Pictures of the Future      Spring 2008

Coal’s Cleaner Outlook

Coal will continue to be a cornerstone of the world’s energy supply for years to come. New technologies are being developed to rid power plant flue gases of carbon dioxide, thus vastly diminishing the environmental impact of our most abundant fossil fuel.

Coal is experiencing a boom. The reasons for this are clear: a growing population and exploding demand for energy. In addition, many countries have their own substantial reserves of coal, making them independent of other sources of energy.

Apart from this, the coal market is characterized by a very stable price structure. While prices for crude oil and natural gas have doubled in the past three years, the price for hard coal has increased by only around 20 %. The drawback to this development is plainly evident: per kilowatt hour generated, the CO₂ emissions from coal-fired power plants are almost twice as high as those produced by natural gas-fired combined cycle power plants.

Nevertheless, at this point in time, the global economy cannot do without coal. Around 40 % of the world’s power is generated in coal-fired power plants—and in China the figure is over 70 %. In 2006 in China alone, 174 coal-fired power plants in the 500 MW-class were connected to the grid. If conditions around the world don’t change, the International Energy Agency (IEA) estimates that global consumption of coal will increase by 73 % between 2005 and 2030.

That means that it is now more essential than ever for utilities, as well as the companies that build power plants, to design and operate coal-fired plants in the most environmentally friendly way possible. "In order to cut CO₂ emissions, it is necessary to increase the efficiency of existing and new power plants on the one hand, and to separate carbon dioxide from power plant emissions and reliably sequester it on the other,” explains Dr. Christiane Schmid, of Business Development at Siemens Fuel Gasification Technology GmbH in Freiberg, Germany, a part of Siemens’ Fossil Power Generation Division.

For several years now, ambitious efforts have been under way worldwide to realize what is called Carbon Capture and Storage (CCS) technology (see CO₂ Sequestration). Depending on the type of power plant, there are three distinct methods of separating carbon dioxide from other gases generated by the combustion of coal:
? ?coal gasification in Integrated Gasification Combined Cycle (IGCC) plants with separation before the combustion stage (pre-combustion capture),
? ?separation of the CO₂ from the flue gas beyond a conventional steam power plant (post-combustion capture),
? ?and the oxyfuel-process intended for steam power plants.

With the oxyfuel concept, instead of using air—as in conventional steam power plants—coal or natural gas are burned with pure oxygen. This prevents large amounts of nitrogen, which makes up three-quarters of the volume of atmospheric air, from being needlessly added to the process and then forming nitrogen oxides during combustion. The flue gas produced is composed mostly of carbon dioxide and water vapor. By simply cooling and condensing the water, the CO₂ can then be separated. With a view to refining this process, power plant operator E.ON has set up a pilot oxyfuel plant in Ratcliff, England. And later this year, power plant operator Vattenfall intends to establish an oxyfuel pilot plant near Dresden, with Siemens supplying all of the plant’s control systems.

Tested Technology. In developing the technology, Siemens has focused on the first two approaches, that is, pre- and post-combustion CO₂ capture. "There are big differences in the current stage of technological development of the three methods. Only the IGCC technology has so far been adequately tested, and there are numerous application examples of CO₂ separation from syngas in the gas-processing industry,” explains Schmid. "We could start immediately with building a full-scale plant in which CO₂ could be separated. Siemens, after all, has been involved in the development of optimized IGCC concepts for years now.”

^{A CO₂ testing laboratory in Frankfurt. Here, Siemens experts investigate CO₂ separation from flue gas. The CO₂ is bound to an absorber (right) by a special scrubbing agent and thus removed}

As long ago as the 1990s, IGCC power plants were built in Puertollano, Spain, and Buggenum, in the Netherlands—where Siemens supplied the power plant section and assisted in the integration of the plants—as well as in Tampa, Florida, and Wabash, Indiana, in the United States. "These plants all demonstrate the feasibility of the IGCC concept. In those days, CO₂ separation wasn’t even on the agenda,” adds Schmid.

The reasons for the fact that there are not yet any large-scale low-CO₂ power plants in operation are many and varied. Guido Schuld, Managing Director of Siemens Fuel Gasification Technology GmbH, points out that: "There are neither legal nor political frameworks in place—and that is true particularly for the sequestration of CO₂. For another thing, the cost situation is not clear for our customers, because it is difficult to project just how expensive IGCC with CO₂-separation is actually going to be.” As a result of all these uncertainties, it will likely be several years before the first IGCC power plant with carbon dioxide separation is built. German companies are gearing up to play a leading role here.

Power plant operator RWE is planning to put a 360-MW plant into service in 2014, and it is budgeting around 1 bn. € for the plant’s construction. In the future, approximately 2.3 mill. t of CO₂ are to be separated there for sequestration in empty gas fields or aquifers. In the U.S., German power plant operator E.ON is one of 12 members of the worldwide FutureGen Initiative, which is planning to realize a 275-MW plant by 2012. Plans call for storing at least 1 mill. t of CO₂ from this power plant annually in deep-lying saline aquifers. E.ON UK is similarly considering construction of an IGCC power plant with CO₂ separation, possibly at a location close to the coast. Locations like these offer the possibility of storing CO₂ in crude oil deposits in the North Sea, thereby improving oil extraction.

"In IGCC power plants without CO₂ separation, our technology makes it possible to achieve an efficiency of over 40 %,” says Schuld. "But in IGCC plants with CO₂ separation, efficiency is generally lower. For economic reasons, a high level of plant availability is extremely important to our customers as well. This is why our new technologies are being subjected to an extended test phase before we launch them on the market.”

Schuld points to the gasifier and the gas turbine as key technologies for IGCC. They form part of the Siemens portfolio, with gasifier technology having been added in mid-2006. At that time, according to Schuld, Siemens acquired "an absolute jewel,” which is today Siemens Fuel Gasification Technology GmbH in Freiberg, near Dresden. Until 1990 it belonged to the German Fuel Institute.

In the early 1970s, in order to use brown coal, the East German government invested in the development of gasification technology. At that time, what was known as the "dry feed system” began to take shape as an ideal solution—and today this is proving to be a decisive competitive edge. The reason is that with this process, almost all types of coal can be used for gasification. Alternatively, coal can be injected into the gasifier in a watery emulsion, which means the ground fuel first has to be mixed with water. "This technology is suitable for expensive anthracite and hard coals, but not at all for brown coal or other coals with low calorific values,” explains Schuld. "But it is precisely these low-grade coal types that are available in large quantities in emerging countries such as China and India, and in the U.S. and Australia as well. Demand for Siemens’ gasifier technology is particularly strong among customers in these countries.”

Omnivorous Plants. Just how great this competitive edge is becomes clearer when we look at the service life of an IGCC power plant. "By the time a customer decides on a gasification plant, he has calculated its anticipated operating costs over a period of between 20 and 25 years,” Schuld points out. "However, a fixed-price delivery contract for coal can be secured for only a few years. Where the coal later comes from, what type it will be, and what it will cost is something nobody can determine in advance. But with our technology, the customer is always on the safe side over the plant’s complete life cycle, because the entire range of coal available around the world can be used and purchased depending on the prices in effect at the time.”

At Siemens’ Freiberg location, experts are currently investigating the behavior of very different coal types in the gasifier. They are examining slag formation in the reactor, and how the gasifier can best be protected from high combustion temperatures. "With our test plant, we have a facility that is unique, one that helps us to establish an economic framework with the customer before a plant is actually built,” says Schuld. But it’s not only fuel behavior that is tested in Freiberg. The gasifier itself is also being enhanced to make sure that the technology is ideally suited to meet future market demands.

"Apart from IGCC plants, this gasification technology is also used in the chemicals industry,” Schuld says. "Syngas can be used to manufacture chemical products, including ammonia, methanol, and dimethyl ether, as well as fuels such as diesel and synthetic natural gas. At the moment, it’s not only the electricity producers who are suffering from increases in the prices of raw material such as oil and gas. That’s why alternative fuels such as coal and even biomass are being looked at closely all over the world.”

In the chemicals field, Siemens’ Freiberg location, with its workforce of 70 employees can point to an industrial-scale technology achievement that has been in operation since 1984: the 200-MW (thermal output) plant at Schwarze Pumpe in the German state of Brandenburg. The plant was originally used for the gasification of brown coal. Most recently, it has been used for converting industrial waste into methanol. "It is our aim to establish it as a reference project in the 500-MW class, so that our customers have even more confidence in this technology and will then work with us in our efforts to realize the next-generation gasification plants,” says Christiane Schmid. Starting in 2009, plans call for five 500-MW gasifiers to enter service at the complex. The facilities will produce polypropylene from coal for the Shenhua Ningxia Coal Industry Group in the Chinese province of Ningxia. The plant will be the largest of its type anywhere, with each of its gasifiers converting 2,000 t of coal every day.

Fuel gasification takes place in a cylindrical reaction chamber at temperatures above the coal-ash fusion temperature. Finely-ground fuel is introduced with a mixture of oxygen, and steam if required, via a burner at the head of the reactor. Within a few seconds the mixture is converted into raw syngas consisting mainly of CO, H₂, CO₂, and H₂O. Part of the liquid clinker solidifies on the cooled wall of the reaction chamber and thus forms a protective coating. In the quenching chamber, underneath the reaction chamber, syngas and liquid clinker are cooled by water injection. Solidified clinker granules are removed via a material lock at the foot of the quenching chamber.

Low-temperature carbon dioxide scrubbing can trap approximately 90 % of the CO₂ content of the flue gases in an absorber using a CO₂ scrubbing agent—a special liquid—and thus remove it. "Then we feed the CO₂-laden agent into a desorber and rid it of the greenhouse gas by raising the temperature, before feeding the regenerated agent back into the absorber. There, the cycle starts again,” explains Schneider, who was previously involved in a range of flue gas scrubbing technologies with Henkel, then with Hoechst, and finally with Siemens spin-off Axiva.

In a laboratory located at the Frankfurt Höchst Industrial Park, Schneider and the members of his team have for the past three years been involved in an intensive study of CO₂ scrubbing agents that bind CO₂ particularly well and release it in response to an increase in temperature, while also remaining stable in the flue gas atmosphere. "In our laboratory we succeeded in mixing all manner of different gases, and we can vary the conditions independent of power plant operation. This means we can, for example, examine the effect of sulfur dioxide on CO₂ scrubbing in exactly the same way as the effect of oxygen,” says Schneider. "And thanks to our laboratory equipment, we are able to thoroughly analyze all the individual aspects of CO₂ scrubbing. The result is that our new chemical CO₂ scrubbing process leaves less scrubbing agent residue in the flue gas and uses less energy than conventional processes. What’s more, it is supported by optimized integration of the scrubbing process into the overall design of the power plant by our plant designers in Erlangen.”

Toward Commercialization. Working in collaboration with E.ON, Siemens intends to push ahead with the new process to make the design of fossil-fuel power plants more climate-friendly as soon as possible.

The company’s initial efforts will be concentrated on hard coal and brown coal power plants. For natural gas power plants, an adapted version is planned for later application. It may be possible to verify the process under realistic conditions as early as 2010, in a pilot facility in an E.ON coal-fired power plant. "The challenge we are now facing is to maintain a high level of efficiency while preventing negative environmental impacts that might arise from traces of harmful scrubbing agent emissions in scrubbed flue gases,” Schneider points out. "Our objective is to further develop the new CO₂ separation process to the point where it will be ready for full-scale commercial operation by 2020.”

So within the next decade, thanks to oxyfuel and pre- and post-combustion capture, technologies will be available that will enable us to burn coal without having a guilty environmental conscience.
                                                                                                           Ulrike Zechbauer

In the IGCC process, the conversion of coal into power can be combined with upstream CO₂ separation. First, coal is converted into a combustible raw gas in a gasifier, at temperatures of between 1,400 and 1,800 °C under pressure. The gas, which is mainly composed of carbon monoxide (CO) and hydrogen (H₂), is then coarsely cleaned and the carbon monoxide is converted, with the help of water vapor, into CO₂ and H₂ in what is called a "shift reactor.” In the next step, sulfur compounds and CO₂ are separated out by means of a chemical or physical scrubbing process. The CO₂ is then compressed and transported to a sequestration area. Separation rates as high as 95 % are projected. The remaining hydrogen is then mixed with nitrogen from the air and burned in the gas turbine, which is connected to an electricity generator. The fuel gas, which is rich in hydrogen, requires specially designed burners that must be able to maintain stable, low-nitrogen oxide combustion. Siemens has acquired experience totaling more than 400,000 operating hours in the combustion of hydrogen-rich fuel gases in various commercial plants. The hot flue gases—and above all atmospheric nitrogen and water vapor—are also used for steam generation. The steam, just as in a classic combined cycle power plant, drives a steam turbine and a second electricity generator.