A review of the current state of biofuels production from lignocellulosic biomass using thermochemical conversion routes

Paola Ibarra-Gonzalez,Ben-Guang Rong

Department of Chemical Engineering,Biotechnology and Environmental Technology,University of Southern Denmark,Campusvej 55,DK-5230 Odense M,Denmark

ABSTRACT The rapid increase in energy demand,the extensive use of fossil fuels and the urgent need to reduce the carbon dioxide emissions have raised concerns in the transportation sector.Alternate renewable and sustainable sources have become the ultimate solution to overcome the expected depletion of fossil fuels.The conversion of lignocellulosic biomass to liquid (BtL) transportation fuels seems to be a promising path and presents advantages over first generation biofuels and fossil fuels.Therefore,development of BtL systems is critical to increase the potential of this resource in a sustainable and economic way.Conversion of lignocellulosic BtL transportation fuels,such as,gasoline,diesel and jet fuel can be accomplished through various thermochemical processes and processing routes.The major steps for the production of BtL fuels involve feedstock selection,physical pretreatment,production of bio-oil,upgrading of bio-oil to transportation fuels and recovery of value-added products.The present work is aiming to give a comprehensive review of the current process technologies following these major steps and the current scenarios of biomass to liquid facilities for the production of biofuels.

Keywords:Biofuels Lignocellulosic biomass Process route Thermochemical conversion Upgrading Separation

1.Introduction

Transport has been the sector most resilient to efforts to reduce CO2emissions due to its strong dependence on fossil energy sources.Oil is the main energy source for transport,it supplies 95%of the sector’s energy consumption [1]and is currently expected to reach depletion in the perspective for 2050,where an objective to reduce the CO2emissions in 80%-95%has been stated[2].Therefore,substitution of oil needs to start as soon as possible and increase rapidly to compensate for declining oil production.The ultimate solution to near full decarbonisation of transport is the substitution of fossil sources by CO2-neutral alternative feedstocks [3].Transportation fuels from renewable sources should be given particular attention,as they can be produced from a wide range of primary energy sources,such as biomass.Biomass is considered to be carbon neutral because the quantity of CO2released during combustion is the same as that absorbed by the plant during photosynthesis[4].

Moreover,biomass has several advantages compared to other renewable energy resources;it can be used for heat,power and production of transportation fuels,and is able to produce continuous energy and be applied to a wide range of scales [4].However,biomass fuels provided only about 5% of the primary energy used in the United States in 2017 and so far this year,as reported by The U.S Energy Information Administration on their monthly review[1].Of that 5%,about 48%corresponded to biofuels derived from food crops,such as bioethanol and biodiesel,41% was from wood and wood-derived biomass,and about 11%was from the biomass in municipal waste[5].Unfortunately,first generation biofuel derived from food crops creates many problems ranging from net energy losses to greenhouse gas emission to increased food prices.Therefore,the development of second-generation biofuels from lignocellulosic biomass has to be explored,due to its many advantages in terms of energy and environmental concerns.Geographical location of the feedstock sources is other of the many favorable factors for the implementation of lignocellulose-based biofuels.These feedstocks are more evenly distributed than the fossil sources enabling to a large extent the security of supply[6].Regarding the total production costs,for first generation biofuels,the main cost factor is feedstock,which accounts for 45%-70%of total production costs,whereas for lignocellulosic biofuels the feedstock costs account for 25%-40%,and the capital cost is the main cost factor accounting for 35%-50% [7].Therefore,extensive investigation on the technologies involved in the production of lignocellulosic biofuels has to be carried out.For instance,important information in terms of capital costs,energy consumptions as well as emissions for different process routes and product portfolios can be obtained when the total production process technologies are evaluated through process synthesis and integration of their corresponding sections (conversion,upgrading and separation) [8].

Production of transportation fuels from lignocellulosic biomass can be achieved by the application of a number of technologies,each with its specific requirements,advantages and disadvantages.The aim of this study is to make an overview of the technologies for the thermochemical conversion of lignocellulosic BtL transportation fuels.To achieve this goal,a review on the BtL technologies including feedstock selection,pretreatment,production of bio-oil,upgrading of bio-oil to transportation fuels and recovery of value-added products is presented.

2.Biomass to Liquid Definition

The biomass source determines the choice of conversion process and any subsequent processing difficulties that may arise.Additionally,the form in which the energy is required influences the choice of biomass source.Thus,the relation between these two aspects enables the flexibility of biomass as an energy source.

The conversion of biomass into energy can be achieved in a number of ways.Biomass can be burned directly or converted to liquid biofuels or syngas that can be burned as fuel[5].Essentially,biomass can be converted into three main types of products:electricity/heat energy,transport fuel,and chemical feedstock [9].

Biomass conversion to liquid transportation fuels can be accomplished through thermochemical and biochemical processes.Thermochemical conversions compared to biochemical conversions are processed at several higher degrees of temperature with or without catalysts to obtain liquid products from different sources [10]and offer low-cost products with some mature technologies [11].In general,thermochemical conversions are much rapid than biochemical conversions [10].

The thermochemical conversion of biomass to synthetic fuels is called biomass-to-liquid(BtL).The objective is to produce fuel components that are similar to those of current fossil-derived fuels and hence can be used in existing fuel distribution systems and with standard engines[12].To produce BtL biofuel,biomass is converted into synthesis gas,which is subsequently processed into synthetic biofuels.BtL fuels also have a very special property:their quality is even better than that of fossil fuels.They excel through significantly lower CO2emissions,zero emission of particulate matter,low NOxemissions and an adjustable product quality (octane and cetane number).As a result they can be used pure or as a blend in vehicles without the engines having to be modified[13].

Thermochemical conversion processes are divided into gasification,pyrolysis,supercritical fluid extraction,and direct liquefaction[14].Each process is chosen based on the types of biomass used,the energy demand,and the applicability in either laboratory setups or industrial applications.Currently,the major BtL production processes are gas-to-liquid conversion through gasification and Fischer-Tropsch and pyrolysis.Both processes employ heat and chemical reactions to convert biomass into fuels,chemicals and power.The products of both processes are cleaner and more efficient than the solid biomass from which they were derived.In addition,these processes are based on well-established commercial technologies.Another benefit is that BtL processes can convert different types of lignocellulosic biomass such as wood and agricultural residues that are difficult to handle using other biofuel production processes[13].

3.Lignocellulosic Biomass as Source for Transportation Fuels Production

From the last several years,due to the worldwide economic and environmental pollution issues there has been increasing research interest in the value of lignocellulosic biomass.Lignocellulosic biomass resources include wood and wood wastes,agricultural crops and their waste byproducts,grasses and energy crops [15].

Lignocellulosic sources are a promising feedstock because of high yields,low costs,abundancy,good suitability for low quality land and the low environmental impacts.They provide a resource for large-scale and cost-effective energy production.Different conversion routes have been applied to lignocellulosic biomass for energy production.For example,wood and wood processing wastes are burned to heat buildings,to produce process heat in industry,and to generate electricity;agricultural crops and waste materials are burnedas a fuel or converted to liquid biofuels;yard and wood waste are burned to generate electricity in power plants[5].

If lignocellulosic biomass is selected as energy source for the production of transport fuels,particular biomass properties become important during the processing.The main properties of interest are moisture content,calorific value,proportions of fixed carbon and volatiles,ash/residue content and alkali metal content[9].

Based on the moisture content lignocellulosic biomass can be classified in two types,high-and low-moisture contents.Most commercial activity has been directed towards the lower moisturecontent types,such as,lignocellulosic wood and agricultural wastes.Dry biomass such as wood chips,is more economically suited to thermochemical conversion processes[9].The proximate and ultimate analyses of wood material are presented in Table 1[16].

Moreover,these materials are mostly composed of three major units i.e.,cellulose,hemicellulose and lignin.Cellulose is a major structural component of plant cell walls,which is responsible for mechanical strength,while hemicellulose macromolecules are often repeated polymers of pentoses and hexoses.Lignin contains three aromatic alcohols (coniferyl alcohol,sinapyl alcohol and pcoumaryl alcohol) produced through a biosynthetic process and forms a protective seal around the other two components i.e.,cellulose and hemicelluloses [6].In general the composition of lignocellulose highly depends on its source whether it is derived from the hardwood,softwood,or grasses.

Lignocellulosic biomass conversion to liquid fuels is the process of interest in this review.Woody and agricultural wastes with low moisture content are explored since they are the most efficient biomass sources for thermochemical conversion to liquid fuels [9].

4.Thermochemical Conversion of Lignocellulosic Biomass to Transportation Fuels

The production of transportation fuels from lignocellulosic biomass is accomplished through a series of steps.The major steps involved are pretreatment,production of bio-oil,upgrading of bio-oil into transportation fuels and recovery of products and byproducts,as depicted in Fig.1.

Fig.1 shows the different technologies discussed in this review.In the next section,physical pretreatment of biomass is discussed.Then,in Section 6,thermochemical conversion of biomass throughdifferent processes is introduced.Section 7 compares the main thermochemical conversion technologies and highlights its differences.In Section 8,upgrading technologies that have been investigated and major findings are presented.In Section 9,the separation needs at any stage of the processes are also discussed.In Section 10,a comparison of the upgraded products properties is presented.Finally,in Section 11,future prospects in process systems engineering methods and tools applied to BtL transportation fuels are discussed.

Table 1 Proximate and ultimate analyses of wood material

Fig.1.BtL transportation fuels production.

5.Biomass Pretreatment

Feedstock preparation techniques are very much essential in the case of solid biomass for any thermochemical method.Thermochemical methods of conversion have certain advantages in terms of processing of biomass.The separation of all the components of biomass is not required and hence,no chemical pretreatment is needed.However,to achieve the high bio-oil yields it is necessary to prepare the solid biomass feedstock in such a manner that it can facilitate the required heat transfer rates.The main properties considered in the pretreatment of lignocellulosic biomass are size distribution,particle shape,moisture content,bulk and particle densities,compressibility and compact ratio[17].Size reduction is needed to obtain appropriate particle sizes.Drying is needed to achieve appropriate moisture so that the process can work efficiently.Densification also may be necessary due to the low density of biomass.

Additionally,particle size of biomass feedstock has a major influence on the heating rate of solid fuel,making it an important parameter.Particles have to be very small to fulfill the requirements of rapid heating and to achieve high liquid yields.Feed specifications range from less than 200 mm for rotating cone reactors to less than 2 mm for fluid beds and less than 6 mm for transported or circulating fluid beds [18].Size reduction is carried out using hammer mills,ball mills,crushers and so on[17].The particle size also controls the rates of drying.

On the other hand,drying is important to reduce the moisture content of biomass.Thermochemical processes such as pyrolysis and gasification yield higher efficiencies when the moisture content of the biomass is lower.Failing to do so consumes energy to vaporize the moisture and all the feed water will be included in the liquid product,thus reducing the efficiency of the process.For the same reason,dried biomass such as wood chips are usually preferred in thermochemical processes [19].

Contrary to fast pyrolysis and gasification processes,in the direct liquefaction process,drying the feedstock is not needed,which makes it especially suitable for naturally wet biomass [20].

6.Bio-oil Production

After feedstock preparation,the subsequent step is the thermochemical conversion of lignocellulosic biomass into bio-oil.Several conversion technologies such as gasification followed by Fischer-Tropsch,pyrolysis,direct liquefaction and supercritical fluid extraction have been investigated and are described in this section.

6.1.Biomass gasification and Fischer-Tropsch synthesis

Gasification is a thermochemical conversion process that involves reacting biomass with air,oxygen,or steam to produce a gaseous mixture of CO,CO2,H2,CH4,N2and a rich spectrum of hydrocarbons,either known as producer gas,synthesis gas or syngas,depending on the relative proportions of the component gases[21].A general objective of gasification is to maximize the yields of gaseous products and minimize the amounts of condensable hydrocarbons and unreacted char.The exact composition of product gas depends on the type of process feeds,their feed ratios,process parameters and the type of gasification reactor used [22].

Syngas has approximately half energy density of natural gas(methane) and can be used for its heat value in steam cycles,gas engines,fuel cells,or turbines to generate power and heat.Syngas is also an intermediate feedstock for producing liquid fuels and a large number of commodity chemicals,including hydrogen,synthetic natural gas,naphtha,kerosene,diesel,methanol,dimethyl ether and ammonia[23].For transportation fuels,the main syngas derived routes to fuels are hydrogen by Water Gas Shift Reaction(WGSR) [24],hydrocarbons by Fischer-Tropsch (FT) synthesis or methanol synthesis followed by further reaction to produce hydrocarbon or oxygenated liquid fuels [25].

For the production of transportation fuels from lignocellulosic biomass via gasification,the main steps that take place include syngas generation,followed by syngas cleanup and Fischer-Tropsch synthesis [26].The syngas generation is carried out in a gasifier.Gasifiers can be designed to gasify almost any kind of organic feedstock,including many types of wood,agricultural residues,and municipal solid waste [23].In a typical atmospheric fluidized bed gasifier,feed together with bed material,are fluidized by the gasifying agents,such as air,oxygen and/or steam,entering at the bottom of the bed.Heat is supplied to the gasifier either directly or indirectly to raise the gasification temperature to 600-1000°C [27].Residences times for the gasification reaction are in the order of 3-4 s [28].The product gas resulting from the gasification process is called raw gas.The raw gas is cooled in a heat exchanger and then fed to a gas-solid separator(i.e.,cyclone)to separate solid particles carried by exhaust gas[29].The raw gas still present some impurities that depend on the carbon source used for its production as well as on the gasification process.Typical impurities include nitrogen compounds,HCl,H2S and COS,which are strong poisons for the WGSR and FT catalysts as they deactivate Fe,Co,and Ni catalysts by forming metal sulfides,and cause reactor corrosion.Therefore,the impurities must be brought down to a very low level.For this purpose,a water scrubber or a solid adsorbent in a packed bed is used as it has a marginal effect on the investment cost [23].

After separating,the raw gas is sent to the WGSR.The WGSR is employed to adjust the H2/CO ratio,in which carbon monoxide reacts with water vapor over a catalyst to produce carbon dioxide and hydrogen,as depicted in Eq.(1).Since the WGSR is reversible,carbon dioxide and water are also formed.

The gas finally obtained is called syngas.The syngas is by passed over supported metal catalysts (Fe,Co,Ru,Rh and Ni) in a FT reactor to produce hydrocarbons.Two main regimes have been used:low-temperature Fischer-Tropsch (LTFT),usually at 200-250°C,that gives long chain molecules,and the hightemperature Fischer-Tropsch (HTFT),at 320-375°C,that gives shorter chain molecules [23].The best form of the reactor to be used depends on the catalyst,the conditions and the distribution of products that is desired.HTFT uses iron catalysts in two phase fluidized bed reactors;LTFT uses either iron or cobalt in three phase slurry reactors or tubular fixed bed reactor [30].

Additionally,all FT applications involve cooling the gases and vapor at the FT reactor outlet to condense and separate the hydrocarbons and water products from the tail gas.Part of the tail gas is usually recycled back to the FT reactor.The remaining gas can be recycled back to syngas production or can be used as fuel for the production of hydrogen or to generate electrical power with a gas turbine.In refineries,syngas is particularly important as a source of hydrogen,which is required for hydrotreating,removal of impurities,hydrogenating olefins,and other hydroprocessing such as catalytic cracking [23].

The FT synthesis produces a syncrude,which is a mixture of linear hydrocarbons (n-alkanes and n-alkenes) with similarities to crude oil.However,it also contains aromatics and oxygenates like 1-alkanols,aldehydes,ketones,carboxylic acids,and therefore,a different upgrading than the crude oil is needed.In addition,the syncrude is not present as a single liquid phase but as a multiphase mixture containing three to four different phases [31].To convert most of the syncrude phases into a single“crude oil”product,some upgrading techniques are required.Various cracking,isomerization,hydrotreating,alkylation,aromatization and oligomerization procedures are used.The purpose of upgrading is to produce a higher quality oil that can then be marketed as a synthetic crude oil to refiners [32].

6.2.Biomass pyrolysis

Pyrolysis is a thermo-chemical decomposition process in which lignocellulosic material is converted into a carbon-rich solid and volatile matter by heating in the absence of oxygen [33].While a wide range of reactor configurations have been operated,fluidized beds and circulating fluidized beds are the most popular configurations due to their ease of operation and ready scale-up [18].

Fast pyrolysis is characterized by high heating rates,short vapor residence times,moderate and carefully controlled temperature;and rapid cooling or quenching of the pyrolysis vapors[34,35].This generally requires a feedstock prepared as small particle sizes and a heat transfer rate to the particle between 600 and 1000 W·cm-2[36].Residence times on the order of seconds to minimize char formation[37].Moderate temperatures,in pyrolysis terms,of around 450-550°C,depending on the species of wood and required product [38].

Moreover,a design that removes the vapors quickly from the presence of the hot solids is needed.If not rapidly quenched to ambient temperature these compounds can crack further into smaller molecular weight fragments and/or polymerize into larger fragments,both at the expense of fragments making up the desired liquid product [39].

From the rapidly quenching of the pyrolysis vapors and aerosols,three product phases are formed;pyrolysis oil,noncondensable gases and char.The yields of each phase depend on the operating conditions,reactor design and feedstock characteristics,including ash content and the relative amounts of cellulose and lignin.The main product,fast pyrolysis liquid,is obtained in yields of up to 80 wt% on dry feed,together with the by-product char and gas,which can be used within the process to avoid waste streams [40].

The fast pyrolysis liquid,also known as bio-oil is obtained after vapor condensation.The product bio-oil is black or dark brown and free flowing at room temperature and typically contains less than 30 wt% of water and hundreds of oxygenated components [41].It is mostly immiscible in hydrocarbon liquids and can be upgraded by hydrotreating to lower the oxygen content and decrease hydrophilicity.The by-product char is primarily composed of carbon and it is separated from the fast pyrolysis vapors and aerosols by a cyclone.This solid product can be used as fuel.Finally,the non-condensable gases are collected during vapor condensation.These gases are recycled internally as fluidizing gas to the fast pyrolysis reactor and/or collected for fuel use [40].

The advantage of fast pyrolysis is that it can directly produce a liquid fuel,which is beneficial when biomass resources are remote from where the energy is required since the liquid can be readily stored and transported.This means that pyrolysis bio-oil is a promising candidate to replace petroleum fuels.

On the other hand,this bio-oil is a complex mixture of water(15wt%-35 wt%),solid particles (0.01wt%-3 wt%) and hundreds of organic compounds (acids,aldehydes,ketones,phenolics,alcohols,ethers,esters,anhydrosugars,furans,nitrogen compounds as well as large molecular oligomers),which make bio-oil a lowgrade liquid fuel,shown as highly oxygenated,acid and corrosive to common materials,thermally and chemically instable,as well as non-miscible with petroleum fuels [42].Upgrading bio-oil to transportation fuels is needed and technically feasible.Nonetheless the upgrading economic is crucial.

6.3.Biomass liquefaction

Direct liquefaction was initially developed for turning coal into liquid fuels.Several single and two-stage processes have been developed,but have not been made commercial in the US.However,China opened a commercial direct liquefaction plant in 2008,the Shenhua plant,which is the largest facility in the world[43].Recently,direct liquefaction has been applied to a number of feedstocks,including woody biomass,agricultural residues,aquatic plants and organic residues,nevertheless,there are no direct liquefaction processes in commercial use [44].

Direct liquefaction,also called hydrothermal liquefaction(HTL)is the thermochemical conversion of biomass into liquid fuels by processing in a hot,pressurized water environment for sufficient time to break down the solid biopolymeric structure to mainly liquid components [45].HTL is conducted under elevated pressure and temperature to keep water in either liquid or supercritical state.The use of water as solvent obviates the need to dry the biomass compared to the other conversion processes and permits reactions to be carried out at lower temperatures in comparison with flash pyrolysis.HTL is usually performed at temperatures between 300 and 400°C,residence times of 0.2-1.0 h(longer than gasification and pyrolysis),operating pressures of 5-20 MPa [20],and with high water to biomass ratio (3:1-10:1) [46].The high pressure enables higher penetration of the solvent into the biomass structure to facilitate fragmentation and decomposition of biomass molecules [47].

Feedstock pre-treatment is required especially for woody biomass for the reasons of reducing the particle size,removing the contaminants,and alkaline treatment to obtain a stable slurry for easy pumping [48].A reducing gas and/or a catalyst are often included in the process to increase the product bio-oil yield and quality[20].Catalysts of various functions are added to the slurry[49].Febased catalysts have been tested for direct liquefaction processes.In addition to the common Fe-based catalysts,Mo,Co and Ru and solid acids have also been tested effective for the liquefaction process[50].Reactor designs can be either batch or continuous.Continuous reactors require feeding system that operate under pressure and include either slurry based pump or lock and hopper systems for large particles [10].When the feedstock is then processed via HTL,phase separation occurs spontaneously at these operating conditions resulting in a gaseous stream of CO2,solid residue(char),biooil and small traces of aqueous phase containing soluble organic compounds [51].However,this process produces a large amount of char [48],leading to relatively low bio-oil yield (20 wt%-60 wt%)compared to the pyrolysis process(80 wt%on dry feed)[20].

The aqueous phase/water can be recirculated to HTL unit to reduce the water requirement and enhance the bio-oil yield.In addition,water generated through HTL process can be treated anaerobically or via catalytic hydrothermal gasification technique to produce methane rich or hydrogen rich syngas.The obtained solid phase material can be directly used as a bio-char/fertilizer.

Concerning the reaction mechanisms,is known that biomass is converted to liquefied products through a complex sequence of physical structure and chemical changes.The changes during liquefaction process involve biomass being first broken up into fragments by hydrolysis,and then degraded into smaller compounds by dehydration,dehydrogenation,deoxygenation,decarboxylation,solvolysis and depolymerization [52].Solvolysis results in micellar-like substructures of the biomass.The depolymerization of biomass leads to smaller molecules.It also leads to new molecular rearrangements through dehydration and decarboxylation.When hydrogen is present,hydrogenolysis and hydrogenation of functional groups,such as hydroxyl groups,carboxyl groups,and keto groups also occur [37].In other words,during the process,the lignocellulosic biomass is decomposed into small molecules.These small molecules are unstable and reactive,and can repolymerize into oily compounds with a wide range of molecular weight distribution [53].Bio-oil product generated in the repolymerization process usually contains acids,alcohols,aldehydes,esters,ketones,phenols and other aromatic compounds [52].However,due to the complexity of the feedstock and HTL products,as well as the seemingly-infinite possible intermediate reactions,the exact pathways of the HTL still remain unclear [10].

In addition to the unclear mechanisms,the presence of large amounts of oxygen [49]and solids that need to be removed from the final product[44]make difficult to operate and to obtain a useful fuel with liquid fuel standards [49].This explains why HTL has not been commercialized yet and is in the transient state from labpilot scale to pilot-industrial scale.Therefore,the study of the mechanisms is important and urgent,since with the support of mechanisms,the development of large-scale units could be more reasonable and high-grade products may be produced,adding a solution to current environmental and energy problems [10].

HTL is a promising route for the production of transportation fuels,nevertheless,the HTL bio-oils are semi-liquid [54],viscous,dark-colored and have a smoke-like smell[55],their typical viscosity is 10-10000 times higher than that of diesel and biodiesel[56]and heating values are not comparable with conventional fuels and biodiesel [57].These properties make HTL bio-oil difficult to use directly as transportation fuels and therefore further upgrading is required.Hydrotreatment can enhance these properties and make it able for commercial utilization.Moreover,complicated reaction mechanisms are not required for the upgrading procedure as the bio-oil obtained through HTL is less in moisture and oxygen content compared to the obtained from other thermochemicalprocesses and hence fine hydrotreatment will enhance the quality of bio-oil.

6.4.Biomass supercritical fluid extraction

Supercritical fluid extraction under supercritical conditions is the thermally disruption process of the lignocellulose or other organic materials in a temperature range of 250-400°C under high pressure(4-5 MPa)[58].It is an analytical process in which extraction and separation of organic compounds from a matrix can be accomplished effectively[59].During the process,a mixture of liquid and gas at equilibrium is heated and the thermal expansion causes the liquid to become less dense.At the same time,the gas becomes denser as pressure increases.At the critical point,the densities of the two phases become identical and the distinction between them disappears.The unique properties at supercritical conditions,such as the strong dependence of the solubility of a material in a supercritical fluid to its density and good contact between oxidants and reactants,make supercritical fluid extraction ideal for separation and extraction of useful products and for oxidation of organic materials [11].

Lignocellulosic biomass is the most difficult one to deconstruct efficiently to produce biofuel due to its heterogeneous and recalcitrant structure[60].Major compounds in biofuels from lignocellulosic biomass are phenols,catechols,guaiacols,syringols,syringaldehydes,syringyl acetone,acids,and esters.Most of these compounds are produced by lignin decomposition in lignocellulose.Furfural and derivatives are produced by the decomposition of cellulose and hemicellulose [59].

The biofuels obtained from biomass using supercritical fluid extraction are significantly affected by extraction parameters such as extraction temperature,extraction time,biomass/solvent ratio,and pressure [59].Organic solvents such as acetone,ethanol,methanol,2-propanol,and 2-butanol have been used to obtain bio-oil and other valuable chemicals [61].Most of the studies regarding supercritical extraction of biofuels from lignocellulosic biomass have been carried out using either ethanol or methanol.These supercritical alcohols have high reactivity,approachable critical temperatures and pressures,and low corrosiveness [62].

Major findings and experimental conditions for the conversion of woody biomass using supercritical fluid extraction have been reported.Brand et al.[63]studied the conversion of red pinewood with temperatures ranging from 280 to 400°C,nitrogen pressures of 0.4-7.5 MPa,residence time up to 240 min and using ethanol as solvent.The highest biofuels yield and biomass conversion were 59.9 wt% and 98.1 wt%,respectively.From the experimental results,it was found that increasing temperature and residence time had a positive effect on both conversions and liquid yields.On the other hand,increasing the nitrogen pressure only increased the biofuel yields and biomass conversions slightly [63].Akal?n et al.[64]used a response surface methodology with central composite experimental design to investigate the supercritical ethanol extraction of bio-oils from beech wood.The experimental process conditions were varied considering temperatures of 265-335°C,residence times of 37-143 min and using ethanol as solvent.The most significant variable was found to be the temperature on the yields of bio-oil and biomass conversion.At optimal conditions of 315.81°C extraction temperature and 123.67 min extraction time,the highest biofuels yield and biomass conversion were 40.4 wt%and 88.5 wt%,respectively [64].Liu and Zhang [65]experimented with the conversion of pinewood at 250-450°C,argon pressures of 1 MPa,residence time of 20 min and different solvents,such as,ethanol,acetone and water.They found that the solvent type had significant effects on biomass conversion as well as biofuels yields.The highest biofuels yield was 26.5 wt%when ethanol was used as solvent and the biomass conversion was the highest (75.9 wt%) in the case of acetone [65].

Compared to the other conversion methods,the supercritical fluid extraction technology presents advantages like fast kinetics,higher biomass conversion,ease of continuous operation and elimination of the use of catalysts.The main concerns are the high temperature and high pressure,which increase the operation costs.In addition,the majority of the papers in the literature focus on the application of this technology as a first stage prior to the lignocellulosic biomass decomposition and subsequent transformation into ethanol[66].Therefore,due to the lack of information regarding its application in the production of biofuels with similarities to diesel,gasoline and jet fuel,this technology is not discussed in following sections.

7.Thermochemical Conversion Technologies Comparison

Lignocellulosic biomass can be converted to bio-oil through the thermochemical conversion technologies discussed before.However,the selection of the appropriate technology depends on several factors,such as the type of biomass,feedstock pretreatment,operation conditions and so on.For instance,HTL and supercritical fluid extraction are more suitable for wet biomass,which means that drying the feedstock is not needed.On the other hand,if rapid biomass conversion to synthetic fuels is required,then pyrolysis or gasification are the most convenient options.Therefore,operation conditions(temperature,pressure and residence times),processing and conversion methods,conversion reaction mechanisms,as well as bio-oil properties should be considered to assess the feasibility and application of these technologies.In Table 2,a comparison of the main thermochemical conversion technologies for bio-oil production as well as their advantages and disadvantages are presented.

8.Bio-oil Upgrading

The unprocessed bio-oil derived from the biomass has low energy density,high moisture content,and its physical form is not free flowing that creates a problem as a feedstock for reciprocating engines [10].In addition,the bio-oil has a high content of oxygen and therefore lowers the stability over the time and lowers the heating value as well[68].These properties make bio-oil a lowgrade liquid fuel,shown as highly oxygenated,acid and corrosive to common materials,thermally and chemically instable,as well as non-miscible with petroleum fuels [42].Upgrading bio-oil is desirable to remove the oxygen and improve the physical and chemical properties in order to be used as an alternative fuel to conventional fossil fuels.Properties such as viscosity,density,heating value,oxygen,nitrogen and sulfur content,and chemical composition can be modified towards meeting fuel standards using technologies such as hydroprocessing,catalytic cracking,catalytic reforming,aromatic alkylation,olefin oligomerization,and so on.

8.1.Upgrading of FT products

Shell and Sasol have already commercialized their FT technology.The refinery designs associated with the industrial application of FT synthesis for the production of transportation fuels and chemicals are quite varied [69].Therefore,for a general analysis of the technologies involved in the upgrading,a commercial FT process can be assumed and the same upgrading conditions can be applied to the BtL plant.

8.1.1.FT tailgas

The C1-C2material is usually not converted to naphtha and the refining depends on the type of FT technology.Its preferred application is as synthetic natural gas [70].

8.1.2.LPG aromatic alkylation

The technology selection depends on the olefin to paraffin ratio.The HTFT derived C3hydrocarbons have a propylene to propaneratio of 87:13 and constitutes about 15%of the FT product,making it the most abundant carbon number in HTFT syncrude.Propylene can typically be converted into high-octane motor-gasoline and jet fuel by aromatic alkylation [70].In order to do so,the C3-C5gases are sent to a solid phosphoric acid (SPA)-catalyzed aromatic alkylation unit to react with some benzene or toluene in the refinery to produce heavier aromatics [71].

Table 2 Comparison of the main thermochemical processes for bio-oil production

8.1.3.Olefin dimerization/oligomerization

Olefin dimerization involves the coupling of two olefins,whereas olefin oligomerization refers to the coupling of more (up to five)olefins(C2-C5hydrocarbons)[23].The main purpose of this technology is to convert the olefinic gases into liquid products.To produce motor-gasoline,a selective technology such as solid phosphoric acid (SPA) based oligomerization,is an appropriate choice[70].Besides SPA,many catalysts types have been reviewed depending on the product type and quality.For production of olefinic gasoline,the use of H-ZSM-22/57 and Ni-based catalyst have been reported.On the other hand,for jet fuel and diesel fuel,HZSM-5 was reviewed [72].Wang et al.[73]performed the oligomerization of biomass-derived ethylene to liquid fuel using HZSM-5 catalyst under atmospheric pressure and found that the ethylene conversion was achieved [73].

8.1.4.Olefin hydrogenation

The conversion of gaseous olefins to specific products with desirable properties,such as high-octane paraffinic motor gasoline and jet fuel blendstock,can be accomplished after hydrogenation [32].The products from biomass olefins oligomerization are mainly C5-C12iso-olefins,which can be further hydrogenated to C5-C12isoalkanes.The hydrogenation of the C5+olefins to alkanes should be promoted since typical gasoline is composed of C5+iso-alkanes[74].

8.1.5.FT naphtha reforming and aromatization

The objective of catalytic reforming is to transform low octane naphtha (C6-C10) into high-octane gasoline.The principal means of obtaining the increase in octane is the conversion of paraffins and cycloparaffins into aromatics[75].The production of mononuclear aromatics from naphtha range paraffins improves the use of these fractions as transportation fuel components [32].To achieve this objective,specialized multifunctional precious metal catalysts are employed in fixed bed or moving reactors [75].Niziolek et al.[76]studied the production of aromatics through several technologies,such as naphtha reforming and aromatization of hydrocarbons via a metal-promoted H-ZSM-5 catalyst.As results they obtained that these technologies can produce benzene,toluene,ortho-,meta-,para-xylene,which are high value chemicals that may increase the profitability of biomass refineries [76].

8.1.6.FT wax hydrocracking

A large fraction of the FT products has boiling points above 370°C,which leads to the necessity of converting the FT wax(C20+) to middle distillate with acceptable cold flow properties.This can be achieved by wax hydrocracking technology,where two main reactions occur in parallel:Hydroisomerization and hydrocracking.Hydroisomerization improves the cold flow properties and the latter catalytically cracks the wax to additional fuel,such as diesel and gasoline range products.Different catalysts have been developed depending on the desired products and the characteristics of the feedstock.Bifunctional catalysts with the presence of acidic sites provide the isomerization/cracking function and catalysts with metal sites provide a hydrogenation/dehydrogenation function.Typical acidic supports are amorphous metal oxides or mixtures of metal oxides.The most common metals used are Pt,Pd,and sulfide bimetallic systems,Co/Mo,Ni/W or Ni/Mo [32].

Rosyadi et al.[77]carried out the hydrocracking of FT wax derived from biomass at reaction conditions of 400°C and initial hydrogen pressure of 5 MPa using NiMo catalyst supported on amorphous silica-alumina (ASA) and achieved the production of diesel fuel with a 59.5 wt% yield [77].

8.1.7.Distillate hydrogenation

The FT distillate is sulfur free and low in polynuclear aromatics,therefore,the traditionalrefining approach would be to hydrogenate this material.After mild hydroprocessing,this material can be sold as diesel fuel,but if maximum gasoline and jet fuel production is desired,hydrocracking is preferable.Some hydroisomerisation-hy drotreating configurations to produce on-specification diesel fuel from HTFT syncrude were reported by Leckel [78].Leckel studied the hydroprocessing of FT distillate at 250°C and 5 MPa and proved that is viable to generate alternative diesel[78].

8.1.8.Aqueous fraction refining

Polar products from Fischer-Tropsch synthesis,such as shortchain alcohols,carbonyl compounds,and carboxylic acids,on condensation,dissolve in the water formed in the Fischer-Tropsch process to form an aqueous product phase [79].The dilute nature of the water product poses challenges to refining,especially for acids [80].Recovery of the organic compounds from this aqueous product phase involves complex separations,and catalytic dehydration of alcohols to olefins is needed to simplify the separations.In the presence of η-alumina catalyst,mixtures could be dehydrated completely at 275°C with selectivity to olefins higher than 95% [79].Alternatively,this oxygenate-rich product can be partially hydrogenated to allow its separation into carbonyl-rich and alcohol-rich products and ease the conversion of the ketones and aldehydes into their corresponding alcohols [23].

8.2.Upgrading of FT products via hydroprocessing

The typical FT product profile consists of high molecular weight paraffinic waxes and FT fuels in the diesel and naphtha boiling range.Three individual hydroprocessing units to upgrade the three fractions are needed;for hydrocracking the wax and for hydrotreating the FT distillate and naphtha product.In the wax hydrocracking unit,the raw FT wax is cracked into naphtha,distillate and fuel gas.On the other hand,the distillate hydrotreater and the naphtha hydrotreater hydrotreat the raw FT distillate and naphtha to stabilize them by saturating the olefins.The distillate product is recovered and then sent to diesel blending.The C7+naphtha product from the naphtha hydrotreater along with the naphtha produced by the wax hydrocracker are sent to the naphtha reforming plant where they are catalytically reformed into a high-octane gasoline blending component and a hydrogen rich gas stream.The stream containing pentane and hexane,which comes from the naphtha hydrotreater is sent to a C5/C6isomerization plant to increase its octane number and produce a high quality gasoline blending component [81].

8.3.Upgrading of pyrolysis bio-oil

Upgrading pyrolysis bio-oil to a conventional transport fuel such as diesel,gasoline,kerosene,and by-products requires full deoxygenation and some conventional refining,which can be accomplished either by hydroprocessing or by catalytic cracking.

8.3.1.Hydroprocessing

Hydroprocessing rejects oxygen as water by catalytic reaction with hydrogen.The process is typically high pressure (up to 200 bars,1 bar=105Pa) and moderate temperature (up to 400°C) and requires a hydrogen supply or source[82].Chemical reactions during bio-oil hydrotreating are very complex.Overall,pyrolysis biooil is almost completely deoxygenated by a combination of hydrodeoxygenation and decarboxylation reactions,with oxygen removed in the form of water and carbon oxides [83].

Single-stage hydrotreating has proved to be difficult,producing a heavy,tar-like product.Multi-stage processing,where mild hydrotreating is followed by more severe hydrotreating,has been found to overcome the reactivity of the bio-oil and prevent catalyst coking[84].Low temperature hydroprocessing can be used to pretreat the unstable bio-oil and reduce the most reactive oxygenated compounds before completely deoxygenating the oil under more severe hydrotreating conditions (higher temperature,lower space velocity) [41].

In the hydroprocessing process,bio-oil is first pretreated in a stabilization bed under relatively mild process conditions,140 to 180°C and 83 bar,followed by processing under more severe hydrotreating conditions in the first and second stage hydrotreating reactors [85].The first stage hydrotreating reactor is designed as a single bed catalytic reactor operated at 180 to 250°C and 138 bar over sulfide CoMo catalysts.The second stage hydrotreating reactor is operated at a higher temperature of 350 to 425°C[40].Sulfided molybdenum based catalysts are commonly used in the modern hydrotreating processes for bio-oil [41].However,this type of catalyst is not as effective in the stabilization bed.Ruthenium-based catalysts work well in the stabilization bed because they can effectively operate at low temperature to convert highly reactive carbonyl groups to less reactive species [86].

Products from the last hydrotreating stage are gas and two liquid fractions.The gas product is primarily non-condensable hydrocarbons (methane,ethane,propane,butane),carbon dioxide and excess hydrogen.The carbon dioxide concentration is smaller compared to the hydrocarbons.Excess hydrogen is recovered by pressure swing adsorption and recycled.On the other hand,the liquid fractions are an aqueous phase and a stable hydrocarbon oil phase.The aqueous phase product is heavier than the hydrotreated oil phase and contains very little carbon[87],typically less than a half percent by weight.The hydrocarbon oil product contains less than 2 wt%oxygen,which is then fractionated into gasoline blendstock,diesel blendstock and heavies.The heavy fraction is sent to the hydrocracking reactor to be catalytically cracked to additional fuel[40].Hydrocracking is a thermal process (with temperatures higher than 350°C)in which hydrogenation accompanies cracking.Relatively high pressure (100 to 2000 psi) is employed,and the overall result is usually a change in the character or quality of the end products[88].The product is a mixture of liquids spanning the gasoline and diesel range.

8.3.2.Catalytic cracking

Catalytic cracking rejects oxygen as CO2,yielding mainly aromatic hydrocarbons as product but with extensive coke deposition on the catalyst [89].The main catalyst used are zeolites,such as HZSM-5.Zeolites are complex,three-dimensional porous structures with varying elemental compositions that exhibit catalytic activity in up to 50%of their volume.Cracking and dehydration are the main reactions seen.Adsorption of the oxy-compound occurs on an acid site.This is followed by either decomposition or bimolecular monomer dehydration,as determined by pore size.Zeolites produce aromatics at atmospheric pressures without H2requirements.The final product generally has a low heating value,due to its low H/C ratio and high O/C ratio as compared to hydrodeoxygenated oils [90].Research is generally conducted at temperatures from 350 to 600°C,atmospheric pressure and residence time of around 15 min.The catalytic cracking is conducted in the presence of N2gas to stabilize the product.The final products are upgraded oil 29 wt%,gases 25 wt%,and aqueous fraction 21 wt%,as well as,excessive carbon production with yields of 18 wt%solids(coke,char,and tar),and thus catalyst coking is presented[91-93].The elemental composition of the fast pyrolysis bio-oil feedstock,as measured by its H/C ratio has been determined to have a large impact on the production of olefins,aromatics,and coke.Experiments have shown that pyrolytic bio-oil feedstocks with a ratio of at least 1.2 or higher perform better in zeolite cracking upgrading[94].

Recently,catalytic cracking has attracted significant attention since it has significant advantages compared to hydroprocessing.It does not require hydrogen,operates at atmospheric pressure and has a lower operating cost [95].

8.3.3.Olefins-rich fuel gas oligomerization

As described before,the main products from biomass catalytic cracking are aromatic hydrocarbons and a gas fraction composed of olefins.This olefins-rich fuel gas can be upgraded to C5-C12iso-olefins and iso-alkanes through oligomerization,which increases the production of gasoline range components.Wang et al.[74]performed the oligomerization of a low carbon olefins mixture (ethylene,propylene and butylene) over amorphous silica-alumina (ASA) catalyst under conditions of 280°C and 3-4 MPa and at these conditions the carbon conversion of ethylene,propylene and butylene reached 19.2 c-mol %,37.3 c-mol % and 58.7 c-mol %,respectively.The bio-gasoline yield obtained was 23.43 c-mol% and the main species of the produced gasoline were C6-C10olefins and alkanes.The results demonstrated that the coupling of biomass catalytic pyrolysis and olefins oligomerization to gasoline is an excellent approach with high efficiency [74].

8.4.Upgrading of HTL bio-oil

8.4.1.Hydrotreating or hydrodeoxygenation

Various biomass-derived oils,containing oxygenates such as alcohols,aldehydes,esters and carboxylic acids can be upgraded to hydrocarbon fuels in the catalytic hydrotreatment process under a high-pressure hydrogen and in the presence of heterogeneous catalysts[96,97].Hydrotreating,also known as hydrodeoxygenation has been proved effective for the upgrading of liquefied biomass.Grilc et al.[98]investigated the hydrotreatment of liquefied lignocellulosic biomass at 300°C under a total pressure of 8 MPa in a slurry reactor over unsupported molybdenum (disulphide,dioxide and carbide) and tungsten (disulphide) catalysts.As result,a high hydrodeoxygenation activity was observed in the mass balance and phase distribution of the upgraded liquid product by reducing tar residue and increasing the yield of oil phase with a gross calorific value of 38 MJ·kg-1and oxygen content below 8.5 wt%[98].

9.Separation Needs and Technologies

Because the reactor effluent is always a complex mixture with multicomponent and multiphase nature,no matter what technological routes are explored,separation and purification technologies to reach the final products are indispensable in the total production processes.More significantly,the separation and purification section accounts for about 50%-70% of the total production costs which is much higher than its counterpart in the petroleum based manufacturing processes (typically 40%-50%) [99].Therefore,synthesis and development of intensified separation processes for biofuels production is significant to reduce the total production costs.For example,for the biofuel production from biological process routes,process synthesis and intensification can achieve novel separation processes for the real fermentation broths with considerable lower capital and energy costs [100-102].

In light of the difficulties in bio-oil production and upgrading through thermochemical-based conversion routes,some primary separation methods must be adopted at any stage of the processes.

9.1.Separation needs in the thermochemical conversion of biomass into bio-oil

The products from the thermochemical conversion of biomass independently of the process are mainly,solid residue (char),the desired product in either gas or liquid phase,non-condensable gases,and an aqueous fraction.Studies on thermochemical conversion of biomass use a number of physical separation methods,mainly to isolate high-value products or facilitate further processing to produce fuel and high-value products.

Solid separators such as cyclones are needed to separate the solid particles carried by the product gas[29].Quench columns and scrubbers are employed for two main purposes,for cooling down the gas and for removing the impurities.In the gasification process,a water scrubber can be implemented to reduce the impurities,such as nitrogen compounds.On the other hand,in the pyrolysis process,a series of quench columns can achieve the separation of the pyrolysis gas from the non-condensable gases and simultaneously rapidly condensate the pyrolysis gas to produce the bio-oil[40].The desired bio-oil can be subjected to various separation methods in order to remove the excess water.Such separation methods may utilize distillation columns and/or total condensers to condense all the bio-oil and water for separation[103].

For HTL,the product bio-oil is already in liquid state and therefore the separation of the by-products is needed.Addition of a solvent to the two-phase product can enhance separation and extraction.The liquid product can be decanted to separate aqueous and oil portions.This crude separation results in an oil fraction with a moisture content of around 5 wt%[104].Several studies have been carried out for the extraction of bio-oil derived from woody biomass using solvents such as acetone [56,105]and diethyl ether [106].Another separation process that can be applied is distillation.Distillation has been used in studies that characterize pyrolysis bio-oils.The temperatures used ranged from 100 to 250°C for atmospheric distillation and 80-230°C for vacuum distillation[107-110].

Solid particles can also be removed by solid-liquid filtration.The general field of solid-liquid separation starts with the knowledge of the particle characteristics and continues with the following stages:(1)pretreatment by chemical or physical methods to increase particle size and filterability,(2)thickening to increase slurry concentration;(3)solid separation by filtration or centrifugation[111].

9.2.Separation needs in the upgrading of bio-oil

The product bio-oil is composed of more than 300 compounds belonging to different functional groups with distinct physical and chemical properties.Although different upgrading techniques have been developed,bio-oil upgrading has not yet been industrialized due to crude bio-oil being too complicated to upgrade directly.Efficient bio-oil separation to achieve the enrichment of compounds in the same family or the recovery of components that are suitable for the same upgrading method is a significant strategy for the utilization of high-grade bio-oil as transportation fuel.For instance,the oil phase obtained from pyrolysis can be divided into different fractions through atmospheric and vacuum distillation units,and then it can be decided whether or not they need further hydrotreating refining according to the properties of its fractions[112].Besides the main fuel components,high-value chemicals can be recovered from the water phase by distillation or extraction.

Moreover,during upgrading other reactants than bio-oil are needed to carry out the reactions.Thus,the final product from upgrading contains upgraded oil compounds,gases,aqueous products,solids and the excess of reactants used,which should be recovered.For example,during hydroprocessing excess hydrogen is required and can be recovered by pressure swing adsorption(PSA).On the other hand,after catalytically cracking the crude bio-oil,the upgraded product in gas phase contains a coke yield of about 18 wt%-20 wt% [113],which should be sent to a cyclone to remove the solid particles and then the remaining product can be cooled down to obtain the upgraded oil.

Concerning the FT products,after FT synthesis,different types of syncrude with different compositions are obtained depending on the FT unit (HT or LT) employed.Therefore,the refinery design must be inherently robust enough to deal with the variations in the bio-oil composition.In this respect,a Fischer-Tropsch refinery is more fortunate,because once the Fischer-Tropsch technology has been selected,the composition of the syncrude can be controlled within a narrow range of variation[70].The syncrude from FT synthesis is a complex product with four different product phases at ambient conditions:gaseous,organic liquid (condensates),organic solid (wax) and aqueous liquid [114].Therefore,in the upgrading section,several separation units are required to recover the different phases before being sent to their corresponding upgrading units.Different refinery configurations can be explored by manipulating the separation and upgrading technologies in an efficient way that maximizes the production of a specific fuel type and meet the relevant fuel specifications.For this matter,the hydrocarbons produced from FT should be separated first into several fractions by distillation before being sent to their corresponding upgrading units [70].For instance,different separation configurations to divide the FT product in several fractions depending on the carbon number distribution can be proposed.Then,after the fractions recovery,different upgrading units depending on the desired product profile can be employed.For illustration purposes,some of the possible configurations for the recovery of the FT fractions are presented in Fig.2.

On the other hand,if instead FT products hydroprocessing is selected,then two distillation columns can be employed.The first distillation column is employed to separate the FT product in a naphtha product stream and a heavy product stream,and the second column is used to separate the distillate from the wax fraction present in the heavy product [8].

9.3.Recovery of final products

The recovery of the BtL transportation fuels,aqueous products,gases and waste streams can be performed with a fractionation column and a flash distillation unit.The upgraded products coming from the fractional upgrading of the FT syncrude and the catalytic cracking of pyrolysis oil are sent to a fractionation column to separate the feed into four fractions:light product (gases and a small amount of naphtha range product),water,naphtha range product,diesel range product and jet fuel blendstock.Then,a flash distillation unit to recover the remaining naphtha can be employed [8].

On the other hand,the finished oils from the hydrotreating and hydrocracking of pyrolysis oil can be separated by distillation or fractionation into gases,gasoline and diesel boiling range blendstocks[40].

10.Comparison of the Upgraded Product Properties

Several studies have demonstrated that the conversion of lignocellulosic biomassinto bio-oil and its further upgrading can produce upgraded bio-oils with similar properties to crude oil.In Table3,the composition and properties of pyrolysis bio-oil,liquefaction biooil,syngas bio-oil,their corresponding upgraded bio-oils and FT fuel are presented and compared with petroleum fuel.

11.Future Prospects

Considerable efforts have been devoted to explore the various technological routes and a large amount of experimental works are being done on the lab-and pilot-scale,mainly focusing on the thermochemical conversion technologies.For instance,pyrolysis and gasification are mature and well stablished technologies,and sufficient data can be collected for further research.HTL and supercritical fluid extraction have also been explored in lab-scale but have not been commercialized due to the complex mechanisms.Moreover,the products from thermochemical conversion need upgrading in order to be classified as compatible with existing transportation infrastructure.Therefore,chemical analysis of bio-oil and fractions should be improved and new methodologies and standards for the chemical and physical characterization of bio-oil fractions should be developed.Better bio-oil characterization will allow to explore novel upgrading technologies.

Fig.2.Separation configurations:FT hydrocarbon fractions’ recovery.

On the other hand,while it is common to use individual reactors to explore the reaction technological routes in lab-and pilot-scale,the separation and purification processes cannot be approached in the same way,because usually many separation methods and units should be used.Experimental work is either too expensive or impossible to approach the real and whole industrial scale separation processes.Conventional separation technologies in the fossil-based chemical processes have the clear high costs in energy and capital investment,and therefore it is crucial to have robust separation technologies to make biorefineries economically viable.Novel separation processes based on intensified equipment are significant to reduce the production costs.Synthesis and evaluation of hybrid unit operations,complex distillation sequences,definition of column sections and reallocation of column sections are important tools to improve the efficiency of the BtL systems.The definition of column sections is important toallow changing the configuration of the separation trains and the generation of subspaces with intensification potential.Moreover,intensification of separation units is not the only possibility;reaction-separation intensification can also be explored.Reactive separation processes can improve the biofuels production efficiency by integrating both reaction and separation into a single unit allowing the simultaneous production and removal of products,therefore overcoming the reaction equilibrium limit,improving the selectivity and productivity,reducing energy use,and intensifying heat and mass transfer.Reactive-distillation is a promising process intensification technology that can lead to high-efficiency systems with better performance and economic benefits [118].In the BtL processes,the isomerization of light naphtha containing C4-C8hydrocarbons to increase the octane number [119]and the upgrading of flash pyrolysis oils to fuels using a high boiling alcohol and acid catalysts could be performed by reactive distillation [120].

Table 3 Composition and properties of bio-oils and upgraded oils

The use of rigorous simulations and experimental data to predict thermodynamic and physical properties,energy consumption,and capital costs,are important for the evaluation of the BtL processes.Process simulation can explore and evaluate process routes at different scales and predict the performance of technologies that are complex and infeasible at lab-scale.For instance,in previous works by Ibarra-Gonzalez and Rong[8,121],different interconnections between the thermochemical conversion,upgrading and fuel recovery technological sections were explored and five process routes for the conversion of lignocellulosic biomass into liquid fuels were proposed and rigorously simulated.The process routes evaluated were Pyrolysis-hydroprocessing-separation (PR-PHS),Pyrolysis-catalytic cracking-fractionation (PR-PCC),Gasification-HTFT-upgrading-fractionation (PR-GHTUF),Gasification-LTFT-up grading-fractionation (PR-GLTUF) and gasification-HTFT-hydropro cessing(PR-GHTH).Fig.3 presents the percentage that each section is accounting in the total capital and energy cost of these process routes.The rigorous simulation and evaluation of these initial BtL processes allowed detecting the process sections that present higher costs and that required improvements and/or intensification.In general,the results showed that the sections that present high capital costs are the upgrading and the fuel recovery due to the several separation units(flashes,distillation columns,fractionation columns and solvent extraction columns) required for the recovery of the carbon fractions to be upgraded and the final fuel products.This means that special attention should be given to these technological sections.

Fig.3.BtL technological sections:impact on the capital and energy cost.

However,it is important to highlight that the conversion of biomass to liquid fuels is not restricted to these process routes.The advantage of the BtL facilities is that besides these alternatives,many other combinations of the technologies described through this review can be explored and different biofuels production systems depending on the desired final product distribution can be proposed.Synthesis and integration of thermochemical conversion,upgrading and separation technologies within the total production processes can contribute to reduce BtL fuel’s manufacturing costs.For example,from rigorous simulations,liquid,gas,and solid emissions can be quantified and process integration techniques can be used to reduce the emissions and the overall energy consumption of the process routes.It might be possible to identify that a process can use the heat rejected by another unit or that a resource can be recovered from a waste stream and be later reused in the process.

Moreover,simultaneous analysis and optimization of different thermochemical routes can explore and propose optimal total biofuels production processes that may minimize the BtL facilities costs and increase the production of a specific type of fuel.

Besides that,better understanding of product requirements and specifications will allow adapting existent petroleum refineries to BtL production facilities.Research and synthesis of new alternatives and refinery configurations for the conversion of biomass via gasification and FT can be accomplished by understanding the operation of industrial plants like Sasol.

In summary,the development and design of optimal industrial scale processes for biofuels production through thermochemical conversion routes can be achieved by systematic methods and tools including process synthesis and integration,process intensification,process modeling and simulation,and process evaluation and optimization.For example,integrated processes have been explored and optimized through a process synthesis framework for bioethanol and biodiesel integration [122],and for coproduction of biodiesel and hydrotreated vegetable oil (HVO)[123].For bioethanol production,process synthesis and integration for comparison and evaluation of different pretreatment methods within the total process technologies [124],as well as specific solvent application and recycle structures in the total process have been studied [125].For thermochemical conversion routes,currently a dual-methodology framework for synthesis and optimization of a processing superstructure based on rigorous simulations is being developed to study the optimal integrated biofuels production processes [8,121].

12.Conclusions

Due to the expected depletion of fossil fuels and the increasing energy demand of the transportation sector,alternate lignocellulosic derived fuels have attracted attention as the ultimate solution to compensate for declining oil production.Moreover,these sustainable feedstocks derived biofuels are the urgent and important alternatives to pursue full decarbonisation of the transport sector.This review has shown BtL as a promising technology for the production of transportation fuels.Several thermochemical technologies for the conversion of biomass to bio-oil have been explored including pyrolysis,gasification,liquefaction and supercritical fluid extraction.Bio-oil has been found as a highly oxygenated product,instable and incompatible with existing fuel distribution systems and with standard engines.The importance of bio-oil upgrading has been highlighted in order to enhance the properties of the fuel and be able to produce fuels with properties similar to fossil fuels.To achieve this goal,upgrading technologies such as,hydroprocessing,catalytic cracking,FT products upgrading via oligomerization,aromatic alkylation,hydrogenation,naphtha reforming,and so on,as well as HTL bio-oil hydrotreating have been described.Additionally,the separation challenges at any stage of the processes and the importance of the separation methods for the recovery of value-added products have been presented.Furthermore,compositions and properties of products have been compared with heavy petroleum fuel properties,presenting these technologies as efficient alternatives for liquid fuels production.Finally,call for systematic methodologies and tools from process systems engineering to support optimal synthesis and development of sustainable biofuels production processes are discussed.

Acknowledgements

The authors acknowledge financial support from CONACYT-The Mexican National Council for Science and Technology(REFERENCE:326204/439098)) and the University of Southern Denmark.