Showing papers by "Sreenivas Jayanti published in 2016"

Evaluation of CO2 gasification kinetics for low-rank Indian coals and biomass fuels

Abstract: 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.

Peclet number analysis of cross-flow in porous gas diffusion layer of polymer electrolyte membrane fuel cell (PEMFC)

Abstract: Adoption of hydrogen economy by means of using hydrogen fuel cells is one possible solution for energy crisis and climate change issues. Polymer electrolyte membrane (PEM) fuel cell, which is an important type of fuel cells, suffers from the problem of water management. Cross-flow is induced in some flow field designs to enhance the water removal. The presence of cross-flow in the serpentine and interdigitated flow fields makes them more effective in proper distribution of the reactants on the reaction layer and evacuation of water from the reaction layer than diffusion-based conventional parallel flow fields. However, too much of cross-flow leads to flow maldistribution in the channels, higher pressure drop, and membrane dehydration. In this study, an attempt has been made to quantify the amount of cross-flow required for effective distribution of reactants and removal of water in the gas diffusion layer. Unit cells containing two adjacent channels with gas diffusion layer (GDL) and catalyst layer at the bottom have been considered for the parallel, interdigitated, and serpentine flow patterns. Computational fluid dynamics-based simulations are carried out to study the reactant transport in under-the-rib area with cross-flow in the GDL. A new criterion based on the Peclet number is presented as a quantitative measure of cross-flow in the GDL. The study shows that a cross-flow Peclet number of the order of 2 is required for effective removal of water from the GDL. Estimates show that this much of cross-flow is not usually produced in the U-bends of Serpentine flow fields, making these areas prone to flooding.

Detailed plant layout studies of oxy-enriched CO2 pulverized coal combustion-based power plant with CO2 enrichment

Abstract: Coal plays a vital role in electricity generation worldwide and is expected to contribute significantly to thermal power generation for the foreseeable future, especially in countries such as India. Carbon capture and sequestration technologies therefore become imperative in order to minimize CO2 emissions. A systematic and comprehensive assessment of the most promising of these technologies—consisting of supercritical boiler parameters, oxy-coal combustion, and low temperature flashing for simultaneous CO2 and SO2 capture—for typical Indian environmental and fuel conditions has been carried out in the present study. The results show that up to 93.3 % of CO2 and 95 % of SO2 generated during combustion can be captured while producing a sequestration-ready CO2 stream of 95.5 mol% purity compressed to a supercritical pressure of 110 bar. The energy penalty for this suite of technologies amounts to about 8.5 % drop in the net power plant efficiency to 33 % despite adopting efficiency measures such as supercritical boiler technologies.

CO2‐Rich Combustion

Abstract: The atmospheric concentration of CO2 is only around 400 ppm, and is often neglected in combustion calculations. CO2-rich combustion arises in two important contexts, namely syngas combustion and exhaust gas recirculation. Syngas is produced through the gasification of coal, biomass, petroleum (pet) coke and other opportunity fuels, and may contain between 10% and 40% CO2 by volume. The CO2 content in exhaust gas from fossil fuel combustion varies in the range of 5% to 15% by volume (on a wet basis), the lower end occurring in gas turbines and the higher end in case of coal combustion. In power plants involving CO2 capture using oxy-fuel combustion, the concentration of CO2 in the exhaust gas can be as high as 80–85%. When this is recirculated to moderate the flame temperature in oxy-fuel combustion, the CO2 concentration in the combustion environment can be in the range of 50–70%. The presence of CO2 in significant quantities in the combustion environment can alter the characteristics of the flame, such as flame length, temperature and appearance, stability, extinction, and soot formation. These aspects are discussed in this chapter, with attention primarily focused on non-premixed flames, which are industrially important. Keywords: Combustion environment; exhaust gas recirculation; flame structure; flame stability; extinction; hydrocarbon fuels; flameless combustion; oxy-fuel combustion