MODELLING CO2 TRANSPORT AND THE EFFECT OF IMPURITIES 1
Thomas Demetriades , Trevor Drage, Richard Graham Efficient Fossil Energy Technologies Engineering Doctorate Centre, University of Nottingham, University Park, NG7 2RD, UK 1
[email protected] Introduction to CO2 Pipelines for CCS Environmental problems caused by the emission of carbon dioxide from fossil fuel power generation have been well documented. The current need for carbon abatement means that within a decade the only feasible sources of energy will be those which are both clean and efficient. As a result of challenges associated with alternatives to fossil fuels, it is likely that the majority of our power will continue to be generated from fossil fuels for the foreseeable future – a reality which does not reconcile with the objective of carbon abatement. Carbon dioxide Capture and Storage (CCS) is a strategy which will allow power generation from fossil fuels as normal, but without the environmental impact. With carbon trading starting to become prominent, as well as the issue of carbon emissions becoming more high profile, CCS will become an important tool, both economically and politically, and there is evidence to suggest that future energy security relies on our ability to successfully implement CCS as soon as possible. If we are to “keep the lights on” as demand for energy continues to increase despite ever more stringent carbon emission regulations, technologies like CCS are highly valuable for a dependable supply of power. Within CCS, economic and technical limitations at the capture stage mean that significant impurity content must be processed along with carbon dioxide, yet the precise effects of these impurities in the various stages of CCS have yet to be fully understood. In order to maximise cost effectiveness, it is necessary for the transport stage of CCS to take place at very high pressure; typically in the range 70 to 150 bar, where carbon dioxide and small amounts of impurity exist in the dense liquid or the supercritical phase. For safety reasons however, the transport must additionally occur within a single phase, but the impurities present can create regimes where liquid and vapour coexist, violating this condition. Given that pipeline transportation of carbon dioxide for CCS is likely to occur very close to these two-­‐phase regions, it is important to be able to identify their exact location to ensure safe and cost-­‐effective operation. This is knowledge which is not widely available, yet the uncertainty generated as a result currently represents a large barrier to the successful implementation of a large scale transport infrastructure network, and in turn to successful operation of CCS, threatening future energy supply. Presented here is work developing a method to predict the two-­‐phase region and critical point of carbon dioxide. A comparison with actual data is made in the hope of identifying where the accuracy of current models can be improved. Physical Properties of Carbon Dioxide In order to design safe pipelines for CCS transport, it is necessary to be able to model the effect of impurities on physical properties, in particular the phase behaviour, of carbon dioxide. The starting point for this analysis is selection of an appropriate equation of state, mixing rules and fugacity equations. Each equation of state is understood to have acceptable accuracy only within certain ranges, so part of the challenge is to bear in mind where the chosen equation ceases to be useful. There is now an increasing industrial demand for a single equation of state which can model a more universal set of conditions with greater accuracy than the current set of equations. Project Mandate It has been established that transport of carbon dioxide in CCS must occur within a single phase. Impurities originating from the combustion stage of CCS can, under certain conditions of temperature and pressure, create a situation where liquid and vapour coexist, violating this condition. This project looks at identifying the location of the two-­‐phase region and its dependence on impurities from the combustion stage of CCS, with increased accuracy over existing methods. The aims are: • To understand and clarify the effect of impurities on phase behaviour of carbon dioxide, proposing a new equation of state if necessary • To clarify the effect of impurities on density and compressibility of carbon dioxide Progression The Peng-­‐Robinson Equation of state (PREoS) has been used to develop the method of identifying phase equilibria. Even though graphical representations of data are widely available for a few well-­‐documented cases, the method for generating numerical data is poorly explained in the literature. It is therefore a part of this project to clarify the mathematical method used for generating predictions of phase behaviour. It will be necessary to create a catalogue of the effects on phase equilibria of a comprehensive range of different impurity scenarios, a task which is expected to take some time. From here however, comparisons with experimentally determined results will allow us to identify where existing equations might be improved. Early Results An understanding of how to solve the mathematical problem underlying this issue has been established and we can now start to generate plots of the expected phase behaviour, according to the PREoS. A few of these are shown below: Pressure (MPa)
9 8 0% 7 1% 6 2% 5 3% 4 4% 3 280.0 5% 290.0 Temperature (K)
300.0 6% Fig 1: Effect of amount of nitrogen impurity on the phase behavior of carbon dioxide. Comparison with experimentally gathered data shows that the PREoS becomes more inaccurate further away from the critical point. It is one of the aims of this project to identify a solution to this problem. Summary It is expected that the method we are currently developing will be of use when it allows us to see the evolution of phase equilibria with varying impurity content, such as shown in Figure 1. It will be of further use when compared to experimentally determined results to highlight the conditions under which the PREoS needs to be modified to increase accuracy. So far: • The method for identifying phase behaviour using the PREoS has been established • The mathematics behind this process have been explored and can be presented to explain this from first principles • We are able to generate graphical representations of this behaviour in two-­‐component cases Looking ahead: • This method will need to be generalised to the multi-­‐component case, as is consistent with CCS contexts • A comprehensive set of predictions needs to be generated • Comparison with experimental data should be made in order to highlight where the PREoS can be improved • Ultimately, a new equation of state should be proposed References 1) Ding-­‐Yu Peng and Donald B. Robinson, A New Two-­‐Constant Equation of State, Ind. Eng. Chem., Fundam., 1976, Vol 15 (1). 2) Antonie Oosterkamp and Joakim Ramsen, State-­‐of-­‐the-­‐Art Overview of CO2 Pipeline Transport with relevance to offshore pipelines, Report Number POL-­‐O-­‐2007-­‐138-­‐A, Polytec, Jan 2008. 3) Element Energy, CO2 pipeline Infrastructure: An analysis of global challenges and opportunities, 2010. 4) DNV Recommended Practice, Design and Operation of CO2 Pipelines, Report Number DNV-­‐RP-­‐J202, 2010. 5) Patricia N Seevam, Dr Julia M Race, and Professor Martin J Downie, Carbon dioxide pipelines for sequestration in the UK: an engineering gap analysis, Journal of Pipeline Engineering, 3rd Quarter, 2007.