| Biomass energy is the world’s fourth largest energy source next to coal,oil and natural gas,accounting for 14%of total world energy consumption,and China is a large agricultural country with abundant straw resources.However,in addition to the disad-vantages of scattered distribution,low energy density and high transportation cost,the phenomena of particulate matter emissions,fouling,slagging,and corrosion caused by the potassium and chlorine elements,and the problem of low calorific value all limit the widespread application of straw resources.As a result,investigation on upgrading process and clean utilization of straw briquettes effectively has strong strategic signifi-cance on the development of renewable energy as an alternative coal,and also is the foundation of technology innovation and industry sustainable development.Densifica-tion,additive modification and pyrolysis are effective measures to solve these problems.Therefore,the transformation and control of potassium,chlorine and sulfur pollutantduring straw briquette combustion were investigated by experiment,simulation and theoretical analysis.Moreover,the productive technology on the basis of mixing addi-tives-briquetting-pyrolysis to upgrade the quality of bio-char briquettes was put for-ward,and the performance of pyrolysis temperatures and additives on physico-chemi-cal,combustion,ash fusion and pollutant emission characteristics of bio-char briquettes were also studied.The main works of the dissertation are listed as following.NH4H2PO4 and Ca（H2PO4）2 were selected as additives,mixing with straws to produce straw briquettes.The influence and reaction mechanism of phosphorous additives on ash fusion temperatures,the ash characteristics and potassium conversion during combustion were studied.The appropriate additive P/K molar ratio should be 1.0～2.0.The ash fusion points increased obviously,and the melting and fusion phenomena were inhibited by phosphorous based additives.The potassium releases were also controlled by additives at 700～1100℃,espcially for NH4H2PO4,that the the increasing rate of potassium fixation ratio were 52.86%for maize straw briquettes（MS）and 64.89%for cotton stalk briquettes（CS）at 1100℃.The phosphorous based additives reacted with with the potassium element in straw briquettes to produce high melting point K-Ca-P products,improving the ash fusion temperature,inhibiting melting problem and potassium release phenomena.The results are benefit for clean combustion of straw briquettes.BaCO3,CaO and CaCO3 were selected as additives,mixing with straws to produce straw briquettes,and the influence of additives on chlorine and sulfur fixation ability were investigated.The results show that the stability of BaCl2 was much better than that of CaCl2 in high temperature.The CaCl2 decomposed obviously at 800℃,while the performance of BaCO3 on chlorine fixation was much better than CaO and CaCO3,es-pecially at 900-1000℃.The appropriate molar ratio of Ba/Cl was 2.5,and the increas-ing rate of chlorine fixation ratio was 5.35 times at 1000℃.The reaction products be-tween KCl,NH4H2PO4/Ca（H2PO4）2 and BaCO3 at 500～1200 0C were simulated by the thermodynamic software.NH4H2PO4 and Ca（H2PO4）2 had good performance on KCl conversion,when the additive ratio of NH4H2PO4 was increased 1.5 times,more than 75%chlorine in KCl was transformed into HCl.When the additive ratio of BaCO3 was increased 1.5 times,only less than 1%gaseous KCl was incresed.Combined with the experimental results,when the phosphorous based additives and BaCO3 were added simultaneously,the appropriate additive molar ratio of Ba/K was 0.5～1,the chlorine fixation ratio were 22.03%and 26.20%at 1000℃.The performance of additives on SO2 control from high to low were BaCO3,CaCO3,CaO.In general,by mixing with BaCO3,chlorine and sulfur pollutant can be controlled,achieving the purpose of clean combustion.To solve the problem of low heating value and difficulty of briquetting process of bio-char,the productive technology of bio-char briquettes on the basis of mixing addi-tives-briquetting-pyrolysis was put forward.The influence of pyrolysis temperature（250～650℃）and phosphorous based additives on physico-chemical and combustion characteristics of bio-char briquettes were investigated.The acquired bio-char bri-quettes had ideal durability rating and water absorption resistance.With increased py-rolysis temperature,the organic functional groups experience fracture,polymerization and reconstruction,and the phosphorus based additives could accelerate the pyrolysis process.The additive grains attached on the fibers,reacting with potassium element to converse it.The heating values of bio-char briquettes were increased obviously,and the energy densities of them were betwee 10～15 GJ/m3,which were better than that of raw biomass briquettes/pellets or bio-char powders.The thermogravimetric analysis results showed the fixed carbon combustion stage got stronger when the pyrolysis temperature increased.As a result,the productive technology of mixing additives—briquetting—pyrolysis can converse and fix potassium in the fuel,and increases the heating values of fuel simultaneously.The influence of phosphorous based additives and pyrolysis temperature on ash fusion characteristics,ash formation characteristics,conversion of potassium,emis-sions of nitrogen and sulfur pollutants during combustion were studied.The ash fusion tendencies of bio-char briquettes were serious.The softening temperature of bio-char briquettes decreased with increasing pyrolysis temperature.The ash fusion tempera-tures of bio-char briquettes with phosphorous additives were much higher,when the pyrolysis temperature was 650℃,the ash fusion temperatures increased 490℃ and 422℃ by NH4H2PO4 and Ca（H2PO4）2 respectively.The potassium release was con-trolled by the additives;the potassium fixation ratio was higher than 60%at 1000℃.And the fusion phenomenon was inhibited.According to experimental and simulation results,the mechanisms of two phosphorous based additives on improving ash fusion characteristics were not totally same.Although the high melting point K-Ca-P com-pounds were produced by both of these two additives,NH4H2PO4 had better perfor-mance on inhibiting potassium release and potassium silicates formation,while Ca（H2PO4）2 could transformed into calcium silicates and calcium phosphates,which finally increase ash fusion characteristics.The SO2 and NO emissions during bio-char briquettes combustion were lower than that of coal.And the bio-char briquettes had lower emission for the same heating value compared with raw biomass.In general,the bio-char briquettes upgraded by phosphorous additives have the advantages of high heating values,high ash fusion temperature and low pollutant emissions,which provide theoretical technical guidance on straw briquette upgrade technology and clean combustion. |
