Production of an upgraded bio-oil with minimal water content by catalytic pyrolysis: Optimisation and comparison of CaO and MgO performances

https://doi.org/10.1016/j.jaap.2019.104751Get rights and content

Highlights

  • Comparison of CaO and MgO for catalytic pyrolysis in an induction-heating reactor.

  • Fractional condensation to obtain an organic-rich bio-oil with low water content.

  • Deoxygenation resulted in dry basis HHVs > 26 MJ/kg with both catalysts.

  • Organic-rich bio-oil at 19.4 wt.% yield and water content of only 7.8 wt.% with CaO.

  • Water gas shift reaction catalysed by CaO led to a 400 % increase in H2 production.

Abstract

Pyrolysis of forest residues using CaO and MgO catalysts was carried out in an induction heating reactor, to produce an upgraded bio-oil with increased higher heating value (HHV). Optimisation with central composite design (CCD) included temperature (444−656 °C) and catalyst concentration (1.7–58.3 wt.%). A fractional condensation system with the first condenser heated to 40 °C was used to collect an organic-rich oil. The optimum HHV of the oil was 26.9 MJ/kg (560.0 °C and 33.8 wt.% catalyst) at 19.4 wt.% yield with MgO and 26.4 MJ/kg (550.0 °C and 30.0 wt.% catalyst) at 18.9 wt.% yield with CaO (based on a desirability function used to obtain significant oil yield). The combination of water-gas shift reaction (WGSR) catalysed by CaO and the fractional condensation system resulted in water content of the oil at the optimum of only 7.8 wt.%. Absorption of CO2 by CaO, together with significant H2 production (400 % increase compared to non-catalytic case) allowed the production of a gas calorific value of 13.7 MJ/kg compared to 5.2 MJ/kg without catalyst.

Introduction

Pulp and paper industries all over the world have been interested in reducing the carbon footprint associated with their processes. In South Africa, carbon emissions from the pulp and paper industry have been reduced by using renewable energy resources such as biomass, that has been generated as a by-product of pulp production processes i.e. black liquor and paper waste sludge [1]. Energy is typically recovered from this waste biomass by combustion in stationary units such as boilers and turbines [1]. However, this cogeneration is limited to onsite use [2]. The fire hazard related to the presence of forest residues (tree tops and branches left behind in the forest after thinning and clear-felling operations) is another problem that the pulp and paper industry faces [3]. However, these forest residues have an untapped energy potential, which could increase energy production from biomass, while lowering carbon emissions from the pulp and paper industry [4,5]. Conversion through pyrolysis has the potential to contribute to the valorisation of forest residues.

Pyrolysis is the thermochemical conversion of biomass in an oxygen limited environment to produce a solid product (char) and volatiles. Part of the volatiles can be condensed under atmospheric conditions to give a liquid product (bio-oil), which can be conveniently stored and transported to a central site for further processing [6], while the uncondensed volatiles remain as permanent gases. Compared to conventional fossil fuels, biomass derived fuels produce fewer greenhouse gas emissions and, assuming sustainable forest management, have the potential to be carbon neutral, since the CO2 released during fuel combustion will be reabsorbed when the biomass is regrown [7,8].

The presence of oxygen in high content (35–40 wt.%) limits the suitability of bio-oils as a feedstock to petroleum refineries, because it gives them undesirable properties such as high acidity, low calorific value and oxidative instability (aging) [9]. Therefore, the quality of bio-oil needs to be improved by lowering its oxygen content, before it can be considered for use as a transportation fuel. Different methods to deoxygenate bio-oils such as hydrotreating, gasification and Fluid Catalytic Cracking (FCC), which can all be done in a dedicated bio-refinery, have been investigated to upgrade bio-oils. Although a dedicated bio-refinery is an attractive option, its construction is expensive and carries high economic risks [6]. A more economic short-term method would be to produce a higher quality bio-oil through an improved single pyrolysis step using catalysts [6]. The upgraded bio-oil could then be co-processed with vacuum gas oil (VGO) in a standard crude oil refinery via FCC to produce transportation fuels containing renewable carbon [[10], [11], [12]]. Capital expenses to modify existing petroleum refineries for bio-oil co-processing are likely to be much less than those of setting up a dedicated bio-refinery, as only the bio-oil feed line to the FCC unit will need to be added [6]. Two critical parameters for co-feeding are the water content of the bio-oil and the oxygen content of the organic fraction (the fuel precursor). To assess the latter, the oxygen content or HHV of bio-oil should be studied on a dry basis. A number of bio-oil co-processing studies have been carried out on both bench and pilot scales [11,13,14]. De Rezende Pinho et al. [12] showed at pilot scale that a raw bio-oil with a 32.8 wt.% oxygen content on a dry basis (31.9 wt.% water content) could be successfully co-processed at a 5 wt.% blending ratio (mass fraction of bio-oil in the feed to the FCC) to produce diesel and gasoline fractions. At a higher blending ratio (10 wt.%), increased deoxygenation of bio-oil would be needed. Thegarid et al. [10] showed that an upgraded bio-oil with oxygen content < 20 wt.% on a dry basis (11 wt.% water content) could be used for co-processing.

The use of bulk metal oxides catalyst during pyrolysis can promote deoxygenation of the volatile organics, resulting in a bio-oil with a lower oxygen content and higher HHV [15]. Among the bulk metal oxides, CaO and MgO have been reported as efficient for bio-oil deoxygenation [[16], [17], [18], [19], [20]]. At pyrolysis temperature in the range of 450−600 °C and catalyst concentration (Ccat) up to 85 wt.%, oxygen content of bio-oils could be reduced to values in the range of 20−25 wt.% [17,19]. Although there have been studies of calcined dolomite (CaO.MgO) as a catalyst [18,19], no direct comparison of MgO and CaO catalysts, including an optimisation work on the same experimental setup, have been previously reported in literature. As a consequence of deoxygenation, the yields of organic compounds were found to be significantly reduced with values lower than 20 wt.% often reported. In most cases, catalytic effect was found to increase water production, resulting in pyrolytic water yields in the range of 20–30 wt.% and bio-oil water contents > 50 wt.%. Therefore, appropriate separation methods are essential to control bio-oil water content, especially for catalytic systems. Isolation of the organic fraction can be done via several steps including centrifugation, solvent extraction, decantation, etc. [17,19,21]. Alternatively, the condensation system for hot volatiles in the pyrolysis set-up can be designed to obtain separate fractions that are rich in water or organics, with the latter more suitable for co-processing in a crude oil refinery without further treatment. Previous non-catalytic pyrolysis studies [22,23] have shown that increasing the temperature of the first volatiles condenser increases the fraction of organics collected, by reducing the water content. A similar fractional condensation system appears essential for catalytic pyrolysis.

While most of previous catalytic pyrolysis studies have used conventional electrical heating, induction heating appears as an interesting alternative. In induction heating, an alternating current of high frequency is passed through induction coils (the electromagnet) generating a magnetic field which heats up any ferromagnetic materials placed inside the induction coil [24]. Induction heating systems are advantageous in that they are highly energy efficient, have fast heating rates, generate heat uniformly and temperature can be controlled with a higher precision [[25], [26], [27]]. In addition, it was observed that, compared to conventional heating, fouling of the catalyst via coke formation could be reduced with induction heating [24].

The objective of this study was the production of an organic fraction with limited water content from catalytic pyrolysis of eucalyptus forest residues. Catalytic conversion of E. grandis using MgO and CaO catalysts was conducted with an induction heating reactor under intermediate pyrolysis conditions. For each catalyst, a rotatable central composite design (CCD) was used for the optimisation of the HHV of the organic-rich liquid phase produced from catalytic pyrolysis as a function of the catalyst concentration (Ccat) and reaction temperature (T). In addition, a fractional condensation system was designed to isolate an organic-rich bio-oil liquid with minimal water content. The effects of the catalysts on pyrolysis mechanisms under induction heating were explored.

Section snippets

Biomass supply and characterisation

Forest residues used in this study were obtained from an E. grandis plantation in the Cape Winelands region of South Africa. Eucalyptus trees are widely grown for the pulp and paper industries due to their relatively fast growth cycle [28]. In South Africa, Eucalyptus grandis is the most widely grown species for the pulp and paper industry [29]. The residues consisted of mainly dry branches, which were mechanically chipped to smaller pieces using a wood chipper. The chipped biomass was then

Results and discussion

The following sections compare the influence of temperature and Ccat on the product yields for both catalysts. Particular attention is given to the fuel properties of the condensable products. The mass balance closure for all experiments was 90–93 wt.%. The missing 7–10 wt. % can be attributed to incomplete condensation of the volatiles due to the relatively high flow rate of the carrier gas (5 SLPM). This was consistent with the presence of oily residues that condensed on the surface of the

Conclusions

Catalytic pyrolysis of forest residues was conducted in an induction heating reactor using CaO and MgO catalysts. Deoxygenation was assessed in terms of the HHV of the organic-rich product. Both catalysts showed significant deoxygenation capabilities, as the organic-rich phase HHV increased by over 20 % compared to the non-catalytic case. With similar effect despite a lower surface area per unit of mass, CaO had greater deoxygenation abilities than MgO. The fractional condensation system used

CRediT authorship contribution statement

Farai Chireshe: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Visualization. François-Xavier Collard: Conceptualization, Methodology, Investigation, Writing - review & editing, Visualization, Supervision. Johann F. Görgens: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declared no conflict of interest.

Acknowledgments

The authors would like to acknowledge the Paper Manufacturers Association of South Africa (PAMSA) and the South African government, through the Department of Science and Technology (DST) for their financial support to this research project.

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