Preeti Aghalayam

Gasification of solid fuels such as coals, lignite and biomasses has been studied using isothermal and non-isothermal thermogravimetric analysis (TG) with CO2 as gasifying agent. Non-isothermal TG of three Indian coals (two bituminous and one sub-bituminous coal), one lignite and two biomass fuels (Casuarina and empty fruit bunches) at a constant heating rate of 20 °C min-1 in the temperature range from 25 to 1200 °C showed a clear separation of DTG peaks associated with pyrolysis and CO2 gasification. Based on these studies, isothermal TG experiments were conducted in the temperature range from 900 to 1100 °C for coals and from 800 to 1000 °C for biomass fuels. These results show that the CO2 gasification rate follows coal rank for the three coals and the lignite. The two biomasses have significantly higher reactivities than the three coals. The higher reactivity of one coal is attributed to the presence of calcium-containing minerals in its inorganic matter. The kinetic parameters for each fuel were extracted from the isothermal TG results using the volumetric reaction model for the coals and a zeroth-order model for biomass fuels. Biomass and lignite are found to have a much higher reactivity index and much lower conversion time than the three coals under identical conditions.

Combustion instability is a nonlinear process with interaction between combustion and acoustics. The nonlinearity in the combustion process is observed in the flame dynamics. In our study of a ducted laminar premixed V-flame, we varied the distance between the flame anchor and the duct exit. We observed that the thermoacoustic system bifurcates from a stable state to a frequency locked state, followed by quasi-periodicity, period 3 oscillations, and finally chaotic oscillations. During the occurrence of these dynamical states, the role of flame dynamics is investigated using high speed imaging. We observed wrinkle formation, its propagation and growth along flame front, rollup of the flame, separation, and annihilation of flame elements during instability. From the present study it is found that each dynamical state is characterized by a particular sequence of flame behavior, highlighting the role of nonlinear flame dynamics in establishing the observed dynamical state.

Underground Coal Gasification (UCG) is considered to be a clean coal technology primarily intended to utilize deep underground (>300 m) coal deposits. In this process, a mixture of reactant gases like air/oxygen and steam are injected directly to an ignited portion of underground coal seam. UCG involves complex interactions of different processes like drying, pyrolysis, chemical reactions and spalling. Spalling is detachment of small coal particles from the coal seam due to interconnection of cracks developed in it. It plays an important role by offering higher surface area to give improved performance. The mechanism of spalling and its characterization are not well understood. Furthermore, there are no well established experimental techniques to measure the spalling rates. This paper studies spalling behavior of a lignite coal, which is characterized by high moisture and volatile matter, and suggests a possible underlying mechanism. The rate of spalling was measured using an experimental setup under the UCG-like conditions. In this setup, a reacting coal block was attached to a load cell and suspended in a UCG-like environment. When the experiments were repeated under similar conditions with different blocks of same coal, it was found that there were variations in the rates of spalling. This might be due to the heterogeneity in coal blocks in the form of originally present fissures or weak regions. A UCG process model was used to explain these experimental results and also to investigate the effect of spalling rate on product gas calorific value. We believe that spalling happens due to formation and extension of cracks. Hence a microscopic crack pattern on a heated coal monolith was examined in different stages of heating to understand the mechanism of spalling. It is concluded that cracks are first formed during the initial stage of drying due to the capillary stresses developed due to removal of moisture from the pores and were further extended due to shrinkage of coal during pyrolysis. The detachment of coal particles happens due to horizontal linking of vertical cracks, which might result out of either horizontal cracks, if any, or available fissures and weak regions or relatively weak interlayer bonding at the bedding planes.

The objective of this work is to elucidate controlling mechanisms in NO x reduction, develop reduced-order reaction models, and analyze the reactor performance using the reduced-order reaction model for the NO-CO reaction. We start with the microkinetic model on platinum, which describes the mechanism of catalytic reduction of NO by CO. The formation of the main product N 2O and the competitive formation of the side product N 2 are accounted for in the microkinetic model. Sensitivity and reaction path analysis have been carried out to determine the rate-limiting steps as well as the most abundant reactive intermediates in the system. Owing to the differences between system performance at high and low temperatures, the model has been analyzed in detail in these temperature regimes. Two closed-form expressions, corresponding to the two global reactions involved, have been derived. The characteristic features of the microkinetic model such as the sharp increase in NO conversion and the selectivity to N 2O are captured well by the reduced model. The reduced-order model has been extended to the rhodium catalyst as well. © 2012 Wiley Periodicals, Inc.

Mesoporous MCM-41 molecular sieves with Si/M (molar) ratios of 100 containing platinum group metals (Pd, Pt, Rh, Ir) were synthesized by the hydrothermal method. These catalysts were systematically characterized by various analytical and spectroscopic techniques, viz., XRD, TEM, DRUV-vis, N2 sorption. XRD analysis confirmed that the presence of platinum metals did not influence the parent structure and phase purity of the MCM-41 catalysts. DRUV-vis spectral studies of MCM-41 indicate the presence of platinum metal ions. The catalytic activities of these catalysts were evaluated for the reduction of NO by CO as a function of temperature. The study revealed that the catalytic activity is, in the order (Rh>Ir>Pd>Pt). Our experimental results are in good agreement with theoretical previous analysis based on the NO* dissociation activation energy. © 2009 Elsevier B.V. All rights reserved.

Formation of nitrogen oxide pollutants is investigated using a homogeneous combustion model for diesel and biodiesel surrogates including-n-heptane, methyl decanoate, methyl-9-decenoate and other oxygenated fuels. The investigations are carried out in a series of detailed simulation studies-ignition and combustion characteristics unique to the fuel are chosen such that comparable engine performance is obtained, and the results compared in terms of emissions. This is followed by an analysis of the NOX formation pathways for the various fuels in a model HCCI engine, operated at conditions where similar temperature profiles are obtained. Different fuel-oxygen ratios including fuel-lean, stoichiometric, and fuel-rich inlet conditions are examined in detail from viewpoint of NOX emissions. Significant NO variations are observed among the fuels at stoichiometric fuel-air ratio, with the oxygenated fuels demonstrating high NO compared to n-heptane. In particular, the NO emissions for MD9D was found to be 2.6 times that for n-heptane, at stoichiometric conditions at practically significant operating conditions. Thermal, prompt, and other pathways for NO formation are evaluated at fuel-lean, stoichiometric, and fuel-rich conditions, for different imposed temperature trends. The thermal pathway is found to contribute >60% of the NO in case of stoichiometric & fuel-lean mixtures. The contribution of the prompt pathway, on the other hand, can be as high as 50% in case of fuel-rich mixtures. Significant insight into the formation of NO in diesel and biodiesel engines is obtained from our studies.

Engine modelling aims at studying the combustion related phenomenon occurring in Internal Combustion (IC) engines. In this regard, a low dimensional mathematical model using first principles has been developed to study Spark Ignited (SI) engines. The resulting equations are Ordinary Differential Equations (ODE) (for volume, pressure, torque, speed and work done) and Partial Differential Equations (PDEs) for temperature and species conservation equations (fuel, CO, CO2, NO). This model utilizes simplified reaction kinetics for the oxidation of fuel in the combustion chamber. A two-step mechanism for the combustion of fuel and the classical Zeldovich Mechanism are used to predict the amount of NO formed during combustion. The model is solved in FORTRAN using LSODE subroutine (for stiff equations) with lumped parameters for thermal properties and diffusion, and invoking the ideal gas assumption. Method of Lines (MOL) has been used to study the axial and transient variation of parameters such as temperature and composition of the exhaust. With fuel composition, RPM, physical engine parameters and amount of fuel as inputs, the model captures the in-cylinder variation of pressure, temperature, exit gas composition (CO, CO2, octane, NO, O2) and work done by the piston with respect to crank angle. The results are comparable qualitatively with experimental data. The simulated data is validated against experimental data of pressure variation versus crank angle and mass fraction of the burned species. Copyright © 2013 SAE International and Copyright © 2013 SIAT, India.

Underground gas gasification (UCG) is a clean coal technology which involves in-situ gasification of deep-seated underground coal. The process can be divided in two phases based on state of coal seam and direction of cavity growth. In phase-I, cavity grows mainly in vertical direction while in phase-II it grows in horizontal direction. The in-house simulator developed for both the phases of UCG has been reported earlier Samdani et al. (2016a,b). It incorporates reaction kinetics, flow patterns, spalling, heat and mass transfer effects. In this work, we take further insight and perform parametric studies to examine the effects of different operating conditions, coal properties and design parameters on key performance indicators i.e. exit gas quality, energy generation rates etc. The investigation revealed that the exit gas quality and rate of coal consumption are strong functions of spalling rates and kinetics of reactions; the coal having very low spalling tendency or less reactivity may not be favorable for the UCG process. An important parameter called critical spalling rate has emerged through this analysis. It is the property of given coal above which UCG is sustainable. In addition, model performance is also sensitive to inlet gas temperature, pressure and composition. Optimum performance of UCG is obtained at a steam to oxygen ratio of 2.5 and at the highest possible inlet gas temperature, operating pressure, and oxygen content in the feed. Among the design parameters, the length of outflow channel is very important as it strongly affects both the exit gas calorific value and its fluctuations with time. The predicted effects of different parameters are in accord with the observations during lab-scale UCG experiments and different field trials. This study demonstrates the importance of a process model to determine the best conditions for UCG process and to evaluate feasibility of the process for a coal seam under consideration.

Global kinetic models capable of capturing experimentally observed features of catalytic reactions are vital in design and optimization studies. In this work, a detailed analysis of the selective catalytic reduction of NO and NO2, particularly in automotive exhaust control is undertaken. The prominent metal based catalysts for this reaction range from Pt, Au & Rh to cheaper options including Cu, Ag & Co supported on Al2O3, SiO2, and occasionally, zeolites. Here, we focus on Ag & Co supported on Al2O3 catalysts, and propose a computationally tractable kinetic model capable of capturing the observed SCR features across a range of catalyst synthesis and reactor operating conditions. In particular, the effects of metal loading on catalyst performance are closely examined. In SCR, an important aspect is the catalytic activity at higher inlet O2 concentrations. The global kinetic model proposed here is shown to predict the trends with respect to reactor temperature and inlet feed compositions (including O2), well, for both catalysts.

High surface area mesoporous solid support has attracted much interest in the development of cost-effective catalysts for NOx abatement at low temperatures. SBA-15, a high surface area mesoporous material, was employed in the preparation of Al, Ti, and Zr incorporated SBA-15 through a direct synthesis approach under long hydrothermal reaction conditions (100 °C for 72 h). Heteroatom incorporation facilitated change in the morphology and textural properties of the material and was applied as a catalytic supports in order to facilitate the development of a MnCu bimetallic catalyst by a facile conventional co-impregnation method. At temperatures as low as 260 °C, the catalyst that was developed displayed superior deNOx activity for SCR of NO with NH3. There have been various physicochemical studies undertaken on the catalysts that were prepared, such as XRD, XPS, BET, HR-SEM, and HR-TEM. The deNOx optimization and deNOx activity over the catalysts are discussed. The exceptional deNOx activity at low temperatures may be attributed to the rod-like morphology, extensive oxygen vacancies, and high ratio of Mn3+ + Mn4+/Mn on 10Mn4Cu/Zr-SBA-15 catalyst. Furthermore, the catalyst also displayed a synergetic interaction between the Mn and the Zr species on its surface, which contributed to the catalyst's superior catalytic recycle ability and its long-term stability on stream stability. A useful approach of preparation is provided in order to accomplish the industrial use of a bimetallic catalyst consisting of manganese and copper at a low cost.