Oxy-fuel Conditions Research Articles

Towards cleaner energy: An innovative model to minimize NOx emissions in chemical looping and CO2 capture technologies

The urgent global challenges of climate change and environmental issues have catalyzed the development of CO2-capture-ready technologies incorporating fluidization, such as fluidized bed (FB) and circulating fluidized bed (CFB) combustion under oxy-fuel conditions, chemical looping combustion (CLC), chemical looping with oxygen uncoupling (CLOU), in-situ gasification chemical looping combustion (iG-CLC), and calcium (or carbonate) looping (CaL) systems. While these technologies have the potential to reduce CO2 emissions significantly, the persistent presence of nitrogen oxides (NOx = NO + NO2) as pollutants remains a critical environmental concern.This paper introduces a comprehensive fuzzy logic-based model for predicting NOx emissions (FuzzyNOx model) from solid fuel combustion in fluidized beds of chemical looping systems. This innovative model takes into account a wide range of operating parameters, including fuel type, fuel particle size, fuel moisture content, air/fuel ratio, and temperature. It also explores advanced coal and biomass combustion modes, including air-firing, oxyfuel, iG-CLC, and CLOU. Furthermore, it delves into the intricacies of two distinct facilities: the 5 kWth dual-fluidized bed Chemical-Looping-Combustion of Solid-Fuels (DFB-CLC-SF) facility at Czestochowa University of Technology, Poland, and the calcium looping dual-fluidized bed (CaL DFB) facility at Niigata University, Japan.Bituminous coal, semi-anthracite, and wood chips are utilized as fuels. Moreover, three various oxygen carriers (OCs) for chemical looping combustion were employed, namely ilmenite, copper oxide (60 % wt.) supported by carbonate waste from ore flotation, and copper oxide (60 % wt.) supported by ilmenite (20 % wt.) and fly ash. Bituminous coal, semi-anthracite, and wood chips are utilized as fuels.The developed knowledge-based fuzzyNOx model allows the optimization of operating parameters to reduce NOx emissions from CO2 capture technologies.

Combustion behavior of solid waste fuels in the vertical tube reactor under different oxy-fuel environments

Oxy-fuel combustion technology has been recognized as one of the promising ways for cost-effective CO2 capture. The co-combustion characteristics of flax straw biomass/bituminous coal mixture (FS/BC) and flax straw biomass/sewage sludge mixture (FS/SS) under air, oxy-fuel (21%/79% O2/CO2) and oxygen-enriched oxy-fuel (30%/70% and 40%/60% O2/CO2) conditions were investigated in the vertical tube reactor. In particular, the study focused on the burning characteristics, the effect of blending ratio, and flue gas emissions. The findings indicated that flax straw biomass exhibited superior ignition performance compared to its blends with sewage sludge and bituminous coal. A higher O2 concentration from 21% to 40 led to a reduction in ignition delay time by 4s at 850 ℃ and by up to 10s at 950 ℃. Moreover, char combustion time was reduced by 50% for FS/SS blends due to the lower fixed carbon content compared to FS/BC blends. Additionally, increasing the O2 concentration resulted in lower emissions of CO by ∽40% and N2O by ∽45% for both blends. However, this also led to a slight increase in SO2 and NO emissions by ∽15% and ∽26%, respectively. The optimal conditions for the tested samples in oxy-fuel environment were found to be 30% O2/70 CO2, which enhanced combustion performance while lowering flue gas emissions.

Experimental study and chemical equilibrium calculation on the mineral transformation of high-alkali coal under oxy-fuel condition

The study aimed to study the mineral transformation of high-alkali coal to address the ash-related issues in oxy-fuel combustion. However, the traditional ashing method alone is inappropriate to study the mineral transformation at the initial stage of coal combustion due to the high ashing temperature. Hence, both the plasma ashing method (<200 °C) and traditional ashing method (600–1200 °C) were employed in this research to prepare the ash samples for characterization. The plasma ashing method is an efficient way to determine the original minerals in coal because it can consume the organic matter at low temperature. The recirculation of flue gas in oxy-fuel combustion can lead to high SO2 and H2O concentrations, while their influences on mineral transformation need to be clarified. In this work, SO2 and H2O were injected into CO2 to simulate the recycled flue gas (RFG). Furthermore, the chemical equilibrium calculation was conducted besides multiple experimental tests to acquire more comprehensive information of mineral transformation. The results show that high sodium and calcium contents are determined in these high-alkali coals, and two of them are rich in iron. In O2/CO2 atmosphere, some sodium (e.g. NaCl) volatilizes into the gas phase and the others converts into aluminosilicates (e.g. NaAlSi3O8) during the heating process. The calcium salts (mainly CaCO3 and CaSO4) decompose and produce silicates, aluminosilicates, and magnesium silicates. The major iron-containing minerals are identified as FeS, FeS2, and Fe2O3. Both FeS and FeS2 are oxidized into Fe2O3 below 600 °C. Moreover, the kaolinite (Al2Si2O5(OH)4) in raw coals decomposes at low temperature (<600 °C). In O2/RFG atmosphere, the additions of SO2 and H2O enhance the sulfation of AAEMs (alkali and alkaline earth metals) and delay the decomposition of sulfates. The productions of some silicates, aluminosilicates, and magnesium silicates of AAEMs are inhibited to varying degrees, and there could be complicated salts containing sulfur (e.g. hauyne). The transformations of iron-containing minerals are little related to the presences of SO2 and H2O. This work obtained the transformations of main minerals in high-alkali coal under O2/CO2 and O2/RFG conditions, which helps to address the ash-related issues.

Advanced orthogonal analysis of multi-factorial interactions influencing co-combustion performance and pollution reduction potential of coal gangue and biomass in oxy-fuel combustion conditions

In the current scenario of increasing energy scarcity and stricter environmental protection requirements, the utilization of coal gangue and biomass energy is gaining attention worldwide. Biomass and coal gangue are complementary in nature, and their co-combustion can improve the burnout efficiency of coal gangue and reduce pollutant emissions. In this study, coal gangue and wheat straw are selected as research subjects, the optimal process conditions for their co-combustion under the coupling of multiple parameters in oxy-fuel atmosphere through orthogonal experiments are investigated. The results indicate that the most significant factor influencing combustion characteristics is the heating rate. When the biomass content is 45 %, O2 concentration is 50 %, heating rate is 30℃/min and particle size is 180–250μm, the flammability and comprehensive combustion indexes of the samples are maximized, which indicates the optimum combustion characteristics. The primary factor affecting pollutant release is biomass content. When the biomass content is 45 %, O2 concentration is 75 %, combustion temperature is 700℃, and particle size is 180–250μm, the pollutant release concentration is minimized.

The characteristics and mechanism NO formation during hydroxylated fuels/NH3 oxidation in oxy-fuel atmospheres

Oxy-fuel combustion, as an important technology to realize CCUS in the field of combustion, needs to control the generation of pollutants such as NOx while realizing CO2 capture, and the conversion of fuel-N to N2 is crucial. Previous experiments have provided insights into the influence of different atmospheres on NO during oxy-fuel combustion of alcohols. However, the specific reaction mechanism underlying how the atmosphere and hydroxyl fuels impact NO generation through the free radical pool remains unclear. In this study, the formation and reduction of NO during the oxidation of CH3OH/NH3 and C2H5OH/NH3 in O2/CO2, O2/H2O and O2/CO2/H2O atmospheres with oxy-fuel condition were investigated by simulation. In the NO generation stage, different H2O, CO2, O2 and fuel types have little effect on the peak of NO although they change the NH3→NO reaction path. In the process of NO reduction, stage 1 is the main reaction phase of NO→N2, and both CO2 and H2O are beneficial for the reduction of NO. But the reduction of NO by H2O was more pronounced. In the case of simultaneous addition of H2O and CO2, the reducing effect of H2O and CO2 alone on NO was attenuated. On the other hand, the reduction effect exerted by H2O and CO2 was stronger in high than in low oxygen-fuel ratio, stronger in C2H5OH than in CH3OH. The reduction of NO by H2O and CO2 is based on the general equation NO→N2+O2. H2O is used to promote the reaction in the positive direction by consuming O2 to convert it to H and OH, so that more of the element O is converted to OH than O2 to inhibit the production of NO. The CO2 contributes to the formation of less O2 during the reduction of N2 from NO by promoting reaction NO + N→O + N2, while the additional OH increases the "capacity" of O2 in the NO→N2+O2 reaction.

Effect of oxy-fuel combustion with different O2/CO2 fractions on the pulverized coal combustion mechanism and heat transfer characteristics in rotary kilns: A numerical simulation study

Oxy-fuel combustion combined with carbon capture and storage is the most effective and direct technology to achieve carbon neutrality in cement industry. In this work, numerical simulation was employed to investigate the effects of air combustion and oxy-fuel combustion with different O2/CO2 fractions on the airflow field, temperature field, pulverized coal combustion mechanism, heat transfer characteristics and NOx distribution in rotary kiln. The results show that the airflow field under different conditions has no significant change. When transitioning from the air condition to the oxy-fuel condition, the ignition point of pulverized coal extends backward by approximately 0.2 m. The maximum temperature decreases from 2306 K to 2052 K, and the temperature becomes more uniform. As the oxygen concentration in the primary air increases, both the maximum and average temperatures gradually rise, and the flame length shortens from 25.4 m to 24.16 m. In the rotary kiln, the locations of char oxidation and gasification reactions differ, but these reaction locations remain consistent under different conditions. Under oxy-fuel conditions, the oxidation rate and oxidation proportion of char decrease, while the gasification rate and gasification proportion of char increase, resulting in a shortening of the burnout distance of coal from 34.5 m to 31.3 m. With increasing oxygen concentration in the primary air, the rates of char oxidation and gasification gradually rise, further shortening the burnout distance. The decrease in temperature at the front end of the rotary kiln under oxy-fuel conditions reduces the total heat transfer in the burning zone by 15 %. When the oxygen content of the primary air is 70 %, it ensures stable calcination of clinker. Additionally, under oxy-fuel conditions, the NOx generation is significantly reduced. With the increase of oxygen concentration in the primary air, the NOx concentration at the outlet decreases slightly from 451 ppm to 406 ppm, due to the increase in kiln temperature that favors the reduction of fuel NOx.

Investigation of the Coupling Schemes between the Discrete and the Continuous Phase in the Numerical Simulation of a 60 kWth Swirling Pulverised Solid Fuel Flame under Oxyfuel Conditions

Investigation of the Coupling Schemes between the Discrete and the Continuous Phase in the Numerical Simulation of a 60 kWth Swirling Pulverised Solid Fuel Flame under Oxyfuel Conditions

Review on Mercury Control during Co-Firing Coal and Biomass under O2/CO2 Atmosphere

Combining biomass co-firing with oxy-fuel combustion is a promising Bioenergy with Carbon Capture and Storage (BECCS) technology. It has the potential to achieve a large-scale reduction in carbon emissions from traditional power plants, making it a powerful tool for addressing global climate change. However, mercury in the fuel can be released into the flue gas during combustion, posing a significant threat to the environment and human health. More importantly, mercury can also cause the fracture of metal equipment via amalgamation, which is a major risk for the system. Therefore, compared to conventional coal-fired power plants, the requirements for the mercury concentration in BECCS systems are much stricter. This article reviews the latest progress in mercury control under oxy-fuel biomass co-firing conditions, clarifies the impact of biomass co-firing on mercury species transformation, reveals the influence mechanisms of various flue gas components on elemental mercury oxidation under oxy-fuel combustion conditions, evaluates the advantages and disadvantages of various mercury removal methods, and finally provides an outlook for mercury control in BECCS systems. Research shows that after biomass co-firing, the concentrations of chlorine and alkali metals in the flue gas increase, which is beneficial for homogeneous and heterogeneous mercury oxidation. The changes in the particulate matter content could affect the transformation of gaseous mercury to particulate mercury. The high concentrations of CO2 and H2O in oxy-fuel flue gas inhibit mercury oxidation, while the effects of NOx and SO2 are dual-sided. Higher concentrations of fly ash in oxy-fuel flue gas are conducive to the removal of Hg0. Additionally, under oxy-fuel conditions, CO2 and metal ions such as Fe2+ can inhibit the re-emission of mercury in WFGD systems. The development of efficient adsorbents and catalysts is the key to achieving deep mercury removal. Fully utilizing the advantages of chlorine, alkali metals, and CO2 in oxy-fuel biomass co-firing flue gas will be the future focus of deep mercury removal from BECCS systems.

Dynamic and optimal ash-to-gas responses of oxy-fuel and air combustions of soil remediation biomass

Given the global agenda of how to best meet carbon targets, how to scientifically and safely dispose post-harvest phytoremediation biomass on a large scale remains to be explored. This study sought to characterize the temperature-dependent ash-to-gas responses of the oxy-fuel (CO2/O2) and air (N2/O2) combustions of Pfaffia glomerata (PG). The two indices of thermal stability (RW) and comprehensive combustion (CCI) were weaker in oxy-fuel combustion, but with the oxy-fuel combustion reducing the activation energy required for the cellulose degradation from 290.07 to 98.83 kJ/mol. Both empirical indices and the SiO2–CaO–K2O method suggested that oxy-fuel combustion intensified the slagging and fouling risk. A combination of thermochemical equilibrium simulations and X-ray diffraction analysis verified the risk through ash-mineral transformations. The oxy-fuel conditions enhanced the stabilization of Zn in PG but without significant effect on Cd. The oxy-fuel combustion inhibited the formation of aromatics but enhanced the emission of gases, such as CO, CH4, C–O, CO, HCl, and SO2. The optimal conditions in terms of energy efficiency and reduced emissions were achieved between 522 and 1000 °C in the oxy-fuel atmosphere. This study can provide new insights into eco-friendly disposal options for soil remediation biomass in modern combustion systems.

Modeling the co-combustion of bi-fuel blends in a drop tube furnace: A numerical approach

The present study focuses on the numerical modeling of the co-combustion of reject coal (RC) and sal leaves (SL) in a drop tube furnace (DTF) under different conditions, including air-fuel and oxy-fuel environments. The numerical results highlight the significant influence of replacing the N2 atmosphere with CO2 on the temperature profiles of individual fuels and their blends. For instance, when exposed to a 21%O2/79%CO2 atmosphere, Blend SL3 and Blend SL5 experience a decrease in temperature from 1372 K to 1559 K to 1327 K and 1395 K, respectively. Conversely, increasing the O2 concentration to 35% leads to a temperature rise. Furthermore, the combustion analysis reveals that the addition of 10% SL to RC in a 21%O2/79%N2 environment enhances the devolatilization rate from 1.41 × 10−12 kg/s to 1.77 × 10−12 kg/s. Additionally, the char combustion process for all blends is completed closer to the DTF inlet at 0.28 m, compared to 0.32 m for RC alone. These findings indicate that blending SL with RC promotes a favorable combustion process. Moreover, increasing the O2 concentration to 35% in oxy-fuel conditions enhances the temperature profiles. The numerical analysis demonstrates good agreement with experimental data within a range of ±5%–11%. This suggests that the developed model can be effectively utilized for optimizing and designing biomass co-firing systems in industrial applications, considering both air-fuel and oxy-fuel conditions.

Pulverized coal combustion in boundary layer turbulence using direct numerical simulation

The present study investigates pulverized coal combustion under oxy-fuel conditions interacting with turbulent boundary layer flows over a flat plate using direct numerical simulation (DNS). The coal particles are tracked in the Lagrangian framework while the flow is solved in an Eulerian way. Three cases with reacting particles, inert particles and single-phase flows were performed. The interplay between pulverized coal combustion, turbulent boundary layer flows and the wall was examined. It was shown that the coal particles first ignite in the outer layer of the boundary layer turbulence, where individual coal particle ignition occurs. The wall-normal velocity in the reacting case is larger compared with that of the non-reacting cases due to flow acceleration induced by combustion. The volatile matter mass fraction is high in the inner layer in the downstream region. However, the heat release rate is low due to the cold wall effect. The strong correlation between the wall heat flux and streak structures in the near-wall region is observed in all cases. The values of radiative heat transfer are similar in the upstream region of both the cases with reacting and inert particles. However, as combustion occurs and particle temperature increases in the reacting case, the radiative heat transfer from particles to the wall increases significantly in the downstream region. Particle accumulation due to turbophoresis in the near-wall region is analyzed. It was shown that the magnitude of streamwise vorticity is attenuated by coal combustion. Therefore, particle wall-accumulation is less prominent in the reacting case compared with the inert case.

Combined flow, temperature and soot investigation in oxy-fuel biomass combustion under varying oxygen concentrations using laser-optical diagnostics

In pulverized oxy-fuel biomass combustion, intricate interaction of complex fluid-mechanical, particle-dynamical, and chemical processes manifests even at intermediate scales. This study performs a comprehensive analysis aimed to understand the processes of flame stabilization, pollutant formation, and combustion behavior of biomass particles, while evaluating the impact of varying oxygen concentration on combustion. The investigation is conducted with a comprehensive data set, encompassing various oxy-fuel operation conditions ranging from 27vol.% to 36vol.% in O2 concentration, along a comparable air flame. These conditions are realized in an optically accessible gas-assisted solid fuel combustor designed to generate flames up to 70kWth. Advanced laser-optical diagnostics are employed to capture the combustion process. These techniques include the use of two-phase PIV/PTV for simultaneous measurements of flow and particle velocities, the application of tomographic absorption spectroscopy to elucidate three-dimensional gas temperature distributions, and the establishment of a quasi-simultaneous LII and LIF experimental setup to capture both PAH and soot occurrences. Additionally, chemiluminescence imaging of CH∗ is incorporated. The analysis extends to both single-phase and two-phase combustion, revealing a pronounced influence of oxygen concentration on both regimes. The inner recirculation zone, characteristic of a swirl flame, decreases with increasing oxygen content, coupled with an acceleration of the main flow downwards due to heightened heat release proximate to the nozzle. In two-phase combustion, a significant combustion delay is observed compared to single-phase combustion. Furthermore, the occurrences of PAHs and soot are clearly separated, and a pronounced influence of local gas composition on soot formation is evident. Comparative examination between oxy-fuel and air combustion show that an oxy-fuel condition with an oxygen concentration just below 33vol.% closely mirrors the flame characteristics of air combustion.

Co-combustion characteristics and kinetics of sludge/coal blends in 21%–50% oxygen-enriched O2/CO2 atmospheres

This research investigated the thermogravimetric analysis (TGA) and combustion kinetics of sewage sludge and Australian sub-bituminous coal and their blends with different mixing ratios (sludge/coal = 100/0, 80/20, 50/50, 20/80, 0/100 by weight) under air atmosphere (O2/N2 = 21/79 by volume), oxy-fuel combustion (O2/CO2 = 21/79, 25/75, 35/65, and 50/50 by volume), and pyrolysis (pure N2 atmospheres). The gaseous products that evolved from the TGA experiments were also analyzed using a Fourier transform infrared (FTIR) spectrometer under pyrolysis conditions. Results showed that the mixed fuel with higher coal blending ratio had higher ignition temperature, lower burnout temperature and larger combustion characteristic index. Furthermore, TGA–FTIR experiments of the 20 % sludge + 80 % coal blended fuel under nitrogen atmospheres showed more gasification components in pure coal than in sludge. The combustion kinetics analytical results via the Flynn–Wall–Ozawa method indicated that the larger the conversion degree, the smaller the obtained activation energy value. Under the oxy-fuel combustion conditions, when the oxygen concentration increased from 21 % to 50 % for the 20 % sludge + 80 % coal blended fuel, the ignition and burnout temperatures decreased, and the combustion characteristic index increased nearly three times. Oxygen enrichment can clearly improve combustion characteristics and flammability.

Numerical study on oxy-fuel combustion characteristics of industrial furnace firing coking dry gas

One of the most promising methods for clean utilization of hydrocarbon fuels and CO2 capture is oxy-fuel combustion. However, report on oxy-fuel burning of industrial gas is still limited. In the study, numerical simulation was conducted to explore the oxy-fuel combustion performance of a coking dry gas fired industrial furnace. A modified weighted sum gray gas (WSGG) model with a four-gray body and fourth-degree temperature polynomial was proposed for both air and oxy-fuel combustion with the introduction of the fourth graybody. The impacts of oxygen content, excess oxygen ratio, oxygen purity, and dehydration efficiency on combustion and heat transfer were mainly investigated. The maximum furnace temperature increases significantly, and the high temperature region inside the furnace shrinks and moves downward as the oxygen content rises. The H2O content increases from ∼20% to ∼37%, and the CO2 content decreases from ∼77% to ∼60%. The furnace temperature is up to 1835 K under air conditions, while the furnace temperature is reduced by 195 K under oxy-fuel condition of 21% O2 content. The composition of flue gas barely changes as the excess oxygen ratio increases from 1.02 to 1.20. Although the CO2 enrichment in flue gas and the improvement of oxy-fuel combustion performance benefit from an increase in oxygen purity, the combustion performance further advances little when it is raised to over 98%. Additionally, when dehydration efficiency grows, radiant heat transfer decreases and the flue gas temperature at the furnace outlet rises. The content of O2 and N2 has little relationship with dehydration efficiency. This study aids in understanding the actual oxy-fuel combustion process of an industrial gas-fired boiler and offers a possible theoretical basis for clean burning of petrochemical combustible gas and carbon capture.

Experimental investigation on the NO formation of pulverized coal combustion under high-temperature and low-oxygen environments simulating MILD oxy-fuel combustion conditions

The NO formation experiments simulating moderate and intense low-oxygen dilution (MILD) oxy-coal combustion conditions were conducted on a laminar diffusion flame burner with the coflow temperatures of 1473–1873 K and the oxygen volume fractions of 5 %–20 % in O2/CO2, O2/Ar and O2/N2 atmospheres. The flame images of pulverized coal combustion were captured to obtain the ignition delay distances, and the axial species concentrations were measured to obtain the variation of NO formation and reduction. The NO yield in O2/Ar atmosphere decreased by nearly 0.2 when the oxygen volume fraction decreased from 20 % to 5 % and by about 0.05 when the coflow temperature decreased from 1873 K to 1473 K. The NO yield in O2/CO2 atmosphere was 0.1–0.15 lower than that in O2/Ar atmosphere. The optimal kinetic parameters of thermal NO and fuel NO formation rate were obtained by a nonlinear fit of nth-order Arrhenius expression. Finally, the relative contribution rates of thermal NO to total NO (Rth) and NO reduction to fuel NO (Rre) were quantitatively separated. Rth decreases with the increase of oxygen volume fraction, below 6 % at 1800 K, 25 % at 2000 K. Rre is almost unaffected by the coflow temperature and affected by the oxygen volume fraction, reaching 30 % at 5 % O2.

Numerical investigation on NOx formation of staged oxy-fuel combustion in a 35 MW large pilot boiler

In this study, high-fidelity simulations are conducted on a 35 MW pulverized coal oxy-fuel boiler to investigate NOx formation characteristics under staged oxy-fuel combustion, including both fuel-staged and oxygen-fuel two-way staged modes. Detailed volatile components are considered by using the chemical percolation devolatilization model. A skeletal reaction mechanisms consisting of 35 species developed for fuel-NOx formation under oxy-fuel conditions are employed for the gas phase combustion and NOx modeling. The radiative property models are also optimized for oxy-fuel combustion. The results show that in the fuel-staged mode, a reduction atmosphere is established in the reburning zone to reduce the NOx concentration in the flue gas. A reduction of 5.6% in the average NO concentration at the furnace outlet is observed relative to the baseline oxy-fuel case. Moreover, the application of oxygen-fuel two-way staged combustion leads to a further reduction in NOx formation. The average NO concentration at the furnace outlet is decreased by 17% compared to the baseline oxy-fuel case. Pathway analysis indicates a reduction in the NO formation pathway involving HCN → NCO → NO, while the NO reduction pathway of NO → HCN is enhanced.

Comparison of the flame stabilities during oxy-methane and air-methane combustion in a two-layer porous burner

The development of the oxy-fuel porous media combustion technique not only takes advantage of the characteristics of porous media combustion, such as a relatively low lean-burn limit and small volume, but also dramatically facilitates carbon capture and storage. In this paper, a two-layer porous burner model is established, in which a two-temperature equation model is adopted for calculation purposes. Differences in combustion behaviour between oxy-fuel and air-fuel conditions are compared. The results show that the difference in physical properties between CO2 and air is the main reason for the significant variation of combustion behaviours under the conditions of oxy-fuel compared to air-fuel. The specific performance is lower combustion temperature and flame propagation speed. The temperature under oxy-fuel combustion conditions is 300 K lower than that under air-fuel combustion conditions at an equivalence ratio of 0.6, and the stable range of oxy-fuel combustion is only approximately 50% of air-fuel combustion. The impact of porous media material parameters on combustion behavior has been investigated. These variables include the equivalence ratio, material thermal conductivity, volume heat transfer coefficient, and extinction coefficient, while the ratio of O2/CO2 remains fixed at 0.21/0.79. The influence of these variables on the stable velocity range and temperature field is consistent, but a large difference in values occurs. Reducing the thermal resistance of the burner by adjusting the properties of the porous matrix can increase the velocity limit of stable combustion. However, even if the thermal conductivity and extinction coefficient are increased by 5 times, there will be no significant impact on the stable range. Reducing the pore size of the combustion zone by 50% will increase the stable range by over 100%.

Air and oxy-fuel combustion characteristics of coal gangue and weathered coal blends

Oxy-fuel combustion technology is suggested as one of the promising technologies for capturing CO2. The combustion characteristics of blends of a coal gangue and a weathered coal under air (21% O2/79% N2), oxy-fuel (20% O2/80% CO2) and oxygen-enriched oxy-fuel (40% O2/60% CO2) conditions were studied using a thermogravimetric analysis (TGA) technique operating at heating rate 10 oCmin−1 to a final temperature 900 °C, focussing on the effect of blending ratio and atmosphere. While the differential thermogravimetric (DTG) profiles of coal gangue in the three different atmospheres were very similar, the weathered coal showed a sharp peak in the temperature range 400–440 °C under oxygen-enriched oxy-fuel condition, which was different from DTG curves under air and oxy-fuel conditions. The blending of the coal gangue into the weathered coal gradually diminished this peak. The negligible deviations between the experimental DTG curves and theoretical DTG curves indicated that interactions were absent for all the coal gangue/weathered coal mixtures under air and oxy-fuel conditions. The obvious interactions between coal gangue and weathered coal were observed under oxygen-enriched oxy-fuel condition and the interaction may be related to thermal effect. The interactions increased as the increasing weathered coal ratio in blends and increasing oxygen concentration under oxy-fuel conditions.

Release of Sulfur and Chlorine Gas Species during Combustion and Pyrolysis of Walnut Shells in an Entrained Flow Reactor

The release behavior of sulfur and chlorine compounds into the gas phase of walnut shell particles (WNS) is studied with an entrained flow reactor. Experiments are carried out in nitrogen (N2), carbon dioxide (CO2) atmosphere and under air and oxy-fuel conditions at different temperatures (T = 1000–1300 °C) and stoichiometries (λ = 0.8–1.1). A total of 98.7% of fuel-bound sulfur volatilizes as sulfur dioxide (SO2), carbonyl sulfide (COS) and hydrogen sulfide (H2S) in the gas phase in N2 atmosphere at 1000 °C. As hydrogen chloride (HCl), 37.0% of the chlorine is released at this temperature. In CO2 atmosphere, a similar total release of sulfur and chlorine is observed (1000 °C). With each temperature increment, the release of SO2, H2S and HCl in the gas phase decreases (N2 and CO2 atmosphere). SO2 forms the major sulfur component in both atmospheres. In CO2 atmosphere, higher concentrations of COS were detected than in N2 atmosphere. Air and oxy-fuel combustion conditions show significantly lower SO2, COS and HCl concentrations as in N2 and CO2 atmosphere. No H2S is detected in the gas phase during any of the combustion trials.

Experimental study on NOx emission characteristics under oxy-fuel combustion

Abstract This study focuses on the emission characteristics of NOx under oxy-fuel combustion conditions. A comparative analysis with air combustion was performed. NOx emission, control measures and influence factors under different working conditions were studied. Experiments were carried out on a 3-MWth test platform and a laboratory platform. The ‘π’-type furnace was adopted, with the furnace width of 2.6 m, depth of 2.0 m and height of 10.5 m for the 3-MWth coal-fired boiler. NOx emissions at different oxygen concentrations and different air distribution were investigated; the effects of H2O and CO2 concentration on denitrification efficiency and SO2/SO3 conversion rate were explored. Experiment results suggest that, compared with air combustion, NO concentration (volume basis) at the furnace outlet under oxy-fuel combustion is higher than that of air combustion, but the amount of NOx emissions in the discharged gas significantly decrease compared to the air combustion conditions. In addition, the formation of NOx can be effectively controlled through staged combustion. Furthermore, the selective catalytic reduction (SCR) denitrification efficiency and the conversion rate of SO2 to SO3 decreases when the CO2 concentration and the H2O content increase, indicating that CO2 and H2O have an adverse effect on the performance of the catalyst. Additionally, compared with CO2 concentration, H2O content has a greater effect on catalyst performance.