The International Symposium on Coal Combustion (ISCC-9) was held at the Qingdao City, China in July, 21–24, 2019. It attracted over 270 engineers and scientists from 16 countries including China, USA, Germany, Japan, Russia, Australia, South Korea, India, South Africa, Turkey, Pakistan, Italy, Colombia, Vietnam, Portugal, and Switzerland. The symposium has strongly reinforced the collaboration and discussion between academic researchers and industry experts, which emphasizes the main theme of the ISCC of establishing an effective bridge between academia and industry. The first ISCC was inaugurated at Tsinghua University, Beijing, China in 1987 under the consistent support of many distinguished scientists/engineers including Prof. Junkai Feng, Prof. Xuchang Xu, Prof. Douglas Smoot, Dr. Ernest L. Daman, Prof. Adel Sarofim, Prof. Klaus Hein, Prof. Ulrich Renz, Prof. Lothar Reh, Prof. Masayosh Sadakata, Prof. Keiji Makino, and Prof. Jianxiong Mao, etc. Since then, the ISCC has been proudly hosted by Tsinghua University for over three decades, once every four years, as a non-profit platform to promote research and development in the field of science and technology of solid fuel combustion, gasification, and the controls of pollutants and CO2 emissions.
The main goal of this special column is to present a set of most recent studies with original scientific contributions to clean and highly-efficient utilization of coal. In this special column, 12 of 175 papers that are more coherent to our theme are carefully selected by two-round reviewing. These remarkable accomplishments are surely to benefit coal utilization and stimulate new ideas worldwide.
Key contributions to this special column:
Cai et al. studied the combustion characteristic of blended coal including lignite and bituminous coal. The different ratios and air-staged combustion experiments are conducted in a drop tube furnace. The combustion characteristic, reductive gases CO and H2, and pollutants of NOx and H2S are studied. Through the air-staged combustion experiments which conducted in an electric heated down-fired furnace (DFF), the relationship between NOx and H2S reduction with CO generation in the fuel-rich zone is identified. Based on this observation, a modeling strategy is proposed: instead of CHi which is difficult to calculate, the CO concentration is used to quantify NO and H2S homogeneous reduction.
Zhang et al. studied the Lu’an bituminous chars and ashes prepared by pyrolysis and combustion processes under N2 and air atmospheres respectively. Scanning electron microscope (SEM) indicates that the number density and physical dimension of the ash spheres exuded from the char particles both gradually increase as the pyrolysis temperature rise, thus the coalescence of ash spheres can be obviously observed above 1100°C. When the pyrolysis temperature increases to 1300°C, some flocculent materials appear on the char particle surface. It can be speculated that b-Si3N4 is generated during the pyrolysis process in N2 with the help of energy dispersive spectrometer.
Liu et al. focused on a circulating fluidized bed (CFB) combustor burning Zhundong (ZD) lignite; the effect of oil shale semi-coke (SC) addition on the ash deposit mineralogy and morphology was studied in a laboratory-scale CFB. 10% and 20% semi-coke of high aluminosilicate was respectively added to ZD lignite and the blend was then combusted in the CFB furnace at 950°C. The mineralogy, morphology, chemistry and size distributions of the deposited ash on both windward and leeward of the probes at the flue path was systematically characterized. When SC is added into ZD, the agglomerates between ash particles decreases, and more coarse-grained discrete particles appear, indicating a decrease in deposit viscosity and the fouling propensity.
Yang et al. conducted the study on ignition of single coal particles under low oxygen concentrations (XO2<21%) using a tube furnace. The surface temperature (Ts) and center temperature (Tc) of the coal particles were obtained from the images taken by an infrared camera and thermocouples respectively. The results indicate the responses of Ts and Tc to the variation of XO2 is different: Ts decreases while Tc keeps nearly constant with increasing XO2 at a low level. In addition, the ignition delay time is less sensitive to the furnace temperature while Ti is more sensitive to Tw at a low XO2 than in air. At a low XO2, buoyancy plays a more significant role in sweeping away the released volatiles during the ignition process.
Alam et al. reviewed the ash content, composition and melting characteristic of ashes in differently ranked coal and biomass, and the slag formation mechanism, characteristics and structure were discussed. In addition, the available methodologies and formulae for slag viscosity measurement/prediction are reviewed and the current limitations and potential applications are summarized. Discussions on the slagging behaviour of the different ranks of coal and biomass by reviewing the viscosity measurement methods, viscosity prediction models and factors that affect the slag viscosity are carried out. This review also show that the existing viscosity models and slagging indices can only predict the viscosity and slagging propensity of high-rank coals satisfactorily.
Yan and Chen et al. studied the gas/particle flow characteristics in a novel swirling pulverized coal burner of a 29 MW pulverized-coal industrial boiler. The results indicate that the mean axial velocity and the particle volume flux in the central region of the burner outlet are negative, which indicates that the central recirculation zone is formed. In the central recirculation zone, the absolute value of the mean axial velocity and the particle volume flux increase when the external secondary air volume increases. However, the size of the central reflux zone is basically unchanged when varying the air volume ratio.
Yan and Sun et al. studied the effects of blend ratio on combustion and pollution emission characteristics for co-combustion of Shenmu pyrolyzed semi-char (SC) and Shenhua bituminous coal (SB) in a 0.35 MW pilot-scale pulverized coal furnace. Gas temperature and concentrations of gaseous species (O2, CO, CO2, NOx and HCN) were measured in the primary combustion zone under different blend ratios. It is found that the stand-off distance of ignition changes monotonically from 132 to 384 mm with the increase of pyrolyzed semi-char blend ratio. Effects on the combustion characteristics may be neglected when blend ratio is less than 30%. Above 30% blend ratio, the increase of blend ratio postpones ignition in the primary stage and lowers burnout rate.
Liu et al. studied the Huangling bituminous coal with different sizes in a drop-tube furnace at 1273, 1473 and 1673 K under air atmosphere. The SEM and cross-section analysis were used to study the physical properties of the collected ash cenospheres and the effects of combustion parameters on the ash film fraction. The results show that the ash film fraction exceeds 0.85 at high temperature, which indicates the majority of ash is distributed at char burnout stage. The ash film fraction increases with increased temperature and carbon conversion ratio, while it decreases when the coal particles have a large size. The high fraction of ash film provides a reasonable explanation for the extinction event at the late burnout stage.
Yang et al. investigated the fuel combustion characteristic under O2/N2 and O2/CO2 atmospheres at fluidized bed combustion conditions, and a one-dimensional single particle combustion model was established. The simulation validation was conducted by comparing the model prediction and the experimental results in the literature to prove the model reliability. Due to the moisture evaporation in the fuel particle, a plateau appears on the particle center temperature curve during the initial period of the thermal conversion process. Different from the cases in the pulverized coal boiler, particle temperature evolutions under different atmospheres are almost the same, indicating that the strong heat and mass transfer control the combustion process in fluidized beds.
Jia et al. obtained the mercury emission by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and CFB boilers. The relationship was obtained between the mercury emission and fly ash adsorption characteristics. The results show that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury is enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor is proposed to characterize the impact of coal property on mercury emissions. The larger the coal factor, the lower the mercury emission to the atmosphere. It is also found that the mercury content of fly ash in CFB boiler is ten times higher than that in PC boiler.
Shen et al. used large eddy simulation (LES) to study the performance of a 660 MW ultra-supercritical boiler under BMCR, 75% THA-100 and 50% THA-100 conditions. The flow, heat transfer and combustion processes are systematically investigated. The predicted gas velocities agree well with the thermal calculation, and the temperature errors are less than 127 K. The simulation results show that the operation load has significant effects on the boiler performance. It is proved that LES can provide guidance for the design and operation of the advanced coal-fired boilers.
Li et al. investigates the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in a 350 MW supercritical down-fired boiler. The experimental results indicate that as the TDA increases from 0° to 15°, the O2 concentration decreases while the CO and NO concentrations increases in the near-burner region. The flame kernel is found to move downward, and the temperature in the hopper region increases simultaneously. The TDA change has little effect on the NOx emissions. In comprehensive consideration of the combustion characteristics and boiler economic performance, the optimal TDA of 15° is recommended.
In general, this special column contains a select 12 of 175 papers presented at 9th ISCC, focusing on the theme of clean and highly-efficient utilization of coal. It is expected that the research contribution from these papers can provide a strong demonstration of the responsibility and active efforts of the international scientific and industrial community to ensure a green and sustainable coal utilization.
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