The Rawmix Suspension Preheater: Complete Technical Guide

The rawmix suspension preheater is the tall cyclone tower in front of the rotary kiln: the heat exchanger of the cement process that preheats the raw meal and, in the modern lines, calcines 90 to 95 percent of it before the material ever reaches the kiln: this article explains the complete technology: the cyclones, the riser ducts, the stages, the heat exchange, the calciner, the instrumentation, the problems and the design numbers that the plant engineer must know.

The Complete Cement Technical Package (931 files including the courses, the books, the Excel tools and the presentations: $249.99 one-time: instant download via the PayPal payment) includes the The Rawmix Suspension Preheater chapter as part of the cement process technology course: the module teaches the preheater tower in the depth that its importance deserves: the function of each stage, the gas and the meal flows, the heat balances, the design and the operation: this article presents the same course content in the written form with the numbers of the industry.

Why the preheater gets its own chapter: the tower is the most efficient heat exchanger of the cement process: without it the kiln would require double the fuel, and with it the plant recovers hundreds of kilocalories per kilogram of clinker from the exhaust gas: the tower is also the most sensitive part of the line to the blockages, the dust and the volatile cycles: the engineer of the pyro section must know the tower in the same depth as the kiln: this chapter is that knowledge.

1. The Principle of the Suspension Preheater: The Gas-Meal Heat Exchange

The suspension preheater works on the simple but powerful idea of the counter-current heat exchange between the hot kiln gas and the cold meal, both in the suspended state:

• The suspension principle: the raw meal is fed into the gas stream of the riser duct, where the gas velocity of 10 to 20 metres per second entrains the fine particles: the meal and the gas mix intimately, the heat transfers from the gas to the particles in fractions of a second, and the hot gas plus the meal rise to the cyclone above;
• The cyclone separation: the cyclone above each stage separates the meal from the gas by the centrifugal force: the meal slides down the cyclone wall into the downcomer and falls to the next stage: the gas with the fine dust passes upward to the stage above: the separation efficiency of a good cyclone is 90 to 98 percent per stage, and the lost dust is caught by the following stages or the filter;
• The counter-flow: the gas moves upward through the stages while the meal moves downward: each stage exchanges the heat between the two streams, so the meal reaches 850 to 900 degrees in the bottom stage while the gas leaves the top at 250 to 350 degrees: the five-stage tower recovers the heat of the exhaust that a single-stage system would waste;
• The heat balance of the tower: the meal at the kiln inlet carries the heat of the tower: the calcination heat of the reaction comes from the gas and after the calciner: the module teaches the heat balance of the tower: the heat input of the gas at the inlet 850-1,100 degrees, the heat output with the meal and the exhaust and the losses;

The principle is the same as the counter-flow heat exchanger, but realized in the dust: every cyclone stage is one heat exchange stage, and the number of the stages is the economical choice between the fuel saving and the tower height: the suspension preheater is one of the two great inventions of the modern pyro process, the other being the precalciner: and this chapter teaches both.

2. The Construction of the Tower: The Stages and Their Order

The modern preheater tower is a concrete or steel structure of 60 to 120 metres of height, carrying the four to six cyclone stages on top of each other:

• The stage layout: the typical new line has five stages, the newest low-consumption lines add a sixth: each stage consists of the cyclone, the riser duct and the feed arrangement: in the double-string towers the flow splits into two parallel strings at the lower stages to reduce the gas velocity and the pressure drop: the module shows the arrangement of the tower of a real modern line;
• The bottom stages (III, IV, V): the hot stages at the bottom: the material from the stage above enters the riser into the hot gas 800-1,100 degrees: the temperature of the feeding to the kiln is reached here: the newest lines put the calciner between the stages: the calcination itself happens in the low stages of the loop:
• The top stages (I, II): the cold stages near the tower top: the meal enters at the top and the gas exits the tower here at 250-350 degrees for the five stages and lower for the six: the top cyclone is the feed of the tower and the target of the filter gas:
• The tertiary air and the split: the ducting of the tertiary air from the cooler enters the calciner: the module draws the air split of the line: the preheater gas, the calciner air, the kiln air: its control is the base of the temperature profiles of the whole tower:

The module walks the tower with the drawings and the number plate of a real line: the reader learns the names of the stages, the direction of the flow and the dimensions of the vessels, so the plant walk on the tower reads like a map: the fuel gas, the meal and the cyclone strings are in each hand:

3. The Function of the Cyclones: The Separation Science

Each stage is a cyclone: the vortex separator that returns the heat to the vote and the dust back to the process: the module teaches the cyclone science honestly:

• The separation mechanics: the gas-meal cloud enters the cyclone tangentially at 12 to 22 m/s: the centrifugal force throw the particles to the wall, the vortex carries the gas up and out through the outlet pipe, the meal falls to the vortex catcher and slides out: the radius of the cyclone and the gas velocity set the separation diameter;
• The efficiency: the real cyclones of the tower carry the collection efficiency of 90 to 98 percent per stage, with the remaining dust escaping with the gas to the next stage: the total carry-over of the tower is ~10-20% of the feed? No: the module gives the real figure: the dust lost to the filter is a few percent of the feed despite the 6 stages: the raw mill and the filter return it to the system:
• The pressure drop: each cyclone and its riser cost pressure drop: generally the tower costs 4,000 to 8,000 Pa (Pa) of the total pressure drop, – the 400-800 mm H2O, which the main fan of the kiln must overcome: the module teaches the pressure profiles of the tower and the influence on the fan power of 1.5-2 kWh/t of clinker:
• The erosion and the wear: the correction of the cyclone wall, the abrasive dust of the raw meal at a high velocity: the wear protection of the cyclones and the risers: the refractories the castables and the wear bricks: the module: the inspection of the housings, the replacement of the liners and the influence on the efficiency: the efficient cyclone must also stay round:

The cyclones look like simple cones, and that is their deception: the module carries the design equations, the efficiency curves and the scale models, so the engineer knows why the cyclone works and why it fails: the separation science of the tower is the same as the dust collection of the filter: the industry’s basic unit.

4. The Heat Exchange and the Temperatures of the Stages

The thermal readings of the tower are the language of the pyro section, and the module teaches the temperature profile of the stages:

• The temperature profile: the typical four-stage tower: the top gas 320-380, the second 450-500, the third 580-650, the fourth 700-800, the bottom (kiln exit) 850-950 of the meal: on a five-stage the top gas drops to 280-330 and each stage temperature falls some dozens of degrees: the module: the table of the stage gas temperatures of the different design lines;
• The measurement points: the temperature probes in the risers and the cyclones, the draft/pressure transmitters at the strategic points, and the historical automation data: the module teaches the standard instrumentation list of the tower: temperature TW, pressure PT, the O2, the flow: each with its purpose in the control loops and its maintenance;
• The heat recovery and the savings: the move from the four to the five-stage tower reduces the exhaust temperature by ~40-60 degrees and the consumption by ~20-30 kcal/kg of clinker: the sixth stage adds another 10-20 kcal: the economic choice of the number of the stages from the trade-off of the fuel price and the capital and the draft energy: the module presents the optimization curve;
• The ph a error: the area: the deviation incidents: the blind zones of the measurement: the normal readings of the tower and the alarms: the operator reads the tower with the low/mid zone temperatures and corrects:

The temperature profile of the tower is the signature of the line: the module trains the reading so the engineer can place the fault from the numbers: a cold bottom stage, a hot cyclone in the calciner loop, or the first stage below the predicted means a different problem each: the profile is the fingerprint.

5. The Calciner: The Second Combustion Chamber

In the modern preheater designs the riser duct of the lowest stage carries the calciner chamber, where 50-65 percent of the line’s fuel burns:

• The calciner principle: the meal enters the calciner at the preheated state, the fuel is injected and burns in the tertiary air stream, and the calcination reaction runs at 850-900 degrees in the temperature range that the flash calcination needs: the calciner is a perfectly stirred reactor of the kiln gas:
• The calcination degree: the meal that leaves the lower stages and enters the kiln is 90-95 percent calcined: the effect of the calciner on the kiln is game-changing: the kiln (about 30-40% of the fuel) only has to sinter: its capacity tripled compared to the kiln-only designs: the module documents the comparison: 3,000-6,000 tpd from the same kiln after the calciner additions:
• The types of the calciners: the in-line and the separated (off-line) layouts: the in-line uses the kiln exhaust gas, the separately fired and the independently controlled: the module covers the designs of the major contractors, the specific ones and the air splits: the combustion staging low-Nox of the calciner: the calc as the NOx and SO2 cleanup: the module: the “calciner as the gas treatment” chapter;
• The control of the calciner: the temperature of the calciner (850-900) is the control variable: the fuel to the calciner is trimmed on that: the poor calcification (the meal enters the kiln with the residual CO2: the kiln gets the extra cal – and the burning zone collapses: the module: the control loops and the response times of the calciner dome and the feed adjustment;

The calciner is the difference that made the modern line possible: the pyre process of today is a two-stage combustion process with two flames (kiln + calc), and the tower is the stage of the second: the module teaches the reader: the shoot and the operation and the whole world line:

6. The Problems of the Tower: The Blockages, The Build-Up And The Dust

The preheater tower has its well-known problems, and the module takes them seriously:

• The heat blockages (the “yorung”): the buildup of the hot meal in the cyclone cones and the risers: the sticky material during the startup, the volatile (alkali, sulfate, chloride) condensation in the upper loops and the coarsening: the restrictions of the exhaust: the module teaches the identification of the blocking: the pressure build, the temperature collapse and the rapping hammer responses:
• The snowman of the coatings: the likening: the bottom cyclone blockages and the kiln inlet: the cleanup with the high-pressure water guns and the jackhammers: the safety of the operations: the module: the procedure of the master: the preheater blockages are among the most dangerous maintenance jobs of the industry: the crew works the enclosure, the falling material, the personnel:
• The high-dust recirculation: the dust of the tower is returned by the filter to the same stages in the air: the recirculation load can reach 20-30 percent of the feed in the volatile cycles: the bypass the volatile: the module: the dust balance of the tower and the filter: the connection with the baghouse and the dedusting:
• The thermal tensions: the expansions and the differential of the cyclones and the risers at the temperatures up to 1,000: the refractory and the castables of the tower: the spring and the iron face: the expansion joints and the steel design: the module of the refractory: a whole chapter of the temperature of the tower: the property of the shells:

The tower problems: the rising and the falling: the module carries the troubleshooting compendium of the tower: the symptom: the cause: the fix: the temperature drop of one stage, the pressure changes, the CO rise in the multiphase: each with the numbered response: this is the operating handbook of the tower that the course gives.

7. The Operation and the Control of the Preheater

Operating the preheater is part of the daily art of the pyro operator, and the module teaches the control:

• The startup of the tower: the meal feed starts gradually, the stages preheat, the cyclones heat slowly to prevent the thermal shock of the refractory: the feed rate ramps with the kiln: the startup sequence of the pyro section: the module: the “c, startup”: the phases, the temperature ramps and the stopped before the feeding: the critical start of the line from the cold of the kiln+ the tower:
• The steady operation: the control of the KPIs: the tower top temperature, the calciner, the O2 at the roof, the draft: the module: the windows of the normal: the limiting factors: the pressure, the filter capacity, the moisture of the raw mix: the night: “standard practice of the kiln operator” not the design pamphlet:
• The feed quality and the tower: the meal feed changes: the moisture raises: the evaporation in the riser chills: the tower reacts with the temperature drop: the frequency of the meal changes: the module: the plant operation at the raw mill variations and the silo blending:
• The shutdown and the purge: the order of the shutdown: the fuel of the calc: the feeding: the purge of the tower with the cold air and the emptying of the cyclones: the un-shutdowns, the two/night: the module: the “waste cycle” of the town:

The operation of the tower is built in the “discipline of the numbers”: the temperature and the pressure on the screens and the sequences: at the same time the operator must remember: the tower’s numbers hide the dust: the process is never visible, only its thermal fingerprint, so the operator working by the numbers is the operator of the tower: the module trains exactly that.

8. The Energy and the Environment of the Tower

The last sections of the module measure the tower with the results: the energy and the environmental duty:

• The energy of the tower: the tower is the fuel saving machine: without it, the kiln consumes 1,100-1,200 kcal/kg instead of the 800-950 of the modern: the module recomputes the contribution of the tower: it is the biggest single: the enthalpy of the clinker out of the exhaust recovery:
• The waste heat: the exhaust gas of the tower: the raw mill drying gas: the module: the integration of the tower: the mills: heating the “WHR” at the bottom and the boiler of the power plant at the top: the modern tower gas after the raw mill: 150-200 degrees above the cil: the dedicated WHR: 20-40 kWh/t: the module: the case study of the WHR in the large setup:
• The emissions: the dust from the tower: the filter and the recycling: the SO2: the kiln gases react with the raw meal and the limestone: the tower is the in-situ of the sulfur: the NOx: from the calciner and the kiln flame: the SCR and the SNCR: the module teaches the emission balance of the pyro: the CO2: the 0.8-1.0 kg per kg, of which the tower shares the proportion of the fuel: the energy of the line:

The tower is not only the reactor; it is the “plug” of the plant in the environmental sense: the gas of the process must be clean at the stack: the module teaches the whole line of the tower: the equipment of the Lie: the bag filter: the optional the WHR and the final emissions: completing the full footprint of the pyro section of the 21st century.

9. The Frequently Asked Questions

Why are the modern preheaters four to six stages?

Each stage recovers part of the heat of the exit gas: the four stage ~350°C: with the five stage ~310°: and the six ~270°: the fuel economy of each additional stage must pay for the additional height, the draft and the emission temperature: the six stage with its capital is justified only in the low-cost fuel regions or when the drying of the raw material needs the colder gas: the module shows the economic choice by the local fuel price.

What is the hot meal residence time in the whole tower?

Briefly: the meal passes through the tower in roughly a minute: the gas residence, because it is entrained at 15-20 m/s in each riser, is a few seconds per riser: the reaction time of the calcination needs only the minutes: the design of the suspension makes the tower the fastest heat-exchanger of the industry per unit of the mass: the inverse of the ball mill geometry.

What does the “calcination degree 95%” really mean?

It means 95 percent of the carbonate in the meal has already released the CO2 before entering the kiln: the remaining 1-2 units and the sintering are finished in the kiln: the high degree of the precalcination is the reason modern lines can reach 5,000-12,000 tpd with a relatively short kiln: below 90 the kiln gas: above 95: risk of the Cl and the overheat of the kiln: the module target: 90-95 %.

How often must the cyclone be inspected?

At least once a year during the major kiln stop, with the internal inspection of the housings, the cyclones and the risers: the inspection of the refractory, the ceramics of the abrasives and the build-up points: the module: the inspection contains the pellets of the wear: the thermocouple readings and the wall thickness: the concrete routes of the inspection routes: the tower is the “tortilla” checked every shift on the outside and every year in the inside.

The tower consumes how much of the plant’s electricity?

The main fan of the Induced Draft (ID) is the big electricity consumer of the pyro: the 30-40% of the mill: the tower alone accounts for about 10-15 kWh/t of the total plant electricity (90-120 kWh/t): the pressure drop of the tower and the filter together 500-700 mbar: the module operates the fan dynamics and the CO2 the real fan power of the modern 5,000 tpd line: 1,000-1,500 kW are the ID fan: part of the fan park of the plant:

Why the fine meal must not fall too quickly between the stages?

The falling in the downcomers is by the gravity, a few meters per second: the fine fractions keep the heat exchange: the dwell holds the hot meal in the gas: the down-pumping rules, the flow-limiting, the aeration: the “lost meal” of too-fast exhaust: the module: the design of the downcomers and the discharge: the falling meal does the job of the heat, but the “storage” of the tower is only a few tens of tonnes + AT THE MOMENT: fine

10. Conclusion

Maybe the tower into one chapter: the suspension preheater is the recovery box of the cement process and the second reactor that divided the fuel bill: the cyclones, the stages, the risers, the heat exchange, the calc and its degree of 90-95, the problems of the blocks: the operation and the energy and the environment: the module of the Complete Cement Technical Package (931 files: courses and books and Excel tools: $249.99 one-time: the PayPal) covers the tower to the depth that no single paper can match: this article has followed the chapter section by section: the reader now owns the mental model of the tower: the exhaust of the platform, the gas and the meal: that model works in the control room, the maintenance or the design office: the tower is the “receiving ven” of the heat: know it and the rest becomes easier.

The next pages of the course, the Clinker Cooling and the Coolers, continue the same line of the module: the purchase of the package: the 931 files: the paypal: the download: The tower was the first upgrade of the kiln, and the cooling is the second: to click is to fire the pyro course of the whole: the future of the engine: all the numbers together under one board: cementequipment.org remembers the young student of the “last wheel”:

12. The Stages of the Hot Gas: The Suspension Sequence

The meal of the preheater does not fall through the towers: it is carried by the hot gases in the suspension: the stages work as the pairs of the contact: the meal disperses in the gas stream, the heat transfers in fractions of the second, the cyclone separates the solid and the gas: the suspension, the separation: four or five times through the tower.

• The top stage: the coldest gas receives the fresh meal: the pre-drying and the pre-heating begin at 300-350°C: the water of the meal leaves as the vapor;
• The middle stages: the meal heats by 100-150°C per stage: the calcination starts at 560-600°C in the upper stages at the low level of the decarbonation; the CO2 gas begins;
• The bottom stages: the meal arrives at 800-850°C with a calcination of 80-95% before it enters the kiln: the tower does the work, the kiln finishes it;
• The gas path: the kiln gases (900-1100°C at the feed end) rise up the tower against the meal stream: the counterflow of the heat: the

  heat of the gas falls to 300-350°C at the top: the efficiency of the counterflow of the five stages: the heat recovery of the system;

The suspension sequence makes the preheater the most efficient component of the line: every stage transfers the heat of the gas to the meal in the seconds of the contact: the tower with the five stages consumes about 700-750 kcal/kg of clinker against the 1000+ of the wet lines, because the meal is hot and half-calcined when it enters the kiln: the suspension preheater is the engine of the modern cement plant’s heat economy.

The preheater numbers of the plant tell the condition of the tower instantly: the gas temperature profile and the pressure drops per stage, the O2 of the exit, the calcination at the kiln inlet: the operators follow these measurements like the kiln itself because the tower is the beginning of the burning: the well-run preheater delivers the meal at 850°C half-calcined, and the kiln of the modern line enjoys the transition to the sintering: the discipline of the suspension, the tower, the lines of the plant.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.