The objective of steam pretreatment, steam explo­sion or liquid hot water, is to solubilize the hemicellu — lose to make the cellulose better accessible for enzymatic hydrolysis and to avoid the formation of inhibitors (Hendriks and Zeeman, 2009). During steam pretreatment parts of the hemicellulose hydrolyze and form acids, which could catalyze the further hydrolysis of the hemicellulose. To avoid the formation of inhibi­tors, the pH should be kept between 4 and 7 during the pretreatment (Hendriks and Zeeman, 2009). The aqueous fractionation of native lignocellulosic mate­rials with hot, compressed water (also known as hydro­thermal processing or autohydrolysis) has been proposed as a fractionation method for biorefineries, as it enables the simultaneous removal of water — soluble extractives and the solubilization of hemicellu — loses, yielding a solid phase enriched in lignin and cellulose (Gullon et al., 2012). Liquid hot water has the major advantage that solubilized hemicellulose and lignin products are present in lower concentra­tions, when compared to steam pretreatment, due to higher water input. These lower concentrations reduce the risk on degradation products like furfural and the condensation and precipitation of lignin compounds (Hendriks and Zeeman, 2009).

Wet oxidation is another oxidative pretreatment method, which uses oxygen as oxidation agent. The sol­uble sugars produced during wet-oxidation pretreat­ment are mainly polymers opposite to the monomers produced during steaming or acid hydrolysis as pre­treatment. Phenolic monomers are no end products during wet oxidation but are further degraded to car­boxylic acids (Hendriks and Zeeman, 2009; Martin and Thomsen, 2007).

Carbon dioxide pretreatment is conducted with high — pressure carbon dioxide at high temperatures of up to 200 °C with duration of several minutes. Explosive steam pretreatment with high-pressure carbon dioxide causes the liquid to be acidic and this acid hydrolyses especially the hemicellulose. Carbon dioxide is also applied as supercritical carbon dioxide (35 °C, 73 bars) for depolymerization of the sugars present in biomass, increasing the glucose yield probably caused by increase in pore size (Hendriks and Zeeman, 2009). This method is considered as a "green" pretreatment because it does not require neutralization or pH adjustment prior to enzymatic hydrolysis (King et al., 2012).

Ammonia fiber explosion (AFEX), ammonia recycled percolation (ARP) and soaking aqueous ammonia (SAA) are alkaline pretreatment methods that use liquid ammonia to pretreat biomass. The difference between AFEX and ARP processes is that the first is carried out in liquid ammonia and the second one in an aqueous ammonia solution.

AFEX is a physicochemical pretreatment process in which lignocellulosic biomass is exposed to liquid ammonia at high temperature and pressure for a period of time, and then the pressure is suddenly reduced (Kumar et al., 2009). The AFEX pretreatment simulta­neously reduces lignin content and removes some hemi — cellulose while decrystallizing cellulose and it has the advantage of ammonia being recyclable due to its high volatility (Yang and Wyman, 2008). AFEX has been shown to complete conversion of cellulose to ferment­able sugars but removes or loses little lignin or hemicel — lulose. In a typical AFEX process, the dosage of liquid ammonia is 1—2 kg of ammonia/kg of dry biomass, the temperature is 90 °C, and the residence time is 30 min (Kumar et al., 2009). However, AFEX pretreat­ment at 40 ° C and longer residence times, up to 8 h, has also been proposed with comparable yields of sugar and ethanol (Bals et al., 2012).

AFEX treatment is a batch process while continuous processing in an extruder is an approach called FIBEX (fiber extrusion) that significantly reduces both the time required for treatment and the ammonia levels required with similar hydrolysis results to those for AFEX (Yang and Wyman, 2008).

ARP is another process based on ammonia, which recycles aqueous ammonia solution (5—15 wt%) through a reactor packed with biomass at elevated temperatures (80—180 °C). Ammonia in aqueous solution and at high temperature breaks down lignin via the ammoniolysis reaction but has virtually no effect on carbohydrates (Geddes et al., 2011). A major challenge for ARP is to reduce liquid loadings to keep energy costs low (Yang and Wyman, 2008). SAA is a modified version of AFEX but it uses moderate temperatures (25—60 °C) to reduce the liquid amount during pretreatment. At ambient temperatures the duration could be up to

10— 60 days while at higher temperatures (150—190 °C) the duration of pretreatment is reduced to minutes (Agbor et al., 2011). The cost of ammonia, and especially of ammonia recovery, drives the cost of the ammonia — related pretreatments (Kumar et al., 2009).