IGCC technology offers fuel diversity, coproduction for competitive generation
Running on coal, heavy oil, orimulsion and waste products, IGCC plants boost their viability by coproducing chemical products
By Ann Chambers
Over the last couple of years, integrated gasification combined-cycle (IGCC) power plants have begun penetrating competitive markets. Japan and Taiwan are applying IGCC technology in their first round of independent power producer bidding. The versatility of this technology is adding to its cachet, as it finds its niche in a variety of market segments and applications.
“Market forces increasingly will dictate that power technology options be driven by economic and environmental criteria. In addition, the flexibility to use certain fuels without environmental concern becomes an even more important criteria for selecting the best power generation system,” said Harry Stoll, GE Power Systems engineer. “Since IGCC is a newly developing technology, it will most likely be accepted for use only where electricity costs are competitive and the risks have been mitigated by operating experience or financial guarantees.”
The initial penetration of IGCC technology occurred in subsidized projects. However, most IGCC projects are now chosen based on economic criteria. The heavy-oil segment of the market, along with some steel-mill applications, are the most advantageous for IGCC use from a purely economic standpoint. Increased use in the coal-powered portion of the market is projected in the next decade, with gas turbine technology advances expected to drive the spread of IGCC into the coal arena.
Economic factors dictate the type of system selected, and IGCC technology can now maintain a power producer`s economic efficiency and output ratings in varied applications. The Schwarze Pumpe plant in Germany is the first operating IGCC plant in the coproduction segment. “The plant allows the electricity supply to vary significantly while maintaining a full write-off on the gasification portion of the plant by producing methanol,” Stoll said. “The plant feeds lignite, waste oils and plastics to a large number of gasifiers, providing a wide variety of syngas compositions.” The Schwarze plant preceded the Shell Pernis-Netherlands plant, with coproduction of hydrogen and a 120 MW output. This technology is now extended to a 550 MW plant at Sarlux, Italy, producing hydrogen and steam for a refinery.
Cost of electricity
Stoll reported on a recent cost of electricity study for IGCC which used published data for a base, exploring various factors affecting IGCC competitive costs. These factors include plant costs such as size, technology level, coproduction options and region. Consideration of operations and maintenance and efficiency factors are also important due to the type of fuel used and system configuration. Finally the cost of electricity, where differential fuel costs and reliability are important, was calculated for various market segments. Many markets now appear to be viable for IGCC, with technology trends leading to an increased acceptance of this form of power generation.
IGCC plant costs are significantly affected by gasifier size and power train. Gas turbine modules usually drive the plant size when there is an unlimited fuel supply. “At present the largest single-train combined cycle is 460 MW for 60 Hz and 550 MW for 50 Hz applications,” Stoll said. “Since oxygen-blown gasifiers can reach this equivalent capacity by either increasing pressure and wall thickness or increasing gasifier vessel size, the lowest cost single-train plant may be in the range of 200 MW due to volume flow, although multi-train gasifiers with a single-train power block are feasible.”
One exception is the conceptual Kobra Plant in Germany, designed for an air-blown fluidized bed at 300 MW because of the high volatility of the specific coal to be used.
Economies of size are thought to be driven by two key factors: the economy of the power train`s size and the economy of multiple power trains. Industry experience shows significant economies of size in the individual power-train sizes, with savings of 20 to 25 percent for every doubling of the power train size in the 200 to 400 MW range. Multiple leverage fixed engineering, procurement and site costs, plus improved productivity of site erection achieves an additional 5 to 10 percent for every doubling of the number of trains.
Advances in gas turbine technology continue to reduce plant costs by both size and efficiency and integration techniques. These factors can lower the $/kW cost by obtaining more output from essentially the same equipment.
Studies show that the plant cost has a larger influence on IGCC economics than efficiency, especially when low cost or waste fuels are used. One rule of thumb is that it takes a 1.6 percent heat rate change to match a 1 percent plant-cost change for using fuel that costs (US)$1.50/million Btus.
Use of heavy oil as feed stock rather than coal can reduce plant cost by elimination of solids handling and crushing and feeding systems. Another cost saver is the lower amount of ash in heavy oil, which reduces fouling in the syngas coolers. This can result in a reduction of (US)$230/kW, according to a recent study published by Texaco. Efficiency is also higher.
Petroleum coke can require 7 percent more oxygen than coal for gasification, boosting plant costs by about 1 percent. Coke also requires grinding for more efficient feeding.
Coproducing chemical products from the syngas can significantly enhance the economics of IGCC generation by using a larger gasification system and dispatching at a higher load factor than what is normally associated with chemical plants. Several IGCC plants coproduce hydrogen for refineries. Coproducing steam leads to increased thermal efficiency benefits.
The Schwarze Pumpe plant is producing methanol as a coproduct from coal, waste plastics and waste oils. The plant added 40 MW of electricity production in fall 1996 to boost economics.
Several early IGCC plants were planned to repower existing steam plants, increasing output while decreasing emissions. Destec and PSI repowered the Wabash River plant in the US with a new gasification system and gas turbine/heat recovery steam generator. The new plant uses coal that is less expensive than that used in the older boilers. PSI reports savings for repowering, when compared to a grass roots plant, of between (US)$100/kW and (US)$150/kW.
The largest IGCC plant currently in operation in Europe is the Buggenum plant in the Netherlands with a net capacity of 253 MW. At this plant, purified gas derived from hard coal is used at an overall efficiency of 43 percent. On the basis of the latest combined-cycle technology, new integrated coal gasification plants can attain efficiencies exceeding 50 percent. Buggenum is undergoing trial operation and has already clocked up more than 5,000 operating hours with coal-derived gas. The plant, operated by Demkolec B.V., a project company of the umbrella organization of Dutch utilities, is scheduled to start commercial operation at the beginning of 1998.
A second demonstration plant, similar to Buggenum, is the Puertollano power plant in Spain. The plant concept is based on the Prenflo gasification process, developed by Krupp-Koppers and Siemens/KWU and being used for the first time in this facility. The design fuel is a mixture of ash-rich Puertollano coal and sulfur-rich petroleum coke from a nearby refinery. This 300 MW power plant will, depending on the fuel used, attain an efficiency of as much as 45 percent. The main components for the plant were supplied by a consortium comprising Siemens/KWU and Babcock Wilcox Espa?ola. A further consortium was responsible for supplying the plant`s instrumentation and controls systems. The combined-cycle section of the plant has been in operation on natural gas since the summer of 1996, clocking around 3,000 operating hours. Elcogas S.A., the operating company formed by eight European utilities and three suppliers, will start operation with coal-derived gas toward the end of 1997.
The power plant consists of three plants jointly designed and integrated into the process. The gasification plant consists of a gasification unit and a gas-cleaning and sulfur recovery unit. The gasification unit uses the process of pressurized entrained flow known as the Prenflo process for coal gasification. The gas is produced by the reaction of coal with oxygen at temperatures reaching 1,600 C. This process is capable of gasifying a variety of types and qualities of coal for the production of a synthetic fuel. In operation the plant will gasify a mixture of 50 percent local coal and 50 percent petroleum coke, measured by weight. The gas-cleaning and sulfur recovery unit treats the gases in the outlet of the gasifier, plus clean any contaminants and solid particles from the fuel before sending it to the gas turbine.
The air separation plant generates oxygen to feed the gasification unit and nitrogen for the pneumatic transportation of the fuels.
As a result of support from the European Union (EU), this plant is operating as a demonstration project during its first three years. The facility will experiment with other types of coal from other countries to provide a better evaluation of the flexibility of this type of power plant.
The project is the first power plant using coal gasification to feed a combined cycle in Spain and is reported to be the biggest power plant of this type in the world. The technological effort made to implement this process, with contributions from several EU countries, is considered a long-term investment to positively affect the future of electric and industrial development in Europe, to increase the value of the coal reserves, and to contribute to cost reduction in new projects using clean coal energy.
The plant`s efficient fuel use results in dramatically reduced gaseous emissions, when compared to other types of coal-fired plants. Its emissions are far below the limits established by the EU standards (see table). Quality of water required and liquid effluents are appreciably lower than in a conventional plant of an equivalent size.
Solid wastes are obtained as vitrified slag, which is insoluble in water, making use or storage environmentally benign. The sulfur is recovered in elementary form and is suitable for commercial use.
The Puertollano IGCC plant in Spain is operating in combined cycle and expected to begin operation with coal-derived gas by the end of 1997.
Puertollano`s cooling tower dominates the vista.
The Puertollano gasification plant is composed essentially by the a gasification unit, and a gas cleaning and sulfur recovery unit.