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Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

Producing Hydrogen and Power Using Chemical Looping Combustion and Water-Gas Shift

[+] Author and Article Information
Niall R. McGlashan

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

Peter R. N. Childs, Andrew L. Heyes, Andrew J. Marquis

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

For all reactions in this paper, three thermodynamic functions are given: ΔGo, ΔHo, and ToΔSo. These are indicative of, respectively, the maximum achievable work output from the reaction, the reaction’s heat release, and the required heat exchange with the environment for a reversible reaction.

In this work, the sink condition is assumed to be standard temperature and pressure (stp), i.e., To=298.15K, Po=1.0bar.

Where ΔSo0, the Qo term in Eq. 3 becomes significant and Eqs. 13,15 no longer apply. Reactions of this type generally involve a change in the number of moles of gas, and consequently pressure changes affect the equilibrium temperature. However, this effect is usually small, so the practical benefit of CLC is maintained.

Equation 15 can also be derived from consideration of the equilibrium temperature of the two CLC redox reactions.

J. Eng. Gas Turbines Power 132(3), 031401 (Dec 03, 2009) (10 pages) doi:10.1115/1.3159371 History: Received March 22, 2009; Revised March 25, 2009; Published December 03, 2009; Online December 03, 2009

A cycle capable of generating both hydrogen and power with “inherent” carbon capture is proposed and evaluated. The cycle uses chemical looping combustion to perform the primary energy release from a hydrocarbon, producing an exhaust of CO. This CO is mixed with steam and converted to H2 and CO2 using the water-gas shift reaction (WGSR). Chemical looping uses two reactions with a recirculating oxygen carrier to oxidize hydrocarbons. The resulting oxidation and reduction stages are preformed in separate reactors—the oxidizer and reducer, respectively, and this partitioning facilitates CO2 capture. In addition, by careful selection of the oxygen carrier, the equilibrium temperature of both redox reactions can be reduced to values below the current industry standard metallurgical limit for gas turbines. This means that the irreversibility associated with the combustion process can be reduced significantly, leading to a system of enhanced overall efficiency. The choice of oxygen carrier also affects the ratio of CO versus CO2 in the reducer’s flue gas, with some metal oxide reduction reactions generating almost pure CO. This last feature is desirable if the maximum H2 production is to be achieved using the WGSR reaction. Process flow diagrams of one possible embodiment using a zinc based oxygen carrier are presented. To generate power, the chemical looping system is operated as part of a gas turbine cycle, combined with a bottoming steam cycle to maximize efficiency. The WGSR supplies heat to the bottoming steam cycle, and also helps to raise the steam necessary to complete the reaction. A mass and energy balance of the chemical looping system, the WGSR reactor, steam bottoming cycle, and balance of plant is presented and discussed. The results of this analysis show that the overall efficiency of the complete cycle is dependent on the operating pressure in the oxidizer, and under optimum conditions exceeds 75%.

FIGURES IN THIS ARTICLE
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References

Figures

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Figure 1

Sankey diagram of a reversible, carbothermic, hydrogen economy: showing fluxes of energy flowing between coupled systems—one producing H2 and the other consuming H2

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Figure 2

Sankey diagram of energy fluxes in a reversible CLC system (units MJ/kmol)

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Figure 3

Schematic diagram of CLC system avoiding indirect heat exchange and using a zinc flash vessel

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Figure 4

Diagram of proposed bottoming steam cycle and WGSR

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Figure 5

Graph showing overall efficiency, ηov, and reducer pressure, Predu, versus Poxi.

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Figure 6

Graph showing key process temperatures versus Poxi

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Figure 7

Sankey diagram showing fluxes of energy in proposed system with Poxi=35.0 bar (W, Q, and Hstr; unit: MJ/kmol)

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Figure 8

Graph showing equilibrium concentrations of species in reducer versus reactor temperature

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