6.3. MECHANISM
6.3.1. Combustion-Reforming Mechanism
mecha-nism and (2) pyrolysis-oxidation mechamecha-nism.
The conversion of oxygenated compounds is believed to proceed through decomposition followed by reforming reactions. These three mechanisms are discussed in detail in the following sections.
6.3.1. Combustion-Reforming Mechanism
This proposed mechanism is similar to the indirect reaction pathway described for catalytic partial oxidation (CPOX)[33e35]. It is speculated that under OSR conditions, the hydrocarbon conversion takes place in two consecutive steps [36e40]. First, part of the fuel undergoes combus-tion with all available oxygen at the top porcombus-tion of the catalyst bed, forming mainly H2O and CO2as products. A very small section of the catalyst bed is utilized during this step, due to the fast kinetics of combustion. If the reaction takes place accord-ing to the stoichiometry, 25% of the fuel is FIGURE 6.6 Illustration depicting the range of operating conditions for the reforming of ethanol;Reprinted from Rabenstein and Hacker[20], Copyright (2008), with permission from Interna-tional Association for Hydrogen Energy.
0 0.5 1 1.5 2 2.5 3
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Equilibrium amount (kmol)
Pressure (MPa)
H2
H2O
CO CO2 CH4
FIGURE 6.7 Equilibrium product distribution as a func-tion of pressure for the OSR of 1 kmol CH4(O/C¼ 0.7, S/C ¼ 1.5, and 800 C). (Calculation performed using HSC Chemistry v6.12[31].)
6. OXIDATIVE STEAM REFORMING
138
converted during this process by the oxygen. The remaining unconverted fuel fragments are then converted into synthesis gas primarily through endothermic steam- and CO2-reforming reactions down the rest of the catalyst bed. Though the endothermic reactions dominate after the initial combustion zone, the equilibrium of the WGS/
RWGS also takes place in this region, and plays a role in synthesis gas selectivity. A representation of the overall combustion-reforming mechanism is depicted inFig. 6.8.
The mechanism in Fig. 6.8 has been sup-ported by several studies in which temperature profiles were measured at multiple points down
the length of the catalyst bed [40e44]. Higher temperatures were observed in the top part of the catalyst bed, followed by lower tempera-tures in the remainder of the catalyst bed. The IR image shown inFig. 6.9 [40]reflects the reac-tion profile of the OSR of CH4 occurring over a commercial Rh/Al2O3. Although temperature is an aggregate measurement, the observed temperatures are consistent with the combus-tion-reforming mechanism.
It has also been found that the type of catalytic metal will influence the shape and magnitude of the peaks in the observed temperature profile [36,40,42]. For first row transition metals, like
Step 1: 2CH4 + O2 + H2O= 0.5CO2 + 2.0H2O + 1.5CH4
Step 2: 1.5CH4 + 2.0H2O or 0.5CO2 = CO + H2
CnHmOp
O2
H2O
Combustion
CnHmOp + O2 H2O + CO2 H <0 Reforming
CnHmOp + H2O H2 + CO H >0 CnHmOp + CO2 H2 + CO H >0 CO + H2O H2 + CO2 H <0
Catalyst Bed H2 & CO
FIGURE 6.8 Reaction scheme describing the com-bustion-reforming mecha-nism for the OSR of a fuel.
FIGURE 6.9 Thermo-graphic image of catalyst bed (Rh/Al2O3) during the OSR of CH4; Reprin-ted from Simeone et al.
[44], Copyright (2008), with permission from International Association for Hydrogen Energy.
MECHANISM 139
Ni, temperature profile results suggest that the reforming zones are almost distinctly separate [42]. Measurements along the Ni catalyst bed show noticeable high-temperature readings at the front, which then decrease as the flow prog-resses down the length of the reactor.
Under oxidizing atmospheres, Ni0 has a strong thermodynamic tendency to be con-verted into Ni2þ[36,45]. It has been found that this oxidized form of Ni is known to be active for combustion reaction, but not for reforming reactions[36,41,42,46]. Therefore, it is speculated that the Ni at the bed inlet is readily oxidized by the feed gas to NiO, which promotes the combustion reaction, and consequently produces a significant temperature rise in that region [41]. In the remainder of the reactor (oxygen-depleted zone), reforming reactions (mainly SR and CO2) occur over reduced Ni.
Noble metals such as Pt are not easily oxidized and are more stable in metallic form even in a highly oxidizing environment[42,47].
Consequently the combustion and reforming zones overlap more for these metals because both sets of reactions can take place on the same catalyst surface [42]. Peak temperatures are therefore reduced due to the overlap between reaction zones, and the resulting temperature profiles are less severe compared with those for Ni-based catalysts[36,42,48,49].
A study by Li et al. [36] found the overlap-ping effects between catalyst zones to be in the order Rh > Pt >> Pd for Group VII metals, with the latter, Pd, showing minimal overlap.
It is speculated that because Rh is a much more active reforming catalyst than oxidation catalyst (compared with other noble metals such as Pt and Pd) a lower temperature gradient was seen during OSR reaction and Rh was therefore reported to be a better OSR catalyst than Pt and Pd[36,44]. The difference in temper-ature profiling for metals like Ni (zones do not overlap) and noble catalysts (zones overlap) can be shown schematically inFig. 6.10.
FIGURE 6.10 Model scheme of the effect of oxygen addition to steam and dry reforming of methane on the temperature profile of the catalyst bed. (a) Ni catalysts, (b) Pt catalyst;Reprinted from Tomishige et al.[42], Copyright (2004), with permission from Elsevier.
6. OXIDATIVE STEAM REFORMING
140
The combustion-reforming mechanism de-scribing the OSR reaction process has also been studied by varying the contact time [37,50].
While running at short contact times (0.15 ms), Sato et al.[50]saw high selectivity to CO2and low fuel conversion (w20%) during the OSR of n-C4H10(O/C¼ 1.0, S/C ¼ 1.0, and 450C) using a Ni/MgO catalyst. Increasing the contact time from 0.15 to 1 ms resulted in a decrease in CO2
selectivity, while improving the selectivities of both H2and CO, and also conversion (w95%).
Collectively these results suggest that the first step in OSR is a highly exothermic combustion reaction, which is then followed by reforming reactions to produce synthesis gas. At the shorter contact times, the high CO2 selectivities indi-cated that combustion was the main reaction.
Meanwhile, the endothermic reactions forming synthesis gas are kinetically restricted at the higher gas velocities because of their much slower reaction rates. However, the observed increase in synthesis gas production and corre-sponding decrease in CO2 selectivity begins to occur as the gas velocities become more suitable for the reforming reactions, suggesting the reac-tions producing H2 and CO occur after the combustion reaction is complete. Also, CO selec-tivity decreased by increasing the contact time (to 10 ms) likely due to water-gas shift and methanation reactions.
Researchers [33,51e55] at the University of Minnesota found that spatially resolved concen-tration profiles coupled with temperature pro-filing can be used to help understand the reaction chemistries occurring during OSR and POX. Results from the spatially resolved species and temperature profiles measured during the catalytic partial oxidation (CPOX) of methane (CPOX results are shown here because of more clarity in H2O and CO2profiles compared with OSR results) over a Rh/Al2O3catalyst (Fig. 6.11) also confirms that the reaction proceeds through two zones in series, which were designated as exo- and endothermic reforming zones. Their results confirm that oxygen was consumed early
and rapidly over a short portion of the bed, roughly 1.3 mm for the Rh catalyst. This length is dependent on the catalyst metal used. Under the same conditions, the length of the oxidation zone was 2.3 mm for a Pt catalyst[53].
However, the observed trends in gas compo-sitions down the length of the bed from the spatial profile suggest the overall reforming chemistry differed slightly than that commonly hypothesized for the combustion reforming mechanism. Their results found the formation, and role of CO2reforming to be minimal. The major products formed through the conversion of CH4 with O2 were H2, CO, and H2O. With FIGURE 6.11 Schematic of mechanism: zone length depend on the type of catalyst;Reprinted from Horn et al.[53], Copyright (2007), with permission from Elsevier.
MECHANISM 141
the oxygen levels depleted, steam was found to be the clear co-reactant with CH4, not CO2. The steam concentration reached a maximum at the end of the oxidation zone, but began to disap-pear, along with CH4, as H2and CO formation increased down the length of the bed.
6.3.1.1. Effect of O/C and H2O/C Ratios Increasing the O/C carbon ratio above>1.0 at a constant S/C ratio in OSR begins to selectively produce combustion products. The oxidation of H2 appears to be more favorable than that of CO, likely because the smaller H2molecule faces fewer diffusional resistances in the boundary layer. The result of H2oxidation is a lower H2/ CO ratio in the reformate. The increase in the reactor temperature accompanying the oxida-tion also facilitates the reverse WGS reacoxida-tion, which further reduces the H2/CO ratio.
Increasing the S/C ratio at a constant O/C ratio will increase the H2/CO ratio for two reasons: (1) higher H2O concentrations favor the WGS reac-tion and (2) excess H2O will increase the IR absorptivity of the gas, thereby lowering the reactor temperature, which is favorable for WGS. Interestingly, increasing either O/C or S/C does not affect the peak temperature location, but rather the peak temperature and the outlet bed temperature [33,40,44]. The water addition increases cooling in the endothermic reforming zone, and it behaves as a diluent in the combus-tion zone. Therefore, the peak temperature loca-tion is not affected by the steam addiloca-tion, but the outlet temperature of the reactor bed decreases as S/C ratio increases. Steam addition also does not affect the O2 conversion in the combustion zone. The combustion zone length and overall CH4conversion were observed to be independent of steam to carbon ratio for methane OSR[33].