Study On Nitrogen Migration Mechanism For Hydrothermal Co-carbonization Of Sewage Sludge And Cornstalk And NOx Formation Characteristics During Hydrochar Combustion

With the rapid increase in the production of sewage sludge(SS)and the growing environmental problems in China,the disposal method of incineration after hydrothermal carbonization(HTC)of SS has attracted much attention due to its advantages of reduction,harmlessness and resourcefulness.However,SS with its high ash content and low heat content still does not meet the standard of fuel coal after HTC.The hydrothermal cocarbonization(co-HTC)of SS with lignocellulosic waste(e.g.cornstalk,CS)can significantly increase the heating value of hydrochar and make it more valuable for combustion.In this paper,a series of co-HTC experiments and constant combustion of hydrochar were carried out to investigate the redistribution and migration mechanisms of nitrogen(N)during the co-HTC process and the NOx generation characteristics during the subsequent combustion of hydrochar.The experimental results will be of great significance for the control of the raw material components and reaction conditions of the co-HTC as well as the control of pollutants during the hydrochar combustion.Firstly,the experimental study of co-HTC of SS and CS was carried out by tuning the blending ratio(SS:CS),reaction temperature and residence time.The chemical composition,functional groups and other physicochemical properties of the hydrochar and the relative content of organic matter in the hydrothermal filtrates were analyzed to investigate the redistribution law and migration transformation mechanism of N during the co-HTC.The results showed that under the hydrothermal condition of 220°C,2 h,and SS:CS=1:1,the high heating value of hydrochar and the energy recovery efficiency respectively reached 15.89 MJ/kg and 71.19%.During the co-HTC process,solid-solid transformations such as rearrangement and condensation cyclisation of amino-N in the solid phase occur,forming pyridine-N,pyrrole-N and quaternary-N.The N components dissolved in the liquid phase(ammonia,amino acids and amine compounds)are combined with sugars and intermediate products(5-hydroxymethyl furfural,etc.)from CS degradation to form heterocyclic-N compounds(including pyridine,pyrrole,pyrazine,etc.)via the Maillard reaction.The heterocyclic-N polymerized and formed melanoidins,which thereafter polymerized with aromatic clusters to form the N-containing polyaromatic char.Therefore,N retention increased and showed a synergistic effect.Increasing the proportion of CS or extending the residence time,the N retention increased due to the fixation of the heterocyclic-N compounds;while increasing the reaction temperature results in a decreasing trend in N retention,there is a phased increase at 220-250°C due to the production of pyrrole-N.Experiments on the NOx generation characteristics of the hydrochar constant combustion process were carried out in a tube furnace to measure and analyze the thermogravimetric characteristics and the NOx generated by combustion.The results showed that the combustion process of the blended hydrochar was smoother as the CS blending ratio or HTC temperature increased.The blended hydrochar had an inhibitory effect on NO production during combustion,which increased with increasing CS blending ratio;the hydrochar obtained at a hydrothermal temperature of 220°C had the significantly lowest NO production concentration.the hydrochar obtained at a hydrothermal condition of SS:CS=1:1 and 220°C-2h had the lowest NO production and N-NO conversion ratio,which were 3.33 mg/g and 7.59%.