Mercury Emission Control Technologies for PPL Montana-Colstrip Testing

Abstract

The Energy & Environmental Research Center (EERC) was asked by PPL Montana LLC (PPL) to provide assistance and develop an approach to identify cost-effective options for mercury control at its coal-fired power plants. The work conducted focused on baseline mercury level and speciation measurement, short-term parametric testing, and week long testing of mercury control technology at Colstrip Unit 3. Three techniques and various combinations of these techniques were identified as viable options for mercury control. The options included oxidizing agents or sorbent enhancement additives (SEAs) such as chlorine-based SEA1 and an EERC proprietary SEA2 with and without activated carbon injection. Baseline mercury emissions from Colstrip Unit 3 are comparatively low relative to other Powder River Basin (PRB) coal-fired systems and were found to range from 5 to 6.5 g/Nm3 (2.9 to 3.8 lb/TBtu), with a rough value of approximately 80% being elemental upstream of the scrubber and higher than 95% being elemental at the outlet. Levels in the stack were also greater than 95% elemental. Baseline mercury removal across the scrubber is fairly variable but generally tends to be about 5% to 10%. Parametric results of carbon injection alone yielded minimal reduction in Hg emissions. SEA1 injection resulted in 20%more » additional reduction over baseline with the maximum rate of 400 ppm (3 gal/min). Week long testing was conducted with the combination of SEA2 and carbon, with injection rates of 75 ppm (10.3 lb/hr) and 1.5 lb/MMacf (40 lb/hr), respectively. Reduction was found to be an additional 30% and, overall during the testing period, was measured to be 38% across the scrubber. The novel additive injection method, known as novel SEA2, is several orders of magnitude safer and less expensive than current SEA2 injection methods. However, used in conjunction with this plant configuration, the technology did not demonstrate a significant level of mercury reduction. Near-future use of this technique at Colstrip is not seen. All the additives injected resulted in some reduction in mercury emissions. However, the target reduction of 55% was not achieved. The primary reason for the lower removal rates is because of the lower levels of mercury in the flue gas stream and the lower capture level of fine particles by the scrubbers (relative to that for larger particles). The reaction and interaction of the SEA materials is with the finer fraction of the fly ash, because the SEA materials are vaporized during the combustion or reaction process and condense on the surfaces of entrained particles or form very small particles. Mercury will have a tendency to react and interact with the finer fraction of entrained ash and sorbent as a result of the higher surface areas of the finer particles. The ability to capture the finer fraction of fly ash is the key to controlling mercury. Cost estimates for mercury removal based on the performance of each sorbent during this project are projected to be extremely high. When viewed on a dollar-per-pound-of-mercury removed basis activated carbon was projected to cost nearly $1.2 million per pound of mercury removed. This value is roughly six times the cost of other sorbent-enhancing agents, which were projected to be closer to $200,000 per pound of mercury removed.« less