Abstract: Method and apparatus for producing synthesis gas comprising the steps of combusting (4) a combustible gas to form hot flue gasses (7), passing the flue gasses (7) through heat storing means (1) so as to heat the heat storing means (1) to an elevated temperature, using the heat thus stored to heat biomass (2) to an elevated temperature so as to form synthesis gas, and withdrawing at least part of the synthesis gas formed, wherein following heating of the heat storing means (1) gaseous medium is circulated through the heat storing means (1), the biomass (2) and back to the heat storing means (1). Furthermore computer program product for performing the method of the invention and the use of the produced biogas are disclosed.