Research Papers: Gas Turbines: Cycle Innovations

Chemical Looping Combustion Using the Direct Combustion of Liquid Metal in a Gas Turbine Based Cycle

[+] Author and Article Information
Niall R. McGlashan

Department of Mechanical Engineering, Imperial College London, South Kensington, London, SW7 2BX, UKn.mcglashan@ic.ac.uk

Peter R. N. Childs, Andrew L. Heyes

Department of Mechanical Engineering, Imperial College London, South Kensington, London, SW7 2BX, UK

Pure Na, in either vapor or liquid state, is unlikely to be present at noticeable levels in molten salt gasifiers due the higher oxygen activity.

J. Eng. Gas Turbines Power 133(3), 031701 (Nov 10, 2010) (13 pages) doi:10.1115/1.4001984 History: Received April 08, 2010; Revised April 26, 2010; Published November 10, 2010; Online November 10, 2010

A combined cycle gas-turbine generating power and hydrogen is proposed and evaluated. The cycle embodies chemical looping combustion (CLC) and uses a Na based oxygen carrier. In operation, a stoichiometric excess of liquid Na is injected directly into the combustion chamber of a gas-turbine cycle, where it is burnt in compressed O2 produced in an external air separation unit (ASU). The resulting combustion chamber exit stream consists of hot Na vapor and this is expanded in a turbine. Liquid Na2O oxide is also generated in the combustion process but this can be separated, readily, from the Na vapor and collects in a pool at the bottom of the reactor. To regenerate liquid Na from Na2O, and hence complete the chemical loop, a reduction reactor (the reducer) is fed with three streams: the hot Na2O from the oxidizer, the Na vapor (plus some entrained wetness) exiting a Na-turbine, and a stream of solid fuel, which is assumed to be pure carbon for simplicity. The sensible heat content of the liquid Na2O and latent and sensible heat of the Na vapor provide the heat necessary to drive the endothermic reduction reaction and ensure the reducer is externally adiabatic. The exit gas from the reducer consists of almost pure CO, which can be used to generate byproduct H2 using the water-gas shift reaction. A mass and energy balance of the system is conducted assuming reactions reach equilibrium. The analysis allows for losses associated with turbomachinery; heat exchangers are assumed to operate with a finite approach temperature. However, pressure losses in equipment and pipework are assumed negligible—a reasonable assumption for this type of analysis that will still yield meaningful data. The analysis confirms that the combustion chamber exit temperature is limited by both first and second law considerations to a value suitable for a practical gas-turbine. The analysis also shows that the overall efficiency of the cycle, under optimum conditions and taking into account the work necessary to drive the ASU, can exceed 75%.

Copyright © 2011 by American Society of Mechanical Engineers
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Figure 1

Graph showing the first law efficiency and work output of a process enacting reaction (1) versus the processes’ total internal and external entropy production

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Figure 2

Schematic diagram of a Na burning: CLC system with auxiliary air separation unit and water-gas shift reactor. The CO2/H2 separation and CO2 compression systems are not shown. (The station numbering is consistent with Appendix ).

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Figure 3

Cross section showing internal details of oxidation reactor. The proposed reactor is of the entrained flow type and uses a blast of oxygen fed through tuyeres at the base to oxidize a spray of metal droplets falling from the top. The reactor’s walls are protected by a layer of metal oxide frozen to an inner membrane wall, which is gas cooled.

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Figure 4

Cross section showing internal details of reduction reactor. The proposed reactor is of the molten bath type and uses a blast of metal vapor to transport solid fuel into a melt of oxides and carbonates. Heat released by condensation of the metal vapor drives the endothermic reduction reaction between the fuel and the bath’s constituents.

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Figure 5

Graph showing the two possible operating regimes of the reduction reaction at equilibrium, separated by the vapor pressure line for Na—at pressures higher than Pvap, no C(s) can exist; at pressures below Pvap, no Na(l) can exist.

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Figure 6

Cross section through proposed reactor/pipe wall design, showing frozen Na2O layer and gas cooling arrangement

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Figure 7

Graph showing: the overall efficiency ηov and second law efficiency ηH2 of the CLC/H2 system with and without a bottoming Rankine cycle and the optimum reducer pressure Predu versus the oxidizer pressure Poxi



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