Flare Tip Right-Sizing: Soot-Free Flaring as APG Production Declines

TNK-Uvat has devised a methodology to achieve soot-free flaring of associated petroleum gas (APG) as its production declines. This has significantly reduced harmful atmospheric pollution and slashed the charges payable for APG flaring. The project won the TNK-BP Chairman’s Award for HSE 2012.

The entire amount of APG produced at the Verkh-Tarskoye and Kalchinskoye fields of TNK-Uvat cannot be put to good use for economic and logistical reasons. Feasibility estimates completed in 2010-2011 to identify potential alternatives to APG utilization at those fields have shown the absence, at this point in time, of appropriate technologies that would be economically viable. Therefore, given the prevailing economic and legal environment, the APG flaring option has been selected.

If soot is discharged when flaring associated gas, it points to substantially incomplete combustion of hydrocarbons, as borne out by statutory estimates of maximum permitted emissions. The sooty combustion mode has been found to result from an inconsistency between the existing flare tip diameters and the actual amount of APG flaring. Originally, the flare tips had been sized for a large volume of gas; however, as field production declines, the velocity of gas leaving the flare tip decreases, critically affecting the combustion mode. At gas velocities of less than 0.2 Mach, the sooty combustion mode kicks in, discharge increases by a factor of 12.5, benzopyrene discharge quadruples, and soot emission rises 100 percent.

Therefore, APG flaring in the prevailing combustion mode results in an excessive discharge of atmospheric pollutants and, consequently, in greater charges payable for adverse environmental impact.

In order to reduce pollutant emissions and slash the APG flaring charges, TNK-Uvat resolved to upgrade the flare systems in the Verkh-Tarskoye and Kalchinskoye fields. The project called for replacing flare tip at existing flare units to achieve soot-free APG combustion across a broad flow range.

New Approach to an Old Problem
To begin with, the type and parameters of new flare tips have been identified in order to achieve soot-free APG flaring across a broad flow range using existing flares.

To enable development of the required technical concepts, the Environmental Section of TNK-Uvat relied on APG flaring records, as well as quantitative pollution measurements in terms of compliance with maximum permitted discharge levels, to painstakingly analyze the combustion mode of existing flares.

According to this research, high-velocity APG flow is the key parameter to ensure soot-free combustion mode. The higher the gas velocity (between 0.2 Mach to 120 m/sec), the less soot is generated by APG flaring. The most affordable method to increase the exit velocity calls for flare tip diameter downsizing.

In case of a decline in APG flaring volumes, flare tip sizing relies on the following calculations:
»    upper bound velocity for the soot-free mode (120 m/sec)
»    temporal range of the soot-free mode, down to the lower bound

In case of an increase in APG flaring volumes, flare tip sizing relies on the following calculations:
»    lower bound velocity for the soot-free mode (0.2 Mach)
»    temporary support for the soot-free mode, up to the upper bound

Benefits
At the Verkh-Tarskoye field, the flare tips were replaced in February 2012, whereas Kalchinskoye flare tips were replaced in late December 2012.

Project implementation costs at the Verkh-Tarskoye field add up to 1.4 million rubles. As a result, over two quarters of 2012, pollutant emissions dropped by 8,300 t (-57.8 percent), and environmental pollution charges decreased by 36.3 million rubles (-60.7 percent).

Given that the project implementation costs at the Kalchinskoye field are budgeted at 4 million rubles, the expectation is for harmful pollution from the existing flares to decline by 11,600 t (-73.5 percent) over 2013-2017, to produce 114.6 million rubles in savings (disregarding perspective changes in environmental laws) (Fig. 1).

Replication Opportunities
In fact, the experience of TNK-Uvat is applicable to any oil and gas producing enterprises facing a lack of gas infrastructure, such as gas processing plants, or gas transportation systems (in-field or trunk pipelines), stricter environmental legislation, and greater charges for harmful emissions associated with APG flaring.

TNK-Uvat has already put the resultant methodology to good use in designing Central Processing Facility (CPF) flare systems for the Ust-Tegusskoye field (Train III) and the Tyamkinskoye field (Train I). In addition, recently, the project was successfully adapted for use at the Kamennoye field of TNK-Nyagan. Following its implementation, the field also achieved soot-free combustion mode against the backdrop of declining oil production and a gas turbine power plant recently put on stream.