FGD and DeNOx
NEWSLETTER       

                                                                                                           May 2003
                                                                                                             No. 301

Pitfalls of SCR Retrofit on Gas Turbines

Kevin Carpenter and Ronald Hitzel of Siemens Westinghouse Power Corporation alerted the Power-Gen 2002 audience to issues relating to SCR retrofit of gas turbines. Flow distribution, backpressure, space, temperature profile, controls and their integration and subsequent cost of the system must be considered. Owners must work with suppliers who understand the entire gas turbine power system, not just the SCR.

In order for an SCR to function properly, the reagent and exhaust must be sufficiently mixed before the catalyst bank. As a result, most catalyst suppliers specify a maximum flow distribution variation with their guarantees. Since gas turbine exhaust is very turbulent, flow modification devices may be required. Addition of any obstruction in the exhaust section of a gas turbine power plant will increase backpressure. This includes the catalyst bank along with any necessary flow modification devices. Backpressure leads to a reduction in power and temperature changes in the turbine. 

The authors include several case studies where problems occurred:

·         An SCR was retrofit to a 100-MW gas turbine with heat recovery steam generator (HRSG). The HRSG was originally built with a spool piece as a provision for an SCR, so space was no problem. However, when the owner later wanted to do a wet compression upgrade, the catalyst could not accommodate the increase in mass flow desired.

·         An SCR was retrofit to an 18-MW gas turbine with HRSG. As space was rather tight, a vertical SCR was constructed between the gas turbine and the HRSG. Once in operation, the SCR functioned properly for the first few days, after which, there was a pressure surge and the doors of the SCR were blown open. The SCR was inspected several times and over a year later, there were still periodic incidents of upset. It appears that the SCR manufacturer did not take into account the variability of gas turbine exhaust and backpressure issues.

·         A “hot” SCR was retrofit to a 100-MW simple cycle gas turbine. A zeolite catalyst was in­stalled. For reasons yet unclear, the SCR stopped functioning properly after about one year.

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