Insight into progress in pre-treatment of lignocellulosic biomass

Accepted Manuscript Insight into progress in pre-treatment of lignocellulosic biomass

Abdul Waheed Bhutto, Khadija Qureshi, Khanji Harijan, Rashid Abro, Tauqeer Abbas, Aqeel Ahmed Bazmi, Sadia Karim, Guangren Yu PII:

S0360-5442(17)30005-1

DOI:

10.1016/j.energy.2017.01.005

Reference:

EGY 10140

To appear in:

Energy

Received Date:

20 May 2016

Revised Date:

01 January 2017

Accepted Date:

02 January 2017

Please cite this article as: Abdul Waheed Bhutto, Khadija Qureshi, Khanji Harijan, Rashid Abro, Tauqeer Abbas, Aqeel Ahmed Bazmi, Sadia Karim, Guangren Yu, Insight into progress in pretreatment of lignocellulosic biomass, Energy (2017), doi: 10.1016/j.energy.2017.01.005

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ACCEPTED MANUSCRIPT

Highlights 

Properties of feedstock LCM and pre-treatment operation play a vital role.



Proper pretreatment method increase access to fermentable sugars thereby improving the efficiency of the whole process.



In this review paper, intensive fundamental and applied research of each pretreatment process is reviewed.

ACCEPTED MANUSCRIPT

Insight into progress in pre-treatment of lignocellulosic biomass Abdul Waheed Bhutto1, 2*, Khadija Qureshi2, Khanji Harijan3, Rashid Abro4, Tauqeer Abbas5, Aqeel Ahmed Bazmi4, Sadia Karim2, Guangren Yu4 1Department

of Chemical Engineering,

Mehran University of Engineering and Technology, Jamshoro, Sindh, Pakistan. 2Department

of Chemical Engineering,

Dawood University of Engineering & Technology, Karachi, Pakistan 3Department

of Mechanical Engineering,

Mehran University of Engineering and Technology, Jamshoro, Sindh, Pakistan. 4Beijing

Key Laboratory of Membrane Science and Technology & College of Chemical

Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China 5

Process and Energy Systems Engineering Center-PRESTIGE, Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan * Corresponding Author

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ACCEPTED MANUSCRIPT Abstract Cost effective pretreatment is key to commercial success of use of lignocellulosic material (LCM) as feedstock for production of bioethanol. The seasonal nature and annual variability of LCM may enforce the use of different biomass sources as feedstock to ensure round the year production. However, different processing requirements of each material may limit the success of such development and impair the use of many feedstocks due to logistic and economic reasons. The selection of pretreatment technology for different LCM sources in cost effective manner is a major challenge. The recent review articles have provided the details of different pretreatment operations. This article has tried to establish a strong connection between pretreatment options and their combination with prior and post pretreatment processes, which is pre-requisite to success of establishing the commercial facility. The key is lessening number of operations and operating cost of each unit of operation. This paper suggests ways towards appropriate pretreatment processes through process intensification leading to the prospects of sustainable biofuel production. Keywords: Abbreviations AFEX

Ammonia Fibre Explosion

DA

Dilute Acid

EBI

Electron Beam Irradiation

EPD

Explosion Power Density

GR

Gamma Irradiation

HMF

Hydroxymethylfurfural

ILs

Ionic Liquids

LCM

Lignocellulosic Material

LSR

Liquid-to-Solid Ratio

LHW

Liquid Hot Water

MWR

Microwave Radiation

2

ACCEPTED MANUSCRIPT OS

Organosolvent

SCF

Supercritical Fluid

SE

Steam Explosion

SER

Specific Energy Requirement

SHF

Separate Hydrolysis and Fermentation

SSF

Simultaneous Saccharifcation and Fermentation

WO

Wet Oxidation

1. Introduction Biomass based fuels currently seem to most suitable alternative of fossil fuel in context of sustainable development [1]. LCM is the most abundant organic materials evenly distributed and easily available throughout the world. The high cost involved in converting LCM into liquid fuel is obstructing large-scale commercial production of biofuels from LCM. Cost competitive conversion of LCM into ethanol requires the fundamental understating and integration of different process [2]. LCM is primarily composed of cellulose (C6H10O5)n (30–50%), hemicellulose (C5H8O4)m (15–35%) and lignin [C9H10O3(OCH3)0.9–1.7]x (10–20%) [3, 4]. Pretreatment is carried out to disrupt the compact structure of LCM and overcome the recalcitrance. Pretreatment of LCM is an essential step prior to its hydrolysis to sugars and fermentation to bioethanol[5]. Pretreatment constitutes for more than 40% of the total processing cost [6]. An ideal pretreatment process avoids the needs for size reduction of biomass, makes the lignocellulosic biomass susceptible for quick hydrolysis with increased yields of monomeric sugars and should limit the formation of inhibitory compounds and minimize energy demands and capital and operational cost requirement[7]. Different pretreatment processes exist which reduce the recalcitrance and cellulose crystallinity of lignocellulosic biomass to open the structure of the material and improve their accessibly for subsequent chemical conversion [8, 9]. Pretreatment is an essential component

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ACCEPTED MANUSCRIPT of converting LCM into liquid fuels. Each pretreatment process has a specific effect on the cellulose, hemicellulose and lignin fraction. Physical and chemical differences between the substrates causes differences during fermentation including: ethanol yield, ethanol titer, fermentation rate, fermentation completion time, mixing, and substrate solubilization[10] which suggest that pretreatment directly influences the operation as well as performance of downstream hydrolysis, fermentation and final product separation. Understanding of these effects helps in choosing proper pretreatment and its configuration with subsequent operations based upon the type of feedstock, hydrolysis and fermentation steps. Pretreatment strongly influences downstream cost by determining fermentation toxicity, enzymatic hydrolysis rate, enzyme loading, and other process variables[11]. Pretreatment step is major technological bottleneck for the cost-effective development of bioprocesses from LCM. Selecting optimum pre-treatment helps in selecting cost-effective strategies for conversion of pre-treated lignocellulose to ethanol. There are many excellent recent review papers on the pre-treatment of lignocellulose to provide basic knowledge and mechanism about biomass composition/structure affecting various pre-treatments. Some recent review articles by Yang and Wyman[12], Mosier et al [13], Taherzadeh and Karimi [14], Hendriks and Zeeman[15], Sun et al[16], Singh et al[8], Chaturvedi and Verma[17], Bensah and Mensah[18], Conde-Mejía et al[19], Chiaramonti et al [20], Brodeur et al[21], Agbor et al[22], Kumar et al[23] and Galbe and Zacchi[24] has provided the detailed overview of different pretreatment operations. These articles have discussed different aspects of pretreatment, provided their comparisons, industrial readiness and economics aspects. The articles written by Yang and Wyman[12], Mosier et al [13], Taherzadeh and Karimi [14], Hendriks and Zeeman[15], Kumar et al[23] and Galbe and Zacchi[24] were published before 2010 and there has been dramatically improvement in knowledge of pretreatment systems in current decade. Sun et al[16] reviewed the structure factors that constrain the digestibility of

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ACCEPTED MANUSCRIPT biomass during the pretreatment. This review specially emphasis on combined pretreatment strategies. Singh et al[8] focused upon the ccomparison of different biomass pretreatment techniques based upon their impact on chemistry and structure. The review by Chaturvedi and Verma[17] has discussed the main principles behind different pretreatment processes. Their emphasis was on processes that provide maximum amount of sugars. Bensah and Mensah[18] reviews chemical pretreatment methods highlighting recent findings and process innovations developed to optimize the operating conditions. Conde-Mejía et al[19] conducted the simulations based on stoichiometric relations and yield data to evaluate the energy requirements of differnet pretreatment method. The study used benchmarks to assess the actual performance of the alternatives. Chiaramonti et al [20] reviewed different options available in pretreatment, however the focus was on new process that used only water and steam as reacting media and compared the results with those achieved by the autohydrolysis and steam explosion processes. Brodeur et al[21] reviews different leading pretreatment technologies focusing on the effects of different technologies on the components of biomass (cellulose, hemicellulose, and lignin) This reviewed also focused on how the treatment greatly enhances enzymatic cellulose digestibility. Agbor et al[22] presented a survey of biomass pretreatment technologies with emphasis on concepts, mechanism of action and practicability. This review also presented the advantages and disadvantages, and the potential for industrial applications of different pretreatment technologies. All these articles have not provided criteria to evaluate the relative performance of these pretreatment technologies. Present study aims at examining current pretreatment options and their combination with prior and post pretreatment processes, identifying main strengths and weaknesses of the different concepts and devises pretreatment solutions suitable for adoption at industrial scale. This review strives to provide quantitative meaningful comparisons of pretreatment options keeping in consideration recent developments in pretreatment methods and their

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ACCEPTED MANUSCRIPT industrial readiness. This study also identifies the areas for further studies in each technology for the researchers and policy makers. Typical goals of pretreatment is the cost effective operation to (1) produce highly digestible solids that enhances sugar yields during enzyme hydrolysis (2) avoid the degradation of sugar derived from both cellulose and hemicellulose (3) minimize the formation of inhibitors for subsequent hydrolysis and fermentation steps and (4) recovery of lignin[21]. Choice of pretreatment depends on raw material characteristics and process integration.

Table 1

summarizes key attributes that should be targeted for integrated pre-treatment processes. All these features are considered in order that pretreatment results balance against their impact cost on downstream processing steps and the trade-off with operational cost, capital cost and biomass cost [8]. Table 2 summarizes the challenges, advantages and recent developments in different pretreatment technologies. Chen et al [25] has reviewed steam explosion and its combinatorial pretreatment as a means of overcoming the intrinsic characteristics of plant biomass, including recalcitrance, heterogeneity, multi-composition, and diversity. Shirkavand et al [26] has provided a feasibility of combining fungal pretreatment with other methods for biofuel production. Economic analyses should be conducted to facilitate the development of combinatorial pretreatment for LCM. Table 1. The key attributes that should be targeted for low-cost, advanced pre-treatment processes Attribute Cost

Remarks Effective pretreatment process should have low capital and operational cost, The use of expensive materials (Catalyst, reagents, solvents, feedstock) during pretreatment and subsequent neutralization should be avoided

Energy performance: Should have a low energy demand. Pretreatment technologies that require size reduction to small size to are undesirable. Technologies that process feedstock

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ACCEPTED MANUSCRIPT of large dimension give better energy performance and the overall process efficiency. Operating

Use of highly corrosive chemicals or performing the operation at high

environment

operating pressure requiring exotic materials of construction.

Presence of inhibitors.The use of chemicals should be reduced as much as possible or even totally avoided, since they interfere during hydrolysis and fermentation. A cost-effectiveness Should result in the recovery of most the lignocellulosic components in a useable form in separate fractions. Pretreatment should yield high fermentable cellulose and hemicellulose sugars. A cost-effective pretreatment process must improve the formation of sugars in the subsequent phase of enzymatic hydrolysis, reducing the degradation of the carbohydrates, and the formation of inhibitors for hydrolysis and fermentation. Process integration

Pretreatment should be effective on a wide range and loading of lignocellulosic

and intensification

material. The rate of pre-treatment should to reasonable high to avoid use of large size reactors. It is highly desirable to eliminate conditioning to reduce costs and to reduce yield losses. The concentration of sugars from the coupled operations of pre-treatment and enzymatic hydrolysis should be above 10% to ensure that ethanol concentrations are adequate to keep recovery and other downstream costs manageable.

2. Process to produce ethanol from LCM LCM

feedstock can be converted into liquid fuels through both thermo chemical and

biochemical route[27]. Different studies has suggested that biochemical route have more cost reduction potentials [13, 28-31]. In the biochemical route overall process consists of (1) feed preparation and reduction of size (2) pre-treatment (3) hydrolysis (4) fermentation (5) product separation/purification (shown in Fig 1 [20]).

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Table 2. Summary of the challenges, advantages and recent developments in different pre-treatment processes Pretreatment

Advantages and recent advances

Challenges

Way forward

process Biological

pre- (i) Biological pre-treatment has the advantages (i) It is a relatively time consuming process (i) Significant research and developmental are needed

treatment

of a low-energy demand [31, 32] and

processes

friendly process[6].

eco- [17].

focusing on understanding the kinetic parameter to

(ii) The large space is required to perform reduce the time.

(ii) No release of toxic compounds to biological pre-treatment which increases the (ii) The other areas to focus include identification of environment and no effluent generation during cost

efficient lignin hydrolysing microbes using advanced

the process[6].

molecular techniques.

(iii) No generation of fermentation inhibitors

(iii) Combining biological pretreatment with other

during the process[6].

pretreatment methods can reduces time necessary for the whole process and improve ethanol production. However, such option may increases the operating cost.

Mechanical

(i) Pulverization of biomass improves the (i) It is energy intensive and highly energy (i) Knowledge of mechanical properties of material are

pretreatment

handling properties as well as efficiency of inefficient operation.

essential for proper design and optimization of biomass

mass and heat transfer during subsequent

size-reduction operation

processes.

(ii) Balance needs to be struck between cost and

(ii) Decreasing of particle size result in higher

efficiency improvement.

hydrolysis yield.

(ii) The location of size reduction also influences the efficiency of the whole supply logistics of LCM. (iii) Careful selection of equipment and final particle size reduces initial investment and energy requirements to improve the process economics.

Gamma radiation (i) Increases rate of enzymatic hydrolysis

(i) Excessive irradiation dose reduces the (i) Combination of the gamma radiation with other glucose yield.

methods such as acid treatment can further accelerate

8

enzymatic hydrolysis but not without additional cost. (ii) It is are very helpful for laboratory analysis.

Beam (i)

Electron

Irradiation (EBI)

Industrially

proven

technology

at (i) Limited penetration, of energy of high-

commercial scale.

energy electrons in relatively thin layers of material [33].

Microwave

(i) Increases reaction rate and reduces reaction (i) Increases degradation of polysaccharides

radiation (MWR) time with high-energy efficiency.

which leads to loss in yield

(ii) Can be carried out in small equipment in short residence time which reduces initial investment.

Ultrasonic energy (i) It

is a green technology with reduces (i) Different LCM responses differently to (i) The cost were significantly reduced when

reaction time.

ultrasonic

treatment

under

the

same ultrasound was combined with other technologies.

(ii) Operation is effective over a range of conditions [35] hence the optimal conditions (ii) particle sizes, although optimization indicates vary significantly between types of LCM.

Ultrasound

has

the

potential

to

augment

pretreatment in organic solvents and ionic liquids[34].

that higher yields are achieved for lower (ii) Key operating parameters determined (iii) A cost benefit analysis of grinding to smaller particle sizes[34].

and optimized for ultrasonic treatment of particle size would also be considered. LCM could only be practical for specific LCM.

Wet (WO)

oxidation (i) Air or oxygen as oxidizing agents are (i) The costs is mainly determined by the (i) When combined with other pre-treatment methods inexpensive and readily available.

reaction conditions. High temperature and gives higher yield of sugars [36-42].

(iI) Capable to mineralize organic material to pressure require expensive materials for the (ii) WO when combined with alkaline pre-treatment CO2, H2O, and inorganic salts

reactor.

reduces formation of product that inhibit hydrolysis and fermentation processes[43]. (iii) Acid soaking prior to WAO helps to hydrolyse the hemicelluloses and

expose more enzyme binding

sites[40].

Liquid hot water (i) Operation at lower temperature minimizing (i) The amount of solubilized product is (i) environmentally friendly technologies as it do not

9

(LHW)

energy

consumption

formation

of

and

degradation

minimizing products

the higher, while the concentration of these use any chemical agents other than water, thus

which products is lower.

reducing expensive recovery costs.

eliminates the need for a final washing step or (ii) Down-stream processing is also more neutralisation.

energy demanding because of the large

(ii) Do not use chemicals other than water, so volumes of water involved. corrosion issues and need for recycling of material is limited[44]. (iii) Results in higher hemicellulose sugar recovery and lower fermentation inhibiting hydrolysates [45, 46]. (iv) life cycle assessment (LCA) suggest LHW is

the

most

suitable

technique

for

the

pretreatment of corn stover[47].

Steam explosion (i) Short residence time and low energy (i) (SE)

Incomplete

destruction

of

lignin- Iogen demonstration plant (Canada), which has an

consumption.

carbohydrate matrix resulting in the risk of average productivity of about 300000 L ethanol per

(ii) No recycling or environmental cost.

condensation and precipitation of soluble year uses modified steam explosion to pretreatment lignin components making the biomass less wheat straw[48]. It is effective for the pretreatment of digestible.

hardwoods and agricultural residues, but less effective

(ii) Destruction of a portion of the xylan in for softwoods[22]. hemicellulose (iii) Possible generation of fermentation inhibitors at higher temperatures, (iv) Need to wash the hydrolysate which may decrease overall saccharification yields by 20–25% of initial dry matter due to removal of soluble sugars[22].

Dilute

acid (i) DA pretreatment can achieve high reaction (i) Produces sugar degradation products such (i) It has the advantage of not only solubilizing rates and significantly improve hemicellulose as furfural and HMF, which are inhibitory to hemicellulose

but

also

converting

solubilized

10

pretreatment

and cellulose hydrolysis by varying the severity the fermentative micro-organisms[49]. of the pretreatment

(ii)

Corrosion

caused

hemicellulose to fermentable sugars thus eliminates or

mandates reduces the need for use of hemicellulase enzyme.

acid

expensive construction material

(ii) At a lower temperature helps in avoiding the formation of inhibitory sugar degradation products. (iii) A two-step steam pre-treatment can result in higher overall sugar yields compared with one-step pretreatment.

Supercritical

(i) Moderate critical temperature of 31.1oC and (i) Low pre-treatment effectiveness

Fluid (SCF)

pressure of 7.4 MPa (73.8 bar).

(ii) High capital cost for high-pressure

(ii) CO2 are inert in nature, non-flammable, equipment. non-toxic, inexpensive, and readily available from the by-products of many industrial processes[50]. (iii) Green technology (based on an inexpensive solvent (iv) No generation of toxins, the use of low temperatures and high solids capacity. Organosolvent (OS) (i) Mild pretreatment temperature and pressure (i) Boiling point necessitates a high pressure (i)) Low boiling point Organic solvents are always easy and a neutral pH condition reduce carbohydrate during pretreatment. degradation into undesired furfural and HMF.

to recover by distillation and are recycled. Hence, the

(ii) The cost of organic solvents used to low boiling point alcohol such as methanol and ethanol

(ii) Low boiling point Organic solvents are pretreat biomass is also very high.

has advantage of lower solvent costs and easy recovery

always easy to recover by distillation and are (ii) Additional cost of solvent recycling of solvents. recycled.

which otherwise inhibit the fermenting (ii) Organosolv pretreatment fractionates biomass into its

(iii) Selective pretreatment method yielding organisms.

components

with

high-purity.

The

future

three separate fractions: dry lignin, an aqueous (iii) Using volatile organic liquid at high development of organosolv pretreatment should be hemicellulose stream, and a relatively very pure temperature cellulose fraction[22].

necessitates

using

of focused on the integrated utilization of biomass

containment vessels thus; no digester leaks components and decrease of the pretreatment costs to

(iv) very effective for the pretreatment of high- can be tolerated due to inherent fire, improve the economics.

11

lignin lignocellulose materials, such as soft explosion hazards, environmental and health woods[22].

and safety concerns[22].

(v) does not require significant size reduction of feedstock to achieve satisfactory cellulose conversion making the process less energy intensive especially with the pretreatment of woody biomass

Ammonia

fibre (i) AFEX is a dry-to-dry process, there is no (i) Use of ammonia requires highly environmental

explosion

ammonia[22].

(AFEX)

wash stream in the process, and no toxic controlled pre-treatment environment chemicals are generated for downstream because of its hazardous, malodorous, and processes. (ii) The ability to retain high cellulose content corrosive properties [21, 55]. after pre-treatment makes this process more (ii) The pretreatment equipment must be attractive [51]. constructed using materials that are not (iii) During AFEX pre-treatment, no sugar loss reactive to ammonia to prevent corrosion in takes places while the production of inhibitory the presence of highly alkaline ammonia– degradation compounds is low and washout or water mixtures[56]. detoxification of downstream biomass is not Compared to other pretreatments, AFEX has

needed [11, 52, 53].

(iv) AFEX process yields high ethanol titers slightly higher capital and utility costs[57]. without

the

need

for

biomass

washing,

with

the

stench

of

Cellulose and hemicellulose are well preserved in the AFEX process, with little or no degradation. Hence, AFEX pre treatmented

LCM are stable for long

periods and can be fed at very high solids loadings in enzymatic hydrolysis or fermentation process. These properties make this processes suitable for AFEX treatment at distributed biomass processing centre prior to transportation to bioethanol plant. Such pretreatment plant can assist in increasing the bulk density, reducing the transportation costs, improving the storability, and creating

detoxification or nutrient supplementation[54].

concerns

better

environment

to

handle

the

be

applied

for

feedstock[58].

(iv) the moderate temperatures (< 100 °C), pH (< 12.0), and short residence time (v) ammonia is a widely used commodity chemical

Alkaline

(i) Lime is a relatively cheap and safer reagent.

(i) Long residence time and difficulties Alkaline

extraction

(ii) Readily remove lignin and xylan side during neutralization are major areas for fractionation of lignocellulosic biomass which is first chains, resulting in a dramatic increase in further studies.

treatments

can

also

step in first step toward realization of bio refinery [59].

12

efficiency of enzymatic hydrolysis [31]. (iii) Alkaline treatment offers option for biomass fractionation and flexibility in utilizing biomass [59]. (iii) Mild reaction conditions

Ionic (ILs)

Liquids (i) ILs are recyclable and reusable because of (i) High cost of IL makes this process very (i) Protic ILs are synthesized through simple their immiscibility with a range of organic expensive.

neutralization of an organic amine with a mineral acid,

solvents.

to yield an IL that does not require purification. Hence one way to reduce the cost for IL production is through the use of Hence protic ILs will be far less expensive than traditional dialkylimidazolium-based salts. (ii) Low temperature IL treatment reduces degradation of both the IL and feedstock and facilitates ionic liquid recycle[60]. (ii) Research to integrate IL pretreatment with subsequent hydrolysis and fermentation has great potential for reducing the cost of pretreatment. (iii) According to Baral et al [61] for IL pretreatment to be economically competitive with sulfuric acid pretreatment, >97% IL recovery, ≤$1/kg IL cost, and >90% waste heat recovery are necessary.

13

ACCEPTED MANUSCRIPT

Fig. 1. Overview of unit operations of lignocellulosic ethanol production [62] and integration of process steps in lignocellulosic ethanol production [16].

Different combinations of individual technological steps have been tested and reported[9]. In separate hydrolysis and fermentation (SHF) processes, all steps are carried out consecutively. In simultaneous saccharifcation and fermentation (SSF) step (2) and (3) are carried out simultaneously. In CBP, step (2), (3) and (4) are carried out in one-step. Regardless of configurations, one of the main bottlenecks of these technologies is the low concentration of sugar in LCM available for fermentation. Large-scale commercial production of ethanol from LCM requires its production technology to be cost-effective and environmentally sustainable. Selection of the suitable combination of unit operations ensures an economically acceptable technology for converting any particular LCM into ethanol.

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ACCEPTED MANUSCRIPT 3. Pre-treatment processes Following section review the recent progress in different pre-treatment processes reported in literature. The study especially focuses on understanding how the changes bring about by pretreatment can be helpful to optimize different strategies involved in the conversion of LCM to ethanol. These pretreatment operations are also primordial steps of lignocellulosic biorefinery. 4. Biological pre-treatment In biological pretreatment microorganism like brown, white and soft rot fungi are used to depolymerize lignin and hemicelluloses in LCM [6, 63]. Brown rots mainly attack cellulose, while white and soft rots attack both cellulose and lignin [64]. Fungi breakdown lignin anaerobically using family of extracellular enzymes collectively termed lignases. Beside the nature and composition of biomass other process parameters like type of microorganism used, incubation temperature, pH, incubation time, inoculums concentration, moisture content and aeration rate affects the performance of biological pretreatment [6]. Sindhu et al[6] has presented an overview of various aspects of biological pretreatment, enzymes involved in the process, parameters affecting biological pretreatment as well as future perspectives. Biological pretreatment processes requires long incubation time for effective delignification which also means large amount of space which limits its industrial application .This can be minimized to an extent by using suitable microbial consortium [6]. Another drawback is subsequent the low hydrolysis rate obtained as compared to other technologies[63]. Lower sugar concentration yields low ethanol. The presence of inhibitory factors necessitates detoxification of hydrolysates[65]. Shirkavand et al [26] suggested developing an efficient and effective combined pretreatment method to make it of commercial interest. An

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ACCEPTED MANUSCRIPT appropriate combination of pretreatment methods can maximize the utilization of lignocellulosic components[16]. 5. Physical pre-treatment Physical pre-treatments include mechanical operations, different type of irradiations and ultrasonic pre-treatment. These methods are briefly discussed in following section. 5.1. Mechanical pretreatment Mechanical pretreatment of LCM present is an essential step to improve particle distribution and densification, enzymatic accessibility, and bioconversion affectivity. It also increases bulk density, improves flow properties, increases porosity, and generates new surface area ideally without the production of toxic side streams[66]. Size reduction of biomass increases the accessible surface area of the material and reduces cellulose crystallinity. Preconditioning after harvesting reduces LCM from logs to coarse sizes of about 10–50 mm. Chipping reduces the biomass size to 10–30 mm while grinding and milling can further reduce the particle size to 0.2–2 mm [22, 67]. Mechanical comminution breaks the LCM down to fine size and reduces the crytallinity of the material. Mills commonly used for such purpose includes knife mill, hammer mill, pin mill, ball mills, vibratory mills, colloid mills, attrition mills, extruders and centrifugal mill [31, 64, 68]. Hammer mills are the most commonly used as they are relatively inexpensive, easy to operate, and produce a wide range of particle sizes [69, 70]. Hammer mills reduce the particle size by shear and impact actions. However, lengthy straw/stalk of biomass may not be directly fed into hammer mills. Such LCM are preprocessed using coarse grinders like a knife mill to allow for efficient feeding in refiner mills without bridging and choking[71]. Bitra et al[71] provided the useful information regarding the energy performance of knife mill for different LCM under varying operating parameters like screen size, mass fee rate, and speed of mill [71]. Igathinathane et al [72] has reported the 16

ACCEPTED MANUSCRIPT effects of moisture contents, knife grid spacing, and bed depths, on ultimate stresses and energies involved in the size reduction of corn stalks in linear knife grid device. Bitra et al [73] carried out the extensive analysis of specific energy and particle sizes to selection of hammer mill operating factors to produce a particular size of switchgrass, wheat straw, and corn stover grind. O'Dwyer et al [74] has presented following correlation between structural features and digestibility for wheat straw during the hydrolysis. Digestibility = 2.04 (𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎)0.99(100 ‒ Crystallinity index) (1) (𝐿𝑖𝑔𝑛𝑖𝑛 𝑐𝑜𝑛𝑡𝑒𝑛𝑡) ‒ 0.39 For dry biomass with moisture contents less than 15% (wet basis) two-roll, attrition, hammer or knife mills are suitable for size reduction When the moisture content of materials is more than 15–20% (wet basis), colloid mills and extruders is suitable choice for comminution[14]. The moisture content of corn stover, soybean stems and leaves, rice straw and sunflower stalk varies between 30% and 70%, while the moisture contents of wheat, oat and barley straw range from 10% to 20% [75]. Analysis of size reduction, specific energy and particle size distribution helps in the selection of suitable mill to produce a particular particle size with optimal performance. Physical properties of material like initial size of the biomass, its moisture content and operating conditions like speed of rotor driving tools and rate of feeding to the milling equipment dictate the energy requirement for comminution operation [31, 66]. As evident from the Fig 2 [76], for several types of milling, material with higher moisture contents require more energy input for comminution to final particle size [76]. Barakat et al [77] while analysing the effect of initial moisture, biochemical and structural proprieties of biomass on energy requirement during the mechanical size reduction concludes that cellulose content, crystallinity, p-coumaric acids and lignin content negatively affected the specific energy requirement (SER) values

whereas the arabinose/xylose ratio and

accessible surface area positively affected the SER values. A comparison of

energy 17

ACCEPTED MANUSCRIPT requirement of mechanical comminution of agricultural lignocellulosic materials with different size reduction is provided in Fig 3[78] and Fig 4[77]. Fig 4(A) shows that the initial moisture content in LCM affects the final particle size obtained after ball milling conducted at constant SER while Fig 4B shows the tendency of the SER variation versus initial moisture content during the ball milling of six different six LCM.

Fig 2. comparison of energy consumption during the comminution of various LCM with hammer and knife mills [76].

18

140 120

Final size (mm)

Knife mill

6.35

2.54

Hammer mill

100 80 60 40 20

2.54

1.6

3.2

1.6

9.5

6.35

3.2

1.6

er Co r

Ha

n

st

w

St

ov

ra

oo

d

w

0

rd

Energy consumption (kWh/ton)

ACCEPTED MANUSCRIPT

Lignocellulosic materials

Fig 3. Energy requirement of mechanical comminution of LCM with different size reduction ( data extracted from [78]).

(A)

Wheat straw Miscanthus Sorghum

SER (Kwh\t DW)

Size (um)

45

(B)

Sunflower stalk Corn stalk Rice straw

40 35 30 25

Wheat straw Miscanthus Sorghum

750

Sunflower stalk Corn stalk Rice straw

650 550 450 350 250

20 0

1

2

3

4

5

6

7

8

150

% moisture content

0

1

2

3

4

5

% moisture content

6

7

Fig 4. Effect of initial moisture, biochemical and structural proprieties of biomass on energy requirement during the mechanical size reduction[77].

19

8

ACCEPTED MANUSCRIPT Bitra et al. [73] established positive strong correlation between hammer mill speed and total specific energy and a correlation between hammer type and SER which help in the selection of hammer mill operating speed to produce a particular particle size of LCM. Gil et al [79] presented population balance model for biomass milling. The model can be used to produce the milling product characteristic under several steady state conditions of the mill (rotor speed, sieve openings and feed rate). The location and time of size reduction operation also influences the efficiency of the whole supply logistics of LCM. Placing size reduction, densification, and torrefactions before transportation and facilitate the uniform handling and transportation of flowable feedstock to end-users with standard equipment and management procedure. During the bioethanol production, a decreasing of particle size involves higher hydrolysis yield of the lignocellulose, however size reduction further than 0.4 mm has little effect on hydrolysis rate[80]. 5.2. Irradiation Irradiation by e.g. gamma rays, electron beam and microwaves is a very convenient tool for the modification of polymer materials through degradation, grafting and crosslinking. 5.2.1. Gamma radiation (GR) Ionizing irradiation results in localized energy absorption within the macromolecules of the cellulose materials and produces long- and short-lived radicals. These radicals initiates the secondary degradation of materials through chemical reactions such as chain scission and cross-linking[81]. Once irradiation is the terminated, the radicals produced in amorphous regions quickly vanish while radicals trapped in the crystalline and semicrystalline regions of the cellulose structure decay slowly over time and cause further degradation of the LCM [8284]. 20

ACCEPTED MANUSCRIPT 5.2.2. Electron beam irradiation (EBI) EBI pretreatment is an energy efficient and environmentally benign method which result in the decrease in the crystallinity and molecular weight and increase in the surface area of LCM. Irradiation of LCM is carriedout under the conditions of 1 MeV and 80 kGy at 0.12 mA[85]. Bak et al [86] found strong correlation between the crystallinity and enzyme digestibility of rice straw samples pre-treated by EBI under different conditions while the study by Karthika et al [87] concluded that the decreased in crystallinity was the major favorable effect of EBI on the hydrolysis of biomass. 5.2.3. Microwave radiation (MWR) MWR accelerate the chemical, biological and physical processes by generating the heat and extensive collisions because of the vibration of polar molecules and the movement of ions[88]. The performance of MWR depends on its dielectric properties (dielectric constant and dielectric loss factor) of LCM. The dielectric constant measures the ability of a material to store electromagnetic energy while dielectric loss factor measure of the ability of a material to convert electromagnetic energy into heat. The overall energy efficiency of MWR is measured by calculating the loss tangent, which is a ratio of the dielectric loss factor to the dielectric constant. Xu[89] reviewed the progress on the use of MWR /water, (2) MWR /alkali, (3) MWR /acid, (4) MWR /ionic liquid, (5) MWR /salt, and (6) other combined MWR -assisted pretreatment. Compared with the conventional alkali pre-treatment, MWR -assisted alkali pre-treatment remove more lignin and hemicellulose from wheat straw in shorter time [90]. Ha et al [91] also reported significant enhancement in release of reducing sugar (both rate and yield) during the enzymatic hydrolysis of cellulose regenerated from IL dissolution pretreatment with MWR. Marx et al[92] carried out the MWR pretreatment and hydrolysis of

21

ACCEPTED MANUSCRIPT grass type biomass to sugar in a single step eliminating the hydrolysis step to make the process economically attractive. Verma et al[93] while pre-treating woody biomass achieved maximum sugar yield of 59.5% MWR at 140°C for 30 min with ammonium molybdate and H2O2. Table 3 summarizes the effects of cellulose conversion by different irradiation pretreatments. 5.2.4. Pre-treatment with ultrasonic energy Ultrasound produces both sonochemical and mechanoacoustic effects. The mechanoacoustic effect alters the surface structure of the biomass while sonochemical production of oxidizing radicals leads to chemical attack of the components of lignocellulose. Fig 5 [94, 95] shows the mechanistic effects of ultrasonic energy in LCM pretreatment. TABLE 3. The effects of cellulose conversion by different irradiation pre-treatments (adopted from [96])

Irradiation

Maximum

Enzymatic

Cellulose Conversion

Hydrolysis

(under standardized

Duration

conditions of

Pretreatment

Wood Species

Dose

Particle Size

(h)

enzymatic hydrolysis)

EBI [97]

Spruce

2 MGy

1–2 mm in

72

0.89

thickness and 10–20 cm2 GR[98]

Softwood

40 kGy

32–42 mesh

8

0.673 ± 0.10

90 kGy

32–42 mesh

8

0.803 ± 0.10

400 W

30–42 mesh

48

0.561

(Triplochiton scleroxylon) Hardwood (Khaya senegalensis) MWR[93]

Beech

22

ACCEPTED MANUSCRIPT

Ultrasonic energy promotes the pretreatment of LCM through its special cavitational effects caused by ultrasound acoustic wave with the frequency range from 10 kHz to 20 MHz[55, 56]. Cavitation is highly dependent on the frequency of ultrasonic energy[57]. During cavitational collapse of huge amount of energy is released to which result in creation of localized hot spots have temperatures of roughly 5000°C, pressures of about 500 atm with a lifetime of a few microseconds[56]. These sever conditions destroy the crystalline state of solid materials, cause solids to melt or fuse solid particles when they collide with each other[58]. Ultrasonic increases the yield of glucose, xylose, and ethanol in downstream processing as well as reduce the treatment time which is attributed to enhanced accessibility and delignification[59].

Fig 5. Mechanistic effects of ultrasonic energy in lignocellulose pretreatment [94, 95]

23

ACCEPTED MANUSCRIPT

For processing of LCM with ultrasound, the selection of ultrasonic parameters such as ultrasonic mode (continuous or pulse), frequency, power, processing temperature, solvent, aeration and the design of reactors with proper geometric construction determines the level and distribution of energy intensity in the system, and thus influence efficiency and reliability of the operation[55]. Operation is effective over a range of particle sizes, although optimization indicates that higher yields are achieved for lower particle sizes[34]. In the design of ultrasonic reactors, some of the design parameters to be considered are reactor types (e.g., bath, probe, flat plate or tube), reactor geometry, transducer design and arrangement, and volume or scale of feedstocks for practical systems. Luo et al [95] examined the recent applications of ultrasonic energy in the pretreatment and conversion of LCM and has also provided the details about the sonochemical reactors that are commonly used in ultrasound-assisted biomass reactions. Enzymatic hydrolysis of sugar cane bagasse was improved with ultrasonic pretreatment, with an increase in glucose yield by 21.3 % [35]. Introduction of a low energy, uniform ultrasound field into enzyme processing solutions greatly improves the effectiveness of enzymatic hydrolysis of LCM by significantly increasing their reaction rate [35]. The ultrasonic pretreatment with choline-based ILs can effectively remove silica, lignin and ash from network structure of rice hulls and destroy their rigid structure to expose the cellulose for the enzymatic hydrolysis, enhancing the efficiency of enzymatic hydrolysis [99]. Ultrasound pretreatment increases degradation of polysaccharides and since lignin is the most accessible component of the lignocellulose and it degrades preferentially which leads to loss in yield[34]. The energy requirements calculated for pretreatment in an autoclave, steam explosion, and ultrasound were 23.3×104 J/g, 9.9×104J/g, and 7.2×104 J/g respectively[100]. Ultrasound is also known to augment oxidization reactions, hence there is potential for

24

ACCEPTED MANUSCRIPT ultrasound to be combined synergistically with oxidizing pretreatments, perhaps with a necessary addition of alkaline [34, 101]. Ultrasonic irradiation has also been used to intensify the fermentation process[102]. However, careful selection of ultrasound parameters (low frequency operation and optimized time and power dissipation levels) is paramount to enhancing the cell uptake as well as improve the mass transfer rates while avoiding the detrimental effect of ultrasound energy on the cells [102]. Ultrasonic processing on a larger scale must be optimized to identify the key operating parameters. The combined enzyme/sonication hydrolysis of LCM could significantly accelerate overall conversion of LCM into biofuels[35]. The technology necessary to scale-up ultrasound pretreatment of lignocellulose can only be developed once the key operating parameters are determined and optimized[34, 103]. 6. Physicochemical Pre-treatments 6.1. Wet oxidation (WO) During WO, LCM is exposed to temperature of 140–210°C, and pressure up to 20.0 MPa using oxygen or air. Upon reaching the target temperature, oxygen is purged into the reactor and LM is treated for a period of time (5–120 min)[104]. Beside gaseous oxygen oxidizing agent like air, or hydrogen peroxide (H2O2) are also employed. However, oxygen/air is preferable due to low cost and it does not require any additional steps such as post treatment detoxification or neutralization or recovery of the chemical. WO open up the crystalline structure of cellulose by solubilising hemicellulose and decomposing lignin into carbon dioxide, water, carboxylic acids and and phenols, [36, 105-107]. In parallel to chemical reaction,

material also undergoes physical rupture[40]. WO has been reported for the

pretreatment of wheat straw[106], sugarcane bagasse[37], rice hulls[37, 105], cassava stalks[37], peanut shells[37], corn stover[108], maize silage, and clover-ryegrass mixture.

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ACCEPTED MANUSCRIPT 6.1.1. Effects of severity factor The severity factor first defined by Overend and Chornet[109] is a common term that describes the combined effects of the temperature, pH and reaction time. Model is based on first order kinetice which obeys Arrhenius law. (𝑇(𝑡) ‒ T𝑅)

𝑅𝑜 = 𝑡𝑒

(2)

14.75

Were, Ro is the severity factor, T is temperature in oC, TR is reference temperature at which no solubilization occurs (100°C), t is the residence time in (min) and constant value of 14.75 is activation energy value for first order system.

Ro measures the combined effect of

temperature and time in a given pretreatment. Following is modified form of above equation which consider temperature in as funcition of residence time in the reactor[110]. t Ro = ∫ o e

(T(t) ‒ T𝑅) 14.75

(3)

dt

Combined severity factor (RCSF) defined in following equation combines the reaction time, pre-treatment temperature, and acid concentration into a single variable [111, 112].

{

𝑅𝐶𝑆𝐹 = log t.e

(T(t) ‒ TR)/14.75

} ‒ pH

(4)

Increasing RCSF increases solubilisation of hemicelluloses during the pre-treatment process [113]. However, this may lead to rapid degradation and production of inhibitory compounds. Plant components and cell wall structure also influence biomass digestibility. 6.1.2. Effects of Temperature In the presence of oxidative or catalytic agents solubilization of hemicellulose starts at 110°C whereas in their absence solubility remains unchanged up to 170°C[104]. At temperatures around 170°C LCM releases organic acids which leads to autohydrolysis[114]. Increasing temperatures and extended residence time results in formation of compounds like organic acids, 5-hydroxymethylfurfural, and furfural, which are inhibitors for both the enzymes and 26

ACCEPTED MANUSCRIPT the fermenting microorganisms. Increased temperature also result in material losses, highenergy consumption[109]. 6.1.3. Effects of pH WO pretreatments start at neutral pH conditions. However subsequent release of organic acids by autohydrolysis tend to lower the pH of the biomass slurry to pH (3–4.5). Acidic condition promotes solubilization of hemicellulose leaving the lignin mostly insoluble. The formation of unwanted byproducts also depends on pH. Acidic conditions promote formation of 5-hydroxymethyl furfural while alkaline conditions favor formation of fragmentation products such as glycolaldehyde and glyceraldehyde [115]. The neutral value of pH also changes with change in temperature [116, 117]. Current research and development activities are centered on identifying the optimal process parameters for different biomass resources[104]. The rate constant for hemicellulose solubilisation are higher than that for lignin, whereas the rate constant for cellulose is slowest [118]. The cellulose recovery (95-100%) was significantly higher than hemicellulose recovery (60%)[118]. At temperatures above 185°C, recoveries decreased due to increased degradation. Presence of Na2CO3 reduces the formation of by-products [106, 119, 120]. Study by Martín et al [119] suggest that WO of bagasse for 15 minutes result in removal of 50% of lignin and 57.4% conversion of cellulose compared to only 35% lignin removal and 48.9% cellulose conversion for steam explosion under the same conditions. Lan Ping et al[121] suggested that WO permit a significant reduction of the klason lignin content of the pulp with a good carbohydrate recovery and improved enzymatic digestibility. WO is an established technique for treatment of industrial wastewater where both pure oxygen and air is used as oxidizing agent. WO is capable to mineralize organic material to CO2, H2O and inorganic salts. The costs of WO are mainly determined by the reaction conditions. High temperature and pressure increase the energy cost. It also requires expensive 27

ACCEPTED MANUSCRIPT materials for the reactor. The amount of by-products formed by WO are also higher compared with steam explosion. 6.2. Liquid hot water (hydrothermolysis) Ruiz et al [122] has reviewed research progress in hydrothermal processing of LCM and aquatic biomass for the fractionation of their main components. Liquid hot water (LHW) used water as a reaction medium at relatively high reaction temperatures (180-220oC). The liquid state is maintained by increasing the pressure[123]. Changing the process temperature and pressure alters the dielectric strength and ionic product of LHW [124] and effectively remove hemicelluloses from the lignocellulosic matrix at 220°C. Increasing Ro increases the yield of undesired byproducts. Hence compromise has to be found between biomass solubilisation and concentration of undesired degradation product[110]. Díaz et al[125] found the best LHW pre-treatment of rapeseed straw at 217.7 °C and 42.2 min, resulting in a 69% of the potential glucose release, although no hemicellulosic sugars were available. If sugar present in the liquid fractions issued from pre-treatment are also taken into account, a more favourable operational conditions of 193°C for 27 min can be used, which allow more than 65% of glucose and 39% of xylose to be available for fermentation. Silva-Fernandes et al[126] demonstrated that mixture of lignocellulosic materials can be efficiently processed by LHW, under similar conditions, and generate consistent product composition independently of the different proportions of each feedstock. Beside this, LHW technologies have the advantages of high efficiency, no additional catalysts, and no intermediate inhibition. However high energy cost derived from high temperature and pressure, the relatively low yield of fermentable sugars, which are the target products of pretreatment and hydrolysis, is the key limiting factor of such processes[127].

28

ACCEPTED MANUSCRIPT 6.3. Steam explosion (SE) Jacquet et al[128] has recently presented review of pretreatment of LCM using steam explosion which has also discussed the effects of SE on lignocellulosic structure and influence on enzymatic hydrolysis yield of the material. SE has been successfully applied to ethanol production from several LCMs including corn stover, poplar trees, sugarcane and switchgrass[128]. SE includes an auto-hydrolysis step and an explosion step after penetration of high-pressure steam into the plant cell wall. In SE, LCM is rapidly heated by high-pressure saturated steam at temperature of 160–260°C and corresponding pressure of 5–50 atm for several seconds up to few minutes. This is followed by is swift release of pressure causing the steam to expand within the lignocellulosic matrix. During the explosion, plant biomass particles are exploded into small pieces, the fibers of the plant biomass are separated, and the ordered structure of the plant biomass is significantly disrupted [22, 23, 129]. SE can be carried out with or without addition of an acid as catalyst. Treatment with 1% acid gives more extensive lignin depolymerisation[130]. The process variable that

influences the efficiency of SE includes residence time,

temperature, chip size and moisture content[131]. Moderate Ro of 3.6 and 3.9 increases the initial rates of hydrolysis and provide a basis for reducing processing times. Ro below 4 limits the thermal degradation of cellulose.

However, Ro greater than 4.24 results in

breakdown of cellulose, formation of formic acid and acetic acid, and loss of xylose [132]. Fig 6 [133] shows the thermal degradation phenomenon of cellulose with the temperature and retention time at Ro of 5.2 and 4.0 respectively. These results allows identification of the pretreatment conditions with minimum degradation of cellulose [14].

29

ACCEPTED MANUSCRIPT 35

SF 5.2

SF 4

Time (min)

30 25

Depolymerization + Dehydratation

Depolymerization

20

Thermal degradation limited

15 10 5 0 150

170

190

210

230

250

270

290

310

Temperature (oC)

Fig 6 : Evolution of the thermal degradation phenomenon of cellulose with the temperature and retention time[133]. (Severity factor 5.2, severity factor 4.0) Since Ro accounts for the temperature and the duration of the pretreatment without any consideration of the explosion conditions, Yu et al [134] describes a new parameter the theoretical explosion power density (EPD) based upon hypothesis of the adiabatic expansion process to express the severity of the explosion phase. EPD = (∆Hs + ∆HI + ∆Hm)/(t ∗ V)

(5)

Where ΔHs, ΔHl and ΔHm signify the enthalpy drop of steam, liquid water, and material, respectively, ‘t’ represents the explosion duration, and ‘V’ represents the volume of the explosion reactor. Different equipment commonly causes the value of ‘t’ to increase by orders of magnitude[134]. Steam is consumed in heating the material and to maintain the pressure inside the pressurized reactor. Lower steam consumption plays the decisive role to reduce the pre-treatment cost. Consumption of steam increases with increase in reaction temperature or increase in initial moisture content of LCM [135]. To remove the degradation products formed during the SE

30

ACCEPTED MANUSCRIPT pre-treatment along with water-soluble hemicelluloses, the pre-treated biomass is normally washed with water[136]. Typically, 20–25% of the initial dry matter is removed by water wash, which decreases the overall saccharification yields [52]. SE is considered the most cost-effective pretreatment method due to short residence time and low energy consumption. Concentrated sugars can be obtained because the biomass is heated rapidly by steam and only small moisture exists in the reactor. It can generate complete sugar recovery while utilizing a low capital investment and low environmental impacts with higher potential for optimization and efficiency. 6.4. Dilute acid (DA) Dilute H2SO4 is commonly employed to pre-treat a wide variety of biomass. It is usually carried out with a very low H2SO4 concentration of 0.2–2.5% w/w, at elevated temperature between 130 °C and 210°C[137]. Pretreatment with dilute acid may results in formation of furfural and HMF [49]. HMF is formed because of dehydration of hexoses along with acetic acid formed during initial decomposition of the hemicelluloses because of hydrolysis of acetyl groups. These compounds are inhibitory to the fermentative micro-organisms. Inhibitors formation increases with increase in temperatures and acid concentrations[138]. 6.5. Supercritical Fluid (SCF) SCF technique is based upon the use of solvent at temperature and pressure above their critical point where heat of vaporization is zero and no distinction exists between the liquid and gas phases[139]. Pre-treatment with supercritical CO2 has been tried on different type of biomass due to its moderate critical temperature of 31.1oC and pressure of 7.4 MPa. Other advantages of CO2 are inert in nature, non-flammable, non-toxic, inexpensive, and readily available from the byproducts of many industrial processes[50]. At super critical conditions

31

ACCEPTED MANUSCRIPT CO2, possess properties (e.g. density, viscosity, and diffusion coefficient) intermediate between those of liquid and gas (see Table 4[50]). Table 4. Density, diffusion coefficient and viscosity of gaseous, supercritical and liquid CO2[50]. State

Density (g/cm3 )

Diffusion (cm2/s)

Viscosity (g/cm s )

Gas

10-3

10-1

10-4

Supercritical

1 to 10 (Liquid like)

103-104(Liquid like)

10-3 to 10-4 (Gas like)

Supercritical

1

<105

10-2

Gas like viscosities accelerates the chemical reaction kinetics. Supercritical CO2 has a zero surface tension and provide a good wetting of the surface to facilitate the surface chemical reactions. It also facilitate a better penetration of the reactants into a porous structure[50]. CO2 is easy to separate from the synthesized materials by releasing pressure and the final products are dry without residuals. Explosive release of CO2 after pre-treatment disrupts the cellulose and hemicellulose structure of LCM to increase the accessible surface area for subsequent hydrolysis. The advantages of SCF with CO2 include the high solid concentrations in pre-treated material, and low pre-treatment temperature which prevents sugar degradation and minimizes the operational cost[140]. Water in the biomass combined with supercritical carbon dioxide generates a carbonic acid mixture which generates a kind of weak acidic environment preliminary promoting the hemicellulose hydrolysis and intensifying the mass transfer conditions [141]. The most important parameters taken into account are the moisture, temperature, pressure and pretreatment time. According to Alinia et al [142], the pre-treatment of dry wheat straw by combined SE and supercritical CO2 at steam temperature and retention time of 200°C and 15 min and supercritical CO2 conditions of 12 MPa, 190°C for 60 min, resulted in the best overall yield

32

ACCEPTED MANUSCRIPT for sugar (234.6 g/kg). The study also suggested that the presence of water molecule as both liquid and steam resulted in carbonic acid production and cellulose de-crystallisation which improve the pre-treatment efficiency. The formation of carbonic acid increases the kinetic constants for the production of either intermediated products or final compounds in comparison to pure water reaction[143]. A small changes in temperature or pressure close to critical point causes 100-fold changes in solubility, therefore separation and recovery of SCF is very easy [144]. 6.6. Organosolvent (OS) OS pretreatment involves mixing an organic liquid and water together in various portions and adding them to the LCM. Zhang et al[145] has recently presently updated review of the previous work on pretreatment of LCM using organic solvent. Commonly mixture of water with alcohols like methanol, ethanol, acetone or glycols in 1:1 ratio are used as organic solvent[76]. This mixture is heated to dissolve the lignin and some of the hemicellulose and leave a relatively pure cellulose residue. Likewise mixture of water with organic acid like acetic acid (30% v/v) and formic acid (30% v/v) is also used as organic solvent[146]. Most of hemicellulose and lignin are solubilized, but the cellulose remains as solid. OS pretreatment yields three separate fractions: dry lignin, an aqueous hemicellulose stream, and a relatively pure cellulose fraction [147, 148]. The process is shown in Fig 7 [145]. Mineral acid is used as catalyst sometime to either reduce the operating temperature or enhance the delignification process. Catalytic OS process can be carried out at 100–150°C to break the hemicelluloselignin bonds. Catalyst increases the rate of delignification and solubilization of the hemicelluloses fraction and higher yield of xylose are obtained[149]. Organic acids such as formic acid, oxalic, acetylsalicylic, and salicylic acid also can be used as catalysts[31].

33

ACCEPTED MANUSCRIPT

Fig7. Flowchart of organic solvent pretreatment of LCM[145].

Low boiling point organic solvents are always easy to recover by distillation and are recycled. Hence, the low boiling point alcohol such as methanol and ethanol has advantage of lower solvent costs and easy recovery of solvents. However, their low boiling point necessitates a high pressure during pretreatment. Acetone pretreatment is similar to low boiling point alcohol pretreatment, but the cost of acetone is higher than cost of methanol and ethanol. However, acetone is the most efficient ketone for delignification [150]. High-boiling polyhydroxy alcohols like ethylene glycol and glycerol permit high temperatures enabling delignification of LCM at atmospheric pressure. However, in such cases the solvent recovery is energy intensive. Pretreatment with formic and acetic acid pretreatment can be performed under atmospheric pressure in presence of mineral acid. Serious corrosion and acetylation of cellulose significantly decrease its attractiveness. OS pretreatment is similar to organosolv pulping, but the degree of delignification for pretreatment is not demanded to be as high as that of pulping[2]. Removal of solvents from the system is necessary because the solvents may inhibit the growth of organisms during subsequent enzymatic hydrolysis and fermentation. Low boiling point alcohol pretreatment seems to be the most promising OS process due to the lower chemical costs and easy recovery of solvents. However, for most easily recovered solvents, the pretreatment process

34

ACCEPTED MANUSCRIPT is always conducted under high pressure with resulting increase of the operational cost. Working with violate organic solvents necessitate extremely tight and efficient control due to inherent fire and explosion hazard. Zhang et al[145] has recommended the optimization of pretreatment conditions in order to develop cost effective organic solvent pretreatment for commercialization. Liquid-to-solid ratio (LSR) is major parameter for optimization to reduce the amount of water and solvent in the system, which reduces capital costs substantially, since smaller tanks and pumps are required for the same quantities of feedstock. Similarly, operating costs are also reduced when low LSR is selected, especially energy for pumping and solvent recovery. OS pretreatment is an emerging way ahead because of its inherent advantages, such as the ability to fractionate LCM into cellulose, lignin, and hemicellulose components with high purity, as well as easy solvent recovery and solvent reuse[145]. Mild pretreatment temperature and pressure and a neutral pH condition reduce carbohydrate degradation into undesired furfural and HMF[151]. 6.7. Ammonia fibre explosion (AFEX) In AFEX process, LCM is treated with liquid anhydrous ammonia under high pressures and moderate temperatures. After a short residence, time pressure is explosively released. Rapid expansion of the ammonia gas disrupts carbohydrate linkage. In AFEX, LCM enters and leaves the reactor in the dry form. Pretreatment parameters like moisture in LCM, ammonia loading, reaction temperature, and residence time are varied to optimize AFEX pretreatment[152]. An optimum combination of these variables is depends upon the recalcitrant nature of the lignocellulosic biomass[56]. During the AFEX process concentrated anhydrous ammonia dissociates into water to form ammonium and hydroxide ions by the following equation.

35

ACCEPTED MANUSCRIPT 𝑁𝐻3 + 𝐻2𝑂→𝑁𝐻 +4 + 𝑂𝐻 ‒ During above reaction heat of formation(∆𝐻𝑁𝐻

4𝑂𝐻

at 25oC = –87.59 kcal/mol ) released

which facilitate the rapidly increase the temperature of the biomass in the reactor[57]. The moisture of LCM can be varied between 60% unto 250% (dry weight basis)[23]. Increasing the moisture content increased the glucan conversion[153]. The ammonia-water mixture equilibrates in the vapor–liquid phase based on the thermodynamic state of the system[154]. Ammonia loading is maintained around one kg weight of ammonia per kg of dry biomass of LCM [155, 156]. After pre-treatment, ammonia is evaporated and recycled [157]. While around 90% of the ammonia will vaporize when the pressure is released, the rest remains soluble in the moisture present within the biomass. In ammonia recycle percolation (ARP) process the aqueous ammonia (10-15 wt %) passes through biomass at elevated temperatures (150-170 °C) with a fluid velocity of 1 cm/min and a residence time of 14 min, after which the ammonia is recovered. Under these conditions, aqueous ammonia reacts primarily with lignin and causes depolymerization of lignin and cleavage of lignin-carbohydrate linkages. After pretreatment ammonia is separated and recycled. The ammonia pretreatment does not produce inhibitors for the downstream biological processes. A small fraction of ammonia left behind in biomass enriches its nutrient content and serves as nitrogen source during fermentation [53, 158]. The moderate temperatures (80–150°C) ensure less energy input and overall costs associated with the process compared to steam explosion process. The residence time varies between 5 min to 30 min depending upon the degree of saturation required for subsequent hydrolysis of different LCM. Harvest type and location also affect the performance of pretreatment, thus, it is necessary to consider an integrated approach between agricultural production and biochemical processing in order to insure optimal productivity. Relative maturities of LCM varieties vary with 36

ACCEPTED MANUSCRIPT harvest time and were harvest location. Relative maturities have an effect on digestibility[152]. For early harvest material, relatively mild pretreatment results in high ethanol yields. However, more severe conditions are necessary for later harvest. Because of seasonal availability of different feedstock supplies pretreatment processing conditions must accommodate specific chemical and structural composition of the various, and variable sources of LCM to maintain year-round production which may has implications for process intensification. The AFEX technology has been used for the pre-treatment of many LCM including alfalfa, wheat straw, switch grass, Bermuda grass, perennial grass, bagasse, corn stover and wheat chaff. Alizadeh et al (2005) [155] carried out the pretreatment of switch grass by AFEX under the optimal pretreatment conditions (temperature, 100°C, ammonia loading of 1:1 kg of ammonia: kg of dry matter with 80% moisture content for 5 min residence). Subsequent hydrolysis and fermentation of optimized AFEX-treated switch grass give ethanol yield of about 0.2 g ethanol/g dry biomass, which was 2.5 times more than that of the untreated sample. Uppugundla et al [159] also reported ethanol yields of 0.20 kg of ethanol per kg of AFEX pretreated corn stover. AFEX process is not very effective for biomass with higher lignin content such as such as woods and nut shells [23]. Much of the capital and utility cost of AFEX is due to the need cost involved in recovery of ammonia [57].The complete operation involves a dryer and distillation after the initial flash. The dryer removes the residual ammonia as well as some moisture, which are separated in the distillation column. Makeup ammonia requirements vary between 15 to 25 g ammonia/kg biomass [51, 53, 155]. An efficient ammonia recovery process is paramount to make this pretreatment economically feasible. Carefully controlling the temperature and vacuum pressure is critical for design[157]. The use of sweep gases, including nitrogen or supercritical ammonia, can also improve the process. In the case of nitrogen is used as sweep gas, a

37

ACCEPTED MANUSCRIPT separate compressor or condenser must be used to remove the ammonia from the nitrogen prior to the distillation column. Aqueous ammonia can be fed to the biomass without any reduction in AFEX treatment quality, which could eliminate the need for a distillation column. Likewise, if the ammonia can be added to the biomass in the gas phase, then the condenser can also be eliminated[57]. Da Costa Sousa et al [160] has proposed pretreatment method using liquid ammonia under conditions that allow conversion of native cellulose to a highly digestible cellulose allomorph and simultaneously extracting a lignin fraction. By removing lignin-caused inhibition of enzymes and increasing enzyme accessibility to structural carbohydrates and by enhancing cellulose reactivity, this method significantly reduced enzyme loadings. 6.8. Alkaline extraction Kim et al[59] has recently reviewed alkaline pretreatment technology for bioconversion of LCM. In alkaline pretreatment, biomass is treated with sodium, potassium or calcium. Alkaline pretreatment involves solvation and saponification[15]. This causes a swelling of biomass with an increase of porosity and internal surface and a concurrent decrease in the degree of polymerization[31] which increases the accessibility of enzymes during subsequent hydrolysis. A neutralizing step to remove lignin and inhibitors (salts, phenolic acids, furfural, and aldehydes) is required before subsequent enzymatic hydrolysis. The gradual release of alkali from the solid portion during washing makes the neutralization a problematic. Alternately, neutralizing lime with CO2 eliminates the solid-liquid separation step [55, 161]. The residence time for low temperatures (50–65°C) alkaline treatment can be up to several days. Increasing the temperatures (85–135°C) reduces the residence time to few hours[162]. Lime pre-treatment did not result in the formation of inhibitors such as furfural and hydroxymethyl furfural[163].

38

ACCEPTED MANUSCRIPT 6.9. Ionic liquids (ILs) Ionic liquids (ILs) are molten salts under 100°C, which comprise organic cations and organic or inorganic anions. ILS are considered “ecofriendly” because of negligible vapor pressure, non-flammability, good thermal, and chemical stability. ILs that contains anions with high hydrogen-bond basicity such as chloride, phosphates, phosphonates and carboxylates are excellent solvents for cellulose dissolution [164, 165]. Recentl ILs have been attracting increasing attention in the field of pretreatment of LCM[166]. For pre-treatment of LCM delignification and reduction in cellulose crystallinity as key factors in the effectiveness of ILs. Enzymatic digestibility is closely correlated with accessible surface area and porosity delignification and reduction in cellulose crystallinity. Pretreating LCM with certain ILs results in structural and chemical changes that make the biomass more digestible by enzymes [167]. Fig 8 [168] shows the schematic presentation of solvation mechanism of cellulose using [C4mim]Cl.

Fig 8: Possible mechanism of cellulose dissolution in [C4mim]Cl [168]. IL pretreatment of LCM typically involves heating finely ground, dry biomass in an excess of the IL at moderate temperatures. A process described in given in Fig 9 [169]. Here water and acetone was used to selectively precipitate cellulose and lignin from softwood and hardwood after the wood is completely dissolved in [C2mim]OAc. The ionic liquid is recycled in the

39

ACCEPTED MANUSCRIPT process. A less volatile solvent may be used to replace acetone for lignin recovery if desired. All these pre-treatment processes may be refined and scaled up.

Fig 9. pre-treatment of LCM with[C2mim]OAc and recovery and reuse of ILS [169]. Under different conditions, the average fermentable sugars yield with IL pretreatment of corn stover and

switch grass were 90%[170] and 80%[171] respectively. Baral et al [61]

identified 1-ethyl-3-methylimidazolium acetate as best at dissolving the LCM. The study also estimated sugar production costs from corn stover, switch grass and poplar were $ 2.7/kg, $3.2/kg, and $3.0/kg respectively. George et al[172] has also identified cost of ILs as main impediments to IL utilization in pretreatment. Residual ILs are toxic to fermentative microbes such as Saccharomyces cerevisiae. 7. Novel biomass pretreatment Raud[173] has presented a novel pretreatment method in which biomass was exposed to a high pressure using N2 gas, and temperature. Highest glucose yield and hydrolysis efficiency were gained at 150°C and 1–3 MPa. The fermentation efficiency was lower at higher temperatures. Nonetheless, the highest ethanol yield was still gained at the same conditions. 40

ACCEPTED MANUSCRIPT No catalysts or chemicals were added in the process thereby, making it economically and environmentally attractive. Wi et al [174] has proposed hydrogen peroxide–acetic acid (HPAC) as effective pretreatment over a wide range of biomass materials which enables year-round operations with maximize utilization of LCM from various plant sources. Pretreatment was carried out at 80°C for 2h using equal volume mixture of H2O2 and CH3COOH. The study reported subsequent fermentation of the hydrolyzates by S. cerevisiae resulted in 412 mL ethanol/ kg of biomass after 24 h, which was equivalent to 85.0 % of the maximum theoretical yield based on the amount of glucose in the raw material. 8. Relevant merits of the leading pretreatment technologies Table 5 and 6 gives quantitative comparisons of different pretreatment technologies and merit and demerits of each technology. The optimum pretreatment conditions are not necessarily the same for all kind of biomasses due to their different composition and lignin contents. Table 7 gives the performance indicators of leading pretreatment technologies. Reaction conditions, particle size and moisture content all influences the performance of pretreatment technology. Difference in chemical composition of LCM also influences the conversion efficiency of pretreatment operation. While Table 8 highlights the effect of different pretreatment technologies on the structure of lignocellulose. Pretreatment processing conditions must be tailored to the specific chemical and structural composition of the various and variable sources of LCM [3]. Other factors used for screening pretreatment methods include consumption of energy, chemicals and water and inhibition and waste minimization. Zhu and Pan (2010)[175] has discussed the pretreatment energy consumption in details. Combinatorial pretreatment is more effective in increasing sugar recovery and yield, decreasing the generation of inhibitors, and shortening operation time. The massive utilization of fuel ethanol in the world requires that its production technology be costeffective and environmentally sustainable. 41

Table 5. Quantitative comparisons of pretreatment technologies Treatment method No pretreatment

Treatment time

Performance

Not applicable

Hydrolysis of LCM without Not applicable any pretreatment yield less than 20% of total sugars[176].

Biological pretreatment

Long residence time, up to 4–8 weeks [178]. ≥ 1min

Low ethanol production with fermentation efficiency ranging from 26 to 52%[65]. Attractive method to increase the yield of hydrolysates from LCM, however the actual performance varies with type of material[66]. Optimal wet oxidation conditions are 10 min at 200°C, 12 bars of oxygen at neutral pH. [182].

Mechanical pretreatment

Wet Oxidation

5–120 min

LHW

15 min

Ozonolysis

15 to 90 min

ILs

30-60 min[190]

Cost

Degree of Development

Additional comments

Not applicable

The sugar yield during the hydrolysis of LCM after pretreatment can reach up to 90% with some pretreatment methods[177] No release of toxic compounds to environment and no effluent generation during the process [6]. Milling has a high energy requirement and is not economically feasible in general compared to others pretreatment[66, 77, 181]

low-cost and eco- Pilot scale process[6] friendly method [179]. Most expensive Commercially processing steps in technology [180] terms of energy and operating costs[66]

proven

Moderate cost[104]

capital BioGasol demonstration plant in Denmark has stlected wet explosion technology for pretreatment of biomass. Straubing demonstration plant has also selected hydrothermal method for pretreatment[183]. sugar recovery up to 95 % with High water Has not been applied beyond high enzyme efficiency[104] consumption and pilot-scale investigation [104] energy input[184]. because of higher water demand and high energy requirement[185]. 42% reduction of lignin High generation Laboratory-scale research[188] content, 53% glucose release costs due to large Full-scale biomass pretreatment and 89% ethanol yield[188] energy demand with ozone has not been during on-site developed yet[65] ozone generation[188] After successive hydrolysis IL are very Relatively new technique and steps, glucose yield of 78 wt% expensive[166] still in the exploratory

Hydrolysis yield obtained by wet oxidation pretreatment is slightly lower than yields obtained in the steam pretreatment[182]

Uses water as solvent, requires no chemical. Two-step pretreatment would be the most adequate process configuration to get a maximum recovery of fermentable sugars[186]. Selection of the appropriate temperature is crucial[187]. Total reducing sugar recovery (99.9%) were determined at 0.8 mm particle size, 40 wt.% moisture content, 75 min reaction time and 105 mL/min ozone flowrate with 19.5% ozone consumption[189] high cost of ILs can be a potential drawback. At least 98% of the ILs

42

can be obtained from cellulose, while the total biomass-tosugars yield is 54 wt%[191].

Steam explosion

≥5 min

DA

20 min

76.5% of hydrolysis yields

SCF

20 min

Under the operation conditions of pretreatment pressure 15 MPa, temperature 180 °C and time 1 h, the optimal sugar yield of 77.8c/o was obtained[197].

stage[104]

Lower cost[185]

Method adopted by BIOLYFE Crescentino biorefinery in Italy and Biocarburantes de Castilla y León, S.A. (BCyL) Iogen demonstration plant, Canada has also adopted modified steam explosion process[183]. Jennings demonstration plant, USA is hydrolysing biomass using steam and mildly acidic conditions[195].

Total utilities costs Laboratory-scale[192, 198] are larger for supercritical pretreatment due to the operational conditions (high pressures and

should be recovered for an economically feasible process[191]. some ILs can compete with the cheapest pretreatment chemicals, such as ammonia, in terms of effectiveness and process cost, removing IL cost as a barrier to the economic viability of ILbased biorefineries[145]. IL pretreatment is far less demanding on processing equipment[192] Energy-intensive[193]. lower environmental impact, and less hazardous process chemicals[194]

Dilute acid pretreatment is commonly applied to solubilize hemicellulose and increase the accessibility of the cellulose in lignocellulose [99]. Pretreatment requires water, chemicals, and high pressure steam. High cost of acid, reactor corrosion problems and formation of inhibitors are major issues. Optimal conditions for pretreating wheat straw were determined as: 140 °C, 10 dm3 m−3 sulfuric acid concentrations and a 30 min reaction time [196]. The formation of inhibitors was thus observed to depend on dilute acid pretreatment conditions. carbon dioxide is a green solvent. The highest glucose yield of 30% was achieved at 3500 psi at 150 °C when corn stover with 75% moisture content was treated for 1 h compared[141]

43

AFEX

OS

5 min

30 min

energy required for carbon dioxide liquefaction)[197] 89% of theoretical ethanol High cost of large yield[199] amount of ammonia[185] High equipment and ammonia costs[16] High cost

Method applied at DuPont's cellulosic ethanol biorefinery pre-commercial facility in Vonore in Tennessee, the US[183].

More effective on the biomass that contains less lignin[185]

Laboratory-scale[192]

In temperature range starting from room temperature to 240 °C, alcohols with low boiling points, such as ethanol and methanol, are favoured because of low cost and ease of recovery[145].

44

Table 6. Quantitative comparisons of leading pretreatment technologies WO

LHW SE

DA SCF OS

AFEX ILs

Temperature and Pressure LCM is exposed to 140–210°C 20.0 MPa

at

180-220oC High pressure saturated steam (0.69–4.83 MPa) and temperature (160–260 °C) are used to treat wet biomass during a short period of time (from seconds to few minutes)[200] 130°C, 10min, 12.5% w/v, 31.1oC and pressure of 7.4 MPa 90°C, 30min, 10% w/v, 1.2% sulfuric acid, ratio of ethylene carbonate to ethylene glycol 4:1 Moderate temperature (60–120°C) and high pressure (1.72–2.06 MPa) for 5–30 min[201] 130°C, 30min, 10% w/v, aqueous BMIMCl solutions containing 1.2% hydrochloric acid[184]

Chemicals Agents Oxygen/ air/ H2O2

Comments Oxygen/air is preferable due to low cost and it does not require any additional steps such as post treatment detoxification or neutralization or recovery of the chemical.

Hot water Steam

Mineral/Organic acid supercritical CO2 mixture of water with alcohols like methanol, ethanol, acetone or glycols in 1:1 ratio are used as organic solvent with liquid anhydrous ammonia

Requires recovery steps

Ionic liquids

Pre-treatment using ILs involves mixing it with the biomass at a ratio of 20:1 and then heating it for 120 °C for 30 min.

Likewise mixture of water with organic acid like acetic acid (30% v/v) and formic acid (30% v/v) is also used as organic solvent[146]. Organic acids such as formic acid, oxalic, acetylsalicylic, and salicylic acid also can be used as catalysts

45

Table 7. Performance indicators of leading pretreatment technologies Pretreatment technologies

Reliance on mechanical pretreatment/particle size reduction to achieve acceptable process efficiency

WO

Size reduction not necessary[185]

LHW

Size reduction of the biomass is not needed[185] For corn stover highest sugar conversion was observed at the particle size of 2.5 cm[204].

SE

DA

Optimum size is 6 mm[175]

Efficiencies for pretreatment of corn stover[202] % cellulose removed 90-100%[203]

5–10[202] 3–5[202]

5–10[202]

% hemicellulose removed

% lignin removed

70%– 90%[203]

68%[203]

40[202]

NA

40[202]

70–75[202]

%release of Glucan and xylan Presence of total sugars conversion after inhibitors when enzymatic hydrolysis* pretreatment % glucan % xylan was followed conversion conversion by enzymatic hydrolysis 79%[182] NA NA Formation of Acetic acid, furfural and HMF[104] NA

91[202]

81[202]

No chemicals and toxic inhibitors.

40–45[202]

75.2%[194]

87[202]

78[202]

Pretreatment at high severity result in higher production of toxic compounds[205].

18[202]

8392.5%[111]

92[202]

93[202]

formation of polysaccharide degradation products that are often inhibitory to downstream fermentation organisms. Formic acid and furfural displayed the highest inhibitor concentrations[196]

Need of additional process steps

Water washing is required to remove both hydrolysis and fermentation inhibitors Additional process steps not required Water washing is required to remove both hydrolysis and fermentation inhibitors[206] Overall ethanol yield improves with removal of inhibitors by water washing [196, 207]

% of theoretical ethanol yield (based upon corn stover as feedstock material ) 83%

88% [159]

92.0% [159]

46

AFEX

Alkaline

Does not require small particle size [185] NA

0[202]

0[202]

0[202]

1–3[202]

30–35[202]

55– 60[202]

NA

96[202]

91[202]

94.2 [208]

94[202]

76[202]

No inhibitors produce [185]

water wash is not required [159, 185]

95%[159]

no inhibitory water wash not NA substances in the required pretreatment liquor [208]. ILs smaller particles (<0.5 1–3 30–35 55–60 54.8%[210] NA NA IL recovery Washing, separation and 93% [159] mm) offered better removal of water need further solubility [209] exploration[211] *Enzymatic hydrolysis was conducted using 15 FPU/g glucan cellulase loading. Glucan and xylan conversions data is based upon the initial composition of corn stover, including both monomeric and oligomeric sugars NA: Data not available

Table 8. Effect of different pretreatment technologies on the structure of lignocellulose[(adapted from [13]). Attribute

Mechanical

SE

LHW

DA

Alkaline

Oxidative

AFEX

SCF

Increases accessible surface area

H

H

H

H

H

H

H

H

Cellulose decrystallization

H

n.a

n.d.

n.a

n.a

n.d.

H

n.a

Hemicellulose solubilization

n.a

H

H

H

L

n.a

M

H

Lignin removal

n.a

M

L

M

M

M

H

n.a

Generation of toxic compounds

n.a

H

L

H

L

L

L

n.a

Lignin structure alteration

n.a

H

M

H

H

H

H

n.a

H: high effect; M: moderate effect; L: low effect; n.d: not determined; n.a: not applicable

47

ACCEPTED MANUSCRIPT

9. Process integration Process integration and intensification plays an important role in establishing commercial facilities [9]. Process Intensification targets improvements in manufacturing and processing by rethinking existing operation schemes into ones that are both more precise and efficient than existing operation. Process intensification leads to a substantially smaller, cleaner, and more energy efficient technology. These reductions can come from shrinking the size of individual pieces of equipment and also from cutting the number of unit operations or apparatuses

involved[212]. Various strategies and methods, including multi-enzyme

complex, non-catalytic additives, enzyme recycling, high solids operation, design of novel bioreactors, and strain improvement are being

explored to improve the efficiency of

subsequent enzymatic hydrolysis and fermentation[213]. These technologies provide significant opportunities for lower total cost, thus making large-scale production of cellulosic ethanol possible. While at the same time, process integration focuses on modelling unit operations adequately evaluated on the bases of the entire pathway which helps in optimization of performance of individual operation as well as whole pathway to improve the performance and reduce the production cost[1].

In convention approval pretreatment of LCM is often followed by

washing, neutralization, nutrient supplementation, and detoxification to improve the fermentability of the hydrolysate. These step may well be prohibitively expensive[214] . Washing or filtration might also result in loss of fermentable monosaccharides. According to Subhedar and Gogate[215] ultrasonic irradiations can be effectively used for the intensification of the enzymatic hydrolysis process for efficient bioethanol production. Xu et al [216] have developed high gravity biomass processing with a one-pot conversion technology that includes ionic liquid pretreatment, enzymatic saccharification, and yeast fermentation for the production of concentrated fermentable sugars and high-titer cellulosic 48

ACCEPTED MANUSCRIPT ethanol using bionic liquids in one-pot high-gravity process able to increase biomass digestibility and obtain ethanol titer yields of 41.1 g/L, which exceeds the production distillation required for industrial ethanol production. Brodeur et al [217] developed solvent based pretreatment of biomass followed by In-situ enzymatic hydrolysis (in the presence of the solvent) as an attractive route to reduces a number of washing steps to remove the solvent and save energy. Work explored the ability of N-methyl Morpholine N-Oxide as solvent for “in-situ” hydrolysis study at high cellulose loadings [217]. Another approach was developed in which simultaneous pretreatment and saccharification of biomass in ILs was performed. Pretreatment has great potential for improvements in efficiency and lowering of costs through further research and development. The biofuels field needs to deeply integrate to move from a single-product production facility to the petroleum production model, in which starting resources are fractionated and all molecular components are used to maximize value generation[218]. If pretreatment are optimized from the lens of co-valorisation with decreased severities and milder pH's, then the economic prospects of the lignocellulosic bio refinery process will increase[219]. Replacing petroleum with renewable sources also requires that current processes be simplified by reducing the number of steps necessary to achieve the final product. 10. Conclusion Properties of feedstock LCM and pre-treatment operation play a vital role in development of substrate properties that govern the efficiency of subsequent enzymatic hydrolysis and fermentation. Proper pretreatment method increase access to fermentable sugars thereby improving the efficiency of the whole process.

This article has described various

pretreatment methods. However, intensive fundamental and applied research of each pretreatment process is particularly vital for crafting its integration with the rest of the processes. The key to the establishment of a commercial ethanol production facility is 49

ACCEPTED MANUSCRIPT lessening number of operations and operating costs of each units of operation. Only proper section of whole process enables processing of various seasonal raw materials in a single plant, enabling round the year continuous operation and assisting in development of its profitability. This requires (i) experimental investigation of physical changes and chemical reactions that occur during pretreatment to develop effective and mechanistic models that can be used for the rational design of pretreatment process (ii) fundamental studies to increase the understanding of how the pretreatment affects the different biomass on a structural and molecular level and how that interacts with the subsequent hydrolysis and fermentation for different feedstock (iii) Consolidation of pretreatment with hydrolysis and fermentation processes involved in production of ethanol from LCM (iv) Use of computational tools such as process modeling and simulation for process development and economic analysis (iv) Economic evaluation of the whole process for production of ethanol from LCM for further optimization (v) Exploitation of by-products. References 1. 2. 3. 4. 5. 6. 7. 8.

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