ABSTRACT

This paper compares and contrasts reaction efficiency findings on properly designed and operated industrial flares with those of rudimentary field flares and shows that the results on the latter hardly apply to the former.  We review the most significant of the contributions to flare emissions research of the last three decades and provide the background perspective of researchers who were directly involved in leading and executing the 1980’s flare efficiency studies that formed the foundation for future studies.  These landmark studies demonstrated that properly designed and operated industrial flares are highly efficient and led to the codification in the United States Environmental Protection Agency’s 40CFR60.18 General Requirements for Flares of the conditions that ensure the proper operation of industrial flares.

THESIS

All studies beginning even before the studies of the early-to-mid-1980s and including the most recent full scale remote sensing field tests have consistently demonstrated the high efficiency of properly designed and operated industrial flares. Exceptions result when flares are improperly operated by being, for example, subjected to liquid carryover, or to over-steaming or to over-aeration; or in an effort to establish the limits of “proper operation” are purposely tested to the verge of extinction.  Any remote field testing to be carried out in the future should include a focus on the issue of wind.  Moderate winds increase the efficiency of industrial flares by enhanced mixing but no one doubts that there exist gale-force winds that are sufficient to blow out any flame or that an unlit flare has zero efficiency.  While the stability of large flares is well known to exceed that of small laboratory-scale model flares, the stability-scaling physics and chemistry are poorly understood.  This important aspect, too, deserves emphasis in any testing to be taken up in the ensuing years.  In short, the efficiency argument resolves itself into what it means to be "properly designed and operated" and whether or not the USEPA's 40CFR60.18 General Requirements for Flares that were intended to ensure “proper design and operation" do in fact ensure the by now more than well established high-efficiency operation of industrial flares.

FOREWORD

Monumental work completed prior to the decade of the 80s provided a prescient foreword to the work in which the authors have been involved over the past two decades.   A slip stream from the relief system at the Oberrheinische Mineralolwerke GmbH (OMW) refinery in Karlsruhe was routed to a 20-cm diameter commercial flare tip mounted on a 5 m tall stack.  A sample probe consisting of a planar array of 40 to   60 sample elements sampled above, within and along the propagation path of the plume. Local combustion efficiencies were calculated ignoring soot.  Steam rates were 0 – 1.5 kg-steam/kg-relief gas and, importantly, a blower simulated crosswinds up to 6 m/sec.  Of the 1296 samples taken, Siegel[80][1] reports that all combustion efficiencies ran >98% and 1292 were >99%.

Since Siegel[80] approximately 200 individual flare efficiency tests in a vast variety of conditions have been reported in the scientific literature.  The reported efficiencies are summarized in Figure 1.

The average combustion efficiency of all tests reported is 96.4% while that of the full scale remote sensing field tests undertaken in more recent years is 98.5%.  All of the outliers resulted either from unintended poor operation or purposeful poor operation while searching for the limits of good operating practice.

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¹ChevronTexaco (Ret); ²DuPont Engineering Technology; ³Natural Resources Canada; ⁴University of  Alberta; ⁵Virginia Polytechnic Institute and State University; ⁶John Zink Company; ⁷Idaho National Engineering and Environmental Laboratory; ⁸Alberta Research Council; ⁹University of Alabama at Birmingham