Technical Challenges and Cost Reduction Potential for Post-Combustion Carbon Capture Gernot Schneider Dr. Rüdiger Schneider Sylvia Hohe

Transcription

1 Technical Challenges and Cost Reduction Potential for Post-Combustion Carbon Capture Gernot Schneider Dr. Rüdiger Schneider Sylvia Hohe Siemens AG, Germany 1

2 1. Abstract CO 2 capture technology was developed to curb greenhouse gas emission into the atmosphere. Technology development reached a stage where it could be applied at large scale. Siemens as a provider of power plant technology offers a solution that has been piloted extensively and which is applicable to gas- and coal-fired power stations. Power generation with carbon capture and storage (CCS) can be evaluated and compared to other technologies by determining the Levelized Cost of Electricity. Even though power generation through photovoltaics and off-shore wind is more costly, CCS introduction is delayed because of the costs it adds to power generation from coal and gas. Thus cost improvement measures as to capital investment and as to operational expenditures need to be in the focus. There are some fields of general nature where costs can be reduced and other fields specifically determined by the nature of the amino acid salt solvent used with the Siemens PostCap capture technology. From today s perspective, a mid-term cost reduction potential of above 20% can be envisaged. 2. Introduction As shown in Figure 1 below, fossil energy sources are forecasted to remain the backbone of power generation worldwide in the coming decades. While the relative portion of fossil power generation is expected to slightly shrink in the future, the absolute value of power generation with fossil fuels will probably rise significantly. This also means that the total CO 2 emissions from power generation are very likely to grow despite worldwide efforts to reduce greenhouse gas emissions with renewables, hydro power, nuclear power and energy efficiency measures. However, there is still a chance to reach CO 2 emission reduction targets and thus the supposed maximum allowable increase in global warming if Carbon Capture, Utilization and Storage (CCUS) is implemented. Figure 1: Forecasted worldwide power generation mix in 2013 (Source: Siemens) Among the three major technology options for carbon capture (oxyfuel, pre- and postcombustion capture), post-combustion capture is the most versatile option because it is not only suitable for new power plants, but can be retrofitted to existing power plants that operate with fossil fuels. Siemens as a provider of integrated power generation solutions offers fuel gasifiers and power plant solutions for pre-combustion carbon capture and has extended its portfolio with a post-combustion carbon capture technology named PostCap TM, which uses an amino acid salt solvent. Siemens fuel gasifiers and power plant solutions are already well- 2

3 positioned in gasification/igcc projects in the USA and in China. This paper focuses on the current status of Siemens post-combustion carbon capture development and the potential for further reduction of investment and operation costs. 3. Market Requirements It is common knowledge that CO 2 capture and sequestration adds additional costs to power generation from fossil fuels. The story is similar to the introduction of emission reduction technologies such as DeNO x and DeSO x in the early 80s of the last century: Additional investment costs coupled with additional auxiliary requirements were adding cost to power generation, and almost no additional products or income streams were generated (with the exception of gypsum). The target with introducing such technologies was to come down with overall costs, thus minimizing the costs of each unit of electricity generated. That needed to be done with regard to capital investments and operational expenses. The capability of technology providers to optimally integrate such systems into the power stations is of great advantage. Particularly in regions with high fuel costs, efficiency is the issue being put in the focus. A comfortable measure of technical and economical efficiency is the definition of levelized cost of electricity (LCOE). The outstanding advantage is that different electricity generation technologies, like solar photovoltaics (PV), solar thermal power, nuclear power, wind power, etc. can be compared utilizing one type of number. One disadvantage lies in the fact that a large variety of parameters and assumptions are to be considered that need lengthy explanations and potentially could distort the interpretation of a comparison. Thus Siemens welcomes the initiative by IEA, GCCSI, EPRI, USDOE and others to define a common method of cost estimation 1. Figure 2 shows a typical comparison of technologies based on LCOE, issued lately by IHS CERA. As mentioned, the individual numbers may be subject to discussion, but obviously there is a relative advantage of fossil fuels with CCS compared tor certain forms of renewable power generation. Figure 2: Comparison of Levelized Cost of Electricity for various fossil and renewable power plant types (source: IHS CERA) 1 A White Paper prepared by the Task Force on CCS Costing Methods. January

4 Figure 2 depicts that next to the operating costs, high investment costs are still a major hurdle for implementation of CCS. Providing that a future power generation infrastructure with CCUS would utilize bundled CO 2 collection, transport and distribution networks instead of point-to-point connection of each CO 2 source with its storage location, then without doubt the major cost contributor in regard to capital investment is the capture plant itself. It must be considered as the main lever for cost reduction. (see Figure 3). Figure 3: Total cost of Carbon Capture and Storage with main cost drivers In addition, this figure points to another method of economic metrics: the allocation of all costs connected with CO 2 capture, transport and storage to each ton of CO 2 actually captured and stored away from the atmosphere. That metric is very helpful for a variety of purposes, e.g.: - The European funding program of CCS demonstration projects, referred to as NER300, intends to allocate the funding in an efficient, competitive way by ranking the projects calculating how many Euros are required for each ton of CO 2 to be safely stored away underground during a period of 10 years. - Utilizing the CO 2 as a commodity to be sold on a market where potential buyers of CO 2 are ready to pay a certain price for each ton of CO 2 received (i.e. Enhanced Oil Recovery). It needs to be well understood which market and which economic background apply for optimizing the CCUS technology. As mentioned above, when low LCOE is required, we are operating in a different market compared to one where a low product price of CO 2 is demanded by potential off-takers. That has implications on the direction and the types of cost reduction measures to be looked at first. 4

5 Further, besides the issue greenhouse gas reduction vs. CO 2 production there are other factors influencing design and optimization issues. These are redundancy and reliability requirements as well as operational concepts suiting an electricity market determined by fluctuating renewable power generation. On the way to cost optimization, appropriate scale-up factors need to be established and agreed. Process engineers are comfortable with high scale-up factors, which are proven and very common in the chemical process industry. However, trust and acceptance need to be built in the power sector where large-scale power generation facilities evolved during decades of careful step-by-step increases in physical sizes of equipment. In this regard, these both industries are worlds apart. Through its chemical process center located in Frankfurt-Hoechst, Siemens is able to reconcile both worlds and is thus capable to provide carefully designed, best suited technologies to its customers. Siemens PostCap TM technology was selected, tested, verified at Siemens laboratory in Frankfurt-Hoechst. Validation and improvements were achieved with its pilot plant at the E.ON power station Staudinger, not far away from the laboratories. Further, through several studies executed for large-scale projects in different regions, the potential for improvements and thus in reduction of investment and operating costs was clearly identified and is further explained in the subsequent paragraphs. 4. Siemens PostCap TM Process Siemens has developed a proprietary post-combustion CO 2 capture technology (PostCap TM ) that is designed for gas- and coal-fired power plants. It utilizes selective chemical absorption of the CO 2 from the flue gas and subsequent desorption, thus gaining nearly pure CO 2. An improved process configuration is applied, resulting in a significant reduction of the energy demand for the solvent regeneration (i.e. 2.7 GJ per ton of CO 2 captured). The chosen solvent allows running the process in the desorber at various temperatures and pressure conditions which provides remarkable design flexibility and potential for optimization. The main targets for Siemens in the development of the PostCap TM process were to meet most stringent environmental requirements without compromising the economics. Therefore Siemens had decided for amino acid salts (AAS) as the basis for the process. The advantages coming with it are e.g. that it is non-toxic, biodegradable, has almost zero vapor pressure, which results in nearly zero solvent emissions, high absorption capacity and temperature stability, low energy consumption and environmental sustainability compared to other aminebased technologies. Further, amino acid salts have ionic structure and therefore low sensitivity to oxygen degradation. Oxygen dissolved in water tends to negative loading and is thus hindered in degrading the dissolved anion of the amino acid salt. This results in a high chemical stability against oxygen, minimizing the need for fresh solvent. Nevertheless due to the fact that in post-combustion processes large quantities of flue gas are treated over a long period of time, even very small amounts of trace components which are contained in the flue gas as secondary components (like sulphur- or nitrogen oxides) could lead to a noticeable accumulation of secondary products in the solvent pump-around. Therefore, a purge stream of the cold solvent is directed to a reclaimer unit, where secondary products are removed. A solvent reclaiming unit ensures that the utilization of solvent gets maximized and solvent residues get minimized, and thus contributes to low operational costs. 5

6 Since the amino acid salt shows almost no vapor pressure, other techniques than thermal processes for solvent reclamation had to be implemented. The special features of the Siemens proprietary reclaimer are described below. One of those features is its capability to convert sulphur oxides contained in the flue gas to a marketable fertilizer product. This goes along with the advantage that the requirements for limiting the SOx levels upstream of absorber are less strict compared to other amine-based capture technologies. The main components in the CO 2 capture plant (i.e. PostCap Plant) are the absorption and desorption columns. Especially the absorber and desorber column internals are essential for efficient and reliable operation. A broad range of column internals has been evaluated during the past years to select the optimal internals for the absorption and desorption process. Great care was applied for setting the design parameters, such as temperature etc. in order to avoid precipitation of solid salts. Throughout over 9,000 hours of pilot plant operation no plugging of trays or packings was observed. Figure 4: Simplified process flow diagram for PostCap TM process (several process elements not shown) The standard desorber design considers atmospheric pressure conditions at the desorber outlet. If required, the design can be changed to elevated pressures and temperatures respectively. The limit is determined by the solvent stability. The aqueous amino acid salt solution exhibits low thermal sensitivity, so that refill requirements due to thermal decomposition are relatively low. Thermal stability of the solvent has been proven in shortand long-term investigations. A CO 2 compressor can bring the gaseous CO 2 up to higher pressure for further processing. CO 2 purity can in most cases be above 99.5 %vol. If there is the need for low water and oxygen traces in the CO 2 stream, for e.g. pipeline transportation and storage, then appropriate units for CO 2 purification are to be added. Such technologies are available from specialized vendors. 6

7 Figure 5: Chemical structure of amino acid salt solvent and inherent properties and advantages Siemens proprietary reclaimer unit includes two different reclaiming steps that are applied independently from each other: the separation of sulphites and sulphates as well as the separation of high-boiling components. A considerable advantage of the reclaimer is its capability to revert the blocking of solvent caused by sulphur oxides typically contained in flue gases from coal fired power stations or refinery off-gases (SO x reclaimer). The blocked solvent can be re-used and the sulphur will be bound in a chemical compound which can be sold as a fertilizer. Thus, higher levels of sulphur oxides in the flue gas upstream of the absorber are acceptable to the Siemens PostCap process than for other post-combustion CO 2 capture processes. In the NO x -Reclaimer secondary products from impurities such as NO x will be removed. The separated amino acid salt will be re-supplied into the CO 2 capture plant. The remaining liquid contains the undesired impurities plus a certain amount of solvent. Figure 6: PostCap TM process with solvent reclaiming 7

8 5. Operational Experience PostCap TM process design and the amino acid salt have been tested, validated and further improved in an automated continuously operating laboratory pilot plant at Industrial Park Frankfurt-Hoechst (Germany). It served well in the solvent selection, in characterization of its chemical behavior and determining its robustness. Besides validation of process design features, corrosion tests with construction materials were done. This lab pilot plant is still in operation for further process improvements. Since September 2009, testing has been executed at a pilot plant which is fed with real flue gas from EON s coal-fired power plant Staudinger in Germany. This PostCap pilot plant at Staudinger represents a fully operational plant at the state of art having been adapted to latest developments. It has been operated for more than 9,000 hours under real flue gas conditions since its commissioning. The measured results are well in line with the expectations derived from earlier phases. The proprietary reclaimer has been set up in the laboratory at the Industrial Park Hoechst, which is only a 30-minute drive away. Solvent to be reclaimed is transported back and forth in barrels. Due to the above mentioned stability of solvent, the amounts to be reclaimed are rather small and make this kind of operation possible. Having located the reclaimer in the laboratory allows for easy optimization. Since November 2012, the pilot plant at Staudinger has been in operation with flue gas coming from a natural gas burner. This extension of the pilot plant was implemented as a part of the Technology Qualification Program (TQP) for Statoil and Gassnova s full-scale CO 2 Capture Mongstad project. The tests aim at demonstrating the good performance of PostCap TM and that the emissions will meet the specified criteria under the flue gas conditions present at Combined Heat and Power Plant Mongstad. The TQP has been completed early April 2013 after 3000 hours of testing. Figure 7: PostCap TM pilot plant: mechanical completion in July

9 Figure 8: PostCap TM pilot plant at EON s Staudinger power station: current configuration with possibility to operate with flue gas from a gas burner package unit Figure 9: PostCap TM reclaimer (pilot scale) set-up in the Siemens lab at Frankfurt Hoechst 6. Cost Reduction Potential and Technical Challenges In the early phases of development of CCS technologies, the focus was on removing CO 2 from coal-burning power generation process. That was justified with the high specific CO 2 emission per unit electricity generated. The environmental benefit reducing 90% of such CO 2 is considerable. Further, the resulting relatively high CO 2 content or CO 2 partial pressure respectively in the off-gas (see Figure 10) is advantageous for any removal technology to be applied. 9

10 Figure 10: CO 2 emissions per kwh produced for power plants operated with natural gas, bituminous coal and lignite Remarkably, post-combustion capture technology is foreseen to be applied to CO 2 capture from gas-fired power plants. That is due to the fact that such power stations can be considered as sources for CO 2 for use in enhanced oil recovery (EOR) projects. Siemens gained valuable design experiences from such a project developed by Masdar, Abu Dhabi. It must be noted that electricity generation from low-carbon fuels like natural gas entails less specific CO 2 emission and lower CO 2 partial pressure in the off-gas compared to coal-based power generation. Figure 11 visualizes major impacts and challenges when implementing CO 2 capture at gas-fired power plants in comparison to coal-fired steam power plants. Figure 11: Major impacts and challenges if applying post-combustion capture to gas-fired combinedcycle power plants (CCPP) vs. steam power plants (SPP) As a result of lower CO 2 and higher O 2 concentrations as well as larger flue gas quantities compared to coal power generation, specific cost for separating a certain amount of CO 2 are significantly increased. In order to be able to implement a capture project successfully in the market, capture cost need to be reduced dramatically. Within own investigations based on FEED studies for various customers, in particular Capital Expenditures (CAPEX) have been identified as the major lever for cost reduction. However, Operational Expenditures (OPEX) also need to be taken into account since both cost shares are associated with each other. 10

11 Several process elements can be improved or modified in order to enhance cost effectiveness. The most promising cost reduction potential has been found in the following upgrades: Optimization of components, chemical reactions, materials and process features New process concepts In the following, the cost reduction potential will be exemplified with certain possible measures. a) Optimizations: Solvent activators: Additional CAPEX savings of up to 5-10 % can be achieved by application of enhanced solvent activators. Due to this, reaction kinetics can be accelerated, which results in lower heights for absorber and desorber columns. First results gained from Siemens laboratory pilot plant tests showed a reduction potential of column heights of about 20 % by application of an enhanced activator dedicated to the AAS solvent used in the PostCap process. Construction materials: Additional 5-10 % of CAPEX savings can be obtained by further optimization of construction materials used in the capture plant. Based on the current pre-fabricated cylinder- experience with pre-fabrication is considered to be a considerable advantage. shape design of absorber and desorber, stainless steel grades are recommended. Comprehensive long-term investigations are under way to judge the applicability of lower grade materials for PostCap including carbon steel. Also, other materials than steel are being qualified as potential construction materials for amino acid solvent based process. Concrete-based designs of towers lined internally with polymers or fiber composites are variants to the cylindrical shaped steel constructions. For the time being, the long-standing With the qualification of a number of materials for different plant parts and equipments, material selection can be optimized with regard to availability and price on the market. Where reasonable, adaptation of certain operating and design parameters of the PostCap process is taken into account in order to apply cheaper materials. Alternative concepts for direct contact cooling of the flue gas: Chemical absorption functions on the principle of a temperature difference between absorption and desorption. Usually flue gases from the combustion process are released from the power generation process at a temperature level above a desired level for the following absorption process. The above-mentioned principle of temperature swing necessitates cooling devices for the flue gas upstream the desorber. There are several ideas investigated how to bring down costs with alternative concepts for cooling. A considerable cost reduction potential has been identified. Details will be published in due time. b) New Concepts Solvent precipitation: Conventionally, absorption/desorption processes will be operated in liquid phase only; i.e. process conditions will be chosen in such a way that no reaction products of the CO 2 with solvent precipitate as solids. However, precipitation enables higher rich CO 2 loadings and more efficient CO 2 desorption. The application of precipitation reduces the energy demand, which may cause direct CAPEX reduction due to smaller desorber diameters and heat 11

12 exchangers. Nevertheless, a process with a precipitation step demands special equipment which might increase CAPEX on the other hand. As a result of a conceptual process evaluation, OPEX can be decreased by about 20 % (mostly thermal energy for stripping) and thus decrease total CO 2 capture cost. Innovative Cycles: In the past, CO 2 capture was mainly foreseen as retrofit to existing concepts of power plants. Siemens as an experienced supplier of power plants and at the same time as technology owner of the PostCap technology, performed comprehensive studies to optimally integrate the capture plant into the power plant environment so that waste heat can be utilized to the most economical extent. However, additional potential for decreasing CAPEX and OPEX was identified by modifying the power plant in such a way that the integration of the carbon capture facilities can be optimized. As a result, total cost for operating the entity of power plant and capture plant can be decreased. Possible concepts hint at a better utilization of sensible heat contained in the flue gas for e.g. desorber heating or increasing flue gas- or CO 2 partial pressure. Following the latter approach, a promising chance for cost reduction was identified in increasing CO 2 concentration in the flue gas. Several concepts have been and will be evaluated, such as flue gas recycling or supplementary combustion of fuel with residual oxygen contained in the off-gas upstream of the absorber. Besides increasing the CO 2 content, this measure reduces the oxygen amount contained in the flue gas and therefore minimizes further the solvent degradation. As a preliminary result, specific CO 2 separation cost at gas fired power plants can be decreased by around 10 % to 20%. 7. Conclusion The levelized cost of electricity (LCOE) of fossil-fired power plants equipped with carbon capture is lower than with off-shore wind or photovoltaic power generation. Through further cost reduction measures and the experience gained in large-scale demonstration projects, LCOE of power plants with CCS are expected to become competitive with other low-carbon power generation options such as on-shore wind and nuclear power plants. Among several available post-combustion carbon capture technologies, Siemens PostCap TM stands out because Siemens uses an amino-acid salt (AAS) solution. Siemens has developed a proprietary reclaiming process for AAS solvents that complements the PostCap TM carbon capture process and significantly reduces operational costs. As a power plant provider, Siemens is well aware of the requirements of power plant operators and could therefore tailor its PostCap TM process to their needs. Specific technologies and competencies are required for developing a suitable carbon capture technology for power plants. Using a post-combustion capture plant with PostCap TM technology, no new safety standards have to be observed by the operating staff, and permits should be easily obtained. Interests of power plant operators have defined the frame for PostCap TM development and will remain in Siemens focus for all process optimization measures. Since the production of CO 2 for Enhanced Oil Recovery (EOR) in oil and gas producing countries becomes more and more important as a driver of carbon capture, cost reduction measures need to concentrate on applications for gas-fired power plants. Considering the specific characteristics of flue gas from gas-fired power plants such as low CO 2 concentration, 12

13 Siemens puts emphasis on such measures that specifically address the requirements of gasfired power plants. Employing cost reduction measures such as additives to accelerate the absorption rate, lowgrade construction materials and innovative integration of the carbon capture plant into the power plant, it is assumed to bring down costs of capture by 20-30%. Such measures are within reach and quite well understood. They do not represent step-changes with potentially unknown risks. However, taking step-change technologies into consideration, like purposely implemented precipitation, further cost reduction potential could be unlocked. 13