Optimisation of the connection of membrane CCS installation with a supercritical coal-fired power plant
Highlights
► The way of integration of a coal-fired unit with a membrane carbon capture installation has been proposed. ► The methodology for determining the optimal division of pressure ratio in a membrane installation has been developed. ► The optimisation of the integration of a membrane installation with supercritical coal-fired unit has been carried out. ► The economic analysis for a coal-fired unit integrated with a membrane installation has been carried out.
Introduction
The European Union accepted the climate-energy package. Under this obligation, the EU member countries took on the realisation of several aims:
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limitation of greenhouse gas emissions by about 20% in relation to the base year (1990),
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reduction of energy consumption by about 20% in comparison to prognoses for EU for the year 2020,
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increase to 20% total electricity production based on renewable energy sources,
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increase of biofuel use in transport fuels up to 10%.
The reduction of CO2 emissions for the year 2020 by about 20% is possible to achieve by the replacement of fossil fuels with renewable sources of energy and through reduced CO2 emission in blocks applying modern technologies of electricity production connected with CO2 capture and storage (CCS) in geological structures. To decrease CO2 emission, improving the efficiency of energy production is also essential [1], [2].
The connection of “clean coal” technology with CCS will permit electricity production from coal with almost zero CO2 emissions [3]. The driving force of the development of the zero-emission technology is the European Trading Scheme concerning allowances for the CO2 emission.
The task of CCS installation involves first the separation of CO2 and further the preparation of carbon dioxide for transportation to the place of storage. There are several different types of CCS systems:
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post-combustion;
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pre-combustion;
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oxy-fuel combustion.
For CO2 separation from flue gases, absorption processes, adsorption processes, cryogenic fractionation and membranes can be used [4]. Absorption processes are well known and are commonly used in the chemical industry. Thus, it is predicted that in the near future, chemical absorption using monoethanolamine (MEA) will play an essential role in CO2 capture from flue gases in power plants [5].
However, separation with the use of MEA requires substantial flue gas cleaning, especially in terms of SO2 and NOx. The removal of these components is connected to an increase in the working time of the solvent (MEA), which has to be regenerated with a great amount of medium- and low-pressure steam [6]. Separation also results in decreased power plant efficiency, as regeneration consumes around 4–6 MJ/kg of removed CO2 [7].
These drawbacks provide a rationale for searching for new, more competitive techniques for CO2 separation from flue gases, including membrane separation. Membrane separation is used both in the petrochemical industry and in the cleaning of natural gas before its transport. The use of polymer membranes in the 1980s contributed to the commercial success of this separation technique in comparison to absorption processes or cryogenic fractionation [8], [9]. On an industrial scale, the membrane process of air separation (O2/N2) was also developed.
Within the last several years, dynamic developments in membrane technology for CO2/N2 separation have occurred. The use of highly selective materials, especially polymers, has allowed the production of membranes with selectivity over 200 and even up to 400 [10], [11].
Several projects are currently underway in which membrane separation of carbon dioxide is applied.
Industrial tests of CO2 capture from flue gases resulting from coal combustion using membranes have been run in the EnBW power plant from 2008, as a part of the METPORE project [12]. These studies were conducted on polymer and cement-type membranes, with the participation of such companies as RWE and EON.
Another project, which has been realised since October 2007, is the MEM-BRAIN project, involving gas separation membranes for zero-emission fossil power plants. It is being realised by twelve research groups and five industrial partners including, among others, Shell and Siemens. The project is based not only on the application of membranes for the capture of CO2 from flue gases but also on the processes of H2/CO2 separation from synthesis gases and the N2/O2 separation in oxy-fuel combustion. In the investigations, polymeric membranes, zeolite membranes and ceramic and hybrid membranes were used [13].
Also, in Norway, the Membrain Research Group (MEMFO) deals with investigations on the use of membranes in industrial installations for the separation of such gases as CO2 and H2. In 2006/2007, MEMFO joined a consortium with 26 European partners from 14 countries and formed a project named NanoGloWa (Nanostructured Membranes against Global Warming) [14]. NanoGloWa is funded by the European Commission (EC) under the 6th Framework Programme.
Section snippets
Description of the reference system
The application of CCS installations in power systems is connected with a significant loss of energy production efficiency due to their high-energy consumption.
In Fig. 1, the structure of a supercritical coal-fired power plant is shown. This unit is considered as a reference solution for Poland after the year 2012. The chosen characteristic parameters of the unit are the following [15]: steam turbine power Nel,REF = 601.55 MW; live steam pressure and temperature 28.5 MPa/600 °C; and superheated
Influence of CCS installation on the efficiency of a coal-fired power plant
The energy consumption of CCS installations and its influence on the decrease of the energy production efficiency in energy systems is determined from the equation [23]:where δ1 is the auxiliary power rate of CCS installation.
Index δ1 is defined as the ratio between the power of the vacuum pump (Nel,VP) and the power of the compressor of separated CO2 (Nel,C) to the power of the reference unit (Nel,REF):
Determined in Ref. [15], δ1 = 0.2103, which
Replacement of low-pressure regeneration. Selection of optimal pressure ratios
If we assume that we close the flow of steam into the low-pressure regeneration (RH1–RH4: Fig. 1) and assume a constancy of enthalpy at the inlet to deaerator and at the outlet of the turbine (points 14 and 70), then the following amount needs to be brought to the condensate heat in the amount:where and h are the mass stream and water enthalpy, respectively, in points marked in Fig. 1.
The thermal power needed for the replacement of the whole low-pressure
Economic analysis
In the conducted economic effectiveness analysis, the net present value (NPV) method was used. NPV is one of the most fundamental and most frequently applied economic coefficients used for the assessment of the economical effectiveness of investments.
NPV can be described by the equation:
To determine the cash flow CFτ, the total investment costs (J), profits from sales (Sel), overall production costs (KPR), income tax (Pd), changes of the working capital (Kobr), amortisation
Discussion and conclusions
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The energy needs for CCS installation with the membrane module may be limited by the selection of optimum pressure ratios in the compressors and vacuum pumps used in the CCS installation and also by the use of gas cooling at the inlet to the mentioned machines. The heat obtained by cooling the flue gases and separating carbon dioxide may be used in the reintegration of the steam cycle of the power block. This reintegration allows the elimination of regenerative heat exchangers and consequently
Acknowledgement
The results presented in this paper were obtained from research work co-financed by the National Centre of Research and Development in the framework of Contract SP/E/1/67484/10 – Strategic Research Programme – Advanced technologies for obtaining energy: Development of a technology for highly efficient zero-emission coal-fired power units integrated with CO2 capture.
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