Lignocellulosic Biomass Pretreatment for Biorefinery : A Review

Lignocellulosic biomass consists of cellulose, hemicellulose, and lignin, which have the potential to produce the so-called green chemicals. Many processes are available at different scales (e.g. labs, pilots, demos, commercials), utilizing lignocellulosic biomass as the raw material. Due to its complicated chemical structures, lignocellulosic biomass must be pretreated before it can be processed further. Simultaneous separation of cellulose, hemicellulose, and lignin from the biomass is an important step to maximize the full potential of the biomass. In this review, several well-known biomass pretreatment technologies are evaluated. These are low pH process (e.g. dilute acid, steam explosion, hot water liquid, and concentrated acid), high pH (e.g. NaOH, lime, NH3), organosolv, ozonolysis, CO2 explosion, and ionic liquids. The objective of this review is to understand how effective the pretreatment process for separating cellulose, hemicellulose, and lignin. Based on this review, organosolv is the only pretreatment process that is capable of separating cellulose, C5 sugars (from hemicellulose), and lignin, simultaneously. This process is currently at pilot scales of 30 – 100 ton per day of biomass feedstock. © 2018 Tim Pengembang Jurnal UPI Article History: Received 11 January 2018 Revised 15 February 2018 Accepted 29 March 2018 Available online 09 April 2018 ____________________ Keyword: Lignocellulosic biomass, Biorefinery, Techno-economic analysis.


INTRODUCTION
Lignocellulosic biomass is considered as future sustainable feedstock for chemicals (Permatasari et al., 2016).It consists of three major components, namely cellulose, hemicellulose, and lignin (Nandiyanto et al., 2016;Nandiyanto et al., 2017).Due to their complicated structures, the biomass must be pretreated before undergo further processing.There are already several biomass pretreatment technologies.Some of them can be categorized into mechanical and thermochemical processes (Cherubini, 2010).Mechanical process aims only to reduce the size of biomass for subsequent processes.

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Thermochemical processes (e.g.pyrolysis, gasification, etc.) produce different products, such as bio-oil or syngas for other applications.These pretreatment technologies, however, do not separate lignocellulosic biomass into its major components.This separation is an important step to maximize the utilization of lignocellulosic biomass into valuable chemicals.Hence, these pretreatment technologies are not considered in this work.
The main goal of biomass pretreatment in biofuel plants is to prepare the cellulose fraction of the biomass for subsequent enzymatic hydrolysis and yeast fermentation processes (Farobie & Hasanah, 2016;Bilad, 2016).The hemicellulose fraction is also sometimes used depending on the technology provider of those subsequent processes (Nandiyanto et al., 2017).Hence, it is basically to make access, provide large surface area, and prepare cellulose and hemicellulose for the enzymes and the yeasts.
In short, this goal is achieved technically by: (i) Increasing surface area (porosity) of the biomass; (ii) Removing inhibitor of yeast growth, e.g.furan derivatives from C5 sugars, which come from the hemicellulose.This is done via controlled hydrolysis of hemicellulose not to produce furan derivatives (via change of operating conditions or reactor design) and removal the hemicellulose from the biomass; (iii) Breaking bonds between lignin and cellulose-hemicellulose via disruption of the lignin structure and dissollution of the lignin itself; (iv) Reduction of crystallinity of the cellulose because this crystallinity feature repels water.Schematically, the pretreatment in this regard is shown in Figure 1.    4 shows the steam explosion pretreatment developed by ENEA.In this particular Crescentino plant, sugars from both cellulose and hemicellulose are fermented to bioethanol, while lignin is used as an energy source to create power (http://www.betarenewables.com/proesa/biorefinery, retrieved on July 2015).3. Biomass pretreatment in pulp & paper industries

Goal of the pretreatment
The main goal of the pretreatment in the pulp and paper industries is to preserve cellulose while removing both hemicellulose and lignin.In the Kraft process, this is done by treating the biomass chips with white liquor (a mixture of Na OH and Na2S).The hemicellulose and lignin are dissolved in the solution giving a black liquor mixture.This black liquor is evaporated and then sent to a recovery boiler to produce green liquor.The green liquor is then treated chemically with lime to produce again the white liquor (http://ipst.gatech.edu/faculty/ra-gauskas_art/technical_reviews/Kraft%20Pulping%20and%20Recov-ery%20Process%20basics.pdf,retrieved on June 2015).This Kraft process is shown in Figure 5.The remaining cellulose is washed and bleached and it is then called pulp.It can be seen in Figure 6.This cellulose pulp is then sent to paper manufacturing plants.

Technologies used commercially
Pulp and paper making industries have existed for years and there are many technology suppliers such as ANDRITZ or Metso.An improved digester (Compact Cooking G2) from Metso is shown in

CONCLUSION
This paper reviews pretreatment technologies for separating cellulose, hemicellulose, and lignin from the biomass.Several well-known biomass pretreatment technologies are evaluated.Many processes are available at different scales (e.g.labs, pilots, demos, commercials), utilizing lignocellulosic biomass as the raw material.These are low pH process (e.g.dilute acid, steam explosion, hot water liquid, and concentrated acid), high pH (e.g.NaOH, lime, NH3), organosolv, ozonolysis, CO2 explosion, and ionic liquids.The objective of this review is to understand how effective the pretreatment process for separating cellulose, hemicellulose, and lignin.
Steam explosion technology combined with dilute acid (Advanced Steam-Ex TM process) of ANDRITZ, which is used in the bioethanol plant of POET-DSM.The technology is shown in Figure 2. In the project Liberty (POET-DSM), sugars from both cellulose and hemicellulose are fermented to bioethanol (http://poet.com/pr/poet-dsm-makes-majortechnology-process-purchase-for-cellulosicbio-ethanol.Retrieved on June 2015).The remaining lignin is sent to an anaerobic digestion treatment to produce biogas.The biogas is used to create power to run the bioethanol plant (http://demoplants.bioenergy2020.eu/projects/info/225, retrieved on July 2015).The schematic of the plant is shown in Figure 3.

Figure 1 .
Figure 1.Schematic goals of pretreatment of lignocellulosic materials (poet.com,retrieved on June 2015) Figure

Figure 2 .
Figure 2. Advanced Steam-ExTM process of ANDRITZ used in the bioethanol plant in Project Liberty POET-DSM (energybiosciencesinstitute.org, retrieved on June 2015).