This work focuses on the 3D CFD isothermal simulation of a 100 kWth air - blown atmospheric circulating fluidized bed gasifier, operating in the slugging regime. Three different computing methodologies are applied for solving the dense gas-solid flow, i.e., i) the conventional two-fluid model (TFM) based on the Eulerian approach, ii) the Discrete Element Method (DEM) and, iii) a new promising approach, the Dense Discrete Phase Model (DDPM). The applied grid for all cases consists of 49332 tetrahedral elements. In the present study, the particles simulated have a mean diameter of 600 ým and their density is 2650 kg/m3. Simulation results are averaged over a time period of approximately 15 seconds and the induced pressure profile is compared against corresponding experimental data, measured at the CPERI's gasifier. In order to decrease the computational cost for the DEM approach, the solid particles are tracked in parcels. Three different number of particles per parcel are tested, i.e. 60, 120 and 240. The comparison with experimental data for pressure distribution revealed that the DEM test case with 120 particles per parcels is the optimum model formulation in terms of computational cost and accuracy. In the case of the DDPM model this value is applied. Compared to the Euler and DEM approaches, the DDPM method is more efficient in terms of computational cost, although it has a moderate accuracy. Finally, the results of DEM and DDPM agree well with the slugging regime, where large pressure fluctuations are expected. The model adopting the TFM formulation failed to reproduce the slugging regime fact that is mainly attributed to the stress tensor and solids wall boundary condition applied.