An engineering-economic model of pipeline transport of CO2 with application to carbon capture and storage
Introduction
Large reductions in carbon dioxide (CO2) emissions from energy production will be required to stabilize atmospheric concentrations of CO2 (Hoffert et al., 1998, Hoffert et al., 2002, Morita et al., 2001). One option to reduce CO2 emissions to the atmosphere is CO2 capture and storage (CCS); i.e., the capture of CO2 directly from anthropogenic sources and sequestration of the CO2 geological sinks for significant periods of time (Bachu, 2003). CCS requires CO2 to be captured from large-scale industrial processes, compressed to high pressures, transported to a storage site, and injected into a suitable geological formation where it is sequestered and kept from the atmosphere. Studies indicate that under appropriate policy regimes, CCS could act as a potential “bridging technology” that would achieve significant CO2 emission reductions while allowing fossil fuels to be used until alternative energy sources are more widely deployed. Moreover, as part of a portfolio of emissions reducing technologies, CCS could substantially reduce the cost of achieving stabilization goals (Herzog et al., 2005).
CCS will have significant impacts on the cost of electricity production and costs in other potential applications. Thus, methods are required to estimate the costs of CCS to evaluate actions and policies related to the deployment of CCS projects. In the last decade the understanding of CCS technologies has increased greatly, as reflected by the recent IPCC Special Report on Carbon Dioxide Capture and Storage (Metz et al., 2005). However, there are still significant gaps in knowledge of the cost of integrated capture, transport, and storage processes. For example, many studies of carbon capture processes have been undertaken (Thambimuthu et al., 2005) and engineering-economic models linking process cost to key engineering parameters have been developed (Rao and Rubin, 2002), but the majority have not yet been linked with transport and storage models to determine the cost of an integrated CCS process. Most cost studies either exclude transport and storage costs or assume a constant cost per tonne of CO2 in addition to capture costs (Metz et al., 2005).
There have been few studies that have addressed the cost of CO2 transport and storage in detail. However, earlier work by Svensson et al. (2004) identified pipeline transport as the most practical method to move large volumes of CO2 overland and other studies have affirmed this conclusion (Doctor et al., 2005). Therefore, this paper focuses on the cost of CO2 transport via pipeline. Skovholt (1993) presented rules of thumb for sizing of CO2 pipelines and estimated the capital cost of pipeline transport. In 2002, the International Energy Agency Greenhouse Gas Programme (IEA GHG) released a report that presented several correlations for the cost of CO2 pipelines in Europe based on detailed case study designs (Woodhill Engineering Consultants, 2002). More recently, an engineering-economic CO2 pipeline model was developed at the Massachusetts Institute of Technology (MIT) (Bock et al., 2003). Results from these and similar studies were summarized in the recent IPCC report (Doctor et al., 2005). However, none of these studies considered the unusual physical properties of CO2 at high pressures (Fesmire, 1983), the realities of available pipeline diameters and costs, or regional differences in the cost of CO2 transportation.
The objective of this paper is to estimate the cost per tonne of transporting CO2 for a range of CO2 flow rates (e.g., reflecting different power plant sizes) over a range of distances, and to also incorporate regional cost differences within the continental US. These cost estimates are embodied in an engineering-economic model that will be presented in this paper. A probabilistic analysis is used to quantify the impact of uncertainty and variability in cost model parameters on CO2 transport cost. This analysis also shows the range of costs associated with a given project and the probability of a given cost for a specific scenario.
Section snippets
Properties of CO2 in pipeline transport
Efficient transport of CO2 via pipeline requires that CO2 be compressed and cooled to the liquid state (Zhang et al., 2006). Transport at lower densities (i.e., gaseous CO2) is inefficient because of the low density of the CO2 and relatively high pressure drop per unit length. Moreover, by operating the pipeline at pressures greater than the CO2 critical pressure of 7.38 MPa, temperature fluctuations along the pipeline will not result in the formation of gaseous CO2 and the difficulties
Pipeline performance model
While there are proven flow equations available for use with high pressure gas pipelines (e.g., AGA fully turbulent equation), these equations can introduce error into the estimation of flow rates in liquid CO2 due to the underlying assumptions made in their development (Farris, 1983). The pipeline performance model used here is based on an energy balance on the flowing CO2, where the required pipeline diameter for a pipeline segment is calculated while holding the upstream and downstream
Pipeline cost model
Detailed construction cost data for actual CO2 pipelines (i.e., as-built-cost including the length and diameter) are not readily available; nor have many such projects been constructed in the last decade (Doctor et al., 2005). For these reasons, the data set used to develop the pipeline capital cost models is based on natural gas pipelines. However, there are many similarities between transport of natural gas and CO2. Both are transported at similar pressures, approximately 10 MPa and greater.
Combining performance and cost
To facilitate calculations, the linked pipeline performance and cost models have been programmed as a stand-alone spreadsheet model using Visual Basic in Microsoft Excel. Implementation of all equations has been validated by comparing spreadsheet results to manually calculated results using the same input parameters. The pipeline model has also been implemented in the Integrated Environmental Control Model (IECM), a power plant simulation model developed by Carnegie Mellon University for the
Case study results
Illustrative results from the pipeline model were developed using parameters representative of a typical coal-fired power plant in the Midwest region of the United States (Table 2). Several parameter values (e.g., capital recovery factor) are default values from the IECM software. Table 2 includes a nominal CO2 mass flow rate and pipeline length, but these two parameters are varied parametrically in the case study results presented here.
For the case study CO2 pipeline, the total levelized cost
Comparison with other studies
A number of recent studies and reports have estimated the cost of onshore CO2 pipelines, including studies by IEAGHG (Woodhill Engineering Consultants, 2002), MIT (Bock et al., 2003), and Hendriks et al. (2003). To the best of our knowledge, however, there are no publicly available CO2 pipeline cost models that have been published in peer-reviewed journals. Results from these and other recent studies were compared in the IPCC Special Report on Carbon Dioxide Capture and Storage (Doctor et al.,
Probabilistic results
To assess the sensitivity of the model to changes in multiple design and financial parameters, uniform distributions were assigned to several parameters of interest and a series of Monte Carlo trials were used to calculate the pipeline transport cost. The uniform distribution was selected to represent uncertainty or variability because there is no prior information that would suggest choosing a more complex distribution (such as a triangular or lognormal distribution). The design parameters of
Conclusions
The model of CO2 transport developed in this paper builds on past work in this area by linking the design of CO2 pipelines with the cost of construction and operation to arrive at the total cost of CO2 transport for six major regions of the United States. For the illustrative example presented here (i.e., 5 million tonnes of CO2 transported annually over 100 km in the Midwest), the estimated cost of CO2 transport was US$ 1.16 per tonne of CO2; however, this cost could vary by over 30% for other
Acknowledgements
Support for this work was provided by the US Department of Energy under Contract No. DE-FC26-00NT40935 from the National Energy Technology Laboratory (DOE/NETL), and by the Carnegie Mellon Electricity Industry Center under grants from EPRI and the Sloan Foundation. The authors are grateful to Howard Herzog, Paul Parfomak, and Hongliang Zhang for their valuable comments on the transport model and to reviewers of an earlier draft of this paper. The authors alone, however, are responsible for the
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