Energy Analysis And Heat Exchanger Network Optimization Of Carbon Dioxide Capture Systems

The total energy consumption of carbon dioxide capture is high because theCO2absorption process is performed at low temperature(40℃), while its desorptioncompletes at much higher temperature(110～140℃). The heat needed in CO2desorption process is applied by the steam extracted from steam turbine in coal-firedpower plants. In addition, CO2is cooled during the procedure of compression toreduce work consumption. The repeated heating and cooling processes result ingreat energy loss. Aiming at solving this problem, the carbon dioxide capture systemis analyzed and optimized to lower total energy consumption and improve itseconomic performance. And the main work is as follows:Firstly, the method of thermodynamic analysis is used to conduct energyanalysis for carbon dioxide capture system. And the thermodynamic analysisapproaches include energy balance calculation and exergy analysis which aremutually coupled and complementary. Considering the desulfurization anddecarburization system as a whole, the enthalpy of each streams in this system arecalculated with black-box model by energy balance method. The results show thatthe heat of the system mainly come from the extracted vapor of the power plant andmost of the energy losses are due to scrubbing solution and cooling water. As for thecalculating process of exergy analysis, the physical and chemical exergy of certainstreams are calculated with a consideration of in chemical reactions. The exergydistribution of the whole system and all units are obtained and the results show thatabsorber, stripper and lean-rich heat exchanger are the preferable energy-savingobjects. In addition, recovering the heat of flue gas can consume less energy andreduce the usage of cooling water.Secondly, pinch technology is utilized to optimize the heat exchange network(HEN) of carbon dioxide capture systems. The temperature (107℃)of pinch pointfor HEN is determined to by use of “problem table” method after choosing cold andhot streams and calculating their heats. And then the hot and cold streams arere-matched according to the standard of pinch analysis. And three new retrofitschemes are obtained: the first one is one-stage HEN for energy recovery of the fluegas which uses the flue gas to heat the makeup MEA solvent; the second one is two-stage HEN for direct energy recovery which recovers the heat of the flue gasdirectly by letting the flue gas heat the rich solvent and the makeup MEA solventsequentially. The last one also belongs to the two-stage HEN, but it recovers the heatof flue gas indirectly, that is, let the flue gas heat water first and then the heatedwater is sent to heat both the rich and the makeup MEA solvent. The three newHENs are compared in terms of energy-saving rate and investment cost, and it isfound that the energy-saving rate of one-stage HEN is the highest; the cost ofone-stage HEN is the lowest. Therefore the one-stage HEN is chosen as the bestscheme.Thirdly, the optimized scheme of carbon dioxide capture systems is simulatedby Aspen Plus V8.2. The flow sheet of the initial carbon dioxide capture system ismodeled in order to compare it with the optimized system. The property method“Electrolyte NRTL” is chosen for the aqueous mixed solvent, the MEA-H2O-CO2module is established and it indicates that its decarburization rate is up to75.3%.Then simulating the flow sheet of reformed scheme, the results show that thetemperature of make-up MEA increases from40℃to93℃and the relative error is8.8%;Finally, the optimized scheme of carbon captured system is evaluated bytechnological economics. Applying the dynamic compensation method of fixednumber of year, the dynamic payback time of taking energy-saving measure is2.6years by conforming lump investment cost of energy-saving measurement, annualnet savings, annual rate of fund and the salvage value of the energy-saving project atthe end of its life cycle. And the data proves the feasibility of the energy-savingscheme on technological economics because this fixed number of year is less thanpayback time of industrial standard and half of designed application years.