SO3 and mercury: The plot thickens
The fact that SO3 impedes mercury capture by native flyash and is detrimental to the performance of activated carbon injection (ACI) systems for mercury removal will become increasingly important as mercury control becomes more stringent. Part 2 of this series presented data from four SBS Injection installations that confirm that reducing the level of SO3 in flue gas to very low levels dramatically increases the capture of mercury by native flyash.
The effect also was observed during recent pilot work conducted by URS in conjunction with Southern Research Institute and EPRI at Southern Company's Mercury Research Center at Plant Crist in Pensacola, Fla. Evidence is emerging from a number of independent sources on the detrimental effect of SO3 on both the mercury removal efficiency and carrying capacity of ACI technology. Both the mercury removal rate and the carrying capacity of activated carbon are enhanced as the flue gas SO3 concentration and temperature are decreased. Removing SO3 upstream of the air heater (allowing the air heater to extract more heat from the flue gas) does more than just lower a unit's heat rate and significantly improve the efficiency and economics of mercury removal via ACI. It also reduces the unit's CO2 emissions and the volume of flue gas downstream of the air heater. Indeed, in the near future the beneficial effect of SO3 reduction on mercury removal may alone justify the use of the process.
It is not the intent of this article to imply that SBS Injection technology is the only way to achieve quick, efficient removal of SO3 upstream of the air heater. However, it is the express goal of the R&D efforts by Codan Development and URS Corp. to demonstrate that the SBS process is a viable means of accomplishing this reduction and delivering its many and very valuable benefits.
Small step, big benefits
Power plants have invested hundreds of millions of dollars in equipment, and they annually spend millions of dollars to control emissions of particulates, SO2, and NOx. Yet the presence of very small amounts of SO3 can cause the stack plume to exhibit a visual discoloration that can extend for many miles (Figure 5). Elevated SO3 concentrations can even reduce the buoyancy of a plume to the extent that it touches down.
The effective removal of SO3 will eliminate several adverse health, environmental, and aesthetic consequences of sulfuric acid aerosol emissions and, in the process, produce significant O&M advantages for operators of coal-fired power plants. This strategy will benefit the public, electric utilities, and the coal industry by further reducing the environmental "footprint" of coal-fired power generation while simultaneously enhancing its efficiency and economics.
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Robert E. Moser is a principal and co founder of Codan Development LLC. He can be reached at
fgdmoser@aol.com.
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