Thermodynamic Carbon Pump Cycle For Solar-assisted CO₂ Adsorption: Fundamental Theory And Experimental Study

Due to global warming caused by greenhouse gases,climate problem has been the focus many countries worry about.However,significant energy consumption is one of the main technical barriers to the large-scale application of CO₂ capture technology.A novel concept---carbon pump---is proposed in this paper to analyze the energy-efficiency of these technologies.The analysis model,which embodies the carbon pump concept,includes the minimum CO₂ separation work and the second-law efficiency.The proposed method is applied to comparative analysis of current capture technologies considering both the quantity of energy consumption and the grade of difficulty level for CO₂ separation.The analyzed results show that the second-law efficiencies of the statistical cases are below 35%.For classical PCC technologies,the high second-law efficiencies are approximately above 15%,such as absorption,TVA,PSA,and RMC;the low second-law efficiencies are approximately10%or lower,which are for processes such as ESA,cryogenic separation,and CMC.A strategy for reducing energy consumption and emissions,which is the effective utilization of renewable energy integrated with CO₂ capture technologies,is proposed based on this analysis.Then,two adsorption technologies,including vacuum-pressure swing adsorption（VPSA）and pressure-temperature swing adsorption（PTSA）,are compared in terms of the minimum separation work and the second-law efficiency.Based on carbon pump,two adsorption cycles can be presented through the process expression in the adsorption isotherm diagram,which is an easy pathway to show CO₂ adsorbed amounts for each step.The influence of process parameters for VPSA and PTSA（4cycle parameters and 4 component parameters）are studied as well.Results show that the maximum values of the second-law efficiency are 24.30%and 19.09%,respectively for VPSA and PTSA with the change of CO₂ concentration from 5%to25%.However,the second-law efficiency for VPSA and PTSA decreases with the increase of the other three factors,including desorption temperature,pressure of the feed and percent of unused bed.For component parameters,when the efficiency of air compressor decreases from 70%to 30%,the second-law efficiency for VPSA and PTSA decreases with the change ofη_com and the second-law efficiency of VPSA is always higher than its PTSA’s counterpart.There is a cross point at the change of the other three factors including efficiency of vacuum pump,temperature difference and fraction of heat recovery.Moreover,a single-stage four-step vacuum swing adsorption（VSA）process,capable of CO₂ separation from the prepared flue gas,is experimentally studied on a fresh perspective of energy-efficiency analysis.Adsorption/desorption cycles utilizing a fixed bed of an adsorbent material made of zeolite 13X are performed under non-equilibrium conditions.Not only the purity-recovery target,but also the second-law efficiency is considered as the main performance criteria in this paper.Thereby,the performance analysis of experimental apparatus is conducted over a range of process parameters,namely CO₂ concentration,flow rate,time duration of pressurization with feed,time duration of feed and time duration of purge.The results show that the second-law efficiency calculated by the theory of carbon pump is1.83-4.27%.Zeolite 13X is able to achieve high second-law efficiency in the single-stage 4-step VSA cycle with high CO₂ concentration in feed gas.In addition,the extension of time duration for the steps of PF and PUR can help to improve the energy-efficiency performance for CO₂ separation,but their impacts on CO₂ purity are opposite.However,the extension of time duration for FD and increasing flow rate of feed gas reduce the second-law efficiency of experimental VSA apparatus.Finally,based on the strategy of the effective utilization of renewable energy integrated with CO₂ capture technologies,a novel system integrating solar Organic Rankine Cycle（ORC）into a power plant with amine-basedchemical absorption for CO₂ capture is proposed and working fluid selection is conduceted as well.From the aspects of technology and economics,a performance analysis is presented to compare the proposed system and three other systems based on a 300 MW_e power plant,in the power generation and emission reductions.Economic evaluation is conducted in terms of levelized costs of electricity（LCOE）and cost of CO₂ removed（COR）.The proposed system achieves a cascade use and matching the quality（temperature）of solar energy with a combined supply of power and heat.Compared to the power plant with PCC,the annual power generation increases 12.1%.From the electricity generation and CO₂ emission reduction,the proposed system is at the second place.From the economic viewpoint,the new system is the largest one at LCOE and COR.Economic evaluations give ussome economic borders for the new system.For instance,when the price of ORC is lower than 1284.46 USD/kW under the conditions that the solar field cost is 120 USD/m²,the proposed system has stronger competitiveness than the system of SOL-CO₂.When the price of ORC is lower than977.63 USD/kW under the conditions that the solar field cost is 120 USD/m²,the new system has advantages over the system of PC-CO₂.