Based on the theory of computational fluid dynamics (CFD), pulverized coal combustion alone, and the co-combustion of pulverized coal and refuse-derived fuel (RDF) in a Trinal-sprayed calciner (TTF) precalciner were simulated. The results revealed that when coal was used as a single fuel, the velocity field in the precalciner had good symmetry, and formed three spray effects and multiple recirculation zones. The main combustion zone was distributed in the lower tertiary air and pulverized coal area, and the highest temperature reached up to 1,500 K. According to the simulation results, the predicted decomposing rate of raw meal was 90.12%, which is in good agreement with the actual measured result. In addition, with the increase in RDF content, the average temperature of the furnace, the decomposition rate of the raw meal, and the NOx concentration all exhibited a downward trend. Under the condition of ensuring the normal operation of the precalciner, blending with 20% RDF is the most reasonable strategy, and the NOx emissions decreased by approximately 16%.
Cement is an indispensable raw material for national infrastructure construction. At the same time, the resource consumption and pollutant emissions of the cement industry are huge. Studies have shown that coal-based fuel combustion pollution from the cement industry is the third largest source of NOx emissions, and coal storage has become lesser [
As the core equipment of the new dry process cement production line, the precalciner is the main place for fuel combustion and raw meal decomposition, and this is also accompanied by a large amount of NOx generation [
Mesh number | 662346 | 884680 | 1013158 | 1262747 | 1421282 |
---|---|---|---|---|---|
Temperature (K) | 1,028 | 1,050 | 1,064 | 1,066 | 1,063 |
The boundary conditions used for the present study all come from the thermal calibration of the cement plant. The inlet boundaries of the tertiary air and flue gas inlet were the velocity inlets. The upper tertiary air speed was 25 m/s at a temperature of 1,200 K, and the lower tertiary air speed was 26 m/s at a temperature of 1,200 K. The flue gas velocity was 27 m/s, and the temperature was 1,350 K. The outlet boundary condition was the pressure outlet, and the pressure value was −1250 Pa. The coal was bituminous from Jiangxi Province [
Sample | Proximate analysis (%) | |||
---|---|---|---|---|
Mad | Aad | Vad | FCad | |
Coal | 1.87 | 14.73 | 28.48 | 54.92 |
RDF | 18.24 | 36.48 | 39.12 | 6.16 |
Note: M means moisture, A means ash, V means volatile matter, FC means fixed carbon and ad means air-dry basis.
Sample | Mole fractions (%) | Lower heating value (MJ/kg) | ||||
---|---|---|---|---|---|---|
Cdaf | Hdaf | Odaf | Ndaf | Sdaf | ||
Coal | 78.01 | 6.98 | 12.53 | 1.72 | 0.76 | 24.91 |
RDF | 55.65 | 7.54 | 34.92 | 0.53 | 1.36 | 9.42 |
Note: daf means dry-ash-free basis.
Considering the characteristics of the geometric model, calculation time and accuracy, the standard
In the standard
where:
The discrete phase model (DPM) is used to solve the trajectory of the particle phase (pulverized coal, RDF and raw meal) in the precalciner [
where:
In comparing the data of pulverized coal and RDF in
where:
After the volatiles were released and burned out, the residual coke would begin to undergo a surface combustion reaction, and the kinetic/diffusion control rate model will be selected. The model assumes that the combustion rate of coke is simultaneously controlled by the kinetic rate
where:
In the research on the decomposition reaction of raw meal in the precalciner, domestic scholars all took the decomposition of CaCO3 as the main research object [
where:
where: the pre-exponential factor of the reaction is
In this equation,
According to the formation mechanism of NOx, NOx is mainly divided into prompt NOx, thermal NOx and fuel NOx. Under the combustion conditions in the precalciner, the proportion of prompt NOx and thermal NOx is less than 5%, which is negligible. The present study only considered the formation of fuel NOx, and the reaction path is shown in
According to the above-mentioned NO reaction mechanism, the generation rate for HCN and NO were respectively used as the source terms to simulate the formation of NO in the precalciner.
The focus of the present research was the TTF precalciner in Yunnan Province, China. The research consisted of two main parts. In the first part, the investigators predicted and analyzed the velocity field, temperature field, composition field, and distribution of NOx in the furnace under separate combustions of pulverized coal, in order to reveal the transport of the gas–solid two-phase flow in the new TTF precalciner. In the second part, under the premise of ensuring that the total heat entering the furnace remains unchanged, the present study simulated five blending situations. Among these, the heat provided by RDF accounted for 10%, 20%, 30%, 40% and 50% of the total heat, respectively. The final ideal blending ratio was used to minimize the NOx concentration at the outlet when the combustion characteristics and decomposition rate of the raw meal were less affected, that is, the precalciner operates normally.
The prerequisite for solving the control equations is to discretize these. In the present study, the controlling-volume method was used to discretize the control equations for the fluid phase, and a second-order upwind difference scheme is used to obtain the difference equations. The semi-implicit method for pressure-linked equations (SIMPLE) algorithm was used to couple the pressure and velocity of the discrete equations. The convergence criterion was that the energy term is less than 10−6, and the remaining residuals were less than 10−3. During the data processing, the burnout rate of fuel was defined as the ratio of the mass loss of unburned coke at the outlet with input fuel mass.
The velocity vector diagrams of the X = 0 and Y = 0 sections in the precalciner are shown in
The temperature contours of the X = 0 and Y = 0 sections in the precalciner are shown in
According to the online measurement, the NOx concentration at the flue gas inlet was approximately 800 ppm.
The comparison between the predicted value and measured value at the outlet of the precalciner is shown in
Temperature | CO2 mass fraction | O2 mass fraction | NOX | |
---|---|---|---|---|
Predicted value | 1064 K | 24.9% | 1.65% | 585 ppm |
Measured value | 1112 K | 25.5% | 1.61% | 560 ppm |
Error | 4.3% | 2.4% | 2.48% | 4.4% |
The burnout rate reflects the degree of mixed fuel combustion in the precalciner. As shown in
The analysis indicates that based on the constant total heat that enters the furnace, when the RDF blending ratio was not less than 30%, the pollutant emissions can be significantly reduced. However, this has a greater impact on the combustion characteristics and decomposition of the raw meal. Therefore, under the condition of ensuring the normal operation of the precalciner, blending with 20% RDF is feasible, and the NOx emission is reduced by approximately 16%, when compared with the use of coal as the only fuel.
In the present study, the numerical simulation of an actual operating TTF precalciner was carried out, and the simulated results were compared with the measured data, in order to prove the reliability of the numerical method. The results indicated that the main combustion zone of the precalciner is distributed in the lower tertiary air and pulverized coal area, and that the maximum temperature can reach up to 1,500 K. The mass fraction of CaCO3 alternately increases and decreases, and the mass fraction of CaO continues to increase. The NOx concentration initially decreases, subsequently increases, and decreases and increases again. Furthermore, the present study also analyzed the mixed combustion of pulverized coal and RDF. With the increase in RDF content, the average temperature of the furnace, the decomposition rate of the raw meal, and the NOx concentration exhibits a downward trend. Considering all these factors, it is feasible and reasonable to mix pulverized coal and RDF at an 8:2 ratio. Furthermore, compared with coal as a single fuel, the NOx emissions at the outlet were reduced by approximately 16%.