Flue gas treatment

Flue gas treatment (FGT) is more effective in reducing NO, emissions than are combustion controls, although at higher cost. FGT is also useful where combustion controls are not applicable. Pollution prevention measures, such as using a high-pressure process in nitric acid plants, is more cost-effective in controlling NO, emissions. FGT technologies have been primarily developed and are most widely used in Japan. The techniques can be classified as selective catalytic reduction, selective noncatalytic reduction, and adsorption. 

TTte most cost-effective methods of reducing emissions of NO are the use of low-NO burners and the use of low nitrogen fuels such as natural gas. Natural gas has the added advantage of emitting almost no particulate matter or sulfur dioxide when used as fuel. Other cost-effective approaches to emissions control include combustion modifications. These can reduce NO emissions by up to 50% at reasonable cost. Flue gas treatment systems can achieve greater emissions reductions, but at a much higher cost. 

Powder Activated Carbon (PAC) - pulverized carbon with a size predominantly less than 0.18mm (US Mesh 80). These are mainly used in liquid phase applications and for flue gas treatment. 

With powder activated earbon, in most cases, the carbon is dosed into the liquid, mixed and then removed by a filtration process. In some cases, two or more mixing steps are used to optimise the use of powder carbon. Powder activated carbon is used in a wide range of liquid phase applications and some specific gas phase applications such as Incinerator flue gas treatment and where it is bonded into filters sueh as fabrics for personnel protection. 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

The R D activities of GRI led to a group of two patents, for gas sweetening (and also useful for flue gas treatment), based on biocatalytic processes for the selective removal of sulfur compounds in the presence of other reactive gases. 

WSA-SNOX A combined flue-gas treatment process which converts the sulfur dioxide to sulfuric acid and the nitrogen oxides to nitrogen. Developed by Snamprogetti and Haldor Topsoe, based on the WSA process. A large demonstration unit was under construction in 1989. 

The control of NO from stationary sources includes techniques of modification of the combustion stage (primary measures) and treatment of the effluent gases (secondary measures). The use oflow-temperature NO,.burners, over fire air (OFA), fiue gas recirculation, fuel reburning, staged combustion and water or steam injection are examples of primary measures they are preliminarily attempted, extensively applied and guarantee NO reduction levels of the order of 50% and more. However, they typically do not fit the most stringent emission standards so that secondary measures or flue gas treatment methods must also be applied. 

Supply two ESPs capable of 70% flue gas treatment that allows the FCC to be brought to a lower rate and one ESP can be taken out of service for any required maintenance. 

These corrections, however, are not so significant in flue gas treatment conditions. 

The dose distribution in the materials is given as a depth-dose curve. An example of the curve is illustrated in Fig. 4 obtained with the irradiation of electron from 0.5 to 1.0 MeV using cellulose triacetate (CTA) film dosimeter [12]. The existence of the maximum dose is an important characteristic of the depth-dose curve. Irradiation from two opposite sides by using two accelerators was proposed in order to give better uniformity in water [13]. The uniform irradiation is also important for flue gas treatment. Better efficiency of NO removal was proved with both-side irradiation by using three accelerators for coal-fired flue gas than single-side irradiation at the same dose [14]. 

Figure 5 Schematic representation of accelerator and reaction chamber for flue gas treatment.

Other important applications of adsorption are the control of greenhouse gases (CO, CH4, N20), the utilization of CH4, the flue gas treatment (SOx, N()x, Hg removal), and the recovery of the ozone-depleting CFCs (Dabrowski, 2001). Activated carbons and hydrophobic zeolites are used for the adsorption of HCFCs (Tsai, 2002). 

Case Studies on Chemical Flue Gas Treatment as a Means of Meeting Particulate Emission Regulations... 

The chemical flue gas treatment business has required the development of marketing skills not normally associated with specialty chemical suppliers. Historically, the major customer, the utility industry, had relied upon mechanical and electrical devices for pollution control. Thus, cyclones, electrostatic precipitators, and,more recently baghouses were the devices considered. The utility industry normally employs a technical staff of skilled mechanical and electrical engineers who operate the boilers and turbines. Thus, these personnel were available to apply their skills to pollution control equipment as well. 

It justified the choice of chemical flue gas treatment by the potential user based upon its capability to bring emissions under control within a short time span at low cost. Both capital and operating costs were shown to be modest when compared to alternate methods, such as a new or retrofitted electrostatic precipitator or baghouse. 

The addition of lime to control acid drainage from mining wastes typically produces calcium arsenates (Pichler, Hendry and Hall, 2001). Bothe and Brown (1999) further concluded that lime precipitates As(V) as a number of hydroxyl and hydrated calcium arsenates (Ca4(OH)2(AsC>4)2 4H2O, CaslAsCLLOH (arsenate apatite), and/or Ca3(AsC>4)2 3H2O) rather than anhydrous tricalcium orthoarsenate (Ca3(As04)2). Calcium arsenates also occur in coal combustion byproducts (Chapter 7). In the flue gas treatment systems of coal combustion facilities, volatile arsenic can readily react with calcium to form the arsenates on the surfaces of flyash and injected lime (Seames and Wendt, 2000 Yudovich and Ketris, 2005, 175). 

Although calcium arsenates may readily precipitate in acid mine drainage and form in flue gas treatment systems, Robins and Tozawa (1982) warn that the compounds may not have long-term stability, which could lead to disposal and environmental problems. Besides dissolving under acidic conditions, the presence of carbonate, bicarbonate, or CO2 may decompose calcium arsenates. When calcium arsenates react with CO2, calcium carbonate forms and the arsenic could be released into the environment (Ghimire et al., 2003, 4946 Jing, Korfiatis and Meng, 2003, 5055-5056). 

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