Avoided Silo Modifications in a Dry Scrubber System

摘要

Dry sorbent injection (DSI) has been developed as a viable option for air pollution control. The mitigation of SO_2 and other acid gasses can be achieved at relatively low capital costs, making the technology an attractive retrofit option. As this technology is implemented on a larger (power plant) scale, certain flow issues have arisen with the resultant sodium / fly ash byproduct. Sodium sulfate is the result of the SO_2 gas and sodium bicarbonate (SBC) powder reaction in the duct, as expressed in Equation 1. Eq. 1: NaHCO_3 + SO_2 → Na_2SO_4 A full scale DSI system was installed at the Northeastern Power Plant Unit 3. The sorbent is injected downstream of an electrostatic precipitator (ESP) and upstream of a pulse jet fabric filter (PJFF). Northeastern is a coal burning 460 MW unit located near Oologah, Oklahoma. It is owned by Public Service Company of Oklahoma; the parent company is American Electric Power. The byproduct is collected via the PJFF. The PJFF employs heated inverted pyramidal hoppers to collect the byproduct shed from the fabric bags above. From the heated hoppers, the byproduct drops down through an air lock to a pneumatic conveying line. From thence it is pneumatically conveyed to the byproduct silo. The byproduct is mainly sodium sulfate (98%+) combined with a small amount of fly ash (<2%). System problems arose at start up when the aerated byproduct silo would not discharge effectively in order to fill trucks used to transport the byproduct to the landfill. This restriction limited the DSI operation since continuing to fill a silo was not desirous while problems with emptying it existed. It should be noted that no flow issues were reported at the PJFF. Laboratory and field tests revealed that the sodium sulfate in the byproduct silo would not fluidize effectively. Initial laboratory testing of the byproduct conducted after system start up indicated a relationship between temperature and flowability. Testing indicated that at lower temperatures, the byproduct cohesive strength would be reduced allowing the material to discharge easier from a bin. However, since at no point was proper fluidization achieved, a proposal was submitted to modify the silo bottom from flat, aerated bottom to a conical shape. At this point in the project, additional resources were requested to review the situation. Sodium sulfate has several transition temperatures that can affect the crystalline structure of the compound. This paper will discuss AEP's efforts to identify the transition temperature (and hence crystalline structure) and its impact on fluidization. This paper will discuss the results of the laboratory scale testing and the full scale implementation of said findings. This paper will also discuss the hygroscopic nature of sodium sulfate and its impact to system design. Testing indicated that the material flow problems were not primarily due to moles of hydration or free water. Rather, the crystalline structure of the material impacted the ability of the material to fluidize and flow. Because of the research conducted, system retrofits were limited to those needed to achieve and maintain the bulk byproduct temperature above a key crystalline transition temperature. Implementation of these relatively minor changes avoided much more costly modifications to the silo design.