"coenzymes nad and fad are found in the vitamin niacin and riboflavin. why is it that excess amounts of these vitamins are removed from the body in the urine? "

The answer is: They are water soluble vitamins and therefore not stored in the body.

While fat-soluble vitamin could be stored inside the fat cells, water soluble vitamin couldn't. The kidney will filtrate the blood and all of the molecule below a certain size will be dumped if there is not mechanism to reabsorb it. Since the vitamin is soluble in water, it will be easily filtrated too. There is no pump to reabsorb the vitamin, so it will be removed with the urine.