Heather Johnstone, Senior Editor
No complete, full-scale carbon capture and storage (CCS) installation for coal fired power plants has yet been built, but the implementation of this important carbon dioxide (CO2) abatement technology has received a boost in recent months.
In January, the European Commission proposed earmarking €1.25 billion ($1.63 billion) to fund the construction of up to 12 CCS demonstration plants at coal fired power plants across Europe. While in the United States, President Obama’s pledged to invest $150 billion as part of his economic stimulus package in developing and deploying clean coal technologies.
Conceptual design of a power plant with integrated post-combustion CO2. Source: Siemens Energy
In April, the UK government announced that all new coal fired power stations to be built in the UK must have CCS demonstration from day one. Alongside its ongoing competition to build a post-combustion demonstrator, a further three projects, including one demonstrating pre-combustion technology, will be funded by a new levy mechanism.
To date there are a number of early stage CCS projects in progress around the world. One of the most important is the first complete demonstration plant, which began operating in September 2008.
Built beside the 1600 MW Schwarze Pumpe power plant in north Germany, the experiment will capture up to 100 000 tonnes of carbon dioxide (CO2) a year, compress it and bury it 3000 m below the surface of the depleted Altmark gas field, located about 200 km from the site.
In the United States, AEP is planning a test project at its Mountaineer plant in West Virginia, using a proprietary chilled ammonia process. AEP, which is one of America’s largest power generators, hopes to capture some of the plant’s CO2 emissions and store them over 3200 m underground in deep saline aquifers.
TransCanada Pipelines Limited is to participate in the development of Project Pioneer, Canada’s first fully integrated CCS plant. When complete it will be one of the largest CCS facilities in the world, and the first to have an integrated underground storage system.
Thus, an array of initiatives is underway, with electric utilities, governments and industry research institutions working closely with power sector original equipment manufacturers (OEM) to capture carbon produced by coal plants.
One such OEM is Siemens AG of Germany, which has been actively pursuing two main approaches to CO2 capture from fossil fuelled power stations. These are integrated gasification combined-cycle (IGCC), also commonly referred to as pre-combustion capture, and post-combustion capture.
CCS would give IGCC a boost
IGCC is a power generation technology that uses proven gasification technology to transform coal into a synthetic gas (syngas), which is essentially a mixture of carbon monoxide (CO) and hydrogen (H2). The syngas is then used to fuel a gas turbine to produces electrical power.
The overall process efficiency is further enhanced by recovering the heat from both the gasification process and the gas turbine exhaust to produce steam. This steam is then used to drive steam turbines to produce additional electricity.
The gasification technology provides the advantage that CO2 can be removed from the syngas prior to combustion. As described above the gasification reaction produces CO and H2, and the CO can be oxidized to CO2 in the presence of steam via a shift reaction resulting simultaneously in a H2 rich gas. The gas mixture is under pressure so the CO2 can be separated using a physical absorption process standard purification processes using Selexol or Rectisol, for example, have been tried and tested on an industrial scale and are readily available.
Siemens has developed its own gasification technology, which has more than 20 years of operating experience. Its gasifier can accommodate a variety of fuels from hard coal to lignite and from biomass to heavy petroleum residues. At its 5 MW (thermal) test facility, located at Erlangen, Germany, both new fuels and processes can be tested and optimized. The company has also more recently up-scaled its coal gasification technology to 500 MW (thermal), and the first of these large gasifiers has already been dispatched to China and is expected to go online next year.
Its coal gasification technology has also been selected for Canada’s first IGCC power plant, featuring CCS, which will be built by EPCOR Power Generation in the province of Alberta. The demonstration plant, with an installed capacity of approximately 270 MW, is scheduled to come online in 2015.
Siemens also has expertize in the so-called power island component of an IGCC plant, i.e. the gas turbine, steam turbine and generator, and it has achieved in excess of 450 000 operating hours of experience with gas turbines running on syngas fuel.
IMPROVED AVAILABILITY OF IGCC PLANTS
Although the early IGCC plants exhibited serious availability problems, the newer IGCC plants are achieving availabilities in excess of 90 per cent. One such plant is Nuon’s 250 MW Buggenum plant in the Netherlands. The Dutch utility is also constructing a second IGCC plant in the Netherlands the 1300 MW Nuon Magnum IGCC power plant, which is scheduled for commissioning in 2011.
The high capital cost of IGCC remains one of the biggest obstacles to its greater integration in the power generation market. However, when you compare the MW/h cost of an IGCC plant and a pulverized coal plant coming online in 2010, it is claimed that IGCC becomes more attractive when you include the costs of carbon capture and storage.
Siemens Energy’s CO2 scrubbing laboratory in Höchst, Frankfurt
With carbon capture, the cost of electricity from an IGCC plant is estimated to increase approximately 30 per cent, while for a pulverized coal plant it would increase by 68 per cent. This potential for less expensive carbon capture therefore seems to make IGCC a viable choice for keeping low cost coal an available fuel source in a carbon constrained world.
According to Dr. Daniel Hofmann, Siemens Energy’s vice president, Carbon Capture and Sequestration, Siemens’ perspective is that the IGCC plant with CCS is ready for the market all the components are there and the CCS technology has been tested and proven on a large scale. Statoil’s Sleipner West field in the North Sea, for example, has been extracting CO2 out of the natural gas and storing it 1000 m underground for 12 years. So all the pieces of the puzzle are there and have been proven to work, so the next step now is to demonstrate that they can function as an overall plant with CCS.
identifying the right solvent
In conventional steam power plants the fuel, usually coal, is combusted with air. The flue gases are cleaned via deNOx, desulphurization and particulate removal systems. The clean gas downstream of these systems in general has a CO2 content of about 14 per cent.
The CO2 can be removed from this off-gas by means of a chemical scrubbing process, which is known as post-combustion capture. Absorption methods using a suitable agent are state-of-the-art today. There are also ideas for adsorption-based and membrane-based separation methods, but these have not yet been shown to be technically feasible.
The CO2 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 extracts the CO2 from the off-gas and the CO2-depleted off-gas is discharged to the atmosphere.
The CO2-laden absorbent is then heated by passing through a heat exchanger before being taken to the desorption column or stripper. 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 before being transported for storage. The regenerated scrubbing agent is returned to the absorption column, and on the way, it exchanges heat with the CO2-laden absorbent.
There are many demands placed on the scrubbing agent, such as it must selectively absorb CO2 from the off-gas, while at the same time require the lowest possible energy input to release the absorbed CO2. It should also exhibit extremely low volatility, so as not to be entrained in the off-gas and carried out of the absorption column, or in the CO2 stream from the stripper. Finally, it should be completely inert to all flue gases, trace gases and all materials used, should not be harmful to the environment and should be available in large quantities and at a low cost.
Siemens is working on a proprietary scrubbing process that uses a solvent based on an amino acid salt formulation, and the company is confident it satisfies all the above criteria. It has been developing this solvent technology over the last three to four years at its Frankfurt Hoechst laboratory in Germany. The laboratory is fully-automated, which means it can run 24/7, and can test a variety of solvents for post-combustion capture in closed loop.
The company demonstrated that with the standard process design for the absorption and desorption of CO2, the efficiency is over ten per cent lower than a reference hard coal fired power plant in a modern steam power plant with an efficiency of 46-47 per cent that would translate into a reduction to between 36-37 per cent in efficiency. The company subsequently evaluated a number of process improvements, and has been able to demonstrate that through advanced process configuration and integration of CO2 compression the efficiency loss can be reduced to 9.2 per cent.
Siemens is now moving forward with its selected solvent into pilot plant testing, which is a pilot CO2 capture plant that it is currently building with E.ON at the utility’s hard coal fired Staudinger plant in Germany. The pilot plant will be operated with part of the flue gas from the Staudinger Unit 5 to demonstrate its lab-proven process can work under real operating conditions. The pilot plant is scheduled to commence operation in August/September of this year.
Siemens intends to run the pilot plant at Staudinger until the end of 2010, but the company is already looking to scale-up its capture technology into a demonstration plant, and is currently exploring opportunities.
Achieving a capture-ready design
The legislative situation as regards the capture and storage of CO2 from coal fired power plants remains unclear, although there are signs that this is changing. There is an EU Directive on CCS now in place, which all EU member states will have to put into law, which says all fossil power plants above 300 MW will have to be built capture ready. However, it does not define what ‘capture ready’ means.
Siemens believes because various important drivers for economy models and the mid-term and long-term operation of power plants are still wide open, it is advisable, with a view to minimizing the risk, to incorporate preparations for later CO2 capture into the design of new power plants from the start. This clearly involves finding a middle-of-the-road, especially in these capital-constrained times, between additional investment cost and the effort required for conversion for future CO2 capture. Furthermore, the operation of the plant must be as efficient as possible, now and in the future.
To address this issue, Siemens has developed a conceptual design for a steam power plant to be built today and converted for CO2 capture at some future stage. This design makes allowance not only for the space requirement for the absorption/desorption plant, but also for the space required within the ‘normal’ power plant to provide facilities for subsequent steam extraction.
Other areas that need to be taken into account are the higher demand for cooling and auxiliary electric power, layout planning, extended flue gas clean-up and switchgear. The German firm also believes it is pertinent that future CO2 capture should also be applied for in the licensing procedure, so as to obtain clarity, for example, on potential water management issues and injection into storage reservoirs, in good time.
It is relatively early days for the capture and storage of CO2 from power plants but momentum is building, especially politically, and it is clear that this technology will play a pivotal role in helping us achieve this goal.