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ENHANCING COKE BULK DENSITY THROUGH THE USE OF ALTERNATE CALCINING

TECHNOLOGIES

Kenneth RIES

KENNETH E. RIES CONSULTING, 15507 NE 48thStreet, Vancouver, WA 98682

Keywords: Coke, Calcining, Bulk Density, Shaft Furnace, Technology

Abstract

The quality of petroleum coke used to make anodes for Aluminumproduction has declined in recent years and this trend is expected

to continue. For example, high volatile green coke can result inlow bulk density calcined coke leading to a corresponding lowapparent density baked anode. This can have a serious negativeimpact on the smelting process.

While most of the coke used in the industry is calcined usingeither rotary kilns or rotary hearth furnaces, there is a notableexception - the widespread use of shaft furnaces for coke

calcining in China. These furnaces have some unique attributesthat result in higher than expected calcined coke bulk densities.This paper will examine the potential for wider use of these shaft

furnaces, and other alternative calcining technologies, to improvecalcined coke density from available green cokes.

Introduction

The worldwide demand for aluminum has increased dramaticallyin recent years and by some projections is expected to almostdouble from the 2007 level of 38-million tons/year to an estimated74-million tons/year in the year 2020. To meet this projected

metal demand over the next 12 years, many new smelters must bebuilt and existing smelter capacity will be increased wherefeasible.

All of this projected growth in aluminum production will require

corresponding increases in supplies of petroleum coke in order toproduce the carbon anodes consumed in the smelting process.Thus, on a global basis, we can expect calcined petroleum cokeusage to produce anodes to increase from the current level of

about 13-million tons/year to about 26-million tons/year.

Currently, the general trend of petroleum coke is towards poorerquality due to decreasing quality trends in the crude oil. Sulfur

and impurities are gradually increasing and, in some instances,where sulfur and impurities are at acceptable levels, the bulkdensity of the calcined coke may be substandard. This isespecially true when the raw petroleum coke (RPC) being

calcined has a volatile content higher than the desired maximum

of 11-12%. When RPC with high volatile levels are calcined in astandard rotary kiln, the resulting calcined coke will typically below in bulk density as measured by vibrated bulk density (VBD).

While the rotary kiln is the worldwide industry standard forproduction of calcined coke, there is an lesser known, alternatecoke calcining technology used very extensively and almost

exclusively in China, called shaft furnace calcining. Shaftcalcining technology has demonstrated the unique ability tosignificantly enhance the bulk density of higher volatile RPC.

This characteristic has been very useful in China where the RPCvolatile content tends to be elevated. The widespread use of shaftfurnaces to calcine higher volatile RPC in China has helped avoidlower calcined coke bulk densities that one might expect if rotary

kilns were utilized for calcination.

Rotary Calcining Technology

Rotary Kiln Design ConceptTo better understand the unique advantages of the shaft calciningprocess, it is necessary to review the design concept and operatingfundamentals of the rotary kiln process. The rotary kiln is

basically a refractory-lined rotating drum on a gentle slope. Rawpet coke is introduced at the upper end of the drum and makes itsway down the kiln by gravity and exits into a cooler at the lower

end of the kiln as seen in Figures 1 & 2.

Figure 1. Typical Rotary Kiln Installation

The fuel (typically natural gas) and combustion air are introduced

at the lower end of the kiln, and together with burning volatilesreleased from the RPC, provide the heat necessary to bring theCPC to its peak temperature. After passing through the cooler,the CPC is stored for shipment to the user. The waste heat from

the gas stream can be used for generating steam and/or electricity.

The gas stream may also be treated in various ways to removeparticulates, residual tars and SO2.

Rotary Calcining ProcessFor most size rotary kilns, the RPC is fed at a rate between 15 and40 tons per hour into the first zone. In this first 25-400C heat-up zone, moisture is released. In the next 500-1000C calcining

zone, devolatilization takes place. Then in the 1200-1400Ccalcined coke zone final densification of the coke and someburning and desulfurization occurs. Typical coke residence time

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Light Metals 2009Edited by: Geoff Bea

TMS (The Minerals, Metals & Materials Society), 20

Light Metals 2009Edited by: Geoff Bea

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Figure 2. Rotary Kiln Process Diagram

in the kiln going from room temperature to 1350C+ is 30-45

minutes. The rate of devolatilization in the 500-600C calciningrange is critical in determining the final VBD of the coke. If

volatile loss is too rapid, puffing of the coke can occur leadingto very low VBD levels.

Most kilns use water spray rotary coolers to reduce thetemperature of coke exiting the kiln so that it can be safelyhandled and stored. The H2S and particulates in the waste gasstream are burned and the heat released can be used to generate

steam and/or electricity or cooled and exhausted.

Rotary Hearth Technology

Rotary Hearth Design Concept

Another commonly used calcining technology is the rotary hearth.A top view showing air and exhaust ducts can be seen in Figure 3.

Figure 3. Top View of a Rotary Hearth Calciner

In Figure 4. the main features of the rotary hearth can be seen.The hearth itself is a large, round rotating table sloped downward

toward the center. In it, there are baffle plates, called rabblesthat are strategically positioned to move the coke stream slightlydownhill. There is a central soaking pit with a discharge tableat the bottom and a rotary cooler similar to what is used on a

rotary kiln. Combustion air is introduced through the roof of thehearth and gases are discharged through the central stack.

Figure 4. Cutaway View of a Rotary Hearth

Rotary Hearth Calcining ProcessRaw petroleum coke is introduced into the outermost point of theround, rotating, sloped hearth. Each of the rabbles diverts the

coke stream slightly down the slope. With each kiln rotation of360, the coke stream encounters the next rabble where it isdiverted again slightly down the slope, and so forth until the coke

finally is deposited in the central soaking pit. There the cokeresides for some time at its peak temperature of 1350C+ beforebeing discharged into the cooler.

Since the RPC initially entering the rotary hearth is immediatelyexposed to high temperatures, volatiles are released very rapidlyleading to lower VBD coke. This is one of the main drawbacks of

the rotary hearth. Unlike the rotary kiln, fines tend to remainentrained in the coke bed in a rotary hearth and must be screenedout later to meet coke quality requirements. In a manner similarto the rotary kiln, waste heat from the rotary hearth can be utilized

to generate steam and/or electricity.

Slot Oven Technology

Slot Oven Design ConceptThe slot oven is actually a derivative of the metallurgical cokebattery. While the metallurgical coke battery is a well-known andcommonly used technology, the use of such slot ovens for

producing calcined petroleum coke is relatively rare. Themetallurgical coke furnace design consists of series of slot ovensvertically arranged side by side in a battery layout as seen inFigure 5.

Figure 5. Example of a Metallurgical Coke Battery

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To produce metallurgical coke, a hopper located on top of thefurnace charges pulverized coal through openings in the top of the

slot oven chamber. Volatiles are burned in the regenerator locatedbeneath the furnace to provide process heat. Off gases arecollected from the top of the slot oven and are directed to betreated in a separate facility. After the coke achieves its final

temperature, the end doors are opened and the met coke is pushed

out using a hydraulic ram. The hot coke is discharged into ahopper car and immediately quenched with cooling water.

Petroleum Coke Slot Oven Design and ProcessTo produce petroleum coke in a metallurgical coke battery,additional upstream and downstream equipment is needed. In this

process a carefully proportioned amount of coal tar pitch (CTP) isblended with the raw petroleum coke before being fed into the slot

oven. To do this blending accurately requires several hoppers tohold the RPC and CTP feedstock which is metered through weighbelt feeders. The RPC/CTP mixture is then added to the slot oven

for calcining in a similar way as metallurgical coke. To meetcustomer sizing requirements, crushing and screening equipmentmust be utilized due to the large size of the calcined coke product.The slot oven configuration can be seen in Figure 6.

Raw Petroleum Coke

Stockpiles

Coal Tar

Pencil Pitch

Crusher Crusher

Slot Ovens

Sizing Plant

Water Quench

Coke Oven Gas

Waste Heat Recovery

To Storage

RPC & CTP Blend

Raw Petroleum Coke

Stockpiles

Coal Tar

Pencil Pitch

Crusher Crusher

Slot Ovens

Sizing Plant

Water Quench

Coke Oven Gas

Waste Heat Recovery

To Storage

RPC & CTP Blend

Figure 6. Slot Oven Coke Process Flow Diagram

Given that the blended mixture of RPC and CTP is heated slowlyover a period of 1-2 days, and that the CTP filling the coke poreshas a much higher coking value than the coke volatiles replaced, a

high VBD calcined coke can be obtained using this process. RPCwith volatile levels of 13-15% can be used in a slot oven andVBD levels of 0.88- 0.90 g/cm3can still be attained. The calcinedcoke product obtained from the slot furnace resembles crushed

baked anode scrap it is extremely hard, typically having aHardgrove Grindability Index (HGI) of 35-38.

Shaft Calcining Technology

Shaft Furnace Design ConceptThe basic design principle of a shaft furnace is fairly simple. Themain component of a single furnace unit consists of a pair of

vertical chambers or shafts, heated on two sides by combustionchannels. The combustion channels are equipped with baffles,similar to those found in anode baking furnace flues, to provide

even heat distribution to the chamber as seen in Figure 7.

Cutaway View of a Single Furnace Chamber Unit

Figure 7. Shaft Furnace Design Concept

The volatiles emanating from the heated RPC are extracted fromthe top area of the furnace chamber into the top portion of the

combustion channel. In the combustion channel, volatiles proceedaround the baffles in a downward serpentine fashion. Controlled

amounts of air are introduced at specific locations in thecombustion chamber to burn the volatiles and maintain the desiredtemperature profile in the calcining furnace area. No auxiliary

fuel is needed to calcine the coke, only the volatiles released fromthe RPC are utilized as combustion fuel. Peak temperatures of1250-1350C. are attained in the combustion channel.

Shaft Calcining ProcessTypically, ten or twelve of these vertical shaft units will beclustered into a single bank of shaft furnaces as seen in figure 8.The raw petroleum coke, after being crushed and blended, is

added through the top of the shaft furnace and moves slowlydownward through the heated areas over a period of 48-72 hours.The RPC is added periodically in small increments eithermanually or automatically. In the final stages of calcination, the

coke attains a peak temperature of 1150-1200C., then is cooledin a water-jacketed bottom cone area and is ultimately dischargedand stored.

Figure 8. Shaft Calciner Process Flow Diagram

Due to the extremely slow heat-up rate of the RPC and the waythe RPC in the upper part of the chamber is immersed in releasedvolatiles, the shaft furnace method produces calcined coke withsignificantly enhanced VBD. Even RPC with volatile levels as

1-Gas Pipe 2-Gas Nozzle 3-Combustion Channel 4-Observation Port

5-Cooling Water Jacket 6-Calcining Furnace 7-Heat Storage Cell 8-Preheated Air Channel

1-Gas Pipe 2-Gas Nozzle 3-Combustion Channel 4-Observation Port

5-Cooling Water Jacket 6-Calcining Furnace 7-Heat Storage Cell 8-Preheated Air Channel

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high as 13% can yield calcined coke VBD levels of 0.86-0.88g/cm3. The shaft furnace operating range for RPC volatiles is 9-13%. RPC with higher volatile levels can be used but must be

blended with lower volatiles RPC or with calcined coke to keepthe volatiles level of the RPC feed below 13%.

The process flow arrangement as seen in Figure 8 represents a

shaft calciner installation with a high level of mechanization;

however, many smaller start-up facilities initially rely on moremanual labor and less mechanization as seen in Figure 9.

Example of a Manually Operated Shaft Furnace

Figure 9. Typical Chinese Shaft Furnace Installation

Because the cost of labor in China is low, new facilities tend toinstall less automation to reduce the initial capital costs. This

approach allows an entrepreneur to enter the coke calciningbusiness with a relatively small investment. Then as the demandfor coke grows, the capacity can be easily expanded incrementallyover time in response to market demand. As the calcining

operation grows and matures, both the RPC input and the calcinedcoke output from the shaft furnaces can be easily mechanized.

In many installations, the waste gas exiting the shaft furnace isexhausted through a natural draft stack. However, more recent

installations are utilizing waste heat recovery units to capture theresidual heat content of the furnace exhaust. The waste heat canbe then used to generate steam for process heating needselsewhere in the facility or the steam can be used to generate

electricity.

Calcining Technology Comparison

Volatile Content Relationship to Coke VBDWhereas all four of the calcining technologies described above are

fully capable of producing good quality CPC for use in carbonanodes, each technology has its own unique advantages and

disadvantages. These considerations must be taken into accountwhen deciding which technology to deploy. One of the mostcritical considerations is the volatile content of the RPC to be usedas feedstock to the calcining process.

For rotary kilns, when the volatile level in the RPC is greater than11%, the resulting calcined coke bulk density will be less than0.84 g/cm3 (as measured by vibrated bulk density (VBD) of the

minus 28 plus 48 mesh size fraction). When used in makingcarbon anodes, low bulk density coke will generally result in lowbaked apparent density (BAD). For a given anode size, lower

BAD may reduce anode shift life and increase overall anode costs.Smelters always prefer to use higher bulk density calcined coke.

RPC Heat-up Rates

For rotary calciners, lower heat-up rates (less than 50C. per

minute) especially in the early devolatilization stage (500C. to600C.), can improve calcined coke VBD. This can be difficult toachieve in practice since the peak coke temperature of 1300C.+

is often achieved from room temperature in less than 30-minutestotal time.

For the rotary hearth technology, because of its design, very rapidheat-up rates of the RPC are a major issue. Rapid heat-up rates

lead to rapid devolatilization, increased coke porosity and poorbulk density of the calcined coke product.

The heat-up rate for shaft calcining furnaces is dramatically lowerthan either the rotary kiln or the rotary hearth technologies. Forshaft calciners, coke residence times in the calcining chamber of2-3 days together with a significantly deeper coke bed combine to

enhance the final coke VBD.

Similarly, the slot oven typically uses a residence time of 1-2 daystogether with a deep coke bed to achieve a high coke VBD. The

coke VBD is further enhanced in a slot oven by the addition ofcoal tar pitch to the RPC resulting in a very dense and hardcalcined product similar in many respects to a baked anode. TheCTP addition makes this technology a special case that results in

the highest coke VBD.

A relative comparison of expected VBD from low (9-10%),medium (10-11%) and high (12-13%) volatile RPC for eachcalcining method can be seen in Table 1.

Low Medium High

Rotary Kiln 0.86-0.88 0.83-0.85 0.80-0.82Rotary Hearth 0.85-0.87 0.82-0.84 0.79-0.81

Shaft Furnace 0.89-0.90 0.87-0.88 0.85-0.86

Slot Oven 0.90-0.92 0.89-0.91 0.86-0.88

Table 1. Estimated VBD for RPC at Various Volatile Levels

for Each Calcining Technology

Advantages of Shaft Calcining TechnologyThere are several reasons for the widespread use of shaft calcining

technology in China. Because of the relative simplicity of theshaft furnace design and operation, the capital cost to install it islow. Unlike the other calcining technologies, it is possible toconstruct a very small scale shaft furnace installation initially and

later on expand the calcining capacity in small increments.

Each shaft furnace unit has an estimated annual CPC productionoutput of 800-1,000 tons/year or 10,000-12,000 tons/year for a

bank of 12-units. The capital cost for a 200,000 ton/year shaftfurnace installation in China is estimated to be about 20% of ansimilar capacity rotary kiln, rotary hearth or even a slot oven cokecalciner installation. Another advantage of the shaft furnace is its

relatively short lead-time to construct. The estimated timerequired to install a shaft furnace facility is between 4-6 months.While the operating labor input is somewhat higher for a shaft

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furnace, depending on the degree of automation employed, it stillonly takes 12-35 people to operate a 200,000 ton/year shaftcalciner compared to 10-20 people to operate a rotary kiln.

A comparison of each type of coke calciner regarding relativeestimated capital costs, operating costs and labor required for ahypothetical 200,000 ton/year facility is displayed in Table 2. The

value 100 is assigned arbitrarily to the costs associated with the

more familiar rotary kiln and rotary hearth technologies.

Capital Operating Labor

Rotary Kiln 100 100 100

Rotary Hearth 100 100 100

Shaft Furnace 20 75 150

Slot Oven 100 150 200

Table 2. Relative Estimated Cost for Installation, Operation andLabor for Each Calcining Technology

Clearly, the very low capital and operating costs associated withshaft calcining furnaces makes it a very attractive option for smallstart-up facilities and why shaft furnaces have become a verysignificant technology in China. The improved VBD of the coke

produced in shaft furnaces makes this option even more attractive.

Raw petroleum coke produced by most Chinese refineries tend tobe high in volatiles. Volatile levels of 11-15% in the RPC are not

unusual even with the use of some heavier imported crudefeedstock in many Chinese refineries. By using shaft calciningfurnaces, RPC with volatile levels in the 9-13% range can beutilized and produce calcined coke with VBD levels of 0.86 g/cm3

and higher. The calciner product recovery is also significantlyhigher than with a rotary kiln while at the same time very littleparticle degradation of the coke product occurs.

Environmental, Health and Safety Considerations

While the economic, technological and coke quality factors makeshaft calcining an attractive alternative to the more familiar rotary

kiln and rotary hearth technologies, there are some concerns in theareas of environment, health and safety. Rotary kilns typically

have very sophisticated environmental systems and a high level ofsafety and health compliance.

Many shaft furnace installations often lack even basic fume

treatment, dust capture or safety controls. It is likely that in thefuture, as shaft furnaces become more mechanized and there isgreater awareness of the need for improved environmental, healthand safety systems, the performance in these areas will improve.

On the other hand, it should be reiterated that shaft furnaces donot need any auxiliary fuel source aside from the volatilescontained in the RPC which will result in less CO2emissions.

Conclusion

Whereas the rotary kiln and to some extent, rotary hearth willcontinue to be the dominant calcining technology used in the

production of anode grade CPC, the less common calciningtechnologies have their place. The shaft furnace is an attractivecalcining option where labor costs are low, where the desire forlow initial capital investment is a priority and where mostly high

volatile RPC is available. Presently shaft furnaces are almostexclusively found only in China due to these factors.

Globally there is a limited availability of RPC with low volatiles,and the high volatiles RPC that is available would normallyproduce low VBD calcined coke using conventional rotary kilns.

Shaft calcining technology is a viable option for futureinstallations in order to meet smelters desire for high bulk densityCPC in a world of decreasing RPC quality (i.e. higher volatiles).It is likely that with technology enhancements and improved

environmental controls, more shaft calciners will be installed

outside China in the future.

Acknowledgement

The author gratefully acknowledges the technical contributionsmade by Great Lakes Carbon, LLC. without whose assistance this

paper would not have been possible.

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