Towards H2-rich gas production from unmixed steam reforming of methane: Thermodynamic modeling

摘要

In this work, the Gibbs energy minimization method is applied to investigate the unmixed steam reforming (USR) of methane to generate hydrogen for fuel cell application. The USR process is an advanced reforming technology that relies on the use of separate air and fuel/steam feeds to create a cyclic process. Under air flow (first half of the cycle), a bed of Ni-based material is oxidized, providing the heat necessary for the steam reforming that occurs subsequently during fuel/steam feed stage (second half of the cycle). In the presence of CaO sorbent, high purity hydrogen can be produced in a single reactor. In the first part of this work, it is demonstrated that thermodynamic predictions are consistent with experimental results from USR isothermal tests under fuel/steam feed. From this, it is also verified that the reacted NiO to CH4 (NiOreacted/CH4) molar ratio is a very important parameter that affects the product gas composition and decreases with time. At the end of fuel/steam flow, the reforming reaction is the most important chemical mechanism, with H2 production reaching ~75 mol%. On the other hand, at the beginning of fuel/steam feed stage, NiO reduction reactions dominate the equilibrium system, resulting in high CO2 selectivity, negative steam conversion and low concentrations of H2. In the second part of this paper, the effect of NiOreacted/CH4 molar ratio on the product gas composition and enthalpy change during fuel flow is investigated at different temperatures for inlet H2O/CH4 molar ratios in the range of 1.2-4, considering the USR process operated with and without CaO sorbent. During fuel/steam feed stage, the energy demand increases as time passes, because endothermic reforming reaction becomes increasingly important as this stage nears its end. Thus, the duration of the second half of the cycle is limited by the conditions under which auto-thermal operation can be achieved. In absence of CaO, H2 at concentrations of approximately 73 mol% can be produced under thermo-neutral conditions (H2O/CH4 molar ratio of 4, with NiOreacted/CH4 molar ratio at the end of fuel flow of ~0.8, in temperature range of 873-1073 K). In the presence of CaO sorbent, using an inlet H2O/CH4 molar ratio of 4 at 873 K, H2 at concentrations over 98 mol% can be obtained all through fuel/steam feed stage. At 873 K, carbonation reaction provides all the heat necessary for H2 production when NiOreacted/CH4 molar ratio reached at the end of fuel/steam feed is greater or equal tol. In this way, the heat released during air flow due to Ni oxidation can be entirely used to decompose CaCO3 into CaO. In this case, a calcite-to-nickel molar ratio of 1.4 (maximum possible value) can be used during air flow. For longer durations of fuel/steam feed, corresponding to lower NiOreacted/CH4 molar ratios, some heat is necessary for steam reforming, and a calcite-to-nickel molar ratio of about 0.7 is more suitable. With the USR technology, CaO can be regenerated under air feeds, and an economically feasible process can be achieved.