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Oxy‐fuel combustion in coal‐fired plants has become one of the most promising technologies for carbon capture and storage (CCS). Because of the obvious difference between the oxy‐fuel combustion medium (O2/CO2) and air in the atmosphere (O2/N2), the characteristics of combustion, heat transfer, etc., under oxy‐fuel combustion have changed greatly from those under air combustion. In this paper, computational fluid dynamics (CFD) simulation was used to investigate pulverized coal combustion in a 200 MWe tangentially fired oxy‐fuel combustion boiler. Improved models for the gas radiative properties and chemical reaction mechanisms were incorporated into the CFD code. Both conventional air‐fired and oxy‐fuel combustion were operated. Different flue gas recycling patterns (dry recycling and wet recycling) were also investigated. The temperature distribution in the furnace, the temperature field, and the CO concentration field in each scheme were analyzed, this being relevant to the design of oxy‐fuel combustion boilers. It was found that combustion could form a good tangential circle and stable temperature field in the furnace either under air‐fired or oxy‐fule combustion conditions. The temperature under oxy‐fuel combustion was lower than with air combustion. The burnout rate under the air condition was lower than that with the oxy‐fuel combustion condition. With oxy‐fuel combustion, it is necessary to pay special attention to the slagging tendency of the primary combustion zone in the furnace. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

There has been little research on volatile releasing characteristics of pulverized coals under moderate or intense low-oxygen dilution (MILD) oxy-combustion (MO) conditions. For the first time, volatile releasing characteristics of bituminous coal and semi-anthracite under both MILD air-combustion (MA) and MO conditions were investigated using a flat-flame assisted entrained flow reactor. Both heating rate (~105 K/s) and residence time (65 ms) were carefully selected to mimic the conditions in typical industrial boilers. The combustion processes and properties of the volatiles were characterized through direct observation and char analysis. The results showed that the lower diffusion rate of the volatile in CO2 resulted in the decreasing of the volatile envelope flame size and a longer volatile burnout time (more than 20%). For bituminous coal (volatile content of 25%), the lower amount of apparent volatile yield under MO conditions reduced the heating value of the volatile. For semi-anthracite coal (volatile content of 7%), the short devolatilization time led to char-CO2 gasification reaction, which increased the apparent volatile yield and the heating value of the volatile by 47% and the volatile-N by 19%. This paper indeed provided new insight into the MILD oxy-combustion of solid fuels. [ABSTRACT FROM AUTHOR]

Pressurized oxy-fuel combustion technology is the second generation of oxy-fuel combustion technology and has low energy consumption and low cost. In this research, a visual pressurized flat-flame reaction system was designed. A particle-tracking image pyrometer (PTIP) system based on a high-speed camera and an SLR camera was proposed. Combining the experimental system and data-processing method developed, the ignition and combustion characteristics of a single coal particle between 69 and 133 μm in size were investigated. The results indicated that at atmospheric pressure, the ignition delay time of ShanXi (SX) anthracite coal was longer than that of ShenHua (SH) bituminous coal, while that of PRB sub-bituminous coal was the shortest. As the pressure rose, the ignition delay time of the PRB sub-bituminous coal and SX anthracite coal showed a continuous increasing trend, while the ignition delay time of SH bituminous coal showed a trend of first increasing and then decreasing. Moreover, pressure also affects the pyrolysis process of coal. As the pressure increases, it became more difficult to release the volatiles produced by coal pyrolysis, which reduced the release rate of volatiles during the ignition stage, and prolonged the release time and burning duration time of volatiles. [ABSTRACT FROM AUTHOR]

Coal-fired power plants are still the way of power generation in most countries, but the CO 2 generated by coal remains high. Pulverized coal co-firing with ammonia is expected to reduce greenhouse gas emissions, which has become a hot topic. This review summarizes the progress on pulverized coal co-firing with ammonia. The effects of ammonia co-firing on the formation and transformation mechanisms of NO x and emission characteristics, as well as the emissions of other pollutants, are analyzed and summarized. This review also focuses on applying the computational fluid dynamics (CFD) sub-models in pulverized coal co-firing with ammonia. CFD is vital in pulverized coal ammonia research because of its speed and low-cost advantages. The sub-models and mechanisms of turbulent flow, heat transfer, combustion, and pollutant generation are summarized to understand better the combustion process of CFD co-combustion of pulverized coal and ammonia. Finally, the influence of pulverized coal mixed with ammonia, the CFD sub-model application, and the areas to be improved are summarized and prospected. Pulverized coal co-firing with ammonia is a feasible method to reduce carbon. This review offers fundamental theoretical support and technical guidance for the subsequent pulverized coal co-firing with ammonia. • Coal-NH 3 co-firing effectively reduces the CO 2 produced by coal combustion. • Computational fluid dynamics plays a vital role in studies on coal-NH 3 co-firing. • Classic models of computational fluid dynamics in coal-NH 3 co-firing are reviewed. • Coal-NH 3 co-firing exhibits good application prospects and some challenges. [ABSTRACT FROM AUTHOR]

In pulverized coal combustion, the participating radiative media includes gas, soot, and coal/char particles; a fundamental understanding of their radiation behavior is essential for the prediction of radiation heat transfer and control of radiation in the furnace. In this study, the individual contributions of gas, soot, and coal particles to the total radiation were examined based on the large eddy simulation of the CRIEPI pulverized coal jet flame. The radiation transfer equation was solved using the discrete ordinate method with state-of-the-art non-gray radiative property models of multi-phase media. The full spectrum correlated k -distributions (FSCK) model was used to calculate the radiative property of the gas-soot mixture. A burnout-based particle radiative property model was applied for coal/char particles. The results showed that the predicted spatial distributions of coal particles, tar, and soot were in good agreement with the experimental data. The gray gas radiation model predicted temperatures approximately 50 K lower in this flame, and the peak soot volume fraction was predicted at approximately 22% lower. For the laboratory scale burner under examination, the individual radiative contributions of gas, soot, and coal/char were found to be 35%, 40%, and 25%, respectively, so that it was necessary to consider all components for accurate prediction. Moreover, these contribution ratios varied with the flame position. Because of the high particle concentration, the individual contribution of coal/char particles accounted for up to 65% in the devolatilization region. Soot radiation played an important role in the pulverized coal flame. In the region where the ensemble-averaged soot volume fraction reached the maximum of 2.7 ppm, the individual contribution of soot was 50% of the total radiation, which accounted for 70% of the total particles (soot and char) radiation. [ABSTRACT FROM AUTHOR]

MILD (Moderate or Intense Low-oxygen Dilution) combustion is a novel technology to reduce NO X emission from combustion. With the development of this technology, it is expected to combine this technology with oxy-fuel combustion to jointly control the pollutants from fossil fuel combustion. The present paper investigates the combustion characteristics of coal under oxy-MILD operation with the aid of CFD simulations. A seven-step global reaction mechanism is used for modeling coal combustion involving gaseous volumetric reaction and particle surface reaction. The predicted results using the adopted models for the reference case agree well with the experimental results. It is revealed that, the combustion temperature level increases with the enhancement of initial oxygen concentration because of the reduced injection momentum and promoted heat release. The in-furnace CO formation increases but the final CO emission reduces when enhancing the oxygen concentration under oxy-MILD combustion. Moreover, oxy-MILD combustion shows a greater potential on NO X formation reduction than air-MILD combustion, and this potential is promoted at higher initial oxygen concentration. [ABSTRACT FROM AUTHOR]

Co-firing residual char and pulverized coal can apply the residual char in pulverized coal combustion equipment, whose difficulties lie in poor ignition performance and low burnout efficiency. For the first time, residual char and bituminous coal fuel blend is experimentally co-fired under moderate or intense low-oxygen dilution (MILD) conditions. MILD co-combustion is achieved with low-temperature preheating air in a pilot-scale furnace. The co-firing experiments include swirl low-NO flame combustion, MILD combustion through symmetrical secondary air jets, and MILD combustion through an asymmetrical secondary air jet. The pilot-scale tests reveal that MILD co-combustion through the asymmetrical single nozzle achieves the lowest peak temperature and the most uniform distributions of temperature, heat flux, and gas compositions in the furnace. The NO emission of MILD co-combustion through the asymmetrical air jet is decreased by 57% compared with swirl low-NO flame co-combustion. The burnout rate of MILD co-combustion through the asymmetrical air jet reaches 94%. Considering more uniform in-furnace distribution, lower NO emission, and higher burnout rate, MILD combustion through the asymmetrical single nozzle shows more obvious advantages than that through symmetrical air nozzles. Overall, the present study further widens the fuel flexibility of MILD combustion even to residual char fuel blend. • Co-firing residual char under MILD combustion is realized for the first time. • MILD co-combustion of residual char can reduce fuel-NO emission by at least 54%. • Fuel flexibility of MILD combustion is even expanded to residual char fuel blend. [ABSTRACT FROM AUTHOR]

In this study, a particle-tracking image pyrometer (PTIP) system, based on a high-resolution high-speed camera and a digital single lens reflex (SLR) camera, was proposed to investigate the ignition characteristics of pulverized coal particles. The goal of the PTIP system was to obtain reliable simultaneous measurements of the velocity, diameter, and temperature of individual burning particles. A modified particle-tracking velocimetry (PTV) algorithm was developed to collect the image sequences of individual particles, and it could weed out defocused particles and other odd particles by low-frequency filtration. Moreover, a template matching method was adopted to distinguish particles undergoing different combustion states (homogeneous volatile combustion or heterogeneous coal/char combustion). The experimental validation was carried out under an O 2 /CO 2 atmosphere in an optically accessible flat flame-supported entrained flow reactor. Particles of Shenhua (SH) bituminous coal, in the size range of 70–133 μm, were combusted in the flue gas with gas temperature of 1400 K and O 2 concentration of 20%. The diameter-temperature patterns of particles along their trajectories were discussed. Several criteria based on statistical analysis were adopted to divide the combustion process and obtain the ignition delay time (IDT) and devolatilization duration time (DDT). • Speed, diameter and temperature of a burning particle are measured simultaneously. • Modified PTV method is used to capture the combustion sequence of single particle. • Different stages of particle combustion are distinguished by particle templates. • Statistical analysis is adopted to obtain characteristic time for combustion. • Combination of micro and macro photography improves the credibility of diagnosis. [ABSTRACT FROM AUTHOR]

Although fuel-NO formation is significant for the combustion of pulverized coal, biomass, and heavy distillate fuels, its mechanism has not been systematically validated under air and oxy-fuel combustion. This is the first study that quantitatively evaluates detailed fuel-NO mechanisms against large experimental datasets from a jet stirred reactor (JSR) under both air and oxy-fuel combustion. The mechanism with the best overall performance is thoroughly reduced and validated. A skeletal fuel-NO mechanism including only 35 species is obtained using methods of directed relation graph with error propagation (DRGEP), DRGEP-aided sensitivity analysis, and computational singular perturbation. The skeletal mechanism is comprehensively validated and agrees well with the detailed mechanism and experiments predicting the ignition delay, temperatures, and concentrations of important species in JSRs, laminar flame speeds, and flame extinction under both air and oxy-fuel combustion. Moreover, the skeletal mechanism is successfully applied to the finite-rate modeling of moderate or intense low-oxygen dilution (MILD) combustion of pulverized coal. Unlike conventional semi-empirical post-processing modeling of fuel-NO production, the fuel-NO calculation in this study is coupled with coal combustion by using the well-validated skeletal mechanism. A highly precise in-furnace distribution of fuel-NO critical precursors (i.e., NH 3 and HCN) and detailed fuel-NO reaction pathway under the MILD combustion are obtained. • Detailed mechanisms for fuel-NO are evaluated for the first time. • A 35-species skeletal fuel-NO mechanism is obtained. • Skeletal mechanism agrees well with detailed mechanism and experiments. • Mechanism is used in finite-rate modeling of coal combustion for NO formation. • Skeletal mechanism is suitable for fuel-NO under air and oxy-fuel conditions. [ABSTRACT FROM AUTHOR]

• CFD results of a swirl oxy-coal flame are compared with detailed experiment. • Nonlinear variation particle radiative property is first used in oxy-coal modeling. • Contributions of gas and particle radiation are illustrated in oxy-coal flame. • Constant model ten times over-predicts the particle absorption after burnout. This paper reports a comprehensive computational fluid dynamics simulation of the IFRF's swirling oxy-fuel flame with pulverized coal using wet flue gas recycle. Special attention is given to the influence of particle radiative property model. Based on the burnout rate of pulverized coal, a recent proposed nonlinear conversion-dependent particle radiative property model (Guo et al., 2018) is used in the simulations. The influences of the high CO 2 concentration on the sub-models are also considered in detail. The gas-phase global reaction mechanism, kinetic parameters of char oxidation and gasification, and gas radiative property model are modified for oxy-fuel condition. The results show the prediction results agree well with the measurements, in terms of the temperature and composition of the flue gas. The particle radiation has an important effect on the flame temperature. Even in the small furnace studied here (optical length is approximately 1.6 m), the radiation in the flame zone is still dominated by particle radiation. Linear variation assumption (Yin et al., 2015) under-predicts the particle absorption in combustion zone. Ignoring the effect of burnout rate on particle radiative property will 10 times over-predict the particle absorption after burnout. [ABSTRACT FROM AUTHOR]