CA1214985A

CA1214985A - Process for the separation of dry fly ash from a gas

Info

Publication number: CA1214985A
Application number: CA000414529A
Authority: CA
Inventors: Maarten J. Van Der Burgt; Annemarie Van Der Rest
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1981-11-19
Filing date: 1982-10-29
Publication date: 1986-12-09
Also published as: ZA828451B; AU552952B2; AU9065382A; DE3242651A1; JPS5889928A; NL8105239A

Abstract

A B S T R A C T

A PROCESS FOR THE SEPARATION OF
DRY FLY ASH FROM A GAS

Process for the separation of fly ash from hot crude synthesis gas by means of a cyclone and a scrubbing system. A fly ash/water slurry derived from the scrubbing system is partly recycled to the hot crude synthesis gas before the cyclone and partly withdrawn.
In conventional gas purification systems there exists a serious chance of the building-up of corrosive materials like halogenides and formic acid, in the water recycle loop(s). This drawback is avoided by the improvement according to the present invention.

Description

A PROCESS FOR THE SEPA.RATION OF
DRY FLY ASH FROM A GAS

The invention relates to a process for the separation of dry fly ash from a gas having a temperature in the range from 100 to 2000C, whereby the greater part of the fly ash is removed from the gas by means of at least one cyclone and the gas is subsequently scrubbed with water in at lPast two steps, the aqueous fly ash suspension obtained in the first step being partly injected into the starting gas.
In principle, all solid particles-containing gases can be purified by the present process. The process is, however, especially suitable for purifying synthesis gas obtained by means of partial oxidation of coal. In addition to hydrogen, 15 carbon monoxide, carbon dioxide, light hydrocarbons and water vapour, said gas also contains fly ash particles and mostly gaseous byproducts, such as sulphur and nitrogen compounds. By means of the present process the fly ash particles can be 20 substantially removed from the synthesis gas.
After the greater part of the solid particles have been removed by means of one or more cyclones, . .

the gas is scrubbed with water in at least two steps. To this end the gas is intimately contacted with water~ This can be effected by means of one or more venturis, clistribution trays, packed columns or sprayersO After every contact step, gas and the resulting aqueous suspension of solid particles are separated from each other. The gas is conducted to the next contact step. The suspension is at least partly conducted to the preceding step.
The gas that has passed the cyclone or cyclones is prefexably first contacted in a venturi with an aqueous suspension partly originating from the second contact step and partly originating from the first contact step. Consequently, the first contact step preferably consists of a venturi. After the gas has passed through this step, the resulting mixture of gas and aqueous suspension is separated in a separator into a partly purified gas and an aqueous suspension of solid particles which is 20 partly recycled to the venturi and is partly injected into the starting gas. The latter contact between hot starting gas and recycled aqueous suspension preferably also takes place in a venturi, after which the watex of the suspension evaporates.
The gas/water vapour/solid particles mixture is subse~uently conducted to the cyclone or cyclones in order to remove most solid particles.
The effluent gas from the separator is preferably scrubbed in a gas scrubber in counter-current with an aqueous suspension of solidparticles. The gas scrubber preferably consists of a packed bed of packing material such as Raschig rings, Pall rings or Berl saddles. The aqueous suspension of solid particles originating from the gas scrubber is preferably partly pumped to the ventuxi and preferably partly cooled. In order to obtain a satisfactory result by means of the present process the water of the aqueous suspension that is injected into the starting gas must substan-tially evaporate after said injection. Thiscondition is fulfilled if the temperature of the starting gas and that of the injected quantity are so balanced tha~ the tempera~ure of the gas/water vapour mixture, after the aqueous suspension of the solid particles therein has completely evaporated, is at least above the dewpoint of said mixture. The temperature of the starting gas is therefore in any case higher than its dewpoint. In practice the temperature of the starting gas will therefore be at least 100C.

The maximum temperature of the starting gas for the present gas purification process is the maximum outlet temperature of -the preceding process s,ep.
In the case of the partial oxidation of coal said temperaturP will be at most 2000C. Since the efficiency of apparatus for the removal of said particles, such as cyclones, increases with falling gas temperature, it is desirable to reduce the gas temperature to temperatures below 1000C, preferably below 500C. Said reduction in temperature is at least partly caused by the evaporation of the injected suspension, as described. Another part of the reduction in temperature can be realized by direct or indirect heat transfer to a cooling medium, for example in a steam boiler or by water injection.
An advantage of recycling an aqueous suspension of the solid particles separated off in the wet gas scrubbing step to the starting gas consists in the possibility to limit the waste water stream from the gas purification system, since the solid particles separated off therein can leave the integrated plant in dry condition. By withdrawing heat from the yas purification system after the dry separation step the outlet temperature of the last gas scrubber can even be set in such a manner that 9~
~ 5 --the quantity of water condensing in the scrubbers is equal to the quantity of water that is recycled with the suspension that is injected into the starting gas. In this case no net water consumption takes place, nor is contaminated water discharged from the gas purification plant. If the present process is applied to a mixture of gas and fly ash origination from an apparatus for the partial oxidation of coal the envisaged use of the synthesis gas is one of the factors determining whether the cooling of the reactor waste gas to the desired cyclone temperature can be more advantageously carried out by means of indirect heat transfer in a steam boiler or by direct water injection.
1~ In the event of a steam boiler being used, minimum waste water production and minimum water consumption will be realized if the temperature in the last part of the gas purification plant lies between 25 and 200C. The purified gas leaving the

2~ gas scrubber is preferably cooled to a temperature in the range from 25 to 120C, in which part of the water vapour remaining therein conden~es. The resulting water is separated from the gas.
If it is useful in the further processing of 25- the ~as to deliver it with a high water vapour 3L?~ 5 concentration - for example to a water gas shift reactor to increase the hydrogen content of the gas at the expense of the carbon monoxide content ~ at least part of the cooling of the reactor waste gas is carried out by direct in~ection of water into said gas. The temperature in the last scrubber ~ill then be 100-360C in order to minimize the conden-sation of water vapour in the scrubbing system. In this case the cooling of this system will be limited or can be omitted entirely~
As already stated, the greater part of solid particles is first removed from the gas by means of at least one cyclone. If the solid particles containing gas originates from a coal combustion or gasification plant, it contains fly ash. The fly ash separated off in the cyclone(s~ is then prefer-ably recycled to the combustion or gasification plant.
An advantage of recycling separated fly ash to the gas combustion or gasification plant is that now non combusted carbon is burnt as much as possible from the fly ash, so that the degree of conversion of the coal increases. If the combustion or gasification plant is of the slag-forming type, the recycled fly ash will leave the combustion apparatus in a considerable degree as a component of the molten slag. The resulting vitreous slag particles or cinders are inert to a variety of leaching liquids, such as rainwater or groundwater, so tha~ any noxious matter present cannot reach the environment. The waste problem of the dusty and sometimes noxious fly ash is consequently reduced, The greater part of the fly ash is separated from the gas by means of one or more cyclones at a temperature between 100 and 2000C, preferably at a temperatuxe of at most 500C, and the remainder of the fly ash is removed from ~he gas by means of scrubbing with water, the resulting aqueous suspension being injected into the starting gas.
Consequently, only purified gas and vitreous slag are obtained as final products of the aforedescribed purification of gas. The recycling of fly ash to the combustion apparatus, however, has the drawback that a variety of volatile components build up in the supply loopO Components that are volatile at the high temperatures in the combustion apparatus, such as some halogenides, cyanides and formic acid, and are therefore not discharged together with the slag, condense or solidify if the temperature of the surrounding gas falls and are invariably partly taken up in the fly ash to be recycled and for the rest in said aqueous fly ash suspension. Consequently, the con-centration of volatile compounds in said streams would invariably increase further. Since condensing volatile compounds play a part in the fouling oE the heat exchangers, said increased concentra-tion of compounds of this type results in a higher ~egree and rate of fouling. Moreover, at elevated concentration said compounds have the result that the apparatus used must be construc-ted of expensive high-alloy steel since plain steel would corrode too quickly. According to the invention said problems are now avoided, since the suspension of solid particles obtained in the first contact step is not en-tirely injected into the starting gas and in said contact step only part of said suspension is removed from the aEoredescribed se~aration system.
The invention therefore relates to a process for the separation of dry fly ash from a gas having a temperature in the range -from 100 to 2000C, in which the greater part of the fly ash is removed from the gas by means oE at least one cyclone and the gas is subsequently scrubbed with water in a scrubbing system, the aqueous fly ash suspension obtained in the scrubbing system being partly recycled to the hot gas to be purified (starting gas) before -the cyclone(s), characterized in that part of the aqueous fly ash suspension obtained in the scrubbing system is dis-charged.

S

Preferably, per kg of gas a quantity of 0.005-2 kg of the aqueous suspension of solid particles obtained in the first contact step is removed from the purification system. This quantity has been found sufficient to avoid the building-up of undesirable components in the recycle loops of the gas purifica-tion process.
It is advantageous to dewater the withdrawn suspension.
After further purification the water can be reused or drained, and the dewatered suspension, consisting of wet fly ash particles, is preferably ground and dried together with coal. The resul-ting homogenized mixture of coal and fly ash is advantageously used as feed for a combustion or gasification plant, in which i-t is com-busted fully or partly, after which the fly ash thus supplied can largely be drained as slag from the combustion or gasification plant. One part of the dewatered suspension is pre-ferably mixed with at least 10 parts by weight (both based on anhydrous and ashless condition) of coal.

Dewatering can be performed in any known manner. For this treatment filtration, flotation or sedimentation are advantageously used. The filtrate can again be used ~ optionally after purification -or can be discharged to the environment afterfurther purification, if desired.
Another suitable method of processing the withdrawn suspension has the advantage that low-temperature heat present in the starting gas ~if desired, after recovery of part of the heat of the starting gas) can be used for drying the solid particlesO In this method the water in the with-drawn suspension is displaced by water containing at least a reduced concentration of said volatile components, for example by dewatering followed by scrubbing and suspending in relatively clean water.
This new suspension is then preferably recycled to the wet scrubbing system or to the recycle loop at the point where the recycle suspension is injected into the starting gas. After purification the displaced water can again be used or discharged to the environment.
In the case of fly ash being recycled from the dry ash separating step, for example one or more 25 cyclones, to the combustion or gasification plant, it is advantageous to choose the temperature in the dry ash separator in such a manner that those components whose presence is desirable in only low concentration for proper operation of the plant upstream of the dry ash separator, for example a boiler, should not be selectively removed from the gas stream. Halogenides, such as chlorides, are well-known examples in this respec~. The temper-ature of the dry ash separator is then preferably chosen higher than 200C~ in particular higher than 250C. Also if it is for other resons desirable to produce a fly ash having low concentrations of said components, it is of advantage to choose this range for the operating temperature of the dry ash separatorts).
The invention will now be further illustrated with reference to a figure giving a diagrammatic representation of a process and apparatus according to the inventionO
In this figure 1 is a reactor in which complete combustion of a feed 24 consisting of coal and recycled fly ash takes place by means of a stream of oxygen 3. A stream of liquid slag 2 is dis charged from the reactor 1. The crude gas 4 is conducted to an indirect heat exchanger 5 from which gas 6 cooled to about 400C is obtained with formatiorl of steam 7 from a water stream 8, The gas 6 i5 separated in a cyclone 9 into a stxeam of fly ash 10, which is recycled to the reactor 1, and an effluent gas 11, which is intimately contacted with a fly ash/water suspension 13 in a yas scrubber 12. A purified gas stream 14 leaves the scxubber 12, which gas stream is freed from fly ash. Stream 15 is a fly ash/water suspension which is partly cooled in a cooler 26 and is then recycled to the top of gas scrubber 12 together with a stream of fresh or recycled water 19 as stream 13. The remainder of the stream 15 is partly concentrated in a rotary filter 16 to form a paste 17, the remaining part being injected into the starting gas to be purified (stream 25). The water 18 liberated in this stage is discharged from the system. The paste 17 is introduced into a pressure-resistant ball mill 21 by means of an extrusion pump 20. While the mill is rotating, the paste is heated therein with the addition of a stream of coal 22 together with which the paste is finely ground and dried. The pressure resulting from the generated steam can be up to 100 kg/cm2.
The pressure chosen is about equal to that in the reactor 1. The temperature in the ball mill 21 can be, for example, 235C. From the ball mill 21 steam flows in which is dispersed a mixture of coal and - 13 ~
fly ash 24, which is passed to the reactor 1. In this manner the fly ash is completely recycled to the gasification process. At the same time the requisite quantity of steam is introduced into the reactor. In this manner fouling problems resulting from the fly ash particles are solved elegantly, while no undesirably high concentrations of halogenides, formic acid, etc., occur in the fly ash recycle system.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the separation of dry fly ash from a gas having a temperature in the range from 100 to 2000°C, in which the greater part of the fly ash is removed from the gas by means of at least one cyclone and the gas is subsequently scrubbed with water in a scrubbing system, the aqueous fly ash suspension obtained in the scrubbing system being partly recycled to the hot gas to be purified (starting gas) before the cyclone(s), characterized in that part of the aqueous fly ash suspension obtained in the scrub-bing system is discharged.

2. A process as claimed in claim 1, characterized in that per kg of gas a quantity of 0.005-2 kg of the aqueous fly ash suspension obtained in the scrubbing system is discharged.

3. A process as claimed in claim 1, characterized in that the discharged suspension is dewatered.

4. A process as claimed in claim 3, characterized in that the discharged suspension is filtered.

5. A process as claimed in claim 3, characterized in that the discharged suspension is dewatered by sedimentation.

6. A process as claimed in claim 3, characterized in that the discharged suspension is dewatered by means of flotation.

7. A process as claimed in claim 3, characterized in that the dewatered fly ash is ground and dried together with coal, after which the resulting homogenized coal/fly ash mixture is combusted or gasified.

8. A process as claimed in claim 7, characterized in that one part by weight of the dewatered fly ash is mixed with at least 10 parts by weight of coal.

9. A process as claimed in claim 3, characterized in that the dewatered fly ash is scrubbed and taken up in clean water, after which the resulting suspension of scrubbed fly ash in water is recycled to at least one of the steps in which the gas is scrubbed or to the starting gas.

15.