SOCIETY OF 

MINING ENGINEERS 

PREPRINT 

NUMBER 

OF 

AI ME 

CALLER NO. 0, LITTLETON, COLORADO 80127 

LOW ENERGY CONCEPT FOR STRAIGHT-GRATE PELLETIZING PROCESS 

K. H. Boss 

W. Hastik 

A. Kilian 

H. Walden 

Lurgie Chemie und Huttentechnik 

Federal Republic of Germany 

For presentation at the SME-AIHE Annual Meeting 

Atlanta, Georgia - March 6-10, 1983 

83-85 

Permission is hereby given to publish with appropriate acknowledgments, 

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of AIME. 

If and when this paper is published by the Society of Mining Engineers of AIME, it 

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These preprints are available for sale. Mail orders to PREPRINTS, Society of Mining 

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MINING ENGINEERING 
83-85 

Low· Energy Concept for Straight-Grate 

1 Pelletizing Process 

I 

K. H. Boss 

W. Hastik 

A. Kilian 

H. Walden 

ERRATA 

Page 6 - Figure 2, bottom part, the legend 

for volume and temperature of direct-recup. 

air was exchanged. 

83-85 

firing temperature on the entire fuel consumption 

will. be demonstrated by the example of a pelletizing plant .which produces blast furnace pellets 

of hematite ore with a basicity of 0.7 at an 

oil consumption of 26.2 1/t of pellets. 

A simulation of this plant (1) resulted in a 

theoretical heat requirement that corresponded to 

an oil input 23.3 1/t of pellets. The difference 

of 2.9 is to be ascribed to losses not recorded 

in the model and, moreover, the calculations were 

based on an ideal center zone of the pellet bed. 

Proposals for simplification and improvement of 

the gas flow pattern (without changing the firing 

zone) resulted in a theoretical oil consumption of 

21.7 1/t of pellets or, considering the losses, 

24.6 1/t. 

Based on this already simplified firing pattern 

of this plant, the effect of a combustion gas temperature change was now investigated. In the preheating zone of the plant, the temperature climbs 

from 380 to 1320°C within 240 seconds. The tested 

firing patterns are sho~m in Figure 1. The 

cooling was always led to the same discharge temperature in the layer center and considered in 

the output computation. The influence of the 

various temperatures of combustion in the range 

of 1220 to 1380°C on the fuel consumption and 

output is shown in Figure 2. 

Raising the firing temperature shortens the 

firing time and reduces thus the needed gas volume to the extent that the enthalpy of the hot 

gas entering into the firing zone falls despite 

a higher temperature. The lower gas requirement 

permits that the cooling zone to be provided for 

direct recuperation can be shorter, which causes 

the direct recuperation temperature to rise 

(Figure 2, bottom). This reduces the heat requirement in the preheating zone which, in turn, 

leads to further reduction of the total heat 

requirement. The most important results of the 

simulation, such as the preheating and firing 

time, gas volumes for firing and direct recuperation, the output and oil consumption are 

summarized in Table 1. 

The model computations indicate that the fuel 

consumption declines when higher combustion gas 

temperatures are applied. The combustion gas 

temperature can be raised only to a certain 

limit beyond which the quality of finished pellets 

would begin to suffer. Particular attention must 

in this context be paid to the decomposition of 

the iron oxide and to slagging. 

The stated capacity increase at higher firing 

temperatures is a consequence of the reduced 

firing times and can be realized only when the 

process zones are newly divided up. In existing 

plants with an established zone division and at 

increased combustion gas temperatures instead of 

increased output, it would be possible to work 

with less suction which would result in a lower 

consumption of electric energy in addition to a 

lower fuel requirement. 

Alternative Patterns for Drying and Preheating 

Slow preheating is known to increase the exhaust gas losses and, hence, the heat requirement. The preheating speed is therefore always 

chosen at the highest level permitted by the dried 

pellets. The preheating speed in the tested plant 

is already very high at approx. 60C/sec(360°C/ 

min). For comparison, an arrangement without pre-
Synopsis 

In modern iron pelletizing plants fuel consumption has been optimized to such an extent 

that only minor savings can be expected in the 

future. Inspite of this fact, different process 

parameters ~ere studied ~ith a mathematical model 

and firing tests. The influence on fuel consumption and productivity ~as established. 

Furthermore, the adoption of simplified firing 

patterns ~as investigated, in particular for coal 

firing. 

In vie~ of high costs of oil and natural gas, 

only substitution of these fuels by coal can substantially reduce production costs. In existing 

plants partial replacement of oil and gas can be 

achieved by addition of coal to the iron ore feed 

and by firing the pellets with pulverized coal. 

Two systems have been developed for the LurgiDravo travelling grate process which enables coal 

to completely replace oil or gas. 

This paper describes in the first part the 

potentials for fuel savings available in the 

Lurgi-Dravo process. In the second part, different techniques for the use of coal as fuel in 

iron or pelletizing plants and in particular the 

recently developed Lurgi Multistage Coal Injection and Combustion Process (LHC) are discussed. Combined with the LMC process, a ~ide 

variety of iron ores and coals - also with high 

ash contents - can be used in the Lurgi-Dravo 

travelling grate process. 

Investigation of Fuel Savings 

Pellet Drying 

In most pelletizing plants, drying is performed in cwo stages, the first one being an updraft drying zone in ~hich the lo~ermost pellets 

are predried and heated to avoid condensation in 

the lower layers during subsequent do~draft 

drying. This complicated gas flo~ was first 

studied by means of model computations in an 

effort to simplify the gas pattern. Thus, it was 

possible to demonstrate that simple do~draft 

drying is possible under the following condidons: 

(a) feeding the green pellets warm (approx. 

40-600C), for instance after dry grinding 

or preheating, 

(b) performing the do~draft drying with a 

temperature profile suitable for the 

entering hot gases. 

In the second instance, a very simple firing 

pattern is obtained that sets in from the top 

already with the preheating ~hen the drying front 

has progressed to about bed center. How energy 

consumption and output are affected is evident 

from examples that will later be discussed in 

detail. 

Pellet Firing 

For pellet firing, the hot air from the 

cooling zone is returned directly into the 

firing zone (direct recuperation) and is there -

by combustion of oil, gas or coal - heated to the 

temperature needed for the firing. For an effective heat transfer to the pellets, the combustion 

gas temperature must be higher than the necessary 

indurating temperature. The influence of the 

83-85 

firing temperature on the entire fuel consumption 

~ill be demonstrated by the example of a pelletizing plant ~hich produces blast furnace pellets 

of hematite ore with a basicity of 0.7 at an 

oil consureption of 26.2 1/t of pellets. 

A simulation of this plant (I) resulted in a 

theoretical heat requirement that corresponded to 

an oil input ·23.3 1/t of pellets. The difference 

of 2.9 is to be ascribed to losses not recorded 

in the model and, moreover, the calculations ~ere 

based on an ideal center zone of the pellet bed. 

Proposals for simplification and improvement of 

the gas flo~ pattern (~ithout changing the firing 

zone) resulted in a theoretical oil consumption of 

21.7 1/t of pellets or, considering the losses, 

24.6 1/t. 

Based on this already simplified firing pattern 

of this plant, the effect of a combustion gas temperature change ~as now investigated. In the preheating zone of the plant, the temperature climbs 

from 380 to !320°C within 240 seconds. The tested 

firing patterns are shown in Figure !. The 

cooling was al~ays led to the same discharge temperature in the layer center and considered in 

the output computation. The influence of the 

various temperatures of combustion in the range 

of 1220 to 1380°C on the fuel consumption and 

output is sho~ in Figure 2. 

Raising the firing temperature shortens the 

firing time and reduces thus the needed gas volume to the extent that the enthalpy of the hot 

gas entering into the firing zone falls despite 

a higher temperature. The lower gas requirement 

permits that the cooling zone to be provided for 

direct recuperation can be shorter, which causes 

the direct recuperation temperature to rise 

(Figure 2, bottom). This reduces the heat requirement in the preheating zone which, in turn, 

leads to further reduction of the total heat 

requirement. The most important results of the 

simulation, such as the preheating and firing 

time, gas volumes for firing and direct recuperation, the output and oil consumption are 

summarized in Table I. 

The model computations indicate that the fuel 

consumption declines when higher combustion gas 

temperatures are applied. The combustion gas 

temperature can be raised only to a certain 

limit beyond which the quality of finished pellets 

would begin to suffer. Particular attention must 

in this context be paid to the decomposition of 

the iron oxide and to slaggingThe stated capacity increase at higher firing 

temperatures is a consequence of the reduced 

firing times and can be realized only when the 

process zones are newly divided up. In existing 

plants with an established zone division and at 

increased combustion gas temperatures instead of 

increased output, it would be possible to work 

with less suction which would result in a lower 

consumption of electric energy in addition to a 

lower fuel requirement. 

Alternative Patterns for Drying and Preheating 

Slow preheating is kno~ to increase the exhaust gas losses and, hence, the heat requirement. The preheating speed is therefore always 

chosen at the highest level permitted by the dried 

pellets. The preheating speed in the tested plant 

is already very high at approx. 60C/sec(360°C/ 

min). For comparison, ~n arrangement without pre-
heating zone was tested with the model, i.e. the 

downdraft drying is followed immediately by the 

firing zone with 13200C firing temperature (Figure I, Pattern 2). Thus it was possible to achieve a capacity increase of approximately 5.6%, 

but the oil consumption increased also by about 

3.3%. In this instance it is a disadvantage that 

the entire hot gas requirement must be satisfied 

by the direct recuperation whereas a slightly 

slower preheating speed would permit to cover 

the requirement partly out of the wind box recuperation. 

Theoretically, it would be possible to try 

omitting the downdraft drying of the previous 

example and to start with the firing immediately 

after the updraft drying (Figure I, Pattern 3). 

Thus, if permitted by the pellets, the output 

could be raised by 11.4% but the energy consumption would then climb, too. This show.s that -

although the hot gas quantity introduced into 

the 38ooc downdraft drying is utilized only with 

a very poor efficiency - some energy can be saved 

in the actual firing, because the pellets need 

only to be heated from 380°C to firing temperature by means of the hot combustion gas. 

Both firing patterns, 4 a and 4 b (Figure I) 

were started immediately in downdraft operation 

without updraft drying. The green pellets of the 

top layer are initially exposed only to approx. 

JOOOC drying temperature (Figure I, Pattern 4 a) 

in order to minimize the condensation in the 

lower pellet layers as much as possible. After 

drying the upper layers, the gas temperature is 

raised to 380°C within three minutes according 

to a temperature profile. This is followed by 

the usual firing pattern.. By means of this very 

simple firing pattern and with a slight increase 

of the energy consumption, the output can be improved by approx. 4.3 % at an increased waste 

gas rate. 

If the available waste gas is to be utilized, 

a second down-draft drying zone with 3800C can 

be provided (Pattern 4 b). Energy savings of 

I liter of oil per tonne of finished pellets 

could thus be achieved at the same output. The 

most important advantage of this firing pattern 

is the simplicity of gas flow which is particularly well-suited for a changeover to coal 

firing. 

The Substitution of 

Oil and Gas by Coal 

Coal Addition to Pellet Feed 

Before the present come-back of coal as a 

substitute for oil or gas, it would be recalled 

that coal was being added as the primary or secondary fuel component even during the early days 

of pelletizing, some 25 years ago. In October 

1956, Cleveland Cliffs Iron Co. started up a 

travelling grate plant at Ishpeming, Michigan, 

USA,treating flotation concentrates from specular hematite ores. The whole operation, except 

for ignition, was carried out in up-draft gas 

flow. Solid fuel, mostly anthracite or coke 

breeze at a rate of about 4 - 5 7., was charged 

in layers onto the surface of green pellets. The 

final product consisted of pellets and a certain 

amount of pellet sinter (2). 

In October 1955, Reserve Mining Co. commis2 83-85 

sioned a travelling grate plant at Silver Bay, 

Minnesota, USA, in which the fuel supply consisted of anthracite (10 - IS 7.) and oil 

(85- 907.)(3~ One year later, the travelling 

grate plant of INCO in Copper Cliff, Ontario, 

Canada, commenced its pellet production (4). The 

only fuel source of this plant was oil. After 

the successful operation of this plant, all 

travelling grate plants were for two decades 

designed exclusively for oil or natural gas 

firing. 

The main reasons for the decision to use oil 

and gas were the rather low costs for these 

fuels at that time, their easy handling properties and the rising product quality requirements. Nowadays, coal addition as a partial replacement of oil or gas is again being seriously 

considered and has been introduced in many travelling grate plants already. Coal prices are in 

many regions attractive even when the costly 

handling of the materials is taken into account. 

Extensive investigations were carried out to 

determine the correct procedure of coal addition. In this respect the primary concern was 

the maintenance of pellet quality which should 

not deteriorate by the addition of coal. 

Detailed studies of coal addition are described in the work of B. Hutter (5). 

Up to 2 % of coals of different reactivity 

were added to five different ores ground to the 

fineness required for green ball formation. The 

pellet diameter was uniformly 10 - !2 mm. Neither 

the green nor the dried pellet strength changed 

in comparison with that of pellets without coal 

addition. 

However, it was noticed that the crushing 

strength of indurated pellets decreased wi-!i~ 

coal addition (as Cfix) of more than 0.5 % ~o\ 

magnetite pellets and the resistance to abras~on diminished with coal addition (Cfix) of above: 

I %. On the other hand, it was possible to considerably raise the capacity of the pelletizing 

travelling grate for hematite pellets. Therefore, the negative and positive influences are 

to be considered and outweighed for each particular case. 

Figure 3 shows the influence of increasing 

coal additions to hematite and magnetite pellets. 

In the case of hematite, the strength rises up 

to about 0.5% coal addition (as Cfix). The 

strength then decreases but is still sufficient 

up to 1.5 % coal addition. This influence is 

explained by partial reduction of hematite to 

magnetite and subsequent incomplete reoxidation 

of magnetite to hematite. Higher coal additions 

may even result in a further reduction towards 

the formation of wuestite which, within the 

time available, does not reoxidize again and 

forms fayalite with the silica present. 

Several plants are currently operated with 

coal addition into the iron ore feed. Hoogovens 

at Ijumiden, Netherlands, add coke breeze in~ 

quantities of about 1.0% (as Cfix) to the iron 

ore mix in the blending yards. The coke breeze 

is thus ground together with the ore mix in 

closed-circuit dry grinding mills. The ore mix 

consists of magnetite and hematite type ores 

in varying ratios. Plant experience shows that 

the coke breeze can partly compensate for the 

oxidation heat of the magnetite. Coal addition 

is also practised in the pelletizing plants in 

Brazil where charcoal is used as the fuel source. 
The SIDBEC plant in Canada replaces part of the 

total fuel by sub-bituminous coal. 

As a result of the serious fuel oil shortage 

there were considerations of designing pellet 

plants for the use of multiple-type fuels in 

order to counteract the situation in future. 

Based on such considerations, our North-American 

partner, Dravo Corporation in Pittsburgh, Pa., 

received 2 orders for pellet plants with the request to design the plants for the use of natural 

gas, for No. 2 and 6 fuel oil as well as for 

pulverized coal. 

Both these plants are located in the center 

of the Mesabi Range in Minnesota, USA, and one 

is owned by the Hibbing Taconite Company and was 

started up in 1976, whereas the other belongs to 

the Inland Steel Company and commenced production 

in 1977. 

The request of the clients for such multifuel-type pellet plants resulted in a significant change from the so far well-proven LurgiDravo straight grate pelletizing system. Extensive research and industrial-scale test work was 

done before the final design of these plants 

started. The diagrams in Figures 4 and 5 illustrate the difference between the new and the 

conventional system. 

Figure 4 shows the conventional system in 

which cold air for cooling the pellets is forced 

up-stream in the cooling zone through the charge. 

The hot air, recuperated during cooling, is collected in a header arranged above the hood. 

The hood is sealed against the moving pallets, 

thus allowing to develop an over-pressure in the 

coo ling hood. 

With the help of this over-pressure the hot 

air collected in the first cooling hood is introduced via ducts and downcommers into combustion 

chambers arranged horizontally in the side walls 

of the preheating and firing hood. Burners located in the,center of these combustion chambers 

heat to the required temperature. The hot air 

collected in the second cooling hood is, by the 

up-draft drying fan, conveyed to the windboxes of 

the updraft drying hood for partial drying of the 

pellets. The air leaving the charge with very 

low temperature is discharged through the stack 

into the atmosphere by an exhaust fan arranged 

above this zone. The downdraft drying, preheating 

and firing zones are operated in the downstream 

direction. All windboxes of these zones are connected to the windbox exhaust and recuperation 

fans. Tll'e recuperation fan is connected to the 

windbox in the downdraft firing zone and therefore handles gases with high temperature. 

The windbox exhaust fan connected to the downdraft drying and preheating zone handles gases of 

low temperature which cannot be reused in the 

process and are therefore led off to the stack 

also. This traditional system works with natural 

gas, fuel oil and in one particular case with a 

combination of coke oven and blast furnace gas. 

Figure 5 shows the system developed for multiple-fuel-type usage. Basically the entire process fan arrangement is the same as in the conventional arrangement. The main difference is 

that no combustion chambers are provided in the 

vertical walls of the respective hoods in the 

preheating and firing zones. The provison of 

several such combustion chambers would create 

unnecessary complications when pulverized coal 

were used as fuel. Two or more vertical! ar3 83-85 

ranged combustion chambers are provided for a 

combustion of pulverized coal or natural gas or 

fuel oil adjacent to the firing zone. When using 

pulverized coal, part of the hot air from the 

second cooling zone is used in the combustion 

system of the coal burner which is located at 

the top of the combustion chamber. The product 

of co~bustion resulting from firing of pulverized 

coal is mixed with the hot air from the first 

cooling zone in the combustion chamber and 

directed into the burner hood through a horizontal duct. 

This new system is now in operation in the 

Hibbing Taconite plant where, due to specific 

commercial conditions, only natural gas had been 

used. The Inland Steel plant had been using pulverized coal over extended periods of time. At 

the present moment this plant is again using 

natural gas due to a very substantial cost advantage offered by the national gas company. 

The operation of the Inland Steel coal fuel 

plant has been described in greater detail in 

other publications (6). It can be stated that the 

developed system has been a complete success with 

the specific ore and pellet quality requirements 

using a low-ash coal which was possible with a 

relatively straight-forJard temperature profile 

and firing pattern. 

LMC Process for High-Ash Coal 

Further investigations were continued by Lurgi 

in order to extend the scope of applicability of 

the process to the use of a wider range of ores 

and concentrates and particularly of coals, including high-ash coals. 

It was observed in the case of oil-fired 

plants that fine ore dust particles from pellets 

entrained in'the recuperated air caused liquid 

slag formation in the combustion chambers. The 

slag build-up led to flame deviations and to subsequent refractory failures. 

In the case of coal firing, an increase in the 

slag-formation was anticipated. This was confirmed by detailed observations, during coalfired operation of industrial plants (7) in Sep.-

Oct. 1981. 

An analysis into the causes of the slagging 

problems in the conventional pelletizing furnaces 

led to the following conclusions: 

Low-oxygen, high-temperature regions in flames 

caused the Fe203 to decompose to FeO which forms 

low-melting eutectics with Sio2 and Alz03 (at 

approx. I IOO'OC). 

Furthermore, due to the high air preheat temperature (800 - I 000°C) there are extremely high 

local temperatures in extended flame areas corresponding to adiabatic temperatures of up to 

2 500 oc. By entrainment of excess air the combustion products are finally cooled down to about 

I 300 oc, the pelletizing process temperature. The 

retention time of slag particles passing through 

the high-temperature flame areas is too short for 

them to reoxidize or cool down and solidify the 

slag impinging onto the refractory. 

As a result of the analysis of slagging problems, the following technological and economical 

preconditions were imposed in order to achieve 

satisfactory operation with high-ash coal as 

fuel:-
!. Technolo~ical Desi~n Requirements: 

The temperature level of the combustion process has to be decreased drastically to prevent 

refracto~; problems from liquid slag formation. 

The local fuel concentration during coal combustion should be as low as possible with 

longest retention times for coal and ash particles in the 02-rich atmosphere to avoid Fe203 

decomposition and build-up of low-melting 

ore/ash eutectics. 

Coal flames using pulverized coal in the conventional sense should be largely avoided due to 

the high intensity of combustion resulting in 

high local flame temperatures. 

II. E:onomic Requirements: 

4 

The coals should be cheap, which are generally 

high-ash coals. 

A minimum of coal preparation operations, e.g. 

grinding. 

Flexibility in the use of different solid fuel 

types as well as in combination with solid, liquid or gaseous fuels. 

In the final development of the L~C process, 

the above criteria have found full application. 

The most significant features of this process are: 

multi-stage· coal injection, 

multi-stage coal combustion 

using 

direct air/gas flow within the furnace, 

aerodynamic size separation of coal. 

Multi-Stage Coal Injection 

As demonstrated in Figure 6, the coal injection takes place through the sidewalls above the 

pellet bed, extended over the first cooling zone. 

The extensive distribution of the coal across 

the pellet bed and across the total first cooling zone breaks the "burner" down into an infinite number of "individual burners" '..Tith "individual coal grain combustion". The pellet bed 

acts as an air distributor, equalizing the air 

flow to the individual coal grains by the flow 

resistance of the pet-lets in the range of 

50 mbar. 

This multi-stage injection ensures low local 

fuel concentration, one of the preconditions for 

high-ash coal firing mentioned before. Furthermore, the amount of coal injected through the 

individual injection points is adjusted according to the temperature and velocity profile of 

the cooling zone and the required process gas 

temperature in the preheating and firing zone. 

~ulti-Stage Coal Combustion 

Figure i illustrates the principles of single 

and multi-stage combustion. In conventional combustion chambers, the combustion of fuel takes 

place under near-stoichiometric conditions, 

producing extremely high temperatures of the 

combustion products. By further entrainment of 

excess air, these products are cooled down to 

the process temperature lev·el of abour l300°C. 

This te~perature evolution is shown in the figure for"single-stage" combustion. The process 

temperature is approached"from above" passing 

the hatchured "critical slagging area". 

83-85 

The contrary is illustrated in the second 

curve for "multi-stage" combustion, Here, the 

final process temperature is approached "from 

below" in infinite combustion stages arranged 

one after the other. The critical slagging area 

is not touched. The temperature-level of the combustion process is lowered drastically, thus 

matching another precondition mentioned before 

for high-ash coal firing. 

As also illustrated in Figure 7, multi-stage 

combustion requires a longer distance to complete combustion, since it proceeds with an 

essentially lower particle temperature and a 

coarser coal size range. The longer distances, 

however, are available by using the direct flow 

pattern, from the cooling directed to the firing 

zone, as shown before in Figure 6, Thus, the 

available distances are on the average about 

10 times longer than in conventional "singlestage" combustion chambers. 

These distances permit the utilization of the 

''aerodynamic size separation effect" for coal 

distribution. This important effect allows the 

final realization of the multi-stage combustion 

principle because it spreads the combustion from 

a two-dimensional in a three-dimensional combustion as illustrated in the Figure 8. The 

aerodynamic effect is created by the lifting 

forces acting onto the coal particles by the 

cooling air leaving the pellet bed. Depending on 

the coal size range, i.e. 0-10 mm, and on the 

final balance of forces, three types of combustion can be distinguished. 

Type I: Bed Combustion 

which takes place on top of the pellet bed 

for a coal particle size range (dp<:dgr)-w,hich 

is not lifted by the air stream with the velocity 

WJ prevailing inside the top layer of the P,ellets. 

dSJ is the diameter of the floating particle for 

the air velocity WJ. 

If the coal to be used leaves a fixed ash 

structure, a high percentage of this ash will be 

transported to the pellet discharge without 

creating any slagging problems. 

In industrial plants a certain percentage of 

coal combustion can be performed this way. 

The retention time available for combustion 

can be calculated by the point of injection and 

the strand speed. The coal combustion should be 

completed before entering the second cooling zone. 

Type II: Fluidized Bed Combustion 

refers to a coal particle size range 

(ds2 :> dp < dg 1) which is lifted by forces cor- responding to air velocity WJ but balanced for 

the lower velocity w2 above the pellet bed. 

The particle retention time for combustion 

here is infinite in theory, but in practise limited by the slow movement of this particle size 

range directed to the firing zone. This movement is created by the change in airflow from 

the vertical direction, leaving the top of the 

pellet bed, to the horizontal direction in the 

transition point from cooling to firing zone. 

Type III: In-Flight Combustion 

refers to the particle size range (O<dp<ds2) 

which is lifted by the force of air corresponding 
to the air velocity w2. 

As also illustrated in the above figure the 

very small coal particles (dp-o) will follow the 

air flow pattern more precisely and faster (t min) 

than coarser particles (dp-ds2) which need more 

time (t max) to be accelerated from the initial 

particle velocity zero to the final velocity. 

Retention time, acceleration, speed and location of coal particles can be calculated approximately on the basis of a mathematical model 

(8) simulating the elliptical flow pattern. 

The principles of the multi-stage coal injection and combustion process LMC can also be 

applied in steps to existing straight-grate 

pelletizing plants. With this concept, used as 

a first step at the Ferteco pelletizing plant 

in Brazil, remarkable fuel oil savings were achieved with comparable low capital investment. 

Conclusion 

The model calculations for energy savings 

by process optimization with the straight-grate 

plants have pointed out the possibilites for 

cost reduction. In existing plants, these possibilities are rather limited in comparison with 

those provided by using relatively cheap coal in 

place of other, more expensive, fuels. In addition to the admixture of coal into the pellet 

feed, it is primarily the direct use of lowergrade, high-ash coal by which the costs for 

pellet induration can be cut substantially. 

First results achieved with a new combustion 

system for coal with highash content and unfavorable ash melting behavior are encouraging. The 

expenditure for coal handling is also low due 

to the preferred coal size range of about 0-10mm. 

The new coal firing system for straight-grate 

pellet plants offers, thus, the currently most 

advantageous means of reducing the fuel costs 

also in existing plants at a relatively low 

capital investment. Concieved as a multi-fuel 

system, it offers the necessary flexibility for 

adaption to the fluctuating conditions of resources and market. 

References 

1. Cappel, F., Hastik, W., MUller, B., and 

Pfaff, E., 1982, "Entwicklungen zur optimalen Brennstoffnutzung bei der Herstellung 

von Eisenerzpellets auf dem Wanderrost," 

(Developments for Optimum Fuel Utilization 

in Production of Iron Ore Pellets on the 

Travelling Grate), 

Stahl und Eisen, Vol. 102 No. 5, pp.207-212. 

2. Violetta, D.C., 1958, "Updraft Pelletizing of 

Specular Hematite Concentrates", J. of Metals, 

No. 10, pp. 118-121. 

3. Haley, K. M., Apuli, W.E., 1962, "Pelletizing 

on a Horizontal Grate Machine, Agglomeration," 

d. Wepper, W.A., New York, Interscience, 

pp. 931-964. 

4. Quenean, P., Bracken, E. H., and Kelly, D., 

1958 "Highgrade Iron Ore at Copper Cliff, 

Ontario, "J. of Metals, No. 10 (8), 

pp. 527-532. 

5 83-85 

S. HUtter, B., 1977, "Die Herstellung gebrannter 

Eisenerzpellets aus GrUnpellets mit geringer 

Brennstoffbeimischung, "(The Production of 

Indurated Iron Ore Pellets from Green Pellets 

with Low Fuel Admixture), Doctoral Dissertation, Aachen, Rheinland Westfalen: Technische Hochschule Aachen (1977) 

6. Leighton, G. P., De Klaver, M. A. A., 1975, 

"Coal Firing of Dravo-Lurgi Travelling 

Grates," presented at Minnesota Section, 

48th Annual Meeting AIME, Jan. 1975. 

7. Queiroga, I. M., et. al., 1982, "Fuel Oil 

Reduction at CVRD Pelletizing Plants", presented at SME-AIME Annual Meeting, Dallas, 

Texas, Feb. 14-18. 

8. Hein, B. C., 1982, "Ein Beitrag zur Berechnung der Laufbahnen von einzelnen Kohlepartikeln in Pelletbrennofen, Theorie und Ergebnisse", (A Contribution on the Calculation 

of Trajectories of Individual Coal Particles 

in Pellet Indurating Furnaces, Theory and 

Results). Unpublished Report, Lurgi Frankfurt, Jan. 4. 

Authors' Biograuhies 

*Boss, K. H., Chemical Engineer, Fachhochschule Frankfurt, Group Manager in Research 

Center, Lurgi Chemie und HUttentechnik GmbH, 

Frankfurt am Main. 

**Hastik, W., Mathematical Engineer, Dipl. Ing. 

Technische Hochschule Darmstadt, Development 

Department Manager for Process Computation 

Lurgi Chemie und HUttentechnik GmbH, 

Frankfurt am Main 

***Kilian, A., Civil Engineer, Dipl. Ing., 

Technische Hochschule Karlsruhe, Manager 

Combustion Department Central Engineering 

Division, Lurgi Chemie und Htittentechnik 

GmbH, Frankfurt am Main 

****walden, H., Metallurgist, Dipl. Ing., Dr. Ing., 

Technische Hochschule Aachen, Senior Manager, 

Prokurist and Asst. Director of Research 

Center, Lurgi Chemie und Htittentechnik GmbH, 

Frankfurt am Main. 
6 

Figure 1. Investigated firing pat~erns 

ILO 1. __ 

l%1 \ 

. ; 

I 

j ; 1 0. I > 

: ! 

tlO 

120 

JO 

'lCO ~Jco ' •J!~ ·1e:o ;•cJ 

~32~ ~>i"J:Mq ;c.s ·~~~· 

Figu=e 2. Influence of :iri~g gas ~empe~ature on 

:uel cons~~pti~n and direct recup. air 

~--------------------------------~.~ ! :.:.TernSM ~:r~rn ~•tttlf'(f! ,.....,.....ia 

:t ~f"S.S\'t ~n tre::Q'Iffrtt':P'--"' ,."'/ 

(Q ~:rtrf't -:1 ~11ft~ / 

lll?rto.Jc:mry :f ~·tt~ ,/ 

,, ... "'/ 

I c ... n ~·. __.....,. 

iltluenC! ~ ::ooi ecdlt:vn en srrength end 

;:ro:JuctMty ct f.r:d CE'.lets GH~EI 

F~gure 3. Inf:uence cf coal addi~icn on 3t=eng~h and 

prcductivit7 of fired ?ellet~ 

83-85 
7 

r:-,_ow SHE:Oi: CONVENTIONAL •NOl.JRA oiNG i'URNACE 

Figure 4. Flow sheet: conventional indurating fur~ace 

~~ 

~ CffYNJ :OO£.,:.fHJ!C~ """ """' 

·.£.>C:N~•!'J'U 

~s;u.tT:::o 

IC82-111L.E 

Figure 5. combustion chamber syste.:n ME))ORC.;, HIBsnrG 

(DAAVO LURGI) 

for startup 

100% COAL FIRED 

PELLET INCURATING FURNACE 

coat 

C82-1112E 

Figure 6. 100~ coal f~red ?ellet indurating furnace 

83-85 
8 

0~.--------------------------------

Figure 7. Comparison single-multi combustion principles 

TYPE l 

>s;.. -.:J 

::lsc;...t.n:: 

_., 

:e:o C0~9LiS"riGN ~;t=:='C:J:A-L ~ ~ 

·-.'. 

••. . >¢~ S ____ , -- ~ J. 

;yp: il 

:::~9LiS7iGN ,,... :I:.·.·.::· ::.:.L ·,.· :'·.I: ".:: ;=:.~~IZE!J 91::::1 @_- ~ 

d~ 2 ~ dp < C51 ~--~~--~ ==_, ~ ~ 

TYPE ;n 

.. ,N .. .=L!GMT'' 

::;~eUS7iCN 

0 <' dp < d,, 

~:" 

:::CLi~G -"IP 

T'tP=:S CF liC"' 1

•

1

'1>=1 

COAL CC~BUSTJCN ! C-- I ~ -, 

Figure 8. Types of coal co~ustion 

BJ-85 
9 83-85 

firing Pattern 110. I 2 3 4 i a I b c d e f 

~ a b : 

Firing Temper•ture 'C 1220 I 1230 1260 1290 1320 1350 1320 1320 1320 1320 

- Preheating and Fl ring I I 

Time s 1372 I 1285 1137 1054 998 9se 927 892 937 983 10 IS 

------ Gas volume rn' /kg 1. 94 1.es I. 70 1.61 1. 55 I. 50 I. 47 1.33 I. 41 I. 55 I. 5 I 

Pellets I '--- ---------i----- Direct Recup.air 

Air volume rn' /kg 1. 7R 1.69 I. 54 1.45 I. 39 1. J4 1.31 1. JJ 1. 41 I. 38 I. 39 

Pellets I 

I ------------ Temperature oc 71D ~5~ 932 960 984 1007 978 g49 959 I 960 

----------· Oil Consumption 1 25.2 1 /t Pel- 34.2 31.8 27.9 25.9 24.6 23.8 23.2 25.4 28.7 23.6 

lets J 

3. --- --------- Firing Capacity t 85.6 92.4 96.8 100 102.4 i 104.4 105.6 111.4 110 99.7 / --- ____ i ---

3 4 Firin~ patterns accordln~ to F1gure 1 

Table I l!esults of Plant s1mulation