GB2085915A

GB2085915A - Method for producing coke and a high calorific gas from coal

Info

Publication number: GB2085915A
Application number: GB8130738A
Authority: GB
Original assignee: Tosco Corp Japan
Current assignee: Tosco Corp Japan
Priority date: 1980-10-22
Filing date: 1981-10-12
Publication date: 1982-05-06
Also published as: AU7522181A; KR830007790A; BR8106788A; IT1143245B; AU530783B2; DE3141407C2; GB2085915B; ZA816333B; JPS5798593A; DE3141407A1; IT8149486A0; AR226925A1

Abstract

The coal is heated in a non- oxidizing atmosphere in first heating zone to partially devolatilize the coal and produce a partially devolatilized char, which is then heated by a mixture of steam and oxygen in a second heating zone to produce a high calorific gas containing volatile vapors and a substantially devolatilized char containing less than 6 weight percent volatile combustible matter. A rigid shaped article having a high crushing strength may be made by mixing the substantially devolatilized char with a binding agent therefor, shaping the mixture, and curing and coking the shaped mixture.

Description

SPECIFICATION Method for producing coke and a high calorific gas from coal For centuries, high grade coke has been produced in so-called coke ovens by the carbonization of coal (including peat). A large percentage of such coke is used in producing iron, e.g., in blast furnaces to reduce iron ore (iron oxide) to iron. In addition, coke is also used to reduce other metal ores, such as copper, etc. However, the production of coke, in coke ovens, is a very expensive process and requires, in general, a particular type of coal which oftentimes is not located in an area near where the users of the coke made from such coal are located.

Because of these and other deficiencies in the coke oven process, it is not surprising that prior art workers have attempted to develop more efficient processes for producing coke as well as processes which can utilize Western coal which is generally considered to be noncaking (non-agglomerating) coal. In this regard, it is noted although there are many different types or ranks of coal, there are generally two broad types, at least in the United States. As noted, one is so-called Western coal, which is generally considered to be non-caking, and the other is Eastern coal, generally considered to be a caking coal. The coke oven process has used, for the most part, the Eastern or caking coal.

Among prior art processes which have attempted to produce coke using Western type coal is United States Patent No. 3,140,241 (Work et al). This patent discloses a six-step process for producing coke from a non-caking type coal. Included within the process are two heating steps. The first heating step is referred to in the patent as the carbonizing stage. In the carbonizing stage dried coal is heated in a fluid bed in the presence of oxygen at a temperature of from 430"C. to 650"C. in order to remove a portion of the volatile combustible matter (VCM) from the coal.

Thereafter, the carbonized char is heated in a second heating step which is referred to in the patent as the calcining stage. In the calcining stage, the carbonized char is heated to a temperature of from 815"C. to 980"C. in a fluidizing atmosphere which is free of reactive gases, such as carbon dioxide and steam.

The Work et al process suffers from a serious disadvantage in that the gases produced both in the carbonizing step and in the calcining step are low calorific gases (i.e., less than 5600 kilojoules/m3). Such low calorific gases cannot be used to supply heat necessary to carry out the process which is a serious, if not fatal, disadvantage.

From the foregoing, it is readily apparent that it is desirable to provide a process for producing form coke of acceptable quality from any type of coal in which during the process, the gas produced from the coal char has a sufficiently high heating value to be used to supply heat in the process thereby rendering the process commercially valuable.

The present invention is predicated on the discovery that a high calorific gas can be produced during the process of making form coke by contacting coal char, from which a certain portion of the volatile combustible matter (VCM) has been removed, with a reactive gas containing oxygen and steam. Surprisingly, contacting the partially devolatilized char with a mixture of steam and oxygen not only produces a high calorific gas but also produces a substantially devolatilized char which, when added to a binding agent, shaped, cured and coked, produces a high quality metallurgical coke which may be used in blast furnaces and the like. The high calorific gas produced has a sufficient number of kilojoules per cubic me,er to be valuable as a fuel gas and can supple heat to the pris-ssess.

The present invention utilizes a twosstage process wherein the coal is first pyroByzed or carbonized in a first heating zone in a nonoxidizing atmosphere, generally at a ten1pera- ture of from 370"C. to 81 5 C., to pyrolyze the coal and form partially devolatilized char and product gases. This is quite different from the prior art methods which employ an oxidizing atmosphere in the carbonizing stage resulting in diluted product gases.

The next step of the present invention involves passing the partially devolatilized char to a second heating zone where the partially devolatilized char is further heated or calcined, generally at a temperature of from 540"C. to 1 370 C., in the presence of steam and oxygen to produce a high calorific gas and a substantially devolatilized char containing less than 6 percent by weight volatile combustible matter (VCM). The high calorific gas resulting from the second heating step will have at least 11 200 kilojoules/m3 and as high as 29850 kilojoules/m3. This gas may be utilized as a fuel gas, for example, to preheat the coal and/or heat the heat-carrying solids which are used to pyrolyze the dried coal.

Additionally, if the process is used to produce coke for use in a blast furnace, the fuel gas may be used for supplying heat to various steps in the iron-making process. The production of such a valuable high calorific gas in the second heating zone is a vast improvement over the prior art which produces only low calorific gases during form coke production. which are of little if any value.

After the second heating stage, the substantially devolatilized char can then be mixed with conventional binders, shaped, cured and coked to provide excellent form coke.

The above features and advantages, and many others, will become apparent as the invention becomes better understood by refer ence to the following detailed description in which all part and percentages are by weight unless expressly stated otherwise.

The method of the present invention produces a high grade coke as well as a gas having a high calorific content from any type of raw coal by a two-step heating process in which dried coal particles are heated (carbonized) in a non-oxidizing atmosphere in a first heating zone, generally at a temperature of from 370 C. to 815"C. to produce a gas containing volatile vapors and a partially devolatilized char and thereafter transferring the partially devolatilized char to a second heating zone and heating (calcining) the partially devolatilized char at a temperature generally of from 540 C. to 1 370 C. in the second heating zone by contacting the partially devolatilized char with a gaseous mixture of oxygen and steam to produce a high calorific gas having at least 11 200 kilojoules/m3 and a substantially devolatilized char containing less than 6 weight percent volatile combustible matter (VCM). The substantially devolatilized char produced in this process is an excellent source of coke. Merely mixing the substantially devolatilized char with a suitable binding agent and forming an article of desired shape from said mixture and then curing and coking the shaped article produces excellent form coke.

Regardless of the type of coal utilized (i.e., Western or Eastern type coal) it is preferred in the present invention that before the coal is processed (i.e., heated to remove the volatile combustible matter) that the raw coal is ground to form small particles therefrom of, for example, less than 1.3 cm to less than 0.3 cm in size.After the coal has been ground, it is also preferred that the moisture be removed from the coal in a preheating step, for example, by heating the ground coal particles at a temperature of from 95"C. to 31 5 C. If the coal is a caking or Eastern-type coal, it is generally preferred that the coal be pretreated either before, after, or during the drying step, to decake the coal by contacting the coal with an oxidizing gas containing from, for example, 1% to 20% by volume, of oxygen as described by Work et al in U.S. Patent No.

3,184,293. In addition to decaking the coal, such a step will remove substantially all the moisture from the coal. After the drying step and/or pretreating step, most coals will contain anywhere from 20% to as high as 50% by weight of VCM and from 75% to 50% fixed carbon and the balance being ash.

In the pyrolysis or carbonizing step of this invention, the gas product contains the tar from the coal and, in general, the amount of VCM contained in the partially devolatilized char after the carbonizing step will be between about 10 weight % and 20 weight %, excluding moisture. The carbonizing step may be conducted anywhere from 370"C. to 81 5 C. with the preferred temperature range being from 430 C. to 650"C. and the most preferred temperature range being from 430"C. to 540 C. The length of time necessary to conduct the carbonizing step will vary greatly depending upon the temperature and the amount of VCM in the raw coal, i.e., the duration increases with a decrease in temperature and/or an increase in the VCM content of the raw coal. If, for example, a temperature of 510"C. is used and the coal contains approximately 40 weight % to 45 weight % of VCM, a five minute residence time is sufficient to produce a partially devolatilized char in which the VCM contained in the partially devolatilized char has substantially no tar therein.

The carbonizing step of the invention is conducted in a non-oxidizing atmosphere and, preferably, in the absence of extraneous gases to that the vapors produced during the carbonizing step will not be diluted.

The dried coal particles preferably are pyrolyzed or carbonized, in the non-oxidizing atmosphere, by contacting the dried coal particles with heat-carrying solids which should be inert to the coal, partially devolatilized char, and the vapors produced during the carbonizing step. The particular type of heat-carrying solids utilized to heat the coal particles may vary widely and have any desired shape. For example, the heat-carrying solids may be metal or ceramic and may have a ball-like shape of approximately 0.6 cm to 1.3 cm diameter. In the preferred exemplary embodiment, the heat-carrying solids are alumina balls of approximately 1.3 cm diameter. In addition, for easy separation, heat-carrying solids should have a size which is substantially different than the size of the partially devolatilized char particles.In general, it is preferred that the heat-carrying solids have a diameter which is greater than the diameter of the partially devolatilized char particles.

In order to conduct heat from the heatcarrying solids to the coal particles, the heatcarrying solids must contact the coal particles to transmit their heat thereto. It is preferred that the carbonizing step be conducted in a rotating retort. The rotational speed of the retort should be sufficient to mix the heatcarrying solids with the coal particles in order to obtain good heat transfer between the coal particles and heat-carrying solids. The specific rotational speed of the retort may vary greatly and is dependent upon the diameter of the retort. After the carbonization step is complete, the heat-carrying solids and partially devolatilized char particles are separated as are the volatile vapors and condensable gases produced during the carbonization step.

The partially devolatilized char is then transferred to a second heating or calcining zone where the char is partially gasified with steam.

The temperature in the calcining zone will be sufficient to react the steam with the carbon in the partially devolatilized char to form a combustible gas consisting predominantly of carbon monoxide and hydrogen and a substantially devolatilized char having a VCM content of less than 6 weight percent. The temperature may vary widely and may range from as low as 540"C. to as high as 1 370 C.

The specific temperature used will depend primarily on whether a gasification catalyst (e.g., sodium or potassium hydroxide, carbonate, oxide, sulfate or sulfide) is added to the gasification zone. If a gasification catalyst is utilized, the temperature will range from 540"C. to 815"C. or 870"C., whereas if the reaction is conducted in the absence of a gasification catalyst (i.e., a thermal reaction) the temperature range will be higher, for example, from 870"C. to 1095"C. or 1370"C.

As is the case with the temperature of the partial gasification reaction, the pressure used to partially gasify or calcine the carbon with steam may vary widely from ambient pressure to 140 kg/cm2, there being no theoretical upper limit on the pressure.

The amount of steam used to partially gasify the partially devolatilized char should be sufficient to substantially reduce the VCM content to below 6 weight percent to form substantially devolatilized char.

The temperature in the second heating or partial gasification zone is maintained by introducing an oxygen-containing gas, such as air, into the zone to oxidize a portion of the carbon contained in the partially devolatilized char, such oxidation reaction being exothermic, the amount of oxygen introduced being sufficient to maintain the temperature at the desired level. The oxygen content of the mixture may be supplied in a number of ways; for example, as pure oxygen or, alternatively, mixed with other extraneous gases and may even be supplied by the introduction of air.

The substantially devolatilized char may be removed from the second heating zone, cooled and then mixed with a suitable binder.

The mixture may be formed into the desired shape (for example, the familiar briquette shape or pellets of a cylindrical shape) and cured and coked to produce excellent form coke.

EXAMPLE 1 Illinois No. 6 caking coal containing approximately 8.84% water, 32.03% volatile combustible matter, 52.46% fixed carbon and 6.67% ash is fed to a preheater and heated to a temperature between 260"C. to 315"C.

with a gas having an oxygen content of 11%, by volume, and a temperature of 675"C. The raw coal is heated for a period of about 5 minutes. The dried coal contains approximately 35 weight percent volatile combustible matter, 50 weight percent fixed carbon, and 7 weight percent ash. The dried coal particles are fed to a rotating retort and contacted with heat-carrying solids having a temperature of 675"C. which raises the temperature of the coal to 500"C. The retort is rotated at a sufficient speed to mix the heat-carrying solids with the coal particles (about 2 rpm). The residence time to reduce the volatile combustible matter to less than about 20 weight percent (in the preferred exemplary embodiment to about 14 weight percent) takes about five minutes.The rotating retort is sealed from the atmosphere and therefore the gases evolved during the pyrolysis or carbonizing step contain no adulterating gases.

Thereafter the partially devolatilized char, which contains approximately 14 weight percent volatile combustible matter, is transferred to a second heating zone where it is contacted with a gas containing 80%, by volume, of steam and 20%, by volume, of oxygen. The partially devolatilized char is heated to a temperature of 870"C. The gases evolved in the second heating zone have a calorific content of between 11 200 kilojoules/m3 and 29850 kilojoules/m3.The thus produced substantially devolatilized char has about 4 weight percent volatile combustible matter with the remainder of the substantially devolatilized char being fixed carbon with a slight amount of ash (about 10%). This substantially devolatilized char can be used to produce coke having excellent properties by mixing the substantially devolatilized char with about a 1 5 weight percent of a coal tar binder and forming pellets from the mixture at a pressure of 20 kg/cm2. The pellets are cured at 230"C.

and coked in a non-oxidizing atmosphere at 815 C.

EXAMPLE 2 In another preferred exemplary embodiment dry Illinois No. 6 caking coal (dried as indicated above) was pyrolyzed in the first heating or carbonizing zone as in the first exemplary embodiment. However, in the second heating of calcining zone the partially devolatilized char is heated to a temperature of 760"C.

using a gas mixture of 70%, by volume, of steam and 30%, by volume, of oxygen at a pressure of 1.2 kg/cm2. The evolved gas from the second zone has a calorific content of between 11 200 kilogoules/m3 and 29850 kilojoules/ m3 and the substantially devolatilized char contains less than about 5 weight percent volatile combustible matter.

The substantially devolatilized char produced in this exemplary embodiment can be mixed with about 17%, by weight, of coal tar binder at a temperature of 100"C. The mixture is formed into pellets 2 cm in diameter and 2.5 cm long under pressure of 20 kg/cm2. The pellets are then cured for two hours at 230"C. in an oxidizing atmosphere and then the cured pellets are coked at 815"C. for 30 minutes using sand and char to cover the pellets to prevent oxidation. The cured and coked pellets have a crushing strength of approximately 1 30 kg/cm2.

Claims

1. A method for producing substantially devolatilized char and gases from coal by heating the coal in a first heating zone to produce a first gas containing volatile vapors and a partially devolatilized char, transferring the partially devolatilized char to a second heating zone and heating the partially devolatilized char to produce a second gas containing volatile vapors and a substantially devolatilized char, characterised by conducting the heating step in the first heating zone in a non-oxidizing atmosphere, and conducting the heating step in the second heating zone by contacting the partially devolatilized char with a gaseous mixture of oxygen and steam to produce a high calorific gas having at least 112to kilojoules/m3 and a substantially devolatilized char containing less than 6 weight percent volatile combustible matter.

2. A method according to claim 1, in which the heating step in the second heating zone is conducted at a temperature of from 540"C to 1370"C.

3. A method according to claim 2, in which the heating step in the second heating zone is conducted at a temperature of from 540"C to 870"C in the presence of a gasification catalyst.

4. A method according to claim 2, in which the heating step in the second heating zone is conducted at a temperture of from 870"C to 1 370 C in the absence of a gasification catalyst.

5. A method according to any preceding claim in which the heating step in the first heating zone is conducted at a temperature of from 370"C to 81 5 C.

6. A method according to claim 5 in which the heating step in the first heating zone is conducted at a temperature of 430 to 650"C.

7. A method according to any preceding claim in which the heating step in the first heating zone is effected by contact with hot nert solids.

8. A method according to any preceding claim, including the additional step of forming =oke from the substantially devolatilized char ay mixing the substantially devolatilized char with a binding agent therefor, forming a shaped article from the mixture and curing and coking the shaped article.

9. A method according to any preceding claim, in which the coal is preheated at a temperature of from 95"C to 31 5 C prior to the first heating step to remove substantially all of the moisture present in the coal.

10. A method according to claim 9, in which oxygen is reacted with caking coal to decake the coal during the preheating step.