Comparative insights into flue gas-to-ash characteristics on co-combustion of walnut shell and bio-oil distillation sludge under atmospheric and oxy-fuel condition

Abstract

The clean recovery of bio-oil distillation sludge (DS) is a pivotal sector to perfect the biomass refinery system in responses to effective waste management and environmentally friendly consensus. This work aimed to comparatively characterize atmospheric and oxy-fuel co-combustion of walnut shell (WS) and DS in terms of (co-)combustion performance, distributed kinetics, flue gas evolution and ash characteristics. The results indicated co-combustion of WS and DS significantly increased the allocated proportion of char combustion stage and improved co-combustion performance, which was reflected in the enhanced four quantitative combustion indices. The fourth pseudo component, corresponded to the char combustion stage, continuously dominated the whole combustion process by adding more fraction of DS to co-combust with WS, whose activation energy was diminished, thereby optimizing the initial reaction energy barrier. In addition, oxy-fuel combustion promoted the recovery and sequestration of CO2, facilitated the formation of CO, and further endowed the conversion of polycyclic aromatic hydrocarbons into phenols rather than aromatics. Although the composition for mixed molten ash presented the decreased contents for easy-slagging minerals, the co-combustion process was still with potential ash fouling risk owing to basically unchanged ash configuration consistent with WS ash. However, the addition of DS to some extent strengthened anti-slagging performance, consequently optimizing the empirical indices of slagging and fouling for mixed ashes. Also, a higher ash content was yielded in oxy-fuel combustion than that in atmospheric combustion owing to the reducibility of CO2, in which the presence of CO2 contributed to the formations of carbonates and low melting-temperature potassium-containing silicates.