The topic of this graduate thesis is Numerical analysis of the flow and heat transfer in the steam generator of the biomass cogeneration plant. The principle of the operation of steam generator is explained and its main components as well as the properties of the cogeneration plant are described. Furthermore, the results of numerical simulation of the flow in the steam generator are shown. In the second part of the thesis, a 3D model of the furnace of steam generator was created in the SolidWorks software package. Based on the given parameters, a mathematical model of the default steam generator was created in Engineering Equation Solver (EES). After the grid of control volumes is created, using the ANSYS Fluent software package, the three-dimensional flow in the steam generator was calculated numerically. k-ε turbulence model was used. During actual work in practice, no one energy plant or system works with the same parameters. Therefore, two modes of operation are simulated in the thesis. The most important and demanding part of the numerical analysis is to generate an optimal mesh. The finite volume method, for the same number of volumes, is more accurate on structured grids than on unstructured grids. Because of the complexity of the geomoetry, sometimes is not possible to generate a structured hexahedral mesh. By applying the finite volume method, in this thesis, tetrahedral and polyhedral control volumes are used. The results of the numerical simulation are compared with the values of the temperature and velocity distribution for each model, using tetrahedral and polyhedral mesh. By comparing the convergence diagram, it is concluded that in the polyhedral mesh it takes much less time to compute than the tetrahedrals in order to reach the convergence of the solution. In addition, convergence properties are much better in computations on polyhedral mesh than in tetrahedral mesh.