关键词: 生物质, 造纸黑液, 木质素, 燃烧特性, 显微结构

Abstract:

In order to utilize the black liquor effectively, lignin was extracted from bagasse soda pulping black liquor by acid precipitation.The bagasse lignin(BL) was then characterized by Fourier transform infrared(FTIR), nuclear magnetic resonance(¹H NMR) and gel permeation chromatography(GPC).The results indicated that BL, which was consisting of syringyl(S) and guaiacyl(G) units and connected together by β-O-4 aryl ether linkages, β-5, β-1 and β-β condensed C-C linkages, was more similar to the broadleaf wood lignin than annual grass lignin.There was large variation among the molecular weight of the BL molecules.The GPC spectrum of BL contained two continuous peaks, and the number-average molecular weight(M_n) of these two peaks was 107106 g·mol^-1 and 11270 g·mol^-1, respectively.Thermogravimetric analysis(TG) was used to investigate the combustion characteristics of BL.The results showed that the combustion process undergoes four periods, i.e. dehydration(30-200℃), volatilization(200-300℃) releasing from cleavage of the C-O linkage and combustion, volatilization(300-500℃) releasing from cleavage of the highly condensed C-C linkage and combustion of coke, and burnout stage(500-800℃).The combustion of BL mainly occurred at the temperature range of 300-500℃ with above 80% mass loss, and the maximum mass loss rate reached its value of -142.75%·min^-1 at the temperature of 477℃(20℃·min^-1).The influences of different heating rates(5, 10, 15, 20℃·min^-1) on combustibility parameters of BL, which consisted of the ignition temperature, peak temperature at maximum mass loss rate, burnout temperature and maximum/mean mass loss rate were analyzed through computing the differential thermogravimetric(DTG) curves.It was shown that the thermal hysteresis would occur and all combustibility parameters were increased along with increasing heating rate.The bagasse lignin residues(BLR) were prepared by burning at 300, 600, 900 and 1100℃, and the surface morphology and elemental composition of those BLR were comprehensively analyzed by using the scanning electron microscopy(SEM) and the X-ray fluorescence(XRF) technique.The results showed that a majority of tight structure of granular with smooth surface were found in the BL, while most of loose and poriferous larger particles, which was caused by the volatilization releasing and melting of the BL, was found in the 300℃ BLR.The sporadically coke particles wrapped in the loose particles still existed in the 600℃ BLR, indicating that the incomplete combustion of the cake occurred.Then, the higher the burning temperature was, the smaller size and homogeneous of the BLR.The well-ordered lamellar particles were the majority surface morphology of BLR when burning temperature exceeded 600℃.The main composition elements of BLR were calcium(Ca), silicon(Si), aluminum(Al), phosphorus(P), sulphur(S) and ferrum(Fe).With the increase of burning temperature the Na and K contents decreased, while the other elements contents not obviously changed.

Key words: biomass, pulping black liquor, lignin, combustion characteristics, microstructure