Study On Removal Mechanism Of NO_x Precursors Formed By Protein In Sludge Added With Calcium Oxide Through Pyrolysis

Sewage sludge,as a by-product of urban sewage treatment,has become an inevitable environmental risk due to its high toxicity,wide sources and low proper disposal rate.Pyrolysis is an effective way to utilize municipal sewage sludge resources,but the nitrogen contained in the sludge is easily converted into NO_X pollutants during the pyrolysis process.Protein-N is the main source of NO_X precursors?NH₃ and HCN?in the gaseous products of sludge pyrolysis.Adding calcium oxide?CaO?is an effective method to control the release of NO_X precursors during sludge pyrolysis,but its mechanism is still unclear at present.Therefore,the influence mechanism of CaO on the conversion of protein components to NO_X precursors during sludge pyrolysis is studied in detail in this essay.Firstly,the protein components in sludge are extracted,the effects of temperature and CaO addition on the pyrolysis of protein components in sludge to generate NH₃ and HCN were determined,and the characteristic temperatures?400?,650?and 750??of horizontal furnace pyrolysis were determined by TG-FTIR experiment.The results shows that between300?and 900?,the inhibition effect of CaO on NH₃ shows the regularity of first increasing and then decreasing and then increasing again;at 400?,the inhibition effect is the most remarkable.For the formation of HCN,the inhibition of CaO is stronger at 650⁹00?.Secondly,the influence of CaO on the kinetic parameters of deashing sludge and protein extraction was studied by AKTS calculation.The results showed that the activation energy of deashing sludge and protein samples increased with the increase of conversion rate.After adding CaO,the activation energy of the two groups decreased first and then increased.Adding CaO has significantly reduces the pyrolysis activation energy of demineralized sludge and protein extraction and its catalytic effect on demineralized sludge is stronger than that on protein.Finally,based on the above results,the pyrolysis experiments are conducted in a horizontal tube furnace using a variety of amino acids as model compounds of proteins under the characteristic temperatures,and the removal mechanism of CaO on NO_X precursors at different characteristic temperatures is studied through analyzes the transformation rules of N-containing functional groups in char by XPS,the transformation rules of nitrogen-containing compounds in tar-N by GC-MS,and the release of NH₃ and HCN by spectrophotometry.The results shows that between 300?and 900?,the inhibition effect of CaO on NH₃ shows the regularity of first increasing and then decreasing and then increasing again;at 400?,the inhibition effect is the most remarkable.The main reason is that CaO reacts with Char-N to produce CaCxNy,which promotes the fixation of pyridine nitrogenpyridine-N and nitrile nitrogennitrile-N in cokechar.Among them,nitrile nitrogen nitrile-N has strong thermal stability and is not easy to react with CaO.The increase of nitrile nitrogen nitrile-N content is mainly caused by the conversion of amine nitrogen amine-N or heterocyclic nitrogen heterocycle-N.At the same time,CaC_x produced by thermal decomposition of CaC_xNy at high temperature?above 700??can effectively promote the fixation of protein nitrogenprotein-N and reduce the formation of NH3.The reason for the poor inhibition at650?may be the formation of NH₃ by hydrolysis of HCN.The main reason for the decrease of HCN release at 650⁹00?is that it reacts directly with CaO,which prevents the nitrile-N in the cokechar from converting to HCN.In this paper,the protein extracted from sludge was studied in a targeted manner,and an effective method for controlling the release of NO_X precursor during sludge pyrolysis was established,which provides a valuable reference for the utilization of sewage sludge.