Development Of Lime Shaft Furnace Briquettes As A Coke Substitute And Study On Its Drying And Combustion Characteristics

Developing a type of briquette as a coke substitute used in the lime shaft furnace is a new project with high application value and wide market prospect,which can alleviate effectively the lack of the coking coal resource and the contradiction between supply and requirement of coke.It will improve the economical benefit for lime production,obtain significant energy saving and protect environment efficiently.This article involved in the development of the briquette as a coke substitute for mechanical mixer lime shaft furnaces of alumina production industry.In the first part,the characteristics,briquetting methods,adhesive mechanism,application and development trend of briquette were presented in detail,the reason of substituting coke with briquette and its feasibility was analyzed.Subsequently,the evaluation indexes for the lime-shaft-furnace briquette used as a substitute for coke was confirmed by contrasting the coke and lump anthracite.The low rank anthracite was selected as the material of the briquette,and a little soft coal was added into it.A compound binder including the magnesium-base curing agent,magnesium-base activator,biomass fiber and water reducing agent was chosen.An orthogonal experiment with L27(3¹³) orthogonal table was carried out.Due to 6 evaluation indexes adopted,a multi-index integrated evaluation method based on weighting factor was used to turn the multiple indexes into one.The optimal component of the compound binder and corresponded key briquetting parameters were obtained with the range analysis and variance analysis in the orthogonal experiment.Then the adhesive mechanism and waterproof performance of the compound binder were qualitatively analyzed.The experimental results showed that the optimal component was 4%of magnesium-base curing agent,1%of biomass fiber,1%of magnesium-base activator,0.12%of water reducing agent,and the key briquetting parameters were 20kN briquetting pressure,17%of briquetting water,and 58%of the anthracite size less than 0.5mm,14%between 0.5mm and 1.0mm,17%between 1.0mm and 2.0mm size and 11%between 2.0mm and 3.0mm size. The mixture ratio of bituminous coal was 10%.The biomass fiber could obviously enhance the cold impact resistance index of briquette.Under a little drying condition,the briquette as a coke substitute has a better waterproof performance.The porosity of the briquette as a coke substitute was measured,and the drying characteristics and the correlation between water content and cold mechanical strength of briquette were tested experimentally.Then,the heat and mass transfer mechanism for the wet briquette convective drying was qualitatively analyzed.The results showed that the porosity of the briquette as a coke substitute was 16.23%,and the convective drying process experienced three stages which include the acceleration,constant and deceleration. The deceleration drying stage was dominated among the three stages.During the whole drying process,the evaporation interface in the briquette moved inward.The cold mechanical strength of the briquette as a coke substitute increased,and then decreased with the decrease of water content,so there was a maximum value.The best water content of the briquette was 2.32%.During the heat convective drying process of briquette,the heat transfer was intercoupled with mass transfer.It was dominated by the external drying conditions during the acceleration and constant drying stages and dominated by the inner conditions during the deceleration drying stage.Based on the Whitaker volume average theory and the Darcy law,the mathematics model for the heat convective drying of briquette as a coke substitute was established from the conservation of mass and energy.The wet and dry regions appearing in the drying process are distinguished with the criterion of irreducible saturation S_ir.The wet and dry region models are coupled at a moving boundary of evaporation front.The two state-variables: saturation S and temperature T were basic parameters in the wet region model,while pressure of vapor P_g and temperature T in the dry region model.With fully implicit finite difference discretization of governing equations,coupling with boundary conditions and dynamic evaporation boundary condition,a non-linear equation group was obtained,which was numerically solved by using an iteration method.VB6.0 was used for program compile and run.The curves of the water content MC and temperature T changing with time during the drying process were obtained.The results were basically identical with experimental results.Then,with this mathematical model,the effect of factors such as the drying medium temperature,briquette size,convective coefficients of mass transfer and heat transfer etc. was simulated and their influences rule was analyzed.The pyrogenation and combustion characteristics of the briquette as a coke substitute were studied by using the thermo gravimetric analysis method.The briquette as a coke substitute was compared with the coke and lump anthracite coal.The results showed that the briquette was characterized by the best igniting capability,stabilized combustion capability and integrated combustion performance,while the intermediate heat released and burnout performance.The combustion reaction of the briquette as a coke substitute belonged to the one order reaction,and the average activation energy was very smaller than that of coke and was slightly higher than that of lump anthracite coal.The combustion rate of single briquette was slightly influenced by the furnace temperature,while was obviously influenced by the size of briquette and air-supply condition.Under the same conditions,the combustion rate of briquette was lower than that of coke,while was higher than that of lump anthracite coal.Experimental results showed that the briquette as a coke substitute could be more suitable for the alternative fuel of the lime shaft furnace than the lump anthracite coal.With all results integrated,it showed that the briquette as a coke substitute could meet the production demand of the mechanical mixer lime shaft furnace on the cold and hot mechanical strength and combustion characteristics,but some necessary technical innovations or modifications must be made for the lime shaft furnace.The research in this dissertation has the theoretical significance and guiding value for substituting coke with briquette in the lime shaft furnace.