Potentially rising oil prices caused by an increasing relative scarcity of mineral oil have farreaching consequences for the transportation sector, the chemical industry and mineral oil companies in particular. As national laws in Germany require biofuels to be mixed into conventional fuel to an increasing extend (BioKraftQuG 2009), mineral oil companies need to identify economically competitive as well as technically feasible biofuel production processes to meet these requirements. A first generation of biofuels was introduced on a large scale but has been criticized for competing with the agricultural production of food and for yielding relatively modest quantities of fuel per hectare of agricultural land. For this reason, 2nd generation biofuel production pathways such as Biomass-to-Liquid (BtL), which convert lignocellulosic material into liquid hydrocarbons using Fischer-Tropsch synthesis, have been developed. While 2nd generation biofuels are superior to their 1st generation counterparts from a yield-per-hectare-perspective and cause less competition for agricultural soils, a significant disadvantage is the considerable investment required for the construction of Biomass-to-Liquid plants. The corresponding investment-related costs affect the competitiveness of 2nd generation biofuels negatively, leaving it in doubt whether BtL fuels could become an economically viable option. A frequently discussed way to improve specific investment-related costs is to increase plant sizes to improve economies of scale. While this improvement has been realized in several conventional kinds of plants like mineral oil refineries, power plants and Coal-to-Liquid plants, the application on BtL plants is complicated by the fact that larger plants are associated with higher specific biomass transportation costs. This is because a higher biomass input requires biomass to be transported over larger distances. The unresolved antagonism between economies of scale and specific biomass transportation costs has so far hindered the realization of BtL plants. The aim of this thesis is to develop a methodology to determine optimal BtL plant sizes by taking nonlinear factors into account. The methodology is required to determine a compromise between minimizing investment-related costs by applying economies of scale and minimizing specific biomass transportation costs by keeping the required transportation distances short. The optimal plant size is however influenced by a third influencing factor. Whether it is advantageous to transport biomass over a certain distance also depends on the value of a plant’s products. Biomass-to-Liquid plants can have a variety of product compositions depending on the catalyst and reaction temperature used in the biofuel synthesis reaction. Depending on which substances are produced and which are upgraded for sale, converted into fuels or combusted for electricity generation, both the value of the products and the required investment may differ considerably. While a number of processes, including biomass treatment and gasification, as well as the Fischer-Tropsch synthesis itself, are required for all considered plant setup alternatives, the choice of upgrading equipment may result in very dissimilar plant setups. By making the capacities of the individual upgrading processes the variables of the optimization model, economies of scale, specific biomass transportation costs and the products’ value are considered simultaneously for the first time. The thesis primarily focuses on the implementation of an optimization model and its application on a variety of scenarios. These scenarios are intended to represent different plant setups and logistics concepts. In order to assess the scale of differences in profitability, the essential influencing factors determining the profitability of BtL plants were included into the model calculations. As the problem at hand is neither linear nor quadratic, it cannot be solved reliably using established solvers for these two classes of problems. Instead, several solvers designed to handle non-quadratic nonlinear multidimensional problems were applied to find the most suitable way to approach the solution of the problem. The objective function has been designed to maximize the annual profit resulting from plant construction and operation. Maximizing this annual profit is subject to a number of primarily technical constraints. These result from the mass balances of the plant, its electricity demand and the specific requirements of individual processes. In addition to securing the validity of the mass balances, these constraints also ensure that the entire Fischer-Tropsch product stream undergoes some kind of upgrading, separation or combustion treatment. The sum of all processes producing salable products is used to approximate the required capacity of the plant as a whole. The total plant capacity then serves to calculate the investment required for the other plant processes and the costs for the purchase and transportation of the required input biomass. Biomass transportation distances are approximated by the radius of an assumed circular area from which biomass is supplied to the plant. Using cost functions that divide transportation costs into fixed and variable parts makes it possible to approximate the effect of rising specific biomass transportation costs in case of increasing plant capacities. The investigated scenario calculations suggest that under the assumed circumstances, fuel oriented low-temperature Fischer-Tropsch-based BtL plants are relatively competitive as long as the tax exemptions in Germany are maintained, but become significantly less attractive without them. By contrast, the combined production of both fuels and chemicals using hightemperature Fischer-Tropsch synthesis appears to be a more promising alternative, as chemicals are expected to earn a higher income in scenarios without tax exemptions. A third option, the production of Substitute Natural Gas, appears to be relatively uncompetitive unless methane prices rise significantly. In addition to comparing the economic attractiveness of different potential product distributions, a number of concepts have been investigated which are intended to improve Biomass-to-Liquid economics. Decentralized pretreatment of biomass, e.g. through fastpyrolysis, leads to larger optimal plant capacities, but the additional investment for the pretreatment units appears to overcompensate the improved economies of scale. By contrast, the combined use of train and road transportation was not assumed to be associated with additional investments. If train transportation is indeed feasible for a given plant location and specific biomass transportation costs are lower than for road transportation, combined traffic concepts should be used whenever possible. The construction of BtL plants in conjunction with mineral oil refineries is a way to reduce investment-related costs instead of transportation costs. While the resulting savings are significant for small BtL plants, they diminish if larger plant sizes are investigated. Cogasification of biomass with another input material is another way to reduce the costly transportation of biomass over large distances. Unless technical requirements significantly increase the cost of the gasification equipment, co-gasification concepts can improve the plant’s profitability even at relatively low quantities of a second fuel. The choice of fuels is however restricted by the Renewable Ener…