Hot Selective Catalytic Reduction, Fast-Starting Combined-Cycle Plants Face Issues

By Barry Cassell

The issues involved with running hot selective catalytic reduction (SCR) systems on large-frame, simple-cycle gas turbines, and with controlling emissions during start-up for fast-starting combined-cycle plants were featured topics at a Dec. 14 sessions at Power-Gen International covering emissions control issues.
Kelly Flannery, Engineering Director at Vogt Power International, tackled the issue of emissions controls for fast-ramping combined-cycle plants.
Flannery said one recent development is the addition of start-up heaters in the ammonia injection systems for SCR installations. This allows more efficient operation of these systems earlier in the start-up cycle as the facility is ramped up to full load.
“For start-up emissions compliance, we need to understand all the emissions control equipment, especially the reactivity requirements at start up when the operating temperatures of the catalyst are lower,” said Flannery. “But we also need to understand the gas turbine and the combined-cycle start up itself; that includes the air permit [requirements]. And we also want to make sure the [heat recovery steam generator (HRSG)] design aligns for optimum catalyst performance.”
He said Vogt Power is seeing more and more projects where the CO catalyst system, traditionally placed upstream in the plant exhaust system, with an SCR immediately downstream, is being moved further upstream into a higher temperature zone, separated by some heating surface between the CO and SCR catalysts.
He noted that fast-ramping is pretty much defined these days as gas turbines going from start-up to full load in 15-20 minutes, with the steam turbine in a combined-cycle plant lagging a bit behind that. At low load, gas turbines are less efficient and put out higher emissions.
Addressing SCR issues for fast-ramp simple-cycle systems was Bob McGinty, a Senior Product Manager at Mitsubishi Hitachi Power Systems Americas. He the most recent 10 years of operating experience for these systems.
McGinty pointed out how wind and solar renewable energy projects, with their highly variable output depending on wind and sun conditions, are creating more demand for fast-ramp, simple-cycle gas systems. He noted how high power demand occurs on the grid in late afternoon and early evening, when people get home from work, around the time the sun sets and before night-time winds pick up.
While ramp rates for simple-cycle plants used to be around 5 MW per minute, that is now up to as high as 50 MW per minute, McGinty said. Simple-cycle turbines are also getting bigger to get economies of scale, he added, with sizes ranging up to 360 MW.
One key to maintaining SCR catalysts in hot operating environments is to run 30% ambient air into the system, cooling the exhaust gas to temperatures that don’t harm the catalysts. Various changes lately in SCR systems to ensure reliability have removed the “fear factor” for plant operators in operating under these high-stress plant conditions.
He said another recent development is that the footprint next to the power generating unit for an SCR has shrunk sharply, since there is a premium on space at many power plant sites.
Development and evolution of catalysts has had a major impact on SCR systems, McGinty said. Vanadium as a catalyst is showing the “best bang for the buck” in terms of efficiency, he added.

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