Flow Analysis Helps Correct Underperformance of Combined Cycle Emissions Equipment

Computational fluid dynamics analysis of the exhaust gas flow distribution in the inlet ductwork of a combined cycle power plant has helped improve both emissions performance as well as lowering plant operating costs. The Turlock Irrigation District Almond Power plant had recently installed an upgraded gas turbine (GT), and replaced both CO oxidation and selective catalytic control (SCR) NOx control catalysts in order to withstand the higher exhaust temperatures of the new gas turbine. After the retrofits, both NOx and CO emissions were within Permitted limits, but were above expected values, and operators found that overspray of ammonia was required to meet required NOx values of 3.0 ppmv at 15% O₂. The need to overspray also resulted in unacceptably high levels of ammonia slip at the stack. Following investigations which included physical flow modeling of the ductwork, TID opted to have the exhaust gas flow numerically modeled to confirm the origin of the emissions problem and identify and implement a long-term resolution. Computational fluid dynamics (CFD) analyses confirmed that the cause of the poor emissions performance was highly biased gas flow distributions at the entrance to the CO catalyst modules and the ammonia injection grid (AIG). Using multiple models to evaluate alternate configurations of flow control devices, a single distribution grid located upstream of the CO catalyst modules was designed to provide improved flow distributions at both the CO catalyst, AIG, and SCR catalysts. Field tests of the plant with the installed distribution grid confirmed that the retrofit significantly improved emissions performance while reducing both required ammonia injection flows and ammonia slip values eight-fold. This not only allowed the plant to meet and exceed required emissions levels, but also reduced ammonia injection flow rates by a minimum of 40%. Additionally, the facility has seen an improvement in the plant’s effective operating range. This is the range at which the plant can operate and still maintain emissions compliance. Prior to the installation of the distribution grid the plant had an effective operating range of 30–50 MW. The plant now has an effective operating range of 20–50 MW. This is beneficial at times when the plant is operated in “load following” mode.