Char Combustion Stage 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.

A thermogravimetric assessment of eco-friendly biochar from oxidative torrefaction of spent coffee grounds: Combustion behavior, kinetic parameters, and potential emissions

This study investigated how temperature and residence time variations impact the combustion behavior, kinetic, and potential emissions of biochar produced through oxidative torrefaction of spent coffee grounds (SCG). The study examined the characteristics of combustion and kinetics by conducting thermogravimetry analysis under heating rates of 10 °C·min−1. The Coats-Redfern model was utilized to calculate kinetic parameters. While the emission indices were approximated using the data obtained from elemental analysis. The results indicated that biochar's comprehensive combustion index (Cci) from oxidative torrefaction was lower than that of raw SCG, suggesting stable combustion behavior. Moreover, with the escalation of torrefaction intensity, the activation energy (Ea) values exhibited an upward trend for the char combustion stage, ranging from 22.08 kJ mol−1 to 38.46 kJ mol−1. Concurrently, the Ea values pertaining to the oxidative pyrolysis stage decreased from 64.26 kJ mol−1 to 52.65 kJ mol−1. Besides, this study emphasized that the ash content of the biochar was lower than that of coal and remained consistent with the ash content of raw SCG (p > 0.05). Moreover, the study revealed that biochar from oxidative torrefaction emitted less CO2 (67.35 g MJ−1) than lignite coal (76.55 g MJ−1). Additionally, biochar exhibited up to 27 times lower dust emissions than bituminous coal, emphasizing its eco-friendly fuel potential.

Insight into the fate of nitrogen during char thermal conversion and the influence mechanism of potassium: A theoretical research

Nitrogen oxides (NOₓ) are primary pollutants produced during biomass combustion. During the devolatilization stage, char nitrogen (char(N)) is formed. In the subsequent char combustion stage, char(N) can decompose directly into NOx precursors or engage in heterogeneous reactions with O2 or NO to form NO and N2. Nonetheless, a comprehensive understanding of the reaction mechanisms and competitiveness of char(N) migration, especially the influence of the alkali metal potassium (K) present in biomass, remains incomplete. Building on the Zigzag char(N) models, the present study delves into the migration reactions of char(N), assessing their competitive dynamics through the integration of density functional theory, electronic structure analysis, and conventional transition state theory. Furthermore, it examines the impact of K on char(N) conversion. The competitiveness of the heterogeneous reactions follows the sequence: heterogeneous reduction of NO to N2 > heterogeneous oxidation of char(N) to NO > decomposition of char(N) to NOx precursors. Moreover, the formation of HCN is more favorable than NH₃ production. The successive conversion from char(N) to NO and then to N2 is the predominant migration route for char(N), with NO generation as the pivotal step. The less preferred char(N) migration route involves decomposition to NH3/HCN, followed by oxidation to NOx within the main combustion zone, which cannot be mitigated by char. K can accelerate NO generation and sustain the primacy of the heterogeneous NO reduction, consequently enhancing the oxidation-reduction process of char(N). As a result, K plays a constructive role in managing NOx emissions during the thermal conversion of char.

Co-hydrothermally carbonized sewage sludge and lignocellulosic biomass: An efficiently renewable solid fuel

The objectives of this investigation were to elucidate the potential use of metal oxide-rich sewage sludge obtained from the treated brewery wastewater into a value-added solid fuel via co-hydrothermal carbonization process (co-HTC). Two residue biomass including spent coffee grounds and bagasse were supplied as co-combustion. The effects of sewage sludge and biomass addition on fuel properties were evaluated to optimize the best condition for biocoal-liked production. The chemical composition and mineral phase of solid product were further analyzed. Combustion kinetics analysis including activation energy (E) and pre-exponential factor (A) were derived from thermogravimetric analysis. It was found that the addition of coffee grounds and bagasse enhanced the fuel properties of the solid products, remarkably increasing high heating value (HHV) along with a low ash content, providing an increased fuel ratio of 0.34 – 1.01 and higher HHV as 14.29 – 22.19 MJ/kg. The highest rate of energy recovery was achieved when combining 75 wt% sewage sludge with 25 wt% spent coffee grounds. A substantial decrease of H/C and O/C atomic ratios was distinguished after bagasse addition compared to commercial lignite coal. It was also noticed that the relationship between the sewage sludge and biomass feedstocks during co-HTC is synergistic by increasing the amount of oxidative carbon during the char combustion stage and enhancing the degree of thermal stability. Moreover, it was also emphasized that during co-HTC some carbon and inorganic contents of sludge and lignocellulosic biomass were partially transferred into a liquid phase confirmed by TOC and ICP-OES analyses. A heavy metal leaching toxicity in a liquid product was also determined according to the USA-EPA standard. The combustion reactivity was improved, especially combustion reactivity of the biomass-sewage sludge-derived hydrochar. Interestingly, the hydrochar product was anticipated to possess enhanced safety and stability. Moreover, the co-combustion of hydrochar and coal improved the devolatilization properties and ignition of coal. This strategy revealed that co-hydrothermal process with biomass is a prospective approach to increase the value-added of sewage sludge feedstock as bicoal-like solid fuel.

Release Characteristics of Potassium during Biomass Combustion

To investigate the release characteristics of potassium during biomass combustion, experimental studies were conducted on three typical biomass fuels in a reactor with a fixed-bed system. The effects of fuel type, combustion temperature, exposure time, oxygen concentration, and water-washing pretreatment were evaluated. The results show that the K release ratio in corn straw increases with the increment in exposure time when the temperature is between 700 and 900 °C. When burned at 900 °C for 40 min, 17.73% of K is released in the volatile combustion stage, while only 2.62% is released in the char combustion stage. When burned at 700–900 °C, the K release ratios in both corn and wheat straw improve slightly with the elevation in oxygen concentration. Water washing significantly reduces the ratio and the amount of K release from corn straw during combustion. The effect of temperature and atmosphere on K release from the water-washed sample is similar to that for the raw sample.

Comparative insights into flue gas-to-ash characteristics on co-combustion of walnut shell and bio-oil distillation sludge under atmospheric and oxy-fuel condition

The clean recovery of bio-oil distillation sludge (DS) is a pivotal sector to perfect the biomass refinery system in responses to effective waste management and environmentally friendly consensus. This work aimed to comparatively characterize atmospheric and oxy-fuel co-combustion of walnut shell (WS) and DS in terms of (co-)combustion performance, distributed kinetics, flue gas evolution and ash characteristics. The results indicated co-combustion of WS and DS significantly increased the allocated proportion of char combustion stage and improved co-combustion performance, which was reflected in the enhanced four quantitative combustion indices. The fourth pseudo component, corresponded to the char combustion stage, continuously dominated the whole combustion process by adding more fraction of DS to co-combust with WS, whose activation energy was diminished, thereby optimizing the initial reaction energy barrier. In addition, oxy-fuel combustion promoted the recovery and sequestration of CO2, facilitated the formation of CO, and further endowed the conversion of polycyclic aromatic hydrocarbons into phenols rather than aromatics. Although the composition for mixed molten ash presented the decreased contents for easy-slagging minerals, the co-combustion process was still with potential ash fouling risk owing to basically unchanged ash configuration consistent with WS ash. However, the addition of DS to some extent strengthened anti-slagging performance, consequently optimizing the empirical indices of slagging and fouling for mixed ashes. Also, a higher ash content was yielded in oxy-fuel combustion than that in atmospheric combustion owing to the reducibility of CO2, in which the presence of CO2 contributed to the formations of carbonates and low melting-temperature potassium-containing silicates.

CFD–DEM modeling of oxy-char combustion in a fluidized bed

The CFD–DEM coupled with heat transfer and chemical reaction sub-models was developed for modeling the oxy-char combustion in a fluidized bed. After validating the model, the char thermal conversion characteristics including the reaction rate, the combustion temperature, and the heat transfer flux and contribution were investigated under various combustion atmospheres, fluidizing velocities, and char properties. Results reveal that the higher temperature of the char floating on the bed surface is induced primarily by the decreasing convection heat transfer. The convection heat and the radiation heat contribute more than 90% to preheat the char particle, with the convection heat being the most significant. The reaction heat contributes approximately 50% to the total heat transfer during the stable char combustion, with comparable contributions of the convection and the radiation found, but the higher char temperature associated with the increasing char combustion rates or the char floating behaviors will further increase the radiation heat contribution. On the other hand, the conduction heat plays an insignificant role regardless of the char combustion stage.

Emission Characteristics of NOx and SO2 during the Combustion of Antibiotic Mycelial Residue.

The antibiotic mycelial residue (AMR) generated from cephalosporin C production is a hazardous organic waste, which is usually disposed of by landfilling that causes potential secondary environmental pollution. AMR combustion can be an effective method to treat AMR. In order to develop clean combustion technologies for safe disposal and energy recovery from various AMRs, the emission characteristics of NOx and SO2 from AMR combustion were studied experimentally in this work. It was found that the fuel-N is constituted by 85% protein nitrogen and 15% inorganic nitrogen, and the fuel-S by 78% inorganic sulfur and 22% organic sulfur. Nitrogen oxide emissions mainly occur at the volatile combustion stage when the temperature rises to 400 °C, while the primary sulfur oxide emission appears at the char combustion stage above 400 °C. Increasing the combustion temperature and airflow cause higher NOx emissions. High moisture content in AMR can significantly reduce the NOx emission by lowering the combustion temperature and generating more reducing gases such as CO. For the SO2 emission, the combustion temperature (700 to 900 °C), airflow and AMR water content do not seem to exhibit obvious effects. The presence of CaO significantly inhibits SO2 emission, especially for the SO2 produced during the AMR char combustion because of the good control effect on the direct emission of inorganic SO2. Employing air/fuel staging technologies in combination with in-situ desulfurization by calcium oxide/salts added in the combustor with operation temperatures lower than 900 °C should be a potential technology for the clean disposal of AMRs.

Emission characteristics and formation mechanisms of PM2.5 and gases from different geological maturities coals combustion

To evaluate the influence of geological maturity on coal combustion pollutant emissions, six chunk coals and corresponding briquettes (volatile matter [VM] content from 8.0% to 39.9%) were selected for simulated residential combustion in a carefully designed chamber, and the formation mechanism involved was investigated. VM was revealed to have a nonlinear effect on the emission factors (EFs) of particulate matter with an aerodynamic diameter of <2.5 μm (PM2.5), polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), and organic carbon (OC), with the highest EFs observed in VM of 15.8% and 26.8%. Coal tar yield and air-supply from VM combustion in its initial stage was found to be a key factor influencing PM2.5 and PAH emissions when compared with the char combustion and burnout stages. The highest PM2.5 and PAH EFs were observed in coal with 15.8% VM. Additionally, significant amounts of hydrogen cyanide (HCN) were detected at stage 1 and were released as volatile-N through the pyrolysis of coal-N. N2O and NOx were also emitted through homogeneous gas-phase HCN reactions. However, only a small proportion of NOx was produced, indicating that the contribution of VM to HCN and N2O emissions is greater than that of NOx. In the char reaction stage (stage 2), char-N reacted with O2 to form large amounts of NO, and the reaction length of the entire coal combustion process exhibited a positive relationship with coal maturity (correlation coefficient 0.69, p < 0.05), suggesting that the contribution of NO from char-N to overall NO emissions increased with the coal’s geological maturity due to the increase in coal-N retention in char. Furthermore, coal briquetting was found to decrease the EFs of pollutants, which may be explained by an increase in MCE (modified combustion efficiency) and the conversion from coal tar to carbon and hydrogen. This study elucidated the effects of geological maturity on pollutant emissions from coal combustion and examined relevant underlying mechanism. These data can serve as the foundation for a clean coal project in China or other counties concerned with coal combustion emissions.

Combustion Characteristics of Coal for Pulverized Coal Injection (PCI) Blending with Steel Plant Flying Dust and Waste Oil Sludge.

Various characterization methods are used to investigate the physical and chemical properties of steel plant flying dust and waste oil sludge, and the combustion characteristics of the mixtures with pulverized coal are studied via thermogravimetric analysis; the catalytic combustion mechanism is also explored. The results show that two types of metallurgical by-products with small particle sizes and developed pores are evenly dispersed in the pulverized coal and are stably combined with it. The additives reduce the ignition temperature and the temperature corresponding to the maximum combustion rate of pulverized coal; simultaneously, they increase the heat released during pulverized coal combustion. During the pyrolysis stage of pulverized coal, the heat generated via organic component combustion in waste oil sludge promotes a cracking reaction and improves the development of the char’s micropore. During the char combustion stage, no catalyst deactivation phenomenon occurs under the ratios of inorganic components in the two types of metallurgical dust and sludge. Two additives markedly reduce the activation energy of the combustion reaction.

Thermogravimetric and kinetic analyses of the Skeletonema costatum microalgae combustion using the fitting method

This work was intended to understand the potential of Skeletonema costatum (S. costatum) microalgae being the solid biofuel feedstock by studying its combustion behavior under a thermogravimetric analyzer (TGA). The following parameters were used during the experiment: about 5 mg of weighed sample was put into a ceramic crucible; the atmospheric air of 100 ml/min was flowed to the chamber to ensure sufficient combustion air; heating was escalated with a rate of 10 °C/min, and combustion was performed at a temperature range from 30 °C to 1200 °C. The results showed that the combustion process of S. costatum microalgae could be divided into four stages of decomposition, including the evaporation stage of moisture content from a temperature of 31.68 °C to 150.93 °C, the devolatilization stage from a 150.93 °C to 505.053 °C, the stage of char combustion from a 505.053 °C to 1018.15 °C and the final stage of ash decomposition from 1018.15 °C to 1200 °C. The kinetic parameters were calculated in the 1st, 2nd, and 3rd stages based on the fitting model of the Coats-Redfern method by selecting reaction mechanisms. The activation energy values for 1st, 2nd, and 3rd stages were 71.18; 39.49, and 56.92 kJ/mol, respectively, and correlation coefficients (R2) of 0.959; 0.988, and 0.978, respectively in the Chemical Reaction Order models.

Co-combustion of wheat straw and camphor wood with coal slime: Thermal behaviour, kinetics, and gaseous pollutant emission characteristics

The disposal of coal slime has attracted increasing attention due to its increasing production and consequent environmental pollution. The co-combustion with biomass is a promising method for the treatment of coal slime. However, there are only a few studies on coal slime and biomass co-combustion. This study investigated the co-combustion characteristics and gaseous pollutant emissions of coal slime with camphor wood and wheat straw. The results showed that there are significant interactions between biomass and coal slime during the co-combustion process. The addition of biomass reduced the ignition and burnout temperature and enhanced the combustion stability of the blends. Meanwhile, the addition of biomass inhibited the emission of NO and SO2 from the blends. Moreover, the kinetic analysis showed that the activation energies of biomass char and coal slime combustion stage decreased with increasing biomass content, indicating that biomass's addition improved the combustion reactivity of the blended fuel. The results provide basic knowledge regarding the thermal behaviour and emission characteristics of coal slime and biomass co-combustion. It was established that co-firing is beneficial for the clean and efficient utilisation of coal slime.

The migration and transformation of sodium carboxylate during Zhundong coal pyrolysis and combustion: A ReaxFF simulation study

In this paper, the migration and transformation of sodium carboxylate during Zhundong coal pyrolysis and combustion was studied by using the reactive molecular dynamic simulation (ReaxFF MD) method. By using the elemental analysis, XPS and 13C NMR spectroscopy, the Zhundong coal model containing sodium carboxylate was constructed. Pyrolysis simulations on the Zhundong coal model were studied at different temperatures ranging from 1200 K to 3000 K. The results show that sodium was mainly released in the form of atom and inorganic substance in the pyrolysis. Atomic sodium were mainly released directly by the H free radicals attacking in the pyrolysis. Furthermore, the nascent char model was constructed to study the migration and transformation of sodium in char combustion stage. Oxygen increased the bonding opportunities between sodium atoms and oxygen atoms. Sodium was first released in the form of atom, and then migrated to the inorganic substance. By using the Arrhenius expression, the kinetic parameter for the release of sodium into various species during pyrolysis and char combustion stages were quantitatively determined. This provides a basis for understanding the migration and transformation of sodium in coal at the atomic level.

Kinetic study of the thermal decomposition of Lippia origanoides bagasse

Combustion is the most widely used process to generate energy from biomass, however, there are still challenges to overcome in terms of efficiency and lack of knowledge of the properties of potential fuels. In the present work, a kinetic study of the thermal decomposition of Lippia origanoides bagasse from the essential oil distillation process is reported. Thermogravimetric analysis was performed on samples of stems, leaves and their mixture and the respective kinetic parameters were calculated from isoconversional methods of Friedman and Kissinger. The results obtained by the previous methods were evaluated by comparing the theoretical and experimental conversion curves, showing a good correlation between them, except for char combustion stage of leaves and mixture samples. Different characteristic combustion parameters were also evaluated; it was found that the stems samples have better combustion properties than the other two samples and that there is a linear relationship between them and heating rates.

Coal/biomass co-combustion investigation by thermogravimetric analysis

In this study, the thermal characteristics and kinetic parameters of coal/biomass blended fuels (75:25, 50:50 and 25:75 wt.%/wt.%) were investigated by using the thermogravimetric technique under atmospheric air. Three types of agricultural waste biomass including cassava root, palm kernel shell and rice husk were used as raw materials. The experiments were performed under different temperatures, ranging from 313-973 K with the heating rate of 5, 10, 20 and 40 K/min. The results show that the thermal decomposition of biomass exhibit three-four stages including moisture and some light volatile removal stage (up to 463 K), volatile oxidation stage (423-663 K), char combustion stage (663-823 K) and inorganic oxidation stage (803-953 K). Lignite on the other hand exhibits only two main peaks during the entire combustion process, corresponding to the moisture removal (up to 433 K) and the decomposition/oxidation (433-833 K), respectively. In addition, it was also found that the blending of biomass residues improved the ignition temperature of the blended fuels, indicating an improvement of devolatilization of coal. Kinetic studies show that the average apparent activation energies of the co-combustion of coal/cassava root, coal/palm kernel shell and coal/rice husk calculated from the Kissinger-Akahira-Sunose method are reported at ca. 105.25, 179.66 and 121.84 kJ/mol, respectively.

Combustion and Kinetics Parameters Investigation of Raw and Torrefied Cocoa Pod Shells by Thermogravimetric Analysis

Torrefaction is one of the techniques to produce solid fuel from cocoa pod shell (CPS). In this research, changing combustion characteristic and kinetics of CPS after torrefaction process was investigated using Coats – Redfern Method. Torrefied CPS processed at temperatures of 200, 250, and 300°C and holding times of 0, 30, 60, and 90 min were used in this investigation. The results proved that the ignition and burnout temperatures shifted to higher value, while the peak mass loss rate tend to lower value when the torrefaction temperature and the holding time increased. The period of volatile release during the combustion process was shorter along with longer holding time and higher temperature of torrefaction. The maximum mass loss rate of raw CPS was 0.12005 g/min, while torrefied CPS has range between 0.046 to 0.114 g/min. The maximum mass loss rates of all samples decrease with increasing fuel ratio (FR). The activation energy increases gradually with increasing severity of torrefaction. The value of activation energy in the devolatilization and volatile combustion stage is greater compared to that of the char combustion stage.

In-situ measurement of temperature and potassium concentration during the combustion of biomass pellets based on the emission spectrum

Here, a calibration method using the K concentration, temperature and the characteristic spectral line radiation intensity in flame is described. In addition, a radiation function model between these three variables, Iλ766T,CK=0.821·Ib(λ766,T)·CK, was established. A Y-type optical fiber was used to connect the spectral signals from the same direction to two spectrometers. The continuous spectra in the 500–800 nm band were used for multi-spectral temperature measurements. The combination of the measured temperature and the K characteristic spectral line radiation intensity at 766 nm was used for measurements of K concentration. The temperature and equivalent ratio of the heating atmosphere were 1300 K (±20 K) and 0.786, respectively. Compared with rice straw pellet, pine wood pellet had a lower K concentration and higher combustion temperature during the entire combustion process. The peak K release concentrations of the pine wood pellet and the rice straw pellet during the volatile combustion stage were 2.09 ppm and 10.03 ppm, respectively, and the peak flame temperatures were 1774 K and 1756 K, respectively. In the char combustion stage, the peak K release concentrations were 27.83 ppm and 110.43 ppm, respectively, and the peak surface temperatures were 1287 K and 1143 K, respectively. The established function model for measuring K concentration is also applicable to the combustion diagnosis of other K-rich fuels.

Determination of alkali release during oxyfuel co-combustion of biomass and coal using laser-induced breakdown spectroscopy

The impetus for clean energy utilization has increased acceptance of biomass as a co-fired fuel with coal, since it reduces carbon emissions. This work utilized time-resolved online LIBS measurement to investigate the release of potassium from oxyfuel co-combustion of fuels. A coal-biomass blended particle consisting of 20% biomass (energy basis) is set as a reference fuel, and its results are compared with native biomass and coal particles. As expected, the potassium release rate was highest with 100% biomass and decreased with reducing biomass content in the blend. Therefore, there are trade-offs associated with potassium-related deposition, fouling and corrosion of a boiler’s heat transfer surfaces when biomass and coal are co-fired compared with biomass alone. Analyses of particle size and surface temperature changes suggest that the evolution of potassium in the gas phase can be characterized by temperature evolution. The online measurement for a 20% blend indicates that about 80% of the releasable potassium is volatilized, mainly during the char combustion stage. The release kinetics model developed obeys an Arrhenius expression. The activation energy reported for a 20% fuel blend was 161.9 kJmol−1 K, and was much lower than that of 100% biomass, but higher than that of 100% coal.

Experimental and numerical investigation of the combustion characteristics and NO emission behaviour during the co-combustion of biomass and coal

As a promising technology for pollutant reduction, biomass-coal co-combustion has attracted worldwide attention. In this study, experiments combined with simulations were conducted to explore the combustion characteristics and NO emission behaviour, as well as their interrelations during the co-combustion process of biomass and coal. The experimental results reveal an obvious decrease in the NO concentration with an increased biomass blending ratio by weight. A maximum NO reduction of 24% was found when 50% biomass was blended with coal, which mainly occurred in the stage of stable char combustion and was attributed to non-catalytic effect related to free radicals. Moreover, the higher biomass blending ratio led to a more violent combustion reaction during the devolatilization stage, while the reaction remained constant during the char combustion stage. As the temperature increased from 800 °C to 1000 °C, the fuel-N/NO conversion ratio increased for the blend with 30% biomass and 70% coal, which resulted from the combined contributions of devolatilization and char combustion. Although the rise in temperature also led to an increase in the combustion rates of the fuel samples, the effect of the temperature on NO emission turned out to be more sensitive than that on the combustion rate. Additionally, when the oxygen concentration increased from 6 vol% to 21 vol%, the NO conversion ratio decreased, which is attributed to the promotion effect of the higher O2 concentration on homogeneous NO reduction. The simulation data were in good agreement with the experimental results, and demonstrated that there were six elementary reactions dominating the NO transformation. Moreover, homogeneous NO reduction plays a vital role in decreasing NOx emissions.

Study on combustion performance of hydrothermally dewatered lignite by thermal analysis technique

The high water content of lignite has raised serious environmental and efficient issues, which are obstacles to its large scale utilization. In this study, hydrothermal dewatering (HTD) method was used to upgrade a lignite from Inner Mongolia in China at 230–330 °C. The combustion behaviors of raw lignite and HTD samples were investigated by thermogravimetry with differential scanning calorimetry (TG-DSC). By changes of mass and heat flow, the lignite combustion process was described from three stages: water evaporation, coal devolatilization and char combustion. Compared with raw lignite, the combustion reactivity of HTD samples upgraded at 230–300 °C was improved, while that of sample upgraded at 330 °C decreased. The temperatures at maximum combustion rate of 230–300 °C samples (below 397.6 °C) were lower and that of 330 °C samples (448.9 °C) was higher than that of raw lignite (399.8 °C). The released heat at char combustion stage increased for HTD samples. Then the physico-chemical structures of lignite before and after HTD process were analyzed to investigate the effect of HTD on coal combustion characteristics. Results showed that HTD process could affect the heat at water evaporation and initial devolatilization stages during coal combustion through removing oxygen functional groups in lignite. The enhancement in aromaticity of lignite after HTD increased activation energy of intrinsic reaction, whereas the large pore volume and high surface area of 230–300 °C samples decreased diffusion activation energy at char combustion stage. The effect of pore structure predominated on combustion reactivity compared with aromaticity for HTD samples.