Problems in ammonia-based desulfurization and improvement methods

This post was last edited by That makes sense on 2019-8-28 00:19. 1. Ammonia escape: Ammonia escape refers to the volatilization of ammonia. The process parameters that affect the amount of ammonia that escapes include factors such as the temperature of the inlet flue gas, the pH value of the absorption liquid, its density, temperature, and the liquid-to-gas ratio. In the ammonia-based desulfurization of sulfuric acid exhaust gases, ammonia escape occurs in 4 stages; reducing or preventing ammonia escape also reduces the formation of aerosols. 1.1 Ammonia escape from ammonia tanks: Ammonia-based desulfurization uses ammonia (including ammonia water, liquid ammonia, and ammonium carbonate) as a raw material; due to the high safety requirements associated with liquid ammonia and the need to dissolve ammonium carbonate, ammonia water is generally used ； Using ammonium carbide solution or waste ammonia solution can reduce the operating costs of desulfurization. Ammonia water is an aqueous solution of ammonia gas; ammonia in it is highly volatile. By lowering the temperature and concentration of the ammonia water, with the w(NH3) level generally kept between 8% and 10%, ammonia escape from the ammonia tank can be effectively reduced. In a certain desulfurization unit, the ammonia tank was originally designed to be made of carbon steel and placed outdoors; the ammonia inlet pipe was not inserted into the ammonia tank. An ammonia water pump is used to add ammonia water to the absorption solution, and a return pipe is installed at the pump outlet; this return pipe is not inserted into the ammonia water either. A vent pipe is placed at the top of the ammonia water tank. These reasons result in a strong ammonia smell at the desulfurization site, with ammonia escaping from the ammonia tank vents. Later, it was modified to be made of fiberglass for use as an ammonia tank; a high-level tank was installed to utilize the pressure difference to add ammonia to the absorption solution. The vent pipe was removed, and a liquid seal was used, thereby solving the problem of ammonia leakage from the ammonia tank.

Even at an ammonia concentration of 8-10%, some ammonia will still escape; does this affect the appearance in any way?

This post was last edited by Yanzhi Youli on 2019-8-28 00:20. In the 1.2 circulation tank, (NH4)2SO3 and NH4HSO3, which are present in the ammonia escape absorption solution, are unstable substances that tend to decompose easily. A high temperature, high density, and high pH of the absorption solution can all cause the decomposition of (NH4)2SO3 and NH4HSO3, resulting in higher vapor pressures of NH3 and SO2 at the surface of the absorption solution, and thus higher ammonia escape from the liquid surface. In a certain desulfurization unit, the ammonia supply pipeline designed originally was not inserted into the absorption liquid. In the other desulfurization unit, although the ammonia addition pipeline is inserted into the absorption liquid, there is only one point for adding ammonia, which results in uneven mixing with the desulfurization liquid. This leads to excessively high pH values in certain areas of the absorption liquid – pH values well above 6, and sometimes even reaching 7 – causing more ammonia to escape from the surface of the liquid. Subsequently, the ammonia addition location was carefully selected, as were the locations for installing the pH meter and density gauge; the pH value was kept between 5.0 and 6.0, while the density of the absorption liquid was maintained between 1,080 and 1,160 kg/m3. A liquid seal was used to isolate the circulation tank at the bottom of the desulfurization tower from the exhaust gas, thereby solving the problem of ammonia escaping from the liquid level in the circulation tank.

The last edit to this post was made by That makes sense on 2019-8-28 at 00:22. Ammonia escape during the spraying process: The currently installed ammonia-based desulfurization units use counter-current air tower atomizing nozzles for spraying; to reduce the power consumption of the desulfurization circulation pumps, a lower liquid-to-gas ratio (1.5–2 L/m3) is chosen. Added to this are issues with process control: the absorption liquid has a high density and contains impurities, which cause scaling to block the nozzles and reduce the spraying density, resulting in gas short-circuiting within the tower. To ensure compliant emissions, the pH of the absorbent solution must be increased to 6.0–6.5, which raises the equilibrium partial pressure of ammonia and increases ammonia escape. On the other hand, the large number of mist droplets generated by the atomization nozzle spray facilitate the absorption reaction, but they also lead to increased ammonia escape. The improvement method involves using a packed tower to increase the liquid-to-gas ratio to 3–5 L/m3. By controlling the pH of the absorption solution between 5.0 and 6.0 and its density at 1080–1160 kg/m3, scaling is prevented and ammonia loss during the spraying process is reduced.

1.4 Ammonia escape during oxidation: The effluent from the tower contains (NH4)2SO3, NH4HSO3, and a small amount of (NH4)2SO4. (NH4)2SO3 and NH4HSO3 have unstable properties and will decompose again under certain conditions; therefore, it is necessary to oxidize (NH4)2SO3 and NH4HSO3 into the relatively stable (NH4)2SO4. The entire oxidation process is influenced by factors such as the concentration of the absorbing solution, temperature, the type and concentration of catalysts, and pH value. Since the oxidation process inside the tower results in a large amount of escaped ammonia and sulfuric acid aerosols in the exhaust gases, external oxidation is generally used. There are two processes for external oxidation in towers: ① Air (or compressed air) is blown into the oxidation tank using a Roots blower, and oxidation is carried out through micro-porous aeration ； ②The ejector oxidation process is employed. When Process ① is used, the Roots blower generates high noise, has high power consumption, is prone to failures, is difficult to maintain, results in low oxygen utilization, and the excess air emitted contains escaped ammonia ； Therefore, the ejector oxidation process is recommended. During the oxidation process, whether ammonia is added first or later, a small amount of NH3 and SO2 will escape. Therefore, the pH value needs to be maintained at an acidic level (pH=4.5~5.5) through automatic ammonia addition; this facilitates the oxidation reaction, reduces the oxidation time, and increases the oxidation rate.