Analysis of a Simulated Moving Bed Configuration for Chemical-Looping Combustion

The impact of greenhouse gas emissions from power generation plants motivates the need for energy production technologies with carbon capture and sequestration. Chemical-looping combustion (CLC) is an emerging method for energy production using fossil fuels with inherent separation of CO₂. CLC utilizes an intermediate metal oxide oxygen carrier and a reduction-oxidation cycle to prevent direct contact of air and fuel during the combustion process. In the reduction step, a hydrocarbon fuel (i.e., CH₄) is oxidized by the oxygen carrier (i.e., NiO), yielding CO₂ and H₂O. In the subsequent oxidation step, the reduced oxygen carrier (i.e., Ni) is regenerated in air, emitting N₂ and unreacted O₂. A pure stream of CO₂ can be obtained after the reduction step after condensation of H₂O without a large energy penalty or additional separation steps.

Multiple reactor designs have been proposed for CLC, including interconnected fluidized bed processes [1], alternating flow fixed-bed processes [2], and moving bed processes [3]. The constraints and implications relating to oxygen carrier particle size, reactor size, and operating conditions vary among these implementations, resulting in different fuel conversions, CO₂ capture efficiencies and additional separation steps required for CO₂ capture. For example, the moving bed reactor utilizes countercurrent flows of fuel and solids to maximize the thermodynamic driving force of the reactions in the CLC reducer, achieving higher efficiencies than fluidized beds. CO₂ selectivity is greatly increased because the hydrocarbon fuel is always introduced to fresh oxygen carrier. However, circulation of solids within a reactor often leads to operational challenges, such as product stream contamination, gas leakage, and high particle abrasion. The simulated moving bed reactor, shown in Figure 1a, is an adaption of the moving bed reactor with stationary solid particles in a fixed-bed configuration. This process consists of switching the inlet and outlet ports simultaneously along the axial dimension of a standard fixed-bed (Figure 1b) to simulate the countercurrent movement of gas and solids. Simulated moving bed reactors have been proven to increase efficiencies and overcome equilibrium restrictions for reactions in absorption, adsorption and extraction processes, such as reactive chromatography [4].

Figure 1: Reactor designs for CLC: (a) simulated moving bed and (b) fixed-bed

In this work, the simulated moving bed design concept is explored as a reactor option for gaseous CLC. A single fixed-bed configuration (Figure 1b) is used as a benchmark for comparison of reactor performance. A simulated moving bed reactor is modeled using multiple fixed-bed reactors in the configuration shown in Figure 1a. A one-dimensional, homogeneous fixed-bed reactor model with axial dispersion, energy balance and momentum balance is used in concert with CLC reduction and oxidation kinetics of NiO with CH₄ and air derived previously, using literature and in-house fixed-bed reactor data [5–7]. The configuration of the baseline fixed bed reactor is representative of existing reactors reported in the literature. The performance of the simulated moving bed reactor is then compared to its fixed bed counterpart in terms of CH₄ conversion, CO₂ capture efficiency, oxygen carrier conversion and selectivity to solid carbon. The reactor temperature profiles are also explored to identify advantages in the proposed setup in terms of heat utilization within and between the reactors.

Figure 2: Fuel conversion (A), CO₂ gas selectivity (B) and solid carbon selectivity (C) vs.

bed NiO conversion for fixed-bed and simulated moving bed (SMB) processes at 900°C

Application of the simulated moving bed technology yields numerous benefits over its fixed-bed counterpart with a negligible sacrifice to fuel conversion, as shown in Figure 2a. First, the SMB achieves higher CO₂ selectivity at high NiO conversions (Figure 2b). As the inlet and outlet ports are switched along the length of the reactor, the inlet feed is constantly introduced to fresh oxygen carrier, which promotes the conversion to CO₂ and suppresses the catalytic reactions that yield partial oxidation products. As a result, the second benefit is the reduction in carbon deposition during the reduction cycle. A comparison to the fixed-bed performance is shown in Figure 2c. By recycling the combustion products (i.e., CO₂ and H₂O) throughout the NiO-depleted regions of the reactor, any solid carbon formed previously is gasified. Another advantage of the simulated moving bed process is the mitigation of cold spots inside the reactor, as shown in Figure 3. The reduction reactions with NiO are generally endothermic, so a cold zone is exhibited when fuel is in contact with NiO. However, in the SMB concept, the reaction zone covers a larger area so the extent of the temperature change is reduced and circulation of hot gases within the converted zones further warms up the bed (Figure 3).

Figure 3: Transient temperature distributions for one CLC reduction step at 900°C

with fixed-bed (left) and simulated moving bed (SMB) (right) processes

 

In summary, the simulated moved bed reactor design is shown to be promising for CLC applications and future work is aimed at implementation for multiple CLC reduction-oxidation cycles and optimization studies of the novel design. In this presentation, a proof of concept analysis will be illustrated with case studies comparing the operation of a fixed-bed process with that of a simulated moving bed, where total reactor size, oxygen carrier loading, methane capacity, temperature and pressure are kept the same.

 

 

 

 

Acknowledgements: This material is based upon work supported by the National Science Foundation under Grant No. 1054718 and the UConn Prototype Fund.

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