DE102008029937A1 - Flue gas cleaning involves contacting flue gas with quicklime-based reagent for deposition of acidic flue gas components at predetermined temperature - Google Patents

Abstract

Flue gas cleaning involves contacting flue gas (1) with quicklime-based reagent (4) for deposition of acidic flue gas components at 140-300[deg] C, preferably 150-240[deg] C, particularly 160-180[deg] CC. The processed gas is contacted with sodium-based reagent (6) for performing further purification process.

Description

The The invention relates to a method for flue gas cleaning, wherein flue gas for the separation of in particular acidic flue gas components in at least a first flue gas purification stage with a lime-based Reactant is brought into contact. The present invention refers in particular to the purification of flue gases that during the incineration of household or special waste as well as in conventional wood and biomass power plants are released. Further fields of application are the cleaning of flue gases, the in process furnaces or in the processing of metal, Glass or ceramics are released.

The in the aforementioned applications released flue gases included in addition to aerial dusts as further undesirable Ingredients usually acidifying gases, heavy metals and organic trace substances such as dioxins and furans. To the legal Maintaining conditions are for the separation of particular acidic gas constituents, such as sulfur oxides and hydrogen halides, known from the prior art method, in which the flue gas is contacted with a lime-based reactant.

Naßwäschen with limestone suspensions reach high degrees of separation for acid-forming gases, but are more heavy metal in the deposition Fine dust only limited efficient; Dioxins and Furans are deposited insufficiently. About that In addition, the cost of investment and ongoing Operation high.

In spray sorption plants lime suspension is finely atomized in a spray absorber and brought into contact with the hot flue gas. The water content of the suspension evaporates and the flue gas components react with the sprayed lime. With spray sorption systems, adequate, medium degrees of separation can be achieved with low stoichiometric factors. The stoichiometric factor is defined as the molar ratio of spent hydrated lime (CA (OH) ₂ ) to crude gas mass flow of hydrogen chloride (HCl), hydrogen fluoride (HF) and sulfur dioxide (SO ₂ ) per unit time. The situation is different for higher degrees of separation of more than 90% HCl and over 75% SO ₂ . In this case, the additive consumption increases disproportionately, ie the degree of utilization and availability of the flue gas cleaning system drops significantly. This applies particularly to waste incineration plants, hazardous waste incineration plants and other plants with strongly fluctuating raw gas parameters or frequent startup and shutdown operations.

Dry Scrubbing are technically and energetically less complicated procedures. they draw characterized by high flexibility compared to raw gas fluctuations out. Prerequisite for acceptable low additive consumption is the use of highly active reagents and process optimization. As lime-based reactants come adapted Products based on high-surface hydrated lime, conventional Hydrated lime and limestone powder for use, optionally together with absorbent materials such as activated carbon, lignite coal dust or hearth furnace coke. Through a mixture of lime-based reagents with lignite coal dust or activated carbon it is possible Dioxins and furans and heavy metals to a high degree.

In addition, known from the prior art, the so-called MKT process, which has the advantages of spray sorption, a relatively simple process technology, a relatively low additive consumption with limitation of HCl or SO ₂ separation rates and dry residues, with those of the conditioned dry sorption , which is characterized by even lower plant and control effort, high flexibility, high reliability and applicability of additive mixtures. In MKT process, a pre-cleaning of the flue gases in a temperature-controlled spray absorber, wherein a lime hydrate is used as a lime-based reactant. In a second step, the residual purification of the acid-forming gases as well as the deposition of dioxins / furans (PCDD / PCDF) or heavy metals, such as. As mercury, by blowing a high-surface hydrated lime. From the prior art lime-based reactants with the trade name Sorbacal (formerly WÜLFRAsorp) of Reinkalk GmbH, a company of the Lhoist Group, known. Lime hydrate can also serve as a base material for mixed sorbents, which in addition to the reduction of acid-forming pollutants allows the deposition of ecotoxic substances, such as heavy metals and dioxins / furans.

All methods described above have in common that for the neutralization of the acid-forming pollutants (HCl, HF, SO ₂ ) are used substantially lime products, namely limestone (CaCO ₃ ), fine lime (CaO) or hydrated lime (Ca (OH) ₂ ). For the separation of ecotoxic pollutants, activated carbon or lignite dust are advantageously used as a mixture with the required lime-based sorbents.

A disadvantage of the flue gas cleaning lime-based reactants is the limited separation efficiency of sulfur oxides, especially at high hydrogen halide concentrations in the flue gas. Moreover, the previously described flue gas cleaning process by a high Consumption of the lime-based reaction agent characterized, which leads to high operating costs in the flue gas cleaning. As a rule, the flue gas is cooled during the cleaning, which leads to a reduction in the output from the purified flue gas heat.

In addition, it is known from the prior art to use sodium bicarbonate (NaHCO ₃ ), also called sodium bicarbonate, in powder form as a reagent for the purification of gases from acidic compounds. Sodium hydrogencarbonate is used in particular for the purification of exhaust gases from sulfur oxides, nitrogen oxides and hydrogen halides. In these applications, the gas to be purified is contacted with the sodium bicarbonate in the form of a finely ground powder. A disadvantage of the use of sodium bicarbonate as a sodium-born reactant is compared to kalkstämmigen reagents significantly higher raw material price, which leads to correspondingly high operating costs in the flue gas cleaning.

task The present invention is a method of the aforementioned To provide kind, which is characterized by a small Additive consumption in flue gas cleaning and the possibility distinguishes, a larger heat flow decouple from the flue gas. Moreover, the procedure should a very high separation of particular acidic smoke gas components and compliance with very low residual concentrations of the deposited Ensure flue gas components in the cleaned flue gas.

to Solution of the above objects is in a method provided of the type mentioned that the flue gas in at least one of the first flue gas purification stage downstream second Flue gas purification stage with a sodium-derived reaction agent is brought into contact. The invention proposes at least two-stage flue gas purification process, wherein in a first Step a lime-based reactant for pre-separation is used in particular by acidic flue gas components. At the first stage is followed by a second flue gas purification stage for the fine or final cleaning of the flue gases, wherein a sodium-derived Reactive agent is used. In the neutralization of the acidic flue gas components arise sodium compounds, such as sodium chloride, Sodium sulfate, sodium fluorite or sodium carbonate by filtration can be separated from the flue gases.

Although sodium-based reactants have higher raw material costs compared to lime-based reactants, the pre-separation of acidic flue gas components in the first flue gas purification stage leads to low consumption of the sodium-born reactant in the second flue gas purification stage, which has a negative effect on the operating costs of the flue gas cleaning. Incidentally, very low clean gas concentrations of the acidic flue gas constituents can be achieved with sodium-based reactants. In principle, it is even possible to completely separate off the acidic flue gas constituents with a correspondingly higher superstoichiometric addition of the sodium-containing reaction agent. The inventive method, it is possible, a concentration of hydrogen chloride in the purified flue gas after the second flue gas purification stage of less than 10 mg / m ³ , a pure gas concentration of hydrogen fluoride less than 1 mg / m ³ and a pure gas concentration of sulfur dioxide of less than 50 mg / m ^{3 to} ensure. In addition, the optimal reaction temperatures in the second flue gas purification stage when using sodium-based reactants are above the reaction temperatures which are usually provided when using lime-based reactants. This allows the extraction of smoke gas heat at a higher temperature level, with a comparatively larger heat flow can be coupled and used. As a result, the use of sodium-based reactants in the second flue gas purification stage allows for a very economical separation of acidic gas constituents from the flue gas while maintaining legal conditions.

The inventive method allows it that in addition to the first flue gas purification stage or alternatively as an alternative to the first flue gas purification stage, a direct separation of flue gas components takes place at the place of origin, for example by injecting a kalkstämmigen reagent in a boiler of a firing plant. The addition of the additive, preferably lime or limestone flour, may be in different places Combustion: either already before burning through Adding to the fuel, by co-injecting with the combustion air into a combustion chamber or by separate introduction in the combustion chamber.

Furthermore It should be noted that the lime-based Reagent used in the first flue gas purification stage is, in addition to Kalkbestandteilen further with respect to a physical and / or chemical reaction with the gas components to be separated may contain reactive components.

In a preferred embodiment of the invention sodium hydrogen carbonate is used as the sodium-born reactant. sodium hydrosulfide Gencarbonate effectively neutralizes the acidic components of the flue gas. The resulting in the neutralization of the acidic flue gas components sodium compounds, in particular sodium chloride, sodium sulfate, sodium fluoride and sodium carbonate can be separated as sodium-containing reaction products by filtration of the flue gas. In principle, it is also possible to use, in addition to or as an alternative to sodium hydrogencarbonate, anhydrous sodium carbonate in the form of a powder having a large specific surface area, whereby the decrease in the reactivity of anhydrous sodium carbonate having a large specific surface area to acid gases can be prevented by a special handling treatment, as it is in the DE 699 08 554 T2 is disclosed.

In the DE 699 08 554 T2 For example, an anhydrous sodium carbonate powder is described which comprises a desiccant and / or is maintained during its handling in an atmosphere having a relative humidity of 0 to 7%. The desiccant and said atmosphere have the function of preventing aging of the anhydrous sodium carbonate powder, which is characterized by a reduction in the reactivity of the sodium carbonate to decompose the acidic gases, in particular hydrogen halides (especially hydrogen chloride and hydrogen fluoride), sulfur oxides ( in particular of sulfur dioxide) and of the nitrogen oxides (in particular of nitrogen oxide).

The Treatment of the flue gas with the sodium-born reactant can preferably be done by dry method. In this case, the reagent introduced in the form of a solid powder in the gas, preferably in the flow stream, and in the absence of a liquid, in particular Water. The powdered reactant is in the flue gas stream injected inside a reaction chamber or conduit circulated. The inventive method leaves in principle however also a half wet treatment of the Flue gas to, wherein the reactant in the gas in the presence such amount of fluid (usually water) is introduced that these in contact with the Gas is completely evaporated. In the treatment semi-wet way, the reactant is generally in the form one dispersed in a liquid, generally in water Powder used. However, according to the invention preferred treatment by dry route.

Becomes the sodium-born reactant in the flow stream in the Flue gas introduced, so is before introduction into the flue gas a milling of the reagent to a particle size from 10 to 50 μm, preferably from 20 to 40 μm, intended. This results in a high reactivity of the reagent ensured in the second flue gas purification stage.

As has already been pointed out at the outset leaves it is the use of a sodium-born reactant, especially sodium bicarbonate, to, the operating temperature in the second flue gas purification stage compared to when used lime-based reaction medium permissible operating temperature to increase. In the neutralization of the acidic components by sodium bicarbonate finds a thermal activation instead, with sodium bicarbonate on contact with hot Flue gases thermally activated and quickly to sodium carbonate with large specific surface and porosity is converted. By the conversion of sodium bicarbonate "activated" sodium carbonate occurs in the second flue gas purification stage including the adsorption of Heavy metals as well as dioxins and furans. However, sodium carbonate is highly hygroscopic. To ensure a high degree of separation, is therefore below the dew point in the second flue gas purification stage by a sufficiently high operating temperature possible excluded. Preferably, in the inventive Process therefore provided that the sodium-derived reactant at a temperature of 140 to> 300 ° C, preferably from 150 to 240 ° C, in particular from 160 to 180 ° C, is brought into contact with the flue gas. The high Allow reaction temperatures in the second flue gas purification stage the extraction of flue gas heat at a high temperature level and thus the decoupling of a comparatively large energetically usable flue gas heat flow.

The The invention further preferably provides that the sodium-derived Reactant at a stoichiometric ratio from 1.0 to 1.5, preferably 1.3, in contact with the flue gas is brought. In this case, high separation efficiency for acid-forming gases from the flue gas at low Equipment costs and low in the second flue gas purification stage be ensured residual amounts. The stoichiometric Ratio describes the ratio of used sodium-derived reactant to the crude gas mass flow the acidic flue gas constituents, namely hydrogen chloride, Hydrogen fluoride and sulfur dioxide, per unit time.

The sodium-derived reactant can be brought into contact with the flue gas free of additives. Pure sodium hydrogencarbonate is preferably used for flue gas purification in the second flue gas purification stage. In principle, however, it is also possible that the sodium-containing reactant together with at least one further reactant in the second Flue gas purification stage is used, wherein preferably before a coal-containing reaction agent may be provided, more preferably an activated carbon or coke-containing reactant. Here, mixed products based on sodium bicarbonate and activated carbon or lignite coal dust can be used. In addition to the reduction of acid-forming pollutants, ecotoxic substances such as heavy metals, dioxins and furans can be separated in the second flue gas purification stage.

to Pre-separation may be wet scrubbing in the first flue gas purification stage the flue gas can be provided with a limestone suspension. Preferably However, in the first flue gas purification stage, a spray sorption takes place with a Kalkmilchsuspension, as in the MKT process of Case is. Finally, in the first flue gas purification stage also a dry sorption with a lime-containing reaction medium be provided. The invention also leaves it to, the pre-separation of acidic flue gas components and, where appropriate the separation of ecotoxic substances, such as heavy metals, Dioxins and furans, multilevel with a lime-derived Reactant and optionally at least one other non-lime Reactants, preferably with activated charcoal or lignite charcoal dust, perform. Also it is possible in the first Flue gas cleaning stage a spray sorption with a lime milk suspension and then a dry sorption with a kalkhydathaltigen Provide reactant. In addition, mixed products from lime-based reactants, such as hydrated lime or limestone flour, and adsorbents such as Activated carbon or lignite coal dust / Herdofenkoks be used. Preferably the pre-separation of acidic flue gas components takes place lime-based reagents but one-stage.

The Reaction temperature in the first flue gas purification stage or the Temperature of the flue gas at the exit from the first flue gas purification stage may be at least 150 ° C, preferably between 160 ° C up to 180 ° C, amount. The temperature of the flue gas at Exit from the first flue gas purification stage should preferably the Temperature when entering the second flue gas purification stage, to heat losses caused by intercooling of the flue gas between the first flue gas purification stage and the second flue gas purification stage to reduce. In particular, this is in terms of thermal decoupling Of flue gas heat of advantage. It is understood that the Pre-separation of acid gas components in the first flue gas purification stage basically also at higher operating temperatures can take place, especially in the deposition of flue gas components by spray absorption with a milk of lime suspension.

Of the Separation efficiency of hydrogen halides in the first flue gas purification stage should be about 80 to 90%, while the degree of separation of Sulfur oxides in the first flue gas purification stage approx. 30 to 50%. These degrees of separation require the addition of the lime-based reaction agent in the first flue gas purification stage with a certain stoichiometric factor, where the Grain size distribution, the specific surface or the pore volume of the lime-based reactant on the one hand and the operating temperature of the pre-separation on the other hand are taken into account. In addition, the purity the lime-based reagent of importance. By Addition of activated carbon or Herdofenkoks can additionally also heavy metals as well as dioxins and furans up to very strict Emission limit values are deposited.

Between the first flue gas purification stage and the second flue gas purification stage and / or after the second flue gas cleaning stage, a dust separation be provided, wherein the deposition with at least one fabric filter, a cyclone or an electrostatic filter can take place.

The inventive method is below With reference to the single figure of the drawing described, wherein the Invention not on the schematically illustrated embodiment is limited.

In the figure is a schematic of a method for flue gas cleaning of flue gas 1 shown when burning waste in a boiler 2 a waste incineration plant is released. The moisture content of flue gases from waste incineration plants is between 10 and 16% by weight, while, for example, the moisture content of flue gases from industrial furnaces may be between 6 and 30% by weight. The flue gas 1 contains flue dust as further undesirable components acidifying gases, heavy metals and organic trace substances such as dioxins and furans. In order to minimize residual emissions, a two-stage flue gas purification process is provided, wherein in a first purification stage 3 a lime-based reactant 4 with the flue gas 1 is brought into contact, in order to come to a pre-separation of acid-forming gases and optionally heavy metals and organic trace substances such as dioxins and furans. Preferably, the first flue gas purification stage 3 designed as Trockensorptionstufe, wherein lime hydrate, optionally in a mixture with lignite coal dust or activated carbon, is used as the lime-based reaction medium.

The flue gas 1 leaves the first flue gas purification stage 3 at a temperature of about 160 to 180 ° C and then enters a second flue gas purification stage 5 one. In the second flue gas purification stage 5 is the addition of a sodium-born reactant 6 provided for the concentration in particular of hydrogen chloride, hydrogen fluoride and sulfur dioxide in the flue gas 1 continue to lower. The sodium-born reactant 6 is added to the flue gas before adding 1 in the second flue gas purification stage 5 ground to a particle size of about 20 to 40 microns. This ensures high reactivity. As a sodium-born reactant 6 Preferably, sodium bicarbonate is used, which is characterized by a high deposition rate and reactivity, with very low residual concentrations of the deposited impurities in the flue gas 1 after passing the flue gas purification stage 5 possible are. The sodium-born reactant 6 is in a dry state in powder form in the flow stream into the flue gas 1 injected. This will be an almost complete reaction of the sodium-born reactant 6 enabled with the gas components to be separated.

As the reaction temperature in the second flue gas purification stage 5 essentially the reaction temperature of the first flue gas purification stage 3 corresponds, so no intermediate cooling of the flue gas 1 between the two purification stages 3 . 5 is provided, is a residual heat utilization of the flue gas 1 possible at a high temperature level, the flue gas 1 first a filter 7 and then a heat exchanger 8th flows through. In the heat exchanger 8th becomes a heat flow 9 from the flue gas 1 decoupled at a temperature level of about 160 to 180 ° C and provided for thermal utilization. Subsequently, the purified flue gas 1 as a clean gas stream 10 discharged through a fireplace.

At the dust collector 7 it can be a fabric filter, a cyclone or an electrostatic precipitator. It is not shown in detail that before the second flue gas purification stage 5 Pre-dedusting can be provided, with a fabric filter, a cyclone or an electrostatic filter can be used for the dedusting. In this case, a large part of the fly ash and in the first flue gas purification stage 3 formed reaction products from the flue gas 1 deposited.

Due to the two-stage nature of the illustrated process, in which in the first flue gas purification stage 3 a pre-separation of acidic flue gas components using a relatively inexpensive lime-based reactant 4 and in the second flue gas purification stage 5 a fine cleaning using a highly reactive sodium-derived reactant 6 is provided, an economic and the legal framework appropriate flue gas cleaning is possible. In particular, the clean gas concentrations required for the gas constituents hydrogen chloride, hydrogen fluoride and sulfur dioxide required in accordance with the 17th BImSchV can be safely met or even exceeded. At the same time, the extraction of heat from the flue gas stream 1 possible at a comparatively high temperature level. By using a low-cost reagent 4 in the first flue gas purification stage 3 and the downstream fine cleaning in the second flue gas purification stage 5 can the additive consumption in the second flue gas purification stage 5 and thus the total operating costs of the flue gas cleaning are reduced. If the process shown is designed as a dry process, no wastewater is produced. Thus, no wastewater treatment is required, which in turn has a cost-reducing effect on the flue gas cleaning. As a result, the illustrated method allows an economical and the legal framework corresponding flue gas cleaning. The illustrated method can be used for example in waste incineration plants as an independent flue gas cleaning or in combination with the existing flue gas cleaning. Also when using locally accumulating high-calorific fuels in energy recovery plants or when using biomass, such. As waste wood, released flue gases provide a field of application for the method described.

It is not shown, moreover, that the addition of a lime-based reaction agent, preferably limestone flour, to the boiler 2 can be provided to integrate pollutants already in the furnace.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• - DE 69908554 T2 [0015, 0016]

Claims (15)

Process for flue gas cleaning, whereby flue gas ( 1 ) for the separation of, in particular, acidic flue gas constituents in at least one first flue gas purification stage ( 3 ) with a lime-based reagent ( 4 ), characterized in that the flue gas ( 1 ) in at least one of the first flue gas purification stage ( 3 ) downstream second flue gas purification stage ( 5 ) with a sodium-born reactant ( 6 ) is brought into contact. Process according to claim 1, characterized in that as a sodium-born reactant ( 6 ) Sodium bicarbonate is used. Process according to Claim 1 or 2, characterized in that the sodium-containing reaction agent ( 6 ) in a dry state in the flow stream into the flue gas ( 1 ) is introduced. Method according to one of the preceding claims, characterized in that the sodium-containing reaction agent ( 6 ) before introduction into the flue gas ( 1 ) is ground to a particle size of 10 to 50 microns, preferably from 20 to 40 microns. Method according to one of the preceding claims, characterized in that the sodium-containing reaction agent ( 6 ) at a temperature of 140 to greater than 300 ° C, preferably from 150 to 240 ° C, in particular from 160 to 180 ° C, with the flue gas ( 1 ) is brought into contact. Method according to one of the preceding claims, characterized in that the sodium-containing reaction agent ( 6 ) at a stoichiometric ratio of 1.0 to 1.5, preferably 1.3, with the flue gas ( 1 ) is brought into contact. Method according to one of the preceding claims, characterized in that the sodium-containing reaction agent ( 6 ) free of aggregates with the flue gas ( 1 ) is brought into contact. Method according to one of the preceding claims, characterized in that the flue gas ( 1 ) in at least one smoke gas purification stage ( 3 . 5 ) is contacted with a coal-based reactant, preferably with activated charcoal or a coke-containing reactant. Method according to one of the preceding claims, characterized in that in the first flue gas purification stage ( 3 ) a wet scrubbing with a limestone suspension is provided. Method according to one of the preceding claims, characterized in that in the first flue gas purification stage ( 3 ) a spray absorption with a milk of lime suspension is provided. Method according to one of the preceding claims, characterized in that in the first flue gas purification stage ( 3 ) a dry sorption with a kalkhydrathaltigen reagent is provided. Method according to one of the preceding claims, characterized in that the temperature of the flue gas ( 1 ) at the exit from the first flue gas purification stage ( 3 ) is at least 150 ° C, preferably more than 160 ° C to 180 ° C, is. Method according to one of the preceding claims, characterized in that the degree of separation of hydrogen halides in the first flue gas purification stage ( 3 ) is about 80 to 90%. Method according to one of the preceding claims, characterized in that the degree of separation of sulfur oxides in the first flue gas purification stage ( 3 ) is about 30 to 50%. Method according to one of the preceding claims, characterized in that between the first flue gas purification stage ( 3 ) and the second flue gas purification stage ( 5 ) and / or after the second flue gas purification stage ( 5 ) Dust separation is provided.