Investigation on the combustion possibility of dry sewage sludge as a pulverized fuel of thermal power plant

Combustion possibility of three dry sludges as pulverized fuel of coal power plant like sub-bituminous Minco coal was studied by thermogravimetric analysis (TGA) and Drop Tube Furnace (DTF). TGA results showed that the fixed carbon contained with minor content in dry sludge was slowly burned than it of Minco coal. The linear regression for the Arrhenius plot to the experimental data is very good, and activation energies for overall combustion of Minco coal and DDSS are 64.382 and 26.799 kJ/mol, respectively. But, combustion patterns of KDSS and SDSS divided into devolatilization and oxidation reaction. It was derived that activation energies for the devolatilization of KDSS and SDSS are 27.127 and 12.571 kJ/mol in reciprocal proportion to volatile matter content, the fixed carbon combustion derives to 45.289 kJ/mol for KDSS, 33.777 kJ/mol for SDSS. Test results show that the volatile content in sludge significantly improved the combustion reactivity whereas the time for the combustion completion delayed. The conversion behavior of the coals and sludge observed in DTF was similar to that reflected in TGA. DTF studies showed that the individual sludge was lower conversion than the Minco coal, but the combustion of most sludge was completed at residence time of around 1 s, set temperature range of 1200 °C similar to commercial coal fired plant. These high IDT of sludge ashes with minimum 1214 °C are not expected to be associated with slagging and fouling in pulverized coal fired systems.     

Gasification rate analysis of coal char with a pressurized drop tube furnace

Two coal chars were gasified with carbon dioxide or steam using a Pressurized Drop Tube Furnace (PDTF) at high temperature and pressurized conditions to simulate the inside of an air-blown two-stage entrained flow coal gasifier. Chars were produced by rapid pyrolysis of pulverized coals using a DTF in a nitrogen gas flow at 1400°C. Gasification temperatures were from 1100 to 1500°C and pressures were from 0.2 to 2 MPa. As a result, the surface area of the gasified char increased rapidly with the progress of gasification up to about six times the size of initial surface area and peaked at about 40% of char gasification. These changes of surface area and reaction rate could be described with a random pore model and a gasification reaction rate equation was derived. Reaction order was 0.73 for gasification of the coal char with carbon dioxide and 0.86 for that with steam. Activation energy was 163 kJ/mol for gasification with carbon dioxide and 214 kJ/mol for that with steam. At high temperature as the reaction rate with carbon dioxide is about 0.03 s⁻¹, the reaction rate of the coal char was controlled by pore diffusion, while that of another coal char was controlled by surface reaction where reaction order was 0.49 and activation energy was 261 kJ/mol.     

The estimation of char reactivity from coal reflectogram

Measurements of the intrinsic reactivity of chars to oxygen are increasingly being sought as an indicator of the combustion potential of fuels. The coal reflectogram has been used to characterize the chemical properties of coal and its resultant char structure. In this study, six Australian coals varying in rank were separated using density separation technique to obtain vitrinite and inertinite rich fractions. Chars were obtained from these density fraction samples in a Drop Tube Furnace (DTF) at 1673 K. The reactivity of the chars was measured non-isothermally in a Thermal Gravimetric Analysis (TGA) in the temperature range of 573–1073 K. The results suggested that with the increase in the coal rank, the maximum reactivity of chars derived from vitrinite rich fractions decreases, while the reactivity of chars derived from inertinite rich fractions decreases with the increase in the inertinite content in samples and has no obvious relationship with rank. The kinetic parameters were derived using data from non-isothermal TGA after accounting for changing in surface area with conversion. The frequency factor is found to decrease with increasing coal FMR, defined as the summation of each reflectance value multiplied by its frequency, for a constant activation energy (E=146 kJ/mol). This suggests that the behavior of a maceral is characterized primarily by its reflectance distribution instead of the type of its parent coal.     

Effect of steam and sodium hydroxide for the production of hydrogen on gasification of dehydrochlorinated poly(vinyl chloride)

Dehydrochlorinated poly(vinyl chloride) (PVC) and activated carbon were pyrolyzed with sodium hydroxide in a flow of steam and nitrogen at 3.0 MPa and 560–660 °C. In both cases, hydrogen and sodium carbonate were the main products, and methane, ethane, and carbon dioxide were minor products. The gasification rate increased with partial steam pressure, and the reaction order with respect to steam partial pressure was 0.69. For both dehydrochlorinated PVC and activated carbon, the gasification rate increased with the NaOH/C molar ratio. However, the rate became saturated at NaOH/C ratios higher than 2.0. The activation energy of gasification of dehydrochlorinated PVC or activated carbon was 178 kJ/mol, assuming first-order reaction rate. These experimental results indicate that hydrogen was produced from the reaction: C+2NaOH+H₂O→Na₂CO₃+2H₂.     

Hydrothermal extraction and hydrothermal gasification process for brown coal conversion

A novel coal conversion process was proposed: the method combines “a hydrothermal extraction of brown coal (HT-Extraction)” and “a catalytic hydrothermal gasification of the extract (CHT-Gasification)” both of which are performed under the exactly same conditions of less than 350 °C and less than 20 MPa. Organic compounds in the aqueous phase, extracted from brown coal, was gasified using a novel Ni-supported carbon catalyst developed by the authors, producing combustible gas rich in CH₄ and H₂. Through this process performed at 350 °C and 18 MPa, an Australian brown coal was almost perfectly converted into 53% of upgraded coal, 23% of methane, and 24% of carbon dioxide on carbon basis. Simultaneously, 4.4 mol of hydrogen was generated from 100 mol of carbon of the coal. This process transferred 97% of energy involved in the raw coal to the products, indicating its effectiveness.     

Thermal characterization of mustard straw and stalk in nitrogen at different heating rates

The pyrolysis of mustard straw and stalk was investigated at different heating rates from the ambient temperature to a temperature of 700 °C in a dynamic nitrogen flow of 40 cc/min. The thermogravimetric (TG) and derivative thermogravimetric analysis (DTG) profiles were examined for the entire degradation zone to determine the order of reaction, pre-exponential factor and activation energy. The orders of reaction were in the range of 0.61–1.02, activation energy in the range of 10.83–21.63 kJ/mol and pre-exponential factor was in the range of 15.67–27.2 min⁻¹. This study of kinetics of pyrolysis of this abundant biomass is helpful in developing the mechanism of a thermochemical conversion process.     

Hydration kinetics modeling of the effect of curing temperature and pressure on the heat evolution of oil well cement

The heat evolution of Class G and Class H oil well cements cured under different temperatures (25 °C to 60 °C) and pressures (2 MPa to 45 MPa) was examined by isothermal calorimetry. Curing pressure was found to have a similar effect on cement hydration kinetics as curing temperature. Under isothermal and isobaric conditions, the dependency of cement hydration kinetics on curing temperature and pressure can be modeled by a scale factor which is related to the activation energy and the activation volume of the cement. The estimated apparent activation energy of the different cements at 2 MPa varies from 38.7 kJ/mol to 41.4 kJ/mol for the temperature range of 25 °C to 40 °C, which decreases slightly with increasing curing temperature and pressure. The estimated apparent activation volume of the cements at 25 °C varies from − 23.1 cm³/mol to − 25.9 cm³/mol for the pressure range studied here, which also decreases slightly in magnitude with increasing curing temperature.     

Air blown partial gasification of coal in a pilot plant pressurized spout-fluid bed reactor

The advanced high efficiency cycles are all based on gas turbine technology, so coal gasification is the heart of the process. A 2 MW_th spout-fluid bed gasifier has been constructed to study the partial gasification performance of a high ash Chinese coal. This paper presents the results of pilot plant partial gasification tests carried out at 0.5 MPa pressure and temperatures within the range of 950–980 °C in order to assess the technical feasibility of the raw gas and residual char generated from the gasifiier for use in the gas turbine based power plant. The results indicate that the gasification process at a higher temperature is better as far as carbon conversion, gas yield and cold gas efficiency are concerned. Increasing steam to coal ratio from 0.32 to 0.45 favors the water–gas and water–gas shift reactions that causes hydrogen content in the raw gas to rise. Coal gasification at a higher bed height shows advantages in gas quality, carbon conversion, gas yield and cold gas efficiency. The gas heating value data obtained from the deep-bed-height displays only 6–12% lower than the calculated value on the basis of Gibbs free energy minimization. The char residue shows high combustion reactivity and more than 99% combustion efficiency can be achieved.     

Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical,structural and petrographic characteristics

An investigation was undertaken to determine the properties of high ash coal–chars derived from South African discards rich in inertinites, for the development of suitable overall reaction rate models at low temperatures (<900 °C). Detailed characterisation results of the parent coal and chars prepared at 700 °C and 900 °C obtained from standard coal analytical methods, petrographic techniques, CCSEM image analysis and a surface adsorption method are presented. The parent coal consisted of 32% by volume of inertite (“pure” inertinite), 7% of vitrite (pure “vitrinite”), and 13% of bi- and tri-macerite, 30% of maceral/mineral mixtures (carbominerite) with 18% of mineral-rich material. Reflectances obtained from measurements taken on vitrinites and total maceral reflectance scans increased dramatically on charring at 900 °C and were accompanied by an extension of vitrinite reflectance class distributions indicating higher molecular ordering. Volatiles were liberated essentially from the original parent vitrinites, creating fine gas pores. Inertinites increased in reflectance but not in porosity and were characterised as dense char fractions in the final charred product, according to a coal form analysis. Structural change due to low temperature thermal stress fracturing (passive deflagration) occurred early on in the temperature regimes, creating increased surface areas and porosity. The chars consist of a high proportion (52%) of extraneous rock fragments together with minerals mainly as fine inclusions in carbon rich particles (13%). The chars had very low porosities and surface areas. These were created by the devolatisation of reactive maceral associations and deflagration. Such materials could introduce intra-particle diffusional effects during gasification and combustion of millimetre size particles at low temperatures.     

Experimental investigation of role of steam in entrained flow coal gasification

Experimental investigations of the influence of excess oxygen coefficient, H₂O/coal mass ratio using high-temperature steam, mean mass diameter of pulverized coal and coal size fraction on basic characteristics of coal gasification were performed. Experiments were carried out on a laboratory scale (0.09 m i.d. × 1.5 m high) coal gasification apparatus with lignite type of coal. Influence of steam was realized through comparison of results obtained from experiments with (H₂O/coal = 0.287 kg kg⁻¹) and without steam addition (H₂O/coal = 0.024 kg kg⁻¹). High values of carbon conversion, obtained both for finely ground and for coarse pulverized coal points to the easiness of lignite gasification, i.e. to its high suitability for gasification.     

基于简单碰撞理论的生物质焦炭气化反应的活化能

利用热重分析仪在800~1000℃及750~1000℃下分别对11种生物质原焦及6种生物质脱灰焦进行了CO2等温气化实验,用碳转化率x=0.2时的瞬时气化反应速率rc,0.2对反应速率rc进行无量纲化处理;根据简单碰撞理论,推导得出了生物质焦炭气化反应速率的表达式,求取了17种生物质焦炭气化反应的活化能;结合催化理论与简单碰撞理论建立了生物质焦炭气化反应活化能的经验预测模型. 结果表明,转化率达0.2后,各焦炭不同温度下无量纲气化反应速率曲线基本重合,表明不同温度下焦炭微观结构在转化过程中具有基本相同的演变规律. 各焦炭的活化能与催化剂所占据的活性位比例存在良好的对数关系. 忽略催化效应的影响,焦炭本征气化反应的活化能趋于某一定值,约为254.35 kJ/mol,而完全催化反应活化能约为66.02 kJ/mol.     

Two-stage master sintering curve applied to two-step sintering of oxide ceramics

Tetragonal (3 mol% Y₂O₃) and two cubic zirconia (8 mol% Y₂O₃) as well as alumina green bodies were used for the construction of the Master Sintering Curve (MSC) created from sets of constant-rate-of-heating (CRH) sintering experiments. The activation energies calculated according to the MSC theory were 770 kJ/mol for Al₂O₃, 1270 kJ/mol for t-ZrO₂, 620 kJ/mol and 750 kJ/mol for c-ZrO₂. These values were verified by an alternative approach based on an analysis of the densification rate in the intermediate sintering stage. The MSCs established from the Two-Step Sintering (TSS) experiments showed at high densities a significant deflection from those constructed from the CRH experiments. This deflection was explained by lower sintering activation energy in the closed porosity stage. A new two-stage MSC model was developed to reflect the change in sintering activation energy and to describe TSS. The efficiency of TSS of four materials under investigation was correlated with their activation energies during the final sintering stage.     

Synthesis,surface characteristics,and electrochemical capacitance of Cu-doped carbon xerogel microspheres

Small-amplitude oscillatory shear tests were used to determine the rheological properties of a copper acetate-doped resorcinol–formaldehyde mixture at between 30 and 40 °C. The apparent activation energy of the sol–gel transition was 76.6 ± 0.6 kJ/mol. Organic gel microspheres were only obtained when the sol was emulsified immediately before the gelation point and not at the gelation point itself, due to the fast gelation kinetics of the copper acetate-doped resorcinol–formaldehyde mixture. The microspherical shape was preserved after carbonization. Cu-doped carbon xerogel microspheres were steam-activated at 840 °C. All samples comprised isolated well-formed microspheres, whose size increased with higher degree of activation. The surface area and porosity varied with the activation degree. Copper was detected as CuO, which acted as gasification catalyst during activation, and its size increased with higher activation degree. Electrochemical measurements were conducted with a three-electrode cell in 1 M H₂SO₄. A very large volumetric capacitance, 146 F/cm³, was found for the 30%-activated Cu-doped activated carbon xerogel, attributable to the high particle density resulting from the very compact packing of the microspheres. This sample also showed the lowest equivalent series resistance, due to its pore texture and high surface Cu content.     

Characterisation of residual carbon from entrained-bed coal water slurry gasifiers

Gasification slag is a by-product of the coal gasification process. To achieve “zero emissions” from coal gasification technology, the environmentally safe utilization of slag by-products from gasifiers must be addressed and developed. In this study, work has been carried out to characterize residual carbon in slag together the morphological study on vitreous material present in slag. The compositions of particles in coal, char and slag were determined by using SEM/EDX. An elemental analyzer (Flash EA 1112) was adopted to analyze CHNS on bulk coal, char and slag. The size distribution of carbon in slag was obtained by analyzing slag sample blocks using an image analysis program developed in this study.It was found that the coarse and fine slags both have a relatively high content of unburnt carbon which hinders their utilization as additives in cement and concrete. There was no evidence that coarse slag is the final destination for coarse char particles since large unburnt chars were found in both the coarse slag and the fine slag. The morphology of the residual carbon in the coarse slag is similar to that found in the fine slag, and is also comparable to partially combusted char produced in a drop tube furnace (DTF) operating at 1300 °C, 600 ms and 2% oxygen. It is also found that the vitreous particles in fine slag tends to be below 150 μm and the majority of residual carbon (75.6 wt%) in fine slag tends to be greater than 150 μm, which implies that, a simple screening operation might make the slag usable in concrete industries.     

Hydrogen production from glycerol by reforming in supercritical water over Ru/Al2O3 catalyst

Supercritical water is a promising medium for the reforming of hydrocarbons and alcohols for the production of hydrogen at high pressures in a short reaction time. Water serves both as a dense solvent as well as a reactant. In this work, hydrogen is produced from glycerol by supercritical water reforming over a Ru/Al₂O₃ catalyst with low methane and carbon monoxide formation. Experiments were conducted in a tubular fixed-bed flow reactor over a temperature range of 700–800 °C, feed concentrations up to 40 wt% glycerol, all at short reaction time of less than 5 s. Glycerol was completely gasified to hydrogen, carbon dioxide, and methane along with small amounts of carbon monoxide. At dilute feed concentrations, near-theoretical yield of 7 mol of hydrogen/mol of glycerol was obtained, which decreases with an increase in the feed concentration. Based on a kinetic model for glycerol reforming, an activation energy of 55.9 kJ/mol was observed.     

Products,pathways, and kinetics for reactions of indole under supercritical water gasification conditions

Indole is commonly reported as a product from hydrothermal processing of algal biomass. The reactions of indole in supercritical water were investigated between 550 and 700 °C in quartz, mini-batch reactors. The indole disappearance rate followed first-order kinetics, and the activation energy was 155 ± 10 kJ/mol. Methane and hydrogen were the most abundant gaseous products under most of the conditions tested, whereas benzene was the most abundant liquid-phase product. Hydrogen and carbon gasification efficiencies (HGE and CGE) exhibited values up to 79% and 20%, respectively. The influence of water density on the yields of H₂, CH₄, and C₂H₆ was negligible at densities above 0.081 g/ml, but the CO₂ yield increased with water density whereas the CO yield decreased. The yield of CH₄ increased significantly as the initial indole concentration increased. The collective results, which showed how the yields of numerous intermediate reaction products responded to changes in the process variables, permitted advancement of a potential reaction network.     

CO2 gasification of Thai coal chars: Kinetics and reactivity studies

Two sized fractions (<75 μm and 150–250 μm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500–900 °C. Gasification of coal chars with excess carbon dioxide was then performed at 900–1,100 °C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conversion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher R _s values than higher rank Lampang subbituminous B coals. Smaller chars from both coals had higher R _s values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various pyrolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.     

Effects of thermal pretreatment in helium on the pyrolysis behaviour of Loy Yang brown coal

A wire-mesh reactor capable of multi-step heating/holding and minimising secondary reactions of volatiles was used to investigate the effects of thermal pretreatment in inert gas on the subsequent rapid pyrolysis behaviour of Loy Yang brown coal. Our results indicate that the presence of small amounts (<10 wt%) of moisture in brown coal has little influence on the tar and char yields from the pyrolysis of brown coal at 1000 K s⁻¹. While the hydrogen bonds between the moisture and the O-containing functional groups in the brown coal have little effects on its pyrolysis behaviour, the hydrogen bonds among the O-containing functional groups tend to induce cross-linking reactions to reduce the tar yields. Preheating the brown coal at >250 °C leads to reduced tar and increased char yields. However, the characterisation of tars using UV-fluorescence spectroscopy indicates that significant decreases in the release of larger aromatic ring systems are only observed after preheating at >380 °C for 30 min. The presence of ion-exchangeable cations (e.g. Ca²⁺) in the brown coal tends to stabilise the carboxylate groups and only preheating at >350 °C would result in changes in pyrolysis yields during the subsequent pyrolysis at 1000 K s⁻¹. These results may be explained by considering the formation of cross-links involving peripheral groups at low preheating temperatures and the formation of cross-links involving aromatic ring systems at elevated temperatures.     

Time-resolved powder neutron diffraction study of the phase transformation sequence of kaolinite to mullite

Mullite formation from kaolinite was studied by means of high-temperature in situ powder neutron diffraction by heating from room temperature up to 1370 °C. Neutron diffractometry under this non-isothermal conditions is suitable for studying high-temperature reaction kinetics and to identify short-lived species which otherwise might escape detection. Data collected from dynamic techniques (neutron diffraction, DTA, TGA and constant-heating rate sintering) were consistent with data gathered in static mode (conventional X-ray diffraction and TEM). The full process occurs in successive stages: (a) kaolinite dehydroxylation yielding metakaolinite in the ∼400–650 °C temperature range, (b) nucleation of mullite in the temperature range ∼980–992 to ∼1121 °C (primary mullite) side by side with a crystalline cubic phase (Si-Al spinel) detected in the ∼983–1030 °C temperature interval; (c) growth of mullite crystals from ∼1136 °C, (d) high (or β) cristobalite crystallization at T > ∼1200 °C and (e) secondary mullite crystallization at T > ∼1300 °C. The calculated activation energy for the kaolinite dehydration was 115 kJ/mol; for the mullite nucleation was 278 kJ/mol and for the growth of mullite process was 87 kJ/mol; finally for cristobalite nucleation the calculated apparent activation energy was 481 kJ/mol.     

Diffusion coefficient of carbon in Fe–Ni alloy during synthesis of diamond under high temperature and high pressure

Synthetic diamond particles were prepared under high temperature and high pressure using arrayed seeds. A dense Fe–Ni alloy shell covered each diamond seed during synthesis; the growth of diamond particles was controlled by the diffusion of carbon through the metallic shell. The diffusion coefficient of carbon through Fe–Ni melt at 1600 K and 5.5 GPa is about 5×10^?6 cm²/s, with an activation energy for diffusion of 336 kJ/mol.