The entire existing fleet of standard Rolls-Royce RB211 and Rolls-Royce Avon gas turbine driven compressors will ultimately be non-compliant with tightening environmental emissions standards without accepting derogations severely restricting running hours or restricting operating life (‘emergency 500 hours use’ or ‘limited life derogation’).

National Grid has sought, in general, to address the issue of environmental improvements through the installation of new electric Variable Speed Drive (VSD) compressor equipment or new Dry Low Emission (DLE) gas turbine drivers. These two technologies have been generically identified as BAT for the gas transmission system for some 10 years, in agreement with the UK environmental regulators. Retrofitting of DLE systems to existing turbines may not be viable because of the age of the assets (up to 40 years) and other alternative emissions control techniques, such as water or steam injection, are periodically reviewed (most recently in 2015) and have limited applicability and potentially introduce a number of operational difficulties.

The use of catalytic technologies for new build gas turbines has previously been considered and rejected as a candidate BAT option for mainstream applications as they are largely unproven in gas transmission applications and do not offer the potential for realising the wider benefits that can be achieved when installing new compressor machinery train (e.g. increased operating efficiency and better compressor matching to site duty).

However, given the cost challenges imposed on National Grid through the RIIO process and scale of the challenge to meet emissions targets, National Grid recognised that catalytic technologies needed to be reviewed again as a potentially cost effective and emissions compliant retrofit option.

Other European Transmission Network Operators (TNOs) have commenced pilot projects to trial catalytic

reduction technologies, and National Grid has commenced its own innovative project to install an oxidation catalyst system for carbon monoxide (CO) control at the Aylesbury compressor station.

The BREF author’s position on catalytic control techniques for mechanical drive gas turbine applications is less clear; there remain a number of challenges to address, in particular relating to the variable load and exhaust temperatures which typify gas transmission operations.

In light of the findings of initial studies carried out by GTAM, the potential environmental benefits which could be realised from an SCR solution, when combined with a potentially lower project cost compared to new build, would indicate that SCR could be a candidate BAT option in some circumstances. It is thus appropriate to consider detailed engineering and environmental assessment to confirm, or otherwise, its potential viability for implementation. However, there is at present insufficient information to address the remaining uncertainties and further study is required.

Benefits

A shortlist of sites / units identified as potentially suitable for FEED.

A SCR conceptual design suitable for application on an NTS compressor site.

Documentation of the understanding of compliance requirements of SCR systems and principal deviations against National Grid’s engineering and safety standards.

A robust BAT model comparing SCR options with ‘do nothing’ and new build DLE scenarios.

Documentation of a clear position which either indicates viability of the SCR technology for application on the NTS or a justified rationale concluding that the SCR technology does not bring superior benefits against other options.

A sound concept design basis for moving rapidly into a full FEED study (assuming viability is proven)

Defined commercial and procurement options for any subsequent pilot project to provide basis for National Grid Global Procurement to be able to seek the best value catalyst solutions (assuming viability is proven).