Etapa 1: Conducta de Entrada Demandas de Aprendizaje

The possible reaction pathways for the combustion of methane with oxygen carrier are already outlined in Section 2.4. One can notice from Eq. (4) to (7) that the combustion of methane can take place either through direct formation of CO2 and H2O (Eq. (4)) or through the formation of intermediate species CO and H2 (Eq. (5)). Chemical species subsequently formed can be oxidized to CO2 and H2O (Eq. (6) and (7)). Therefore, special attention to the primary products of CH4 conversion is required, with these primary products being a strong function of the metal oxide used as an oxygen-carrier (Adanez et al. 2012).

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Abad et al. (Alberto et al. 2010) assumed that the conversion of methane takes place via H2 and CO species. This path was considered adequate to predict the gas composition of a 10 kW CLC unit (Adánez, et al. 2006a) operated with Cu-based oxygen carriers. During the reduction of Fe-based and CaSO4 oxygen carriers, the major combustion products were CO2 and H2O with variable amounts of CO and H2. (Abad et al. 2007; Corbella et al. 2007; F. He et al. 2007; Johansson et al. 2004; Leion et al. 2008; Mattisson et al. 2004; Mattisson et al. 2001a; Pröll et al. 2009; Song et al. 2008, 2009).

The same approach was also taken by Illuita (Iliuta et al. 2010) for their studies on NiO reduction with CH4 in a fixed bed reactor. These authors found that the reduction steps were more selective toward the formation of CO2 than CO at the temperatures of interest in CLC. As a result, these authors assumed that the primary products of the CH4 reacting with NiO were H2 and CO2.

The direct reaction (Eq. (4)) of CH4 to CO2 and H2O was also reported by several others (Dewaele et al. 1999; Hossain et al. 2010, 2010b; Sedor et al. 2008) using NiO with no CO and H2 detected at the exit gas stream of a FR. This assumption was also found adequate to predict gas distribution in a 120 kW CLC unit (Abad et al. 2010a). Similarly, this one step reduction process was observed (Abad et al. 2006; Johansson et al. 2006) and modeled (Mahalatkar et al. 2011) using a Mn-based oxygen carrier.

Thus, different mechanisms for methane conversion using Ni-based materials can be adopted based on the species present at the exit of a reduction reactor. Figure 35 and Figure 36 present the exit gas concentration of the reduction runs conducted with HMF and MF oxygen carriers. One can notice the presence of both CO and H2 from the very beginning of the reduction processes. Therefore, it was felt in this study that the combustion of CH4 to CO2 and H2O has to be considered through the formation of CO and H2 as the intermediate species. Moreover, the rate of change of CO and H2 concentrations after 20s is low. This suggests either the possible development of equilibrium reactions catalyzed by reduced Ni or the depletion and/or low reactivity of the lattice oxygen of the carrier particles (Adánez et al. 2006; Chandel et al. 2009; Dueso et al. 2010; Mattisson et al. 2006b; Ryu et al. 2003).

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Figure 35: Gas product distribution at the exit of CREC Riser Simulator with HMF oxygen carrier for the temperature range of 550 to 650 °C while the contact time was

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Figure 36: Gas product distribution at the exit of CREC Riser Simulator with MF oxygen carrier for the temperature range of 550 to 650 °C while the contact time was varied up to

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One should mention that the mechanism for CH4 conversion via methane reforming (Eq. (8) to (10)) can be of higher relevance when H2 and CO are the main unconverted products (Adanez et al. 2012). The conversion of methane by reforming, decreases with the increase in the total pressure (Jin et al. 2002) and also depends on the degree of reduction of NiO particles. However, the occurrence of the reforming reaction under the operating condition of this study is not kinetically feasible enough for consideration. Similarly the decomposition reactions (Eq. (11) to (14)) are ignored as they are favored when the degree of reduction is above 80% (Cho et al. 2005) or less than 25% of the required oxygen is supplied for complete combustion of CH4 (Jerndal et al. 2006).

A maximum particle conversion was observed 75% in the present study. Moreover, the outlet gas analysis from the carrier re-oxidation cycle (regeneration) showed negligible amounts of CO and/or CO2, with this being an indicator of little carbon formation during the reduction cycle. Therefore, ignoring the contribution of decomposition reactions is consistent with the experimental observations.

The water-gas-shift (WGS) is the only catalytic reaction (Eq. (15)) that appears relevant and feasible within the operating condition of this study. This reaction is relatively fast and can be considered to be at equilibrium (Abad et al. 2010a). On the other hand, the reaction with H2 is relatively rapid enough when compared to CO conversion. Thus, the disappearance of H2 with NiO particles is compensated by the H2 generated via WGS. However, this study and other reported experimental data, both provide evidence (Abad et al. 2007) that the gas composition is far from thermodynamic reaction equilibrium. Therefore, including the relevant WGS, the following reaction mechanism is proposed and used for kinetics investigation in the present study:

(49) (50) (51) ₍₅₂₎

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