Study On The Novel Chemical Heat Pump Based On Ammonium Carbamate Reaction For Waste Heat Recovery

Due to the huge displacement of low-grade waste heat,a large amount of energy is wasted.In order to improve the energy utilization efficiency,heat recovery technology has been studied a lot,among which,chemical heat pump technology has been widely concerned because of its high energy density and low heat loss.However,most chemical heat pumps are used in the range of 227-927℃.The IAH（isopropanol/acetone/hydrogen）heat pump can absorb waste heat at 80℃,but the COP is very low.Therefore,high-efficiency chemical heat pump to recover low-grade waste heat is still in lack.Ammonium carbamate（AC）is the intermediate of urea production and can react at low temperature and pressure with large thermal effect.Based on the reaction of AC,a new heat pump system which combines chemical reaction with absorption process and mechanical compression is proposed.Low-grade heat is recovered by AC decomposition and NH₃ and CO₂ is produced.After being boosted,NH3 and CO₂ synthesis and release heat.At 20～100℃,the reaction heat is 2010k J/kg,which is around 13.4 times and 1.8 times of latent heat of R134a and NH3,respectively,which indicates that AC has great potential for application in heat pumps.In order to evaluate the system performance and provide guidance of the novel heat pump system,the following research is carried out:First of all,a novel selective absorption-compression composite heat pump system based on AC chemical reaction is proposed.And the following requirements should be met:（1）the maximum temperature in reaction side should not exceed 125℃to prevent AC reacts into urea and water;（2）the reactants circulating environment should keep water-free to avoiding the side reactions of（NH₄）₂CO₃ and NH₄HCO₃;（3）the absorption temperature should be lager than the reaction equilibrium temperature corresponding to the inlet gas pressure to prevent reactions in absorber.In addition,the circulating fluids is determined:（1）Ethylene Glycol（EG）is chosen to dissolve AC to enhance the thermal conductivity of AC in the reactors;（2）NH₃/Li NO₃ solution is chosen to selectively absorb NH3 in absorber because of its high absorbability.Secondly,the thermodynamic properties of NH₃/Li NO₃ and AC/EG solution is investigated and the AC solubility in EG is simulated by Aspen plus.Then,the operating conditions of the system were determined according to the P-T relationship between the reaction and absorption side and the crystallization line of NH₃/Li NO₃solution.To evaluate the performance,a thermodynamic model under steady-state conditions is established and the first and second law of thermodynamics analysis is conducted.Results show that:（1）the COP of the AC heat pump is about 1.4 times that of the Li Br/H₂O absorption heat pump,and 1.8 times that of the NH₃ absorption heat pump（NH₃/H₂O,NH₃/Li NO₃）.（2）Operating parameters affect the COP.As the generation temperature rises,the maximum COP can reach 2.7 and the maximum temperature rise of reaction is 35°C.In addition,the AC concentration and reaction conversion rate can also affect the COP.At low AC concentration,increasing the AC concentration can effectively improve COP.（3）While for irreversible loss（ΔS）,absorber and generator account for the largest proportion of the entire system,it’s about 54.5%.Besides,the generation and absorption temperature will have an impact onΔS,but the effect is not significant.While the reaction temperature difference has opposite effect onΔS and COP,ΔS reaches the minimum value of 1.26 k J/K atΔT=30℃,but COP is small under the same condition.Therefore,the optimization of the operating conditions of the system should consider both the efficiency of energy quantity and quality.Then,the selective absorption process in the absorber is simulated by establishing a falling film absorption model.The absorption process consists of two steps:（1）NH₃ in gas phase overcome the CO₂ resistance and diffuse to the gas-liquid interface;（2）the NH₃ at the interface is driven by the mass concentration difference between interface and solution.Through energy and mass balance laws,three ordinary differential equations are obtained,which can be solved by the 4th Runge-Kutta method to obtain the NH₃ absorption rate j and outlet CO₂mole fraction.It was found that for every20%increase in the inlet CO₂ mole fraction,the maximum NH₃ absorption rate decreased by 58.20%,and the difference of inlet and outlet CO₂ mole fraction decreases by 7.48%.When the inlet temperature difference between solution and cooling water increased by 5°C,the NH₃ absorption rate increased by 27.28%and the outlet CO₂ mole fraction increased by 24.65%.While the inlet velocity has the opposite effect on the selectivity and absorbability of NH₃,which is the result of residence time of NH₃ on liquid film.Through performance evaluation,it is believed that the system has industrial application value,but it still needs to optimize the components structure and working conditions.Through selective absorption simulation,it is concluded that under the condition of small inlet gas flow rate,increasing the heat transfer temperature difference can strengthen the NH₃ absorption process and multi-stage absorption can effectively increase the outlet CO₂ concentration.This article provides new research ideas for the development of chemical heat pumps in the middle to low temperature range,and provides theoretical support for the practical application of AC heat pump systems.