Preheated Coal Research Articles

Evolution of pore structure and changes in oxidation characteristics of coal after heating freezing treatment

To study the spontaneous combustion characteristics of preheated coal treated by liquid nitrogen (LN2), surface structure test, thermal analysis experiment and chemical characteristic analysis experiment were used to study the change characteristics of surface structure, oxidation characteristic and chemical structure of coal under the action of LN2 quenching. The results showed that with the increase of preheating temperature, the fractal dimension, specific surface area, pore volume and average pore diameter of the LN2 treated coal (quenched-coal) increase. With the increase of preheating temperature, under the oxidation condition, the CO and CO2 produced at ST of 230 ℃ are 1.19 times and 1.21 times higher than those at ST of 70 ℃. The CO/CO2 emission of quenched-coal is significantly increased, and the secondary oxidation capacity is stronger. Due to the more developed pore cracks of coal under pyrolysis and quenching conditions, the natural characteristics of coal are stronger. The oxygen adsorption capacity and apparent activation energy of pyrolysis coal in the stageⅠ affected by LN2 are stronger. Oxidized coal shows stronger oxidation and heat release ability in the stageⅡ under the action of LN2, resulting in higher apparent activation energy in this endothermic stage. The addition of LN2 blocked the oxidation reaction of preheated coal and sealed the structure of -C = O, –COOH and –OH. The bridge bond –CH2- structure of pyrolysis coal is further destroyed, and the ratio of –CH2/–CH3 is reduced. The increase of alkyl free radicals in pyrolysis coal and alkyl oxygen free radicals in oxidation coal accelerates the spontaneous combustion oxidation of coal. The research results have important guiding significance for the occurrence and prevention of coal reignition disaster after LN2 fire quenching.

Technology for Producing Composite Solid Fuel From Coals of Kyzyl-Bulak and Kozho-Kelen Deposits and Heated Bitumen

The paper presents the results of a study of the production of composite solid fuels (CSF) from compositions of coal from the Kyzyl-Bulak and Kozho-Kelen deposits and a bitumen binder. The possibilities of producing CSF from a mixture of bitumen and preheated coals have been studied. To determine the dependence of the quality of the resulting CSF on the grinding size, the above coal was crushed into three fractions: a fraction with a particle size of 0-1mm, a fraction with particles of 0-3 mm and a fraction of 0-5 mm. These 3 fractions of coal were subjected to sieve analysis and the granulometric composition was determined, then each fraction individually was subjected to briquetting on a hydraulic press, in a mold with a diameter of 40 mm. Based on the task, pressing took place at 3 parameters, namely: at a pressure of 10 MPa, 15 MPa and 20 MPa. The use of different pressures made it possible to trace the dependence of the quality of the resulting CSF on the pressing pressure and determine the optimal pressure parameter. Briquetting was also carried out at different concentrations of the charge and the mechanical properties of the CSF were determined at a binder concentration of 6, 8, 10%. It has been established that the use of a bitumen-like product (bitumen) as a binding material, obtained as a result of thermal dissolution of coal with oil waste at a temperature of 320-380°C, produces mechanically strong briquettes that have compression resistance up to 106 kg/cm2 and bending resistance up to 7.5 kg/cm2. It has been shown that the mechanical strength of briquettes increases with increasing degree of coal grinding, and also an increase in pressing pressure increases the mechanical strength of briquettes. As the moisture level of coal increases, the mechanical strength of the briquette decreases. The optimal parameters for producing CHP have been established: coal size 0-3 mm: humidity 6.5%, pressing pressure 200 kg/cm2.

Experimental study on municipal sludge/coal co-combustion preheated by self-preheating burner: Self-preheating two-stage combustion and NOx emission characteristics

To achieve efficient and low-pollution sludge treatment, this study explored the first-time application of self-preheating combustion technology in sludge/coal co-combustion, and subsequently investigated the influences of sludge blending ratio (BR) and tertiary air types on self-preheating two-stage combustion and NOx emission characteristics in a 30 kW self-preheating combustion system with a novel two-stage down-fired combustor (DFC). This system was well adaptable for fuels, and still ran steadily even if BR was as high as 50 %. During preheating, properly increasing BR improved carbon microcrystalline structure of preheated carbon-based fuel and increased its specific surface area, but its carbon microcrystalline structure was gradually deteriorated at BR>20 %. Fuel properties of preheated coal gas were deteriorated with increasing BR. During combustion, combustion efficiency (η) increased first and then decreased with increasing BR, while NOx emission increased. For tertiary air nozzle located at top of burnout region, the structures of center symmetry and ring hedge had less effect on η and NOx emission, both of which remained at high levels, exceeding 98.5 % and 160 mg/m3. Notably, properly shifting injection position down reduced NOx emission significantly, albeit at the expense of reduction in η. Excessively low injection position had less effect on further NOx emissdddction.

Experimental study on the flexible peak shaving with pulverized coal self-preheating technology under load variability

The integration of renewable energy sources into power grids has led to power fluctuations, necessitating coal-fired power units to provide flexible peak shaving. The pulverized coal self-preheating technology can potentially provide broad-load regulation and rapidly respond to load changes. In this study, the impact of the self-preheating technology on fuel preheating modification, combustion characteristics, and NOx emissions are investigated. Load-stabilization bases are proposed based on preheating temperature, preheated coal gas composition, combustion temperature, and flue gas emissions. Load change rates are calculated using these bases to character the load response. The results demonstrate that the preheated fuel has similar properties as the highly reactive gaseous fuel and exhibits excellent ignition and combustion performances at 25 % load. Preheating treatment improves combustion efficiency, and burnout ratio and reduces NOx emissions. The NOx concentration approaches the ultra-low emission standard at 50 % load. Four load-stabilization bases, covering the preheating and combustion sides, comprehensively characterize the load response performance. The rates based on preheating indicators and combustion indicators maximize to 3.30 %/minute and 2.71 %/minute, respectively. The load change rate in the ramp-up phase is higher than that in the ramp-down phase, which is related to the effective internal heat storage carrier and external thermal insulation conditions. The driving force formed by high load step ranges and the deviation from the full load capacity are conducive to increasing the load change rates.

Experimental study of deep and flexible load adjustment on pulverized coal combustion preheated by a circulating fluidized bed

Peak shaving of coal-fired power plants is a key to promoting renewable energy generation in a grid system. In this study, the deep and flexible load regulation characteristics of pulverized coal preheating combustion are explored. The determination criteria of the variable load related to the preheating temperature, preheated coal gas, combustion temperature and main flue gas (NO, N2O, CO and O2) emission concentrations are established, and the temperature hysteresis coefficient is introduced to characterize the hysteresis of the combustion process. The results show that this system has a deep load regulation capability and can operate stably over a wide load range from 25% to 113%. The chemical reactivity of the pulverized coal is enhanced with thermal modification, which ensures stable combustion in the combustion chamber at ultralow loads. Moreover, the optimal rate of variable load is achieved when the load step is 25%, and the maximum rates of variable load for the preheating process and combustion process are 4.17%/min and 3.13%/min, respectively. Furthermore, an excessively high load step will trigger a large amount of combustion hysteresis, which negatively affects the variable load process.

Experimental research on effects of multi-layer injection of the high-temperature preheated pulverized coal on combustion characteristics and NOx emission

Under the background of carbon peak and carbon neutrality, cleanandefficientutilization ofcoal is imperative in China. As a novel combustion technology, the coal self-preheating combustion technology has broadprospectsforapplication, nevertheless, previous studies have mainly focused on limiting NOx generation by simply applying air staging method in the combustor, which made further NOx reduction difficult to achieve. Combining the advantages of reburning technology, this study firstly realized the multi-layer injection of the high-temperature preheated fuel (PF), and investigated the effects of the reburning fuel fraction (RFF), the location of the reburning fuel injection port (RFL) and the coal type on combustion characteristics and NOx emissions. The results revealed that the generated high-temperature PF could continuously and stably enter the down-fired combustor (DFC) for subsequent combustion. In the intensely reducing atmosphere inside the self-preheating device, the released fuel-N was mainly converted to N2, and the combustion performance of the preheated coal char which could be used as an excellent fuel close to gas reburning effect was greatly modified compared with raw coal. To simultaneously achieve efficient and clean combustion of pulverized coal, the favorable RFF and RFL were 10.62%∼15.48% and 600 mm ∼ 800 mm, respectively, corresponding to the residence time in the primary combustion zone and the reburning zone in the range of 0.91 ∼ 0.99 s and 0.87 ∼ 1.11 s, respectively. What’s more, under the same reburning parameters, Alxa lignite coal had the largest NOx reduction rate (32.15%), with NOx emission of 51.36 mg/m3 (@6%O2), while Jincheng anthracite coal had the lowest NOx reduction rate (5.34%) and the highest NOx emission (149.91 mg/m3, @6%O2), and Shenmu bituminous coal achieved ultra-low NOx emission (45.75 mg/m3, @6%O2) with the NOx reduction rate of 28.05%, suggesting that on the basis of air staging, multi-layer injection of PF had more advantages for clean and efficient combustion of high volatile coal.

Preheating and combustion characteristics of anthracite under O2/N2, O2/CO2 and O2/CO2/H2O atmospheres

In this study, the anthracite was preheated by a circulating fluidized bed under O2/N2, O2/CO2 and O2/CO2/H2O atmospheres. And the preheating and combustion characteristics of anthracite were studied. Compared with the O2/N2 atmosphere, the structural characteristics of preheated char was improved more obviously under the O2/CO2 and O2/CO2/H2O atmospheres when the oxygen concentration was 31%, which mainly reflected in the increase of BET surface and the increase of small aromatic rings. HCN was mainly NOx precursor during the preheating under different atmospheres. In addition, a novel comprehensive evaluation of ignition and stable combustion characteristics for preheated fuel was proposed. And the minimum thermal power required for preheated char combustion and the combustion heat of preheated coal gas were main factors affecting on the ignition and stable combustion characteristics. The highest combustion efficiency of anthracite was 98.85% under the O2/N2 atmosphere (the oxygen concentrations of primary gas, secondary gas and tertiary gas were 31.0%, 36.0% and 35.0%), and the lowest NO emission in tail flue gas was 24.24 mg/MJ under the air atmosphere.

Mechanism of aliphatic hydrogen on the caking property of 1/3 coking coal during rapid preheating

It is of significance to reveal the mechanism of rapid preheating to enhance the caking property of coking coal and expand high-quality coking coal resources. In this study, a 1/3 coking coal was preheated at two low and one rapid rates, then raw coal and heat-treated samples were comprehensively analyzed. The results revealed that the aliphatic H in coal formed volatiles at the early stage of pyrolysis (≤ 380 °C) under low-rate preheating. In the thermoplastic stage, the bonding phase was confirmed to be formed by a part of aliphatic H. The decreases amount of volatile and hydrogen contents and H/C and O/C atomic ratios of rapidly preheated coals were less than those of low-rate preheated coals, indicating that its thermal decomposition reaction degree was lower. This was considered to be the basis for keeping G values of rapidly preheated coals from decreasing. The aliphatic H concentration and CH2/CH3 ratios of rapidly preheated coals were higher and lower than those of raw coal respectively, which contributed to the fact that their G values were basically close to or higher (∼349–401 °C) than that of raw coal. Rapid preheating weakened the consumption of aliphatic H by volatiles formed during low-rate heating. Moreover, it forced weak bonds in the molecular of coal to break, such as bridge bonds and intermolecular forces that connect structural units. The breaking of bridge bonds between alkyl side chains or cycloalkanes and aromatic groups enhanced hydrogen supply. The weakening of intermolecular force resulted in loose macromolecular network structure and enhanced molecular dynamic properties. These evolutions of coal molecular structure strengthened the combination of hydrogen and free radicals and generated more bonding phases. Finally, a mechanism which provided a new explanation for the rapid preheating to alter the molecules structure and enhance the caking property of coking coal was proposed, and it will be significant valuable to enlarge the utilization of weakly-caking coal in coking process.

Comparison of oxy-fuel preheated combustion characteristics of high- and low- volatility carbon-based fuels

A comparative evaluation and analysis study of combustion characteristics, main pollutant emission behaviors, and fuel-N conversion paths of high- and low- volatility carbon-based fuels are conducted under O2/CO2 atmospheres in a preheating combustion reactor, i.e., CFB. Compared with conventional air combustion, the oxy combustion with high oxygen concentrations from 21% to 27.3% is studied. The studies show that the net calorific value of the preheated coal gas of semi-coke in O2/CO2 is up to 2 times that in air, and the net calorific value of the preheated coal gas of bituminous coal in O2/CO2 is up to 1.3 times that in air. During the preheating process of semi-coke, CH4 and HCN occupy a discreet proportions of preheating coal gas. Fuel characteristics and preheating atmosphere have no effect on the particle size of preheated coal char. Under an O2/CO2 atmosphere, the highest BET surface area of preheated coal char of bituminous coal is up to 30 times that of raw coal and that of semi-coke is up to 40 times that of raw coal. The carbon structure modification ratio of preheated coal char during the preheating process of bituminous coal is larger than that during the preheating process of semi-coke. With the increase in the concentration of oxygen in the primary stream, the NO emission decreased during the bituminous coal combustion process but increased during the semi-coke combustion process. In addition, the main generation path of NO in the reduction zone in O2/CO2 is NH3-NH2-NH-HNO-NO. In O2/CO2, the total promotion effect of reactions on NO formation in the combustion process of semi-coke preheated fuel is larger than that of bituminous coal.

Experimental study on pulverized coal combustion preheated by a circulating fluidized bed: Preheating characteristics for peak shaving

With the continuous growth of renewable energy, the electricity market is increasingly targeting flexibility and deep-peaking capabilities for coal-fired generation. In this study, the low-load operation and rapid load-change characteristics of pulverized coal combustion are explored on a 1 MWth test platform, based on a promising preheating-combustion technology. At loads of 25% and 35%, excellent gas–solid flow characteristics and stable operating parameters such as temperature and pressure drop can be achieved in the pre-circulating fluidized bed. After preheating, pulverized coal is converted into preheated coal gas with a high calorific value and preheated char with a high physical sensible heat and ample porous structure. These greatly improved the low-load ignition and combustion stability in the combustion chamber. Moreover, two bases for determining the stabilization after load change were discussed: the preheating temperature and the preheated coal gas. The load-change rates at ramp-up were generally higher than those at ramp-down within the same step ranges. The optimal step range for a higher load-change rate during load ramp-up/down was found, and the maximum load-change rate of 3.75%/min was achieved. Furthermore, the step range of the fuel mass flow rate had a positive effect on counteracting the thermal inertia at the load ramp-down. This study verifies a feasible technology for peak shaving of coal-fired power generation, and it can provide certain theoretical guidance for future engineering applications.

Experimental investigation on fluidized modification in gasification of preheated coal using oxygen and steam

A new coal gasification technology integrating advantages of a circulating fluidized bed (CFB) preheater and an entrained bed gasifier is proposed by gasification of preheated coal. Improvements of morphological properties and gasification reactivity can be gained for coal from the fluidized modification in the CFB preheater, which enhances the gasification reaction in the gasifier. In this study, the characteristics of the fluidized modification were investigated experimentally using oxygen and steam as gasifying agent on a bench-scale test rig. Effects of the air replacement by O2-steam in gasifying agent, modification temperature and steam to coal ratio on gas and modified char of the preheater were illustrated. Thermogravimetric analyses and Raman spectroscopy were used to characterize the gasification reactivity and carbon structural order of the modified char, respectively. The steam gasification reaction in the preheater is enhanced after air is replaced by O2-steam in the gasifying agent. The enhancement contributes to the formation of active carbon sites in the modified char and improves the gasification reactivity. The steam gasification reaction is also enhanced when the modification temperature increases from 858 to 952 °C, but active carbon sites in the modified char are first formed and then consumed. The highest gasification reactivity of modified char is obtained at 893 °C. The steam gasification reaction is enhanced by the increased steam to coal ratio from 0.55 to 1.07 as well and the gasification reactivity of modified char first increases and then decreases with the highest value at the steam to coal ratio of 0.87.

Experimental and Theoretical Investigations on High-ash Coal-air Flames in High-speed Jets Stabilized Recirculating Flow

ABSTRACT In this paper, experimental and theoretical investigations on the stability of coal-air flames in high-speed jets stabilized recirculating flow are presented. The current work reports the operational limits of high-ash coal-air flames stabilized by large velocity differentials. Experiments are performed to investigate the effects of particle size distribution, flow rate of primary and high-speed jets, and feed rate of coal on the stability of recirculating coal-air flames. Temperature measurements of the primary reaction zone are used to assess the stability of the reactor under various operating conditions. Experiments are carried out with two different particle size distributions, namely: (1) the fine (355 m), and (2) the coarse ( m) distributions. Investigations are carried out with a range of primary jet velocities (4 m/s to 9 m/s), high-speed jets mass flow rates (1.24 to 4.94 kg/s), and feed rates (1.11 to 2.1 kg/s). A nondimensional ignition index () which is a ratio of the particle flow time () to particle ignition time () is shown to indicate the stability of recirculating coal-air flames. Temperature measurements indicate that the operating conditions with τ>0.8 are stable whereas those with lead to reactor quenching. Force balance on particles with numerically resolved velocity profiles is used to estimate the particle flow time. Particle ignition times are quantified by the extended unified ignition-devolatilization model. Results indicate a strong influence of primary jet velocity on the stability of the flame. Only a marginal increase in stability can be attributed to the high-speed jets. Particle size distribution, along with the feed rate plays a crucial role in determining the stability of recirculating coal-air flames. Temperatures with the fine distribution exceed ash-fusion temperature causing slag deposition in the reactor. Model-based inferences on the increase in stability of recirculating coal-air flames with preheated coal are presented.

Pilot-scale study on NO emissions from coarse coal combustion preheated by circulating fluidized bed

Achieving stable low-NOx combustion of coarse pulverized coal could improve the energy efficiency of the combustion system. In this study, a pilot-scale experiment on coarse coal (0–1 mm) combustion preheated by circulating fluidized bed is conducted. The coarse coal could achieve a long-term stable preheating, and the assisted combustion by the high-calorific preheated coal gas (3.06 MJ/Nm3) and the higher physical sensible heat (754 °C) greatly improved the ignition performance and the combustion stability of the preheated fuel. Meanwhile, the release ratio of fuel-nitrogen during the preheating was 36.56%, while the concentrations of NOx in the preheated coal gas were all 0 ppm. During the stable combustion, the volume-based average temperature and the volume-based temperature fluctuation coefficient in the main combustion zone was approximately 1112 °C and 8.89%, and the combustion efficiency was approximately 95%. The NOx concentrations in the strong reductive zone were almost 0 ppm. As the reaction atmosphere along the combustion path gradually varied from reducing to oxidizing, fuel-nitrogen was mainly oxidized to NO, and the NO emissions was approximately 68 ppm (114 mg/m3 @6%O2). In addition, the effects of burnout air positions in strong reductive zone on NO emissions could be ignored.

Preheating Combustion Characteristics of Ultra-Low Volatile Carbon-Based Fuel

Pulverized coal combustion technology with preheating solid fuel in a circulating fluidized bed was used for the combustion test of ultra-low volatile carbon-based fuel. This paper first validated the feasibility and advantages of applying the combustion technology to this kind of fuel. The carbon-based fuel could achieve a stable preheating process in this test system. After the preheating, the apparent sensible heat of the fuel was significantly increased. This provided a necessary condition for the stable ignition and efficient combustion of the carbon-based fuel in the post-combustion chamber. The relative proportions of CO, H2, and CH4 in preheated coal gas were very low, and the effect of high-temperature coal gas at the entrance of the post-combustion chamber was greatly impaired, indicating that the combustion process in post-combustion chamber was mainly the combustion of preheated char. At the same time, the strong reducing atmosphere in the circulating fluidized bed also facilitated the reduction of fuel-nitrogen into N2, which resulted in low NOx emissions. On this basis, with the combination of preheating combustion technology and air-staging combustion technology, the NOx emissions had drastically decreased when the burnout air distribution position moved down or varied from a single-layer distribution to a multi-layer distribution system. The lowest original NOx emissions were 90.6 mg/m3 (at 6% O2), and the combustion efficiency exceeded 97%, which ultimately achieved efficient and clean combustion of ultra-low volatile carbon-based fuel.

Gasification of preheated coal: Experiment and thermodynamic equilibrium calculation

A new process integrating a circulating fluidized bed (CFB) reactor and an entrained bed reactor was proposed for gasification of preheated coal. The CFB reactor as a preheater was successfully used in clean coal combustion. In this study, gasification of preheated coal was tested in a bench-scale test rig, which consisted of a CFB preheater and a down flow bed (DFB) gasifier. The effects of operating parameters of the preheater and gasifier were revealed via thermodynamic equilibrium calculations. A stable preheating process was obtained in the CFB preheater at the O2/C molar ratio of 0.31 and higher gasification reactivity was gained in preheated char owing to the improvement in intrinsic reactivity, specific surface area and total pore volume. Effective gasification of preheated char was achieved in the DFB gasifier at 1100 °C and the total O2/C molar ratio of 0.67, meanwhile the CO + H2 yield and carbon conversion increased. Thermodynamic equilibrium calculations revealed when the gasification reaction rates varied little above 1100 °C and the same carbon conversion was achieve in gasifier, lowering the temperature would lead to an increase in cold gas efficiency and a decrease in O2 demand.

Carbothermal Reduction of Quartz and Carbon Pellets at Elevated Temperatures

In this study, the carbothermal reduction of pellets composed of quartz and carbon at temperatures between 1898 K and 1948 K (1625 °C and 1675 °C) are investigated. The main product from this reaction is silicon carbide (SiC). The reduction of quartz with carbon black, charcoal, coke, coal, and pre-heated coal in the pellet were compared to investigate the different carbon resources used in silicon production. Charcoal and coke have high SiO reactivity, while carbon black and coal (pre-heated coal) have low SiO reactivity. Charcoal and carbon black show better matching between quartz/carbon reactivity and SiO reactivity, and will lose less SiO gas than coke and pre-heated coal. Coal has a high volatile content and is thus not recommended as a raw material for the pellets.

Study of coal oxidation behaviour in re-opened sealed heating

Possible changes in the oxidation behaviour of coal in the spontaneous combustion site of re-opened sealed heating were studied. Two samples of bituminous coal and three types of coal pre-treatment procedures were applied to simulate in situ conditions at the “spon-com” site: i) pre-oxidation of coal, ii) pre-heating of coal under inert gas, and iii) immersion of pre-heated coal in liquid water. Pre-treated samples were then examined for the production rate of the indication gases evolved during oxidation and for oxidation heat effects. Two main conclusions were drawn with respect to oxidation of the coal in re-opened sealed heating: i) carbon monoxide and unsaturated hydrocarbons maintain their relevance as spontaneous combustion indication gases, however, increased production of both carbon oxides can be expected; ii) coal in the extinguished spontaneous combustion site gains increased susceptibility to the self-heating process.

ＥＳＲによる急速加熱炭の石炭構造解析

Coal was heat-treated up to the softening temperature under the 2 types of heating conditions (10°C/min, 104°C/min). Spin concentration by high temperature Iusitu ESR, and the saturation behavior of the ESR spectrum were measured. Based on that result, effect of rapid preheating on coal molecular structure was examined.(1) Because the change in coal chemical structure by the cracking reaction is taking place, spin concen-tration of slow preheated coal is higher than that of rapidly preheated coal and raw coal. At 300°C or more, the spin concentration of rapidly preheated coal is same profile as raw coal. In the case of rapid preheating, it is indicated that the degree of coal structure change by cracking reaction is small.(2) The following results were obtained as the saturation power of ESR was measured. At 250°C or more, the saturation power of slow preheated coal declines as the temperature rises. One side, it confirmed that the saturation power of rapidly preheated coal became large at 370°C. The increase in saturation power indicates that the relaxation of coal structure progressed about coal molecular structure part that can be observed with ESR.

微粉炭の成形性に及ぼす熱間成形条件の影響

微粉炭の成形性に及ぼす熱間成形条件の影響

熱間成形が高温炭装入時のキャリーオーバーに及ぼす影響

熱間成形が高温炭装入時のキャリーオーバーに及ぼす影響