| Biomass is kind of resource with huge amount, environmental friendly, and renewable, which is highly required and applied. However, biomass still has some drawbacks like high moisture content, hygroscopicity, low density, high oxygen content, low heating value and energy density, not easy to store and transport. The drawbacks result in the high cost of transportation, low rate of energy conversion, and low added value of products. Biomass torrefaction has been recognized as a good method to solve the issues of high moisture content, hygroscopicity, high oxygen content, low heating value and energy density. However, drawbacks of low density and not easy to store and transport are still exist. Biomass densification could improve the aforementioned drawbacks. In addition, the ignitin point of biomass increased after torrefaction. Combustion addictive should be an effective method to solve the problem. Therefore, in order to obtain high quality biomass solid fuel, torrefaction without an inlet medium in a semi-closed system, adding lignin/inorganic salts in biochar to improve their combustion kinetics when combustion, and adjusting biochar moisture content or/and adding enzymathc hydrolysis lignin when pelletization of biochar were proposed in the thesis. Methods of elemental analysis, TG, XRD, SEM, FTIR, TG-FTIR were used to study the changes of fuel properties, thermal properties and physical-chemical structures.The mechanism of torrefaction was revealed. The effect of KMnO4, Ba(NO3)2, and lignin on the combustion properties and kinetics of torrefied biochar was explored, and the optimized conditions of torrefied biochar pelletization was obtained. The main contents and results are as follow:(1) The traditional torrefaction is mostly conduced in nitrogen medium, while the use of inert medium, like nitrogen, increases the cost of torrefaction process. In the thesis, torrefaction without an inlet medium in a semi-closed system was adopted. The mass yield, energy yield, elemental composition and heating value of raw poplar and cultivation residue of Auricularia auricula-judae (CRAA), biochar of poplar and CRAA were analyzed and compared to that in nitrogen medium system. Results showed that the mass yield and energy yield decreased, and the carbon content and heating value increased with the increasing of torrefaction temperature and extension of residence time. Except the torrefaction condition of 320℃ at 60 min, biochar obtained in semi-closed system is of similar quality to the biochar ontained in mitrogen medium system. Thus, torrefaction in semi-closed system could reduce the cost of torrefaction process.(2) The performance of weight loss of biochar when subjected with heat is studied through the thermogravimetric analysis of poplar and CRAA biochars obtained from ssemi-closed system and nitrogen medium system in different temperatures and residence times. The thermokinetices of biochars was also determined. Results showed that with increasing of torrefaction severity, the characteristic peak of hemicelluloses was gradually eliminated, while the characteristic peak of lignin was enhanced. The ingnin point and burnout temperature increased, and the char percentage is enhancen gradually. Then the biochar could burn for a longer time. The peak temperature of biochar obtained in semi-closed system is reduced. However, the peak temperature of biochar obtained from nitrogen medium system is almost the same. The reaction activation energy of biochar obtained from semi-closed system is reduced and less than that of raw biomass. The reaction activation energy of biochar obtained from nitrogen medium system is less than that of raw biomass when torrefaction temperature is 320 ℃. The reaction activation energy of biochar obtained from semi-closed system is smaller than that of biochar obtained from nitrogen medium system. Comparing to poplar biochar, CRAA biochar has smaller reaction activation energy and pre-exponential factor, which reveal a lower energy requirement when reaction.(3) In order to reveal the changing reasons of fuel properties and thermal properties during torrefaction, micro- and crystal-structures and chemical structures of biochar obtained from different torrefaction temperature and residence time were analyzed. The biochar formation mechanism during torrefaction was studied. Results showed that, the degradation of hemicellulose, cellulose and lignin in biomass during the torrefaction process cause the shrinkage of wood cell walls. The higher oxidation level of biomass, the more "corrosion" like microstructure the biochar is. The unstable changes of cellulose crystallinity is due to the untable degree between broke of molecular in cellulose crystalline region and recrystallization in cellulose amorphous region. The cellulose crystalline degree decrease when the oxidation degree of branched molecular is higher. In nitrogen medium, the hemicellulose, cellulose, and lignin of biochar mainly underwent broken and recombination between molecule chains. However, in the oxidative medium of semi-closed system, the biomass underwent the oxidation of aliphatic hydroxyl in lignin and hydroxyl methyl groups of glucose units in cellulose side chains. The most oxidation occurred at torrefaction temperatures of 280℃ and 320℃, which has accelerated the cracking at o-position β-1,4 glycoside bonds of cellulose, as well the ’β-O-4’ alkyl-aryl ether bonds of lignin, thus reduced the peak temperature and reaction activation energy of biochar when subjected with heat.(4) In order to improve the high ignition point of torrefied biochar, exploratory trial of adding KMnO4, Ba(NO3)2, and lignin into biochar was studied. The ignition point, burnout temperature, thermo weight loss, and changes of gaseous products were tested with different addictive. Effect of inorganic salt of KMnO4 and Ba(NO3)2 and lignin on the combustion properties was analyzed. Results showed that adding of inorganic salts (KMnO4 and Ba(NO3)2) reduced the ignition temperatures of torrefied biochar. The best adding content of both inorganic salts is 3%. When single inorganic salt or lignin added, the weight loss rate of cellulose and lignin with low polymerization degree decreased. When adding KMnO4, Ba(NO3)2 and lignin together, the inorganic salt can activate the lignin which is easy to response at lower temperatures, thus accelerate the reaction rate, then has positive effect on the combustion. Adding different content of inorganic salt and/or lignin could reduce the activation energy at lower temperature reaction region. Besides,20% adding content of lignin gained the most reduction of activation energy in these addictives.(5) In order to obtain high quality pellet, torrefaction in semi-closed system and adding enzymatic hydrolysis lignin were used to improve the quality of pellet. The density, energy density, moisture uptake of raw pellet and torrefied pellet formed at different conditions were compared and analyzed. Results showed that when same particle size was applied, the density of raw pellets is larger than that of biochar pellets. Both raw pellets and biochar pellets could reach their largest density when the particle size is between 20 meshes to 40 meshes. Both raw pellets and biochar pellets could reach their largest density and energy density when the lignin adding content is 20%. The more lignin added, the higher equilibrium moisture content is after water uptakeing. When 10% to 15% of water added, the density of biochar pellets exceeded that of raw pellets, and biochar pellet got it lowest equilibrium moisture content when adding 10% of water. The density of biochar pellet increased obviously when die temperature is 170℃, the energy density of biochar pellets at this condition is the largest at 22.56 GJ/m3 in average. |
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