**3. Current status of hydro-fractionation**

### **3.1. Patent filing of hydro-fractionation technologies**

In **Figure 7**, it showed that numbers of filed patents in fields of subcritical and supercritical water extraction and steam explosion technologies have increased from 2007 to 2015. The trend of patent filing of subcritical water extraction decreased in 2016 and was the same number until 2017. On the other hand, more patents were filed in supercritical water extraction and steam explosion after 2015. Quantitatively, it can be seen that the number of filed patents for supercritical water extraction is a lot greater than that of steam explosion and subcritical water extraction. This could be explained by the fact that supercritical water extraction has more versatile applications than the other two technologies. Since this method is not only employed in biomass fractionation, it could be used in coal, oil, polymer, organic and inorganic compounds, nanomaterial, and waste-recycle applications [25].

from the model compounds and the fine conditions including temperature, pressure, heating rate, and residence time were published [26–29]. It is worth mentioning that in subcritical extraction, temperature, heating rate, and residue time enormously affected the reaction behavior and desired products, while the role of pressure is to maintain water in a liquid state and increases the rate of reaction. On the other hand, the effect of pressure on the reaction and kinetics was gained at the supercritical water state [30]. The first component after the degradation of biomass under hot compressed water is hemicellulose at a temperature above 453 K. Typically, at a suitable temperature, a random cleavage between monomeric sugar bonds took place and hemicellulose oligomers were extracted when the chain was cleaved until shorter chains were obtained. And if the reaction time is high enough, another reaction called deacetylation occurred and gave acetic acid. With higher temperature, the product yield was increased but the selectivity dropped [31, 32]. Moreover, if the temperature was raised above 513 K, the oligomer of cellulose from the amorphous part in cellulose was generated, leading to the reduction of the solid yield [33]. After the temperature reached 553 K, the products derived from the hydrolysis reaction of cellulose were 5-hydroxymethylfurfural, levulinic acid, formic acid, and lactic acid [34–36]. For the extraction of lignin, there was a handful of evidence that indicated that the decomposition temperature of lignin without the addition of a catalyst was above 623 K and provided phenols, cresols, guaiacol, catechol, and methyl dehydroabietate as its degrading products [37, 38].

Hydro-Fractionation for Biomass Upgrading http://dx.doi.org/10.5772/intechopen.79396 77

The steam explosion process offers several attractive features for biomass fractionation technologies. Obviously, this process has low environmental impacts and mild operating reaction conditions, no chemical is required except water, and moist biomass can be used as feedstock; the higher the moisture content, the longer the steam pretreatment time [39]; it provides high sugar yield and small amounts of by-products and offers low capital investment. However, some unwanted degradation compounds occur when the operating condition is excessive (high temperature and pressure). For example, xylose obtained from hemicellulose could be degraded to furfural, and glucose obtained from cellulose could be degraded to 5-hydroxymethyl furfural, respectively. These two by-products are undesirable compounds since they could inhibit some microbial activities. Therefore, some detoxification methods should be determined prior to enzymatic hydrolysis. During the process, heat transfer can generate the issue of overcooking at the surface of the larger biomass particles and an incomplete pretreatment of the interior region [39], so optimization size of the feedstock is also a crucial step to achieve high sugar conversion and low production cost. Steam explosion can be performed as a process either in a batch or as a continuous reaction with the most important operational conditions as residence time, temperature, and particle size; a combination effect of these parameters that depend upon feedstocks has been operational for steam explosion such as Salix [40], orange peel [41], wheat straw [42] and barley straw [39]. In recent years, there have been a good number of researchers who gained interest in the underlying work of water responsibility. Boluda-Aguilar et al. studied the steam explosion pretreatment of lemon (*Citrus limon* L.) citrus peel wastes to obtain bioethanol, galacturonic acid, and other coproducts [43]. The steam explosion pretreatment showed an interesting effect on lemon peel wastes for obtaining ethanol and galacturonic acid. The simultaneous saccharification and fermentation (SSF) processing of steam-exploded lemon citrus peel wastes with low enzymatic

**3.3. Steam explosion**
