Search

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]

Based on the pseudo-steady-state approach, the combustion effect of high-reactivity char (i.e., HR-char) on low-reactivity char (i.e., LR-char) is investigated numerically in a hot O 2 /CO 2 atmosphere under two arrangements (LR-LR case : the upstream and downstream particles are LR-char, HR-LR case : the upstream particle is HR-char and the downstream particle is LR-char). This paper is aimed to study the influence of oxygen concentration (from 2% to 30%) and ambient temperature (1300 K, 1400 K, and 1500 K) on the combustion effect under the condition with dominated oxidation reaction and weak gasification reaction, and explain the mechanism in aspects of the temperature, specie concentration, and reaction rate. The numerical results are carefully validated against experimental data published in the literature. The results show that under these studied ambient temperatures, the char-O 2 oxidation-reduction brings a more pronounced effect than the char-CO 2 gasification enhancement for char consumption owning to the dominant role of the char oxidation reaction. The upstream HR-char particle combustion under the HR-LR case produces a higher temperature, lower oxygen, and higher CO 2 atmosphere ahead of the downstream char than under the LR-LR case, which significantly inhibits the downstream LR-char particle combustion. In addition, lower oxygen concentration or higher ambient temperature decreases the difference in temperature, oxygen, and CO 2 between the HR-LR and LR-LR cases ahead of the downstream char, resulting in the lower difference in the downstream LR-char oxidation consumption rate between the HR-LR and LR-LR case. Consequently, the weaker combustion inhibitive effect of the high-reactivity char on the low-reactivity char is observed. The findings shed light on the interaction effects observed during blended coal combustion in O 2 /CO 2 mixtures. • Combustion effect of high-reactivity char on low-reactivity char is investigated. • Effect mechanism is studied with spatially and chemically resolved simulations. • Lower oxygen or higher ambient temperature corresponds to weaker combustion effect. [ABSTRACT FROM AUTHOR]

The combustion interactions during blended coal combustion affect their combustion characteristics, and the combustion interactions under air–fuel conditions have been well studied. However, oxy–fuel combustion considerably differs from air–fuel firing. Significantly, the char–CO2 gasification reaction under oxy–fuel and high-temperature conditions substantially affects the combustion behaviors, which may influence the combustion interactions. This paper is to study the effect of the char oxidation and gasification reaction competition on comprehensive interactions in O2/CO2 mixture. The results show that under high-temperature and low-oxygen (O2<4%) conditions, an intense gasification reaction is observed. The competition between the char–CO2 gasification and char–O2 oxidation reactions influences the comprehensive interactions during blended chars combustion. As the char–O2 oxidation reaction is strengthened and the char–CO2 gasification reaction is weakened, the comprehensive interactions during blended chars combustion gradually decrease. Moreover, under the competition between the char–CO2 gasification and char–O2 oxidation reactions, only enhancing the char–CO2 gasification reaction or enhancing the char–O2 oxidation reaction weakens the comprehensive interactions. The weaker comprehensive interactions result in the smaller nonadditive behavior of the comprehensive combustion index. The findings shed light on the interaction effects observed during blended chars combustion, which can provide useful information to improve the combustion behaviors of the blended coals in oxy–fuel conditions. [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 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]

Down-fired boilers are commonly used to burn low-rank coals for power generation. However, they often have operational problems such as low char burnout and high NO x emissions. This paper comprehensively investigates how to mitigate such problems in a 300 MW e down-fired utility boiler burning a low-volatile coal by optimizing the staged-air windbox structure. First, experiments and simulations are conducted on a 1:1-scaled model of the staged-air windbox to study the cold air flow in the original and two new windbox structures for understanding the effect of different staged-air windbox structures. Then, combustion simulations are performed for the utility boiler to further study the influence of the newly designed staged-air windboxes on the boiler performance, based on which the optimization scheme of the staged-air windbox in the boiler is more reliably concluded. Finally, the utility boiler is retrofitted by using a newly designed staged-air windbox. Performance tests are made for the boiler before and after the retrofit. The new staged-air windbox is confirmed to greatly improve the performance of the utility boiler, e.g., NO x reduction by 20.7% and thermal efficiency increase by nearly 2 percentage points (from 88.76% to 90.48%) at full load. The idea of the new staged-air windbox design can be readily implemented into down-fired boilers of this kind for more efficient and environmentally friendly firing of low-rank coal for heat and power generation. • Down-fired systems commonly used to burn low-rank coal but with operation problems • Modelling and testing done on both testrig and boiler for effect of staged-air box • A simple and smart optimization of staged-air box improves downshot-fired system • At full load, NOx reduction by 21% and efficiency increase by 2 percentage points. [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]

Pressurized oxy-coal combustion in conjunction with carbon capture and sequestration is a promising technique for reducing carbon emissions. Biomass is a renewable energy source of zero carbon emissions. Therefore, negative carbon emissions can be achieved with pressurized oxy-biomass combustion, which is an appealing method for carbon peaking and carbon neutralization. NOx is an acidic gas produced from combustion that has the potential to corrode pipelines, demanding strict control. However, the evolution of NOx precursors and NOx formation in pressurized oxy-biomass combustion is still unclear. In this study, the NOx emissions in pressurized oxy-combustion and nitrogen migration characteristics of biomass in a pressurized CO 2 atmosphere were studied in a pressurized fixed bed reactor. The combustion results indicate that as the pressure increases, NO emission decreases while N 2 O formation slightly increases. The pressurized pyrolysis/gasification results show that the release of NH 3 and HCN in CO 2 atmosphere significantly reduces with increasing pressure, while the other-N species are reduced to N 2 by char and CO at high pressure. NH 3 formation peaks at 700 °C while HCN emission increases significantly with increasing temperature. NH 3 yield is considerably higher than HCN yield when the temperature is below 800 °C. The correlation analysis shows that NO formation increases with the increase of the total amount of NH 3 and HCN released in the pyrolysis/gasification process. This study provides new insight into the evidence that the suppression of volatile-N under elevated pressure is one important reason for the decrease of NOx in the pressurized oxy-biomass combustion. [ABSTRACT FROM AUTHOR]

[Display omitted] • As 2 O 3 adsorption characteristics of mineral oxide sorbents at 300–900 °C were studied. • The order of As 2 O 3 adsorption at 300–700 °C is CaO > Fe 2 O 3 > Na 2 O > MgO > Al 2 O 3 > SiO 2. • The mechanism of As 2 O 3 adsorption was revealed by DFT analysis. As a highly toxic element of coal, arsenic (As) is mainly converted into As 2 O 3 vapor during coal combustion. Mineral oxides are effective and abundant sorbents to control As 2 O 3 emissions. In this paper, the characteristics of As 2 O 3 adsorption by mineral oxides were studied in a two-stage reactor, and the density of states, adsorption sites, and adsorption energies for the adsorption reaction were investigated using molecular simulation. The results demonstrate that CaO is an excellent As 2 O 3 sorbent. The As 2 O 3 adsorption effect of CaO, MgO, and Al 2 O 3 was found to become better with increasing temperature within the range of 300–900 °C. The order of As 2 O 3 adsorption at 300–700 °C follows CaO > Fe 2 O 3 > Na 2 O > MgO > Al 2 O 3 > SiO 2 , and SiO 2 hardly adsorbs any As 2 O 3. At 900 °C, the melting of Na 2 O and Fe 2 O 3 greatly reduces their As 2 O 3 adsorption capacity. The O atoms in Na 2 O/CaO/MgO/Al 2 O 3 /SiO 2 and the Fe atoms in Fe 2 O 3 are the adsorption active sites for As 2 O 3. The order of the calculated adsorption energy follows Fe 2 O 3 > Na 2 O > CaO > Al 2 O 3 > MgO > SiO 2 , and the As 2 O 3 adsorption involves chemical adsorption. The adsorption products are Ca 3 (AsO 4) 2 , FeAsO 4 , AlAsO 4 , etc., for which As exists in the form of As3+ and As5+, with As3+ transformed into As5+ with increasing adsorption temperature. [ABSTRACT FROM AUTHOR]

Coal combustion is one of the major pathways by which lead (Pb) enters the ecological environment. An effective method to control Pb emissions in a furnace is to transform the Pb from vapors to particles using Si-Al-based sorbents. However, the high price, poor thermal stability, and difficult molding of sorbents limits their application. Fly ash cenospheres (FACs) prepared from the waste of coal-fired power plants, which are mainly composed of SiO 2 and Al 2 O 3 , are ideal materials for Pb sorbents because of their high strength, light weight and low price. In this study, the adsorption and reaction mechanisms of FACs and several Si-Al-based sorbents for Pb vapors were investigated in a two-stage fixed-bed reaction system, and the thermal stability and regeneration characteristics of FACs were tested. The results indicate that the adsorption capacity for Pb vapors is FAC > kaolin > SiO 2 > γ-Al 2 O 3. The acidic sites, such as Si-O and Al-O bonds in FACs, were the main reaction sites for Pb adsorption and captured Pb in the form of stable Pb compounds (Pb2+). As the temperature increased from 700 to 1200 °C, the Pb adsorbed by FACs increased, and 55.21% of the Pb in the simulated flue gas was removed at 1200 °C. Kaolin is an excellent sorbent for Pb vapors. The adsorption capacity of kaolin for Pb first increases and then decreases with increasing temperature, and the maximum removal rate occurs at 1000 °C, reaching 33.46%. In addition, FACs are thermally stable and reusable. Therefore, the high-temperature application of FACs to remove Pb vapors has good prospects. [ABSTRACT FROM AUTHOR]