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The Moving Bed Fuel Reactor Process

Andrew Tong Mandar V. Kathe Dawei Wang and Liang-Shih Fan

The Ohio State University, Department of Chemical Engineering, 151 W. Woodruff Ave, Columbus, OH, 43210, USA

1.1 Introduction

Chemical looping refers to the use of a chemical intermediate in a reaction-regeneration cycle to decompose one target reaction into two or more sub-reactions. The decomposition of the target reaction with a reactive chemical intermediate can decrease the process irreversibility, and, thus, increase the recoverable work from the system yielding a higher exergy efficiency. Further, when one or more of the reactant feedstocks consist of an inert substrate, the chemical looping reaction pathway is designed to prevent the direct contact of the inert with the desired product, minimizing the product purification steps required [1-3]. In 1987, Ishida et al. was the first publication to use the term, "chemical looping," referring to the use of a metal oxide as the chemical intermediate to perform oxidation-reduction reaction cycles for power generation applications [4]. However, Bergmann's invention of a calcium carbide production process using manganese oxide redox reaction cycles with carbonaceous fuels suggests that the chemical looping concept was in development as early as 1897 [5]. Table 1.1 summarizes the early developments of chemical looping processes in the twentieth century [6-9, 12-21]. Though several achieved pilot scale demonstration, no early chemical looping processes were able to achieve widespread commercial realization due to limitations in the oxygen carrier reactivity, recyclability, and attrition resistance and the reactor design for maintaining, continuous high product yield.

Table 1.1 Summary of early chemical looping process development.

With growing concerns of greenhouse gas emissions, a renewed effort in developing chemical looping processes occurred at the start of the twenty-first century as reflected in the exponential growth of research publications [1]. As of 2012, over 6000 cumulative hours of operation of chemical looping processes for power generation with CO2 capture have been demonstrated over fuel processing capacities ranging from 300?Wth to 3?MWth [22]. Nearly all chemical looping processes at the pilot scale demonstration have adopted a fluidized bed reactor design for the conversion of the fuel source to CO2/H2O, or the fuel reactor [23]. Recent developers are investigating fixed bed reactors to perform the cyclic oxidation-reduction reactions with chemical looping oxygen carriers for power generation and chemical production applications [24-27]. Alternatively, chemical looping processes utilizing a moving bed fuel reactor are under development for full and partial fuel conversion for CO2 capture/power generation and syngas production, respectively [23, 28, 29]. This chapter describes the use of moving reactors for chemical looping processes with specific application to syngas and power production with CO2 capture using metal oxide materials as oxygen carrier chemical intermediates. Two modes of moving bed operation are discussed and their application for full and partial fuel oxidation. Reactor thermodynamic modeling combined with experimental results are provided.

1.2 Modes of Moving Bed Fuel Reactor Operation

As illustrated in Figure 1.1, the moving bed fuel reactor can be operated in the counter-current or co-current mode based on the gas-solid flow contact pattern with Fe-based oxygen carrier as the exemplary chemical intermediate [1]. The counter-current moving bed fuel reactor in Figure 1.1a achieves a high oxygen carrier conversion while maintaining high CO2 product purity. The oxygen carrier conversion, as defined in Eq. (1.1), is the mass ratio of the amount of oxygen used from the oxygen carrier exiting the fuel reactor relative its maximum available oxygen.

Figure 1.1 Conceptual design of a moving bed chemical looping processes with a counter-current (a) and co-current (b) fuel reactor for full fuel conversion to CO2/H2O and for fuel gasification/reforming to syngas, respectively.

where mox and mred refer to the mass of the fully oxidized and the reduced sample at the outlet of the fuel reactor, respectively, and refers to the mass of the sample at the fully reduced state (e.g. metallic iron for Fe-based oxygen carriers).

Figure 1.1b shows the co-current moving bed fuel reactor for partial oxidation of the solid or gaseous fuel source to syngas. The co-current process allows for accurate control of the oxygen carrier and fuel residence times, ratios, and distribution to maintain continuous high purity syngas. The present section discusses the advantages of each mode of the moving bed reactor operation and considers several applications for solid and gaseous fuel conversion for each.

1.2.1 Counter-Current Moving Bed Fuel Reactor:

In a counter-current moving bed operation of chemical looping process, the gas species in the fuel reactor travel the opposite direction relative to the solids flow. Further, the gas species operate below the minimum superficial gas velocity and, thus, travel only through the interstitial spaces of the packed moving bed of oxygen carrier solids. For full fuel conversion, the counter-current moving bed design is capable of maintaining high CO2 purities and reducing the oxygen carrier to a low oxidation state, ideal for metal oxides with multiple oxidation states such as iron [30, 31]. Figure 1.2 is an example of operation lines for moving bed chemical looping fuel reactor and steam reactor. The figure illustrates the phase equilibrium of a Fe-based oxygen carrier particle at varying partial pressures (i.e. conversions) of the reducing gas at 850?°C. In the figure, the solid line represents the phase equilibrium of iron. The dashed line in the fuel reactor region represents the counter-current reactor operation while the dotted line represents the fluidized bed/co-current operation. The slope of the moving bed and fluidized bed operating lines are determined based on the oxygen balance between the oxygen carrier and the gas species. In the case of fluidized bed operation with iron-based oxygen carrier, the maximum oxygen carrier conversion achievable is 11% (i.e. reduction from Fe2O3 to Fe3O4), as a higher oxygen carrier conversion will result in a decrease in product purity from the fuel reactor. Further, the high extent of reduction of the iron oxide oxygen carrier achieved in the counter-current fuel reactor allows for thermodynamically favorable reaction of Fe/FeO with H2O to produce H2 via the steam-iron reaction. High purity H2 production from a third reactor, i.e. the steam reactor, increases the product flexibility of the processes and can serve as an advanced approach for H2 production with minimal process operations for product separation compared to traditional steam-methane reforming ().

Figure 1.2 Operation lines for moving bed chemical looping fuel reactor and steam reactor.

Figure 1.3 illustrates the design of the counter-current fuel reactor for solid fuel conversion to CO2. Here, the fuel reactor is divided into two sections [32-34]. Once the solid fuel is introduced to the high temperature fuel reactor, it devolatilizes and the solid char species travel downward co-currently with the flow of oxygen carrier solids into the char gasification section. The volatiles travel upward counter-currently with the flow of the oxygen carrier. In the lower bed, the solid char is gasified using an...