Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Understanding the Secondary Reactions of Flash Pyrolysis Vapors inside a Hot Gas Filtration Unit Miguel Ruiz,*,† Eric Martin,†,§ Joel̈ Blin,†,† Laurent Van de Steene,†,⊥ and Francois Broust†,§ †
CIRAD, UPR BioWooEB, F-34398 CIRAD Montpellier, France CIRAD, UPR BioWooEB, F-97743 Saint-Denis, Reunion, France ⊥ CIRAD, UPR BioWooEB, Hanoï, Vietnam §
ABSTRACT: Ex situ hot gas filtration (HGF) has been shown to be a simple and robust technique to upgrade the quality of flash pyrolysis oils. In this study, the secondary reactions inside the HGF unit and their impact on product yields and the chemical composition of bio-oils were investigated in a total of 18 experiments conducted at both pilot and bench reactor scales (1 and 0.1 kg/h), with beech wood (BW) and sunflower stalks (SFS). The impact of HGF on yields was found to be dependent on the extent of secondary reactions which, in turn, were determined by three parameters: (a) HGF temperature, (b) HGF char cake thickness, and (c) AAEM content of the raw feedstock. Nevertheless, independently of the conditions used, the drop in the organic yield was less than 10 wt % with both BW and SFS feedstocks. Our results demonstrated that (1) cracking reactions mainly took place in the homogeneous gas phase and (2) dehydration, coking, and decarboxylation reactions took place in the HGF char cake, probably catalyzed by the high AAEM content and the small size of the HGF char particles. Based on detailed chemical analysis of bio-oils, we propose several secondary reaction pathways to explain the interaction between the HGF char and the pyrolysis vapors, such as (1) ketonization of low Mw carboxyl functions via decarboxylation, (2) the formation of simple anhydro sugars by the selective cleavage of the glycosidic bond of the carbohydrate oligomers, and (3) the depolymerization of lignin oligomers. Further, our results suggest the interdependence of two factors which determine the impact of HGF: (a) the physical and chemical properties of HGF char and (b) the reactivity of the vapors.
1. INTRODUCTION
It is known that the AAEM species originally present or unnaturally added to biomass have a substantial impact on the primary reaction pathways of holocelluloses and lignin model compounds or on raw biomass particles.28−37 In ex situ HGF systems, the primary TAR compounds in the form of gas and aerosol (hereafter referred to as “vapours”) leaving the pyrolysis reactor could undergo secondary reactions in homogeneous and heterogeneous phases inside the HGF unit. Secondary homogeneous gas-phase reactions can take place when pyrolysis vapors are exposed to additional residence time and high temperatures inside the HGF vessel. The influence of residence time and temperature on the extent of these reactions has been extensively investigated.38−43 Using a downstream tubular reactor, Hoekstra and co-workers44 observed temperature-dependent asymptotic behavior of product yields provided that long residential times were applied. However, reported GCMS results show that, although the overall bio-oil yield was only marginally affected at 400 °C, the chemical composition of the bio-oil was modified with increasing residence time. Secondary heterogeneous reactions are expected to take place promoted by the interaction between the pyrolysis vapors and the HGF char. Several TAR abatement studies45−50 conducted at high temperatures (600−1000 °C) have proven that the pyrolytic char itself displays catalytic activity. At these temperatures, the interaction between the pyrolysis vapors and
Lignocellulosic biomass is a CO2-friendly material which can be converted into liquid intermediates (bio-oils), via flash pyrolysis (FP), with a high liquid yield. Conventional bio-oil is a complex microemulsion of water dispersed in an oily oxygenated organic phase,1,2 whose composition mainly depends on the nature of the feedstock and on the conditions inside the reactor. The quality requirements imposed by end-user applications hamper the commercialization of FP oils, notably, in terms of time stability, low heating value, and fouling issues.3,4 Previous studies reported that alkali and alkaline earth metallic (AAEM) species, inherently present in the biomass,5 concentrate in the solid char fraction6−12 and catalyze bio-oil aging reactions,6,13−16 hence, reducing the solid content of bio-oils is a necessary step in upgrading their quality. In this context, hot gas filtration (HGF) has proven to be a simple and robust technique for overcoming some of the aforementioned quality deficiencies. It is now widely accepted that introducing a HGF unit in (in situ)17 or downstream (exsitu)6,9,18−27 from the pyrolysis reactor considerably decreases the solid content of bio-oils and hence its AAEM content. However, it can be generally stated that introducing a HGF unit in a conventional pyrolysis system causes a 5% to 10 wt % drop in liquid yield.6,21,22,25 Conflicting results17,26,27 concerning the impact of HGF on yields are probably related to interlaboratory differences in HGF critical parameters, such as the HGF temperature, the thickness of the HGF char cake, and the properties of the HGF char particle, particularly its particle size and its AAEM content. © XXXX American Chemical Society
Received: September 27, 2017 Revised: November 13, 2017
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DOI: 10.1021/acs.energyfuels.7b02923 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
experiments, the BW sawdust was used as supplied (750−2000 μm), while the SFS feedstock was ground in a cut mill to
