The Research Of Water Leaching And Additives Pre-treatment Effects On Wheat Straw Ash Melting Temperature Characteristics

Biomass solid fuels are becoming more competitive by the improvement of biomass briquetting technology and application,making it one of the largest proportion of energy resources in the future.As a kind of biomass solid fuel,straw biomass has great energy potential.However,it is easy to cause ash-related problems such as ash sintering,which adversely affects the boiler combustion efficiency.Consequently,the application of straw biomass is not extensive.Therefore,it is of great practical significance to improve the ash melting temperature of straw biomass solid fuel and improve fuel properties.In this thesis,wheat straw is taken as an example to analyze the properties of wheat straw fuel.Pretreatments such as water leaching,additives,water leaching–additives synergistic means were carried out to investigate the effects of pretreatments on ash melting temperatures.Moreover,a prediction model of wheat straw ash melting temperature which can correctly predict the ash melting temperature after such pretreatments.Firstly,the proximate constituent（including ash content,moisture content,volatile content,and fixed carbon content）,ultimate constituent（including C,H,O,N,S,Cl,etc.）,heating value,ash melting temperature were tested.The main influencing factors of wheat straw ash melting temperature were analyzed.The results show that:（1）Wheat straw is a suitable material for burning as it has relatively low moisture content（6.57%）and ash content（4.14%）,as well as considerable volatile content（78.97%）and higher heating value（18.95～19.38 MJ/kg）.（2）The shrinkage starts temperature（SST）is 700℃,which is lower than normal biomass boiler temperature（1000～1200℃）,making the wheat straw ash easily melted.（3）Ash content is the main reason that affects the ash melting temperature,the higher Si O₂ and K₂O content in the ash,the lower the ash melting temperature of the wheat straw.Secondly,the effects of water leaching pretreatment and additive pretreatment（including kaolin,dolomite and slaked lime）on the ash melting temperature of wheat straw were analyzed.X-ray diffraction analysis（XRD）,scanning electron microscopy（SEM）coupled with energy dispersive X-ray spectrometer（EDX）,phase diagram analysis methods were used to study the mechanism of water leaching and additive pretreatment effects on the ash melting temperature of wheat straw.The results show that:（1）The ash removal rate of water leaching pretreatment is 26.09%,and the K₂O removal rate is 63.09%,which increase the ash melting temperature and reduce the ash sintering degree.（2）Additive pretreatment（kaolin,dolomite,slaked lime）can significantly increase the ash fusion temperature of wheat straw and improve the ash sintering characteristics.Kaolin has the best effect,followed by slaked lime and dolomite.（3）Water leaching decreases the fusible potassium salts,moreover,the rest potassium salts are reacted to KCa₃P₅O₂₃,K₃Ca P₂O₈ and K₂Mg₂（SO₄）₃,etc.minerals whose melting temperature are high.（4）The kaolin additive forms high–melting temperature minerals such as KAl Si O₄ and KAl Si₃O₈,etc.in wheat straw ash;high–melting temperature minerals such as K₂Mg Si₅O₁₂ and Ca₃Si₃O₉;slaked lime additive produces Ca₃Si₃O₉,Ca Mg（Si O₃）₂ and other high–melting temperature minerals,thereby increasing the wheat straw ash melting temperature.Then,combined with XRD,SEM–EDX,and phase diagram analysis methods,the effects of synergistic pretreatment of water leaching and additive（kaolin,dolomite and slaked lime）on the ash fusion temperature of wheat straw were studied.The results show that:（1）The synergistic pretreatment further improve the ash melting temperature of wheat straw（SST is greatly increased）,which effectively alleviated the sintering degree of wheat straw ash.The water leaching–slaked lime pretreatment（SST increased by 260°C）works the best,followed by the water leaching–kaolin pretreatment（SST increased by 240°C）and the water leaching–dolomite pretreatment（SST increased 220°C）.（2）The mechanism of synergistic pretreatments are:refractory K-Al-silicates such as KAl Si₂O₆,KAl Si₃O₈,and KAl Si O₄ are formed by the water leaching–kaolin pretreatment;Ca₃Si₃O₉,Ca Mg（Si O₃）₂ and K_2.62Mg₄Si₄O₁₄ are produced by water leaching–dolomite pretreatment because of the abundant content of Ca and Mg;the water leaching–slaked lime pretreatment makes Ca and Si as the main ash component,which contributes to the formation of Ca₃Si₃O₉ and K-Ca-phosphates（such as K₂Ca P₂O₇）.Lastly,according to the influence mechanism of each pretreatment method on the ash melting temperature,a suitable prediction model of ash melting temperature（SST）of biomass solid fuel after pretreatments were established by analyzing partial least squares（PLS）regression method and cuckoo search（CS）BP neural network method.The results show that:（1）The PLS regression prediction model cannot accurately predict the SST of pretreated biomass solid fuel.The indexes which are ash composition show no significant linearity to SST.（2）Indexes of Si,Cl,acid-base ratio（B/A）,Babcock index（R_s）,erosion index（R_f）,ash viscosity index（G）,potassium ratio（R_k）are significantly correlated to SST,which are chosen as optimized indexes.（3）The CS–BP neural network prediction model which is established by optimized indexes can accurately predict the SST after pretreatment of biomass solid fuel.The research work in this thesis shows that:（1）Water leaching,additives,and synergistic pretreatments can significantly increase the ash melting temperature of wheat straw.The water leaching pretreatment can reduce the low-melting potassium salt in ash,while the additive pretreatment generates refractory mineral components.The synergistic pretreatment decreases fusible salts and generates higher melting temperature components both,which gain the most significant improvement of ash melting temperature of wheat straw.（2）The CS-BP prediction model based on the optimized indexes can accurately predict the SST of biomass solid fuels after pretreatment,and the prediction error meets the application requirements.