Firstly, lignocellulosic material has to be size-reduced and pretreated for alteration and removal of structural and compositional impediments (physical disruption) to hydrolysis. Therefore, the rate of enzymatic hydrolysis as well as yields of accessible, fermentable sugars from cellulose and hemicellulose can be increased. Through pre-treatment, cellulosic biomass is made more accessible to enzymes, which can then convert carbohydrate polymers into fermentable sugars.
A successful pre-treatment may improve the following requirements:
- Formation of sugars and/or the ability to form sugars in the hydrolysis process
- Prevention of degradation or loss of carbohydrates
- Hindrance of by-products-formation, which may be prejudicial to hydrolysis and fermentation process
- Improvement of cost effectiveness
Because of having significant implications on the configuration and efficiency, as well as on economics of the whole production process, a major challenge of developing cost effective pre-treatment technologies can be observed. Numerous studies have shown, that pretreatment is the most significant determinant of successfully produced cellulosic bioethanol, as it defines the extent to and cost at which the carbohydrates of cellulose and hemicellulose can be converted.
There are different possibilities of carrying out pre-treatment of cellulosic biomass:
- Mechanical pre-treatment
- Steam explosion
- Ammonia fiber explosion
- Supercritical CO2 treatment
- Alkali or acid pre-treatment
- Ozone pre-treatment
- Biological pre-treatment
For a comparison of those pre-treatment possibilities concerning temperature, pressure, reaction time, xylose yield, downstream enzymatic effect, costs and availability, have a look at table 1.
Table 1: Comparison of numerous pre-treatment possibilities (lignin removal and hemicellulose hydrolysis) (source: adapted from Hamelinck et al. 2005, 2003).
In addition to aqueous separation and hot-water systems, "Steam explosion" or "autohydrolysis" is one of the biomass fractionation processes. Commercial products of biomass fractionation include levulinic acid, xylitol, and alcohols.
Main fractionation chemicals from biomass ingredients are:
Figure 1: List of main fractionation chemicals from biomass ingredients (source: adapted from Balat et al. 2008).
Steam explosion was developed by Stake Technology Ltd., Canada. With this technology, biomass is extruded at high temperature and pressure. Whereas, peroxide extrusion (developed by Xylan Inc., USA) uses chemical pre-treatment in addition with extrusion for breaking down the biomass fiber structures.
In detail, lignocellulosic material (e.g. agricultural residues) is given into a sealed chamber. High-pressure and high-temperature steam is introduced. After 1 – 5 minutes, pressure is released, inducing, that the steam expands within the lignocellulosic matrix, therefore separating individual fibers.
Uncatalyzed steam explosion is done without addition of any chemicals, where lignocellulosic biomass only is rapidly heated by high-pressure steam. After a few minutes, the biomass-steam mixture is terminated by an explosive decompression.
During steam explosion pre-treatment, hemicellulose is hydrolyzed by released acetic and other acids. As steam explosion induces chemical effects and reactions, acetic acid is generated from hydrolysis of acetyl groups (associated with hemicellulose) and further catalyzes hydrolysis and glucose or xylose degradation (figure 2). Steam explosion pre-treatment with addition of acid catalyst (e.g. H2SO4 or SO2), may produce high sugar yields. Also water acts as an acid at high temperatures. Therefore, the yield of hemicellulose sugars, as well as the enzymatic hydrolysis of the solid fraction can be increased. H2SO4 is a strongly acting catalyst, which improves the removal of hemicellulose. On the other hand, it produces also a number of inhibitory substances. SO2 is a milder catalyst, which produces less inhibitors, but also a less extended hemicellulose hydrolysis.
Figure 2: Schematic demonstration of pre-treatment steps. Conversion between crystalline (C) amorphous cellulose (C*) is reversible. Both forms may yield oligosaccharides, which in turn form glucose. Glucose (G) degradation can then occur to form fermentation inhibitors. K is equilibrium constant and k is rate constant (Source: Mosier et al. 2005).
Pre-treatment by only water and steam (so-called batch steam explosion technique) is conceptually simple, relying on the release of natural acids from hemicellulose to break down the hemicellulose. Rapid pressure release causes reaction-quenching and therefore a disruption of the fibrous structure. In spite of being conceptually simple, the yields of sugars from hemicellulose are low (> 65 %).
Summing up, steam explosion treatment of lignocellulosic material provide a few effects, namely:
- Increase of crystallinity of cellulose (crystallization of the amorphous portions)
- Better hydrolysis of hemicellulose
- Possible promotion of delignification
- Low energy requirements (compared to mechanical break-up)
- No involvement in recycling or environmental cost
This pre-treatment technique requires liquid ammonia and steam explosion. Pre-wetted lignocellulosic material is placed in a pressure vessel with liquid ammonia (NH3). Pressures more than 12 atm are needed to operate at ambient temperature. This process is conceptually simple and time-saving. For feedstocks with higher lignin content, the AFEX process is less effective. Through AFEX-pre-treatment of lignocellulosic materials, the polymers (hemicellulose and cellulose) are made more accessible for being converted enzymatically into monomeric sugars.
An ammonia evaporation-recovery after pre-treatment is advisable, because of the high costs of ammonia.
Those pre-treatment procedures, including either diluted or concentrated acids, e.g. sulphuric acid, hydrochloric acid, peracetic acid, nitric acid or phosphoric acid may obtain high yields of sugars from lignocellulosic biomass. The dilute acid pre-treatment is one of the most studied and widely used one.
Dilute acid pre-treatment processes are divided into two general types:
- Low solids loading (5-10% [w/w]), high-temperature (T>433 K), continuous-flow processes
- High solids loading (10-40% [w/w], lower temperature (T<433 K), batch processes
Generally, higher temperatures and shorter reactor residence times during pre-treatment process provide higher soluble xylose recovery contents and better enzymatic cellulose digestibility. Additionally, higher temperature dilute acid pre-treatment show an increased cellulose digestibility of pre-treated residues (about 80 – 95 % of hemicellulosic sugars can be obtained).
With hot-wash process, which is a variant of the dilute acid pre-treatment, a re-precipitation of lignin and/or xylan (which may exist solubilised under pre-treatment conditions) can be avoided. A possible re-precipitation may be negative for subsequent enzymatic hydrolysis.
With alkaline pre-treatment, lower temperatures and pressures are used. This pre-treatment is accomplished at ambient conditions and pre-treatment time is measured in terms of hours or days. Advantageous is, that with alkaline pre-treatment lignin can be removed, without having big effects on other components, compared to e.g. acid-catalyzed pre-treatment, where some alkali are converted to irrecoverable salts or incorporated as salts into the biomass during pre-treatment reaction.
Treatment of lignocellulosic biomass with NaOH leads to an increase in the internal surface area, water molecules can penetrate to the inner layer, the degree of crystallinity decreases, therefore disrupting the lignin structure by breaking the bonds between hemicellulose and lignin-carbohydrats.
Generally, alkali pre-treamtent is accomplished with diuted NaOH, being more economic than the concentrated NaOH pre-treatment. However, a combination of dilute NaOH treatment with other treatments (e.g. combined with irradiation) can cause a significant increase in obtained glucose amount.
Biodelignification is the biological degradation of lignin using fungal organisms, which solubilize the lignin. Firstly mentioned in 1984, this technology could enormously simplify pre-treatment, but currently there is only little experience with this approach.