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The semi-dry & dry lime processes have been developped to compete and offer an alternative to classical wet technology. The process development allow to offer currently similar removal efficiencies than wet systems (up to 98%) and consequently, to comply in most of the case to EU emission limits. The process is based on the reaction bewteen hydrated lime and the flue gases. The lime-based reagent is injected under the form of milk of lime (in the case of lime spray dryer or semi-dry process) or (humidified) powder (in case of dry processes). The flue gas remaining heat dries the reagent and the solid reaction product is collected by a downstream dedusting equipment (ESP or baghouse filter). High recirculation rate of this product in the process is key to reduce specific consumption as illustrated on the belows flowcharts.
Flowchart illustrates a Lime Spray Dryer system (LSD or semi-dry process) with by-products recirculation
Flowchart illustrates a Circulating fluidized bed FGD system (CFB or dry process) with by-products recirculation
Those processes, as all FGD processes, have some advantages and disadvantages.
Advantages of dry processes on classical wet technologies
The (semi-)dry FGD process has the following advantages when compared to wet limestone FGD technology:
The absorber vessel can be constructed of unlined carbon steel, as opposed to lined carbon steel or solid alloy construction for wet FGD. Typically, for small and medium units, the capital cost is lower than for wet FGD while for larger units, multiple module requirements causes the dry FGD process to be less attractive.
Pumping requirements and overall power consumption are lower than for wet FGD systems.
Waste CaSO3, CaSO4, and calcium hydroxide are produced in a dry form and can be handled with conventional pneumatic fly ash handling equipment.
The waste is stable for landfilling purposes and can be disposed of concurrently with fly ash.
The dry FGD system uses less equipment than does the wet FGD system, resulting in fixed, lower operations and maintenance (O&M) labor requirements.
Molecular weight of limestone higher than for lime. The logistic cost of the reagent delivery is thus much higher for comparable SO2 removal efficiencies.
The pressure drop across the absorber is typically lower than for wet FGD.
High chloride levels improve (up to a point), rather than hinder, SO2 removal performance.
Sulfur trioxide (SO3) in the vapor above approximately 140-150°C, which condenses to liquid sulfuric acid at a lower temperature (below acid dew point), is removed efficiently with a dry FGD + baghouse system. Wet limestone scrubbers capture less than 25% to 40% of SO3 and would require the addition of a wet electrostatic precipitator to remove the balance or hydrated lime injection. The emission of sulfuric acid mist, if above a threshold value, may result in a plume visible after the vapor plume dissipates.
Flue gas following a dry FGD is unsaturated with water (up to 10°C above dew point), which reduces or eliminates a visible moisture plume. Wet limestone scrubbers produce flue gas that is saturated with water, which requires a gas-gas heat exchanger to reheat the flue gas to operate as dry stack. Due to the high costs associated with heating the flue gas, recent wet FGD systems have used wet stack operations as well.
Dry FGD systems have the capability of capturing a high percentage of gaseous mercury in the flue gas if the mercury is in the oxidized form. Further, due to the nature of the filter cake present in the fabric filter associated with dry FGD, this configuration will tend to capture a higher percentage of oxidized mercury than would dry FGD equipment with an electrostatic precipitator. The major constituent that will influence the oxidation level of mercury in the flue gas has been identified as chlorine. Considering the typical level of chlorine in coals is thus also key to evaluate capture efficiency of Mercury.
There is no liquid waste from a dry FGD system, while wet limestone systems produce a liquid waste stream. In some cases, a wastewater treatment plant must be installed to treat the liquid waste prior to disposal. The wastewater treatment plant produces a small volume of waste, rich in toxic metals (including mercury) that must be disposed of in a landfill. A dry FGD system provides an outlet for process wastewater from other parts of the plant when processing residue for disposal.
Disadvantages of dry processes versus classical wet technologies
The dry FGD process has the following disadvantages when compared to limestone wet FGD technology:
For large units size, multiple absorber modules will be required. This will also result in large inlet and outlet ductwork and damper combinations.
The process uses lime instead of limestone-based FGD systems. The availability of this reagent as to be analyzed case-by-case and the reagent handling as to be adapted (e.g. Lime has to be stored in a steel or concrete silo).
Reagent utilization is slightly lower than for wet limestone systems to achieve comparable SO2 removals. The lime stoichiometric ratio is higher than the limestone stoichiometric ratio (on the same basis) to achieve comparable SO2 removals.
Dry FGD produces a waste, which does not have many uses (unlike gypsum). Nevertheless, new and local applications could be identified where the dry FGD waste can be used.
Combined removal of fly ash and waste solids in the particulate collection system precludes commercial sale of fly ash if the unit is designed to remove FGD waste and fly ash together. In some cases, FGD could be backfit after the existing electrostatic precipitator, which would allow the sale of fly ash.
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