Free radical involvement in the oxidation of Victorian brown coal

Electron spin resonance spectroscopy has been used to investigate free radical involvement in the oxidation of Victorian brown coal. Fresh, wet Yallourn brown coal exhibits two superimposed e.s.r. signals due to organic free radicals. The free radicals responsible for the narrow signal interact reversibly with atmospheric oxygen while the broad signal changes intensity dramatically during prolonged exposure to air. Coal:oxygen complex formation may explain these observations. Results also suggest that the free radical content of brown coal increases as a consequence of vacuum drying.     

Free-radical involvement in the drying and oxidation of victorian brown coal

The technique of electron spin resonance spectroscopy has been used to study the changes in free-radical nature and content of brown coals as a result of vacuum drying the coal at various temperatures, and exposing the dried coal to either air at room temperature or oxygen at 105 °C. Free-radical concentrations increased during the vacuum-drying process and this is thought to be a result of organic functional group breakdown. Free-radical concentrations in dry coals following air or oxygen exposure have been found to vary with exposure time. The observations can be explained in terms of free-radical involvement in the oxidation process.     

Kinetic studies on Victorian brown coal hydroliquefaction

Reaction kinetics of the liquefaction of Victorian brown coal in a process development unit (PDU) having three reactors in series have been studied at temperatures of 430–470°C, and pressures of 15–25 MPa. It is shown that the rate of hydrogen consumption can be expressed as a function of the concentrations of coal and catalyst, hydrogen partial pressure, reaction temperature and residence time, and is controlled by the rates of hydrogenation of polynuclear aromatic components, and the rates of formation and stabilization of radicals. The relative contribution of these reactions, at any temperature, determine the influence of the hydrogen partial pressure on the rate of the hydrogen consumption. The kinetics of the decomposition reactions of brown coal to preasphaltene, asphaltene and to oil also have been studied. The apparent activation energies determined are 25 kJ mol^?1 for the brown coal to preasphaltene, 50 kJ mol^?1 for preasphaltene to asphaltene, 76 kJ mol^?1 for asphaltene to oil, and 184 kJ mole^?1 for oil to gases.     

Variation in Victorian brown coal characteristics and hydrogenation potential

A detailed study of the variation of properties and characteristics of Victorian brown coal both within seams and between fields has been carried out. Within a seam, the organic coal properties are lithotype dependent whilst the exchangeable inorganic elements, Na, Mg, Ca and Fe, show no relation to lithotype and, hence, are believed to be of post-depositional origin. Considerable variation in coal properties between different brown coal fields has been demonstrated by analyses of 219 samples from 10 coal fields. The coal properties which correlate best with hydrogenation reactivity are primarily associated with the hydrogen content of the coal. The mineral matter also influences the hydrogenation reactivity, as demonstrated by the occurrence of the value for ash level in multiple linear regression equations.     

Measurement of volatile matter in Victorian brown coal

The effects of using one-stage and two-stage heating, using air-dried coal and coal oven-dried under nitrogen, and using final temperatures of 850°C and 900°C on the measured volatile matter for twelve Victorian brown coals have been examined by the use of a statistically controlled experiment. The repeatability of the two-stage method was found to be significantly better than that of the one-stage method (0.7% abs. compared to 0.9% abs.). The volatile matter of coals at the front of the furnace was found to be significantly different from that of coals at the back of the furnace for the one stage method but not for the two-stage method. A two-stage heating method (7 min at 400°C and then 7 min at 900°C) using oven-dried coal is recommended for the routine determination of volatile matter in brown coal.     

低温氧化破粘对煤炭地下气化热解特性的影响

粘结性烟煤在地下气化过程中容易造成通道堵塞,气流分布不均匀进而导致地下气化过程环境的恶化,使气化反应停止。粘结特性在热解过程中表现得尤为突出,研究了低温氧化破粘的方法对地下气化热解特性的影响。结果表明随着氧化时间的增加,最大失重速率降低,煤气中H2,CH4体积分数以及热值在部分热解温度区间均低于原煤,从而增加了煤炭地下气化热解惰性。     

Degradation of a Victorian brown coal in sub-critical water

Conversion characteristics of a Victorian brown coal in sub-critical water were investigated. Pulverized brown coal was heated up to 623 K in flowing sub-critical water pressurized at 25 MPa. The total conversion of the coal into extract and non-condensable gas reached over 70 wt%-daf, which was appreciably higher than the maximum conversion (50 wt%-daf) with a sub-critical non-hydrogen donor solvent, 1-methylnaphthalene (MN). Laser-ionization-desorption mass spectrometry showed that the sub-critical water extract was richer in lower-molecular-mass compounds than the sub-critical MN one. Thus, degradation of the coal occurred more extensively in sub-critical water than in MN. Along with the conversion in sub-critical water, both the total contents of hydrogen and phenolic hydroxyls in the whole products remained nearly unchanged. This suggests comparable and simultaneous formation and decomposition of hydroxyls through hydrolysis of ethers/esters and dehydration condensation between hydroxyls/carboxyls, respectively. For detecting the hydroxyl formation, the coal was first heated at 623 K under an inert gas atmosphere until the formation of water and the other volatiles was completed. Then, the heat-treated coal (LY-H) was exposed to flowing sub-critical water. As expected, the net formation of phenolic hydroxyls from LY-H was detected as 0.8 mmol-OH/g-LY-H while that of hydrogen as 2.3 mmol-H/g-LY-H. Approximately a half of the hydrogen gain was explained as phenolic hydroxyls gain, suggesting the importance of hydrolysis of esters and ethers that formed carboxyls and alcoholic hydroxyls as well as phenolic hydroxyls.     

Evidence of poly-condensed aromatic rings in a Victorian brown coal

An attempt was made for obtaining UV/VIS absorption and fluorescence spectra of a whole Victorian brown coal and then examining the presence of aromatic ring systems (ARSs) with poly-condensed rings in the coal. Loy Yang brown coal was subjected to an alkali-promoted depolymerization in an aqueous solution of sodium hydroxide at 473 K and then dissolved nearly completely into the solution. The solution of the solubilized coal was analyzed by UV/VIS absorption and fluorescence spectroscopies. The spectra of the solubilized coal were compared with those of a tar that was produced from the rapid pyrolysis of the coal in a wire-mesh reactor. Absorbance of the solubilized coal per unit molar concentration of aromatic carbon and unit light path length was averaged over 50 nm wavelength intervals in a range from 250 to 600 nm. The averaged absorbance of the solubilized coal was much greater than those of mono-aromatic compounds and lignin at intervals from 400 to 600 nm while comparable to those of tetra- to hexa-aromatic compounds, suggesting that the coal is abundant of ARSs with poly-condensed rings. The fluorescence spectra of the solubilized coal also give indication of the presence of ARSs with three to six condensed rings. Furthermore, the presence of even larger ARSs is evidenced from appreciable difference in apparent quantum yield of the solubilized coal and that of the tar. By comparing the absorption and fluorescence spectra of the solubilized coal with those of the tar, it was revealed that the tar is richer in relatively small poly-ARSs than the solubilized coal while the larger poly-ARSs are abundant in the solubilized coal. Smaller poly-ARSs were thus released from the coal preferentially to larger poly-ARSs during the pyrolysis.     

Prediction of BP neural network and preliminary application for suppression of low-temperature oxidation of coal stockpiles by pulverized coal covering

We have developed a new method to suppress spontaneous combustion of coal piles by covering the surface of coal piles with pulverized coal. Experimental studies of three type of coal samples from China (YJL, CYW, and SW) with particle size ratio of 10:1 were performed to investigate the low-temperature oxidation of coal pillars. In this work, we have also demonstrated that the distributions of oxygen concentration, the temperature field, as well as the spontaneous combustion of three typical Chinese coal samples can be predicted accurately using back-propagation neural network (BPNN) by MATLAB. Pearson correlation analysis showed that temperature and oxygen concentration highly depend on the ratio of pulverized coal thickness to coal piles thickness, activation energy, void ratio, wind speed, and low-temperature oxidation time. Three-layer BPNN models with five input factors were developed to predict the low-temperature oxidation process under pulverized coal. The prediction data of BPNN are fitting better with our experimental data, which confirms that BPNN modelling can accurately predict the low temperature oxidation process of coal.     

Attrition of Victorian brown coal during drying in a fluidized bed

Analyzing the attrition of Victorian brown coal during air and steam fluidized bed drying, the change in particle size distribution over a range of initial moisture contents (60% to 0%) and residence times (0 to 60 minutes) was determined. Dried at a temperature of 130°C with a fluidization velocity 0.55 m/s and an initial particle size of 0.5–1.2 mm, both fluidization mediums show a shift in the particle size distribution between three and four minutes of fluidization, with a decrease in mean particle size from 665 µm to around 560 µm. Using differential scanning calorimetry (DSC), the change in particle size has been attributed to the transition between bulk and non-freezable water (approximately 55% moisture loss) and can be linked to the removal of adhesion water, but not to fluidization effects. This is proved through the comparison of air fluidized bed drying, steam fluidized bed drying, and fixed bed drying—the fixed bed drying is being used to determine the particle size distribution as a function of drying. The results show the three drying methods produce similar particle size distributions, indicating that both fluidization and fluidization medium have no impact upon the particle size distribution at short residence times around ten minutes. The cumulative particle size distribution for air and steam fluidized bed dried coal has been modeled using the equation P_d = A₂ + (A₁ ? A₂)/(1 + (d/x₀)^p), with the resultant equations predicting the effects of moisture content on the particle size distribution. Analyzing the effect of longer residence times of 30 and 60 minutes, the particle size distribution for steam fluidized bed dried coal remains the same, while air fluidized bed dried coal has a greater proportion of smaller particles.     

Rheological behaviour of Victorian brown coal

As part of an investigation into the use of extrusion as a feed mechanism for processing very wet, soft, brown coals, a parallel plate plastometer was used to obtain absolute rheological data for Loy Yang coal which had been previously work-softened by shearing for various lengths of time. A Bingham model was found to describe the flow behaviour of the material adequately. Most of the resistance to deformation was found to be produced by viscous and not plastic stresses, from which it was concluded that the work-softening effect is a result of a decrease in Bingham viscosity. The fine structure of the coal is broken down by shearing and the water released as a consequence acts as a lubricant between the particles. The ruptured bonds do not remake on standing. It is concluded that any investigation of coal rheology, or the design of any industrial handling or processing operation in which coal rheology might be important, should take account of not only the total water content of the coal but also the disposition of that water. For example, coals with low water contents which would not be thought suitable for extrusion because of their apparent stiffness may be suitable if water can be liberated from the structure by shearing.     

Pyrolysis—gas chromatography of Australian coals. 1. Victorian brown coal lithotypes

Pyrolysis—gas chromatography (Py—g.c.) has been shown to be a useful technique for characterizing Victorian brown coal lithotypes. The pyrograms show marked changes in the predominance and distribution of specific molecular classes as a function of lithotype. Hydrogen-rich triterpenoid components are predominantly associated with the lighter lithotypes, whereas hydrogen-deficient phenolic components are more abundant in pyrolysates from the darker lithotypes. Carbon preference indices (CPIs) are >1 for the alkanes but <1 for the alkenes released by pyrolysis. All CPI values generally increase with darkening lithotype. Correlations of the components released with the maceral composition of the lithotypes have also been established. This information is used to establish some details of the mechanism of pyrolysis and the probable relative liquefaction behaviour of these lithotype samples.     

Further studies of coal oxidation

The low-temperature oxidation of coal has been investigated by Fourier transform infrared spectroscopy (FT-i.r.). Samples were oxidized at 140 and 60 °C for intervals of a few hours to days. These FT-i.r. results were correlated to changes in the thermoplastic properties of the coal measured by a Gieseler plastometer. The loss of Gieseler fluidity as a function of oxidation time corresponds to loss of aliphatic CH groups. Curve-resolving methods were used to distinguish between various C = 0 functional groups. At both oxidation temperatures there was an overall increase in carbonyl and carboxyl groups, but in different relative proportions depending on the oxidation temperatures.     

Supercritical gas extraction of Victorian brown coals: The effect of coal properties

A selection of fifteen Victorian brown coals, which varied in lithotype but only slightly in rank, were subjected to supercritical gas extraction with toluene. Seven of these coals were also extracted with 5% tetralin/toluene under the same conditions of temperature and pressure (400 °C and 10 MPa). The overall conversion, the extract yield and the yield of toluene solubles (oil and asphaltene) were correlated with more easily obtained coal properties using simple linear regression analysis. Good correlations were obtained between the total conversions and the volatile matter content of the coals, and for the toluene extractions between both the extract yield and the yield of toluene solubles and the H/C atomic ratio. For the toluene solubles from the toluene extractions, the aromaticity decreased and the molecular weight increased as the H/C atomic ratio of the coal increased. Inorganic constituents of the coals did not appear to have a marked effect on total conversion and liquid yields. Removal of the cations from two coals increased conversion and liquid yields in one case and decreased these in the other, but in both instances the changes were not large.     

多因素影响下的煤低温氧化特性试验研究

为研究氧浓度、粒度和挥发分对采空区遗煤低温氧化过程的影响,利用油浴式程序升温试验装置设计了煤低温氧化试验,研究了不同影响因素条件下煤的耗氧速率和放热强度,验证了氧浓度、粒度和挥发分等对煤低温氧化特性的影响,通过对试验数据分析,得到了煤低温氧化特性受各因素影响的变化规律。试验结果表明:在一定范围内,氧浓度的增加会加快煤的耗氧速率,提高反应的放热强度,促进煤的氧化,但不会改变氧化反应的趋势;粒度较大的煤耗氧速率低,反应释放的热量小,氧化过程缓慢且强度较低;煤的挥发分降低后,耗氧速率和反应的放热强度也随之减小,其自燃临界温度将显著提高。     

Low-temperature oxidation of brown coal. 1. Changes in internal surface due to oxidation

The low-temperature oxidation of a Yallourn brown coal was found to decrease markedly the internal surface area of the coal. The surface areas were determined by applying the Dubinin-Polanyi approach to carbon-dioxide isotherms measured gravimetrically at 0 °C.     

Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char

Catalytic gasification of a woody biomass, Japanese cypress, was investigated under a prepared nickel-loaded brown coal (LY-Ni) char in a two-stage fixed-bed reactor. The nickel-loaded brown coal was prepared by ion-exchange method with a nickel loading rate of 8.3 wt.%. Nickel species dispersed well in the brown coal, and the LY-Ni char via devolatilization at 600 °C showed a great porous property with a specific surface area of 382 m² g^− 1.The LY-Ni char was confirmed to be quite active for the Japanese cypress volatiles gasification at a relatively low-temperature range from 450 to 650 °C. For example, at 550 °C, 16.6 times hydrogen gas and 6.3 times total gases were yielded from the catalytic steam gasification of Japanese cypress volatiles under the LY-Ni char, compared with the case of non-catalyst. The biomass tar decomposition showed a dependence on catalyst temperatures. When the catalyst temperature was higher than 500 °C, Japanese cypress tar converted much efficiently, high gas yields and high carbon balances were obtained.     

Mild oxidations of coal: 1. Hydrogen peroxide oxidation

Mild oxidation of Pittsburgh bed coal was carried out to investigate the removal of organic sulphur. It was postulated that the organic sulphur in coal might first be oxidized to sulphonic acids and then removed through known desulphonation methods. The coal was oxidized with hydrogen peroxide or mixtures of hydrogen peroxide and acid. After oxidation, the coal was treated with water or phosphoric acid at reflux. Under the mild conditions used, coal oxidation led to slight changes in the organic sulphur content. The ensuing treatment of the oxidized coal with water or phosphoric acid again affected the organic sulphur content, generally decreasing the organic sulphur level. Pyritic sulphur removal varied considerably depending upon the reagent system and reaction conditions.     

Low-temperature oxidation of soot

The oxidation rate of propane soot over the temperature range (770–1250 K) has been measured using two methods. These were direct measurements of the burn-out of soot produced by a laminar diffusion flame, and a thermal gravimetric technique using collected soot. The reaction order with respect to oxygen (over the range 1–20 kPa) was determined at a variety of particle temperatures and found to lie between 0 and 0.65. The intrinsic reaction rate at an oxygen partial pressure of 101 kPa was found to be given by ρ_i = 1.05 exp(− 143.5/RT) units where the activation energy is given in kJ/mol. Some values for the oxidation rate of methane soot are also given.