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"Precalciners"
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Carbon and air pollutant emissions from China's cement industry 1990–2015: trends, evolution of technologies, and drivers
China is the largest cement producer and consumer in the world. Cement manufacturing is highly energy-intensive and is one of the major contributors to carbon dioxide (CO2) and air pollutant emissions, which threatens climate mitigation and air quality improvement. In this study, we investigated the decadal changes in carbon dioxide and air pollutant emissions for the period of 1990–2015 based on intensive unit-based information on activity rates, production capacity, operation status, and control technologies which improved the accuracy of the cement emissions in China. We found that, from 1990 to 2015, accompanied by a 10.3-fold increase in cement production, CO2, SO2, and NOx emissions from China's cement industry increased by 627 %, 56 %, and 659 %, whereas CO, PM2.5, and PM10 emissions decreased by 9 %, 63 %, and 59 %, respectively. In the 1990s, driven by the rapid growth of cement production, CO2 and air pollutant emissions increased constantly. Then, the technological innovation in production of replacing traditional shaft kilns with the new precalciner kilns equipped with high-efficiency control facilities in the 2000s markedly reduced SO2, CO, and PM emissions in the cement industry. In 2010, nationwide, 39 % and 31 % of the nationwide PM2.5 and NOx emission were produced by 3 % and 15 % of the total capacity of the production lines, indicating the disproportionately high emissions from a small number of the super-polluting units. Since 2010, the growing trend of emissions has been further curbed by a combination of measures, including promoting large-scale precalciner production lines and phasing out small ones, upgrading emission standards, installing low NOx burners (LNB), and selective non-catalytic reduction (SNCR) to reduce NOx emissions, as well as adopting more advanced particulate matter control technologies. Our study highlights the effectiveness of advanced technologies on air pollutant emission control; however, CO2 emissions from China's cement industry kept growing throughout the period, posing challenges to future carbon emission mitigation in China.
Journal Article
Numerical Modeling of Hydrogen-Blended Pulverized Coal Combustion in RSP-Type Cement Calciner
To address the issue of high carbon dioxide (CO 2 ) and nitrogen oxides (NOx) emissions from cement precalciner kilns, this study employs CFD numerical simulation to investigate the co-combustion of pulverized coal and hydrogen in an RSP-type cement precalciner of a production line with a raw meal processing capacity of 3800 tons per day (t/d). The combustion behavior of the fuel (pulverized coal and hydrogen), the decomposition of raw meal, and the generation of CO 2 and NOx within the precalciner were analyzed under different Thermal Substitution Ratios (TSR) of hydrogen and varying numbers of hydrogen injection pipes. The research demonstrates that: As the TSR increases, the decomposition rate of CaCO 3 slightly decreases, while the precalciner outlet temperature experiences a minor increase. Concurrently, the concentrations of CO 2 and NOx inside the precalciner exhibit a decreasing trend. Consequently, the CO 2 mass flow rate and NOx concentration at the precalciner outlet progressively decrease. Under the condition of TSR = 50%, the number of hydrogen injection pipes has no significant effect on the CaCO 3 decomposition rate, precalciner outlet temperature, outlet CO 2 mass flow rate, or outlet NOx concentration.
Journal Article
Optimization of Fuel In-Situ Reduction (FISR) Denitrification Technology for Cement Kiln using CFD Method
Nitrogen oxides (NO
x
) from cement industry have drawn more and more attention and the existing denitrification technologies can hardly meet the increasingly stringent emission requirements in China. In our previous work, fuel in-situ reduction (FISR) method was proposed to cut cement NO
x
emission. With the pilot-scale precalciner in the previous experiment as objection, optimization of FISR method was conducted using CFD method. The results demonstrated that NO
x
emission decreased by 69.86% after adopting FISR method. The effects of initial concentrations of NO and O2 in kiln gas, feeding location of the first-stage tertiary (tertiary air-I) and cement raw meal (CRM) were further investigated. With increasing initial NO concentration, NO
x
emission increased linearly, while the reduction rate of NO in kiln gas maintained above 80%. When O
2
content in kiln gas is more than 4%, oxygen would more significantly promote the formation of NO
x
and inhibit the reduction of NO. The dimensionless locations of tertiary air-I and CRM were introduced. The simulation results showed that the optimal dimensionless locations are 0.6 and 1.6 for tertiary air-I and CRM, respectively. The outputs achieved in this study will provide a strong support for the practical application of FISR method in cement industry.
Journal Article
Research on the Prediction of Cement Precalciner Outlet Temperature Based on a TCN-BiLSTM Hybrid Neural Network
As the global cement industry moves toward energy efficiency and intelligent manufacturing, refined control of key processes like precalciner outlet temperature is critical for improving energy use and product quality. The precalciner’s outlet temperature directly affects clinker calcination quality and heat consumption, so developing a high-accuracy prediction model is essential to shift from empirical to intelligent control. This study proposes a TCN-BiLSTM hybrid neural network model for the accurate prediction and regulation of the outlet temperature of the decomposition furnace. Based on actual operational data from a cement plant in Guangxi, the Spearman correlation coefficient method is employed to select feature variables significantly correlated with the outlet temperature, including kiln rotation speed, high-temperature fan speed, temperature A at the middle-lower part of the decomposition furnace, temperature B of the discharge from the five-stage cyclone, exhaust fan speed, and tertiary air temperature of the decomposition furnace. This method effectively reduces feature dimensionality while enhancing the prediction accuracy of the model. All selected feature variables are normalized and used as input data for the model. Finally, comparative experiments with RNN, LSTM, BiLSTM, TCN, and TCN-LSTM models are performed. The experimental results indicate that the TCN-BiLSTM model achieves the best performance across major evaluation metrics, with a Mean Relative Error (MRE) as low as 0.91%, representing an average reduction of over 1.1% compared to other benchmark models, thereby demonstrating the highest prediction accuracy and robustness. This approach provides high-quality predictive inputs for constructing intelligent control systems, thereby facilitating the advancement of cement production toward intelligent, green, and high-efficiency development.
Journal Article
Coal-Biomass Preheating Combustion Characteristics in Cement Precalciner. Part 1: Preheating of Coal/Biomass
The co-combustion of biomass and coal can positively impact the environment and reduce the cost of power generation. However, biomass fuels have many limitations. Circulating fluidized bed (CFB) preheating combustion is suitable for co-combusting coal and biomass because of better fuel adaptability. In the cement industry, fuel combustion and raw meal decomposition in precalciners affect cement quality and cause pollutant emissions. The preheating combustion method used in precalciners can improve combustion performance and reduce NO
x
emissions. This study investigated the preheating characteristics of a coal-biomass mixed fuel in a cement precalciner. The effects of load, biomass type, and biomass proportion on the preheated fuel and the conditions of the CFB were investigated. The results indicated that a lower load reduces the combustible components in gaseous and solid preheated fuels. However, due to the gas volume remains constant under different loads, a lower load also increases temperature and intensifies the reaction. The carbon chain and microscopic structural activities of preheated fuels are considerably enhanced, facilitating their combustion in precalciners and reducing nitrogen oxides in rotary kilns. Furthermore, adding biomass can improve the reactivity of a fuel subjected to preheating. Thus, biomass fuels (e.g., rice husks) exhibit high combustion efficiency, and thus high energy utilization. The present study achieved better pore structure and molecular activity using preheated fuel from a CFB preheater. In addition, the improvement of pore structure and molecular activity increases with the proportion of the biomass.
Journal Article
Modeling De-NOx by Injection Ammonia in High Temperature Zone of Cement Precalciner
The quantity of NOx emission from cement production is second only to thermal power generation and vehicle exhaust. In this paper, a phenomenon found by Taniguchi is used to achieve NOx reduction in the cement precalciner. Based on his results, it is proposed to reduce NOx that ammonia is injected in the high-temperature and lean-oxygen zone (HT-DeNOx) during pulverized coal combustion. For a large cement precalciner (3200 t/d), numerical simulation is used to evaluate the technology of HT-DeNOx combined with the traditional selective non-catalytic reduction (SNCR) method. The results indicate that NH3 and HCN in HT-DeNOx can reduce NO during the reaction process. With very little ammonia injection (normalized stoichiometric ratio NSR=0.1, the normalized stoichiometric ratio), the efficiency of NO reduction by HT-DeNOx is 27.72%. Combining SNCR (NSR=1.1) and HT-DeNOx (NSR=0.1), the reduction efficiency will be improved to 60.05%, compared with 50.83% efficiency when using only SNCR at NSR=1.2.
Journal Article
Effects of Co-Firing Biomass and Pulverized Coal on NO Reduction in Cement Precalciner
With increased awareness of the large-scale CO
2
emissions from the cement industry, there has been growing focus on greenhouse gas reduction strategies. Among all these strategies, fuel substitution using biomass fuel is extensively used to achieve CO
2
zero-emission in cement production. Due to the avoidable high-temperature-generated thermal nitrogen oxides during cement production, research on the impact of biomass application on nitrogen oxide emissions shall be carried out. Three types of biomass fuel and bituminous coal were used to investigate the NO reduction characteristics under different O
2
concentrations on experimental benches. It was found that the change in oxygen concentration from 9% to 1% increased the reaction time in the reactor from 555 s to 1425 s, which means the increase in oxygen concentration can lead to shorter reaction time, and correspondingly, the existing time of nitric oxide in the flue gas is also shortened, but the peak value of nitric oxide rises, during the process of O
2
concentration changing from 1% to 9%, the peak NO concentration in the flue gas increased from 5.4×10
−5
to 1.05×10
−4
. An increase in O
2
concentration greatly reduces the total reduction of NO and the minimum change in NO concentration. The peak NO concentration during the combustion process of corn stalk is 4.56 ×10
−4
, which is approximately 7 times higher than that of coal, and it is caused by the high amount of N in corn stalk. The addition of raw meal has an inhibitory effect on the reduction of NO: after adding raw meal, the effective reduction time of NO by fuel decreased by about 20%, but adding raw meal raises CO
2
concentration of fuel gas in the early stage of reaction.
Journal Article
Study on the influencing factors of removal of NOX from cement kiln flue gas by sewage sludge as a denitration agent
Experimental study on the influencing factors of using sewage sludge as a denitration agent for cement industry was carried out on a self-made laboratory-scale fluid-bed reactor. Results indicate that sludge combustion at 900 °C shows an ideal NO
X
(the sum of NO and NO
2
) removal activity under simulated working conditions of cement precalciner. The optimal removal efficiency of NO
X
can reach 70.36 ± 3.59% in the presence of cement raw meal (CRM) at a sludge particle size range of 0.18–0.25 mm and the sludge dosage of 0.75 g/min. Besides, the NO
X
removal efficiency increases to 76.94 ± 5.02% in the absence of CRM, indicating that cement raw meal inhibits the NO
X
removal. This phenomenon may be attributed to the fact that CRM has promotion effect on NH
3
produced and obvious inhibitory effect on CO produced; while NH
3
and CO play a leading role in NO
X
reduction, the combined effect leads to the decrease of NO
X
removal. Moreover, the relationship between the composition of CRM on the inhibition of NO
X
removal is MgO < CaCO
3
< CRM < Al
2
O
3
Journal Article
Numerical Simulation of Selective Non-Catalytic Reduction Denitrification Process in Precalciner and the Effect of Natural Gas Injection on Denitrification
Cement production is the third largest source of nitrogen oxides (NOx), an air pollutant that poses a serious threat to the natural environment and human health. Reducing NOx emissions from cement production has become an urgent issue. This paper aims to explore and investigate more efficient denitrification processes to be applied in NOx reduction from precalciner. In this study, firstly, the flow field, temperature field, and component fraction in the precalciner are studied and analyzed using numerical simulation methods. Based on this, the influence of the reductant injection height and amount on the SNCR was studied by simulating the selective non-catalytic reduction (SNCR) process in the precalciner. The effect of natural gas on the NOx emissions from the precalciner was also investigated. The simulation results showed that, with the increase in height, the NOx concentration in the precalciner decreased, then increased, then decreased, and then increased again. The final NOx concentration at the exit position was 531.33 ppm. In the SNCR denitrification process, the reductant should be injected in the area where the precalciner height is 26–30 m so that the reductant can fully react with NOx and avoid the increase of ammonia escape. The NSR represents the ratio of reductant to NOx, and the results show that the larger the NSR is, the higher the denitrification rate is. However, as the NSR approaches 2, the denitrification rate slows down and the ammonia escape starts to increase. Therefore, according to the simulation results, the NSR should be kept between 1 and 1.6. The denitrification rate reached the maximum value of 42.62% at the optimal condition of 26 m of reductant injection height and 1.6 of NSR. Co-firing of natural gas with pulverized coal can effectively reduce the NOx generation in the furnace. The denitrification rate reached the maximum value of 32.15% when the natural gas injection amount was 10%. The simulation results of natural gas co-combustion and SNCR combined denitrification showed that combined denitrification was better than natural gas co-combustion or SNCR denitrification. Under the condition of NSR of 1 and natural gas injection of 10%, the denitrification rate increased by 29.83% and 31.64% compared to SNCR-only or co-combustion-only denitrification, reaching 61.98%, respectively. Moreover, less reductant is used in co-denitrification, so the problem of excessive ammonia emissions can be avoided. The results of this study provide useful guidance for denitrification process development and NOx reduction in cement production.
Journal Article
Numerical Simulation and Optimization of Staged Combustion and NOx Release Characteristics in Precalciner
In order to study the combustion characteristics in a precalciner, the temperature and composition field in a typical Trinal-sprayed calciner were numerically analysed. The results obtained by simulation were compared to actual measurements and the simulated results were in good agreement with the measured ones. The results indicated that the aerodynamic flow field in the precalciner is satisfactory, and a symmetrical reflux occurs in the shrinkage zone of the precalciner because of air staging, which can increase the residence time of the solid particles. The temperature distribution in the furnace is uniform, and the average temperature is greater than 1200 K, which can satisfy the conditions for the pulverised coal combustion and raw material decomposition. The mass fraction distribution of oxygen, carbon monoxide, and carbon dioxide in the precalciner is closely related to the temperature distribution. The concentration of nitrogen oxides (NOx) exhibits a trend of increasing, decreasing and then increasing, and finally tending to a stable level. Within a certain velocity range, the average temperature in the precalciner and the decomposition efficiency of the raw material increase as the flue gas velocity increases. When the flue gas velocity is 24 m/s, the overall performance of the precalciner is optimal.
Journal Article