Chemical looping tar reforming using La/Sr/Fe-containing mixed oxides supported on ZrO2

https://doi.org/10.1016/j.apcatb.2015.10.047Get rights and content

Highlights

  • La/Sr/Fe containing mixed oxides were supported on ZrO2 by dry impregnation.

  • Materials exhibited favourable properties for use in chemical looping tar reforming.

  • High benzene and ethylene conversion by steam reforming were observed.

  • Benzene conversion could be further increased by selective oxidation.

Abstract

Biomass gasification gas contains condensable hydrocarbons usually referred to as “tars”. The use of chemical-looping reforming (CLR) has been proposed as a downstream technology for tar removal from the hot raw gasification gas. In this work two different ZrO2 support materials impregnated with La, Sr, Fe and mixtures thereof have been investigated as bed material for this proposed CLR process, with benzene and ethylene as tar surrogates. It was found that only combinations of La and Fe yielded significant catalytic activity for benzene conversion that could be further improved by adding Sr. Over this material, the benzene conversion reaction was found to be of first order with respect to benzene, and a simple kinetic model indicates that a high degree of benzene conversion can be obtained at reasonable residence times when the reactor temperature is sufficiently high (T = 850 °C). It was also observed that this material exhibited some activity for selective catalytic oxidation of benzene, which could further increase the tar conversion when either the bed material provided oxygen to the gas or a small stream of molecular O2 was added to the gasification gas feed. XRD analysis of the used bed materials revealed that a pyrochlore phase and SrZrO3 perovskite were formed during the experiment.

Section snippets

Introduction and background

Gasification of biomass presents a promising route to generate carbon-neutral synthesis gas (CO + H2), which can then be further processed into valuable gaseous and liquid fuels such as Substitute Natural Gas (SNG), Dimethyl Ether (DME) or others [1], [2], [3]. Apart from the desired major syngas compounds, raw gasification gas usually contains impurities and contaminants such as sulphur compounds, ammonia, non-condensable hydrocarbons and condensable hydrocarbons usually referred to as tars [4].

Experimental setup and procedure

Experiments were conducted in a small-scale fluidized bed reactor and the experimental setup is illustrated in Fig. 2. A detailed description of this experimental setup can be found in a previous publication [11]. Experiments were conducted with 9 g of bed material sieved to a size range of 125–250 μm. Additionally, empty reactor experiments were conducted to determine the extent of homogeneous gas phase reactions which could not be attributed to the bed material. To emulate the circulation of

Gas concentration profiles

In Fig. 3 the recorded gas concentration profiles are shown with ZTECH_LaSrFe as a bed material and 1.4 % C6H6 in the gasification gas at T = 800 °C. In the beginning, the fully oxidized bed material is exposed to gasification gas and thus reduced. It oxidizes the gasification gas, resulting in a peak in H2O and CO2 concentrations. Once these concentrations stabilize the material has reached its thermodynamically stable state and the bed material does not provide oxygen to the gas any longer.

Discussion

The results presented in Fig. 4 together with the results of the XRD analysis of the materials indicate that only the impregnation with La and Fe yields materials with a significant catalytic activity for benzene reforming; however this catalytic activity does not seem to require the existence of a crystalline La1-xSrxFeO3 phase, since the ZTECH_LaSrFe material exhibits high catalytic activity without such a phase being present during the experiment. The high catalytic activity could originate

Conclusions

In this study ZrO2-supported bed materials were investigated with respect to their tar removal capabilities in a chemical looping reforming system with benzene and ethylene as tar surrogates. ZrO2 was impregnated with individual metals La, Fe and Sr and combinations thereof corresponding to LaFeO3 and La0.8Sr0.2FeO3 perovskites, and it was found that only combinations of La and Fe yield significant conversion of benzene. The addition of Sr to La and Fe proved to be very beneficial for benzene

Acknowledgments

This work has been supported by the Swedish Gasification Center and the Chalmers Energy Area of Advance.

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