A pilot plant putting the amino acid salt concept to the test in real operating conditions is due to start up this summer at E.On’s Staudinger 5 hard-coal-fired plant in Grosskrotzenburg near Hanau. The slipstream pilot plant, which will be operated with a portion of unit 5’s flue gas, is expected to operate until the end of 2010. Staudinger 5 is equipped with selective catalytic reduction, electrostatic precipitation and flue gas desulphurisation.

This project is being sponsored by the German Federal Ministry of Economics under the terms of the COORETEC initiative. It is part of the German federal government’s 5th Energy Research Programme, “Innovation and New Energy Technologies”, which promotes research and development in the field of low-CO2 power plant technologies.

“As a major contribution toward climate protection E.On is planning industrial-scale CO2 capture and storage for coal-fired power plants starting in 2020. Operation of this pilot plant…is a major step in this direction,” said Gerhard Seibel, technical director responsible for new units at E.ON Kraftwerke.

In another recent key development Siemens has announced that, under a project with innovative state owned Norwegian utility Statkraft, it is adapting its amino acid salt based CCS technology so that it can be used for carbon capture at combined cycle plants. This project kicked off in January 2009 and is scheduled to be completed within two years, with technology then becoming “available for industrial-scale applications.” The Statkraft generation portfolio includes the 420 MWe Karsto combined cycle plant in Norway (joint owned with StatoilHydro and designed to be capture ready from the outset) together with two new combined cycle stations in Germany, Knapsack (800 MWe) and Herdecke (400 MWe, jointly owned with Mark-E), plus two older German combined cycle plants, Robert Frank (487 MWe) and Emden (452 MWe), which were acquired recently under an asset swap arrangement with E.On.

Siemens has developed a rigorous simulation model for its amino acid salt based process and has built a fully-automated absorption plant at laboratory scale to perform extensive testing and validation of the simulation model. This has been operating for nearly four years at Hoechst Industrial Park and has been used to analyse absorbent properties.

The laboratory scale plant is characterised by continuous operation of the complete absorption and desorption process under a wide range of operating conditions. Besides the model validation, the laboratory scale plant has delivered valuable information on the behaviour of the different flue gas byproducts with the solvent.

Long-term observations have confirmed the superior stability of the solvent when combined with oxygen. Since the laboratory scale plant runs on synthetic flue gas, the influence of gases such as NO2 and SO2 has been observed separately in order to get a deeper understanding of the underlying absorption mechanisms of these gases.

Furthermore, corrosion experiments using typical plant materials have been carried out inside the columns. Due to the fact that the plant is partly made of glass, process behaviour can be easily observed during operation. Extensive operational experience has been gained over about 5000 operating hours.

Why amino acid?

Solvent selection is a key issue 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, Siemens says.

Beyond minimal environmental impact, the Siemens 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.

These considerations led Siemens to an amino acid salt solution for the chemical absorption process.

This substance group has the advantage of negligible vapour 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 vapour pressure, they are not inflammable. Furthermore, the solvent exhibits low thermal sensitivity and so 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 in terms of process design, ie, the absorption and desorption process can be performed under a wide range of temperatures and pressures.

Also, “this second-generation solvent is well adapted to operational requirements”, says Siemens. Handling of the solvent for operation and storage is easy as it is not flammable, not hazardous, not toxic 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 that for MEA (monoethanolamine).

Considering these benefits, “the Siemens solvent is well adapted for CO2 capture from the flue gas of fossil-fired power plants,” the company says.

Process design

The capture process follows the principle of an absorption/desorption cycle. The off-gas is first cooled down slightly and then its pressure is increased slightly to transport it through the downstream scrubbing process. In the scrubber vessel, the absorbent (amino acid salt) extracts the CO2 from the off-gas. The CO2-depleted off-gas is discharged to the atmosphere. The CO2-laden absorbent is heated by passing through a heat exchanger and taken to the desorption column (stripper or reboiler). There the scrubbing agent is heated further to drive out the absorbed CO2. The heat for this step is provided by steam extracted from the power plant cycle. The CO2 leaving the absorbent is cooled and compressed. The regenerated scrubbing agent is returned to the absorption column. On the way, it exchanges heat with the CO2-laden absorbent.

The amino acid salt solution captures at around 45-55°C while the desorption (regeneration) occurs at about 120°C.

As well as the identification of a suitable capture solvent and its improvement, Siemens development activities are also focused on determining an optimum capture process configuration considering the given boundary conditions and interfaces involved in the power generation process (eg, flue gas properties).

The operating costs of the capture plant are mainly determined by the energy demand for 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 (eg, 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, about 50 different improvement options for the flow scheme were identified and rated according to qualitative criteria. From these 50 options, approximately 30 promising process variants were selected and calculated using the simulation model, and the operating conditions were optimised for each process scheme. The results were ranked based on energy consumption, investment and operational costs. In addition, combinations of the most promising process variants were evaluated. The preliminary results indicated that the energy consumption of the process could be reduced from 3.5 GJ/ton to 2.4 GJ/ton of separated CO2 by using an advanced process configuration. At the same time, costs per ton of CO2 avoided could be reduced by about 15%.

Additional improvements have been identified for the solvent, process optimisation and for the 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 in capital expenditures and operating costs. In particular with the reduction in the heat requirements for the regeneration step it should be possible to reduce the size of the equipment, piping and overall plant arrangement.

Siemens estimates that it should be possible to reduce the efficiency penalty associated with the process from 10.4 percentage points to 9.2 percentage points.

Another issue to be addressed will be plant dynamics when shifting from base load to transient load conditions.

Looking beyond the Staudinger coal pilot, a second pilot plant is being contemplated using another fuel, eg natural gas or lignite.

Capture readiness

As various important factors determining the economics of carbon capture, eg, the CO2 price itself, are still very uncertain, when designing new power plants it is advisable to make provisions from the outset for later incorporation of capture to minimise risk.

This involves finding a middle way, striking a balance between additional investment costs and the amount of effort required to accommodate carbon dioxide capture in the future, with one overriding requirement of course being that the plant should be as efficient as possible.

To address the issue, Siemens has developed a conceptual design for a coal fired power plant constructable today, but which is designed to be converted to accommodate CO2 capture at some point in the future. This design incorporates the space needed for the absorption/desorption plant, as well as the space required within the “normal” power plant to provide facilities for subsequent steam extraction. Additional issues to be taken into account are the higher demand for cooling and auxiliary electric power, layout planning, increased flue gas clean-up, additional switchgear, etc. The capture-ready steam plant owner should also apply for the appropriate licences to obtain clarity, for example, on possible water management issues and matters relating to injection of carbon dioxide into storage reservoirs.

Siemens has defined all the measures needed for its SSP5-6000 hard-coal fired plant, a reference power plant layout with a gross plant installed capacity of 600 MWe, efficiency about 46%, SST5-6000 steam turbine, Benson once-through boiler (tower type), and steam conditions 285 bar/600°C (HP), 60 bar/620°C (IP), 5.5 bar/269°C (LP).

Other options

As well as its own amino acid based post combustion capture technology, Siemens also has an alliance with Powerspan on that company’s ECO2 post combustion technology, which employs aqueous ammonia as the solvent and is based on Powerspan’s ECO multipollutant concept (for NOx, SO2 and Hg). The Siemens scope includes: absorber design/supply; process mechanical; and detail design/purchase/supply.

ECO2 post-combustion technology promises advantages, especially if the synergies between ECO and ECO2 can be realised in practice. A pilot ECO2 unit is in operation at the Burger power plant and the first FEED studies for the implementation of the technologies are already being executed.

One of the key advantages of post combustion as perceived by Siemens is that as well as being applicable to new build it can also be backfitted to conventional fossil plants. It is also scaleable and can be introduced via demo projects using slipstreams at existing coal plants (as at Staudinger). In addition, multi-train concepts can mitigate the risks of upgrading risks.

However, Siemens is also pursuing the carbon-dioxide-capture-before-combustion (pre-combustion) option through its relatively recent entry into the gasification arena via the acquisition from Sustec of Future Energy and its GSP entrained flow gasification technology developed in the former GDR (as described in Modern Power Systems, October 2007, pp 19-23). The company says the Siemens IGCC technology, with multi-fuel capabilities, is ready for implementation and FEED studies are currently being pursued.

Sources: Daniel Hofman and Hermann Kremer, Siemens Energy Sector