Natural Resources - Clean Future
Clean Future
Zero emission coal-fired power plants? The dream may soon become a realitythanks to integrated coal gasification technology.
A future with coal? State-of-the-art technologies for low-emission combustion could herald a renaissance for coal-fired power plants. Such technologies transform hard coal into a synthesis gas that generates electricity in a gas turbine
Natural gas or coal? Until now, purely in terms of investment costs and efficiency, its been easy for energy providers to make that decision. Combined-cycle plantsi.e. a combination of gas and steam turbineshave an efficiency rating of around 58 %, while coal-fired plants only attain an efficiency of slightly more than 48 %. Moreover, at 400 /kW of power, the investment costs for combined-cycle plants are much lower than for coal-fired plants, which cost more than 700 /kW.
Its therefore no surprise that only combined-cycle power plants have been built in the U.S. in the last four years. An EU study entitled "World Energy, Technology and Climate Policy Outlook," which predicts that the amount of electricity produced from natural gas worldwide will triple between now and 2020, also confirms the trend toward combined-cycle plants. The situation is nevertheless being reconsidered, especially in the U.S. "Lately, theres been increased interest in clean coal-fired plants," says Frank Bevc, Director of New Technologies at Siemens Power Generation (PG) in the U.S. The reason for this is that natural-gas prices have increased sharply since September 11, 2001from $2.60 to $5.60 per gigajoule. Unlike Germany, where power plants with a combined output of approximately 40 GW will need to be updated by 2020, the U.S. still has excess capacity, so it wont have to begin investing heavily in new plants until 2006. If investors continue to opt for natural gas, there will be no way to avoid imports. The U.S. currently produces nearly all of the natural gas it requires, but forecasts say that imports will account for at least 25 % of total consumption by 2030. The gas will have to be transported from other continents in liquid form. This will be expensiveand theres also the danger of terrorist attack.
Coal, on the other hand, is available in sufficient amounts around the world. Furthermore, the EU predicts that coal prices will hold steady until 2030. The U.S. Department of Energy therefore launched the Clean Coal Power Initiative in 2002, which will provide half of the investment costs for new coal-fired power plants over a ten-year period, regardless of which technology is employed. The only condition is that the facilities fulfill strict environmental and efficiency criteria. "Siemens is prepared for a renewed increase in demand for coal-fired plants," says Dr. Georg Rosenbauer, an energy expert in strategic planning at Siemens Power Generation (PG) in Erlangen. Particularly encouraging is the fact that the gasification technology Siemens has already tested enables hard coal to be used in gas turbines or combined-cycle processes. And, when it comes to reducing pollutant emissions, the technology can even compete with the clean and highly efficient combined-cycle power plants that use natural gas.
From Coal to Gas. A gasification and combined-cycle unit is referred to by specialists as an Integrated Gasification Combined Cycle (IGCC) power plant. At such a facility, a liquid or solid fuel (e.g. hard coal) is converted into a gas and then burned in a gas turbine. The advantage of this process is that pollutants can be separated before combustion beginsor never form in the first place. IGCC plants that use hard coal still arent as economical as conventional steam power plants. Nevertheless, "this young technology harbors substantial potential for improvement in terms of both technology and economy," says Jürgen Karg, who is responsible for IGCC Marketing at Siemens PG.
IGCC facilities that are currently commercially operated as power plants or used in the petrochemical industry in Europe already offer one big benefit. Theyll eat anythingwhether its a mixture of coal and biomass, as is the case in Buggenum, Holland; coal and petroleum coke in Puertollano, Spain; or liquid refinery residue (e.g. asphalt) in Priolo, Italy. They can digest nearly any fuel and convert it into a hydrogen-rich synthesis gas that can be used to operate a specially designed gas turbine or a fuel cell. Whats more, refineries can use the synthesis gas to create hydrogen for their own needs.
According to Siemens market analyses, IGCC units could play an important role in the near future when it comes to using refinery residue, especially since gasification of liquid residue is much less expensive than coal gasification. Siemens PG has estimated that by 2010, up to 120 GW of additional output could be produced worldwide by upgrading existing refineries and building new ones. The key factors driving this development are more stringent environmental regulations and demand for ever better product quality. The residue created as a byproduct of crude-oil processing must either be disposed of properly or else further processed. One solution is offered by gasification and IGCC technology. Siemens PG and its partners have developed a concept for a standardized IGCC facility in the 500-MW performance class, for which there is a particular need. Siemens is providing the power-plant components, while the partner companies are responsible for the gasification process and the subsequent purification of the gas.
This IGCC power plant in Spain processes coal and petroleum coke
Whether, and when, IGCC power plants that use coal will pay off in terms of directly generating electricity depends on legislative decisions regarding pollutant emissions. Restrictive legislation would play into the hands of IGCC proponents, while conventional coal-fired plants that burn powdered coal and purify exhaust gases would retain their appeal in the event of less restrictive legislation. IGCC units can separate pollutants such as sulfur or vanadium from the synthesis gas and then concentrate and reutilize them. And CO2 can be more easily separated from the compressed synthesis gas than from flue gas (see Deep-Sixing Carbon Dioxide). However, Rosenbauer believes that IGCC plants with CO2 separation wont be competitive until 2020.
Breaking Efficiency Records. Until then, engineers will attempt to gradually increase the efficiency of conventional power plants. Replacing old facilities with new technologies would offer the potential for reducing CO2 emissions by 40 million tons per year in Germany alone13 % of the total CO2 emissions from power plants. Upgrading activities are already well advanced for brown-coal facilities, which account for 26 % of total electrical output in Germanythe highest among all fossil-fuel energy sources. New, highly efficient power plants are already operating in central and eastern Germany, and the RWE Group opened the worlds largest brown-coal power plant near Niederaußern in 2002. The facility is equipped with a Siemens steam turbine that has an efficiency rating of more than 43 % at a net output of 965 MW. Thus, the plant not only saves on fuel but also emits three million tons less CO2 per year compared with a 600-MW block built in 1974, whose efficiency rating is only 35 %. But Niederaußem is just the beginning of a series of brown-coal plants that are equipped with optimized technology (known as BoA facilities). Plans call for efficiency to be raised to over 50 % through two measures.
- The first involves drying out the brown coal, which can contain up to 60 % water. Water is currently removed with flue gas at 1,000 °Ca process requiring a lot of energy. A new fine-grain drying procedure should increase efficiency in BoA-Plus power plants by four percentage points to over 47 % beginning in 2015. This procedure efficiently employs energy from evaporated water for the drying process.
- The second measure is to raise steam temperature, which would yield an additional four percentage points in efficiency.
New Materials for Efficient Turbines. The laws of physics tell us that the greater the temperature difference between the steam flowing in and out of the process, the higher the efficiency. Steam at the Niederaußem facility enters the high-pressure turbine at 600 °C and a pressure of just under 300 bar. The problem here is that "conventional materials cannot withstand temperatures above 620 °C," according to Uwe Hoffstadt, who is responsible for turbine design at Siemens PG in Mülheim an der Ruhr.
In the Komet 650 project, which ended in 2002, several German companies and institutes attempted to find materials that could withstand temperatures 50 degrees higher than usual. To this end, a high-temperature test facility was built at the VEW power plant in Westphalia. "Nickel-based materials performed best over 16,500 hours of full-capacity operations," says Christian Stolzenberger from VGB Powertech, the European association for power-generating utilities, which coordinated the project. The European project AD700, which will become a VGB initiative in 2004, has an even more ambitious goal: to achieve operation at 700 °C.
RWE also studies the possibility to realize CO2-free brown-coal power plants. The problem here is that the associated efficiency losses, and consequently the costs of producing electricity, are very high. The strategy for the next few years is therefore clear for Dr. Johannes Ewers, who is responsible for the further development of power-plant technology at RWE Power AG: "Our top priority is to boost efficiency."
Tim Schröder
How an IGCC Facility Transforms Coal into Gas and Electricity
The conversion of a liquid or solid fuelsuch as hard coal or refinery residueinto a so-called synthesis gas is conducted in several stages:
- Air separation (1): Pure oxygen is needed for the gasification process. To liquify oxygen, air is compressed to between ten and 20 bars with the help of the gas turbines (7) compressor (2) or a separate compressor. The oxygen is separated through distillation at a temperature of approximately -200 °C.
- Gasification (3): Chemically speaking, this is a combustion process with pure oxygen. However, it involves less oxygen than would be necessary for complete combustion. The process produces a crude gas consisting mainly of carbon monoxide (CO) and hydrogen (H2). Steam is then used to transform CO into carbon dioxide (CO2) and additional hydrogen. There are three procedures for gasifying solid fuels such as coal or petroleum coke. IGCC technology primarily utilizes what is known as the entrained flow gasification process. Here, coal dust is pressurized and fed into a burner along with nitrogen as a carrier gas. It is then converted into a synthesis gas in the gasification unit using oxygen and steam.
- Crude gas cooling (4): The synthesis gas has to be cooled before further processing. This cooling process creates steam, which is used to generate electricity in the combined-cycle plants steam turbine.
- Cleaning (5): After the gas has been cooled, filters are used to capture ash particles, after which carbon dioxide can also be removed, if required. Other pollutants such as sulfur and heavy metals are also captured using chemical and mechanical processes. This achieves the fuel purity necessary for operation of the gas turbines.
- Combustion: The hydrogen-rich gas is mixed with nitrogen from the air separation process, or else with steam, before entering the gas turbines combustion chamber (6). This reduces the combustion temperature and largely suppresses the formation of nitrogen oxides. The flue gas created through combustion with air flows into the gas turbine blades (7). This gas mainly consists of nitrogen, CO2 and steam. Because of this combination with nitrogen or water the gass specific energy content is reduced to approximately 5,000 kJ/kg. Natural gas has ten times that, which is why the fuel-mass flow through the gas-turbine burner in an IGCC power plant has to be around ten times higher than in a natural-gas unit.
- Exhaust cooling (8): After the flue gas has settled in the gas turbine and heat has been recovered in the steam generator, the exhaust gas is released into the atmosphere. The steam flows resulting from crude-gas and exhaust-gas cooling are combined and fed into the steam turbine (9). After settling, the steam is sent back through the condenser and the feeder water tank into the water/steam cycle again. The gas and stream turbines are connected to a single generator (10) that produces electrical energy.