In a large, heavy-bottomed stock pot, combine oil, achiote powder and chopped garlic. Saute over medium heat for about one minute, until garlic has softened (but is not browning).
Add uncooked rice to the pot. Stir until rice is evenly coated with oil.
Add vegetable broth and orange zest (if using) to the pot. Stir well. Bring to a boil, reduce heat to low, cover with lid and simmer for 20-25 minutes, until rice is tender and has absorbed all the broth.
Remove the pot from heat. Leaving the lid in place, allow the rice to steam in the pot for 5 minutes. Remove the lid and fluff with a fork.