Hcn Cn H2663

The radicals then react with the NO to form molecular nitrogen:

An oxygen-deficient atmosphere is critical for Reactions (6-61) through (6-63) to occur. If oxygen levels are high, the NOZ reduction reactions will not occur, and the following will predominate:

To complete the combustion process, air is introduced above the reburn zone. Some NOZ is formed from conversion of HCN and ammonia compounds; however, the net effect is to significantly reduce the total quantity of NOZ emitted from the boiler. The reactions with HCN and ammonia are:

Reburn offers the advantages of being able to operate over a wide range of NOZ reduction values using a variety of reburn fuels. A reburn system can be varied from relatively low levels of reduction (25 to 30%) using an overfire air system without any reburn fuel to higher levels of reduction (~70%) when reburn fuel is added [21,30]. This allows for fine-tuning to meet emissions limits.

Concerns regarding the use of reburn technology are similar to those for other combustion modification processes. This includes concerns about incomplete combustion (i.e., CO and hydrocarbon production and unburned carbon in the fly ash), changes in slagging and fouling characteristics, different ash characteristics and fly ash loadings, corrosion of boiler tubes in reducing atmospheres, higher fan power consumption, and pulverizer constraints (if pulverized coal is used as the reburn fuel).

Cofiring Cofiring is the practice of firing a supplementary fuel, such as coal-water slurry fuel (CWSF) or biomass, with a primary fuel (i.e., coal) in the same burner or separately but into the main combustion zone. This technology was originally developed to utilize opportunity fuels; however, various levels of NOZ reduction were achieved and provide an option for NOZ reduction without investing in a post-combustion system when the emissions are near the regulatory requirements. This technique is not currently used as a commercial means for NOZ reduction; however, it is briefly discussed in this section because several demonstrations of this technology have been conducted, with a few still ongoing. In addition, it is considered a viable option for NOZ trimming especially if used in conjunction with legislation that mandates a percentage of electricity be generated from renewable/sustainable sources. Such legislation has been seriously discussed in the United States and has been included in congressional bills although they have not yet passed.

The CWSF technology was originally developed as a fuel oil replacement and underwent considerable research and development from the late 1970s to the late 1980s. During the late 1980s and early 1990s, coal suppliers and coal-fired utilities began to evaluate the production of CWSF using bituminous coal fines from coal cleaning circuits in an effort to reduce dewa-tering/drying costs and/or to recover and utilize low-cost impounded coal fines [35,36]. This marked a philosophical change in the driving force behind utilizing CWSF in the United States as well as the CWSF characteristics of these two fuel types, as cofire CWSFs are quite different from fuel oil-replacement CWSFs: Cofire CWSFs have a low solids content (50%) and no additive package to wet the coal, provide stability, and modify rheol-ogy, whereas fuel oil-replacement CWSFs have a high solids content (~70%) and an expensive additive package. Extensive testing performed by several companies and universities culminated in waste impoundment characterizations and several utility demonstrations in pulverized coal-fired boilers (both wall- and tangentially-fired units) and cyclone-fired boilers. Funding for these demonstrations was provided by industry, the DOE, the Electric Power Research Institute (EPRI), and state agencies. Penn State provided fuel support in all but one of these demonstrations, as summarized in a CWSF preparation and operation manual prepared by Morrison et al. [37], where the CWSFs were being developed to provide coal preparation plants a means for utilizing difficult-to-dewater fines, cleaning up waste coal impoundments to reduce coal mine liability, and supplying utilities with a low-cost fuel that also serves as a low-cost NOZ reduction technology. NOZ reductions were achieved that varied from ~11% in cyclone-fired boilers [38] to ~30% in wall-fired boilers [39,40] to ~35% in tangentially-fired boilers [41]. Several mechanisms were responsible for the NOZ reduction, including lower flame temperature from the addition of the water, staged combustion from cofiring in low-NOz burners, and the CWSF acting as a reburn fuel when injected in upper level burners.

Biomass cofiring has been demonstrated and deployed at a number of power plants in the United States and Western Europe using a variety of materials, including sawdust, urban wood waste, switchgrass, straw, and other similar materials [42]. Biomass fuels have been cofired with all ranks of coal: bituminous and subbituminous coals and lignites. The benefits of biomass cofiring include reduced NOZ, fossil CO2, SO2, and mercury emissions.

Cofiring biomass, particularly sawdust and urban wood waste but also switchgrass to a lesser extent, in large-scale pulverized coal-fired and cyclone-fired units has been demonstrated at several utilities with seven commercial installations in the United States [43,44]. Many of the demonstrations were conducted to achieve NOZ reductions, which can vary significantly but can be as high as ~35%. Tillman [42] noted that the dominant mechanism for NOx reduction is to support deeper staging of combustion when staging has not been particularly extensive. When biomass can introduce or accentuate staging by early release of volatile matter, then NOx reduction can be significant [42,45]. A secondary mechanism for NOx reduction is the influence of cofiring on furnace exit gas temperature (FEGT). Data indicate that cofiring has minimal impact on flame temperatures but can have a pronounced impact on FEGT, thereby reducing NO x emissions. A third influence is the reduction in fuel nitrogen content when a low-nitrogen fuel such as sawdust is used.

Process Optimization Several software packages have been developed or are under development that apply optimization procedures to the distributed control system of the boiler to provide tighter control of plant operation parameters [30]. The combustion process is optimized, resulting in lower NOx emissions and improved boiler efficiency while maintaining safe, reliable, and consistent unit operation. Also, combustion optimization approaches have been developed where advanced computational and experimental approaches are used to make design and operational modifications to the process equipment and boiler as a whole [46].

The main software packages are the ULTRAMAX Method, Generic NOx Control Intelligent System (GNOCIS/GNOCIS Plus), Boiler OP, Quick-Study, and Smart Burn [30,46]. The use of these packages has resulted in NOx reductions of 10 to 40%, reduced unburned carbon levels by 25 to 50%, increased boiler efficiencies by 1 to 3%, and increased heat rates by 0.5 to 5%.

Flue Gas Treatment Flue gas treatment technologies are post-combustion processes to convert NOx to molecular nitrogen or nitrates. The two primary strategies that have been developed for post-combustion control and are commercially available are selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). Additional concepts are under development, including combining SCR and SNCR technologies (known as hybrid SCR/SNCR) and rich reagent injection; however, these are not extensively used at this time. Of these technologies, SCR is being identified by utilities as the strategy to meet stringent NOx requirements. These technologies are discussed in the following sections, with an emphasis on SCR.

Selective Catalytic Reduction Selective catalytic reduction of NOx using ammonia (NH3) as the reducing gas was patented in the United States by Englehard Corporation in 1957 [47]. This technology can achieve NOx reductions in excess of 90% and is widely used in commercial applications in Western Europe and Japan, which have stringent NOx regulations, and is becoming the post-combustion technology of choice in the United States. Stringent NOx regulations in Western Europe essentially mandate the installation of SCR, and approximately 40 GW of generating capacity are fitted with secondary NOZ reduction systems, the majority of which utilize SCR with only a few boilers using the SNCR process [48]. Similarly, SCR technology was introduced into commercial service in Japan in 1980 and has been applied to more than 23 GW of coal-fired generating capacity in 61 plants. U.S. utilities initially deployed SCR for coal-fired units for new and retrofit applications in 1991 and 1993, respectively [49]. SCR units have been installed on ~26 GW of generating capacity in the United States, but by the year 2007 more than 200 SCR installations with overall capacity greater than 100 GW are anticipated to be in place to meet NOZ targets mandated by the SIP-Call [48,49].

The SCR process uses a catalyst at approximately 570 to 750°F to facilitate a heterogeneous reaction between NOZ and an injected reagent, vaporized ammonia, to produce nitrogen and water vapor. Ammonia chemisorbs onto the active sites on the catalyst. The NOZ in the flue gas reacts with the adsorbed ammonia to produce nitrogen and water vapor. The principal reactions are [50]:

A small fraction of the sulfur dioxide is oxidized to sulfur trioxide over the SCR catalyst. In addition, side reactions may produce the undesirable by-products ammonium sulfate ((NH^SO4) and ammonium bisulfate (NH4HSO4), which can cause plugging and corrosion of downstream equipment. These side reactions are [47]:

The three SCR system configurations for coal-fired boilers are highdust, low-dust, and tail-end, which are shown schematically in Figure 6-14 [50]. In a high-dust configuration, the SCR reactor is placed upstream of the particulate removal device between the economizer and the air preheater. This configuration (also referred to as hot-side, high-dust) is the most commonly used, particularly with dry-bottom boilers [30], and is the principle type planned for U.S. installations [48]. In this configuration, the catalyst is exposed to the fly ash and chemical compounds present in the flue gas that have the potential to degrade the catalyst by ash erosion and chemical reactions (i.e., poisoning); however, these can be addressed by proper design as evidenced by the extensive use of this configuration.

In a low-dust installation, the SCR reactor is located downstream of the particulate removal device. This configuration (also referred to as hot-side, low-dust) reduces the degradation of the catalyst by fly ash erosion; however, a) High-dust System

I Stack 220°F

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