EP0376971B1 - Process and installation for recovering reusable gas from waste through pyrolysis - Google Patents

Abstract

According to a process for recovering reusable gas from waste through pyrolysis, the previously comminuted waste is transformed into fluff, granulates or pellets and introduced into a degassing drum (16), in which low temperature carbonization gas is generated and separated from the residual matter. The low temperature carbonization gas is decomposed into combustion gas in a gas converter (19) and cleaned in a subsequent gas washing installation (21-24 and 47-51) with circulating washing water. Part of the water of the circulation system of washing water is withdrawn and replaced with fresh water in order to limit its concentration of toxic substances. The pyrolysis residues to be withdrawn from the low temperature carbonization drum are withdrawn through a water bath (72). At least part of the quantity of liquid withdrawn from the circulation system of washing water of the gas washing installation (21-24) is introduced in the water bath (72).

Description

The invention relates to a method for recovering usable gas from waste by pyrolysis according to the preamble of claim 1. A system for carrying out the method for this is described in the preamble of claim 11.

Methods and systems of this type are known, for example, from DE-A-33 47 554 and DE-A-35 29 445. There is a very low level of wastewater recovery of usable gas from waste, which is why for the various Stages of necessary water, in particular the amount of water required for the gas scrubbing system of the cracked gas processed in the gas converter, is circulated. In order to keep the pollutant level in the gas washing plant constant and to ensure gas purity, it was necessary to withdraw a certain part of the water cycle and replace it with fresh water. The withdrawn portion required post-treatment, because its residues posed a disposal problem. It had also turned out that fluctuating concentrations of cyanides, phenols and ammonium made it difficult to introduce them directly into a biogas plant that possibly worked with the pyrolysis plant. The fluctuating concentrations caused reduced performance of the microbes and in extreme cases even a culture destruction. However, neutralization by adding chemical oxidants and flocculants increased the amount of residues to be disposed of as special waste.

From US-A-3 862 887 a process for the recovery of gas from waste by pyrolysis is known, in which the carbonization gas is led to a water bath via a clarifier. However, the carbonization gas is only freed of the contaminants in a combustion chamber and then only washed in the car wash.

The object of the present invention is to achieve a further reduction in environmentally harmful substances requiring disposal while maintaining a low-wastewater treatment process, with a further improvement in the efficiency of the system possibly being achieved at the same time.

According to the invention, this object is achieved in that at least a portion of the amount of liquid withdrawn from the wash water circuit of the gas washing system is introduced into the water bath through which the pyrolysis residues are discharged from the degassing drum, the impurities of the carbonization gas contained in the wash water being bound by the pyrolysis residue.

Surprisingly, it was found that the pyrolysis residue absorbs more than 140 percent by weight of its own weight in liquid with a sufficient residence time. This means that of the wash water, which can consist in part or in whole of the water cycle of the gas scrubbing system, can bind to a high degree. It was found that this fluid intake the greater the carbon content of the pyrolysis residue, the greater. In addition, a sufficient dwell time for total wetting should be maintained, which can be achieved, for example, by a screw conveyor system with a correspondingly slow speed.

To increase the carbon content in the pyrolysis residue, it can therefore be provided in a further improvement of the invention that the garbage entered in the degassing drum is previously removed by sorting out inert substances. This can be done using suitable sorting devices such as, for example, comb roller sorters.

It has now also been shown, unexpectedly, that the pyrolysis residue treated in this way obtains an activated carbon structure with chemosorbing properties, so that it can be used as activated carbon for any application.

In an advantageous development of the invention, it can be provided that the pyrolysis residue is introduced as an activated carbon filter into a filter device behind the drying tower of the gas washing system.

Sulfur components of the pyrolysis gas, which mainly depend on the sulfur content of the coke used in the gas converter, are known to be difficult to remove from the pyrolysis gas. If you now connect a filter device to the gas scrubber, sulfur or fluorine compounds that have not been fully washed out can be absorbed by the activated carbon filter. Because the pyrolysis residues are used for this purpose, there are no significant additional costs and the pyrolysis residues are even put to sensible use in this way. This also means that the sulfur content in the exhaust gas is reduced cost-effectively far below the prescribed air limit values, which means that coke with a somewhat higher sulfur content can also be used inexpensively as a further advantage in the gas converter.

Due to the high absorption capacity of the pyrolysis residue, if necessary, pollutant components from other productions can advantageously be disposed of in such a way that they are mixed with the liquid portion withdrawn from the water cycle. This means that in addition to the company's own almost pollution-free treatment of waste, the method according to the invention can still be used to remove pollutants from other plants.

It was found as a further advantage of the method according to the invention that the amount of wastewater from a pyrolysis gas plant that processes household waste is reduced by more than 50% and that in general, less than 100 liters of wastewater per ton of household waste disposal. This amount of waste water is usually so polluted that untreated discharge is ecologically undesirable. However, it can be achieved a significant and cost-effective reduction in pollutant pollution if it is provided in a further development according to the invention that the amount of liquid withdrawn from the water circuit of the gas washing system is pretreated in batches by ozone injection so that after the treatment the concentration of cyanide ≦ 10 g / m³ and or phenols <40 g / m³.

If the pyrolysis residue is treated in this way, it can be easily introduced into biogas plants. Such a pyrolysis residue, which has a liquid content of approx. 60 percent by weight, is a particularly nutrient-rich carrier substance for anaerobic methane gas generators, which convert the absorbed biologically convertible groups of substances into useful gas, due to the balanced load values of phenols and cyanides due to the high carbon and ammonium content from gas scrubbing. The ozone-controlled homogenization of the pollutants minimizes the risk of overdoses and thus the destruction of culture. Experiments have shown that a short ozonization, which is generally less than a quarter of a full ozonization time, is sufficient to achieve this homogenization effect.

To further reduce the pollution caused by waste water, it can be provided that the excess water not bound by the pyrolysis residue, which generally corresponds to less than 50% of the portion withdrawn from the water cycle of the gas washing system, is fed to a rest ozonization. This can be done either in parallel with a use in a biogas plant or instead. By total ozonization of the excess water, the COD can be reduced to below 400 mg oxygen per liter, the BOD to approx. 60 mg / m³ and the proportion of cyanide and phenols to generally below 0.1 g / m³. Because of the substantial Binding the amount of partial liquid withdrawn from the water cycle to the pyrolysis residue, but only a significantly small proportion of the waste water has to be treated in this way, only a minimal amount of energy is required for ozonization of the remaining amount. In contrast to the previously known methods, there is no longer any disposal problem with the method according to the invention.

It has now been further determined that, as an alternative to using the pyrolysis residue treated in accordance with the invention in biogas plants - or in parallel - this pyrolysis residue can be used for the energetic supply in lime kilns. For this purpose it can preferably be pressed into egg briquette-like pellets. It has been surprisingly found that if a portion of the inert substances is pre-separated before the degassing, its calorific value may correspond approximately to that of lignite, which is sufficient for a smelting process. If the correct mixing ratios are observed, the inert substances are calcined or ceramized, which in particular also includes heavy metals.

Experiments have shown that with a corresponding compression between 400 and 550 kg / cm² to egg-briquette-like pellets in a mixed product of calcium-containing compounds such as lime, lime kiln filter dust, lime hydrate or lime hydrate waste with a fuel component such as coal dust as well as substances containing sulfur, chlorine, fluorine and heavy metals at approx. 1200 degrees Celsius and a residence time of 45 to 120 minutes in this temperature range, a burnout residue occurs in which almost 100% of the total chlorine present and the sulfur content, as well as the metals are ceramized or incorporated into the ashes, almost in a non-leachable manner. This means that environmentally friendly removal of pollutants, especially heavy metals, is achieved in this way.

The only requirement for the ceramization of the metals is that the molar ratio of SiO₂, Al₂O₃, CaO. ZnO, Fe₂O₃ and / or MgO on the one hand to the total molar proportion of the metals lead, chromium, manganese, cadmium, berilium, barium, selenium, arsenic, vanadium, antimony, bismuth, strontium and / or zircon is at least 6: 1.

A prerequisite for the integration of sulfur, chlorine and fluorine compounds is that the molar ratio of calcium, magnesium and sodium on the one hand to the total sulfur, chlorine and fluorine is at least 2: 1. The mixing product should contain a maximum of 40 percent by weight of liquid and the calorific value should be at least 100 kilocalories / kg so that the fuel, for example, placed in a lime kiln, with the necessary dwell time and temperature can burn up, whereby the introduced metals oxidize and are taken up in the melts of CaO and MgO, where they form relatively inert ceramic complexes. Sulfur compounds are absorbed as sulfites or sulfates, liquid chlorine and fluorine are bound to CaCl₂ and CaF₂.

Experiments carried out with pyrolysis residue have shown that this can take over the function of the fuel component for temperature generation. The pollutant components absorbed in it, which can also come from other sources than the pyrolysis gas plant itself and have been integrated into the water bath of the pyrolysis residue discharge, can be disposed of in an environmentally friendly manner. The pyrolysis plant does not leave any residues that require final landfill.

It was also found that after deducting the own energy requirement for the operation of the plants, the available useful energy can increase by more than 5% through a reduction in the own electricity requirement for the ozone plant for gas cleaning and through the improved energetic use of the pyrolysis residue.

If the pyrolysis residue is used in biogas plants, their economic efficiency for the thermal conversion of waste materials can be increased.

The residues requiring final storage in the method according to the invention are several times smaller than the residues in a waste incineration plant, which are also highly toxic and must be disposed of as special waste.

A system according to the invention for carrying out the method is described in principle below with reference to the drawing.

Since the basic structure of such a system is already known from DE-A-35 29 445, only the parts essential to the invention are described in more detail below. A discharge of the pyrolysis residues from the degassing drum via a screw device is described in DE-OS 33 47 554, see in particular FIGS. 7 and 8. Of course, other discharge devices are also possible instead of the screws shown. The only requirement is that there is intensive wetting of the pyrolysis residue in the water bath for a corresponding period of time.

The garbage is conveyed via a conveyor belt 1 into a primary crusher 2 for coarse crushing, a conveyor trough 3 and a conveyor belt 4 with a magnetic separating device 5 Garbage in a sorting device 6. In the sorting device 6, the heavier, wet, vegetable fraction is separated and falls into an underlying container or onto a conveyor belt 7. Likewise, inert substances can also be separated off in order to increase the carbon content of the pyrolysis substance. The remaining waste fraction is fed to a thermal screw press 14 via a further conveyor belt 8, a further shredding device 9 and a downstream hydrocyclone 10, in which heavy parts are separated again via a line 11. Via the line 11, the waste together with the separated waste from the container 7 is fed via a feed line 12 to a biogas plant 13, which can be preceded by a fluid classifier for the pre-selection of inert heavy parts, which works, for example, according to the flushing method.

In the thermal screw press 14, the formation of granules in a rapidly degassable structure and a size of approximately 3 to 50 mm is achieved in a known manner by frictional pressure at approximately 110 to 150 degrees Celsius.

Instead of in granular form, fluff, ie a loose waste form, or pellets, ie waste constituents compressed into briquettes, can also be produced in the specified size, but these generally require longer degassing times. The waste components crushed in this way pass through a rotary valve 15 or other entry devices, such as stuffing screws in a degassing drum 16, in which carbonization gas is generated in a known manner at temperatures of 450 to 600 degrees Celsius, which is introduced into a high-temperature gas converter 19 via a discharge line 17 and a dust separating device 18. The processing or conversion of the carbonization gas takes place in the gas converter 19 over a coal or coke bed. A gas converter of this type is described, for example, in DE-A-33 17 977.

After passing through a heat exchanger 20, the gas passes into a gas scrubbing system which essentially consists of a water spray tower 21, a blower 22 and a cleaning cylon 23 and a droplet separator 24. Via a gas line 25, the cleaned gas reaches a gasometer 26, in which excess gas can be supplied to a flaring device 28 if too much gas is supplied via a secondary line 27.

The gas normally passes from the gasometer 26 to a gas engine 29, which is connected to a generator 30, the burned exhaust gases being introduced into a chimney 32 via an exhaust gas line 31.

The gas converter 19 receives water via a line 33 and 34 coke via a coke inlet. Ash and slag is over a discharge line 35 is discharged. Possibly. To save energy, a coke return line 36 can also be provided for the coke freed from the slag.

A branch line 37 branches off from the gas line 25 and leads to a gas burner 38 which serves to supply heat to the degassing drum 16. During the start-up phase of the system, an oil burner 39 or a separate gas burner is used to heat the smoldering drum. Subsequently, during operation, however, the heat required for the degassing drum 16 can be completely covered by the burner 38.

The scrubbing water obtained during gas cleaning passes into a scrubbing water tank 40 and then into a filter device 41 (usually a settling tank). Separated solids in the filter device are introduced into an ash container 43 via a line 42. The residues are removed from the ash container 43 via a discharge line 44 and introduced into the smoldering drum 16 again via the insertion device, possibly a cellular wheel sluice 15.

The cleaned washing water returns from the filter device 41 via a return line 45 after passing through a cooling tower 46 back into the spray tower 21 of the gas washing system. A part of the cleaned wash water is in a Wash water neutralization system 47 is introduced, into which the vapor vapor condensate from the thermal screw press 14 is also introduced via a line 58, provided that it is not fed via line 65 to the reservoir 53 of the biogas system.

From the wash water neutralization system 47, the wash water reaches the spray tower 21 via a return line 78 for circuit treatment, while a partial quantity reaches a circuit water batch treatment system 48 via a partial quantity removal line 79. In a known manner, the washing water is chemically cleaned therein by appropriate chemicals, which are entered via line 49, provided that the chemical oxidation is not replaced by ozonization, as a result of which the residue-increasing additive addition can be greatly reduced. The washing water is partially removed via a circuit line 50. passed through an air filter 51 to remove foams, exhaust gases being blown off through a line 52 over the chimney 32, and partly. there is a return via a line 80 into the spray tower 21.

The chemically and mechanically cleaned water passes from the circulating water batch treatment system 48 via a line 71 into the preliminary tank 53 of the biogas system. If necessary, sewage sludge, raw compost can also be placed in the storage tank or the like. What is indicated by the arrow "54". The vapor vapor condensate, if it has not been passed through the circulating water batch treatment system 48, can be introduced directly into the preliminary container 53 via a line 65.

The substances to be worked up in the biogas plant 13 arrive in a hydrolysis stage or a hydrogenator 56 from the pre-tank 53. A countercurrent heat exchanger 57 connects to the hydrolysis stage 56 and receives its heat through a hot water line 62, which comes from the cooling tower 46 of the wash water cleaning system branches. In a septic tank floor 67, a coil heating system ensures a temperature rise in the methane area of the septic tank of 33 to 37 degrees Celsius. In this way, the excess heat from the pyrolysis plant is used for the biogas plant 13.

The biogas plant 13 is constructed in the usual way. As a phase-separated biogas plant, it can have a normal acid phase in the upper region in the middle shaft 63, while an acetic acid phase is present in the lower region. The methane gas formed is drawn off via a methane gas line 59 and fed to the gas line 25 or the gas washing system of the pyrolysis system for cleaning via a buffer 60 and a compressor 61. The digestate is discharged through a suction line 66 and a pre-dewatering device 68 fed, whereby it is brought to about 20% dry matter. The solids contained in the fermentation residue can be brought to approx. 85% dry matter via a dry press 69. The remaining fermentation water is collected in a lagoon 70 and, if necessary, fed to the treatment plant 48 or fed directly into the sewage system.

The substances separated in the circulating water batch treatment plant 48 can be fed to a sewage treatment plant via a line 64.

Pyrolysis residues are discharged in the degassing drum 16 via a water bath 72, the discharge e.g. can take place via screw conveyors. The water bath 72 is supplied with the liquid required for wetting the pyrolysis residue via a partial liquid line 73. The partial quantity line 73 branches off from the batch treatment system 48.

The pyrolysis residue wetted with liquid in this way is discharged via a line and can optionally be fed to the biogas plant 13 after treatment, for example ozonization or other cleaning. The biogas plant 13 is of course only mentioned for example. It is not necessary for the invention itself. Instead of one Introducing the pyrolysis residues into the biogas plant 13, the pyrolysis residues can, if necessary, also be transported via a line 74 A to a lime kiln 77.

Likewise, the pyrolysis residue can be introduced via a line 74 B after it has dried into a filter device 75 as an activated carbon filter. The filter device 75 is located between the gas washing system and the gasometer 26. In this case, however, the activated carbon obtained from the pyrolysis residue should be at least largely free of pollutants. This means that one operates in batches and introduces fresh water through a fresh water line 81 into the water bath instead of a water supply via the partial liquid quantity line 73 with contaminated circulating water.

If necessary, the liquid portion removed from the water cycle can also be treated in an ozone injection system 76. The ozone system 76 is connected to the batch treatment system 48. If the partial quantity withdrawn from the circuit via the partial quantity extraction line 79 is appropriately cleaned before it is introduced via the liquid partial quantity line 73 into the water bath 72 in the ozone system 76, the pyrolysis residue can also be used as an activated carbon filter in the filter device 75 in this procedure.

The function and mode of operation of a lime kiln 77, into which the pyrolysis residue is introduced via line 74 A, are generally known, which is why it is not discussed in more detail here. The pyrolysis residue may be used as fuel together with other fuel components. Of course, it is not necessary that a direct line 74 A is present between the water bath 72 and the lime kiln 77. The system according to the invention is designed so that not all units have to be in one place. For example, the biogas plant 13 and the lime kiln 77 are at a different location, and the substances can be transported there in any way.

The partial liquid quantity which has not been completely absorbed by the pyrolysis residue in the water bath 72 can, if necessary, also be passed through the ozone system 76, if necessary for full ozonization, before it is introduced into a sewage treatment plant.

Of course, it is not absolutely necessary for the partial liquid line 73 to branch off from the wash water neutralization system 47; rather, the removal can also take place at another point in the circulating water supply if required. The same applies to the activation of the Ozone injection system 76.

Claims (13)

1. Process for recovering reusable gas from waste through pyrolysis, in which the previously crushed waste is worked up to fluff, granules or pellets of 1 to 50 mm in size, is brought to a solids content of at least 75 % and subsequently is introduced into a heated degassing drum, in which carbonisation gas is produced and is separated from the residual matter, such as ash and other constituents, and in which the carbonisation gas is decomposed to combustion gas in a gas converter and is cleaned in a subsequent gas-washing installation with circulating washing water, part of the water of the circulation system being removed from the washing water circulation system of the gas-washing installation and being replaced by fresh water in order to limit its concentration of noxious substances, and the pyrolysis residue to be withdrawn from the carbonisation drum being withdrawn via a water bath, characterised in that at least part of the quantity of liquid removed from the washing water circulation system of the gas-washing installation (21-24 and 47-51) is introduced into the water bath (72), the impurities of the carbonisation gas contained in the washing water combining with the pyrolysis residue.

2. Process according to claim 1, characterised in that inert substances are first removed by sorting from the waste introduced into the degassing drum (16) in such a quantity that the pyrolysis residue withdrawn from the degassing drum contains at least 25 per cent by weight of carbon.

3. Process according to claim 1, characterised in that the pyrolysis residue treated with the quantity of washing water from the circulation system is used as an activated carbon filter.

4. Process according to claim 3, characterised in that the pyrolysis residue is used as an activated carbon filter in a filtering apparatus (75) after the gas-washing installation (21-24).

5. Process according to one of claims 1 - 4, characterised in that harmful constituents from other productions are mixed with the circulating water concentrate removed from the washing water circulation system for the disposal thereof.

6. Process according to one of claims 1 - 5, characterised in that the quantity of liquid removed from the water circulation system of the gas-washing installation is pretreated batchwise by ozone injection so that after the treatment the concentration of cyanide ≦ 10 g/m³ and/or phenols < 40 g/m³.

7. Process according to one of claims 1 - 5, characterised in that the quantity of liquid removed from the water circulation system of the gas-washing installation not absorbed upon moistening of the pyrolysis residue is subjected to further batchwise ozonisation until its COD < 400 mg of O₂/litre.

8. Process according to claim 1, characterised in that the pyrolysis residue moistened with the quantity of liquid is introduced into clarification plants or biogas installations (13).

9. Process according to claim 1, characterised in that the pyrolysis residue moistened with the quantity of liquid is fed to lime kilns (77).

10. Installation for carrying out the process according to one of claims 1 - 9, comprising a waste grinder, a drying apparatus and a degassing drum comprising an inlet for the previously crushed waste, an outlet for solid pyrolysis residue and a carbonisation gas discharge pipe, to which a gas converter for recovering cracked gas is connected, and comprising a gas-washing installation connected after the gas converter and having a water bath connected to the outlet for the pyrolysis residue for the withdrawal thereof, characterised in that a pipe (73) for the quantity of liquid leads from an apparatus (48) of the gas-washing installation to the water bath (72).

11. Installation according to claim 10, characterised in that a sorting apparatus (6) for sorting inert substances is arranged in front of the degassing drum (16).

12. Installation according to claim 10 or claim 11, characterised in that a filtering apparatus (75) is arranged after the gas-washing installation (21-24 and 4751) into which the pyrolysis residue can be placed as an activated carbon filter.

13. Installation according to one of claims 10 - 12, characterised in that an ozone injection apparatus (76) is provided for the quantity of liquid removed from the water circulation system of the gas-washing installation (21-24).

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