Life cycle assessment of the Fischer-Tropsch biofuels production by supercritical water reforming of the bio-oil aqueous phase

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published by Elsevier in Energy, Vol. 210, on November 2020, available at: https://doi.org/10.1016/j.energy.2020.118648 © 2020 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND 1 Life cycle assessment of the Fischer-Tropsch biofuels production by supercritical water reforming of the bio-oil aqueous phase F. J. Gutiérrez Ortiz a, B. Alonso-Fariñas a, F. J. Campanario a, A. Kruse b a Department of Chemical and Environmental Engineering, Technical School of Engineering, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain b Department of Conversion Technologies and of Biobased Products, Institute of Agricultural Engineering, University of Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany Corresponding author email: “Francisco Javier Gutiérrez Ortiz” [email protected] Abstract This paper is aimed at performing an environmental evaluation regarding biofuel production. The process combines four sections: biomass fast pyrolysis to bio-oil with two phases, oil-phase upgrading by hydrotreating, using H2 obtained by steam reforming, and a new process to produce Fischer-Tropsch biofuels from supercritical water reforming of the aqueous phase. This phase can be valorised in the latter process entirely so natural gas is reformed to H2 (casestudy 1), or partially so a fraction of this aqueous phase is reformed (case-study 2), or all this phase is reformed to H2 (case-study 3). The two former can include CO2 storage and aqueous phase concentrations were 15, 25 and 35 wt.% organic compounds. At 25 wt.%, the global warming potential is 11 g CO2-eq/MJ-biofuel for the case 2 with CO2 storage, while it was 11.3, 12.6, 27.7 and 34 g CO2-eq/MJ-biofuel for the cases 3, 2 without CO2 storage, 1 with and without CO2 storage, respectively. Thus, the case-study 2 with CO2 storage gives the minimum global warming potential, allowing significant reductions with respect to the use of fossil fuels. For the other categories, the case-study1 presents the lowest impacts. Similar trends are found at 15 and 35 wt.%. Keywords Life Cycle Assessment, Fischer-Tropsch Biofuels, Supercritical Water, Bio-oil, Global Warming, Greenhouse Emissions 2 1. Introduction Climate change, global warming and greenhouse gases (GHG) have become common terms in everyday life. Despite rising prices and apparent depletion of resources, fossil fuel consumption has increased and led to higher GHG emissions, thus contributing to global warming. The use of biology-based renewable fuels (biofuels) makes it possible to reduce the dependence on fossil fuels and permits to decrease the GHG and, hence, the global warming. Biomass can be converted into more transportable forms of fuel/energy such as liquid biooils through fast pyrolysis, which involves thermal decomposition reactions that occur in a few seconds in the absence of oxygen and around 400 - 500 ºC. Fast pyrolysis results in a singlephase bio-oil with a water content of 15-30 wt.%. By adding water, bio-oil can be separated into a valuable oil-phase and an aqueous phase. The former needs to be upgraded for its use as a transportation fuel (a mixture of gasoline-range products and diesel-range products), and it can be performed by a number of techniques, although hydrotreating seems to be the most efficient way to upgrade the oil-phase [1]. This upgrading is necessary due to the corrosive nature of bio-oil, as well as to its high viscosity, limited volatility and chemical instability, which make it unsuitable for direct use as a fuel. The reason is the high content of oxygenated compounds, which sometimes is greater than 50% [2], so oxygen must be removed. The aqueous phase still contains organic compounds (even up to 20-30 wt.%), although the water content of this phase may be as high as 80 wt.% [3]. This high-water content is a high drawback to re-use this fraction and, in fact, the bio-oil aqueous phase has been frequently considered as a waste to be sent to wastewater treatment [4]. Despite this, some valorisation attempts are found, but instead of applying a high-cost extraction technique due to low content of the organic compounds, hydrogen production via catalytic reforming has been mostly explored to valorise energetically the bio-oil aqueous phase [5], [6]. In addition, supercritical water reforming is an emergent technology that can be also 3 used to produce H2, as previously verified [7]. Likewise, there are several works about how to use the aqueous phase of bio-oil for oil-phase upgrading, from the aforementioned hydrogen production. This is of interest because process off-gas and supplemental natural gas (but not the aqueous phase of bio-oil) are mostly feedstocks to be reformed with steam in order to produce the hydrogen required for hydrotreating performed in the oil-phase upgrading [1]. Thus, Hübner et al [8] examined the effect of digesting the bio oil aqueous phase on biomethane production. The hydrogen required for the oil-phase hydrotreating may be obtained by steam reforming of the aqueous phase, as Bridgewater mentioned [9], who pointed out that this topic had been considered earlier, but disregarded due to the high cost associated with the high-pressure requirement and catalyst coking as a result of the poor C-to-H ratio. However, Zhang et al [10] proposed again a process where the aqueous phase was directly used to produce hydrogen by catalytic steam reforming, so this hydrogen was used in the oil-phase upgrading. Likewise, Shemfe et al [11], [12] analysed a process for the biofuel production and electric power generation through biomass fast pyrolysis and bio-oil hydroprocessing, so the required hydrogen was generated by reforming 40 wt.% bio-oil aqueous phase and supplementary natural gas. Similarly, Dang et al [13] used 35 wt.% aqueous phase of bio-oil to produce hydrogen by steam reforming. Life Cycle Assessment (LCA) is a useful tool to compare, in terms of environmental sustainability, alternative production options by quantifying the environmental impacts involved throughout the life cycle of products [14]. There are a number of works on LCA regarding gasification and Fischer-Tropsch synthesis to produce gasoline and diesel from peat [15], electricity generation using fast pyrolysis bio-oil [16], bio-syngas produced from lignocellulosic biomass gasification [17], or sugar cane bagasse [18]. Likewise, there are quite LCA studies on pyrolysis-based pathways [19], and recently a review on techno-economic and LCA for lignocellulosic biomass thermochemical conversion technologies, including pyrolysis 4 and gasification, was published [14]. However, the number of studies on the LCA of bio-oil upgrading is smaller. In this way, Hsu [20] performed the LCA of the production of gasoline and diesel from forest residues by fast pyrolysis and further hydroprocessing, by considering from the feedstock production to the end-use of the fuel in a vehicle, and compared his results with the GHG emissions obtained from gasoline and from ethanol produced via gasification. Later, Iribarren et al [21] carried out another work on the LCA of a fast pyrolysis of poplar biomass in a circulating fluidized bed reactor followed by bio-oil upgrading through hydrotreating and hydrocracking to produce gasoline and diesel, thus identifying direct emissions to atmosphere and the production of natural gas and electricity as the largest contributions to the environmental impact. Likewise, Snowden-Swan and Male [22] and Peters et al [23] conducted studies on LCA of pyrolysis with hydrotreating of poplar residues. In these studies, the authors did not valorise the aqueous fraction of bio-oil. In this regard, Heng et al performed LCA studies on conversion of the aqueous phase into polyol fuel and esters through supercritical methanol catalytic esterification [24] and on H2 production via iron-based chemical looping process using non-aqueous phase bio-oil as fuel [25], so H2 coming from this latter was used for the former process, just the opposite to the overall process dealt with in the present paper, as shown below. Apart from these studies, there are other two LCA works that took into account the bio-oil aqueous phase, one performed by Shemfe et al [12], who quantified the GHG emissions due to the upgrading of bio-oil coming from fast pyrolysis of Miscanthus, and another one by Dang et al [13], who obtained the environmental impacts of the bio-fuel production via fast pyrolysis of corn stover and subsequent hydroprocessing. 2. Aims and scope In the present study, the bio-oil aqueous phase, as a waste stream coming from the bio-oil separation, is reutilized and valorised by a novel liquid carbon from waste to liquid (WTL) process for biofuels production. This process consists of the supercritical water reforming 5 (SCWR) of the bio-oil aqueous phase to obtain a syngas that is converted into biofuels by a Fischer-Tropsch (FT) loop at low temperature (LTFT), so it is termed as SCWR-LTFT and has been already techno-economically assessed [26]. Therefore, the aim of this paper is to carry out an environmental evaluation of the impacts regarding the SCWR-LTFT process by an LCA study to complete the research about the new proposed process. To achieve this purpose, LCA study involves bio-fuels production considering four processes: (1) the bio-oil production from biomass fast pyrolysis and subsequent separation into aqueous- and oilphases, (2) the oil-phase upgrading by hydrotreating, (3) the hydrogen production needed for hydrotreating by steam reforming, as the most common industrial technology used to produce H2, and (4) the aqueous-phase valorization. Figure 1 shows graphically a simple overview of the overall process dealt with in this paper. The first three processes are well-known, but the fourth process is new. The first and second processes are invariable in this study, but the third process affects the fourth one. Thus, the objective of this study is to obtain and analyze the environmental impacts in a comparative way considering three case-studies, depending on the valorization option chosen for the aqueous-phase, thus processing the entire aqueous-phase for FT biofuels production via the proposed process and using external natural gas as feedstock to produce the needed H2 (case-study 1), a fraction of the aqueous phase to produce FT biofuels and the other fraction to be reformed with steam to obtain the needed H2 (case-study 2), and the aqueous-phase for hydrogen production via steam reforming, so no FT biofuels are produced at all (case-study 3). In this way, we have used a similar structure as the aforementioned works [12] and [13] in which LCA was performed in a comparative way. In the scope of the paper, apart from the comparative LCA among the three aforementioned case-studies, three different concentrations of organic compounds in the bio-oil aqueous phase (15, 25 and 35 wt.%) are considered to examine its effect on the environmental impacts of the 6 three cases studies. In addition, as the SCWR-LTFT process allows carbon capture, case-studies 1 and 2 are carried out without and with CO2 capture. Fig. 1. Overview of the overall process with the four relevant sections. Therefore, the study is focused on estimating the cradle-to-gate environmental impacts, especially the global warming impact (GWP), regarding the biofuel production by the SCWRLTFT process applied to the bio-oil aqueous phase along with the biofuels produced by oilphase upgrading. As far as we know, no other study like this has been previously performed, because the SCWR-LTFT process is a novel process and the comparative LCA allows getting insight on the environmental advantages of using the bio-oil aqueous phase for both FT biofuel production and oil-phase upgrading versus reforming all the aqueous phase with steam to produce H2, as a more conventional process, or using natural gas for the oil-phase upgrading. 3. Methodology This section includes the design of the aforementioned four processes to produce biofuels from biomass fast pyrolysis, and the steps that LCA follows in order to evaluate de Biomass (pine wood chips) Fast pyrolysis Phase separator Hydrotreating Distillation Gasoline & Diesel SCWR (+WGS) Distillation Gasoline, Diesel, & Jet fuel FT synthesis PSA SR + WGS Hydrogen Natural gas Fast pyrolysis section SCWR-LTFT section Oil-phase upgrading section Aqueous phase Bio-oil Oil phase H 2 production section Water Aqueous phase Hydrogen H 2 surplus Water 7 environmental impacts according to the purpose and objectives of the paper. As an LCA requires income and outcome mass and energy flows, simulation was used to obtain the mass and energy balance of the four processes because, in contrast to the agricultural production including biomass production, there is no available data from a database. 3.1. Process design The overall process consists of four sections. The fast pyrolysis (FP) section includes drying of the wet biomass, pyrolysis, separation of solids, char combustion, quench of the gases and vapours, and separation of the pyrolysis liquid in two phases, as previously stated [27]. In the FP reactor, the fluid residence time is about 2 s and the reaction temperature (500 °C) is reached almost instantaneously. The char combustion is used to provide the heat required for the pyrolysis and drying. The other sections are described below. Figure 2 depicts the concept of the SCWR-LTFT process (section) in a simple way using a block flow diagram; further details and specifications of the main units as well as the complete flowsheet can be found elsewhere [28]. The process is divided into four parts: (1) SCWR of the bio-oil aqueous phase, (2) syngas upgrading to increase the molar flow rate of H2 and CO, and to achieve the H2/CO molar ratio required at the FT reactor inlet, by water-gas-shift (WGS) and DR reactors, as well as by two PSA systems, (3) Fischer-Tropsch synthesis loop, and (4) refining and upgrading of FT products through distillation and hydrocracking. The material and heat integration of the designed process makes it energy self-sufficient, so no external fuel is needed as a heat source. The bio-oil aqueous phase is pumped and heated, entering the supercritical water reforming reactor operating at 240 bar and 800 ºC. The syngas leaving this unit is expanded to 40 bar, thus generating electrical power. Then, a fraction of syngas enters the high-temperature water-gas shift (HWGS) reactor operating at 350 ºC. The gas leaving this unit is mixed with the by-passed 8 gas and cooled down to 35 ºC in order to separate the condensate (mainly, water) from the noncondensable gas. Fig. 2. Block flow diagram of the SCWR-LTFT process/section. The gas is separated into streams of pure H 2 and pure CO, as well as a CO 2 +CH 4 -rich stream, by two PSA systems. A fraction of the H 2 -rich stream is sent for hydrocracking and the rest goes to the FT synthesis loop along with the CO-rich stream. The CH 4 and CO 2 rich stream is depressurized to 1 bar and heated to 600 ºC before entering the dry reforming reactor. This reactor allows increasing the flow rate of syngas fed into the FT synthesis loop and reduces the CO 2 emissions to atmosphere. In fact, a stream of CO 2 with a purity higher than 95 %, suitable for CO 2 storage, is obtained. The FT reactor operates at 20 bar, 220 ºC and with an inlet molar ratio H 2 /CO of 2.0, so CO conversion per pass is about 50 %. The liquid phase leaving the FT loop enters a decanter to separate water from heavier hydrocarbons, which are expanded and sent to the distillation subsystem. The light stream leaving the first column is composed of jet 15 (3) The impact of catalysts is accounted for in the SCWR-LTFT process to produce biofuels, although they are rarely included in LCA of biofuel production, as recently pointed out [39]. However, this impact was not considered for the other sections since the oil-phase upgrading and steam reforming (SR) were simulated in the same way for all case-studies and, hence, all consumption was always the same. Due to lack of data, the recovery of the spent catalysts was not incorporated but considered a waste to send to a landfill for treatment. System expansion was applied to handle all the co-products as recommended in the guidelines of ISO 14040/44:2006, but also an energy allocation was used to compare the results by applying a different method. In the former, all the impacts are assigned to the main product -biofuels- and credits are subtracted to account for the impacts avoided from the production of co-products through a conventional process. In the latter method, the percentage of total impacts corresponding to a percentage of the total energy output is assigned to the main product, in such a way that thermal energy is multiplied by 1.1, electricity by 2.6 and fuels by 1.0 (H2 must be considered as a fuel –with an LHV of 120 MJ/kg–, even if it has other uses), as indicated by the European Union for the calculation of energy equivalent (MJ), and generally used [40]. 3.2.2. Life cycle inventory (LCI) LCI includes all the data required to quantify the relevant inputs and outputs associated with the production of the functional unit (FU), and will be shown as a result of this study in section 4 from the mass and energy balances performed by Aspen Plus. 3.2.3. Life cycle impact assessment (LCIA) The ReCiPe2016 midpoint (H) V1.02 is the applied method for environmental impact characterization. The following impacts have been considered: Global warming potential, stratospheric ozone depletion, ionizing radiation, ozone formation, fine particulate matter formation, ozone formation, terrestrial acidification, freshwater eutrophication, marine eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, human 16 carcinogenic toxicity, human non-carcinogenic toxicity, land use, mineral resource scarcity, fossil resource scarcity and water consumption. However, since the main product is biofuel, the discussion is mostly focused on the global warming potential (GWP). 4. Results and discussion In this section, life cycle inventory is obtained from process simulation, as described in the previous section, and using Ecoinvent, so as to perform the evaluation of the environmental impacts according to the aims of the paper, mainly but not only focused on GWP. 4.1. Mass/Energy balances and inventory First results are related to the mass balance for the fast pyrolysis (FP) unit, which is fed with biomass (pine wood) and the bio-oil phase separator where water is added. The FP section is the same for all case-studies (same feedstock and produced bio-oil). The required wet biomass (50 wt.% H2O) is 295.9 t/h that is dried to achieve 8 wt.%-wet biomass (160.8 t/h), before entering the FP unit. This biomass has 20.2 wt.% char and an ash-to-char mass ratio of 29.1 wt.% [27]. The biomass drying is performed by the flue gas coming from the combustion of the char in the FP section. The bio-oil flowrate is 100 t/h (62.19 % dry biomass), with 75 wt.% organic compounds and 25 wt.% water. The flowrates of char and ash result in 32.4 and 9.4 t/h, respectively. The rest is fuel gas. Table 3 shows the flowrates of the oil and aqueous phases as well as of the water added for the phase separation so as to the organic content of the aqueous phase of bio-oil ranges from 15 to 35 wt.%. This Table is based on the diagram by Oasmaa and Peacocke [41] to compute organic and water mass fractions in each phase of bio-oil. Table 4 shows the life cycle inventory (LCI) for the three case-studies at 25 wt.% organic compounds in the bio-oil aqueous phase. Regarding the FP section, the water for separation of bio-oil phases is negative, and it means that external water is not needed. In fact, 59.6 m3/h 17 must be added to the separator for the three case-studies, but the water separated in the oilphase upgrading and SCWR-LTFT section related to bio-oil aqueous phase, which depends on the case-study, overcomes that value. Table 3. Mass balance in the bio-oil phase separator for the three concentrations of organic compounds in the aqueous phase. Syngas from SCWR by-passes the HWGS reactor for aqueous phase concentrations of 15 and 25 wt.%, because the flow rate of CO, which determines the FT liquid biofuel production, is low. However, at 35 wt.%, a fraction of CO must be converted into H2 by HWGS reactor, so the by-pass is reduced from 100 % to 57 %. This implies the use of another reactor and catalyst. The amount of water produced by the oil-phase hydrotreating decreases from the case at 15 wt.% to the one at 35 wt.%, although, in the former case more water leaves the reactor because a more diluted feedstock is fed. Likewise, the concentration of organic compounds at the hydrotreating reactor outlet increases with the concentration of the aqueous phase, so more diesel and gasoline are produced. At 35 wt.%, more heat is needed for distillation since a higher mass of organic compounds is fed to the column, although the heat required in the heater before entering the reactor decreases from the case at 15 wt.% to the one at 35 wt.% due to the amount of water. Aqueous phase Oil-phase Total Bio-oil (from FP) Added water Mass flow (k g /h) 84 880 74 694 159 574 100 000 59 574 Or g anic mass f low (k g /h) 21 220 53 780 75 000 75 000 0 Water mass f low (k g /h) 63 660 20 914 84 574 25 000 59 574 wt.% or g anic compounds 25 72 47 75 0 wt.% wate r 75 28 53 25 100 Mass flow (k g /h) 159 198 75 177 234 375 100 000 134 375 Or g anic mass f low (k g /h) 23 880 51 120 75 000 75 000 0 Water mass f low (k g /h) 135 318 24 057 159 375 25 000 134 375 wt.% or g anic compounds 15 68 32 75 0 wt.% wate r 85 32 68 25 100 Mass flow (k g /h) 55 875 73 436 129 310 100 000 29 310 Or g anic mass f low (k g /h) 19 556 55 444 75 000 75 000 0 Water mass f low (k g /h) 36 319 17 992 54 310 25 000 29 310 wt.% or g anic compounds 35 75.5 58 75 0 wt.% wate r 65 24.5 42 25 100 18 Table 4. Life cycle inventory per 1 MJ biofuel (functional unit, FU) at 25 wt.%. SECTION DESCRIPTION TYPE Unit per FU (MJ biofuel) CASE 1 CASE 2 CASE 3 Unit Fast pyrolysis Biomass (wet) 50 % Input 2.45E-01 3.23E-01 3.65E-01 kg Fast pyrolysis Biomass waste (ash) Waste 7.83E-03 1.03E-02 1.16E-02 kg Fast pyrolysis Water for the separation Input -4.65E-02 -3.56E-02 -3.58E-02 kg SCWR-LTFT FT-diesel Main product - - - - SCWR-LTFT FT-jet fuel Main product - - - - SCWR-LTFT FT-gasoline Main product - - - - SCWR-LTFT Water for district heating Co-product 9.94E-02 2.48E-02 0.00E+00 MJ SCWR-LTFT Electricity Co-product* 6.72E-03 1.68E-03 0.00E+00 kWh SCWR-LTFT Ni-SCWR Catalyst Input 7.33E-08 1.83E-08 0.00E+00 kg SCWR-LTFT Fe-WGS Catalyst Input 0.00E+00 0.00E+00 0.00E+00 kg SCWR-LTFT Ni-DR Catalyst Input 3.02E-08 7.55E-09 0.00E+00 kg SCWR-LTFT Co-LTFT Catalyst Input 9.68E-08 2.42E-08 0.00E+00 kg SCWR-LTFT Pt-HYDR Catalyst Input 5.28E-08 1.32E-08 0.00E+00 kg SCWR-LTFT Waste catalysts (to landfill) Waste 2.53E-07 6.33E-08 0.00E+00 kg SCWR-LTFT (with CO2 storage) Biogenic CO2 Emission -6.51E-03 -1.63E-03 0.00E+00 kg SCWR-LTFT (with CO2 storage) Electricity Input* 7.70E-04 1.92E-04 0.00E+00 kWh Oil-phase upgrading High temperature heat Input* 5.90E-02 2.47E-01 3.66E-01 Oil-phase upgrading Green gasoline to use Main product 1.36E-03 5.69E-03 8.43E-03 kg Oil-phase upgrading Electricity Input 4.59E-03 5.32E-03 7.17E-03 kWh Oil-phase upgrading FT-diesel Main product - - - - Oil-phase upgrading FT-gasoline Main product - - - - Oil-phase upgrading Emitted fossil CO2 Emission 2.59E-02 0.00E+00 0.00E+00 kg Oil-phase upgrading Natural gas Input 8.57E-03 0.00E+00 0.00E+00 m3 Oil-phase upgrading Hydrogen Co-product 0.00E+00 0.00E+00 8.17E-04 kg Supplementary Total exported electricity* - - Without carbon storage (case -A) 2.13E-03 -3.64E-03 -7.17E-03 kWh With carbon storage (case -B) 1.36E-03 -3.84E-03 - kWh Total direct emissions of CO2 Without carbon storage (case -A) 2.59E-02 0.00E+00 0.00E+00 kg With carbon storage (case -B) 1.94E-02 -1.63E-03 - kg CASE 1: Everything goes to SCWR-LTFT; biofuel production is 1 205 519 MJ/h but burning 8 058 kg/h natural gas. CASE 2: A fraction is sent to SCWR-LTFT; biofuel production is 914 778 MJ/h. CASE 3: Nothing is processed by SCWR-LTFT; biofuel production is 811 682 MJ/h but producing 663.5 kg/h H2 surplus. * Electricity would be a co-product when the net balance (total exported electricity) results in a positive value. When a negative value is reported, electricity becomes an input. In case of using a fraction of the aqueous phase to produce the required hydrogen (casestudy 2), the rest may be processed in the SCWR-LTFT system: 55.0 t/h at 15 wt.%, 16.1 t/h at 25 wt.%, and 3.2 t/h at 35 wt.%. These decreasing values are due to the higher flowrates of aqueous phase at lower concentrations. 19 4.2. Environmental characterization of the system In this study, the individual impacts in each sub-section or sub-system is not provided due to the high level of energy integration among all sections in the overall process, especially in the SCWR-LTFT process; to be coherent in the LCA study, it was not performed for any subsystem. This affects mainly the electricity and heat recovery, which cannot be broken down. However, the catalyst impact is known for the different parts of the SCWR-LTFT section (catalytic reactors). The other consumption is due to biomass. Waste comes from the use of catalysts, but without any further distinction because it would be very speculative to calculate the impact of each source separately. Thus, it was considered to be proportional to the total amount of catalyst and the amount coming from each one is known. Likewise, waste comes also from biomass ash. The way to proceed is similar to the focus given in other studies, such as that by Iribarren et al [17], where the results of the environmental characterization of the system and the distribution of the environmental impacts among the products were presented without mentioning individual impacts in each sub-section, and as the work recently performed by Vienescu et al [42], where environmental impacts were given globally for several scenarios related to different methods of oil-phase upgrading, but used without distinguishing among the different sub-sections in each scenario. Next, the results are commented and discussed using a concentration of 25 wt.% organic compounds in the aqueous phase as a base value. In addition, the possibility of CO2 storage was considered for case-studies 1 and 2, so label ‘A’ means without CO2 storage, while label ‘B’ denotes that CO2 storage is included. Finally, results obtained for the two other concentrations (15 and 35 wt.%) are also presented and compared with the base concentration. In Table 4, biogenic CO2 is mentioned, which corresponds to CO2 emission coming from processing the biologically-based feedstock, so it cannot be accounted for as fossil carbon. Biogenic CO2 comes from the CO2 capture by a PSA system implemented in the SCWR-LTFT 20 process, and it involves negative figures, which is positive from an environmental viewpoint. Biogenic CO2 is higher for the case-study 1 (-6.51ꞏ10-3 kg/MJ) since it processes all the bio-oil aqueous phase by SCWR-LTFT, while the case-study 2 only processes a fraction of the aqueous phase (-1.63ꞏ10-3 kg/MJ) and the case-study 3 does not process aqueous phase at all by SCWRLTFT, so biogenic CO2 is null. Likewise, because of the larger amount of water present in the bio-oil aqueous phase, the high temperature heat needed in the oil-phase upgrading section increases from the case-study 1 to the cases 2 and 3. As a consequence, for an energy selfsufficient process, a part of the produced biofuels must be used in the own process, and green gasoline was chosen because of its higher production and, hence, larger availability. 4.2.1. Global warming potential (GWP) Figures 5-7 illustrate the results regarding the GWP by system expansion, as the most significant environmental impact when dealing with biofuels as opposite to fossil fuels. In these Figures, a legend allows distinguishing some aspects considered in this study. Thus, electricity, heat recovery (water for district heating) and H2 are potential co-products, and they are taken into account as credits corresponding to negative impacts. The two formers may be present if the SCWR-LTFT process is operating (case-studies 1 and 2), while the latter one is only present for the case-study 3, where there is a H2 surplus and 8.17ꞏ10-4 kg H2/MJ is obtained. Total exported electricity is the co-produced electricity (only for case-studies 1 and 2) minus the input due to the oil-phase upgrading. When CO2 storage is added, total exported electricity decreases as more electricity is needed (input). It can be seen as electricity becomes an input for the case-studies 2 and 3. Similarly, water for district heating (heat recovery) is higher for the case-study 1 (9.94ꞏ10-2 MJ/MJ) since it processes the entire bio-oil aqueous phase, while the case-study 2 only processes a fraction of the aqueous phase (2.48ꞏ10-2 MJ/MJ) and the case-study 3 does not process aqueous phase at all, so heat recovery from hot water is null. 21 Figure 5. Global warming potential for all case-studies at 25 wt.% organic compounds in the aqueous phase (system expansion). Likewise, direct CO2 emissions come from the burned natural gas (2.59ꞏ10-2 kg CO2 eq for the case-study 1A); even considering carbon storage (case-study 1B), there is a positive-value impact regarding CO2 emissions (1.94ꞏ10-2 kg CO2 eq). Moreover, credits coming from electricity (-7.32ꞏ10-4 kg CO2 eq and -4.67ꞏ10-4 kg CO2 eq for the cases 1A and 1B) and heat recovery (-5.22ꞏ10-3 kg CO2 eq for both cases 1A and 1B) cannot overcome the high impact due to the direct CO2 emissions. On the other hand, in the case-study 2B, direct CO2 emissions have negative values (-1.63ꞏ10-3 kg CO 2 eq) thanks to the carbon storage and the higher proportion of biogenic CO2. Finally, the term ‘process’ refers to the impact of catalyst and ash, natural gas supply, and even electricity if this is consumed, while ‘feedstock’ denotes the impact of pine wood fed into the process: 9.56ꞏ10-3, 1.26ꞏ10-2 and 1.42ꞏ10-2 kg CO2 eq for the casestudies 1, 2 and 3, respectively. On each bar in these graphs, the total GWP is written. 0.0340 0.0277 0.0126 0.0110 0.0113 -0.02 -0.01 0.01 0.02 0.03 0.04 0.05 Case 1A Case 1B Case 2A Case 2B Case 3 GWP (kg CO 2 eq.) Hydrogen Heat (water) Electricity Direct CO2 emissions Process Feedstock 22 Figure 6. Global warming potential for all case-studies at 15 wt.% organic compounds in the aqueous phase (system expansion). Figure 7. Global warming potential for all case-studies at 35 wt.% organic compounds in the aqueous phase (system expansion). 0.0288 0.0251 0.0100 0.0081 0.0105 -0.03 -0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 Case 1A Case 1B Case 2A Case 2B Case 3 GWP (kg CO 2 eq.) Hydrogen Heat (water) Electricity Direct CO2 emissions Process Feedstock 0.0373 0.0279 0.0125 0.0120 0.0124 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 Case 1A Case 1B Case 2A Case 2B Case 3 GWP (kg CO 2 eq.) Hydrogen Heat (water) Electricity Direct CO2 emissions Process Feedstock 23 At 25 wt.%, the GWP for the case-study 1A and 1B is 253 % and 151 % higher than for the case study 2B, while GWP for the case-studies 2A and 3 is 14 % and 2 % higher than for the case study 2B. Similar trends are found at 15 wt.% and 35 wt.%, although the percentages decrease when the concentration is reduced, especially for the case-study 3 that is 29 % higher than for the case study 2B. This is because of the larger amount of biogenic CO2 and coproduced electricity and hot water related to the case-study 2B at the lowest tested concentration of bio-oil aqueous phase (15 wt.%) despite the GWP increases with this concentration. Figure 8 shows the GWP results based on an energy allocation. The percentages of energy allocation were the following for the fifteen scenarios studied in this manuscript:  At 15 wt.%: 72.98 %, 73.20 %, 87.03 %, 87.20 % and 72.24 % for case-studies 1A, 1B, 2A, 2B and 3, respectively.  At 25 wt.%: 88.55 %, 89.12 %, 97.34 %, 97.34 % and 91.07 % for case-studies 1A, 1B, 2A, 2B and 3, respectively.  At 35 wt.%: 85.88 %, 92.50 %, 99.43 %, 99.43 % and 98.30 % for case-studies 1A, 1B, 2A, 2B and 3, respectively. As can be seen, the case-study 2B is again the option with the lowest GWP, but now the second-best option is the case-study 2A (13 % higher at 15 and 25 wt.%, and 4 % at 35 wt.%). At 15 wt.%, both the higher impacts due to the feedstock and electricity in the case-study 3 justify the third position regarding the GWP (80 % higher than for the case-study 2B). At 25 wt.%, electricity is the main reason (27 % higher than for the case-study 2B), while at 35 wt.% there is not a clear main reason but the sum of all the aspects (7 % higher than for the casestudy 2B). In all cases, biogenic CO2 along with a balanced electricity requirement are crucial to boost the case-study 2B to the best position related to GWP. Therefore, the best option would be to perform the SCWR-LTFT process with a fraction of the bio-oil aqueous phase while using the rest to produce the H2 required for hydrotreating the oil-phase. Likewise, it should be noted 24 that whereas the GWP increases for the case-studies 1A and 1B as the concentration of organic compounds in the aqueous phase increases (154, 194 and 196% higher for case-study 1A than for the case-study 2B, at 15, 25 and 35 wt.%, respectively), the opposite takes place for the rest of case-studies (2A, 2B and 3), as shown above. This is mainly due to the increasing percentage of energy allocation for both case-studies 1 and the decreasing amount of water that reduces not only the heat recovery but also the produced electricity; in fact, this latter becomes a requirement at 35 wt.%. Figure 8. Global warming potential for all case-studies at 15 (in red), 25 (in blue) and 35 (in green) wt.% organic compounds in the aqueous phase (energy allocation). Whichever allocation method is used to assess the global warming potential, the use of SCWR-LTFT process provides the lowest contribution to GHG, especially if carbon storage is considered. This is due not only to the efficient biofuel production but also to the electricity production as well as to the process energy design that involves cogeneration and CO2 storage. 0.0326 0.0299 0.0145 0.0128 0.0231 0.0353 0.0298 0.0135 0.0120 0.0153 0.0362 0.0294 0.0127 0.0122 0.0131 0.00 0.01 0.02 0.03 0.04 0.05 GWP (kg CO 2 eq.) 31 FT: Fisher-Tropsch FU: Functional Unit GWP: Global Warming Potential HWGS: High-temperature Water-Gas Shift LCA: Life Cycle Assessment LCI: Life Cycle Inventory LHV: Lower Heating Value LTFT: Low-Temperature Fisher-Tropsch PEM: Proton-Exchange Membrane (fuel cell) PSA: Pressure Swing Adsorption SCW: SuperCritical Water SCWR: SuperCritical Water Reforming SR: Steam Reforming UNIQUAC: UNIversal QUAsiChemical (thermodynamic method) WGS: Water-Gas Shift WTL: Waste-To-Liquid References [1] Jones S, Valkenburg C, Walton C, Elliot D, Holladay J, Stevens D, et al. 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