(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 800oC for CO2 emission even in nitrogen (N2). Since equilibrium shift can reduce the reforming temperature from the conventional 800oC to 600oC, 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 200oC. 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.
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