Gas phase SO3, sulfuric acid vapor and sulfuric acid mist in flue gas are concerns for many utilities due to both an emissions and operational perspective. Hydrated Lime (Calcium Hydroxide) has been proven to be an effective alkaline reagent used to remove SO3 and H2SO4.

Hydrated Lime Benefits:

• Effective at removing SO3
• Low material cost compared to other alkaline reagents
• Hydrated lime designed of FGT applications has low NSR
• can reduce wet fgd reagent use
• Proven effectiveness
• Low capital cost & reliable material handling
• Dry injection systems are cost effective
• Proven, time tested material handling system available
• Robust product with good material handling characteristics
• Good availability
• Significant US capacity
• Local/regional production
• Supply chain is simple, short and reliable
• FGD system compatibility
• Injection can be performed at multiple system locations
• Excess reagent can be removed by the particulate control device or wet scrubber system
• Does not generate CO2 in the mitigation process
• Not soluble
• Compatible with fly ash sales requirements
• Solubility issues won't gum up injection equipment or injection ports
• Ease of handling

SO3 formation

SO3 in the flue gas is formed from the oxidation of SO2. This happens both in the combustion of sulfur containing coals and across the selective catalytic reduction (SCR) units. This SO3 vapor becomes a sulfuric acid vapor (H2SO4) as it combines with water in the flue gas. This vapor becomes a mist as it falls below the sulfuric acid dew point based on the temperature and acid concentrations.

SO3 and the resulting H2SO4 mist can create both emissions issues and operational challenges in a power generation facility. SO3 emissions and the resulting sulfuric acid vapor can result in visual plume opacity and this situation may increase the impact of plume touchdown. H2SO4 poses a significant corrosion risk on all back end equipment and operating changes to avoid this may be expensive.

A variety of options exist for operators to reduce SO3 levels in flue gas treatment systems. Switching to low sulfur coal, installing a wet ESP installations, optimizing catalyst performance, and injecting alkaline reagents are all potential options to reduce SO3 concentrations. Fuel switching can be costly, wet ESP installations require expensive capital and improved catalysts may not be fully effective.

Hydrated lime (calcium hydroxide) and other alkaline reagents are the preferred option for SO3 mitigation in many situations. High percentages of SO3 removal are possible with hydrated lime. The capital cost of a dry injection system is very low compared to other available SO3 mitigation options.

Hydrated Lime with the proper characteristics is an effective SO3 mitigation reagent with many benefits compared to other alkaline reagent options. Care must be taken to avoid selecting the proper hydrated lime for use in flue gas applications.

Hydrated Lime having the proper characteristics, is low cost compared to other alkaline reagents, is compatible across the FGT system and has multiple benefits including the potential to reduce wet-scrubber FGT reagent use.

Mississippi Lime is ready to assist in both product trialing and full system design and implementation.