Catalytic upgrading of pyrolytic oils over HZSM-5 zeolite: behaviour of the catalyst when used in repeated upgrading–regenerating cycles

doi:10.1016/S0016-2361(00)00063-6

Fuel

Volume 80, Issue 1, January 2001, Pages 17-26

https://doi.org/10.1016/S0016-2361(00)00063-6 Get rights and content

Abstract

The behaviour of HZSM-5 zeolite in the upgrading of a wood pyrolysis oil produced in the ENEL fast-pyrolysis plant located in Bastardo, Italy, was studied in repeated upgrading–regenerating cycles. The HZSM-5 zeolite performs a catalytic activity by its acidic sites that, through a carbonium ion mechanism, promote deoxygenation, decarboxylation and decarbonylation of the oil constituents, as well as cracking, oligomerisation, alkylation, isomerisation, cyclisation and aromatisation. As a consequence of the catalytic process, coke and tar were also obtained as undesirable by-products. The continued regeneration of the zeolite, consisting of removal of the coke deposits by air at 500°C, reduced the effectiveness of the catalyst in converting biomass pyrolysis oils to an aromatic product, until an irreversible deactivation was observed. By the analysis conducted on the catalyst it was possible to assess that the loss of activity is mainly connected to the disappearance of a significant amount of acidic sites, mainly the stronger ones, due to the thermal cycling to which the catalyst was submitted. Even if the regeneration was conducted at 500°C, localised raisings of temperature above 500°C due to the combustion of coke may have caused dehydroxylation of the Brønsted acid sites that predominate in zeolites activated at 500°C with formation of Lewis acid sites. Thus, the active acid sites in the upgrading reactions are presumed to be preferentially Brønsted acid sites, which were gradually deactivated by the repeated regeneration treatments.

Introduction

The oils produced by the pyrolysis of biomass, due to the high levels of oxygen, may be highly viscous and corrosive, relatively unstable, and may exhibit a poor heating value [1], [2], [3], [4], [5], [6]. The upgrading of pyrolytic oils, a necessary process before they can be used as a regular fuel, essentially involves the removal of oxygen. The low-pressure upgrading of biomass pyrolytic oils over acidic catalysts, such as zeolites or silica–alumina [7], [8], [9], [10], [11], [12], is deriving an increasing interest as the alternative route to the catalytic hydrotreating with hydrogen, or hydrogen and carbon monoxide under high pressure, and/or in the presence of hydrogen donor solvents [2], [13], [14], [15], [16]. This study follows a preliminary investigation on the capability of HZSM-5 zeolites in deoxygenate wood fast pyrolysis oils [17]. The HZSM-5 zeolites perform a catalytic activity by their acidic sites that, through a carbonium ion mechanism, promote deoxygenation, decarboxylation and decarbonylation of the oil constituents, as well as cracking, oligomerisation, alkylation, isomerisation, cyclisation and aromatisation. A mostly aromatic product was obtained in higher yields at 450°C. As a consequence of the catalytic process, coke and tar were also obtained as undesirable by-products.

In this work, specific attention was focussed on the effect of regeneration on the activity and selectivity of HZSM-5 catalysts, that were used in repeated upgrading–regenerating cycles. This was accomplished by determining the trend of the yield and composition of the upgraded product with time, and by observing the change of the catalysts specific surface area and acidic sites content in the different cycles up to their complete deactivation.

Section snippets

Raw bio-oil

The raw pyrolytic bio-oil was sampled from the ENEL fast-pyrolysis plant located in Bastardo, Italy, which is based on the ENSYN RTP process [18]. The flow-sheet of the pyrolysis plant is reported in Fig. 1. The biomass fed to the pyrolyser was dried Swedish pine (residual moisture about 3% by weight). The operating conditions of the pyrolyser were the following: pyrolysis temperature, 456°C; first condenser temperature, 47°C; second condenser temperature, 42°C; pyrolyser velocity, 4.2 m/s.

The

Characterisation of the raw bio-oil

The properties of the raw bio-oil studied, reported in Table 1, appears to be in line with the typical composition of flash-pyrolysis oils, characterised by quite high water and oxygenated compounds contents and, as a consequence, by a low heating value, a significant acidity and a relatively high viscosity.

The IR spectrum, shown in Fig. 3, confirms the presence of alcohols (O–H stretching at 3400 cm⁻¹, O–H out-of-plane bending at 610 cm⁻¹, C–O stretching at 1051 cm⁻¹); and of ketonic functional

Conclusions

Pyrolytic oil upgrading over HZSM-5 zeolites produced a highly deoxygenated-mainly aromatic oil; the best oil yield was obtained at 450°C by using HZSM-5/50 catalyst. The deposition of coke and tar on the catalyst caused a gradual decrease of the activity of the catalyst up to complete poisoning. Beginning from the first regeneration, a gradual decrease of the regenerated catalyst activity was observed up to an irreversible poisoning after the fifth upgrading–regenerating cycle. By the analysis

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Catalytic upgrading of pyrolytic oils over HZSM-5 zeolite: behaviour of the catalyst when used in repeated upgrading–regenerating cycles

Abstract

Introduction

Section snippets

Raw bio-oil

Characterisation of the raw bio-oil

Conclusions

Fuel Processing Technology

Fuel

Fuel

Fuel

Journal of Catalysis

Coal Science Technology

Thermochemical Biomass Process. Bio-Oil Prod. Util

Biomass Bioenergy

Fuel Processing Technology