Bitter orange (Seville orange) peel is commonly found in Belgian Wit styles as well as other wheat beers.
Orange peel is added to beers for flavor and its aromatic qualities, generally in the last few minutes of wort boiling and should be strained out before bottling. Some brewers add coriander and some other flavoring to the primary fermentation vessel to get a stronger flavor.
Bitter Orange peel is thick, rough, and generally is green, yellow, or gray colors. The flesh of the orange is dry and very bitter. The peel, which is used in brewing beer is less bitter, has a mild orange flavor. Bitter orange gives beers a citrusy and herbal flavor and aroma.
Bitter orange is generally added to boiling wort during the last 20 minutes of cooking. The amount varies by the style of beer being brewed from about grams per gallon to 5.5grams or about .3 OZ to 1 OZ of dried bitter orange peel. A Belgium Ale would use the low end of this range while a stronger holiday beer may use more.
When brewing with malt extracts, the orange peel and other spices can be added to a muslin bag for easily removal before the wort is put into the primary fermenter.