SCONOX reduces CO emissions by oxidizing CO to CO2. When CO is reduced from 11.51 ppm (gas turbine with duct burner firing) to 1 ppm, the cost effectiveness is $21 706/T of CO removed.
In the United States, combustion turbines and duct burners are subject to the federal NSPS, but the regulations provide no CO emission limits. The following sections assess the control strategies that are potentially feasible for decreasing CO emissions from the facility.
Catalytic Oxidation
The rate of formation of CO during natural gas combustion depends primarily on the efficiency of combustion. The formation of CO occurs in small, localized areas around the burner where oxygen levels cannot support the complete oxidation of carbon to CO2. Efficient burners can minimize the formation of CO by providing excess oxygen or by mixing the fuel thoroughly with air. CO emissions resulting from natural gas combustion can be decreased via catalytic oxidation. The oxidation is carried out by the well‐known overall reaction:
Several noble metal‐enriched catalysts at high temperatures promote this reaction. Under ideal operating conditions, this technology can achieve an 80% reduction in CO emissions. Prior to entering the catalyst bed where the oxidation reaction occurs, the exhaust gas must be preheated to about 400–800 °F. Below this temperature range, the reaction rate drops sharply, and effective oxidation of CO is no longer feasible. Moreover, there is an energy loss because of the reduction in performance due to the pressure drop across the CO catalyst.
Sulfur and other compounds in the exhaust may foul the catalyst, leading to decreased activity. Catalyst fouling occurs slowly under normal operating conditions and may be accelerated by even moderate sulfur concentrations in the exhaust gas. The catalyst can be chemically washed to restore its effectiveness, but eventually, irreversible degradation occurs. Catalyst replacement is usually necessary every 5–10 years depending on the type and operating conditions.
An economic analysis for the catalytic oxidation of CO emissions based on vendor information estimates the cost at $5084/T of CO removed. This cost level is considered to be economically infeasible for BACT. In addition to cost, catalytic oxidation would lead to increased downtime for catalyst washing and would present hazardous waste concerns during catalyst disposal. Due to the high cost and concerns with downtime and hazardous material disposal, catalytic oxidation is not selected as BACT for control of CO emissions from the turbines and duct burners.
Table 6.18 CO emissions, control effectiveness, economics, and environmental impacts.
| Technology effectiveness | CO emissions reduction (TPY) | Capital cost ($) | Annualized cost ($) | Cost effectiveness ($/T) | Adverse environmental impacts |
| SCONOX (1 ppm) | 251 | 14 922 733 | 5 444 139 | 21 706 | Yes |
| DLN + CO catalyst (3.4 ppm) | 194 | 1 302 514 | 2 208 461 | 5 084 | Yes |
Good Combustion Practices
Clearinghouse (RBLC) data show that the majority of BACT determinations for CO relied on the use of good combustion practices. Since add‐on controls for CO were shown to be economically infeasible, the proposed BACT for CO emissions is the use of good combustion practices.
Combustion Control
Because CO results from the incomplete combustion of fuel, combustion control is an inherent design feature of combustion turbines and duct burner. Control is accomplished by providing adequate fuel residence time and high temperature in the combustion zone to ensure complete combustion. These control methods, however, also result in increased emissions of NOx. Conversely, a low NOx emission rate achieved through flame temperature can result in higher levels of CO emissions. Thus, a compromise is needed to set the flame temperature at the level that will achieve the lowest NOx emission rate possible while keeping CO emissions rates at acceptable levels.
Summary of CO BACT for Turbines and Duct Burners
Table 6.18 provides information on the emissions, control effectiveness, economics, and environmental impacts associated with control of CO. The analysis was performed on a unit (turbine and duct burner) basis.
SCONOX provides a higher level of CO reduction than a combination of DLN combustors and CO oxidation. The adverse impacts include an energy loss from the performance loss due to the pressure drop across the CO catalyst and emissions of sulfates condense as additional PM10 or PM2.5. However, at a cost of $5084/T of CO removed, and the second option is more than four times as cost‐effective as a control option.
Combustion control is selected as BACT for CO control with the limit of 9 ppm at 15% O2 (annual average) without duct burners firing and 11.5 ppm at 15% O2 with duct burner firing (annual average).
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