Ammonia injection

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia sHp can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

Selective catalytic reduction (SCR) is cmrently the most developed and widely applied FGT technology. In the SCR process, ammonia is used as a reducing agent to convert NO, to nitrogen in the presence of a catalyst in a converter upstream of the air heater. The catalyst is usually a mixture of titanium dioxide, vanadium pentoxide, and hmgsten trioxide. SCR can remove 60-90% of NO, from flue gases. Unfortunately, the process is very expensive (US 40- 80/kilowatt), and the associated ammonia injection results in an ammonia slip stream in the exhaust. In addition, there are safety and environmental concerns associated with anhydrous ammonia storage. 

Ammonia Injection rate controlled on pH of condensate in accumulator... 

Chang et al. conducted pilot scale tests in a corona radical shower system with ammonia injection for simultaneous removal of NO and S02 from coal boiler flue gases [61]. The corona radical shower system used had dimensions of 2.1 x 1.8 x 2m3 and consisted of 20 parallel flow channels with five multiple nozzle corona electrodes per... 

Current soil injection equipment owes much to the development of narrow tines for injecting anhydrous ammonia and to improvements in soil loosening equipment (6, 7, 8). Ammonia injection causes little soil disturbance and is adequate for incorporating volumes of only 1.0 to 2.5m3/ha but the amounts of slurry which are normally applied are up to 100 times greater and much greater soil disturbance is required to ensure complete incorporation. 

Ammonia injected into the upper furnace is used to promote SNCR reactions to reduce emissions. The optimum temperature for... 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia... 

The ductwork shown in Figure 17.19 weighs 13 tons and contains guide vanes, static mixers, and the ammonia injection lances. 

FIGURE 17.19 Duct containing static mixers and ammonia injection lances for SCR. (With permission from Haldor-Topsoe, Inc.)... 

SCR is a process by which NO gases in diesel exhausts can be reduced to levels that will meet future legislation. SCR is based on the reduction of NO in the catalytic converter by the injection of ammonia or urea into the exhaust gases before they enter the catalytic converter, where NO and NHj react to form and H O. The ammonia injection process may be controlled by measuring either the ammonia or nitric oxide slip after the catalytic converter. Such an ammonia sensor should be able to tolerate contaminants such as particles in the exhaust gases and should show very low cross sensitivity to NO and HC. Typical diesel exhaust contains 3-9% CO 50-250 ppm CO, 6-12% O, 200-1,000 ppm NO, and 130-260 ppm HC. Furthermore, the response to NH3 should have a time constant in the order of 1 second. 

Utilities using post-combustion SCR-supported ammonia injection for NOx control as well as those using ammonia conditioning to improve electrostatic precipitator performance will produce fly ash that contains ammonia compounds. The ammonia is primarily physically adsorbed onto the fly ash particles as sulphate and bisulphate species. In many cases, the residual ammonia levels are quite low (<50ppm) however, elevated concentrations can occur as the catalyst ages or due to mechanical problems with the ammonia injection system. While elevated ammonia concentrations in fly ash do not negatively impact pozzolanic properties, it can reduce ash marketability due to odour concerns. For this reason, several processes have been developed to remove or reduce the amount of ammonia in fly ash. 

Fig. 32. Measured (symbols) and simulated (lines) NOx conversion and NH3 slip for (a) Mode 6 (385°C), (b) Mode 7 (285°C) and (c) Mode 10 (500°C) of the ESC cycle with progressively increasing ammonia injection rate.

Fig. 33. Plot of calculated fractional ammonia coverage for the three ESC modes at the front (1), middle (2) and rear (3) of the catalyst brick, with progressively increasing ammonia injection rate.

In cases in which the space charge across the electrical field was insufficiently intense for proper charge buildup, it was found that ammonia injection improved the capturability of the fly ash (14)- Other studies confirmed the applicability of sulfuric acid, ammonium sulfate and ammonium bisulfate as useful conditioners (15). 

Shimizu, T., Inagaki, M., and Furusawa, T. Effects of Sulphur Removal and Ammonia Injection on NO, Emission from a Circulating Fluidized Bed Combustor, in Circulating Fluidized Bed Technology III (Basu, P., Horio, M., and Hasatani, M., eds.), pp. 393-398. Pergamon Press, Oxford (1991). 

Upon leaving the reactor, the effluent enters a drtim where ammonia injection neutralizes the catalyst, forming salts subsequent water injection dissolves the salts removing them from the system. The scale of this washing operation is small because the catalyst concentration is low-- with only about 15 gpm of process water required for 1000 BPSD of isohexenes product. If required, a subsequent small treating step can be supplied with the unit to remove anions from the wash water. 

Turk, A., Sakalis, E., Lessuck, J., Karamitsos, H. and Rago, O., Ammonia injection enhances capacity of activated carbon for hydrogen sulfide and methyl mercaptan. Environ. Sci. Technol. 23 (1989) 1242. 

Prediction of the dynamic behavior of the catalyst is essential for controlling the ammonia injection in SCR systems applied to stationary diesel engines... 

Chu P., Downs B. and Holmes B., Sorbent and ammonia injection at economiser temperatures upstream of a high-temperature baghouse. Environmental Progress 9 149 (1990). 

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