WO1998039082A1

WO1998039082A1 - Process for eliminating pollutants from combustion gas

Info

Publication number: WO1998039082A1
Application number: PCT/EP1998/001005
Authority: WO
Inventors: Bernhard Thoene; Wolfram Klee; Hans Beisswenger
Original assignee: Metallgesellschaft Aktiengesellschaft
Priority date: 1997-03-06
Filing date: 1998-02-21
Publication date: 1998-09-11
Also published as: DE19709095A1

Abstract

A process is disclosed for eliminating pollutants from combustion gas from waste incineration plants and power plants. The combustion gas is treated with an aqueous Ca(OH)2 suspension in a spray absorber, then supplied to a solid separator. The process is characterised in that the Ca(OH)2 suspension consists of 5 to 23 % by weight Ca(OH)2, 15 to 44 % by weight of the loaded absorbent produced in the solid separator, and water, which makes up the remaining percentage, so that the Ca(OH)2 suspension contains maximum 50 % by weight solid matter. The process is also characterised in that the loaded absorbent is added to the Ca(OH)2 suspension before the suspension is introduced into the spray absorber. This process also provides for the combustion gas to be at least partially dedusted before being treated with the Ca(OH)2 suspension, and for the loaded absorbent added to the Ca(OH)2 suspension to be totally or partially replaced by the fly ash produced during dedusting.

Description

Process for the separation of pollutants from combustion gases

description

The invention relates to a method for separating pollutants from combustion gases from waste incineration plants and power plants, in which the combustion gases are treated with an aqueous Ca (OH) ₂ suspension in a spray absorber and then fed to a solids separator.

When solid and liquid fuels are burned in power plants and when household waste, special waste and sewage sludge are burned in waste incineration plants, not only is energy generated, but combustion gases are also produced in large quantities, which contain solid and gaseous pollutants that are separated as much as possible from the combustion gases before the exhaust gases can be released into the atmosphere. With the solid pollutants, which in the Combustion fumes are included, it is dusty ashes and heavy metal compounds. S0 ₂ , S0 ₃ , HC1, HF, N0 _X and CO are present as gaseous pollutants in the combustion gases. Metallic mercury, hydrocarbons, halogenated dibenzodioxins and halogenated dibenzofurans are also contained in the combustion gases as gaseous pollutants in small quantities. However, the dioxins and furans can be partially adsorbed on the dust particles, which are suspended in the combustion exhaust gases. The pollutant levels of the combustion exhaust gases depend on the composition of the fuels and the various types of waste, and they fluctuate within wide limits. For example, the combustion exhaust gases generated in power plants contain a lot of S0 ₂ (e.g. 2700 ppm) and little HC1 (e.g. 60 ppm), while waste incineration generally contains a lot of HC1 (e.g. 750 ppm) and little S0 ₂ (e.g. 290 ppm) arises. Since the combustion gases can contain different types of pollutants in different amounts and because the requirements for the purity of the combustion gases to be released into the atmosphere are different, in the past a variety of very different working methods for cleaning combustion gases have been proposed, all of which are optimal for the performance Separation of some pollutants under certain conditions.

The aim of the present invention is to provide a method for separating the pollutants S0 ₂ , S0 ₃ , HC1, HF, heavy metals and heavy metal compounds and halogenated dibenzodioxins and halogenated dibenzofurans, which is based on the known spray absorption with downstream solid separation. The spray absorption is a semi-dry one Exhaust gas purification process in which the sorbent, in particular Ca (OH) ₂ , is used as a solution or as a liquid suspension and the reacted sorbent is separated from the exhaust gas stream in a dry form (see H. Michele, Chem. -Ing. -Tech 56, 1984, pages 819 to 829). The aim of the invention is not the separation of NO _x , CO and hydrocarbons.

The cleaning of the combustion exhaust gases by spray absorption and subsequent solid separation is known and is used in practice. It is carried out in such a way that the dust-containing or the at least partially dedusted combustion exhaust gas enters a spray absorber in which a Ca (OH) ₂ suspension is sprayed and the water introduced with the suspension is evaporated. The calcium hydroxide reacts in particular with the acidic pollutants of the combustion exhaust gas to form CaS0 _{3 /} CaS0 _4; CaF ₂ and CaCl ₂ , whereby heavy metals, heavy metal compounds, dioxins and furans are partially physically attached to the dust particles. The dry absorbent loaded with the pollutants is suspended in the exhaust gas stream and leaves the spray absorber with the exhaust gas stream. The solids suspended in the exhaust gas are then separated off in a solids separator. The spray absorption has the disadvantage that it requires a considerable excess of calcium hydroxide, because only this excess of the absorbent can ensure the largely quantitative separation of the acidic pollutants. Malfunctions in spray absorption result in particular from the fact that there are temperature fluctuations in the area of the spray devices used for spraying the Ca (OH) ₂ suspension, which promote the formation of deposits on the spray devices and thereby the flow of the Prevent Ca (OH) ₂ suspension. Temperature fluctuations also frequently occur in the spray absorber, as a result of which the dew point is undershot and the solids leaving the spray absorber have a more or less high water content, which considerably hinders the subsequent solids separation by causing in the solids separator

Corrosion problems or the formation of very solid solid deposits on the filter fabric.

The object of the invention is to avoid the known disadvantages of spray absorption and to improve their operational safety. In particular, it should be achieved that the spray absorption with a

Pollutant-Ca (OH) ₂ ratio of 1: 1.2 to 1.4 works reliably, that the loaded absorbent is separated in dry, free-flowing form and that in addition to the acidic pollutants, the heavy metals, heavy metal compounds, dioxins and furans are largely quantitative and reliably separated from the combustion exhaust gases, whereby even at high inlet concentrations of the gaseous pollutants HC1 and S0 ₂ very low final concentrations of these pollutants in the clean gas must be observed, namely <10 mg HCl / Nm ³ and <50 mg S0 ₂ / Nm ³ .

The object on which the invention is based is achieved in that the Ca (OH) ₂ suspension consists of 5 to 23% by weight of Ca (0H) ₂ , 15 to

44% by weight of the loaded in the solids separator

Absorbent and the rest of the water is that the

Ca (OH) ₂ suspension has a maximum solids content of 50 wt .-% and that the loaded absorbent of

Ca (OH) ₂ suspension before introducing the suspension into the

Spray absorber is added. It has been surprising It has been shown that the viscosity of the Ca (OH) ₂ suspension is significantly reduced if it contains 15 to 44% by weight of the loaded absorbent obtained in the solid separator. This means that even suspensions with a high content of Ca (OH) _{2 are} relatively thin and can therefore be sprayed easily in the spray absorber using a spray device, although their solids content is high. In addition, the partial return of the loaded absorbent to the spray absorber improves the utilization of the Ca (0H) ₂ , because the solid that leaves the spray absorber still contains unused absorbent. In addition, the Ca (0H) Cl contained in the loaded absorbent forms both CaCl ₂ and free Ca (0H) ₂ when the loaded absorbent is returned to the Ca (OH) ₂ suspension, as a result of which the pollutant Ca (OH) ₂ Ratio is advantageously shifted to low values. Finally, it has been shown that heavy metals, heavy metal compounds and halogenated dibenzodioxins and halogenated dibenzofurans are adsorbed largely quantitatively on the solids, which is probably due both to the good adsorption capacity of the fly ash and to the unburned carbon content of the recycled solids. If the recycled solid has an unburned carbon content of> 2% by weight, the heavy metals, dioxins and furans are separated quantitatively, and no additional measures are required to separate these pollutants. The Ca (OH) ₂ suspension must have a maximum solids content of 50% by weight so that it can be evenly distributed over the spray disk in the spray absorber. The solids content - ie the content of Ca (OH) ₂ and the solids returned to the spray absorber - may not be more than 50% by weight. The recycled solids also surprisingly improve the evaporation behavior of the Ca (OH) ₂ suspension, so that fine-particle, dry particles are formed in the spray absorber, which can be separated from the exhaust gas stream without difficulty in the solids separator.

According to the invention it is further provided that the combustion exhaust gases are at least partially dedusted before treatment with the Ca (OH) ₂ suspension and that the loaded absorbent, which is added to the Ca (OH) ₂ suspension, is wholly or partly by the at the fly ash that has been dedusted is replaced. Surprisingly, it has been shown that the fly ash very strongly reduces the viscosity of the Ca (OH) ₂ suspension and also adsorbs some of the heavy metals, heavy metal compounds, dioxins and furans.

According to the invention, it has proven to be expedient for the dedusting to be carried out in a cyclone and / or an electrostatic filter. These process devices can be handled easily and are very reliable.

According to the invention, it has furthermore proven to be expedient for the Ca (OH) ₂ suspension to contain a surfactant in an amount of 0.1 to 1% by weight, the addition of the surfactant before the suspension is introduced into the spray absorber he follows. The surfactant has the task of further reducing the viscosity of the Ca (OH) ₂ suspension. Nonionic detergents are particularly suitable as surfactants. According to the invention, it is particularly advantageous if the suspension is ground in a colloid mill after adding the loaded absorbent and before introducing it into the spray absorber. This creates a particularly fine suspension, which is distributed very evenly in the spray absorber and which is very reactive.

According to the invention it is provided that the Ca (OH) ₂ suspension is introduced into the spray absorber at a temperature which is above the dew point of the combustion gases entering the spray absorber, the Ca (OH) ₂ suspension used for separating pollutants from waste combustion gases at a temperature> 55 ° C and the Ca (OH) ₂ suspension used for separating pollutants from power plant emissions is introduced into the spray absorber at a temperature> 45 ° C. This procedure ensures that no deposits are formed on the spray disc nozzles, which preferably consist of CaS0 ₄ x 2H ₂ 0.

According to the invention, the waste incineration gases enter the spray absorber at a temperature of 200 to 250 ° C and the power plant gases at a temperature of 130 to 170 ° C. They are cooled there by the water introduced with the Ca (OH) ₂ suspension or by the evaporation of this water to such an extent that the temperature in the spray absorber during the removal of pollutants from waste gas from the combustion of waste gases is at least 60 ° C above the dew point of the exhaust gases and at

Pollutant separation from power plant emissions is at least 10 ° C above the dew point of the exhaust gases. In the method according to the invention, the spray absorber can thus advantageously be operated at a temperature that is significantly above the dew point the combustion exhaust gases. This also suppresses the formation of Ca (OH) Cl and promotes the formation of CaCl ₂ , which ultimately results in low Ca (OH) ₂ consumption. According to the invention, it has proven to be expedient for the temperature in the spray absorber to be 120 to 170 ° C. for pollutant separation from waste gas from waste incineration and 70 to 90 ° C. for pollutant separation from power plant exhaust gases. It has been found that the working temperature of the spray absorber should be at least 10 ° C. above the dew point when cleaning combustion gases with an HC1 content of <60 ppm, while it is still higher for combustion gases with a higher HCl content is to be raised above the dew point. The term "dew point" here means the dew point of the water.

Finally, according to the invention it is provided that an electrostatic filter or a fabric filter is used as the solid separator. As is known, the fabric filter ensures a very reliable separation of the solids, and the solid layer (filter cake) that forms on the fabric filter also acts as an absorption or adsorption agent against the pollutant residues in the combustion exhaust gases. The tissue filter is cleaned periodically using known methods. For safety reasons, the loaded absorbent which leaves the fabric filter should still have a Ca (OH) ₂ content of about 3% by weight.

According to the invention, it is expedient if a scrubber is connected downstream of an electrostatic precipitator used as a solid separator, in which the combustion exhaust gases with a treated aqueous solution containing NaOH and / or NaHC0 ₃ . The scrubber has the task of chemically binding residual acidic pollutants and removing the solid particles not separated by the electrostatic filter from the exhaust gas stream. This is because the electrostatic filter does not achieve the high solids separation rates of the fabric filters and the possible separation of residual gaseous pollutants on the fabric filter. The washing solution loaded with pollutants is fed to the spray absorber via a separate line. It is not possible to add the washing solution to the spray absorber together with the Ca (OH) ₂ suspension via the spray device (spray disc), since CaS0 ₄ crystals are formed in the nozzles of the spray device and block the nozzles.

It is known that fly ash contains a more or less large amount of unburned carbon. In the process according to the invention it has now been shown that the unburned carbon, which either enters the spray absorber through the loaded, recycled absorbent or through the fly ash added to the Ca (OH) ₂ suspension, increases the cleaning performance of the process according to the invention, because the unburned carbon acts particularly against heavy metals, heavy metal compounds, dioxins and furans as adsorbents and against S0 ₂ as an oxidation catalyst. It has been found that the sulfur removed from the combustion gases is largely present in the loaded absorbent as calcium sulfate and that the adsorption of heavy metals, heavy metal compounds, dioxins and furans is favored by a content of unburned carbon. The object of the invention is explained in more detail below with reference to the drawing and an exemplary embodiment.

The drawing shows the flow diagram of the method according to the invention.

Household waste is burned in the waste incineration plant (1). The combustion exhaust gases from the waste incineration plant (1) are first cooled to a temperature of approx. 210 ° C on heat exchanger surfaces, which is not shown in the drawing, and then reach the spray absorber (3) at this temperature via line (2). The combustion exhaust gases contain fly ash, which has an unburned carbon content of approximately 4 to 5% by weight. A Ca (OH) ₂ suspension is fed to the spray absorber (3) via the line (14) and consists of 5% by weight Ca (OH) ₂ , 30% by weight of the loaded absorption medium and in the fabric filter (5) The rest of the water is there. The suspension is sprayed in the spray absorber (3) through a spray disc, not shown in the drawing, and has an inlet temperature of 57 ° C. The water contained in the suspension evaporates in the spray absorber (3), resulting in a temperature of approx. 150 ° C. This working temperature is far above the dew point of the combustion gases, so that the solid particles formed in the spray absorber (3) have a water content of <1% by weight. The gas-solid suspension leaves the spray absorber (3) via line (4) and is passed to a fabric filter (5) which is designed as a bag filter. The combustion exhaust gases freed from the fly ash and the gaseous pollutants are fed via line (6) into a chimney (not shown in the drawing) and released into the atmosphere. The part of the loaded absorbent that is already separated in the spray absorber (3) passes through the line (16) into the Vorra sbunker (8). The fabric filter (5) is cleaned periodically, and the part of the loaded absorbent obtained is fed via line (7) into the storage bunker (8), from which the majority of the loaded absorbent is removed via line (15) and then deposited. Part of the loaded absorbent is fed via line (9) into the mixing device (10), to which water is fed via line (11) and a concentrated Ca (OH) ₂ suspension is fed via line (13) Lime extinguishing system (12) is generated from CaO and water. A nonionic surfactant can be added to the water carried in line (11) in an amount of approximately 1% by weight. In the mixing device (10), the Ca (OH) ₂ suspension required to carry out the process according to the invention is produced with a solids content of 35% by weight, the viscosity of which (according to Brookfield) is 1190 mPa-s and which therefore is easily introduced into the spray absorber (3) can be introduced. The suspension passes through the line (17) into the colloid mill (18) and then via the line (14) into the spray absorber (3).

The following table contains the process parameters of the process according to the invention shown in the drawing. The table shows that the process according to the invention has a very good cleaning performance with regard to the gaseous and solid pollutants. The method according to the invention can also be operated without interruption in the case of fluctuating combustion performance of the waste incineration plant (1), since the quantity entered into the spray absorber (3) via the line (14) Ca (OH) ₂ suspension can be easily regulated via the HC1 or S0 ₂ content of the pure gas and the dew point in the spray absorber (3) is never undercut. The entry temperature of the Ca (OH) ₂ suspension into the spray absorber (3) is determined by the heat content of the concentrated Ca (OH) ₂ suspension produced in the limescale extinguishing system (12) and / or by the heat content of the loaded absorbent and / or by a arranged in the line (14) arranged additional heater. The carbon content of the fly ash ensures that the loaded absorbent has a comparatively high sulfate content, because in particular the unburned carbon is responsible for the catalytic oxidation of S0 ₂ to S0 ₃ by the oxygen contained in the combustion exhaust gases. In addition, both the heavy metal content and the dioxin and furan content of the combustion exhaust gases in the spray absorber (3) were largely separated, which is attributed in particular to the presence of the unburned carbon.

It is possible to at least partially dedust the combustion exhaust gases carried in line (2), which is expediently carried out in an electrostatic filter. Part of the fly ash separated in the electrostatic filter can be fed to the mixing device (10), and there it replaces part of the recycled, loaded absorption medium. The fly ash causes a significant reduction in the viscosity of the Ca (OH) ₂ suspension, which is particularly the case with high

Ca (OH) ₂ concentrations is very advantageous because concentrated calcium hydroxide suspensions have a high viscosity, which leads to considerable problems when they are conveyed. The method according to the invention thus opens up the possibility of reducing the amount of water used to carry out the spray absorption by introducing the loaded absorption medium or the fly ash into the Ca (OH) ₂ suspension, as a result of which the dew point in the spray absorber is undershot and the problems that arise as a result can be avoided.

A partial recirculation of the flue gas, which takes place via the line (19) provided with a blower, can reduce the oxygen content of the flue gas, which is normally approximately 11%, to 6 to 8%, which increases the economy of the method according to the invention .

Should the method according to the invention shown in the drawing result in a breakdown of the spray absorber (3) and / or the fabric filter (5), it is possible to discharge the combustion exhaust gases in the waste incineration plant (1) into a scrubber, which is operated with a washing liquid containing NaOH and / or NaHC0 ₃ . In this washer, the combustion exhaust gases are cleaned of solid and gaseous pollutants during the brief shutdown. In the event of a brief shutdown, the waste incineration plant (1) does not have to be switched off. The washing liquid, which is loaded with pollutants during the shutdown, is collected in a collecting container and, after the malfunction has ended, is introduced into the spray absorber (3) via a separate line, care being taken that the washing liquid and the Ca (OH) ₂ suspension supplied to the spray adsorber (3) separately be otherwise undesirable deposits in the common supply line and in the nozzles of the spray device.

Nm = m exhaust gas at 1 bar and 20 ° C = standard cubic meter

Claims

claims

1. A process for the separation of pollutants from combustion gases of waste incinerators and power plants, wherein the combustion gases with an aqueous Ca (OH) treated ₂ suspension in a spray absorber, and then a solids are fed, characterized in that the Ca (OH) ₂ - Suspension of 5 to 23 wt .-% Ca (0H) ₂ , 15 to 44 wt .-% of the im

Solid absorbent accumulating absorbent and the rest of water is that the Ca (OH) ₂ suspension has a maximum solids content of 50% by weight and that the loaded absorbent is added to the Ca (OH) ₂ suspension before the suspension is introduced into the spray absorber becomes.

2. The method according to claim 1, characterized in that the combustion gases before treatment with the

Ca (OH) ₂ suspension are at least partially dedusted and that the loaded absorbent, which is added to the Ca (OH) ₂ suspension, is completely or partially replaced by the fly ash obtained during dedusting.

3. The method according to claim 2, characterized in that the dedusting takes place in a cyclone and / or an electrostatic filter.

4. The method according to claims 1 to 3, characterized in that the Ca (OH) ₂ suspension contains a surfactant in an amount of 0.1 to 1 wt .-%, the addition of the surfactant before the introduction of the suspension in the spray absorber is done.

5. Process according to claims 1 to 4, characterized in that the suspension is ground in a colloid mill after the addition of the loaded absorbent and before being introduced into the spray absorber.

6. The method according to claims 1 to 5, characterized in that the Ca (OH) ₂ suspension is introduced into the spray absorber at a temperature which is above the dew point of the combustion exhaust gases entering the spray absorber.

7. The method according to claim 6, characterized in that the Ca (OH) ₂ suspension used for separating pollutants from waste incineration plants with a temperature> 55 ° C and the Ca (OH) ₂ suspension used for separating pollutants from power plant emissions with a temperature> 45 ° C is introduced into the spray spray.

8. The method according to claims 1 to 7, characterized in that the waste incineration gases with a temperature of 200 to 250 ° C and the power plant gases with a temperature of 130 to 170 ° C enter the spray absorber.

9. The method according to claims 1 to 8, characterized in that the temperature in the spray absorber at the pollutant separation from waste gas at least 60 ° C above the dew point of the exhaust gases and at

Pollutant separation from power plant emissions is at least 10 ° C above the dew point of the exhaust gases.

10. The method according to claim 9, characterized in that the temperature in the spray absorber in the pollutant separation from waste incineration gases from 120 to 170 ° C and in the pollutant separation from power plant exhaust gases is 70 to 90 ° C.

11. The method according to claims 1 to 10, characterized in that an electrostatic filter or a fabric filter is used as a solid separator.

12. The method according to claims 1 to 11, characterized in that an electrostatic filter used as a solid separator is followed by a scrubber in which the combustion gases are treated with an aqueous solution which contains NaOH and / or NaHC0 ₃ .