This article will highlight the importance of residue upgrading processes for modern refineries in light of declining fuel oil demand and increasingly stringent environmental regulations. Carbon rejection processes are ideal for converting residue – it yields liquid products with higher hydrogen-carbon ratios, clean-burning fuel, known as flexigas, and a byproduct coke with reduced hydrogen-carbon ratio. ExxonMobil's carbon rejection process technology, FLEXICOKING™, integrates thermal conversion with air coke gasification to produce flexigas. To highlight how this technology works, the article will conclude with a case study which illustrates how FLEXICOKING helped improve the Hellenic Petroleum Elefsina Refinery’s economic and environmental performance.