Biomass and Bioenergy 151 (2021) 106171
Available online 10 July 2021
0961-9534/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Impact of hydrogenation on miscibility of fast pyrolysis bio-oil with refinery fractions towards bio-oil refinery integration
A. Dimitriadis
a, D. Liakos
a, U. Pfisterer
b, M. Moustaka-Gouni
c, D. Karonis
d, S. Bezergianni
a,*aChemical Process & Energy Resources Institute – CPERI Centre for Research and Technology Hellas – CERTH, 6th km Charilaou-Thermi Rd, Thermi-Thessaloniki, 57001, Greece
bBP Europa SE, Bochum, Germany
cAristotle University of Thessaloniki (AUTH) Greece, School of Biology, Greece
dSchool of Chemical Engineering, National Technical University of Athens, 15780, Athens, Greece
A R T I C L E I N F O Keywords:
Ablative fast pyrolysis Pyrolysis oil Hydrotreatment Hydrodeoxygenation Hybrid fuel Green fuel
A B S T R A C T
In particular, a straw-based Ablative Fast Pyrolysis (AFP) oil was upgraded via hydrotreatment (HDT) in order to be used as a blending component with fossil-based intermediates. The raw pyrolysis oil is characterized by H/C
=0.14, O/C =0.58 and C =57.73 wt%, while the HDT pyrolysis oil has a H/C =0.14, O/C =0.016 and C = 85.85 wt%. Based on the density, viscosity and composition, and in association with a typical refinery, the possible petroleum refinery entry points for HDT pyrolysis oil include Straight Run Gas-Oil (SRGO), Atmospheric Gas-oil (GO), Fluid Catalytic Cracking Light Cycle Oil (LCO) and Heavy Cycle Oil (HCO), as well as Light Vacuum Gas-Oil (LVGO). To that aim, this work examined raw and HDT bio-oil blends with the aforementioned candidate petroleum streams at a 30:70 v/v ratio (raw or HDT bio-oil/petroleum stream) with respect to their miscibility, by comparing the blend components’ properties, microscopic observation, and interfacial tension analysis. From the microscopic observation, the addition of HDT pyrolysis oil in all examined petroleum candidates renders a homogeneous mixture where the two phases cannot be distinguished. In addition, no interfacial tension was observed in the examined blends, while the blend of HDT pyrolysis oil with the different petroleum streams not only did not deteriorate the fuel characteristics over the original petroleum stream, but also it improved them, in some cases. In general, the HDT pyrolysis oil was found to be miscible with all the examined refinery streams, extending the potential further investigation of stabilized pyrolysis oil integration over oil refineries.
1. Introduction
The instability in the world energy and economic sectors, as well as the concern over global climate change, have led the interest in renewable energy sources. Among various renewable energy sources, biofuels along with hybrid fuels are going to play an important role in the coming decades in the transportation sector. Biomass, as the main feedstock for the production of biofuels, includes a wide range of ma- terials such as plant matter (including agricultural crops and residues), animal waste, municipal waste and industrial effluents [1]. The advantage of biomass is that it is characterized by insignificant sulfur content, leading to lower SO2 emissions compared to conventional fossil fuels. In addition, as the CO2 can be recycled by plants through photo- synthesis, quantitatively, the net emission of CO2 is very low [2].
Until now, various technologies for the production of biofuels and
hybrid fuels have been developed, such as transesterification, Fischer–Tropsch, hydroprocessing, pyrolysis etc. [3]. In general, pyrol- ysis is a well-established technology that involves the thermal decom- position of biomass by heat in the absence of oxygen and, in some cases, in the presence of a catalyst, which results in the production of solid, liquid and gaseous fuel products [4,5]. The main liquid product from biomass pyrolysis, is often referred to as pyrolysis oil, bio-oil, pyrolysis liquid, pyrolytic oil, liquid wood, liquid smoke, etc. [6]. However, py- rolysis oil faces many drawbacks, limiting its usability as a direct fuel, because of its low heating value due to high oxygen and water content, high ignition delay, high acidity, low thermal stability, high viscosity, poor lubrication, and formation of engine deposits, attributed to its complex mixture of oxygenated compounds (from short-chain organic compounds to polymer) [7,8].
One way to use pyrolysis oil as a fuel is to blend it with diesel fuel. It
* Corresponding author.
E-mail address: sbezerg@cperi.certh.gr (S. Bezergianni).
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journal homepage: www.elsevier.com/locate/biombioe
https://doi.org/10.1016/j.biombioe.2021.106171
Received 16 December 2020; Received in revised form 11 June 2021; Accepted 20 June 2021
was reported that the addition of diesel can mitigate the problems of acidity and high viscosity, and the emulsions containing up to 10%
pyrolysis oil could be used [9]. However, the composition of pyrolysis oil which contains high amounts of acids, phenols, ethers, ketones, al- dehydes and esters, makes it not miscible with nonpolar hydrocarbons of diesel (paraffin, naphtha and aromatic) and affects the combustion of the final blend. Thus, an upgrading step of pyrolysis oil is considered important prior to its use as a liquid fuel or chemical feedstock. There are several reviews on the upgrading of pyrolysis oil and various tech- nologies have been developed for this purpose [10], hydro- deoxygenation of pyrolysis oil is among them [11]. In fact, hydrodeoxygenation allows the nearly full deoxygenation of the pyrol- ysis oil by a combination of hydrodeoxygenation and decarboxylation reactions [12]. The resulting product is a two-phase liquid consisting of an aqueous and an organic phase. In most cases, aqueous phase is up to 40%. Once stabilized pyrolysis oil is produced, it can be further pro- cessed into conventional fuels or sent to a refinery as an intermediate feed for co-processing with petroleum streams [11,13].
In general, hydrotreatment of pyrolysis oil is performed at a tem- perature range of 573–723 K under high pressure hydrogen with the use of heterogeneous catalysts [14]. However, many pyrolysis oil compo- nents when heated-up become very reactive leading to coke formation on the surface of the catalysts and thus leading to ΔP build up and product deterioration [15–18]. To overcome this drawback, Baker and Elliot [19] suggested a first mild-hydrotreatment step from 523 to 553 K in the presence of noble metal catalysts (Pd/Cs), in order to lead to a stabilized pyrolysis oil that could be further upgraded via a 2nd more severe hydrotreatment step to useful transportation fuels or to be used as an intermediate stream for co-processing with fossil fractions in a typical refinery [20,21]. In general, the rate of pyrolysis oil upgrading depends on the severity of the process (hydrotreating pressure and temperature).
However, more severe conditions require higher processing costs [22]. A major advantage of co-processing partially stabilized pyrolysis oil into the existing petroleum refining infrastructure is to leverage existing capital instead of constructing new parallel infrastructure for standalone pyrolysis oil hydroprocessing to final fuel blend. The main target of co-processing research is to investigate the minimum amount of pre- treatment required to stabilize the raw pyrolysis oil to a possible inter- mediate stream that a refinery can successfully co-process with petroleum fractions. However, the integration of stabilized pyrolysis oil into a refinery stream has some negative effects on H2 consumption.
Hydrogen utilization of 490–710 m3/m3 (standard temperature and pressure of 298 K, 101.15 KPa) of dry pyrolysis oil is required to end up to a low oxygen product. This is larger compared to the upper end of hydrogen consumption that is reported (350 m3/m3) for petroleum streams hydrotreating [22]. Thus, cost associated with the use of H2
should be balanced by improving H2 efficiency and product quality. This is a hot topic that needs further investigation and it is beyond the scope of this study.
Recent results indicate that the stabilization of pyrolysis oil by low severity hydrodeoxygenation might be sufficient to allow co-processing using refinery processes like fluid catalytic cracking (FCC) or hydro- processing [20,23–25]. Yet, even when much of the oxygen is removed from raw pyrolysis oil via an upgrading processes, the resulting hydro- carbon product still demonstrates differences to petroleum-derived fuels. Christensen et al. [26] pointed out important chemical constitu- ents within upgraded pyrolysis oils of various oxygen-containing species that are not present in petroleum-derived cuts. In particular, aromatics appear in the upgraded oil fraction equivalent to kerosene cut, while paraffins are more abundant in the heavy naphtha cut and less abundant in the light naphtha cut. Thus, if the target is a compatible blend feed- stock, further downstream processing of HDO in a refinery is necessary to meet the qualifying qualities of existing fuel grades. From all the above it is important for the upgraded pyrolysis oil to be miscible with the petroleum candidate streams for co-processing in an existing re- finery infrastructure.
According to literature, very few studies examined the effectiveness of blending and consequently co-processing of pyrolysis oil with petro- leum fractions and most of them are using model compounds such as guaiacol with atmospheric gas oil to simulate the effect of inserting a pyrolysis oil in an existing petroleum refinery [27,28]. The most com- mon petroleum candidate stream is straight run gas-oil [28–30]. Pinho et al. have used raw fast pyrolysis oil originating from pinewood chips for co-processing with vacuum and atmospheric gas-oil (VGO and AGO) in an FCC (fluid catalytic cracking) unit to produce second generation biofuels. They have noticed that the two liquids are not miscible and their blend cause coke formation. In order to overcome the problem of miscibility between the pyrolysis oil and the VGO, they have used two independent feed lines (one for VGO and one for pyrolysis oil) to send the two feeds on the reactor [31,32]. Their work shows the importance of miscibility study, although their co-processing tests were successful.
However, the study did not include a miscibility analysis for the mate- rials blended, which is essential for understanding the fluids interactions.
At the moment, the literature investigating the criteria for selecting a particular petroleum stream for obtaining a homogenous biocrude blend is rather limited. To that aim, the ultimate goal of the current research is to investigate the potential compatible refinery entry points for a hydrotreated pyrolysis oil (HDT pyrolysis oil). In particular, a straw- based Ablative Fast Pyrolysis (AFP) oil produced by Fraunhofer UMSICHT [33,34] was upgraded via hydrotreatment at CERTH to be used as a blending component with fossil-based intermediates, in order to define the HDT pyrolysis oil possible refinery entry points. Based on the properties of density, viscosity, oxidation stability and composition, in association with BP refinery, the possible petroleum refinery entry points of HDT pyrolysis oil include Straight Run Gas-Oil (SRGO), At- mospheric Gas-oil (GO), Fluid Catalytic Cracking Light Cycle Oil (LCO) and Heavy Cycle Oil (HCO), as well as Light Vacuum Gas-Oil (LVGO).
The innovative part of the manuscript is that for the first time the po- tential entry points of an existing refinery are investigated for their ability to form homogenous blends with an upgraded pyrolysis oil via microscopic observation and interfacial tension analysis.
2. Materials and methods
2.1. Feedstock materials
The pyrolysis oil utilized in the original version involved the organic phase product of the ablative fast pyrolysis of lignocellulosic biomass and specifically of a mixture consisting of barley and wheat straw at 50%
mass fractions each (Picture 1) [33,34]. The straw mixture was supplied by the company Erhard Meyer, under the trade name “Strohfix-Gerste”. The pyrolysis oil was produced via ablative fast pyrolysis in a cycle reactor, at Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT [34]. During the process, the reactor, cyclone and piping were operated at 723 K to avoid condensation of pyrolysis liquids prior to the condenser. One stage condensation was used to cool the pyrolysis vapor at around 288 K. Further details of the process can be found elsewhere [34].
The pyrolysis oil was stored in a refrigerator to reduce its decom- position. According to Picture 1 and Table 1, pyrolysis oil (feedstock) is a dark brown liquid with high density (1.14 Mg/m3) and high viscosity (106.1 mm2/s). In addition, it is characterized by high water content (32.8% v/v) combined with high oxygen content (33.25 wt%).
To that aim, in an authors’ previous work [35], a hydrotreatment step was performed at CERTH, in a small hydrotreating continuous flow pilot plant with a capacity of 60 cm3/h, in order to upgrade the raw pyrolysis oil to a suitable intermediate refinery stream towards hybrid transportation fuels production. During the process, the raw pyrolysis oil was mixed with a small quantity of DMDS (Dimethyl Disulphide) as it is necessary to maintain a certain sulfur level in the hydrotreating feed- stock when a sulphiding catalyst is employed (NiMo/Al2O3) [36]. The
hydrogen consumption of the process ranged from 450 to 720 m3/m3 (all gas volumes measured at standard temperature and pressure of 298 K, 101.15 KPa) depending from the operating conditions of the process.
The hydrotreatment of pyrolysis oil was performed at 603 K reaction temperature, at 7 MPa reaction pressure, at 1 hr−1 LHSV (Liquid Hourly Space Velocity) and at 843 m3/m3 H2/pyrolysis oil ratio [35]. However, as the upgrading step of raw pyrolysis oil was performed and deeply described in an author’s previous work [35], no further details about the process will be presented in the original manuscript. The liquid product collected from the hydrotreatment step consisted of an organic dark liquid 55–63% volume fraction as the top layer and the balance is an aqueous phase on the bottom layer that were separated via gravity (Picture 1). In general, 1 dm3 of pyrolysis oil could give 0.580 dm3 of organic phase and 0.389 dm3 of water phase. During catalytic hydro- deoxygenation, water separation occurs. Water is formed during hydrogenolysis of C–O bonds and dehydration reactions [37]. Further- more, the carbon content of the raw pyrolysis oil was increased from 57.73 wt% to 85.85 wt% after the hydrotreatment step. Thus, the carbon conversion was calculated based on the following equation:
%HDC=X0− X X0
×100% [27]
where X0 is the initial carbon content of the raw pyrolysis oil (wt%) and X is the carbon content of the hydrotreated pyrolysis oil (wt%). From the results, the carbon conversion is 48.7%, showing the increase of carbon content after the hydrotreatment step. From the two phases, the organic phase was considered as the main product and was further analyzed. The organic phase will be called “HDT pyrolysis oil” and will constitute the product that was used as a blending component with petroleum streams in the original manuscript.
During the study, different petroleum streams (see Picture 2) were investigated in terms of their miscibility with the HDT pyrolysis oil as appropriate candidates for hybrid transportation fuel production. The
proposed petroleum streams are described below, while their properties are listed in Table 1:
➢ Straight run gas-oil (SRGO): A complex combination of hydrocarbons produced by the distillation of crude oil. It consists of hydrocarbons having carbon numbers predominantly in the range of C11 through C25 and boiling point in the range of approximately 478–673 K [38].
SRGO is characterized by density close to diesel EN 590 specs (846 kg/m3). In addition, it has low viscosity (3.16 mm2/s) and low water content (50 mg/kg). In general, SRGO presents similar properties to HDT pyrolysis oil and thus it is can be characterized as a well promising feed for co-processing with HDT pyrolysis oil.
➢ Atmospheric gas-oil (GO): Gas-oil is a distillation cut derived from an atmospheric distillation unit, and it is primarily made up of mole- cules with 14–20 carbon atoms. The GO distillation has a boiling range between 488 and 616 K. The primary use of atmospheric gas- oil is as a blend stream to make diesel or heating oil [39]. According to Table 1, GO presents almost similar density (0.867 g/ml) and elemental composition to HDT pyrolysis oil, however, its viscosity (16.09 mm2/s) is more than two times higher compared to HDT pyrolysis oil (7.54 mm2/s).
➢ Light cycle oil (LCO): One of the products of an FCCU (Fluid Catalytic Cracking Unit) is light cycle oil (LCO), a highly aromatic diesel boiling range material. LCO is characterized by almost similar den- sity (0.943 g/ml) to that of HDT pyrolysis oil (0.918 g/ml).
Furthermore, the viscosity of LCO is the lowest (2.19 mm2/s) among all the examined petroleum candidates. However, LCO has presented the refiners with limited options for direct use in more valued finished products such as road diesel [40,41].
➢ Heavy cycle oil (HCO): Heavy cycle oil (HCO), produced from FCC units typically set in the distillation range between 623 and 773 K, is also composed of highly polynuclear aromatics, often in excess of 40%, a significant portion of which features tetracyclic and Table 1
Properties of various tested feedstocks.
Units Barley/Wheat Straw Biomass
Pyrolysis-oil HDT pyrolysis-oil SRGO GO LCO HCO LVGO
Density at 288 K g/cm3 1.129 1.138 0.918 0.846 0.867 0.943 1.081 0.896
Viscosity at 313 K mm2/s 110 106.1 7.54 3.157 16.09 2.193 239.4 17.49
Surface tension mN/m – 36.4 32.1 27.6 29.4 30.9 34.2 30.7
C wt% 69.22 57.73 85.85 85.53 85.87 88.95 89.43 85.91
H wt% 7.31 8.23 11.84 13.98 13.5 9.82 8.17 12.99
N wt% 0.82 0.74 0.87 0.19 0.25 0.11 0.24 0.20
S wt% 0.07 0.047 0.039 0.18 0.35 0.9 1.87 0.46
O wt% 22.58 33.25 1.40 0.12 0.03 0.22 0.29 0.44
Water mg/kg 9.16 480 50 55 105 75 45
Water % v/v – 32.8 – – – – – –
Refractive index – – 1.5304 1.500 1.4697 1.4940 1.5460 1.5720 1.4962
Oxidation stability min – 16.1 35.3 1039.8 1011.2 239.0 169.5 1028.2
Picture 2.Petroleum candidate streams.
pentacyclic aromatics [42]. This stream is usually recycled in the riser reactor of an FCC unit without pretreatment. In residue fluid catalytic cracking (RFCC) units, the recycling ratio of HCO to fresh feed is about 0.2. As such, cracking both HCO and fresh feedstock could result in retarded cracking performance and poor conversion quality [42]. According to Table 1, HCO is characterized by high density (1.081 g/ml) almost close to the raw pyrolysis oil feed (1.138 g/ml), presenting it as the heaviest petroleum feedstock among the ones examined. In addition, the viscosity of HCO is also the highest (239.4 mm2/s) of all examined petroleum feeds but also it is even higher compared to the raw pyrolysis oil feed (106.1 mm2/s) rendering its blending perspective difficult.
➢ Light vacuum gas-oil (LVGO): Vacuum gas-oil is used as a raw ma- terial in lube oil production or for preparing lighter fractions by hydrocracking or catalytic cracking [43]. LVGO is characterized by low density (0.896 g/ml) and viscosity (17.49 mm2/s) (Table 1), presenting it as a promising candidate for co-processing with HDT pyrolysis oil.
2.2. Analyses
2.2.1. Dynamic interfacial tension measurements
Dynamic interfacial tension measurements were performed using the pendant drop/axisymmetric drop shape analysis method. CAM200 (KSV) was used and analysis was performed via the Young-Laplace equation using One-Attension Software (version 1.8 BiolinScientific).
The measurements were performed after forming an aqueous phase pendant drop in the sample phase contained in a quartz cell (Hellma Analytics, Müllheim). For the dynamic interfacial tension measure- ments, adsorption times of approximately 2 s and up to 7000 s were used. Dynamic interfacial dilatational rheology measurements were performed using the piezoelectric PD200 module of the same instru- ment. Harmonic oscillations were applied. Measurements of interfacial rheology properties were performed once the interface attained equi- librium as verified by interfacial tension measurements. All measure- ments were performed at 293 ±1 K. The methodology is described more thoroughly in another study [44].
2.2.2. Microscopic observation
A comparative analysis among the blends of pyrolysis oil at 30%
volume fraction with the petroleum streams and HDT pyrolysis oil also at 30% volume fraction with the petroleum streams was carried out in order to examine the effectiveness of pyrolysis oil hydrotreatment on its miscibility with the potential refinery entry points streams. To that aim, 2 cm3 sample of each blend was examined in sedimentation chambers using the Nikon Eclipse SE 2000 inverted microscope [45]. The micro- scopy analysis started with a scan of the entire chamber bottom at low magnification (X45) to give an overview of the sample. This was fol- lowed by analysis at higher magnification (X100) in order to determine the existence of one or two separate liquid phases. Micrographs were taken using the Nikon DS-L1 microscope camera.
2.2.3. Fuel properties analytical measurements
In order to evaluate the potential for utilizing the pyrolysis oil and HDT pyrolysis oil as intermediate blending components with petroleum streams, 1 dm3 of each sample was sent to the Fuels and Lubricants Technology Laboratory of the National Technical University of Athens (Greece). Analyses of the samples were made by employing mainly the test methods described in the EN 590 standard for automotive diesel fuel. Specifically, the density (at 288 K) was determined via ASTM- D4052 [36], kinematic viscosity (at 313 K) via the ASTM-D7042 [46].
The sulfur content was measured via ASTM-D4294 [47]. Nitrogen, hydrogen and carbon content were measured via ASTM-D5291. Once the carbon, hydrogen, sulfur and nitrogen mass fractions were deter- mined, the oxygen concentration was calculated by difference assuming a negligible concentration of all other elements in the product. The
dissolved water volume fraction of pyrolysis oil is measured via the ASTM-D95, while the dissolved water mass fraction from the organic phase of the HDT pyrolysis oil was measured via the ASTM-D63 [48].
The refractive index and the surface tension were measured by ASTM-D1331 [49]. Finally, the oxidation stability was measured via the ASTM D7475.
3. Results
3.1. Microscopic observation results
A homogeneous binary blend is formed according to the miscibility of the two blending components, which is associated with their inter- molecular interactions (polarity). As water is a relatively polar molecule, it tends not to be miscible or be poorly soluble with organic products (hydrocarbon derivatives) which are usually non-polar molecules [50].
One way to evaluate the miscibility of the bio- and fossil-based blending components under examination is microscopic observation.
The results of the light microscopy observation tests on raw pyrolysis oil with different petroleum fraction blends are shown in Picture 3. The first and second rows display the blend samples at X45 magnification, where the different color spots emphasize the variation across the sur- face of the sample. The third row depicts the same blend samples at X100 magnification, indicating the immiscibility of the binary blends.
As it is obvious from Picture 3, the raw, untreated pyrolysis oil was not miscible with any of the five petroleum fractions tested. In every binary blend sample, the presence of two distinct phases between the pyrolysis oil and the tested petroleum stream was clearly detected, as well as the boundaries between the two phases. There were significant differences between these five mixtures to the size of the second phase area, but none of them could be qualified as miscible. The main reason is that raw pyrolysis oil is a mix of organic compounds with high proportion of water and significant amounts of heterocyclic compounds (high polar- ity) [35].
On the contrary, the five binary blends of the HDT pyrolysis oil with petroleum streams were perspicuous and the HDT pyrolysis oil was clearly miscible with all the refinery intermediates that were tested, as is depicted in Picture 4. The first two rows display the blend samples at X45 times magnification, exhibiting homogeneity across the surface of the sample, while the third row display the same blend samples at X100 times magnification, confirming homogeneity and presence of a few very small particles, which is not related to the miscibility under examination.
It is apparent that the hydrotreatment converts all the non-soluble- to-hydrocarbons pyrolysis oil components into compatible and miscible compounds that can be mixed with several refinery in- termediates without any consideration. It is also noteworthy that non- hydrotreated AFP pyrolysis oil has problematic miscibility with all pe- troleum fractions tested. This information expands the exploration of HDT pyrolysis oil as a potential refinery co-feed of multiple petroleum fractions.
3.2. Interfacial tension comparison
Interfaces separating two not miscible fluids such as oil and water which are common to a large number of industrial processes. Interfacial tension was measured for all the blends with pyrolysis oil/HDT pyrolysis oil and petroleum candidates. The results from the measurements of interfacial tension are presented in Table 2, where it is observed that the blends of raw pyrolysis oil with petroleum fractions present high interfacial surface tension. Picture 5(A) presents an example of interfa- cial tension measurement between pyrolysis oil and SRGO. It is observed that the addition of some hundreds of mm3 pyrolysis oil in SRGO does not cause any cloudy effect in the blends neither any change in the color for a measurement time of more than 1 h. Furthermore, with the use of a micro-camera, no mass transfer was detected between the two fractions.
These findings show that the pyrolysis oil is not miscible with the pe- troleum streams, as similar results were also observed with all the pe- troleum candidates, however for brevity reasons, only tests with SRGO are presented in the manuscript.
On the other side, the HDT pyrolysis oil is characterized by almost zero oxygen and water mass fraction (Table 1). Furthermore, according to an author’s previous research [35], the purpose of this HDT step is also to convert the aldehydes, ketones, and sugars to their corresponding alcohols and make the raw pyrolysis oil feed more stable by consisting of phenols and hydrocarbons. As a result, during the measurements of interfacial tension of HDT pyrolysis oil and its blends with petroleum streams, the formation of drop-shaped interfaces was not possible in order to perform the measurement due to almost zero (below the detectable limit) surface tension. A similar methodology with pyrolysis oil was also applied in the case of HDT pyrolysis oil. The interfacial tension measurement between HDT pyrolysis oil and LCO is juxtaposed in Picture 5(B). In that case, the insertion of some hundreds of mm3 of HDT pyrolysis oil in LCO leads to one phase liquid after a few minutes and the HDT pyrolysis oil is totally mixed with the LCO. Similar are the results also with the other petroleum streams. It is observed that, after the complete removal of oxygen and water phase from raw pyrolysis oil via hydrotreatment, the interfacial tension is almost zero (below the detectable limit).
3.3. Properties investigation
The appearance of the HDT pyrolysis oil was improved, and partic- ularly its handling over the raw pyrolysis oil. From the HDT pyrolysis oil analysis (Table 1), it is observed that several properties of the raw py- rolysis oil were improved post hydroprocessing, such as viscosity that was reduced from 106.1 to 7.54 mm2/s and density that was reduced from 1.14 to 0.92 Mg/m3, the oxidation stability that was increased from 16.1 to 35.3 min. In addition, the oxygen mass fraction was reduced from ~33.25 to ~1.4 wt%, and the dissolved water of the raw pyrolysis oil was almost nulled, while a second aqueous phase was formulated, leading to a significant decrease of organic phase polarity (Picture 1).
Blends of 30% volume fraction pyrolysis oil or HDT pyrolysis oil in petroleum streams were prepared in order to evaluate the miscibility effectiveness of the raw pyrolysis oil as well as of the HDT pyrolysis oil.
The properties from the blends of pyrolysis oil with petroleum streams
(30/70 v/v) are presented in Table 3, while the properties from the blends of HDT pyrolysis oil and various petroleum streams (30/70 v/v) are presented in Table 4.
In general, density is normally determined at 288 K and depends on the composition of the fuel and more specifically from the hydrocarbon compounds. However, density is also strongly correlated with other fuel parameters, particularly the cetane number, aromatics content, viscosity and distillation. The energy content of the fuel is approximately pro- portional to the mass of the fuel injected. Thus, density is an important parameter for the evaluation of a liquid fuel. The density for the raw petroleum streams compared to blends with raw pyrolysis oil and HDT pyrolysis oil is depicted in Fig. 1 while the absolute values of densities are presented in Table 1, Tables 3 and 4 respectively. The results clearly demonstrate that the addition of 30% v/v pyrolysis oil on the petroleum streams raises the density of the blends in all examined petroleum fractions with the exception of LVGO. However, as the raw pyrolysis oil is not miscible with petroleum streams, the determination of density is very difficult, as few ml sample is taken from the blend in order to perform the analysis of density. In case of LVGO the sample that was used for the determination of density didn’t contain any pyrolysis oil and thus the absolute value of density between the LVGO and the LVGO/
pyrolysis oil blend is similar. On the other hand, in the case of HDT pyrolysis oil, its addition in some fractions leads to an increase of den- sity, such as SRGO, GO and LVGO while in some other heavier fractions it leads to a decrease of density, such as LCO and HCO. This shows that when heavier petroleum feeds will be used for co-processing with HDT pyrolysis oil, the addition of 30% v/v HDT pyrolysis oil could help to reduce the overall density of the blend. Once again, the favorable effect of the first hydrogenation step on improving the properties of the raw pyrolysis oil is confirmed in most of the cases.
Another important property that was evaluated was the viscosity, which presents the resistance of a liquid to flow [51]. According to a research review [52], the maximum value of viscosity to allow crude oil transportation ranges between 250 and 400 mm2/s at 311 K. The vis- cosity of the raw petroleum streams compared to blends with raw py- rolysis oil and HDT pyrolysis oil is depicted in Fig. 2, while the absolute values of viscosities are presented in Table 1, Tables 3 and 4 respec- tively. According to the results, in most of the petroleum streams, the addition of pyrolysis oil which is characterized by a viscosity of 106.1 mm2/s leads to an increase in the viscosity of the blend as the petroleum streams have lower viscosity (from 2.1 to 17.49 mm2/s) compared to raw pyrolysis oil. An exception is in the case of HCO which has a vis- cosity of about 239.4 mm2/s and thus, the addition of pyrolysis oil leads to a reduction to the viscosity of the blend. On the other hand, in the case of HDT pyrolysis oil which has a viscosity of 7.54 mm2/s, when it is blended with GO, HCO and LVGO it leads to a reduction while when it is blended with SRGO and LCO leads to a small increase of the respective viscosity of the blends. From the results, it is observed that in all cases, the blends either with pyrolysis oil or HDT pyrolysis oil and petroleum streams, are within the specifications for crude oil transportation (<250 mm2/s). Therefore, no additional measures are required to ship them via Table 2
Interfacial tension of pyrolysis oil and HDT pyrolysis oil when blended with petroleum streams.
SRGO GO LCO LVGO HCO
Bio-oil 2.2 ±0.2 1.3 ±0.3 0.4 ±0.0 1.0 ±0.1 a
HDT bio-oil b b b b a
aDue to the very dark color, the measurement wasn’t possible.
b The creativity of drop-shaped interfaces was not possible to perform the measurement due to zero surface tension.
Table 3
Blends of 30% v/v pyrolysis oil with 70 %v/v petroleum streams.
Units Pyrolysis oil/SRGO Pyrolysis oil/GO Pyrolysis oil/LCO Pyrolysis oil/HCO Pyrolysis oil/LVGO
Density at 288 K g/cm3 0.934 0.950 1.002 1.098 0.896
Viscosity at 313 K mm2/s 6.429 25.87 4.547 187.1 27.79
Surface tension mN/m 29.5 30.6 31.8 34.6 31.3
C wt% 75.41 75.81 78.25 79.62 76.04
H wt% 11.88 11.49 9.46 8.22 11.3
N wt% 0.39 0.42 0.32 0.41 0.41
S wt% 0.13 0.24 0.61 1.31 0.32
O wt% 12.19 12.04 11.36 10.44 11.93
Water mg/kg – – – – –
Water % v/v 9.8 10.0 10.2 9.6 10.0
Refractive index – 1.477 1.496 1.537 1.573 1.499
Oxidation stability min 198.2 182.6 71.6 68.2 191.3
pipelines for co-processing at the refinery. However, in the case of HDT pyrolysis oil, the improvement in the viscosity of the blend is remarkable.
The elemental composition analysis was also performed for all
examined binary blends, as presented in Tables 3 and 4. The O/C content versus H/C content for the blends of pyrolysis oil, as well as for the blends with HDT pyrolysis oil and the various petroleum streams, are presented in Fig. 3. As it is expected, the HDT pyrolysis oil blends present Table 4
Blends of 30% v/v HDT pyrolysis oil with 70 %v/v petroleum streams.
Units HDT Pyrolysis oil/SRGO HDT Pyrolysis oil/GO HDT Pyrolysis oil/LCO HDT Pyrolysis oil/HCO HDT Pyrolysis oil/LVGO
Density at 288 K g/ml 0.861 0.896 0.935 1.012 0.903
Viscosity at 313 K mm2/s 3.168 12.020 2.954 25.85 12.94
Surface tension mN/m 28.2 29.4 30.6 33.1 30.9
C wt% 85.71 85.91 88.22 88.54 86.03
H wt% 13.22 12.93 10.28 9.18 12.71
N wt% 0.41 0.45 0.35 0.42 0.42
S wt% 0.14 0.26 0.66 1.39 0.33
O wt% 0.52 0.45 0.49 0.47 0.51
Water mg/kg 180 185 220 200 175
Water % v/v – – – – –
Refractive index – 1.475 1.494 1.532 1.573 1.496
Oxidation stability min 263.4 232.2 109.8 83.8 247.6
Fig. 1. Density (at 288 K) of petroleum streams and blends with pyrolysis oil and HDT pyrolysis oil.
Fig. 2. Viscosity (at 313 K) of petroleum streams and blends with pyrolysis oil and HDT pyrolysis oil.
Fig. 3. O/C ratio vs H/C mass ratio for the blends with pyrolysis oil and HDT pyrolysis oil with various petroleum streams.
Picture 1. On the left (A) is the raw pyrolysis oil feed and on the right (B) is the upgraded pyrolysis oil after mild-hydrotreatment (organic phase and water phase).
Picture 3.Blends of pyrolysis oil (30%) and petroleum streams (70%) via microscope.
much lower O/C content as a result of the deoxygenation step that oc- curs during the upgrading of the raw pyrolysis oil, leading to an HDT pyrolysis oil with negligible water content. Clearly the blends of HDT pyrolysis oil with petroleum streams are more stable compared to the blends of raw pyrolysis oil with petroleum fractions due to its lower oxygen and water content of HDT pyrolysis oil. Furthermore, the oxygen content of the blends is an important factor for co-hydroprocessing as oxygenates are absorbed on the catalyst surface in the areas that in other cases would absorb the hydrocarbon molecules [53]. In that case, as the oxygenates consists of large size molecules, they cannot enter the mes- opores and thus, they turn into coke in the catalyst surface leading to pore blocking due to coke formation [36,54]. Another important effect of oxygenates during co-processing is that when commercial catalysts that are supported via γ-Al2O3 are used during co-processing, they are strongly influenced by the oxygenates. The reason is that the high moisture content which is a result of water formed during the co-process
of a biomass-based feed such as pyrolysis oil, negatively affects the thermal stability of γ-Al2O3. As a result, the formation of coke deposition is high, and it leads to a reduction of the surface area and of the pore volume of the commercial catalysts. Finally, it is observed that the addition of both raw pyrolysis oil and HDT pyrolysis oil decrease the oxidation stability of petroleum feeds. However, in case of raw pyrolysis oil the reduction of oxidation stability is higher compared to HDT py- rolysis oil.
From all the above analysis, the conclusion is that the hydrotreat- ment step is a valuable step to transform the raw pyrolysis oil with high oxygen content to a high-quality HDT pyrolysis oil with almost zero oxygen content which can be a perfect intermediate for co- hydroprocessing with various petroleum streams. According to the elemental composition results, the blends with HDT pyrolysis oil and petroleum streams are by far more stable feeds for the refinery as they have limited oxygen concentration compared to the blends with the raw Picture 4.Blends of HDT pyrolysis oil (30%) and petroleum streams (70%) via microscope.
Picture 5. Interfacial tension measurement between (A) pyrolysis oil and SRGO and (B) HDT pyrolysis oil and LCO.
pyrolysis oil and the petroleum streams (Table 3, Table 4 and Fig. 3). In addition, regarding the results of O/C and H/C ratio of HDT pyrolysis oil and the selected petroleum fractions, the following descending order in values is reported: SRGO <GO <LVGO <LCO <HCO.
The surface tension of pure liquids and liquid mixtures is often required in calculations such as those involving flow through porous media or boiling heat transfer [55]. The surface tension also influences the relative gas/liquid phase permeabilities. The surface tension of the blends is presented in Table 3 for pyrolysis oil with petroleum streams and in Table 4 for HDT pyrolysis oil with petroleum streams respec- tively. Homogeneity of mixtures is related to similarities in physico- chemical properties that reflect the type of intermolecular forces (adhesive and cohesive forces) [39]. The oxygen-containing compounds are related to the increased surface tension. It has been noticed in many cases that fractional components of a mixture affect surface tension without correlating well with the amount of that component [56]. From the results, it is observed that the surface tension between the blends of pyrolysis oil and HDT pyrolysis oil with petroleum streams are very close to each other. This is due to the fact that the difference of the surface tension between the pyrolysis oil and HDT pyrolysis oil is no more than 4 mN/m and thus, the addition of 30% v/v of these bio-crude feeds on the petroleum streams does not influence more than 1–2% the initial surface tension of the petroleum streams. However, considering the re- sults from the surface tension of the blends, the descending order of the petroleum streams is as follow: SRGO <GO <LCO <LVGO <HCO.
4. Discussion
The use of pyrolysis oil as a transportation fuel is limited due to its properties. According to literature [6], the use of pyrolysis oil in blends with fossil fuel, biodiesel or additives is an alternative way of using the pyrolysis oil in transportation sector. However, the results of the study, have shown that the raw pyrolysis oil is not miscible with all the examined petroleum streams. The microscopic observation, has shown that in all cases the two phases of raw pyrolysis oil and petroleum fraction were easily observable. In addition, the findings from the interfacial tension analysis have shown that the blends of raw pyrolysis oil with petroleum fractions present high interfacial surface tension.
Furthermore, with the use of a micro-camera, no mass transfer was detected between the two fractions which clearly proves that the raw pyrolysis oil is not miscible with the petroleum streams. Finally, the properties of the blends with pyrolysis oil have confirmed the negative effect of pyrolysis oil on petroleum candidates in terms of density, vis- cosity and oxygen content. The results of the study come in agreement with the literature as according to Garcia-Perez et al. [57] who has tested blends of pyrolysis oil at a ratio from 10 to 50 wt% with biodiesel, the organic phase proved to be more soluble in biodiesel than the aqueous phase but solubility was limited overall. In addition, the den- sity, viscosity and water content increased slightly. Garcia-Perez et al.
[57] suggested that pyrolysis oil is not soluble in biodiesel, however, it can be improved by supercritical reforming or by aqueous phase cata- lytic processes (APPs). Doll et al. [56] studied blends of pyrolysis oil and biodiesel with high and low sulfur petroleum diesel fuel. According to their results, pyrolysis oil is a viable alternative for biodiesel as a blending component with high and low sulfur petroleum diesel fuel [56]. From a recent review of Han et al. [58] in co-processing of py- rolysis oil with petroleum intermediates, it is concluded that the crude pyrolysis oil is not miscible with non-polar liquid hydrocarbons such as diesel fuel or other petroleum intermediates due to its high polarity and hydrophilic nature and a further upgrading of pyrolysis oil is very important prior to its use as a blending component in a refinery [59].
One way that has been examined in the literature to overcome the immiscibility of pyrolysis oil with diesel fuel is the use of co-solvents.
Alcohols, ethanol and n-butanol are some of the suggested miscibility enhancers of pyrolysis oil and diesel, however according to Weer- achanchai et al. [60] large amounts of alcohol are required for phase
stability. Another way of improving the miscibility of pyrolysis oil is the emulsification with petroleum diesel [61,62]. Ikura et al. [63] emulsi- fied the aqueous top phase (10–30 wt%) of pyrolysis oil with a diesel fuel. According to their findings, the emulsions were more corrosive than diesel fuel but less corrosive compared to neat pyrolysis oil. In general, emulsification has many drawbacks as a method such as the need of a surfactant with high cost, the high energy demands, the corrosion in engines, the poor thermal stability etc. Hydro- deoxygenation was proved to be an alternative way to upgrade the properties of raw pyrolysis oil. However, the literature on miscibility of hydrotreated pyrolysis oil with petroleum streams is rather limited.
Miguel Mercader et al. [13] have investigated the co-processing of an upgraded HDO pyrolysis oil with FCC feed in an FCC unit. Their results have shown that after mixing and heating to 75 ◦C the HDO pyrolysis oil was miscible in the Long Residue FCC feed.
The results of the current investigation confirmed the statements of Han et al. [8] as they have shown that in all examined petroleum can- didates, the addition of HDT pyrolysis oil creates a homogeneous mixture where the two phases cannot be distinguished via microscopic analysis. In addition, no interfacial tension was observed in the current blends. While, from the property’s analysis, it was observed that the addition of HDT pyrolysis oil on the petroleum streams not only did not decline the properties of the initial feed, but also improved them in some cases. This finding is very important as it puts the basis for future research in co-processing of HDT pyrolysis oil with various petroleum fractions. Until now the research of co-hydroprocessing with petroleum fractions was limited mostly to heavy gas-oil (HGO) or straight run gas-oil (SRGO) while the literature on miscibility effect of HDT pyrolysis oil with various petroleum intermediates was very poor [27–29].
5. Conclusions
The results of the original study are summarized below:
• The raw pyrolysis oil is not miscible with all the examined petroleum streams. In all cases, two distinct phases of pyrolysis oil and petro- leum fraction were easily observable.
• The addition of HDT pyrolysis oil in all examined petroleum streams leads to a homogeneous mixture, where the two phases cannot be distinguished via microscope.
• No interfacial tension was observed in the blends of petroleum streams with HDT pyrolysis oil.
• The density in case of HCO and LCO was reduced with the addition of 30% v/v HDT pyrolysis oil. In addition, the viscosity was improved in the case of GO, HCO and LVGO while, the carbon content was increased in the case of SRGO and HCO.
• A first hydrotreatment step of raw pyrolysis oil can lead to a stabi- lized HDT pyrolysis oil which is miscible with all the examined pe- troleum streams.
Acknowledgement
The authors wish to express their appreciation for the financial support provided by European Union’s Horizon 2020 research and innovation program under grant agreement No 727463 for the project
“BioMates”.
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