The Mechanisms Of The Effects Of Calcium-based Additives On Migration And Transformation Of Phosphorus During Sludge Pyrolysis

With the sharp increase in sludge production in recent years,how to properly handle and treat it has become an urgent problem to be solved.At the same time,the sharp reduction of non-renewable phosphorus resources also forced us to pay attention to the utilization of sludge,which called‘the second P source’.The characteristics of P migration and transformation during the sludge pyrolysis process are currently the focus of research.This paper used standards,measurements,and testing protocol（SMT）methods to determine these characteristics of various P forms in sludge at different pyrolysis temperatures.The effects of calcium-based additives on the content and evolution of the P forms had been systematically studied.The reaction kinetic models of the pyrolysis process of P-containing model compounds was constructed,which clarifies the mechanism of the effect of calcium-based additives on the characteristics of P migration and transformation during fast pyrolysis of sewage sludge.The results show that the P in sludge will be enriched in its coke after fast pyrolysis,and the degree of enrichment will increase first and then decrease with pyrolysis temperature.The total phosphorus content in the char of Tangxunhu sludge（ST）/Dali sludge（SD）/Longwangzui sludge（SL）which pyrolyzed at 650^oC was as high as 21.53/183.16/83.90 mg P/g dry sludge,respectively.The P content is 43/48/95%respectively higher than the sludge sample before pyrolysis.The organic phosphorus（OP）transformed to IP with temperature,and there were only IP existed when the temperature is higher than 850^oC.The ratio of AP to TP increased with temperature at intermediate and low temperature,this ratio in the char of ST/SL that pyrolyzed at 650^oC is 23.8/94.3%respectively,which increased by 11/49%compare to that of unheated.The effect of calcium-based additives on P-fixation during sludge pyrolysis at intermediate and low temperature is not obvious,but it can significantly promote the formation of AP.With the addition of Ca O,the recovery of AP of ST/SL/SD which pyrolyzed at 450^oC respectively increased by 153.8/7.3/7.1%and increased by 1200/85.6/35.8%that pyrolyzed at1050^oC.Ca Cl₂ performed better than Ca O in promoting the AP formation when the pyrolysis temperature is lower than 850^oC,and Ca O is better at high temperature.Ca Cl₂and Ca O promoted the recovery of AP of ST/SD respectively by 40.1/80.9%and 2.8/8.0%at intermediate and low temperature.The results of dynamic analysis show that,the addition of calcium-based additives can inhibit the volatilization of Al-NAIP in sludge and promote the conversion of OP and Fe-NAIP to AP,thereby achieving the effect of fixing phosphorus and improving the recovery rate of AP.The activation energy of the corresponding characteristic peak of the IP model compounds Al PO₄ at 500～600^oC increased from 80.99 k J/mol to 101.94 k J/mol after adding Ca O.After adding Ca O and Ca Cl₂,the activation energy corresponding to the newly characteristic peaks of Fe PO₄ at 400～650^oC are 66.69 k J/mol and 58.27 k J/mol,respectively.After mixing with Ca O or Ca Cl₂,the activation energy of the characteristic peak of the OP model compound,ATP,at high-temperature pyrolysis section,decreased from 96.22 k J/mol to 51.77 k J/mol and55.94 k J/mol.The experimental results of the model compounds revealed the chemical reaction process of Ca O and Ca Cl₂ reacting with OP and IP during pyrolysis to form AP such as Ca₂P₂O₇,Ca₃（PO₄）₂,Ca₂PO₄Cl and so on.