Influence of water/acid washing pretreatment of aquatic biomass on ash transformation and slagging behavior during co-firing with bituminous coal

Highlights: • Ash transformation and slagging of co-firing aquatic biomass with coal were studied. • Pretreatment strategies of water washing and acid washing were applied. • Water washing was more proper for Ulva lactuca to alleviate slagging propensity. • SCK and Factsage estimations are not fully consistent with the analytical results. • There is a synergistic effect on reducing SO₂ emission during co-firing process. In this study, bituminous coal and two aquatic biomasses, Ulva lactuca and Hydrilla verticillate, were selected as the raw materials for studying the ash slagging behavior. Furthermore, Ulva lactuca was pretreated by washing with water and acid; the influence of pretreatment on ash transformation was subsequently studied. Co-firing tests were performed in a drop tube furnace and the SO₂ content in the flue gas was determined using a flue gas analyzer. The chemical compositions and mineral phase properties of the collected ash were studied using several analytical techniques, including ICP, XRF, SEM-EDS, and XRD, as well as thermochemical software Factsage 7.2. The results indicated that the most extreme agglomeration of coal co-fired with Ulva lactuca was due to the increased content of alkali metals, especially K. Pretreatment can effectively reduce the slagging trend of co-fired ash. Acid washing had a higher removal rate of S and alkali and alkaline earth metals but did not show a lower slagging tendency when co-firing with coal. The SO₂ emissions during co-firing were less than the value calculated by linear interpolation but much higher than the SO₂ emissions when coal was burned alone.