Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion

Oxygen/carbon dioxide recycle coal combustion is actively being investigated because of its potential to facilitate CO₂ sequestration and to achieve emission reductions. In the work reported here, the effect of enhanced oxygen levels and CO₂ bath gas is independently analyzed for their influence on single-particle pulverized coal ignition of a U.S. eastern bituminous coal. The experiments show that the presence of CO₂ and a lower O₂ concentration increase the ignition delay time but have no measurable effect on the time required to complete volatile combustion, once initiated. For the ignition process observed in the experiments, the CO₂ results are explained by its higher molar specific heat and the O₂ results are explained by the effect of O₂ concentration on the local mixture reactivity. Particle ignition and devolatilization properties in a mixture of 30% O₂ in CO₂ are very similar to those in air.     

Fundamental investigation of NOx formation during oxy-fuel combustion of pulverized coal

NO_x formation was measured during combustion of pulverized coals and pulverized coal char in N₂ and CO₂ environments under isothermal and nearly constant oxygen conditions (i.e. using dilute coal loading). Three different oxygen concentrations (12% O₂, 24% O₂, and 36% O₂) and two representative US coals were investigated, at a gas temperature of 1050 °C. To investigate the importance of NO reburn reactions, experiments were also performed with an elevated concentration (550 ppm) of NO in the gases into which the coal was introduced. For low levels of background NO, the fractional fuel-nitrogen conversion to NO_x increases dramatically with increasing bath gas oxygen content, for both N₂ and CO₂ environments, though the fuel conversion is generally lower in CO₂ environments. Char N conversion is lower than volatile N conversion, especially for elevated O₂ concentrations. These results highlight the importance of the volatile flame and char combustion temperatures on NO_x formation. For the high background NO_x condition, net NO_x production is only observed in the 36% O₂ environment. Under these dilute loading conditions, NO reburn is found to be between 20% and 40%, depending on the type of coal, the use of N₂ or CO₂ diluent, the bulk O₂ concentration, and whether or not one considers reburn of volatile-NO_x. This dataset provides a unique opportunity to understand and differentiate the different sources and sinks of NO_x under oxy-fuel combustion conditions.     

PM10 formation during the combustion of N2-char and CO2-char of Chinese coals

The formation of PM₁₀ (particles less than or equal to 10 μm in aerodynamic diameter) during char combustion in both air-firing and oxy-firing was investigated. Three Chinese coals of different ranks (i.e., DT bituminous coal, CF lignite, and YQ anthracite) were devolatilized at 1300 °C in N₂ and CO₂ atmosphere, respectively, in a drop tube furnace (DTF). The resulting N₂-chars and CO₂-chars were burned at 1300 °C in both air-firing (O₂/N₂ = 21/79) and oxy-firing (O₂/CO₂ = 21/79). The effects of char properties and combustion conditions on PM₁₀ formation during char combustion were studied. It was found that the formation modes and particle size distribution of PM₁₀ from char combustion whether in air-firing or in oxy-firing were similar to those from pulverized coal combustion. The significant amounts of PM_0.5 (particles less than or equal to 0.5 μm in aerodynamic diameter) generated from combustion of various chars suggested that the mineral matter left in the chars after coal devolatilization still had great contributions to the formation of ultrafine particles even during the char combustion stage. The concentration of PM₁₀ from char combustion in oxy-firing was generally less than that in air-firing. The properties of the CO₂-chars were different from those of the N₂-chars, which was likely due to gasification reactions coal particles experienced during devolatilization in CO₂ atmosphere. Regardless of the combustion modes, PM₁₀ formation in combustion of N₂-char and CO₂-char from the same coal was found to be significantly dependent on char properties. The difference in the PM₁₀ formation behavior between the N₂-char and CO₂-char was coal-type dependent.     

A DNS study on temporally evolving jet flames of pulverized coal/biomass co-firing with different blending ratios

Biomass co-firing within the existing pulverized coal boiler is thought as a practical near-term way of biomass utilization, while its detailed combustion characteristics and pollutant formation have not yet been fully understood. In the present study, we report a Carrier-phase Direct Numerical Simulation study coupled with detailed mechanism to provide a deep insight into the coal/biomass co-firing (CBCF) jet flames under different blending ratios. It is found that compared with the pure coal flame, the CBCF could (i) prompt the volatiles ignition, produce higher H₂O and similar CO₂ mass fractions at blending ratios of 20% and 40%, and obviously reduce the gas temperature and CO₂ mass fraction at the blending ratio of 50%; (ii) prompt the coal devolatilization and char burnout at blending ratios of 20% and 40%, while the char burnout is reduced when blending ratio is 50% due to the local enrichment of large particles and lack of oxygen; (iii) reduce the thermal, prompt, NNH and N₂O-intermediate routes of NO formation, but show limited effect on the NO-reburning route of NO destruction, therefore, resulting in an obvious NO reduction.     

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.     

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.     

Homogeneous ignition of CH4/air and H2O and CO2-diluted CH4/O2 mixtures over Pt; an experimental and numerical investigation at pressures up to 16 bar

The homogeneous ignition of CH₄/air, CH₄/O₂/H₂O/N₂, and CH₄/O₂/CO₂/N₂ mixtures over platinum was investigated experimentally and numerically at pressures 4 bar p 16 bar, temperatures 1120 K T 1420 K, and fuel-to-oxygen equivalence ratios 0.30 0.40. Experiments have been performed in an optically accessible catalytic channel-flow reactor and included planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous (gas-phase) ignition and one-dimensional Raman measurements of major species concentrations across the reactor boundary layer for the assessment of the heterogeneous (catalytic) processes preceding homogeneous ignition. Numerical predictions were carried out with a 2D elliptic CFD code that included elementary heterogeneous and homogeneous chemical reaction schemes and detailed transport. The employed heterogeneous reaction scheme accurately captured the catalytic methane conversion upstream of the gaseous combustion zone. Two well-known gas-phase reaction mechanisms were tested for their capacity to reproduce measured homogeneous ignition characteristics. There were substantial differences in the performance of the two schemes, which were ascribed to their ability to correctly capture the p–T– parameter range of the self-inhibited ignition behavior of methane. Comparisons between measured and predicted homogeneous ignition distances have led to the validation of a gaseous reaction scheme at 6 bar p 16 bar, a pressure range of particular interest to gas-turbine catalytically stabilized combustion (CST) applications. The presence of heterogeneously produced water chemically promoted the onset of homogeneous ignition. Experiments and predictions with CH₄/O₂/H₂O/N₂ mixtures containing 57% per volume H₂O have shown that the validated gaseous scheme was able to capture the chemical impact of water in the induction zone. Experiments with CO₂ addition (30% per volume) were in good agreement with the numerical simulations and have indicated that CO₂ had only a minor chemical impact on homogeneous ignition.     

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Investigating particle and volatile evolution during pulverized coal combustion using high-speed digital in-line holography

Devolatilization is an important process in pulverized coal combustion because it affects the ignition, volatile combustion, and subsequent char burning and ash formation. In this study, high-speed digital in-line holography is employed to visualize and quantify the particle and volatile evolution during pulverized coal combustion. China Shanxi bituminous coal particles sieved in the range of 105–154 µm are entrained into a flat flame burner through a central tube for the study. Time-resolved observations show the volatile ejection, accumulation, and detachment in the early stage of coal combustion. Three-dimensional imaging and automatic particle extraction algorithm allow for the size and velocity statistics of the particle and stringy volatile tail. The results demonstrate the smaller particle generation and coal particle swelling in the devolatilization. It is found that the coal particles and volatiles accelerate due to the thermal buoyancy and the volatiles move faster than the coal particles. On average, smaller particles move faster than the larger ones while some can move much slower possibly because of the fragmentation.     

Multi-parameter diagnostics for high-resolution in-situ measurements of single coal particle combustion

To study volatile combustion processes of single coal particles non-intrusive simultaneous multi-parameter measurements were performed. The experiment was carried out in a fully premixed flat flame burner with well-defined boundary conditions. For flame visualization high-speed luminescence imaging was combined with high-resolution high-speed OH-PLIF. To address particle size and shape a stereoscopic high-resolution backlight-illumination system was set up. Due to simultaneous recording of individual particle events the volatile combustion duration related to particle size, shape and velocity was measured. A comparison of luminescence imaging and OH-PLIF for flame visualization was investigated to define their application areas in coal combustion. The stereoscopic backlight-illumination setup was benchmarked to a well characterized bituminous coal. With a pixel resolution of ～2.5 µm fine particle contours were resolved. The particle diameter and eccentricity were evaluated by an ellipse approximation. The experimental setup can be used to investigate different coal ranks and biomass in N₂/O₂ and CO₂/O₂ atmospheres in future.     

Experimental study of influence of temperature on fuel-N conversion and recycle NO reduction in oxyfuel combustion

Coal combustion in O₂/CO₂ environment was examined with a bituminous coal in which the gas-phase and char combustion stages were considered separately. The effects of temperature (1000–1300 °C) and the excess oxygen ratio λ (0.6–1.4) on the conversion of volatile-N and char-N to NOx were studied. Also, the reduction of recycle NOx by fuel-N was investigated under various conditions. The results show that fuel-N conversion to NO in O₂/CO₂ is lower than that in O₂/N₂. In O₂/CO₂ atmosphere, the volatile-N conversion ratios vary from 1–7% to 15–24% under fuel-rich and fuel-lean conditions, respectively. The char-N conversion ratios are 11–28% and 30–50% under fuel-rich and fuel-lean conditions, respectively. The influences of temperature on the conversion of volatile-N to NO under fuel-rich and fuel-lean conditions are contrary. A significant difference for char-N conversion in fuel-rich and fuel-lean conditions is observed. The experimental data of recycle NO reduction indicate that the reduction of recycle NO by gas-phase reaction can be enhanced by volatile-N addition in fuel-lean condition at high temperature, while in fuel-rich condition, the volatile-N influence cancelled out and the overall impact is small. NO/char reaction competes with the conversion of fuel-N to NO at higher temperatures.     

Ignition and volatile combustion behaviors of a single lignite particle in a fluidized bed under O2/H2O condition

O₂/H₂O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O₂/H₂O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H₂O atmosphere was darker than that in the N₂ atmosphere because the heat capacity of H₂O is higher than that of N₂. With the increase of oxygen concentration, the flame temperature in the O₂/H₂O atmosphere was dramatically enhanced rather than that in the O₂/N₂ atmosphere, where the diffusion rate of oxygen in O₂/N₂ atmosphere became the dominant factor.     

A new technique for the measurement of the product CO/CO2 ratio at the surface of char particles burning in a fluidized bed

A new experimental technique is proposed to measure the product CO/CO₂ ratio at the surface of spherical char particles during fluidized bed combustion. It is based on the measurement of the burning rate of a single char particle under low oxygen concentration conditions and on the use of an accurate prediction of the particle Sherwood number. This technique was applied to spherical char particles obtained from a bituminous coal which is characterized by a low attrition and fragmentation propensity. The product CO/CO₂ ratio was measured at a bed temperature of 850 °C and at a fluidization velocity of 0.3 m/s in a lab-scale apparatus operated with a bed of 0.5–0.6 mm sand. The char particle size was varied between 2 and 7 mm and the inlet oxygen concentration between 0.1% and 2.0%. Results showed that under the experimental conditions investigated carbon was mostly oxidized to CO₂ within the particle boundary layer, with a maximum fraction of carbon escaping as CO of 10–20% at the lowest oxygen concentrations and largest particle sizes.     

Single particle ignition and combustion of pulverized pine wood,wheat straw,rice husk and grape pomace

This work examines the combustion behavior of single pulverized biomass particles from ignition to early stages of char oxidation. The biomass residues investigated were pine wood, wheat straw, rice husk and grape pomace. The biomass particles, in the size range 224–250 µm, were injected upward into a confined region with hot combustion products, produced by a flat flame McKenna burner, with a mean temperature of 1610 K and a mean O₂ concentration of 6.5 vol%. Temporally and spectrally resolved images of the single burning particles were recorded with an intensified charge-coupled device camera equipped with different band-pass spectral filters. Data are reported for CH*, C₂*, Na* and K* chemiluminescence, and thermal radiation from soot and char burning particles. The data on CH* and C₂* chemiluminescence and soot thermal radiation permits to identify important differences between the ignition delay time, volatiles combustion time and soot formation propensity of the four biomass residues, which are mainly affected by their volatile matter content. The Na* and K* emission signals follow the same trends of the CH* and C₂* emission signals until the end of the volatiles combustion stage, beyond which, unlike the CH* and C₂* emission signals, they persist owing to their release from the char burning particles. Moreover, during the volatiles combustion stage, the Na*/CH* and K*/CH* ratios present constant values for each biomass residue. The CH* and thermal radiation emission data suggest that all biomass char particles experienced heterogeneous oxidation at or immediately after the extinction of the homogeneous volatiles combustion.     

Oxidation and combustion of stabilized lithium metal powder (SLMP)

Power systems based on combustion are of interest for space missions where the use of solar and nuclear energy is impractical. A novel design of such systems involves the so-called filtration combustion of metal powders with oxygen supplied by a chemical oxygen generator. In missions to Mars and Venus, the atmospheric CO₂ could be added to the oxygen flow. A stabilized lithium metal powder (SLMP) is a promising fuel for this application, but its oxidation and combustion in O₂ and CO₂ has not been studied yet. In the present work, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been used to investigate high-temperature oxidation of SLMP in O₂/Ar and CO₂ environments. Further, combustion of SLMP in vertical quartz tubes with two open ends at natural infiltration of O₂ and CO₂ has been studied in a laser ignition facility. The TGA has revealed the formation of lithium peroxide (Li₂O₂) in addition to lithium monoxide (Li₂O) at temperatures below 400 °C. Scanning electron microscopy has shown that the oxidized particles are hollow shells, which implies that the oxidation process includes growth of a solid oxide layer on the surface of the lithium droplet and simultaneous growth of a cavity inside the droplet. The laser ignition of SLMP in O₂ results in vigorous combustion of the top layer followed by the downward propagation of a counterflow combustion wave. The TGA and DSC have shown that the reaction of SLMP with CO₂ is a multi-stage process, which includes the formation of lithium oxide and its subsequent conversion into lithium carbonate (Li₂CO₃). Self-sustained combustion of SLMP was not achieved in a CO₂ environment, apparently because the formed products hinder the transport of CO₂.     

An intrinsic kinetics model to predict complex ash effects (ash film,dilution, and vaporization) on pulverized coal char burnout in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

During coal combustion, char chemical reaction is the slowest step, particularly in the last burnout stage, where the char consists of small amounts of carbon in a predominant ash framework. Existing kinetics models tend to deviate from experimental measurements of late char burnout due to the incomplete treatment of ash effects. Ash can improve pore evolution through vaporization, hinder oxygen transport by forming an ash film, and reduce active carbon sites and available surface per unit volume by penetrating into the char matrix. In this work, a sophisticated kinetics model, focusing on these three ash evolution mechanisms (ash vaporization, ash film, and ash dilution) during pulverized coal (PC) char combustion, is developed by integrating them into a thorough mechanistic picture. Further, a detailed comparison of the three distinct ash effects on PC char conversion during air (O₂/N₂) and oxy-fuel (O₂/CO₂) combustion is performed. For the modeled coal, the mass of ash vaporization is approximate 3 orders less than the mass of ash remaining, which participates in ash dilution and ash film formation, both in O₂/N₂ and O₂/CO₂ atmospheres. The influence of these phenomena on burnout time follows the order: ash dilution > ash film > ash vaporization. The influence of ash vaporization on burnout time is minor, but through interactions with the ash dilution and ash film forming processes it can have an impact at high extents of burnout, particularly in O₂/CO₂ atmospheres. In O₂/N₂ atmospheres the residual ash predominately exists as an ash film, whereas it mainly exists as diluted ash in the char matrix in O₂/CO₂ atmospheres. The residual ash particle is encased by a thick film when the ash film forming fraction is high (low ash dilution fraction). These results provide in-depth insights into the conversion of PC char and further utilization of the residual ash.     

Counter-current flame propagation in a cylindrical bed of activated carbon saturated with artificial volatile matter in an upward flow of O2/N2 mixture

A counter-current flame propagation process, or traverse combustion, in a cylindrical bed packed with activated carbon (AC) saturated with artificial volatiles was studied experimentally to appreciate the effect of homogenous volatile oxidation in the combustion process of packed solid. The experiments were conducted with an upward flow of a 50/50 v/v O₂/N₂ mixture. Flame propagation velocity and total carbon consumption during traverse combustion period, from ignition to the time when the flame propagated to the bed bottom, were determined. Ethanol, isooctane and benzene, were used respectively as the artificial volatile. Results show that compared to the case of AC alone, the flame propagation velocity was reduced while the combustion duration extended in the presence of volatile. The visible gas phase flame due to homogeneous volatile combustion traversed in the opposite direction of the oxidizing gas stream, competing with heterogeneous carbon oxidation for available oxygen. The proportion of oxygen consumed by homogeneous oxidation was the highest for ethanol, up to 87–94%, while isooctane and benzene accounted for 30–65% and 27–63%, respectively. Despite the competing homogeneous volatile combustion, the total amount of carbon consumed in the heterogeneous carbon oxidation was even increased in the cases of isooctane and benzene due to the extended combustion durations and relatively higher proportions of O₂ consumed by heterogeneous carbon oxidation than in the ethanol case.     

High-temperature transformation of pyrite in CO2: Effects of residence time and the presence of O2

To understand the behavior of pyrite (a key slag-inducing coal mineral) in oxyfuel combustion (O₂/CO₂ combustion), our previous work firstly investigated the effects of pure CO₂ on fixed bed reactors and at low temperatures (<1273 K) and long residence time (order of minutes). As a consecutive contribution, this work explores pyrite transformation in CO₂ with the presence of O₂ on a drop tube furnace (DTF) and at a high temperature of 1573 K. These conditions are more relevant to practical oxyfuel combustion. Pulverized pyrite samples of sizes between 63 and 75 µm were tested in CO₂ with varying O₂ from 0 to 3%, mimicking the conditions near the boiler burners. The changes of pyrite transformation with residence time were investigated by examining the intermediate products, collected by a high-temperature sampling probe. For a clear clarification of the effects of CO₂, tests were also carried out in O₂/N₂ for comparison. The solid products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS). The gas products were analyzed online by a gas analyzer. Results showed that, in the absence of O₂, CO₂ chemically accelerated pyrite decomposition and oxidation when compared with N₂. This finding was consistent with that in previous work performed on fixed-bed reactors. In the presence of the same O₂ concentration, the positive roles of CO₂ persisted, enhancing the formation of magnetite (Fe₃O₄) and SO₂. In either O₂/CO₂ or O₂/N₂, increasing O₂ concentration also favored the formation of magnetite and SO₂. Significant changes of SO₂ and S in pyrrhotite (FeS_x) were observed between 0.5–0.7 s, and longer residence time did not seem to have significant effects. However, the amount of magnetite formed increased with increasing residence time. The obtained knowledge is new and very useful to further understanding of pyrite behavior in real oxyfuel combustion systems.     

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion

Pyrolysis of sewage sludge under conditions relevant to applied smouldering combustion was carried out in this study to investigate the influences of gas flow rate, oxidative atmosphere, and inert porous medium involvement on the properties of products. The experiments were carried out at 300–600 °C under atmospheres of N₂, 5% O₂/95% N₂, 10% O₂/90% N₂, and 15% O₂/85% N₂, with Darcy flow rates of 1.0 and 3.5 cm/s, respectively, with dried sewage sludge loaded individually or as a mixture with sand. As a result, both the increment of gas flow rate and involvement of sand leaded to lower yields of char and higher yields of bio-oil and gas under N₂ at temperature of ≤500 °C, due to the enhanced efficiency of pyrolysis reaction and gas transportation. However, when temperature increased to 600 °C, the influencing trends on product distributions changed due to the mechanisms of secondary cracking reaction and volatile-char interaction. The involvement of oxygen in fraction of ≤15 vol% at temperatures of 400–500 °C would lead to the intense decreasing yields of char and bio-oil, and increasing yield of the gaseous (dominated by CO₂ and CO), due to the involved oxidation reaction during pyrolysis. Both increment of temperature and oxygen fraction would lead to the delay of ignition and the increase of activation energy of the produced char, except for that of char produced at 400 °C under 5% O₂/95% N₂, whose calculated activation energy was lower and volatile content was higher compared to that of char produced from pyrolysis at 400 °C under N₂. The bio-oil from pyrolysis under N₂ was dominated by aliphatic acids, phenols, steroids, amides, and indoles, etc., and the involvement of partial oxidation would lead to the weakened formation of aromatics, phenols, and S/Cl/F-containing compounds in bio-oil.