Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
1156S82
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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
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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
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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-
~;
1156S82
,
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
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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
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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-
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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.
1 156582
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
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