) 

) 

) 

SOCIETY OF 

MINING ENGINEERS 

CALLER NO. 0, LITTLETON, COLORADO 80127 

PREPRINT 

NUMBER 

86-128 

DESIGN AND PERFORMANCE OF THE NATIONAL STEEL PELLET PLANT 

HIGH TEMPERATURE HEAT RECUPERATION SYSTEM 

E. F. Draskovich 

National Steel Pellet Company 

Keewatin, Minnesota 

D. J. Englund 

M. A. Hanna Research Center 

Nashwauk, Minnesota 

W. C. Dailey 

National Steel Pellet Company 

Keewatin, Minnesota 

For presentation at the SME Annual Meeting 

New Orleans, Louisiana -March 2-6, 1986 

Permission is hereby given to publish with appropriate acknowledgments, excerpts or summaries not to exceed one-fourth of the entire text of the paper. Permission to print in more extended form subsequent to publication by the Society 

must be obtained from the Executive Director of the Society of Mining Engineers. 

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

embody certain changes made by agreement between the Technical Publications 

Committee and the author, so that the form in which it appears is not necessarily 

that in which it may be published later. 

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

Mining Engineers, Caller No. D, Littleton, Colorado 80127. 

PREPRINT AVAILABILITY LIST IS PUBLISHED PERIODICALLY IN 

MINING ENGINEERING 
ABSTRACT 

In 1984 a high temperature heat 

recuperation system was installed in 

the pellet plant of the National 

Steel Pellet Company located in 

Keewatin, Minnesota. The system is 

similar to those of Cleveland Cliffs 

Empire and U.S. Steel's Minntac 

operations. It recovers hot process 

air from the cooler by induced flow, 

and directs it to an intermediate zone between the drying and preheat 

zones on the grate machine. This 

system recently underwent further 

modifications in April 1985, 

:following initial operation in 1984. 

The M. A. Hanna Company, which 

manages the property, developed the 

design criteria and made the 

subsequent recommendations for 

modification through its research 

facility in Nashwauk, Minnesota. 

INTRODUCTION 

National Steel Pellet Company 

(NSPC) is jointly owned by National 

Steel Corporation and Nippon Kokan 

Steel Corporation. The plant, 

located near Keewatin, Minnesota, on 

the Mesabi Iron Range, commenced 

operation in 1969. At that time, it 

consisted of a concentrator with 2.44 

Mt/yr capacity (2.4 MM lt/yr) and an Allis Chalmers Grate-Kiln pelletizing 

system. In 1976, a second Allis 

Chalmers Grate-Kiln system (Phase II) 

with 4.06 Mt/yr (4.0 MM lt/yr) pellet 

capacity was added, and the concen- trator was expanded proportionately. 

In 1979, the original indurating line 

was closed permanently. Today, the 

plant employs 570 people and has 

increased its capacity to 4.65 Mt/yr 

(4.58 MM lt/yr) through modifications made to the indurating system. 

The plant is managed by M. A. Hanna 

Company. 

M. A. Hanna Company also 

maintains a Research Center at 

Nashwauk, Minnesota, for support of 

its mining concerns. · Hanna Research 

Center came into existence in 1929 as 

a beneficiation lab for the lean ores of the Mesabi Range. Since then, the 

laboratory has grown to include full 

support services for the iron mining 

industry as well as research covering 

beneficiation of sulfide and oxide 

ores, treatment of industrial 

minerals, air and water pollution 

sampling and analysis, pilot plant 

testing of industrial processes and 

upgrading of energy resources. The 

Research Center has fully equipped 

chemical and mineralogical laboratories to support its research programs in the aforementioned areas. 

1 

The NSPC Phase II grate 

machine was originally designed 

with two process zones consisting 

of eight windboxes for drying 

followed by six windboxes for 

pellet preheating. The machine 

had a "dedicated fan" type heat 

recuperation system. Subsequent 

operation showed the recuperation 

fan to be too large, since it 

frequently pressurized the drying zone on the grate. In 1980, the 

fan wheel was removed, and the 

flow throught the duct was induced by the process waste gas 

fans which exhaust from the 

drying zone. Gas flow through 

this duct was tempered by 

bleed-in air to maintain a 399"C 

(750"F) operating temperature. 

In 1980, Cleveland Cliffs 

Iron Company introduced the idea 

of a high temperature heat 

recuperation system at their 

Empire III unit. The high 

temperature recuperation system 

recovered process air from the same zone of the cooler as before, but no longer tempered 

the flow with bleed-in air. 

Depending on operating conditions 

the temperature of this air could 

reach as high as 815"C (1500"F). 

This hotter recoup gas was 

introduced into an intermediate or secondary drying zone situated 

between the primary drying and 

preheat zones on the grate. 

In general, this secondary 

drying zone could be made up 

from windboxes taken from the 

last half of the original drying zone and/or one or two of the 

preheat zone windboxes. The use of a high temperature heat 

recuperation system reduces the 

amount of process air required by 

utilizing recovered process heat 

without dilution from ambient 

air. The advantages as applied 

to a retrofit such as at NSPC 

include 1: 

1. Reduction in fuel consumption 

through more efficient use of 

process heat. 

2. Reduction in fan power as a 

result of decreased tempering air 

and decreased drying load. 

3. Increased oxidation of 

magnetite on the grate resulting 

in additional heat released to 

improve fuel consumption and 

decrease preheat fan power. 
4. Less magnetite entering the 

cooler further lowering the cooler 

air requirements and reducing fan 

power. 

5. More complete recovery of heat 

from the cooler and less heat lost up 

the stack. 

6. Decreased waste gas fan load as the equivalent drying heat is now moved with less air. 

For new plants, such a system can result in the employment of a shorter kiln and fewer or smaller 

process fans 1. Today, variations 

of this system are in operation at 

Cleveland Cliff's Empire facility, 

u.s. Steel's Minntac facility, 

National Steel Pellet Company and 

LKAB's KP-79 plant in Sweden. 

RESEARCH PROGRAM 

The decision to evaluate the 

application of a high temperature 

recoup system at National Steel 

Pellet Company was made in 1981. The 

scope of the program required 

definition of: 

1. The best windbox split configuration for maximum fuel savings. 

2. The impact on indurating capacity 

using the recommended windbox con- figuration. 

Based on the results of the test 

program, plant personnel would eval- ~.:ate the economic incentives for installing the proposed system. 

The Hanna Research Center performed and evaluated all tests and 

conditions involved for redesign of 

the machine. The various grate 

machine configurations were si~ulated 

using an Allis-Chalmers pot-grate 

furnace. The furnace is equipped with a 0.093 m2 (l ft2) grate area and 

standard instrumentation for controlling and recording gas temperatures, 

bed differential pressures, process 

gas oxygen content, and bed temperature profiles. All tests were performed with plant production greenballs, made possible by the proximity 

of the Research Center to the plant. 

Table lists the drying zone configurations that were examined. 

These zones were defined as a combination of windboxes within the original 

eight. This new zone division would 

only be installed above the grate, 

both primary and secondary drying zones would exhaust into a common header below the grate. 

2 

The major portion of the 

test program was performed 

simulating the plant at design 

capacity of 525 t/hr (517 lt/hr). 

The process conditions of gas 

temperature and pressure drop in 

each zone were determined based on actual plant data, anticipated 

conditions, and discussions with 

Allis Chalmers personnel. Two 

drying zone temperatures were evaluated at 343 •c and 398 •c 

(650"F & 750"F), in combination 

with six recoup gas temperatures 

ranging from 510"C to 649"C 

(950"F to 1200"F). The preheat 

inlet gas temperature was held 

constant at 1037"C (1900"F) in 

order to limit the number of test 

variables under consideration. 

A series of pot-grate 

tests were also performed at 

increased production rates of 

5%, 10%, and 15% above the design rate. From these test 

results, the anticipated increase 

in throughput would be determined 

based on bed temperature profiles, mass flow rates of gas, 

and pellet quality. 

A series of pot-grate tests were performed to evaluate each 

configuration. These tests 

generated data regarding greenball drying rates, magnetite 

oxidation rates, and pellet 

strengths at the end of the grate 

cycle. Approximately 10 tests were performed for each configuration. If a process gas temperature was changed, a new series 

of tests was performed, omitting 

any duplication of tests when 

possible. The data from these 

tests yielded temperature 

profiles of the pellets at three 

points in the bed, a gas/solids mass ratio by zone, and all 

appropriate off-gas temperatures 

by zone. This information was organized into data files and 

processed using an energy and 

mass balance computer program 

developed at the Research Center. 

Table 2 gives a complete listing 

of the input required· by the 

program to perform a balance. 

The computer program performs a straight forward mass and energy balance on the 

indurating system based on the 

grate machine configuration. A 

complete air, water and heat 

balance is done around each zone on the grate. The kiln is 

treated as one zone and the 

cooler as three zones. For 
these tests, the kiln and cooler 

conditions remained relatively con- stant. 

For the sake of comparison, all 

balances were performed with an assumed greenball mineral composition 

of 80% magnetite and 5% iron car- bonate minerals. Where not specifically stated, magnetite content was 80% and carbonate content was 5%. 

The program starts with an assumed fuel input into the kiln and 

proceeds to calculate heat requirements zone by zone on the grate 

followed by calculations for heat 

recovered from each of the cooler zones. The cooler heat is proportioned to three zones based on a solids cooling curve as affected by 

airflow through the zone, incoming 

solids temperature, and the amount of 

magnetite remaining in ·the pellets. 

Additional constraints are placed on the system to maintain a minimum 

cooler stack gas temperature and a specified recoup gas temperature. 

The balance around the secondary drying zone is completed when the 

calculated cooler recoup gas flow and 

heat content match the calculated 

heat and air requirements in the 

secondary drying zone. In other 

words, the required mass of air 

corning out of the cooler has to carry 

the requisite amount of heat, thus 

being at the specified gas temperature. Since the cooling curves varied from test to test, a perfect 

match was rare. 

The program has two methods 

to achieve a balance. The first is 

based on satisfying the second drying zone heat requirement using less 

cooling air than required (actual 

recoup gas temp > specified gas 

temp). In this case, bleed-in air is 

used to temper the mix to the correct 

temperature. 

Secondly, if the recoup gas 

temperature falls below the specified 

temperature, the air requirement by 

the second drying zone is satisfied, 

but not the heat content. Thus, the 

correct mass of air is moved through 

the duct, but with insufficient heat 

to match the simulated conditions. 

In this case, the program calculates 

the additional heat required and 

labels it as a heat imbalance. As 

long as this value remains less than 

5% of the total system fuel input, 

the balance is considered an acceptable simulation that can 

be attained in the plant. Any 

3 

test condition that produced an imbalance in excess of 5% was considered to have insufficient 

heat recovered in the cooler to 

meet the demand of the secondary 

drying zone. This represents an unattainable plant condition. 

The 5% limitation is an arbitrarily chosen value. In such cases, 

the pot-grate series was repeated 

with a lower recoup gas temperature or a smaller area in the 

secondary drying zone to achieve 

balanced conditions. 

All told, twenty-two series 

of tests were carried out in the 

initial phase of the research 

program. Only the pertinent ones will be discussed here. 

RESULTS OF THE TEST PROGRAM 

Figure illustrates the 

effect of the secondary drying zone size on the predicted fuel 

savings. As can be seen, fuel 

savings increased as the size of 

the zone increased. Figure 2 

illustrates the relationship 

between the required gas/solids mass ratio and the size of the 

secondary drying zone. In 

looking at Figure 2, it can be seen that waste gas fan capacity 

would be exceeded when the secon- dary drying zone is increased to 

four or more windboxes in length. 

The maximum fan flow limit is 

indicated by the broken lines. 

·Also shown is the maximum flow as measured by plant tests. The 

measured flow rates were lower 

than the simulated rates due to 

leaks in the system that could 

not be accounted for in the bal- ance. 

With the original design 

(low temp recoup), the plant 

operated with a preheat 

gas/solids ratio in the range 

of 0.8 to 1 .0. Based on the 

experiences at Empire, it was predicted that when in the 

modified mode, the gas/solids 

ratio would drop into the range 

of 0. 5 to 0. 7. The excess waste 

gas fan capacity thus created 

would be used to induce additional recoup flow. 

The Phase II machine design 

included the use of preheat 

by-pass ducts. These ducts 

routed kiln exit gas around the 

preheat zone to the preheat fans, 

to boost the preheat off-gas 

temperature to 399 •c ( 750 •p). 
The high temperature recuperation 

system presented an alternative, the 

replacement of the preheat by-pass duct 

with a recoup by-pass duct. This duct 

would direct hot recoup gas to the 

preheat fans as a replacement for the 

preheat by-pass system. This alternative presented a means for more efficient utilization of process heat. 

The by-pass heat required could 

also be minimized by making the 

secondary drying zone as large as possible. By increasing the size of 

the secondary drying zone, the 

resultant temperature of the solids 

entering the preheat zone also increased, and there was a corresponding 

decrease in the amount of by-pass heat 

required to boost off-gas temperature. 

These factors work in unison to reduce 

the load on the preheat fans, decreasing their power consumption. 

As stated earlier, the incorporation of a secondary drying zone utilizing hotter recoup gas should 

have increased the extent of magnetite 

oxidation occurring on the grate. The 

pot-grate tests indicated a relatively 

minor improvement in oxidation. 

Reasons for this trend were not clear, 

although it was felt that the oxidation 

rate was dependent on both temperature 

and time at temperature, and that the 

combinations of conditions tested did 

not actually vary enough to affect the 

oxidation cycle. Figure 3 illustrates the effect of magnetite content on predicted fuel consumption with 5% 

total iron carbonates present in the 

concentrate. This figure indicates 

that the machine, when operating in the 

low temperature recoup mode, was more sensitive to magnetite content than it 

would be when modified. 

The effects of concentrate carbonates on fuel consumption were also 

examined. Carbonate levels in the 

plant typically range from 1 to 3% by 

weight. For these simulations, 5% 

represented the maximum level expected 

with future operation. Figure 4 

illustrates the effect of carbonate 

content on predicted fuel consumption 

for a concentrate containing 80% magnetite. 

The test results indicated a 

maximum fuel savings when the primary 

drying zone was reduced to three 

windboxes. However, the subsequent 

analysis of airflow distribution on the 

machine demonstrated that the primary 

drying zone should contain at least 

four windboxes. If the secondary 

drying zone equalled or exceeded five 

windboxes (primary zone <3WB), the 

induced flow in the recoup duct became 

too great for the waste gas fans to 

4 

handle. The research program 

concluded that an even split in 

the original drying zone of four 

windboxes each for the primary 

and secondary drying zones would 

reduce fuel consumption by 78 

MJ/t (75,000 BTU/lt). The alternate configuration was the 5/3 

split. This combination was predicted to yield a fuel savings of 

62 MJ/t (60,000 BTU/lt). The 

prime factor in reaching these 

recommendations was the amount of 

air to be moved through the re- coup duct. As will be explained 

shortly, the 5/3 windbox split was chosen by the plant personnel. 

Based on the decision to use the 5/3 windbox split, a series of pot-grate tests evaluated the anticipated effects on grate capacity. These tests 

measured airflow and pellet 

strengths at 5%, 10% and 15% 

increases in the design capacity. 

By comparing bed temperature 

profiles and pellet strengths in 

conjunction with the fan flow 

constraints, a maximum 9% in- crease in capacity was considered 

possible. This information was then turned over to the plant 

maintenance and production en- gineers for preparing an economic 

evaluation of the project. 

INSTALLATION OF THE SYSTEM 

Plant personnel opted for 

the 5/3 windbox split after 

studying the findings. They 

wanted to be sure that the wastegas fans would be able to handle 

the predicted airflow through 

the bed and felt that the 5/3 

split offered a more reasonable 

fan load particularly if production was to be pushed. For 

either split, a major portion of 

the original recoup duct would be 

reused. This prevented an additional cost of $300,000 in material and labor. 

The installation commenced on September 13, 1983, with a 24 

hour shutdown to blank off both 

ends of the recoup duct. The 

plant was allowed to operate 

without heat recuperation while 

ductwork construction progressed. 

All work that could be performed 

while the plant operated was completed within a 90 day period. 

In mid November, the plant went 

down for scheduled maintenance. 

At this time, the final tie-ins 

of ductwork and associated equipment were completed. 
This system has three stainless 

steel dampers located in the duct 

inlets to the grate machine. These 

dampers serve to block the flow of hot 

air from the cooler during emergency 

shut-downs to prevent fires in the 

grate area. The plant with new recoup 

system started up in January, 1984 

wit!)out problem. 

PROCESS OPERATING RESULTS 

At the end of the 1984 operating 

year, a fuel savings of 57 MJ/t (55,000 

BTU/lt) had been realized. This sav- ings at the fuel rate of $3.15/GJ 

($3.33/MMBTU) equates to $0.180/t 

( $0. 183/1 t) and an annual savings of 

$790,000 at 4.39 Mt/yr (4.322 MM 

lt/yr). In addition, the plant capacity 

increased by approximately 327,000 t/yr 

(322,000 lt/yr) from 4.06 to 4.39 Mt/yr 

(4.00 to 4.322 MM lt/yr). This resulted in a net savings in production 

costs of $0. 65/t or $2,853,500 at the 

4.39 Mt/yr production rate. The total 

indurating power consumption also 

dropped by 0.98 KWH/t (1.00 KWH/lt) or $52,300 annually in 1984. Figure 5 

illustrates the change in fuel con- sumption by comparing 1983 with 1984 

operating,conditions. Because of shutdowns in 1983 and 1984, the months in 

which start-ups occurred have not been 

included in the plot. Similarly, Fig. 6 

compares average monthly production 

rates for 1983 and 1984. Using the 

design capacity of 4.06 Mt/yr as the 

base, production increased by 8.1% in 

1984. 

PLANT AIRFLOW SURVEYS 

Table 3 illustrates the major 

points in the process where airflow 

measurements were taken during operation. The simulation compared remarkably well to the actual plant perfor- mance in 1984. In most instances, 

actual flows came within 10% of the 

predicted values. The simulation 

differed significantly from the actual 

operation in predicting the recoup gas 

temperature. When the plant achieved 

steady-state conditions, recoup temperatures exceeded 788"C (1450"F) at 

design throughput. The simulation 

predicted a maximum of 565 •c ( 1050 "C) 

for comparable conditions. 

Operation of the system in 1984 

revealed two important limitations in 

the process. While the predicted rate 

of recoup airflow was achieved, inplant tests showed that moving more air 

could further reduce fuel consumption. 

These tests were conducted because of excess available heat in the cooler. 

Since the existing recoup duct was re- used to lower installation costs and 

gain approval for the project, the 

5 

duct cross sectional area was fixed. Estimates of pressure dr~ps and airflow in the duct 

pr1or to installation concluded 

that the duct was sufficient in 

size to handle the volume flow. 

However, because of the hotter 

recoup gas temperature and the 

larger volume of gas available in 

the cooler, the duct cross sec- tional area became a limiting 

factor to. flow, particularly at 

the higher throughput rates. 

Since the duct cross sec- tional area was fixed, the only 

other means available to increase 

flow was to increase the pressure 

differential on the duct. The 

plant tests mentioned above 

demonstrated the impact of pres- sure differential on flow and 

fuel consumption. By doubling the 

pressure differential from 

310 Pa to 620 Pa (1.25 to 2.5 in 

H20), an approximate 23% in- crease in gas flow was gained, 

which decreased fuel consumption 

by an additional 26 MJ/t (25,000 

BTU/lT). Data from these tests are shown in Table 3 under the 

heading of high suction tests. 

These tests demonstrated the need 

to move the maximum amount of air 

through the recoup duct. It 

should be noted that production 

rate significantly affected the 

pressure differential on the 

duct. The plant process fans 

could achieve the 620 Pa differential at design capacity, but as throughput exceeded this rate, 

the differential decreased. 

The second limiting factor was grate casting temperature. 

Since the pellet bed was hotter 

and the preheat zone retention 

time remained constant, casting 

temperatures began to climb. The 

only way to maintain an acceptable casting temperature was to 

cut back on the preheat fan flow. 

When this was done, the kiln 

frequently became dusty. It was thought that this action resulted 

in excessive amounts of underfired pellets near the bottom of 

the bed. Process operators chose 

to limit the recoup gas temperature to 677"C (1250"F) to 

alleviate this problem. However, 

to achieve this temperature, 

additional tempering air was required which displaced air from 

the cooler forcing larger volumes 

of air up the cooler stack. 

A 

correct 

proposal 

proposal •11as made 

these situations. 

considered making 

to 

The 

the 
preheat zone smaller by increDsing the 

size of the secondary drying zone. The objective was to limit the 

retention time in the p~eheat zone so that the preheat fans could be opened 

up more without burning up the 

castings. At the same time, the 

capacity of the waste gas fans would 

be increased 7% by changing fan wheels 

to increase the gas flow through the 

recoup duct. An increased flow would 

lower the average recoup gas temperature while recovering more heat 

from the cooler. Also, since the 

preheat zone size would be decreased, 

the primary cooling zone would be 

scaled down to balance it, limiting 

flow through the kiln to that required 

in the preheat zone. This move would 

shift more heat into the secondary 

cooling zone which the larger fans were expected to handle. Pot-grate 

testwork on this program was carried 

out in the early spring of 1985. 

Five series of pot-grate tests were performed to evaluate the 

enlargement of the secondary drying zone. Again, computer simulated heat 

and mass balances were performed on the pot-grate data. The five series were comprised of three series at the 

original design capacity and two 

series at a higher production level. 

The wall separating these zones would 

only be moved above the grate. Moving 

the wall would allow the preheat fans 

to exhaust either one or two windboxes 

of the secondary drying zone. The 

last two test series generated dat-a 

for heat balances at a 20% production 

increase from design capacity for both 

of the proposed windbox splits. This 

increase was chosen on the basis of 

plant performance in 1984. It 

actually represented a maximum feed 

rate that could be attained in the 

plant under optimum conditions, but 

generally not held for more than 24 

hours. 

1985 TEST RESULTS 

Since the decision was made to 

increase the size of the waste gas fan 

wheels, the heat and mass balances were evaluated on the predicted 

capabilities of the fans. The 5/4/5 

windbox configuration was chosen as the better of the two modes tested. 

The simulations at design capacity 

indicated that both modes would 

perform equally well. However, the 

amount of air required to be moved 

through the recoup duct increased 

significantly with each additional 

windbox in the secondary drying zone. This trend is illustrated in Fig. 7; 

also shown is the original operating 

fan flow range for this duct as well as the predicted maximum limit. 

6 

These simulations clearly indicated that the 5/5/4 configuration placed too much dependence on the waste gas fans to exhaust 

this zone, particularly at high 

capacity. The simulations 

predicted a maximum fuel savings 

of 46.7 MJ/t (45,000 BTU/lt). 

Another simulation was car- ried out using a model developed 

by Dr. Mark Cross, a consultant 

to the M. A. Hanna Company. This 

simulation investigated decreasing the size of the primary cooling zone to match air flow in 

the kiln withthe reduced requirement of the preheat zone. It was concluded that the wall could be 

moved two windboxes. This move would fit into place with the 

other modifications that were being recommended. It would 

shift more air and heat into the 

secondary cooling zone which 

would be exhausted into the 

recoup c1uct by the larger waste 

gas fans. 

In the 1984 high-suction 

plant tests conducted at low 

tonnage rates, the differential 

pressure on the recoup duct was increased from the typical 248 Pa 

(1 in H20) to 620 Pa (2.5 in 

H20). These tests were intend- eo to simulate the effects of the 

larger fan wheels. They demonstratec1 that fuel consumption 

could drop by as much as 26 MJ/t 

(25,000 BTU/lt) c1epending on plant conditions. Figure 8 illustrates the impact of differential pressure on measured flow 

in the recoup duct. Figure 9 

plots the impact of recoup gas 

flow on overall fuel consumption. 

this figure includes the results 

of simulations made in 1981, 1982 

and 1985 and the corresponding 

plant data. It should be noted 

that modifications were made to 

the computer program in 1984 in 

order that the 1985 simulation more closely approximate the 1984 

plant data. However, as can be 

seen, the plant data for 1985 

appears to fall into a reg ion of 

its own. The reason for this 

will be explained shortly. 

1985 PLANT PERFORMANCE 

In April 1985, the plant 

went down for scheduled mainten- ance. Special tasks included 

installing the new waste gas fan 

wheels, moving the preheat zone baffle wall one windbox into the 

preheat zone, and moving the 

primary cooler curtain wall two 
windboxes into the primary cooling zone. 

Plant operation started on April 28, 1985 \'lith the new system. 

Comparing plant operation in 1985 

with 1984 results indicated a fuel 

savings of 10 MJ/t (9600 BTU/lt) was achieved. This represents a $.0314/t 

savings or an annual savings of 

$146,100 at the 4.65 Mt/yr production 

rate achieved in 1985. This production 

rate represents an increase of 6% over the 1984 production rate and a 14. 5% 

increase from the design capacity of 

4. 06 Mt/yr. Power consumption in the 

pellet plant was reduced by an additional 3.94 KWH/t (4 KWH/lt). The 

total savings projected for the 1985 

production rate was $3,545,000 based on the decrease in fixed costs per ton of 

pellets. 

The predictions for fuel savings 

indicated a potential for up to 72 MJ/t 

(70,000 BTU/lt). Airflow surveys made 

in May and June identified the probable reason for not fully achieving the 

predicted fuel savings. Apparently, 

the cooler wall was moved too far, 

shifting more heat and air into the 

secondary cooling zone than could be 

recovered via the recoup duct. 

Refering to Table 3, one can note the 

significant difference in cooler stack 

flow when comparing the 1985 plant 

results to the 1985 predicted flow data or to the 1984 high suction plant test 

results. Also, one should note the 

significant increase in the recoup 

tempering air used to bring the recoup 

gas temperature to set-point in the 

recoup duct. 

As more tempering air is injected 

into the recoup duct under a constant 

flow condition, less air can be recovered from the secondary cooling zone. Tempering air displaces hot gas 

from the secondary cooling zone resulting in an increased flow up the cooler 

stack. This is the reason why the 1985 

data shown in Figure 9 did not achieve 

the predicted fuel savings. Although 

the total flow in the duct was increased, the amount of heat and air 

recovered from the cooler decreased. 

If the tempering air were subtracted 

from the 1985 plant results in Figure 9 

and the data replotted, the 1985 re- sults would be shifted over into the 

1984 operating range. Figure 10 

illustrates the relationship for airflow under approximately constant pres- sure differential and temperature 

conditions. The cooler modification 

made in 1985 resulted in a net decrease 

in cooler air recovered in the recoup 

duct when compared to similar operating 

conditions in 1984. Plans are currently underway to move the cooler 

7 

wall back one windbox in the 

hopes of correcting this situation. 

CONCLUSIONS 

Taken as a whole, using 1983 as the base year, the modifications have: 

1. Decreased fuel consumption 

by 63 11J/t, a decrease of 

14% 

2. Increased throughput by 

589,000 t/yr, a 14.5% 

increase over the rated 

capacity. 

3. Decreased pellet plant power 

consumption by 4.92 KWH/t, a 17.9% decrease. 

ACKNOWLEDGEMENTS 

The authors wish to acknowledge the contribution of Msrs. 

G.F. Dolinar, R.D. Frans, J.S. 

Wakeman, and T.M. Wolke to the 

program. 

REFERENCES 

1. Skoronski, G.J. and 

Woida, L.J., "Recent Design 

and Improvements in the 

Grate Kiln System" presented 

at the SME-AD1E Fall Meeting, Salt Lake City, 

October, 1983, Preprint 

No. 83-355 
Table 1 

NSPC He~t Recuperation Program: Grate Cycle Test Conditions 

Windbox Splits Inlet Gas Temp,oC Product 

001/002/PH 001)002/PH t/Hr 

Base Case - Original Machine 

8 - 0 - 6 398-NA-1038 525 

8 - 0 - 6 343-NA-1038 525 

8 - 0 - 6 398-NA-1038 593 

8 - 0 - 6 343-NA-1038 593 

High Temperature Recoup Conditions 

3 - 5 - 6 343-565-1038 

4 - 4 - 6 398-565-1038 

4 - 4 - 6 343-593-1038 

4 - 4 - 6 343-538-1038 

4 - 4 - 6 343-510-1038 

5 - 3 - 6 343-621-1038 

5 - 3 - 6 343-56.5-1038 

5 - 3 - 6 398-565-1038 

5 - 3 - 6 398-565-1038 

5 - 3 - 6 398-565-1038 

5 - 3 - 6 343-538-1038 

6 - 2 - 6 343-648-1038 

6 - 2 - 6 343-593-1038 

001 - first downdraft drying zone 

002 - second downdraft drying zone 

PH - preheat zone 

NA - not appropriate 

525 

525 

525 

525 

525 

525 

525 

593 

572 

551 

525 

525 

525 

Rate 
9 

Table 2 

Data Input Required by Simulation Program 

1. Mass of dry pellets in pot firing. 

2. Mass of water contained in pellets. 

3. Percent weight magnetite in pellets. 

4. Percent weight carbonate in pellets expressed as, 

a. percent Ankerite, Ca(FeMg)(C03)2 

b. percent Siderite, FeC03 

5. Percent weight silicate minerals. 

6. Percent of total water removed in each drying zone. 

7. Estimated temperature of moisture remaining in pellets after 

first drying zone. 

8. Percent of total magnetite oxidized in each zcne including 

kiln and cooler. 

9. Ambient air humidity, mass water/unit mass dry air. 

10. Averaged pellet bed temperature at end of each zone on grate. 

11. Averaged off-gas temperature at end of each zone on grate. 

12. Inlet gas temperature in each zone on grate. 

13. Kiln exit gas temperature (assumed equal to preheat zone inlet 

gas temp). 

14. Estimated solids discharge temperature from kiln. 

15. Simulated production rate. 

16. Averaged casting temperature by zone on grate. 

17. Type of fuel to be used in simulation; i.e, coal, gas or oil. 
10 

Table 3 

Comoarison of Simulated Gas Flow with Plant Measured Flows 

* 

Kiln 

* 1985 Wa11 Locations 

Simulated Actual 1984 High Simulated 

Airflow Measurement 1984 1984 Suction 1985 

(Includes Water Vaoor) Conditions Operation Ooeration Conditions 

T • "l'aste Gas Fans G/S 1. 7082 1. 6161 1. 7240 1.5396 

Kg/Min 14951 14649 15627 13475 

•c 93 119 130 52 

2. Preheat Fans G/S 1.0222 0.6946 0.5472 0.7049 

Kg/Min 8947 6296 4959 6169 

·c 343 388 393 372 

3. Total Flcr.r G/S 0.5391 0.4810 0.5942 0.6470 

Recoup Duct Kg/Min 4718 4360 5385 5663 

·c 537 788 800 678 

... " Recoup Tempering G/S 0 0.0737 0.0399 0 

Air Kg/Min 0 668 361 0 

5. Recoup By-Pass Air G/S 0 0.0601 0.0207 0.1025 

Kgfttin 0 545 259 897 

•c 0 788 800 678 

6. Primary Cooling Air G/S 0.7034 0.5776 0.4944 0.4440 

Kg/Min 6156 5235 4482 3886 

7. Secondary Cooling Air G/S 0.7740 0.8825 0.7500 0.9003 

(Including Recoup Kg/Min 6774 7727 6798 7880 

Tempering Air) 

8. Cooler Stack G/S 0.2351 0.3851 0.0888 0.2530 

Kg/Min 2057 3490 804 2214 

•c 270 332 173 293 

9. Proouction Rate t/hr 525 544 544 525 

G/S Refers to 9as:solids mass ratio. 

8 

Actual 

1985 

Operation 

1. 8239 

17624 

129 

0.7226 

6449 

388 

0.6157 

5950 

793 

0.1550 

1497 

0.0279 

269 

793 

0.5978 

5773 

1.0558 

10249 

0.4199 

4056 

354 

580 
FIGURE 1 

EFFECT OF ZONE SIZE ON FUEL SAVINGS 

120 ~------------------------------------------------------~ 

'Z' 110 

a 

~ 100 ~ 

--Ill 90 ~ 

Q 

·~ 

~ 80 

(/) 

.... 

Q) 

~ 

li. 

'0 

Q) 

+) 

0 

•M 

'1:1 

Q) 

1-4 

11. 

~ 

0 

Q) 

QJ) 

Q 

aj 

~ 

70 

60 

50 

40 

30 

20 

10 

Production Rate 52 5. mt/hr 

Recoup Gas Temp 538..;565 C 

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

2 3 4 5 

Wind boxes in Secondary Drying Zone 

..... 

..... 
FIGURE 2 ,. EFFECT OF ZONE SIZE ON RECOUP FLOW 

0.8 ~-------------------------------------------------------. 

~ 

~ 0) 

()~ 

::I ....... A Ol 

~~ 

~'-' 

0 0 () += 

d) ((j' 

~a: 

~ (/) 

..... (/) 

0.75 

0.7 

0.65 

0.6 

ct1 ~ ~' 0.55 

0 

...... (/) 

~"0 

'"d :=: 0.5 

d) 0 . () (/) 

:;j' '"d (/) ~ ct1 0.45 

H<.!J 

0.4 

0.35 

Production Rate 52 5 mt/hr 

Recoup Gas Temp 538-565 C 

-------- Max Plant Flow prior to Modification 

2 3 

Max Simulated Plant Flow 

4 

Wind boxes in Secondary Drying Zone 

5 

/~. 

....... 

N 
FIGURE 3 

EFFECT OF MAGNETITE ON FUEL CONSUMPTION 

520 

510 

-

+) 

a 500 

~ )! 490 -

~ 

0 480 •1"'1 

+) 

p. 

a 470 

------- ------- I ..... I w ::;; 

Ul 

~ 460 

0 

() 

1"""1 450 

Q) 

::;; 

~ 440 

"0 

Q) 

...,; 430 

0 

·1"'1 

"0 

Q) 420 

1-4 

n.. 

410 --t Production Rate 525 mt/hr 

400 Recoup Gas Temp 538-565 C 

I I 

75 ~ 80 85 

Percent Weight Magnetite in Concentrate 
FIGURE 4 

EFFECT OF CARBONATE ON FUEL CONSUMPTION 

510 

500 

--+) 490 s 

~ 480 .._... 

~ 

~ 470 

0 

•r-1 

.j.J 460 

Po. 

s 450 l 

----- -------' 1-' 

~ ~ 

{I) 

d 440 

0 

C) 430 

.-4 

cl) 

~ 420 

f;L. 

"d 410 

(1) 

-+) 

0 

...... 400 

"d 

cl) 390 J.c 

P-e 

380 ~ Production Rate 525 mt/hr 

Recoup Gas Temp 538-565 

370 . . . 1 3 5 7 

Percent Weight Carbonate in Concentrate 

--
550 

540 

-...., 530 

s 520 

~ 510 ~ ........ 500 ~ 490 0 

..... 480 .j.) 

Ill 

s 470 

;j 460 Ul 

Q 450 0 

u 440 ...-1 

Q) 430 ::1 

~ 420 

l>l 410 ...-1 

,.d 

+' 400 ~ 

0 390 

~ 380 . 

Q) 370 p. 

< 360 

350 . 

JAN 

FIGURE 5 

AVERAGE MONTHLY FUEL CONSUMPTION 

~ 

: 

'8 

(ij ~ 

i () 

8-

1 

~ 

FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 

0 1984 

Operating Month 

+ 1983 0 1985 

1-' 

U1 
450 

- 440 

....,> 

-~ ...._, 430 

s:1 420- 0 

•1""1 

+' 

0 410 ~ 

"d 400 0 

M 

Ill 390 

+' 

Q) 380 ...-1 

...... 

Q) 

n.. 370 

~ ...-. 360 

..ct 

+> 

~ 350 

0 

~ 340 

Q) 

330 ttl) 

~ 

H 

d> 320 I> 

~ 310 

300 . 

JAN 

FIGURE 6 

AVERAGE MONTHLY PELLET PRODUCTION 

FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC 

0 1984 

Operating Month 

+ 1983 <> 1985 

..... 

0'1 
0.74 

0.72 ...., 

0 0.7 ::s 

AI"""\ 

p. ~- 0.68 

p ' 

g ~ 0.66 

11) '-' 

~ 0 I 0.64 ~ ·.;::: .... ~ 0.62 . G) 

~ ~ 0.6 

(tj ctJ 

~ ::2!. 0.58 

~ C/) 

.9 ~ 0.56 ~ 0 

ret ~ 0.54 

(\.) C/) 

...., ctJ 

.~ CJ 0.52 

"0 

Q) 0.5 Jot 

~ 

0.48 

0.46 I 

2 

FIGURE 7 

EFFECT OF DRYING ZONE SIZE ON GAS FLOW 

Max Estimated Flow Range 

with New Fan Wheels (1985) 

3 4 

Original Fan Wheel 

Flow Range (1984) 

Production Rate 525 mt/hr 

Recoup Gas Temp 677 C 

5 

Windboxes in Secondary Drying Zone 

6 

...... 

........ 
FIGURE 8 

RECOUP FLOW VS PRESSURE DIFFERENTIAL 

0.6~------------------------------------~~--------~ 

0.59 

Q) 0.58 

b.& 

~ '"""' 0.57 

Q) 

~ ~ 0.56 ~ Q) 

~ ~ 0.55 rx.c'-' 

~ .~ 0.54 

g ~ '0.53 

() 

~ ~ 0.52 

ctS 

~ 2: 0.51 

It! C/) 

.-1 '0 0.5 

A-4 = 

'd ~ '0.49 

d) ...... 5 C/) 0.48 

Cl} ctS 

It!- CJ 0.4 7 

11) 

~ 0.46 

0.45 

Production Rate 525 mt/hr 

Recoup Gas Temp 704-873 C 

0.44 ,_~.---.--.---.---.--.---.---.-~--~--~--~--~~--~--~ 

300 340 380 420 460 500 540 580 620 

Diff. Pressure Across Recoup Duct CPa) 

t-> 

co 
480 

470 

460 

450 

-+> 440 

s 430 ~ 

._... 

~ 420 

~ 410 

0 

:p 400 

At 

s 390 

p 

00 380 ~ 

0 370 C) 

r-1 360 4) 

~ 350 ~ 

340 

330 

320 

310 I 

0.3 

,96 

~.b 

FIGURE 9 

EFFECT OF RECOUP FLOW ON FUEL 

'se, ... 19e 

11. ~ lflgh 1': 

lleollp G'*" ""'P lieeo,P 

. 1" ""'P li 7 o~6.,98~11/'*tlo,a 

~ ~. 'q'o e~ ~e~ 0~ ~~ ' 

199 /Jjlll)t ~ 0 

0.35 

QQ)~ ~~ 

~ II?~~ ~.0 4.9 

>o 

0.4-

"r, 

<9.> 

.a() 

0.45 0.5 

lieeollp Gea 'P:.,.'*t/,9 lili/J9e 

~?'Bac 

0.55 0.6 

1985 Simulations with 

shortened Preheat Zone 

Recoup Gas Temp 732 C 

~ ~ 

0.65 0.7 

Recoup Flow from Secondary Cooling Zone 

Gas/Solids Mass Ratio (Kg/Kg) 

0.75 

1-' 

U) 
FIGURE 10 

RECOUP TEMPER AIR VS FLOW FROM COOLER 

0.19 --r---------------------L--------, 

0.18 

0.17 

r-. 0.16 

Ol 

~ 0.15 

J.t Ol 

:< ~ 0.14 t..J 

~ .~ 0.13 ...... +"" 

l-4 m 

d) cc 0.12 ~ 

s ~ 0.11 

d) «S 

~ ~ 0.1 

PI CJ) :J "0 0.09 

0 

0 0 0 08 Q) (J) • 

~ ~ 0.07 

«S 

(j 0.06 

0.05 

0.0·1-

Production Rato 525 mt/hr 

Recoup Gas Temp 732 C 

Plant Measurements 

& 1985 

m 1984 

0.03 I I I I I I I I I I I I I ., 

0.42 0.44 0.46 0.48 0.5 0.52 

Recoup Flow from Secondary Cooling Zone 

Gas/Solids Mass Ratio (Kg/Kg) 

0.54 

N 

0