2013 Theses Doctoral

Utilization of Char from Biomass Gasification in Catalytic Applications

Klinghoffer, Naomi

Utilization of biomass as an energy source is likely to increase in the near future. One way to recover energy from biomass is via gasification, which enables the production of electricity, heat, chemicals, or fuels such as synthetic natural gas or gasoline. The desired product from gasification is synthesis gas, which is a mixture of CO and H2; however by-products such as tar and char are formed. The tars must be decomposed or removed, as they can cause clogging in downstream equipment. Tars are most commonly decomposed catalytically or thermally. However, thermal decomposition requires high temperatures, and catalyst deactivation takes place during catalytic decomposition. This thesis focuses on the utilization of char as a catalyst for tar decomposition. Char has a surface area that is higher than many typical catalysts, and contains catalytic minerals and metals which are well dispersed on the surface. Using char in this application would eliminate the need for purchasing expensive catalysts, and deactivation would not be a concern since deactivated char could be easily replaced by fresh char which is produced inside the gasifier. In addition, it provides a useful application for the char, which would otherwise be considered to be a low value product. In this work, poplar wood was gasified in a fluidized bed reactor under steam and CO2 at 550, 750, and 920C for different periods of time. The char was recovered from the fluidized bed, and its properties were studied. The BET surface area of the char ranged from 429-687 m^2 g^-1 and increased with increasing gasification temperature or time. In addition, micropores were observed in char that was made in CO2, but not in char that was made in steam. Gasification was also done in an ESEM under air, steam, and CO2. ESEM results showed sintering of the metals and minerals on the char surface during gasification in air and steam, but sintering was not observed during gasification with CO2. This showed that the properties of char depend on the gasification conditions. Catalytic activity of the char was demonstrated for decomposition of methane, propane, and toluene, which is a major component of gasification tar. The light off temperature for methane decomposition using a char catalyst was 100C lower than the light off temperature when a commercial Pt/Al2CO3 catalyst was used. Higher surface area char had higher catalytic activity. However, microporous char had lower catalytic activity than non-microporous char with a similar surface area, indicating that diffusion limitations occur in the micropores, reducing access to these catalytic sites. Deactivation was observed during catalytic cracking of CH4. A 20% reduction in surface area and 33% reduction in mesopore volume were observed when comparing the used char catalyst to the fresh sample. This indicates that deactivation occurs via pore blocking. Kinetic analysis of the data showed a steeper deactivation function for mesoporous char that was made in H2O compared to microporous char that was made in CO2. A steeper deactivation function is indicative of a higher number of catalyst sites per pore, since once a pore becomes blocked all of the catalytic sites within the pore will become inaccessible. Therefore, char made in steam, which is mesoporous, has more accessible catalyst sites per pore. The char morphology influences its catalytic activity, which increases with increasing accessible surface area. The accessible surface area of the char depends on both the surface area and the porosity of the char. Carbon based materials such as chars have been used in low temperature catalytic applications. In these applications, the catalytic activity is attributed to the presence of oxygen groups on the surface. Therefore, in this thesis the role of oxygen groups in the catalytic activity of the char for high temperature applications was investigated. Temperature programmed desorption (TPD) was used to identify the types of oxygen groups on the char surface and both acidic (lactone, carboxylic) and basic (pyrone, quinone) groups were identified. There were no significant differences in the concentration and type of surface oxygen groups amongst the different char samples. In order to understand the role that these compounds play in the catalytic activity of the char, oxygen was added to the surface of a char sample via nitric acid treatment and its catalytic performance was compared to the raw char. However, when the sample was heated in nitrogen to the reaction temperature (850C) prior to utilization for methane decomposition, the oxygen groups desorbed, and the catalytic activity of the oxygenated char was the same as the raw char. Therefore, the char has catalytic activity even when the acidic surface oxygen groups have been removed. The role of metals in the catalytic activity of the char was studied. Metals were removed via acid washing, and the catalytic activity of the acid washed char was compared to the untreated char. The catalytic activity of the acid washed char was 19% lower than the untreated char, which demonstrated that the presence of metals increases the catalytic activity of the char. The metals were found to be dispersed on the surface of the char. When the char was heated to 1000C, and was then used to catalyze the decomposition of CH4, the catalytic activity of the char was lower than the untreated sample. Therefore, the gasification process preserves the high dispersion of inorganic elements in the char, which improves the catalytic performance of the char. Char is often considered to be a by-product of gasification processes. However, this work has shown that char is a valuable product that has the potential to be used in catalytic applications. It has a surface area which is higher than many commercial catalysts, and contains metals and minerals which are catalytically active and are well dispersed on the char surface.