Design and simulation of a utility oilfield flare in Iraq/Kurdistan region using CFD and API-521 methodology

This paper aims at reviewing and analyzing the operation and design of a utility flare in an oilfield in the Iraq/Kurdistan region. The flare supports a gas separation unit that separates 100 MMSCFD of natural gas from other liquid compounds in petroleum refining. The actual flare dimensions are 50 m high and 0.6 m diameter and works in summer where the crosswind speed is 9 m/s and a flow of 1.2 MMSCFD of treated natural gas is flaring through it. At the beginning, the flare design was performed using the API-521 recommended approach based on full operating capacity of the unit and composition of the gas to be flared. The API-521 based design resulted in a flare with a 0.76 m diameter and 48.19 m height. The effects of stack height on heat radiation in case of full capacity firing showed that as the flare height increases from 42.34 m to 133.05 m, the heat radiation decreases from 15.8 kW/m^2 to 1.6 kW/m^2 within 45.7 m dimeter. Furthermore, the relation between stack height and heat radiation was studied for the actual firing rate 1.2MMSCFD using simulation, where the results showed that as the stack height increasing from 10 m to 50 m the heat radiation decreasing from over 1000 w/m^2 to around 150 W/m^2. In fact, CFD code C3d was used to analyze flare performance at normal firing condition during summer operation of 1.2 MMSCFD with a flare diameter and height of 50 m and 0.6 m, respectively. The code was able to predict the flame shape and size during actual flare operation. The results of the simulation demonstrated by defining four locations in the domain to measure the average temperatures and emissions, and to calculate the Combustion Efficiency (CE) and Destruction and Removal Efficiency (DRE). These points were 6 m, 8 m, 10 m, 12 m far from the tip on x-axis and at height of 52 m. The results showed that the average temperature at 6 m far from the flare is 658 K and it decreasing to 490 K at 12 m away from the tip. The CO and CO2 also decreased from 7.27E-5 and 0.033 mass% to 4.53E-6 and 0.027 mass%, respectively. Generally, soot formation was low but at points 8 m and 10 m from the tip the soot formation was considerably lower, respectively at 6.16E-5 and 8.71E-5 mass%. The emissions of C1, C2, C3 and C6+ were measured at 7.46E-9, 5.39E-9, 5.13E-9 and 4.35E-9 mass% at 6 m away from the tip. The emissions increased slightly at 8 m and 10 m from the tip but at 12 m they were observed to decrease. The flare CE and DRE were estimated to be 98% and 100%, respectively. Analysis results confirmed that the flare design was safe and the flare operation was highly efficient with very little smoke produced as indicated by the predicted CE and DRE.