Role of catalysts in biomass gasifcation with carbon dioxide
Gasification of biomass with CO2 is a clean technology to produce energy and bio-fuels while recycling the greenhouse gas CO2. However, reaction between char (C) and CO2 is endothermic and requires high temperatures. The motivation behind this work is to improve gasification rates by using catalysts to make CO2 recycling via gasification energy efficient. Catalytic gasification with CO2 can become viable if energy efficient processes can be designed. Availability of biomass without affecting the food supply and regeneration of tillable soil are two of the major practical challenges that are not explicitly considered in this work. Instead, the focus is on improving efficiencies once the availability of biomass is ensured by selection of locally available nonedible materials: (1) pine wood and (2) corn stover. The available biomass must first be dried and pyrolyzed for reduction of CO2. The CO2 reduction process can occur with or without a catalyst. The catalyst may be added before or after pyrolysis of biomass (charification). Pinewood gasification experiments are used to investigate the effects of the method of sample preparation on gasification rates. Four different methods of sample preparation are used—1) pyrolyzing sawdust without a catalyst (baseline); (2) impregnating sawdust with catalyst, pyrolyzing and then removing catalyst by leaching with distilled water; (3) impregnating sawdust with catalyst and then pyrolyzing; and (4) pyrolyzing sawdust and then impregnating the char with catalyst. The results show that the catalyst enhanced the gasification rates whether it is added before or after charification. Therefore, the process of adding the catalyst before charification is selected in order to minimize the inefficiencies associated with cooling of the char for addition of the catalyst. Corn stover CO2 gasification rates at 1.25 atm pressure and 950, 1000, and 1050 K temperatures and 10 atm pressure and 1000, 1060, and 1110 K temperatures with and without catalysts are examined. Five different samples are prepared by milling and screening corn stover. Four of the five samples are treated with a catalyst using the wet impregnation technique (method 3 described above in the context of pinewood gasification). Enhancement of gasification rates by four different catalysts: K2CO3, Na2CO3, Ca(OH)2 and Fe(NO3) 3 is studied by comparison with the baseline case of gasification rate without any catalyst. Char-sample conversion, relative initial reactivity and apparent reaction rates are calculated to compare reactivity and effectiveness of the different catalysts. Activation energies based on apparent reaction rates are estimated. The results of the present work show that CO2 can be recycled using biomass gasification particularly with the use of an appropriate catalyst. Furthermore, the process can be effectively conducted at atmospheric pressure. While K2CO3 is found to be the most effective catalyst, Na2CO3 is recommended for further investigation since it is easily and plentifully available. The catalyst Ca(OH)2 was found to be the least effective for the present conditions. The catalyst Fe(NO 3)3 exhibited an initial high activity period but showed significant reduction in activity over the overall gasification period. Future work may involve an examination and prevention of the deactivation making widely available Fe based catalysts viable. Similarly, methods to reduce CO 2 in the presence of H2O should be investigated in future studies. Demonstration of Na2CO3 at practical scales is the most logical next step.
Gore, Purdue University.
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