Elsevier

Energy

Volume 183, 15 September 2019, Pages 1114-1122
Energy

Catalyzed pyrolysis of SRC poplar biomass. Alkaline carbonates and zeolites catalysts

https://doi.org/10.1016/j.energy.2019.07.009Get rights and content

Highlights

  • Poplar biomass containing 49% of cellulose yielded 53% of pyrolysis bio-oil.

  • Na2CO3, MgCO3, FCC and HZSM5 were used as catalysts to improve bio-oil quality.

  • FTIR spectra of bio-oils allowed a fast characterization of functional groups.

  • Carbonates were less effective than zeolites in lowering bio-oil acidic compounds.

  • FCC catalyst was the most effective to lessen the acidity of produced bio-oil.

Abstract

Poplar biomass of nine different genotypes, from short rotation coppice, was pyrolyzed in a fixed bed reactor using several solid catalysts. Pine bark was used as reference for uncatalyzed pyrolysis. Pyrolysis tests were performed for temperatures in the range 425–500 °C, selected from the thermal degradation profiles obtained by thermogravimetry under N2 flow. All the analyzed poplar genotypes showed similar pyrolysis behavior, with the highest bio-oil yield (53% average value) being obtained for the highest tested temperature (500 °C). In analogous conditions, the pine bark resulted in higher bio-char yields than poplar biomass, due to its larger lignin content.

Catalyzed pyrolysis carried out at 500 °C using 10% of catalyst (Wcat/Wbiomass) for H-ZSM5 and FCC (spent catalyst from Fluid Catalytic Cracking unit) zeolites, and Mg and Na carbonates, showed improved gasification with slightly lower liquid production. The FCC catalyst promoted the lowest depreciation of bio-oil yield with the highest decrease of acid functional groups. All the used catalysts were effective to lessen the acidic components of the produced bio-oils thus having a beneficial effect on the pyrolysis liquid products.

Introduction

Reducing net greenhouse gas emissions from the transportation sector is a priority for climate change mitigation, and thus the production of carbon neutral fuels from sources such as lignocellulosic biomass is of growing relevance [1]. Short rotation coppice (SRC) plantations on agricultural lands have a great potential to increase biomass supply for renewable energy. In Europe, poplar is the main woody SRC due to characteristics as high productivity and great potential for genetic improvement [2]. According to Sannigrahi et al. [3], hybrid poplars are among the fastest growing temperate trees in the world and a promising lignocellulosic feedstock for biofuels and other value-added products. Poplar trees can be grown on forestlands or on economically marginal crops lands [3]. Recently, Mateus et al. reported that the liquefaction of poplar biomass from SRC crops could contribute to the transition to a carbon neutral global economy [4].

Several processes can be used to convert biomass into biofuels: bio-chemical, thermo-chemical, and physio-chemical. Among thermo-chemical conversion, the three main processes are pyrolysis, gasification, and liquefaction [5,6]. Biomass pyrolysis has been used for more than a thousand years and pioneering research studies in biomass pyrolysis were initiated in 19th century [7], however little progress was made until the 1980′s.

Pyrolysis is a complex process in which organic matter is depolymerized in absence of oxygen [8]. It is generally accepted that the pyrolysis of biomass consists of three main stages: (i) initial evaporation of free moisture, (ii) primary decomposition followed by (iii) secondary reactions (oil cracking and repolymerization) [9]. Dehydrogenation, depolymerization, and fragmentation are the main competitive reactions during the primary decomposition of biomass. Pyrolysis produces liquid (bio-oil), gaseous (pyrogas) and solid (char) products with proportions depending on both the pyrolysis conditions and the technology used, as indicated in Table 1. The composition of the biomass (lignocellulosic and inorganics) also affects the product yields [10].

All products, liquid, solid and gas, obtained by pyrolysis of biomass can be used for energy purposes (Fig. 1). The liquid phase, bio-oil, can be upgraded into transportation fuels [11].

Bio-oil, or pyrolysis oil, is a dark and high viscous liquid with low pH and high content of oxygenated compounds such as acids, alcohols, aldehydes, esters, ketones, phenols and lignin-derived oligomers [12]. Such characteristics raise several technical issues for bio-oil commercialization and improvements of bio-oil quality are highly desired. Liu et al. [13] underlined that oxygen must be removed before bio-oil can be used as a replacement of diesel and gasoline. Bio-oil upgrading can be achieved by catalyzed pyrolysis. The main role of the catalyst is to remove selectively the bio-oil oxygen and to crack large molecules, converting undesired compounds into stable and useful molecules [13]. Thus reactions such as cracking, aromatization, ketonization/aldol condensation, hydrodeoxygenation and steam reforming must be prompted by catalysts during (in-situ) or after pyrolysis (ex-situ) [13].

Several materials can be used as pyrolysis catalysts. Liu et al. [13] reviewed the behavior of soluble inorganic salts, metal oxides and micro and mesoporous materials during the fast pyrolysis of lignocellulosic biomass underlining that catalyst development is the key factor for improving the pyrolysis process. The authors [13] underlined that the stability of pyrolysis catalyst and the use of multifunctional catalysts with optimized morphology, acid-base properties and metallic functionalities must be addressed in future research. Recently, Duo et al. [14] underlined the role of affordable catalysts (Ni-based) to be used for enhancement of hydrogen production from biomass, through thermochemical pathways.

In order to contribute to the knowledge on lignocellulosic biomass catalyzed pyrolysis the sections below present data obtained by fixed bed uncatalyzed and catalyzed pyrolysis of SRC poplar biomass (425–500 °C). Catalytic tests were carried out using, in-situ, carbonates and zeolite catalysts. The liquid product was the focus of this study and was characterized by FTIR, allowing to analyze the effect of the used catalysts on the chemical functional groups.

Section snippets

Materials and methods

Lignocellulosic biomass samples were obtained from poplar SRC sites in Portugal (Santarém, PT) and Belgium (Lochristi, BE). In Portugal, the 3.5 ha plantation had a 3 year cycle and in Belgium with a 14.5 ha plantation, the cycle was of 2 years. For further details of the Belgium site, refer to Ref. [15]. The samples corresponded to 9 different genotypes of commercial clones of poplar (genus Populus or P.) and are characterized in Table 2 (designations in this work, country of origin, parentage

Results and discussion

Data in Table 2 show biomasses with different compositions depending on the genotype and geographic origin, which corroborate the finding of A. Rodrigues et al. [15]. The authors claimed a relationship between soil chemical composition and the poplar biomass quality.

All the poplar genotype samples were analyzed by thermogravimetry in order to evaluate the lignocellulosic composition and to infer about the temperature range of pyrolysis. As reported before, the analyzed samples [16] showed

Conclusions

Short rotation coppice poplar wood was used to produce bio-oils by uncatalyzed and catalyzed pyrolysis. Using a fixed bed pyrolysis reactor, bio-oil yields in the range 30–53 wt% and char yields around 21 wt% were obtained for temperatures in the range 425–500 °C, in uncatalyzed pyrolysis. The maximum bio-oil yield was obtained for the highest pyrolysis temperature of 500 °C. Catalytic tests were performed at this temperature to assess the potential for removal of acidic chemical groups from

Acknowledgements

The authors thank (FRego, FRosa, MCasquilho) CERENA (“Centro de Recuros Naturais e Ambiente”, Centre for Natural Resources and the Environment), Project UID/ECI/04028/2013; and (ARodrigues) INIAV, I. P. (“Instituto Nacional de Investigação Agrária e Veterinária”, National Institute for Agrarian and VeterinarianResearch), Ministry of Agriculture, Portugal, and MARETEC (“Centro de Ciência e Tecnologia do Ambiente e do Mar”, Environment and Marine Science and Technology Centre). The authors also

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