Ash formation and deposition during combustion of pulverized torrefied wood and coal in a 100 kW downflow combustor

Torrefied wood originating from beetle-killed trees is an abundant biomass fuel that can be co-fired with coal for power generation. In this work, pulverized torrefied wood, a bituminous coal (Sufco coal) and their blended fuel with a mixing ratio of 50/50 wt.%, are burned in a 100-kW rated laboratory combustor under similar conditions. Ash aerosols in the flue gas and ash deposits on a temperature-controlled surface are sampled during combustion of the three fuels. Results show that ash formation and deposition for wood combustion are notably different from those for coal combustion, revealing different mechanisms. Compared to the coal, the low-ash torrefied wood produces low concentrations of fly ash in the flue gas but significantly increased yields (per input ash) of ash that has been vaporized. All the mineral elements including the semi- or non-volatile metals in the wood are found to be more readily partitioned into the PM₁₀ ash than those in the coal. The inside layer deposits sticking to the surface and the loosely bound outside deposits exposed to the gas both show a linear growth in weight during torrefied wood test. Unlike coal combustion, in which the concentration of (vaporized) ash PM₁ controls the inside deposition rate, wood combustion shows that the formation of porous bulky deposits by the condensed residual ash dominates the inside deposition process. Co-firing removes these differences between the wood and coal, making the blended fuel to have more similar fly ash characteristics and ash deposition behavior to those of the bituminous coal. In addition, results also show some beneficial effects of co-firing coal with torrefied wood, including reduction of the total deposition rate and the minimization of corrosive alkali species produced by wood.     

Realization of oxyfuel combustion for near zero emission power generation

Oxyfuel combustion is one of the promising carbon capture and storage (CCS) technologies for coal-fired boilers. In oxyfuel combustion, combustion gas is oxygen and recirculating flue gas (FGR) and main component of combustion gas is O₂, CO₂ and H₂O rather than O₂, N₂ in air combustion. Fundamental researches showed that flame temperature and flame propagation velocity of pulverized cloud in oxyfuel combustion are lower than that in air with the same O₂ concentration due to higher heat capacity of CO₂. IHI pilot combustion test showed that stable burner combustion was obtained over 30% O₂ in secondary combustion gas and the same furnace heat transfer as that of air firing at 27% O₂ in overall combustion gas. Compared to emissions in air combustion, NOx emission per unit combustion energy decreased to 1/3 due to reducing NOx in the FGR, and SO_x emission was 30% lower. However SO_x concentration in the furnace for the oxyfuel mode was three to four times greater than for the air mode due to lower flow rate of exhaust gas. The higher SO₃ concentration results that the sulphuric acid dew point increases 15–20 °C compared to the air combustion. These results confirmed the oxyfuel pulverized coal combustion is reliable and promising technology for coal firing power plant for CCS.In 2008, based on R&D and a feasibility study of commercial plants, the Callide Oxyfuel Project was started in order to demonstrate entire oxyfuel CCS power plant system for the first time in the world. The general scope and progress of the project are introduced here. Finally, challenges for present and next generation oxyfuel combustion power plant technologies are addressed.     

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NO_x emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NO_x emissions by reducing the production of char-NO_x and fuel(NH₃)-NO_x simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NO_x emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NO_x emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH₃/air and the prominent denitration effect of NH₃ in the vicinity of the NH₃ downstream injection location. In addition, NO_x emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NO_x emissions is closely associated with the denitration efficiency of the NH₃. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO₂ reduction without extensive retrofitting works. And the NO_x emission can be reduced by producing a locally NH₃ flame zone with a high equivalence ratio as well as ensuring adequate residence time.     

The roles of added chlorine and sulfur on ash deposition mechanisms during solid fuel combustion

The focus of this paper is on effects of chlorine and sulfur on coal ash deposition rates, under practically relevant but systematically controlled combustion conditions. This problem is important, not so much for coal, but to understand and predict deposition rates for biomass combustion where chlorine contents can be high. To this end, ash deposition rates on a controlled temperature surface were measured for controlled amounts of chlorine and sulfur added to a pulverized coal, doped with potassium and burned in a 100 kW rated combustion rig. Previous work with 35 tests on 11 coal, biomass and petroleum coke fuels burned under a range of operating conditions had strongly suggested that the deposition rate of the tightly bound inside deposits was independent of the ash aerosol composition, and depended only on PM₁ in the flue gas. The loosely bound outside deposition rate was dependent primarily on the total alkali content in the flue gas. The new results using chlorine added to the fuel (in the form of ammonium chloride) required these previous conclusions to be drastically revised. They showed that chlorine, not alkali alone, had large effects on the deposition rate of the inside deposits, which now were orders of magnitude higher than without chlorine addition, and did not fit previous (multi-fuel) correlations with PM₁. Sulfur addition, together with chlorine, did not affect deposition rates much, although it did lower the chlorine content of the deposit. These results are interpreted in terms of the ash aerosol size segregated composition, which was also measured, and potential sulfation reactions within the deposit.     

Experimental and modeling study of H2S formation and evolution in air staged combustion of pulverized coal

High-concentration H₂S formed in the reduction zone of pulverized coal air-staged combustion can result into the high temperature corrosion of water wall tube of boiler, so it is of great importance to accurately predict H₂S concentration for the safe operation of boilers and burners. H₂S formation and evolution depends on two steps: the sulfur release from coal conversion and gas-phase reactions of sulfur species. In this study, the sulfur release characteristics from the pyrolysis of 17 coals, including 5 lignite, 9 bituminous coals and 3 anthracites, are investigated in a drop tube furnace (DTF). Sulfur release model is developed to describe the relationship between sulfur release and coal types. A global gas-phase reaction mechanism of sulfur species composed of ten reactions is used to calculate and predict the formation and evolution of H₂S, COS and SO₂ in the reduction zone of pulverized coal air-staged combustion. A wide range of air-staged combustion experiments of 17 coals are conducted in the DTF at different temperatures and stoichiometric ratios to validate the developed model. The results show that the prediction errors of sulfur species, including SO₂, H₂S and COS, are within ± 30%, which indicates that the developed prediction model of sulfur species is of great assistance for CFD modeling of actual engineering application.     

Interactions of NiO particles with polycyclic aromatic hydrocarbons and acetylene generated from the pyrolysis of a model fuel

Experiments have been conducted to study the effects of NiO, a prevalent form of nickel in combustion-generated ash particles, on polycyclic aromatic hydrocarbons (PAH) and other hydrocarbons in a fuel product mixture. The fuel product mixture is generated from the gas-phase pyrolysis, in N₂, of the model fuel catechol in a quartz tube reactor at 1100 K and 5 s, a condition that ensures full conversion of the catechol and produces a mixture—of PAH, hydrocarbon gases, and oxygen-containing species—that is representative of the products of practical liquid and solid fuels in pyrolysis and fuel-rich combustion environments. Once formed, the product mixture is passed, at the same temperature, through a bed of ultrafine NiO particles held in place by quartz wool, in the contact zone of the reactor. The products exiting the reactor are quenched, collected, and analyzed by non-dispersive infrared analyzers, gas chromatography, and high-pressure liquid chromatography with diode-array ultraviolet–visible absorbance detection. The results from the experiments at 1100 K show that—compared to the case of no inorganics in the contact zone—when NiO is present: PAH yields are reduced 86% (from 10.8% to 1.48% fed carbon); all of the highly mutagenic 5- and 6-ring PAH are eliminated; and all of the acetylene, the highest-yield hydrocarbon product when NiO is absent, and other hydrocarbons with carbon–carbon triple bonds are eliminated from the gas phase. Most of the surface-bound carbon is released as CO. Similar experiments at 1275 K show that—except for the release of the surface-bound carbon as CO—the selective surface effects of NiO bring about similar results at higher temperature: 89% reduction in PAH yield, elimination of mutagenic 5- and 6-ring PAH, removal of acetylene and acetylenic species, as well as a decrease in the production of solid carbon (not formed at 1100 K).     

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O₂ in both N₂ and CO₂ diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N₂ versus CO₂ diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO₂ diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO₂ on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O₂ on coal particle ignition results from its effect on the local mixture reactivity. CO₂ decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO₂ mixtures, whereas higher O₂ concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.     

The progressive formation of submicron particulate matter in a quasi one-dimensional pulverized coal combustor

In this work, we aim to investigate the formation mechanisms of submicron particulate matter (PM₁) by observing progressive changes of collected samples at different combustion stages. A 25 kW quasi one-dimensional down-fired pulverized coal combustor was used, where PM₁ was collected from the furnace centerline through the desired sampling ports by using a nitrogen-aspirated, water-cooling isokinetic sampling probe followed a 13-stage electric low pressure impactor. First, the mass concentration particle-size-distributions (PSD) of PM₁ sampled at coal flame zone clearly exhibit two distinct modes separated by a fraction of 0.173–0.267 μm, ultrafine mode and intermediate mode. However, the ultrafine peak around 63 nm greatly decreases and becomes flat as coal combustion further progresses along axial length. Then, the contributions of either organically bounded minerals or inherent minerals to these two modes at different stages are analyzed. Finally, the evolution of sulfur-concentration PSD reveals the effects of pyrite decomposition and the sulfation reaction on PM₁ formation in the combustion system.     

Fragmentation of pulverized coal in a laminar drop tube reactor: Experiments and model

Fragmentation during pulverized coal particles conversion shifts the particle size distribution of the fuel towards smaller particle sizes, affecting both conversion rates and heat release. After pyrolysis of a high volatiles Colombian coal in CO₂ atmosphere in a drop tube reactor at 1573?K, solid carbonaceous particles of different size, from 100?µm of the particle feed down to the nanometric size, have been observed. A fragmentation model has been used to predict the fate of Colombian coal particles under the experimental conditions of the drop tube experiment and predict the particle size distribution (PSD). Model and experimental results are in very good agreement and indicate that in the DTR experiment the coal underwent almost complete pyrolysis and that fragmentation generated a 36?wt% population of particles with size close to 30?µm. The close match between the PSDs obtained from experiments and from the fragmentation model is an important novelty. It demonstrates that fragmentation occurs not only under fluidized bed conditions but also under the conditions of pulverized coal combustion. Experimentalists are warned against the fact that the fine particulate sampled at the outlet of laminar flow reactors and boilers is not always composed of soot only. Char fragments can be misidentified as soot. The implementation of fragmentation submodels in pulverized fuel combustion and gasification codes is highly recommended.     

Char characteristics and particulate matter formation during Chinese bituminous coal combustion

The characteristics of char particles and their effects on the emission of particulate matter (PM) from the combustion of a Chinese bituminous coal were studied in a laboratory-scale drop tube furnace. The raw coal was pulverized and divided into three sizes, <63, 63–100, and 100–200 μm. These coal samples were subjected to pyrolysis in N₂ and combusted in 20 and 50% O₂ at 1373, 1523, and 1673 K, respectively. Char samples were obtained by glass fiber filters with a pore size of 0.3 μm, and combustion-derived PM was size-segregated by a low pressure impactor (LPI) into different sizes ranging from 10.0 to 0.3 μm. The characteristics of char particles, including particle size distribution, surface area, pore size distribution, swelling behavior and morphology property, were studied. The results show that, coal particle size and pyrolysis temperature have significant influence on the char characteristics. The swelling ratios of char samples increase with temperature increasing from 1373 to 1523 K, then decrease when the temperature further increases to 1623 K. At the same temperature, the swelling ratios of the three size fractions are markedly different. The finer the particle size, the higher the swelling ratio. The decrease of swelling ratio at high temperature is mainly attributed to the high heating rate, but char fragmentation at high temperature may also account for the decrease of swelling ratio. The supermicron particles (1–10 μm) are primarily spherical, and most of them have smooth surfaces. Decreasing coal particle size and increasing the oxygen concentration lead to more supermicron-sized PM formation. The influence of combustion temperature on supermicron-sized PM emission greatly depends on the oxygen concentration.     

Effect of CO2 gasification reaction on oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, as is commonly employed, it is recognized that elevated CO₂ levels affect radiant transport, the heat capacity of the gas, and other gas transport properties. A topic of widespread speculation has concerned the effect of the CO₂ gasification reaction with coal char on the char burning rate. To give clarity to the likely impact of this reaction on the oxy-fuel combustion of pulverized coal char, the Surface Kinetics in Porous Particles (SKIPPY) code was employed for a range of potential CO₂ reaction rates for a high-volatile bituminous coal char particle (130 μm diameter) reacting in several O₂ concentration environments. The effects of boundary layer chemistry are also examined in this analysis. Under oxygen-enriched conditions, boundary layer reactions (converting CO to CO₂, with concomitant heat release) are shown to increase the char particle temperature and burning rate, while decreasing the O₂ concentration at the particle surface. The CO₂ gasification reaction acts to reduce the char particle temperature (because of the reaction endothermicity) and thereby reduces the rate of char oxidation. Interestingly, the presence of the CO₂ gasification reaction increases the char conversion rate for combustion at low O₂ concentrations, but decreases char conversion for combustion at high O₂ concentrations. These calculations give new insight into the complexity of the effects from the CO₂ gasification reaction and should help improve the understanding of experimentally measured oxy-fuel char combustion and burnout trends in the literature.     

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 PM_1–10 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.     

An exploratory study of phosphorus release from biomass by carbothermic reduction reactions

Phosphorus (P) from biomass can cause operational problems in thermal conversion processes. In order to explore the release mechanism of P to the gas phase, carbothermic reduction of meta-, pyro-, and orthophosphates of ash elements commonly found in biomass; sodium, potassium, magnesium, and calcium was investigated. Mixtures of each phosphate and activated carbon were heated to 1135 °C in a laboratory-scale reactor, with the CO and CO₂ evolving from the sample monitored, and the chemical composition of selected residues analyzed to quantify the release of P. Thermal gravimetric analysis was also performed on selected samples. The alkaline earth phosphates were reduced in steps, following the sequence meta → pyro → ortho → alkaline earth oxide. However, the alkali metaphosphates appear to be reduced in one step, in which both alkali and P are released. Alkali pyro- and orthophosphate appear to undergo a two-step process. In the first step, mainly alkali is released and in the second step both alkali and P. An intermediate is produced in the first step, which has a K:P:O atomic ratio of about 2:1:2.7, indicating it might be a phosphite with the overall stoichiometry; K₄P₂O₅. The reduction of alkaline earth phosphates could be interpreted using available thermodynamic data, whereas thermodynamic equilibrium calculations for the alkali phosphates did not correspond well to the experimental observations. Kinetics were derived for the different reduction reactions, and can be used to compare the reactivity of the phosphates. The work suggests that carbothermic reduction reactions are important for the release of P in the temperature range 850–1135 °C and relevant for biomass combustion, pyrolysis and gasification.     

Mechanisms of the central mode particle formation during pulverized coal combustion

Ash particles produced from pulverized coal combustion are considered to be tri-modally distributed. These include the well-known ultrafine and coarse modes, and a central mode that is less reported but attracts increasing attention. This work presents a preliminary study on the formation mechanisms of the central mode particles during pulverized coal combustion. Experiments of four sized and density-separated coal samples were carried out in a laboratory drop-tube furnace under various controlled conditions. Experimental data show that the ash particle size distributions have an evident central mode at 4 μm for all coal samples. Increasing combustion temperature leads to an increase in the central mode particle formation, which is thought to be due to enhanced char fragmentation. The small-size coal sample produces a larger amount of the central mode particles, reasonably due to abundant fine particles in the parent coal sample. Under similar combustion conditions, both the Heavy (>2.0 g/cm³) and Light (<1.4 g/cm³) coal fractions produce a central mode, indicating that not only the included minerals but also the excluded minerals contribute to the formation of the central mode particles.     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

Etching and oxidation of InAs in planar inductively coupled plasma

The surface of InAs (1 1 1)A was investigated under plasmachemical etching in the gas mixture CH₄/H₂/Ar. Etching was performed using the RF (13.56 MHz) and ICP plasma with the power 30–150 and 50–300 W, respectively; gas pressure in the reactor was 3–10 mTorr. It was demonstrated that the composition of the subsurface layer less than 5 nm thick changes during plasmachemical etching.A method of deep etching of InAs involving ICP plasma and hydrocarbon based chemistry providing the conservation of the surface relief is proposed. Optimal conditions and the composition of the gas phase for plasmachemical etching ensuring acceptable etch rates were selected.     

Experimental and numerical investigation of a stagnation pulverised coal flame

A multi-stream Flamelet Progress Variable (FPV) model, specifically developed for coal combustion, is proposed. The model accounts for the different fuel streams associated with the volatile and char burnout products. The applicability of the new FPV model is investigated in a laminar stagnation pulverized coal flame. The flame considered is a premixed mixture of CH₄, O₂ and N₂, carrying pulverized coal particles, stabilized in an impinging wall. Spontaneous emissions of OH*, CH* and C${}_2^{*}$ are measured to identify the flame. The 1D numerical simulations of the experimental conditions are able to reproduce the main features of the flame. The applicability of the multi-stream FPV model to coal combustion is further evaluated with the a posteriori analysis of the FPV results, comparing the results with a reference model, where the species are fully transported and the chemistry directly evaluated. Then, with the budget analysis, the influence of the control variables used to build the look-up table is assessed by examining the conditional contributions to the overall transport terms of scalar quantities (e.g. species, temperature). The results of both analyses show that the proposed multi-stream FPV model can accurately predict the main features of coal combustion, with only minor issues related to the manifold used to build the look-up table.     

Effects of pressure on flame propagation in a premixture containing fine fuel droplets

Combustion experiments on fuel droplet–vapor–air mixtures have been performed with a rapid expansion apparatus which generates monodispersed droplet clouds with narrow diameter distribution using the condensation method. The effects of fine fuel droplets on flame propagation were investigated for ethanol droplet–vapor–air mixtures at various pressures from 0.2 to 1.0 MPa. A stagnant fuel droplet–vapor–air mixture, generated in a rapid expansion chamber, was ignited at the center of the chamber using an ignition wire. Spherical flame propagation under constant-pressure conditions was observed with a high-speed video camera and flame speed was measured. Total equivalence ratio, and the ratio of liquid fuel mass to total fuel mass, was varied from 0.6 to 1.4 and from zero to 56%, respectively. The mean droplet diameter of fuel droplet–vapor–air mixtures was set at 8.5 and 11 μm. It was found that the flame speed of droplet–vapor–air mixtures less than 0.9 in the total equivalence ratio exceeds that of premixed gases of the same total equivalence ratio at all pressures. The flame speed of fuel droplet–vapor–air mixtures decreases as the pressure increases in all total equivalence ratios. At large ratios of liquid fuel mass to total fuel mass, the normalized flame speed (the flame speed of droplet–vapor–air mixtures divided by the flame speed of the premixed gas with the same total equivalence ratio), increases with the increase in pressure for fuel-lean mixtures, and it decreases for fuel-rich mixtures. The outcome is reversed at small ratios of liquid fuel mass to total fuel mass; the normalized flame speed decreases with the increase in pressure for fuel-lean mixtures, and increases for fuel-rich mixtures. The results suggest that the increase in pressure promotes droplet evaporation in the preheat zone.     

Behavior of coal-chlorine in chemical looping combustion

This work focuses on two aspects: (i) the behavior of coal-derived chlorine in chemical looping combustion (CLC); (ii) the potential adverse impacts of primary gaseous chlorine (i.e., HCl) on Cu-based oxygen carrier (OC). The inactivation mechanism of the sol-gel-derived CuO/Al₂O₃ OC is investigated. Systematic experiments are conducted in a batch fluidized reactor. First, in CLC of coal, chlorine distribution including HCl, Cl₂, Cl adsorbed in the outlet tube and Cl in solid phase is studied under various bed inventories, temperatures and gas atmospheres. The main gaseous Cl from coal is HCl, which shows a high reactivity towards CuO and is partially physically adsorbed by Al₂O₃. Unconverted HCl is 15.63 ± 0.20%, which could result in corrosion of the CO₂ transportation line and compression equipment. What's more, the coal ash exhibits a dechlorination function by forming KCl and CaCl₂. The CO₂ atmosphere and high temperature in fuel reactor show a promotion on the conversion of coal-Cl to HCl. Then, the corrosion of various OC components is evaluated by a mixture gas with 400 ppm HCl, i.e., Cu-Al (whole OC), CuO (active phase) and Al₂O₃ (inert support phase). It is found that a part of HCl is converted to Cl₂ via the Deacon reaction (4HCl + O₂ = 2H₂O + 2Cl₂) and oxidized by CuO (2CuO + 4HCl = 2CuCl + Cl₂ + 2H₂O). At the high concentration of HCl (400 ppm) atmosphere, CuO is partially lost from the OC, producing the gaseous copper chlorides, i.e., CuCl and (CuCl)₃, which are found to be condensed in the outlet tube. Besides, the solid-phase copper chlorides also degrade the oxygen donation capacity of the OC. Finally, the migration path of coal-chlorine during CLC is summarized. This work will contribute to the development of Cl-resistance OCs and control approaches for Cl emission.     

Experimental and modeling assessment of sulfur release from coal under low and high heating rates

Coal combustion releases elevated amounts of pollutants to the atmosphere including SO_X. During the pyrolysis step, sulfur present in the coal is released to the gas phase as many different chemical species such as H₂S, COS, SO₂, CS₂, thiols and larger tars, also called SO_X precursors, as they form SO_X during combustion. Understanding the sulfur release process is crucial to the development of reliable kinetic models, which support the design of improved reactors for cleaner coal conversion processes. Sulfur release from two bituminous coals, Colombian hard coal (K1) and American high sulfur coal (U2), were studied in the present work. Low heating rate (LHR) experiments were performed in a thermogravimetric analyzer coupled with mass spectrometry (TG-MS), allowing to track the mass loss and the evolution of many volatile species (CO, CO₂, CH₄, SO₂, H₂S, COS, HCl and H₂O). High heating rate (HHR) experiments were performed in an entrained flow reactor (drop-tube reactor – DTR), coupled with MS and nondispersive infrared sensor (NDIR). HHR experiments were complemented with CFD simulation of the multidimentional reacting flow field. A kinetic model of coal pyrolysis is employed to reproduce the experiments allowing a comprehensive assessment of the process. The suitability of this model is confirmed for LHR. The combination of HHR experiments with CFD simulations and kinetic modeling revealed the complexity of sulfur chemistry in coal combustion and allowed to better understand of the individual phenomena resulting in the formation of the different SO_X precursors. LHR and HHR operating conditions lead to different distribution of sulfur species released, highly-dependent on the gas-phase temperature and residence time. Higher retention of total sulfur in char is observed at LHR (63%) when compared to HHR (37–44%), at 1273 K. These data support the development of reliable models with improved predictability.