Make sure you use Coke Oven Bricks (made using the recipe below) and not just vanilla bricks.
The Coke Oven is a 3x3x3 brick structure, hollow in the middle, for converting Coal into Coal Coke or Wood into Charcoal and producing Creosote Oil as a byproduct. The Oven is built out of 26 special bricks made from Brick and Sand. If broken down, it takes 104 Clay and 130 Sand to build one Coke Oven. When building a Coke Oven, you will know the Oven is complete when the window appears in the middle block on the sides. They cannot be built adjacent to each other; there must be at least one tile in between ovens.Converting Coal into Coke or Wood into Charcoal is a slow process, it takes three minutes to convert one piece of Coal into Coke. You will probably want to build several Ovens if you plan to make a
Two Coal produces two Coal Coke and one Creosote Oil (now found in the form of a liquid which must be pumped out using buildcraft pipes). Four wood produces four Charcoal and one Creosote Oil.
The Oven can be loaded and unloaded via Loaders or Buildcraft pipes. The entire 3x3x3 Oven is treated as a single block, similar to a Furnace. Coal or Wood is loaded into the top, Coke or Charcoal is unloaded from the sides, Creosote Oil must be pumped out with waterproof pipes and engines accordingly.
It is highly recommended that you do not use Coke Oven Brick for decorative purposes. Incomplete Ovens periodically check if they form a valid Oven, in mass quantities this could lead to performance glitches and framerate drops
Building the Coke Oven
Click to enlarge.
Coke Oven Build
Coke Oven with transport pipes