Thermodynamics study on the generation of electric

Authors: Heping Xie Wen Jiang Yufei Wang Tao Liu Ruilin Wang Bin Liang Yang He Jinlong Wang Liang Tang Jinwei Chen

Publish Date: 2015/08/23

Volume: 74, Issue: 8, Pages: 6481-6488

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Abstract

Carbon dioxide CO2 is the principal cause of the greenhouse effect which is due to the excessive consumption of fossil fuels Several methods have been proposed for reducing CO2 emissions in the atmosphere but none has been completely successful Recently a novel technique called the CO2mineralization cell CMC technique based on CO2 mineralization and utilization CMU was proposed by our group This converts chemical energy from the CO2 mineralization reaction into electricity while also producing highly valuable chemical products However some confusion and doubts still exist about its theoretical feasibility Herein a thermodynamics study and analysis of the CMC were conducted and the feasibility of generating electricity by CO2mineralization reaction confirmed theoretically The corresponding theoretical electromotive force and the maximum electric energy production per 1t carbon dioxide completely consumed under standard conditions were also calculated and their influential factors fully discussedGlobal atmospheric carbon dioxide levels are increasing sharply with the excessive use of fossil fuels and aggravate the greenhouse effect which is a serious threat to the human environment Mikkelsen et al 2010 Xie 2010a Data produced by the International Panel on Climate Change IPCC indicate that the content of CO2 in the atmosphere could increase to 570 ppm resulting in a temperature rise of 19 °C around the world by the end of twentyfirst century Yang et al 2008 At the same time the growing demand for energy is another global issue as yet unresolved Hence it is essential to develop new forms of energy before the fossil fuels are depletedAs is well known there are several methods of reducing CO2 emissions in the atmosphere such as the CO2 capture and storage CCS technology or by utilizing CO2 as a chemical raw material Aresta 2010 Barrow 1973 Hou et al 2014 Klotz and Rosenberg 1950 Liu et al 2014 Sheng et al 2015 Xie et al 2014a 2015a Zhang et al 2015b To mitigate the adverse effect on the environment and ensure the sustainable use of fossil energy numerous studies have been carried out to sequestrate CO2 underground Kolditz et al 2015 Zhang et al 2015a The socalled geological sequestration technology has been researched and trialed globally as a way of storing carbon Hou et al 2015 The feasibility of this solution has especially been the focus of a detailed research project by a European group under conditions close to those beneath the North Sea Holloway et al 1996 As reported recently CO2 has also been injected into deep unminable coal seams both as a means of storage and to increase the exploitation of methane using a method similar to that for the enhanced oil recovery EOR programs in the Texas oilfields in the United States Hu et al 2010 Pearce et al 1996 Although carbon dioxide can be sequestered in porous reservoir rocks below a certain depth using the present technology the problems of storing it in the subsurface cannot be resolved fundamentally due to the possibility of longterm geological disasters when it leaks to the surfaceIt is well known that the mineralization of CO2 is a chemical process between carbon dioxide and alkaline minerals to generate carbonated compounds which are stable under natural conditions Seifritz 1990 Xie et al 2013 To date several experimental and theoretical studies on carbonation reactions have been carried out under different conditions using a variety of natural minerals eg serpentine forsterite and wollastonite in an attempt to reduce CO2 levels in the atmosphere Lackner et al 1995 However direct experimental carbonation processes are usually extremely slow and difficult under room temperature as the carbonate generated will decompose when the temperature is too high Therefore the traditional carbonation process is not a satisfactory solution to carbon dioxide problemsAlthough carbon dioxide exists stably as an inert gas in the atmosphere under certain conditions it can also react with specific alkaline compounds in a few minutes or hours in a process called accelerated carbonation Pan et al 2012 During this progress carbon dioxide may interact with an alkaline material including natural silicateminerals and industrial residues to form a stable carbonate in a short time under specific temperature and humidity conditions Alkaline solid wastes have been proved to be effective raw materials to react with CO2 Nyambura et al 2011 Wang and Yan 2010 However the necessary conditions for these reactions are always harsh and the loss outweighs the gain from an economic point of view as the process would consume additional energy and generate more CO2 emissionsMeanwhile a new method known as CO2 mineralization and utilization CMU has been proposed by our research group to resolve CO2 problems Under this technique the captured CO2 is utilized as a raw material to manufacture high value carbonation products from both an economic and an environmental point of view Xie 2010b Xie et al 2012 2013 2015b c During this process carbon dioxide is carbonated by natural minerals and solid waste under conditions of low energy consumption and low cost Unlike the CCS method this is an effective way to reduce carbon dioxide emissions while also creating economic valueA novel technique to generate electricity using a CO2mineralization cell CMC was reported recently Xie et al 2014c in which portlandite CaOH2 and CO2 are the raw materials H2 gas is catalytically used up at one electrode and regenerated at the second electrode to introduce electrons or an H2induced current in the external circuit of the cell This design allows a stable current and electric energy to be generated by the CMC cell However some doubt still exists about its theoretical feasibility to be a revolutionary method to deal with carbon dioxide especially as the mechanism for the process of electricity generation via carbonation is currently not well known At the same time the corresponding electromotive force and the maximum electric energy produced under different conditions are also unknown In this paper the thermodynamics of generating electricity by CO2 mineralization has been studied and a theoretical analysis and feasibility of the relevant reactions conducted In addition the corresponding theoretical electromotive force and the maximum electric energy produced at standard conditions were theoretically calculated and the influential factors discussedChemical thermodynamics is the interrelation between work and heat which may change after physical changes of state or chemical reactions and must obey the three laws of thermodynamics Thermodynamic functions eg internal energy U enthalpy H entropy S Gibbs free energy G are obtained by experience and verified by rigorous logical reasoning and only involve the beginning and end of the change in state regardless of the process Chemical thermodynamics can only solve problems of a system in a state of equilibrium Klotz and Rosenberg 1950 Ott et al 2001

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