Elsevier

Energy

Volume 35, Issue 6, June 2010, Pages 2729-2733
Energy

Gas hydrate formation process for pre-combustion capture of carbon dioxide

https://doi.org/10.1016/j.energy.2009.05.026Get rights and content

Abstract

In this study, gas hydrate from CO2/H2 gas mixtures with the addition of tetrahydrofuran (THF) was formed in a semi-batch stirred vessel at various pressures and temperatures to investigate the CO2 separation/recovery properties. This mixture is of interest to CO2 separation and recovery from Integrated Gasification Combine Cycle (IGCC) power plants. During hydrate formation the gas uptake was determined and composition changes in the gas phase were obtained by gas chromatography. The impact of THF on hydrate formation from the CO2/H2 was observed. The addition of THF significantly reduced the equilibrium formation conditions. 1.0 mol% THF was found to be the optimum concentration for CO2 capture based on kinetic experiments. The present study illustrates the concept and provides thermodynamic and kinetic data for the separation/recovery of CO2 (pre-combustion capture) from a fuel gas (CO2/H2) mixture.

Introduction

The emission of carbon dioxide from the burning of fossil fuels has been identified as the major contributor to the greenhouse emission and subsequent global warming and climate change. Particularly, fossil fuel electric power plants are producing about one third of CO2 emission worldwide [1]. There are three pathways for CO2 separation: pre-combustion de-carbonization, oxyfuel combustion and post combustion CO2 separation from flue gases. Carbon dioxide can be captured in power station, the flue gas stream (post-combustion capture) or an integrated gasification combined cycle process (pre-combustion capture). At present, the capture of CO2 from flue gases is done using amine solvents [2]. Pre-combustion carbon capture, involves the removal of all or part of the carbon content of fuel before burning it. The basic idea with pre-combustion de-carbonization is to convert a fossil fuel to a synthesis gas that contains mostly hydrogen and carbon dioxide. In pre-combustion process, the fuel gas produced contains a high concentration of CO2 (25–40%), and comes out at a total pressure of 2.5–5 MPa [3]. And, the separated hydrogen can be used in a gas turbine or a fuel cell [2]. Chemical–physical absorption and solid physical adsorption are processes employed for the separation of CO2 mixtures [4]. Since the energy cost is a major factor, new processes are being considered. One such process is based on gas hydrate crystallization [3], [5].

Gas hydrates are non-stoichiometric crystalline compounds formed when “guest” molecules of suitable size and shape are incorporated in the well-defined cages in the “host” lattice made up of hydrogen-bonded water molecules [6], [7]. These compounds exist in three distinct structures I (sI), structure II (sII), and structure H (sH), which contain differently sized and shaped cages. The sI and sII hydrates consist of two types of cages, whereas the sH hydrate consists of three types of cages [7]. CO2 is known to form structure I(sI) hydrate at moderate pressures, in the range of a few MPa [7]. According to Mao et al. [8], H2 forms structure II (sII) hydrate at the high pressure of 200 MPa or at the low temperature of about 80 K. Sugahara et al. [9] reported that hydrogen does not occupy any of the cages in the resultant sI hydrate structure based on a Raman spectroscopy study on single hydrate crystal of CO2/H2 hydrate. Kim and Lee [10] used both Raman spectroscopy and NMR to determine the occupancy of hydrogen in CO2/H2 hydrate cages. They suggested that it is difficult to detect the hydrogen peak through Raman spectra and only NMR was able to provide two different chemical shifts of hydrogen peaks with clear distinction. They reported that hydrogen molecules are occupied in the small 512 cages of structure I. When hydrate crystals are formed from a gas mixture, the composition of the hydrate is different to the original mixture. In other words, hydrate phase is enriched with one component. This is the basis for utilizing gas hydrate crystallization as a separation process for CO2 capture [11], [12].

Linga et al. [11] presented a two stage hydrate/membrane process for separating CO2 from a fuel gas mixture (40% CO2/60% H2). The authors also presented two metrics namely, CO2 recovery and separation factor to evaluate the hydrate based separation process. The operating pressure for the hydrate process presented by Linga et al. [11] is relatively high, the first stage operates at a pressure of 7.5 MPa. Hence there is a continuous interest on using additives to reduce the operating pressure without compromising the separation efficiency or CO2 recovery of the process. Some of the promising additives for the fuel gas mixture are propane [13] and tetrahydrofuran (THF)[14]. Based on kinetic experiments coupled with compositional analysis, Kumar et al. [15] proposed a two stage hydrate/membrane process with an addition of 2.6 mol% propane into the fuel gas mixture. The first stage of the process would operate at 3.8 MPa without compromising the CO2 recovery compared to the high pressure process [11], [15]. Moreover, Recently, Kumar et al. [16] based on molecular level studies found out that CO2/H2 mixed hydrate exhibits a self preservation effect similar to that of CO2 hydrate.

Hashimoto et al. [14] presented thermodynamic data showing that addition of 3 mol% of THF in the water lowers the hydrate formation pressure from 9.1 to 1.0 MPa for a 40%CO2/60%H2 gas mixture at 280.1 K. Moreover, it has been demonstrated that the presence of small amount of THF (1.0 mol%) significantly reduces the operating pressure of the hydrate process for CO2 capture from a flue gas mixture [5], [17].

The objective of the present study is to investigate the effect of THF concentration on separation of CO2 from CO2/H2 gas mixture via hydrate crystallization. More specifically, thermodynamic and kinetic data are presented for the 0.5, 1.0 and 3.0 mol% THF concentrations.

Section snippets

Experimental

The composition of the gas mixture was chosen so that they represent industrial composition. In pre-combustion, treated synthesis gas coming out of an IGCC power station consist of approximately 40% CO2 and 60% H2 mixture at a total pressure of 2.5–5 MPa [3]. Thus, the gas compositions used in this study were determined to be 39.9% CO2 and 60.1% H2 by gas chromatography.

The experimental apparatus used in this study is shown in Fig. 1. It consists of reactor (R), which is a high pressure vessel

Hydrate equilibrium for the CO2/H2/THF system

Phase equilibrium data was determined for CO2/H2/THF system at temperatures in the range of 273.0–283.6 K and the overall results are summarized in Table 1. Aqueous solutions containing 0.5, 1 and 3 mol% of THF were used to form the mixed CO2 and H2 hydrates. The phase equilibrium data is shown in Fig. 2 along with experimental data available in the literature for CO2/H2 mixture in pure water. As shown in Fig. 2, the equilibrium dissociation pressure gradually decreased as increasing the amount

Conclusion

This study investigated the effect of THF concentration on separation of CO2 from CO2/H2 gas mixture via hydrate crystallization. Also, the thermodynamic as well as kinetic data are presented for the 0.5, 1.0 and 3.0 mol% THF additions. The equilibrium dissociation pressure gradually decreased as increasing the amount of THF addition and the addition of a small amount of THF to water expanded hydrate stability region. For example, the equilibrium hydrate formation pressure was abruptly shifted

Acknowledgement

The authors gratefully acknowledge the financial support from the Ministry of Knowledge and Economy (MKE) in Korea, and this work is the outcome of a Manpower Development Program for Energy & Resources and New Renewable Energy Development Program.

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