Whole-chain CCS system modelling

Jul 22 2013

With many diverse stakeholders involved it is important to have a common basis for making decisiosn that works across the whole CCS value chain. In a UK Energy Technologies Institute (ETI) commissioned project, PSE has developed enabling technology to help accelerate commercialisation and manage technology risk. By Mark Matzopoulos, Process Systems Enterprise Ltd

With the commercialisation of carbon capture & storage (CCS) given significant impetus by the announcement of the shortlist for the £1bn DECC award, UK CCS stakeholders will be required to address and resolve design and operability details from generation to storage at a commercial scale for the first time.
While CCS seems like ‘new technology’, there is little that is new in terms of individual CCS chain components. Indeed, conventional power stations, amine-based CO2 capture plants, pipelines and compressors have been part of the power and process industry world for decades and are well understood. Although it is spread out geographically, a CCS chain has less complexity than a small chemical plant. 
Challenges: interoperability at a system level
There are, however, still significant challenges in the commercial implementations of CCS. Many of these arise from the fact that the whole chain – and, eventually, whole CO2 transportation network – needs to be considered as a single system in order to make design and operation decisions that satisfactorily address the commercial imperatives and risk requirements of the various stakeholders along the chain. Even a cursory analysis shows that design and operating decisions at the power plant can have a significant effect on storage providers at the other end of the chain, and vice versa. 
Many challenges relate to the difficulty of investigating these system-wide interactions, resolving the techno-economic trade-offs that inevitably arise – which are affected by both design and operating decisions, and may require both steady-state and dynamic analysis – and proving that the whole system will work satisfactorily under a range of current and future scenarios. Underpinning all of this is the challenge of getting stakeholders with very different natures and commercial interests, working practices, technologies and tools – for example, power generation and oil companies – to work together.  
Many decisions come down to trade-offs between these diverse stakeholders. For example, power generators and storage providers both maintain that their requirements are simple. Generators simply want all their CO2 to be removed as they produce it. Storage providers, on the other hand, want a steady flowrate of CO2 with constant composition. Technically, it would be possible to achieve both of these requirements, although at the considerable cost of building a very large buffer storage at some central location and routing all CO2 via this. Obviously this is not a practical solution. 
One (capital-intensive) option is to provide sufficient installed compression capacity to handle all flowrate eventualities. An alternative is to provide ‘rich solvent’ buffer storage at the capture plant in order to peak-shave by allowing some processing to take place at off-peak generation time, for example during the night; depending on peak electricity prices, the cost of solvent and the amount of buffering required, this might improve overall generation economics. 
Another option would be to allow fluctuation in flowrates within certain bounds while imposing stringent limits on impurities, moving the onus from compression and storage providers to upstream gas treatment. Each of these options could involve significant capital and operating costs for one of the parties involved, depending on the specifics of operation.
It is evident from this single example that the decision space is complex. Not only is it necessary to determine realistic costs in each case in order to negotiate suitable commercial terms, but important technical considerations also need to be taken into account in these decisions. What is the effect of strongly fluctuating flow on well operation, for example? Does the consequent likelihood of two-phase flow impose risks of its own? How much buffering will a given CO2 pipeline allow, given pressure limits and taking into account other likely demands? It is no use resolving the economics at the expense of creating new technical risks; indeed it is almost impossible to separate the technical and economic aspects of such decisions.
A common basis for techno-economic decisions across the chain
What is clear is that some common basis for providing decision support is absolutely essential. Robin Irons, from E.ON’s CCS Innovation Centre, sums up the situation when he says: “we need tools that enable us to look at the whole system simultaneously so that we can answer questions and make decisions based on accurate numbers”. 
This was echoed by Andrew Green, CCS Programme Manager at the Energy Technologies Institute (ETI), whose members comprise BP, Caterpillar, EDF, E.ON, Rolls-Royce and Shell and the UK government. “Our members identified system modelling as a key enabling technology for the commercialisation of CCS”, he says.  “It is an essential tool for understanding interoperability of the components in the system, managing trade-offs and mitigating risks”. 
Investigation quickly shows that there is a lack of tools that cover the whole system to an adequate level of fidelity. Many simulation tools – such as PROATES®, PROMAX®, Aspen Plus® and OLGA® – have been and are currently used in power station design, amine plant design, compressor design, and analysis of transmission and injection operations. While very effective in resolving the current set of challenges, they are used mostly for single areas within the chain. 
A challenge for interoperability analysis is that different companies along the CCS chain tend to use different simulation software packages, some of which have a very industry-specific focus. Even if the underlying assumptions and approach of these software packages were compatible, the software architecture of most simulation software does not allow easy interoperation. This is particularly true for dynamic simulation, which is a key requirement for analysis of flexible power generation.
The ETI responded by setting out to commission a project to produce a ‘CCS system modelling tool-kit’. Says Green, “We asked respondents for proposals to create a commercially-supported CCS system modelling package capable of being used not only by individual stakeholders along the chain, but also for government policymakers and others that need to quantify options based on the whole chain or networks.” 
CCS system modelling tool-kit project
The result was a £3m project commissioned and co-funded by the ETI and project participants, comprising E.ON, EDF, Rolls-Royce, Petrofac (via its subsidiary CO2DeepStore), Process Systems Enterprise (PSE) and E4tech. The project is aimed at delivering a robust, fully integrated tool-kit that can be used by CCS stakeholders across the whole CCS chain. In parallel PSE is developing the commercial software product, known as gCCS, which will ensure that the outputs from the project will be made widely available and supported and further developed into the future.
Industrial input is from E.ON and EDF on the power generation side, Rolls-Royce for compression, and Petrofac/CO2DeepStore for the transmission and injection aspects. The project is managed by energy consultancy E4tech. PSE’s gPROMS advanced process modelling platform provides the modelling software basis, with the company providing much of the modelling expertise for creation of models that cover operations across the chain. 
Because of the requirement for many stakeholders to be able to model areas of the chain beyond their own specific processes (for example, a key requirement for power generators is the ability to investigate the effects on their operation of amine and compression systems attached to their plants), an early conclusion was that it is necessary for such a tool to provide process models for the whole chain. This is also useful – indeed essential – for groups who require a whole-chain view, such as engineering companies or government departments who need to quantify policy decisions.
Thus gCCS is being constructed with a full complement of models for conventional generation (pulverised coal and combined cycle gas turbine), new generation (gasification and oxyfuel), solvent-based carbon capture, compression, transmission and injection. In addition, it is possible to incorporate models of other plants, such as air separation units, using existing capabilities of the gPROMS platform, or to create custom models that can be incorporated within the environment. Using the tool it will be possible to look at single areas such as amine plants in detail; investigate partial-chain operations – for example, power generation, capture and compression; or analyse interactions across the whole chain and eventually – when this becomes necessary – whole network.
The individual process models are mostly implemented to what conventional process simulators consider to be medium-to-high fidelity: multicomponent streams; equilibrium methods for vapour-liquid separation; comprehensive compressor models allowing multi-stage, multi-section compressors with manufacturers’ curves; and distributed pipeline models for construction of pipeline networks that can take elevation into account. The capture models use rate-based techniques for accurate quantification of chemical and physical capture in order to quantify energy penalties accurately and allow meaningful analysis of transient operations. 
Says Chief Architect of gCCS Prof. Costas Pantelides, MD of PSE and a Professor of Chemical Engineering at Imperial College, “When you are seeking to make design and operating decisions, assess interoperability across whole CCS chains and perform economic optimisation, it is no use working with approximate or empirical models. We are now at the stage where high-fidelity models are required in order to deliver true commercial value and provide accurate quantification of risk.”
gCCS also allows external packages to be interfaced to the core environment, in order to simplify interoperability analysis while allowing companies to preserve existing workflows where necessary. For example, E.ON can use models of power plants constructed in its PROATES® package directly within gCCS. 
“This allows us to integrate our proprietary models – most already validated at commercial scale – into gCCS to accurately represent the operation of a complete CCS chain within a much wider framework (e.g. transportation networks connecting various CO2 sources and sinks). This will provide valuable information to evaluate potential opportunities and challenges for future CCS projects, in particular those where multiple CO2 sources (i.e. CO2 clusters) are located” says David Peralta-Solorio, who is managing E.ON’s contribution to this project on behalf of the CCS Team at E.ON New Build & Technology.
Accelerating new technology for efficiency improvement
While the individual components along the chain may be well understood, in many case there is much scope for improvement in the individual systems. Solvent-based CO2 capture may be well understood, but there is a significant body of ongoing work to design and prove new solvents that improve process economics and reduce energy penalty. 
Because of the inherent challenges of solvent-based processes, much R&D effort is being put into developing alternatives. These need to be scaled up, demonstrated and proven industrially; model-based techniques can accelerate all of these, including management of technology risk inherent in new designs. Likewise there are many questions around the design of pipelines and compression system for CO2 with significant impurities, as well as aspects of gasification processes that need to be proven in power generation scenarios.  
A key to the gCCS framework is that it provides and advanced custom modelling capability, meaning that it is easy to add models of new processes and combine them with existing flowsheeting components. 
Prof. Pantelides adds “We are working with universities and SMEs throughout the UK to provide services and tools to accelerate development of capture processes, and bring models of these into the gCCS framework. In the future we also hope to persuade vendors of capture processes, gasifiers and other equipment that it makes sense to have models of their equipment and processes that can be used for rapid analysis of integration issues, and also as sales tools.”
Current status
Half-way through the 30-month project, much has been achieved. The project team recently demonstrated a whole-chain model, built from a pulverised-coal power plant model constructed by E.ON, with compressions system input by Rolls-Royce, coupled with other components such as transmission and injection systems. Work is currently progressing with EDF on a dynamic CCGT demonstration, and the software is being rolled out to the initial users within the participating organisations.
Alfredo Ramos, head of PSE’s CCS strategic business and project manager of the gCCS development team, says “the fact that we are starting from a well-established process modelling platform is an enormous advantage. This means that the flowsheeting framework, steady-state and dynamic modelling, custom modelling and optimisation capabilities are already there. Our main task is to develop fit-for-purpose model content and tailor the environment for CCS users.”
“Given the challenges of this type of simulation”, says Ramos, “we were pleased to see that following a short training course E.ON was able to build a model of the power plant, and Rolls-Royce a model of the compression train, and then the two groups were able to combine these and create a whole-chain model within a few days.”
The future
The ETI-commissioned  project is due for completion in March 2014, after which gCCS will be maintained, supported and further developed by PSE. 
“We intend to create a broad system modelling ‘ecosystem’ for CCS”, says Pantelides. “The more people who are using the environment the better for the whole industry”. 
PSE is engaging with universities to provide tools for the considerable academic research activity into CCS currently underway in UK universities. “We see strong mutual benefits: CCS PhD students will be able to concentrate on their real research topics rather than spend the first 6 months creating a power plant model from scratch; we can also provide a route for dissemination of their results to industry. In return they provide an early test bed for the gCCS tool before we go out to industry on a large scale.” 
A test for success will be the application of gCCS on the DECC competition FEED studies. “We are talking to several of the projects”, says Ramos, “and look forward to seeing the benefits of the system modelling approach being proven in real industrial applications.” 


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