# Modeling of the Thermal Efficiency of a Whole Cement Clinker Calcination System and Its Application on a 5000 MT/D Production Line

^{1}

^{2}

^{*}

## Abstract

**:**

^{i}); a thermal efficiency model of the whole system φ

^{QY}was thus established by correlating the relationship between φ

^{i}and φ

^{QY}. The thermal efficiency model of the whole system showed that φ

^{i}had a positive linear correlation with φ

^{QY}; it was found that the thermal efficiency of the decomposition and clinker calcination unit (φ

^{DC}) had the greatest weight on φ

^{QY}, where a 1% increase in φ

^{DC}led to a 1.73% increase in φ

^{QY}—improving φ

^{DC}was shown to be the most effective way to improve φ

^{QY}. In this paper, the developed thermal efficiency model was applied to one 5000 MT/D production line. It was found that its φ

^{QY}was only 61.70%—about 2.35% lower than a representative line; such decrease was caused by its low φ

^{DC}and φ

^{P}which, as disclosed by model, were derived from the low decomposition rate of calcium carbonate in preheated meal put into a calciner and the high excess air coefficient of secondary air. Controlled parameter optimization of this 5000 MT/D production line was then carried out. As a result, the φ

^{DC}and φ

^{P}of the production line were increased from 30.03% and 64.61% to 30.69% and 65.69%, respectively; the φ

^{QY}increased from 61.70% to 62.55%; the clinker output of the production line increased from 5799 MT/D to 5968 MT/D; the heat consumption of clinker was reduced from 3286.98 kJ/kg·cl to 3252.41 kJ/kg·cl.

## 1. Introduction

_{2}emissions [1,2]. Global process emissions during cement production in 2017 were 1.48 ± 0.20 GT of CO

_{2}, equivalent to about four percent of the total global fossil fuel emissions [3]. With the development of suspension preheaters and pre-calciner technology, the capacity of single-cement production lines has been greatly increased at a much-reduced energy consumption rate [4,5]. The maximum production scale of a cement production line can reach 12,000 MT/D at a heat consumption of as low as 2900 kJ/kg·cl [6]. Benefiting from continuous technological advances, the modern cement industry has become more conscious of the energy efficiency of the whole production line while pursuing increased capacities of cement kilns.

## 2. Scope and Objectives of the Whole System

- Raw Material Grinding and Boiler of Suspension Preheater (SP) unit (R&SP) for handling raw material and cogeneration.
- Preheating unit as row feed preheaters.
- Decomposition and Clinker Calcination unit (D&C) for the formation of clinker.
- Clink Cooling unit (grate cooler) for the rapid cooling of hot clinker.
- Coal Grinding and Boiler of Air Quenching Cooler (AQC) unit (C&AQC) for handling raw coal and cogeneration.

## 3. Effective Heat and Thermal Efficiency

#### 3.1. Definition of Effective Heat and Thermal Efficiency

_{i}is the thermal efficiency of the whole system or its subunit (%), where a larger thermal efficiency value relates to higher heat utilization efficiency of the subunit or the whole system. E

_{u}is the effective heat (kJ) and E

_{in}is the total input heat (kJ). The symbol “i” in Equation (1) refers to different objectives, RS for R&SP unit, P for Preheating unit, DC for D&C unit, C for Cooling unit, CA for C&AQC unit and QY for the whole system. For instance, φ

_{RS}is the energy efficiency of the R&SP unit, and ${\mathrm{E}}_{\mathrm{U}}^{\mathrm{CA}}$ is the effective heat utilized by the C&AQC unit.

#### 3.2. Assumptions

- The whole cement clinker system is stable, i.e., in a steady state.
- All calculations are based on one kilogram of clinker.
- Ambient temperature (20 °C) was used as the reference temperature; therefore, sensible heat of normal temperature materials and air entering the system, such as raw materials, coal and cooling air, can be ignored.
- No air seepage occurred in the whole system.

#### 3.3. Effective Heat of Subunits

#### 3.3.1. Raw Material Grinding and SP Unit (R&SP)

_{YQ}) from the Preheating unit and the sensible heat of fly ash contained in the flue gas (Q

_{FC}) constitute the input heat of the R&SP unit (${\mathrm{E}}_{\mathrm{in}}^{\mathrm{RS}}$). That is:

_{CSP}

_{.}Flue gas discharged from the SP boiler is then divided into two routes, 90% of which enters the raw mill (route A in Figure 2) to dry the wet raw material while the other 10% (route B in Figure 2) enters the process with the flue gas and powdered dry raw mill discharged from the raw mill for further environmental protection treatment, where the powdered raw mill can be collected. Heat consumed to evaporate the moisture in the raw material constitutes the second effective heat of the R&SP unit and is denoted as Q

_{SLQH}; thus, the effective heat${\mathrm{E}}_{\mathrm{U}}^{\mathrm{RS}}$ of the R&SP unit can be calculated by Equation (3):

#### 3.3.2. Preheating Unit

_{1A1/A2}, C

_{2A}, C

_{3A}, C

_{4A}, C

_{1B1/B2}, C

_{2B}, C

_{3B}and C

_{4B}for convenience (see Preheating unit in Figure 1). The hot flue gas exiting from the D&C unit is streamed evenly into columns A and B through each upward cyclone, and the raw feed is fed into cyclones C

_{1A1/A2}and C

_{1B1/B2}, respectively.

_{C5F}).

_{SC1}) and belongs to effective heat. The formula of Q

_{SC1}is as follows:

_{C4}) and the formation heat of clinker in the Preheating unit (see Equation (5).

_{1A1/A2}and C

_{1B1/B2}and enters the SP boiler in the R&SP unit, where the sensible heat can be recycled. Then, the sensible heat of the flue gas and fly ash at the preheater (denoted as Q

_{YQ}and Q

_{FC}) outlet can be classified as transfer heat. The only lost heat in the Preheating unit is the heat dissipation of the Preheating unit surface (denote as Q

_{YRS}). Heat flow modeling of the Preheating unit is shown in Figure 3.

#### 3.3.3. Decomposition and Clinker Calcination Unit (D&C unit)

_{5A/B}, tertiary air ducts and a rotary kiln. Most cement enterprises use pulverized coal as fuel, 30–40% of which (M

_{TM}in Figure 1) is fed into the rotary kiln and the other 60–70% (M

_{WM}in Figure 1) into the calciner; under the action of heat and oxygen provided by high temperature air from the Cooling unit, pulverized coal burns and releases heat, which can be denoted as Q

_{TM}and Q

_{WM}, respectively. Exothermic combustion of pulverized coal, as well as the sensible heat of high temperature tertiary air and secondary air from the Cooling unit (denoted as Q

_{3C}and Q

_{2C}, respectively) and heated material/feed from the Preheating unit (denoted as Q

_{C4}), are composed of the input heat in the D&C subunit.

_{3}S, C

_{2}S, C

_{3}A and C

_{4}AF in Equation (8) are the percentages of various minerals in clinker, which can be concluded from the composition analysis of clinker.

_{C5F}) can be reused as the heat source for preheating raw meal, and the sensible heat of red-hot clinker (denoted as Q

_{HS}) can also be recycled in the Cooling unit, both of which are classified as transfer heat of the D&C unit.

_{5A/B}, tertiary air duct and rotary kiln (denoted as Q

_{DCS}). It is worth noting that Q

_{DCS}is usually considerable, greater than that of all other subunits because of the high surface temperature of the D&C unit (the surface temperature of the rotary kiln is much higher than that of all other thermal equipment in the whole system).

#### 3.3.4. Cooling Unit

_{HS}), most of which is utilized as the sensible heat of hot cooling air after heat exchange. Cooling air discharged from the front of the grate bed has a high temperature and is sent to the D&C subunit as “secondary air” and “tertiary air” to provide oxygen and heat for coal combustion; their heat is denoted as Q

_{2C}and Q

_{3C}, respectively. Cooling air discharged from the middle and back section of the grate bed is generally used for waste heat power generation and drying raw coal for its low temperature; its heat is denoted as Q

_{ZWF}.

_{LQS}) and the sensible heat of cooled clinker at the grate cooler outlet (denoted as Q

_{CS}). The heat flow of the Cooling unit is shown in Figure 5.

#### 3.3.5. Coal Grinding and AQC Unit

_{ZWF}) as the input heat. The heat balance of the C&AQC unit is given in Table 2, where heat input, effective heat and lost heat (Heat outflow) have been identified.

#### 3.3.6. Effective Heat of the Whole System

_{SC}, which is the sum of Q

_{SC1,}Q

_{SC2}, and Q

_{SC3}. About 30% of ${\mathrm{E}}_{\mathrm{in}}^{\mathrm{QY}}$ is consumed in the form of sensible heat of medium and low temperature flue gas at the outlets of both the Preheating and Cooling units, which can be reused for drying raw coal and materials and power generation. Lost heat of the whole system includes surface heat loss during heat utilization and recovery, and the sensible heat of the exhaust gas and cooled clinker discharged from the R&SP and C&AQC units. There is no transfer heat when taking the whole system as the research object.

## 4. Modeling of the Thermal Efficiency of the Whole Cement Clinker System

^{QY}has a positive linear correlation with the thermal efficiencies of all five subunits, meaning that an increase in the thermal efficiency of any subunit will lead to an increase in φ

^{QY}.

^{C}will reduce the air temperature entering the D&C and C&AQC units and affect the combustion of both calciner coal and kiln coal, and the heat exchange effect of the coal mill and AQC boiler, φ

^{DC}and φ

^{CA}, will be reduced. A poor combustion of pulverized coal will reduce the quality and output of clinker, which will further reduce the heat brought into the Cooling unit by hot-red clinker and the heat exchange effect between cooling air and clinker and cause further reductions in φ

^{P}.

^{i}(as shown in Table 3). To quantitatively analyze the influence of the individual thermal efficiencies of each subunit on the whole system φ

^{QY}, the mutual influence of the thermal efficiency between subunits must be controlled—it is hereby assumed that the change in thermal efficiency of one subunit will not affect the thermal efficiency of another subunit. On this basis, the thermal efficiency of one subunit φ

^{i}was maneuvered with an increase or decrease by an increment of 2%, and the contribution of such changes to the thermal efficiency of the whole system φ

^{QY}was calculated according to the thermal efficiency model as given in (11); the result is shown in Figure 7.

^{DC}carries the most influence on φ

^{QY}, where every 1% increase in φ

^{DC}leads to a 1.73% increase in φ

^{QY}. Next are the Preheating and Cooling units, which have similar effects on φ

^{QY}where a 1% increase in φ

^{P}and φ

^{C}increases φ

^{QY}by 0.59% and 0.52%, respectively; ${\mathsf{\phi}}^{\mathrm{RS}}$ and φ

^{CA}have the smallest impacts on φ

^{QY}, as a 1% increase in φ

^{P}and φ

^{CA}only led to increases of 0.17% and 0.15% in φ

^{QY}, respectively. Such results are consistent with most efforts and achievements of cement practitioners. One may conclude that efforts to improve efficiency should focus on the Decomposition and Clinker Calcination unit (D&C unit).

## 5. Application of the Thermal Efficiency Model

#### 5.1. Calculation of Thermal Efficiency Before Optimization

#### 5.2. Comparative Analysis of Thermal Efficiency and Optimization Suggestions

^{CA}; as a result, the φ

^{QY}of the production line A is only 61.70%, which is 2.35% lower than the representative line.

^{DC}and φ

^{P}were analyzed according to the parameters of the production line A (as shown in Table 4) as follows.

- (1)
- Low decomposition rate of preheated meal into the rotary kiln.

- (2)
- Excess secondary air volume.

- (3)
- Low heat transfer efficiency between gas and solid.

^{P}would be increased. Since both φ

^{DC}and φ

^{P}would increase, the thermal efficiency of the whole kiln system of production line A can be improved.

#### 5.3. Calculation of Thermal Efficiency After Optimization

^{QY}of production line A increased from 61.70% to 62.62% after parameter optimization. Measurements also confirmed that its clinker output increased from 5799 MT/D to 5968 MT/D, whereas the heat consumption of clinker was reduced from 3286.98 kJ/kg·cl to 3252.41 kJ/kg·cl; all of which shows that the thermal efficiency model plays a positive role in improving the thermal efficiency of the cement production line.

#### 5.4. Summary of Thermal Efficiency Application

^{QY}lies in its low φ

^{DC}and φ

^{P}. φ

^{DC}was improved by increasing the decomposition of preheated meal into the kiln, while φ

^{P}was increased by reducing the excess air coefficient of secondary air as diagnosed by the model. The values of φ

^{DC}and φ

^{P}in the production line increased from 30.03% and 64.61% to 30.69% and 65.69%, respectively; as a result, φ

^{QY}increased from 61.70% to 62.55%, and the clinker output and heat consumption of clinker were slightly optimized.

## 6. Conclusions

- The thermal efficiency of the whole system (φ
^{QY}) is linearly correlated with the thermal efficiencies of its subunits. Increases in the thermal efficiency of each subunit lead to increases in φ^{QY}. - The thermal efficiency of the D&C subunit, φ
^{DC}, provided more influence than other subunits on the whole system. It was found that a 1% increase in φ^{DC}led to a 1.73% increase in φ^{QY}, followed by the Preheating unit and Cooling unit, where a 1% increase in φ^{P}and φ^{C}led to increases in φ^{QY}of 0.59% and 0.52%, respectively; ${\mathsf{\phi}}^{\mathrm{RS}}$ and φ^{CA}had the lightest impact on φ^{QY}where 1% increases in φ^{P}and φ^{CA}might only lead to increases of 0.17% and 0.15% in φ^{QY}, respectively. - Thermal efficiency of the whole system (φ
^{QY}) of one 5000 MT/D production line was 61.70% as calculated, which was 2.35% lower than the representative line. Application of the model revealed that this was due to low φ^{DC}and φ^{P}values derived from the low decomposition rates of calcium carbonate in the preheated meal in the kiln and the high excess air coefficient of secondary air. - After control parameter optimization based on the model, φ
^{DC}and φ^{P}values of the production line increased from 30.03% and 64.61% to 30.69% and 65.69%, respectively, and as a result, the φ^{QY}increased from 61.70% to 62.55% and the clinker output and heat consumption of clinker were slightly optimized at the same time.

## Author Contributions

## Funding

## Conflicts of Interest

## Nomenclature

Symbol | Content | Unit |

A_{1} | Volume of flue gas at preheater outlet per kg clinker | Nm^{3} |

A_{2} | Volume of exhaust gas at R&SP outlet per kg clinker | Nm^{3} |

C_{1} | Specific heat of flue gas at preheater outlet | kJ·(kg K) |

C_{2} | Specific heat of fly ash at preheater outlet | kJ·(kg K)^{−1} |

C_{3} | Specific heat of exhaust gas at R&SP outlet | kJ·(kg K)^{−1} |

C_{4} | Specific heat of raw materials at raw mill outlet | kJ·(kg K)^{−1} |

C_{5} | Specific heat of water vapor at SP inlet | kJ·(kg K)^{−1} |

C_{6} | Specific heat of water vapor at SP outlet | kJ·(kg K)^{−1} |

M_{1} | Fly ash at preheater outlet per kg clinker | kg |

M_{2} | Raw materials at raw mill outlet per kg clinker | kg |

M_{3} | Water in raw materials at raw mill outlet per kg clinker | kg |

M_{4} | Water vapor of SP boil per kg clinker | kg |

M_{5} | Magnesium carbonate contained in raw materials to preheating unit per kg clinker | kg |

M_{6} | Calcium carbonate contained in raw materials per kg clinker | kg |

T_{0} | Ambient temperature | °C |

T_{1} | Temperature of flue gas at preheater outlet | °C |

T_{2} | Temperature of fly ash at preheater outlet | °C |

T_{3} | Temperature of exhaust gas at R&SP outlet | °C |

T_{4} | Temperature of raw materials at raw mill outlet | °C |

T_{5} | Temperature of R&SP surface | °C |

T_{6} | Temperature of water vapor at SP inlet | °C |

T_{7} | Temperature of water vapor at SP outlet | °C |

ξ_{1} | Heat transfer coefficient of R&SP surface | kJ·(m^{2}·K)^{−1} |

S_{1} | Superficial area of R&SP surface | m^{2} |

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Input Heat | |||

Symbols | Description | Formula Used | |

Q_{YQ} | Sensible heat of flue gas at preheater outlet | A_{1}·C_{1}·(T_{1}-T_{0}) | |

Q_{FC} | Sensible heat of fly ash at preheater outlet | M_{1}·C_{2}·(T_{2}-T_{0}) | |

${\mathrm{E}}_{\mathrm{in}}^{\mathrm{RS}}$ | Total input heat of R&SP unit | ΣQ_{i} | |

Output heat | |||

Item Symbols | Description | Formula used | |

Lost heat | Q_{FQ} | Sensible heat of exhaust gas at R&SP outlet | A_{2}·C_{3}·(T_{3}-T_{0}) |

Lost heat | Q_{S} | Heat dissipation of R&SP surface | ξ_{1}·S_{1}·(T_{5}-T_{0}) |

Lost heat | Q_{SL} | Sensible heat of raw materials at raw mill outlet | M_{2}·C_{4}·(T_{4}-T_{0}) |

Effective heat | Q_{SLQH} | Heat consumption of water evaporation | M_{3}·2496 |

Effective heat | Q_{CSP} | Absorbed heat by SP boiler | M_{4}·(C_{6}·T_{7}- C_{5}·T_{6}) |

${\mathrm{E}}_{\mathrm{out}}^{\mathrm{RS}}$ | Total output heat of R&SP unit | ΣQ_{i} |

Heat Input | ||

Symbols | Description | |

Q_{ZWF} | Sensible heat of middle and low temperature air | |

${\mathrm{E}}_{\mathrm{in}}^{\mathrm{CA}}$. | Total heat input of C&AQC unit | |

Heat Out | ||

Symbols | Description | |

Lost heat | Q_{YTF} | Sensible heat of exhaust gas at the C&AQC outlet |

Lost heat | Q_{YTS} | Heat dissipation of the C&AQC surface |

Lost heat | Q_{MF} | Sensible heat of coal at coal mil outlet |

Effective heat | Q_{AQC} | Absorbed heat by AQC boiler |

Effective heat | Q_{MFQH} | Heat consumption of water evaporation in coal |

${\mathrm{E}}_{\mathrm{out}}^{\mathrm{CA}}$. | Total heat output of C&AQC unit |

Item | Units | R&S Unit | Preheating Unit | D&C Unit | Cooling Unit | C&A Unit | Whole System |
---|---|---|---|---|---|---|---|

E_{in} | kJ/kg·cl | 533.98 | 1865.83 | 5498.71 | 1662.59 | 484.16 | 3187.29 |

Total effective heat | kJ/kg·cl | 152.71 | 1266.03 | 1761.76 | 1577.07 | 79.60 | 2041.59 |

φ^{i} | % | 28.60 | 67.85 | 32.04 | 94.86 | 16.44 | 64.05 |

Parameters | Unit | Value | |
---|---|---|---|

Before Optimization | After Optimization | ||

Kiln coal | kg/kg.cl | 0.0651 | 0.0518 |

Calciner coal | kg/kg.cl | 0.0714 | 0.0832 |

Secondary air | °C | 1100 | 1109 |

Nm^{3}/kg.cl | 0.423 | 0.293 | |

Tertiary air | °C | 902 | 912 |

Nm^{3}/kg.cl | 0.455 | 0.549 | |

Flue gas discharged from Preheating unit | °C | 323.6 | 313.8 |

Nm^{3}/kg.cl | 1.455 | 1.413 | |

Dust discharged from Preheating unit | °C | 323.6 | 313.8 |

g/Nm^{3} | 80.32 | 84.59 | |

Flue gas discharged from D&C unit | °C | 860 | 860 |

Nm^{3}/kg.cl | 1.457 | 1.416 | |

Cooled clinker | °C | 125 | 123 |

Decomposition rate of preheated meal into kiln | % | 90 | 95 |

Flue gas discharged from rotary kiln | °C | 1270 | 1150 |

Nm^{3}/kg.cl | 0.563 | 0.549 | |

Flue gas into AQC boiler | °C | 339 | 369 |

Nm^{3}/kg.cl | 0.860 | 0.860 | |

Flue gas into coal mill | °C | 250 | 250 |

Nm^{3}/kg.cl | 0.114 | 0.114 | |

Flue gas discharged from PH boiler | °C | 220 | 220 |

Nm^{3}/kg.cl | 1.457 | 1.416 | |

Flue gas discharged from raw mill | °C | 115 | 110 |

Nm^{3}/kg.cl | 1.656 | 1.607 |

Item | Units | R&S Unit | Preheating Unit | D&C Unit | Cooling Unit | C&A Unit | Whole System |
---|---|---|---|---|---|---|---|

E_{in} | kJ/kg·cl | 684.97 | 2153.69 | 5852.42 | 1700.53 | 399.63 | 3286.98 |

Total effective heat | kJ/kg·cl | 157.11 | 1391.52 | 1757.21 | 1620.97 | 66.45 | 2028.19 |

φ^{i} | % | 22.94 | 64.61 | 30.03 | 95.32 | 16.63 | 61.70 |

Item | Units | R&S Unit | Preheating Unit | D&C Unit | Cooling Unit | C&A Unit | Whole System |
---|---|---|---|---|---|---|---|

E_{in} | kJ/kg·cl | 642.51 | 2090.87 | 5727.65 | 1662.86 | 435.83 | 3252.41 |

Total effective heat | kJ/kg·cl | 152.46 | 1373.50 | 1757.74 | 1585.01 | 79.10 | 2036.73 |

φ^{i} | % | 23.73 | 65.69 | 30.69 | 95.32 | 18.15 | 62.62 |

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## Share and Cite

**MDPI and ACS Style**

Yao, Y.; Ding, S.; Chen, Y.
Modeling of the Thermal Efficiency of a Whole Cement Clinker Calcination System and Its Application on a 5000 MT/D Production Line. *Energies* **2020**, *13*, 5257.
https://doi.org/10.3390/en13205257

**AMA Style**

Yao Y, Ding S, Chen Y.
Modeling of the Thermal Efficiency of a Whole Cement Clinker Calcination System and Its Application on a 5000 MT/D Production Line. *Energies*. 2020; 13(20):5257.
https://doi.org/10.3390/en13205257

**Chicago/Turabian Style**

Yao, Yanfei, Songxiong Ding, and Yanxin Chen.
2020. "Modeling of the Thermal Efficiency of a Whole Cement Clinker Calcination System and Its Application on a 5000 MT/D Production Line" *Energies* 13, no. 20: 5257.
https://doi.org/10.3390/en13205257