Article pubs.acs.org/EF
Production of Renewable Hydrocarbons from Thermal Conversion of Abietic Acid and Tall Oil Fatty Acids Ehsan Jenab, Paolo Mussone, Goeun Nam, and David Bressler* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada ABSTRACT: The primary objective of this study was to investigate the pyrolytic conversion of crude tall oil, a byproduct from the Kraft process in the pulping industry, to hydrocarbon products for use as renewable hydrocarbons and fuels. Abietic acid, the main resin acid in crude tall oil, and tall oil fatty acids, the main distillate fraction of crude tall oil, were pyrolyzed separately using a batch micro reactor at 370, 410, and 450 °C for 2 h. The gas and liquid reaction products were identified and quantified using gas chromatography equipped with a flame ionization detector, a thermal conductivity detector, and mass spectrometry. At all temperature regimes, the pyrolysis of abietic acid resulted in the formation of aromatic compounds characterized by up to three benzene rings. While the pyrolysis of tall oil fatty acids at lower temperatures resulted in alkanes and alkenes and negligible amounts of aromatic compounds, higher temperatures promoted the formation of relatively shorter chain alkanes and alkenes and detectable amounts of aromatics. The presence of both CO and CO2 in the gas fractions suggested that deoxygenation of the initial compounds proceeded through decarbonylation and decarboxylation, but the favored mechanism is dependent upon the structure of the initial substance. This work demonstrates the feasibility of producing renewable hydrocarbons through the pyrolysis of the fatty-acid-rich distillate fraction of crude tall oil. fatty acids3 is not as well-established as pyrolysis of lignocellulosic biomass sources, despite the simpler, more hydrocarbon-like, molecular structure. Because of these structural differences, the resulting chemistries and product bio-oils from these two sources are completely different.11,12 Tall oil soaps (TOS) are a byproduct stream of the Kraft process in the pulping industry.13 TOS are acidified using concentrated sulfuric acid to produce crude tall oil (CTO), which may be subsequently fractionated to light oil, fatty acids, resin acids, distilled tall oil (DTO), and pitch residue.14 The relative compositions of fatty acids in CTO are highly dependent upon the age, geographical location, and species of conifer trees. In general, CTOs are composed of two main acid classes: mono- or diunsaturated straight-chain 18 carbons with distribution of cis or trans configurations, in which the degree of unsaturation is dependent upon the operating parameters of the Kraft pulping as well as acidulation process, and resin acids, which are diterpene carboxylic acids, on the basis of an alkyl-substituted perhydrophenanthrene ring structure, including abietane- and pimarane-type diterpenoids.15 Despite the low cost and abundance of CTO, the scientific literature dealing with the use of CTO or fatty acids in CTO as a potential feedstock for the production of biofuels and renewable hydrocarbons is sparse and dated. In fast pyrolysis of purified and neutralized CTO and neutralized DTO at 750 °C for 20 s using pyrolysis gas chromatography equipped with a mass selective detector and a flame ionization detector (Py−GC/MSD and FID),16 a wide range of volatile aliphatic and aromatic compounds were identified with some chemically bound oxygen formed in the
1. INTRODUCTION The U.S. Energy Information Administration’s (EIA’s) International Energy Outlook 2013 estimated that the world use of petroleum and other liquid fuels will grow from 87 million barrels per day in 2010 to 97 million barrels per day in 2020 and 115 million barrels per day in 2040. While technology advancements in the extraction of conventional and unconventional fossil fuels are expected to contribute toward meeting this rapidly rising demand, societal concerns over long-term environmental sustainability continue to drive significant research and development investments in advanced renewable fuels compatible with the existing petrochemical infrastructure. Oils, fats, and fatty acids derived from animal and plant sources constitute an ideal biomass feedstock for conversion into renewable hydrocarbons and fuels, owing their widespread availability and energy density. However, the possibility of creating undue competition for land and water used for food production as well as their sustainable production are under review. These concerns can be addressed by the production of biofuels from agricultural and forest residue and non-food crop feedstocks.1,2 Three main processes for conversion of fatty acids or oils/fats to biofuels and linear hydrocarbons are pyrolysis or thermal cracking,3 transesterification,4 and hydroprocessing.5 The transesterification process focuses on an esterification reaction between an alcohol, usually methanol, and triglycerides or fatty acids. This process has several drawbacks, including substantial methanol usage, which itself is a product from fossil-based sources, and glycerol production as a byproduct of the reaction, which is difficult to remove from the desired products and derived process salts.6 Hydroprocessing also has several drawbacks, including the requirement for high temperature and pressure conditions and its cost-wise efficiency.7 Alternatively, pyrolysis is increasingly drawing interest as a potentially robust and cost-efficient approach to the conversion of biomass to biofuels and renewable hydrocarbons.8−10 However, pyrolysis of © 2014 American Chemical Society
Received: August 4, 2014 Revised: October 2, 2014 Published: October 20, 2014 6988
dx.doi.org/10.1021/ef501746b | Energy Fuels 2014, 28, 6988−6994
Energy & Fuels
Article
used for sampling from the gas fraction, and the collected gas sample was transferred and maintained in 5 mL vacutainer tubes (BD, Franklin Lakes, NJ) before further analysis. N2, CO, CO2, and H2 in the gas samples were quantified by injecting 100 μL aliquot gas samples into an Agilent 19091P-MS4 packed column in gas chromatography (GC, 7890A, Agilent Technologies, Fort Worth, TX) equipped with a thermal conductive detector (TCD). The temperature of the injector was set at 170 °C, and the temperature of the detector was set at 200 °C. The GC oven temperature program was set at 40 °C for 2 min, increased to 170 °C at a rate of 10 °C min−1, and then held for 4 min with a total run of 20 min. The carrier gas for analysis of N2, CO, and CO2 was helium with a constant pressure of 20 psi, and it was switched to argon for the analysis of H2. The standard curve for individual gases was obtained by injecting 2.5, 10, 40, 70, and 100 μL of each pure gas component for triplicates at the same conditions. GC (7890A, Agilent Technologies, Fort Worth, TX) equipped with a flame ionization detector (FID) was used to determine hydrocarbons in the gas fractions of the pyrolysis products. A total of 100 μL of the sample from the gas fraction was injected onto a CP-Al2O3/Na2SO4 Varian capillary column (50 m × 320 μm × 5 μm, Varian, Inc., Lake Forest, CA). The temperature of injection port was kept at 170 °C, and the temperature of detector was kept at 230 °C. Helium was used as the carrier gas at a flow rate of 1.6 mL min−1. The initial oven temperature was held at 70 °C for 0.67 min, subsequently increased to 170 °C at 3 °C min−1, and held for 26 min. A standard curve for methane for hydrocarbon quantification was obtained by injections of 2.5, 10, 40, 70, and 100 μL for triplicates at the same conditions. Then, in each chromatogram after quantifying methane, the response factors of the other hydrocarbons in the gas products were considered the same as methane for further analysis. Figure 1a depicts a typical GC−FID chromatogram of the hydrocarbons in the gas fraction product. 2.3.2. Liquid Product Analysis. After venting, the liquid product in the micro reactor was recovered using diethyl ether for abietic acid pyrolysates and pentane for TOF pyrolysates. To quantify the main products in the liquid fractions, methyl behenate was used as an internal standard for abietic acid pyrolysates and methyl nonadecanoate was used as an internal standard for TOF pyrolysates. An aliquot of 10 mL of solvent containing 1−1.5 mg mL−1 internal standard was poured into the micro rector containing the liquid fraction, and then the content was thoroughly mixed with a glass agitator. After mixing, the micro reactor was left for around 15 min, and then the extract was poured into a glass vial. Any solid residual left in the reactor was considered as the solid residue. The amount of solid residue was measured by storing the micro reactor containing the solid residue under a fume hood for a whole day to evaporate the solvent. The weight difference between the micro reactor containing the solid residue and the clean reactor was considered as the weight of the solid residue. GC (6890N, Agilent Technologies, Fort Worth, TX) equipped with a FID in split mode was used for determination of the main components in the liquid fraction extracts of the pyrolysis reaction. The liquid fraction extracts were methylated using diazomethane prior to GC injection. Aliquots (1 μL) of each sample were injected onto a HP-5ms column (30 m × 250 μm × 0.25 μm, Agilent Technologies, Fort Worth, TX). Helium was used as the carrier gas at a flow rate of 1 mL min−1. The injector was set at 300 °C, and detector temperatures was set at 350 °C. The initial oven temperature was held at 35 °C for 0.1 min, then increased to 280 °C at 10 °C min−1, and held for 5.4 min to reach the total time of 30 min. A typical GC−FID chromatogram of the main products in the liquid fraction of the pyrolysis reaction has been shown in panels b and c of Figure 1. Mass spectrometry (MS) analysis was conducted on the same GC system described above coupled to an Agilent 5975B inert XL EI/CI MSD operated in electron ionization (EI) mode. The mass spectra were matched to the National Institute of Standards and Testing (NIST) spectral library and recognition of the fragmentation pattern of mass spectra. The peaks with a quality match of 90 and higher were considered as identified peaks. After the mass spectra were matched with their related FID chromatograms, the area of the peaks were obtained
pyrolysates. Neutralized mixtures of tall-oil-derived fatty acids and resin acids were separately pyrolyzed using Py−GC/MSD and FID at 750 °C for 20 s by Lappi and Alen.17 In the previous studies, the neutralized forms of the feedstocks and not the acid forms were pyrolyzed through fast pyrolysis for the production of renewable hydrocarbons and chemicals16,17 without consideration of the dominant chemical pathways involved in the pyrolysis mechanism. Therefore, the main objective of this work was to investigate the dominant chemical pathways in the pyrolytic conversion of abietic acid, the main resin acid of CTOs, and tall oil fatty acids (TOF) recovered from CTO through chemical characterization of the gaseous and liquid product streams. Batch reactions conducted over a range of temperatures lower than the temperatures in fast pyrolysis with a longer time demonstrated the feasibility of using pyrolysis as a means of converting low-value forestry residue into renewable hydrocarbons and fuels.
2. MATERIALS AND METHODS 2.1. Materials. Abietic acid (≥75%) was purchased from SigmaAldrich (St. Louis, MO) and TOF (SYLFAT FA1) was generously provided by L.V. Lomas (Calgary, Alberta, Canada). The product composition of TOF is shown in Table 1. Diethyl ether and pentane
Table 1. Composition of TOF Used for the Pyrolysis Reaction composition moisture ash resin acid unsaponifiable fatty acid total non-conjugated linoleic acid conjugated linoleic acid oleic acid saturated fatty acid other fatty acid a
(%, w/w)a
