Method of producing direct reduced iron with fluid bed coal gasification

A method of producing direct reduced iron with fluid bed coal gasification in which a portion of cooled, recycled gas is used as coolant in the gasification chamber and a second portion of the cleaned recycled gas is heated and mixed with the hot, dust-free gasification gas to form reducing gas for the direct reduction process. Limestone is preferably mixed with the pulverized coal feed to the gasification chamber to desulfurize the gas.

BACKGROUND OF THE INVENTION 
The direct reduction of iron ore and pelletized iron oxide by a high 
temperature reducing gas prepared from natural gas and recycled spent 
reducing gas has become a significant commercial route toward the 
production of steel. The high temperature reducing gas prepared from 
natural gas has a high concentration of reducing constituents, carbon 
monoxide and hydrogen, as compared to the oxidizing constituents, steam 
and carbon dioxide. The ratio of these constituents (reducing to 
oxidizing) is called reducing ratio. As natural gas declines in 
availability and increases in cost, alternate routes are needed to produce 
a high temperature reducing as from coal and other fuels such as oil. The 
growing overdemand for oil, resulting in excessive prices, makes coal the 
fuel of choice for future processes to produce direct reduced iron from 
hot reducing gas. 
Three basic types of coal gasification are conventionally available on a 
commercial scale, the entrained bed process, the fixed bed process, and 
the fluidized bed process. 
The entrained bed process produces a reducing gas at about 1500.degree. C. 
and atmospheric pressure by the concurrent reaction of oxygen and steam 
with entrained coal dust. The gas from the process has a reducing ratio of 
about 5 but must be cooled before the gas can be compressed to the 
approximately two atmospheres of pressure required for direct reduction 
and also to allow removal of carbon dioxide, water and gaseous sulfur 
compounds before heating and use in direct reduction. Such cooling, 
cleaning and subsequent reheating is too costly in both equipment 
investment and energy loss to make the process highly attractive. 
The fixed bed coal gasification process features a descending bed of coal 
moving countercurrent to an ascending gasification gas stream. The 
gasification gas is initially oxygen and steam at the bottom grate of the 
gasifier. As the gasification gases pass up through the descending bed of 
coal several zones are encountered. The first zone at the bottom 
discharges ash almost entirely free of carbon from the gasifier. In the 
next zone gasification gases oxidize the char from the coal to form 
hydrogen, carbon monoxide and carbon dioxide. In the next higher zone 
devolatilization of the organic content of the coal takes place as well as 
some gasification reactions. The gasification gas, now being rich in 
hydrogen and CO as well as methane and higher hydrocarbons such as 
naphthas and tars, passes to the next higher level in the bed where 
dewatering and preheating of the coal bed takes place. The discharge gases 
therefore contain large quantities of vaporized water, CO.sub.2, CO, 
hydrogen, some methane, naphthas and tars. Before such a gas can be used, 
the sulfur compounds, steam, CO.sub.2, naphthas and tars must be removed. 
This is best accomplished through the use of low temperature or ambient 
temperature removal systems. The cooling equipment, the cleaning and 
subsequent reheating of the gas is expensive, both in equipment investment 
and energy loss. 
As is known to those skilled in the art, the only fully commercial fluid 
bed process presently available for the gasification of coal operates at 
atmospheric pressure and produces a gas which contains high concentrations 
of oxidizing constituents, steam and carbon dioxide. Before this gas can 
be used in direct reduction it must be cleaned of dust, compressed and 
then cleaned of carbon dioxide and sulfur compounds. Not only are the 
cooling and reheating steps expensive, but the gasification process itself 
is unable to utilize a large fraction of the carbon fed to the gasifier. 
Char is produced as a by-product and must be used in other processes. 
Newly developing fluid bed processes carry out the gasification under 
pressure and one process has a hot zone within the fluid bed where ash is 
agglomerated and allowed to fall from the bed. Char and ash removed from 
the discharge gas by a cyclone system is returned to the hot zone to 
obtain high utilization of the char and to remove the ash in agglomerated 
form. The purpose of the system is to obtain a high conversion of coal to 
gas by minimizing the withdrawal of char from the system. Because of the 
cyclone return system, the process offers the ability to accept fines in 
the coal feed. Even under the best of conditions however, the gas quality 
has not exceeded a ratio of 2, primarily because of the need to feed 
excess steam into the gasifier to cool the char in the fluid bed to 
prevent agglomeration. As a consequence the gas cannot be used without 
cooling, purification and reheating. These processes are undesirable for 
investment and energy reasons. 
Note that reducing gas quality is commonly expressed as the ratio of 
reductants (CO+H.sub.2) to oxidants (CO.sub.2 +H.sub.2 O) in the gas 
mixture. In order to take full advantage of the chemical efficiency of a 
counterflow shaft direct reduction furnace, the qualify of the hot 
reducing gas introduced to the furnace should be at least about 8. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide a method of 
producing direct reduced iron with gas produced by fluid bed coal 
gasification. 
It is also an object of this invention to eliminate the necessity for 
reheating gasification gas to reduction temperature prior to entry into a 
direct reduction furnace. 
It is a further object of this invention to provide a process which is 
capable of accepting coal fines in the feed material.

SUMMARY OF THE INVENTION 
This invention is a method of producing direct reduced iron with fluid bed 
coal gasification by fluidized bed gasification of coal, cleaning the 
gasification gas, mixing it with hot, cleaned top gas, injecting the 
mixture as reduction gas into a direct reduction shaft furnace, 
withdrawing reacted reducing gas from the furnace, and cleaning it to form 
a reductant-rich recycled gas, heating a portion of the recycled gas prior 
to mixing it with a gasification gas and injecting a second unheated 
portion of the cooled, cleaned recycled gas into the gasifier to cool the 
gasification reaction. 
By utilizing recycled gas from the direct reduction process as a coolant in 
the fluidized bed gasification chamber the steam feed required by the 
gasifier can be drastically reduced and in some cases eliminated. With a 
concomitant reduction in the percentage of oxidants in the gasification 
gas product, by increasing the residence time of the coal in the gasifier, 
using highly reactive coals and utilizing recycled gas as coolant, a high 
quality reducing gas is produced suitable for direct reduction of iron 
without any further necessity for upgrading its quality; that is the 
oxidants need not be further reduced. Lime or some other sulfur acceptors 
such as calcined dolomite can be pulverized and fed to the fluidized bed 
along with the pulverized coal to desulfurize the gases formed in the 
fluidized bed. Thus the gas need not be cooled below the reduction 
temperature required in a direct reduction furnace. This reduces the power 
requirement as well as the cost of investment for such a plant. 
DETAILED DESCRIPTION 
Referring now to the FIGURE, coal which has been crushed to a particle size 
sufficiently small to obtain good fluidization is fed from bin 10 into 
fluidized bed gasification chamber 12 via feedpipe 13. All of the coal 
must be minus 10 mm to obtain good fluidization. If desired, pulverized 
limestone or other sulfur acceptor is fed from bin 15 through feedpipe 13 
to gasification chamber 12. The presence of lime in the bed reduces the 
sticking tendency of the particles in the bed, allowing higher operating 
temperatures, which results in better carbon utilization and a higher 
quality reducing gas product. Oxygen moderated as necessary with steam is 
fed from source 16 into the bottom of the chamber 12 and upwardly into the 
fluid bed to fluidize and gasify the material in chamber 12. The gas 
produced in the gasifier is removed via an internal cyclone 18. Entrained 
particles in this gas are removed by the cyclone and returned to the lower 
portion of the gasifier by return pipe 20. The gasifier gas is removed 
from cyclone 18 by pipe 22 and undergoes further cleaning in any desired 
number of gas cleaners 24. Particulate materials from gas cleaners 24 are 
returned to the hot zone 25 at the bottom of the fluid bed by return 
system 26. The cleaned gasifier gas enters pipe 27 wherein it is mixed 
with heated recycled gas from a direct reduction furnace to form a 
reducing gas of suitable temperature for the reduction of iron oxide. The 
reducing gas mixture is introduced to direct reduction shaft furnace 28 at 
inlet 30. The shaft furnace has an iron oxide feedpipe 32 at its upper end 
and a metallized product removal means 34 at its lower end. The operation 
of the latter causing gravitational flow of the feed material or burden 
through the furnace. The reducing gas moves in counterflow relation 
through the burden in the furnace, the reductants carbon monoxide and 
hydrogen reacting with the oxygen in the iron oxide to chemically reduce 
the iron to a highly metallized product and forming a top gas containing 
principally CO.sub.2, H.sub.2 O, H.sub.2, N.sub.2, CH.sub.4 and CO. The 
reacted top gas is withdrawn from the furnace at outlet 40, undergoes dust 
removal and substantial water removal in cooler scrubber 42, from whence 
the major portion of the dust-free cooled top gas is conducted through 
line 43 to acid gas removal unit 44 in which it is scrubbed to remove a 
substantial portion of CO.sub.2. Steam or some other source of thermal 
energy 46 passes through the acid gas removal unit 44 for regeneration of 
the scrubbing fluid. Exhaust acid gases such as CO.sub.2 and H.sub.2 S are 
removed from the system through pipe 48. The acid gas removal process 
produces a recycled gas rich in hydrogen and carbon monoxide which is 
divided, a portion entering pipe 50 which is fed back to the bottom of the 
gasification chamber to control the temperature of the bed by absorbing 
the exothermic heat of reaction between the oxygen and coal in the bed. 
The remaining portion of the cooled recycled gas enters line 52 for mixing 
with gasification gas in pipe 27 to form reducing gas. Part of the 
recycled gas is heated in heater 54 while the remaining part bypasses the 
heater in line 55 then is recombined with the hot recycled gas to control 
the temperature of the reducing gas. The temperature of the gas is 
measured by thermocouple 56 which is connected to valve 58 for controlling 
the amount of cooled gas in pipe 55 to temper the reducing gas stream. 
Heater 54 is fired by burner 60 which uses a portion of the dust-free 
spent reducing gas from line 62 as burner fuel. 
In the gasification chamber 12 the upwardly flowing oxygen from source 16 
reacts with the coal to produce a reducing gas. Oxygen, moderated as 
necessary with steam, from source 70 is injected into the ash removal 
system and reacts with the char formed from the hot coal to form hot zone 
25 where the ash particles agglomerate under controlled conditions. As the 
agglomeration continues, the particles form agglomerates of a size 
sufficient to fall from the bed into discharge system 68. 
As an alternative to the fines return from cyclones 24 to hot zone 25 as 
discussed above, all or part of the fines from pipe 26 may be directed 
through pipe 72 into the upflowing stream of oxidants from source 70. The 
location of fines injection is controlled by valves 74 and 76. 
The invented method of fluid bed coal gasification production of reducing 
gas for direct reduction of iron has significant advantages over the 
commercially available fluid bed gasification process wherein gasified 
gases are cooled, cleaned of carbon dioxide, steam and hydrogen sulfide, 
then reheated along with cleaned, cooled spent top gas from a direct 
reduction furnace to form the reducing gas for direct reduction. In the 
present method a significant portion of the recycled gas is reheated while 
acting as a coolant for the fluid bed gasification reaction in the 
gasification chamber. The present process is capable of obtaining up to 
95% coal utilization as measured by loss of carbon in the ash and in the 
discharge gases from the cyclone system. Since there is no need for 
cooling the gas produced in the gasifier or for purifying the gasifier 
gases to remove carbon dioxide, hydrogen sulfide or water, such cooling 
and purification equipment is eliminated with an attendant savings in 
equipment investment. Since the gasification gases are not cooled, 
equipment for reheating gasification gases is unnecessary. The two sources 
of heat for bringing the reducing gas to reduction temperature are the 
gasification chamber itself and recycle gas reheater 54. The recycle gas 
reheater is much smaller than reheaters used in commercially available 
processes because so much less gas is required to be reheated in the 
present process. The invented method is also capable of accepting fines in 
the coal feed because the fluidized bed and cyclone system are readily 
capable of handling them. Fines are normally eliminated from coal 
gasification systems. 
EXAMPLE 
As an example, Table I compares the process flow rates using purified 
recycle top gas from pipe 50 as a coolant in the fluid bed gasifier (Case 
A), with the use of steam as a coolant (Case B) for the production of one 
tonne of direct reduced iron having a 92% metallization. 
In both Cases the process conditions are as nearly alike as possible and 
the raw gas in conduit 27, as produced from the gasifier 12, is at 
1010.degree. C. The hot raw gas is used without removal of carbon dioxide. 
TABLE I 
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FLOW RATES 
GASIFIER (NCM is defined as cubic meters of gas at 0.degree. C. and a 
pressure of 760 mm of Hg) 
CASE A 
CASE B 
__________________________________________________________________________ 
Coal Feed 
(Gross heating value kg 436 540 
6943 Kcal/kg) 
Steam NCM 43.6 432.0 
Oxygen NCM 241.7 289.3 
Recycle NCM 305.2 0.0 
Raw Gas Product NCM 1092.9 
1337.6 
GAS PROCESSING AND REDUCTION 
Bustle gas (into 30) NCM 1859.1 
3126.6 
Top gas (from 40) wet NCM 1886.6 
3166.9 
Scrubbed top gas (from 42) 
NCM 1777.1 
2795.4 
Top gas fuel (in 62 and to 
NCM 290.7 522.8 
generate steam for 
CO.sub.2 removal) 
To CO.sub.2 removal (to 44) 
NCM 1486.5 
2272.6 
CO.sub.2 removed (in 48) 
NCM 354.7 408.1 
Product from CO.sub.2 removal 
NCM 1055.2 
1749.1 
(from 44) 
Recycle to gasifier (in 50) 
NCM 305.2 0 
Steam to CO.sub.2 removal 
kg 674.0 775.1 
(in 46) 
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For simplicity the top gas used as fuel to produce the steam 46 for 
CO.sub.2 removal is not shown in the figure. 
The savings in coal, recycle gas handling and oxygen are readily apparent. 
In the examples raw gas is prepared from a coal having an ultimate analysis 
of 72.2% C, 4.5% H, 1.3% N, 6.8% O, 3.1% S and 12.1 weight percent ash on 
a dry basis. The gross heating value is 6943 Kcal/kg. The raw gas produced 
from the gasifier has a composition as shown in Table II. 
TABLE II 
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RAW GAS COMPOSITION (vol. %) 
CASE A CASE B 
______________________________________ 
CO 59.56 33.41 
CO.sub.2 3.88 14.97 
H.sub.2 28.14 29.86 
H.sub.2 O 2.41 17.61 
CH.sub.4 3.90 3.30 
N.sub.2 + Ar 2.11 0.86 
Total 100 100 
##STR1## 13.9 1.9 
______________________________________ 
Note the significant improvement in quality of the raw gas in conduit 27 of 
Case A over that of Case B. 
The N.sub.2 +Ar in the oxygen feed to the gasifier was assumed to be 2%. 
The temperature of the raw gas in conduit 27 is 1010.degree. C. in both 
cases and the bustle gas temperature in inlet 30 is adjusted to 
815.degree. C. by the heater 54 and cold gas in line 55. 
From the foregoing, it is readily apparent that we have invented a method 
for producing direct reduced iron by utilizing a fluidized bed coal 
gasification system which is readily able to accept finely divided coal 
and which more efficiently utilizes the heat in the system.