Selection, Preparation, and Performance of High Temperature Novel Co2 Sorbents

Kwang B. Yi1, Jong-Nam Kim1, Julien Meyer2, and Dag Eriksen2. (1) Separation Process Research Center, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-gu, Daejeon, 305-343, South Korea, (2) Environmental Technology Department, Institute of Energy Technology, P.O. Box 40,, Kjeller, 2027, Norway

Sorption-enhanced reforming (SER) accomplishes reforming, shift, and purification in a single processing step. The reactions occur simultaneously in the presence of reforming catalyst and a CO2 sorbent (designated as A). The simultaneous and overall reactions are:

Reforming: CH4(g) + H2O(g) = CO(g) + 3H2(g)

Shift: CO(g) + H2O(g) = CO2(g) + H2(g)

CO2 removal: A(s) + CO2(g) = ACO2(s)

Overall: CH4(g) + 2H2O(g) + A(s)= ACO2(s) + 4H2(g)

Removal of CO2 takes very important role in this reaction. It shifts the normal equilibrium limits of the reforming and shift reactions and permits high CH4 conversion with almost complete removal of CO and CO2. Recently, importance of selecting this high temperature CO2 sorbent has gained attention with respect to performing long-term multi-cycle process. Currently, most widely used high temperature CO2 sorbent is dolomite. It is a natural mineral and its one of the attractive features is cheap price. However, natural dolomite normally contains small amount of sulfur, which is enough deactivate catalyst completely within few cycles. In order to utilize dolomite, therefore, energy intensive pretreatment has to be performed prior to loading. In addition, it has tendency to decrease its CO2 capacity significantly through multi-cycle. Recently, new generation of high temperature CO2 sorbents such as Li4SiO4, Li2ZrO3, K doped Li2ZrO3, and Na2ZrO3 were developed sorbents. However, multi-cycle durability has not been proved using Li4SiO4. In addition, Li2ZrO3 has showed extremely kinetics. Also, when K is doped on Li2ZrO3, one may obtain faster kinetics but will lose CO2 capacity due to the inert portion occupied by K2CO3. Na2ZrO3 showed fast kinetic and stable weight gain but original weight is not obtained after first regeneration. In this study, new CO2 sorbents including those listed above were selected through thermodynamic screening process using commercial software (HSC chemistry). Alkali metals such as Li, Na, and Ca were combined with other metal oxides and resulted compounds were put in condition of SER. Then, hydrogen yields were obtained through thermodynamic calculations. The compounds providing more than 95% of hydrogen yield in the range of 500oC to 700oC were selected as potential candidates. The selected materials were Li2ZrO3, Na2ZrO3, Na2Fe2O4, Na4SiO4, Ca2Al2O5, and Ca3Al2O5. These six materials were prepared in various ways in our laboratory. Absorbents were prepared by two major methods. One is solid-solid reaction method; the other is liquid based method with various precursors. Most of materials were prepared successfully and its chemical compositions were identified with XRD. The surface morphology was identified using SEM. Then, prepared materials were tested in a thermogravimetric analyzer (TGA). While Li2ZrO3 prepared using solid-solid reaction showed very slow kinetics, Li2ZrO3 prepared using liquid based method showed faster kinetics (38 mgCO2/gmin), large capacity (26g CO2/ 100g sorbent) and stable multi-cycle results. Its CO2 uptake rate was faster than any other Li2ZrO3 reported. Also, small particle size of Li2ZrO3 enabled avoiding doping the absorbent with K. Na2ZrO3 showed similar CO2 uptake rate with the both preparation methods. However, liquid based preparation provided full recovery of original weight with fast kinetics after regeneration while the one with solid based preparation method showed 10% of loss of its capacity. Furthermore, rests of materials were tested and test results showed general favor of liquid based preparation method.