| In this paper,the grade and heat recovery potential of waste heat are analyzed and evaluated,which takes a large amount of waste heat produced during the production process of natural gas purification plant for research object.According to the principle of "temperature counterparts,cascade utilization",multi-grade organic rankine cycle power generation(MG-ORC)and multi-grade Kalina power generation(MG-Kalina)schemes are proposed for recycling the waste heat.A multi-objective optimization function is established based on the unit working net output power,enthalpy efficiency,UA value of heat exchanger and system heat emission which are regarded as the key performance indicators,and the performance of multi-objective is compared and analyzed.The research and analysis show that the performance of R-600 organic solvent in the MG-ORC power generation scheme is better than that of the other working medium;the subcritical MG-ORC power generation scheme has a better temperature change curve than the supercritical MG-ORC power generation scheme in matching the curve of residual heat temperature changes;the MG-ORC power generation scheme using R-600 as the working fluid has better comprehensive performance than the MG-Kalina power generation scheme.When analyzing the interaction between the effluent flow rate and evaporating temperature in the MG-ORC power generation scheme,the result indicates that the optimum evaporating temperature is constant at 104 ? with the increase of working fluid flow in a certain range;as the flow rate of the working fluid continues to increase,the optimum evaporation temperature increased from 104 ? to 108 ?,the multi-objective optimization function curve presents a trend of increasing first and then decreasing,showing a maximum at a working fluid flow of 27400 kg/h.When analyzing the interaction between the evaporating temperature and the compressor outlet pressure of the MG-ORC,it is found that at the same evaporating temperature,the overall performance will be better with the higher compressor outlet pressure;the multi-objective optimization function curve exhibits a small fluctuation when the evaporating temperature is between 100 ? and 105 ?,the multi-objective optimization function value is significant reduced with the increasing of evaporating temperature,and the maximum value is obtained at an evaporation temperature of 105 ?.It is discovered that the influence of evaporator minimum heat transfer temperature difference on the MG-ORC system can be reduced through improving the circulating fluid flow rate of the system by studying the effect of evaporator minimum heat exchange temperature difference on the system performance.In addition,the process parameters such as compressor outlet pressure,condensing temperature and superheat have a strong influence on system performance.In the analysis of MG-ORC power generation system,it was found that the system's main exergy loss was in the preheating,evaporation and condensation.A regenerative multi-grade organic rankine cycle power generation process(R-MG-ORC)and a semi-reheat multi-grade organic rankine cycle power generation process(SR-MG-ORC)are proposed in this paper to reduce exergy loss and improve energy efficiency.Both SR-MG-ORC and R-MG-ORC can effectively reduce the exergy loss of the system.The waste heat emissions of R-MG-ORC process is up to 478.6 kW,which means insufficient utilization of waste heat.The net output of the SR-MG-ORC process is 678.7 kW,which is higher than R-MG-OR process,and its thermodynamic efficiency is as high as 65.5%.Therefore,the SR-MG-ORC power generation scheme is determined to be the waste heat recovery and utilization scheme for the natural gas purification plant. |
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