FI113249B - Method for purifying the waste gases from a waste incineration plant - Google Patents

Description

113249

The present invention relates to a process for purifying the waste gases of a waste incineration plant by using dry or quasi-dry sorption by adding basic additives containing calcium to separate the acid impurities from the waste gas by forming a not too wet reaction product. In particular, the present invention relates to processes for purifying flue gases on a dry end product directly from the flue gas stream (dry method, apparent dry method, sprays adsorption method, see A. Kristensen et al., " Vol. 7, pp. 737-767, 1983, EF-Verlag fur Umwelttechnik, edited by Prof. Dr. J. Ing. Karl J. Thome-Kozmiensky, Berlin).

In the dry and quasi-dry cleaning of waste gases from the incineration of waste, alkaline additives containing calcium, such as calcium hydroxide powder or powdered lime milk, are formed. . hygroscopic CaCl2 with HCl. Process heat- • · ·. *; when the conditions are too low, this CaCl2 quickly becomes * * · [· overheated and starts to get wet, which can lead to cake formation or even stoppage of activity.

Therefore, in the treatment of flue gases in waste incineration plants using limestone, a high exhaust gas temperature is selected such as in the case of CaCl2 spray drying that the hydrated water content remains clearly below the saturation threshold for dry and moist CaCl2.

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Lella. For this reason, plants of this type * · ** 'are normally used at relatively high end-temperatures of * 150 to 180 ° C, particularly to protect the hose filters used to separate the product. 35 due to rotting and clogging of products, which are too moist, ···, and to ensure trouble-free product handling »» 2 113249 ly ·

On the other hand, however, this dry mode of operation results in increased additive consumption and / or a lower recovery rate of these harmful gases in the case of HC1, HF, SO2, and SO3 absorbers. The reason is the too low humidity of the flue gas and the reaction product, which is crucial for the recovery process. In particular, the chemical absorption of SO 2 and SC> 3 on limestone is dependent on the intermediate step whereby sulfuric acid and / or sulfuric acid is formed due to the reaction of these acidic gases with water.

At present, the increasing requirements for recovery rates and emissions in the conventional mode of operation in dry and / or quasi-dry methods, lead to 15 limestone at relatively high temperatures, to limestone consumption that is no longer economically acceptable, or to emission limits at all. In addition, as the amount of additive used increases, the amount of product to be destroyed increases significantly with respect to the precipitated calcium compounds, which is contrary to the claims. . minimizing residues.

»· · • * ·, *) Flue gases from waste incineration plants

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Depending on the operating conditions, the temperature varies between • 25 and about 200 - 300 ° C. In previous plant operation ... ”, the flue gas temperature is cooled by flushing • · · • V in the flue gas stream. Conventional temperatures in this range are from about 150 ° C to about 180 ° C, with the temperature being controlled by a control circuit that adjusts the water spray to a constant value in this range. The consequence of this is that. ···. the moisture content of the flue gas is too low at concentrations of water vapor below the average value of 12 - 14 vol.% and above this level it is too moist, especially when the waste is wet ... or when the humidity of the combustion air is high. :. ·. 35 high. The maximum range of flue gas humidity, ···, therefore constitutes a constant hazard in flue gas cleaning methods that are dry-finished and that are dependent on the dry but nevertheless highly reactive mode of operation in the end separator (tissue filter, electric filter).

Thus, DE 3 915 915 A1 discloses that good resolution can be achieved for harmful acidic gases in the operating temperature range of the purification plant to about 120-130 ° C. But at these temperatures, as mentioned above, there is always the risk that the filters 10 necessary for solids separation will become clogged.

Therefore, in the process described in DE 3 915 934 A1, chemical impregnation is carried out at temperatures above about 170 ° C, but with the addition of surfactants more reactive calcium hydroxide additives are used, thereby compensating for the higher temperatures and the lower moisture they cause. reduced resolution.

It is therefore an object of the present invention to provide a process for the purification of waste gas from waste incineration plants which can be used at lower temperatures than those conventional in the art and which. . regardless of the flue gas temperature and humidity • * · *) bones ensures optimum degree of separation of harmful substances and, on the other hand, avoids excessive moisture and harm caused by the products of the separation reaction. The optimum separation rate is achieved when the additive is added to the flue gas and the fly ash and harmful substances contained therein::: Receives moisture that favors chemical absorption, whereby harmful substances are removed from the flue gas as much as possible, and the absorbents used the isolated reaction product contains only a small amount of unreacted additives.

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The problem solved by the present invention is that the method comprises the steps of measuring temperature,. 35 water vapor concentration and volume flow at the outlet of the exhaust gas purification system, here calculates 113249 4 the amount of water required for evaporative cooling of the exhaust gas by adjusting the temperature to a preselected, substantially constant temperature of not more than 40 ° C or more, the water vapor content to a preselected value, essentially a constant range of not more than 25 volume percent below the water-vapor concentration as determined by the saturation curve of calcium chloride dihydrate, and an included amount of 10 water is introduced.

The CaCl2 formed in the reaction between limestone [Ca (0H) 2] or other calcium compounds such as CaO or CaCC3 and flue gas HCl is of great importance for the moisture of the separated reaction products since both its hydrates and its solutions are hygroscopic .

Figure 1 shows part of the temperature-humidity diagram of the CaCl2 * 2H20 phase system. Line D corresponds to a saturation curve of calcium chloride dihydrate, where the saturation curve defines the values for temperature and water vapor concentration in which the calcium chloride is present as the dihydrate. The area to the right of the saturation curve is calcium. . chloride contains less crystalline water and is dry, on the left side the calcium chloride begins to l1 be surrounded by free liquid water, becomes a * 25 · cm-1, and finally is present in the water vapor concentration .. !! * when growing as a saturated calcium chloride solution containing

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• ', · solid base body CaCl2 "2H20. This means that the heat:: temperature and water vapor content that must be controlled at the flue gas scrubber and are the result of the flue gas water vapor concentration in, after combustion and from the water vapor. added during the evaporative cooling must be to the right of the saturation curve of said calcium chloride dihydrate, as otherwise excess moisture and caking of the reaction products will result.

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However, as the moisture content of an alkaline additive (e.g., calcium hydroxide) increases, i.e., the water vapor content of the flue gas, it tends to well used benefits.

It has been found that the saturation curve of calcium chloride dihydrate determines the optimum working range with respect to temperature and water vapor concentration, not only absorbing HCl but also other harmful gases such as SO 2, SO 3 or HF.

These facts are explained by the following borderline cases:

Point A in Figure 1 indicates the temperature and humidity (absorbed water vapor content by volume) in the flue gas resulting from incineration of relatively dry waste with the addition of dry air. By spraying the water 20, cooling to a temperature favorable for chemical absorption of the harmful flue gas substances occurs, whereby. . the water vapor content increases due to the added water.

; | Therefore, after cooling off the exhaust gas temperature and water vapor concentrations, point C in Figure 1 is indicated.

• * *: 25 Point C is on the right side near the saturation curve of calcium chloride dihydrate, i.e. the calcium-containing reaction product precipitates relatively dry and is easy to::: separate.

If the composition of the waste changes, for example if 30 wetter waste is incinerated or if the supplied air contains * »* *, · * ·. As the humidity increases, the water vapor content and temperature of the flue gas released change (point A in the 'figure 1'), and in particular the water vapor content of the flue gas increases. If now purely by spraying with water, the adjustment to the same temperature T as before is achieved, ···, to C, to the left of the saturation of the calcium chloride hydrate »113249 6, i.e. the formed product containing calcium chloride is super-humidified.

In order to avoid this problem, the impregnation plant is conventionally maintained at a safe temperature range of 5 with respect to the saturation curve of calcium chloride dihydrate to come at Τ ', dat B and B '). Thus, in fact, a dry separation product is obtained which does not tend to clump and form a cake, but an increased addition of the additive is necessary due to the rise in temperature and low humidity, the plant 15 operates under conditions of reduced reactivity.

Water spraying results in almost Adia-based evaporative cooling. This adiabatic cooling is shown in the calcium chloride dihydrate temperature / humidity graph as a straight line having a negative gradient of decrease relative to the direction of the calcium chloride dihydrate saturation curve, as shown, for example, in straight lines. . lines E and E 'in Figure 1 and H, H' and H '' in Figure 2.

, *; Figure 2 illustrates the method of the present invention in a temperature / humidity diagram of calcium chloride dihydrate.

> · '25 Here the flue gas entering the purification plant is not cooled to a constant temperature T or Τ' (Figure 1), but added thereto, depending on the relevant flue gas inlet temperature::: and the incoming water vapor concentration (points F, F 'and F' ') the amount of water needed to lower the temperature of the waste gas 30 to a preselected value above the temperature defined by the saturation curve of calcium chloride dihydrate.

"'Since the temperature in the seemingly adiabatic process is directly related to the water vapor content of the waste gas, the weather:. In this case, the water vapor content is also determined to a predetermined value, * ·>, below the value defined by the saturation curve of calcium chloride dihydrate 113249 7. Starting from the given inlet temperature of the flue gas and the water vapor concentration (point F in Fig. 2) per saturation curve of calcium chloride dihydrate, a point G is obtained which is characterized by a predetermined temperature difference DELTA T and a difference of water vapor

As process conditions change, such as can occur when incinerating waste with varying humidity or as the moisture content of feed air varies, the flue gas inlet temperature and inlet humidity change very rapidly under certain conditions. The point F 'in Figure 2 (analogous to point A' in Figure 1) defines a state of flue gas having an increased humidity of 15 relative to F, F '' defines a state of elevated temperature relative to F '. The method of cooling the flue gas according to the present invention only injects water as needed to reach a point G 'or G1' that differs from the inlet conditions at point 20 with respect to temperature and water vapor concentration but takes into account substantially the same temperature difference Δ T and water vapor concentration Δ W kalsiumklorididihydraa-

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; ] versus the saturation curve.

In this way, even with various input parameters, the optimal process flow for the separation of harmful substances is achieved, characterized by the "action line" I, which is represented in the diagram by the saturation line of calcium chloride dihydrate. on the right hand side, and runs in one direction relative to the latter. This line 30 has all the values for which the flue gas temperature and cos, ··. the cooling is adjusted after cooling, as shown, for example, in Fig. 2 by the points G, G 'and G' '.

For an optimal degree of separation of the undesirable substances, it is preferable that the action line I runs at a minimum of 35 intervals from the saturation curve D of calcium chloride dihydrate. In practice, a value in the range of 1132498 ° C to 100 ° C and 0-40 vol. % in water vapor concentration, has been found to be recommended. Particularly preferred is the temperature difference in the range of 5 to 40 ° C and the difference in water vapor concentration in the range of 5 to 25% by volume, respectively. The value once preselected in the process of the present invention is maintained substantially the same, i.e., the distance of the action line I from the saturation curve of calcium chloride dihydrate remains constant as the inlet parameters of the flue gases change.

Since temperature and water vapor concentration are directly related through the CaCl2'H2O temperature / humidity diagram, it is equally possible to monitor the water vapor content of the flue gas as a set value and adjust it at a preselected distance relative to the saturation curve of calcium chloride dihydrate.

Therefore, in Figure 2, of the many parameter combinations (depending on the waste humidity, the mode of operation of the digester, the load of the digester as well as the contamination of the digester, the composition of the fuel), three selected examples show that both with the same inlet temperature; (F and F ') that inversely with different inlet temperatures • »·! ! la, but with the same inlet humidity (F 'and F' '), a very different amount of water must be sprayed for cooling • · · • 25 water into the evaporator cooler and / or spray absorber, and · .. then passing along the adiabatic line. to achieve a substantially constant moisture content of the product and therefore stable separation conditions.

Now that the various flue gas humidities and ·> · 30 flue gas temperatures, which depending on the composition of the waste, the load on the digester, the contamination of the digester • f or the humidity of the supplied air, may change to a greater or lesser degree quickly or slowly. to follow the entire field of parallel cooling: 35 curves, is the temperature of the flue gas and the water vapor • · i ». ···. • before 113249 9 va. As the amount of water needed for cooling also depends on the amount of flue gas passing through the plant, in addition to temperature and water vapor concentration, the volume flow of the flue gas must be measured as an input parameter.

5 Since the cooling curves run parallel and approximately directly over a wide range of temperature and water vapor contents, and since the saturation curve of calcium chloride dihydrate also runs linearly in the range conventional for waste incineration operations, n in the temperature / humidity diagram in Figure 3.

The parameters used in Figure 3 and in the following mathematical treatment have the following meanings: 15 D = saturation curve of calcium chloride dihydrate I = action line = function f (t) m = rising gradient of function f (t) b = ordinal path of f (t) K = angle constant 20 ti = flue gas entry temperature before cooling 12 = calculated flue gas temperature after cooling • · ·! ! wi = the incoming water vapor content of the flue gas before • »y cooling • W · = the calculated water vapor content of the flue gas after cooling * · · * * · · V '<Mathematical line: f (t) = m- 1 - b (1) •: · 30 Δ W = f (t) - w = d (2) sin β z = c --------- (3) sin y Δ t = z sin a ( 4) ^ \ 35 is placed in equation (3) (4): •> »* * 113249 10 sin β Δ t = c --------- sin a (5) sin y Equations (2) and (5): 5 sin β Δ t = [f (t) - wi] c --------- sin a (6) sin y Y = m - 90 ° - a 10 is placed equation (1) ( 6) to: sin (90 ° -m) Δ t = [m-ti-b-wi] ------------- sin a (7) sin (90 ° -ma) 15 sin (90 ° -m) -------------- sin a = K (= constant) sin (90 ° -ma) Δ t = ti - t2 20 t2 = ti - Δ t ( 8). . The coordinates of the destination point are obtained by locating • * »! ) equation (7) to equation (8) and by placing equation (9) equals- »* · / stroke (1): · 25 t2 = ti - [(m · ti-b-wi) -K] (9) W2 = m * t2 - b (10) t * »•» · • ·

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: Placing the above dependencies on a control plan is described below.

• h 30 A cascade rule,, ** was selected for this purpose. that works with the so-called "forward feed" •> principle. The "forward" signal is generated by the algorithm defined in equation (9) and reflects the calculated temperature. Cascade control for: 35 reasons that two controllers are connected in series. The first controller in the signal stream, ···, is named the controller 113249 11, and the next controller, respectively, is called the sequence controller.

In order to determine the actual amount of water to be fed, the difference between the calculated temperature and the 5 instantaneous temperature before the evaporator cooler is determined. This temperature difference is then multiplied by the volumetric flow rate of the flue gas and the constant which combines the heat capacities and the evaporation enthalpies.

The "back tune" signal consists of the difference between the calculated temperature 10 (according to equation (9)) and the temperature measured after the water spraying. This difference is corrected additively by the PI behavior (controller) to the temperature difference that cooling is intended to achieve. This ensures that the calculated / actual differences with optimized tuning parameter tuning and given tuning elasticity are very small.

The sequence controller then sends the amount of water thus determined to the next setting element.

The humidity measuring instruments located in the flue gas stream are monitored by a washability control in the form of a comparison of the water calculated using equation 9. · ,; vapor concentration and before and / or after the tissue filter • i »! ! measured water vapor concentration. The line of action I, which is then formed as a linear function (equation 1) in the CaCl 2 / H 2 O system, can be replaced by a parallel curve above the orb of the ordinate.

:, · Thus, it is possible to pre-select a safe distance from the saturation curve of calcium chloride dihydrate and set it manually depending on the requirements - '· 30 and the type of plant.

In addition, it is possible, instead of the calculated 1 t temperature, to express a fixed preselected temperature value, eg in the event of a measurement error. Selecting the maximum value for the calculated value · · 35 ensures that the fixed pre-set minimum temperature is not exceeded.

113249 12

Similarly, selecting the minimum amount of water to be fed prevents the feeding of too much water.

As a basic calcium-containing additive for sorption of harmful substances into the flue gas, any such additive conventionally used for this purpose can be added. Here, limestone, CaO and CaCC> 3 are preferred. Additives such as surfactants are possible to increase the degree of separation, but they are not necessary.

The calcium-containing additive is fed to the flue gas in a finely divided form following conventional procedures after cooling and / or adjusting the water vapor content. Optionally, the calcium compound may be added in dissolved and / or suspended form, eg as lime milk. Here, the additive solution itself can be used partially or completely for cooling and controlling the humidity of the flue gas. The amount of additive to be added must be measured to ensure safe and largely complete separation of the harmful compounds.

Because of the need for cooling and humidity control, ·; the creep water is calculated from the flue gas parameters its f i ·! on arrival at the purification plant (temperature, water vapor content, air flow and volumetric flow), virtually a check is made on the flue gas temperature and water vapor content after cooling, and optionally to adjust it to 4 · · • by spraying with more water.

'/ ·' · Flue gas volume flow measurement is no longer necessary as corrections need only be made to a known temperature 30 and / or a known water vapor concentration.

Figure 4 shows a sketch of a flue gas cleaning device suitable for the process of this invention.

The flue gas leaving the digester 1 is led to the reaction · *. 35 to 6 via channel 2. Before entering reactor 6, temperature 3, water vapor concentration 4, and volume 113249 13 flow 5 are determined. Flue gas passes from bottom to top in reactor 6. In the lower part, the flue gas is cooled by spraying 14 water through a suitable nozzle system. Then, spraying of basic calcium compounds 5 to 13 is performed, depending on the corresponding amount of harmful gases.

Optionally, the two steps (13 and 14) may also be combined and the spraying may be performed in the form of a suspension. After the flue gas leaves the reactor 6, it passes through a duct 7 to a dust separator 10, which is designed as a tissue filter, where the reaction products and chemicals added to the flue gas, other chemicals and fly ash are separated. The fan 11, which is above the dust separator, is used to overcome the total pressure drop in the plant, and it directs the purified flue gas through the smoke 12 to the atmosphere.

The amount of water required to cool the flue gas temperature to a temperature at a preselected temperature difference from the saturation curve of calcium chloride is calculated by calculating a calculated value 20 as described above, depending on temperature 3, water vapor concentrations 4 and 9 and subsequent temperature compensation. in unit 19 together with temperature 8, as well as. *! additional connection between flue gas flow rates 5 and 18.

The regulator 17 dispenses the required amount of water by means of a valve 15. The water volume measurement value 16 is sent as a true quantity to the control unit 17.

EXAMPLE • f i::: The process of the present invention is compared in the table below to conventional methods for controlling the humidity of process 30 in incinerators.

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Table

Departmental ABC

without adjustment with this batch adjustment according to invention 5

inlet temperature in km. 202 ° C km. 205 ° C km. 246 ° C

input moisture - - about 10-15t-%

exit temperature km.l47 ° C km.l39 ° C 128-134 ° C

exhaust moisture km.17.2 t-% km.16.9 t-% n.15.5-20 t-% 10 HCl in pure gas <16mg / m nt <12mg / m nt <5mg / m nt S02: a pure gas <91mg / m3nt <50mg / m3nt <10mg / m3nt product moisture * em2> km. 0.31 wt% km. 1.35% w / w 15 Cl ~ in e.m. km.17.4 wt% km.21 wt%

Note: * Measured as weight loss at 110 ° C in a dry chamber e.m. = unmeasured 20 km = average nt = normal mode • · t I t • I · i Conventional operating modes with fixed setting temperature * »* t; 'the farm led, for example, to the plant shown in the table; · '25' marked A or B during operation at somewhat reduced temperature during the test where the vessel had to be repeatedly closed because the filter hoses were clogged. As a result, a non-chemical safety distance had to be reintroduced, · 30 reactivity, and operating at 150 ° C.

In installations to be installed and which have to meet the limit values of Regulation 17 BlmSchV, operation at a preset high temperature for safety reasons is not possible because, to achieve these limits, acceptable limestone consumption relatively high flue gas humidity 113249 15 are required, and therefore relatively low operating temperatures in the range of about 120 ° C to 140 ° C, which, however, results in the operating problems described above, taking into account the inevitable fluctuations in the raw gas humidity.

Using the process of the present invention, it is possible to operate the flue gas purification plant at average temperatures lower than what is conventional in the art so far, since it is not necessary to maintain such a high temperature safety interval to avoid harmful clogging of separation products. On the contrary, the optimum humidity for chemical reactions can be adjusted in a simple manner while maintaining the technically controllable physical behavior (hardness, viscosity, caking tendency) of the solid reaction products. This method works by increasing the separation rate and minimizing the amount of residues in an environmentally sound and economical way and also leads to increased operational safety in incineration plants. In addition, other additives, e.g. 20 surfactants, are not required to increase the degree of separation.

; There is no need for the operation of this invention! Vita expensive rearrangements except installing the regulator. Due to the reduced consumption of the additive and the • good resolution at the same time, this method allows for the extremely economical use of the plant.

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Claims (6)

16 113249 A process for purifying the waste gases of a waste incineration plant using dry or quasi-dry sorption by adding basic additives containing calcium to separate the acid impurities from the waste gas by forming a not too moist reaction product containing calcium chloride, characterized in that the process comprises: - calculating the amount of water required for evaporative cooling of the exhaust gas by adjusting the temperature to a selected, substantially constant temperature range of not more than 40 ° C above the temperature determined by the addition of calcium chloride or to a value substantially constant, not more than 25 ti-20% by volume below the water vapor content The saturation curve of the chloride dihydrate is defined by, and: a calculated amount of water is introduced. i · 2. A method according to claim 1, characterized in that the temperature is preselected to a value between 5 and 40 ° C above the saturation curve of the calcium chloride dihydrate or the water vapor content is preselected to a value. , which is in the range of 2-25 vol.% below the saturation curve of calcium chloride dihydrate. 3. A method as claimed in claim 1, characterized in that the basic additive containing calcium is fed after adjusting the water vapor content. Process according to Claim 1, characterized in that the basic additive containing calcium is selected from the group consisting of lime, CaO and CaCO3. Method according to one or more of Claims 1 to 4, characterized in that the basic additive, 17,13249 containing calcium, is partially or completely dissolved or suspended in a quantity of water necessary for controlling the water vapor content. Method according to one or more of Claims 1 to 5, characterized in that the amount of water required to control the target temperature and the water vapor content is re-adjusted as a function of the exhaust gas temperature and the water vapor content as it exits the exhaust gas purification system. i «» • • * * • • • * I * »· •« * ♦ • t · • * • I »» »•» I I> ft »» 1β 1 13249