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TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

Integration of Gas Turbines Adapted for Syngas Fuel With Cryogenic and Membrane-Based Air Separation Units: Issues to Consider for System Studies

[+] Author and Article Information
John G. Wimer, Dale Keairns, Edward L. Parsons, John A. Ruether

 National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, WV 26507-0880

A study by Air Products and Chemicals (4) concluded that compared to a highly integrated, elevated-pressure cryogenic ASU, their membrane-based ASU would “decrease the installed specific cost ($mTPD) of air separation equipment by 35%. Such savings decrease the installed capital cost of the overall (CGCC) facility by 7%… while the efficiency also improves by one percentage point.” A similar comparison by Praxair (5) reached very similar conclusions.

Also known as dry low-NOx combustion (General Electric), dry-low emissions process (Rolls-Royce/Allison) and SoLoNOx process (Caterpillar/Solar Turbines).

However, DOE is currently funding the development of new combustors that will enable lean-premix combustion technology to eventually be used in future syngas applications to control NOx emissions to 3 ppmvd or less (at 15% excess O2).

Parts per million (dry volume basis).

See Sec. 3. Injected nitrogen contributes to a net increase in the turbine-expander flow rate only if it was not separated from air extracted from the turbine-compressor. (Nitrogen in extracted air is simply diverted from the turbine, separated from the air, and returned.)

When the turbine flow is choked (sonic) at the first-stage nozzle, it cannot be increased by adjusting its downstream pressure.

An axial compressor surges when its flow decreases enough to cause a momentary flow reversal. The flow reversal occurs when the compressor can no longer overcome its discharge pressure.

Substantially increasing the area of the stage-one nozzle would require a major redesign, essentially equivalent to developing a new gas turbine model.

Syngas-fired gas turbines must be operated on an alternative fuel, typically natural gas, during certain off-design conditions, e.g., during startup and shutdown.

Two CGCC plants in Europe experienced delays in startup and increased operating complexity as a result of employing full integration. Consequently, there is a perception that full integration is now out of favor with the gasification industry, especially when project economics value reliability over efficiency (11).

GE has adapted its Model F machines for syngas such that up to 20% of the air from the compressor discharge can be extracted without affecting the cooling air system (15).

In the typical two-step distillation process, the relatively impure oxygen produced in a high-pressure column is further purified in a low-pressure column.

The benefit of elevated-pressure operation is diminished as a greater fraction of the byproduct nitrogen must be vented because it is not required by the gas turbine. In some cases, it may be worthwhile to utilize an EP ASU coupled with an expander to extract work from the vented nitrogen. In other cases, it may be better to simply employ an LP ASU.

This guideline is more or less corroborated by Table 2 which shows that the two European CGCC plants fully supply their EP ASUs by extracting 16%–18% of the air discharged by their turbine-compressors. GE cites a similar range of 11%–20% air extraction for fully integrated gasification systems with cryogenic ASUs (15).

I.e., the minimum degree of integration when the turbine-compressor is not throttled down. Although throttling would allow all of the ASU nitrogen to be injected at a lower degree of integration, i.e., less air extraction, FWC determined that this strategy actually decreases system efficiency.

A report by FWC (16) recommended expanding the extraction air to the pressure at which air would be extracted when the gas turbine is operating at 50% load.

APCI and Praxair are developing membrane-based air separation technology under the names Ion Transport Membrane (ITM) and Oxygen Transport Membrane (OTM), respectively.

Depending on future application needs and necessary technology advances, APCI and Praxair may revisit steam-purged concepts at a later time.

To achieve purities >95%, a cryogenic ASU would require the addition of more distillation columns which substantially increases its cost.

Praxair (5) is attempting to reduce the operating temperature of the membrane-based ASU they are developing from 900 to 750°C. A lower operating temperature could reduce the capital cost of the ASU and its associated equipment.

When operated on natural gas, “F” class gas turbines have firing temperatures between 1300°C and 1400°C and “G” and “H” class gas turbines have firing temperatures of 1400–1500°C.

Some applications may deviate from this general guideline. For example, to increase the oxygen recovery fraction, the partial pressure of oxygen on the permeate side may be decreased to less than one-seventh of that on the nonpermeate side. Praxair (5) demonstrated that the oxygen flux through the membrane they are developing has a logarithmic dependence on the oxygen partial pressure ratio across it. The oxygen flux targeted by Praxair corresponds to an oxygen partial pressure ratio of 3.5—about half the ratio implied by the APCI heuristic cited above.

J. Eng. Gas Turbines Power 128(2), 271-280 (Jan 13, 2005) (10 pages) doi:10.1115/1.2056535 History: Received May 07, 2004; Revised January 13, 2005

The purpose of this paper is to aid systems analysts in the design, modeling, and assessment of advanced, gasification-based power generation systems featuring air separation units (ASUs) integrated with gas turbines adapted for syngas fuel. First, the fundamental issues associated with operating a gas turbine on syngas will be reviewed, along with the motivations for extracting air from the turbine-compressor and/or injecting nitrogen into the turbine expander. Configurations for nitrogen-only and air-nitrogen ASU integration will be described, including the benefits and drawbacks of each. Cryogenic ASU technology will be summarized for both low-pressure and elevated-pressure applications and key design and integration issues will be identified and discussed. Finally, membrane-based ASU technology will be described and contrasted with cryogenic technology in regard to system design and integration.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Elements of a gas turbine

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Figure 2

Nitrogen-only integration (simplified view)

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Figure 3

Air-nitrogen integration (simplified view)

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Figure 4

Integration of a membrane-based ASU

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Figure 5

Operating pressure envelope for membrane-based ASUs

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