By Ryan Hensel, SCR Product Line Manager, Babcock & Wilcox
With the increased popularity of biomass and other renewable fuels in the generation portfolio, new emissions control challenges are emerging, especially for selective catalytic reduction (SCR) systems, where biomass can have much higher concentrations of catalyst poisons – most notably sodium, potassium, and phosphorus – than other fuels. The elevated levels of these catalyst poisons results in an increase in required catalyst volume, as well as a decrease in the expected catalyst life, reducing the duration between outages. In order to maintain an acceptable operating period between outages, it may be beneficial to locate the SCR on the “clean side,” downstream of the particulate collector.
This low dust arrangement lessens the exposure of the catalyst to ash, which decreases the potential of plugging the catalyst, eases concerns of any unburned carbon in the ash sintering the catalyst, and mitigates catalyst deactivation due to detrimental constituents in the ash. It also allows other catalyst poisons that precipitate at lower flue gas temperatures to be collected by the particulate collection device prior to the catalyst and no longer pose a threat to deactivation of the catalyst. Removing ash prior to ammonia injection also eliminates concerns over ammonia being absorbed by the ash and released after disposal.
The minimum ammonia injection temperature for an SCR is a function of the SO2 and SO3 concentrations in the flue gas and the SO2 to SO3 oxidation across the catalyst bed. The more SO2 and SO3 present, the higher the minimum ammonia injection temperature. A typical coal-fired, high dust or “dirty side” SCR is located upstream of the air heater, where the flue gas temperature exceeds the minimum ammonia injection temperature to prevent ammonium salt formation. A typical clean side SCR is located where the operating temperature is below the minimum ammonia injection temperature, which means the SO3 in the flue gas can react with ammonia to form ammonium sulfate or ammonium bisulfate (ABS), the latter of which can block the active sites of the catalyst. Therefore, in a clean side SCR system, SOx mitigation is likely required.
On a recent project that fired wood biomass, a lower sulfur fuel with relatively low variability, B&W utilized a dry sorbent injection (DSI) system in combination with the fabric filter to achieve SOx concentration in the flue gas low enough for the SCR to operate at temperatures in the range of 400-450°F. On a project that fired municipal solid waste, a higher sulfur fuel with much higher variability, utilizing only a DSI system with the fabric filter would have resulted in SOx concentrations consistent with a minimum ammonia injection temperature of nearly 600°F, which would require a significant amount of re-heating of the flue gas. To reduce the amount of re-heat required, B&W located the SCR downstream of the flue gas desulfurization (FGD) equipment so the SO2/SO3 in the flue gas was minimized. In this case, utilizing a spray dryer absorber (SDA) in combination with the fabric filter removed more than 90 percent of the SO2 and almost all of the SO3 in the flue gas, lowering the minimum ammonia injection temperature to the desired range of 400-450°F.
Maintaining flue gas temperature in the proper operating range can be accomplished several different ways. Where DSI/fabric filter can reduce the SO2/SO3 concentration sufficiently, the economizer surface could be modified to remove surface prior to the DSI/fabric filter/SCR and surface added downstream of the SCR. This increases the inlet temperature to the SCR without sacrificing boiler efficiency. If the fuel’s sulfur content requires an FGD system before the SCR, the SCR inlet temperature can be increased by adding a heater to the SCR inlet fluework. If this additional heater is selected, given the operating costs associated with re-heating the gas prior to the SCR, it’s typical to include a gas to gas heat exchanger (GGH) near the SCR reactor. This GGH uses the heated gas leaving the SCR to help heat the flue gas entering the SCR, greatly reducing operating cost.
These are only a couple of scenarios for SCR flexibility in the wide world of fuels in use today. For each unique opportunity, there is an equally unique solution to achieve reliable emissions compliance and optimize operating costs.