Bale V. Reddy

Second law analysis of a waste heat recovery steam generator

In recent years a great deal of attention is focussed on the efficient utilization of energy resources with minimum heat loss. There is a growing interest on second law analysis to minimize the entropy generation in various thermal units and thereby to improve and optimize the design and performance. In the present work, a waste heat recovery steam generator is considered, which consists of an economizer, an evaporator and a super heater. The unit produces superheated steam by absorbing heat from the hot flue gases. A general equation for the entropy generation has been proposed, which incorporates all the irreversibilities associated with the process. By using suitable non-dimensional operating parameters, an equation for entropy generation number is derived. The effect of various non-dimensional operating parameters, on the entropy generation number are investigated. The role of gas specific heat, non-dimensional inlet gas temperature difference ratio (τ), heat exchanger unit sizes (NTUB, NTUS, NTUE) on entropy generation number are also reported. The results will help to understand the influence of different non-dimensional operating parameters on entropy generation number, which in turn will be useful to optimize the performance of the unit.

Thermodynamic Equilibrium Model and Exergy Analysis of a Biomass Gasifier

Biomass gasification involves the production of a gaseous fuel by partial oxidation of a solid fuel. Clean synthesis (syn) gas, produced from partial combustion of biomass, can be burnt in a gas turbine combustion chamber to run a biomass based combined cycle power plant. A thermochemical model has been developed to predict the gas composition and performance of a biomass gasifier based on thermodynamic equilibrium concept for different biomass materials. A simplified numerical method is applied to solve the thermochemical equilibrium reactions. The system consists of a pressurized circulating fluidized bed to produce the syn gas from the biomass. The effect of the relative air fuel ratio (RAFR), steam fuel ratio (SFR), and gasifier pressure has been examined on the gas composition, gasifier temperature, lower heating value of syn gas, and exergy efficiency of biomass gasifier to obtain a high yield from the biomass. It has been found that at lower values of RAFR and SFR, the heating value of the syn gas and the exergy efficiency of gasifier is high.

Review of underground coal gasification technologies and carbon capture

It is thought that the world coal reserve is close to 150 years, which only includes recoverable reserves using conventional techniques. Mining is the typical method of extracting coal, but it has been estimated that only 15% to 20% of the total coal resources can be recovered in this manner. If unrecoverable coal is considered in the reserves, the lifetime of this resource would be greatly extended, by perhaps a couple hundred years. Mining involves a large amount of time, resources, and personnel and contains many challenges such as drastic changes in landscapes, high machinery costs, elevated risk to personnel, and post-extraction transport. A new type of coal extraction method, known as underground coal gasification (UCG), that addresses most of the problems of coal mining is being investigated and implemented globally. UCG is a gasification process applied to in situ coal seams. UCG is very similar to aboveground gasification where syngas is produced through the same chemical reactions that occur in surface gasifiers. UCG has a large potential for providing a clean energy source through carbon capture and storage techniques and offers a unique option for CO2 storage. This paper reviews key concepts and technologies of underground coal gasification, providing insights into this developing coal conversion method.

Parametric simulation of steam injected gas turbine combined cycle

Abstract In the current work, thermodynamic analysis has been conducted for a steam injected gas turbine (STIG) in the combined cycle system with the dual pressure heat-recovery steam generator. Effect of operating variables such as low-pressure (LP) steam temperature ratio, steam reheat pressure ratio, steam turbine inlet pressure, gas cycle pressure ratio and combustion chamber temperature on the efficiency of the combined cycle has been investigated. Exergy efficiency of the cycle is compared with and without the steam injection with respect to the above-mentioned parameters. Maximum mass ratio of steam injection to fuel has been examined as 6 kg/kg fuel with the complete combustion of the fuel due to excess air supply in the combustion chamber and gas reheater. Results are plotted from the case of without steam injection to the maximum possible steam ratio. LP temperature ratio is identified as a dominant parameter having impact on the efficiency of the combined cycle as the steam is injected at this pressure. Component exergetic losses of the combined cycle as a fraction of exergy of fuel are compared with and without the steam injection. The results showed that the major exergetic loss in the combustion chamber decreased with the steam injection.

Exergy analysis of a natural gas fired combined cycle power generation unit

Combined cycle power generation units are becoming popular due to their higher energy conversion efficiency and reduced pollutants and greenhouse gas emissions. In the present work, exergy analysis of a natural gas fired combined cycle power generation unit is performed to investigate the effect of gas turbine inlet temperature and pressure ratio on exergetic efficiency for the plant and exergy destruction/losses for the components. For a fixed gas turbine inlet temperature, an optimum pressure ratio exists where the exergy destruction is minimum. The exergy analysis provides the details on exergetic efficiency, exergy loss and destruction for individual components in the plant and for the overall plant and their variation with operating parameters.

A model for heat transfer in a pressurized circulating fluidized bed furnace

A mechanistic model based on cluster renewal approach is proposed to predict bed to wall heat transfer coefficient in a pressurized circulating fluidized bed (PCFB). The model takes into account the effect of pressure on cluster density, cluster thermal conductivity and particle convection heat transfer coefficient. The effect of pressure and bed temperature on bed to wall heat transfer coefficient is investigated. Published information on cluster velocity and bed hydrodynamics are used in the formulation of the model. The effect of cross-sectional average volumetric solids concentration on bed to wall heat transfer coefficient is also reported. The model predictions are validated against the experimental data obtained from a PCFB riser 52.4 mm in diameter and 2020 mm high. The unit is heated by electrical resistance heaters. The experimental results are reported for pressures up to 600 kPa and bed temperatures up to 650°C. The experimental data and model predictions are in reasonable agreement with each other. The model predictions are also validated with the data from the published literature, and a fair agreement is observed.

Effect of riser exit geometry on bed hydrodynamics and heat transfer in a circulating fluidized bed riser column

This paper reports the variation of suspension density along the riser column and the effect of riser exit geometry on bed hydrodynamics and heat transfer in the upper region of a circulating fluidized bed (CFB) riser column. The experiments are conducted in a CFB riser column which is 102 mm × 102 mm in bed cross-section (square), 5.25 m height, with a return leg of the same dimension. The unit is made up of interchangeable plexiglass columns. The superficial primary air velocity is varied between 4.2 and 6.4 m/s. The suspension density profile along the riser height is influenced by the exit geometry. With a 90° riser exit geometry, the suspension density profile in the upper region of the CFB riser column increases towards the riser exit. This particular trend has been observed for about 2 m length in the top region of the riser. The change in suspension density profile in the top region influences the variation of heat transfer coefficient. With a 90° riser exit geometry, the suspension density increases towards the riser exit, which in turn increases the heat transfer coefficient. The effect of riser exit geometry on hydrodynamics and heat transfer is significant for about 2 m length in the upper region of the riser column. Copyright

Axial and radial heat transfer studies in a circulating fluidized bed

The axial and radial variation of the heat transfer coefficient in a circulating fluidized bed riser column, and the effect of operating parameters thereon, are investigated. The experimental set-up consists of a riser column of 102 mm×102 mm in bed cross-section, 5·25 m in height with a return leg of the same dimensions. The unit is fabricated with plexiglass columns of 0·6 m in length which are interchangeable with one another. Two axial heat transfer test sections of 102 mm×102 mm in cross-section, 500 mm in height, and made of mild steel, are employed for the axial heat transfer study and one horizontal tube section of 22·5 mm OD made of mild steel is employed for the radial heat transfer study. The primary air velocity is varied between 4·21 and 7·30 m s−1. Local sand of mean size (dp) 248 μm is used as the bed material. One empirical model with the help of dimensional analysis has been proposed to predict the heat transfer coefficient to a bare horizontal tube in a CFB riser column and the model results are validated with the experimental data; good agreement has been observed.

Performance Simulation of Combined Cycle with Kalina Bottoming Cycle

ABSTRACT The Kalina cycle has potential for improved performance regarding electrical efficiency, specific power output and cost of electricity compared with conventional technology because the mixture of working fluids enables efficient energy recovery. Thermodynamic analysis has been carried out for combined cycle with the Kalina bottoming cycle. In this work, the identified key parameters for the Kalina cycle are turbine inlet condition (pressure, temperature and concentration), separator temperature and ambient temperature. The effect of these parameters on exergy efficiency of combined cycle is examined. The combined cycle efficiency increases with the increase in the turbine inlet pressure, and the same decreases with increases in ambient temperature, turbine inlet temperature and its concentration. Heat recovery from exhaust decreases with increases in the separator temperature, and it does not alter the output of the combined cycle. The efficiency of the cycle is very sensitive to the turbine inle...

Effect of lateral and extended fins on heat transfer in a circulating fluidized bed

Abstract Experimental investigations are made in a hot circulating fluidized bed unit to study the effect of lateral, and lateral and extended fins together on heat transfer coefficient and on the rate of heat transfer. The circulating fluidized bed (CFB) unit consists of a riser column of 102 × 102 mm in bed cross-section (square), 5.25 m in height with the return leg of the same dimensions. Both liquid petroleum gas and coal are used as the fuel. A mathematical model is also proposed to predict the heat transfer coefficient to a water-wall test section with lateral and extended fins. The model results are compared with the present experimental data and also with those of published literature and a fair agreement has been observed.