In the United States, selective catalytic reduction (SCR) systems for post-combustion NOx control are installed downstream of most large-scale industrial and utility gas turbines (simple and combined-cycle) to meet stack NOx permit requirements. These systems use ammonia as a reagent and are designed to handle gas turbine (GT) outlet (that is, SCR inlet) NOx levels in the range of 10–25 ppmv (at 15% O2, dry) with the GT firing natural gas. They are generally capable of providing 90% NOx reduction with less than 5 ppmv of unreacted ammonia (also called ammonia slip) as a byproduct. This allows GTs to comply, in some cases, with NOx permit levels as low as 2 ppmv at the stack.