Research On The Pyrolysis And Gasification Characteristics Of Wheat Straw Soda Pulping Black Liquor And Formation Rules Of Products

Black liquor gasifcation (BLG) has been suggested as an alternative means ofTomlinson-type recovery and the new process has been developed for convertingblack liquor into high H2and CO synthesis gas.This study focused on the pyrolysis and gasification products characteristics ofwheat straw black liquor (BL) and the three organic components (alkali lignin (AL),polysaccharides (PLS) and lignin-carbohydrate complexes (LCC)), and the influenceof sodium slats on the pyrolysis and gasification behavior. Achievements of this studyhave important significances to reveal the composition characteristics of tar and thegasification reactivity of char, and clear the catalytic gasification mechanism oforganic bound sodium groups and inorganic sodium salts.The physical and chemical characteristics of BL, AL, PLS and LCC werecomprehensively investigated by using elemental analysis, FT-IR,¹H-NMR,¹³C-NMR and thermogravimetric analysis (TGA). The results revealed that ALcontained more guaiacyl and syringyl propane unit. The elemental analysis showedthat the carbon and hydrogen content of AL was significantly higher than PLS andLCC. TG results showed that the mass loss stages during BL pyrolysis were clearlycorresponded with the three organic components.The pyrolysis products component and distribution of black liquor and its threeorganic components, and the effect of Na salts on the formation of pyrolyzate wererevealed by means of small tube furnace pyrolysis apparatus. The pyrolysis productsof BL, AL, PLS and LCC were analyzed by using gas chromatography (GC), gaschromatography-mass spectrometry (GC/MS), scanning electron microscopy-energyspectrum (SEM-EDS) and FT-IR. The forming laws of pyrolysis products wereobtained by a thermogravimetric analyzer coupled with Fourier transform infraredspectrometry. It was found that the releasing of CH4during AL pyrolysis wassignificantly higher than PLS and LCC. The generation of CO₂from PLS pyrolysiswas significantly higher than AL and LCC. The main component of AL pyrolysis tarwas phenolic compounds; the main compositions of tar from low temperature pyrolysis (400°C) were4-methyl guaiacol (21.59%) and guaiacol (20.69%); the maincomponent of high temperature pyrolysis (800°C) tar was phenol (20.99%). The maincomponent of PLS pyrolysis tar were furans and ketones; the main component of LCCpyrolysis tar were acids and phenolic compounds. Phenolic compounds of BLS tarwere attributed to AL and LCC pyrolysis, ketones compounds were corresponded withPLS and LCC, ethers compounds were mainly due to the inorganic sodium saltcatalyzed AL and LCC pyrolysis. Organic bound Na inhibited the formation of furancompounds e.g. furfural, containing methyl phenols e.g.4-methyl guaiacyl phenol and3-methy-4-methoxy phenol, ketones compounds e.g.2-cyclopentenone and2-methyl-2-cyclopentenone during catalyzed AL pyrolysis. However, organic boundNa increased the formation of guaiacyl phenol, phenolic compounds, aromaticscompounds e.g.1,2-dimethoxy benzene and1,2,3-trimethoxy benzene. Theinorganic sodium increased the formation of naphthalene and indene and otherpolycyclic aromatic hydrocarbons.The steam and CO₂gasification reactivity of black liquor and its three organiccomponents char, and the effect of Na salts on char gasification reactivitywere carriedout on a thermogravimetric analyzer. With TG-FTIR, temperature programmeddesorption-gas chromatography (TPD-GC), the mechanism that sodium salt catalyzedalkali lignin char gasification reaction was discussed. The results show that PLS charhas the highest CO₂gasification reactivity; AL char has the lowest CO₂gasificationreactivity. Steam gasification reactivity of AL char was significantly better than itsCO₂gasification reactivity; however, steam gasification reactivity of PLS and LCCchar was lower than that of CO₂gasification. At800°C, CO₂and steam gasificationreactivity of BLS char was low. As the temperature rose to900°C, CO₂and steamgasification reactivity of BLS Char were improved significantly. The catalyticgasification mechanisms of organic and inorganic Na were different.At lowtemperature (600°C), organic bound Na catalyzed AL char gasification mainlydepended on the reaction between catalyzed activity complex-CO₂Na and C. Atmedium temperature (600°C-800°C), it relied mainly on-CO₂Na and-CONaactivity complex reaction with char C. At high temperatures (800°C), it was the reaction of-CONa complex with C. The catalytic activity complex could be formedtill600°C during inorganic Na catalyzed AL char gasification, and the number ofcatalytic activity complex were significantly lower than the organic bound Na ofsodium. It may be the root reason that the catalytic effect of organic bound Na wasbetter inorganic Na.