In the presence of air and various cofactors and enzymes, fatty acids are converted to acetyl-CoA. The pathway is called beta-oxidation. Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms and produces one equivalent each of acetyl-CoA, NADH, and FADH. The acetyl-CoA is metabolized by the citric acid cycle to generate ATP, while the NADH and FADH are used  by oxidative phosphorylation to generate ATP.  Dozens of ATP equivalents are generated by the beta-oxidation of a single long acyl chain.