| Driven by the energy crisis and the goal of achieving carbon neutrality,renewable energy in China has been developed vigorously.Additionally,the use of energy-saving,conversion alternatives by different energy-consuming technologies has been frequently promoted.As key factors in the field of energy conservation,utilization of waste heat and pressure have been given increasingly more attention in various applications across the industry.Cogeneration units,being important sources of electricity and heat in China,are also large carbon emitters and have always been the target of energy conservation and consumption reduction efforts.As in other industries,waste heat from cogeneration units is usually emitted at a medium-low temperature.Efficient utilization of the medium-low temperature waste heat and pressure in cogeneration units is of great importance both in terms of improving the efficiency of the units and promoting carbon neutralization in China.This study combined temperature matching and cascade utilization techniques,as well as using equipment modeling,system simulation,and practical engineering analysis;the work has been divided into three parts:utilization of residual pressure from extraction steam,utilization of energy emitted from cold-end systems of steam turbines,and utilization of latent heat from flue gas.First of all,considering the fact that the transient performance of twin-screw expanders is not fully understood,and the performance analysis under different working conditions is deficient.The dynamic process of twin-screw expanders was calculated by CFD.The characteristics of the flow field were analyzed and parameters under different operating conditions and different meshing clearances were investigated.It is found that the leakage in the rotor cavity is mainly caused by the leakage from the contact line and the leakage from the blowhole.At the same time,the key parameters of the rotor fluctuate periodically with changes in the rotation angle.The output torque of the female rotor is only 27%of that of the male rotor.Under varying operating conditions,over-or under-expansion occur,resulting in the increase of outlet flow fluctuation unevenness and the decrease of adiabatic efficiency.In addition,the analysis of different meshing clearances shows that the adiabatic efficiency decreases rapidly and the exhaust steam temperature increases with the increase of meshing clearance.Secondly,to recover the extraction steam pressure energy of cogeneration units caused by the mismatch of heating parameters,a cascade utilization system of extraction steam based on screw expanders was proposed.Taking a 330MW unit as an example,the proposed scheme was compared to two other schemes,namely,the spray desuperheating scheme and the heat pump heating scheme,and the energy-saving mechanism was explored.The results show that,as a common regulating method,spray desuperheating has a negative impact on the unit.Under the rated heating load,the standard coal consumption rate of the expander heating scheme is reduced by 4.74g/kW·h,and the exergy efficiency of the heating process is increased by 17.29%,Under off-design working conditions,there is an optimal operating range for the expander heating scheme.In addition,the equivalent energy saving index was proposed to evaluate the energy-saving benefit.And the calculation results show that,for general cogeneration units,the energy-saving benefit of the heat pump heating scheme is better than that of the expander heating scheme,with the heat recovered by the heat pump being 3.81 times higher than the power output of the expander.Third,the residual heat and pressure of cogeneration units are interrelated,but there is little research on their coupling utilization.To solve this problem,based on the above research on the heating characteristics of the heat pump and the screw expander,a cascade utilization system of residual heat and pressure coupled with heat pumps and expanders was proposed.Absorption heat exchange,heat pumps,and expanders were combined to generate heat.Through joint simulation of MATLAB and EBSILON,the system was calculated and compared with four reference schemes.The results show that,under the rated operating condition,the standard coal consumption rate of the new system is reduced by 13.84g/kW-h,and the exergy efficiency of the heating process is increased by 13.93%.The thermoelectric decoupling analysis shows that the power load operating range under the rated working condition is increased by 23.34%,and the maximum heating load is increased by 18.32%.In addition,analysis of off-design operating conditions shows that the new system has advantages in both coal consumption rate and heating efficiency,and can effectively recover residual heat of exhaust steam and realize the near-zero loss at the cold end of steam turbine.Then,to solve the problem of insufficient latent heat utilization of wet flue gas in a boiler,a new system of collaborative deep recovery of flue gas waste heat and water,combining water spray and solution dehumidification,has been proposed.The system combines the cascade utilization scheme of waste heat and residual pressure designed above to recover steam residual pressure and flue gas waste heat.The heat and mass transfer processes of parallel-flow spray and solution dehumidification were modeled.Parameter analyses and calculations regarding changing conditions in the system were carried out by means of joint simulation.The results show that there is a critical tower height and a critical droplet size for each liquid-gas ratio in the spray tower.Among all the parameters,the liquid-gas ratio has the greatest influence on the performance of a spray tower and packed tower,and the increase of liquid-gas ratio weakens the influence of other parameters.The system analysis shows that,under the rated operating condition,the new system recovers 38.46MW of flue gas waste heat and 55.81 t/h of water,the coal consumption rate is reduced by 8.91g/kW·h,and the exergy efficiency of the heating process is increased by 12.12%.Finally,an engineering case of the screw expander-based extraction steam residual pressure utilization system was introduced.The performance of the screw expanders was analyzed based on operational data.The results show that a 600mm diameter screw expander can reach its rated power output of 720kW,with an internal efficiency of about 68%.Placing cascade utilization of medium-low temperature waste heat and residual pressure in cogeneration units as the primary focus,this study combines system structure designs with modeling and explores system operation characteristics and key parameter characteristics through simulation and engineering data analysis.A technological foundation for the promotion and implementation of medium-low temperature waste heat and residual pressure utilization of cogeneration units may be provided in this work. |
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