  The growth of solid particles towards meter sizes in protoplanetary disks has
to circumvent at least two hurdles, namely the rapid loss of material due to
radial drift and particle fragmentation due to destructive collisions. In this
paper, we present the results of numerical simulations with more and more
realistic physics involved. Step by step, we include various effects, such as
particle growth, radial/vertical particle motion and dust particle
fragmentation in our simulations. We demonstrate that the initial dust-to-gas
ratio is essential for the particles to overcome the radial drift barrier. If
this value is increased by a factor of 2 compared with the canonical value for
the interstellar medium, km-sized bodies can form in the inner disk <2 AU
within 10 thousand years. However, we find that solid particles get destroyed
through collisional fragmentation. Only with the unrealistically high-threshold
velocities needed for fragmentation to occur (>30 m/s), particles are able to
grow to larger sizes in low turbulent disks. We also find that less than 5% of
the small dust grains remain in the disk after 1 Myrs due to radial drift, no
matter whether fragmentation is included in the simulations or not. In this
paper, we also present considerable improvements to existing algorithms for
dust-particle coagulation, which speed up the coagulation scheme by a factor of
10 thousand.
