Pretreatment of biomass to facilitate conversion to bioenergy or biofuels

Overview

Before energy crops or organic wastes can be converted to biofuels, typically some form of pretreatment is required, by physical (mechanical), chemical, thermochemical or biochemical methods.

Mechanical processing

Generally, the first step is to mechanically process the feedstock (for example wood waste, straw, etc) to reduce size by shredding and grinding. Size reduction by mechanical processing (milling, grinding, extrusion, irradiation, etc):

  • faciltates handling
  • increases surface area
  • decreases crystallanity
  • reduces degree of polymerization
  • improves the efficiency of enymatic or acid hydrolysis, and processes such as pyrolysis and torrefaction (outlined below).

Energy densification

Pretreatment helps to improve the energy density of the biomass, allowing it to be more efficiently transported from the point of production (field, forest or bioindustrial facility) to the point of use. See Biomass Densification for Energy Production, Fact Sheet produced by Ministry of Agriculture, Food and Rural Affairs, Ontario, Canada, June 2011.

Biomass may be compressed into pellets, cubes or pucks (similar in size and shape to the eponymous ice hockey pucks), or treated with heat and pressure (e.g. pyrolysis, torrefaction) to create 'bio-coal' or 'bio-oil' as outlined in more detail below.

Pelletisation

Compressing fractionated biomass to form fuel pellets improves handling and creates a product that has similar flow properties to plant grains. Pellets can be more easily delivered to homes, businesses and power plants and, due to their consitent size and shape, can be automatically fed into advanced pellet boilers in a controlled and callibrated way. Wood pellet production has more than doubled between 2006 and 2010 to over 14 million tons [Source: IEA Task 4].

Fractionation

Biomass fractionation (most commonly by steam explosion) converts lignocellulosic feedstocks into three discrete fractions of lignin, cellulose and hemicelluloses. The yield and purity of the three fractions is a key advantage in downstream applications [Source: NNFCC, UK]. This allows the fractions to be more efficiently processed within a bioefinery (where a number of different products are produced on the same site in a cascading process that maximises the value of the biomass feedstock e.g. production of heat and power, fuel additives, ethanol, biochemicals, etc).

Steam explosion

"Steam Explosion (SE) is the most commonly used pretreatment of biomass and uses both physical and chemical methods to break the structure of the lignocellulosic material through an hydrothermal treatment. The biomass is treated with high pressure steam at high temperature for a short time, then it is rapidly depressurized and the fibrils structure is destroyed by this explosive decompression. This defibration and the remarkable autohydrolysis significantly improve the substrate digestibility and bioconversion as well as its reactivity toward other catalytic reactions. The successive sudden decompression reduces temperature, quenching the process."

Steam explosion also produces some toxic compounds which must be removed before fermentation. Dilute acids may be used in combination with steam explosion.

[Source: Anna Maria Raspolli Galletti, Claudia Antonetti, University of Pisa, Department of Chemistry and Industrial Chemistry, from presentation at EUROBIOREF Summer School on Biomass pre-treatment: separation of cellulose, hemicellulose and treatment: separation of cellulose, hemicellulose and lignin. Existing technologies and perspectives, September 2011].

Similar processes to steam explosion include:

Ammonia Fibre Explosion AFEX - Biomass is treated with liquid ammonia at high temperature and pressure. When the pressure is released the ammonia vapizes and can be recovered. For example, see Michigan State University AFEX technology.

Super-critical Carbon dioxide explosion - See Pretreatment of Lignocellulosic Biomass Using Supercritical Carbon Dioxide as a Green Solvent, Tingyue Gu, 2013

Alkaline hydrolysis

Alkaline hydrolysis inolves treatment of biomass with an high concentration of alkaline at a lower temperature for a longer time period. See Hydrolysis of Lignocellulosic Biomass: Current Status of Processes and Technologies and Future Perspectives, Alessandra Verardi et al, ENEA, Italy. This paper also compares the pros and cons of various per-treatment methods.

Liquid hot water (LHW)

LHW (also known as aqueous fractionation, aquasolv or hydrothermolysis) involves immersion of biomass in water at 180-230° C) at high pressure. This releases acid compounds in the biomass that break it down via 'autohydrolysis'.

Use of Ionic liquids (ILs)

ILs are compounds composed solely of ions with immeasurable combinations of anions and cations. They possess widely
tuneable properties, such as hydrophobicity, polarity and solvent power. Various ILs have been investigated for pre-treatment of biomass as discussed in the paper Ionic liquids as a tool for lignocellulosic biomass fractionation, Andre M da Costa Lopes et al, 2013.

In August 2014, researchers (Aaron Socha et al) at US Do Joint BioEnergy Institute published a paper on "Efficient biomass pretreatment using ionic liquids derived from lignin and hemicellulose" in Proceedings of National Academy of Sciences PNAS. The aim of the research is to replace conventional ionic solvents with a renewable product - tertiary amine-based ionic liquids synthesised from aromatic aldehydes in lignin and hemicellulose.

Plantrose process (supercritical hydrolysis)

Renmatix Inc., has developed the patented 'Plantrose process', which uses supercritical hydrolysis to produce high volume cellulosic sugars at low cost. Renmatix investors include BASF, Kleiner Perkins Caufield & Byersa and Total. Renamtix has acquired the Mascoma manufacturing facility in Rome, New York. The new feedstock processing facility (FPF) opened officially on 20 April 2015.

Low Temperature Steep Delignification (LTSD)

LTSD, developed by Bio-Process Innovation Inc, uses low inputs of non-toxic chemicals, oxygen and base, at mild reaction conditions to break down lignocellulosic feedstocks. In February 2015, the company announced the start-up of a one ton pilot plant in Indiana, following ten years of process optimisation. The technology is now commercially available for use in biorefineries.

Co-solvent Enhanced Lignocellulosic Fractionation CELF

CELF uses renewable, water-miscible tetrahydrofuran (THF) with dilute sulfuric acid to fractionate cellulosic biomass and achieve high yields of sugars for fermentation or furfural, 5-hydroxymethylfurfural, and levulinic acid for catalytic conversion into fuels and chemicals. The technology developed by the University of California, has been licensed by CogniTek.  A new company "MG Fuels" is being set up to commercialize the technology.

Organosolv process

Oragnosolv was originally developed as an alternative to the KRAFT process for pulping, and uses organic solvents (such as ethanol, methanol, butanol, acetic acid, etc) to solubilise lignin and hemicellulose. It is now being developed for biorefinery systems and cellulosic ethanol production.

American, Science and Technology AST has developed a patented Organosolv process (at pilot scale, using sulphuric acid, butanol and other organic solvents) to convert lignocellulosic biomass into sugars, pure lignin, pulp and added value biochemicals. The fractionation process is operated at 140-180 C under autogenous to 50-60 psi added pressure (200-280 psi at the peak temperature). The hydrolysis process works at 40-60 C and 4-5 psi. The fractionation process yields 40-55% cellulose, 20-30% lignin, and 20-25% hemicelluloses. This process is very efficient in producing more than 95% yield to sugars. Lignin is also more than 95%.

Lignol Energy Corporation, similarly, used an ethanol-based organosolv process to yield high-quality lignin. As per the transaction announced on March 9, 2015, Lignol Energy Corporation has been acquired by Fibria Celulose SA.

Similar technologies with vaious solvents have been developed e.g. Organocell (methanol), Compagnie Industrielle de la Materière Végétale CIMV (acetic acid/formic acid) and Chempolis (formic acid).

Ozonolysis

Pretreatment of biomass with ozone prior to enzyme hydrolysis has been investigated, for example as outlined in the paper Association of wet disk milling and ozonolysis as pretreatment for enzymatic saccharification of sugarcane bagasse and straw, Rodrigo da Rocha Olivieri de Barrosa et al, Bioresource Tecnology, May 2013.

Pyrolysis

Pyrolysis is the chemical decomposition of organic matter by heating. Flash pyrolysis involves rapid heating (1-2 seconds) of fine material up to 500°C. Thermochemical conversion uses superheated water to convert organic matter to bio-oil. This may be followed by anhydrous cracking/distillation. The combined process is known as Thermal depolymerization (TDP).

Please see the page on 'biocrude' for further information about recent demonstrations of pyrolysis technology to convert lignocellulosic biomass to intermediate liquids (bio-oil) for further upgrading to advanced biofuels for transport, or for use as heating fuels or refinery feedstocks.

Torrefaction

Torrefaction is a thermochemical process typically at 200-350 °C in the absence of oxygen, at atmopsheric pressure with low particle heating rates and a reactor time of one hour. The process causes biomass to partly decompose, creating torrefied biomass or char, also referred to as 'biocoal'.

Please see the torrefaction page for further informaiton about recent research and demonstrations in Europe and the United States.