Fundamental Study On Transformation Behavior Of Inorganic Components During Thermochemical Conversion Of Biomass And Ash Fusion Characteristics

With the development of biomass energy technologies, more and more attentions have been focused on the relevant scientific issues. The transformation behavior and fusion characteristics of biomass ash not only affect the thermo-chemical utilization route of biomass, but also affect the operating conditions optimization of the plant and the high value-added utilization of biomass ash. With the support of National Natural Science Foundation, the transformation behavior and fusion characteristics of typical biomass ash were systematically studied using X-ray fluorescence (XRF), X-ray diffraction (XRD), HSC chemistry simulation, ash melting point test apparatus, phase diagrams in silicate ceramics and thermal gravimetric analyzer (TGA). The high value-added utilization of biomass ash was also preliminarily explored. The main results of this study are as follows.The existence form of Cl was determined using low-temperature plasma ashing and ion chromatography. Results show that KCl is the main form of Cl, so the potassium could be divided into two types (KCl and K2O). The main components, crystal structure and transformation behavior of different biomass ash were studied through experiment and chemical simulation. Different K-bearing compounds show different transformation behaviors. The calcium element in calcium-rich ash mainly exists in the form of calcium carbonate at low temperature and decomposes into calcium oxide or converts into calcium silicate at high temperature. The proportion of water-soluble potassium compounds gradually decreases with temperature. The main water-soluble potassium compound is K2SO4, while insoluble components are mainly in the forms of potassium silicates. The water-soluble calcium is Ca(OH)2, which results from the reaction of CaO and water.In this thesis a new evaluation method of fusion characteristic was proposed through the density variation of biomass ash and comprehensive use of ternary diagrams. Experimental studies show that the bulk density of silicon and potassium-rich soft straw ash sharply increases with temperature increasing, while the true density of ash has little change. SEM images show that this type of biomass ash occurs significant melting at the high temperature. The melting point of biomass ash obtained by multi-ternary diagrams is more appropriate for industrial running, which overcomes the shortcomings of evaluation by single phase diagram. Results show that deformation temperature is more suitable to predict the melting point of biomass ash than softening temperature.Reaction atmosphere can significantly affect the transformation behavior of biomass ash. As to the calcium-rich cotton stalk ash, the CO2atmosphere can restrict the decomposition of calcium carbonate remarkably. The largest amount of potassium silicate obtained in potassium-rich biomass ash is under1200℃in CO2atmosphere, while1050℃in oxidizing atmosphere. The soft straw ash, which has high potassium and silicon contents, shows more obvious amorphous structure with the presence of CO2, and the volatilization of potassium is also inhibited. Under the steam-gasification condition, the existence form of ash constituents is also different from the combustion condition. Some of the potassium tends to exist in the form of KOH, while, the other proportion prefers to combine with SiO2, generating high amount of silicates. The presence of H2O can slow down the volatilization of KC1. The effect of pressure on the transformation of biomass ash is mainly reflected in the inhibition of the decomposition process of calcium carbonate and the silicification process of potassium carbonate. High pressure promote the conversion of K2O·SiO2to K2O·2SiO2in potassium and silicon-rich ash.The effects of different potassium compounds (KC1, K2CO3and K2SO4), CaCO3and Al2O3on the conversion of biomass ash and fusion characteristics were systemically studied. Results show that KCl tends to evaporate at high temperatures and plays an insignificant role in the complex reactive process. K2CO3is easy to react with SiO2, generating high amount of potassium silicate, which is the major factor in reducing biomass ash melting point. K2SO4is stable during the heating process. However, it also reacts with SiO2at temperatures higher than1200℃. This is another factor affecting potassium silicate generation. CaCO3is easier to react with SiO2than K2CO3and K2SO4. The corresponding calcium silicate products ususlly have relatively high melting points, thus increase the melting point of biomass ash significantly. Meanwhile, with the presence of high amount of CaCO3, K2CO3and K2SO4directly decompose at high temperatures, resulting in the volatilization of K into the gas phase greatly. The addition of Al2O3leads to the formation of high amount of Al-bearing species. which have high melting points, such as potassium nepheline. kyanite. potassium aluminum oxide and other compounds, thus greatly improve the biomass ash melting point. Analysis of ash fusion characteristic using melt theory shows that, the presence of alkali and alkaline earth metal oxides mainly play a key role of free oxygen in the SiO2grid. Then the viscosity of the system is observably reduced. Meanwhile, the viscosity of the system is also affected by the ion potential. With the influence of the dual effects, the contributions of metal ions in reducing ash melting point follow the subsequent order:K+> Na+> Ca2+> Mg2+> Al3+.The catalytic effect of biomass ash on pyrolysis and CO2-gasification was carried out on a TG analyzer, with potassium-rich wheat straw ash and calcium-rich cotton stalk ash as the typical samples. The study found that the catalytic effect of wheat straw ash and cotton stalk ash under600℃is better. Other ashes show unconspicuous effect mainly due to K2CO3or CaCO3gradually transformed into glassy silicate, which led to the weakeness of catalytic effect. Ashes obtained at lower temperatures show little effect, which is mainly because of the residual unburned char in ash. The addition of both types of biomass ash can slightly increase the char yield and the maximum weight loss rate of pyrolysis, while the temperature at the maximum weight loss rate does not change significantly. The addition of ash can also promote the release of CO2and CO slightly, and a substantial increase of volatile organic substances. Catalytic CO2gasification experiments on TGA reveal that biomass ash can reduce the activation energy of the reaction dramatically. Meanwhile, the reaction temperature is accordingly lowered, thus improve the condition of gasification. This catalytic effect of biomass ash on gasification can bring many benefits. Finally, combined with the high value-added utilization of biomass ash, this thesis presents a novel biomass gasification system, with cyclic utilization of biomass ash and reduction of the potassium volatilization and fusion possibility of biomass ash.