NOVELTY - Elimination of nitrogen from biomass DIG0 and reuse for energy purposes, involves: (A) alkaline conditioning of the biological starting material DIG0, adding calcium oxide (CaO) to it and forming a first biomass DIG1 and free ammonia (NH3) in gaseous form; (B) stripping of the NH3 from the first DIG1 biomass; (C) introduction of the gaseous fluid GAS1 into an internal combustion cogenerator fuelled by biogas or other fuel; (D) purification, from nitrogen oxides, of the gaseous fluid GAS2.    USE - The process is useful for the elimination of nitrogen from biomass DIG0 and reuse for energy purposes (claimed).    ADVANTAGE - The process can be efficiently used in the extraction and recovery for energy use of the nitrogen contained in biomass such as livestock manure, biological sewage sludge and residual biomass.    DETAILED DESCRIPTION - Elimination of nitrogen from biomass DIG0 and reuse for energy purposes, involves: (A) alkaline conditioning of the biological starting material DIG0, adding calcium oxide (CaO) to it and forming a first biomass DIG1 with pH greater than 10 and free ammonia (NH3) in gaseous form; (B) stripping of the NH3 from the first DIG1 biomass resulting from step (A) via injection of air, with generation of a second biomass DIG2, free from ammonia, with low nitrogen content and alkaline pH, and a gaseous fluid GAS1 containing air, NH3 and water vapor; (C) introduction of the gaseous fluid GAS1 resulting from step (B) into an internal combustion cogenerator fuelled by biogas or other fuel, with use of the NH3 contained in the GAS1 for energy purposes, by combustion together with the main fuel, and reduction of the nitrogen oxides present in the exhaust gas (GAS2) of the cogenerator by catalytic converters; (D) purification, from nitrogen oxides, of the gaseous fluid GAS2 containing nitrogen oxide (NOx), water (H2O), carbon dioxide (CO2) and nitrogen (N2) generated during step (C) by: injection of the gaseous fluid GAS2 into the biomass DIG2 resulting from step (B); combination of the nitrogen oxides (NOx) of the gaseous fluid GAS2 with the alkaline mass of the biomass DIG2 with formation of calcium nitrite (Ca(NO2)2) and calcium nitrate (Ca(NO3)2) and simultaneous lowering of the pH of the biomass DIG2; production of a gaseous fluid GAS3 devoid of NOx and a resultant biomass DIG3 with pH lower than the pH of DIG2. An INDEPENDENT CLAIM is included for a plant for the extraction of ammoniacal nitrogen in gaseous form from a biological starting material DIG0, reuse for energy purposes and/or the purification, from nitrogen oxides, of a gaseous fluid GAS2 containing NOx, H2O, CO2 and N2, comprises: a first reactor for alkaline conditioning of the biological starting material DIG0 and formation of an intermediate biomass DIG1 containing free NH3 in gaseous form and a resultant biomass DIG2, with pH higher than neutral, to be extracted from the first reactor and a gaseous fluid GAS1 containing NH3, H2O, air; and unit for loading the biological material DIG0 and unit (102a) for introducing calcium oxide (CaO) into the reactor; and unit for injecting air, and unit for heating the biomass DIG0; and an internal combustion cogenerator fuelled by biogas and/or other fuel, with a catalytic converter on the exhaust for reduction of the NOx, which uses as combustive agent at least the air contained in the GAS1, besides any atmospheric air, and, as fuel, the ammonia contained in GAS1, in addition to the biogas and/or other conventional fuel, and outputs a gaseous fluid GAS2 containing N2, NOx, H2O and CO2; and unit for extracting the fluid GAS1 from the first reactor and feeding the same to the cogenerator, a second reactor for treating the gaseous fluid GAS2 containing NOx, H2O, CO2 and N2, and the biomass DIG2, with generation of a gaseous fluid GAS3 containing CO2, N2, NH3 and a biomass with pH which is alkaline, but lower than that of DIG2.