An investigation on the rheological and sulfur-retention characteristics of desulfurizing coal water slurry with calcium-based additives

Desulphurizing coal water slurry is a kind of new clean coal water slurry(CWS), which has good performance on SO₂ emission during combustion and gasification process. But, the addition of sulfur-retention agents have some effects on the stability and fluid characters of the coal water slurry. In this paper, the viscosity, stability and rheology of Xinwen coal water slurry have been studied by adding different kinds of calcium-based sulfur-retention agents and different dosage. The results show that the sulfur-retention agents have little effect on rheological nature of CWS, which still presents pseudoplastic fluid. The addition of sulfur-retention agents will increase the viscosity of CWS, but the stability will decrease a little. The results also show that inorganic calcium has less negative effect on the performance of CWS than the organic calcium. The viscosity of the CWS with organic calcium agent keeps 1000–1200 mPa s when Ca/S molar ratio is 2. Sulfur release of the CWS with CaCO₃ reduces to 52% at Ca/S = 2 compared to original of 98%.     

Supercritical water oxidation of coal: Investigation of operating parameters' effects, reaction kinetics and mechanism

Supercritical water oxidation (SCWO) of coal was conducted in a continuous tubular reactor under various reaction conditions. Our experimental results show that the removal rate of chemical oxygen demand (COD) had no significant dependence on the temperature variations. Effect of residence time was less significant as exceeded fixed values. Free radical mechanism of SCWO reaction may be a possible explanation for the relative low conversion rate of coal at the range of tested oxygen excess. Compared with other parameters, effect of pressure was less significant. A global power-law rate expression was regressed from experimental data. The reaction orders for coal slurry and oxidant were 1.79 and 0.28 respectively. The reaction activation energy E_a was determined to be 112.3 kJ mol⁻¹, and the pre-exponential factor k₀ was 412 (mol/L)^−1.07 s⁻¹. The deviation between the model and experimental data was within ± 9%. Free radical mechanism, oxidation and hydrolysis mechanisms and phenolic hydroxyl oxidation mechanism were considered to be the possible mechanisms for the SCWO process of coal.     

Three-dimensional numerical simulation of a horizontal rotating fluidized bed

Three-dimensional numerical simulations of a horizontal rotating fluidized bed (RFB) containing glass bead particles (d_s = 82 μm, ρ_s = 2450 kg/m³) and washed alumina (d_s = 89 μm, ρ_s = 1550 kg/m³) were performed. FLUENT 6.1 software was used to carry out our simulation. The numerical results were compared with the experimental data of Qian and Pfeffer et al. [G.H. Qian, I. Bagyi, I.W. Burdick, R. Pfeffer, H. Shaw, Gas-Solid Fluidization in a Centrifugal Field.” AIChE J. 47 (5) (2001) 1022-1034]. The rotating speed of the RFB was set at 325 rpm (34 rad/s), which is equivalent to a centrifugal acceleration of 7 g.The flow behavior of the solid particles was analyzed; the bed thickness and the calculated pressure drop were compared with the experimental results. Our calculated pressure drop agreed very well with the experimental results.     

Effects of pore fractal structures of ultrafine coal water slurries on rheological behaviors and combustion dynamics

The ultrafine coal water slurry (CWS) with the particle size of 1-10 μm, ash content of 1-2%, solid concentration of 50% is a promising substitute fuel for diesel oil. The effects of pore fractal structures of three ultrafine CWSs on their rheological behaviors and combustion dynamics were studied in this paper. When the pore fractal dimensions of Yanzhou, Huainan and Shenhua ultrafine CWSs increase, their apparent viscosities all increase and the increase extents gradually enlarge with decreasing shear rates, while their ignition temperatures and apparent activation energies all decrease. For example, when the pore fractal dimension of Yanzhou coal increases from 2.31 to 2.43, the CWS apparent viscosity at a low shear rate of 12 s⁻¹ increases from 75 mPa s to 2400 mPa s, and that at a high shear rate of 100 s⁻¹ increases from 80 mPa s to 820 mPa s. Meanwhile, the ignition temperature of Yanzhou CWS decreases from 445 °C to 417 °C at a heating rate of 12.5 °C/min, and the apparent activation energy decreases from 104 kJ/mol to 32 kJ/mol.     

Thermogravimetric analysis and kinetics of coal/plastic blends during co-pyrolysis in nitrogen atmosphere

Investigations into the co-pyrolytic behaviours of different plastics (high density polyethylene, low density polyethylene and polypropylene), low volatile coal and their blends with the addition of the plastic of 5 wt.% have been conducted using a thermogravimetric analyzer. The results indicated that plastic was decomposed in the temperature range 438–521 °C, while the thermal degradation temperature of coal was 174–710 °C. The overlapping degradation temperature interval between coal and plastic was favorable for hydrogen transfer from plastic to coal. The difference of weight loss (?W) between experimental and theoretical ones, calculated as an algebraic sum of those from each separated component, was 2.0–2.7% at 550–650 °C. These experimental results indicated a synergistic effect during plastic and coal co-pyrolysis at the high temperature region. In addition, a kinetic analysis was performed to fit thermogavimetric data, the estimated kinetic parameters (activation energies and pre-exponential factors) for coal, plastic and their blends, were found to be in the range of 35.7–572.8 kJ/mol and 27–1.7 × 10³⁸ min^− 1, respectively.     

An investigation of the causes of the difference in coal particle ignition temperature between combustion in air and in O2/CO2

The ignition temperatures of a Loy Yang brown coal and a Datong bituminous coal were investigated in a wire-mesh reactor where the secondary reactions of the evolved volatiles were minimised. An increase in the average particle ignition temperature of 21 °C was observed for the brown coal when air (21% O₂ + 79% N₂) was replaced with a mixture of 21% O₂ + 79% CO₂. Combustion was also carried out in the mixtures of 21% O₂ + 79% argon and 21%O₂ + 79% helium in order to determine the effects of heat transfer on the observed particle ignition temperature. It is concluded that the thermal conductivity of gas atmosphere surrounding the particles greatly influences the observed particle ignition temperature while the effects of the heat capacity of the gas atmosphere was very minor under our experimental conditions. The structure of char and the reactions involving the char (char-O₂ and char-CO₂) can greatly affect the observed particle ignition temperature. In particular, the char-CO₂ reactions were largely responsible for the observed difference in particle ignition temperature in air and in 21% O₂ + 79% CO₂. Alkali and alkaline earth metallic (AAEM) species in the brown coal also significantly affect the observed particle ignition temperature.     

Coal dust/air explosions in a large-scale tube

Coal dust/air mixture explosions under weak ignition conditions have been studied in a horizontal experimental tube of diameter 199 mm and length 29.6 m. The experimental tube is closed at one end and open at the downstream end. An array of 40 equally spaced dust dispersion units was used to disperse coal dust particles into the experimental tube. The coal dust/air mixture was ignited by an electric spark. A constant-temperature hot-wire anemometer was used to measure the gas velocity during the dispersion process. Kistler piezoelectric pressure sensors were used to measure the propagation of the pressure wave during the explosion process. The maximum overpressure of the coal dust explosion under the weak ignition conditions in the tube was 70 kPa and the propagation velocity of the pressure wave along the tube was approximately 370 m/s. The minimum concentration for obtaining a coal dust explosion that propagated along the tube was 120 g/m³. The suppressing effects on the coal dust explosion of two different kinds of suppressing agents have been studied.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Performance of an entrained-flow gasification technology of pulverized coal in pilot-scale plant

Performance of an entrained-flow gasification technology of pulverized coal in pilot-scale plant is introduced. The gasifier was operated for a throughput of 30–45 t coal per day at pressures of 1–3 MPa. Dense-phase pneumatic conveying was employed for coal's feeding to the gasifier using nitrogen and carbon dioxide as carrier gas, respectively. Effects of the operating conditions including oxygen/carbon ratio and steam/carbon ratio on gasification results were investigated, and the concentration of (CO + H₂) in gaseous products reached up to about 97% (vol., dry basis) when CO₂ was employed as carrier gas. Moreover, performances of some important instruments in the conveying system of pulverized coal, such as the level indicator and the solid mass flow meter, were also investigated. The typical operating results in this plant such as (CO + H₂) concentration, oxygen consumption, coal consumption, carbon conversion and cold gas efficiency were almost as good as those of some well-known dry-fed entrained-flow coal gasification plants.     

High pressure hydropyrolysis of coals by using a continuous free-fall reactor

Rapid hydropyrolysis of coal was carried out at temperatures ranging from 923 to 1123 K and H₂ pressures up to 7 MPa by using a continuous free-fall pyrolyzer. The effects of the reaction conditions on product yields were investigated. Carbon mass balance was fairly good. It was revealed that a large amount of methane was produced due to the hydrogenolysis of higher hydrocarbons and the hydrogasification of char. The influence of pyrolysis temperature was significant on both reactions while H₂ pressure mainly affected the latter. A considerable amount of reactive carbon was formed during hydropyrolysis of coal. It was converted to methane at high temperatures and high H₂ pressures, while the hydrogasification of reactive carbon takes place relatively slowly at low temperatures and low H₂ pressures, resulting in a low overall carbon conversion. The coal conversions observed in the present study were much higher than those obtained with using reactors where the contact between coal particles and H₂ is insufficient.     

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t_v = Ad_vⁿ. The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.     

Vaporization behavior of boron from standard coals in the early stage of combustion

Boron-containing compounds have been listed as one of environmentally hazardous substances in Japan since 2001, and known to condense in coal fly ash particles during coal combustion and coal fly ash formation in coal-fired electric power stations. So far, the authors have revealed that the speciation of boron-containing compounds in coal fly ash particles is mostly a calcium orthoborate or pyroborate. In this research, the speciation of boron compounds in standard coals and their char generated by laboratory-scale combustion test has been investigated by using a microwave-assisted acid digestion method and a Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS-NMR) in order to reveal the vaporization behavior of boron in standard coals during combustion at relatively low temperature. Three isolated peaks are observed in ¹¹B MAS-NMR spectra of standard coals, and all of them are attributed to four-oxygen-coordinated boron atom. Around 50% of boron vaporizes even though heating condition is 200 °C and O₂ = 25%, and the percentage of vaporization reaches higher value than 80% at 400 °C and O₂ = 25%. The remaining boron contents in ash components are relatively small, and it suggests that most of boron in standard coals exist with relatively volatile carbon contents, and they volatilize in the very early stage of coal combustion.     

Co-pyrolysis of biomass and coal in a free fall reactor

An experimental study on co-pyrolysis of biomass and coal was performed in a free fall reactor under atmospheric pressure with nitrogen as balance gas. The coal sample selected was Dayan lignite, while the biomass used was legume straw. The operation temperature was over a range of 500-700 °C, and the blending ratio of biomass in mixtures was varied between 0 and 100 wt.%. The results indicated that there exist synergetic effects in the co-pyrolysis of biomass and coal. Under the higher blending ratio conditions, the char yields are lower than the theoretical values calculated on pyrolysis of each individual fuel, and consequently the liquid yields are higher. Moreover, the experimental results showed that the compositions of the gaseous products from blended samples are not all in accordance with those of their parent fuels. The CO₂ reactivities of the chars obtained from the co-pyrolysis under the higher blending ratio (around 70 wt.%) conditions are about twice as high as those of coal char alone, even higher than those of biomass alone.     

Prediction of coal grindability based on petrography,proximate and ultimate analysis using multiple regression and artificial neural network models

The effects of proximate and ultimate analysis, maceral content, and coal rank (R_max) for a wide range of Kentucky coal samples from calorific value of 4320 to 14960 (BTU/lb) (10.05 to 34.80 MJ/kg) on Hardgrove Grindability Index (HGI) have been investigated by multivariable regression and artificial neural network methods (ANN). The stepwise least square mathematical method shows that the relationship between (a) Moisture, ash, volatile matter, and total sulfur; (b) ln (total sulfur), hydrogen, ash, ln ((oxygen + nitrogen)/carbon) and moisture; (c) ln (exinite), semifusinite, micrinite, macrinite, resinite, and R_max input sets with HGI in linear condition can achieve the correlation coefficients (R²) of 0.77, 0.75, and 0.81, respectively. The ANN, which adequately recognized the characteristics of the coal samples, can predict HGI with correlation coefficients of 0.89, 0.89 and 0.95 respectively in testing process. It was determined that ln (exinite), semifusinite, micrinite, macrinite, resinite, and R_max can be used as the best predictor for the estimation of HGI on multivariable regression (R² = 0.81) and also artificial neural network methods (R² = 0.95). The ANN based prediction method, as used in this paper, can be further employed as a reliable and accurate method, in the hardgrove grindability index prediction.     

A new fluidization–suspension combustion technology for coal water slurry

Slagging is a major operating problem in application of the atomization–suspension combustion technology for burning coal water slurry (CWS) fuel in small and low height industrial boilers. The fluidization–suspension combustion is a new alternative for replacement of oil, which is capable of solving the slagging problems. In addition, it can be successfully applied to CWS-fired boilers with capacity smaller than 35 t/h. About 530,000 medium and small scale industrial boilers with low boiler efficiency in China provide the technology a very promising prospect. The principles and contents of CWS fluidization–suspension combustion technology are introduced in detail in this paper. And a new type of 14 MW fluidization–suspension CWS-fired boiler was developed, the performance of which showed that boiler efficiency was 91.53%. Emission of SO₂ and NO_x was 346.1 mg/m³ and 469.5 mg/m³, respectively. From the application, the CWS-fired boiler showed good features such as high efficiency, low pollutant emission, good load regulation, good CWS quality adaptability, steady operation and convenient maintenance.     

Ignition behaviour of different rank coals in an entrained flow reactor

An experimental study to determine the temperature and mechanism of coal ignition was carried out by using an entrained flow reactor (EFR) at relatively high coal feed rates (0.5 g min⁻¹). Seven coals ranging in rank from subbituminous to semianthracite, were tested and the evolved gases (O₂, CO, CO₂, NO) were measured continuously. The ignition temperature was evaluated from the gas evolution profiles, and it was found to be inversely correlated to the reactivity of the coal, as reflected by the increasing values of the ignition temperature in the sequence: subbituminous, high volatile bituminous, low volatile bituminous and semianthracite coals. The mechanism of ignition varied from a heterogeneous mechanism for subbituminous, low volatile bituminous and semianthracite coals, to a homogeneous mechanism for high volatile bituminous coals. A thermogravimetric analyser (TGA) was also used to evaluate coal ignition behaviour. Both methods, TGA and EFR, were in agreement as regards the mechanism of coal ignition. From the SEM micrographs of the coal particles retrieved from the cyclone, it was possible to observe the external appearance of the particles before, during and after ignition. The micrographs confirmed the mechanism deduced from the gas profiles.     

Emission of suspended PM10 from laboratory-scale coal combustion and its correlation with coal mineral properties

Four pulverized coals were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate matter smaller than 10 μm (PM₁₀) and to study the correlation of PM₁₀ emission with mineral properties of the coals. Combustion conditions of 1200 °C, 2.4 s and 20% atmospheric oxygen content were used and all the carbon was consumed under given conditions. The properties of PM₁₀ were studied including its concentration, particle size distribution and elemental composition. Two typical sizes were also subjected to Computer controlled scanning electron microscopy (CCSEM) analysis for determination of chemical species within them. To investigate the influence of coal mineral properties, the metallic elements in the raw coals were divided into three parts: organically bound, included inorganic particles and excluded ones. The results indicated that during coal combustion, about 0.5-2.5 wt% of inherent minerals changed into the suspended PM₁₀. With an increase in the coal ash content, the concentration of PM₁₀ increased proportionally. The resulting PM₁₀ had a bimodal size distribution with two peaks around 2.5 and 0.06 μm, respectively. SiO₂ and Al₂O₃ dominated the large mode around 2.5 μm, which is formed by the direct transformation of inherent minerals. On the other hand, SO₃ and P₂O₅ were prevalent in the small mode around 0.06 μm, which is formed by vaporization of these two elements. For other metals found in PM₁₀, the refractory metals were enriched in the large mode, with concentrations proportional to their content in the excluded minerals in the raw coal. Volatile metals were however enriched in the small mode since, they react with gaseous SO₂ and P₂O₅ to form sulfates and phosphates in the solid phase. The study showed that experimental observations agree with thermodynamic equilibrium considerations.     

Characterization of electrooxidized Pittsburgh No. 8 Coal

A combination of spectroscopy methods (SEM, EDX, XRD, IR, and XPS) was used to study the morphology of Pittsburgh No. 8 Coal after electrolysis in 1.5 M H₂SO₄/100 mM Fe²⁺/100 mM Fe³⁺. The experiments were performed galvanostatically at different operating temperatures (40, 60, and 80 °C) in a continuous coal electrolytic cell. Pt-Ir-Rh plated on carbon fibers was used as anode. The results showed that the activity of the coal appears to increase with the temperature. Deactivation of electrooxidized coal seems to be associated with the formation of products on the surface of the coal. Differences in morphology and mineral composition were found in the coal electrolyzed by using SEM-EDX analysis. The minerals identified in the original coal sample by XRD, FTIR, and XPS were quartz, kaolinite, pyrite, and hematite. After electrooxidation illite and siderite were identified on the surface of coal.     

Study of the characteristics of the ashing process of straw/coal combustion

An experimental study was performed to examine the ashing process during straw/coal co-combustion to determine the effect of the blending ratio on ash products. A total of eleven blending samples with coal contents varying from 5 wt.% to 90 wt.% along with pure wheat straw and pure coal samples were tested to determine the ash fusion points, oxide contents and mineral contents. Blends with coal contents between 5 wt.% and 15 wt.% were able to inhibit ash and reduced ash quantity. Blends with coal contents greater than 20 wt.% promoted ash and increased ash quantity. Thermal decomposition processes during the combustion and ash pyrolysis of the blends with 10 wt.% and 40 wt.% and the samples of pure coal and pure wheat straw were simulated using a Thermal Gravity Analyser. The results indicated that the ashing processes of the blends were influenced by the coupling reactions of the minerals in the straw and coal. When using a blend of 10 wt.% coal, more potassium (K) was accelerated into gaseous products during the volatile releasing and firing stage, which caused an ash quantity reduction effect. K₂O content was lowest in this sample, and a minimum amount of K compounds was detected. With a blend of 40 wt.% coal, because the coupling reactions of Ca and Al produced stable minerals of CaAl₈Fe₄O₁₉ and KSi₃AlO₈, less CaCO₃ and CaSO₄ were produced. Thermal decomposition at the ash pyrolysis stage was very weak and resulted in much less gaseous products than what would be expected at high temperatures; therefore, more ash residues remained.     

Studies of relationship between petrography and elemental analysis with grindability for Kentucky coals

The effects of macerals, ash, elemental analysis and moisture of wide range of Kentucky coal samples from calorific value of 23.65-34.68 MJ/kg (10,170-14,910 (BTU/lb)) on Hardgrove Grindability Index (HGI) have been investigated by multivariable regression method. Two sets of input: (a) macerals, ash and moisture (b) macerals, elemental analysis and moisture, were used for the estimation of HGI. The least square mathematical method shows that increase of the TiO₂ and Al₂O₃ contents in coal can decrease HGI. The higher Fe₂O₃ content in coal can result in higher HGI. With the increase of micrinite and exinite contents in coal, the HGI has been decreased and higher vitrinite content in coal results in higher HGI. The multivariable studies have shown that input set of macerals, elemental analysis and moisture in non-linear condition can be achieved an acceptable correlation, R = 90.38%, versus R = 87.34% for the input set of macerals, ash and moisture. It is predicted that elemental analysis of coal can be a better representative of mineral matters for the prediction of HGI than ash.