Energy-Tech Magazine

In addition to formation of NO_X, the combustion process generates numerous other oxides, some of which are produced as a result of incomplete oxidation of fuels in the combustion zones with reduced availability of oxygen (O₂). An example of these oxides is carbon monoxide (CO) that is very stable and highly toxic to humans. The limits established for CO emissions might require more than 90 percent reduction of raw CO emission levels. Such high conversion levels are possible by oxidizing CO to carbon dioxide (CO₂). The oxidation of CO to CO₂ is facilitated by the oxidation catalyst, which also reduces concentration levels of unburned hydrocarbons (UHC) and volatile organic carbons (VOC). The CO oxidation reaction stoichiometry is:

CO + 1/2O₂ → CO₂ (4)

Whereas many techniques have been developed for reduction of emissions by modifying turbine combustion characteristics, only post combustion exhaust gas cleaning technologies are capable of reducing NO_X, CO, UHC and VOC below 5 ppm.

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To comply with environmental standards, simple-cycle power plants can be equipped with catalytic systems consisting of the oxidation catalyst to control CO and TOC emissions, and the selective reduction catalyst to control NO_X emissions. The catalytic treatment of the exhaust gas is generally considered as a Best Available Control Technology (BACT) that represents the most stringent emission control process to be technologically feasible and cost effective.

Problem formulation
Depending on the turbine design, temperature of the flue gas exiting the turbine system can exceed the allowable temperature range for the emission control catalysts. As a result, temperature of the exhaust gas must be controlled for optimal catalytic performance. Typically ambient air (tempering air) is injected at the turbine discharge into the exhaust system. Because tempering air is injected at one location upstream of the emission control catalysts, the exhaust flue gas is cooled down and both catalysts (CO and NO_X) are maintained at the same operating temperature. However, these catalysts were developed to control different pollutants and operate efficiently at different operating temperatures. By exposing all catalysts to the same temperature, the catalytic processes are not efficient. Additionally, when injecting cooling air at the turbine discharge, the overall system pressure losses can increase due to additional pressure drop over perforated plates or other flow straightening devices installed in the exhaust ductwork.

The study presented here describes a new arrangement of the tempering air injection system, maximizing the efficiency of CO and SCR catalysts, and discusses the effects of the location of the tempering air injection point on the overall system efficiency.

Tempering air system arrangements
In a typical configuration of the exhaust system of the simple cycle power plant (see Figure 1), the exhaust gas leaves the gas turbine system through the expansion joint. Next, the exhaust gas enters the tempering air system. Traditionally, tempering air is injected and distributed across an interconnecting section of ductwork through a system of round pipes, sometimes equipped with dampers. A better view of the tempering air system is shown in Figure 2. In many applications, a significant amount of ambient air is added to reduce temperature of the exhaust gas.

Following the tempering air injection system, the mixture of ambient air and exhaust gas passes through the perforated plate, the oxidation catalyst, the ammonia injection grid and the reduction catalyst.