Ethanol production from lignocellulosic biomass has to include a pretreat­ment more intensive than those used in processing sugar and starch-rich biomass in order to release the sugar compounds contained in the biomass. Agricultural residues like wheat straw or other types of biomass derived from plant material contain lignin, which is constructed to resist microbial attack and to add strength to the plant. Pretreatments are used to open the biomass by degrading the lignocellulosic structure and by partially hydrolysing the substrate. Current pretreatment methods, however, contribute to 30-40% of the total costs of bioethanol production from lignocellulosic biomass. The National Renewable Energy Laboratories (NREL) estimates that in an Nth generation plant (mature technology), feedstock handling and pretreat­ment would account for approximately 20% of the total ethanol production costs [15].

Several pretreatment methods have been developed [16] (see also Zac — chi, this volume). However, in all methods the biomass concentrations need to be higher than 20% dry weight to ensure a suitable ethanol concen­tration for the subsequent distillation process. A new patented pretreat­ment process, Wet-ox explosion (WE), has been developed in our labora­tory combining steam-explosion and wet oxidation using small amounts of oxygen [17]. The optimal combination of process parameters such as tem­perature (170-200 °C), pressure (12-30 bar), amount of oxygen addition, and residence time (2-15 min) has been tested. Depending on the biomass mate­rial used, the method will yield variable sugar yields but overall the results show that the method will be efficient and cost-effective for opening of most major biomass materials such as straw, corn stover, bagasse, and woody materials. Table 1 shows the apparent advantages and disadvantages of this pretreatment method.

Table 1 Advantages and disadvantages of Wet-Ox-Explosion

Disadvantages

Fast and efficient Requires water supply

No emission products Advanced technology

Low heat consumption No standard equipment

No detoxification Only tested on pilot scale

Easily convertible substrates No waste products

4.2