Nitrogen Conversion And Control Of Nitrogen-containing Compounds During Microwave Pyrolysis Of Sewage Sludge

Microwave pyrolysis is considered as a potential alternative technology due to itseffective reduction of sludge, stability of heavy metals and high added value ofbyproducts. However,50%-80%of nitrogen in the sludge is converted to thenitrogen-containing gases during microwave pyrolysis of sewage sludge, reducing thequality of bio-gas and causing secondary pollution of environment. This paper discussedthe influences of process conditions on the formation of NH₃and HCN, built thenitrogen-containing model compounds meeting sludge characteristics and investigatedthe nitrogen conversion pathways during microwave pyrolysis of sewage sludge. Theinhibition mechanism of inorganic additives on the formation of NH₃and HCN was alsoconducted and the strategies for the selective conversion of NH₃and HCN into N2wereproposed.The thermogravimetric analysis was conduced by the batch experiments, indicatingthat the thermal decomposition reactions of protein, carbohydrate and lipid occurredsimultaneously in the microwave pyrolysisin comparison with the conventionalpyrolysis system. FTIR analysis showed that NH₃and HCN were the majornitrogen-containing gases accounting for about70%and10%of total gas-N yields,respectively. The influences of temperature, moisture content of sludge, heating rate andmicrowave absorbers on the formation of NH₃and HCN during microwave pyrolysis ofsewage sludge were stuied. The results showed that NH₃and HCN yields increased withthe increase of temperature, heating rate, moisture content of sludge during pyrolysis.XPS analysis showed that the inorganic ammonium nitrogen, protein nitrogen andheterocyclic nitrogen （pyrrole and pyridine） were the main nitrogen functionalities inthe sludge. The steam distillation and Kjeldahl nitrogen determination analysis revealedthat the protein nitrogen was the major nitrogen form in the sludge, accounting foraround90%of total nitrogen in sewage sludge. PITC-RP-HPLC analysis showed thatthe contents of nine kinds of amino acids, such as arginine, glycine, proline and so on,accounted for above82%of the total. A theoretical model compound of arginine: lysine:proline: threonine: alanine: leucine: glycine=15:6:6:16:21:6:21:9was establishedaccording to their pyrolysis characteristics. Meanwhile, an organic polymeric peptidechain model compound soybean protein was selected according to the similar sludgeamino acid compositions. Microwave pyrolysis of two nitrogen-containing modelcompounds showed that the soybean protein was more proper as the sludge-N modelcompound. Three important intermediates, including the amine-N, heterocyclic-N andnitrile-N compounds, were identified during microwave pyrolysis of soybean proten,which contributed to more than97%of the total （NH₃+HCN） productions. The nitrogen distributions and evolution of nitrogen functionalities in the char, tarand gas fractions were conducted by XPS and GC-MS. The nitrogen conversions inrelation to NH₃and HCN were investigated during microwave pyrolysis of sewagesludge. At temperatures below300℃, the thermal decomposition of inorganic nitrogencompounds contributed to the release of NH₃. In the temperature ranges of300-500℃,the thermal cracking of labile proteins contributed to the formation of amine-Ncompounds in the tars further generating NH₃. The cracking of nitrile-N andheterocyclic-N compounds resulted from the dehydrogenation and polymerization ofamine-N compounds led to the formation of NH₃and HCN at temperatures of500-800℃. The analysis on nitrogen conversin mechanism revealed that the secondarycracking reactions was the major pathway for the production of NH₃and HCN duringthe sludge pyrolysis. Specifically, the cracking of amine-N, heterocyclic-N and nitrile-Ncompounds contributed to above80%of the total （HCN+NH₃） productions.Consequently, it was considered that it might be able to reduce the NH₃and HCNemissions by controlling the production of three nitrogen-containing intermediates.Conparing the release of gas-N by pyrolysis of sewage sludge and demineralizedsludge, the inhibition of the intrinsic minerals on the formation of NH₃and HCN wasconfirmed. The experiments on the added mineral species, mineral/sludge ratio andloaded methods demonstrated that addition of20%Fe2O3and20%CaO in the sludgecould inhibited the formation NH₃and HCN by approximately50%and80%,respectively. Additionally, compared with the direct mixing method, the chemicalprecipitation loading of Fe salt possessed better inhibitory effect on thenitrogen-containing gases. GC-MS analysis on the tar-N compositions produced frommicrowave pyrolysis of sewage sludge and minerals added sludge showed that mineralsdid not inhibit the formation of amine-N compounds （300-500℃）, while inhibit theformation of heterocyclic-N and nitrile-N compounds （500-800℃）. Thus, this resultedin the significant decrease of heterocyclic-N and nitrile-N compounds, leading to thedecline of HCN yields during pyrolysis. The formation of N2was ascribed to theintermediate mechanism in the presence of inorganic minerals （such as the interstitial Feand Ca nitride species）. The non-thermal microwave effects directly coupled to energymodes within the molecules and caused the reactive molecular oscillation and rotationreducing the reaction activation energy. As a consequence, the microwave irradiationpromoted higher productions of N2.