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    Engineering Applications of Computational FluidMechanics

    ISSN: 1994-2060 (Print) 1997-003X (Online) Journal homepage: http://www.tandfonline.com/loi/tcfm20

    Parametric Study of Ethylene Flare OperationsUsing Numerical Simulation

    Kanwar Devesh Singh, Preeti Gangadharan, Tanaji Dabade, Varun Shinde,Daniel Chen, Helen H. Lou, Peyton C. Richmond & Xianchang Li

    To cite this article: Kanwar Devesh Singh, Preeti Gangadharan, Tanaji Dabade, Varun Shinde,Daniel Chen, Helen H. Lou, Peyton C. Richmond & Xianchang Li (2014) Parametric Studyof Ethylene Flare Operations Using Numerical Simulation, Engineering Applications ofComputational Fluid Mechanics, 8:2, 211-228, DOI: 10.1080/19942060.2014.11015508

    To link to this article: http://dx.doi.org/10.1080/19942060.2014.11015508

    Copyright 2014 Taylor and Francis GroupLLC

    Published online: 19 Nov 2014.

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  • Engineering Applications of Computational Fluid Mechanics Vol. 8, No. 2, pp. 211228 (2014)

    211

    PARAMETRIC STUDY OF ETHYLENE FLARE OPERATIONS USING

    NUMERICAL SIMULATION

    Kanwar Devesh Singh

    #, Preeti Gangadharan

    #, Tanaji Dabade

    ^, Varun Shinde

    #

    Daniel Chen#*

    , Helen H. Lou#, Peyton C. Richmond

    # and Xianchang Li

    ^

    #Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA

    ^Department of Mechanical Engineering, Lamar University, Beaumont, TX 77710, USA

    *E-Mail: daniel.chen@lamar.edu (Corresponding Author)

    ABSTRACT: In addition to CO2 and H2O, industrial flares may also release Volatile Organic Compounds (VOCs), NOx, and CO among others. Since experimental measurements of these emissions are expensive, rigorous

    computational fluid dynamics (CFD) simulations and the accrued correlations are viable tools to understand and analyze factors affecting flare operations. In this paper, parametric studies of air and steam assisted ethylene flares

    based on CFD modeling were employed to investigate important flare operating parameters such as vent gas

    velocity, crosswind velocity, stoichiometric air ratio, steam-to-fuel ratio and heat content of the vent gas. The CFD

    modeling utilized a 50-species reduced mechanism (LU 1.1) based on rigorous combustion chemistry. Validation

    results of LU 1.1 are also presented. The destruction/removal efficiency and the combustion efficiency (DRE & CE)

    were computed along with HRVOCs/VOCs/NOx emission rates to quantify the flare performance. Correlations

    between DRE/CE and major parameters (crosswind, jet velocity, and combustion zone heating value) were

    developed using the results obtained from the case studies. A modified combustion zone heating value definition was

    proposed to compute a comprehensive heating value in the combustion zone.

    Keywords: C2H4, air/steam assisted flares, flare efficiency/emissions, combustion mechanism

    1. INTRODUCTION

    Flaring is widely used in the upstream energy,

    refining, and chemical process industries to

    relieve pressures, vent unwanted gases, and then safely dispose them to the environment. This open

    air combustion system oxidizes the fuel gases into

    carbon dioxide and water vapor and hence avoids the contamination of air with harmful gases that

    cause air pollution and climate change. However,

    complications arise due to the significant effects

    on flare performance of a wide range of parameters such as the fuel to air and fuel to

    steam ratios (Castieira and Edgar, 2006), jet

    velocity, net heat content of the fuel, crosswind velocity (Castieira and Edgar, 2008), etc. When

    flare performance deteriorates, incomplete

    combustion takes place which produces more combustion byproducts such as CO, aldehydes,

    HOx, and NOx (Seinfeld and Pandis, 2006). The

    oil and gas industry processes millions of cubic

    feet of hydrocarbon gases every day so a slight decrease in flare performance means a release of

    tens of thousands of cubic feet of such byproducts

    into the atmosphere. The common indicators used to quantify flare performance are Destruction and

    Removal Efficiency (DRE) and Combustion

    Efficiency (CE) (Baukal and Schwartz, 2001).

    A common industrial practice (American Petroleum Institute, 2008) for calculating VOC

    emissions from flaring events is to assume 98%

    DRE. According to EPA regulations, a 98% DRE

    or higher (McDaniel, 1983) can be achieved if the flares are operated according to 40 CFR Section

    60.18 (EPA 1986). A flare not complying with

    these regulations may not achieve a 98% or higher DRE (Pohl, 1984/1985). But recent flare

    studies done by the University of Texas

    (UT/TCEQ/John Zink, 2008; UT Austin, 2011)

    suggest otherwise. The flare field tests, conducted in Tulsa, Oklahoma (John Zink Hamworthy

    Combustion facilities), used different

    combinations of fuel heat content/LHV and flow rates. The final report (UT Austin, 2011) showed

    DREs lower than 98% even when the flare was

    operated in compliance with the EPA regulations. The comprehensive flare study covered various

    tests simulating flare operations in a standby

    mode for which the vent gas flow rates were kept

    very low. The flares during the tests were conducted at a tiny fraction (0.1 - 0.25% ) of the

    full capacity. Other operating modes like startup,

    shutdown, or emergency are not represented by such low jet velocities.

    To achieve the goal of a 98% DRE and to ensure

    the proper operation of flares, the effect of many

    Received: 7 Feb. 2013; Revised: 14 Oct. 2013; Accepted: 4 Dec. 2013

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  • Engineering Applications of Computational Fluid Mechanics Vol. 8, No. 2 (2014)

    212

    operating parameters needs to be well understood.

    To the authors' knowledge, for example, the effect of vent gas velocity under different crosswind

    velocities has not been quantified, and neither has

    the air/steam-to-fuel ratio. Clearly, operating a

    flaring system under the most favorable conditions can help reduce the emissions into the

    atmosphere and may even save the use of

    supplemental fuel (e.g., methane) and steam. In the past, experimental setups (Poudenx and

    Kostiuk, 1999; Johnson and Kostiuk, 2002;

    Kostiuk et al., 2004) and CFD modeling (Barlow et al., 2001) have been used to study high jet

    velocity flares. This paper will summarize the

    effects of different operating and meteorological

    conditions on flare performance using a commercial CFD package ANSYS FLUENT

    13.0.

    Flares are classified by the flare tip height (ground or elevated) or by the method of

    enhancing mixing at the flare tip (i.e., steam-

    assisted, air-assisted, pressure-assisted, or non-assisted). Various flare designs from a simple

    stack to a complex steam assisted flare with

    multiple steam nozzles are used to optimize

    combustion. Two of the most commonly used types, air- and steam- assisted flares, are studied

    in this work. As suggested by the name, these

    types of flares mix air or steam with the fuel to accomplish smokeless (perceived as satisfactory)

    combustion.

    1.1 Air-assisted flares

    Air-assisted flares, the simpler of the two, use

    assist air which is either premixed with the fuel or sent through a ring shaped configuration

    (explained below). The air-assist ensures the

    availability of sufficient air for complete combustion. The air-assist also provides

    additional turbulence to ensure adequate mixing

    and hence better combustion.

    1.2 Steam-assisted flares

    The more complex of the two, steam-assisted flares use steam during the combustion process.

    The steam can either be premixed, non-premixed

    or a combination of the two. The steam-assisted flares use nozzles at the flare tip to inject non-

    premixed steam. Better mixing of fuel, steam and

    air caused by high speed injection results in a

    more complete combustion. Besides creating a turbulent flame, steam also interacts with the

    combustion chemistry. Smoke formation is

    drastically reduced when water vapor reacts with

    the hydrocarbons and forms CO and CO2. Also,

    injecting steam lowers the combustion zone temperature and prevents thermal cracking of

    hydrocarbons. In the present study, a simple,

    cylindrical flare is used and the fuel and steam are

    premixed prior to combustion.

    1.3 Flare efficiencies

    The two parameters, DRE and CE, used to

    monitor the flare performance are discussed

    below 1) DRE (Destruction and Removal Efficiency) DRE represents the percent of the fuel (ethylene

    was used in this work, except in some cases,

    where it was diluted with nitrogen to lower the CZHV value of the fuel) destroyed relative to the

    amount of fuel actually sent to the flare. DRE can

    be written as:

    2) CE (Combustion Efficiency) CE, on the other hand, indicates the

    conversion of fuel into CO2 rather than other intermediate radicals. It is defined as: