Char Combustion Reactivity Research Articles

NO emission during the co-combustion of biomass and coal at high temperature: An experimental and numerical study

Biomass/coal co-combustion presents a promising avenue for reducing carbon dioxide emissions. However, the mechanism of NO generation during co-combustion in pulverized coal-fired furnaces remains unclear. Herein, this study investigates NO emission characteristics at elevated temperatures, emphasizing mechanistic understanding. The experiments and simulations were conducted across temperatures ranging from 1000 °C to 1600 °C. In addition to exploring how fundamental conditions (temperature, oxygen concentration, and biomass blending ratio (BBR)) affect NO generation, the study also investigates the interactions between biomass and coal, as well as between fuel NO and thermal NO. The results indicate that the change in NO production with increasing temperature is non-monotonic, attributable to the combined contribution of fuel NO and thermal NO. Moreover, the interaction between biomass and coal manifests predominantly during the char combustion stage, owing to the disparate combustion characteristics and reactivity of biomass char and coal char. Numerical simulations were highly consistent with the experimental findings, underscoring a reduction in the rate of production (ROP) for major elementary reactions leading to NO formation as the BBR increased, consequently mitigating NO emissions. By integrating experimental insights with reaction kinetics calculations, this study proposes pathways for fuel NO and thermal NO generation during co-combustion at high temperatures, involving precursors such as NCO, HNO, and NH. These findings offer a deeper understanding of the mechanism of fuel NO and thermal NO emission during co-combustion at high temperatures.

Mechanism of CO2 and H2O Action on Char in Oxy-fuel Combustion with Wet Flue Gas Recirculation

The mechanism of the combined action of the C–CO2 and C–H2O reactions has yet to be clarified in the study of oxy-fuel combustion with wet flue gas recirculation (WFGR). The ultrahigh temperature (1573 K) and atmosphere of an industrial boiler are simulated in the flat flame burner reaction system (FFB-RS). The measurement of surface reactive groups and combustion reactivity of char uses Fourier transform infrared spectroscopy (FTIR) and a microfluidized bed reaction analyzer (MFBRA). The results show that char combustion reactivity has an excellent positive correlation with an O-containing reactive group. Above a 35% H2O concentration, its physical properties dominate, while below 35%, its chemical properties play a significant role. When the ratio of H2O/CO2 is close to 1, there is a synergistic relationship between the C–CO2 and C–H2O reactions, which did not occupy the same active sites, promoting the formation of O-containing reactive groups (33.83%) and alkyl complexes (19.6%) with a combustion reaction rate of 0.225%/s.

Experimental investigation on effect of external circulation system on preheating characteristics of pulverized coal

To explore whether self-preheating combustion systems ran steadily after loop seal failed, we comparatively investigated the influence of primary air ratio (λp) and extra heat input ratio (Cehi) on self-preheating of pulverized coal preheated in a 30 kW self-preheating burner with or without an external circulation (EC) system. For temperature distribution, lack of EC system greatly raised the temperature in the middle-lower part of the riser, and this temperature was more than 150 °C higher than that with the EC system. Moreover, extra heat input extended this influence throughout the riser. As λp and Cehi increased, the temperature in different positions always rose. For coal gas component and fuel component release, lack of EC system improved fuel composition conversion rates but reduced combustible gas yields. These two indexes increased simultaneously as λp and Cehi elevated properly, and there was an optimal λp with the EC system. Lack of EC system was inconducive to reducing particle size, improving specific surface area, promoting micro-pore generation on the surface and reducing graphitization extent, meaning the macrostructure and microstructure tended to be deteriorated in this condition. Nevertheless, the microstructure in this condition may be improved with extra heat input, and was even better than that with the EC system. The macrostructure and microstructure were improved as λp and Cehi increased properly, and there was an optimal λp with the EC system. The combustion reactivity of coal char was mainly influenced by the macrostructure, followed by the microstructure. Hence, absence of EC system tended to deteriorate it, and it was improved as λp and Cehi increased properly.

A study on pyrolysis of wood of different sizes at various temperatures and pressures

In this work, three different particle sizes (0.5 ∼ 1 mm, 2 mm and 5 mm) of pine wood were pyrolyzed at the temperature range between 350 and 800 °C and pressures between 1 and 8 bar in a pressurised fixed bed reactor. A monolayer of different particle sizes of biomass samples was also pyrolysed at various pressures to assess by comparison the effect of intra-particle vs inter-particle secondary reactions on char formation. The relatively small particle and reactor size as well as the slow heating, enable assessing the effects of particle size and pressure in the absence of heat transfer limitations. The liquid and solid products were characterized by various techniques, including gel permeation chromatography (GPC), gas chromatography with mass spectrometry detection (GC–MS), elemental analyser and Raman spectroscopy. The combustion reactivity of chars obtained at various conditions was tested by thermogravimetric analysis (TGA). Higher temperatures promote tar and gas yields at the expense of solid residues. The chars produced at higher temperatures show greater aromaticity and lower combustion reactivity. The larger particle size can decrease tar formation at 350 °C, while smaller particle sizes decrease tar yield through the inter-particle secondary reaction within the hot char bed at 500 °C and above. For larger particle sizes, a stronger intra-particle secondary process can make the chars more ordered, thus leading to a lower combustion reactivity. Higher pressure can hinder the vaporization of fragments and promote the formation of secondary chars, thus less tar and more char were obtained. Chars produced at higher pressures show a lower combustion reactivity due to the higher aromaticity of secondary chars. Therefore, higher temperature, larger particle size and higher pressure can all produce more ordered chars with lower combustion reactivity.

Dynamic investigation on potassium migration and transformation during biochar combustion and its correlation with combustion reactivity

For agricultural biomass combustion, K migration and transformation is a key factor affecting boiler operation and reaction characteristics. In this study, dynamic K migration and transformation characteristics during rice straw char combustion process under different O2 concentrations were studied using chemical fractionation analysis (CFA), X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive X-ray (SEM-EDX). Furthermore, the correlation between K migration/transformation and char combustion reactivity was discussed. The results showed that most of K in rice straw char was H2O-soluble K. And the main forms of H2O-soluble K and insoluble K are KCl and KAlSi3O8, respectively. As reaction proceeded, the contents of H2O-soluble K and insoluble K decreased but those of NH4Ac-soluble K and HCl-soluble K increased. Most of the released K is H2O-soluble K. H2O-soluble K would be transformed into NH4Ac-soluble K and HCl-soluble K, and some insoluble K would be transformed into H2O-soluble K. The increase of O2 concentration would stimulate the transformation of H2O-soluble K into NH4Ac-soluble K, HCl-soluble K and released K. However, the reaction to form K-aluminosilicate was hardly found during the combustion process at 400 ℃. Ultimately, it is found that H2O-soluble K is the key occurrence form K affecting the combustion characteristics of rice straw char.

Combustion reactivity of chars pyrolyzed from low-rank coal using a fixed bed reactor installed with internals

Pyrolysis of low-rank coal could contribute toward the goal of upgrading coal quality, chemical feedstock and fuel production. This research investigated the combustion reactivity of char pyrolyzed from a low-rank sub-bituminous coal using a conventional fixed bed reactor and a novel reactor with internals at 0 kPa and − 40 kPa gauge pressures. Proximate and ultimate analyses showed that the novel reactor reduced char’s fuel ratio by 46% and 54% at 0 kPa and − 40 kPa respectively. The chars derived from the novel reactor also had a higher order of carbonaceous structure and active sites, as supported by the increment in both IG/IALL and ID3/(IG+ID2+ID3) area ratios obtained from a Raman spectrometry analysis. From a non-isothermal combustion analysis, it was found that the ignition temperature of char derived from the novel reactor was 24–40 °C and 30–45 °C lower at 0 kPa and − 40 kPa respectively. A micro fluidized bed reactor analyzer was used for an isothermal differential analysis and it showed that chars produced from the novel reactor had an average combustion activation energy of around 77.3 kJ·mol−1, which was 23% and 10% lower than chars obtained from the conventional reactor at 0 kPa and − 40 kPa respectively. Overall, the novel reactor produces chars with more desirable combustion quality and, therefore, provides the opportunity for more low-rank coal to be upgraded and utilized more effectively as a fuel source for energy sustainability.

Effects of the T-abrupt exit configuration of riser on fuel properties, combustion characteristics and NOx emissions with coal self-preheating technology

NOx generated from coal combustion brings about severe environmental problems. Among many combustion technologies, coal self-preheating combustion technology is valued for its great potential of low NOx combustion, while in-depth studies on the structure of the self-preheating device (SPD) have not been carried out, which makes it difficult to further reduce NOx. In addition, disposal of coal gasification fly ash (CGFA), a kind of solid waste, is still mainly dissipated in landfills, resulting in the pollution of soil, water and wastes energy. In order to optimize the SPD structure for further reducing NOx and provide technical support for the engineering utilization of CGFA, the properties of preheated fuel (coal gas and coal char), the behavior characteristics of combustion and NOx emission under different He/Hr (the ratio of projected height to the riser height) were investigated on a 30 kW bench-scale test rig. Stable and high efficient MILD combustion was achieved without manifest flame front. The results revealed that appropriate increase of He/Hr ratio was conducive to the release of fuel-N during preheating, facilitating the conversion of fuel-N to N2. For BC and CGFA, the pre-removal effect of fuel-N in the SPD reached the best when He/Hr ratio was 3/8 and 1/4, respectively. The increase of He/Hr ratio was not salutary to improving the combustion reactivity of BC, while for CGFA, the modification effect on the combustion reactivity of coal char could reach the best when He/Hr ratio = 1/4. Both NOx emission and the conversion ratio of fuel-N to NOx of BC and CGFA could reach the optimal value when He/Hr ratio was 1/4.

Experimental study on devolatilization characteristics of lignite during pressurized oxy-fuel combustion: Effects of CO2 and H2O

Pressurized oxy-fuel combustion (POFC) is currently considered one of the advanced coal utilization technologies for large-scale CO2 capture but causes distinct coal conversion behavior, particularly more complicated and unclear with wet recycle. To explore coal conversion mechanisms and structural evolution of chars during the initial stage of POFC, lignite devolatilization was conducted in an isothermal horizontal tube furnace at 900 °C in individual and blending CO2 and H2O atmospheres (CO2/N2, H2O/N2, and H2O/CO2) under different pressures of 0.1 to 0.9 Mpa. The pore and chemical structures of resulting chars were characterized using N2 adsorption and Raman spectroscopy, and oxy-fuel combustion reactivities of chars were evaluated by thermogravimetry. The results indicate that coal char conversion in coexisting H2O/CO2 presents three different interactions of addition, synergy, and inhibition, corresponding to mesoporous surface area evolution of chars with partial and total pressures of H2O. CO2 chars primarily consist of micropores while chars formed in H2O-rich atmospheres contain more mesopores. These dominant micro-/mesopore numbers increase with CO2/H2O concentrations but decrease at 0.9 Mpa. The accelerated condensation of aromatic rings in chars induced by CO2 and H2O gasification also follows different pathways. H2O gasification dominates the carbon skeleton structure evolution of H2O/CO2 chars but the interaction between H2O/CO2 results in the complex variations of O-containing structures. CO2/N2 chars have the highest oxy-fuel combustion reactivities, follow by H2O/CO2 chars, and H2O/N2 char reactivities are the lowest. The combustion reactivities of H2O/CO2 chars show a more obvious attenuation trend than that of H2O/N2 chars and the disparities narrow with H2O concentration. Elevated total pressure decreases the reactivities of all chars along with the growth of aromatic ring sizes. The correlations between oxy-combustion reactivities and Raman band area ratios highlight the significance of the devolatilization stage to subsequent char combustion.

Biomass-coal reburning: Competitive mechanism of gas-solid product activation coal char

Reburning technology combining coal and biomass can effectively reduce NOx emissions and promote carbon neutrality in coal-fired systems. This work is based on the flat-flame burner test bench that decoupled the process of biomass-coal reburning effect on coal in the initial stage. The changes in surface functional groups and carbon skeleton structure of coal char were analyzed using FTIR, XPS, and Raman detection methods. The char's combustion reactivity and NO reduction ability were analyzed based on the micro-reactive fluidized bed test bench. The results showed that biochar and syngas would react with oxygen in the transport medium, and the resulting reactive species would activate the char. The activated effect of syngas is significantly better than that of biochar. The syngas reacts with oxygen while protecting the unnecessary consumption of the coal particle surface, and the generated active substances are enriched on the surface of the coal char to enhance its reactivity. The disordered structure of char was enhanced, and the number of small aromatic ring clusters increased by about 28%. The total alkane group content on the char surface increased by nearly 140%, and the total oxygen-containing group content decreased by nearly 17.8%. The combustion reactivity of char is increased by nearly 44.6%, and the reduced ability of NO is increased by nearly 23.4%. Theoretically, the reburned pulverized coal is reduced by 23.4%.

Experimental study and modelling of pressurized oxy-coal combustion: Effect of devolatilization conditions on the physical structure and combustion characteristics of char particles

Pressurized oxy-coal combustion has been regarded as a promising carbon capture and storage (CCS) technology, and the combustion characteristics of char particles under pressurized oxy-fuel combustion conditions have been extensively investigated. However, most previous studies used the char samples prepared under the atmospheric N2 condition rather than under the pressurized CO2 condition, which may lead to the misunderstanding of the combustion characteristics. In this study, the effects of devolatilization atmosphere and pressure on the physical structure of char particles were experimentally investigated. The char particles produced under different devolatilization conditions were then used for the oxy-char combustion experiments conducted in a fluidized bed with the operating pressure between 0.1 and 0.5 MPa. In view of the limitations of the experiment, an experimental verified particle-scale char combustion model was further used for predicting the char particle’s combustion reactivity and temperature, and the influence of physical structure on the combustion characteristics was analyzed. The results obtained from the experiments demonstrated that higher devolatilization pressure led to larger mean pore sizes and specific surface areas of char particles. The char particles obtained with CO2 atmosphere had larger specific surface areas but smaller mean pore sizes than those obtained with N2 atmosphere. Besides, increasing devolatilization pressure resulted in shorter burnout times of char particles. The simulation results showed that using the char samples prepared under the atmospheric N2 condition for the pressurized oxy-fuel combustion experiments led to the underestimations of char combustion reactivity and peak temperature.

Screening of Potential Additives for Alleviating Slagging and Fouling during MSW Incineration: Thermodynamic Analysis and Experimental Evaluation

The formation of slagging and fouling during municipal solid waste (MSW) incineration not only significantly affects heat transfer, but also results in shortened operating cycles. In order to solve the issues, the effect of different additives on the migration and transformation patterns of alkali/alkaline earth metals (AAEM) and chlorine during MSW incineration is screened based on the Gibbs energy minimization method. The effect of potential additives on the ash fusion temperature and combustion reactivity of MSW char is subsequently verified and evaluated by experimental methods. The thermodynamic equilibrium analysis shows that Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 have great potential to increase the ash fusion temperature. The experimental investigation confirms that the addition of Al(NO3)3, Ca(NO3)2, and Mg(NO3)2 significantly increases the ash fusion temperature. The order of increasing the ash fusion temperature by different additives is Mg(NO3)2 > Ca(NO3)2 > Al(NO3)3. The addition of Mg(NO3)2 significantly increased the initial deformation temperature, softening temperature, hemispheric temperature, and flow temperature of ash from 1180, 1190, 1200, and 1240 °C to 1220, 1230, 1240, and 1260 °C, respectively. The addition of Cu(NO3)2, Fe(NO3)3, and KMnO4 significantly decreases the temperature at the maximum weight loss rate of MSW char, while increasing the maximum weight loss rate. Additionally, Cu(NO3)2 shows the best performance in improving the combustion reactivity of MSW char. The addition of Cu(NO3)2 evidently increases the maximum weight loss rate from 0.49 to 0.54% °C−1. Therefore, it is concluded that Mg(NO3)2 and Cu(NO3)2 are supposed to be the most potential candidates for efficient additives. This study presents an efficient and economical method to screen potential additives for alleviating slagging and fouling during MSW incineration.

Enhanced cyclic redox reactivity of Fe2O3/Al2O3 by Sr doping for Chemical-Looping combustion of solid fuels

Aiming at the problems of low redox activity and easy sintering deactivation of iron-based oxygen carriers (OCs), Sr was used to modify Fe2O3/Al2O3 in chemical looping combustion (CLC) of solid fuels. The Sr doped Fe2O3/Al2O3 possessed high fuel combustion reactivity with different solid fuels (coal, coal char and biomass). During consecutive cycles, high cycling durability and morphological preservation were observed for Sr doped Fe2O3/Al2O3, signifying that the addition of strontium effectively enhanced the anti-sintering ability and inhibited the phase segregation of Fe2O3/Al2O3 during cyclic reactions. By comparing the coal char combustion reactivity of Fe2O3/Al2O3 samples doped with different SrO content (1 wt%, 5 wt% and 10 wt%), the optimum additive amount of SrO was 5 wt%. In addition, the characterization analysis proved that the Sr dopant suppressed the outward diffusion of Fe cations and maintained the surface micro-morphology of Fe2O3/Al2O3 in the process of CLC. This study presented important guiding principle and strategy for the synthesis of iron-based OCs with high cyclic redox reactivity in the CLC of solid fuels.

Study on the pyrolysis behavior of coal-water slurry and coal-oil-water slurry

In this work, different concentrations of coal-water slurry and coal-oil-water slurry were prepared and pyrolyzed at various temperatures of 800–1000 °C. The chemical characteristics and combustion reactivities of chars and the formation mechanism of main gas components of CO, CO2, CH4, and H2 were investigated. With the water content of the coal-water slurry increasing, the formation of CO and CH4 were hindered, while CO2 and H2 were promoted. For the coal-oil-water slurry, oil promoted CH4 and H2 production but inhibited CO and CO2. Moreover, the variation of the nitrogen-containing pyrolytic gas during pyrolysis was studied for various concentrations of coal-water and coal-oil-water slurries at different temperatures. The relative content of HCN and NH3 gradually increased with the increase of pyrolysis temperature and water content. For coal-oil-water slurry, the waste lubricating oil promoted the formation of HCN and NH3, with the relative content of HCN being much higher than that of NH3. To study the mechanism of nitrogen migration during pyrolysis, Lammps was used to simulate pyrolytic behavior of pyrrole and pyridine under N2 and steam atmospheres. The increase in temperature promoted the decomposition of pyrrole and pyridine, thereby promoting the production of HCN and NH3. Finally, the properties and the combustion reactivities of the chars were analyzed. The water and oil content in the coal-water slurry increased the structural ordering degree of chars but decreased the combustion reactivities.

Prediction and minimization of NOx emission in a circulating fluidized bed combustor: A comprehensive mathematical model for CFB combustion

A comprehensive 1-dimensional/1.5-dimensional hybrid mathematical model is developed for predicting NOx emission of a circulating fluidized bed (CFB) combustor under broader operating parameters. In this model, the local gas–solid fluidization state and gas/heat transfer conditions in different regions of a CFB combustor are specifically considered. Some two- or three-dimensional problems, such as bubble breakage over dense bed surface, secondary air injection, core-annular flow structure, and particle clusters in freeboard, are also taken into account in 1-D/1.5-D modeling. The detailed chemical kinetic mechanism is creatively used to describe the homogeneous reaction system towards CFB combustion simulation. In addition to operating parameters and fuel-specific inputs, no other model parameters can be trimmed from case to case. This integral CFB model is validated against the field test data obtained from three commercial CFB boilers with different capacities, some of which are first disclosed. Favorable comparisons are obtained between the predicted and measured results, involving particle size distributions, temperature and pressure profiles, and NOx/SO2 emissions. The final NO emission, as well as gas profiles, are somewhat different among the cases, which may be attributed to the discrepancy in boiler structure, fuel properties, and operating conditions. Further sensitivity analysis indicates that the proportion of volatile-N in total fuel-N, char combustion reactivity, and char-NO reactivity significantly impact the NOx emission for CFB combustion. Meanwhile, the gas–solid fluidization state also plays an essential role in the NOx emission and the in-furnace combustion efficiency, such as the gas flow distribution between phases, bubble size, secondary air penetration depth, etc. However, the NOx emission seems insensitive to the particle external gas mass transfer coefficients.

Physicochemical characteristics of biomass‐coal blend char: The role of co‐pyrolysis synergy

AbstractCo‐pyrolysis synergy between cornstalk and bituminous coal was investigated based on the physicochemical properties of the char. The char was prepared by directly putting the individual or blended materials into a heated resistance furnace at 600°C or 800°C. The performance of surface morphology and mineral matters was not notably changed by co‐pyrolysis, indicated by scanning electron microscope and X‐ray diffraction. However, N2 and CO2 adsorption‐desorption isotherms confirmed that surface area and pore structure were strongly influenced, particularly in large micropores and mesopores. Ultimate analysis and inductively coupled plasma optical emission spectrometer measurement revealed excessive depositions of carbon, alkali, and alkaline‐earth metal species in co‐pyrolysis char. Energy‐dispersive X‐ray spectrometer analysis examined the elemental surface distribution, suggesting that co‐pyrolysis under moderate temperature strongly favored inhibiting potassium release. The non‐isothermal thermogravimetry‐differential scanning calorimeter experiments under air atmosphere were conducted for char combustion reactivity, which was expected to be instructive for deeper understanding about co‐pyrolysis synergy and reutilization of the char. The thermal behaviors during combustion of co‐pyrolysis char clearly resembled a single fuel, which completely varied from those of the char blends. Besides, co‐pyrolysis chars and char blends were superior in total heat release compared with the simple addition of calculated values from individual char. Based on these observations, key factors of the synergy were supposed to have consisted of free radical reaction, secondary char formation, and the migration of alkali and alkaline‐earth metal species.

Performance evaluation of biomass pretreated by demineralization and torrefaction for ash deposition and PM emissions in the combustion experiments

Biomass combustion is a promising method in combustion facilities to reduce CO2 emissions. Biomass fuel has unfavorable characteristics, such as low temperature ash minerals and high particulate matter (PM) emission in the combustion facility, which influences operation and creates environmental issues. In this work, different types of empty fruit bunch (EFB) and palm kernel shell (PKS) biomass samples, such as raw biomass, demineralized biomass, and its biomass pretreated by torrefaction were studied to identify their char combustion characteristics by thermogravimetric analysis (TGA) and their ash deposition behaviors and PM emissions in a drop tube furnace (DTF) experiment. Regarding the char combustion characteristic results obtained by TGA, demineralization pretreatment did not clearly influence the char combustion reactivity compared to that of raw biomass, but torrefaction pretreatment lowered the char combustion reactivity at low temperatures. In the DTF experiments, torrefaction pretreatment led to increasing ash deposition and PM emissions during biomass combustion. However, demineralization pretreatment clearly lowered the amount of ash deposition and PM emissions because potassium (K), chlorine (Cl), and sulfur (S) were significantly removed by demineralization pretreatment. In particular, scanning electron microscopy with dispersive X-ray spectroscopy (SEM-EDX) analysis results showed that the demineralized biomass samples having low K, Cl, and S components emitted relatively large PM compared to untreated biomass. Therefore, demineralization pretreatment might be a good method to produce cleaner biomass fuel.

Influence of cooling rate on structure and combustion reactivity of char during char preparation process

Different chars with various cooling rates were obtained in the novel concentrating photothermal reactor to investigate the influence of cooling rate on the structure and combustion reactivity of char. The chars were prepared at 900°C with holding time of 10s, 30s and 120s, respectively. Subsequently, the cold (ex-situ) chars were prepared from the hot (in-situ) chars by controlling different cooling rates of 180°C/s (fast cooling rate), 30°C/s (sub-fast cooling rate), 6.8°C/s (moderate cooling rate) and 1.3°C/s (slow cooling rate). The ex-situ chars were then employed for the combustion in 30%O2/70%CO2 atmosphere at 900°C for 180s. For comparison, the in-situ char was conducted by instantaneously switching the reaction atmosphere. For the chars with holding time of 10s, the mass fraction of in-situ char was higher than the char yield of ex-situ char, and the char yield was decreased with the decrease of cooling rate. The char yield with slow cooling rate was decreased by 11.16% than that with fast cooling rate. The maximum and average mass loss rate of char in O2/CO2 was decreased as the cooling rate decreased. Reduced influence of cooling rate on the aromatic ring systems of char with holding time of 30s and its reactivity was displayed, while almost no difference of those with holding time of 120s was illustrated. The reactivity was closely related to the chemical structure of the char, but no positive correlation was found between the combustion reactivity and the specific surface area of char. The residues proportion of volatile components was the decisive factor of char reactivity.

Effects of Vitrinite in Low-Rank Coal on the Structure and Combustion Reactivity of Pyrolysis Chars.

Pyrolysis is a highly promising technology for the efficient utilization of low-rank coal. The structure of coal plays an important role in its utilization. In this paper, the evolution of the char structure during heat treatment (200–800 °C) of Naomaohu coal and its different vitrinite-rich fractions was studied. The functional group structure, aromatic ring structure, and crystallite size of chars were determined by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray diffraction (XRD) spectroscopy, respectively. The results indicated that minerals inhibit the condensation reaction of aromatic rings during pyrolysis. The high vitrinite content in coal is conducive to the formation of larger char crystallite average sizes (La). The relationship between La (1.69–3.10 nm) and the Raman band area ratio A(GR+VL+VR)/AD or AD/Aall was established. In addition, the combustion performance and kinetics of chars were also investigated. The results showed that the char from high contents of the liptinite fraction has lower combustion reactivity, and demineralization treatment has significantly reduced the combustion reactivity of char.

Influence of pyrolysis pressure on structure and combustion reactivity of Zhundong demineralized coal char

The investigation on evolution of coal char structure during pressurized pyrolysis can reveal the combustion reactivity of coal char in thermal utilization at elevated pressure. In this study, Zhundong subbituminous coal was demineralized and a pressurized drop tube reactor (PDTR) was used to prepare coal char under different temperature and pressure conditions. The physicochemical structures of raw and demineralized coal chars were characterized by the application of nitrogen adsorption analyzer, automatic mercury porosimeter, and Fourier transform infrared spectroscopy (FTIR). The change mechanism of char infrared structure with pyrolysis pressure is revealed on the molecular level in this paper. The results show that the N2 adsorption quantity of raw coal char increases with the increase of pyrolysis temperature, while that of demineralized coal char decreases. Because of the difference in molecular volume and steric hindrance between aliphatic and aromatic structure in char, the increasing pressure has less inhibition effect on the escape of the former than the latter. With the increase of pyrolysis pressure, the combustion reactivity of char is related to the infrared structure at 700 and 800 °C while to macropore structure at 900 and 1000 °C.

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

AbstractIn an attempt to achieve the clean and efficient utilization of lignite, drying pre‐treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%‐35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%‐19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from Lc1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.