Low Temperature Catalytic NOx Control under High Sulfur Environment
NOx is a well known hazardous gas which can not only form acid rain and destroy ozone, but also cause respiratory diseases of human body. There are multiple methods developed currently to remove NOx. Among them, selective Catalytic Reduction, SCR, represents the most widely used and efficient post-combustion technique for mitigation of NOx emissions from stationary combustion sources. The most common SCR process for coal-fired power plants is the high-dust (HD) configuration, in which the SCR catalyst is located between downstream of economizer and upstream of the precipitator or other particle collection devices and processes the full dust loading leaving the boiler. The reason for HD configuration is because of the desired reaction temperature of 500-700 °F at this configuration. However, HD configuration can cause significant catalyst deactivations by mechanism of masking, fouling, and poisoning. If a catalyst that could efficiently remove NOx at a relatively lower temperature 350 °F, the typical temperature downstream of particle devices, and can tolerate high concentration SO2 as well as H2O, then fast deactivation problems at HD configuration could be easily avoided. However, such a low temperature SCR catalyst development still remains a big challenge nowadays faced by the entire catalysis field.
Although the amount of flying ash from coal combustion is significantly reduced after the electrostatic precipitator (ESP), the low temperature makes a notably negative impact on the achievement of similar catalytic activity as that obtained at hot side. This type of catalyst deactivation is physical and catalyst can therefore be regenerated easily by simply washing or heating up to slightly higher temperature. Furthermore, the formation of ammonium sulfate or sulfite on the catalyst surface has a higher tendency at low temperature end than at hot end, which will cover the active sites of the SCR catalyst, resulting in the catalyst deactivation. In addition, the presence of high concentration water vapor will create more unfavorable influence on the maintenance of high activity at cold side than at hot side due to pore condensation, although its influence is reversible. If a chemical bond is formed between the active metal containing in the catalyst and adsorbed SOx, the formed metal sulfite or sulfate will permanently poison the catalyst and is very difficult to be removed from the catalyst surface even under high temperature.
Based on aforementioned challenges to do NOx removal in flue gas from high sulfur coal-fired power plant at low temperature, the conventional high temperature SCR catalyst is not suitable to address all the difficulties facing at cold side. It is critical to develop a new catalyst which can remain active under such harsh environment.
