Predictions of the impurities in the CO2 stream of an oxy-coal combustion plant

doi:10.1016/j.apenergy.2010.04.014

Applied Energy

Volume 87, Issue 10, October 2010, Pages 3162-3170

https://doi.org/10.1016/j.apenergy.2010.04.014 Get rights and content

Abstract

Whilst all three main carbon capture technologies (post-combustion, pre-combustion and oxy-fuel combustion) can produce a CO₂ dominant stream, other impurities are expected to be present in the CO₂ stream. The impurities in the CO₂ stream can adversely affect other processes of the carbon capture and storage (CCS) chain including the purification, compression, transportation and storage of the CO₂ stream. Both the nature and the concentrations of potential impurities expected to be present in the CO₂ stream of a CCS-integrated power plant depend on not only the type of the power plant but also the carbon capture method used. The present paper focuses on the predictions of impurities expected to be present in the CO₂ stream of an oxy-coal combustion plant. The main gaseous impurities of the CO₂ stream of oxy-coal combustion are N₂/Ar, O₂ and H₂O. Even the air ingress to the boiler and its auxiliaries is small enough to be neglected, the N₂/Ar concentration of the CO₂ stream can vary between ca. 1% and 6%, mainly depending on the O₂ purity of the air separation unit, and the O₂ concentration can vary between ca. 3% and 5%, mainly depending on the combustion stoichiometry of the boiler. The H₂O concentration of the CO₂ stream can vary from ca. 10% to over 40%, mainly depending on the fuel moisture and the partitioning of recycling flue gas (RFG) between wet-RFG and dry-RFG. NO_x and SO₂ are the two main polluting impurities of the CO₂ stream of an oxy-coal combustion plant and their concentrations are expected to be well above those found in the flue gas of an air-coal combustion plant. The concentration of NO_x in the flue gas of an oxy-coal combustion plant can be up to ca. two times to that of an equivalent air-coal combustion plant. The amount of NO_x emitted by the oxy-coal combustion plant, however, is expected to be much smaller than that of the air-coal combustion plant. The reductions of the recirculated NO_x within the combustion furnace by the reburning mechanism and the char-NO reactions are the main reason for a smaller amount of NO_x emitted by the oxy-coal combustion plant. The concentration of SO₂ in the flue gas of an oxy-coal combustion plant can be up to six times to that of an equivalent air-coal combustion plant if the recycling flue gas is not desulphurized. The flue gas volume flow rate of an oxy-coal combustion plant is much smaller (<20%) than that of an equivalent air-coal combustion plant, which is a significant advantage for the purification of the flue gas.

Introduction

Carbon capture and storage (CCS) is regarded as one of the future carbon abatement methods to stabilise CO₂ concentrations in the atmosphere [1]. It involves the separation and capture of CO₂ from major CO₂ emitters such as large-scale power plants, transportation and storage of the capture CO₂ at a storage location for long-term isolation from the atmosphere. CCS is not a new technology and CO₂ has been captured, transported and stored from naturally occurring CO₂ sources since 1970s for enhanced oil recovery (EOR) in oil fields [2]. However there is little experience of fully integrated CCS systems with anthropogenic CO₂ sources and therefore further pilot studies and demonstrations have to be performed before a CCS system can be installed and utilized with a large-scale power plant.

At the present time, there are three main capture methods of CO₂ applicable to large-scale power plants: post-combustion, pre-combustion and oxy-fuel combustion [3], [4], [5]. With these CO₂ capture methods, a highly concentrated CO₂ stream could be obtained from the power plant processes. However, the impurities found in the flue gases of large-scale power plants have the potential to pass through each step of the CCS process and could interact adversely with the carbon capture process, the compression equipment, the phase properties of the CO₂ stream, the purification process, the transportation pipelines and the geological storage site [6], [7], [8], [9], [10]. Both the nature and the concentrations of potential impurities expected to be present in the CO₂ stream of a CCS-integrated power plant depend on not only the type of the power plant but also the carbon capture method used. In particular, the concentrations of potential impurities in the CO₂ stream of an oxy-coal combustion plant can vary greatly depending on the operating conditions of the oxy-coal combustion plant.

Oxy-fuel capture of CO₂ involves the combustion of a fuel in oxygen rather than air, producing a lower volume of flue gas with a much higher concentration of CO₂. The flue gas consists mainly of water vapour, high concentrations of CO₂ and excess O₂ which is needed to ensure complete combustion of the fuel. This flue gas composition makes the separation of CO₂ from the gas stream easier and less energy demanding than post-combustion capture as water vapour can be easily removed by cooling and condensing the gas stream, leaving a high concentration of CO₂-rich gas [3], [4], [5], [11], [12], [13]. To ensure the combustion temperature and heat transfer inside the oxy-fuel combustion boiler comparable to that of coal-air combustion, the O₂ needed for combustion and supplied by air separation units (ASU) is diluted with the recirculated flue gas (RFG) which is mainly of CO₂, with the O₂ concentration in the oxidizer kept at about 30–35% [14], [15], [16]. An additional benefit of recycling the CO₂ back into the boiler before undergoing purification and compression, is that the size of the combustion plants can be reduced by approximately 20% when compared to conventional, air-combustion systems [3]. After the flue gas stream is dried, compressed and purified if necessary, the gas stream composed of mainly CO₂ is ready for transportation and storage.

Oxy-fuel combustion technologies have been used in metallurgical and glass industries but are yet to be fully demonstrated or commercialized for CCS purposes with fossil fuel-fired power plants [3], [5], [13]. However, in recent years, oxy-coal combustion for CCS purposes has advanced rapidly from laboratory studies (e.g. [14], [15], [16], [17]) to pilot-scale demonstrations (e.g. [18], [19]). The 30 MW pilot-scale CCS demonstration plant of Vattenfall [18] which was designed as an oxy-coal combustion plant started its operation from September 2008. In July 2009, Doosan Babcock started the demonstration of the world’s largest OxyCoal™ Clean Combustion system on a full-size 40 MWth burner in Scotland, UK [19].

Although the flue gas of an oxy-coal combustion plant is mainly of CO₂, it also contains various impurities, such as H₂O, N₂, NO_x, SO_x and O₂. In addition, the flue gas recirculation of oxy-coal combustion could result in ‘accumulation’ of impurities in the furnace if the impurities in the flue gas are not removed prior to the recirculation. This means that the concentrations of some impurities such as SO₂ and NO_x in the flue gas of an oxy-coal combustion plant can be significantly higher than those in the flue gas of an air-coal combustion plant. The concentrations of various impurities present in the flue gas of an oxy-coal combustion plant depend on many factors such as the purity of oxygen generated from the air separation unit and the operating conditions of the plant (fuel properties, stoichiometry of combustion, flue recirculation arrangement etc.). Although some studies have listed the expected concentrations of various impurities [3], [10], [20], few have predicted the concentrations of impurities in the CO₂ stream of an oxy-coal combustion plant under different operating conditions. The results of such predictions could provide realistic data on the concentrations of impurities likely to be present in the CO₂ stream to those researchers who are investigating the impacts of impurities on the thermodynamic properties, compression, transportation and storage of the CO₂ stream.

The main objective of the present study is to predict the concentrations of CO₂ and its main impurities namely, H₂O, N₂/Ar, O₂, SO₂ and NO_x in the flue gas of an oxy-coal combustion plant under various conditions.

Section snippets

Predictions of the impurities in the CO₂ stream of an oxy-coal combustion plant

A block diagram for an oxy-coal combustion plant to be modelled is presented in Fig. 1, which is the similar to that of EPRI [20]. The flue gas desulphurisation (FGD) unit of the diagram presented by EPRI [20] was omitted in this study. According to EPRI [20], if the SO₂ concentration in the flue gas is higher than 3000 ppmv, the recirculated flue gas (RFG) should first pass through an FGD unit which follows the particulate removal device, such as an ESP or a baghouse. High concentrations of SO₂

Conclusions

The present study focused on the predictions of the impurities within the CO₂ stream of an oxy-coal combustion plant. The modelling results show that:

(1)
The flue gas composition of oxy-coal combustion depends on a number of factors, particularly the coal properties, the O₂ purity of ASU, the stoichiometry of the boiler, the O₂ concentration in the oxidizer and the partitioning of RFG between dry-RFG and wet-RFG. The flue gas compositions obtained under standard modelling conditions with three

Acknowledgments

The author would like to acknowledge the financial support to the multidisciplinary research project ‘Influence of Impurities on CO₂ Transport and Storage’ of the Bridging the Gaps fund of EPSRC (EP/E018580/1) via University of Nottingham. The other investigators of the research project including J. Billingham, M. Clarke, T.C. Drage, R.S. Graham, E. Norris and Michel Whitehouse are acknowledged for their contributions to the discussions on the background knowledge of the present paper. Deborah

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Predictions of the impurities in the CO2 stream of an oxy-coal combustion plant

Abstract

Introduction

Section snippets