Scholarly article on topic 'Optimized post combustion carbon capturing on coal fired power plants'

Optimized post combustion carbon capturing on coal fired power plants Academic research paper on "Mechanical engineering"

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{"Post combustion capture" / Coal / Amine / "Power station" / Optimization}

Abstract of research paper on Mechanical engineering, author of scientific article — Brian Stöver, Christian Bergins, Jürgen Klebes

Abstract Worldwide coal contributes to over 40% of the electricity generation today and its share is expected to increase steadily over the coming decades. The continued dominance of coal in global energy structure and the growing concern of climate change necessitate accelerated development and deployment of new technologies for clean and efficient coal utilization. Coal fired power plants with CO2 capture and sequestration (CCS) are widely expected to be an important part of a sensible future technology portfolio to achieve overall global CO2 reductions required for stabilizing atmospheric CO2 concentration and stopping global warming. Within the last decades the efficiency of coal fired power plants was significantly raised by different technical steps as running the steam generator on higher temperatures and pressures. So an efficiency (based on LHV) of higher than 45% for hard coal and 43% for lignite coal are more or less the technical standard for modern supercritical coal fired power plants. The amine scrubbing carbon capturing technology, which is planed to be added for fossil fuel fired power stations, will raise the self consumption of the plant and so lower the efficiency drastically. To minimize this effect it is necessary not only to find solvents with a small heat duty for the carbon dioxide scrubbing process, but also to optimize the heat re-integration to the water steam cycle and reduce the electrical self consumption of the scrubbing plant. This can be successfully designed with a detailed review of all key components as the turbine, LP–and HP–preheaters, the steam generator, the flue gas cleaning systems including CO2 scrubbing equipment and the cooling system of the plant. By improving the internal heat recovery in the power station the efficiency penalty of carbon capture (including the compression) can be reduced to values of 8 percentage points instead of 12 or more percentage points for systems without heat recovery and turbine modifications. This optimization is shown for different stages of heat reintegration from the CO2-compression unit, CO2 capture unit to the water-steam and flue gas side of the power station. Hardware modifications of components like the steam turbine are as well discussed as the additional components to be inserted in the boiler and water-steam cycle. As a global technology and equipment provider for complete thermal power plants, Hitachi has the knowledge and capability to address the above challenges of CCS.

Academic research paper on topic "Optimized post combustion carbon capturing on coal fired power plants"

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Energy Procedía 4 (2011) 1637-1643

Energy Procedía

www.elsevier.com/locate/procedia

GHGT-10

Optimized Post Combustion Carbon capturing on Coal fired Power

Plants

Brian Stöver*a, Christian Berginsa, Jürgen Klebes b 1

aHitachi Power Europe GmbH, Dept. New Technologies, Schifferstraße 80, 40759 Duisburg, Germany Hitachi Power Europe GmbH, Dept. Thermal Design Engineering, Schifferstraße 80, 40759 Duisburg, Germany

Abstract

Worldwide coal contributes to over 40 % of the electricity generation today and its share is expected to increase steadily over the coming decades. The continued dominance of coal in global energy structure and the growing concern of climate change necessitate accelerated development and deployment of new technologies for clean and efficient coal utilization. Coal fired power plants with CO2 capture and sequestration (CCS) are widely expected to be an important part of a sensible future technology portfolio to achieve overall global CO2 reductions required for stabilizing atmospheric CO2 concentration and stopping global warming.

Within the last decades the efficiency of coal fired power plants was significantly raised by different technical steps as running the steam generator on higher temperatures and pressures. So an efficiency (based on LHV) of higher than 45 % for hard coal and 43 % for lignite coal are more or less the technical standard for modern supercritical coal fired power plants. The amine scrubbing carbon capturing technology, which is planed to be added for fossil fuel fired power stations, will raise the self consumption of the plant and so lower the efficiency drastically. To minimize this effect it is necessary not only to find solvents with a small heat duty for the carbon dioxide scrubbing process, but also to optimize the heat re-integration to the water steam cycle and reduce the electrical self consumption of the scrubbing plant.

This can be successfully designed with a detailed review of all key components as the turbine, LP- and HP-preheaters, the steam generator, the flue gas cleaning systems including CO2 scrubbing equipment and the cooling system of the plant. By improving the internal heat recovery in the power station the efficiency penalty of carbon capture (including the compression) can be reduced to values of 8 percentage points instead of 12 or more percentage points for systems without heat recovery and turbine modifications. This optimization is shown for different stages of heat reintegration from the CO2-compression unit, CO2 capture unit to the water-steam and flue gas side of the power station. Hardware modifications of components like the steam turbine are as well discussed as the additional components to be inserted in the boiler and water-steam cycle.

As a global technology and equipment provider for complete thermal power plants, Hitachi has the knowledge and capability to address the above challenges of CCS.

© 2011 Published by Elsevier Ltd.

* Corresponding author. Tel.: +49-203-8038-1750; fax: +49-203-8038-611750. E-mail address: b_stoever@hitachi-power.com.

doi:10.1016/j.egypro.2011.02.035

Keywords: Type your keywords here, separated by semicolons ; post combustion capture; coal; amine; power station; optimization

1. Requirements of PCC Integration

To minimize the effect of the efficiency drop due to the CO2 removal with post combustion capture it is necessary not only to find a solvent with a small heat duty for the carbon dioxide scrubbing process, but also to optimize the heat re-integration to the water steam cycle and the electrical power consumption of the scrubbing plant

The implementation of CO2-scrubbing technologies into a power plant implies enormous challenges to the design ofthe power plant itself and the post combustion capturing plant respectively. Apart from the heat requirement for solvent regeneration, an optimised integration into the power plant process seems to be the key for further reduction ofthe energy losses for carbon capture. Several interfaces have to be considered and optimised which necessitate modifications to the plant components [1]. The following figure 1 shows these main interfaces.

Optimized flue gas cleaning (dust, NOx, SOx)

Process cooling and heat recovery (CO2-Compressor, Amine-cycle, fluegas cooling)

Clean Gas

Scrubber

Process heat for Desorber and Amine regeneration by steam extraction

Electrical power for CO2-Capture process, integration of me asurement and control

Energy supply of

CO2 Com-ressor

0X1—

Absorber

Regenerator

Figure 1: Interfaces between PCC-components and the power station [1]

Amine based washing solutions are known to react strongly with SO2 and SO3. Heat stable salts are formed which leads to the degradation of the solvent. From today's knowledge, SO2-emissions ofmuch less than 20 mg/Nm3 are needed to optimise the overall process. The target should be below than 5 mg/Nm3. To reduce sulphur dioxide emissions to this level Hitachi has developed an advanced FGD technology for CCS requirements. Furthermore the additional cooling demand for process integration is to be considered. Besides, the flue gas is cooled in described advanced FGD down to approx. 40 °C. Carbon dioxide compression and CO2 absorption intercoolings are essential for an efficient process. Moreover, nearly water-saturated CO2 from regenerator must be cooled and dewa-tered preliminary it can be compressed. A large part of this rejected heat must be led away in an enlarged cooling circuit. Another requirement is the energy supply for the desorbtion unit. This is realised by a steam extraction from the turbine. Besides, the steam must have a pressure that enables to use the condensation warmth. Otherwise the required mass flow is too large. The power demand for additional pumps, compressors and the compression is also huge.

B. StOver et al. /Energy Procedia 4(2011) 163! 7-1643

Steam turbine modifications

The large amount of heat for the regeneration of the post combustion carbon capturing process makes modifications to the standard steam turbine design necessary. For 90 % CO2 separation approximately 40 - 60 % of the steam flow in the cross over pipe between IP- and LP -turbine has to be extracted. Depending on the possibilities for steam extraction at the steam turbine the following design considerations are required: The blades of the HP- and IP-turbine must be designed for the increased pressure/enthalpy drop across all stages. The casted outer casing of the IP- turbine must be designed according to the increased mass flow of the steam extraction for the CCS process. The LP turbine must be able to accommodate large flow variations due to the process steam extractions. The length of the last stage blades (LSB) of the LP- turbines must be optimized according to the new exhaust steam flow requirement, (which is less with CCS). Operation with CCS will require shorter LSBs to avoid excessive exhaust losses due to ventilation and low load operation. Optionally a crossover valve between the IP- and LP-turbines can be used to reach the required steam pressure for the supply of the heat quantity for the CCS process. The crossover valve maintains a constant steam pressure on IP- turbine outlet and the extraction stub, which would minimize the modifications required for HP and IP turbine design. However, the crossover valve itself creates its own design challenges that need to be considered and moreover, throttling losses of the crossover valve will decrease the cycle efficiency.

Figure 4: Modifications of the steam turbine for an optimized power plant with PCC

The above mentioned design requirements can be considered in the planning phase for new power plants. In case o f existing plants, the required modifications at the steam turbine for the steam extraction might be executed with a turbine retrofit.

3 The reference plant

A supercritical pulverized coal-fired power station with a post combustion carbon capturing plant and CO2-compression is selected as the reference case for a waste heat integration study. As shown in Figure 2, it has a single reheat steam cycle with a main steam temperature of 596 °C and reheat temperature of 608 °C. The PCC-plant is designed for a treatment of 100 % of the flue gas and a 90 % CO2-separation. The used solvent is a Hitachi-solvent named H3 with an advanced specific regeneration heat of 2800 kJ/kgCO2. As a reference there is also used MEA with a specific regeneration heat of 3600 kJ/kgCO2. For the compression of the CO2 to 120 bar there is a five staged compressor line used. The overall-plant with an optimized steam turbine for this case will have a net efficiency between 35.4 - 37.3 % depending on the used solvent. This come up to an efficiency loss of 9.6 - 11.5 % compared to a state-of-the-art power plant without CCS.

49,9 bar

Figure 2: 800 MW state-of-the-art hard coal fired power plant with added PCC-plant and compression

4 Process with optimized waste heat integration

To further increase the efficiency, an optimisation of the overall process is needed. This concerns the integration ofthe waste heat from regenerator and compression into the water steam cycle and air-preheating as well as the integration of the waste heat into a district-heating-grid. The following figure 3 shows different options for an optimised water steam cycle of a power plant with post combustion capture plant and compression.

B. StOver et al. /Energy Procedia 4(2011) 16317-1643

Figure 3: Different options of waste heat integration into the overall-process, the numbers refer to figures 4 and 5 and indicate if the heat exchange options (letters) are used in the respective options

The modifications of the water steam cycle are as follows:

• The condensate (C) from the reboiler heating is reintegrated into the main condensate line downstream the LP-preheater.

• The CCS-process has a considerable amount of waste heat from regenerator column (A) and from the CO2-compressor (B). A part of this waste heat can be used to warm up the condensate upstream the feedwater tank. As a result, the LP- heaters no. 1-3 can be bypassed and unloaded respectively.

• Another possibility is a modification of the air preheating and flue gas system which allows to use the waste heat of the CCS-process to warm up the feedwater as well as main condensate. A part of the waste heat from the CO2 cooling at the desorber outlet (A) and the waste heat of the CO2-compressor (B) are used for air preheating before entering the main air heater. Since these waste heats are used for air preheating, a part of the flue gas heat can be shifted to the feedwater line by using a heat exchanger in parallel to the main air heater. The remaining heat amount of the flue gas downstream the main air heater can be used for main condensate preheating (E).

• The preferred location for the steam extraction is the crossover pipe (S) between the IP and LP- steam.

• Further alternative for the integration of waste heat is a district-heating-grid. The heat from the regenerator (A) and the CO2-compressor (B) can integrate up to 117 MW. Another potential for district heating is the reflux of the heat-exchanger for air-preheating with the compressor waste heat (F).

As a result of the modifications for the water steam cycle and the steam turbine, the loss of net efficiency is reduced to only 8.0 %-points for the H3-solvent. This optimized heat integration in Option 3 leads to an efficiency recovery of 1.6 %-points.

Figure 4: Efficiency-loss for the options of waste heat integration (H3 as solvent)

50 45 40 35 30 25 20 15 10 5 0

compression

no CCS

with CCS

with CCS Option 1

with CCS Option 2

with CCS Option 3

with CCS Option 4

with CCS Option 5

Figure 5: Efficiency-loss for the options of waste heat integration (MEA as solvent)

B. Stöver et ar. /Energy Procedia 4(2011) 163,7-7677

The technology of the post combustion capture for coal fuel fired power plants on one hand will lead to a drop in efficiency on the other hand this technology is able to drop the CO2 emissions to the lowest of the fossil fuel power plants. Further development of the steam generation technology also will recover some more of the efficiency loss. It seems to be in a conceivable range to develop fossil fueled power plant technology with a future efficiency of 45 % in 2020 with CO2 emissions of lower than 100 gCO2/kWhel. This will lead us to a new age of power generation [2]

[1] W. Schreier, G. Boon: Consequences of CO2 Capture for Concept and Performance of Power Plants, VGB Congress, Power Plants 2009, Lyon, France, September 23-25, 2009.

[2] C. Bergins et. al.: Optimized Post-Combustion Carbon Capture for Coal-fired Power Plants; The 35th International Technical Conference on Coal Utilization & Fuel Systems, June 6-10 2010, Sheraton Sand Key, Clearwater, Florida, USA