D. N. Saulov

Exergy optimisation of reverse combustion linking in underground coal gasification

Abstract Underground coal gasification (UCG) is a gasification process carried on in non-mined coal seams using injection and production wells drilled from the surface, which enables the coal to be converted into product gas. A key operation of the UCG is linking the injection and production wells. Reverse combustion linking (RCL) is a method of linking the process wells within a coal seam, which includes injection of an oxidant into one well and ignition of coal in the other so that combustion propagates towards the source of oxidant thereby establishing a low hydraulic resistance path between the two wells. The new theory of the RCL in typical UCG conditions has been recently suggested. The key parameters of the RCL process are determined using the technique of intrinsic disturbed flame equations. The present study is concerned with extending the results of the RCL theory to incorporate hydrodynamics of air injection and flow during RCL operation to derive mass flow rate of air to the combustion front a...

Conditional model for sorption in porous media with fractal properties

In this study, the conditional moment closure approach, which is proven to be very useful for modelling of reactions in turbulent flows, is extended to characterise adsorbing, desorbing or reacting flows in porous media. A complete specification of the porous distance conditioned moment closure model, which is formulated in terms of single-conditioned expectations, is presented. The closure of the model equations is obtained assuming the diffusion approximation for fluxes of the reactive species. The model simulates complex multi-cascade processes of convective and diffusive transport of species between pores in a continuous and consistent manner and is a generalisation of dual (or triple) porosity concept. The model addresses the major difficulty of describing transport, entrapment and sorption processes in porous media with fractal properties, where distant transport occurs in the largest pores or fractures, while the adsorbing or desorbing surface is mainly allocated in small pores. The model is able to simulate various regimes of methane replacement by CO2 in a coal sample, which makes it useful for optimising the design and parameters of enhanced coal bed methane recovery operations. It is demonstrated that the power-low decrease in downstream methane concentration, which has been observed experimentally, can be accurately reproduced by the model.

Coupling the porous conditional moment closure with the random pore model: applications to gasification and CO 2 capture

Gasification of coal or biomass with in situ CO2 capture simultaneously allows production of clean hydrogen at relatively low cost and reduced emission of CO2 into the atmosphere. Clearly, this technology has a great potential for a future carbon constrained economy. Therefore, the development of a comprehensive, physically-based gasifier model is important. The submodels that describe reactive transport processes in coal particles as well as in particles of CO2 sorbent material are among the key sub-models, which provide a necessary input for an overall gasifier model. Both coal and sorbent are materials that have complicated pore structures. The porous conditional moment closure (PCMC) model proves to be adequate for modeling reactive transport through porous media with fixed pore structure. Consumption of coal in the heterogeneous gasification reaction, however, widens the pores and reduces the surface area available for this reaction. At the same time, formation of a carbonate layer narrows the pores in the sorbent material and reduces the reaction rate of CO2 sorption. In both cases the pore structures are affected. Such changes are not taken into account in the existing PCMC model. In this study, we obtain the parameters of the diffusive tracer distribution based on the pore size distribution given by the widely applied random pore model (RPM), while coupling PCMC with RPM. Such coupling allows taking into account changes in pore structure caused by heterogeneous reactions and thus improves the accuracy of these key sub-models.

Modified associate formalism without entropy paradox: Part II. Comparison with similar models

The modified associate formalism is compared with similar models, such as the classical associate model, the associate species model and the modified quasichemical model. Advantages of the modified associate formalism are demonstrated.

Modified associate formalism without entropy paradox: Part I: Model description

A modified associate formalism is proposed for thermodynamic modelling of solution phases. The approach is free from the entropy paradox described by Luck et al. [R. Luck, U. Gerling, B. Predel, Zeitschrift fur Metallkunde 80 (1989) 270–275]. The model is considered in its general form for an arbitrary number of solution components and an arbitrary size of associates. Asymptotic behaviour of chemical activities of solution components in binary dilute solutions is also investigated.

General approach for modelling of reactive transport in porous media

This work presents a relatively new approach designed for modelling reacting flows in porous media by using conditional expectations. Similar methods, aimed at obtaining, closing and using conditional expectations in reacting fluid flows, were previously developed for and successfully used in turbulent combustion (e.g. conditional moment closure or CMC) are now generalised and adapted to perform simulations of reacting flows in porous media. Different versions of the porous models PCMC (porous CMC) variations of PDCMC (distance conditioned moment closure) have been proposed and are now summarised in this work. These approaches utilise single-conditioned expectations and the closure of the equations is obtained by using diffusion approximations conventional in CMC. Fractal properties of a porous medium can be used to evaluate the coefficients of the conditional equations. A new approach for investigating transport phenomena in irregularly-connected pore networks and obtaining corresponding transport coefficients has also been suggested. This approach combines a generalised effective medium approximation with a macroscopic continuum model and allows us to explicitly obtain analytical expressions for the transport coefficients for both unconditional and conditional models. As demonstrated, the proposed general approach is capable of emulating various regimes of reactive transport in porous media, while permitting accurate reproduction of the experimental results.

Numerical simulation of coal gasification with CO2 capture based on two-dimensional fluidized bed model

Numerical model of a fluidized bed gasifier with in-situ CO2 capture is presented. The model is implemented using the software package MFIX. Simplified chemical reactions are used to describe evaporation and devolatilization processes, while the carbonation reaction is modelled by the grain model. Initial and steady state stages of the gasification process are simulated. The simulation results demonstrate that a substantial increase in hydrogen content in product gas can be achieved by the addition of the CO2 sorbent material. The simulation results are in agreement with the experimental data reported in the literature.

Underground fire prospective technologies

Abstract In this work, we present a brief review of underground fires, which are generally characterized by slow, heterogeneous combustion (or smoldering) of porous combustible materials. Due to low temperatures and small propagation velocities of the smoldering processes, underground fires are difficult to detect, especially in their initial stages. Furthermore, the estimation of the extent of the detected underground fires is problematic as well. Very few diagnostic techniques provide adequate information on these fires. Underground fires are also extremely difficult to extinguish. Such fires can last for a very long period of time, posing a serious safety threat and having substantial adverse environmental and economic consequences. Current technologies used in controlling and extinguishment of underground fires either are costly or usually do not result in satisfactory outcomes. We also discuss the technologies developed over the years in underground coal gasification and possibilities to utilize these technologies for controlling underground fires and decreasing their harmful impact.

Adsorption Thermodynamics in the Framework of the Modified Associate Formalism

A new thermodynamic model of adsorption of gas mixtures on microporous adsorbents is suggested. The model is based on Vacancy Solution Theory, while the recently proposed Modified Associate Formalism is used for adsorbed phase. The proposed model allows prediction of co-adsorption isotherms based on fitting those for pure gases.