(133f) Comparison of Sodium and Potassium Carbonates as Lithium Zirconate Modifiers for High-Temperature Carbon Dioxide Capture From Biomass-Derived Synthesis Gas

After cleaning and conditioning, residual carbon dioxide in biomass derived process syngas must be removed before its use in mixed alcohol or other fuel synthesis. Previous studies have shown that CO2 removal is expensive with the capital cost of removing CO2 using an amine-based scrubbing system estimated at 20% of the total plant cost with an even larger associated energy cost. For thermochemical biomass to ethanol, about 25 tons of CO2 must be removed each hour requiring 100 mmBtu/h of process heat. Since process heat is provided by burning raw syngas, the yield of ethanol is impacted since less syngas is available for conversion. Solid-phase sorbents (SPS) can remove CO2 with potentially much smaller energy and yield penalties using temperature-swing processes. Depending on the solid used, CO2 removal can be done at relatively low or high temperatures. The current thermochemical ethanol process has locations where high and low temperatures are available for CO2 removal. Within this process context, novel solid-phase sorbents were investigated for the removal of CO2 from a N2/CO2 gas stream using a CO2 concentration similar to that found in a biomass gasification process. Thermal gravimetric analysis was used to evaluate the absorption rates from sorbents composed of lithium zirconate (Li2ZrO3), as well as mixtures of Li2ZrO3 with potassium carbonate (K2CO3) and sodium carbonate (Na2CO3). Results showed that Li2ZrO3 had a low CO2 absorption rate, but sorbents containing combinations of Li2ZrO3 and the K2CO3 and Na2CO3 additives had high uptake rates. The CO2 absorption and regeneration stability of the solid-phase sorbents were also examined. A sorbent composed of Li2ZrO3 and Na2CO3 was shown to be stable, based on the consistent CO2 uptake rates. Sorbents prepared with a mixture of Li2ZrO3, K2CO3, and Na2CO3 showed instability during regeneration cycles in air at 800°C. Sorbent stability improved during regeneration cycles at 700°C. In summary, it was shown that Li2ZrO3 mixed with Na2CO3 gives a CO2 uptake that is comparable to that of the Li2ZrO3 and K2CO3 mixture. It was also shown that there is an optimum mixture of both carbonates that gives a better uptake rate than either carbonate by itself. These results support the use of solid-phase sorbents as a way to remove CO2 from syngas.