Study On The Forming Mechanisms, Physical Properties And Combustion Characteristics Of Briquettes Made Form Caragana Korshinskii Kom

Caragana korshinskii Kom is a drought-tolerant、cold-resistant and saline-alkaline-enduring shrub which could fix the sand to protect the eco-environment. It has been extensively cultivated in Inner Mongolia, Shaanxi, Gansu and Yanmenguan ecological zones of Shanxi province. Re-vitalization is necessary in its growth period, the stem of first three yeas could be processed into feed with high nutritional value, but the perennial Caragana korshinskii Kom can’t be processed for high quality feed because of lack of nutrition caused by lignification, compressing it into solid fuel will make it get good transformation and application because of its properties such as hard woodiness, high fiber strength and good oiliness. Because the physical properties of the stalk which are very different with other agricultural materials such as straw, sawdust,etc. it is necessary to study the physical properties of Caragana korshinskii Kom and molding process parameters for designing its special molding equipment. Furthermore, the basic research is not enough on the combustion characteristics and combustion kinetics characteristics of the briquette for development of combustion equipment. The aim of the study was to analyze pointedly the forming mechanism, physical properties and combustion characteristics of Caragana korshinskii Kom solid fuel, mainly investigated the influence of the main process parameters (pressure and temperature) and raw material characteristics (moisture content and particle size) on the deforming effect of Caragana korshinskii Kom briquette, analyzed physical characteristics, formation mechanism and combustion characteristics of the briquette. Finally, we provided the basis for designing equipment related to processing and utilization. The main contents and conclusions were as follows:(1) The stalk of Caragana korshinskii Kom was carried out proximate analysis, chemical composition analysis, elemental analysis and calorific value determination. The characteristics of raw material are high volatile content, high lignin content, and low sulfur content (only 0.18%), moreover, the calorific value of the 1 Kg Caragana korshinskii Kom is equivalent to the value of 0.54 Kg high quality bituminous coal. Caragana korshinskii Kom is a high quality clean fuel because it is easy forming, ignitable, stable burning property and low harmful volatile components.(2) The paper studied the physical properties of Caragana solid forming fuel and the influence factors on briquette. The influence curve and the relational expression between the factors and density, durability were deduced, the range of forming parameters were determined based on univariate statistical analysis. Using the regression analysis, the function relationship between the physical properties (density, durability, compression strength, shear strength, impact resistance and resistance to water) and moisture content, particle size, pressure and temperature was deduced.The results showed that the small particles, plastic and intensity of fuel will be worse, reduced particle size, low moisture content and high temperature are, in this order, the most important factors in determining an increase in the density and durability of briquette, pressure is of marginal importance and represents the least correlated factor with density and durability. The necessary moisture content for producing good quality briquettes ranges from 5%-13%(w.b.), and the optimum moisture content ranges from 8%-9%. In order to produce high physical properties briquettes, reduced particle size, process temperature of at least 90℃ and pressures of at least 50MPa should be reached, from energy saving and low cost point of view, the two factors need not exceed 150℃ and 1 lOMPa, respectively.(3) Using the Taguchi method, the study analyzed the optimum conditions for preparing the solid briquette of the Caragana korshinskii Kom and the percentage contribution of each experimental parameter to the briquetting. The results showed that the best parameter combination was as follow:particle size was <0.63 mm, moisture was 8%(w.b.), temperature was 130℃, pressure was 120 MPa. The confirmation experiment was carried out according to the optimum conditions and the density of briquette was the maximum 1.152 g-cm-3. Moreover, the percentage contribution of each controllable factor to densification was determined by the analysis of variance (ANOVA). According to their magnitudes, the rank order of the percentage contribution of each factor was particle size (56.7%), moisture content (12.8%), pressure (11.3%), temperature (10.7%).(4) The paper analyzed the microstructure, forming mechanism and failure mechanism under the light microscope and scanning electron microscope. Good fuel adhesion, which results from Van der Waals’ force and hydrogen bond, was obtained due to smaller particle size; although the density of the fuel from small size particle was higher, compression strength and shear strength were lower. The adhering method for large size particle was mechanisms interlocking or folding and over-lapping microstructure of the fuel made from large size particle enhanced the compression strength. The main adhering mechanism under lower temperature was due to Van der Waals1 force, hydrogen bond and mechanical interlocking and higher temperature and pressure promote solid fuel forming, while higher moisture content significantly restrained the effect of pressure and temperature. The temperature of lignin glass transition ranged from 50℃ to100℃. Solid bridge bond of particle were shaped when natural bonded components were squeezed out from particle. Because caragana contained large amounts of lignin, forming effect of Caragana solid fuel was fine at the appropriate temperatures and pressures. Cohesive failure and interface failure were observed in bonding failure mechanism, other failure mechanism was likely to appear at the same time.(5) The combustion characteristics and dynamics of solid fuel were studied by thermal analysis. The combustion kinetics formula was deduced and factors affecting combustion speed were optimized. The Caragana korshinskii Kom briquette possessed lower igniting temperature, higher combustion speed, is easy to ignite, and bottom ash was infusible and not slagging. Higher furnace temperature, larger burning rate were beneficial to ignite and burn, while excessive amount of wind would decrease furnace temperature which was unfitable for burning. In order to release more heating, higher heating rate and blowing rate were required for larger fuel particle, but for smaller particle, lower blowing rate was needed for complete combustion. Based on the Taguchi method, the optimal parameters of solid briquette made from Caragana korshinskii Kom burned were 900 ℃ furnace temperature,1.5m3/h blowing rate and particle size less than 0.63mm. Meantime, the percentage contribution of each controllable factors to combustion speed was determined by the analysis of variance (ANOVA) and the rank order of the percentage contribution of each factor was particle size (42.1%), furnace temperature (28.5%), blowing rate (0.9%). Particle size and furnace temperature were the major factors for burning rate.The results not only further enriched and refined the densified theory of solid biofuel, provided the basis for the choice of processing technology and the design of processing and burning equipment for Caragana korshinskii Kom solid biofuel, but also provided reference for the development of biomass energy conversing and utilizing equipment from other types of shrubs and forest waste.