(120b) A Multi-Period Process Integration to the Management of Process Flares with Consideration of Fuel Substitution and Cogeneration Systems

A wide variety of process upsets can lead to flaring. Such flaring activities cause environmental emissions and result in economic losses. A possible approach to mitigating these emissions and losses is to utilize the flared gases in combustion systems (boilers and industrial furnaces) that can be used to produce utilities such as steam. Additionally, the generated steam can be further used in a cogeneration system (e.g., through steam turbines) to produce power and heating. This paper introduces a process integration approach to the effective utilization of flared gases into combustion and cogeneration systems. Since the flared gases differ in composition and characteristics from the fuel that is typically used in boilers, there is a need to make sure that the fuel quality is in the acceptable range for the boiler. Various constraints may be used to assess usability of flared gases as part of the fuel blend fed to the boiler. For instance, the Wobbe index (WI) may be used to study fuel properties sent to the boilers. A multi-period optimization formulation is developed to determine the optimal usage of flare gases in the boiler and cogeneration systems. An operational period is discretized into multiple periods corresponding to different operating modes of the process (process episodes). Economic objectives are used while including technical and environmental constraints. Additionally, the cost-benefit analysis for adding a cogeneration system with an optimal size is carried out. A case study using an ethylene plant is used to demonstrate the various aspects of the process.