Tramper cargo handling at port is done on a first-come and first-served basis in most cases. A tramper has to voyage at high velocity to the port for early turn of cargo handling and after arrival waits for a long time for her turn to come. The act is inefficient and leads to severe increase of CO₂ emission. Port reservation system can mitigate the problem. If ships can get the rights of loading at a future date, the ships can voyage to the destination port at slower speed. In this paper tramper transportation simulator is developed which can re-create demurrage distribution and estimate CO₂ emission generated by ship. Firstly, basic port reservation system based on auction is estimated and that the reservation system favors smaller ships over bigger ships is revealed. Secondly, a modification of the reservation rule is introduced and it is shown that its effect is enough to mitigate the potential problem.