Response Surface Models for SET POINT Determination of Smokeless Steam and Air Assisted Flares

Electronic code of Federal Regulations §63.670 requires flare operators to specify smokeless design capacity for flares and operate flares with no visible emissions when regulated vent gas is sent to flares with flow rate less than the smokeless design capacity. Alternatively, 96.5% combustion efficiency (CE) or 98% destruction efficiency (DE) must be achieved using site specific operating limits to demonstrate the compliance in lieu of threshold values of minimum combustion zone net heating value (NHV_cz) as 270 BTU/scf during any 15–minute average period, maximum flare tip velocity (V_tip) less than 400 ft/s and net heating value dilution parameter (NHV_dil) as 22 BTU/ft². Site specific operating limits include threshold values for NHV_cz, V_tip and NHV_dil as well as vent gas flow rate, vent gas composition and steam/ air assist flow rate. This study aims at developing robust response surface models to express CE and opacity as a function of operating variables and to determine the set points for steam/air and make up fuel under smokeless flaring conditions. Multivariate response surface models were developed for steam and air assisted flares burning propylene, propane, natural gas, methane, ethylene and typical refinery vent gas mixtures. In this study, soot emissions, visible emission scale, DE, and CE test data from 1983 to 2014 (including 1983/1984/1985 EPA, 2010 TCEQ, 2009/2010 Marathon Detroit/Texas City, and 2014 Carleton University) were analyzed. The input variables include measurable variables like velocity, lower heating value, flare tip diameter, vent gas composition (π-bond, carbon to hydrogen molar ratio and molecular weight), and operating variables (NHV_cz, NHV_dil, steam/air assists, and fuel). Minitab and Polymath were used to develop models and solve them to determine set points. Response surface models developed for CE and opacity showed high coefficient of determination (R²) values 0.90 and 0.94 respectively for steam assisted and 0.91 and 0.93 respectively for air assisted flare test data.