Accumulation of inorganic impurities on HZSM-5 during catalytic fast pyrolysis of switchgrass

Authors: Mullen, Charles; Boateng, Akwasi

Submitted to: Journal of Industrial and Engineering Chemical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2013
Publication Date: 12/19/2013
Citation: Mullen, C.A., Boateng, A.A. 2013. Accumulation of inorganic impurities on HZSM-5 during catalytic fast pyrolysis of switchgrass. Journal of Industrial and Engineering Chemical Research. 52:17156-17161.

Interpretive Summary: In order to meet the renewable fuels standards set by the US government, 21 billion gallons of advanced and cellulosic bio-fuels will need to be used in transportation fuels by 2022. The largest source of feedstock to produce these advanced biofuels is lignocellulosic biomass, including woody materials, herbaceous grasses (e.g. switchgrass) and crop residues (e.g. corn stover, straws). One promising process to convert biomass to a liquid is fast pyrolysis which produces a product called pyrolysis oil (bio-oil) which can be refined to “green” gasoline and diesel fuels that are indistinguishable from those produced from petroleum. However, pyrolysis oil is currently incompatible with petroleum for refining because it is corrosive and unstable, meaning its viscosity greatly increases during storage creating processing problems. Chemically, these unfavorable properties are due to the fact that pyrolysis oil, unlike petroleum, consists of reactive oxygenated molecules. One method to reduce the oxygen content of pyrolysis oil is to incorporate a catalyst material in the pyrolysis process. The catalyst acts to change the chemistry to remove the oxygen from the molecules that make up the pyrolysis oil resulting in hydrocarbons, the types of compounds found in petroleum. However, catalysts can be poisoned by certain components in biomass including inorganic components such as calcium, potassium, etc. Catalyst poisons reduce the usable life of the catalyst, meaning it will need to be replaced more often adding costs to the process. We studied the tolerance of a popular pyrolysis catalyst to the calcium, copper, potassium, iron, magnesium, sodium and phosphorus in switchgrass. We found that these metals accumulate on the catalyst quickly. This information is important to those designing catalytic pyrolysis systems so they can design systems to minimize exposure of pyrolysis catalysts to these components of switchgrass or other biomass feedstocks.

Technical Abstract: The fate of inorganic species present in switchgrass during fluidized bed catalytic pyrolysis over HZSM-5 catalysts was studied with emphasis on their accumulation on the catalyst. Five catalytic pyrolysis experiments were performed in two series, reusing the catalyst after each sample. Catalysts were regenerated from deactivation due to carbon deposits via their removal by combustion at regular intervals. The fates of seven inorganic elements were tracked: Ca, Cu, Fe, K, Mg, Na, and P. Potassium accumulated with the HZSM-5 catalyst the fastest while iron was the metal that accumulated most preferentially on the catalysts, with about 42% of the Fe content of the biomass remaining with the HZSM-5. The total amount of these seven elements accumulated on HZSM-5 in a linear fashion during continued use of the same catalysts sample. As the catalyst was exposed to more switchgrass and accumulated more inorganic containments, its effectiveness at deoxygenating the pyrolysis liquids decreased, and selectivity for aromatic hydrocarbons also decreased, indicating that deactivation of HZSM-5 by exposure to inorganic contaminants in biomass should be a consideration for biomass deoxygenation by catalytic fast pyrolysis.