Jolanta BIEGAŃSKA ? Department of Technologies and Installations for Waste Management, Silesian University of Technology, Gliwice

Abstract:
Exergy is a term introduced to thermodynamics to determine ability to perform work in the environment with relation to human activity. Manufactured industrial explosive  materials are destined mainly to get unmined stone. Hence, their use is connected with the realization of the effective work. The exergy loss related to obtaining solid stone  in mining where explosive materials are used should be taken into consideration.

Please cite as: CHEMIK 2013, 67, 13, 47-52

Introduction

In thermodynamics, an ability to perform work under natural conditions, within the area of human production is a touchstone for determining quality of energy. This criterion of quality was called exergy by Z. Rant [1]. The notion of exergy is related to the irreversibleness of thermodynamic processes. A numerous bibliography of over 2500 items concerning suggestions how to use exergy was collected by Sciubba and Wall [2]. In Poland, professor Szargut [3÷11] dedicated a lot of works to the issue of exergy. He defined it as the maximum efficiency of work in relation to the surrounding natural environment. Analysis of exergy is based on the second law of thermodynamics, which states that all the macroscopic processes are irreversible. Each exergy loss results in a decrease of the efficiency of given process or an increase of wear of elements driving the process. Analysis of the exergy losses makes it possible to define possibilities how to improve heating processes.

An example of the irreversibility of processes which allow for determining exergy loss is explosive process. The aim of use of industrial explosives (EM) is to obtain material using mining methods consisting on detonation. Extraction is realized at the cost of energy included in the explosives. The ability of explosives to accomplish work is defined by calculating energy included in the material, i.e. its potential. It may also be determined on the basis of experiment. Exergy loss may be considered as related to heating processes which lead to exchange of heating energy included in the blasting material into effective work, i.e. mechanical energy.

Explosives ? energy

?Explosives are considered to be [12] solid or liquid chemical substances or mixes of substances able to undergo chemical reaction with emission of gas of such temperature and pressure that they can cause damages to the surrounding environment.? Energy emitted during a process of explosion is a basic and characteristic parameter describing effects that the explosion may have on the environment. The amount energy emitted during a process of explosion is defined by the following equation:

Value of the heat of explosion is a difference between internal energies of the products which undergo change and initial material. In order to define numerical value of the heat of explosion the following dependence is applied:

Heat of explosion is an amount of heat which will be emitted during explosive decomposition of 1 mol or 1 kilogram of the explosives. This is a very important feature of explosives . Higher value of the heat of explosion means that more efficient work can be done. Once the heat of explosion is defined, it is possible to calculate temperature of gas products during explosion. Volume of gas products which are created during explosion exceeds 700 ? 900 times the volume of material in the initial state. These gases are heated to the temperature of 2500 ? 3500 K, which increases their pressure even more. Gas expands in all the directions and constitutes a basic impact force of explosion, capable of damaging objects located within its reach. This is a result of a very high pressure. Value of pressure of expanding products of explosion decreases as it recedes from the center of explosion and reaches a value of atmospheric pressure or lower. Moving away from the source of explosion, kinetic energy of the shock wave turns into heat energy and larger masses of medium are set in motion; the amplitude of pressure and the velocity of the created shock wave decrease. During explosion the energy is turned into mechanical work with significant losses, such as chemical loss (30%), heat loss (40%) and mechanical loss (30%).

Chemical losses result from the fact that the reaction of chemical change is not complete. The smaller is the diameter of the load the bigger is the loss. The whole energy of blasting material, except for chemical loss, is emitted at the moment of explosion in the form of heat and constitutes actual heat energy of the explosion. The explosion process may be considered as adiabatic work of expansion of the explosion products, i.e. such an expansion where there is no heat exchange between the products of explosion and the explosives. A complete exchange of heat into mechanical work is possible when the process of expansion is realized until infinite large volume. Such a process is realized when using blasting materials in a quarry.

Meaning of exergy and the way it is expressed

Exergy is not a real work but a maximum ability to execute work, which could be realized depending on the limits of irreversibility of particular real changes. Differences between energy and exergy are listed [13] in table 1.

Stream of heat and works can be connected by the exergy. The amount of exergy of all the incoming and outcoming streams in given system can be calculated [14] and total balance of exergy can be determined (Fig. 1).

Total exergy of the system can be expressed as a total of physical exergy Ef, kinetic exergy Ek, potential exergy Ep, and chemical exergy Ech:

Exergy described in such a way is thermo-mechanical exergy.
Chemical exergy for various fuels can be determined on the basis of the following dependence:

The loss of exergy is expressed through product of the generated entropy and ambient temperature.

Role of explosives ? realization of work

Explosives are chemical compounds or mixes which under impact of an external impulse are able to undergo a sudden chemical reaction which is accompanied by emission of large amount of gas products which are further able to accomplish mechanical works. The essence of detonation is fast chemical reaction in effect of which, the initial substance (explosives) is transformed into gas products characterized by high temperature and pressure. There is a reaction between gas detonation products and environment around place where high energy process is going on. Products of detonation perform in the environment mechanical work, which is defined as ability of explosives to accomplish work. During these processes the environment is damaged and deformed.

Assuming ideal conditions for explosion (adiabatic process), maximum ability to work for the products of explosion can be expressed through the following dependence:

For crushing explosives (defined as industrial explosives: dynamite, metanite, emulsion explosives, ANFO, etc) the ability to do work is crucial. Experimental method of defining the ability of explosives to accomplish works is a lead block expansion test, so called Trauzl test [15]. This method tests expansion inside the lead block resulting from the explosion of a definite amount of the explosives. This test does not give absolute values but makes it possible to compare capacity for the work of particular explosives in relation to other given explosives. List of usability parameters of explosives, including ability to accomplish work is presented in table 2.

The dynamites have the highest heat of explosion and the highest value of concentration of energy. Their capacity to accomplish work tested in Trauzl block is also the best. In other explosives these value differ insignificantly.

Summary

Considering exergy as a qualification of the ability to perform work in the environment, related to the human activity, it may be stated that explosives used in mining fall within such definition. In order to calculate exergy for industrial explosives with this method, one needs to know composition of the explosive, which is a trade secret of factories which produce these materials. Taking into account the amount of energy required to deform the mass of the lead, in order to undertake a research of exergy there should be taken a research on the strict definition of work performed in Trauzl?s attempt. The action of explosives, where the quantity of useful work done is observed and measurable, results in crushing rocks and also quantity of excavated material, which is obtained as a result of using explosives in opencast crushing of rocks or underground mining works .

Translation into English by the Author

Literature
1. Rant Z.: Exergie, ein neues Wort für ?technische Arbeitsfähigkeit?. Forschung auf dem Gebiet des Ing.-Wesens. 1956, 22(1), 36-37.
2. Sciubba E., Wall G.: A brief commented history of exergy from the beginnings to 2004. International Journal of Thermodynamics. 2007, 10(1), 1-26.
3. Szargut J.: Bilans potencjonalny procesów fizycznych wynikający z II zasady termodynamiki. (Potential balance of physical processes resulting from II law of thermodynamics )Archiwum Budowy Maszyn. 1956, 3(3).
4. Szargut J.: The notation of exergy in contradistinction to energy and the possibility of practical application of exergy. Energetyka Przemyslowa. 1962, (10)11, 374-378.
5. Szargut, J.: Classification of exergy notations. Zesz. Nauk. Politechniki Slaskiej, Energetyka. 1964, (14)104, 5-11.
6. Szargut J.: Die Exergie von typischen Rohstoffen und Produkten der Hüttenindustri. Neue Hütte. 1965, (10)5, 266-275.
7. Szargut J.: Depletion of the unrestorable natural exergy resources. Bulletin of the Polish Academy of Sciences, Technical Sciences. 1997, 45, 241-250.
8. Szargut J.: Anthropogenic and natural exergy losses (exergy balance of the Earth?s surface and atmosphere). Energy, the Intern. Journal. 2003, 28, 1047-1054.
9. Szargut J.: Termodynamika techniczna. (Technical thermodynamics) Wydawnictwo Politechniki Śląskiej, Gliwice. 2005.
10. Szargut J.: Egzergia; poradnik obliczania i stosowania. Editorial of Silesian University of Technology, Gliwice. 2007.
11. Szargut J.: Energia czy egzergia. (Energy or exergy) Rynek Energii. 2010, 5(90), 3-5.
12. Act on explosives used for civil purposes, 21.06.2002, Journal of Acts no. 117, item 1007.
13. Gong M. and Wall G.: On exergetics, economics and optimization of technical processes to meet environmental conditions. TAIES?97, June 10-13, Beijing, China. 1997.
14. Shukuya M., Hammache A.: Introduction to the Concept of Exergy ? for a Better Understanding of Low-Temperature-Heating and High-Temperature-Cooling Systems. Otamedia Oy, Espoo. 2002.
15. PN-C-86037: Explosives. Determining ability for work in lead block. 2000.

Jolanta BIEGAŃSKA ? (Ph.D., Eng), Professor at the Silesian University of Technology, graduated from the Faculty of Chemistry at Silesian University of Technology in Gliwice (1981). She defended her doctoral thesis in 1998 and earned habilitation in 2008. She has been working on the Faculty of Power and Environmental Engineering at Silesian University of Technology since 1996. Since 2009 she is the Director of the Department of Technologies and Installations for Waste Management. Main field of interests: special and dangerous wastes and technology of explosives.