Catalytic pyrolysis of olive mill wastes towards advanced bio-fuels and bio-chemicals using metal oxide catalysts

Abstract

Among the environmental challenges that Mediterranean countries have to address are the abundant amounts of olive mill wastewater (OMWW) produced by olive oil agro-industry, such as those found in Greece and Tunisia, which lead to significant land and water pollution. The most common disposal method of olive mill wastewater, includes storage ponds, where most of the water is evaporated, while the residual sludge (olive mill wastewater sludge, OMWWS) is sent to landfills. Fast pyrolysis of OMWWS was investigated as an alternative approach of valorizing this waste, leading to the production of high heating value biofuel (bio-oil). Extraction of polyphenolic compounds with bioactive properties from OMWWS was additionally explored in order to provide further added value to OMWWS, considered as a source of bioactively important polyphenolic compounds, prior its use as pyrolysis feedstock towards the production of green fuel. Pyrolysis of the treated OMWWS (after extraction) led to a considerable reduction of the bio-oil yield, however in both cases the bio-oil produced exhibits a very interesting composition (predominance of aliphatic hydrocarbons and small fractions of aromatic hydrocarbons and phenols) and sufficiently low oxygen content. Exploring the effect of either an acidic (ZSM-5) or a basic (MgO) catalyst on the yield and composition of the bio-oil produced during the catalytic pyrolysis of OMWWS we observed that yield and quality of the produced bio-oils strongly depended on the catalyst used. Thus, acidic ZSM-5 zeolite catalyzed cracking and aromatization reactions, resulting in a bio-oil enriched in aromatics, while basic MgO catalyzed decarboxylation of fatty acids yielding a bio-oil composed mostly of aliphatic hydrocarbons. In addition, Co-promoted catalysts (5% Co either on ZSM-5 or MgO) led to additional variation of bio-oil quality, mostly enhancing ketonization reactions. The effect of a series of Fe and Mn oxide catalysts, based on alumina, silica and magnesia supports, was also studied for the catalytic pyrolysis of OMWWS, mainly focusing on using low-cost catalytic materials. In addition, ferric oxide, aluminum oxide, magnesium oxide, and silicon oxide, are among the major components of red mud, an effective catalyst used in the pyrolysis of OMWWS. All of these Fe- or Mn- materials exhibit very promising in situ upgrading of pyrolysis vapors (retaining organics’ yield and improving produced bio-oil). Most of these materials also tend to favor ketonization reactions, a more or less preferred route among the studied catalysts, which seems to mainly relate with their multifunctional (both acidic and basic) character.