Radoslaw Gnutek of Nuon Business Development & Projects gives the lowdown on the Dutch utility’s carbon capture and storage demonstration project at Buggenhum, which is already home to a pioneering integrated gasification combined-cycle facility.
Radosław Gnutek, Nuon, the Netherlands
Construction of the CO2 Catch-up pilot plant in Buggenum
Nuon, part of the Vattenfall Group, operates an integrated gasification combined-cycle (IGCC) plant in Buggenum and is developing a multi-fuel IGCC with CO2 capture and storage (CCS) at the Nuon Magnum site in Eemshaven, the Netherlands.
The IGCC concept is especially attractive in a market of restricted availability of natural gas and of stringent environmental constraints. The technology allows for converting fuel into electricity at high efficiency with emissions, i.e. NOx, SOx and particles, below those of conventional coal fired plants and matching those of natural gas, except for CO2.
By operating the plant in Buggenum since 2001, Nuon has acquired vital experience of IGCC technology allowing it to ‘cross the bridge’ of its commercialization. Although the plant in Buggenum was far from perfect in its initial design – it required around 5000 modifications and improvements over the years of operation – the unit proved this technology can be used for highly efficient and reliable electricity production.
The only weak point of IGCC’s environmental performance, as with all other coal fired plants, is the large quantity of CO2 emitted to the atmosphere: approximately double that emitted from natural gas fired plant per kilowatt-hour generated. It is generally believed that extensive anthropogenic emissions of CO2 and other greenhouse gases are causing a steady increase in atmospheric temperature.
The international community, under the leadership of the United Nations, declared reduction of greenhouse gases emissions by committing to the Kyoto Protocol initially adopted in 1997. Although no further concrete action was taken, the recent Copenhagen Summit in December 2009 sustained the course.
However, in December 2008 the European Union committed its member states to an ambitious target of reducing the contribution to global warming and ensuring reliable and sufficient supplies of energy. Its target is to achieve a 20 per cent reduction in its greenhouse gas emissions by 2020 compared with 1990 levels. The multiple options to reduce CO2 emissions include CCS, which could make a substantial contribution.
Unfortunately, the application of CCS imposes a significant penalty on net plant efficiency, which significantly offsets the advancements made in the past 30-40 years of technology development. Therefore, the main challenge is to minimize the efficiency penalty. According to the Intergovernmental Panel for Climate Change (IPCC), the combination of IGCC with the CCS leads to the lowest efficiency penalty compared to the other leading carbon capture options1.
Although CO2 capture has never been applied in combination with an IGCC unit, many of the required process elements have been proven in the chemical industry for years, yet in a slightly different configuration than the one foreseen by Nuon. The difference lies in another purpose of the CO2 capture process. The usual objective is to obtain chemical products at certain precisely defined specifications. In the power industry, the aim is to remove majority of the CO2 from the treated stream at minimal cost. In addition, the mode of operation in the chemical industry is different than that in the power sector; in the latter the operating point of the water-gas shift and CO2 capture units should be able to comply with the load-following mode of the power plant.
Finally, the syngas composition when gasifying coal and biomass may differ somewhat from the composition when gasifying coal and refinery residues as performed in the chemical industry.
CO2 CATCH-UP PROJECT CHARACTERISTICS
The new aspects of the CO2 capture application to IGCCs have forced Nuon to take steps towards better understanding the nuances of the technology and to mitigate technical risks by first studying the concept in a pilot project called ‘CO2 Catch-up’. As such, the Buggenum plant is a perfect location. The pilot is a simplified, smaller version of the CO2 capture unit planned for the Nuon Magnum IGCC unit at Eemshaven, designed to treat 0.8 per cent of the syngas produced by the main power plant and to capture approximately 10 kt per annum of CO2.
The overall project objective is to generate knowledge and tools in order to better design Nuon Magnum and future IGCC plants with CO2 capture, with a strong focus on efficiency improvement. Apart from the improved design process, the pilot plant will allow for operator training and evaluating operating procedures and strategies. It will also help to predict the maintenance requirements of future full-scale plants and help in preparation of a maintenance approach.
As there is no suitable reservoir in the vicinity of Buggenum, the CO2 released from the solvent is compressed and mixed together with the hydrogen-rich stream emerging from the top of the absorber. It is then fed back to the Willem-Alexander power plant to the syngas line and then to the gas turbine.
After the pilot plant (see Figure 1) is commissioned in August 2010, a new phase of the project starts: plant operation and research, which is foreseen to last 18 months. The research programme is carried out by Nuon together with Delft University of Technology (TU Delft), Energy Research Centre of the Netherlands (ECN) and KEMA.
|Figure 1. Visualisation of the newly built Nuon Magnum IGCC plant in Eemshaven.|
The plant consists of five sections: syngas conditioning, water-gas shift, condensate recovery, CO2 absorption and solvent regeneration, and CO2 compression (see Figure 2). The syngas slip-stream is taken from the main syngas line downstream of the H2S removal (Sulfinol) unit and upstream of the saturator in the Willem-Alexander plant.
|Figure 2. Simplified process flow diagram of the CO2 Catch-up pilot plant|
The critical components, the water-gas shift section and the CO2 absorption and regeneration section, are essentially identical to those of the full-scale plant. The pilot plant, however, does not include the heat integration of the full-scale plant.
Syngas conditioning section: In the first section, the syngas is treated to bring it up to the specification required by the water-gas shift section, i.e. traces of Sulfinol solvent are removed in a water wash column, then the desired ratio of H2O:CO in the syngas is set by injecting reaction water to the stream, and finally the temperature is increased to the level required for the shift reaction. The water is injected in a liquid form and subsequently evaporated.
The heat is supplied with electric heaters instead of heat exchangers, as they would be applied in a full-scale plant, and the reactor effluent is cooled with forced-draft air coolers and cooling water. In this way the dependency of two streams (feed-effluent) is avoided and the precise control of temperature is possible. This simplifies the operation, extends operational flexibility and avoids process fluctuations that could influence the reliability of several tests.
Water-gas shift section: In the water-gas shift section the CO contained in the syngas feed is converted into CO2 in catalytic reactors. The following exothermal chemical reaction takes place between H2O and CO in the gas phase:
Although the reaction in theory is equimolar, an excess of water vapour is needed for forcing equilibrium to the right of the equation and preventing catalyst deactivation by limiting the adiabatic temperature rise and hotspots. It also prevents coke formation, which catalyses a Fischer-Tropsch reaction and reduces catalyst strength.
The water-gas shift reactor configuration is maintained as in the full-scale design with three reactors in series and a gas quench after the first reactor. The reactors were designed for the same inlet and outlet temperatures, H2O:CO ratio and space velocity as the full-scale plant. They are filled with an iron oxide catalyst typical for high temperature shift.
Condensate recovery section: In the condensate recovery section the shifted syngas is cooled down to condense the excess water in a liquids separator. The condensate is then recycled as reaction water to the syngas conditioning section.
CO2 absorption and solvent regeneration: The shifted syngas is subsequently introduced to the CO2 absorption and solvent regeneration section. First, the shifted syngas is compressed in a booster compressor to overcome the pressure drop over the upstream units bringing syngas back to the initial pressure. Then the stream is cooled and fed to the CO2 absorber where in contact with a physical solvent, i.e. dimethyl ether polyethylene glycol (DEPEG), CO2 is removed, resulting in hydrogen–rich gas.
In the absorber design process, one needs to account for separation efficiency and hydrodynamic behaviour (flooding, pressure drop). The pilot plant is designed for the same number of theoretical stages as the full-scale plant. Since the full-scale tower would be too large for the pilot, it was decided to use a finer packing to achieve the same separation efficiency and reduce the height. The packing should also be suitable for the high liquid loads typical for CO2 separation using DEPEG.
CO2 is recovered by depressurization of the rich solvent using three flash drums connected in series. The flash vessels operate at the same pressures as in the full-scale plant, with the exception of omitting a CO2 recycle stream from the first flash drum to the top of the absorber. It still contains some combustible components that normally should be recovered. The stream composition is analysed though and it is mixed with other CO2 streams from the second and third flash vessels.
CO2 compression section: The CO2 is released at atmospheric pressure in the solvent regeneration system and needs to be compressed in the CO2 compression section in order to feed it back to the power plant. For that purpose an electric-motor-driven two-stage reciprocating compressor is used with an intermediate water cooler, which is fundamentally different from the multi-stage integrally-geared centrifugal compressor foreseen for the large-scale application. The resulting CO2 product is mixed together with the hydrogen–rich gas from the top of the absorber and fed back to the main syngas line of the Willem-Alexander plant.
Instrumentation and analyzers: Since the main objective of the project is to generate knowledge in the form of useful data, which in turn is used to develop state-of-the-art tools that can be applied in the design process, extensive instrumentation is installed throughout the pilot plant. A large number of flow, temperature and pressure meters allow for automatically composing a full mass and energy balance.
A number of gas chromatography analyzers measure CO, CO2, H2, H2O, N2, H2S and COS in the key points of the installation. Gas phase as well as liquid phase samples are measured. Apart from automatic composition measurements, the plant is also equipped with manual sample-taking points to measure trace elements and organic by-products in gas and liquid streams.
An extensive research programme is also part of the project. Researchers from the partnering institutions are studying details of the pilot plant and trying to derive scale-up relations for the future full-scale plants, based on fundamental knowledge rather than engineering experience. The research programme objectives are:
- Testing a range of parameters representative for full-scale in a pilot plant
- Verifying the technology performance and operational window in the field environment
- Identifying and mitigating potential risks associated with the novel application of the selected technology
- Optimizing the design and performance.
In the research programme, a set of sophisticated mathematical models representing the processes taking place in the pilot plant will be developed. These models represent various levels of detail, allowing for application in various stages of the plant design, i.e. from conceptual design of the system to detailed design of the key components.
The models will be validated against measurements taken during operation of the pilot plant and subsequently extrapolated to full scale conditions. The research programme is divided into four work packages dealing with various aspects of the plant design and operation.
‘Work Package 1’ encompasses performance validation and optimization, water balance control, plant start-up, shut-down, and the impact of dynamics and off-design operation on the plant performance. In this work package, the main plant parameters – CO conversion, CO2 capture efficiency, CO2 product quality, specific energy consumption – will be analysed and optimized. In addition, the impact of such control parameters as H2O:CO ratio, water-gas shift temperature, absorption and flash temperature, flash pressure, absorber gas and liquid flow will be studied, optimised and verified in the pilot plant.
Also the influence that variations in the feed syngas composition have on the CO2 capture unit operation is of interest. Moreover, dynamic plant operation analysis is part of this work package. Regular transient operation will be simulated and optimised. The plant start-up and shut-down procedures shall be tested and optimized.
The main issues to be studied in ‘Work Package 2’ are the reaction kinetics, H2O:CO ratio, the impact of impurities and dynamics on the catalyst (activity, stability, selectivity). The kinetics are required for the modelling work. Rate constants can be derived from CO conversion measurements. In laboratory tests, catalyst performance can be measured.
Reducing the H2O:CO ratio prescribed by catalyst vendors can lead to reduced consumption of steam. However, as mentioned before, it may also lead to coke formation causing catalyst deactivation. In this work package, optimum operation conditions will be studied. Catalyst activity tends to be sensitive to certain syngas impurities originating from coal used in the gasifier. Finally a detailed mathematical model allowing for steady-state and dynamic analysis of the reactor will be developed in this work package.
The purpose of ‘Work Package 3’ is to optimize the operating parameters of the absorber as well as the solvent regeneration unit and to search for better solvent alternatives. Using measurements of CO2 concentration in both liquid and gas phases, a mass transfer coefficient will be estimated. Also, several packings will be tested in the absorber to enable optimal selection for the full-scale application. In addition a detailed model of the absorber will be developed within this work package allowing for studying both steady-state and dynamic phenomena.
In ‘Work Package 4’, fouling and corrosion will be investigated. Corrosion probes are installed to monitor the condition of the equipment in essential points of the installation. This work package is closely related to the overall maintenance concept development and plant operation.
The CO2 Catch-up project will allow Nuon to gather necessary knowledge and experience for full-scale CO2 capture applications in the Nuon Magnum project and future IGCCs. The test and research programme planned for 18 months after the pilot plant commissioning in August 2010 will generate detailed process insight and state-of-the-art tools. This knowledge will help Nuon to reduce the risk from the novel application of this technology and get one step closer on the way towards clean electricity production from fossil fuels.
1. Carbon Dioxide Capture and Storage, IPCC, 2005 – Bert Metz, Ogunlade Davidson, Heleen de Coninck, Manuela Loos and Leo Meyer (Eds.), Cambridge University Press, UK.