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TG-FTIR Method for the Characterization of Bio-oils in Chemical Families Filip Stankovikj, and Manuel Garcia-Perez Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03132 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017
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Energy & Fuels
TG-FTIR Method for the Characterization of Bio-oils in Chemical Families Filip Stankovikj1, Manuel Garcia-Perez1* 1
Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164-6120
(Paper to be submitted to Energy and Fuels)
Abstract: Pyrolysis oils are complex mixtures of hundreds of compounds that can be described as a mixture of few chemical macro-families. This description is highly desirable for modeling biooil combustion, cracking, separation and evaporation. In one of our previous papers we recommended the deconvolution of bio-oil DTG curves as the basis for characterizing bio-oils in chemical families. However, the positioning of the peaks was based on the shape of the DTG curve. More specific guidelines are needed to position the peaks in this deconvolution strategy. In this paper we developed and tested a methodology to identify the position and shape of the peaks in the DTG curves. The first strategy uses the FTIR spectra of the TG evolved pyrolysis oil vapors. The second consists on doping the bio-oil with model compounds and with its fractions. The IR studies provided information on the position and width of the fitting curves describing the behavior of the very light volatile compounds (400°C). The range between these two temperatures was fitted based primarily on the shape of the DTG curve and the results from the oil doping studies. Contribution of the chemical families
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towards formation of coke residue was examined based on the doping studies, and rules for coke assignment were developed. The developed DTG deconvolution method was applied to ten pyrolysis oils produced from compositionally diverse biomass feedstocks and production technologies. The chemical composition of the oils obtained by our new DTG deconvolution strategy was compared with other well accepted characterization methods. Our results show that the method proposed could be used to estimate the changes in the content of water, light volatile compounds, water soluble (WS) and water insoluble (WIS) fractions.
Keywords: Fast pyrolysis, characterization, chemical families, TG-FTIR, TG/DTG.
*Corresponding author: Manuel Garcia-Perez Associate Professor Biological Systems Engineering Department, Washington State University e-mail:
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Energy & Fuels
1.
Introduction
Increased demand for liquid transportation fuels becomes more than obvious nowadays, when most of the World’s industrial development is fueled by fossil derived fuels and chemicals.1–4 The global warming, unstable crude oil prices and the gradual depletion of known oil resources arises the need for finding alternative ways to fuel our transportation.5–7 Biomass happens to be our most viable renewable source of carbon based fuels and chemicals.8–11 Sustained increase in oil demand, stagnant supply, political instability in oil-producing countries represent factors that can certainly boost the prices of oil on a long term and can jeopardize the economies of oil dependent nations.2,4 Developing advanced biofuels may not only prepare us for this forthcoming crisis, but may provide an incentive for spurring the development in the rural regions.12 Furthermore, biomass is a CO2 neutral source of energy, in some cases carbon negative, and using biomass derived fuels could contribute to mitigating the effects of the global warming.13,14
Fast pyrolysis of biomass is one of the most promising technologies to convert solid non-edible biomass into liquid bio-oil.4,8,15–20 Wood, agricultural residues, perennial grasses, or dedicated energy crops can be converted to crude pyrolysis bio-oil with yields of up to 70 wt.% (on biomass dry basis). This high bio-oil yield is due to the high heating rates that can be achieved when small biomass particles are heated rapidly in absence of oxygen at temperatures of about 450–550°C. The other products are non-condensable gases and char that can be used for thermal applications, as a soil amendment and for CO2 sequestration
The pyrolysis oil is a very complex mixture of about 400 major identified compounds.21,22 They cover a wide range of molecular weights and functionalities derived from the biomass basic 3 ACS Paragon Plus Environment
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building blocks (cellulose, hemicellulose and lignin).23 Depending on the biomass feedstock composition (amounts of lignin, hemicellulose, cellulose, mineral content, extractives, etc.), pyrolysis reactor type and reaction parameters, the resulting bio-oil can have varying composition that in many cases determines the post-treatment needed to improve its physical and chemical properties before it can be used as a liquid transportation fuel.19 The pyrolysis oil has compounds that can be classified as acids, alcohols, aldehydes, esters, ketones, phenols, guaiacols, syringols, sugars, furans, alkenes, aromatics, nitrogen compounds and other miscellaneous oxygenates that are found in low concentrations. There has been a significant effort in the past decades to characterize pyrolysis oils in terms of their composition, chemical and physical properties, and many different approaches have been suggested.24–27 Several analytical techniques need to be used in tandem in order to completely characterize and understand pyrolysis oils, and although to date there is no one standardized approach for analyzing bio-oils,28 there have been many efforts to standardize particular analytical techniques.29–32
Common approach to characterize pyrolysis oils has been fractionation by using various organic solvents (reviewed by Mohan et al.)18 All those separation schemes depend on solvents’ polarity and acidity as driving force for separation, and the schemes have been adjusted so that they are more efficient in targeting specific chemicals that one wants to focus on in the further analysis. The separation of bio-oil in water soluble and water insoluble fractions is among the most useful characterization methods so far developed.24,33
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Another approach that was proposed for meaningful comparison of pyrolysis oils is based on describing the bio-oil as a set of few chemical macro-families.34 Based on the distribution of molecular masses or boiling points of the constituent compounds, by utilizing gel permeation chromatography (GPC) and thermogravimetry (TG) respectively, one could describe the whole bio-oil in few groups of compounds. Thermogravimetry measures reliably the weight loss, which depends on the volatility and thermal stability of the constituent compounds. This approach has been used in the past to characterize vacuum pyrolysis oils.34–37 Deconvolution of the DTG curves was used to determine the composition of those bio-oils in terms of macro-chemical families. The bio-oils and their fractions were described as mixtures of eight chemical macrofamilies. The TGA and GPC were used to quantify the compounds belonging to the macrofamilies and that was compared with the results obtained by GC. The main weaknesses of this method is the limited experimental evidence for determination of the position and shape of the deconvolution peaks which belong to certain chemical macro-families. In this paper we hypothesize that by: (1) using FTIR of the evolved gases, (2) doping the bio-oil with model compounds, and (3) doping the bio-oil with its fractions, we would be able to obtain sufficient amount of information to support the deconvolution of the DTG curves and in this way improve the DTG characterization method.
TG-FTIR as a tool to analyze evolved gases from bio-mass and bio-oils has been used in the past, most commonly for quantification and for obtaining kinetic parameters.38–42 The FTIR spectra consist of multiple overlapping IR absorption bands, which makes the qualification of the evolved species a challenge, and the quantification becomes very unreliable. However, the FTIR spectrum gives information on certain functional groups, and one may be able to discern when
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certain groups of compounds, belonging to a particular chemical macro-family start and end their evolution from the analyzed pyrolysis oil. Moreover, it has been shown that compounds belonging to certain chemical families are grouped in narrow ranges, in both the GC-MS and GPC.33,34
The main objective of this paper is to improve an existing DTG method for the characterization of bio-oils in chemical families by using the FTIR spectra of evolved gases, by doping the bio-oil with model compounds and its fraction. The results of the deconvolution methods is compared with the data obtained by well-established analytical techniques.43
2.
Material and Methods
2.1. Pyrolysis oils The source of the ten oils studied in this paper have been described elsewhere.43 Briefly, the BTG bio-oil was produced from pine wood using a rotating cone reactor (http://www.btgbtl.com/). The average particle size was 3 mm, the average reactor temperature 510°C, gas residence time
