System Integration And Analysis Of An Absorption Compression Heat Pump System In Recovery Of Waste Heat From Low Temperature Flue Gas

Large amount of the consumed energy in industrial process is wasted in low-temperature flue gas,which has resulted in severe energy wastage and thermal pollution.The AHP(absorption heat pump)can recover energy in low grade and transform it into higher-grade energy.It is one of the effective ways to reuse low-temperature waste heat.Generally,the working fluid of the AHP system is LiBr/H2O.The technology is usually applied for recovering heat of hot water from 80 ?to 100 ? and the output of the system is below 150 ?.However,the coefficient of performance is low,because the system must operates at negative-pressure,which consumes lots of energy.Therefore,there are many limitations for the application of AHP technology in industrial area.Founded by National Basic Research Program of China,"Fundamental research on significant and common challenge of efficient and multipurpose utilization of industrial waste heat",research on HACHP(hybrid absorption-compression heat pump)recovering low temperature sensible heat was conducted.The aim of this research focuses on system integration,analysis and optimization of thermodynamic performance of the HACHP system.Furthermore,this technology was applied into the micro-turbine CHP(combined heat and power)system,and the thermodynamic performance of the integrated HACHP and micro-turbine CHP system was also investigated.In this paper,HACHP system was integrated to recover low temperature sensible heat,typically flue gas below 200? that cannot be used by HRSG(heat recover steam generator).Driven by flue gas below 200 ? and a small proportion of power,saturated steam at pressure of 0.5 MPa could be produced by the HACHP system.The work fluid of the HACHP system is NH3/H2O and the system can operate at positive pressure.The compression process of working fluid is the combination of mechanical compression and thermal compression.The high-temperature portion of heat drives the thermal compression process,which can achieve high pressure ratio with lower power consumption by substituting electricity with heat.The low-temperature portion of heat is used to evaporate liquid ammonia.Furthermore,by adopting energy cascade utilization,the flue gas successively goes into the reboiler and the evaporator,as a result,the thermal efficiency of the system is improved significantly.To evaluate the thermodynamic performance of the proposed system,a new evaluate criteria,coefficient of heating performance,is defined.It can be used to evaluate the ability of the system converting the low-temperature flue gas into heat in high grade.In the base case,the simulation of the proposed system is conducted using commercial software Aspen Plus and the thermodynamic parameters of every state point of the proposed system are calculated.When the amount of heat recovered from the flue gas is 100 kW,the required power is 4.81 kW and the output of higher-grade heat capacity is 25.33 kW,which can be used to produce saturated steam at weight of 40.6 kg per hour.The temperature lift of the system is up to 66 ?.The exergy efficiency and the coefficient of heating performance is 28.7%and 0.198,respectively.Compared with the traditional coal-fired boiler,the HACHP system can save 53%of the primary energy to produce the same saturated steam.Based on the process simulation,an exergy balance analysis was adopted to reveal the methods for the thermodynamic performance improvement and the distribution of the exergy destruction.Besides,this paper investigated the effects of the mass fraction of ammonia water,temperature of the ammonia vapor and the generation pressure on the performance of the system,providing theoretical basis for the experimental research.Targeting the relatively high temperature of the waste flue gas of the micro-turbine CHP system,the HACHP system is integrated with the micro-turbine CHP system.The thermodynamic performance of the proposed CHP system is compared with that of the traditional micro-turbine CHP system.The results shows that the primary energy ratio and energy saving ratio of the new system are 67.3%and 23.56%,respectively,which are 11.2%and 14.76%higher than these of the reference system,respectively.The generated power of the new system is 4.4%lower,while the heat output of the new system is about 34%higher than that of the reference system.The exergy analysis shows that the exergy loss in the waste flue gas could be lowered by 78%in the new system.Though the power output of the new system is declined by the HACHP system,more heat in higher grade could be recovery from the waste flue gas of the micro-turbine CHP system.Therefore the energy saving effect of the new system is better.This study provides a new approach for waste heat recovery in the micro-turbine CHP system.