Using another pot, boil the 2 1/2 cups sugar with the 1 1/2 cup water over strong heat, stirring constantly until the sugar melts and the mixture boils for a couple of minutes. Add the dry orange peels, stir well so that they are covered in syrup, lower the heat, half-cover the pot and let it simmer for about an hour or until almost all of the syrup is absorbed, stirring from time to time.