DE102010001859A1 - Exhaust air purification system useful for eliminating formaldehyde from a corresponding contaminated exhaust gas stream of a biogas combustion device - Google Patents

Abstract

Exhaust air purification system for eliminating formaldehyde from a corresponding contaminated exhaust gas stream of a biogas combustion device (10), is claimed. The exhaust gas stream coming from the combustion device is introduced directly into a chamber of a belt drying device (20), further passed through the belt drying device as processing air, and withdrawn at the end as drying exhaust air. The drying exhaust air is conducted at least via first and second gas purification systems (40, 50) formed as wet scrubbers, in which at least ammonia and formaldehyde is washed out respectively. Exhaust air purification system for eliminating formaldehyde from a corresponding contaminated exhaust gas stream of a biogas combustion device (10), is claimed. The exhaust gas stream coming from the biogas combustion device is introduced directly into a chamber of a belt drying device (20), further passed through the belt drying device as processing air, and withdrawn at the end as drying exhaust air. The drying exhaust air is conducted at least via a first gas purification system (40) formed as a wet scrubber, in which at least ammonia resulting in the drying exhaust air during the drying process is washed out, and is conducted at least via a second gas purification system (50) formed as the wet scrubber, in which at least formaldehyde present in the drying exhaust air emanating from the exhaust gas is washed out.

Description

The present invention relates to an exhaust air purification system for the elimination in particular of formaldehyde from a correspondingly loaded exhaust gas stream of a biogas incinerator.

Biogas is a combustible gas produced by fermenting biomass in biogas plants and used to produce bioenergy.

• – vergärbare, biomassehaltige Reststoffe wie Klärschlamm, Bioabfall oder Speisereste;
• – Wirtschaftsdünger (Gülle, Mist);
• – bislang wirtschaftlich ungenutzte Pflanzen bzw. Pflanzenteile (z. B.
• Zwischenfrüchte, Pflanzenreste, etc.); sowie
• – gezielt angebaute Energiepflanzen (Nachwachsende Rohstoffe wie insbesondere Mais, Getreide, Gras, Klee etc.).

Suitable starting materials for the technical production of raw biogas are in particular:

• - fermentable, biomass-containing residues such as sewage sludge, biowaste or leftovers;
• - manure (manure, manure);
• - hitherto economically unused plants or plant parts (eg
• Catch crops, plant remains, etc.); such as
• - Targeted energy crops (Renewable resources such as corn, cereals, grass, clover, etc.).

With the exception of the latter option, apart from transport and other ancillary costs, these are basically free starting materials.

Raw biogas before its preparation is a water-saturated gas mixture with the main components methane (empirical formula: CH ₄ ) with an average of 60%; Carbon dioxide (empirical formula: CO ₂ ) with an average of 35% and vaporous water (empirical formula: H ₂ O) with an average of 3.1% share of the gas composition. Most traces of nitrogen are also present (empirical formula: N ₂ ) with an average of 1%; Oxygen (empirical formula: O ₂ ) averaging 0.3%; Hydrogen (empirical formula: H ₂ ) averaging less than 1%; Ammonia (empirical formula: NH ₃ ) with an average of 700 mg / m ³ and hydrogen sulfide (empirical formula: H ₂ S) with an average of 500 mg / m ³ share of the gas composition.

Just like biogas, landfill gas and sewage gas are produced during the anaerobic decomposition of organic material called fermentation or digestion. Therefore, these gases are occasionally summarized by their origin under the terms biogas or biogas - both terms are used interchangeably below.

For the utilization of the biogas, the proportion of methane (CH ₄ ) contained between 45% and 70%, depending on the starting materials used and the operating mode of the biogas plant, is the most valuable, since it releases energy during combustion as an oxidizable compound. The higher its proportion, the higher the energy of the gas. Not usable are the carbon dioxide (CO ₂ ) and the water vapor. In the raw biogas disturbing are above all hydrogen sulfide (H ₂ S) and ammonia (NH ₃ ). They are usually removed during biogas upgrading prior to incineration to prevent corrosion in engines, turbines and downstream components such as heat exchangers, catalysts or mufflers, etc.

In Germany, the incineration of processed biogas in combined heat and power plants (CHP) is the most common. The engine or turbine of a CHP drives a generator that usually produces electricity for supply to the public grid. The Renewable Energy Sources Act (EEG 2009) currently in force in Germany ensures a feed-in tariff that is higher than conventional electricity and guaranteed for 20 years.

The combustion of biogas in combustion equipment such as gas engines or gas turbines produces formaldehyde (scientific name: methanal, structured formula: HCHO, empirical formula: CH ₂ O). Depending on the engine manufacturer, engine settings, etc., the values or concentrations found in the exhaust gas are between 80 mg / m ³ or less and about 300 mg / m ³ . The formaldehyde concentrations are dependent on the boost pressure, the combustion air ratio and the ignition timing.

Formaldehyde occurs in the combustion chamber very early during combustion of the biogas in the cold areas of the flame at 400 K to 800 K. It is the result of incomplete oxidation of methane contained in the biogas with an average of 60% and forms in particular when extinguishing the flame front of the Combustion chamber walls and columns. Furthermore, it arises in the exhaust system on the hot components of the exhaust system during the valve overlap.

Formaldehyde is a harmful gas for the human body. At concentrations greater than 1 ppm (parts per million), it causes nasopharyngeal irritation, tear irritation at 4 to 5 ppm, and more than 20 ppm causes eye burning and heavy lacrimation, respiratory complaints and severe coughing. Values> 20 ppm are considered dangerous and can lead to lung damage. Furthermore, studies have confirmed the carcinogenic effects of formaldehyde.

The German legislator has therefore set the limit value for emissions from engines or technical systems to 60 mg / m ³ in the exhaust gas, for example in the Technical Instructions (TA) for Clean Air (TA Luft). About 75% of all current plants do not reach this level without additional measures.

As an incentive to reduce emissions well below this value, the so-called "Air Purification Bonus" amounting to EUR 0.01 / kWh el for biogas plants was also introduced in the EEG 2009, if it was implemented in accordance with the Federal Immission Control Act (BImSchG ) and have an emission value of max. 40 mg / m ³ formaldehyde in the exhaust gas can be demonstrated by measurements of an independent and according to § 26 BImSchG recognized measuring institute permanently (resolution of the country working group Immissionsschutz (LAI) of November 2008). In the meantime, this regulation also applies to biogas plants that did not require approval under the BImSchG and that have only a building permit (mostly plants with a capacity <500 kW el.).

It is known to be able to purify a formaldehyde-loaded exhaust gas stream of a biogas incinerator with various technical retrofitting measures. These known cleaning systems are installed directly on the combustion device in the exhaust system and sometimes attract considerable additional investments.

So come as emission control method usually either the so-called. "Thermal afterburning (TNV)" in very expensive TNV plants or the "catalytic exhaust gas purification" in so-called oxidation catalysts used, the latter due to the impurity of the engine exhaust gases (especially harmful gases such Sulfur dioxide (empirical formula SO ₂ ) and nitrogen oxides (abbreviation: NO _X )) wear out very quickly and become ineffective. In the cumulative use of activated carbon filters for the purpose of flue gas desulfurization before the oxidation catalyst, the exhaust gas flow must first be cooled to temperatures below 40 ° C in order not to burn the filter - this is usually used electrically operated air cooler with high power consumption.

In addition to the guaranteed feed-in tariff for electricity generated from renewable energies, the EEG 2009 also sets a bonus for combined heat and power (CHP), ie for the additional use of the waste heat of a biogas incinerator, so that the sale of the heat and the CHP bonus will generate additional heat Income can be generated. So far, however, only a small part of the heat is used in most agricultural biogas plants especially for lack of heat demand. In this case, the waste heat could be used effectively for drying in particular fermentation residues, wood chips, sewage sludge and / or the like more in so-called belt drying systems that convert the feedstock in a continuous drying process to dry material that is thermally utilizable or agricultural fertilizer available.

In a typical belt drying plant, the material to be dried in the inlet part by means of a, possibly special, feeding device is given as uniform as possible heap on a dryer belt. After being distributed on the conveyor belt, the material to be used is transported gently and statically. By avoiding friction of the material, both the mechanical wear of the drying plant is reduced, and minimizes the formation of dust and thus avoids the risk of explosion. The drying zone is preferably divided into individual drying chambers, which are flowed through with warm drying air (100 ° C-140 ° C). A modular design in chambers allows easy capacity expansion by inserting additional drying chambers. The humidified in the dryer air is withdrawn by means of an exhaust fan from the dryer and performed in a first known embodiment, for example by a series of heat exchangers: the first lowers the temperature of the drying air, in the second stage, the condenser, the air is further cooled and characterized the vapors condensed out. Subsequently, in the third stage, the circulating air reheated. Such a system allows a substantial reuse of the dryer exhaust air, as well as a reduction of the energy requirement through internal heat recovery.

It is known to indirectly and / or directly heat strip drying systems.

Indirect heating is particularly suitable for a dryer location with an existing energy carrier network, such as steam or thermal oil of a heat exchanger unit operatively connected to the exhaust gas of the biogas incinerator. The heat input into the system takes place here by arranged in the belt drying system heater.

In the case of direct heating, the heat energy required for evaporation is supplied to a stream of air by gas or oil burners. This hot air stream is then introduced into the chambers of the belt drying system for heating.

Direct heating is also known, in which the approximately 450 ° C. hot exhaust gas (A) coming from the gas engine of a CHP is conducted into the exhaust gas mixing chamber of a belt drying plant for sewage sludge after purification in an oxidation catalyst and mixed with preheated air. The forwarding of this conditioned to 300 ° C air in a chamber of the belt dryer finally gives a process air (P) for the passage through the dryer of 100 ° C to 140 ° C. Temperature and heat supply in the dryer are automatically controlled by sensors. The moist, cooled to 70 ° C air is withdrawn at the other end of the dryer in a known manner and passed through a wet scrubber. In the process, the vapor, ie air saturated with water vapor, which is formed when solids are dried and ammonia (NH ₃ ) formed during the drying process is washed out and the further cooled air is passed into a biofilter. There still containing odors are bound. The wastewater from the scrubber is ideally fed to the sewage treatment plant whose sludges serve as input material in the belt drying plant. However, the use of oxidation catalysts to reduce, inter alia, the formaldehyde (CH ₂ O) in the exhaust gas has the already described above rapid wear and the costs associated with the necessarily regular replacement of the catalyst costs to the detriment.

On this basis, the present invention has the object to provide a the disadvantages of the prior art avoiding exhaust air purification system for the substantial elimination in particular of formaldehyde (CH ₂ O) from a correspondingly loaded exhaust gas stream of a biogas incinerator, which to avoid heat losses from the biogas Combustion device is introduced directly into at least one chamber of the belt drying plant. Although the term "elimination" in the context of the present invention, the usual sum of all processes is understood, which contribute to the decrease in concentration of a substance in a mixture, in particular of formaldehyde (CH ₂ O) in the exhaust stream, with the present invention In particular, a system can be provided which ensures formaldehyde values of well below 10 mg / m ³ , based on 1 m ^{3 of} original exhaust gas (undiluted).

This object is achieved by an exhaust air purification system for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly loaded exhaust gas stream of a biogas combustion device having the features of patent claim 1. Advantageous embodiments and developments, which can be used individually or in combination with each other, are the subject of the dependent claims.

The exhaust air purification system according to the invention for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a biogas incinerator is characterized in that the exhaust stream (A) coming from the biogas incinerator directly, ie without the interposition of formaldehyde ( CH ₂ O) eliminating devices such as oxidation catalysts or thermal Nachverbrennungsvorrichtungen, in at least one chamber of a belt drying plant and introduced as process air (P) by the belt drying plant forwarded and at the end as dryer exhaust air (T) from the belt drying plant is deductible.

The withdrawn from the belt drying plant dryer exhaust (T) is inventively at least a first, designed as a wet scrubber gas cleaning plant, in which at least in the dryer exhaust air (T) during the drying process ammonia (NH ₃ ) preferably below the limit specified in the TA air limit of 60 mg / m ^{3 is} washed out, as well as at least a second, also designed as a wet scrubber gas purification system, in which in the dryer exhaust air (T) at least containing, from the exhaust gas (A) originating formaldehyde (CH ₂ O), the belt dryer without undergoes further reactions and also the first gas purification plant usually happens without significant reactions, preferably at least below the limit specified by the LAI of 40 mg / m ³ based on 1 m ^{3 of} original exhaust gas (undiluted) washed out or oxidatively eliminated.

The introduction of the coming from the biogas combustion device exhaust stream (A) in the belt drying plant can be done in a chamber, which also represents a first drying chamber of the belt drying plant.

Alternatively or cumulatively, however, it has proven useful to first introduce the exhaust stream (A) coming from the biogas combustion device into an exhaust mixing chamber of the belt drying plant, in which the exhaust gas (A) with air (AIR), which is usually too hot for the drying process, is preferably in the ratio 1 2, so that the resulting process air (P) containing about 33.33% of exhaust gas (A) and about 66.66% of air (AIR) in the belt drying plant is automatically controllable by sensors, especially when the Air (AIR) by means of waste heat of the combustion device and / or the exhaust gas (A) is preheated.

As far as the location of an exhaust air purification system according to the invention equally easy access to fresh water as waste water easy to eliminate allowed - such as in construction near a sewage treatment plant - has proven, the first primarily for washing at least ammonia (NH ₃ ) provided gas cleaning plant with water as in particular cleaned wastewater from the drainage of the sewage treatment plant, preferably in a continuous process to operate.

In this as well as other cases can prove itself, a first Gas purification system upstream of a vapor condensing system, which allows further heat recovery.

As an alternative to a water-operated first gas purification plant, it has proven useful to operate it with dilute, preferably 37.5%, sulfuric acid (empirical formula: H ₂ SO ₄ ), which is advantageously independent of a constant supply of fresh water in a closed recirculation process.

According to the invention, the first gas purification system downstream and also designed as a wet scrubber second gas cleaning plant primarily for the elimination of formaldehyde (CH ₂ O), preferably in a Umwälzverfahren with dissolved in water potassium hydroxide (KOH) - hereinafter trivially referred to as dilute potassium hydroxide - on the washer floor and with dilute hydrogen peroxide solution (H ₂ O ₂ ) in the exhaust air stream (T) operated.

In particular, means for supplying dissolved in water hydrogen peroxide (H ₂ O ₂ ) in the dryer-exhaust air flow (T) have proven, which are preferably arranged between the first and the second gas cleaning system such that a reaction time of the hydrogen peroxide (H ₂ O ₂ ) remains with the formaldehyde (CH ₂ O) in the dryer exhaust air stream (T) of at least one, preferably two, seconds before the exhaust air stream (T) reaches the second gas purification system.

This results in the following chemical reaction of formaldehyde (CH ₂ O) in the dryer exhaust air (T) with the sprayed, aqueous hydrogen peroxide (H ₂ O ₂ ): HCHO + H ₂ O ₂ => HCOOH + H ₂ O

Formed as reaction products are formic acid (scientific name: methanoic acid, structured formula: HCOOH, empirical formula: CH ₂ O ₂ ) and water (H ₂ O). The formic acid (CH ₂ O ₂ ) is neutralized by the dissolved potassium hydroxide (KOH) at the scrubber bottom. This leads to the following reaction: HCOOH + KOH => CHOOK + H ₂ O

This produces water (H ₂ O) and potassium formate (structured formula: CHOOK; empirical formula: CHO ₂ K), which, together with the dried starting materials, can preferably be utilized as fertilizer component.

Formaldehyde values (CH ₂ O) of significantly less than 10 mg / m ³ , based on 1 m ^{3 of} original engine exhaust gas (undiluted), can advantageously be achieved with the second gas cleaning system operated according to the invention, preferably in a closed recirculation process.

As far as the location of an exhaust air purification system according to the invention should be odorless, it has finally proven to lead the dryer exhaust air (T) after passing through the second gas purification plant by a biofilter, in which still in the dryer exhaust air (T) existing odors (especially odorous particles and aerosols).

With realization of the present invention can be advantageous to conventional emission control methods such as the so-called. "Thermal afterburning (TNV)" in very expensive TNV plants or the "catalytic exhaust gas purification" in so-called oxidation catalysts, which due to noxious gases such as sulfur dioxide (SO ₂ ), Nitrogen oxides (NO _X ) and other impurities in the exhaust gases (A) from the biogas or digester gas combustion device wear very quickly and become ineffective, be dispensed with.

Additional details and further advantages of the invention will be described hereinafter by way of example with reference to two embodiments, to which the present invention is not limited, and in conjunction with the accompanying drawings.

In it show schematically:

1 the flow diagram of an exhaust air purification system for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a biogas incinerator, such as this can be constructed, for example, in the immediate vicinity of an agricultural fermentation plant; and

2 the flow diagram of an exhaust air purification system for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a digester combustion device, as this example, can be erected in the immediate vicinity of a municipal sewage treatment plant.

In the following description of two preferred embodiments of the present invention, like reference characters designate like or similar components.

1 shows the flow diagram of an exhaust air purification system 1 for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream A of a biogas incinerator 10 like these, preferably can be built in the immediate vicinity of a farm fermentation plant.

This is the from the biogas incinerator 10 upcoming exhaust stream A directly, ie without the interposition of currently used in the biogas industry formaldehyde (CH ₂ O) eliminating facilities such as either fast-burning oxidation catalysts or expensive TNV plants, a belt drying plant 20 fed, in which mainly the fermentation residues, such as these for the production of biogas for a biogas incinerator 10 are indispensable, dried, for example, to be used as fertilizer in agriculture.

The biogas incinerator 10 associated belt drying system 20 can thus directly with the exhaust stream A from the biogas combustion device 10 be heated (so-called direct heating). This is the tape drying 20 in addition to the primary drying of the fermentation residues from the agricultural fermentation plant and a temperature sink for the exhaust gas A biogas combustion device 10 represents.

The dryer exhaust T, in which the exhaust gas A from the biogas incinerator 10 about one third is represented (mixture of 2/3 ambient air (AIR - supplied via a supply line 70 ) and 1/3 flue gas) is in the digestate drying in the absence of access to a fresh water source and service water in rural areas preferably directly into a first trained as a wet scrubber gas purification plant 40 passed, which with dilute, preferably 37.5%, sulfuric acid (H ₂ SO ₄ - fed via a supply line 71 ) is operated in a closed recirculation process, wherein gaseous pollutant components such as ammonia (NH ₃ ) are washed out of the dryer exhaust air T.

About the exhaust A from the combustion device 10 in the dryer exhaust air T additionally existing harmful gases such as nitrogen oxides (NO _X ) and formaldehyde (CH ₂ O) are in some cases already chemically bound and are so far already removed from the exhaust air A to a proportion.

2 shows the flow diagram of an exhaust air purification system 1 for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream A of a digester gas combustion device 10 How this can be built, for example, in the immediate vicinity of a municipal sewage treatment plant.

In this case, the from the digester gas combustion device 10 incoming exhaust stream A in turn directly, ie without the interposition of currently in the biogas industry usual formaldehyde (CH ₂ O) eliminating facilities such as either fast-burning oxidation catalysts or expensive TNV plants, a belt drying plant 20 in which primarily the sewage sludge, such as these for the production of digester gas for a digester combustion device 10 are inevitable, dried, for example, to be used thermally as a substitute fuel in industrial plants such as cement plants or waste incineration plants.

The digester gas combustion device 10 associated belt drying system 20 can thus directly with the exhaust stream A from the digester gas combustion device 10 be heated (so-called direct heating). This is the tape drying 20 in addition to the primary drying of sewage sludge from the municipal sewage treatment plant and a temperature sink for the exhaust gas A of the digester gas combustion device 10 represents.

The dryer exhaust T, in which the exhaust gas A from the Faulgas combustion device 10 about one third is represented (mixture of 2/3 ambient air (AIR - supplied via the supply line 70 ) and 1/3 flue gas) is in sewage sludge drying, adjacent to which usually the access to fresh water sources and service water disposal is unproblematic, preferably initially via a vapor condensation plant 30 and then a first, designed as a wet scrubber gas purification system 40 led, in which the dryer exhaust air T with about 60 ° C after the condensation in the vapor condensing plant 30 entry. Unlike exhaust air purification systems 1 for example for rural biogas incinerators 10 may be the first gas purification system 40 an exhaust air purification system 1 for municipal biogas combustion equipment without addition of chemicals only with water, which via a supply line 72 supplied and via a discharge line 73 be discharged, operated in an open cycle process, whereby gaseous pollutant components such as ammonia (NH ₃ ), but also the dust particles typical in sewage sludge drying in the dryer exhaust air T are washed out.

About the exhaust A from the combustion device 10 in the dryer exhaust air T additionally existing harmful gases such as nitrogen oxides (NO _X ) and sulfur dioxide (SO ₂ ) and formaldehyde (CH ₂ O) go into the aqueous solution and are so far already removed from the exhaust air A to a proportion.

For the almost complete elimination mainly of formaldehyde (CH ₂ O) from the dryer exhaust air T see both in 1 and 2 described exhaust air purification systems 1 before that Dryer exhaust air T after the first gas scrubber designed as a wet scrubber 40 a second, also designed as a wet scrubber gas purification plant 50 fed, which with water-dissolved potassium hydroxide (KOH - supplied via a feed line 75 ) or trivially expressed with dilute potassium hydroxide solution on the scrubber bottom 61 and operated with dilute hydrogen peroxide solution (H ₂ O ₂ ) in the exhaust air flow T in a recirculation process.

The hydrogen peroxide solution (H ₂ O ₂ ) can be used in the gas purification plant 50 arranged distribution system 64 be introduced into the exhaust air flow T. It has been found, however, that far better reaction results can be achieved if dissolved in water, hydrogen peroxide (H ₂ O ₂ ) in such a way in the dryer exhaust air flow T between the first 40 and the second 50 Gas purification system via a supply means or a supply line 74 is introduced, that a reaction time of the hydrogen peroxide (H ₂ O ₂ ) with the formaldehyde (CH ₂ O) in the dryer exhaust air flow T of at least one, preferably two seconds remains, before the exhaust air flow T the second gas purification system 50 reached.

Both the first 40 as well as the second 50 trained as a wet scrubber gas purification system preferably operate on the countercurrent principle. In this case, we the exhaust air T to be cleaned in the lower part of the column 62 the scrubber 40 . 50 blown and simultaneously from above via a distribution system 64 sprayed with the specified detergent. The exchange of exhaust air T and detergent occurs at about the middle of the column 62 arranged packing 63 , This achieves a specific surface interface and efficient absorption.

As far as chemical substances are used as detergent either this will be by means of a circulating pump 65 through a recirculation line 76 into the distribution system 64 pumped and passes over the packing 63 back into a storage container 66 ,

As far as only water is used as a detergent is this constantly fresh the distribution system 64 fed and from the storage container 66 pumped into a sewage treatment plant inlet.

All processes can be controlled automatically and thus react to the immediate reaction to peak loads of the exhaust air T. Overall, with the two gas purification systems 40 and 50 almost complete absorption levels between 98% and 99% of the registered substances can be achieved.

As far as the location of an exhaust air purification system 1 According to the invention should also be odorless after testing has finally proven, the dryer exhaust air T after passing through the second gas purification system 50 still through a biofilter plant 80 to be conducted, in which odors still present in the dryer exhaust air T are neutralized as described below: For pretreatment, the exhaust air T to be cleaned is first moistened and into the lowest layer of the filling 81 the biofilter plant 80 blown. The inert substance ensures an ideal air distribution over the filter surface and the formation of uniform aerobic conditions. The exhaust air T to be cleaned is then passed through a biologically active medium 82 It is made of bark, active compost and other ingredients that are used in special applications. The aim of this biological filter is to retain particles and aerosols, while the biologically active substances are adsorbed and absorbed in the air, so that primarily pure air R, the biofilter plant 80 leaves. The odor-laden substances retained therein are biologically converted to carbon dioxide (CO ₂ ) and water (H ₂ O).

Finally, the present invention is also suitable for use with belt drying systems for applications other than those mentioned above, particularly in connection with belt drying systems for wood shavings, industrial sludges or the like.

The benefit for the operator of an agricultural biogas plant in particular is conventional exhaust gas purification methods such as the so-called "thermal afterburning (TNV)" in very expensive TNV plants or "catalytic exhaust gas purification" in so-called oxidation catalysts, which due to harmful gases such as sulfur dioxide (SO ₂ ), nitrogen oxides (NO _X ) and other impurities in the exhaust gases A wear out of the biogas or digester combustion device very quickly and become ineffective to be able to do without.

At the same time he receives an extremely economical solution to his emissions problem, since the well-known exhaust air wash 40 for the elimination of ammonia (NH ₃ ) anyway solid delivery component of each belt drying plant 20 is and the extension of new or retrofit already existing drying systems 20 with a second gas purification stage according to the invention 50 for the targeted elimination of formaldehyde (CH ₂ O) from the exhaust A of a biogas incinerator 10 after its use as process air P in the belt drying plant 20 in any case economically in relation to the result achieved and very favorable with the clean air bonus.

With a second, preferably made of corrosion-resistant plastic components, gas cleaning system 50 equipped exhaust air purification systems 1 for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly loaded exhaust stream A of a biogas internal combustion engine 10 are also characterized by an advantageous long life and high reliability.

In particular, however, the use of the added chemicals for the elimination of formaldehyde (CH ₂ O) to below 95% of the original value durchsoierbar, so that even no conformity problems are to be expected if the formaldehyde limit by the LAI or in the TA air future possibly lowered further.

After all, the chemicals used and the formaldehyde (CH ₂ O) converted into potassium formate (CHO ₂ K) after gas purification can be used in particular as fertilizer in agriculture, so that is why an extremely environmentally friendly and economical exhaust air purification system 1 provided with the present invention.

LIST OF REFERENCE NUMBERS

1

Exhaust air purification system for the elimination in particular of formaldehyde from a correspondingly polluted exhaust stream A of a biogas or digester gas incinerator 10

10

Biogas or biogas incinerator

20

Band dryer

21

Flue gas mixing chamber or first drying chamber

30

Brüdenkondensationsanlage

40

first, designed as a wet scrubber gas purification system

50

second, designed as a wet scrubber gas purification plant

61

Scrubber bottom of the gas purification plants 40 respectively. 50

62

Column of gas purification plants 40 respectively. 50

63

Packing of the gas purification plants 40 respectively. 50

64

Distribution system of gas purification plants 40 respectively. 50

65

Circulation pump of gas purification plants 40 respectively. 50

66

Reservoir of gas purification plants 40 respectively. 50

70

Supply line for air (AIR) as process air component

71

Supply line for sulfuric acid (H ₂ SO ₄ ) as detergent

72

Feed line for water (H ₂ O) as detergent

73

Discharge line of contaminated water (H ₂ O)

74

Feeding agent or line for hydrogen peroxide (H ₂ O ₂ ) as a detergent

75

Supply line for dissolved potassium hydroxide (KOH) as detergent

76

circulation line

80

biofilter

81

formed from inert substances bottom layer of the filter filling

82

biologically active layer of the filter filling

A

Exhaust gas, exhaust gas stream of the biogas or digester gas combustion device 10

P

Process air for the passage in the belt drying plant 20

T

Dryer exhaust air, exhaust air flow from the belt drying plant 20

R

clean gas

AIR

Air, ambient air

Claims (10)

Exhaust air purification system ( 1 ) for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly loaded exhaust gas stream (A) of a biogas incinerator ( 10 ), in which the exhaust gas flow (A) - from the biogas incinerator ( 10 ) coming directly into a chamber ( 21 ) a belt drying plant ( 20 ) and - as process air (P) through the belt drying plant ( 20 ) and - is withdrawn at the end as a dryer exhaust air (T); wherein the dryer exhaust air (T) - at least via a first, designed as a wet scrubber gas purification plant ( 40 ) is carried out, in which at least in the dryer exhaust air (T) during the drying process ammonia (NH ₃ ) is washed out, and - at least a second, also designed as a wet scrubber gas purification plant ( 50 ), in which in the dryer exhaust air (T) at least containing, from the exhaust gas (A) originating formaldehyde (CH ₂ O) is washed out. Exhaust air purification system ( 1 ) according to claim 1, characterized in that the one chamber ( 21 ) of the belt drying plant ( 20 ) is a first drying chamber and / or an exhaust gas mixing chamber for mixing the process air (P) from the exhaust gas (A) and a, preferably preheated, air (AIR). Exhaust air purification system ( 1 ) according to claim 2, characterized in that the process air (P) in the belt drying plant ( 20 ) consists of about 33.33% exhaust gas (A) and about 66.66% air (AIR). Exhaust air purification system ( 1 ) according to one of the preceding claims 1 to 3, characterized in that the first gas purification plant ( 40 ) With Water, preferably in a continuous process, operated. Exhaust air purification system ( 1 ) according to one of the preceding claims, characterized in that the first gas cleaning device ( 40 ) a vapor condensation plant ( 30 ) is connected upstream. Exhaust air purification system ( 1 ) according to one of the preceding claims 1 to 3, characterized in that the first gas purification plant ( 40 ) with dilute, for example, 37.5% sulfuric acid (H ₂ SO ₄ ), preferably in a recirculation process, is operated. Exhaust air purification system ( 1 ) according to one of the preceding claims, characterized in that the second gas purification plant ( 50 ) is operated with dilute hydrogen peroxide solution (H ₂ O ₂ ) in the exhaust air stream (T). Exhaust air purification system ( 1 ) according to claim 7, characterized in that between the first ( 40 ) and the second ( 50 ) Gas cleaning plant means ( 74 ) for supplying water-dissolved hydrogen peroxide (H ₂ O ₂ ) into the dryer exhaust air stream (T) are preferably such that a reaction time of the hydrogen peroxide (H ₂ O ₂ ) with the formaldehyde (CH ₂ O) in the dryer exhaust air stream (T) of at least one, preferably two, seconds remains before the exhaust air flow (T) the second gas cleaning system ( 50 ) reached. Exhaust air purification system ( 1 ) according to one of the preceding claims, characterized in that the second gas purification plant ( 50 ) with dilute potassium hydroxide solution, ie dissolved potassium hydroxide (KOH), on the scrubber bottom ( 61 ) is operated. Exhaust air purification system ( 1 ) according to one of the preceding claims, characterized in that the second, preferably operated in a recirculation, gas purification plant ( 50 ) a biofilter plant ( 80 ) is subordinate.

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