This category includes methods in between, or a mixture of, purely physical and chemical methods. Steam pretreatment is one of the most widely used methods for pretreatment of lignocellulosics. This pretreatment method used to be called steam explosion, since it was believed that an “explosive” action on the fibres was necessary to render a material suitable for hydrolysis. It has been shown that it is more likely that the effect of steam pretreatment is due to acid hydrolysis of the hemicellulose, which is the reason why some cellulosic materials are easier than others to break down [30,31]. In particu­lar, agricultural residues and some types of hardwood contain organic acids, which act as catalysts for the hemicellulose hydrolysis. Using steam pretreat­ment the raw material is usually treated with high-pressure saturated steam at a temperature typically between 160 and 240 °C (corresponding to a pres­sure between 6 and 34 bar), which is maintained for several seconds to a few minutes, after which the pressure is released. During pretreatment some of the raw material, predominantly hemicellulose, is solubilized and found in the liquid phase as oligomeric and monomeric sugars. The cellulose in the solid phase then becomes more accessible to the enzymes. It is in some cases dif­ficult to find conditions that result in high yields of both hexose and pentose sugars, and at the same time also create a cellulose fraction which is easy to hydrolyse to glucose. This may call for steam pretreatment using two steps, where hemicellulose sugars are recovered at lower severity, while the cellulose fraction is subjected to pretreatment at higher severity.

Steam pretreatment can be improved by using an acid catalyst, such as H2SO4 or SO2. The acid increases the recovery of hemicellulosic sugars, and also improves the enzymatic hydrolysis of the solid residue. The use of an acid catalyst in steam pretreatment results in an action similar to dilute acid hydro­lysis but with less liquid involved. It is especially important to use an acid catalyst for softwood, since softwood in general is more difficult to degrade.

Steam pretreatment with addition of a catalyst is the pretreatment method for hydrolysis and enzymatic digestibility improvement that is closest to commercialization. It has been widely tested in pilot-scale equipment, for example, in the NREL pilot plant in Golden, CO (USA) [32] and in the SEKAB pilot plant in Ornskoldsvik (Sweden) [33], and is also used in a demonstration-scale ethanol plant at Iogen in Ottawa (Canada) [34].

Hydrothermolysis, or liquid hot-water (LHW) treatment, involves treat­ment in water at high temperature. This method is similar to steam pre­treatment, but lower temperatures and lower dry matter (DM) content are used, and thus more poly — and oligosaccharides are recovered [35,36]. A cat­alyst, such as an acid, can be added, making the method similar to dilute acid pretreatment. Since the water content is much higher than in steam pre­treatment, the resulting sugar solution is more diluted and thus causes the downstream processes to be more energy demanding. In the range 1-10 wt % DM virtually no difference in ethanol yield was found when bagasse was treated at 220 °C, after which SSF was performed using S. cerevisiae [37].

Wet oxidation pretreatment involves the treatment of the biomass with water and air, or oxygen, at temperatures above 120 °C, sometimes with the addition of an alkali catalyst. This method is suited to materials with low lignin content, since the yield has been shown to decrease with increased lignin content, and since a large fraction of the lignin is oxidized and solubi­lized [38]. As with many other delignification methods, the lignin cannot be used as a solid fuel, which considerably reduces the income from by-products in large-scale production. As discussed in the “Process Economics” chapter, it is extremely important to recover as much as possible of the lignin fraction (Sassner et al., in this volume).

Ammonia fibre explosion (AFEX) is also an alkaline method which, sim­ilarly to the steam pretreatment process, operates at high pressures. The biomass is treated with liquid ammonia for about 10-60 min at moderate temperatures (below 100 °C) and high pressure (above 3 MPa) [39,40]. Up to 2 kg of ammonia is used per kg of dry biomass. The ammonia is recycled after pretreatment by reducing the pressure, as ammonia is very volatile at atmospheric pressure. During pretreatment only a small amount of the solid material is solubilized, i. e. almost no hemicellulose or lignin is removed. The hemicellulose is degraded to oligomer sugars and deacetylated [41], which is a probable reason for the hemicellulose not becoming soluble. However, the structure of the material is changed, resulting in increased water holding cap­acity and higher digestibility. Like the other alkaline pretreatment methods AFEX performs best on agricultural waste, but has not proven to be efficient on wood due to its higher lignin content [42,43]. According to Sun et al. the AFEX process does not produce inhibitors that may affect downstream bio­logical processes [44].

Another type of process utilizing ammonia is the ammonia recycle perco­lation (ARP) method [45,46]. In the process aqueous ammonia (10-15 wt %) passes through biomass at elevated temperatures (150-170 °C), after which the ammonia is recovered. ARP is an efficient delignification method for hardwood and agricultural residues, but is somewhat less effective for soft­wood.

3.4