(380g) Improvement of Methane Steam Reforming by Using Equilibrium Shift with Lithium Silicate

The major process for hydrogen (H₂) production is the conversion of methane (CH₄) via steam reforming. However, growing demand for H₂ in various sectors means that a more efficient process is required. Equilibrium shift is thought to be effective for promoting H₂ production by CH₄ steam reforming. There are two methods of bringing about the shift: one uses membrane for H₂ removal and the other uses CO₂ absorbent for selective removal of by-product CO₂ from the reaction zone. In the latter method, a packed bed reactor with a mixture of reforming catalyst and absorbent is used. In this case, following CO₂ capture in the reforming step, absorbent is required to emit CO₂ in the regeneration step. Calcium oxide (CaO), a well-known absorbent, requires higher temperature of around 800^oC for CO₂ emission even in nitrogen (N₂). Since equilibrium shift can reduce the reforming temperature from the conventional 800^oC to 600^oC, another absorbent which has a lower CO₂ emission temperature is required in order to improve the total energy efficiency of the H₂ production system.

On the other hand, Toshiba has developed a lithium silicate (Li₄SiO₄) absorbent that has the properties of absorbing and emitting CO₂ quickly. In particular, in the case of Li₄SiO₄, CO₂ emission occurs at considerably lower temperature than in the case of CaO absorbent. The difference in CO₂ emission temperature between the two absorbents is approximately 200^oC. Therefore, using Li₄SiO₄, which brings about both reforming and CO₂ emission at lower temperature, a highly efficient H₂ production system is expected to be established.

The effect of equilibrium shift on CH₄ steam reforming by using Li₄SiO₄ has already been confirmed at atmospheric pressure, resulting in the H₂ concentration of ca. 94 vol% and CO concentration of ca. 0.2 vol%. For a practical purified H₂ production system, pressurized H₂ gas is required in order to connect to purifier, for example, by pressure swing absorption (PSA).

In this study, the effect of equilibrium shift with Li₄SiO₄ at pressurized condition was investigated. The strong influence of pressure on the equilibrium shift was observed. The concentration of H₂ increased to ca. 97 vol%, and that of CO decreased to ca. 0.1 vol% at 300 kPa. It was found that higher H₂ concentration and much lower CO concentration were obtained even under pressurized condition.