Carbon Burnout Research Articles

Experiment investigation of coal MILD-Oxy combustion integrated with flue gas recirculation at a 0.3 MWth furnace

This paper presents an experimental study of coal MILD-Oxy combustion at 0.3mw pilot-scaled facility in Huazhong University of Science and Technology (HUST), which contains an entirely flue gas recycle system. By increasing the jet velocity to 100m/s, the MILD combustion was achieved without high temperature preheating. Meanwhile the experiment was operated under Oxy-fuel atmosphere that was established by the flue gas recirculation. A swirl Oxy-fuel combustion was also tested as a comparison. The CO2 concentration in flue gas could sustain a high level during the combustion. The major conclusions include: The coal MILD-Oxy combustion can be achieve by high-velocity jet in some extent. But the full dilution and homogenization of heat and reactants can't be achieved under non-premixed combustion because of the separation of fuel and oxygen, which means the ideal MILD-Oxy combustion can't be achieved but only be approached. The reaction zone was enlarged to the vast area of furnace, in which the combustion temperature and reaction rate decreased. The carbon burnout rate decreased due to the reduction of residence time and reaction rate. The NOx emission decreased nearly 40% because of the lower flame temperature.

Evolution of temperature of a droplet of liquid composite fuel interacting with heated airflow

The macroscopic patterns of a temperature change at the center of a droplet of three-component (coal, water, petroleum) composite liquid fuel (CLF) were studied using a low-inertia thermoelectric converter and system of high-speed (up to 105 frames per second) video recording during the induction period at different heating intensity by the air flow with variable parameters: temperature of 670−870 K and motion velocity of 1−4 m/s. The studies were carried out for two groups of CLF compositions: fuel based on brown coal and coal cleaning rejects (filter cake). To assess the effect of liquid combustible component of CLF on characteristics of the ignition process, the corresponding composition of two-component coal-water fuel (CWF) was studied. The stages of inert heating of CLF and CWF droplets with characteristic size corresponding to radius of 0.75−1.5 mm, evaporation of moisture and liquid oil (for CLF), thermal decomposition of the organic part of coal, gas mixture ignition, and carbon burnout were identified. Regularities of changes in the temperature of CLF and CWF droplets at each of identified stages were identified for the cooccurrence of phase transitions and chemical reactions. Comparative analysis of the times of ignition delay and complete combustion of the droplets of examined fuel compositions was performed with varying droplet dimensions, temperatures, and oxidant flow velocity.

Investigation on coal moderate or intense low-oxygen dilution combustion with high-velocity jet at pilot-scale furnace

An experiment of coal MILD combustion was carried out in a 0.3MWth down-fire furnace. By increasing the jet velocity to 112m/s, a sufficient internal recirculation was obtained inside the furnace. Therefore the coal MILD combustion could be achieved without highly temperature preheating. Under same condition, a numerical investigation was also carried out as a comparison, and the modeling data can fit the measured data well. Under MILD condition, the peak value of temperature decreased, and the temperature distribution became uniform. Due to high velocity jet, the reaction zone was largely extended and moved toward the exit, in which the reaction rates were decreased for all oxidation reactions. The flame appeared as an integration of dark spark and flame fragment. The NOx emission largely decreased due to the lower flame temperature and deoxidized atmosphere. The carbon burnout rate decreased due to the reduced particle residence time, in turn the CO and oxygen concentration in the exhaust gas also increased. However, the ignition delay increased, and the flame stability decrease without the highly temperature preheating.

Performance Evaluation of South African Coals under Oxy-Fuel Combustion in a Fluidized Bed Reactor

In this study, the experimental results of the oxy-fluidized combustion of three different South African coals (sub-bituminous) are presented. The coal samples were denoted Coal A, B, and C. Three combustion atmospheres—air, oxy (21% O2/79% CO2), and oxy (30% O2/70% CO2)—were studied. A total of 18 tests were conducted in a bubbling fluidized bed reactor at 850 and 925 °C. The results obtained showed that the highest carbon burnout was obtained at 30% O2/CO2, followed by air, and last at 21% O2/CO2. Coals A and B had a higher carbon burnout than Coal C. There was a marginal difference of 1% in the carbon burnout for both Coals A and B in all the conditions except at 21% O2/CO2 at 925 °C, when the carbon burnout of Coal A was 5% higher than that of Coal B. Carbon burnout in Coal C was about 20% lower than those of Coals A and B. Carbon burnout had a good correlation with the char particle temperature at all the combustion conditions studied for the three coals. The carbon burnout only had a good correlatio...

The evolution of coal char structure under the oxy-fuel combustion containing high H2O

Oxy-fuel wet recycle system with a higher energy efficiency exhibits more attractive. Compared to conventional pulverized coal combustion, influences of gasification in oxy-fuel combustion is more complicated, especially in wet recycle system. Meanwhile, gasification has a significant impact on char structure, which can directly affect the combustion characteristic of char. The evolution of coal char structure under the oxy-fuel combustion containing high H2O was investigated. Char samples were prepared in CO2/H2O atmosphere using a drop-tube furnace (DTF), N2, CO2 and N2/H2O atmosphere as the control group. Carbon content, surface morphology, pore structure, carbon structure and characteristics temperature of the coal char were analyzed. Experimental results show that carbon burnout rate increases with the content of volatile matter increase. Char particles from a containing-H2O atmosphere formed around less fine particles than the char particles under no-H2O atmosphere, because fine coal particles in the atmosphere containing-H2O are easy reacted and taken away from the airflow. The remarkable reduction of char pores under an atmosphere containing CO2 or H2O caused by carbon consumption during gasification reaction, which can not only produce a more porous structure but also diminish the proportion of large pores. Amorphous carbon in coal could be quickly reacted in a containing-H2O atmosphere. Atmosphere containing H2O dramatically reduces char characteristics temperature at 21% O2 concentrations. In conclusion, char combustion reactivity affected by carbon reactivity largely than the specific surface area in oxy-fuel atmosphere containing high H2O.

Pore fractal structures and combustion dynamics of cokes derived from the pyrolysis of typical Chinese power coals

The pore fractal structures and combustion dynamics of cokes derived from the pyrolysis of 18 typical Chinese power coals were investigated to use the said cokes for boiler combustion and power generation after pyrolysis gas extraction. When the total contents of volatile matter and moisture in raw (air-dried) coals increased from 15.22% to 39.49%, the pore fractal dimensions of the pyrolyzed cokes gradually increased from 2.30 to 2.84 because of the decrease in the activation energies of coal pyrolysis. The average pore diameters of the coke particles gradually decreased, and the peaks of the differential specific surface area formed at a pore diameter of 3.7nm gradually increased. Accordingly, the ignition temperatures of cokes gradually decreased from 617°C to 486°C. Their activation energies also gradually decreased, which resulted in an increase in fixed carbon burnout efficiencies from 84% to 91%. This indicated that cokes derived from lignite pyrolysis had larger pore fractal dimensions and therefore had lower ignition temperatures and higher burnout efficiencies than those from lean coal and anthracite pyrolysis.

Fabrication of large alumina foams by pyrolysis of thermo-foamed alumina–sucrose

Abstract

Maximum burning rate and fixed carbon burnout efficiency of power coal blends predicted with back-propagation neural network models

Back-propagation (BP) neural network models were developed to accurately predict the maximum burning rate and fixed carbon burnout efficiency of 16 typical Chinese coals and 48 of their blends. Early stopping method was used to prevent the BP neural network from over-fitting. The generalisation performance and prediction accuracy of the neural network thus became significantly improved. Pearson correlation analysis results showed that the maximum burning rate was most relevant to coal calorific value as well as carbon and ash content. Fixed carbon burnout efficiency was most relevant to coal volatile matter, fixed carbon and calorific value. Accordingly, three-layer BP neural network models with three input factors were developed to predict the combustion characteristics of power coal blends. The BP neural network used to predict the maximum burning rate gave a relative mean error of 1.97%, which was considerably lower than that given by the quadratic polynomial regression (7.06%). Moreover, the BP neural network used to predict the fixed carbon burnout efficiency gave a relative mean error of 0.91%, which was significantly lower than that given by the quadratic polynomial regression (4.03%).

Mathematical Modeling of the Carbon Burn-out Process from the Shale Semi-coke Surface of Aerofountain Technological Furnace in the Solid Heat-carrier Oil Shale Semi-coking Scheme

The mathematical describing of the shale semi-coke carbon burn-out process in the aerofountain technological furnace (ATF) is represented in the paper. Even carbon distribution over the semi-coke particles ash surface is accepted as a working hypothesis.

Synergistic effect of co-firing woody biomass with coal on NOx reduction and burnout during air-staged combustion

Hybrid technologies combining fuel blending and air-staging processes have been applied to a pulverized coal fired furnace to reduce NOx emissions. In this study, an Australian bituminous coal (Whitehaven), an Indonesian sub-bituminous coal (Adaro), and an Indonesian woody biomass were selected as fuel with blending ratio of 10%, 20%, and 30% of the low-rank fuels, and the air-staging levels were set to 235mm, 390mm, 585mm, and 760mm. The purpose of this study was to investigate the synergistic effect of woody biomass co-firing on the level of NOx emissions and the degree of carbon burnout under air-staged conditions. For single coals, sub-bituminous coal was more favorable than that for bituminous coal to reduce NOx emissions due to low fuel-N composition. This tendency was more dominant with increasing the air staging levels. In the co-firing of woody biomass with coal, as the biomass is highly volatile but has a low carbon content, it could be successfully applied to low-NOx combustion under air-staged conditions. In addition, the degree of carbon burnout and the flame temperature both increased. As a result of this research, we determined that hybrid NOx reduction technologies have the potential to reduce exhaust gas emissions and enhance combustion performance. A dominant synergistic effect on NOx reduction and carbon burnout was observed when woody biomass co-firing with coal was applied to air-staged combustion.

Nitric oxide emissions and combustion performance of nontraditional ring-fired boilers with furnace geometries

The flow, combustion and emission characteristics of nontraditional ring-fired-type furnaces were investigated numerically to evaluate the effects of incorporating additional inner water walls on the heat transfer, carbon burnout and emissions of nitrogen oxides. Both tangentially and ring-fired-type furnaces were considered and the ring-fired-type furnaces were divided into four inner-water-wall cases: without, normal type, radiant expended type and both radiant and convective expended type. The presence of the inner water wall led to an improvement of approximately 50% in the heat flux. In particular, the reduction in nitrogen oxide emissions was approximately 30%, whereas the carbon burnout was kept constant.

발전용 역청탄 및 아역청탄의 파일롯 연소특성 평가

With the depletion of high grade coal, it is indispensable to be used co-combustion of low rank coal with bituminous coal in pulverized coal-fired power plants. This study describes the detailed measurements of combustion characteristics of bituminous and subbituminous coal in a 0.7MWth pilot-scale test facility. This experimental works include the measurement of gas temperature, gas concentrations along with the reactor axial and radial distance at the condition of excess air ratio of 1.2. The solid sampling was carried out and analyzed with the combustion of bituminous coal. The main reaction zone of coal flame in a reactor was formed about 1 m from the swirl burner, and at downstream, the fully developed temperature and species distribution was observed. The sampled particles of bituminous coal in a reactor revealed the complete carbon burn-out was achieved just after an main combustion zone.

Effect of an internal reaction on the characteristics of high-temperature heat and mass transfer of gas suspensions of carbon particles

The high-temperature heat and mass transfer of a gas suspension of carbon particles has been simulated physically and mathematically with allowance for the chemical reaction in their pores. It is established that the internal reaction leads to a significant decrease of the ignition delay times and critical parameters determining the ignition and extinction of particles of a gas suspension. The range of the initial particles sizes and mass concentrations, corresponding to the most complete burnout of carbon fuel, is found; the effect of the internal reaction on the limits of this range is analyzed. It is shown that the way of varying the mass concentration affects qualitatively the dependences of the ignition delay on the mass concentration.

Experimentation and Simulation of the Combustion of Biomass Briquettes in Southern China

The thermogravimetry (TG) of typical biomass briquettes used as fuel in southern China was analysed to investigate the influences of fuel grain size and heating rate on combustion. The results suggested that grain size and heating rate exerted little influence on combustion. In accordance with the data and the TG results obtained from the fuel, a biomass grate incinerator process was numerically simulated using fluid dynamics-based incinerator code (FLIC) software to obtain the solid phase temperature distribution of the fuel along the bed length, the spatial temperature distribution of flue gas, and the underlying variation laws of the primary components. A comparison of the mass-loss curves from the numerical simulation to the TG analysis demonstrated that the two curves exhibited consistently staged variations, including dehydration and drying, fast pyrolysis and combustion of volatiles, and the burnout of residual carbon. The specific characteristics of the fuel obtained from these tests improved the accuracy of the numerical simulation, while the variations in temperature and components obtained were conducive to optimising the combustion process of a biomass incinerator.

Co-firing coal with biomass in oxygen- and carbon dioxide-enriched atmospheres for CCS applications

Biomass combustion and carbon capture and storage (CCS) individually represent significant options for decarbonising the power generation sector and when combined may permit a carbon–neutral or even carbon-negative process. Despite this potential, little research has been published that examines the combustion of biomass in atmospheres with application in CCS processes. This work reports on bench- and laboratory-scale testing of biomass and coal combustion in atmospheres enriched in O2 and CO2 which are relevant for retrofitting power stations for oxyfuel combustion. At bench-scale, thermogravimetric analysis shows substituting N2 as the combustion diluent with CO2 has little impact on the combustion properties of the fuels but that increasing the O2 concentration accelerates combustion of coal and biomass chars. Results from cofiring three biomasses with coal at 20kW scale suggest substitution of N2 with CO2 significantly reduces temperatures, carbon burnout and emissions of NO while combustion in O2-enriched conditions has the opposite effects. Emissions of NO and SO2 were found to reduce compared to air in combustion atmospheres enriched with O2 and CO2 while combustion temperatures and carbon burnout slightly increased.

The experimental study of fly ash recirculation combustion characteristics on a circulating fluidized bed combustor

Fly ash recirculation combustion (FARC) technology is an effective way to improve the combustion performance due to the improvement of carbon burnout as well as the utilization of calcium oxide. In order to study the effect of FARC on operation and emission characteristics of CFB boilers, a series of experiments were carried out on a lab-scale CFB combustor by using two types of fly ash (desulfurization and non-desulfurization fly ashes). Results show that with FARC, the temperature distribution in the furnace becomes uniform. The difference between the two different types of fly ash is mainly reflected in the emissions of SO2 and NOX. Desulfurization fly ash would increase the NOX emission while decrease the SO2 emission, and the optimum desulfurization temperature is 880°C. On the contrary, non-desulfurization fly ash would increase SO2 emission while decrease NOX emission. FARC could effectively improve the combustion performance, as the recirculation rate increases from 0 to 0.3, the combustion efficiency increases by 2% for both two fly ashes and the CO emission decreases, but the particulate matter emission increases. For desulfurization fly ash, Ca/S molar ratio should be appropriately reduced for inhibiting the NOX emission when limestone was used for in situ desulfurization.

Straw Mixed Combustion Characteristics and Kinetic Analysis

The mixed materials come from Beijing Daxing District, corn stalks as the main raw material, peanut shells and wheat straw as affixation. Combustion characteristics of mixed materials are studied by using Thermogravimetric analyzer and combustion dynamics studied by using Coats-Refern method to offer practical and theoretical data for crop straw combustion and Densified . Studies have shown that Combustion curve of three samples have two distinct peaks, divided into water evaporated to dryness, and the combustion of volatiles, fixed carbon combustion and burnout four basic stages and lied a larger overlapping area in the entire combustion stage, That combustion characteristics and kinetic parameters of mixed materials are influenced by adding peanut shells, and less affected on adding wheat straws.

Thermochemical Wear of Refractory Linings Associated With Gas Firing in Metallurgical Plants

The present paper deals with the corrosive interaction between the atmosphere and the refractory lining as well as in one case the steel shell for two industrial facilities. In one of the following cases a magnesia carbon lining is partly decarburized by the atmosphere during preheating of a steel ladle. In the other case, the flue gas from blast furnace gas heating of a hot blast stove causes a nitric attack on the cold side of the refractory lining at the steel shell. Investigation of these mechanisms rely on computational fluid dynamics (CFD) simulations which quantify the flow and temperature field as well as the species transport during heat release from the gas combustion in both plants. Furthermore, the heat exchange between hot gas and refractory is estimated and thereby associated chemical reactions with/within the refractory material. In both CFD case studies a non‐premixed approach was applied to model the combustion process and to characterize the flue gas composition. The follow‐up reactions with refractory lining and the steel casing are discussed specifically for each case. The calculated oxygen content in flue gas during ladle preheating is estimated to 15%vol. The maximum heat transfer between hot gas and refractory takes place in the bottom region of the ladle due to convection. The radiative heat exchange plays the most important role in the zone of magnesia‐carbon lining. Within the investigation of carbon burnout kinetics limiting factors like diffusion and mass transfer coefficient of oxygen were defined and (partially) determined. The NO formation during the blast stove heating period strongly depends on the dome temperature which is an important blast process control value. Calculated NO emission loads showed promising agreement with real plant data and literature trends.

Highly active PdCeOx composite catalysts for low-temperature CO oxidation, prepared by plasma-arc synthesis

The plasma-arc method was adapted for the synthesis of composite palladium–ceria catalysts, which were utilized for the low-temperature oxidation of carbon monoxide. The Pd/CeO2 catalysts were synthesized in two steps: step 1 – direct synthesis of palladium–cerium–carbon composite PdCeC in the plasma-arc chamber, and step 2 – calcination of the composite with carbon burnout in air at 600, 700, and 800°C. Catalytic testing using temperature-programmed reaction demonstrated the high efficiency of the synthesized catalysts during the oxidation of carbon monoxide and their ability to oxidize CO at temperatures as low as room temperature. In comparison with a catalyst that had similar morphology and palladium content but was prepared by coprecipitation, the synthesized catalyst showed that the calculated TOF value for the composite catalyst was two–three times higher than that of the catalyst prepared by chemical methods.A variety of physical methods (HRTEM, XRD, XPS, Raman spectroscopy and others) were used to examine the microstructure, composition and electronic state of the composite components in detail after the high-temperature calcination stage of catalyst synthesis. It was shown that high catalytic activity was provided by the formation of a high-defect fluorite structure of PdCeOx solid solution and highly dispersed PdOx nanoparticles. The XPS data suggest that carbon nanostructures mixed with palladium and cerium in the initial PdCeC composite prevent sintering during high-temperature calcination and play a key role in the formation of a high-defect solid solution of palladium in the CeO2 structure, which has a high concentration of Ce3+ and oxygen vacancies.

Computational Fluid Dynamics Modeling on the Air-Firing and Oxy-fuel Combustion of Dried Victorian Brown Coal

The numerical modeling of the combustion of air-dried Victorian brown coal in O2/N2 and O2/CO2 mixtures with 21–30% O2 has been conducted via the use of computational fluid dynamics (CFD), ANSYS FLUENT 13.0, with the refined weighted-sum-of-gray-gases model (WSGGM), the single-film model with multiple surface reactions (i.e., char-O2, char-CO2, and char-H2O) for char particle, and the refined two-step mechanism for the oxy-firing of methane to mimic the volatile oxidation. The purpose of this study is to verify the experimental observations in a lab-scale drop-tube furnace (DTF) and to promote the understanding on the details underpinning the combustion characteristics of Victorian brown coal, the youngest coal in the world and the single largest source for power generation in Victoria, Australia. As confirmed, the modeling results show good agreement with the experimental measurements on the particle temperature, coal ignition delay photographed by a high-speed camera, carbon burnout rate, and particle velocity. The air-dried Victorian brown coal bears an extremely high reactivity for devolatilization and char-O2 and char-CO2 reactions. The inherent moisture in coal was released with volatile matter simultaneously rather than as that predicted by the CFD wet combustion module for pulverized coal with surface moisture. Increasing the secondary gas temperature greatly narrowed coal ignition delay caused by the substitution of 21% O2 balanced in CO2 for air. At the furnace temperatures of 1073 and 1273 K, the contribution of char-CO2 to coal burnout reached approximately 10 and 25% in the oxy-fuel mode, respectively, which, in turn, reduced the coal particle temperature by a maximum of 300 K. To achieve an identical flue gas temperature with the air-firing case, the use of 30% O2 in CO2 is essential. However, the radiation heat flux match can be achieved by the substitution of 27% O2 in CO2 for air.