EP0412587B1 - Process for purifying raw combustible gas from solid-fuel gasification - Google Patents

Description

The invention relates to a method for cleaning the raw gas from the gasification of solid fuels, in particular carbon-containing waste, with oxygen-containing gasification agent, wherein a carbon oxides, hydrogen and methane and as contaminants dust, NH₃ and hydrogen halide containing raw gas with temperatures of about 500 to 1200 ° C generated becomes. Garbage can also be given to gasification.

The gasification of coal, carbon-containing waste and also waste is known and e.g. in U.S. Patent 4,032,305. The problem here is the purification of the raw gas from the gasification, for which there are known methods, but additional waste materials are generated.

The invention has for its object to remove from the raw gas NH₃, HCl, HF and dust in a simple manner so that the amount of impurities to be removed is as small as possible. In the process mentioned above, this is achieved by cooling the raw gas to temperatures of 150 to 400 ° C, the cooled raw gas with an NH₃ content of at least 0.1 vol .-% in a spray drying zone and in contact with sprayed return water brings, the return water is completely evaporated, that you pass the steam containing raw gas from the spray drying zone at temperatures of 80 to 250 ° C through a filter, in the filter at least 80 wt .-% of the solids contained in the raw gas in the filter inlet and at least 50 wt. -% of the halogen compounds are separated dry, that the raw gas is passed from the filter through a saturation zone in which the raw gas is brought into direct contact with sprayed water, the raw gas is saturated with water vapor and cooled to temperatures of 50 to 90 ° C. the Saturation zone derives saline and solids-containing water with a pH of 7.5 to 9.5 and gives the spray drying zone as return water, and that the raw gas from the saturation zone is aftertreated to remove dust and saline liquid droplets.

In the process according to the invention, the raw gas entering the spray drying zone has a considerable NH 3 content of at least 0.1% by volume up to about 1% by volume. This NH₃ content usually results in the raw gas without further notice that the fuels are not burned but gasified, the energy required for this being supplied by partial oxidation. In addition to oxygen, air or air enriched with oxygen, steam is also used as the gasifying agent, which can be partially replaced by CO₂.

The calorific value of the gas produced during gasification can be in an incinerator and e.g. can also be used in a power plant. It is important that the raw fuel gas is cleaned in a cost-effective manner and yet with sufficient intensity. In the method of the invention, the cleaning of the raw gas does not lead to polluted wastewater that would have to be cleaned again. The process can be carried out completely without the addition of chemicals, if only it is ensured that the NH₃ content in the raw gas in the entrance to the spray drying zone is sufficiently high. In the gasification of waste and municipal waste, this is usually the case without the addition of external NH₃.

It is very important for the process that temperatures of 80 to 250 ° C are maintained in the spray drying zone. Under these conditions, NH₃, HCl and HF form ammonium halides directly from the gas phase and are deposited in solid and dry form by desublimation and attachment to existing solid particles. The agglomerates thus formed can then be easily filtered out.

Heavy metals or heavy metal compounds also desublimate, accumulate preferentially in the spray drying zone on the desublimated halogen particles and on the dust brought in with the raw gas and can thus also be removed from the gas to a sufficient extent.

Due to the sufficiently high NH₃ content in the raw gas and the ammonia introduced into the return water, the pH in the water is 7.5 to 9.5, so that cheap carbon steel can be used for the systems and pipes. A cost reduction through energy savings is also given by the fact that one can manage with a relatively small amount of water both in the spray drying zone and in the saturation zone.

The gas from the saturation zone mainly contains dust and saline liquid droplets, so that an aftertreatment is necessary. A wet electrostatic filter, a wet scrubber, a droplet separator are particularly suitable for this aftertreatment, the latter also being able to be connected downstream of a wet scrubber, or a condenser. It is important to remove the dust and the saline liquid droplets without adding chemicals. The water that contains dust and salt is also fed into the spray drying zone.

Further details and refinements of the method are explained with the aid of the drawing.

In the gasification reactor (1), solid fuels, for example coal or biomass, or also carbon-containing wastes, which are introduced in line (2), are gasified with air from line (3). The waste can be municipal waste, for example. As gasifying agent, the reactor (1) can also be given water vapor coming from line (4). The gasification air can be enriched with oxygen, you can only work with technically pure oxygen together with water vapor. Im in the The process shown in the drawing shows that the fuel or waste is gasified in the circulating fluidized bed, but the gasification can also take place in the fluidized bed, in the entrained flow or in a fixed bed. The deck furnace is also possible as a gasification reactor.

A mixture of raw gas and solids passes through the channel (6) to a cyclone (7), where the majority of the solids are separated from the gas. Instead of just one cyclone (7), you can also connect several cyclones in parallel or in series. The solids are then partly fed back through lines (8) and (9) to the gasification reactor (1) and an excess can be drawn off in line (10). Ash is drawn off directly from the reactor (1) through line (5).

The dust-containing raw gas, which contains combustible components and NH₃, HCl and usually also in small quantities HF, leaves the cyclone (7) in line (12) at temperatures in the range of about 500 to 1200 ° C. A first indirect cooling takes place in the heat exchanger (13). can be used to preheat the gasification air in line (3). For the sake of clarity, this possibility was not shown in the drawing. Further cooling can take place in the heat exchanger (14) if this is expedient.

From the heat exchanger (14), the pre-cooled raw gas at temperatures of 150 to 400 ° C in the line (16) to a spray dryer (17). Care is taken to ensure that the NH₃ content in the raw gas of line (16) is at least 0.1% by volume and is present in a sufficient amount for the reaction with the hydrogen halides in the raw gas. Usually the raw gas from the waste gasification contains enough NH₃, but if the NH₃ content is too low, foreign NH₃ can be added. In the hot raw gas coming from line (16) is sprayed in Spray dryer (17) return water from line (18). This water carries halogen compounds, heavy metal compounds and dust with it, which have been washed out of the raw gas elsewhere. The pH value of the return water is therefore in the range from 7.5 to 9.5. Also in the spray dryer (17), water is sprayed from the line (19) which contains hardly any impurities. Particular care is taken to ensure that no further chemicals are added to the spray dryer (17). Such an addition of chemicals would only lead to the need to dispose of large amounts of waste.

In the spray dryer (17), all the water introduced is evaporated, with NH 3, HCl and HF reacting in the presence of the recycled solids and salts to form agglomerates containing ammonium halides. At the same time, the conditions in the spray dryer mean that aerosols are avoided, which is caused in particular by the supply of condensation nuclei with the return water from line (18).

At a temperature in the range of 80 to 250 ° C, a mixture of raw gas and solids passes through the line (20) to a filter (21). This filter, in which the solids are separated from the gas dry, can be, for example, a bag filter, a candle filter, an electrostatic filter or one or more cyclones. It is important that at least 80% by weight of the solids introduced in the raw gas in line (20) and at least 50% by weight of the halogen compounds which are removed in line (22) are separated off in the filter (21). Usually at least 50% by weight of the heavy metals are also separated off in the filter (21). The solids in the line (22), the amount of which has not been increased by avoiding the addition of foreign chemicals, must be removed and deposited.

The partially dedusted raw gas now flows in line (24) to a saturation zone (25), into which water is sprayed through line (23a). As a result, the raw gas is cooled further, saturated with water vapor and partially dedusted again. Halogen and heavy metal compounds in particular are effectively removed by the sprayed water. Salt and solids-containing water which has a pH in the range from 7.5 to 9.5 is drawn off from the sump (25a) of the saturation zone. This water is led through lines (18 a) and (18) back to the spray dryer (17). The gas leaving the saturation zone (25) in line (26) has only a temperature in the range from 50 to 90 ° C.

For the now necessary aftertreatment, a wet electrostatic filter (27) was provided in the drawing, which is acted upon with water from the line (28). Water containing dust and salt is drawn off in line (29) and added to the return water in line (18). Deviating from the drawing, the wet electrostatic filter (27) can be replaced by a wet scrubber and / or by a droplet separator.

Dust-free, water vapor-containing gas passes from the electrostatic filter (27) in line (30) to a condenser (31) in which the water vapor is partially condensed by indirect cooling and thus removed. The condensate, which is fairly clean water, is discharged in line (32) and distributed to lines (23) and (19), a partial stream can also be returned via line (33) and as rinsing water passed through the condenser. In the condenser (31), which can also be omitted behind a wet electrostatic precipitator, its calorific value can be regulated by adjusting the water vapor content in the gas. In the line (35), purified fuel gas is available, which can be used, for example, in a Power plant can use. Before use, the gas can be heated by indirect heat exchange with the raw gas from line (12), for example in the heat exchanger (14). By means of the cleaning method of the invention, it is readily possible to provide a fuel gas in line (35) which has maximum HCl values of 10 mg per Nm³ and dust, HF and heavy metals of at most 1 mg each.

example

C

31,1 Gew.-%

H

4,6 Gew.-%

O

19,0 Gew.-%

N

0,6 Gew.-%

S

0,2 Gew.-%

Cl

0,4 Gew.-%

H₂O

28,0 Gew.-%

Asche

16,05 Gew.-%

Schwermetalle

0,05 Gew.-%

In a plant corresponding to the drawing, waste consisting of a mixture of municipal waste, commercial waste and sewage sludge is gasified. The lines (4), (10) and (33) shown in the drawing are missing in the system. 6,500 kg of waste are gasified per hour with 8,000 Nm 3 of air supplied to line (3), which has been preheated to 600 ° C. in heat exchanger (13). The basic analysis of waste is as follows:

C.

31.1% by weight

H

4.6% by weight

O

19.0% by weight

N

0.6% by weight

S

0.2% by weight

Cl

0.4% by weight

H₂O

28.0% by weight

ash

16.05% by weight

Heavy metals

0.05% by weight

CO

13,6 Vol.-%

H₂

10,4 Vol.-%

CO₂

7,1 Vol.-%

CH₄

1,6 Vol.-%

CnHm

0,7 Vol.-%

N₂

42,8 Vol.-%

H₂S + COS

0,05 Vol.-%

NH₃ + HCN

0,3 Vol.-%

HCl + HF

0,1 Vol.-%

H₂O

23,35 Vol.-%

The waste has a calorific value of 12,600 kJ / kg. The gasification takes place in the circulating fluidized bed at a pressure of 1.35 bar, the gasification reactor (1) having an inner height of 14 m and an inner diameter of 2 m. Two cyclones (7) are connected to the channel (6). The first in line (12) Heat exchanger (13) with a temperature of 950 ° C and raw gas flowing in at 15,000 Nm³ / h is made up of the following main components:

CO

13.6 vol .-%

H₂

10.4 vol .-%

CO₂

7.1 vol%

CH₄

1.6 vol%

CnHm

0.7 vol%

N₂

42.8 vol%

H₂S + COS

0.05 vol .-%

NH₃ + HCN

0.3 vol%

HCl + HF

0.1 vol .-%

H₂O

23.35 vol .-%

This raw gas contains 10,000 mg of inert dust and 34 mg of heavy metals per Nm³. When cleaning this raw gas, the following temperatures occur in the different lines:

The following amounts of water flow through the pipes:

The spray dryer (17) has a height of 6 m and a diameter of 1.5 m, the downstream filter (21) is designed as a bag filter. In the line (22) 180 kg of filter dust accumulate per hour, which together with 1,200 kg / h of ash from the line (5) are to be disposed of from the gasification reactor (1). The flowing out in the line (35), cleaned fuel gas contains less than 1 mg of dust, 5 mg of NH₄Cl and 1 mg of heavy metals and 1.5 g of NH₃ per Nm³. The gas is heated to 500 ° C in the heat exchanger (14) and burned in a power plant boiler. In the boiler, which works on the principle of the circulating fluidized bed and is also fed with coal, the sulfur compounds are bound into the boiler ash in a known manner using limestone.

Claims (3)

1. A method for purifying the crude gas from the gasification of solid fuels, in particular carbon-containing waste, with oxygen-containing gasification agent, in which a crude gas containing carbon oxides, hydrogen and methane, and also dust, NH₃ and hydrogen halides as impurities, is produced at temperatures of about 500 to 1200°C, characterised in that the crude gas is cooled to temperatures of 150 to 400°C, the cooled crude gas having an NH₃-content of at least 0.1% by volume is passed into a spray-drying zone and is brought into contact with recycled water sprayed therein, the recycled water being completely evaporated, that the crude gas containing water vapour is passed out of the spray-drying zone through a filter at temperatures of 80 to 250°C, at least 80% by weight of the solids contained in the crude gas at the entrance to the filter and at least 50% by weight of the halogen compounds is separated off in the dry state in the filter, that the crude gas is passed out of the filter through a saturation zone in which the crude gas is brought into direct contact with sprayed water, the crude gas in so doing is saturated with water vapour and is cooled to temperatures of 50 to 90°C, water containing salts and solids and having a pH value of 7.5 to 9.5 is withdrawn from the saturation zone and is fed to the spray-drying zone as recycled water, and that the crude gas from the saturation zone is subsequently treated to remove dust and salt-containing droplets of liquid.
2. A method according to Claim 1, characterised in that the subsequent treatment of the crude gas from the saturation zone is performed in a wet electrostatic precipitator, a wet scrubber, a mist eliminator and/or in a condenser, and water produced thereby is fed to the spray-drying zone as recycled water.
3. A method according to Claim 1 or 2, characterised in that the subsequently-treated gas is cooled, water vapour is condensed out and at least a portion of the condensate is passed into the saturation zone.

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