Research On Briquetting Mechanism And Its Characteristics Of Bio-briquette Made From Lignite & Biomass

Lignite is abundant and widely distributed solid fossil resource in the world. However, its current industrial utilization (i.e. lignite fired power generation) is inefficient and has been causing environmental pollution, due to the lignite disadvantages of high moisture content, high ash content, low calorific heating value, low ash fusion temperature, easy weathering, self-ignition and high elemental contents of sulfur and nitrogen. Therefore, it is nessessary to upgrade liginte quality before utilization. One potential technology is to blend lignite with carbon-neutral biomass resouce for combution, which can reduce not only fossil-based CO₂ emission but also the emissions of other pollutants such as SOx and NOx. In this dissertation research, a process based on Mechnical and Thermal dewatering system was developed and studied to produce fuel briquettes from lignite-biomass blends. The binding mechanisms between lignite and biomass and the co-pyrolysis of lignite and biomass were investigated and discussed, respectively. Specifically, this dissertation research was divided into 4 sections as follows.Firstly, the briquetting processing conditions and the optimization of processing parameters for bio-briquette were studied. Two Chinese lignite (Xianfeng & Xiaolongtan) were blended with the biomass samples of rice stalk, wheat straw and spruce sawdust, respectively. The effects of briquetting pressures, briquetting temperatures, pressure holding times, the mass ratios of biomass to lignite and particle sizes on the strength of the bio-briquettes were studied. The mechanical strength of obtained briquettes was evaluated with respect to compressive strength, impact strength and drop strength. The relation between processing conditions and mechanical strength of bio-briquette was further studied. The order of these factors in terms of importance was briquetting pressure, biomass/lignite mass ratio, treatment temperature and treatment time. The interaction effects between briquetting pressure, biomass/lignite mass ratio and compressive strength and impact strength were significant. But, the interaction effects between temperature, treatment time and compressive strength was not statistically significant.Secondly, the binding mechanisms between lignite and biomass were investigated. With micro-structural analysis, it was found that biomass fibers with different lengths interconnected and interlocked each other. As a result, lignite particles well connected together, forming complicated network structure. Under the conditions of high pressures and temperatures, a glass transition was observed during the formation of biomass particle. Liquid bridges were formed between glass transitional materials, water and melted light materials. Then the liquid bridges in the briquette changed to be in the form of solid, after cooled and moved out of the mold. These solid bridges increased the strength of bio-briquettes. Also, the plastic properties of briquettes was improved and the relax properties (elasticities) decreased, due to the high pressure and temperature treatments. The presence of the mechanical interlocking bonds resisted the disruptive forces caused by elastic recovery from compression. These above positive changes improved the drop strength of bio-briquette and avoided cracking. Moreover, the mechanical strength and water resistance of bio-briquettes further improved with the biomass being pretreated with weak alkaline solutions, which acted as a binder between lignite and biomass particles. On the other hand, the particle characteristics and electrical potential characteristics were also investigated. The Zeta potential of lignite and biomass surfaces was always negatively charged, which helped prevent particle agglomeration. Though the mechnical and thermal process increased the contact angles of bio-briquette surface, the water resistance was not improved significantly. The pore volume, specific surface area and porosity were reduced by the process too.High mechanical strength of bio-briquette was the result of several binding mechanisms. Among different mechanisms, the form of structural bridges connecting particles was the dominant one. By comparison, the mechanical bond force and physical/chemical bond forces (i.e. chemical bonds, electrostatic, Van der Waals'forces et al) were negligible.Thirdly, a pilot scale test of bio-briquette was examined. A regression equation of the relation between biomass/lignite mass ratio, treatment time, briquetting temperature and drop strength was established. The optimal pilot-scale processing condition was rice husk mass ratio of 27-31 %, treatment time of 21-22min, briquetting temperetue of 146-153℃. The optimal conditions for xiaolongtan lignite/wheat straw were adjusted to be wheat straw size of 0.16-0.38 mm, briquetting temperetue of 140℃, treatment time of 20 min and briquetting pressure of 15 MPa.Finally, the co-pyrolysis characteristic of lignite with biomass was preliminarily investigated. It revealed that the co-pyrolysis process consisted of 4 stages. The third stage was considered the most important phase, causing 40%-50% mass loss during pyrolysis. Furthermore, based on the analysis of TG curves, it was noted that the co-pyrolysis of lignite/rice stalk blends could be the overall result of pyrolysing lignite and rice stalk independently. In addition, by the method of Coats-Redfern, reaction kinetic model for each stage of the pyrolysis was developed in this dissertation research .The dissertation has 65 figures, 25 tables and 146 references.