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.

Underground coal gasification is an in situ coal utilization technique that has immense potential as a future clean coal technology. UCG possesses a number of advantages including the ability to use deep and unmineable coals. The most important component of UCG is the underground “cavity”—which serves as a chemical reactor with rich interplay of kinetics and transport. Field and laboratory-scale experiments have revealed several interesting features of the UCG cavity. Modeling studies on the UCG cavity involve fundamental models and CFD simulations. In this chapter, we will discuss various experiments and models of UCG cavities, with a focus on the effects of reaction chemistry and thermomechanical spalling on cavity evolution.

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.

Exhaust Gas Recirculation (EGR) is a potential option for controlling in-cylinder NOx in automobiles. This paper aims to study the effect of EGR on NOx emissions and in Compression Ignition (CI) engines at various conditions. To this end, low dimensional models are developed using a first principles approach without recourse to empirics. Fuel oxidation represented by a three-step global kinetic model coupled with the Zeldovich mechanism for NOx formation is used to predict the composition of the entire spectrum of engine-out gases. Solution of the conservation equations using MATLAB provides the in-cylinder variation of parameters like volume, pressure, torque, speed and work done and species (Fuel, CO, CO2, NOX, etc.) with respect to Crank Angle Displacement (CAD). The inlet conditions are the fuel-air equivalence ratio, engine specifications and the inlet air temperature. The simulations are validated against experimental pressure profiles from the literature. External EGR is then implemented to study its effect on engine-out emissions under cold start conditions. The effect of EGR at various combinations of engine operating conditions is examined in detail.

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.

Propulsion systems such as gas turbines are susceptible to combustion instability, when operated at lean equivalence ratio [1]. During combustion instability, there is a nonlinear interaction between combustion and acoustics leading to large amplitude acoustic oscillations. These large amplitude oscillations are detrimental to the stability of the combustor and can cause damages to the structural integrity of the combustor, flame flash back or blow off. The main source of nonlinearity is in the heat release rate caused due to the velocity perturbations at the flame holder [2]. The heat release rate fluctuations are due to the variation in the flame surface area. Hence there is a need to understand the flame dynamics that contributes to the heat release rate fluctuations. The present study aims in understanding the stability of a V - flame combustor by varying the flame location inside an acoustic resonator. By varying the flame location the instability regimes of the combustor are identified. At the flame locations where the system exhibits combustion instability, acoustic pressure oscillations are acquired simultaneously with high speed images of the flame front fluctuations so that a correlation can be made between them. Tools from dynamical systems theory are applied to the pressure signal to quantify different dynamical states of the system during combustion instability. Moreover the flame dynamics at each dynamical state are investigated. It is observed that combustion instability is characterized by interesting dynamical states such as frequency locked state, quasi-periodic oscillations, period 3 oscillations and chaotic oscillations. High speed imaging of the flame reveals different interesting patterns of flame behavior during combustion instability. Flame wrinkling, roll up of flame elements, separation as islands of the flame elements and mutual annihilation of flame elements were some of the interesting flame behavior observed. This study helps in understanding the role of nonlinear heat release rate mechanism in establishing different dynamical states during combustion instability.