Effects Of Alkali Metals And Alkaline Earth Metals On Nitrogen Transformation During Pyrolysis Of Biomass

NOx precursors such as NH3,HCN and HNCO will be produced during biomass pyrolysis process,which will eventually release NOx and seriously pollute the environment.Alkaline metals(Na)and alkaline earth metals(Mg)have important effects on the formation of NOx precursors during the pyrolysis of amino acids.Effective control of the release of NOx precursor during biomass pyrolysis is the key to realize its efficient and clean utilization.In this paper,the effects of alkali metals(Na)and alkaline earth metals(Mg)on the the release and mutual evolution of of nitrogen-containing products from the pyrolysis of biomass amino acids(phenylalanine,glutamic acid,proline and aspartic acid)were studied by means of TG-IR,tubular furnace,gas chromatography-mass spectrometry(GC/MS),in-situ infrared(In-situ DRIFTS)experiments and quantum chemical simulation.The main results are as follows:A horizontal tubular furnace was used to study the pyrolysis of amino acid samples,and the nitrogen content of pyrolytic coke was studied by elemental analysis.The results showed that the addition of alkali metal Na and alkali earth metal Mg greatly reduced the nitrogen element in pyrolytic coke,and the phenylalanine,glutamic acid and aspartic acid decreased by about 8%to 10%.The main components of phenylalanine pyrolysis oil are hydrocarbons.The largest content of glutamic acid pyrolysis oil is oxygenated compounds,up to 34%,mainly pyrrolidone and esters.During the pyrolysis of glutamic acid,NH3 is the main nitrogen-containing gas,followed by HCN.NaCl and MgCl2 inhibited the release of NH3 and HCN from the pyrolysis of glutamic acid.The addition of NaCl inhibited the pyrolysis process of both proline and aspartic acid,and inhibited HCN and NH3 equally.Fourier transform infrared spectroscopy(FTIR)was used to analyze the changes of pyrolysis functional groups of four ammonia acids.Based on the distribution of amino acid pyrolysis gas products and the evolution process of nitrogen-containing functional groups in pyrolysis coke,combined with previous research results,the pyrolysis reaction paths of four kinds of amino acids were designed.Phenylalanine continues to be aromatization due to its benzene ring structure,and is transformed into polycyclic aromatic nitrogen-containing compounds.The side chains after fracture are directly decomposed,and will continue to decompose into small molecular gases after condensation into rings.During phenylalanine pyrolysis,NaCl reduced the reaction rate of the loop formation process.MgCl2 catalyzes the conversion of NH3+ groups and reduces the reaction rate of the formation of aromatic nitrogen compounds.Glutamic acid pyrolysis is mainly direct decomposition,and glutamic acid is also prone to condensation reaction,and then ring formation and aromatization.NaCl changes the transformation process of NH2 groups,but has little effect on other groups.MgCl2 reduces the initial formation temperature of NH3+ groups and the reaction rate of this process.Proline is decomposed into various gas products on the basis of ring-opening reaction in different ways.NaCl catalyzed the formation of aromatic compounds,while MgCl2 greatly catalyzed the transformation of NH2 groups and reduced the reaction rate of the formation of aromatic nitrogen compounds.Aspartic acid is mainly composed of two amino acids,which are dehydrated and condensed into a ring and decomposed into various gases.In the pyrolysis process of aspartic acid,NaCl and MgCl2 had little effect on the gas release in the first stage,but had a great effect on the release amount in the second stage.Both of them had a strong inhibitory effect on the release amount of NH3 in the second stage,which was the main reason for the reduction of the release amount of NH3.TG-IR method was used to study the distribution of gas products from amino acid pyrolysis,and to explore the effect of alkali(earth)metals on the release of nitrogen-containing gas from amino acid pyrolysis.During the pyrolysis of phenylalanine,NaCl catalyzed the first stage of HCN release and inhibited the second stage.MgCl2 also reduces the reaction rates of the two stages,lengthening the temperature range of HCN release and increasing the total release amount.There are two release stages of HCN,NH3 and HNC during glutamic acid pyrolysis process.Both NaCl and MgCl2 reduced the total release of NOx precursors.During the pyrolysis of proline,there is only one release peak of nitrogen-containing gas,which is mainly due to its high pyrolysis reaction rate and the whole reaction process is concentrated in a small temperature range.During aspartic acid pyrolysis process,NaCl and MgCl2 both reduce the release of NOx precursors and have little impact on the release in the first stage of aspartic acid pyrolysis,but have a great impact on the release in the second stage,which has a strong inhibitory effect on the second stage of NH3 release,which is the main reason for the reduction of NH3 release.The effects of alkali(earth)metal on the nitrogen conversion reaction mechanism during the pyrolysis of four amino acids were investigated by means of quantum chemical simulation,and the kinetic and thermodynamic parameters of different reaction processes were obtained.The results showed that NaCl reduced the competitiveness of the cyclic formation,HCN release and HNCO release,and reduced the reaction rate during the pyrolysis of phenylalanine.MgCl2 has strong inhibition on the formation of aromatic compounds,HNCO formation and ring formation.During glutamic acid pyrolysis process,NaCl enhanced the competitiveness of the formation of aromatic compounds,the formation of HCN and the formation of ring,but also increased the energy barrier of the reaction rate control step of these processes.MgCl2 showed a similar effect to NaCl on the pyrolysis of glutamate,and the effect was more obvious.During proline pyrolysis process,NaCl enhanced the competitiveness of the formation process of aromatic compounds and the cyclization process,but also reduced the reaction rate of these processes.MgCl2 reduced the competitiveness of aromatic compounds,but reduced the energy barrier of the reaction rate control step.During the pyrolysis of aspartic acid,NaCl enhanced the intermolecular decarboxylation process and promoted the release of NH3 in this process.NaCl and MgCl2 strongly inhibited the polymerization of aspartic acid molecules and the release of amide,reducing the competitive reaction of releasing HCN and NH3.