Pragmatic approach to retrofitting cracking furnace firing systems for improved safety

摘要

This paper presents a pragmatic approach to retrofitting and safe operation of firing systems for existing cracking furnaces. Cracking furnaces are the heart of the ethylene plant. The furnaces convert hydrocarbon feedstock into ethylene, propylene and other high valuable cracked gas products by thermal cracking in the presence of steam. These reactions require a lot of energy which is supplied to the furnace with a firing system. The firing system needs to be accurately controlled in order to maximize the profit. With respect to safety, the firing system needs to work inside a defined operating window. This has to be ensured at all times in order to avoid mal operations like: uncontrolled combustion, under-stoichiometric combustion, unstable combustion and over-firing. Operation outside the operating window could occur because of operational error, improper safety controls or malfunctioning of the equipment. Most furnace safety incidents including explosions occur during special operations like burner light-off and decoke operations. The European standard EN 746-2 specifies the safety requirements for combustion and fuel handling systems. Not everyone agrees that this standard should be applicable for cracking furnaces, although the cracking process is listed in the annex of the standard. In Europe and many other non-European countries the goals of the EN 746-2 standard are adhered to when designing the combustion system for cracking furnaces. A survey amongst the European ethylene producers revealed that most of the cracking furnaces constructed before 2000 do not fully comply with EN 746-2. When these non-compliant furnaces are retrofitted, authorities usually insist that the firing system is brought into compliance with EN 746-2. This paper addresses how the firing system of an existing furnace can be retrofitted such that it complies with the intent of the EN 746-2 or equivalent standard.