Oxy-fuel Atmosphere Research Articles

Deeper insights into the devolatilization mechanism of biomass fixed-bed gasification under various atmospheres

In recent years, biomass oxy-fuel gasification has prompted increased interest for its ability to produce a gas with low tar content and to assist in carbon reduction. It is noteworthy that the type of atmosphere is a crucial factor influencing the devolatilization process in biomass gasification and the specific mechanisms by which an oxy-fuel atmosphere influences the devolatilization process remain inadequately explored. This work aimed to investigate the effect of various reaction atmospheres on devolatilization behavior, reaction kinetics, intermediate product formation, and biochar functional groups (BFGs) evolution. In comparison to an inert atmosphere, the oxy-fuel atmosphere facilitated the release of volatiles, and enhanced energy self-sustainability within the devolatilization system. The presence of O2 and CO2 promoted the generation of potential anhydrosugars while suppressing the formation of phenolic compounds. The synergistic effect present in the oxy-fuel atmosphere augments the exothermic effect and promotes the release of volatile and alkane gases during the devolatilization process. Furthermore, as oxygen concentrations increased in the reaction atmosphere, the sequential response of BFGs primarily followed the order: C-O → aromatic ring → C-OH/C–H/COO– → C=O. Moreover, advanced characterization analyses revealed that the oxy-fuel environment was more effective in promoting the aromatization and pore structure of biochar than an inert atmosphere. Collectively, these findings have substantial implications for reaction modeling in biomass fixed-bed gasification under real gas atmospheres and guide future research on tar control strategies.

Oxy-fuel co-combustion properties and N-containing pollutants release characteristics of biomass/coal blends

Thermal properties and NO release characteristics during oxy-fuel co-combustion of pine sawdust (PS) and Shenmu coal (SM) were investigated based on the effects of various parameters, including atmosphere, temperature, blending ratio, and oxygen concentration in this research. The results indicate that the release of NO decreased with the increasing temperature during decomposition process of PS and SM. The content of N-Q and N-X in semicoke (SC) increased at higher torrefaction temperature. The N-Q and N-X content for SC at the temperature of 1000 °C was 36.9 % and 23.2 %, respectively. When 40 % PS was added to the SM, the NO release amount was significantly reduced by 16.3–23.1 % in the oxy-fuel atmosphere compared to the O2/N2 atmosphere, and the ignition time was reduced from 4.0s to 0.66s. The NO emission initially increased and then decreased as the combustion temperature increased from 800 to 1000 °C. Furthermore, increasing the oxygen concentration from 10 % to 40 % shortens the combustion time and increases the emissions of NO. Conversely, increasing PS blending ratios from 10 % to 40 % decreased NO emission and the conversion ratio. These findings emphasize that adding biomass can effectively improve coal ignition, increase combustion rates, reduce NO emissions, and address the air pollution problems associated with NOx emissions.

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.

Radiative Heat Flux Measurement in a Semi-Industrial Oxyfuel Combustion Chamber with Biomass and Coal

Oxyfuel is a combustion technology where the oxidant consists mainly of oxygen and carbon dioxide instead of oxygen and nitrogen. Since carbon dioxide has strongly absorbing bands in the thermal spectrum, the radiation properties of the flame change in an oxyfuel atmosphere compared to conventional combustion. When retrofitting an existing air-fired combustion system to an oxyfuel process, the oxygen content in the oxidant must be adjusted so that similar values for heat transfer by radiation are achieved. This measure allows the system to be operated with otherwise unchanged parameters. In this work, the thermal radiation of natural gas, pulverised walnut shells and lignite under an air and oxyfuel atmosphere is investigated in a semi-industrial combustion chamber with water-cooled membrane walls, at different oxygen concentrations and combustion parameters. While the radiative heat fluxes for natural gas with an oxygen content of 28 vol% in the oxidant are significantly higher than those for firing with air, the values for lignite are still below the air-firing, even with an oxygen content of 30 vol%. For walnut shells, the oxyfuel results are close to the air case for all oxygen concentrations between 27 and 33 vol%. The walnut shells show higher radiative emissions than the lignite at the same thermal output. For non-swirled flames, the radiative heat flux is lower than for swirled flames.

Comprehensive experimental investigations on Co-combustion dynamics and multi-pollutant formation characteristics of coal gangue and biomass in oxy-fuel atmosphere

Based on the co-combustion principle of coal gangue and biomass, the combustion characteristics, kinetic mechanisms, and pollutant emission characteristics of mixed coal gangue and wheat straw in oxy-fuel atmosphere were investigated by conducting experiments with a thermogravimetric analyzer and a tube furnace in this paper. The results indicated that the comprehensive combustion characteristics, reaction rate and burnout rate of the samples can be improved with the increase of O2 concentration in oxy-fuel atmosphere. When O2 concentration is 50 %, the emission peaks of pollutants SO2 and NOx are the highest. The increase of biomass content can accelerate the combustion reaction rate, increase the total weight loss rate, enhance the comprehensive combustion characteristics of the samples significantly, and reduce the release of SO2. A faster heating rate corresponds to higher ignition temperature, burnout temperature, and the maximum weight loss rate temperature, leading to the improvement of comprehensive combustion characteristics and flammability. The combustion temperature has a significant effect on pollutant release, and the release of CO and SO2 is relatively low under the condition of 900∼1000 °C. Therefore, the use of oxy-fuel co-combustion technology holds the potential to significantly improve the combustion performance of coal gangue, reduce pollutant emissions, and provide a theoretical guidance for the resource utilization of coal gangue and biomass.

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.

Chemical reactor network modeling in the context of solid fuel combustion under oxy-fuel atmospheres

A multi-phase chemical reactor network (CRN) is developed and applied to investigate solid fuel combustion under oxy-fuel atmospheres in a laboratory-scale combustion chamber. The development of a novel solid-gas plug flow reactor (sPFR) allows the full coupling of solid and gas phase processes to increase the fidelity in CRN modeling. Together with the recently developed solid-gas perfectly stirred reactor (sPSR), the new sPFR is applied within a multi-phase reactor network to model the complex features of the investigated application. To design an initial reactor network, a hybrid experimental and computational fluid dynamics (CFD)-driven approach is being pursued, taking into account both global and local information on flow and temperature fields. Initially, an in-depth investigation of the physical processes is conducted, covering solid fuel conversion processes such as devolatilization and char conversion, along with the thermochemical characteristics of the flue gas. In particular, the prediction of carbon monoxide (CO) emissions is analyzed in detail by means of sensitivity analyses. Based on the findings of the sensitivity study, the network complexity of the initial reactor network is increased to enable an accurate prediction of the CO formation in comparison to the experiments. Finally, the formation of nitric oxides (NOx), sulfur oxides (SOx), and aromatic pollutants is discussed, highlighting the importance of detailed chemistry in solid fuel combustion at technically relevant scales.

Microplastics as emerging contaminants in textile dyeing sludge: Their impacts on co-combustion/pyrolysis products, residual metals, and temperature dependency of emissions

As emerging contaminants in textile dyeing sludge (TDS), the presence and types of microplastics (MPs) inevitably influence the combustion and pyrolysis of TDS. Their effects on the co-combustion/pyrolysis emissions and residual metals of TDS remain poorly understood. This study aimed to quantify the impacts of polyethylene (PE) and polypropylene (PP) on the transports and transformations of gaseous emissions and residual metals generated during the TDS combustion and pyrolysis in the air, oxy-fuel, and nitrogen atmospheres. Thermal degradation of the MPs in TDS occurred between 242–600 °C. MPs decomposed and interacted with the organic components of TDS to the extent that they increased the release of VOCs, dominated by oxygenated VOCs and hydrocarbons under the incineration and pyrolysis conditions, respectively. The presence of PE exerted a limited impact on the concentration and chemical form of metals, while PP reduced the residual amount of most metals due to the decomposition of mineral additives. Also, PP (with CaCO3 filler) reduced the acid-extractable content of cadmium, copper, and manganese in the bottom slag or coke but increased that of chromium. This study provides actionable insights into optimizing gas emissions, energy recovery, and ash reuse, thus reinforcing the pollution control strategies for both the MPs and TDS.

Analysis of the Impact of Rdf Combustion on Cement Clinker Quality in a Superstoichio-Metric Oxy-Fuel Atmosphere by Cfd Simulations

ABSTRACT Superstoichiometric oxygen combustion (i.e. the oxygen content during combustion is larger than required for the complete conversion of the fuel), also called pure-oxy-fuel, is a novel concept to reduce CO2 emissions from the cement clinker production. In the current contribution, we examine in a simulation study the influence of superstoichiometric combustion in combination with the use of refuse-derived fuel (RDF) of different composition and particle size on process parameters and cement clinker quality in the rotary kiln. A comparison with a standard fuel, namely lignite, and a conventional air-fired kiln is presented. The specific models developed to determine the flight and combustion behavior of RDF particles, which are of complex shape and significantly larger than pulverized coal, are described. The simulations conducted show that some RDF fractions react only partially in the gas phase due to their relative large size. Unburnt RDF particles reach the clinker bed, where they continue to react. In addition, for RDF, gas-phase temperature and the clinker temperature are different along the kiln length compared to coal, influencing the local mineralogical reaction in the clinker bed. However, in summary, similar clinker qualities could be obtained also compared to a standard air-fired kiln.

Ash fouling behavior during the combustion of bituminous coal and high-Ca pyrolytic biochar under air and oxyfuel atmosphere

Co-combustion of coal and pyrolytic biochar under oxy-fuel atmosphere is a promising option for addressing CO2 problems in power plants, where the issue of ash fouling during combustion was considered in this study. A high-Ca pyrolytic biochar from pilot-scale trail and a high-Si Utah bituminous coal were selected for single- and co-combustion on a drop tube furnace (DTF) coupled with fouling sampling system under the atmosphere of air and oxy50 (CO2:O2 = 50:50 vol%) at 1300 ℃. The fouling deposits were collected and characterized by various techniques to investigate the formation mechanism of ash deposits. The results indicated that the combined effects of impaction and capture resulted in the different deposition propensity from coal and biochar combustion under both air and oxy50 conditions. Besides, co-firing can inhibit the formation of ash deposits during coal combustion under air atmosphere when the blended biochar was greater than 50%, while the deposition propensity of the blend with 80% biochar rapidly aggravated under oxy50 atmosphere. The fine particles act as “glue” to fix coarse particles from impaction were the main source of the inside deposits from coal combustion, whereas the inside deposits from biochar combustion were dominated by agglomerated and sintered fine particles. Co-firing can weaken the “glue effect” from coal combustion, and finally presented as mitigated deposition propensity of inside deposits. Switching to oxy50 condition, the enhanced sulfidation of deposits and scavenging of alkali metal suppressed the growth of inside deposits from coal combustion, while that had minor impact on biochar. The inhibitory effect of co-firing on inside deposits diminished with the increased biochar under oxy50, where the deposition propensity of the blend containing 80% biochar was higher than that in air-firing.

Co-combustion of pulverized coal and walnut shells in air and oxy-fuel atmospheres: Thermal behavior, synergistic effect and kinetics

In this work, the combustion characteristics of pulverized coal, walnut shells, and their blends in the oxy-fuel atmosphere (21% O2/79% CO2) and air atmosphere (21% O2/79% N2) were investigated by the thermogravimetric analyzer (TGA). The interaction between the blends was quantified by the overlap ratio (OR) and the difference between theoretical and experimental weight loss (ΔW). The results showed that the combustion characteristics of walnut shells were better than coal attributed to its higher volatile components, whereas co-combustion of walnut shells with coal could improve the reactivity of coal. The replacement of the air atmosphere with the oxy-fuel atmosphere had a negligible effect on the ignition temperature of the selected materials but generated an increase in the burnout temperature to some extent. The activation energy was calculated by using Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods, and the order of reaction and pre-exponential factor of the selected samples was determined by the master plots method. The results of this study confirmed the feasibility of the co-combustion of walnut shells with coal under the oxy-fuel atmosphere and provided certain theoretical guidance to realize the operation of existing boilers in the oxy-fuel atmosphere.

Process Simulations of High-Purity and Renewable Clean H2 Production by Sorption Enhanced Steam Reforming of Biogas

Renewable clean H2 has a very promising potential for the decarbonization of energy systems. Sorption enhanced steam reforming (SESR) is a novel process that combines the steam reforming reaction and the simultaneous CO2 removal by a solid sorbent, such as CaO, which significantly enhances hydrogen generation, enabling high-purity H2 production. The CO2 sorption reaction (carbonation) is exothermic, but the sorbent regeneration by calcination is highly endothermic, which requires extra energy. Biogas is one of the available carbon-neutral renewable H2 production sources. It can be especially relevant for the energy integration of the SESR process since, due to the exothermic sorption reaction, the CO2 contained in the biogas provides extra heat to the system, which can help to balance the energy requirements of the process. This work studies different process configurations for the energy integration of the SESR process of biogas for high-purity renewable H2 production: (1) SESR with sorbent regeneration using a portion of the produced H2 (SESR+REG_H2), (2) SESR with sorbent regeneration using biogas (SESR+REG_BG), and (3) SESR with sorbent regeneration using biogas and adding a pressure swing adsorption (PSA) unit for hydrogen purification (SESR+REG_BG+PSA). When using biogas as fuel (Cases 2 and 3), these configurations were studied using air and oxy-fuel combustion atmospheres in the sorbent regeneration step, resulting in five case studies. A thermodynamic approach for process modeling can provide the optimal process operating conditions and configurations that maximize the energy efficiency of the process, which are the basis for subsequent optimization of the process at the practical level needed to scale up this technology. For this purpose, process simulations were performed using a steady-state plant model developed in Aspen Plus, incorporating a complex heat exchanger network (HEN) to optimize heat integration. A comprehensive parametric study assessed the effects of biogas composition, temperature, pressure, and steam to methane (S/CH4) ratio on the process performance represented by the selected key performance indicators, i.e., H2 purity, H2 yield, CH4 conversion, cold gas efficiency (CGE), net efficiency (NE), fuel consumption for the sorbent regeneration step, and CO2 capture efficiency. H2 with a purity of 98.5 vol % and a CGE of 75.7% with zero carbon emissions can be achieved. When adding a PSA unit, nearly 100% H2 purity and CO2 capture efficiency were achieved with a CGE of 77.3%. The use of oxy-fuel combustion during regeneration lowered the net efficiency of the process by 2.3% points (since it requires an air separation unit) but allowed the process to achieve negative carbon emissions.

Oxyfuel Cofiring Characteristics of Biomass with Ultralow Volatile Carbon-Based Fuels

Biomass from agricultural production is a renewable energy source with a high-volatile content. Semicoke (SC) and gasification residual carbon, which are ultralow volatile carbon-based fuels (LVFs), are by-products of the coal chemical industry, which are over capacity and urgently need to be cleanly and effectively consumed. Biomass and LVFs exhibit complementary fuel characteristics, particularly the volatile content, which can potentially be coprocessed efficiently. Furthermore, oxyfuel combustion technology can not only realize carbon dioxide (CO2) capture and benefit from carbon neutrality, but also effectively reduce NOx emissions. The cocombustion of biomass and ultralow volatile fuel under oxyfuel conditions can both use individual benefits and effectively control pollutant emissions. However, the cofiring characteristics and interaction mechanisms of LVF with biomass in an oxyfuel atmosphere are yet to be fully understood. In this study, three ultralow volatile fuels and one typical biomass were selected to explore the oxyfuel cocombustion characteristics via thermogravimetric experiments. The experimental results indicated that blending with wheat straw (WS) improved the overall combustion performance, the ignition point was close to that of WS, and the burnout temperature was close to the average value of the two individual fuels. An increase in the oxygen content can effectively enhance the combustion feature of ultralow volatile fuels while it has little effect on the burnout performance of biomass. In the presence of high-content CO2, the decomposition of carbonate in ultralow volatile fuel is impeded and the decomposition temperature is slightly increased. The present investigation can promote the efficient co-utilization of inferior LVFs and biomass, which also benefits CO2 capture, carbon neutrality, and even negative carbon emissions.

Research on Co-Combustion Behaviors of Binary and Ternary Blends of Coal, Walnut Shell, and Biochar by TGA

In this paper, the co-combustion behavior of the walnut shell, biochar, and coal, and the respective binary and ternary blends were investigated by thermogravimetric analysis (TGA) in the oxy-fuel atmosphere (21% O2/79% CO2). The combustion reactivity of coal was similar to biochar but lower than that of the walnut shell. The presence of the walnut shell improved the combustion performance of coal and biochar. The interaction between only biochar and coal was inhibited in the binary blends. The presence of the walnut shell in the ternary blend covered up the inhibition between biochar and coal. The average activation energy of each sample was obtained using the Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) kinetic models. The results of this study provide a reference for the rational utilization of biomass and biochar and the practical improvement of the thermal conversion efficiency of coal.

Phase-locked absorption tomography for retrieving 5 kHz time-resolved tracer profiles in solid fuel combustion

Turbulent mixing processes and species transport govern many important properties like stability and pollutant formation of reactive systems. We propose spectrally resolved absorption tomography as a viable tool to quantitatively assess species transport by imaging tracer pulses at high time resolution. Measurements on a gas-assisted solid-fuel combustor operating in air and oxy-fuel atmospheres are presented. The measurements are conducted using a scanning tomographic imaging system that has previously been validated for gas temperature measurements. As the scanning system itself is not able to achieve the necessary time resolution, a phase-locked tomographic scheme is applied, i.e. each measurement beam is acquired separately for several tracer pulses and phase-averaged. Hence, the full 5 kHz time resolution of the underlying tunable diode laser spectroscopy system can be utilized, granting insight into species transport in the recirculation zones as well as the flame brush. Despite a possible influence of the tracer injections on the inner recirculation zone, it is shown that qualitative differences between methane combustion and gas-assisted solid-fuel operating conditions can be detected, and influences of devolatilization as well as volatile combustion become visible.

Interactions of potassium vapor with reactor tubes made of different materials and their impacts on particulate matter emission during pulverized biomass combustion

During the combustion of biomass in drop-tube furnace (DTF) systems, the released alkali metal (e.g., potassium, K) inevitably reacts with reactor tube at high temperatures, affecting the experimental results on the emission of particulate matter with aerodynamic diameters of <10 μm (PM10). This study reports the interactions between K vapor and tube reactors made of silicon carbide, corundum, and mullite and their impacts on PM10 emission. Demineralized wood samples loaded with potassium chloride (KCl) or ion-exchanged K respectively were combusted in a DTF at 1300 °C under air or oxy-fuel atmosphere. Another series of experiments was conducted to collect and analyze the PM10 from the combustion of KCl-loaded wood, K-exchanged wood, and two typical biomass samples (cotton stalk and wheat straw) in the three reactor tubes under air atmosphere. Experimental results show that 4.1‒72.5% of K is retained in the three tubes when burning the KCl-loaded wood in air, and the combustion in oxy-fuel atmosphere slightly increases the K retention. For K-exchanged wood combustion in air, only 3.7‒23.6% of K is released from the reactor tubes. In all conditions, the reactivity of the reactor tubes with K vapor follows a sequence of mullite > corundum > silicon carbide. The retained K is unstable, 49.0‒64.8% of which can be re-released during polyvinyl chloride combustion. In addition, the results demonstrate that, compared with silicon carbide tube, the use of corundum and mullite tubes leads to a 16.2‒54.3% decrease in PM1 yields and a significant drop in fine mode peaks in PM10 during the combustion of biomass samples in air, while the PM1–10 yields and the coarse mode peaks remain largely unchanged. These are attributed to the enhanced retentions of alkali metals in corundum and mullite tubes, which reduce the yields of Na, K, and Cl in PM10, but has negligible effect on those of refractory elements such as Mg and Ca.

Particle temperature and composition measurements in the ignition phase of single coal particles and particle groups under conventional and oxy-fuel atmospheres

The transition from single to group particle combustion is experimentally investigated in a laminar flow reactor. Optical techniques including two-colour pyrometry and a sampling probe provide data on surface temperature and composition of high-volatile bituminous coal particles during ignition, volatile combustion, and char burnout. Adjustable particle number densities (PND) permit studying the transition from single to group particle combustion. An optical imaging pyrometer provides simultaneously acquired data on the soot particles during the volatile combustion and the particle temperature, particle size and position in the 3D particle streak. These parameters are investigated as a function of the PND, which is used to differentiate between single and group particle combustion. Additionally, particle sampling provides particle composition data, which together with the particle temperatures extends a previously published data set from the same reactor. The results show a direct relation between the particle temperature during char burnout and the PND. As particles in group combustion absorb a significant amount of thermal energy from the surrounding atmosphere and reduce the oxygen content in the group, the measurements prove a lower temperature for particles that burn in group compared to single particle. Furthermore, sampling at several residence times indicates a notable difference in the burnout process between single particle (SP) and group particle (GP) combustion. While the effect of different atmospheres impacts the results of SP combustion, GP combustion is significantly less affected. The results of the current study are jointly discussed with previously acquired data of the same burner for a thorough interpretation.

Flamelet LES of a 40 kW[formula omitted] pulverized torrefied biomass furnace in air and oxy-fuel atmospheres

For the first time, large-eddy simulations are conducted for a 40 kWth self-sustained pulverized torrefied biomass furnace in air and oxy-fuel atmospheres using an extended flamelet model. A recently developed chemical reaction mechanism specifically for oxy-fuel combustion (117 species and 840 elementary reactions) is employed. The simulation results are compared to the experimental data, including the axial and tangential velocities and selected species mass fractions. In addition to this, certain characteristics are investigated: the flame structure, thermo-chemical variable distributions and the NOx emissions in different atmospheres. The flame in the oxy-fuel atmosphere is predicted to be longer and broader than that in the air atmosphere. The main reason for this is the significantly different flow dynamics in different atmospheres that are specifically designed to achieve the same oxidizer-to-fuel ratio near the burner. Two external recirculation zones (ERZs) and a central internal recirculation zone (IRZ) exist in the air atmosphere due to the designed boundary condition with a strongly swirled flow. In contrast, only a single ERZ and a weak IRZ can be observed in the oxy-fuel atmosphere. These different flow dynamics significantly influence the torrefied biomass combustion characteristics for the two atmospheres. Overall, the combustion characteristics in the air atmosphere exhibit features which are expected for a strongly swirling flow, while the thermo-chemical variable distributions in the oxy-fuel atmosphere are similar to those of a jet flame. It is also found that the atmospheres have significant effects on the NOx emissions, particularly for the furnace studied, which features staged oxidizer combustion.

Bluff-body MILD combustion regime for semicoke and bituminous coal mixtures with effect of oxidizer O2 concentration in O2/N2 and oxy-fuel (O2/CO2) atmospheres

The present study numerically investigates the effects of the O2 content (XO2) in the oxidizer on the MILD combustion behaviours of large-proportion semicoke mixtures, such as those involving temperature, MILD regime recognition, char surface consumption, homo/heterogeneous Damkhöler number, and NOx emission, under O2/N2 and O2/CO2 atmospheres. The results indicate that by increasing XO2 from 15 % to 30 %, the peak temperature increases significantly due to the increased char consumption rate, the high temperature region shrinks, and the axial length of the MILD regime region slightly increases from ∼1 to ∼1.4 m because of enhanced flue gas recirculation under O2/N2 or O2/CO2 atmosphere. The reduction effects of the char-O2 reaction proportions and the promoted effects of the char-CO2 reaction proportions due to the increased XO2 are both obviously weakened when N2 is substituted for CO2. A high O2 content and an O2/N2 atmosphere reduce the char burnout time. With increasing XO2, the maximum value of the turbulent Damkhöler number Dat increases from 6.79 to 10.16 under the O2/N2 atmosphere and from 3.99 to 10.6 under the O2/CO2 atmosphere, implying that a low O2 dilution and a CO2 environment promote the establishment of the homo-MILD regime. The maximum values of the surface Damkhöler number (Das-CO2 and Das-H2O) of the two gasification reactions (Char-CO2 and Char-H2O) have the same order of magnitude and are much lower than those of the surface Damkhöler number (Das-O2) of the Char-O2 oxidation reaction, i.e., Das-H2O < Das-CO2 ≪ Das-O2 < 1. The maximum values of the three surface Damkhöler numbers all increase with increasing XO2, while they decrease with the replacement of N2 by CO2, indicating that a high O2 content and an O2/N2 atmosphere could facilitate the establishment of an ideal hetero-MILD combustion regime to some extent. Low NOx emissions can be realized under moderate O2 and CO2 dilution conditions by regulating the oxy-MILD combustion regime of semicoke blends.