In this article, the binding forms of two lignite samples are determined by thermal desorption using a high-temperature furnace. Each mercury compound, such as HgCl2, has a specific binding strength whose decomposition requires a certain thermal energy. Hence, the release of mercury from pure substances and lignite samples was analyzed in a high-temperature furnace. The released mercury is determined with a Mercury Vapor Monitor. The obtained characteristic temperature range and peak of the mercury release were compared between lignite samples and mercury pure substances. For the lignite samples investigated, the binding form of mercury was then identified as Humic Acid. These organic compounds vaporize at lower temperatures. About half of the mercury bound in the lignite was already released at 350 °C. Furthermore, the question arises whether mercury is already released during the grinding-drying process in the coal mill of a power plant. At two power plants, lignite samples were taken simultaneously at the feeder before entering the coal mill and at the dust line afterwards. The samples were analyzed for mercury concentration. The results show that up to one third of the mercury was already released in the coal mill. The vaporized mercury enters the combustion chamber detached from the lignite. The stated analysis methods and the results presented in this article contribute to the understanding of the mercury binding forms in lignite. It also shows the potential of thermal coal pretreatment as a favorable alternative mercury separation technology to others such as activated carbon dosing.