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NOx Control Technologies

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Cutting SCR Cost for NOx Control

Selective Catalytic Reduction (SCR) is a very effective but expensive means to control NOx emission. Capital costs for installing SCR on new industrial boilers range from 6,000 to 10,000 $/MMBtu/hr. In retrofit application, capital costs are about 30 to 50% higher. The increase in cost is primarily due to modifications to existing ductwork, the cost of structural steel and reactor construction, auxiliary equipment costs (e.g., additional fans, ammonia vaporizer, air compressor), and engineering costs. In addition, significant demolition and relocation of equipment may be required to provide space for the reactor. Thus, there is a great deal of interest in exploring innovative means to reduce both the size and the cost of SCR. Significant saving in SCR costs can be obtained by reducing the catalyst and ammonia requirements.

In the SCR process, NOx emission is reduced by reaction of NOx with ammonia vapor over a bed of catalyst. To maximize NOx reduction and minimize ammonia slip, a 1:1 molar ratio of ammonia and NOx is used. The optimum temperature depends on both the type of catalyst utilized in the process and the flue gas composition. For the majority of commercial catalyst (metal oxides), the optimum temperature is in the range of 480 to 800oF and is shown in Figure 1. NOx control efficiency of over 90% can be achieved with SCR. However, the temperature zone to capitalize on this high reduction is relatively small. Thus, maintaining high efficiency may not always be practical from a cost standpoint.

 


Figure
1. NOx control efficiency as a function of flue gas temperature (metal oxide catalyst).

Another factor that results in increasing the cost of SCR is the catalyst requirement at high reduction efficiency. Figure 2 indicates that as the NOx control requirement is increased above 80%, the catalyst volume increases exponentially, resulting in a substantial increase in costs. Thus, significant cost savings can be achieved by operating the SCR at efficiencies in the range of 60 to 85%.

 


Figure 2. Relative catalyst volume as a function of NOx reduction.

 

One way to reduce cost is by combining ETEC's Flue Gas Recirculation technologies with SCR. For gas and light oil fired combustion units planning to install SCR, ETEC offers Slip-Stream FGR technology (patent pending) as a hybrid approach to reduce cost of SCR. In Slip-Stream FGR process, a slip-stream from a fan downstream of the combustion unit such as a SCR fan or an induced draft fan is recirculated back into the flame zone to obtain NOx reduction. FGR based processes are very efficient in reducing NOx. ETEC has installed over 30 FGR based NOx control systems and NOx reductions as high as 85% have been obtained. Since Slip Stream FGR uses the SCR fan to drive the flow, incremental cost for Slip Stream FGR process is minimal.

A major advantage of Slip Stream FGR process is that it decreases NOx concentration in flue gas stream resulting in a substantial decrease in ammonia usage and catalyst requirements. With Slip Stream FGR, SCR costs can be reduced significantly. To demonstrate this viewpoint, assume a baseline emission rate of 0.2 lb/MMBtu (typical range is 0.1 to 0.45 lb/MMBtu). To meet a target emission limit of 0.01 lb/MMBtu, the amount of reduction required is [(0.2-0.01)/0.2] or 95%. The SCR vendor would need to guarantee a reduction of 95% based on inlet NOx concentration of 0.2 lb/MMBtu. Now, if an ETEC's FGR system were installed resulting in a NOx reduction of 75%, the inlet NOx to SCR would be 0.05 lb/MMBtu. Therefore to achieve the target level of 0.01 lb/MMBtu, NOx reduction required by SCR would be 80%, and not 95% (required reduction from an SCR only system). Based on our kinetic model, this difference in reduction requirement would indicate about 80% decrease in the catalyst requirement (see Figure 2). Lower catalyst usage results in a smaller SCR reactor leading to substantial savings in equipment, space and structural steel requirements. Furthermore, with ETEC's FGR technologies, ammonia requirements can be expected to decrease by 50 to 80%, resulting in reduced storage, ammonia vaporizer and air compressor requirements. Additional savings can also be realized from lower operating costs in terms of reduced ammonia consumption and lower energy requirement to vaporize ammonia. Furthermore, with ETEC's Slip Stream FGR, the flue gas temperatures would be increased by about 20 to 35oF. This increase in flue temperature will improve the performance of the SCR. Depending on the size of the unit, savings in ammonia usage alone could payback Slip-Stream FGR cost in less than 6 months. Figure 3 summarizes results of our cost analysis as a function of the amount of NOx reduction.

In Figure 3, the relative cost refers to the cost of control technology relative to the cost of LNB technology for 50% reduction. Conventional technology refers to use of LNB for NOx reductions up to 50%, and, for higher levels of NOx reduction it refers to SCR technology. Hybrid technology refers to use of IFGR for NOx reductions up to 65%, and for higher levels of NOx reduction hybrid technology refers to a combination of Slip-Stream FGR with SCR technology. The results for conventional application indicates that as the amount of NOx reduction is increased to levels above 50%, there is an exponential increase in the control technology capital cost. Our analysis indicates that the cost can be reduced by as much as 65% using the “hybrid” approach of Slip-Stream FGR and SCR. If NOx control level of 90% is desired, ETEC offers a hybrid of IFGR with C-Mods and it is more cost effective when compared to a hybrid of Slip-Stream FGR and SCR. For NOx control levels above 90%, the hybrid of Slip-Stream FGR and SCR is more cost-effective when compared to SCR only installations.

 

Figure 3.  Relative Control Technology Cost as a function of percent NOx reduction.

In the HGA area, several companies are retiring boilers and planning to import steam from a third party operated cogen facility. Depending on a third party for plant utilities not only limits operating flexibility, but eliminates the option of burning waste gas stream that were burned in the boilers. Cost-effectiveness of ETEC's systems make it possible to continue operating the boilers while achieving significant NOx reduction with minimal cost implications.




TYPICAL COSTS OF SELECTED NOX REDUCTION

TECHNOLOGIES FOR GAS AND OIL FIRED COMBUSTION UNITS.

(INCLUDES FIRED HEATERS AND BOILERS)

 

Combustion-Unit Size

 

40 million Btu/hr

100 million Btu/hr

740 million Btu/hr

(75 MW)

3.400 million Btu/hr

(320 MW)

Capital Cost $

IFGR

66,000

104,000

238,000

424.000

LNB

124,000

269,000

921,000

2,045,000

SCR

a

689,000

2,250.000

6,400,000

IFGR + SCR

a

587,000

1,812,000

4,904,000

Total annual cost (TAC), $:

IFGR

12,100

19,200

52.300

93,300

LNB

26,100

56,600

193,900

430,600

SCR

a

19I,400

625,000

1,778,000

IFGR + SCR

a

117,300

362,500

980,800

Typical Cost Effectiveness, $ / ton NOx Reduced:

IFGR

1,500-7,000

1,200-4,500

800-1,600

250-700

LNB

4,500-15,000

4,000-12,000

2,600-3,500

900-2,600

SCR

a

10,000-27.000

3,600-10,000

1,000-8,000

IFGR + SCR

a

6,000-15,000

2,000-5,500

550-4,000

a = Not cost effective. Compliance can be achieved by other cost-effective alternatives.

b = Combustion modifications alone may not be sufficient to meet compliance

 

Note: Cost estimates are highly variable, and accurate estimates can only be made on a case-by-case basis.