Individual particles of pulverized coals exhibit strikingly different combustion phenomena depending on their rank (anthracite, semi-anthracite, bituminous, sub-bituminous and lignite). Herein a ...concise review is presented on pertinent findings in the literature to contrast ignition and combustion behavior of such fuels at the particle-level. Emphasis is given to recent investigations performed in the laboratory of the authors, where combustion of a variety of coal particles of the same size-cut took place in the same apparatus under identical operating conditions. Such behaviors were then compared to those reported in the literature to verify their replication under often different, but yet relevant, conditions. The objective has been to relate the effect of coal rank to a number of key qualitative and quantitative parameters, such as modes of ignition and combustion, ignition temperatures, ignition delay times, combustion temperatures and burnout times (both those encountered in the volatile and the char combustion phases), volatile flame sizes as well as extents of particle fragmentation. Besides reviewing combustion behaviors in air, analogous behaviors under simulated dry oxy-combustion conditions were also highlighted. Then the coal rank dependence of the required oxygen mole fraction in dry O2/CO2 blends to match the intensity of air-fired combustion was examined.
This paper deals with experimental and modelling studies carried out for different Equivalence Ratio (ER). Seven different ER (0.24–0.386) were selected to investigate the downdraft gasifier ...performance. The lignite and sawdust briquette (70:30, %wt.) were used as feedstock in a 10 kWe atmospheric pressure downdraft gasifier. The fuel consumption and air consumption were observed to exist in the range of 0.54 kg h−1 to 11.1 kg h−1 and 0.59 kg h−1 to 19.5 kg h−1 respectively. The Lower Heating Value (LHV) and gas yield were found to be 4.91 MJ Nm−3 and 2.99 Nm3 kg−1 respectively at 0.386 ER. Tar and Particulate Matter (PM) were found in the range of 516.3 mg Nm−3 to 565.23 mg Nm−3 and 47.86 mg Nm−3 to 57.29 mg Nm−3 respectively for different ER. The CGE observed was 80.03% whereas the tar and PM were reported as 516.3 mg Nm−3 and 47.86 mg Nm−3 respectively at ER of 0.386. A thermo-equilibrium model was developed to estimate the producer gas compositions and LHV. The results were compared with the experimental results from the literature and good agreement between the two was observed.
•Air Gasification with lignite was carried out with seven Equivalence Ratios (ER).•Lower Heating Value and cold gas efficiency were found maximum at 0.386 ER.•Tar and particulate matter significantly reduced at higher reaction temperature.•Thermo-equilibrium model offered satisfactory results for validating experiment results from literature.
•Adding >4wt% silica is essential in promoting the agglomeration of ash.•Formation of friable calcium sulfate is favored by the use of silica additive.•The capture of Na led to the transformation of ...Na into coarse bottom ash.•A portion of Fe was precipitated as discrete grains out of slag matrix.•Mg was highly crystallised within the slag.
In this paper, we, for the first time report the properties of ash deposits collected from the combustion of lignite mixed with silica additive, in a 30MWth pulverised-fuel boiler with sub-critical steam conditions. The combustion test was carried out continuously over ten months, and ash deposits were sampled from various locations in the boiler. As has been confirmed, the ash-related slagging and fouling have been controlled effectively. Apart from capturing volatile alkali and alkaline earth metals, silica added in an excess amount is beneficial in mixing with molten liquidus to promote their agglomeration, rather than adhering to the vicinity of coal burners for slagging. Compared to the formation of abundant Fe2+-slag that is prone to stick to coal burner vicinity in the raw coal case, the formation of friable calcium sulfate in fireside deposit is favored upon the mixing of silica with coal, which is even predominant in the fireside ash deposit. The calcium sulfate formed is highly layered in a plerospheric structure in which Ca-silicate is present as the core with calcium sulfate being the shell. The physical adsorption of free Ca on silicate surface is supposed to enhance its specific surface area for a quicker sulfation. The capture of Na by silica is also efficient, leading to the transformation of a remarkable portion of Na into coarse bottom ash with less being sulfated. This in turn alleviates the sulfate – relating fouling propensity on the water tube surface in the convection zone. A noticeable portion of Fe was noticed to remain as discrete grains outside the Ca-rich slag matrix. In contrast, the majority of Mg is highly crystallised as sub-micrometer Ca/Mg-silicate and oxide particles residing within slag matrix, which in turn increase the viscosity and decrease the sticking propensity of the molten slag.
A dynamic CFD model, which is based on the inertia impaction, the thermophoresis and the direct alkali vapour condensation incorporating the influence of the heat transfer to the tube, has been ...developed for predicting the ash deposition formation in Zhundong lignite combustion in a pilot-scale furnace. The results show that particle deposition from the inertia impaction and the thermophoresis dictates the ash deposition formation under high furnace temperatures. The deposition caused by the direct alkali vapour condensation is less significant. As deposition time increases, particle impaction efficiency decreases and sticking efficiency increases due to the thermophoresis and the local temperature conditions, which result in the time-dependent behaviour of the deposition growth. In addition, the ash deposition characteristics are influenced under different furnace temperatures, due to the change in the particle impaction and sticking behaviours. Qualitative agreement is obtained between the predicted results and the measurements for the heat flux to the tube and the ash deposition growth.
•The free and bound water in lignite were quantified before and after HTD.•The removal of free water was associated with the collapse of macropore.•The bound water was reduced because of the removal ...of oxygen functional groups.•DFT calculation on the interaction energy between lignite and water were carried out.
High moisture content greatly restricts the large-scale utilization of low-rank coals (LRCs). Hydrothermal dewatering (HTD) is a promising technique for dewatering and upgrading LRCs. Chinese lignite from XiMeng Mine in Inner Mongolia was upgraded by HTD. The effects of pore structure and oxygen functional group content on moisture distribution were investigated. Thermogravimetric analysis, mercury intrusion porosimetry, and chemical titration were conducted to characterize the physicochemical properties of coal samples. Results show that a substantial amount of moisture was removed, the coal composition was modified, and the energy density was significantly improved after HTD. Both free-phase and bound-phase moisture in lignite were significantly removed. Free water removal was associated with the collapse of macropore structures caused by shrinkage forces. The bound water was remarkably reduced as a result of the removal of oxygen functional groups, such as phenolic hydroxyl and carboxyl. During HTD, the oxygen functional groups were decomposed, and the hydrogen bonding between water and hydrophilic sites was destroyed, thereby leading to a weakening of the water-holding capacity of lignite. Four representative model molecules with different polarities were used to study the intermolecular interactions between lignite and water. The interaction energies of model molecule⋯water complexes were determined by density functional theory (DFT) calculation. The types of non-covalent interactions of both hydrophobic and hydrophilic sites in lignite with water were vividly demonstrated using color-mapped reduced density gradient isosurface. The carboxyl and hydroxyl groups are extremely liable to interact with water via hydrogen bonding with large interaction energy and exhibiting strong hydrophilicity. On the other hand, the alkanes and benzene rings interact with water via van der Waals attractions and show hydrophobic effects.
This research examined the reduction of combustion-generated sulfur dioxide and nitrogen oxide emissions from a pulverized high-sulfur bituminous coal by co-firing it with a lignite coal and by ...substituting air with O2/CO2 gases, of compositions pertinent to dry once through oxy-fuel combustion conditions. Oxygen mole fractions were in the range of 21–30%. The neat bituminous and lignite coals as well as a 50–50wt% blend thereof were burned in a laboratory-scale electrically-heated drop-tube furnace under fuel-lean conditions. Coal particles were in the size range of 75–90μm and the furnace was operated at 1400K. Results showed that the SO2 emissions from the lignite coal were drastically lower than those from the bituminous coal, whereas the NOx emissions were only mildly lower. Co-firing the high-sulfur bituminous coal and the low-sulfur lignite coal reduced the SO2 emissions, under both air and oxy-combustion conditions, to values well-below those predicted by linear interpolation of the respective emissions of the two neat coals. This observation, in conjunction with ash analysis shows that the alkali-rich ashes of the lignite coal acted as sulfur sorbents for the copious SO2 emissions of the bituminous coal. This behavior was contrasted to SO2 sorption when the bituminous coal was co-injected with calcium- and sodium-based sorbents at analogous alkali to sulfur ratios. Co-firing of the coals reduced the NOx emissions only mildly, whereas the substitution of the background N2 gas with CO2 resulted in much more extensive NOx reductions.
•Digital image technique was used to monitor the morphology of sintered ash samples under oxy-fuel combustion atmosphere.•The change in height and area of sintered ash samples with time was obtained ...through image processing system.•Layer structure of the sintered samples with different colors was analyzed.•Mineralogy, chemical compositions, and microstructure of layer structure of sintered samples were analyzed.
This paper was addressed to evaluate the sintering behavior of coal ash at different temperatures under oxy-fuel combustion (O2/CO2 combustion) atmosphere. A lignite (Zhundong coal) characterized by high sodium level has been applied as the fuel material. The raw ash was heat treated at four different temperatures: 1350, 1300, 1250, and 1200°C, respectively. In addition, two atmospheres has been applied to as the reaction atmosphere: (1) 30vol.% O2/70vol.% CO2 (oxy-firing); (2) air (reference). Meanwhile, the shape of the sintered ash samples during the sintering process was monitored by charge coupled device (CCD). The shrinkage of height and area of sintered samples can be obtained by the image processing system. In addition, the microstructure and chemical compositions of the sintered samples were analyzed by a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry (EDX). Moreover, the distribution of mineral phases in the sintered samples was identified by X-ray diffraction (XRD). The results show that the sintering degree of the samples increases with temperature. In addition, all the samples are characterized by layer structure with different colors. Meanwhile, the XRD results reveal that the oxy-fuel combustion atmosphere did not change dramatically the kinds of mineral phases, but did influence the relative amount of crystalline phases.
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•A mathematical model for explosion suppression of coal by inert powder is built.•Suppression effect of FAC on SM bitumite is greater than that of ZT lignite.•This model is suitable ...for different coal species.•Relative errors between the predicted and experimental values are <0.1.•50 wt% FAC basically achieves the complete suppression of coal explosion.
This study aims to predict the severity of coal dust explosion in confined space and to provide guidance for effective suppression of explosion. Based on kinetic analysis of devolatilization and heat transfer mechanism of inert particle-coal particle-pyrolysis gas in confined space, a mathematical model for suppressing coal dust explosion by inert powder was established. Compared with the experimental results of 20 L spherical explosion tank, relative errors of the maximum explosion pressure (Pmax) and the maximum rise rate of explosion pressure ((dP/dt)max) predicted by the model were both <0.15. The particle size of inert powder was concentrated in 0.1–10.0 μm, while 90 vol% coal particle was in the size range of 0–23 μm. The convective of pyrolysis gas and coal was weaker than the radiation of coal and inert particle. When the proportion of inert suppressant increased to 50 wt%, (dP/dt)max was reduced from 50.14 MPa/s of raw coal to 6.70 MPa/s, and Pmax was reduced from 0.78 MPa of raw coal to 0.16 MPa, which basically achieved the complete suppression of explosion. Due to the difference in inherent composition of coal, the Pmax and (dP/dt)max of ZT lignite were higher than that of SM bituminite. This study can provide a basic reference for the severity of coal dust explosion and suppression prediction in industry.
•Effect of acid washing on the structure of coal was revealed.•Decomposition of chemical bonds during coal pyrolysis process was evaluated.•Influence of catalysts on the distribution of light species ...was investigated.•Mechanisms of catalytic pyrolysis were explored by XRD experiments.
The catalytic effects of three metal chlorides, including KCl, ZnCl2, and MnCl2, on the pyrolysis of lignite were investigated by thermogravimetry (TG) coupled with Fourier transform infrared (TG-FTIR) analysis from 105°C to 1000°C at a heating rate of 10°C/min. The procedure of acid washing and the addition of metal chlorides had little effect on the structure of coal. The catalytic effect of inherent mineral became obvious only when the temperature was higher than 660°C. The addition of metal chlorides promoted the pyrolysis process when the temperature was lower than 400°C and higher than the critical temperature. DTG curve was fitted into six sub-curves to further analyze the pyrolysis process especially the breakage of chemical bonds. All three metal chlorides had a good catalytic effect on the quantity of CH4 production but the opposite effect on CO production. MnCl2 was the most favorable metal chloride for the production of CH4. The catalytic mechanism of KCl was different from that of MnCl2 and ZnCl2 on coal pyrolysis.
Several reactions parallel happened during co-pyrolysis of coal and biomass, such as primary volatiles of coal and biomass will contact and react with the nascent char from coal or biomass ...themselves' pyrolyzed. To clarify the function of bio-char and investigate if bio-char has the catalytic effect on pyrolysis products of Hulunbuir lignite, a two-stage reactor is designed and used in this study. The effects of bio-char preparing temperature, char-making method and usage amount of bio-char were discussed. In comparison with the alone coal pyrolysis at 650 °C, the portion of light tar (n-hexane soluble) from pyrolysis of coal with bio-char (20 wt% of coal), increased by 30.31%, gas volume yield increased by 9.05% and the volume yield of hydrogen and methane increased by 6.36% and 5.64%, respectively. For the most efficient RSCR-700 against the test with quartz, the H/C molar ratio in RSCR-700's produced tar increased by 22.06%, while the contents of N and S elements decreased by 27.88% and 25.0% accordingly. The role of bio-char in the coal tar upgrading process was further explored through the instruments characterization. Besides the thermal cracking, the bio-char also can provide catalytic active sites to crack heavy fraction of coal tar.
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•Bio-char has catalytic cracking function in lignite pyrolysis.•Amount of light tar increased and element N and S in the resulting tar lowered.•Surface O, K, and Na etc. of bio-char contribute to coal tar upgrading.
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