Static mixing retrofit for Selective Catalytic Reduction in a 4 × 600  MW coal‐fired power plant under ultra‐low emission constraints

Selective catalytic reduction (SCR) retrofits in large coal‐fired units increasingly operate under ultra‐low emission (ULE) limits and frequent load‐following, where inlet‐flow and ammonia‐to‐NOx (NH 3 /NOx) maldistribution can cause local over/under‐dosing, elevated outlet NOx, ammonia slip, and downstream ammonium bisulfate (ABS) deposition that degrades air‐preheater performance. This manuscript reports a full‐scale engineering demonstration of a coupled static mixer and redesigned ammonia injection grid (AIG) retrofit implemented on a 4 × 600 MW coal‐fired power plant. A three‐dimensional computational fluid dynamics (CFD) co‐design workflow was used to diagnose baseline non‐uniformities, optimize the mixer–AIG configuration under multi‐load operating scenarios, and enforce an engineering constraint on incremental pressure drop. Field validation was then conducted using plant instrumentation and routine performance testing under representative loads. The optimized configuration reduced the coefficient of variation of the NH 3 /NOx molar‐ratio distribution at the first catalyst inlet plane from 7.46% to 3.26% at full load (a 56.3% reduction), while maintaining robust improvements at part load. Post‐retrofit tests confirmed stable compliance with ULE outlet NOx targets and improved SCR operational consistency, with concurrent benefits to air‐preheater pressure‐drop behavior consistent with reduced ABS‐related fouling. The study provides a replicable, plant‐ready template for SCR inlet‐duct retrofits that jointly improve mixing quality, emissions robustness, and downstream heat‐exchanger operability.