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A novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuel-lean nozzles from the arches to the front/rear walls and rearranging staged air as well as introducing separated overfire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13 to 4.30%.

Cofiring of coal slime in existing utility boilers is a promising approach to the disposal of coal slime. However, reports about cofiring coal slime with anthracite in the down-fired utility boiler are still few. This work presents a comprehensive study on the combustion characteristics, slagging potential, and NOx emissions of cofiring coal slime and anthracite through lab-scale experiments, numerical simulations, and full-scale industrial measurements in a 600MW down-fired utility boiler. The lab-scale experiments show combustion interactions between anthracite and coal slime, which have both a promotive effect on the ignition of anthracite and inhibitive effect on the combustion and burnout of anthracite. Especially, the inhibitive effect is greater than the promotive effect when the blending ratio of coal slime is above 5%. The addition of coal slime can effectively improve the ash fusion temperature of blended samples. In addition, simulations and full-scale industrial measurements of anthracite cofiring with coal slime are performed in a down-fired utility boiler. The results show that as the blending ratio of coal slime increases, the carbon content in fly ash increases and the boiler efficiency and NOx emissions decrease. A 5% ratio is validated to be a reasonable cofiring ratio of coal slime in the actual operation without no slagging phenomenon. These results provide more insights into the cofiring behavior of coal slime and anthracite and guide the application of blending coal slime in a down-fired boiler. [ABSTRACT FROM AUTHOR]

Coal blends are commonly used in thermal power plants and oxy-fuel combustion also attracts great concerns recently. However, little has been done on oxy-fuel combustion of coal blends. In this paper, combustion tests are performed in a drop tube furnace for various coal blends under O 2 /CO 2 mixtures, which are also reproduced numerically. Strong combustion interactions between the parent coals in a blend are observed. The ignition of the low-volatile coal is promoted due to the rapid combustion of the volatiles from the high-volatile coal. However, the char burnout of the low-volatile coal is compromised due to the rapid O 2 consumption by the large amount of volatiles. The interactions under oxy-fuel conditions are more sensitive to the excess O 2 , inlet oxidizer temperature and coal particle size. 1) The increase in the excess O 2 tends to weaken the combustion interactions, in which both the ignition promotion and burnout inhibition effects on the low-volatile coal become less apparent. 2) The increase in the inlet oxidizer temperature further promotes the ignition of the low-volatile coal whereas weakens the burnout inhibition of the low-volatile coal. 3) The effects of reducing particle size on the combustion interactions are similar to those of increasing inlet oxidizer temperature. Since biomass often contains a high volatile content, the findings may shed light on the synergistic effects in oxy-fuel co-combustion of biomass and coal. [ABSTRACT FROM AUTHOR]

Blending coal combustion technology is commonly used in coal-fired power stations, and the combustion interactions (i.e., the ignition promotion and burnout inhibition) during blended coal combustion affect their combustion characteristics. The combustion interactions under conventional air-fuel condition have been well studied. Oxy-fuel firing considerably differs from air-fuel combustion and the combustion interactions of blended coals in O 2 /CO 2 atmospheres have been investigated in our previous research. However, the H 2 O concentration in actual oxy-fuel condition is relatively high, which may influence the combustion interactions. This study further investigates the effect of H 2 O on the combustion characteristics and interactions of blended coals in O 2 /H 2 O/CO 2 atmospheres. The results show that adding H 2 O into the oxy-fuel atmosphere remarkably affects the combustion characteristics with the more notable ignition delay, faster burnout and better comprehensive combustion characteristics (CCI). In addition, the presence of H 2 O strengthens the ignition promotion, the burnout inhibition and the comprehensive interactions, resulting in the more obvious non-additive behaviors of CCI. In O 2 /H 2 O/CO 2 mixtures, enhancing the H 2 O/CO 2 ratio slightly strengthens the ignition promotion, the burnout inhibition, and the comprehensive interactions, which increases the non-additive behaviors of CCI. Furthermore, both of increasing O 2 /H 2 O or O 2 /CO 2 ratios significantly weaken the ignition promotion, strengthen the burnout inhibition and the comprehensive interactions, which increases the non-additive behaviors of CCI. The findings shed light on the interaction effects observed during blended coal combustion in O 2 /H 2 O/CO 2 mixtures, which can provide useful information to improve the combustion characteristics of the blended coals in the real oxy-fuel atmospheres. • Effect of H2O on interactions of blended coals oxy-fuel combustion is studied. • H 2 O increases promotion, inhibition and CCI non-additive behavior in O 2 /H 2 O/CO 2. • Higher H 2 O/CO 2 ratio increases promotion, inhibition, CCI non-additive behavior. • Higher O 2 /H 2 O or O 2 /CO 2 ratios show weaker promotion and stronger inhibition. • Higher O 2 /H 2 O or O 2 /CO 2 ratios present more obvious CCI non-additive behavior. [ABSTRACT FROM AUTHOR]

Coal blending is an effective method to reduce the slagging tendency. The mineral transformation within blended coal ashes has an important influence on the slagging behaviors, which have been well studied under conventional air-fuel condition. Oxy-fuel combustion is recognized as one of the most promising CO 2 reduction technologies. However, the mineral transformation within blended coal ashes in oxy-fuel condition has yet to be fully understood. This work is aimed to comprehensively study the influence of CO 2 atmosphere on Ca -containing mineral transformation behaviors during a Ca dominated coal co-firing with a Si/Al dominated coal in O 2 /CO 2 condition by TG-DSC, XRD and HSC simulation. These results show that the atmosphere significantly affects on the transformation of Ca -containing mineral and the final existing form of Ca in the blended coal ashes. When the high content Ca coal is blended with the high content Al/Si coal in the O 2 /CO 2 atmosphere, the CO 2 promotes the transformation of Ca into CaCO 3 and Ca under low temperature mainly exists in the form of CaCO 3 (little CaSO 4) in the blended coal ashes. CaCO 3 under high temperature is decomposed to produce highly active CaO, and CaO reacts with Al/Si to form the low melting point minerals, which may aggravate the slagging tendency. The present study can provide useful information to reduce the slagging behavior of the blended coals in the O 2 /CO 2 atmosphere. • Ca -containing mineral transformation of blended coal oxy-fuel combustion is studied. • The atmosphere affects the final Ca- existing form within blended coal ashes. • The CO 2 in O 2 /CO 2 promotes the transformation of Ca into CaCO 3. • CaCO 3 is decomposed to produce highly active CaO under high temperature. • CaO– Al/Si reaction produces low melting point mineral to aggravate the slagging. [ABSTRACT FROM AUTHOR]

The combustion of a biomass with coal may be one of the most effective methods to improve energy utilisation efficiency. This study aims to investigate the combustion characteristics of distillers' grains with bituminous coal using a thermo‐gravimetric analyser. The results demonstrate that increasing the proportion of distillers' grains in coal results in a reduced ignition temperature (Ti) and burnout temperature (Tb), and activation energy (E) in the combustion stage. Therefore, the comprehensive combustion index (CCI) improves. In an O2/CO2 atmosphere, Ti increased by ∼15°C, while Tb did not change much. However, E and CCI declined relative to an O2/N2 atmosphere. Under the O2/CO2 atmosphere, with an increase in O2 concentration from 10 to 40%, Ti changed over a small range, but Tb declined from 745.8 to 641.4°C, while E and CCI improved. As the heating rate rose from 10 to 30°C/min, the values of Ti decreased (from 281.4 to 244.9°C), but the values of Tb increased significantly (from 662.8 to 745.8°C); the value of E reduced, and CCI also improved. This research could provide useful information as a practical reference for heat and power production. [ABSTRACT FROM AUTHOR]

The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO 2 emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O 2 /N 2 mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O 2 /CO 2 mixture. In the O 2 /CO 2 mixture, both increasing the O 2 concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O 2 /CO 2 atmosphere. • Ignition promotion and burnout inhibition exist during blended coal combustion. • Weaker promotion, inhibition and interaction are shown in O 2 /CO 2 than in O 2 /N 2. • Higher oxygen shows weaker promotion, stronger inhibition and interaction. • Smaller particle size shows weaker promotion, stronger inhibition and interaction. • Higher heating rate shows stronger promotion, weaker inhibition and interaction. [ABSTRACT FROM AUTHOR]

Coal combustion is the major contributor to global toxic selenium (Se) emissions. Inorganic elements in coals significantly affect Se partitioning during combustion. This work confirmed that the calcium (Ca) in ash had a stronger relationship with Se retention at 1300 °C than other major elements. Ca oxide chemically reacted with gaseous Se, and its sintering densification slightly affected Se adsorption capacities (44.45 −1840.71→35.17 −1540.15 mg/kg) at 300 − 1300 °C. Therefore, Ca in coals was identified as having potential for hindering gaseous Se emissions, and coals with increased Ca contents (2.74→5.19 wt%) were used in a 350 MW unit. The decreased Se mass distribution (3.54%→2.63%) in flue gas at air preheater inlet (320 −362 °C) confirmed the effectiveness of increased Ca content on gaseous Se emission reduction. More gaseous Se further condensed and was chemically adsorbed by fly ash when passed through an electrostatic precipitator, resulting in a significant increase in the Se content of fly ash. Additionally, the corresponding Se leaching ratio decreased from 4.88 − 35.74% to 1.87 − 26.31%, indicating enhanced stability of Se enriched in fly ash. This research confirmed the feasibility and environmental safety of sequestration of gaseous Se from flue gas to fly ash by increasing the Ca content in coals. [Display omitted] ● Se retention at 1300 °C is associated with Ca content in ash. ● Ca oxide removes more gaseous Se via chemical reaction during flue gas cooling. ● Coals with higher Ca contents will release less gaseous Se into flue gas. ● A higher Ca content in fly ash can enhance the stability of Se within. ● Electrostatic precipitator induces Se emission reduction with increased Ca content. [ABSTRACT FROM AUTHOR]

Currently, the severe fluctuation of steam temperature in double-reheat (DR) ultra-supercritical (USC) units has become a major problem that constrains their efficient and flexible operation, wherein the deviation of coal calorific value from the design value is one of the major reasons for this issue. However, the effect of coal calorific value variation on the steam temperature characteristics during the flexible operation of DR USC units remains unclear. A dynamic model of a 1000 MW DR USC unit is developed with primary principles and validated with real operation data. The effect of the coal calorific value variation on the steam temperature characteristics is revealed. Results show that as the calorific value of the coal increases, the main steam temperature can always maintain at the design value with attempering water adjustments, while the two reheat steam temperatures decrease. The steady-state deviations of the reheat steam temperatures are positively correlated with the magnitude of the coal calorific value variation, where the deviation of the secondary reheat steam temperature is larger. Meanwhile, as the load increases, the steady-state deviations of reheat steam temperatures increase, but the response time decreases. Finally, the effect of calorific value variation on the boiler efficiency is studied. As the coal calorific value increases, the boiler efficiency firstly rises caused by the decrease in the loss of the flue gas and then falls caused by the limited furnace temperature increase. This study provides guidance on the DR USC boiler operation under coal variation. • A dynamic model of a 1000 MW double-reheat ultra-supercritical unit is developed. • The effect of the coal calorific value variation on the steam temperature characteristics is revealed. • The effect of calorific value variation on the boiler efficiency is revealed. • This study provides guidance on the double-reheat ultra-supercritical boiler operation under coal variation. [ABSTRACT FROM AUTHOR]

Sodium, calcium, sulfur and chlorine in the flue gas of coal-fired boilers are the main cause of ash deposition/fouling in heat exchangers. However, there are still many debates on the mechanism of ash deposition/fouling due to the lack of direct micro evidence. This paper aims to clarify the initial deposition/fouling characteristics of a high-alkali coal by computer-controlled scanning electron microscopy (CC-SEM). The results demonstrate that temperature has an influence on the deposition/fouling compounds. The crystalline compounds in the samples at 900 °C were mainly CaSO 4 and Ca 2 Al 2 SiO 7 , while the compounds in the samples at 500 °C were mainly NaCl. Sulfur had a significant adsorption effect on sodium and calcium at 900 °C, while chlorine had a significant adsorption effect on sodium at 500 °C. A 3-layer microstructure of deposition/fouling was found after the samples were stripped by molecular force. Additionally, deposition/fouling could be divided into three stages: the formation of sodium salt precursors, the growth of loose structural aluminosilicates and the adhesion of particulates on the surface. Sulfates, especially Na 2 SO 4 and CaSO 4 formed a precursor for the initial deposition/fouling of high-alkali coal. • The initial deposition/fouling characteristics of a high-alkali coal were studied. • Samples were stripped by molecular force. • A 3-layer structure of deposition/fouling was observed. • The mechanism of the influence of sulfur and chlorine on deposition/fouling was proposed. • A series of detailed microscopic maps of the deposition/fouling process was obtained. [ABSTRACT FROM AUTHOR]