Nonisothermal Stage Research Articles

Kinetic analysis of solid fuel combustion in chemical looping for clean energy conversion

Chemical looping combustion (CLC) offers an advanced, eco-friendly method for converting solid fuels into energy with inherent CO2 capture, presenting a cost-efficient solution. The kinetics of solid fuel CLC, crucial for reactor design, are not well-understood, limiting its application and optimisation. Conducting a comprehensive kinetic analysis of solid fuel CLC is crucial, as it examines the combustion of both volatiles and fixed carbon, thereby addressing existing knowledge gaps and advancing clean conversion of solid fuels. In this study, a comprehensive kinetic analysis method for the first time has been applied to investigate the kinetics of CLC of low-volatile semi-anthracite coal with CuO at temperatures of 700–950 °C under varied oxygen excess ratios (0.5, 1.0, and 2.0). The results show that the combustion of coal in CLC can be divided into three distinct stages. The combustion of coal with CuO initiates with the combustion of volatiles interacting with solid CuO at 450–580 °C, where the combustion efficiency varies between 3–6 wt%. This is followed by a complex simultaneous mechanism involving gas-phase volatiles and solid-phase CuO, as well as gas-phase oxygen and solid-phase fixed carbon under non-isothermal conditions. The combustion efficiency is ranged 19–71 wt% at the temperature ranges from 580 °C to the isothermal temperatures of 750–950 °C. The activation energy for the combustion of volatiles was determined as Ea = 119 kJ/mol, whereas the initial combustion of fixed carbon in the non-isothermal stage ranged between Ea = 39–53 kJ/mol. The rate of combustion is initially limited by the oxygen diffusion rate from CuO, but with additional oxygen carriers and increased temperature, the reaction becomes constrained by first-order kinetics. Upon reaching the isothermal stage, the final combustion phase between fixed carbon and gas phase oxygen occurs, and the combustion efficiency increases to 77–96 wt% at 750–950 °C. The activation energy for fixed carbon combustion under the isotherm stage was approximately Ea = 234 kJ/mol, with a reaction rate constant (k0) of 7.84 × 109 min−1. This pioneering study not only clarifies the multi-stage kinetics of solid fuel CLC, bridging significant gaps in current knowledge but also sets the foundation for the enhanced design and efficiency of CLC systems for cleaner energy production.

Microstructure Formation of LiTiZn Ferrite Ceramics during Radiation-Thermal Sintering

The effect of intensification of the compaction rate of ferrite compacts under irradiation conditions with a high-power electron beam both in the heating regime and in the isothermal stage of sintering was established. The compaction mechanisms of the compacts are different at each of these stages. The intensification of compaction at the non-isothermal stage in radiation-thermal conditions is due to processes involving the liquid phase. The role of bismuth oxide in the compaction of the material at the isothermal stage of sintering is unessential, but its influence is significant in recrystallization processes. Under of Ivensen’s phenomenology, compaction curves are explained by the deceleration of annealing of structural defects responsible for the fluidity of the material. Dislocations are the most probable type of defects, satisfying the detected regularities.

Optimal control of a two-stage reactor system

AbstractOptimal operation policies were investigated for a batch reactor system with two different operation stages. At the end of the first nonisothermal stage one of the reactants was added. Since that moment the reactor was operated isothermally. In each stage behavior of the reactor was described by a set of differential equations. The maximum conversion problem was investigated subject to various operating constraints. Dynamic optimization based on the control vector parametrization was used to find the optimal control profile. Gradients of the resulting nonlinear programming problem were obtained by adjoint method based on the optimal control theory.

Effect of Rotation in Cloud Core Collapse

The collapse of rotating clouds is investigated. At first, isothermal collapse of an initially uniform-density, uniform-rotating, molecular cloud core with pressure and self-gravity is investigated to determine the conditions under which a cloud is unstable to fragmentation. A semianalytic model for the collapse of rotating spheroids is developed with the . method of characteristics for inwardly propagating rarefaction waves. Three-dimensional self-gravitating hydrodynamical calculations are performed for the initially uniform-density rigid-rotating sphere. Both investigations show that the criterion for fragmentation is modified from the one in the literature if the property of the non-homologous collapse is taken into account. It is shown that the central flatness, that is, the axial ratio of the isodensity contour in the central region, is a good indicator for the fate of the cloud. We derive the criterion for the fragmentation considering the evolution of the flatness of the central core. If the central flatness becomes greater than the critical value ∼ 4π, a collapsing cloud with moderate perturbations is unstable for fragmentation, while if the central flatness stays smaller than the critical value, it does not fragment at least before adiabatic core formation. Warm clouds (α 0 ≥ 0.5) are not expected to fragment before adiabatic core formation almost independent of the initial rotation (β 0 ) and the properties of the initial perturbation. The effect of the initial density central concentration is also investigated. If it exists, distortion or flattening of a cloud core is suppressed even if α 0 ≤ 0.5 in small rotation cases due to stronger nonhomologous property of the collapse. We conclude that the binary fragmentation is difficult during isothermal stage if a core collapse had started from near spherical configurations with moderate thermal energy or small rotation. We suggest that the close binary fragmentation may be possible in the nonisothermal stage by rapid growth of a nonspherical first core. Second, to mimic fragmentation processes of primordial clouds, the equation of state is approximated by a simple polytropic relation with γ ∼ 1.1. A series of numerical and semianalytical calculations of the rotating collapse of an initially spherical cloud shows a criterion for fragmentation of rotating polytropic cloud cores with γ=1.1. Fragmentation during core collapse is not expected to take place if the cloud thermal energy is greater than 0.3 times its gravitational energy at the initial stage of runaway collapse. The collapse of the central small core will not be halted by centrifugal force since a nonaxisymmetric waves will appear and the flow will converge to a self-similar flow until γ exceeds 4/3. Finally, we take into account the detailed non-equilibrium chemical reactions for primordial gas that consists of pure hydrogen. The parameters of the collapse and the condition of the fragmentation are compared with those of isothermal clouds. It is shown that the geometrical flatness of the central region of the disc is a good indicator for predicting whether the clouds fragment or not. If the flatness is greater than the critical value, ∼ 4π,a cloud fragments into filaments and blobs.

Dynamics of the evolution of volatile products during laser‐induced polymer destruction

AbstractThe dynamics of evolution of volatile products arising during destruction of poly(vinyl alcohol) and polyacrylnitrile films under the action of cw laser radiation were investigated by means of time‐of‐flight mass spectrometry. It was found that the evolution of volatiles occurs in two stages, one isothermal and the other nonisothermal. Depending on laser focusing and power, the nonisothermal stage comprises three dynamic regimes of desorption of volatile products: regime of single ejection, oscillating regime and chaotic behaviour of evolution. It is shown that the various regimes of desorption are due to inhomogeneity of laser heating and nonuniform propagation of the destruction wave from the centre of the laser beam to the periphery.

Hot purge gas regeneration of adsorption beds with solute condensation: Experimental studies

AbstractActivated carbon beds with adsorbed n‐octane are regenerated using hot air. Effects of pressure, feed temperature, and initial loading on temperature and concentration breakthrough curves are studied. For moderate and high values of these variables, effluent vapors are found to be saturated with hydrocarbon during part of the heating step, indicating the presence presence of liquid within the bed. Regeneration behavior is predicted with reasonable accuracy using a nonisothermal stage model.

Two-stage ignition under engine conditions parallels that at low pressures

A direct experimental comparison of both the physical properties and the chemical natures of cool flames (i.e., of the first nonisothermal stage of two-stage ignition) under engine conditions and at sub-atmospheric pressures has been made. The physical conditions appropriate to an engine were simulated using a rapid-compression machine; low-pressure combustion experiments were made in a conventional Pyrex reactor. The similarity of the chemical processes involved in these two regimes has been strikingly demonstrated. Forty-two molecular organic products of the cool-flame combustion of 2-methylpentane were estimated gas chromatographically. Of these, no less than 39 are common to both regimes. The variation with pressure of cool flame intensity is represented by the equation Δp=k1p0m+k2, and that of the induction period by the equation τ-c1p0−n+c2. These relationships are again common to the low-and high-pressure studies, m being virtually the same (1.67) and n at high pressure corresponding to the lowest value of n determined at low pressure. Despite the very wide differences in physical conditions, it is evident that a common, predominantly homogeneous, chemical mechanism operates. The nature of this is discussed.