Preeti Aghalayam

Synergetic and inhibition effects in carbon dioxide gasification of blends of coals and biomass fuels of Indian origin.

The present study investigates the enhancement of CO2 gasification reactivity of coals due to the presence of catalytic elements in biomass such as K2O, CaO, Na2O and MgO. Co-gasification of three Indian coal chars with two biomass chars has been studied using isothermal thermogravimetric analysis (TGA) in CO2 environment at 900, 1000 and 1100°C. The conversion profiles have been used to establish synergetic or inhibitory effect on coal char reactivity by the presence of catalytic elements in biomass char by comparing the 90% conversion time with and without biomass. It is concluded that both biomasses exhibit synergistic behavior when blended with the three coals with casuarina being more synergetic than empty fruit bunch. Some inhibitory effect has been noted for the high ash coal at the highest temperature with higher 90% conversion time for the blend over pure coal, presumably due to diffusional control of the conversion rate.

Role of flame dynamics on the bifurcation characteristics of a ducted V-flame

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.

Understanding NO emissions in diesel and biodiesel based engines

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.

Cavity Models for Underground Coal Gasification

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.

Improving efficiency of CCS-enabled IGCC power plant through the use of recycle flue gas for coal gasification

CO2 capture from coal-fired power plants is necessary for continued use of coal as a fuel. Proven CO2 capture techniques such as amine absorption and oxyfuel combustion entail significant energy penalty leading to considerable decrease in the net thermal efficiency of the power plant. Recent studies of high-ash Indian coals show that CO2 has sufficient reactivity for coal gasification in temperature ranges of interest to IGCC. Against this background, we analyse in the present study, a new power plant layout which uses part of the sequestered flue gas stream for high-pressure gasification of the coal within the framework of an IGCC power plant with CO2 capture. Detailed thermodynamic calculations of the new plant layout, referred to here as Oxy-RFG-IGCC-CC, using commercial power plant simulation software show that the optimized Oxy-RFG-IGCC-CC plant with CO2 capture produces power at an overall thermal efficiency of 34.2%, which is nearly the same as that of current generation of pulverized coal boiler-based power plants without CO2 capture or that of a conventional IGCC with post-combustion capture. The proposed simpler layout is also 1.9% more efficient than a comparable CO2-capture-enabled IGCC plant that uses steam for coal gasification.

Investigation of Flame Dynamics in a V - Flame Combustor During Combustion Instability

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.Copyright

CFD - A virtual platform for teaching concepts in non-ideal Chemical Reactors

This paper proposes the use of Computational Fluid Dynamics (CFD) techniques as an instructional tool for the topic of flow non-idealities in Chemical Reactors. The residence time distribution (RTD) and velocity contours obtained from CFD simulations will enable visualisation of crucial reactor flow patterns. The tool is expected to augment conventional teaching of the undergraduate course on Chemical Reaction Engineering.