Method for preparing a fuel resulting from biomass, comprising: a) a step of providing an acid oil resulting from the refining of one or more oils chosen from a vegetable oil and/or an animal oil, the acid oil having a water content less than or equal to 1wt%, or even less than or equal to 0.8wt%, in particular from 0.1wt% to 0.7wt%, this step a) comprising: a1) a step of extraction of the fatty acids present in a soap stock resulting from the saponification of one or more oils chosen from a vegetable oil and/or an animal oil, this extraction step being carried out in an acidic medium in conditions effective for forming an aqueous phase and an organic phase comprising said fatty acids, a2) a step of separation during which said organic phase previously formed is separated and it is recovered, the organic phase recovered in step a2) forming an acid oil having a water content less than or equal to 3wt%, subjected to step b), optionally after a step a3) of centrifugation when the water content is greater than 1% but less than or equal to 3%, said centrifugation step allowing to obtain a water content less than or equal to 1wt%, or even less than or equal to 0.8wt%, in particular from 0.1wt% to 0.7wt%, b) a step of reduction of the content of ash, sulphur, phosphorus, calcium and sodium in the acid oil obtained in step a), this reduction step comprising one or more sub-steps chosen from a filtration and a precipitation, carried out in conditions effective for obtaining a combustible acid oil comprising a content of - ash less than or equal to 0.1%, ideally less than 0.05wt%, - sulphur less than or equal to 150mg/kg, preferably less than 100mg/kg, - calcium less than or equal to 5mg/kg, preferably less than 1mg/kg, - phosphorus less than or equal to 150mg/kg, - sodium less than or equal to 10mg/kg, preferably less than lmg/kg. a) a step of providing an acid oil resulting from the refining of one or more oils chosen from a vegetable oil and/or an animal oil, the acid oil having a water content less than or equal to 1wt%, or even less than or equal to 0.8wt%, in particular from 0.1wt% to 0.7wt%, this step a) comprising: a1) a step of extraction of the fatty acids present in a soap stock resulting from the saponification of one or more oils chosen from a vegetable oil and/or an animal oil, this extraction step being carried out in an acidic medium in conditions effective for forming an aqueous phase and an organic phase comprising said fatty acids, a2) a step of separation during which said organic phase previously formed is separated and it is recovered, the organic phase recovered in step a2) forming an acid oil having a water content less than or equal to 3wt%, subjected to step b), optionally after a step a3) of centrifugation when the water content is greater than 1% but less than or equal to 3%, said centrifugation step allowing to obtain a water content less than or equal to 1wt%, or even less than or equal to 0.8wt%, in particular from 0.1wt% to 0.7wt%, a1) a step of extraction of the fatty acids present in a soap stock resulting from the saponification of one or more oils chosen from a vegetable oil and/or an animal oil, this extraction step being carried out in an acidic medium in conditions effective for forming an aqueous phase and an organic phase comprising said fatty acids, a2) a step of separation during which said organic phase previously formed is separated and it is recovered, the organic phase recovered in step a2) forming an acid oil having a water content less than or equal to 3wt%, subjected to step b), optionally after a step a3) of centrifugation when the water content is greater than 1% but less than or equal to 3%, said centrifugation step allowing to obtain a water content less than or equal to 1wt%, or even less than or equal to 0.8wt%, in particular from 0.1wt% to 0.7wt%, b) a step of reduction of the content of ash, sulphur, phosphorus, calcium and sodium in the acid oil obtained in step a), this reduction step comprising one or more sub-steps chosen from a filtration and a precipitation, carried out in conditions effective for obtaining a combustible acid oil comprising a content of - ash less than or equal to 0.1%, ideally less than 0.05wt%, - sulphur less than or equal to 150mg/kg, preferably less than 100mg/kg, - calcium less than or equal to 5mg/kg, preferably less than 1mg/kg, - phosphorus less than or equal to 150mg/kg, - sodium less than or equal to 10mg/kg, preferably less than lmg/kg. - ash less than or equal to 0.1%, ideally less than 0.05wt%, - sulphur less than or equal to 150mg/kg, preferably less than 100mg/kg, - calcium less than or equal to 5mg/kg, preferably less than 1mg/kg, - phosphorus less than or equal to 150mg/kg, - sodium less than or equal to 10mg/kg, preferably less than lmg/kg.