Optimization Design And Performance Analyses Of The Heat Recovery Exchangers And Systems On Rotary Kilns

Cement production is one of the most important part in the construction industry and is also one of energy intensive sectors in the industrial processes.The rotary kiln is one of the core components in a cement plant and affets the quality of the process of clinker formation.Heat loss from the rotary kilns accounts for a large proportion of total energy consumption in cement production.The energy consumption of the rotary kiln is mainly due to the exhaust gas and heat transfer from the kiln shell.Some findings show that these energy loss accounts for 30-40%of the total heat input in cement production.Therefore,heat recovery from the rotary kilns has become an alternative promising method for energy conservation in cement industry.A novel heat recovery exchanger and some related waste heat recovery systems are proposed to preheat water or generate steam in this paper.Numerical simulations and experimental measurements are carried out to investigate the flow and heat transfer characteristics of the heat recovery exchangers and related systems.Then,some mathematic models are analyzed to yield the relations which connect the design requirements to the design parameters of the waste heat recovery exchangers and systems,i.e.the mass flow rate and heat transfer areas or the thermal conductance.Based on these analyses,with the aid of the optimization algorithms in Matlab,the optimal design parameters are obtained to provide some good selections for the engineers considering the specific requirements.The main research work and results obtained are as follows:1.Considering the problems of lacking the analyses of the heat transfer processes in rotary kilns in the design of the heat recovery exchangers,a mathematic model is proposed to analyze the shell temperatures and heat loss rates of several regions on rotary kilns.Furthermore,the experimental measurements are carried out to determine the required temperatures and heat transfer rates of the heat recover systems,which provide the boundary conditions for the optimal design of the structural and operational parameters in heat recovery systems.The design methods of the heat recovery exchangers could be summarized based on the experimental results.In the clinkering reaction section which has a shell temperature of 200-300 ?,the heat recovery exchangers are to warm the kiln shell surfaces.So there is no need to have much more heat transfer surfaces here.In the regions which the shell temperature is over 300 ?,the heat recovery exchangers require more surface to absorb more heat.On the other hand,the function of the heat recovery exchangers is to absorb certain heat loss rates from the shell and make the shell temperature uniformly distributed along the kiln.2.Considering the low efficiency and complicated manufacturing of existing heat recovery devices,two novel heat recovery exchangers which installed on the shell surfaces are proposed.One has hexagonal water tubes which are easily manufactured.The other has water tubes and coiled pipes,which function as the radiative and the convective heat transfer surfaces,respectively.Through analyses of the fluid flow and heat transfer characteristics of the heat recovery exchangers,several mathematic optimization models are formed which connect the design requirements and the design parameters,i.e.the tube length,the tube numbers and the tube diameter.The thermal resistances of the heat recovery exchangers based on the entransy theory are also deduced.The heat transfer area,the pressure drop and the modified entropy generation numbers are set as objective functions.With the aid of genetic algorithm in Matlab,the optimized design parameters are obtained.The optimization results show that for the heat recovery exchanger with water tubes,the optimized heat transfer area is decreased by 30%compared with the value before optimization.What's more,in the cases of different pressure drop,the design parameters of the heat recovery exchanger change while the required heat transfer area remains the same.For the heat recovery exchanger with water tubes and coiled pipes,the optimized heat transfer areas of water tubes and coiled pipes are decreased by 15%and 20%,respectively.The corresponding pressure drop is significantly decreased after optimization.The modified entropy generation numbers are decreased due to fluid friction,while the modified entropy generation numbers remain unchanged due to heat transfer.The Multi-objective optimization method is more suitable for the structural design of the practical heat recovery exchangers.The optimization results show that the heat transfer area and pressure drop are decreased by at least 18%and 10.8%,respectively.The Pareto front solutions provide various selections for designers to choose according to the specific requirements and constraints.3.For the optimization design of the parallel and series-parallel heat recovery systems,several mathematic models are established to deduce the relations between the design requirements and the structural and operational parameters,i.e.the heat transfer areas and mass flow rates.Based on the relations,two optimization problems with the total heat transfer area and power consumption set as objectives are fomed to deduce the optimization equation groups by the conditional extremum principles.Solving the equation groups gives the optimal arrangement of the design parameters.Furthermore,the total heat transfer area,the total power consumption and the entropy generation due to heat transfer and fluid flow are set as the objective functions in four multi-objective optimization cases of the parallel and series-parallel heat recovery systems.With the aid of the genetic algorithms in Matlab,the optimized design parameters are obtained.The optimization results illustrate that the optimization method based on thermal resistance analyses can obtain the optimal area allocation and mass flow rate distribution of each heat recovery exchanger compared with the traditional method such as the control variate method.The total heat transfer area and power consumption are reduced by at least 10.8%and 12.1%,respectively.Besides,the economic analyses of the practical heat recovery system are proposed and the results show that the system is profitable with a saving of 41126.1(?)per year.The construction cost will be recovered in 10 months.4.A waste heat power generation system is proposed which contains a SP boiler,an AQC boiler,a condenser,a turbine and a heat recovery system with nine heat recovery exchangers.Integration of the thermodynamic and thermal resistance analyses yields physical model relations between the system requirements and the design parameters,i.e.the thermal conductance and the mass flow rates,which form the constraint equation group for a global optimization model.With the aid of the conditional extremum principles,the equation group is solved to obtain the optimal structural and operational parameters for the system.The optimization results show that the global optimization method can obtain the optimal design parameters for each component in the system.The required thermal conductance of the heat recovery system could reduce by 22%compared with the values before optimization.The vaporization temperature will decrease as the total mass flow rate and the work output of the turbine increase.Besides,the total thermal conductance will increase as the inlet temperature of the cooling water in the condenser and the work output in the turbine increase.