| Energy shortage is a serious problem in social development.Energy recycling has become the focus of the energy industry.Heat recovery steam generator(HRSG)is a kind of heat exchanger,which re-collects waste heat from hot exhaust gas or liquid in order to generate steam.It makes the possibility of reaching more energy utilization efficiency and less emission.Hence,the analysis and optimization of the HRSG operation has become the focus of many industries.In addition,with the complexity of the HRSG structure and the variation of the working condition,it is of great significance to study the dynamic characteristics and transient working conditions for the safe and efficient operation of the HRSG.Since exhaust gas or liquid is a multicomponent mixture,which contains a large amount of ash particles,fouling and slagging on the tube surfaces over time is a common phenomenon for HRSGs.However,the deposition of ash particles may cause serious problems,such as low heat transfer efficiency,low unit-load operation or shutdown.Therefore,it is of great significance to study the characteristics of heat transfer enhancement,dynamic response analysis,structural design and optimization of the HRSG.Taking the U type once-through HRSG as the research object,this paper adopts theoretical analysis,numerical simulation and experimental study method to analyze the heat transfer,gas flow,particle deposition and dynamic characteristics.Researches and main results are as follows:1.According to the theoretical analysis of the movement and force of particles in the flue gas,we solved the numerical solution of the particle motion equation and studied the influence of each force on the particle motion.Based on the numerical results of particle motion and the critical velocity parameters of particle collision and deposition,we have established a new theoretical model of loose particle deposition for low temperature heating surface,which can accurately predict the amount,location and morphology of ash deposition.2.Numerical simulation of the HRSG was carried out based on the Fluent 15.0,extended by user-defined functions(UDFs).According to the full-scale numerical simulations of the HRSG,characteristics of heat transfer and gas flow are obtained,which are useful for the analysis of heat transfer performance and influence factors in the HRSG For the discrete-phase-model simulation of single heating surface,the multi component characteristics of flue gas are studied.The characteristics of deposition and distribution of particles with different diameters and velocities are analyzed based on the mathematical model of particle motion,collision,deposition and desorption.Comparison has been done between different tube arrangements of novel aligned,traditional aligned and staggered,and the effect of heat exchanger tube structure on heat transfer,flow and ash deposition was analyzed.Results show that particle diameter and gas velocity are the two dominant factors which influence the deposition ratio.Small high-speed particles have strong deposition propensity and the decrease of velocity promotes the sticking of large particles.Compared with the results of staggered and aligned heating surfaces,Nu of the novel aligned cross-flow one increases by 9.8%?23.2%,and fly ash deposition decreased by 4.9%?15.6%.With such tube arrangement,ash deposition is alleviated greatly so that heat transfer has been enhanced to a large extent at the expense of increasing pressure drop and flow resistance.3.A once-through HRSG experimental platform was designed and built,which could be used for low temperature heat resource recovery.Experiments have been done in a cement plant under different conditions to study its thermal performance,flow and ash deposition characteristics and verified the rationality of numerical simulation.The experiment of reclaiming the residual heat of different flue gas temperature,flue gas amount,feed water temperature and water supply conditions was studied by using the HRSG.The effects of different working conditions on the steam temperature and steam output of waste heat boiler were analyzed.Exergetic and economic analyses of the HRSG have been performed and it is proved that the HRSG is efficient in recovering energy from low temperature heat source.For further recycling of the waste heat of high temperature water produced by HRSG operation,a flash tank was implemented in this study.On the basis of theoretical analysis,HRSG-flash system model was constructed.Optimized researches were carried out to find the best exergy efficiency of the HRSG-flash system,through the selection of different water flow and diversion water temperature.Results show that when the flue gas amount is around 32000 Nm3/h,the highest exergy efficiency of the generating system and the HRSG can be up to 44.43%and 54.61%,respectively,by setting the water flow rate around 15.90 m3/h and the diversion water temperature 409.67 K.The annual income of HRSG-flash system after optimization can be 8.61×105 RMB and the construction cost can be recovered in 1.1 years.4.A dynamic simulation model was built for different heating surfaces of the HRSG.It can accurately simulate the phase change process of feed water in the heat exchange tubes,and make up for the lack of experimental observation.It is built in the Matlab/Simulink environment based on the thermodynamics principles and conservations of mass,momentum,and energy.Gradient algorithm(GA)and particle swarm optimization algorithm(PSO)are applied as parameter identification techniques to adjust the model parameters.After optimization,the dynamic model is matched well with the experimental data,and the root mean square error of the model is 2.97-4.43.The fitness function convergence values of the two optimization methods are approximately.In the convergence iteration,particle swarm algorithm finishes a little faster.Real-time on-site measurement data of start-up,off-design and variable operation for different conditions are used for model testing and validating purpose.Heat storage equation of metal tubes is added into the model,which considers feed water side delay in the system response because of the thermal inertia of the heat exchanger tubes.The model can be used to represent the real export parameter changes of heating surface for different operating conditions.By using this model,the most suitable operating conditions can be obtained under the premise of ensuring the service life of the metal tube.The results show that HRSG shutdown time is about 2 times of the start-up time.In the process of start-up and shutdown,the change of water temperature is the same as that of flue gas temperature,but it has a certain delay.Whether to and when to reach the saturation state depend on the combined effect of feed water and flue gas. |
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