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Coal to Natural Gas

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Coal to Natural Gas Firing Conversion Technologies

In the present operating climate, a number of electric utilities are considering converting some of their units from coal to natural gas firing. Along with this conversion, new natural gas fired NOx emissions requirements will be established. Fuel gas fired NOx emissions are formed during the combustion process by thermal fixation of the combustion air nitrogen. One of the most effective combustion NOx control technologies is flue gas recirculation (FGR), which, as a standalone technology may effectively reduce the NOx emissions below the new requirements. If not, then FGR may be augmented with a stoichiometric NOx control technology commonly referred to as Burner-Out-Of Service (BOOS) operation to further reduce NOx emissions. It is very likely that the combination of FGR and BOOS will reduce NOx emissions below required levels. IFGR and Burner Out of Service (BOOS) operation provide a very effective means for controlling the thermal and stoichiometric NOx formation, respectively. A description of IFGR and BOOS are provided below.

Induced Flue Gas Recirculation (IFGR)

Recirculating flue gas back to the combustion zone has been one of the most effective methods of reducing NOx emissions from gas and oil fired units since the early 1970’s. Flue Gas Recirculation (FGR) reduces NOx formation by reducing peak flames temperatures. In conventional applications, the recirculated flue gas is typically extracted from the outlet duct upstream of the air heater. The flue gas is then returned through a separate duct and fan to the combustion air duct that feeds the windbox. The recirculated flue gas is mixed with the combustion air with airfoils or other mixing devices in the duct. This technology is known as windbox FGR (WFGR). WFGR systems require installation of a separate FGR fan to move flue gas from the exit to the air supply ducting at the windbox inlet.

ETEC offers a far less costly version of the FGR technology that eliminates the need for a separate FGR fan and windbox mixing devices. This patented technology, known as Induced Flue Gas Recirculation (IFGR) is based on ETEC’s "Extractor/Inducer" technology that uses the existing forced draft fan (FDF) to pull (induce) flue gas from the exhaust duct, via the extractor, into the combustion air at the fan inlet. The fan also serves as the mixing device.

This process reduces NOx emissions, primarily by reducing peak flame temperature levels. The exhaust gases mixed with the combustion air absorb heat during the combustion process, thereby reducing peak flame temperature and NOx emissions formation (as illustrated in CFD model results shown in Figure 1-1).

ETEC engineers pioneered the application of IFGR technology to electric utility boilers in the 1990's. The very first electric utility IFGR project was conducted for the client through an EPRI tailored collaboration program, designed to demonstrate the viability and cost-effectiveness of IFGR technology in reducing NOx emissions. Since then ETEC has designed and installed over 50 IFGR systems. Test data obtained from various ETEC IFGR systems’ implementation test programs, showing the relationship of NOx emissions reduction as a function of %IFGR, is presented in Figure 1-2.

The before and after pictures for a typical electric utility boiler IFGR system are shown in Figure 1-3. Each IFGR duct is equipped with control dampers that regulate the amount of flue gas recirculation to the forced draft fan inlet.

Combustion Modification (BOOS)

In most electric utility projects, ETEC has combined IFGR operation with combustion modifications via BOOS to achieve dramatic NOx emissions reductions. Implementation of BOOS operation is a practical and effective means of achieving staged combustion, i.e., modifying burner stoichoimetry, on an existing boiler. The technique of BOOS operation involves terminating the fuel flow to selected burners while leaving the air registers open. The remaining burners operate fuel-rich, thereby limiting oxygen availability, lowering peak flame temperatures, and reducing NOx formation. The un-reacted combustion products combine with the air from the terminated-fuel burners to complete burnout before exiting the furnace. BOOS operation can be a very effective NOx reduction technology, depending on the degree of staging, as shown in Figure 1-4.

A list of ETEC IFGR system projects is presented in Table 1-1. For most of the 50+ IFGR systems, ETEC has also implemented BOOS operation, in conjunction with IFGR, to achieve maximum combustion NOx emissions control. Our clients have stated that this combination of technologies is by far the most cost-effective approach in their NOx control strategy. The costs of alternate NOx control technologies are an order of magnitude higher, with results only equal to or slightly improved over ETEC's technologies.

Figure 1-1. CFD Model- NOx Emissions Reductions with IFGR

Predicted Gas Temperature contours (ºF)
IFGR Predicted Gas Temperature contours

Predicted NOx Concentration contours (ppm)
(CFD Model Results).
Predicted NOx Concentration contours

ETEC IFGR Systems NOx Emissions Reductions
ETEC IFGR Systems NOx Emissions Reductions

Figure 1-2. NOx Emissions Reductions with IFGR

ETEC Before IFGR

ETEC After IFGR

Figure 1-4. Stoichiometry Modification (BOOS) NOx Reduction

ETEC BOOS Performance

Table 1-1. ETEC IFGR Systems Client List

etec clients