Bogdan Z. Dlugogorski

Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling

Coal oxidation at low temperatures (i.e. <100 °C) is the major heat source responsible for the self-heating and spontaneous combustion of coal and is an important source of greenhouse gas emissions. This review focuses on the chemical reactions occurring during low-temperature oxidation of coal. Current understanding indicates that this process involves consumption of O 2, formation of solid oxygenated complexes, thermal decomposition of solid oxygenated complexes and generation of gaseous oxidation products. Parameters, such as mass change, heat release, oxygen consumption, and formation of oxidation products in the gas or solid phase, have been used to qualitatively and quantitatively describe the oxidation process. Reaction mechanisms have been proposed to explain the characteristics of consumption of O2, and formation of oxidation products in the gas and solid phases. Various kinetic models have also been developed to describe the rate of oxygen consumption and the rates of formation of gaseous oxidation products in terms of the rate parameters of the relevant reactions, oxidation time, temperature, and initial concentration of oxygen in the oxidising medium. Further research emphasis should be placed on the formation of the complete reaction pathways proceeding in the oxidation process and on the development of kinetic models applicable for predicting the self-heating and gas emission in a coal seam or stockpile.

Analysis of the mechanism of the low-temperature oxidation of coal

The mechanism of the oxidation of coal at low temperatures, i.e., below 100°C, was examined using measurements of the gases emitted from a bed of coal in an isothermal flow reactor. Employing an online two-column micro gas chromatograph, transient rates of production of CO2 and CO were monitored during desorption and oxidation experiments. A bituminous coal was milled into three nominal top size classes: 0-0.5 mm, 0-1 mm, and 0-2 mm. Desorption experiments with unoxidized coal samples at 20-70°C indicated that even an unoxidized coal incorporates oxygenated complexes in its structure. The threshold for thermal decomposition of these oxygenated species was found to be between 50 and 70°C. Carbon oxides liberated from oxidizing coal were compared with those from the thermal decomposition of coal oxidized at the same temperature, suggesting that two parallel reaction sequences contribute to the emission of carbon oxides during oxidation. A multi-step reaction mechanism was also proposed to describe low-temperature oxidation of coal and to explain the phenomena observed during the desorption and oxidation experiments.

Bubble size distribution and coarsening of aqueous foams

This paper characterises the bubble size distribution of aqueous foams, produced by a compressed-air foam generator. The time evolution of bubble size distribution in aqueous foams is experimentally measured using a CCD video camera. A computer model, which predicts the change in bubble size distribution with time, is used to simulate the coarsening and disproportionation of aqueous foams. As an extension of previous computer simulations, our model incorporates the variation in liquid fraction during the foam aging process, thereby enabling the simulation of both wet and dry foams. It is found that a Weibull-type distribution best approximates the narrow bubble-size distribution produced by a compressed-air foam generator. The modelled predictions show good agreement with the experimental measurements and a sensitivity analysis indicates a significant dependence of the model on drainage (hence on the thickness of the lamellae), the Henry constant, the gas diffusivity and the surface tension of the foam solution. This paper characterises the bubble size distribution of aqueous foams, produced by a compressed-air foam generator. The time evolution of bubble size distribution in aqueous foams is experimentally measured using a CCD video camera. A computer model, which predicts the change in bubble size distribution with time, is used to simulate the coarsening and disproportionation of aqueous foams. As an extension of previous computer simulations, our model incorporates the variation in liquid fraction during the foam aging process, thereby enabling the simulation of both wet and dry foams. It is found that a Weibull-type distribution best approximates the narrow bubble-size distribution produced by a compressed-air foam generator. The modelled predictions show good agreement with the experimental measurements and a sensitivity analysis indicates a significant dependence of the model on drainage (hence on the thickness of the lamellae), the Henry constant, the gas diffusivity and the surface tension of the foam solution.

Thermal decomposition of solid oxygenated complexes formed by coal oxidation at low temperatures

Solid oxygenated complexes formed by coal oxidation play an important role in low-temperature oxidation of coal. Using an isothermal-flow reactor, the decomposition behaviour of solid oxygenated complexes was examined under pure nitrogen, at temperatures between 60 and 110 °C. The production of CO2 and CO during thermal decomposition of the complexes was quantified by an on-line dual-column micro GC. Experiments show that the production rates of CO2 and CO depend on temperature, but are independent of the particle size of the samples, indicating that the thermal decomposition process is dominated by chemical kinetics rather than diffusion. It was also found that the rates of formation of carbon oxides follow the Elovich equation and the activation energies for the production of CO2 and CO are 52.1 ± 6.3 and 72.0 ± 5.8 kJ/mol, respectively, indicating two separate reaction pathways proceeding in the decomposition of solid oxygenated complexes.

Theoretical analysis of reaction regimes in low-temperature oxidation of coal

This paper examines the low-temperature oxidation of coal using pore model resembling ordinary tree structures, where the trunk of each effective pore reaches the exterior of the coal particle. Theoretical analysis shows that, at low temperatures and atmospheric pressure, the mean diffusivity of oxygen in a coal particle is related to the porosity and particle size, and varies between 10-8 and 10-6 m2/s. When the particle size is large (more than 1 mm in diameter), the coal oxidation is controlled by continuum diffusion, while for a very fine particle the reaction regime switches to Knudsen-diffusion controlled (for active coal) or kinetically controlled (for less active coal). With increasing porosity of fine coal particles, the trend for the reaction regime to be kinetically controlled becomes more significant. For the less active coal with high porosity and particle size of several tens of microns, the reaction regime is almost entirely kinetically controlled. The rate of oxygen consumption of coal usually shows a dependence on particle size, but in the case of the less active coal and a particle size of a few tens of microns, the rate of oxygen consumption is virtually independent of the particle size. The independence of the rate of oxygen consumption of the particle size is also observed for larger particles (even around 500 μm in radius), when the coal reactivity is sufficiently low. The predictions from the present model are in agreement with published experimental findings, and have application to the modelling of spontaneous combustion of coal.

Rheology of fire-fighting foams

This paper examines the rheological properties of compressed-air foams and contains velocity profiles of foams flowing through straight horizontal tubes. It is shown that a master equation can be derived from the experimental data to account for a range of expansion ratios and pressures normally encountered during pumping of polyhedral-in-structure fire-fighting foams. The experimental data come from a Poiseuille-flow rheometer consisting of three stainless steel tubes 6.95, 9.9, 15.8 mm in diameter, with foam generated by mixing a pressurised solution of Class A foam with compressed air. Results are corrected for wall slip following the method of Oldroyd-Jastrzebski, which implies the dependence of slip coefficients on the curvature of the tube wall. The experimental results demonstrate the applicability of the volume equalisation method to the more expanded, polyhedral (e>5) and transition, bubbly-to-polyhedral (5⩾e⩾4) foams. (The method of volume equalisation was introduced by Valko and Economides to correlate the viscosity of low expansion foams (e<4), characterised by spherical bubbles.) The present results indicate that all data points align themselves along two master curves, depending on whether the foam consists of bubbles or polyhedral cells.

A comparative study of drainage characteristics in AFFF and FFFP compressed-air fire-fighting foams

Drainage measurements are commonly used for assessing the quality, water-retention ability and stability of aqueous foams used in fire-fighting applications. A new experimental technique is proposed in this paper, for measuring the drainage rate of liquid from compressed-air fire-fighting foams. The procedure outlined here provides advancement in precision over that prescribed by the standard for low expansion foams (NFPA 11, Standard for evaluating low expansion foams, NFPA, Quincy, MA, 1998). A comparative analysis of drainage characteristics in two commonly used Class B fire-fighting foams was undertaken, from theoretical and experimental perspectives: (i) aqueous film forming foam and (ii) film forming fluoroprotein foam. It is demonstrated that even though both the foam solutions exhibited similar fundamental physical properties, the disparities in surface rheological properties cause the resulting foams to have remarkably distinct drainage and coarsening characteristics. In addition, a drainage model is outlined, which allows the explicit prediction of the time evolution of liquid holdup profiles and drainage rates in fire-fighting foams. The existing drainage model is extended to simulate fire-fighting foams made from protein based and synthetically produced surfactants.

A mechanistic and kinetic study on the formation of PBDD/Fs from PBDEs.

This study presents a detailed mechanistic and kinetic investigation that explains the experimentally observed high yields of formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) from the polybrominated diphenyl ethers (PBDEs), commonly deployed in brominated flame retardants (BFRs). Theoretical calculations involved the accurate meta hybrid functional of M05-2X. The previously suggested pathways of debromination and generation of bromophenols/bromophenoxys/bromobenzenes were found to be unimportant corridors for the formation of PBDD/Fs. A loss of an ortho Br or H atom from PBDEs, followed by a ring-closure reaction, is the most accessible pathway for the production of PBDFs via modest reaction barriers. The initially formed peroxy-type adduct (RO₂) is found to evolve in a complex, nevertheless very exoergic, mechanism to produce PBDDs. Results indicate that, degree and pattern of bromination, in the vicinity of the ether oxygen bridge, has a minor influence on governing mechanisms and that even fully brominated isomers of BFRs are capable of forming PBDD/Fs. We thoroughly discuss bimolecular reactions of PBDEs with Br and H, as well as the Br-displacement reaction by triplet oxygen. The rate of the Br-displacement reaction significantly exceeds that of the unimolecular inititiation reactions due to loss of ortho Br or H. Results presented herein address conclusively the intriguing question of how PBDEs form PBDD/Fs, a matter that has been in the center of much debate among environmental chemists.

Yield stress measurements of aqueous foams in the dry limit

This paper reports measurements of yield stress of aqueous foams approaching the dry foam limit using a pendulum device. Traditionally, the vane rheometer has been used to measure the yield stress in liquids that exhibit wall slip. However, using the simple and inexpensive pendulum technique, shear rates many orders of magnitudes lower can be achieved. The pendulum was used to observe the change in yield stress for the foam as the gas fraction and bubble size increased. The local gas fraction in the foam was found by measuring the sonic velocity, and the bubble size was determined photographically. Strong support is found for the existence of a true yield stress in aqueous foams at the dry foam limit. Yield stress results, once scaled by σ/〈R〉, agree well with data from previous studies.

Kinetic modeling of low-temperature oxidation of coal

A kinetic model has been developed for determining the rate of oxygen consumption and production of carbon oxides during the oxidation of coal at low temperatures (i.e. <100°C), based on current understanding of the mechanism of coal oxidation. The chemical reactions considered in the model include two parallel sequences consuming oxygen and two thermal decomposition pathways producing carbon oxides. The resulting rate expressions reflect the contributions of various reactions consuming oxygen and producing carbon oxides and predict the effects of temperature, oxidation time and [O2] in the gas phase. The general form of the rate expressions confirms that chemisorption is relatively fast, only playing an important role at the early stage of coal oxidation. With the formation of stable and unreactive oxygenated complexes in a coal’s structure, the oxidation of coal is dominated by thermal decomposition of oxygenated complexes.