Abstract

At present, the domestic alternative fuels available for large-scale use of cement kilns are mainly derived fuels (hereinafter referred to as RDF) prepared from domestic waste after crushing, screening and drying and other pretreatment processes. Although RDF has a certain calorific value, it still has the characteristics of large particle size, high moisture, high ash content, poor uniformity and so on, which is quite different from coal, in the gas-solid multi-phase environment of cement kiln, there are still problems such as unstable combustion, low burnout rate and large fluctuation of system working conditions, which seriously restrict the application of RDF in cement kiln. To this end, this paper systematically carried out RDF combustion characteristics and combustion dynamics analysis, in this paper, the development of a large proportion of cement kiln using alternative fuels of the new decomposition furnace, successfully solved the non-fossil fuels in the cement kiln decomposition furnace large substitution ratio of the key problems.

Industrial Analysis of 1.1 RDF

The industrial analysis results of RDF and pulverized coal used in cement kilns are shown in Table 1. As can be seen from Table 1, RDF has high volatile content, high ash content, low fixed carbon content, and its heat mainly comes from volatile substances. The volatile fraction of pulverized coal is low and the fixed carbon content is high, so the combustion speed of coke in pulverized coal particles is the main factor to determine whether it can be burned out.

Table 1 Industrial analysis of RDF and coal

Surface Pore Structure of 1.2 RDF

The specific surface area of RDF particles is less than 1m2/g, which is much smaller than that of pulverized coal, while the average pore size is larger than that of pulverized coal, which indicates that the internal and external pores of RDF particles are far less abundant than that of pulverized coal, especially the number of small pores and micropores are less than that of pulverized coal. In the reaction system controlled by the chemical reaction rate, the specific surface area has a decisive influence on the reactivity of the fuel. The larger the specific surface area, the richer the pores, the larger the contact surface with oxygen, the stronger the combustion reaction, and the more complete the combustion. Therefore, under the same circumstances, RDF is more difficult to burn out than pulverized coal. The surface pore structure parameters of RDF and pulverized coal particles are shown in Table 2.

Table 2 Surface pore structure parameters of RDF and pulverized coal particles

Ignition characteristics of 1.3 RDF

The ignition temperature of RDF and pulverized coal is shown in Figure 1. As can be seen from Figure 1, the ignition temperature of RDF is lower (about 260°C) than that of pulverized coal, indicating that RDF can start ignition and combustion earlier. The ignition stability discrimination index of RDF and pulverized coal combustion is shown in Figure 2. The average value of the flammability discrimination index C of the three RDF is 7.29, and the average value of the stable combustion discrimination index M is 5.94. By comparing the combustible index and the stable combustion index of various fuels, it can be seen that RDF belongs to the extremely stable zone. Compared with pulverized coal, RDF has better flammability and subsequent combustion stability than pulverized coal.

Fig.1 Ignition temperature of RDF and pulverized coal

Fig.2 Fire stability discrimination index of RDF and pulverized coal

After RDF particles are put into the decomposition furnace, their composition changes with the increase of temperature, which means that the activation energy E and frequency factor K of RDF pyrolysis stage are different at different temperatures, so E and K should be functions of RDF particle temperature Tp, I .e. E (or K)= f(Tp). At a given temperature, RDF particles may have several volatiles released at the same time, so at each moment of the entire volatilization process, there is an average E and K value, that is, E (or K)= f1(Tp). In fact, Tp is unknown to RDF particles, so it is difficult to give a specific expression. In order to simplify the calculation, Tp is replaced by the final temperature T∞ of RDF particles, and the kinetic parameters of RDF in the whole pyrolysis process are replaced by equivalent E and K. According to the law of mass action, the relationship of E (or K)= F(T∞) is obtained.

(1)RDF particle pyrolysis kinetics equation:

Where:

V -- Percentage of volatilized mass;

R-Ideal gas constant;

T-thermodynamic temperature;

E, K-the equivalent activation energy and equivalent frequency factor at a given temperature T∞, respectively, and the values of E and K are only related to the final temperature and volatilization rate of RDF particles;

V∞-the final content of volatilization analysis at a certain temperature and reaction time t → ∞, determined by experiment.

(2) Energy equation of RDF particles:

Where:

ρc-density of RDF;

Cpc -- Specific heat at corresponding temperature;

r-radius of RDF particle;

qv -- latent heat of pyrolysis;

Vp-RDF particle volume;

Gv-the rate of volatile release per unit volume of RDF particles.

Since the porosity of RDF particles increases during pyrolysis, its thermal conductivity will decrease and approach the thermal conductivity of gas. For simplicity, λ = λ(T∞) and Cpc = Cpc(T∞) are assumed, representing the average thermal conductivity and the average specific heat during pyrolysis of RDF particles. According to the magnitude analysis, some terms in equation (2) are much smaller than others and can be ignored, so the equation can be simplified:

The boundary conditions are:

The initial conditions are:

(3) Mass equation of RDF particles:

Solving equations (1), (3), and (4) together, the equation V = V(t) of the mass percentage and time of the RDF particle volatilization analysis can be obtained.

The content of fixed carbon in RDF is very small, and the calorific value mainly comes from volatile matter. The pyrolysis and gasification of RDF particles releases small combustible volatiles, which can carry out rapid homogeneous combustion reaction with oxygen in the decomposition furnace, so its combustion speed mainly depends on the gasification cracking speed.

When RDF particles are fed into the decomposer, they have a certain initial velocity and direction of movement, and move under the combined action of their own gravity, gas buoyancy and flowing gas resistance. At the same time, the particles are continuously cracked, gasified and burned in the high-temperature flue gas, and their particle size is continuously reduced until they burn out. The velocity equation of the particles in the gas flow is as follows:

Where:

m-mass of particles;

u-Relative velocity of particles in airflow;

V-volume of particles;

Ps-particle density;

Pg-air density;

Φ -- shape coefficient;

D-particle diameter;

C- Resistance coefficient.

According to the combustion model of RDF, the relationship between the particle size diameter and time D = D(t) is known. The velocity equation u = u(t) of RDF particles in the decomposition furnace can be obtained by integrating the above equations, and the motion trajectory equation S = S(t) of RDF particles in the decomposition furnace can be obtained by integrating the velocity equation. S(t) is given:

The size of RDF particles initially fed into the decomposer is large, and the gravity is greater than the gas resistance at this time, and the particles accelerate downward and move in the opposite direction of the gas flow. In the process of falling, due to the continuous cracking, gasification and combustion of the particles themselves, the particle size becomes smaller, and the gas resistance per unit mass of the particles increases continuously. When the gas resistance per unit mass of the particles begins to be greater than the residual gravity, the particles begin to decelerate and fall. When the particles further shrink to a certain size, the particles finally move in the same direction with the airflow.

Due to the large particle size of RDF particles and the cylinder wind speed of the decomposing furnace is generally about 8~10 m/s, some large particles of RDF directly short-circuit into the kiln without full combustion in the decomposing furnace, resulting in reducing atmosphere and crust in the kiln. In order to ensure the full combustion of RDF particles, a multi-layer jet shrinkage is designed in the height direction of the decomposition furnace. Through the impact of the high-speed jet of air flow, the coarse RDF particles are continuously churned and mixed in the decomposition furnace, thereby prolonging the residence time of the particles in the furnace, increasing the mixing effect of gas-solid two-phase, and ensuring their complete combustion.

Under the dual action of the initial velocity vector and the airflow in the decomposition furnace, the RDF particles do not move up and down along the vertical plane in the decomposition furnace. In order to study the variation H of the RDF particles on the height of the decomposition furnace, the component of the trajectory equation on the vertical Z axis is solved. H is given:

For RDF particle groups composed of multi-size particle sizes, the equation of motion in height can be written as H = SZ(ρ,D,t), that is, the range of motion height of the particle groups in the decomposition furnace is also related to the density and size of each particle. Therefore, according to the coupling combustion between pulverized coal and RDF of different particle sizes, as well as the longitudinal distribution of oxygen concentration in the decomposition furnace, the multi-point partition feeding of pulverized coal, heterogeneous RDF and raw material is proposed to realize the efficient gradient combustion of pulverized coal and heterogeneous RDF.

Due to the difference in combustion characteristics of RDF after sorting, the RDF decomposer with large substitution ratio adopts a new design concept:(1) triple air double rotary cutting enters the vortex chamber;(2) Three-jet design;(3) Multi-point partition feeding of heterogeneous RDF, raw meal and pulverized coal;(4) Gasification and denitrification of RDF and pulverized coal at the bottom, see Figure 3 for the specific structure.

The two tertiary winds enter the vortex chamber at the bottom of the decomposing furnace in the same rotation direction, which can enhance the rotational momentum of the tertiary winds, and the vortex airflow formed strengthens the mixing effect of the gas-solid two-phase in the radial direction of the decomposing furnace, and improves the effective furnace capacity utilization rate of the decomposing furnace. In the initial stage when RDF is fed into the vortex chamber, the agglomerated RDF enters the tertiary wind under the action of gravity, and is then scattered and accelerated by the tertiary wind rotating at high temperature and high speed. Part of RDF preheats, gasifies and burns with the rotation of the tertiary wind, and decreases in size. Finally, the tertiary wind enters the middle of the decomposing furnace and continues to burn. The other part of RDF is caused by centrifugal force, in the process of rotation, it collides with the inner wall of the vortex chamber, and along with the raw meal powder that is cut to the wall, it slides down to the lower constriction of the decomposition furnace. Under the action of the high-speed kiln gas in the lower constriction, this part of RDF and raw meal enter the swirl flow field of the vortex chamber again to continue to burn.

Fig.3 Structure diagram of large substitution ratio RDF decomposing furnace

The raw meal entering the calciner is divided into three layers, from low to high, which are the top of the vortex chamber, the bottom and the middle of the main furnace body of the calciner. The axial temperature field of the decomposition furnace is controlled by gradient feeding of raw material, a controllable high temperature zone is formed in the vortex chamber, and the RDF, the combustion rate of pulverized coal and the decomposition rate of raw material in the vortex chamber are controlled to improve the reduction efficiency of NOx, which can ensure the formation of a uniform and stable temperature field in the main furnace of the decomposition furnace.

Using CFD numerical simulation technology to analyze the flow field in the large substitution ratio RDF decomposition furnace, light RDF, small size heavy RDF, large size heavy RDF show different coupling combustion methods.

Combustion of 4.1 lightweight RDF

The upper part of the vortex chamber and the bottom of the decomposing furnace are coupled combustion decomposition zones of light and high calorific value RDF, combustion air, pulverized coal and raw meal. Part of the pulverized coal and light high calorific value RDF are fed to the top of the vortex chamber, and this part of the pulverized coal and RDF enter the vortex chamber and rotate at high speed with the tertiary wind and start burning. Light RDF reduces the temperature in the upper part of the vortex chamber due to its high moisture content, large particle size, long drying and preheating time, and large heat absorption before ignition; while pulverized coal can quickly precipitate volatile components in high-temperature three winds to start combustion, which can increase the temperature in this area, and finally realize the formation of enhanced coupling combustion between pulverized coal and light RDF. At the same time, a part of C4 raw meal is introduced at the entrance of the tertiary air, and the raw meal rotates at a high speed along the tertiary air to form a curtain on the inner wall of the vortex chamber to protect the refractory material of the vortex chamber. The vortex chamber radially forms a thick-thin-thick three-phase zone from the outside to the inside. The outer wall is the dense-phase zone of raw meal, and the carbonate absorbs heat and decomposes. The middle is the dilute-phase high-oxygen combustion zone of tertiary air, pulverized coal and light RDF, and the fuel combustion releases heat. The inner part is the dense-phase zone of highly mixed kiln gas, tertiary air, raw meal, pulverized coal and light RDF and high dust. After spouting at the outlet of the vortex chamber, it enters the bottom of the decomposing furnace to continue to complete the combustion of pulverized coal and light RDF. The motion trajectories of light RDF, pulverized coal, and swirl chamber raw meal are shown in FIGS. 4, 5, and 6.

Figure 4 Lightweight RDF trace

Fig. 5 Trajectory of pulverized coal

Combustion of 4.2 small size heavy RDF

Heavy high calorific value RDF has high density, compact structure, large settling velocity in the decomposition furnace and slow combustion velocity. After the heavy high calorific value RDF is fed into the decomposition furnace, it gradually accelerates downward movement because the gravity is much greater than the resistance and buoyancy of the airflow. In the process of downward movement, RDF particles are continuously gasified and burned, the particle size becomes smaller, the speed increases, and the acceleration decreases. As the RDF particle size continues to decrease, the resistance continues to increase, and the force balance is broken at a certain moment, and the motion begins to decelerate gradually. When the settling velocity of RDF particles is less than the velocity of rising airflow in the decomposition furnace, the airflow resistance at this time is greater than the effective gravity, and the particles have the acceleration in the same direction as the airflow movement direction, and finally move in the same direction with the airflow until they burn out. Part of the heavy and high calorific value RDF is large in size, and after reaching the central constriction of the decomposer, it is carried by the high-speed jet airflow and enters the central combustion zone of the decomposer for the second time, entering the next combustion sedimentation cycle. The motion trajectories of the small size heavy RDF and the raw meal of the main furnace body are shown in Figure 7 and Figure 8.

Fig. 6 Raw material trajectory of vortex chamber

Fig.7 Small size heavy RDF trajectory

Combustion of 4.3 large-size heavy RDF

The settlement height of large-size heavy RDF is large, and the combustion process in the decomposing furnace is similar to that of small-size heavy RDF, but the feeding position is higher, and the upper necking is set to perform multiple sprays to ensure the burnout of large-size heavy RDF in the decomposing furnace. Its movement trajectory is shown in Figure 9.

Fig. 8 Raw Material Track of Main Furnace Body

Fig.9 Large size heavy RDF trajectory

5.1 performance evaluation

From the production experience and cold simulation analysis, the large substitution is more uniform than the gas-solid mixing of the three-jet and double-swirl suspension flow field in the decomposition furnace, the residence time of solid and gas in the furnace is as high as 6.3 than Kt, which is much larger than that of ordinary decomposition furnace, and the CO concentration at the outlet of preheater C1 is less than 0.05, which has better adaptability to the efficient combustion of alternative fuels, as shown in Table 3.

5.2 application practice

The kiln tail of the cement clinker production line in Huaxin Huangshi Factory adopts a large substitution ratio decomposing furnace. After calibration by a third-party authority, under the condition that the clinker production capacity reaches 12 000 t/d, the actual use of RDF reaches 2 094 t/d, the substitution rate of the decomposing furnace is 80%, the heat substitution rate of the whole kiln system is 38.9, and the comprehensive energy consumption of clinker is 67 kgce/t, the carbon emission per unit clinker is 683.2 kg/t, and the NOx emission is reduced to 249 mg/Nm3 (without SNCR and SCR).

(1) Compared with coal, RDF as an alternative fuel for cement kilns has excellent characteristics such as high volatile content, low ignition temperature and stable combustion, but due to its large particle size and small pore surface area, it will lead to long combustion duration and difficult to completely burn out.

(2) Based on the coupling model of physical field and chemical field in the decomposition furnace, the large substitution ratio decomposition furnace adopts multi-point feeding and gradient combustion system to ensure the efficient combustion of RDF in the furnace.

(3) The large substitution ratio decomposition furnace has excellent adaptability to multiple heterogeneous alternative fuels, which can greatly increase the use of alternative fuels in cement kilns and reduce energy consumption and carbon emissions per unit of clinker.