DE112011100511B4 - Exhaust air purification process and system for the elimination in particular of formaldehyde from a corresponding polluted exhaust gas stream of a biogas incinerator - Google Patents

Abstract

Exhaust air purification process for the elimination in particular of formaldehyde (CH2O) from a corresponding polluted exhaust stream (A) of a biogas combustion device (10), wherein the exhaust stream (A) - from the biogas combustion device (10) coming directly into a chamber (21) Convection drying plant (20) introduced and - forwarded as process air (P) by the convection drying plant (20) and - withdrawn at the end as a dryer exhaust air (T); wherein the dryer exhaust air (T) - at least over a first, designed as a wet scrubber gas cleaning system (40) is performed, in which at least in the dryer exhaust air (T) during the drying process ammonia (NH3) is washed out, and - at least a second, also designed as a wet scrubber gas cleaning system (50) is performed in which in the dryer exhaust air (T) at least containing, from the exhaust gas (A) derived formaldehyde (CH2O) is washed out.

Description

The present invention relates to an exhaust air purification method and system for eliminating in particular formaldehyde from a correspondingly polluted exhaust 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 residues, 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 clean a formaldehyde-loaded, usually about 450 ° C hot, 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 pull in part. considerable additional investment.

So come as a cleaning method for about 450 ° C hot exhaust gas 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 the impurity of the engine exhaust gases (especially noxious gases such as sulfur dioxide (molecular 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, especially agricultural, biogas plants for lack of heat demand on site. 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 convection drying systems such as belt or fluidized bed drying plants in particular, convert the feedstock in a continuous drying process to dry, usable thermally or agriculturally as fertilizer is.

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.

In a typical fluidized bed drying plant, the material to be dried is usually piled up on a perforated sheet - the so-called inflow base - and passed through with warm drying air or comparable hot gas against the force of gravity. The gas velocity is chosen so that particles of the material are indeed lifted, but not discharged. This creates a fluidized bed that behaves like a liquid due to the free movement of the solid particles in the gas stream.

It is known to heat such Konvektionstrocknungsanlagen indirectly and / or directly.

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 in operative connection with the exhaust gas of the biogas incinerator. The heat input into the system takes place here by arranged in the convection 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 in the chambers of the Convection drying plant introduced 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 passed, after cleaning in an oxidation catalyst, into the exhaust gas mixing chamber of a belt or fluidized bed drying plant for sewage sludge and mixed with preheated air. The forwarding of this heated to 300 ° C air in a chamber of the dryer finally gives a process air (P) for the passage through the dryer of 100 ° C to 140 ° C. Temperature and heat in the dryer are automatically controlled by sensors. The moist, cooled to about 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 forms during drying of solids and ammonia (NH ₃ ) formed during the drying process, is washed out and the further air cooled to approximately 55 ° C. is passed into a biofilter. There still containing odors are bound. The wastewater from the scrubber is ideally fed to the treatment plant whose sludge serve as feedstock in the belt or fluidized bed 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.

Finally, in addition to the noxious gases formaldehyde (CH ₂ O) and ammonia (NH ₃ ), exhaust air that is directly heated with exhaust gases from a biogas incinerator can also be expected to have considerably odor-laden exhaust air. Especially with non-rotted sewage sludge this odor load is often so great that with a merely downstream biofilter the limit value of 500 odor units (GE) valid in Germany can not be reached.

On this basis, the present invention has the object, a the prior art avoiding the disadvantages of the exhaust air purification process and system for the substantial elimination in particular of formaldehyde (CH ₂ O), ammonia (NH ₃ ) and the odor load from a corresponding polluted exhaust stream of a biogas To provide incinerator, which is introduced to avoid heat loss from the biogas combustion device coming directly into at least one chamber of a convection drying plant. Although in the context of the present invention, the term "elimination" the usual sum of all processes is understood, which for the decrease in concentration of a substance in a mixture, here primarily of formaldehyde (CH ₂ O), ammonia (NH ₃ ) and the odor load in Contribute exhaust gas flow, should be provided with the present invention, in particular an exhaust gas purification method and system, which in particular formaldehyde values of well below 10 mg / m ³ , based on 1 m ^{3 of} original exhaust gas (undiluted), sure.

This object is achieved by an exhaust air purification method and by an exhaust gas purification system for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust gas stream of a biogas incinerator with the features of claim 1 or 12. 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 method 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 afterburning, in at least one chamber of a convection drying plant introduced and as process air (P) by the convection drying system forwarded and at the end as dryer exhaust air (T) from the convection drying plant is removable.

According to the invention, the dryer exhaust air (T) withdrawn from the convection drying plant is at least via a first gas scrubber designed as a wet scrubber, in which ammonia (NH ₃ ) produced at least in the dryer exhaust air (T) during the drying process is preferably below the limit of 20 mg / m ^{3 is} washed out - whereby the concentration value is lowered so that also the additional limit of TA air for the emission load with max. 100 g / h is maintained, and at least over 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 (A) originating formaldehyde (CH ₂ O), the convection drying plant 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 exhaust stream (A) coming from the biogas incinerator into the convection drying plant can take place in a chamber which at the same time constitutes a first drying chamber of the convection drying plant.

Alternatively or cumulatively, however, it has proven useful to first of all introduce the exhaust stream (A) from the biogas incinerator into an exhaust gas mixing chamber of the convection 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 waste gas (A) and about 66.66% of air (AIR) in the convection drying plant is automatically controllable by sensors, especially if the Air (AIR) by means of waste heat of the combustion device and / or the exhaust gas (A) is preheated.

As far as the implementation of an exhaust air purification method according to the invention equally simple. Access to fresh water as wastewater easy to eliminate allowed - as for example in the construction of the required for the implementation of the process system components near a sewage treatment plant - has proven to be the first primarily for washing at least ammonia (NH ₃ ) provided gas cleaning plant with water such as in particular purified wastewater to operate from the flow of the treatment plant, preferably in a continuous process.

In this case, as well as other cases, it may also prove useful to connect a vapor purification plant upstream of a first gas purification plant, which permits further heat recovery.

As an alternative to a water-operated first gas purification plant, it has proven useful to operate with dilute, preferably 10-30 or sometimes 37.5% strength (regular operation: 10%), sulfuric acid (empirical formula: H ₂ SO ₄ ), which is advantageous in a closed circulating process is operable independent of a constant supply of fresh water.

According to the invention, the second gas purification system arranged downstream of the first gas purification system and likewise designed as a wet scrubber is preferably used for the elimination of formaldehyde (CH ₂ O), preferably in a circulation process, with potassium or sodium hydroxide dissolved in water (KOH or NaOH, especially 10-30 10%) - hereinafter also referred to trivially as dilute potassium or caustic soda solution - operated on the scrubber bottom and with dilute hydrogen peroxide solution (H ₂ O ₂ ) in the exhaust air stream (T).

In particular, means for supplying dissolved in water, preferably 5-10%, hydrogen peroxide (H ₂ O ₂ ) and optionally other chemicals such as in particular sodium (NaOH) or potassium hydroxide (KOH) in the dryer exhaust air stream (T ), 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 ) 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) 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 (KOH) or sodium hydroxide (NaOH) at the scrubber bottom. This leads to the following reaction: HCOOH + KOH => CHOOK + H ₂ O or: HCOOH + NaOH => CHOONa + H ₂ O

This produces water (H ₂ O) and potassium or sodium formate (structured formula: CHOOK or Na; empirical formula: CHO ₂ K or Na). Preferably used in agricultural. Biogas plants in the second gas purification plant potassium hydroxide solution as an alkaline medium, since the resulting potassium formate (Na) together with the dried feedstocks (fermentation residues, etc.) are preferably used as fertilizer component and can satisfy a part of the necessary Kalibedarfs to be fertilized soils.

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.

For extensive, preferably simultaneous, elimination of the first gas cleaning system passing unpleasant odor loads (mainly volatile organic acids and mercaptans, etc.) from the dryer exhaust air (T), the second gas cleaning plant with basic basic a, for example, 10-30% potassium or sodium hydroxide solution (KOH or NaOH) are operated, which advantageously removes or binds up to 90% and more of the odors from the exhaust air stream (T).

As far as the location requires that the implementation of the exhaust air purification process according to the invention should turn out to be 100% odorless, it has finally proven to still lead the dryer exhaust air (T) after passing through the second gas purification plant by a biofilter, in the remaining, about 10% in the dryer exhaust air (T) existing odors (especially odorous particles and aerosols) are neutralized.

The present invention also relates to an exhaust air purification system for carrying out the previously described exhaust air purification process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a biogas incinerator. The exhaust air purification system according to the invention comprises according to the invention at least means with which the exhaust stream (A) coming from the biogas incinerator directly into a chamber of a belt drying plant and as process air (P) through the belt drying plant forwarded and at the end as dryer exhaust air (T) can be deducted; wherein the dryer exhaust air (T) at least over a first, designed as a wet scrubber gas purification plant is feasible, in which at least in the dryer exhaust air (T) during the drying process ammonia (NH ₃ ) is washed out, as well as at least a second, also designed as a wet scrubber gas purification system is feasible, in which in the dryer exhaust air (T) at least containing, derived from the exhaust gas (A) formaldehyde (CH ₂ O) is leachable.

Advantageous embodiments and further developments of the exhaust air purification system, which can be used individually or in combination with one another, are the subject of the dependent claims 13 to 16.

With realization of the present invention can be advantageous to conventional exhaust gas purification 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 process for the elimination in particular of formaldehyde (CH ₂ O) from a corresponding polluted exhaust stream (A) of a biogas incinerator, such as this can be built, for example, in the immediate vicinity of an agricultural fermentation plant; and

2 the flow diagram of an exhaust air purification process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a digester combustion device, such as this can be constructed, for example, 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 process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream A of a biogas combustion device 10 how this can preferably be built in the immediate vicinity of an agricultural 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, preferably as a belt drying plant 20 trained convection drying plant fed, in which mainly the digestate, such as this 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 fermentation residues from the agricultural fermentation plant also an advantageous temperature sink for the exhaust gas A biogas combustion device 10 is, without which the exhaust gas A could not be fed to a trained as a wet scrubber gas purification plant, as about 450 ° C hot exhaust A evaporate the water of the scrubber and any chemicals dissolved therein and would quickly put the wet scrubber out of action.

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 10-30% (standard rate: 10%), 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 Additionally, harmful gases such as nitrogen oxides (NO _x ) and formaldehyde (CH ₂ O) pass through the sulfur-free first scrubber in the dryer exhaust air 40 and remain as harmful exhaust components in the exhaust air T, which before entering a biofilter 80 or to the atmosphere are still to be eliminated. The odor freights in the first gas purification plant are also experienced 40 still no scheduled elimination.

2 shows the flow diagram of an exhaust air purification process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream A a. Digester gas combustion means 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 used in the biogas industry formaldehyde (CH ₂ O) eliminating facilities such as either fast-burning oxidation catalysts or expensive TNV plants, preferably as a belt drying plant 20 trained convection drying plant fed, in which mainly the sewage sludge, as this 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 the sewage sludge from the municipal sewage treatment plant and an advantageous temperature sink for the exhaust gas A of the digester gas combustion device 10 represents, without which the exhaust gas A could not be fed to a trained as a wet scrubber gas purification plant, as about 450 ° C hot exhaust A evaporate the water of the scrubber and any chemicals dissolved therein and so would put the wet scrubber after a few minutes out of action.

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 processes or systems 1 for example for rural biogas incinerators 10 may be the first gas purification system 40 an exhaust air purification process or 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 T to a proportion. By contrast, the odor freights experience in the first gas purification plant 40 still no targeted elimination.

The almost complete elimination primarily of formaldehyde (CH ₂ O) from the dryer exhaust T both look in 1 and 2 described exhaust air purification process or systems 1 before, the dryer exhaust air T after the first designed as a wet scrubber gas purification system 40 a second, also trained as a wet scrubber Gas cleaning plant 50 fed, which with dissolved in water potassium or sodium hydroxide (KOH or NaOH - fed via a supply line 75 ) or trivially expressed with dilute potash or caustic soda on the scrubber bottom 61 and with dilute, z. B. about 5-10%, hydrogen peroxide solution (H ₂ O ₂ ) is operated in the exhaust air flow T in a recirculation process.

For extensive, preferably simultaneous, elimination of the first gas purification system 40 Passing unpleasant odor loads from the dryer exhaust T can be the second in each case in 1 and 2 shown gas purification system 50 additionally basic with a, for example, 10-30% potassium or sodium hydroxide solution (KOH or NaOH) are operated, which advantageously removes up to 90% or more of the odors (predominantly volatile organic acids and mercaptans, etc.) from the exhaust air flow T. or binds.

The potassium (KOH) or sodium hydroxide solution (NaOH) as well as 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. However, it has been shown that far better reaction results can be achieved if, in particular dissolved in water, hydrogen peroxide (H ₂ O ₂ ) 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, the exhaust air T to be cleaned is 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.

To carry out the abluftreinigungsverfahren described above, an exhaust air purification system 1 proven, which at least comprises means with which the exhaust stream A from the biogas combustion device 10 coming directly into a chamber 21 a preferably designed as a belt drying plant convection drying plant 20 can be introduced and as process air P through the convection drying plant 20 forwarded and at the end as Trocknerabuft T is removable; wherein the dryer exhaust air T at least over a first, designed as a wet scrubber gas purification system 40 is feasible, in which at least in the dryer exhaust air T produced during the drying process ammonia (NH ₃ ) is leachable, as well as at least a second, also designed as a wet scrubber gas purification system 50 is feasible, in which in the dryer exhaust air T at least containing, originating from the exhaust gas A formaldehyde (CH ₂ O) is leachable.

The means not shown in detail for the initiation, forwarding and / or withdrawal of the exhaust stream A, the process air P and / or the dryer exhaust T are composed in particular of known per se exhaust stacks, air ducts, flaps and / or blower.

Preferably, the one chamber 21 the preferred use belt drying plant 20 a first drying chamber and / or an exhaust mixing chamber for mixing the process air P from the exhaust gas A and a, preferably preheated, air AIR be.

It may also be appropriate, the first gas cleaning device 40 a vapor condensation plant 30 vorzulagern.

Furthermore, it may be preferable between the first 40 and the second 50 Gas purification plant means 74 for supplying hydrogen peroxide (H ₂ O ₂ ) dissolved in water and / or potassium (KOH) or sodium hydroxide (NaOH) into the dryer exhaust air stream (T).

As far as the location of an exhaust air purification system 1 After the invention in addition to 100% odor-neutral should finally turn out proven, the dryer exhaust air T after passing through the second gas cleaning system 50 still through a biofilter plant 80 to be led, in which still remaining, about 10% in the dryer exhaust air T existing odors are neutralized as described below, with respect to the neighborhood non-odorous operation of an exhaust air purification method according to the invention and -system 1 To ensure: 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 the last particles and aerosols contained in the dryer exhaust air T, while the biologically active substances are adsorbed 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 and / or fluidized-bed drying systems for applications other than those mentioned above, in particular in connection with belt drying systems etc. for wood chips, 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 any convection 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 carrying out an exhaust air purification process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream A of a biogas internal combustion engine 10 are also advantageous for a 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.

Finally, the chemicals used and the using of potassium hydroxide in the second scrubber stage to potassium formate (CHO ₂ K) converted formaldehyde (CH ₂ O) can be used after the gas cleaning in particular as fertilizer in agriculture, so that therefore a highly environmentally friendly as economical exhaust air purification process and system 1 provided with the present invention.

LIST OF REFERENCE NUMBERS

1

Exhaust air purification system for carrying out an exhaust air purification process for the elimination in particular of formaldehyde from a corresponding 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 of polluted with pollutants 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 (16)

Exhaust air purification process for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted 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 convection drying plant ( 20 ) and - as process air (P) through the convection 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 method according to claim 1, characterized in that the one chamber ( 21 ) of the convection 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 purification method according to claim 2, characterized in that the process air (P) (in the convection drying 20 ) consists of about 33.33% exhaust gas (A) and about 66.66% air (AIR). Exhaust air purification method according to one of the preceding claims 1 to 3, characterized in that the first gas purification plant ( 40 ) is operated with water, preferably in a continuous process. Exhaust air purification method according to one of the preceding claims, characterized in that the first gas purification device ( 40 ) a vapor condensation plant ( 30 ) is connected upstream. Exhaust air purification method according to one of the preceding claims 1 to 3, characterized in that the first gas purification plant ( 40 ) with dilute, for example 10-30 or 37.5%, sulfuric acid (H ₂ SO ₄ ), preferably in a recirculation process, is operated. Exhaust air purification method according to one of the preceding claims, characterized in that the second gas purification system ( 50 ) is operated with dilute hydrogen peroxide solution (H ₂ O ₂ ) in the exhaust air stream (T). Exhaust air purification method according to one of the preceding claims, characterized in that the second gas purification system ( 50 ) is basically operated with a, preferably 10-30%, potassium (KOH) or sodium hydroxide solution (NaOH) in the exhaust air stream (T). Exhaust air purification method according to claim 7 or 8, characterized in that between the first ( 40 ) and the second ( 50 ) Gas cleaning plant means ( 74 ) for supplying dissolved in water hydrogen peroxide (H ₂ O ₂ ) and optionally other chemicals such as in particular potassium (KOH) or sodium hydroxide (NaOH) in the dryer exhaust air stream (T) are preferably such that a reaction time of 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 stream (T), the second gas cleaning system ( 50 ) reached. Exhaust air purification method according to one of the preceding claims, characterized in that the second gas purification system ( 50 ) with dilute potassium or sodium hydroxide solution, ie dissolved potassium (KOH) or sodium hydroxide (NaOH), on the scrubber bottom ( 61 ) is operated. Exhaust purification method according to any 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. Exhaust air purification system ( 1 ) for carrying out the method according to one or more of the preceding claims for the elimination in particular of formaldehyde (CH ₂ O) from a correspondingly polluted exhaust stream (A) of a biogas incinerator ( 10 ), at least comprising means with which the exhaust stream (A) - from the biogas incinerator ( 10 ) coming directly into a chamber ( 21 ) a convection drying plant ( 20 ) and - as process air (P) through the convection drying plant ( 20 ) and - at the end as dryer exhaust air (T) is removable; wherein the dryer exhaust air (T) - at least via a first, designed as a wet scrubber gas purification plant ( 40 ) is feasible, and - at least a second, also designed as a wet scrubber gas purification plant ( 50 ) is feasible. Emission control system ( 1 ) according to claim 12, characterized in that the one chamber ( 21 ) of the convection 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 12 or 13, 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 claims 12 to 14, characterized in that between the first ( 40 ) and the second ( 50 ) Gas cleaning plant means ( 74 ) are provided for supplying dissolved in water hydrogen peroxide (H ₂ O ₂ ) and / or potassium (KOH) or sodium hydroxide (NaOH) in the dryer exhaust air stream (T). Exhaust air purification system ( 1 ) according to one of the preceding claims 12 to 15, characterized in that the second, preferably operated in a recirculation, gas purification plant ( 50 ) a biofilter plant ( 80 ) is subordinate.

Images