CA1156582A - Process for the carbonization of coal - Google Patents

Abstract

Abstract:

Coal particles are carbonized by heating them in a series of fluidized beds to produce fuel gas, tars and calcined char while using recycle calcined char to provide carbonization heat and recycle car-bonizer gas as the fluidizing atmosphere for the car-bonization. A high BTU by-product gas is obtained and increased yields of oils and tars.

Description

11~6~82 PROCESS FOR THE CARBONIZATION OF COAL
This invention relates to the carbonization of coal, and in particular to a method for effecting low temperature carbonization of coal by the fluidized solids technique whereby there is obtained increased yields of oils and tars while providing high BTU by-product gas.
In U.S. Patents 3,140,241 and 3,140,242 to Work et al, there is described a form coke product which is manufactured by briquetting a mixture of a reactive coal calcinate and pitch binder, curing the resulting green briquettes in an oxidizing atmosphere and then coking the cured briquettes in an inert atmosphere to reduce the volatile content to less than about 3%.
The product is a highly reactive, physically strong carbonaceous material eminently satisfactory for use in metallurgical furnaces including iron smelters.
For instance, during commercial blast furnace runs, these briquettes proved generally equal in performance to conventional high temperature oven coke. In fact, they were superior to oven coke in that their uniform size and shape tend to facilitate furnace operation.
The reactive coal calcinate used in preparing the form coke briquettes aforesaid is a particulate amorphous carbonaceous material, the surfaces of which are peculiarly susceptible to the formation of strong carbon to carbon bonds with carbon derived from coal tar pitch or other such bituminous binders with con-sequent formation of strong internal three-dimensional bonds. It is this surface affinity which accounts for the exceptional strength and uniformity of the briquette.
The reactive coal calcinate aforesaid is obtained by heating coal particles in three distinct stages under conditions which evolve tar-forming vapors.
Desirably, the heating is carried out in fluidized beds. In the first or catalyzing stage, the coal is 1 12~6582 heated in a fluidized bed in the presence of oxygen to a temperature below which tar-forming vapors are evolved. Heating of the coal particles in the flu-idized bed may be effected by burning a small portion of the coal, by sensible heat in the fluidizing medium or by indirect heat exchange. The catalyzed coal is then heated in a second or carbonization stage to evolve tar-forming vapors and produce char particles.
The fluidizing atmosphere contains just sufficient oxygen to provide the desired temperatur~, usually no higher than about 649C, by partial combustion of the coal. The oxygen is admitted to the bed in the form of air as a component of the fluidizing medium, the remainder of which may be any gas which is not reactive with the coal particles in this stage. The carbonized coal or char is then heated in a third or calcining stage to give the final reactive coal cal-cinate. Desirably, calcination is effected in a flu-idized bed operated at a suf~icient temperature, that 20 is, about 760C to about 871C to reduce the volatile combustible matter in the end product to below about 5%; heat is provided by limited combustion of the char.
According to the Work et al patents, the fluidizing atmosphere is an essentially inert gaseous medium except for the presence of oxygen only in such amount as is demanded by that oxidation rate of the char necessary to supply the heat demands of this stage.
Preferably, the fluidizing atmosphere is air or a mix-ture of ai~ and flue gas.
The binder for the form coke briquettes is prefer-ably obtained by processing the tars recovered from the coal carbonization aforesaid. Such processing consists in air blowing the tar to reduce its water content to about 0.5% and increase its softening point to about 55C to 65C (ASTM Ring and Ball).
The form coke of the Work et al patents can be produced from a wide range of coal types including /

1 156~82 noncoking coals such as lignite, thereby providing the metallurgical industry with an alternative to conventional oven coke. This feature is particularly attractive to coke users in areas having no metallurgi-cal coal but where there are deposits of noncokingcoal. But even when metallurgical coal is readily available, processing it in conventional high tempera-ture coking ovens may not be economically feasible because of the need for expensive environmental control systems. In fact, pollution abatement has added so much to the capital costs of coking operations that steel makers in the affected areas are resorting in-creasingly to importing coke rather than invest huge sums in new plants and equipment capable of meeting the stringent emission standards.
By contrast, the process of the Work et al pa-tents is inherently non-polluting thereby minimizing the need to install costly cleanup devices. Even now, form coke briquettes are currently being manufactured by this process in a plant which meets all U.S. Federal and State environmental regulations and very likely would comply with the standards of most other coun-tries.
Despite its manifestly excellent metallurgical properties the Work et al form coke has not met with the commercial acceptance it would seem to warrantr This can be ascribed almost entirely to the existence of two manufacturing deficiencies. One of these is due to the low heat content of the by-product gases - about 130 to 170 BTU/SCF (484 x 104 to ~33.4 x 104 J/m3) compared with about 550 BTU/SCF (204.9 x 105J/m3) for medium BTU coke oven gas. This condition results from using air as the fluidizing and oxidizing atmos-phere in the various stages of the process. The nitro-gen in the air, being inert, passes through the flu-idizing vessels and appears as a component of the by-product gases, the heating value of which is greatly reduced by the presence of the nitrogen.
Up to the time of the energy crisis, the low BTU by-product gas would not have been a significant factor in determining whether to replace a substantial percentage of conventional coke capacity with the Work et al form coke. Natural gas and oil could be pur-chased at low cost in virtually unlimited amounts to meet a steel mill's fuel needs. However, at today's cost and availability of energy, there is a need to provide a by-product gas of increased heating value in conjunction with the manufacture of form coke.
The other criticism directed at the Work et al form coke is that the process generally does not pro-duce sufficient tar to meet the internal binder needs of the plant. When this occurs, a suitable outside binder pitch such as high temperature pitch must be procured to make up the internal binder deficiency.
The yield of binder is, of course, dependent on the type of coal being processed. For instance, certain of the bituminous B rank coals such as the Elkol-Ada-ville Seam from Remmerer, Wyoming are generally self-sufficient in binder. Other common bituminous B coals such as Illinois No. 6, give lower yields of tars and do not provide enough in-house binder. On such oc-casions, the binder deficit must be made up from ex-ternal sources, commonly high temperature oven pitch.
Just as the low BTU by-product gas was not a significant factor in assessing the Work et al form coke, prior to the energy crisis, neither was the need to obtain supplemental binder. Rowever, the price of coal derived hydrocarbons has risen precipitously and pitch binder now sells for about $200 per ton (907 kg) compared to $20 to $30 per ton prior to the energy crisis.
Manifestly, in today's energy climate such an-cilliary factors as the thermal value of by-product gas and binder yield are important in assessing the 1156~82 commercial prospects of form coke technology.
In accordance with the present invention there is an improve~ent in the fluidized bed pyrolysis of coal to produce a reactive calcinate, tar and high 5 BTU gas, wherein:
A. coal particles are heated in a first flu-idized bed under oxidative conditions to a temperature above 121C and below that at which substantial amounts of tar-forming vapors evolve to give a product which is non-agglomerating in step B;
B. the product of step A is heated further in a second fluidized bed to carbonization temperatures to effect evolution therefrom of substantially all tar-forming vapors to produce tar, char and carbonizer gas and C. the char from step B is heated in a third fluidized bed to reduce the volatile content of the char to less than about 5% to produce reactive cal-cinate and calciner gas, is improved in that high BTU
carbonizer gas and increased tar yield can be obtained by introducing, at a temperature below its thermal decomposition, recycle carbonizer gas into step B to provide the fluidized atmosphere and introducing hot recycle calcinate into step B to provide the carbon-ization heat.
It is accordingly the principal advantage ofthe present invention to provide a process for the low temperature carbonization of coal under fluidized conditions wherein the coal is devolatized to produce a reactive calcinate, increased tar yields and high BTU by-product gas. A further advantage of the in-vention is to provide a process for the fluidized bed carbonization of coal from the rank of bituminous, sub-bituminous and lignitic coals wherein the coal is de-volatized to produce a reactive calcinate and at leastsufficient tar to meet the binder needs for producing metallurgical form coke from said calcinate while 11~6~82 generating high BTU by-product gas. Other advantages will become apparent in the ensuing description.
In carrying out the invention, coal is ground, for example, in a hammer mill, to fluidizable size and conveyed into a first fluidized bed where it is heated under oxidizing conditions to reduce its agglomerating tendencies. The fluidizing media is desirably steam or flue gas containing air to provide the requisite oxidation. ~eating of the coal particles in the flu-idized bed may be effected by burning a small portionof the coal, by sensible heat introduced in the flu-idizing medium or by direct heat exchange. The flu-idized bed is normally maintained at a temperature of about 121C to 260C for noncoking coals and for coals possessing coking and caking characteristics the temperature ranges from about 260C to 427C.
Maximum temperature is where tar vapors begin to be evolved.
The coal particles from the first fluidized bed are conveyed to a second fluidized bed heated to carbon-ization temperatures until all tar-vapors are evolved.
The lower temperature is that at which the coal begins to evolve tar-forming vapors in ~uantity and this tem-perature is the same as the upper limit of the first stage heating, that is, about 427C for coking coals and about 260C for noncoking coals. The upper limit temperature is that temperature above which the ex-panding coal particles form cracks, fissures and bub-bleæ to such an extent that retraction to the size and shape of the original coal particle cannot occur.
This upper temperature limit is approximately 621C
to 649C. In general, the higher the temperature of carbonization (within the lower and upper limits), the greater the quantity of tar produced.
The char particles from the second or carbon-ization stage are further heated to reduce the amount of volatile matter therein to below about 5%. Desir-~;

, ably, thîs calcination is achieved in a fluid bed op-erating at that minimum temperature necessary to achieve this reduction, that is, from about 760C to 871C
and for a residence time of from about 7 minutes to about 60 minutes. The fluidizing atmosphere should be free of reactive gases, such as carbon dioxide or steam and oxygen can be tolerated only in such an amount as is consumed by the oxidation of the char in supplying heat to this stage.
As previously pointed out, carbonization heat in the Work et al process, is supplied by oxidation of the carbonaceous components in the reactor with oxygen, usually in the form of air as part of the flu-idizing atmosphere. Consequently, the by-product gas recovered from the carbonizer contains considerable nitrogen and thus is undesirably low in heating value, less than about 150 BTU/ft3 ~558 x 104 J/m3). Morever, oxygen in the fluidizing medium reacts preferentially with the tar-forming hydrocarbon volatiles and thereby reduces tar recovery to 30 to 50% of the quantity available from a given coal. For most coals, tar recovery falls below the amount needed to provide the requisite amount of binder when manufacturing metal-lurgical coke from the reactive calcinate.
By means of the present invention, the disad-vantages aforesaid can be overcome by carrying out the Work et al process but modifying it wherein hot recycle calcinate is used to convey heat to the car-bonization stage of the coal pyrolysis while utilizing recycle carbonizer gases in the fluidizing media.
Heating of the recycle calcinate is conveniently im-plemented by withdrawing some of the low BTU calciner gas and burning it in a heater through which an oxi-dizing gas is passed and the heated gas then introduced into the calciner where the resulting heat of combus-tion provides calciner heat plus the heat load for the recycle calcinate. Preferably, the oxidizing gas is air heated to 816C.
The use of the herein process results in an increase of tar amounting to about 50% to 100% over that of the previous prior art process aforesaid, and at the same time supplies a by-product gas in the form of a carbonizer gas having a heating value of 900 to 1000 BTU per cubic foot (335.3 x 105 to 372.6 x 10 J/m3). This compares with 150 BTU per cubic foot (558 x 105 J/m3) when air is used to provide heat for the carbonization. The total heating value of the gen-erated by-product gas can also be viewed as an increase from about 900M BTU (9.5 x 105 Joules) per ton (907.2 kg) of dry coal to about 1.5MM BTU (1.6 x 106 Joules).
In an example of the process of the invention, 2000 lbs. (907.2 kg) of coal is pretreated by feeding it into a first fluidized bed after being crushed to a size suitable for fluidization, typically it will pass through a No. 14 sieve (U.S. Sieve Series ASTM
E-ll). The coal is fluidized in this stage with a mix-ture of flue gas and air. The temperature in the firstfluidized bed is maintained at about 288C. ~eat for this stage is provided by the sensible heat in the flue gas plus oxidation of the coal with oxygen in the fluidizing atmosphere. Oxidation of the coal serves to diminish its agglomerating properties and is limited to that necessary to prevent agglomeration of the coal particles when subjected to higher tempera-tures in succeeding stages of treatment. About 1%
of non-aqueous volatile matter is removed from the coal during a residence time of about 30 minutes.
The pretreated coal amounting to about 1,980 lbs. (898.1 kg) is next fed into stage 2 where it is subjected to a carbonization temperature of 49~c and maintained at such temperature until essentially all tar-forming vapors are evolved, typically for a period of about 20 minutes.
After carbonization, the resuitiDg ~har amounting , ~ l~B582 -- g to about 1,540 lbs. (698.5 kg) is introduced into the third or calcining stage where it is heated at a temperature of about 816C to produce calcined char or calcinate having a maximum volatile content of about 5%. The yield of calcinate is about 1200 lbs. (544.3 kg).
A portion of the calciner gas is withdrawn from the main stream and split into two parts for internal use. One part is burned with air in a heat exchanger which heats a recycle stream of carbonizer gas to 538C which is used to fluidize the car-bonization stage 2 and to provide part of the carbonization heat.
The other part of the calciner gas is burned with air in a sec-ond heater through which a stream of air is passed and is heated to 816C and then conveyed into the calciner vessel where the heated air combusts with fuel values therein to provide calciner heat plus the heat for the recycle calcinate. The ratio of recycle calcinate to coal feed is preferably about 0.5 to about 0.7. Together the recycle carbonizer gas and recycle calcinate meet the heat requirements for the carbonization stage 2.
The carbonizer overheads from stage 2 are passed through a condenser which removes tars leaving high BTU carbonizer gas which is recovered. Flue gas from the heaters can be introduced into stage 1 to provide heat and fluidizing atmosphere.
Reference is now made to Table I which shows the results of manufacturing one ton of form coke from calcinate and bitu-minous binder produced by the pyrolysis of Illinois No. 6 sub-bituminous B coal using the Work, et al. process of U.S. Patent 3,140,241 on the one hand and as modified by the carbonization process of the inve-ntion on the other. It will be observed that the process of the invention yields 0.26 tons (235.9 kg) of binder material compared to 0.12 ton (108.9 kg) obtained by the prior Work, et al. process. Thus, whereas Work, et al.
operates with a binder deficiency of 0.06 ton (54.4 kgj, the invention provides a binder surplus of 0.08 ton (72.6 kg).
Whereas the low BTU car-~156582 bonizer gas of Work et al has a value (coal equ~valent) of ~1.50, that of the invention is essentially equiva-lent to pipeline gas. The only deficit arising from carrying out the herein process is a reduction in the 5 output of calciner gas due to its being used as the fuel for heating the recycle calcinate and carbonizer fluidizing gas. It is readily apparent, however, that such use of a low quality by-product is a minor penalty to achieve an overall cost advantage of $27.50 per 907.2 kg of coke provided by the process of the in-vention.
It is to be pointed out that all of the heat for effecting the fluidized bed coal pyrolysis of the invention is derived from the burning of the calciner off gas, even the heat for pretreatment stage can be supplied by the flue gas from the calciner gas burners.
No additional calcinate is consumed in heating the recycle calcinate, the extra heat burden coming from the combustion of calcinate with the 316C air. In both the process of the Work et al patent and the here-in process, about 1200 lbs. (544.3 kg) of calcinate was obtained from one ton of coal. Thus, the provision of high BTU by-product gas and increased tar yields by the herein process is not achieved at the expense of a reduction in the output of fixed carbon values.

TkLLE I

Carbonizer GasPrior Art* Present Invention Quantity 1.5MM BTU 2.5MM BTU
(1.6 x 106J) (2.63 x 106J) Quality 120-150 BTU/ft3 900 BTU/ft3 (447-559 x 104 J/m3) (335 x 105 J/m3) Value Equivalent to coal Pipeline gas Credit (Cost) $1.50 $10.00 Calciner Gas Quantity 4MM BTU 2MM ~lU
(4.22 x 106J) (2.11 x 106J) Quality 150-170 BTU/ft3 150-170 ~TU/ft3 (559-633 x 104 J/m3) (559-633 x 104 J/m3) Value Equivalent to coal Equivalent to coal Credit (Cost) $4.00 $2.00 Liquids (Binder) Required 0.18 Ton (163 kg) 0.18 Ton (163 kg) Produced 0.12 Ton (108 kg) 0.26 Ton (235 kg) Excess or Deficiency (0.06) Ton (-54.4 kg) 0.08 Ton (72.5 kg) Credit (Cost) ($9.00) $12.00 at $150/Tbn New Value or (Cost) ($3.50) $24.00 Advantage Present over Prior - $27.50/Ton (907.2 kg) Coke *U.S. Patent 3,140,241 to Work et al hc67B136 em73

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The fluidized bed pyrolysis of coal to produce a reactive calcinate, tar and carbonizer gas wherein A. coal particles are heated in a first flu-idized bed under oxidative conditions to a temperature above about 121°C and below that at which substantial amounts of tar-forming vapors evolve to give a product which is non-agglomerating in step B;
B. the product of step A is heated further in a second fluidized bed to carbonization temperatures to effect evolution therefrom of substantially all tar-forming vapors to produce tar, char and carbonizer gas and C. the char from step B is heated to a still higher temperature in a third fluidizied bed to reduce the volatile content of the char to less than about 5% to produce reactive calcinate and calciner gas, characterized by providing high BTU carbonizer gas and increased tar yields by introducing, at a tempera-ture below its thermal decomposition, recycle carbon-izer gas into step B to provide the fluidized atmos-phere and introducing hot recycle calcinate into step B to provide the carbonization heat.

2. The process according to claim 1 characterized in that heat for the hot recycle calcinate is derived from the combustion heat of burning calciner gas.

3. The process according to claim 1 characterized in that the coal is selected from the class consisting of bituminous, sub-bituminous and lignitic coals.

4. In the fluidized bed pyrolysis of a bituminous coal to produce a reactive calcinate, tar and carbonizer gas wherein A. particles of the coal are heated in a first fluidized bed under oxidative conditions at a temperature of from 121°C to 427°C to give a product which is non-agglomerating in step B;
B. the product of step A is heated further in a second fluidized bed to a carbonizing temperature not exceeding about 649°C until substantially complete removal of tar-forming vapors is effected to produce tar, char and carbonizer and C. the char from step B is heated to a still higher temperature in a third fluidized bed to reduce the volatile content of the char to less than about 5% to produce reactive calcinate and calciner gas, characterized by providing high BTU carbonizer gas and increased tar yields by introducing, at a tempera-ture below its thermal decomposition, hot recycle carbonizer gas into step B to provide the fluidized atmosphere and introducing hot recycle calcinate into step B to provide the carbonization heat.

5. The process according to claim 4 characterized in that the temperature of step A is about 288°C, that of step B about 496°C while the heat for the hot recycle calcinate is derived from the combustion heat of burning calciner gas.

6. The process according to claim 5 characterized in that the heat load for the carbonization is derived by withdrawing calciner gas, one portion of which is burned in a heat exchanger to heat the carbonizer gas and another portion of which is burned in a heat ex-changer to heat oxygen containing gas entering the fluidized bed of step C wherein sufficient calcinate is oxidized to provide calciner heat and the heat load for the recycle calcinate.

7. The process according to claim 6 characterized in that the oxygen containing gas of step C is air which is heated to about 816°C.

8. The process according to claim 7 characterized in that the ratio of recycle calcinate to coal feed is about 0.5 to about 0.7.