(167h) Utlilization of Char From Biomass Gasification for Catalytic Processing of Tars

Char is a by-product of gasification and has the potential to be used in catalytic applications.  It is a promising candidate for catalyzing the destruction of tar, which is another by-product of gasification.  Tar is typically decomposed catalytically, however, tar reforming causes rapid catalyst deactivation.  For this reason, it is expensive to use commercial metal catalysts for treatment of tars, since the catalysts will need to be replaced or regenerated frequently.  Char, however, is produced on site (in the gasifier) and therefore can easily be replaced as it deactivates.  Gasification conditions (co-reactant, temperature, residence time, etc.) can vary significantly, which will influence the physical and chemical properties of the char. This research focuses on demonstrating the catalytic activity of char from biomass gasification and understanding which char properties give rise to its catalytic activity. 

The char studied in this work was produced from gasification of poplar wood chips.  The biomass was gasified under CO₂ and steam at temperatures ranging from 550-920^oC in a fluidized bed reactor.  The char was recovered and its catalytic properties and performance were investigated.  The char catalyzed the decomposition of methane, propane, and toluene, which is a major component in gasification tars.  The catalytic performance of the char was compared to a commercial precious metal catalyst and the char had a lower light off temperature and higher extent of reaction than the commercial catalyst.

The char is primarily composed of carbon (~85%) and also contains oxygen (~7%) and trace amounts of many inorganic elements (ex.  Ca, P, K, Na, Fe, Ni, Al, etc.).  The influence of each of these components on the catalytic activity of the char was studied.  The role of inorganics was studied by removing the inorganics from the char via an acid washing treatment (no other properties were modified from this treatment).  It was found that removal of inorganic elements reduced the catalytic performance of the char, so the inorganics play a role in its catalytic activity.  In addition, the inorganic elements were found to be highly dispersed, and lowering the dispersion decreased the catalytic activity. The presence of oxygen-based functional groups on the surface of the char was observed, and these functional groups are often considered to be important in the catalytic activity of carbon based catalysts (such as activated carbon).  However, the oxygen functional groups desorbed at temperatures below 675^oC which is lower than the temperature where hydrocarbon cracking reactions take place.  Therefore, the catalytic performance was not influenced by oxygen functional groups for the reactions tested here.  In order to understand the role of carbon, the carbon was removed (burned off), creating ash, and the catalytic activity of the ash was compared to the raw char.  The activity of the ash was lower than the char.  Therefore, the catalytic activity is improved by the presence of carbon, as well as the highly dispersed inorganic elements in the char.

Gasification conditions influenced the morphology of the char.  The surface area and micropore volume were measured for char samples that were made in different gasification conditions.  Surface area ranged from 429-687 m²g^-1.  Higher gasification temperature and longer residence times created chars with higher surface areas.  In addition, the gasification environment influenced the porosity.  Gasification in CO₂ produced char with more micropores (with diameter <1nm) compared to gasification with steam.  As expected, the catalytic activity increased with increasing surface area.  However, diffusion limitations were observed in the pores, leading to lower catalytic activity for chars with micropores, compared to non-microporous char with the same surface area.  After the char was used as a catalyst the surface area was reduced by 20% and the pore volume was reduced by 30%, indicating that pore blocking is a mechanism of catalyst deactivation. 

Calculations were done to determine if the quantity of char produced from a gasifier would be sufficient to reform all of the tar from the process. Based on the demonstrated catalytic activity of the char, and the deactivation profile of the char, the quantity of char is sufficient to reform all of the tar from a given gasifier.  Therefore, utilization of char as a catalyst for tar reforming presents an economical and convenient solution for the reforming of tars, which remains one of the most significant issues with gasification processes.   The activity of the char is attributed to the high surface area of the char, and the presence of carbon with dispersed inorganic elements.