SO3 Measurement for Mercury Control


Federal regulations covering the capture and removal of mercury from generating station stack gas take effect in 2010. An increasing number of states are accelerating that deadline, some as early as 2008. Significant industry research suggests that injection of powdered activated carbon (ACI) into the flue gas prior to the particulate collection device (ESP/FF) will allow utilities to meet the near term regulations.

Figure 1The level of mercury removal, while site specific, is generally related to the amount of carbon injected. The amount injected is usually defined in terms of pounds (of ACI) per million actual cubic feet of flue gas (Lb/MMacf).

Figure 1 is from an EPRI presentation on mercury capture in ESPs using both standard ACI and chemically pre-treated ACI. Today this generally means pre-treatment with bromine. Both ADA-ES and Sorbent Technologies provide this type of technology.

A review of the graph shows that mercury removal rates of 35% – 55% can be achieved with ACI and 55% – 75% with brominated ACI at an injection rate of 5 Lb/MMacf. These are fairly broad ranges, but give some insight into what is generally expected with activated carbon technology.

Figure 2 shows the results of ACI injection with high levels of SO3 (sulfuric Figure 2acid) present in the gas stream. This data is from a published ADA-ES demonstration at Mississippi Power’s Plant Daniel. While the source of the SO3 is an ESP Flue Gas Conditioning system, the results are typical of a gas stream with high SO3. Note that at 5 Lb/MMacf only a 22% removal was achieved and the max level seems to plateau at around 32%.

What is happening is this: the sulfuric acid (SO3) is highly reactive and competes with the mercury for the active sites on the carbon particle. The higher the SO3 level, the lower the effectiveness of activated carbon for the capture of mercury.


Placement of the AbSensor-SO3 probe downstream of the air heater, or the SO3 injection system, provides direct measurement of flue gas sulfuric acid level. Once the level of SO3 is known, a control process can be implemented to keep its level at a minimum.

High levels of native SO3 can be found in the following situations:

  • Plants burning high sulfur coal
  • Plants burning oil or petroleum coke
  • Plants with SCRs using older catalyst

In any of these situations, the native SO3 may be high enough to preclude ACI as a viable means for mercury control. The solution is to inject an SO3 sorbent after the air heater, but ahead of the ACI injection point. This will allow the sorbent to lower the SO3 levels to a point suitable for ACI. The probe is used to provide a feedback for controlling the sorbent injection. The reasons for the control include:

  • Reduction of Sorbent usage costs (sorbents typically cost between $75 and $150/ton delivered. Proper control can cut the usage levels in half.
  • Reduction of sorbent based ESP load. Marginal ESPs may not be able to handle the extra particulate loading. Proper control can keep the ESP viable.
  • Reduction of sorbent based duct and perf plate fouling.

Where high native SO3 is the problem, the probe should be placed after the air heater, and either before or after the sorbent injection point depending on plant preference.

Flue Gas Conditioning is required to keep the ESP operational in low sulfur fuel environments. However, with an SCR in operation, the levels of native SO3 increase. Also, depending on flue gas temperature and ash resistivity, the level of injected SO3 required will vary.

Each site will required specific testing to determine exactly what level of FGC based SO3 injection is required for ESP performance after ACI is employed. Once that level is known, the AbSensor probe can provide feedback to the FGC skid to keep the sulfuric acid level in control.

In this application, the probe should be placed after the flue gas conditioning but before the activated carbon injection point.

Site testing:

Because the dynamics between SO3, activated carbon and mercury capture are site specific it is important that any plant considering activated carbon injection execute a thorough study of localized SO3 levels. Additionally, recent guarantee statements from the suppliers of ACI systems require that SO3 levels be maintained below a specified level. Currently, the AbSensor-SO3 system is the only available, reliable, method of on-line, automatic detection of flue gas SO3.