Janez Oman

Efficiency of a combined gas-steam process

Abstract The equations for the determination of the overall energy and exergy efficiency of a combined gas-steam cycle process are determined and discussed. The cogeneration in the gas and in the steam cycle, the reduction of power due to increasing the heat flow in the steam process, the supplementary firing at the gas turbine exhaust, the heat recovery boiler efficiency as well as the heat exchanger efficiency in the gas and steam cycle were taken into consideration. The presentation of the results is based on the thermodynamic derivation, the calculations of a typical example, and graphic diagrams.

Influence of lignite composition on thermal power plant performance: Part 2: Results of tests

Abstract The results of measurements and experiments on two power plant units are presented. The treated coal is lignite with lower heating values ranging between 9 and 10 MJ/kg and with contents of ash about 20% and moisture about 38%. The influences of significant lignite composition components on heating value, boiler losses, boiler specific production of steam, own consumption of electric power, power consumption for coal grinding and flue gas desulphurisation etc. are discussed. The results of analyses and measurements are graphically presented.

Solutions to the problem of waste deposition at a coal-fired power plant

This paper deals with the impact on the environment of the combined operation of a power plant and a coal mine located in Velenje, Slovenia. The consequences of intensive deep underground mining are the sinking of the surface and the filling up of the emerging hollows with water. As far as the various consequences of coal combustion are concerned, only the harmful effects of the surface deposition of ash and slag are discussed, with the poisoning of the lake and the river water being considered the most harmful. The integrated system of waste treatment at the power plant is presented. The idea of the system is to use the waste materials such as ash, slag, gypsum, contaminated water, etc. as the building materials or to apply them in the process of improving the ash dump and the environment. The system needs to be closed as much as possible, so that the waste cannot influence the surroundings of the dump. The physical and chemical properties of the waste materials from the power plant, the results of ecological measurements and the impact of the ecological damage are presented. In addition, we present: a description of the applied methods; the activities and the technical measures; the obtained results and the difficulties; and the problems that need to be overcome in order to normalise the ecological conditions.

Volumetric absorption in gas—properties of particles and particle-gas suspensions

This paper deals with the volumetric absorption of solar radiation in particle-gas suspensions. The optical properties of different carbonaceous particles in connection with solar radiation on the basis of Mies scattering theory are discussed. An efficiency factor for reflection was introduced to determine the solar radiation scattered backwards in a hemispherical angle towards the source of radiation. Projections of the relevant optical properties of individual small particles of black and brown coal, anthracite, carbon black and coke are illustrated and compared. Extinction experiments were performed and are presented, and experimental and theoretical results are compared. Some practical experiences of particle production and particle-gas suspension characteristics are presented. The results of experiments and analysis show which substances and particle sizes are most appropriate for the absorption of solar radiation in particle-gas suspensions.

Influence of lignite composition on thermal power plant performance: Part 1: Theoretical survey

Conversion of energy with the highest possible efficiency is of primary importance in processes with high intensity of energy flows, among which the most significant are those in thermal power plants. The paper deals with the influence of coal composition on the conversions taking place in power plants. The analysis of energy conversions treats, above all, the heat loss due to the exhaust flue gas. The analyses of the power plants use of electric powers are focused on the processes of coal pulverising and exhaust flue gas cleaning. The results of the analyses are graphically presented.

Overfeed fixed-bed combustion of wood

Abstract Research on the combustion of wood on a fixed grate with a separate supply of combustion air is described in this paper. From the layer of the wood on the grate, only fixed carbon was burned in the primary combustion chamber in the presence of primary air, whereas the volatiles were burned subsequently in the secondary combustion chamber in the presence of secondary air. A one-dimensional mathematical model describing the combustion in the overfeed layer of wood chips is given. Theoretical results were confirmed by the experiments carried out in the experimental furnace. The scheme of the experimental device and the results of experiments and calculations are presented. It was confirmed that the conditions in the layer and the quality of combustion were highly effected by the ratio of the fuel mass flow and mass flows of primary and secondary air. It was found that the ratio between primary and secondary air could be determined on the basis of the measured concentration of CO 2 in flue gases leaving the burning layer. The optimum ratio of primary and secondary combustion air at any excess air was determined.

A cross-correlation optical velocimeter

This study presents a newly developed approach to velocity measurements of moist air in a natural-draught cooling tower (CT). It is based on the cross-correlation of two signals acquired from infrared (IR) light-to-voltage optical sensors. The air flowing through the CT becomes supersaturated in CT packings and thus consists of small water droplets of size 4?12 ?m. The same air usually contains droplets resulting from drift, which drags along too small droplets in the spray zone. When the moist-air droplet pattern crosses the beam emitted by IR light-emitting diodes (LEDs), the optical sensors detect a certain signal. The cross-correlation technique gives the time delay of the measured signals. Therefore, by knowing the distance between the sensors, it is possible to calculate the velocity of the moist air. The method has been developed for dynamic velocity measurements of moist air and has achieved a satisfactory precision. The optical velocimeter is a very practical device that makes it possible to carry out measurements on a local basis across the entire plane area of the CT. With the use of simultaneous measurements of velocities and temperatures, the exact location of problems with the packings, nozzles and water distribution system can be thus determined.

Influence of temperature profile in an annular shaft kiln on calcination

The paper discusses an experimental measurement of the vertical temperature profile and kiln gas composition in an annular shaft kiln. The results obtained confirm that calcination in the kiln begins at a temperature between 820°C and 830°C. From energy balance, kiln gas composition and the samples of partially burned stones from the kiln the average heat transfer coefficient from kiln gases to the stones is determined, as well as the calcination distribution within the kiln. Modelling of the calcination process in an annular shaft kiln was performed on the basis of the measured temperature profile. The results of this model were in good agreement with those for samples of partially calcinated stones taken from the area of the lower and upper burners.

Stromabfall infolge der Wärmeerzeugung bei der Kraft-Wärme-Kopplung

ZusammenfassungMit dem energetischen Wirkungsgrad der Kraft-Wärme-Kopplung wird mechanische Arbeit und Wärme in gleicher Weise bewertet, was nicht dem zweiten Hauptsatz der Thermodynamik entspricht. Aus diesem Grund hat man in der Vergangenheit oft versucht, den Einfluß der Wärme durch eine sinnvolle Abänderung des Wirkungsgrades zu mildern. Somit sind mit der Zeit verschiedene Definitionen des energetischen Wirkungsgrades entstanden, welche in diesem Beitrag vorgestellt und mit dem exegetische Wirkungsgrad verglichen werden.In vielen Heizkraftwerken wird mit steigendem Wärmestrom die elektrische Leistung geringer. Diese Tatsache läßt sich mit der Stromabfallkennziffer berücksichtigen, welche das Verhältnis von erzeugter Wärmeleistung zur auftretenden Stromminderung darstellt. Es zeigt sich, daß die Stromabfallkennziffer für die Bewertung verschiedener Definitionen des energetischen Wirkungsgrades bei der Kraft-Wärme-Kopplung sehr nützlich ist. Führt man sie in die verschiedenen Definitionen für den Wirkungsgrad ein, so läßt sich folgendes erkennen.Sämtliche abgeänderten Definitionen des Wirkungsgrades der Kraft-Wärme-Kopplung können entfallen, denn sie sind sinnlos bzw. technisch nicht vertretbar. Verblieben sind zwei Definitionen: energetischer Wirkungsgrad (Nutzungsgrad) und exergetischer Wirkungsgrad der Kraft-Wärme-Kopplung. Die erste ist anzuwenden, wenn der Einfluß der Wärme weniger wichtig erscheint oder wenn man für die erzeugte Wärme einen besseren Verkaufspreis zu erzielen versucht. Die zweite dann, wenn man thermodynamisch korrekt handeln will.AbstractIn the equation of overall energy efficiency of cogeneration, the heat and power are considered as equal terms, what is not in accordance with the second law of thermodynamics. For this reason many attempts were made in the past with intention to reduce the influence of heat in the equation. Therefore several definitions of energy efficiency were set up in course of time, some are presented in this paper and compared with the exergy efficiency.In cogeneration plants the increase of heat production usually causes lower production of electric power. This fact can be described by the heat-to-power-reduction ratio, which is defined as the quotient of the gained heat and decrease in produced electric power. The application of the heat-to-power-reduction ratio is very convenient when evaluations of different definitions of energy efficiency of cogeneration plants are performed. When inserting the heat-to-power-reduction ratio into different definitions of the energy efficiency, we draw the following conclusions:All adapted definitions of the energy efficiency of co-generation plants are redundant and have no sense, some of them are even technically unacceptable. Two definitions remain: energy and exergy efficiency of cogeneration plants. When the influence of the heat is not very significant and when we want to attain a higher price of the heat, the first definition should be used. From a thermodynamic stand of point correct consideration of the processes can be performed only be application of the second definition.