| Grate cooler is a key equipment in new dry cement production technique.The main function of it is quenching the clinker and recover the waste heat.It includes fixed grate plates and moving grate plates.The high-temperature clinker calcined in rotary kiln falls onto the grate plates and clinker layer is formed due to the grate palte moving.Cooling air flow across clinker layer vertically to cool.The exhaust air formed is utilized gradually by rotary kiln,decomposing furnace,heat recovery boiler,and coal mill.Reducing the energy consumption during clinker cooling process,minimizing the irreversible loss in energy transfer process and improving the quality of waste heat recovered are important ways for energy conservation and emission reduction.Therefore,there are great significance to obtain the temperature distribution of heat transfer process in grate cooler and improve the efficiency of cooling system for energy saving and consumption reducing.The heat transfer principle in grate cooler is a forced convection problem in moving particles packed bed.The boundary conditions of clinker cooling process are varying at all time.Then the paper conducts theoretical researches based on fundamental convective heat transfer equations,convective energy equation in stationary packed beds,entropy generation minimization analysis and genetic algorithm.Moreover,multi-parameter and multi-objective optimization design are applied to study the cooling air distributions.Firstly,based on energy differential equation for single-phase flow,combining the energy two-equation model in stationary packed beds,an energy mathematical model for heat transfer process in grate cooler considering the effect of clinker movement is established.The model is appropriate for most gas-solid coupled heat transfer in moving packed bed.And boundary conditions are defined on the premise of reflecting the operating conditions.The partial derivatives in energy two-equation are discretized via Finite Difference Method based on Taylor series expansion.The first order upwind scheme is applied to convective terms and central difference scheme is applied to diffusion terms.The boundary conditions are discretized by heat balance method.Then difference equations set is obtained.For obtaining the values of coefficient and boundary conditions,an experimental research for the operating parameters of the grate cooler in a 5000t/d cement plant are carried out.The measuring contents include the cooling air distributions,mass flow rate of clinker,clinker temperatures of inlet and outlet of grate cooler,and temperatures of secondary air,tertiary air,high-temperature air,middle-temperature air and low-temperature air.Temperature distribution in the grate cooler and related operating data are understanded basically.Valuations of the equations set are carried out based on the experimental measurements and empirical formula.Jacobi iteration algorithm is applied to solve the discrete equations under the coding environment of MATLAB 2015a.Then the numerical solutions of clinker cooling process are obtained and steady gas and solid temperature distributions in clinker layer are obtained.The solutions are validated by the experimental values.Then,an analysis about the numerical solutions of clinker cooling process is conducted.The results show that the average gas-solid temperature difference is above 200? in rectangle calculated clinker cooling domain.Furthermore,the more close to lower-left corner,the larger temperature difference is.The largest temperature difference located in lower-left corner,which is about 1200?.It also verifies the necessity of applying the thermal non-equilibrium conditions;under the condition of same thickness of clinker layer,high temperature zone of clinker and cooling air become larger with clinker speed increasement and the most appropriate speed of clinker is 0.008m/s,which can ensure both the temperature of secondary and tertiary air and clinker cooling requirement;Thirdly,according to gas-solid heat transfer and flow empirical equations in packed beds,applying entropy generation analysis and genetic algorithm,an entropy generation analysis model of heat transfer and flowing process in grate cooler is established with cross-flow heat exchanger analogy.The design variables are superficial velocities of different air chambers and thicknesses of clinker layers.Then a multi-parameter optimization method for cooling air distribution based on modified entropy generation number is proposed.The qualitative and order of magnitude analysis on entropy generation calculation equation show that superficial velocity of cooling air plays opposite roles on modified entropy generation number caused by heat transfer and modified entropy generation number caused by pressure drop.Furthermore,the modified entropy generation number caused by pressure drop is much less than that caused by heat transfer.As a result,the modified entropy generation caused by pressure drop is easier to be "silenced" in the process of optimization.Thus,a multi-objective optimization method for cooling air system of grate cooler is proposed.The two independent objectives are modified entropy generation caused by heat transfer and pressure drop respectively.A serial Pareto optimal solutions are obtained via the multi-objective optimization and the analysis on Pareto front shows that entropy generation caused by heat transfer and entropy generation caused by pressure drop play the equal roles during the optimization process.The optimal air distribution is selected from Pareto front based on energy consumption of cooling fans minimization.And cooling fans energy consumption decrease about 45.52%compared with operating data.Moreover,a series of Pareto optimal solutions can be obtained by multi-objective optimization,which is more flexible on final solution selection than single-objective optimization.Sensitive analysis about average clinker particles diameter,air chamber quantity and cooling air inlet temperature are followed for the multi-objective optimization model of cooling air distribution of grate cooler.In particular,the analysis about cooling air inlet temperature is based on two heat recovery schemes,which are inducing the exhaust air into front and middle part of the grate cooler respectively to cool the clinker again.Calculating the multi-objective optimization model under different parameters and the optimal cooling air distributions are selected from Pareto front based on cooling fans energy consumption minimization.Then,the optimal cooling air distributions of different parameters are compared and analyzed on energy consumption of cooling fans,heat recovery and thermal efficiency of grate cooler.The reasons of energy consumption of cooling fans variations with average clinker diamenters,energy consumption of cooling fans and heat recovered variations with air chamber quantity,and energy consumption of cooling fans variations with cooling air inlet temperature are discussed.The final goal of analysis on optimal cooling air distributions of different parameters is to find the more benificial operating parameter for grate cooler.The comparisons and analysis show that average clinker particles diameter of 0.02m is the most beneficial parameter;it is the most effective and economic while the air chambers are 9;inducing exhaust air into the front part of grate cooler is more economic.Finally,experimental platform for studying convective heat transfer in cement soheres packed channel considering the wall effect is built and experimental research is conducted for convection heat transfer mechanism.Through experimental study for cement spheres with different sizes and Reynold numbers,a convective heat transfer correlation is fitted according to the experiment data,which considering the wall effects.Rationality and difference are discussed comparing with the correlation in reference.Then,convective heat transfer enhancement in the packed channel with different size spheres is studied.The analysis on the wall temperature shows that smaller spheres perform better than larger spheres on heat transfer enhancement. |
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