Pet Coke Grinding and Firing of Lime Recovery Kilns – An Overview

E.A. (Gene) Golebiowski

Marvin Weakly

Phoenix Process Engineering, Inc. St. Peters, MO USA ABSTRACT The rising and unstable cost of oil and natural gas coupled with continual pressure on mill management to reduce production costs, has resulted in a re-kindling of interest in firing lime regeneration kilns with petroleum coke or other lower cost solid fuels. Prior to 2004 there were three pulp and paper mills with the capability to co-fire ground pet coke in their limekilns. In 2005 and 2006 six pulp and paper mills added this capability. In 2007 three more mills have already added this capability with an additional ten in the evaluation, design or construction phase. This paper will review the chemical impacts on the pulping process, emissions characteristics of firing pet coke and the technical and engineering aspects of grinding and firing pet coke, including a review of four different types of weigh feed systems. Commercial lime kilns and cement kilns have been fired by pet coke and coal for many years. The technology for grinding and firing is well established and applicable to firing lime re-generation kilns. The capital cost of a complete system, while not inconsequential, typically can be recouped in 2 to 3 years when the cost of natural gas or oil exceeds the cost of pet coke by US$3.20 a million kJoules (about US$3.00 per million BTU’s). Depending on transportation distance from oil refinery to paper mill, current price spread between pet coke and natural gas is typically greater in many locations. INTRODUCTION

Pulp and paper mills, being large consumers of energy, have been concerned with the historical spikes and subsequent declines in fuel prices. In recent years these spikes have been much higher and prolonged and have taken longer to abate. Some contributors to these cycles are the demand for oil from very large growing economies like China and India, forces of nature such as hurricanes, and the economic instability of certain Middle East countries. Those mills that have lime recovery kilns have an option to reduce their energy costs by replacing up to 85% to 90% of the oil or natural gas used to fire these kilns with petroleum coke (“pet coke”). The purchased cost of “raw” pet coke from the refinery is increased by two factors. These are freight to the pulp and paper mills from the oil refineries and grinding or pulverizing costs. The pet coke has to be ground to a high fineness before firing in the kiln and this can add US$0.35 to US$0.55 per GJ. The justification for the capital cost of the grinding and firing system is based on the difference in fuel costs offset by additional mill costs. These can include additional caustic and lime purchases and increased dead load circulation and evaporation costs. On several recent project evaluations we have completed for US mills, the cost differential results in very favorable discounted cash flow internal rates of return. Pet coke is an economic alternative to natural gas or oil as a fuel in the lime kiln. Prior to 2004 there were three pulp and paper mills with the capability to co-fire ground pet coke in their limekilns. In 2005 and 2006 six pulp and paper mills added this capability. In 2007 three more mills have already added this capability with an additional ten in the evaluation, design or construction phase or pending permits and final authorization to proceed. PET COKE CHARACTERISTICS

Pet coke suitable for fuel is dependent on two characteristics: grindability and sulphur content. Grindability is measured in terms of Hardgrove Grindability Index or HGI, where a higher number means easier grinding. Other characteristics, such as moisture, ash, trace metals, as delivered particle size, are manageable in the design of the grinding system and the kiln process. Table I summarizes the characteristics and typical values.

Table I: “Raw” Pet Coke Characteristics

Characteristic Typical Range Fuel Specification

Hardgrove Grindability 20 to 90 HGI 35 to 55 HGI

Sulphur 3.5% to 7% As low as possible

Moisture 3.5% TO 12 % 6% or less

Ash 0.5% to 1.5% As low as possible

Metals (V, Ni, Fe, etc) 100 – 5000 ppm Not a constraint

Particle Size, as received 3 mm to 200 mm 100% less than 50 mm

Energy Content (gross) 32.5-33.5 x 106 Joules/kg 32.5 x 106 Joules/kg

There are several types of pet coke, the nomenclature dependent on source and to some degree common usage. The characteristics and the ability of the process to absorb sulphur and metals, not the name, determine use as a fuel. Grinding to a high fineness, a minimum of 90% passing 200 USS mesh per ASTM E-11 or a BSI or Tyler mesh of 200 is needed. This average particle size is 75 microns and the energy content of this reduced moisture, less than 1.0%; ground pet coke is increased to 34.8 x 106 Joules/kg. CHEMICAL IMPACTS AND EMISSIONS

Chemical Impacts

The primary concern in substituting pet coke for natural gas and fuel oil is in increased operating costs in the chemical recovery cycle and emissions, principally NOx, SO2 and high volatility metals. Effects to the chemical recovery cycle are addressed in four areas:

• Lime kiln.

• Slaker.

• Causticizer.

• Post white liquor or dead load. Lime Kiln

There have been different opinions where the sulphur reports when pet coke is fired in a lime kiln. Some published emissions factors indicate at least 97% of the sulphur, generated as SO2, is converted to calcium sulfate by anhydrite reaction with the lime. A similar high conversion, approaching 100%, has been reported by some pulp and paper mills after actual testing. Mills evaluating pet coke firing in lime kilns should be able to consider a sulphur conversion to calcium sulfate in the high 90% range. Items such as the length and diameter of the kiln and whether NCG’s or SOG’s are also burned affect the degree of sulphur conversion. The generation of calcium sulfate will displace some calcium oxide production. Fuel per ton of product will remain the same but fuel needed per ton of calcium oxide will increase due to this reduced availability. Various opinions have been expressed, some based on theoretical analyses, how much additional lime product needs to be processed. Real mill data indicates a mill will need to process about 3.5% more lime product and makeup lime requirements will increase. Two other lime kiln concerns that have been expressed are ring formation and reduced refractory life due to the vanadium content of the pet coke. Burning pet coke will aggravate ring and ball formation. The sulphur is oxidized to SO2 and reacts with lime and residual sodium (which readily converts to Na2O in the kiln) to form CaSO4 and Na2SO4. The sulfate deposits can form rings in the kiln, which can form blockages although there is a tendency for these rings to stabilize. This stabilization reduces the frequency rings have to be shot out with a kiln gun. However, there are other items that also influence ring formation. There are mills that have two kilns burning pet coke, one of which sees ring and ball formations, the other does not.

Vanadium can reduce refractory life. In alumina based refractory the concentration of the vanadium and hot face refractory temperature determine if reduced refractory life will be seen. Mills burning pet coke that use higher service temperature low cement castable refractory in the kiln hot zone have not reported any refractory issues. One additional lime kiln system related concern is about outages required to remove calcium sulfate deposits in a scrubber. This has not been a significant issue to-date. Slaker

The calcium sulfate produced in the lime kiln has a low solubility in the slaker. Calcium sulfate will carry over to the Causticizer. Although a lot is not known about vanadium in the liquor cycle it is felt vanadium and nickel, the principle heavy metal elements, are controlled by elimination in the grits of the Slaker and the green liquor dregs system. Purging lime mud on a steady basis can also help and this purge is necessary to keep the inerts in the lime at a reasonable level (85-88%). Although these activities help purge the vanadium a significant amount has been found in the white liquor at one mill. Causticizer

Calcium sulfate carried over from the Slaker will react, over time, with sodium carbonate to form calcium carbonate and sodium sulfate. The disassociation constant of calcium sulfate is about ten times less than that of calcium carbonate. Sodium sulfate generation results in a reduction of sodium hydroxide formation or a make-up caustic requirement. The impact is mill specific depending on existing sources of sulphur, sources and cost of caustic and other factors. If the mill is burning a relatively high sulphur fuel oil and the pet coke is low in sulphur, the impact may be negligible. Those mills using waste caustic, for which they are paid a disposal fee or receive at no cost, have an advantage. The advantage or lack thereof is dependent upon the sulphur content of the displaced fuel. Dead load

Sodium sulfate generated reports to the white liquor as dead load. This impacts the solids to the recovery boiler and the solids to the evaporator increase. For bleached mills this increased sodium sulfate dead load from pet coke might be offset by a reduced introduction of salt cake to the black liquor. For brown mills the increased sulphidity may be desirable. Emissions

NOx Emissions NOx formation in kiln flames is by both thermal and fuel routes. In natural gas fired kilns, NOx formation is through the thermal route as fuel nitrogen is absent. Because natural gas flame temperatures are typically higher than those generated by pet coke, coal and fuel oil, NOx formation is significant. Additional mechanisms that contribute to thermal NOx formation are in-flame oxygen concentration (due to primary air levels and fuel-air mixing rates) and the residence time in the high temperature zones of the kiln. . Due to the 1% to 5% nitrogen in pet coke, one can predict 50% to 100% conversion to NOx emissions from this source. There are opportunities to control or reduce NOx emissions from lime recovery kilns.

• Decreasing the primary air flow through the kiln burner to 10% to 15% of the total combustion air will lower NOx.

• Managing the excess kiln oxygen at a level just below the onset of significant carbon monoxide generation would reduce NOx concentration and increase fuel efficiency.

• The contribution of the swirl air component of the primary air can affect NOx concentrations.

The use of Computational Fluid Dynamics (CFD) modeling of pet coke firing and experience of burner manufacturers specific to pet coke firing with access to actual kiln measurements will ensure that NOx emissions are minimized and, where possible, remain within required mill limits. The use of CFD modeling of pet coke firing could in the future ensure that NOx emissions are minimized. CFD is currently the only tool available to predict NOx in lime recovery kilns and will ultimately be used to provide the best predictions for permitting. However, it is yet to be validated with mill data and, hence, is still “work-in-progress”. Because of limitations in design and lack of experience of solid fuel firing, conversion to pet coke and the application of optimized multi-fuel burners from an experienced burner manufacturer are recommended to control the increases in NOx emissions. SO2 Emissions

Sulphur input to the kiln occurs through fuel (Pet Coke and Fuel Oil), NCG’s, and SOG’s. As previously mentioned, an order of 97% of the sulphur is captured by the lime in the kiln with the remaining removed in the scrubber and to some extent in the ESP. Those kilns with only an ESP are likely to emit some SO2, although if they are presently firing high sulfur fuel oil, there may be no increase. Particulates

There is some indication that those kilns with only scrubbers could experience an increase in particulates from firing pet coke. Historically, low energy types of scrubbers were characterized as being less efficient than high energy scrubbers. There are now low pressure drop multi-stage scrubbers that are very efficient in removing particulate. Metals

The low volatility metals report to the Slaker grits and green liquor dregs in the slaker. The higher volatility metals condense and are captured in the feed end of the kiln or scrubber. Some are retained in the white liquor.

ENGINEERING DESIGN OF PET COKE GRINDING SYSTEMS

There are 3 distinct areas of engineering expertise required for the introduction of pet coke to a kiln:

• Pet Coke receiving from the refinery, unloading and storage.

• Pet Coke grinding, transportation and storage of the finely ground product, weigh feeding and transport to the burner.

• Burner design and operation. Solid Fuels such as coal and pet coke have been fired in cement and commercial lime kilns for over 70 years. The technology is well established and experience with these systems is employed in the engineering design of firing pet coke in pulp and paper mill lime kilns. The challenge is coming up with a lower cost system than is typically found in the other industries because capacities of pulp and paper mill lime kilns vary and are typically much smaller. The elements of the system are:

• Unloading rail cars or trucks of pet coke received from the refinery.

• If required, crushing the pet coke to minus 50 mm.

• Conveying this “raw” pet coke to a storage bin.

• Withdrawing of the pet coke from the bin and weigh feeding to the grinding mill.

• Drying and grinding in the grinding mill.

• Collecting the ground pet coke in a fabric filter.

• A hot air supply for drying the pet coke in the mill.

• Conveying ground pet coke from the fabric filter to a storage bin.

• Weigh Feeding ground pet coke out of the bin.

• Conveying the ground pet coke to the kiln burner.

• Firing the pet coke through a multi-channel burner designed for pet coke.

• Power and Controls to regulate the system.

• Fire and Deflagration suppression and control systems.

In the past few years in the United States some major energy services companies have built large pet coke grinding facilities. These facilities receive raw pet coke, grind it to the desired fineness, store it, and ship it to different pulp and paper mills. While regenerative limekilns at pulp and paper mills are candidates for pet coke firing, some mills with a single lower capacity kiln may be “borderline” for an investment in a grinding circuit. Reasons for this “borderline” status could include a relatively low pet coke usage and a high internal rate of return threshold. Some mills in this category opt to receive ground pet coke rather than raw pet coke. Eliminating the grinding mill reduces the capital investment but at the expense of a higher cost for the ground pet coke. Basics of Grinding Pet Coke Pet coke and other solid fuel grinding systems are typically configured in one of three ways:

• Direct grinding and firing.

• Semi-direct grinding and firing.

• Indirect grinding and firing. The direct grinding and firing is a system in which the solid fuel is swept out of the grinding mill by the primary air and piped directly to the burner. The I.D. fan is often coupled to the drive shaft of the mill but can be independently driven. The semi-direct system installs a cyclone downstream of the mill followed by the I.D. fan. This fan discharges the air to two paths, one to the cyclone discharge and the burner, the other back to the inlet of the mill. The solid fuel is discharged from the cyclone through a rotary feeder/airlock into the pipe and is blown to the burner either by the I.D. Fan or a primary air fan depending on burner requirements. In both systems, hot air for drying is typically drawn from the kiln hood. The indirect system has a fabric filter in place of the cyclone and a ground solid fuel storage bin and weigh feeding system to meter fuel to the burner. Direct grinding systems result in 30%-40% primary air. A semi-direct system reduces primary air to 15%-20% and indirect systems offer a further reduction to 12% to 15%. The indirect system offers much better flame and firing control and less NOx formation. In firing lime recovery kilns with pet coke, the indirect system is used because the amount of air to the burner has to be restricted, the system has to respond to a fuel trip and the re-start of firing after a fuel trip has to be quick and safe. In both direct and semi-direct systems the grinding mill shuts down on a fuel trip. Re-start is an involved and slow process. In all of the above systems, the classic belt type weigh feeder is used to convey pet coke or other solid fuel to the grinding mill and a rotary airlock at the mill inlet limits ambient air infiltration. In lime recovery applications, hot air for drying comes from a natural gas or No.6 oil fired hot air generator. Types of Grinding Mills There are four types of grinding mills that can be used. They are summarized along with their characteristics in table II.

Table II: Grinding Mills

Mill Type Characteristics

Ball Mills • Highest purchase and installation cost.

• Much lower efficiency.

• Limited drying capacity.

• Requires separate classifier and extensive materials handling systems.

Ring Roller Mills • Lowest cost option. Reconditioned equipment with capacities suitable for lime recovery kilns readily available.

• Retrofitted with modern dynamic classifiers to achieve required pet coke fineness.

• Efficient Grinding Power, Acceptable Drying Capacity.

Vertical Roller Mills • Built-in high performance dynamic classifiers provide superior drying capacity and pet coke fineness control.

• Most Efficient Grinding Power, greatest drying capacity.

• Typically run for a year between maintenance shutdowns.

• Higher cost than other types of roller mills, longer delivery times.

Roller Mills • Lowest cost option of new mills.

• Efficient Grinding Power, good drying capacity.

• Now available with modern dynamic classifiers.

Fabric Filters

Essentially there are two types that are used, the classic pulse jet and the newer medium pressure controlled air jet cleaning units. The critical consideration is whether to apply the 3.4 bar (50 psi) rated versions or filters with explosion vent panels to comply with safety requirements. The filters must be insulated in locations where temperatures can drop below 10o C. Ground Pet Coke Conveying

The finely ground pet coke collected in the filter is pneumatically conveyed to the ground pet coke storage bin. Conveying can usually be done with an eductor type system because the grinding rates are modest, the storage bin is typically located near the firing floor, and conveying pressures typically do not exceed 1000 mm wc. Conveying velocity and conveying air to pet coke ratios are critical parameters. Where possible, elbows should be long sweep type. Pipe runs should be a combination of horizontal and vertical runs. If the lower cost option of a centrifugal blower is selected, provision to monitor pipeline air velocity and adjust the blower speed to maintain constant velocity is necessary. The alternative is a Positive Displacement blower. Ground Pet Coke Storage

Capacity, appropriate fire and deflagration protection, and attention to discharge flowability are the key parameters. The planned mill operating program, adequate time for mill and fabric filter maintenance and available excess capacity dictate bin capacity. Ground pet coke has difficult flow characteristics, which dictate bin discharge geometry. Explosion vents are an economical means to comply with fire and deflagration regulations; however fire suppression requires either CO2 or other approved systems. Ground Pet Coke Weigh Feeding and Conveying

There are four systems in use for ground solid fuel weigh feeding, which are summarized in Table III.

Table III: Ground Pet Coke Weigh Feeding Systems

Type of Weigh Feeder Characteristics

Weigh Bin on Load Cells 2%, typically better, accuracy, reliable

Impact or Momentum Flow Meter 1% accuracy claimed, prone to flushing, requires weigh bin on load cells for calibration.

Schenck Coriolis Feeder 0.5% accuracy claimed, reliable

Pfister Feeder 0.5% accuracy claimed, reliable

The Coriolis and Pfister feeders offer the highest accuracy but at a considerable cost, up to about 8-10 times the cost of a weigh bin on load cells system. Mills should determine if this high level of accuracy is justified or required to operate their lime recovery kilns effectively. ENGINEERING DESIGN OF MULTI-FUEL BURNERS

Lime recovery kilns present a significant challenge to burning pet coke or other solid fuels. Due to the low volatile content of pet coke, the distinct flame characteristics and the need for NOx control, the burner selection is critical. There are a very limited number of suppliers with the expertise to meet the challenges of lime recovery kilns. The heat flux profile for pet coke differs significantly from natural gas and to a lesser extent fuel oil, which must be taken in to account when designing the firing system. In lime recovery kilns, particularly those without coolers, secondary air temperatures are relatively low. This makes pet coke, with its low volatiles content, slow to ignite. Due to its high ignition temperature, >1000° C, and the fact lime recovery kilns operate at a lower temperature than cement or pebble lime kilns, a co-fuel such as natural gas or oil is needed to stabilize and maintain combustion.. Flame Emissivity

Radiation is the dominant mechanism of heat transfer in the burning zone of a rotary kiln, with over 95% of the heat transferred in this way. The rate at which heat is transferred to the bed or from a flame is controlled predominantly by the radiant exchange in the combustion chamber. The factors that affect this exchange are the temperatures, emissivity and relative geometry of the flame and surroundings. The emissive properties of a flame are a function of the concentrations of the spectrally emissive and absorptive gases (CO2, CO, H2O) from the combustion process, and the particulates burden in the flame. Flames in rotary kilns fired with pet coke have a high particulates burden and hence a high emissivity, while natural gas flames have a low particulates burden and low emissivity. Natural gas flames are therefore at a disadvantage because the low emissivity results in a peak heat flux further down the kiln (Figure 1). Physical modeling of the fuel/air mixing and mathematical modeling of the heat transfer along with an optimized burner design can minimize this effect, but not eliminate it; everything else being equal, a pet coke flame will produce a more effective combustion profile, compared to natural gas.

FIGURE I: HEAT FLUX CURVES FOR DIFFERENT FUELS

Burner Design

For any given kiln, the flame length and heat transfer are determined by the fuel characteristics, the fuel/air mixing rate and the quantity of excess air. The fuel/air mixing rate is primarily dependent on the ratio of the burner momentum and that of the secondary air. Aerodynamic and combustion modeling is the best method for assessing optimal burner engineering given the unique design characteristics of each kiln and the specific firing properties of the fuels used. In lime kilns, the high temperatures in the burning zone allow combustion to take place immediately where fuel and oxygen meet. Flame shape is controlled by the rate of mixing of fuel and air rather than the kinetic rates of reaction. Pet coke requires less air for combustion due to its lower hydrogen content in the fuel. This ultimately leads to lower kiln gas volume flows and lower feed-end temperatures. Due to improved heat transfer from pet coke firing, lower overall fuel requirements and a further reduction in the flue gas volume are realized. The combined effect is to reduce load on the kiln ID fan - an important feature for ID fan limited kilns. STEPS TO IMPLEMENT A PET COKE GRINDING AND FIRING SYSTEM

A Feasibility or Planning Study is the preferred first step to determine if a grinding circuit or a ground pet coke system is the economically preferred solution. Some mills have decided to proceed directly with detailed engineering for a ground pet coke system without a planning or feasibility study to justify the investment due to anticipated very favorable economics. Otherwise, the objectives of this study are to:

• Evaluate the type and capacity of grinding mill and system applicable to the Pulp and Paper Mill.

• Evaluate the pet coke receiving and unloading system and storage requirements.

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• Prepare conceptual arrangements and determine the capital cost. This capital cost will include the ground pet coke system, which is a part of the grinding circuit.

• Determine the pet coke supply cost and operating and maintenance costs so the Rate of Return can be established.

• Determine the emissions characteristics and parameters for permit application. Following evaluation of this study and a decision to go forward with either a ground pet coke system or a raw pet coke handling and grinding system, the next step is the environmental permit application. Once the permit application is submitted and Management has confidence a permit will be forthcoming, a Process Audit by the Burner Manufacturer is needed. This will establish important kiln operating features, potential fuel savings, assessment of effects on the pulping process and other factors. Four to six months prior to award of a permit, which will allow construction to begin, specifications and purchase of equipment and engineering design must begin. Construction will begin about three months after the start of engineering design and typically can be completed in about four months. You start construction at your own risk if you do not have a permit in hand. Some states will not allow construction to begin by not issuing a construction permit until the environmental permit is issued. Prior to start up, operator and maintenance training is required. Some mills insist on preparing an Operating and Maintenance Manual. This is very helpful in the training program and in subsequent operations. During the first 12 to 24 hours of start up, one can expect to operate at a 25-50% replacement of natural gas or No.6 fuel oil with pet coke. As the operators gain more confidence the substitution rate is raised to 80-85+%. We do not expect lime recovery kilns to achieve 100% firing of pet coke that is routine in the higher temperature cement and commercial dolomite lime kilns. Can other solid fuels, such as coal, be fired in lime recovery kilns? This is quite possible. Remember the make-up lime you currently buy has been produced from coal and pet coke fired kilns. CONCLUSION

The use of ground pet coke as a fuel for regenerative lime kilns offers pulp and paper mills a lower and more stable fuel cost in comparison to natural gas or fuel oil. However, this savings in fuel cost needs to be reduced by individual mill specific costs that address the undesirable effects resulting from burning pet coke. Lime kiln availability will decrease by 3-4 % and burning ground pet coke will aggravate ball and ring formation. If NCG’s and SOG’s are also burned ball and ring formation can become worse. Sulfur will be added to the chemical recovery cycle. Every ton of sulfur added to the system via the pet coke route will require 2.3 tons NaOH addition to the liquor system. In brown mills this increased sulfur load may be beneficial. Mills that adhere to following the basic chemistry balances for controlling the increased sulfur load have seen the best savings due to lower or reduced costs to control the increased sulfidity. Regardless of the cost of resolving dead load and other adverse process impacts substantial fuel cost savings exist. No mill that has converted to burn ground pet coke has stopped. ACKNOWLEDGMENTS

The authors wish to acknowledge the assistance of the following in the preparation of this paper: Mr. Richard Manning, Director, Kiln Flame Systems Ltd., High Wycombe, UK and Mr. John D, Macfadyen, PE and President, Phoenix Process Engineering, Inc., St. Peters, MO USA.

Pet Coke Grinding and Firing of Lime Kilns An Overview

Presented by E A (Gene) GolebiowskiPhoenix Process Engineering, Inc.

October 2007TAPPI Engineering Conference

Jacksonville, Fla.

Topics

Technical Aspects of Pet Coke SubstitutionBackgroundConversion of Ground Pet Coke System to an Indirect Grinding and Firing SystemTypes of Grinding MillsTypes of Weigh Feed SystemsPneumatic Conveying IssuesImportance of Burner Design to Pet Coke Substitution Rate

Emissions and Operating IssuesThe Engineering Process

Engineering Design and Construction ServicesProject Management

Why Use Pet Coke to Fire Lime Kilns?

Well established in firing cement and pebble lime kilns.Supply of pet coke is less prone to price variations than oil or natural gas. World production of pet coke is in excess of 65 million DMT, and growing.Several Pulp and Paper Mills are now burning pet coke.Substitutes 75%+ of natural gas or oil in firing lime kilns.Requires specially designed burner.Pet coke must be ground to a high fineness.Ground pet coke requires a Storage and Weigh Feed System.

Natural Gas Spot Prices

Natural Gas Futures Prices

Crude Oil Spot Prices

Crude Oil Futures Prices

Some Key Issues for Successful Replacement of Natural Gas or #6 Oil

Drying (for grinding mills)Fineness of grindUniformity and control of flow from storage bin(s)Uniformity of solid fuel delivery rate to the burnerUniformity of conveying air flow rate to the burnerConsistent GJ or BTU contentBurner designPreventing product contamination

Pet Coke Characteristics

Several Different TypesGreen CokeFuel CokeDelayed Process CokeFlexicokeFluid Coke

CharacteristicsGrindability varies with type, 20 to 90 HGISize varies with type, 3 mm BB’s, Fines, to 200 mm chunksSulphur = 3.5% to 7%Moisture = 3.5% to 12%100-5000 ppm Metals such as vanadium, iron, nickel0.5% to 1.5% ash and 1% to 5% nitrogen

Grinding of Pet Coke

Ball mills, ring roller mills, and vertical roller mills have all been used

Targeted fineness is > 90% passing 200 mesh (75 micron)

Grinding mill capacity is dependent on solid fuel hardness of grind (HGI), desired fineness of grind, moisture content, degree of wear on grinding components, and classifier efficiency

Ring Roller Mills

Vertical Roller Mill

Roller Mill

Pulverized Pet Coke Storage, Weigh Feed, and Conveying System - Basic Flow Sheet

Note: System must comply with applicable fire prevention codes.

Indirect Grinding and Firing System

Indirect Grinding and Firing System

Lowest primary air ratio and highest amount of preheated secondary airGrinding mill operation independent of kiln firing needsGrinding mill operates as required to maintain an inventory of ground solid fuel and can be oversized for future needsMultiple kilns can be fired with the output of a single grinding millEasy to stop/start ground solid fuel firingA ground pet coke storage system can be converted to include a grinding circuit in the future

Highest capital cost, increased maintenance costs, and highest electric power costsRequires attention to details to comply with regulations for fire and explosion suppression

Weighing and Flow Control

Flow characteristics of ground pet coke vary significantly and require special bin discharge systemsAttention to detail is justified as this is an automatic and unattended processGround pet coke has a low bulk densityGround material can settle out in conveying lines if the system is not designed, installed, and operated correctly

Weighing and Flow Metering

Weigh feed systems are available with degrees of accuracy from about 2%, or better, to within 1%.

Weigh bin on load cells – low cost, +/- 2%, or better, accuracyFlow meter with screw conveyor feed – has unfortunately proven to be inconsistent in performance, some working from day #1 and others require several months attention before stable flow and metering can be achieved, +/-1% accuracy claimed is less substantiated than the reproducibility once set up correctly, proper application requires weigh bin on load cells to calibrate flow meterSchenck Coriolis feeder - +/-0.5% accuracy claimed, expensivePfister feeder - +/-0.5% accuracy claimed, expensive

How accurate do you need to be to realize an acceptable operation?

Weigh Bin on Load Cells

Weigh Bin on Load Cells-Loss in Weight Trace

Impact Flow Meter

Impact Flow Meter with Screw Conveyor Feed

Schenck Coriolis Feeder

Pfister Feeder

Pneumatic Conveying Issues

High velocity conveying with a low solids to air ratio is neededto achieve a uniform solids loadingConveying distance and density of the solids conveying rate (t/h) dictates the pressure, flow rate, and type of pneumatic pumpCentrifugal blowers, positive displacement blowers, vane blowers/compressors, and screw compressors can all be used, depending on the applicationAdjustable speed motors are needed with centrifugal blowers, the air flow must be measured so the volume of air remains constant as the pressure variesIncreased conveying distances result in an increased chance of the ground pet coke dropping out, providing an inconsistent solids flow rate to the burner, an undesirable condition.

Multi Fuel Burners

Few burner manufacturers have pet coke experienceImportant to have a positive relationship with the burner manufacturer. Success hinges on the burnerBurner manufacturer needs to conduct a kiln audit to size the burnerModeling of the combustion process is advisableCo-firing with a volatile fuel (No. 6 oil, natural gas) is necessaryBurner design should ensure NOx emissions are minimized and, where possible, remain within required mill limitsStart up services by burner manufacturer are necessary

Burner Design & Pet Coke Firing

Low levels of volatiles in pet coke make it slow to ignite and limits substitution rate to below 90%Burner should allow for co-firing of either natural gas or oilTo ensure complete burnout, a specifically designed optimisedburner is critical

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Burner System Design

Recognition that the individual characteristics of different fuels will have an effect on combustion performance

NATURAL GAS

PETCOKE

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Emissions

For natural gas firing, fuel NOx is absent and all NOx is produced through the thermal NOx routeIn high temperature environments, >1600° C, thermal NOx is generally the dominant reactionHigher nitrogen solid fuels do not necessarily translate into a proportionate increase is NOx emissions, compared to natural gas, due to the lower flame temperaturesThere will be an increase in NOx emissionsSolid fuels will introduce SO2, most of which is absorbed in the kilnLime kilns with caustic wet scrubbers have the highest capture rate of SO2

Some Operating Impacts

Changes sulphur balance in the processMost mills need to add NaOHMud filters will need periodic chemical treatmentAn increase in mud moisture will occur in kilns with wet scrubbers. More attention to filter operation parameters is neededSO2 formed will react to form CaSO4 and Na2SO4, which can contribute to ring formation in kilns. NCG’s and SOG’s also contribute sulphurLow volatility metals are principally controlled by elimination in the slaker grits and green liquor dregsSolids to the recovery boiler and evaporator may increaseLime availability will drop 3-4% (CaSO4)

The Engineering Process

Client Project Objectives & ScopeProject Deliverables & Plan of ExecutionPersonnel Selection & Project OrganizationCommunication ProceduresSchedule Development & UpdatingProject Review & Progress MonitoringClient Communications & ReviewsEngineering DesignConstruction Site Services

Engineering Design & Construction Site Services

Engineering DesignRefine flow sheets and GA’sIssue specifications for equipment quotationsAssist in purchase of equipmentDetail engineering drawings – all disciplinesIssue drawings and specifications for installation/contractor bids

Construction ServicesCheck out prior to start upTrainingStart upPost start up follow up

Conclusions

Sulphur will be added to the chemical recovery cycle. Mills that adhere to following the basic chemistry balances for controlling the increased sulfur load have seen the best savings due to lower or reduced costs to control the sulphidity.In brown mills this increased sulphur may help.Every ton of sulphur added to the system via the pet coke route will require 2.3 tons NaOH addition to the liquor system.Burning pet coke will aggravate ball and ring formation in lime recovery kilns. If NCG’s and SOG’s are also burned ball and ring formation can become worse.However, pet coke still provides a substantial fuel savings, even after costs have been assigned to resolve dead load and other adverse process impacts.No mill that has converted to burning pet coke has stopped burning it.