CA1076809A - Pre-treatment of water-containing lignite for use in a gasifier operating under pressure - Google Patents

Abstract

Abstract of the Disclosure Water-containing solid fuels, particularly brown coal or lignite, must be dried before conveying to a gasification reactor operating under pressure. A
process is provided for treating the coal whereby water or steam is added to the coal to yield a mix-ture having a ratio of water to coal (computed on a dry basis) of approximately 2.5-3.5 to 1. The coal-water moisture is then maintained at a temperature of approximately 200-300°C and a pressure higher than that of the gasifier and in the range of about 15-110 bars until the coal has substantially lost its colloidal structure and the water separates from the coal in liquid form. After separation from the water, the coal is transported to the gasifier.
Preferably, the solid fuel used has a granular par-ticle size of about 0.1 to 10 mm and the coal-water mixture is pre-heated prior to pressurization. Op-tionally, the dried coal may be heated and/or lowered in pressure prior to entering the gasifier, and residual adhering water may be removed by post-treating with heated or unheated inert gas.

Description

76~9 1 Background of the Invention The present invention relates to a pre-treatment of watercontaining solid fuel for use in a gasifier operating under pressure. ~ore particularly, the invention is directed to a proc-cess for drying water-containing lignite (brown coal) and con-ducting the resulting low water-content product into a gasifica-tion reactor operating under pressure.
With the gasification under pressure of water-contain-ing solid fuels, it is necessary to dry the fuels thoroughly be-fore conducting them into the actual gasifier. Further, it is necessary to bring the fuels under pressure which at least corres- `
ponds to the pressure at which the gasifier is to be operated.
In general, the drying of the fuel is carried out either in a rotary dryer without pressure or by suspension drying with steam or hot gases. This manner of drying leads to extremely ~ dust-laden dryer exhaust vapors which must undergo costly dust ;` removal in order to comply with air pollution requirements.
~;~ In addition, this permits the high degree of vaporization heat required for fuel moisture to escape unused into the atmosphere.
To pressurize the fuel prior to its conduction into the gasifier, the use of a pressure-lock system has been described.
According to this system, the fuel is loaded without pressure into a primary lock chamber which is subsequently closed and brought to ~he gasification pressure by the addition of its own gasification sas. By opening a connecting valve, the fuel is transported into a second lock-chamber, already under pressure, from which it is then steadily conducted into the gasifier.
After the intermediate valve is closed, the pressure in the first chamber is then reduced to a normal level and the chamber is then reloaded with fuel.

- 2 - ~, 8~9 1 The disadvantages of this mode of operation are primarily the costly, large pressure chambers and the need to recompress the decompressed "lock-gas" to its original pressure.
The ratio of this lock-gas to the entire gas produced increases significantly as the gasification pressure rises.
To eliminate the disadvantages mentioned above, pro-posals and experiments exist for the gasification of fine-grained fuels, such as mixing the fuel with a liquid to form a pumpable ~ r paste, bringing the paste to the required pressure with a liquid pump by simple means, and then forcing the paste into the gasifier ;
system. As liquids oil, on the one hand, and water on the other -~ -hand, have been proposed.
For example, the Institute of Gas Technology in Chicago uses a mixture of oil and coal in its Hygas-Process (see The Oil And Gas Journal, February 11, 197~). The coal is dried without pressure to a moisture content of approximately 10% and is then mixed with oil to form a blend of 45% coal and 55% oil. In this form, the mixture is brought to gasification pressure by means of piston pumps. Prior to conduction into the gasifier itself, the oil is separated from the coal by evaporation in a prelimi-nary stage, is recovered in the further course of the process and is reused for mixing of fresh coal with oil. To cover the unavoidable losses, some fresh oil must be continuously supplied. `
A further difficulty in using a coal-oil paste lies primarily in the fact that upon evaporation of the oil cohesion of the coal easily occurs.
The use of a coal-water paste is described, for example, in DT-OS (German Offenlegungschrift) 2,350~953. The finely-dis-persed fuel is mixed with water into a pumpable paste. By means of adequate pumps, the paste is brought to a pressure higher than , , . ,: :: . i.

i~7~8~9 1 that of the gasification pressure desired and is mixed with the hot crude gases from the gasifier. The mixture of coal and water vapor thus produced is separated in a cyclone into dry coal and a gas-steam mixture. The gas-vapor mixture is then subjected to the usual processes for heat recovery, gas purification and gas refining, such as conversion and methanization. The dry coal separated in the cyclone is conducted into the gasifier according to known methods. A particular disadvantage of this process is the fact that a considerable quantity of water must be evapor-ated from the coal-water paste. Due to the high heat of vapor-i~ation for water, a large quantity of heat must be supplied.
It is an object of the present invention to avoid the disadvantages of known processes including, among others which exist, that on the one hand the process for the removal of water and on the other hand the process for placing the coal under pressure are not coordinated with each other. Therefore, the economy of the process can be improved, and indeed particularly ~;
through reduction of the costs for employment of the previously mentioned processes, without experiencing an increase in the operation costs. On the contrary, the latter can also be de-creased.
It is a further object of the invention to modify the drying and conveying process so that the two essential process steps, namely removal of water and placing the coal under pres-sure, become adapted to each other. The water removal should be carried out so that the operating conditions established thereby can be used together for both conveying and placing the solid matter under pressure, in order to assure a sirnple transition between the process step of water removal and the process step of placing coal under pressure.

10768~9 Essential pro~isions for accomplishing the gasification should already be fulfilled by carrying out the drying pTOceSS and should be maintained during the transition from this to the next following step.
SummarY of the Invention , The present invention provides in a process for preparing water-containing solid fuel for gasification, including the steps o~ drying the fuel and conducting the resulting reduced water content pToduct into a gasification reactor operated at a pressure of approximately 15 to 110 bars, the improvement compTising adding to a water-containing solid fuel sufficient ~ter and/or water vapour to yield a fuel-water mixture having a water to fuel Iatio of approximately 2 5 - 3~5 to 1 ~computed as dry f~el), subjecting - the mixture to pressures higher than the pressure selected for the reactor - .:
but within the above mentioned range, subjecting the mixture under said pressure to temperatures of approximately 200 - 300C until the fuel has ~ufistantially lost its colloidal struc~ure and wateT initially associated with the solid fuel prior to formation of said fuel-water mixture, is present in liquid form and can be separated from the fuel, then recove~ing the fuel as a product fuel having a reduced water content and feeding said pToduct fuel into the gasification Teactor, while maintaining said product fuel at 2Q a pressure at least equal to the pressure prevailing in the reactor.
The aboYe described disadvantages relating to prior art proposals and in accordance with present day technical levels may be avoided in applying the mode of operation accosding to the presen* invention. For example, in ; accordance with th~e invention, solid, water-containing lignite ~brown coal) must be dried and transported into a gasification reactor operating under approximately 15-110 bars pressure. To accomplish this according to the invention, water and/or steam or water vapor are added to water-containing lignite to yield a ratio of water to coal ~computed on a dry basis) of approximately 2.5-3~5 to 1~ This coal-water mixture is then subjected to pressures higher than the pressure selected for the reactor, but in the y ~B ~ -5-1~68~39 abovementioned range, and temperatures of approximately 200~300C, until the fuel has substantially lost its colloidal structure and until the ; ~ater, uhich separates in liquid form from the fuel, has been largely : removed~ The fuel, thus separated fTom the water, is then conducted into the gasification reactor. This process can be carried out continuously or ;~. discontinuously~
. Depending upon the proposed met:hod of gasification, powdered coal dust or coal haying a granulation size of approximately 0.1-10 mm is mixed uIth water into a pumpable paste. The quantity of water required depends upQn the kinds of lignites or brown coal, but in general, the ratio of coal ~ :`
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-~` Ccalculated as uater~free) to water should be brought to approximately ~ 1 to 2~5 3~5~ For example, to brown coal having .. ''. ~
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1 a water content of 59~, it is advisable to add enough water so that the mixture consists of approximately 1 part coal (cal-culated as water-free) and 3 parts water. Instead of water, water vapor or steam can be added.
It is advisable to pre-heat the mixture prior to the processing. The mixture is then brought to a pressure higher than that of the gasifier, which is generally operating at a - pressure of approximately 5-110 bars. At the increased pressure, still no water evaporation will take place from the mixture.
The coal-water mixture is then treated at temperatures of ap-proximately 200-300C until the coal has substantially lost its colloidal structure and the formerly combined water is set free in liquid form. At the same time, depending upon the kind of coal, carbon dioxide may also be released from the humic acids contained in the coal. The water may be separated from the coal in any known manner, for example, by decanting or centrifuging.
The dried coal thus obtained, which may still possess some residual moisture content, can be conducted into the gasi-fier in the customary manner, for example, by means of endless screws. Advantageously, the coal may be freed from residual -~
adhering water by processing with an inert gas, which may op-tionally be preheated. Further heating of the coal prior to conduction into the gasifier can also be advantageous. With these methods of treatment, it can be of further advantage to preliminarily decrease the pressure on the coal, but not below the pressure prevailing in the gasifier.
Brief Description of the Drawings For the purpose of illustrating the invention, there are shown in the drawings forms which are presently preferred;
it being understood, however, that this invention is not limited 1~7~8~g 1 to the precise arrangements and instrumentalities shown.
Figure 1 is a schematic flow diagram illustrating one ` embodiment of the present invention in which the process of the invention is carried out in a continuous manner.
` Figure 2 is a schematic flow diagram illustrating another embodiment of the present invention in which the process of the invention is carried out in a discontinuous manner.
Detailed-DescriE_ion of Preferred Embodlments -~
As used in the present specification and claims, un-- 10 less otherwise specified, all ratios and percentages are on a weight basis. Referring to Figure 1, lignite having a high com- ~
bined water content (for example in the range of 35-65%) from ~
conduit 1 and water from conduit 3 are blended in mixing vessel 2 with the help of an agitator 4 to form an easily pumpable -~ paste. By means of pump 5, the mixture is brought to a pres-sure higher than that prevailing in dryer 16 and gasifier 17, respectively. The paste is continuously pumped via conduit 6 through pipes 7a of heater 7 and then via conduit 10 into a settling chamber 11.
Heat can be supplied to heater 7, for example, with all or part of the hot crude gas from conduit 19 via conduit 8.
In addition, or as a matter of choice, a different hot gas can also be supplied via conduit 28. The cooled heating gas leaves heater 7 via conduit 9 and can, for example, be recycled to the hot gas in conduit 19 Eor further processing.
The coal-water mixture is brought to temperatures of approximately 200-300~C in heater 7. Any carbon dioxide which may possibly be formed thereby is removed via conduit 27.
The present mixture consisting of water and coal con-taining little or no combined water separates in settling ~L076~309 1 chamber 11. The water, having low solid matter content, is carried off via conduit 12. The coal, with low water content, which has accumulated in the bot:tom of chamber 11, is fed by means of screw 13 either directly to gasifier 17 via conduit 15 or to dryer 16 via conduit 14. In dryer 16, the residual water is vaporized by the heat of the crude gas coming from gasifier 17 via conduit 26.
Two or more decanting chambers 11 can also be pro-vided for and operated in known manner with alternate filling.
The mechanical separation of the water from the dry coal can also be carried out in other ways besides decanting, for example, by the use of hydrocyclones, centrifuges or rotary filters.
Direct conduction of the coal into gasifier 17 can be particularly advantageous if gasification is carried out with oxygen or air and water vapor supplied via conduit 24. Thus, in this case, tbe gasification steam, which is otherwise sup-plied via conduit 29, does not have to be produced separately.
If the coal is conveyed into dryer 16, suspension dry-ing or fluidization drying can be carried out, depending on - 20 whether the crude coal was supplied in a powdery or a fine-'~ grained state. In the former case, the dry coal is separated as coal dust in cyclone 18, and is then transported to gasifier 17 via conduits 20 and 21. In the latter case, the dry coal goes into gasifier 17 via conduit 25. In this case, only a small amount of fine solid matter accumulates in the cyclone and can be returned to the gasifier via conduits 20 and ?1 or to mi~ing vessel 2 via conduits 20, 22 and 1 for reblending.
It is also possible for solid matter to be removed via conduit 23 together with the slag or residual coke coming from the gasifier.

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1 An after-drying step especially for the removal of residual surface water from the :Low water content mixture pro-duced in accordance with the invention may be advantageously employed when a hydrogenizing gasification of coal, by hydrogen or mixtures rich in hydrogen, is to be carried out in gasifier 17.
Figure 2 illustrates the possibility of discontinuous or batch operation of the invention. The device for drying and ~` pressurizing of moist coal dust consists of three pressure chambers 40a, 40b and 40c, all provided with the same supply and discharge conduits. The chambers are timed so that overall continuous feeding of coal to the pressure-type gasifier 48 is effected. The number and size of the pressure chambers 40 can be selected by choice, depending on the size of the gasifier unit, the kind of fuel, structural factors, etc. Pressure chamber 40a will illustrate the general mode of operation.
Water containing crude coal, which may be in powdery, fine-grained or even lumpy form, is supplied through conduit 41. Subsequently, via conduit 43, saturated or super-heated steam is forced in at a pressure and temperature sufficiently high to bring the entire load of coal to the desired tempera-ture of approximately 200-300~C. As a result, with the steam condensate and the release of the previously colloidally com-bined water, the coal forms a mixture of water and coal which is now low in combined water.
The water accumulates in the bottom of chamber 40a.
As soon as this has occurred, the water becomes liquid and is drawn off under pressure through conduit 44. Coal with residual adhering water remains in chamber 40a.

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1 In order to dry this coal, hot crude gas, for example, may be blown in through conduit 45 and returns through conduit 46 to crude gas pipe 53. Finally, the crude gas is replaced by an auxiliary gas, carbon dioxide for example, which is sup-plied from conduit 56. The pressure of the auxiliary gas is somewhat higher than that of gasifier 48. After conduit 46 is shut off, the pressure existing in chamber 40a is therefore high enough so that the dry coal can now be transported to gasifier 48 via conduit 47 by known methods.
As soon as chamber 40a is empty, there is a switch-over to one of the other chambers 40b or 40c, which in the mean-time, have been readied for the feeding of coal into gasifier 48. For its part, chamber 40a has its pressure reduced to a normal level by means of conduits 46 and 57, the cover of the chamber is removed and the chamber is refilled with coal.
The remaining conduits and apparatus shown in Figure 2 have the same functions as the corresponding conduits and apparatus of the embodiment according to Figure 1. Thus, con-duit 49 supplies the gasification medium to gasifier 48. Con-duit 50 supplies steam. Via conduit 51 the crude gas produced in gasifier 48 is conveyed to cyclone 52 and from there goes to -~
conduit 53. The latter leads to the place where the product gas will be used. Via conduit 54, the dust separated in cyclone 52 is removed. Conduit 55 drains slag and residual coke from gasifier 48.
Example I
Every hour 1.802 kg of crude brown coal, having 59%
moisture content, and 1.153 kg of water were blended in a mixing vessel to yield 2.955 kg of a pumpable coal-water paste. The paste 30 was pumped through a heater at a pressure of 50 bars and was heated to 2505C by hot crude gas. The mixture was then ~`

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7~;~309 1 sent to a settling chamber from which 1.842 kg of decanted water was drawn off hourly. A mixture of 739 kg of the coal (computed as water-free) and 374 kg of water were conducted to a gasifier which was maintained at a pressure of 45 bars.
Using 413 Normal cubic meters (Nm3) of oxygen gas and a temp-erature of 1500C, gasification took place yielding 1.707 Nm3 per hour of crude gas having the following composition:

C2 12.53%
CO 36.55%
H2 22.06 CH4 0.10~
H2S 0.14%

N2 0.33%
H2O 28.29%

Simultaneously, 76 kg of slag in fluid form resulted.
Exam~le II
A load of 4 cubic meters of crude brown coal (equiva-lent to 2 tons), which was processed in the same manner as the crude brown coal in Example I, was charged to a vessel having a volume of 6 cubic meters. The overall system consisted of 4 such individual vessels. For pressurization, 750 kg of 40 bar - steam with a temperature of 480DC were introduced. As a result, 800 kg of moisture in liquid form were obtained from the coal, together with approximately 650 kg of steam condensate, so that 1450 kg of 250~DC hot condensate under 40 bar pressure were ob-tained. The heat of this condensate can be used in a conven-tional manner. The coal still had a residual moisture content of approximately 30%. In the further course of the process, the pressure was decreased to 30 bars and hot process gas ~:

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~07G8~)9 ` 1 was added whereby the water still adhering was evaporated.
After termination of this cleansing process, the coal was re-moved from the vessel and transported to the gasifier. Subse-quently, the compartment was completely re-emptied so that the process of coal reloading could be resumed. In this case, with the utilization of the resulting condensate heat, only 200 to 300 kcal per kg of released water was required for heat.
The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specifica-tion as indicating the scope of the invention.
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Claims (10)

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

1. In a process for preparing water-containing solid fuel for gasification, including the steps of drying the fuel and conducting the resulting reduced water content product into a gasification reactor operated at a pressure of approximately 15 to 110 bars, the improvement comprising adding to a water-containing solid fuel sufficient water and/or water vapour to yield a fuel water mixture having a water to fuel ratio of approximately 2.5 - 3.5 to 1 (computed as dry fuel), subjecting the mixture to pressures higher than the pressure selected for the reactor but within the above mentioned range, subjecting the mixture under said pressure to temperatures of approximately 200 - 300°C until the fuel has substantially lost its colloidal structure and water initially associated with the solid fuel prior to formation of said fuel-water mixture, is present in liquid form and can be separated from the fuel, then recovering the fuel as a product fuel having a reduced water content and feeding said product fuel into the gasification reactor, while maintaining said product fuel at a pressure at least equal to the pressure prevailing in the reactor.

2. A process according to claim 1 wherein the solid fuel comprises lignite having a granular size of 0.1 - 10 mm.

3. A process according to claim 1 wherein the fuel-water mixture is pre-heated prior to pressurization.

4. A process in accordance with claim 1 wherein the pressure on said product fuel is reduced to a level which is still higher than the pressure of the reactor, and residual adhering water is removed by treating said product fuel with heated or unheated inert gas prior to feeding said product fuel into the gasification reactor.

5. A process according to claim 2 wherein the pressure on the product fuel is reduced to a level which is still higher than the pressure of the reactor, and residual adhering water is removed by treating said product fuel with heated or unheated inert gas prior to feeding said product fuel into the gasification reactor.

6. A process according to claim 3 wherein the pressure on the product fuel is reduced to a level which is still higher than the pressure of the reactor, and residual adhering water is removed by treating said product fuel with heated or unheated inert gas prior to feeding said product fuel into the gasification reactor

7. A process according to claim 1 wherein the product fuel is heated before being conducted into the gasification reactor.

8. A process according to claim 2 wherein the product fuel is heated before being conducted into the gasification reactor.

9. A process according to claim 3 wherein the product fuel is heated before being conducted into the gasification reactor.

10. A process according to claim 4 wherein the product fuel is heated before being conducted into the gasification reactor.