Development of Post-combustion Technology
Future Technology - The long-lasting resources of coal and the worldwide growing energy demand suggest that coal will continue to play an important role in the world's energy supply for decades to come. To comply with global emission reduction targets, the development of carbon capture and storage technologies (CCS) is necessary and represents a "no regret" option for clean coal usage. Efficiency increases of steam power plants have been a key factor in making power plants more economical and will continue to be crucial going forward, particularly when considering the impact that fossil-fueled power plants have on climate and the environment.
For several years, Siemens has been intensively developing its own post-combustion technology based on amino acid salt formulations. Successful process development requires reliable data of the process synthesis, optimization and the equipment design. Therefore, Siemens has developed a rigorous simulation model for the process and has built a fully-automated absorption plant in laboratory scale to perform extensive testing and validation of the simulation model operating at the Hoechst Industrial Park. The laboratory-scale plant ("lab plant") for CO2 capture can be used to analyze absorbent properties. This lab plant is characterized by continuous operation of the complete absorption and desorption process under a wide range of operating conditions. Besides the model validation, the lab plant has delivered valuable information on the behavior of the different flue gas by-products with the solvent. Long-term observations confirmed the superior stability of the solvent when combined with oxygen. Since the lab plant runs on synthetic flue gas, the influence of gases such as NO2 and SO2 have been observed separately in order to gain a deeper understanding of the underlying absorption mechanisms of these sour gases. Furthermore, corrosion experiments with the potential plant materials were carried out inside the columns. Due to the fact that the plant is partly made of glass, process behavior can be easily observed during operation. Last but not least, extensive operational experience has been gained during the more than 8,000 plant operating hours.
Amino Acid Salt Is the Basis of the Siemens Solvent
Solvent selection is essential because the solvent directly influences the energy demand and the environmental impact of the CO2 scrubbing process. Mastering the environmental risks of CCS is a precondition for its implementation because the advantage of decelerated global warming through reduced CO2 emissions should not be impaired by other environmental risks that might result from CO2 capture. Beyond minimized environmental impact, the priorities for solvent choice are high selectivity for CO2, low degradation, low energy demand, high CO2 capture rate and high purity of the CO2 stream. This is why Siemens uses an amino acid salt solution for the chemical absorption process. This substance group has the advantage of negligible vapor pressure so that, given an appropriate demister on top of the absorption column, the solvent emissions will be nearly zero. Amino acid salts have an ionic structure and are less sensitive to oxygen. As salts have no vapor pressure, they are not inflammable. Furthermore, the solvent exhibits low thermal sensitivity, thus refill requirements are expected to be very low, which has a direct impact on the operating costs of the CO2 capture plant. Thermal stability of the solvent also provides increased flexibility with the process design, i.e. the absorption and desorption process can be performed under a wide range of temperatures and pressures.
This second-generation solvent is well adapted to operational needs. Handling of the solvent for operation and storage is easy as it's not flammable, not hazardous, nontoxic and has good biodegradability. In addition, it is a registered chemical substance with available safety data. Tests have shown that the heat requirement for solvent regeneration is considerably lower than for MEA.
Considering these benefits, the Siemens solvent is well adapted for CO2 capture from the flue gas of fossil-fired power plants.
Standard Process Design for Absorption and Desorption Of CO2
Besides the identification and improvement of a suitable capture solvent, Siemens' development focuses on determining an optimum capture process configuration considering the given boundary conditions and interfaces from the underlying power generation process (e.g. defined flue gas properties). It is known that the operating costs of the capture plant are mainly caused by the energy demand for the regeneration of the solvent. This energy demand can be reduced by applying mature technologies that have been used for absorption and distillation systems in the chemical industry (e.g. heat integration concepts, withdrawal of side streams). Therefore, the identification of suitable process configurations was supported by an extensive survey in the open literature and in databases in print and online. The optimal process configuration was developed systematically.
During process development, approx. 50 different improvement options of the flow scheme were identified and rated according to qualitative criteria. From these approximately 30 promising process variants were selected and calculated using the simulation model, and the operating conditions were optimized for each process scheme. The results were ranked based on energy consumption, investment and operational costs. Additionally, combinations of the most promising process variants were evaluated. The preliminary results indicated that the energy consumption of the process can be reduced from 3.5 GJ/ton to 2.7 GJ/ton of separated CO2 by using an advanced process configuration. At the same time, costs per ton of CO2 avoided can be reduced by about 15%. Additional process improvements can be indentified for the solvent, process optimization and design and manufacturing of the required plant equipment. All of the identified improvement areas will be incorporated in the plant concept and should lead to a reduction of capital expenditures and operational expenses. With the reduction of the heat requirement in the regeneration step, we will be able to reduce the size of the equipment, piping and overall plant arrangement.
Next Major Steps
The Siemens amino acid salt post-combustion process has major advantages and offers improvement potential for a commercially viable CCS plant. Additional process improvements in several development areas, such as equipment improvement, process optimization and solvent optimization, are ongoing to further minimize the efficiency decrease to 9.2 percentage points. E.ON and Siemens have started up a pilot CO2 capture plant at the E.ON power plant Staudinger in Grosskrotzenburg near Hanau. The inauguration ceremony took place September 18, 2009. The two companies are thus pushing further ahead with the development of a process geared toward climate-friendly coal-based power generation. The lab-proven process is to be employed under real operating conditions at the power plant's hard-coal-fired Staudinger Unit 5.
Here the process will be validated in a slipstream pilot installation under real operation conditions. Siemens will also work on the plant dynamics from base load to transient load conditions.
The pilot plant will be operated with part of the flue gas from Unit 5. E.ON and Siemens intend to run the facility until the end of 2010. The results achieved and the operating performance of the pilot plant will serve as the basis for large-scale demonstration of the technology, which is scheduled to start operation in the middle of the next decade.
Reference Steam Power Plant Layout
Siemens has the in-house competence for the entire process chain, from engineering to design to implementation. Many years of experience in the construction and operation of power plants are supplemented by chemical process and plant engineering expertise, acquired with the acquisition of the process engineering from the former chemical giant Höchst.
As various important drivers for economy models, e.g. CO2 price development, are still wide open, it is advisable to incorporate preparations for later CO2 capture into the design of new power plants from the beginning to minimize risk. This involves finding a middle-of-the-road solution between additional investment cost and the effort required to accommodate CO2 capture in the future. Furthermore, operation of the plant should be as efficient as possible -- now and in the future. To address this issue, Siemens has developed a conceptual design of a steam power plant to be built today, which can be converted to accommodate CO2 capture at some point in the future. This design incorporates the space requirement for the absorption/desorption plant, as well as the space required within the "normal" power plant to provide facilities for subsequent steam extraction. Additional areas to be taken into account are the higher demand for cooling and auxiliary electric power, layout planning, extended flue gas clean-up, switchgear, etc. The capture-ready steam plant owner should also apply for the appropriate licenses to obtain clarity, for example, on possible water management issues and injection into storage reservoirs. Siemens has all measures defined for the SSP5-6000, a reference Siemens steam power plant layout.
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