The need for reliable utility-scale coal fired power plants that include cost-effective carbon mitigation systems is paramount. Alstom is developing several carbon capture technologies, including post-combustion capture, which will cover the specificities and different needs of the coal power plant market.
The technology described in this article is the promising oxycombustion process. Alstom is confident that its large oxycombustion integrated carbon capture solutions will be commercially available around the middle of the next decade. This is because the processes used in oxycombustion are based on existing systems developed for other industrial applications; and the way forward consists in adapting and scaling up these systems to power plant processes.
Commercially available oxycombustion is developed through rational, step-by-step validation. Alstom has already validated the first bench-scale step and has reached the second step in two small oxycombustion demonstration projects of 30 MW: one in Germany for Vattenfall, currently in the testing phase; and another one in France for TOTAL, where boiler retrofit work is nearing the end of construction.
The results of these first steps reinforce Alstom’s faith in oxycombustion technology as one of the most promising answers to carbon capture challenges. This article discusses oxycombustion based on pulverized fuel (PF) boilers.
Key features of oxycombustion
A summary list of the key features of oxycombustion carbon dioxide (CO2) capture technologies is presented below, and that are as follows:
- Rapid scale-up to large size units, up to the 1000 MWe range, can be envisioned in the time-frame needed to meet objectives for CO2 emission reduction
- Reliability: components and systems used in the oxy-chain already exist in other fields of application and have only to be adapted and integrated in power plants and scaled-up
- The existing various types and configuration of boilers and firing systems are adaptable to oxycombustion, including the ability to burn a wide range of fuels
- The technology can address the existing installed base, i.e. retrofit/conversion of the boiler island to oxycombustion
- High efficiency and competitiveness of supercritical/ultra-supercritical cycles and large unit size are key benefits
- A wide panel of utilities, institutes, universities, suppliers and manufacturers worldwide are participating in the development of oxycombustion technology à‚— this contributes to reaching consensus and minimizing uncertainties.
Oxycombustion principle for power plants
The oxycombustion boiler is derived from the air fired boiler. In an oxyfuel boiler, a mixture of pure oxygen and re-circulated CO2 replaces the air (79 per cent nitrogen (N2) / 21 per cent O2). The resulting flue gas comprises primarily CO2 and H2O vapour along with some limited O2, N2 and other trace gases (e.g. sulphur oxide, SOx) and can then be processed easily.
Remaining traditional pollutants (e.g. particulates, SOx,) are extracted through conventional air quality control system (AQCS) components, then most of the H2O of the flue gas is condensed to obtain a concentrated CO2, which is compressed and purified to minimize all the remaining trace gases.
In the end the purified CO2 can be pumped, transported and finally injected in geological storage.This principle can be applicable to all types of boilers for thermal power plants, with the addition of systems utilized in other industrial processes.
CO2 is unloaded from the trucks and into storage tanks at the Altmark site, before being injected into the ground with the aid of CO2 booster pumps Source: Vattenfall
Compared to an air fired boiler island chain, an oxy fired power plant requires as part of the boiler island chain: an air separation unit (ASU) to produce oxygen; a coal boiler fired with oxygen (oxyfuel-boiler); a flue gas condenser (FGC) to separate the water from the CO2; and a CO2 compression and purification process (GPU) down stream of the flue gas condenser, which gives the CO2 necessary characteristics for its transport and storage or use for other applications.
Oxyfuel firing requires tempering of the combustion taking place in the boiler. Recirculating a part of the cooled flue gas back to the furnace controls the furnace temperature.This maintains normal temperatures without added heat transfer surfaces in the furnace, an advantage for retrofit applications.
The development and the introduction of a new technology requires a progressive scale-up with several validation steps. During the first phase à‚— small pilot scale (0.5 MW to 5 MW) à‚— following the research studies and bench-scale tests, the general principles of the oxygen combustion concept are validated for the boiler component.
Up to now several tests at scales up to 0.5 MW have been performed for PF boilers with different coals. Regarding oxyfuel circulating fluidized bed (CFB) technolgy, three pilot scale test campaigns have been run in Alstom’s 3 MW (9.9 MMBtu/h) CFB facility, which is located in Windsor, Connecticut, USA. In general, the laboratory and pilot-scale tests have not indicated any technical barriers for the development of the oxyfuel technology.
Step two is medium pilot plants (30-50 MW). This phase, which includes Vattenfall’s Schwarze Pumpe and the Lacq project of TOTAL allows the validation of the full process at small-scale, that is to say all the integrated oxy-chain components highlighted in the figure on the previous page à‚— ASU, boiler, FGC and CO2 processing), and to acquire data for further performances definition.
Relative to the boiler component itself, the firing system (burner and O2 mixing conditions), the furnace thermodynamic and the material specificities are tested.
Oxycoal carbon capture units feature an air separation unit that removes nitrogen so that the coal is burned in pure oxygen. The CO2 emitted is then compressed for piping away. Source: Vattenfall
Step three is pre-commercial demonstration. This phase is based on intermediate size utility power plants (150 MWe to 350 MWe gross) and allows the validation of the CO2 specific systems of the oxy-chain (for example, the ASU, the boiler with its burners and the CO2 compression and purification), as well as their full integration, at the significant size needed for commercial release.
It also includes validation of the anticipated performances and optimization of the interfaces between systems, as well as overall operability. This step represents the first tests of the technology close to nomal commercial conditions, which proves the major technical and economical parameters before commercial offering.
Schwarze Pumpe demonstration project
Alstom was awarded the contract by Vattenfall to build the steam generator and electrostatic precipitator (ESP) for the first 30 MW oxycombustion plant in the world, designed to operate on air, as well as on oxygen mode. The pilot plant is located at the Schwarze Pumpe power station in Brandenburg, Germany, and is at present in the final phase of commissioning. i.e. performance testing.
The 30 MW pilot plant is intended to validate and support the technical concept and serve as the main step towards the construction of a 200 MWe to 300 MWe demonstration power plant generating “near-zero CO2” electricity under commercial conditions by 2015 or earlier.
At the oxyfuel pilot plant, the complete process chain is implemented as is necessary for usage at large-scale power plants. This includes, besides the steam generator, everything from the ASU for the supply of oxygen to the steam generator, the ESP, the flue gas desulphurization plant (FGD), the FGC and the CO2 treatment plant. This approach allows testing each component of the complete capture technology chain and the interaction of the different components in terms of startup, shutdown, and switch from air to oxy-fuel combustion, as well as load change behaviour. The only exception here is the lack of a steam turbine, where no significant changes are anticipated compared to today’s technology.
Nevertheless, for a full scale application the integration of new heating sources and sinks will be of major importance.
As Vattenfall has a wide operational experience on steam turbines and generators, it is not necessary to implement these components at the oxyfuel pilot plant. The steam generated by the pilot plant will be fed to the existing steam-cycle of the Schwarze Pumpe power station.
The steam generator of the oxyfuel pilot plant has a thermal output of 30 MW and generates about 40 tonnes/h (t/h) of steam at 330 à‚°C and 25 bar. To do this, 5.2 t/h of pulverized lignite and 10 t/h of oxygen are required. At full load operation, approximately 9 t/h of liquid carbon dioxide are produced. The steam parameters were selected in order to supply the steam to the 18 bar auxiliary steam network of the Schwarze Pumpe industrial site (paper mill and briquette factory).
The multi-pass drum steam generator is equipped with indirect firing where the pulverized coal burner, providing an output of 30 MW is located on the ceiling of the combustion chamber. The pulverized coal burner is ignited by a gas firing system.
The steam generator is designed by Alstom and planned in a way that it can be operated on air, as well as on oxygen plus recirculated flue gas.
For both operating methods, comparative studies at a load range between 50 per cent and 100 per cent are possible, which allows a scaling of the results applicable for the oxyfuel operation (operation with pure oxygen) in a demonstration plant.
Pulverized coal is fed from the fuel silo to the burner via pneumatic transport. The carrier gas can be either air or, when running in oxyfuel mode, cold re-circulated flue gas.
The ash from the boiler and the ESP is transported through the respective openings in the hopper to gas-proof discharge screw conveyors and from there via conveyor belts and the bucket elevator to the ash silo. The supply of fuel and disposal of ashes is by trucks.
Approximately 70 per cent of the hot flue gas volume is re-circulated downstream of the electrostatic precipitator. A major part of the necessary flue gas and oxygen flow is mixed upstream of the burner.
Additionally, there is a possibility to increase the oxygen concentration in single burner oxidant inlets by adding pure oxygen, or to feed pure oxygen directly into the combustion chamber through the burner.
The heat of the flue gas is recovered by conveying it through a gas/gas heat exchanger for increasing the oxidant temperature and through four economizer-packages.
For meeting emission limits like nitrogen oxides (NOx), sulphur dioxide (SO2) and dust, some primary and secondary procedures have been planned. NOX reduction is achieved in the combustion chamber by staged oxidant combustion at the burner and in the combustion chamber.
An additional DeNOx plant can be retrofitted in order to consider later the behaviour of the catalyst in an oxyfuel atmosphere. A reduction of SO2 emissions can be achieved by spraying lime hydrate through nozzles into the furnace and by the flue gas desulphurization plant.
The pressure parts of the steam generator basically consist of the drum, the evaporator which functions as combustion chamber plus two superheater-surfaces and an economizer in the second pass. At the change of direction to the third pass, the flue gas branch to the optional DeNOx plant is provided.
Operation of the plant
For commissioning the pilot plant, all regulations and standards valid for Vattenfall’s large-scale plants were applied. There was a commissioning period for the air operation of the plant and one for the oxyfuel operation. The oxycombustion pilot plant is operated directly from the central control room in the switchgear building.
In order to ease the operation of a research plant, the control and communication system is designed in a way that modifications and adjustments can easily be implemented.
Generally, the control and communication system is operated in either ‘standard operation’ mode or in ‘expert operation’ mode. When running in ‘expert operation’ mode, several settings that will not be accessible otherwise can be altered here, such as the oxygen concentration at the burner.
For the time of test operation, a comprehensive test programme has been scheduled. For example, the steam generator will be fired both with lignite of different quality and different moisture content, as well as with hard coal. The oxygen surplus at the end of the boiler will be differently adjusted and the flue gas recirculation will be varied in amount, temperature and oxygen content. Other components of the pilot plant will be comprehensively tested as well.
A major issue during test operation will be the interaction between the air separation unit and the boiler. In the units for dry and wet flue gas cleaning, the effects of CO2 rich flue gas on the separation efficiency will be analyzed. In the whole plant, electrochemical corrosion sensors and material probes are installed to give detailed information on the durability of each single component.
For future large-scale plants, exact material requisition for different parts of the plant can be defined in this way. Another major objective of the investigation is the achievable CO2 quality at the end of the process chain. All tests are aimed at finding ideal operation modes for combustion, flue gas cleaning and CO2 treatment. The overall objective remains the definition of benchmark data for oxyfuel operation, and the drawing of conclusions regarding full and part-load behavior of large-scale plants.
General operating experience will also be studied such as load changes and dynamic interaction between the different systems, in particular the re-circulation system operation, the ASU system and the CO2 compression and purification systems. These results will serve as a basis for defining further equipment testing to provide the input for designing the commercial-scale plant.
A major milestone was achieved on 9 September 2008, when the pilot plant commenced operation. The plant is planned to run for at least ten years. Through a cooperative agreement signed between Gaz de France and Vattenfall, the CO2 captured will be used for enhanced gas recovery and storage at Europe’s second largest onshore gas field, Altmark. CO2 will be forced 1000 m below the soil into porous rock.
Next step : Large demonstration plants
Large demonstration boiler islands and power plants are the next steps in the development of oxycombustion after the above medium size pilots.
With these demonstration projects, typically ranging in size from 150 MWe to 350 MWe gross, main equipment and systems are designed to allow further scale-up directly to large commercial units from 450 MW up to 1000 MW à‚— the remaining improvement being the global integration and cycle efficiency improvement that is similar to air-firing.
With the large demonstration units, some parameters are still to be validated, but the power plant will be closed to normal commercial operating conditions and the full process will be validated before the full commercial release. Commercialization of the first large sized oxyfuel plants is expected by the middle of the next decade.
- Tests with hard coal and lignite
- Variation of moisture content in lignite (10.5 à‚— 20 per cent)
- Variation of excess oxygen content (1 à‚— 5 per cent)
- Variation of flue gas recirculation (flow, temperature, O2 content)
- Variation of oxygen content at different burner registers
- Combustion performance
- Ash qualities
- Flue gas composition
- Heat transfer
- Combustion characteristics
- Flame characteristics
- Corrosion potential
- Identification of optimal configurations