Tar Combustion Research Articles

ReaxFF simulations on the transformation pathway of nitrogen elements in the heavy tar under oxy-coal combustion

Heavy tar is a crucial intermediate product during coal combustion. To explore the transformation pathway of N atoms in heavy tar under oxy-coal combustion, a comprehensive molecular model of heavy tar with typical N-containing functional groups was constructed. Different temperatures and chemical equivalence ratios were set for the oxy-coal combustion. The ReaxFF was employed to study various products' distribution and molecular numbers. The reaction network among different precursors and NOx was extracted, and the NO to N2 conversion mechanism was summarized. The results indicated that, like char combustion, the proportion of heavy tar gradually declined, the proportion of light tar and organic gas first rose and then gradually declined, and the proportion of inorganic gas continuously rose during heavy tar combustion. As the temperature increased, the proportion of cyanide precursors decreased, while the proportion of amine precursors and NOx increased. The oxidation of N-containing intermediates became more intense as the O2 content rose, but this oxidation effect was inhibited, and the NOx generation was reduced as the O2 content further increased. NO could bond with NHi, HNO, CN, and activate NO, decomposing to produce N2O, and N2O reacted with H radical to produce N2.

Fire zone forefront residual tar combustion characteristics and functional group transformation mechanism: Appling the based on thermogravimetric and in-situ high-temperature FTIR

When preventing spontaneous combustion disasters in extinguished coal fire zones, the influence of the tar re-ignition phenomenon is often overlooked. Therefore, in this study, using thermogravimetric analysis and high-temperature (630 °C) in-situ infrared technology, the combustion reactivity and real-time evolution mechanism of key functional groups for tar were deeply discussed under oxygen-lean conditions. Additionally, a novel two-step baseline drift correction method for spectral graphs at high temperatures is proposed. The results showed that tar combustion could be divided into two stages: pyrolysis-volatilization mass loss and crosslinked structure oxidation combustion. Reduced oxygen concentration limits the violent and stable rapid combustion of tar-crosslinked structures and reduces the re-ignition tendency of tar. Among, with a decrease in oxygen concentration, H (burnout performance), Cb (the reaction ability in the early stage of ignition) and S (overall combustion performance) decreased, while Ti (mass loss endpoint temperature), Th(burnout temperature), and HF (combustion stability) increased. This provides information for the selection of a reasonable process to prevent tar re-ignition. The contents of methylidene and aldehyde carbonyl in tar increased by approximately 2 and 3.7 times, respectively, compared to those of raw coal. In the first thermal decomposition stage of tar, the aromatic hydrocarbons were relatively stable under the action of the structural framework, whereas aliphatic hydrocarbons continued to decrease. Before burnout, a decrease in oxygen concentration increased the crosslinking ability of tar and delayed the tar burnout process. The contents of aromatic hydrocarbons and aldehyde groups are highly sensitive to the evolution of the oxygen concentration. Specifically, the Kubelka–Munk value of the aromatic hydrocarbons increased from 0.015 to 0.10, while that of aldehyde carbonyl groups increased from 0 to 0.23. Additionally, in the early stage of tar combustion, the volatile mass loss was basically not affected by the time-scale effect, and mass loss endpoint residual mass was approximately 48.11 %. In the later stage of tar combustion, owing to the cumulative of time-scale effect, the tar re-ignition tendency increased, H, Cb, and S increased, and HF decreased from 1.388 to 1.098. This research is helpful guiding disaster prevention and control of tar re-ignition in a fire area, which leads to reburning in the surrounding area.

Process development for a novel polygeneration purpose based on tars produced by a biomass gasification unit; feasibility study from the thermodynamic, economic, and environmental viewpoints

The tar produced by biomass gasification can be challenging to handle in processing units. To address this issue, a new polygeneration structure based on tar combustion has been designed in this study. This structure has been developed using Aspen HYSYS software and consists of subsystems, including the tar combustion section integrated with a gas turbine cycle, a low-grade temperature section, a Goswami utility section, and a hydrogen production section. The products of this structure are 7920.65 kg/s of hot water, 33.52 kg/s of cold water, 0.56 kg/s of hydrogen, 4.444 kg/s of oxygen, and 222100 kW of power. The system is examined from various perspectives: energy, exergy, environment, and economics. The overall energy and exergy efficiencies are 89.72% and 54.53%, respectively. The exergy analysis exhibits a total irreversibility of 859,492 kW, in which the low-grade temperature section has the highest exergy destruction with a share of 72%. Accordingly, the environmental analysis demonstrates that the new process has zero indirect carbon dioxide emissions, and its footprint is equivalent to 0.22 kgCO2/kWh. Also, the products have been produced according to economic estimates with a cost of energy of 0.0068 $/kWh and a total unit cost of products of 0.0274 $/GJ.

Flue gas analysis for biomass and coal co-firing in fluidized bed: process simulation and validation

Coal-conversion technologies, although used ubiquitously, are often discredited due to high pollutant emissions, thereby emphasizing a dire need to optimize the combustion process. The co-firing of coal/biomass in a fluidized bed reactor has been an efficient way to optimize the pollutants emission. Herein, a new model has been designed in Aspen Plus® to simultaneously include detailed reaction kinetics, volatile compositions, tar combustion, and hydrodynamics of the reactor. Validation of the process model was done with variations in the fuel including high-sulfur Spanish lignite, high-ash Ekibastuz coal, wood pellets, and locally collected municipal solid waste (MSW) and the temperature ranging from 1073 to 1223 K. The composition of the exhaust gases, namely, CO/CO2/NO/SO2 were determined from the model to be within 2% of the experimental observations. Co-combustion of local MSW with Ekibastuz coal had flue gas composition ranging from 1000 to 5000 ppm of CO, 16.2%–17.2% of CO2, 200–550 ppm of NO, and 130–210 ppm of SO2. A sensitivity analysis on co-firing of local biomass and Ekibastuz coal demonstrated the optimal operating temperature for fluidized bed reactor at 1148 K with the recommended biomass-to-coal ratio is 1/4, leading to minimum emissions of CO, NO, and SO2.

Gasification of Biomass: The Very Sensitive Monitoring of Tar in Syngas by the Determination of the Oxygen Demand—A Proof of Concept

A novel method for quasi-continuous tar monitoring in hot syngas from biomass gasification is reported. A very small syngas stream is extracted from the gasifier output, and the oxygen demand for tar combustion is determined by a well-defined dosage of synthetic air. Assuming the total oxidation of all of the combustible components at the Pt-electrode of a lambda-probe, the difference of the residual oxygen concentrations from successive operations with and without tar condensation represents the oxygen demand. From experiments in the laboratory with H2/N2/naphthalene model syngas, the linear sensitivity and a lower detection limit of about 70 ± 5 mg/m3 was estimated, and a very good long-term stability can be expected. This extremely sensitive and robust monitoring concept was evaluated further by the extraction of a small, constant flow of hot syngas as a sample (9 L/h) using a Laval nozzle combined with a metallic filter (a sintered metal plate (pore diameter 10 µm)) and a gas pump (in the cold zone). The first tests in the laboratory of this setup—which is appropriate for field applications—confirmed the excellent analysis results. However, the field tests concerning the monitoring of the tar in syngas from a woodchip-fueled gasifier demonstrated that the determination of the oxygen demand by the successive estimation of the oxygen concentration with/without tar trapping is not possible with enough accuracy due to continuous variation of the syngas composition. A method is proposed for how this constraint can be overcome.

Combustion characteristics and kinetic analysis of heavy tar from continuous pyrolysis of camellia shell

The objective of this research was to study the combustion kinetics of heavy tar from continuous pyrolysis of camellia shell. TG-DSC was used for combustion study with heating rates of 10, 20, and 30 °C/min, temperatures from 26 to 800 °C, and air flowrate of 100 mL/min. Results showed that the combustion process, with three stages, includes low-boiling components volatilization, light components decomposition and combustion, and heavy components and char combustion. The correlation among heating rate, maximum mass loss rate and mass loss ratio and the combustion behaviors at each stage with different heating rates were analyzed. The activation energy and frequency factor of each stage were obtained by Friedman and Ozawa methods. The reaction mechanisms are the random nucleation and growth, and the kinetic parameters were obtained by fitting experiment data with theoretical models. The mechanisms and kinetic parameters obtained could be used to describe the heavy tar combustion.

Kinetics of the Catalytic Combustion of Coal Tar

Coal tar combustion behavior was investigated in coexistence with ferric oxide and kaolin as catalysts. Thermogravimetry and differential thermal analysis experiments were performed using heating rates of 10, 20, and 30°C/min. The apparent activation energy (E) and pre-exponential factor (A) for combustion kinetic were obtained. The results showed that addition of ferric oxide and kaolin can improve tar combustion properties efficiently.

Self-sustaining smouldering combustion of coal tar for the remediation of contaminated sand: Two-dimensional experiments and computational simulations

This study presents the development and validation of a computational model which simulates the propagation of a smouldering front through a porous medium against unique experiments in coal tar and sand. The model couples a multiphase flow solver in porous media with a perimeter expansion module based on Huygens principle to predict the spread. A suite of two-dimensional experiments using coal tar-contaminated sand were conducted to explore the time-dependent vertical and lateral smouldering front propagation rates and final extent of remediation as a function of air injection rate. A thermal severity analysis revealed, for the first time, the temperature–time relationship indicative of coal tar combustion. The model, calibrated to the base case experiment, then correctly predicts the remaining experiments. This work provides further confidence in a model for predicting smouldering, which eventually is expected to be useful for designing soil remediation schemes for a novel technology based upon smouldering destruction of organic contaminants in soil.

Polycyclic aromatic hydrocarbons and their quinones modulate the metabolic profile and induce DNA damage in human alveolar and bronchiolar cells

The release of particulate pollutants into the air through burning of coal, crude oil, diesel, coal tar, etc. raises concerns of potential health hazards to the exposed human population. Polycyclic aromatic hydrocarbons (PAHs) are major toxic constituents of particulate matter (PM), which upon ingestion get metabolized to even more toxic metabolites such as quinones. The PAHs levels were assessed in both respirable particulate matter (RSPM, <10μM size) and suspended particulate matter (SPM, >10μM size) of urban ambient air (UAA) and that of major contributors viz. diesel exhaust particles (DEPs) and coal tar combustions emissions (CTCE). Seven US Environmental Protection Agency (USEPA) prioritized PAHs in RSPM and 10 in SPM were detected in UAA. Ten and 15 prioritized PAHs, respectively, were also detected in diesel exhaust particles (DEP) and coal tar combustion emission (CTCE) evidencing their release in the air. These PM associated PAHs for UAA, DEP and CTCE showed significant increase (p<0.05) in mutagenicity and mammalian genotoxicity in the order CTCE>DEP>UAA. Human lung alveolar (A549) and bronchiolar (BEAS-2B) cells when treated with PAH-metabolites viz. 1,4-benzoquinone (1,4-BQ), hydroquinone (HQ), 1,2-naphthoquinone (1,2-NQ), 1,4-naphthoquinone (1,4-NQ) and 9,10-phenanthroquinone (9,10-PQ) showed metabolic modulation in these cell lines with significant depletion of principal cellular metabolites viz. NADP, uracil, asparagines, glutamine, and histidine and accumulation of di-methyl amine and beta-hydroxybutyrate, identified using (1)H NMR spectroscopy. These results suggest that PAH-quinones induce genotoxic effects by modulating the metabolic machinery inside the cells by a combined effect of oxidative stress and energy depletion. Our data for metabolic profiling of human lung cells could also help in understanding the mechanism of toxicity of other xenobiotics.

Combustion Kinetics Characteristics of Dry Distillation Coal Tar

In order to improve burning-out characteristic of dry distillation coal tar, the sample of dry coal tar was investigated by thermo-gravimetric analyzer(TGA), and the sample’ properties of kinetics and burning-out were studied. The results show that the dry distillated tar combustion is mainly dynamic combustion with diffusive combustion as a supplement. The combustion activation energy was 30 kJ/mol above, but the activation energy, frequency factor and reaction order appeared change on the subsequent combustion. The dry distillated coal tar is easy to be ignited, but difficult to burn out, so its maximum burn out temperature is higher.

Effect of the addition of cornstalk to coal powder/coal tar combustion

Thermogravimetry (TG) and differential thermal analysis were used to investigate combustion behavior with the addition of cornstalk to coal powder and coal tar at three different heating rates: 5, 10, and 15 °C min−1. Different reaction kinetic mechanisms with the classical Arrhenius model were used to treat TG data. The first-order combustion model fitted the data well. The combustion characteristics of the two masses were analyzed according to combustion characteristics parameters such as ignition temperature, peak temperature at maximum weight loss rate, and burnout temperature, among others. By calculation, a uniform trend of decreasing activation energies was observed with the addition of cornstalk. The rate of coal powder and coal tar combustion process was also greatly improved.

Characterization of tar content in the syngas produced in a downdraft type fixed bed gasification system from dried sewage sludge

Tar yields in the syngas produced in a pilot-scale downdraft type fixed bed gasification system from dried sewage sludge have been quantified and characterized to identify the effect of equivalence ratio (ER of 0.29–0.36). The increase of ER resulted in higher temperature of oxidation zone because air promoted the combustion reaction. High ER and high temperature also enhanced cracking and combustion of tar. Lower tar mass was observed while increasing ER. The change in tar composition with the change of ER was also observed by using the size exclusion chromatography (SEC). The SEC results showed that heavier molecular tar (in the molecular weight range of 300–500 u) formed whereas lighter molecular tar decreased under the higher ER conditions. Tar removal performances of the gas cleaning system (the venturi scrubbers and the sawdust adsorbers) were also investigated. The tar removal efficiency of the gas cleaning system depended on gasification conditions, tar components and the amount of tar. Tar content in the syngas was reduced to 26–53% and 14–36% (by weight) at the exit of the scrubbers and sawdust adsorbers, respectively. By the action of this gas cleaning system, about 44% of light aromatic hydrocarbon tar was removed while no light PAH tar was detected at the exit of the gas cleaning system.

Kinetics of Perovskite Catalyzed Biomass Tar Combustion Studied by Thermogravimetry and Differential Thermal Analysis

Biomass tar combustion behavior was investigated with perovskite (CaTiO3, Ca2Fe2O5) as catalysts using heating rates of 5, 10, 15, 20, and 30 °C min−1. The addition of 10 wt % of CaTiO3, Ca2Fe2O5 greatly improved the tar combustion characteristics. For the combustion process, a three stages model, volatilization, low-temperature oxidation (LTO), and high-temperature combustion (HTC), was proposed and applied. The starting temperature of HTC stage was lowered by 30−60 °C, and the combustion residue decreased by nearly 10 wt % resulting from the catalyst’s presence. Different reaction kinetics mechanisms using the classical Arrhenius equation were applied to treat TG data, the results showing the first-order combustion model fitted the data best. Volatilization and HTC stages had large apparent activation energy (E), and a uniform trend of decreasing E was observed in the presence of CaTiO3, Ca2Fe2O5.

Study on the combustion kinetic characteristics of biomass tar under catalysts

TG and DTA experiments were performed to investigate the biomass tar combustion behavior in coexistence of dolomite and mayenite at two different heating rates as 5 and 15°C min−1. Different reaction kinetic mechanisms with the classical Arrhenius model were used to treat TG data, and showed that the first-order combustion model fitted the data well. Three stages combustion model was proposed and applied for the calculation of kinetics parameters successfully. The starting temperature of high temperature combustion stage moved up near 100°C because of the coexistence catalysts, and the combustion amount of biomass of the stage also improved nearly 10 mass%. By calculation a uniform trend of decreasing activation energies was observed with the addition of dolomite and mayenite, and also greatly improved the amount and speed of tar combustion process.

AN EXPERIMENTAL STUDY OF THE TREATMENT OF ALUMINUM-CELL GASES FOR POLLUTION CONTROL USING FLUIDIZED-BED TECHNOLOGY

This paper describes the two-stage treatment of tars and hydrogen fluoride contained in emitted aluminum-cell gases for pollution control and recovery of useful components. The treatment involves combustion of tar and then chemisorption of hydrogen fluoride on alumina particles in a fluidized-bed reactor of 25 cm inner diameter. Calcined coke was used to support the combustion of tar. Smelter-grade alumina was used as the bed material with average particle size of 66.71 μm. Stability and temperature uniformity of the bed were confirmed for different operating conditions. The tar was removed with combustion efficiency of up to 98% at a bed temperature of 700–900°C. Heat removal from the bed by water cooling tubes was achieved with a heat transfer coefficient as high as 450 W/m2 · K. The hydrogen fluoride was captured by chemisorption with a scrubbing index of up to 0.75 kg−1 depending on the bed temperature, distributor design, fluidizing velocity, and bed height. Good reactor performance could be obtained with a distributor fitted with 113 nozzles each having six inclined holes.

Volatile matter release, particle/cloud ignition, and combustion of near-stoichiometric suspensions of pulverised coal

The ignition characteristics of a cloud of pulverised coal flowing in a drop-tube furnace are examined.Cloud ignition is defined at the minimum gas temperature at which significant oxygen consumption occurs. This is accompanied by the generation of carbon dioxide and oxides of nitrogen, and the formation of a visual flame. Cloud ignition occurs at several hundred degrees above that for the ignition of some single coal particles. For three coals the cloud ignition temperature decreased with oxygen partial pressure, but was not sensitive to fuel equivalence ratio or particle size (as has been observed for single particles). For the three coals examined, pyrolysis measurements show the existence of a tar/light hydrocarbon/oxygenmixture at the cloud ignition temperature. On combustion the accompanying temperature increase results in coal burnoffs from 35 to 65% even though the combustible volatile matter prior to the ignition event corresponds to less than 6.5% burnoff. Different trends of tar and methane combustion and soot formation are observed in the experiments as gas temperature and oxygen levels were changed; however, significant tar combustion was always observed at cloud ignition. A mechanistic model is proposed to explain the observations.