Method for the thermal pretreatment of organic material, in particular biomass, for conversion to energy, said method incorporating a first preheating step with a preheating tank, a hydrolysis step with a hydrolysis reactor and a pressure reduction step with a pressure reduction tank, characterised in that the method comprises the following steps: - in the preheating step, supply organic material with a dry matter content of 5 - 40%, preferably 10 - 25 % to a preheating tank, and heat the organic material in the preheating tank with flash steam from the pressure reduction tank, - provide a vacuum in the hydrolysis reactor by the supply of cold water in the hydrolysis reactor, - open the supply valve between the preheating tank and the hydrolysis reactor, and transfer the heated organic material from the preheating tank to the hydrolysis reactor with the help of vacuum and gravity, so that the hydrolysis reactor becomes essentially fully filled, - supply steam to the top of the hydrolysis reactor so that surplus organic material flows back from the hydrolysis reactor to the preheating tank via a pipe fitted with a valve, said pipe connecting the preheating tank with the hydrolysis reactor, said pipe sticking some distance down in the hydrolysis reactor, the lower end of said pipe defining the required filling level in the hydrolysis reactor - measure the pressure in the hydrolysis reactor with the help of a sensitive high-frequency pressure sensor that measures the pressure and analyses pressure oscillations in the hydrolysis reactor to detect when the required filling level in the hydrolysis reactor is reached, - close the supply valve between the hydrolysis reactor and the preheating tank, - supply new organic material to the preheating tank, - heat the organic material in the hydrolysis reactor by the supply of steam and maintain the desired temperature for a sufficient length of time, - open a blow valve between the hydrolysis reactor and the pressure reduction tank and transfer the treated organic material from the hydrolysis reactor to the pressure reduction tank and obtain a steam explosion as a consequence of the pressure difference between the hydrolysis reactor and the pressure reduction tank, - lead the flash steam that is released in the steam explosion in the previous step via a steam return line to under the liquid level in the preheating tank and recover the heat by steam condensation in the organic material in the preheating tank, - close the steam return line and open a steam return line from the pressure reduction tank to above the liquid level in the preheating tank to further reduce the pressure difference between the pressure reduction tank and the preheating tank beyond the pressure which the liquid column in the preheating tank represents. - in the preheating step, supply organic material with a dry matter content of 5 - 40%, preferably 10 - 25 % to a preheating tank, and heat the organic material in the preheating tank with flash steam from the pressure reduction tank, - provide a vacuum in the hydrolysis reactor by the supply of cold water in the hydrolysis reactor, - open the supply valve between the preheating tank and the hydrolysis reactor, and transfer the heated organic material from the preheating tank to the hydrolysis reactor with the help of vacuum and gravity, so that the hydrolysis reactor becomes essentially fully filled, - supply steam to the top of the hydrolysis reactor so that surplus organic material flows back from the hydrolysis reactor to the preheating tank via a pipe fitted with a valve, said pipe connecting the preheating tank with the hydrolysis reactor, said pipe sticking some distance down in the hydrolysis reactor, the lower end of said pipe defining the required filling level in the hydrolysis reactor - measure the pressure in the hydrolysis reactor with the help of a sensitive high-frequency pressure sensor that measures the pressure and analyses pressure oscillations in the hydrolysis reactor to detect when the required filling level in the hydrolysis reactor is reached, - close the supply valve between the hydrolysis reactor and the preheating tank, - supply new organic material to the preheating tank, - heat the organic material in the hydrolysis reactor by the supply of steam and maintain the desired temperature for a sufficient length of time, - open a blow valve between the hydrolysis reactor and the pressure reduction tank and transfer the treated organic material from the hydrolysis reactor to the pressure reduction tank and obtain a steam explosion as a consequence of the pressure difference between the hydrolysis reactor and the pressure reduction tank, - lead the flash steam that is released in the steam explosion in the previous step via a steam return line to under the liquid level in the preheating tank and recover the heat by steam condensation in the organic material in the preheating tank, - close the steam return line and open a steam return line from the pressure reduction tank to above the liquid level in the preheating tank to further reduce the pressure difference between the pressure reduction tank and the preheating tank beyond the pressure which the liquid column in the preheating tank represents.