NOVELTY - Thermal-mechanical pretreatment of biomass, comprises: subjecting biomass feedstock having fibers containing cellulose, hemicellulose and lignin, to thermal reaction under conditions exceeding atmospheric pressure, at temperature exceeding ambient temperature, at predetermined moisture content and for predetermined amount of time; reducing pressure of thermal reaction under conditions resulting in explosive decompression of biomass; and subjecting biomass to axial shear forces to mechanically reduce size of fibers of biomass to obtain treated biomass.    USE - The process is useful for thermal-mechanical pretreatment of biomass (claimed), which is useful to produce ethanol. The ethanol is useful as a part of the supply of motor fuel.    ADVANTAGE - The process: provides the biomass with a high level of enzymatic digestibility and a low concentration of degradation products; is an environmentally friendly process, as the process converts biomass (preferably wood and wood waste materials) to ethanol or other products derived from monomeric sugar fermentation using little or no acids, hence has a significantly lower impact on the environment as compared to other technologies being used for the production of ethanol from biomass; and achieves over 25% solids, hence saves significant operating and capital costs, compared to conventional acid and steam explosion processes which is limited to 8-15 wt% solids based on the total weight of the slurry. In the process: the biomass is pretreated so that the cellulose and hemicellulose contained, is easily and efficiently converted to ethanol using enzymatic hydrolysis without any significant formation of biomass conversion inhibitors (furfural) that are potentially toxic to yeast or other fermentation organisms; and the pretreatment eliminates the need for corrosion resistant equipment necessary for acid hydrolysis and similar processes, and utilizes materials which are inexpensive, easily handled, and environmentally safe in order to exclude the need for neutralizing the process and disposing of neutralization byproducts.    DETAILED DESCRIPTION - Process for the thermal-mechanical pretreatment of biomass, comprises: subjecting a biomass feedstock including fibers containing cellulose, hemicellulose and lignin, to thermal reaction under conditions exceeding atmospheric pressure, at a temperature exceeding ambient temperature, at a predetermined moisture content and for a predetermined amount of time; reducing the pressure of the thermal reaction under conditions resulting in explosive decompression of the biomass; and subjecting the decompressed biomass to axial shear forces to mechanically reduce the size of the fibers of the biomass to obtain treated biomass, where the treated biomass has a high level of enzymatic digestibility and a low concentration of degradation products.    DESCRIPTION OF DRAWING(S) - The figure is a process flow diagram showing a thermal-mechanical pretreatment process.    Stream (1)    Chip bin (2)    Plug feeder (3)    Thermal reactor (4)    Twin screw pretreatment compounder (8)