We have developed and applied an Eulerian-Lagrangian model for the transport, formation, break-up, deposition and re-entrainment of particle agglomerates. In this paper, we focus on agglomeration and break-up. Simulations were carried out to investigate what changes in the turbulent flow are inflicted by the presence of the agglomerates. Also, the dependence of the properties of the agglomerates on the Reynolds number of the flow and on the strength of the bonds between the primary particles is studied. The presence of the agglomerates attenuates the turbulence and thereby lowers the Reynolds stresses. As a result, the flow rate increases at constant pressure drop when agglomerates are formed (up to a certain dimension). If the agglomerates surpass this dimension, long-distance viscosity effects become dominant and a flow rate decrease occurs. The characteristics of the agglomerates are largely insensitive to the Reynolds number, provided the flow is turbulent. The agglomerates have an open and porous structure, and a fractal dimension of 1.8-2.3. Their mean mass scales exponentially with the strength of the internal bonds. Contrary to assumptions that are typically made in engineering models in the literature, agglomerates do not preferentially break into two fragments of similar size.