Reduction of CaSO4 oxygen carrier with coal in chemical-looping combustion: Effects of temperature and gasification intermediate

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Abstract

Chemical-looping combustion (CLC) has been suggested as an energy efficient method for the capture of carbon dioxide from combustion. Thermodynamics and kinetics of CaSO4 reduction with coal via gasification intermediate in a CLC process were discussed in the paper, with respect to the CO2 generating efficiency, the environmental factor and the surface morphology of oxygen carrier. Tests on the combined process of coal gasification and CaSO4 reduction with coal syngas were conducted in a batch fluidized bed reactor at different reaction temperatures and with different gasification intermediates. The products were characterized by gas chromatograph, gas analyzers and scanning electron microscope. And the results showed that an increase in the reaction temperature aggravated the SO2 emission. The CO2 generating efficiency also increased with the temperature, but it decreased when the temperature exceeded 950 °C due to the sintering of oxygen carrier particles. The use of CO2 as gasification intermediate in the fuel reactor had a positive effect on the sintering-resistant of oxygen carrier particles. However, increasing the steam/CO2 ratio in gasification intermediate evidently enhanced CO2 generating efficiency and reduced SO2 environmental impact.

Introduction

Chemical-looping combustion (CLC) has been suggested among the best alternatives to achieve an increase in power station efficiency simultaneously with low energy penalty carbon capture because CO2 is inherently separated in the process (Lyngfelt et al., 2001, Richter and Knoche, 1983). Fig. 1 shows a schematic diagram of a CLC process of coal using CaSO4 oxygen carrier. The CLC typically employs a dual fluidized bed system: an air reactor and a fuel reactor. The oxygen carrier circulates between the air reactor and the fuel reactor, transferring oxygen from air to fuel. In the fuel reactor, coal is firstly pyrolyzed and gasified with steam or CO2 or both into mainly syngas, and then the syngas is oxidized by the CaSO4 to CO2 and H2O. Limestone may be added for the removal of sulfur in the coal. Sulfur in the coal reacts with the limestone to form CaS, which then upon oxidation becomes a part of the CaSO4 stream. In the air reactor, air is supplied to oxidize the reduced oxygen carrier CaS to CaSO4, where oxygen is transferred from air to CaSO4. In this way, the stream from the air reactor is composed of atmospheric N2 and residual O2; and the stream from the fuel reactor is mainly consisted of CO2 and water. After the water condensation, almost pure CO2 can be obtained.

Starting from the introduction of CLC in 1983, the majority of studies and process development have been intensively concentrated on gaseous fuels (such as methane and natural gas) and metal oxides NiO, Fe2O3, CuO, Mn2O3, and CoO based oxygen carriers (Bolland and Undrum, 2003, Lyngfelt et al., 2008, Sedor et al., 2008). However, the methane and natural gas supply can not fully meet the energy demand for the long term. Although some studies have been published on CLC of solid fuels and showed many promising results (Abad et al., 2006, Berguerand and Lyngfelt, 2008a, Berguerand and Lyngfelt, 2008b, Cao and Pan, 2006, Cao et al., 2006, Gao et al., 2008, Johansson et al., 2006, Leion et al., 2007, Leion et al., 2008, Scott et al., 2006, Shen et al., 2008a), it is still urged to develop suitable oxygen carriers for large scale application of CLC (Anthony, 2008, Hossain and de Lasa, 2008). For example, the metal oxygen carriers may be used limitedly because of the high cost, bad environmental sound and sulfur-poisoning.

Recently calcium sulfate is becoming an attractive oxygen carrier for the commercial application of CLC because of its easy availability and low price. Alstom Power Inc. has proposed the CaSO4-based oxygen carrier for CLC of coal, yet no further result has been published (Jukkola et al., 2005). In our previous study, both the reactions of CaSO4 reduction with gas fuels (CO and H2) and CaS oxidation with O2 have been investigated by a thermo-gravimetric analyzer (TGA) coupled with Fourier Transform Infrared (FTIR) spectroscopy (Shen et al., 2008b). The results show that the CaSO4 oxygen carrier can be an interesting alternative oxygen carrier for the CLC of gaseous fuels. An increase in the reaction temperature enhances the reduction and regeneration reactions of the CaSO4-based oxygen carrier. However, the SO2 is released during the periodic shifts between CaSO4 reduction and CaS oxidization. And the level of SO2 formation increases with the reaction temperature both at the reducing and oxidizing conditions. Furthermore, our research group has also discovered that the CaSO4 oxygen carrier has high reduction reactivity and stability in a long-time reduction/oxidation test by using the gaseous fuels in a laboratory fixed/fluidized bed reactor (Song et al., 2008a, Song et al., 2008b, Song et al., 2008c). Although a fraction of SO2 is produced during the periodic shifts between CaSO4 reduction and CaS oxidization, it could be recaptured and then recycled to CaSO4 by adding a small amount of fresh limestone into the CLC system. Wang and Anthony (2008) proposed a CO2-based gasification coupled chemical-looping process for the clean combustion of solid fuel with CaSO4-based oxygen carrier, and presented some promising simulation results. However, most of the progresses for CaSO4-based oxygen carrier focus on the gas fuels, rather than the solid fuels. Besides, in the studies of solid fuels, the data available in literature (Wang and Anthony, 2008) are just from numerical simulation, and none of these studies has been done on the reaction kinetic analysis.

In this paper, the thermodynamics and reactivity of CaSO4 reduction with coal during the first cycle were investigated at varied temperatures (from 850 to 975 °C) and with varied gasification intermediate ratios of steam to CO2 in a batch fluidized bed reactor. The CO2 generating efficiency, the surface morphology of oxygen carrier and the sulfur release during the reduction reaction of CaSO4 with coal were discussed.

Section snippets

Thermodynamic analysis

With the standard Gibbs free energy changes, the equilibrium constant can be calculated at different temperatures. The relationship between the equilibrium constant K and reaction temperature Temp is expressed as:lnK=1R(ΔHTempθTemp+ΔSTempθ)where ΔHTempθ and ΔSTempθ are the standard heat change of formation, the standard entropy change of formation at the corresponding reaction temperature respectively, and R is the universal gas constant. On the basis of the related thermodynamic parameters (

Kinetics of the CaSO4 reduction with coal via gasification intermediate

The reduction of CaSO4 by coal was discussed at different temperatures and with different gasification intermediates, with respect to the CO2 generating efficiency, the environmental factor (E-factor) and the surface morphology of oxygen carrier. Tests were conducted in a batch fluidized bed reactor. Coal was gasified in the bed of anhydrite ore. The products were characterized by Gas Chromatograph (GC), gas analyzers, and Scanning Electron Microscope (SEM). Some experiments, where the oxygen

Conclusions

Thermodynamics and kinetics of CaSO4 reduction with coal via gasification intermediate in a CLC process have been investigated in the present work. The effects of reaction temperature and gasification intermediate on the CO2 generating efficiency, the environmental factors (E-factors) and the surface morphology of oxygen carrier are discussed. Some useful results are achieved, as follows:

  • (1)

    High concentration of CO2 product is thermodynamically feasible in the reduction of CaSO4 oxygen carrier

Acknowledgements

This work was supported by the National Natural Science Foundation of China (90610016, 50976023).

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