Research and analysis

Review of differential absorption LiDAR flare emission and performance data: summary

Published 22 October 2019

1. Chief Scientist’s Group research summary

This project reviewed data on how well waste industrial gases are broken down by burning in flares. Flares use flames to burn flammable waste gases under controlled conditions. Different types of flare include enclosed flares, for example inside a chimney, and open flares that are visible at the end of a pipe.

The Environment Agency uses data on the combustion performance of different types of flare to regulate flares at industrial sites. However, information on the effectiveness of flares is limited. For this project the National Physical Laboratory (NPL) reviewed its database of industrial gas emission measurements, made using a particular LiDAR (Light Detection and Ranging) remote sensing technique known as DIAL (Differential Absorption LiDAR). The database was specifically checked for additional information on the effectiveness of flares in breaking down gases.

1.1 Use and emissions of flares

Flares have been widely used to burn off waste gases at regulated sites including landfills, chemical plants and refineries. The Environment Agency has previously assumed that flaring is best practice for protecting the environment, but the evidence to support this is limited. For example, there are few measurements available to evaluate the combustion efficiencies of different types of flare under different operating conditions. The efficiencies are a measure of how much unburnt gas is emitted. Flares can emit combustion gases that present a risk to human health, and inefficient flares can emit unburnt gases that can harm the environment and may contribute to global warming.

1.2 Monitoring of flare emissions

The high temperatures and design of flares make them difficult to monitor so there is limited information on their emission performance. Flares can be monitored with remote sensing devices, but this is often expensive. DIAL is an established technique for optical remote sensing that can quantify and map three-dimensional emissions of gases. It shines a laser into the flare and records the wavelengths, intensities and timings of the reflected light which indicate the composition, concentration and position of the gas.

1.3 Analysis of flare data

By analysing DIAL measurements and data on the hydrocarbons supplied to a flare, it is possible to evaluate the flare’s combustion efficiency and the amount of unburnt methane and other air pollutants released to atmosphere. NPL has over 30 years’ experience of using DIAL systems for industrial emission measurements, and the review covered 69 separate campaigns between 1983 and 2015.

The review identified 14 campaigns with flare measurements, covering 29 individual flares. In many cases, the flare was measured as just one feature of the overall site with no particular details being recorded of:

• the type of flare;
• the flow and composition of hydrocarbons supplied to it;
• its operational status during the DIAL measurements.

1.4 Case studies and conclusions

Although details were lacking for many cases of DIAL measurements, the review identified 4 case studies that had sufficient details and were relevant to OOG sites. The review confirmed that:

• Combustion efficiencies can be calculated when there are relevant data
• DIAL is particularly suited to the direct measurement of emissions from flares

The case studies did not provide enough examples for a comprehensive analysis of flare emissions performance covering different flare types, flows, compositions of supplied hydrocarbons, and operating conditions.

However, the following conclusions can still be drawn regarding flare measurement and performance.

• DIAL remote sensing is effective for direct measurements of flare emissions, as shown for a range of gases and flares on complex industrial sites.
• When DIAL emission measurements are combined with data on the flow and composition of the hydrocarbons supplied to a flare, various aspects of flare performance can be determined including combustion efficiency.
• Flare emissions and combustion efficiency can vary (for example, combustion efficiency varied across the case studies from ~85% to ~99%) and there was a tendency for higher emissions of unburnt hydrocarbons to occur under conditions of low flow.
• Gas valves may leak so that hydrocarbons are released during non-operational periods.
• In order to evaluate flare performance, it is not only necessary to make DIAL measurements of flare plumes, but also to keep full records of the relevant flare types, flows, inlet gas compositions and operating conditions.
• A template is needed for keeping such full records for flares during future DIAL campaigns, and the report has developed one.

1.5 Publishing details

This summary relates to information from project SC150026, reported in detail in the following output(s):

Report:

SC150026

Title:

Review of differential absorption LIDAR flare emission and performance data

October 2019

Project Manager:

Roger Timmis, Research, Analysis and Evaluation

Research Contractor:

NPL (National Physical Laboratory)
Hampton Road,
Teddington
TW11 0LW

Tel. 0208 977 3222

This project was funded by the Environment Agency’s Research, Analysis and Evaluation group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: [email protected]

© Environment Agency