CS265208B2 - Process for preparing gases containg methane - Google Patents

Abstract

Ash-slagging fixed bed gasification of carbonaceous materials with steam and oxygen is effected by substituting at least a portion of the steam with an aqueous liquor. The liquor may be water per se eg. boiler-feed water or an effluent liquor comprised of condensed organic compounds and unreacted steam and solubilised salts which liquor is obtained by treating the product gas. Although oxygen consumption is slightly increased, the economic effect is offset by not having to expend energy in steam raising.

Description

The present invention relates to a process for gasifying coal to produce methane-containing gases.

The gasification of coal with water vapor and oxygen produces an aqueous liquid containing mainly dissolved phenolic compounds, ammonia, hydrogen sulfide, hydrogen cyanide, alkali metals and salts. This aqueous liquid cannot be discharged into the sewer without treatment because its biological oxygen demand is too high to be able to cope with the surrounding natural environment without serious environmental damage. This problem is even more serious in ash-grinding processes than in dry ash-based processes, since in the former the amount of water vapor required is generally lower and the waste water liquid formed as a by-product is more concentrated.

Typically, this waste aqueous liquid is treated to remove phenols and ammonia prior to biological treatment. Alternatively, the untreated or partially treated aqueous waste liquid may be incinerated to yield a solid as waste. All of these methods of treating aqueous waste liquid have disadvantages and are associated with technological difficulties.

Recycling of the aqueous waste liquid resulting from the gasification of coal by steam and oxygen is known. For example, PCT Publication No. WO 80/00974 discloses a process in which an aqueous liquid falling off in a fixed bed gasification of coal in a fixed bed process is fed to an entrained bed gasifier operating with ash grinding. Similarly, U.S. Pat. No. 3,971,636 discloses the recycling of aqueous condensate separated from the synthesis gas obtained by gasification of coal in a fluidized bed by an ash scaling process. In these examples, the aqueous liquids are used as carriers for conveying fine pulverized coal in the form of a slurry to the gasifier.

Although in the above methods, the primary gasifier is an ash-disintegrating gasifier, they are characterized by a completely different mode of operation. In either case, the ash-slagging gasifier does not have a fixed bed structure, with the obvious advantages that the non-fixed bed arrangement allows the aqueous liquid to heat up rapidly, thereby eliminating the potential for rapid reaction termination.

As can be seen from the above-mentioned PCT Publication No. WO 80/00974, the waste aqueous liquid obtained from the fixed bed gasifier is recycled only to the entrained bed gasifier.

It would not be expected from the prior art that waste aqueous liquid can be successfully recycled to fixed bed gasifiers operating with ash grinding.

Surprisingly, it has now been found that coal gasification can be successfully performed in a fixed bed gasifier, operated by ash grinding, by direct injection of an aqueous liquid together with other gasifying agents, and that where the liquid is aqueous waste, there is a solution to the problem of its removal. very relieved.

Accordingly, the present invention provides a process for producing a methane-containing gas, wherein a feedstock comprising a solid carbonaceous material is gasified in the presence of water vapor and oxygen as fixed bed gasifiers under ash grinding conditions and the resulting gas is treated to remove unreacted water vapor, water-soluble compounds and condensable The process is characterized in that at least a portion of the water vapor required for the reaction is supplied by the addition of an aqueous liquid in admixture with the other gasifying agents.

Preferably, the aqueous liquid is aqueous waste obtained by treating the gas produced as a product. However, the aqueous liquid may also be purer forms of water, such as water used for cooling or boiler feed.

The recycled aqueous waste is injected into the gasifier at the point of introduction of the other gasifying agents, i.e., exhaust pipes. In practice, the outlet and steam baffles will be adapted to supply aqueous liquid or water at the point of steam injection.

The addition of water or aqueous liquid offers a number of surprising advantages, although it would rather be expected that such addition would adversely affect either the gasifier operation or the slag discharge. However, no problems were observed in any way. It was found that oxygen consumption (based on the amount of carbonaceous material consumed) increased slightly, but this was more than offset by cost savings by avoiding the need to produce steam. A further considerable advantage is that, if the liquid to be supplied is aqueous waste, not only considerable savings can be achieved by reusing the water, but also by disposing of dissolved and suspended substances in the reactor. And, of course, the organic materials are utilized as reactants.

Thus, by recycling the aqueous liquid in the manner described, the water vapor requirements required for gasification are greatly reduced, for example up to about 50% of virtually all the aqueous waste normally generated by the treatment of tar coal is recycled. Up to 50% of the total required water of all types for the gasification reaction can be supplied with an aqueous liquid, either as water per se or as recycled aqueous waste. Another advantage is that the inorganic substances contained in the aqueous waste pass into the slag and are removed therefrom from the manufacturing process. The heavy metal content of the slag, such as zirconium, is increased compared to the non-aqueous waste injection process. For processing, it may be advantageous to separate a portion of the aqueous waste to remove chlorides and other salts when these compounds begin to accumulate in the recycled aqueous waste. The aqueous waste may contain some organic substances in dissolved or emulsified form. In addition, however, the aqueous waste can be used as a carrier for conveying suspended solids, such as finely pulverized coal, to the reactor.

The invention is illustrated by the following example:

Example

Fixed bed coal gasification apparatus operating in an ash scaling process, such as described in The Chemistry of Coal Utilization, II. Supplement, 1981, edited by John Wiley and Son, has been modified to inject aqueous liquid of all types by steam and oxygen blowers. The plant was put into operation according to the usual steam and oxygen gasification mode at the Marky Mains Coal plant at an operating pressure of 2,478 MPa. The amounts of reactants and gas produced per hour are shown in the following table. Column A lists the amounts of the reactants prior to injecting the aqueous liquid. The amount of injected aqueous liquid increased during 21 hours of operation, as indicated in columns В, C and D.

AND В С D Quantity of coal supplied (number of hoppers.h 3.89 3.90 3.83 3.7 6 amount of water vapor supplied (m ³ .h Ь 5 418 4 827 4 482 4 182 AND В С D amount of injected aqueous liquid (kg.h N) 0 440.38 708.24 817.20 total ^ O / O ^ ratio 1.16 1.17 1.18 1.16 the amount of gas 21 165 21 672 20 961 20 482

Composition of produced gas (mol%)

WHAT

h ₂ s other

54.9

28.9

7.1

3.4

0.5

5.2

Claims (6)

SUBJECT OF THE INVENTION A process for producing a methane-containing gas, wherein a feedstock comprising a solid carbonaceous material is gasified in the presence of water vapor and oxygen as gasifying agents in a fixed bed under ash grinding conditions and the resulting gas is treated to remove unreacted water vapor, water-soluble compounds and condensable organic compounds characterized in that at least a portion of the water vapor required for the reaction is supplied by the addition of an aqueous liquid in admixture with other gasifiers. 2. A method according to claim 1, wherein the aqueous liquid is water as such. 3. The process of claim 1 wherein the aqueous liquid is aqueous waste comprising condensed unreacted water vapor, water-soluble compounds, and condensed organic compounds obtained by treating the product gas. 4. A process according to claim 2 or 3, wherein up to 50% of the water of different types required for the gasification reaction is supplied by said aqueous liquid. 5. The method of claim 1 wherein said aqueous liquid serves as a carrier for the material suspended therein. 6. The process of claim 5 wherein said suspended material is finely pulverized charcoal.