The objective of the present paper is to propose a new variant of the oxy-fuel combustion for carbondioxide (CO2) sequestration in which steam is used to moderate the flame temperature. In this process, pure oxygen is mixed with steam and the resulting oxidant mixture is sent to the boiler for combustion with a fossil fuel. The advantage of this method is that flue gas recirculation is avoided and the volumetric flow rates through the boiler and auxiliary components is reduced by about 39% when compared to the conventional air-fired coal combustion power plant leading to a reduction in the size of the boiler. The flue gas, after condensation of steam, consists primarily of CO 2 and can be sent directly for compression and sequestration. Flame structure analysis has been carried out using a 325-step reaction mechanism of methane-oxidant combustion to determine the concentration of oxygen required to ensure a stable flame. Thermodynamic exergy analysis has also been carried out on SMOC-operated CO2 sequestration power plant and air-fired power plant, which shows that though the gross efficiency increases the absolute power penalty of ∼8% for CO2 sequestration when compared to air-fired power plant. © 2010 Elsevier Ltd. All rights reserved.

Sreenivas Jayanti

Department Of Chemical Engineering

CO2 SEQUESTRATION

Concentration of

Efficiency increase

Exergy analysis

Flame structure

FLAME TEMPERATURES

FLUE GAS RECIRCULATION

Oxyfuel combustion

POWER PENALTY

PURE OXYGEN

reaction mechanism

Volumetric flow rate

Boilers

Coal

Coal industry

exergy

Flammability

Flue gases

FLUES

Fossil fuel power plants

Fossil fuels

Fuels

Methane

Oxidants

Oxygen

recycling

Steam

Steam condensers

Steam power plants

Coal combustion

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Energy Conversion and Management

Of the various clean combustion technologies with carbon capture and sequestration (CCS) possibilities, chemical-looping combustion (CLC) promises to be an efficient and attractive method for oxidizing fuels without the energy penalty required for oxygen separation from air. The present work reports on a detailed thermodynamic analysis of 1,500 MWth, syngas-fueled, CLC-based power generation system which includes a provision for CCS. Taking account of the exothermic nature of the reaction of syngas with the selected oxygen carrier, NiO, in the fuel reactor, operating temperatures of air and fuel reactors are fixed at 900 and 908 °C, respectively. The CLC reactor system operates at atmospheric pressure on fuel/air side, and generates supercritical steam. An overall plant lay-out has been prepared such that the steam side, which is rated at 240 bar/538/552/566 °C, is very similar to that of a conventional thermal power plant making retrofitting a distinct possibility. A detailed analysis of the ideal cycle shows that a highly promising gross cycle efficiency of 41.22 % and net cycle efficiency of 36.77 % can be achieved after accounting for the energy cost of CO2 compression to 110 bar to facilitate CCS. © 2014 Springer-Verlag.

Clean Technologies and Environmental Policy

Syngas-fueled, chemical-looping combustion-based power plant lay-out for clean energy generation

Emission of carbon dioxide from fossil fuel-fired thermal power plants is a major concern for energy providers all around the world. In this context, carbon capture from thermal power plants and the energy penalty incurred in the process are important issues. The present work reports on a detailed analysis of gas-fired power plant layouts with in-built carbon capture, i.e. the flue gas from these power plants contains mainly carbon dioxide and water vapour. Four layouts, two of which are based on pressurized oxyfuel combustion and two on chemical looping combustion (CLC), have been considered. Based on detailed mass balance, energy and thermodynamic analyses of the power plant layouts, the net efficiencies for each plant have been computed. After accounting for thermodynamic irreversibilities and CO2 compression to 110 bar, these have been found to vary between 31 and 52 % for the four plants. Despite the technological maturity of oxyfuel combustion, it is concluded that CLC-based plants would be future-ready in the sense that they can readily accommodate CCS with only 2 % loss in overall thermal efficiency for CO2 capture. © 2015 Springer-Verlag.

Comparative analysis of four gas-fired, carbon capture-enabled power plant layouts

Coal is the mainstay of electrical power generation in India. In this study, an experimental and theoretical assessment is made of the possibility of retrofitting an existing pulverized coal boiler for operation in oxy-coal mode. Experiments have been conducted using non-isothermal thermogravimetric analysis for a typical Indian coal to assess its reactivity under simulated oxy-coal combustion. Computational fluid dynamics simulations of a tangentially fired, 210 MWe pulverized coal boiler of a typical Indian design have been carried out with a typical Indian coal composition to study the relative effect of oxy-coal mode operation on the flow and heat transfer characteristics within the boiler. A process flow diagram has been developed for a 662 MWt power plant to include a retrofitted boiler and CO2 sequestration. It is concluded that a 70:30 volumetric mix of CO2 to O2 in the oxidizer composition will leave the essential features of the flow, temperature and heat transfer characteristics more or less unchanged, thus enabling retrofitting. An overall thermal efficiency loss of 11% is estimated for a retrofitted plant which delivers a flue gas which does not require chemical means of CO 2 sequestration. © IMechE 2012.

Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy

Assessment of retrofitting possibility of an Indian pulverized coal boiler for operation with Indian coals in oxy-coal combustion mode with CO2 sequestration

Underground coal gasification (UCG) is a potential clean coal technology which converts coal into combustible gas in situ. The syngas produced from the UCG process using dry air contains more steam, tar and higher hydrocarbons compared to the conventional gasifiers. The focus of the present work is to use solid oxide fuel cells (SOFCs) to produce electrical power efficiently. To this end, a UCG system, based on air alone as the gasifying medium, is integrated with the SOFC system taking advantage of the high temperature exhaust from the latter to reform the syngas for producing hydrogen. Additional power is produced in the conventional way from a steam turbine making use of the unutilized fuel in SOFC. A detailed energy analysis of this system shows more than 4% thermal efficiency gain over an electricity generating system based entirely on a steam turbine cycle. © 2012 Elsevier Ltd.

Applied Energy

Underground coal-air gasification based solid oxide fuel cell system

Underground coal gasification (UCG) is a promising clean coal technology. Typically, the syngas obtained from UCG is used for power generation via the steam turbine route. In the present paper, we consider UCG as a hydrogen generator and investigate the possibility of coupling it with a solid oxide fuel cell (SOFC) to generate electrical power directly. We show, through analysis, that integration with SOFC gives two specific advantages. Firstly, because of the high operating temperature of the SOFC, its anode exhaust can be used to produce steam required for the operation of UCG as well as for the reforming of the syngas for the SOFC. Secondly, the SOFC serves as a selective absorber of oxygen from air which paves the way for an efficient system of a carbon-neutral electrical power generation from underground coal. Thermodynamic analysis of the integrated system shows considerable improvement in the net thermal efficiency over that of a conventional combined cycle plant. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

International Journal of Hydrogen Energy

Integration of underground coal gasification with a solid oxide fuel cell system for clean coal utilization

A numerical study of oxy-fuel combustion has been carried out in the pressure range of 0.1-3 MPa with methane as the fuel and carbondioxide-diluted oxygen with trace amount of nitrogen (termed here as c_air) as the oxidant. The flame structure and NO generation rate have been calculated using the flamelet model with the detailed GRI 3.0 mechanism for two oxygen concentrations of 23.3% and 20% by weight in the oxidant at a strain rate of 40 s-1 (corresponding to a scalar dissipation rate of 1 s-1). It is observed that, for the reference case of 23.3 wt.% of oxygen, as the pressure increases, the peak temperature of the flame increases rapidly up to a pressure of 0.5 MPa, and more gradually at higher pressures. The concentrations of important intermediate radicals such as CH3, H and OH decrease considerably with increasing pressure while NO concentration follows the same trend as the temperature. Reducing the oxygen concentration to 20% by weight leads to an order of magnitude reduction in NO concentration. Also, for pressures greater than 0.3 MPa, the NO concentration decreases with increasing pressure in spite of the increasing peak flame temperatures. This can be attributed to the increasing domination of recombination reactions leading to less availability of the intermediate radicals H and OH which are necessary for the formation of NO by the thermal route. It is concluded that a stable, low NOx oxy-fuel flame can be obtained at high pressures at slightly increased dilution of oxygen. © 2008 Elsevier Ltd. All rights reserved.

Flame structure and NO generation in oxy-fuel combustion at high pressures

Oxy-fuel combustion, in which a conventional hydrocarbon fuel is burned in presence of oxygen diluted with carbon dioxide (called henceforth as oxy-fuel), is an emerging technology that accommodates CO2 sequestration while offering the prospect of low emissions. Dilution by CO2 (from flue gas recirculation) prevents high peak temperatures thereby reducing material damage, high NOX formation etc. Studies [1] reveal that CO 2-diluted flames are unstable as compared to N2-diluted flames and that a higher molar fraction of oxygen, 30% by volume, is necessary to ensure stable combustion. Nevertheless, as the heat transfer properties of CO2/O2-70:30 mixtures are different to that of normal air, the flow profiles and heat flux distributions might vary within the furnace. While considering the retrofit of existing normal air operated coal fired furnaces with oxy-fuel mode, it is imperative to investigate these flow variations in detail. In the present study, Computational Fluid Dynamics (CFD) based simulations have been done for a typical 210 MW Indian pulverized coal fired furnace under normal air combustion and under oxy-fuel combustion (CO2/O2-70:30 volume %) mode and the results in terms of CO2 concentration, temperature distribution, velocity of flue gas, particle trajectories and devolatilisation characteristics have been compared. Copyright © 2008 by ASME.

Journal	Energy Conversion and Management
ISSN	01968904
Open Access	No