Coal, Gas

Gasifier Reliability and Flexibility

Issue 7 and Volume 110.

Key to IGCC’s Future in the Power Sector

By Steve Blankinship, Associate Editor, Power Engineering

In future power plants, gasification islands will replace the gas pipelines feeding gas turbine combined cycle facility. The gasification island also can replace the conventional pulverized coal power plants, putting a chemical plant on the front of the thermal island to clean up the fuel instead of on the back of the boiler to clean up its exhaust.

Gasifiers, combustion turbines and the equipment associated with them comprise the major difference between a pulverized coal (PC) plant and an integrated gasification combined cycle (IGCC) design. In a traditional utility-scale PC plant, coal is delivered to the plant’s coal yard by rail or barge to be crushed and pulverized into progressively finer consistencies until it resembles a fine powder. This powder is then injected to a boiler and ignited to produce steam by heat recovery from the exhaust. The steam’s thermal and potential energy are transformed to electrical energy in a steam turbine generator. Then precipitators, scrubbers and other equipment are used to attempt to clean the exhaust gas before it is released into the atmosphere.

In an IGCC plant, the coal is converted to synthesis gas (syngas) in the gasifier, then cleansed of impurities in a series of chemical processes. The syngas is then fired in a combustion turbine (CT) to produce electricity. The large amount of heat exiting the CT is retained and applied to a second (combined) cycle, producing steam to spin a steam turbine. It is the gasifier on the front end of the plant – ahead of the CT – along with an array of associated equipment, that represent the most essential differences in the engineering, design and operation of an IGCC plant.

Major players in the gasifier industry include ConocoPhillips, whose E-Gas Technology can be used with gas turbine and steam power generation in an advanced IGCC configuration to produce electric power while co-producing syngas, hydrogen and steam in flexible combinations. A front-end engineering design (FEED) package is being developed in conjunction with Fluor, incorporating the E-Gas Technology with the Siemens SGT6-5000F combustion turbine. Nearly 180 SGT6-5000Fs are operating in North America on natural gas. Conversion to this IGCC application will be modeled on similar Siemens hardware that have operated for 125,000 hours in conjunction with the E-Gas Technology gasification system at other facilities.

The Wabash River Syngas Facility, built on ConocoPhillips’ E-Gas™ Gasification Technology, went into service in 1995 and, to date, has processed more than three million tons of coal and petroleum coke to produce electricity. Photo courtesy of ConocoPhillips.
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The first IGCC project based on the E-Gas Technology-Siemens configuration will be Excelsior Energy’s Mesaba Energy Project in northeastern Minnesota. The first phase of the project will include two Siemens combustion turbines and have a net generation of over 600 MW. Scheduled for commercial operation in 2011, the project has filed air permits and is currently undergoing regulatory review with the Minnesota Public Utility Commission. Earlier this year, the project executed a cooperative agreement with the U.S. Department of Energy to demonstrate a high availability, fuel-flexible IGCC facility.

“If we have a customer who wants to purchase a gasifier and a power island separately, the customer or his engineer can integrate them separately,” says Edward Lowe, general manager of gasification for GE Energy.

Another area of interest comes from customers with natural gas-fired combined cycle plants who are looking to install a gasification island to provide syngas “over the fence.” Lowe cautions, however, that since many combustion turbine facilities were designed as dedicated natural gas plants, the space needed for gasification isn’t always available.

Shell Global Solutions is another supplier of solid fuel gasification technology. The Shell Coal Gasification Process is designed specifically for coal and petroleum coke and is also applicable to low-rank coals, including lignite. Shell’s gasification technology is running in two utility scale coal-fueled power plants in Europe, the Nuon facility in Buggenum, Netherlands, and the EldoGas plant in Puertallano, Spain. The Nuom plant has been operational since 1993, the Puertallano plant since 1998.

AEP has announced plans to build more than 1,200 MW of IGCC capacity within the next decade. Sites under consideration are in Ohio, West Virginia and Kentucky. Monty Jasper, director of new generation development projects for AEP, says building and operating utility power IGCCs will require a shift in thinking.

“For us, an IGCC is a power plant and not a component in a refinery or chemical plant. There’s nowhere to put the gas if it isn’t being consumed. So the output of the gasifier must always match the load.”

He notes that in the industries where IGCC technology evolved, no one could conceive of anything but running gasifiers wide open all the time producing as much syngas as possible except during maintenance shutdowns. “Even though we expect our plants to dispatch early in the queue, we have to allow for turndown capability because no baseload power plant in an interconnection such as we operate in (PJM) is going to run full load all the time.”

That fact of life in the utility world makes operating profiles significantly different than those for refineries and chemical plants. Not only must the gasifiers that provide fuel to the power island operate in cycling modes, the ancillary pieces that support them, including acid gas removal (AGR) systems and air separation units (ASUs) must cycle up and down as well. Historically, such front end components have not been required to operate in high cycling patterns. Sulfur recovery units, for example, are content to run at continuous throughput. On the other hand, the power island of an IGCC utility facility should be expected to perform with virtually the same flexibility as any other combined cycle plant.

Some people would like to characterize an IGCC power plant as being prototypical or cutting edge, says Jasper. “In terms of overall size, it is. But each train will be about the same size as existing power-producing IGCCs. And lessons learned from IGCC plants worldwide will be incorporated at the plants to be built. So while we may be putting the pieces together a little differently and having multiple sets of those pieces, there’s very little here that hasn’t been done before.”

The Wabash River Coal Gasification Repowering Project, at Duke Energy’s Wabash River Station in West Terre Haute, Ind., has been demonstrating IGCC technology on a commercial basis since 1995, supplying customers with power on the Cinergy (now Duke Energy) grid. The 262 MW plant uses high-sulfur coals or petroleum coke to produce hydrogen-rich synthesis gas for a gas turbine. Duke owns and operates the combined cycle power block, which receives steam and syngas over the fence from the syngas facility, which is owned and operated by SG Solutions LLC. SG Solutions is a joint venture of the Wabash Valley Power Association and Global Energy Inc.

Dwight Lockwood, vice president of regulatory affairs for Global Energy Inc., says the facility has generated nearly six times the annual power output of the conventional coal unit it replaced and still has reduced SO2, NOx and particulates by thousands of tons each year it has operated.

The Wabash River facility, a single train gasification unit, averages about 75 percent syngas availability year to year, which was the target set in 1991 when design began.

“With the lessons learned in 10 years of operations at Wabash and advances in materials and control technologies, the next generation of gasification plants will have annual availabilities of 80 percent to 85 percent per train,” says Phil Amick, Commercial Director for ConocoPhillips gasification business. “Some of this is already becoming reality, and we’ve seen the best operation since start-up at Wabash over the last six months.”

Gasification technology providers propose adding extra gasifier trainage to increase annual availabilities to near 95 percent. Since the gasifier and its immediately surrounding equipment are only a small part of the overall IGCC facility, this additional equipment isn’t as cost prohibitive as adding extra boilers would be in a conventional PC plant.

High availability is expected because providers of gasification technology and hardware have taken into account the unique demands placed on gasification and syngas treatment systems that are posed by operating in the baseload power sector.

Turndown may be the current Holy Grail for the gasification industry. IGCC plants like Wabash River have demonstrated load reductions to 70 percent and 80 percent of overall plant electrical output for weeks at a time, but not yet the daily turndown capability sought by some utility clients. These plants run baseload in their utility systems because of high efficiency and low fuel costs coupled with environmental benefits. True turndown capability will be sought in the next generation of IGCC plants, however. Shell and ConocoPhillips, whose designs have multiple burners which will facilitate load changes, already are touting this capability to their gasification technology customers.

The ultimate flexibility for the IGCC plant, though, is its inherent ability for future carbon capture capability. Using conventional technologies currently in use in refineries and chemical plants, the syngas can be shifted to hydrogen and stripped of its carbon components before combustion. There are nominally 10 process units in an IGCC facility; modifying just one of them will produce a carbon dioxide stream for ultimate use in applications like enhanced oil recovery or even, sometime in the future, geologic sequestration.