As already mentioned, the main cell-wall components are cellulose, hemicelluloses, and lignin. Another polysaccharide found in the biomass, in relatively low concen­tration is pectin. In addition to these, the biomass contains extractives, mainly tall oil and volatile terpenes, ash, and some nitrogen containing molecules.

The main monomeric sugars in the high percentage polysaccharides, that is, cel­lulose and hemicelluloses, are: glucose, xylose, mannose, galactose, and arabinose.

7.3 Acid Catalyzed Hydrolysis of Biomass

Various methods for the hydrolysis of lignocellulosic materials have recently been described [9]. The dilute acid process is conducted under high temperature and pres­sure. For example, using a dilute acid process with 1 % sulfuric acid (H2SO4) in a continuous flow reactor at a residence time of 0.22 min and a temperature of 237 °C with pure cellulose provided a yield over 50 % sugars. Dilute acids lead to a limited hydrolysis called prehydrolysis. Dilute-acid hydrolysis is carried out using mineral acids such as H2SO4 or HCl, at temperatures of 122-202 °C [10]. The chief ad­vantages to using HCl over H2SO4 are that HCl permeates the wood more easily than H2SO4 and is a volatile compound, which assists in the crucial acid recov­ery steps. The Udic-Rheinau process was an attempt to make the Bergius-Rheinau process economically advantageous. The latter process will be further discussed. In the improved process, the wood was first prehydrolyzed in 1 % HCI at 130 °C to remove the hemicelluloses. The wood was then dried and subsequently hydrolyzed with 40 % HCl at 12 °C for 10 h. After washing the lignin residue with dilute HCl, the HCl was recovered by vacuum distillation. This process was more economical than the Bergius-Rheinau process [11]. The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing [12].

To summarize, acid catalyzed hydrolysis of biomass can be performed for example by using HCl or H2SO4. The H2SO4 catalyzed hydrolysis process was developed in the US to yield sugar monomers, however, it seems that the sugar yields are relatively low, due to the formation of relatively high percentage of degradation products. Additional drawbacks are relatively high acid left in the lignin, high energy consumption, and formed gypsum as by-product.