Effects Of Leaching Pretreatment On Combustion And Ash Formation Of Aquatic Biomass

Biomass energy has attracted widespread attention as carbon–neutral renewable energy that can be used to replace traditional fossil fuels to reduce carbon dioxide emissions.Aquatic biomass has the advantages of fast growth,high yield,no need for arable land and freshwater,and no seasonal restrictions.It can complement land–based biomass and effectively expand the utilization of biomass energy.However,the high content of alkali metal elements(mainly potassium and sodium)and chlorine elements in aquatic biomass may cause serious ash–related problems(sedimentation,slagging,etc.),which severely limits the resource utilization of aquatic biomass.In this paper,two kinds of aquatic biomass,Ulva lactuca and Hydrilla verticillate,were pretreated by water washing and acid washing,and the combustion and kinetic characteristics of aquatic biomass before and after pretreatment,mineral migration and transformation characteristics of aquatic biomass ash and slagging characteristics when pretreated aquatic biomass was co–fired with bituminous coal were studied.Water washing and acid washing can increase the content of volatiles and fixed carbon in aquatic biomass and greatly reduce the ash content.In addition,the contents of alkali metal and chlorine elements in the aquatic biomass were also greatly reduced after pretreatment.Acid washing had more impact on aquatic biomass,and in addition to the higher removal rate of alkali metal elements,most of the alkaline earth metal elements were also removed.The study of combustion and kinetic characteristics showed that pretreatment had a more direct effect on Hydrilla verticillate,the comprehensive combustion characteristics increased by more than 2 times,the activation energy required for combustion decreased,and the combustion characteristics were significantly improved.However,the effect of pretreatment on Ulva lactuca was complicated,and the change of combustion characteristics was small,with different increases and decrease.The combustion characteristics of Ulva lactuca after water washing decreased slightly,while the combustion characteristics of Ulva lactuca after acid washing increased.The study of mineral migration and transformation characteristics showed that due to different mineral compositions and characteristics in two kinds of aquatic biomass,the serious slagging caused by Ulva lactuca was related to the existence of alkali(Na and K)sulfate,while the existence of silicate in Hydrilla verticillate had more effect on slagging.It indicated that the removal of alkali metals(especially Na and K)and chlorine from aquatic biomass by leaching pretreatment are conducive to alleviating aquatic biomass slagging.Furthermore,alkaline earth metal elements rich in aquatic biomass can inhibit slagging by generating high melting points of alkaline earth metal silicates or phosphates.Although water washing and acid washing can alleviate slagging,the comparative investigation showed water washing of aquatic biomass had a better effect than acid washing.The co–firing experiments of pretreated aquatic biomass and bituminous coal showed that the most extreme agglomeration of coal co–fired with raw aquatic biomass was due to the increased content of alkali metals,especially K.Pretreatment can effectively decrease the slagging trend of co–fired ash.Acid washing had a higher removal rate of alkali and alkaline earth metals,but it did not show a lower slagging tendency when co–firing with coal.The SO2 emissions during co–firing was less than the value calculated by linear interpolation,but much higher than the SO2 emissions when coal was burnt alone.According to the results of the experiment and considering the economic benefits,water washing was considered to be a more suitable treatment method for aquatic biomass.Compared with other types of biomass,aquatic biomass was also more suitable for washing because of its delicate and thin structure.