acid radical in metal salt（\rmSO_4^2- , \rmNO_3^- , \rmCO_3^2- , \rmHCO_3^- , AC⁻ and \rmPO_4^3- ）on the compound distribution of bio-oil was systematically investigated. The results show that in the process of ash removal pretreatment, with the deepening of the acidity of the ash removal solution, the removal rate of AAEMs gradually increases. According to the selective removal law of AAEMs in the process of cascade ash removal, their morphology in biomass can be divided into the following three groups, namely the water-soluble metal (K), the ion-exchanged metals (Ca and Mg), the acid-soluble metal (Na). The removal of AAEMs promoted the formation of levoglucosan (LG), while restrained the formation of ketones and furans. However, the incorporation of AAEMs in biomass presented an opposite variation trend. The AAEMs would act as catalyst during biomass pyrolysis which promoted the secondary cracking of LG, leading to the reduction of LG and increase of ketones and furans. In addition, different acid roots in potassium salt also have remarkable influence on the secondary cracking reaction of LG and the rupture of the aryl ether bond and the phenolic hydroxyl group in lignin. The influence of the acid roots on the secondary cracking reaction of LG was in the order of \rmHCO_3^- >\rmCO_3^2- >AC⁻>\rmPO_4^3- >Cl⁻>\rmNO_3^- >\rmSO_4^2- , while the influence of acid roots on the rupture of the aryl ether bond and the phenolic hydroxyl group was in the order of \rmCO_3^2- >Cl⁻>\rmHCO_3^- >\rmPO_4^3- ≈AC⁻>\rmSO_4^2- ≈\rmNO_3^- .