In entrained flow partial oxidation processes of carbonaceous, ash containing feedstock, the slag behaviour is important to ensure a safe and reliable operation. Inside the reactor, the inorganic content of the fuel is converted into slag which is deposited at the walls of the cooling screen where a stationary and a mobile slag layer are formed. For design and operation, a detailed understanding of the slag layers as well as the slag physical properties is important. In order to reduce the amount of CO2 emissions during the process and to generate renewable syngas, biomass can be applied as feedstock. However, the slag properties and the amount of ash are different for biomass compared to what is known for coal. The bioliq (R) demonstration plant has been erected to demonstrate sustainable, high quality fuel production from biomass. The gasification unit is a 5 MWth entrained flow reactor equipped with a cooling screen. Experiments at 40 bar(a) with a plant load of approximately 5 MWth and an ash content of the feedstock lower than 5 wt% were performed. Results of the local heat flux to the cooling screen, slag thickness at the reactor wall and thermophysical properties of the slag were applied in the 'slag model' to evaluate the slag flow under different operating conditions. Compared to results from coal gasification reported in literature, a very low slag thickness has been determined for biomass that is in the range of 3mm at the bottom of the cylindrical part of the cooling screen. The slag flow for different positions of the cooling screen is calculated with the 'slag model'. The sensitivity analysis shows that three variables have the strongest effect on the slag mass flow: the heat flux to the cooling screen, the total slag thickness and the interface viscosity temperature (Tint), defined as being the temperature at the interface between the stationary and mobile slag layers. According to our study, a Tint corresponding to a slag viscosity in the range of similar to 1000 Pa.s is identified to best represent the slag mass flow. Even for very low ash contents of the feedstock, a complete and homogeneous coverage of the cooling screen can be obtained even at the top section which exhibits very low depositions rates according to the model.