Quantifying Emissions from Gas Flaring Operations

The purpose of this document is to provide the reader with guidance on the metering of flare and vent gas, the efficiency of the flare tip and the analysis of the emissions from its combustion.

Accurate flare and vent gas metering is important to report the correct quantities of natural gas flowing to the vents and / or flares. This is important to meet regulatory requirements, but to also ensure that the flare systems are sufficiently purged to prevent the formation of combustible mixtures within the stack which could result in flashback or burn-back issues. Furthermore, an incorrect meter reading could also lead to sub-optimal operation of the flare system and lead to higher quantities of greenhouse gases being released into the atmosphere. Therefore, flaring and venting system flowrates must be accurately measured, operate above their minimum safety flow rate and, in the case of flaring, achieve a high combustion and destruction efficiency to reduce emissions.

In the case of venting, a reduction in emissions can be achieved by reducing the flow rate of gas leaving the vent. However, operating these systems at such low levels may be significantly outside their original design specification. Additionally, in the case of flaring, the combustion efficiency can be inherently linked to meteorological conditions such as the wind which dilutes and cools the combustion zone.

Unfortunately, accurate metering of flaring is particularly challenging because of multiple factors. For example, the calibration of the flow meters is not usually possible due to the scale of the installation, the range of flow rates to be metered requires a large turndown ratio and fluid properties need to be known in real time. It is therefore necessary to correct the measured flow rates taking into account the actual installation, fluid properties and operating conditions.

CFD (computational fluid dynamics) modelling can provide detailed insight on the combustion process for the specific flare installation and can cover the full range of operating conditions of interest. It can also be used to guide design modification to the combustion system to improve the combustion process and the safety of the operation. Compared to the physical measurement of the emissions, numerical modelling can be a more practical solution as a range of operating conditions can be analysed a priori instead of having constant monitoring.

This report by TÜV SÜD National Engineering Laboratory was prepared for the Department for Science, Innovation & Technology (DSIT). TÜV SÜD National Engineering Laboratory is the UK’s Designated Institute for Flow Measurement, under contract from DSIT, and part of the UK’s National Measurement System.