The Use of Exergy for Evaluating Environmental Impact of Processes

Transcription

1 The Use of Exergy for Evaluating Environmental Impact of Processes *Stavropoulos, G.G 1 1, 2, 3 and Skodras, G. 1 Lab of Chemical Process Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece 2 Laboratory of Environmental and Energy Processes, Chemical Process Engineering Research Institute, Thessaloniki, Greece 3 Institute for Solid Fuels Technology and Applications, Ptolemais, Greece *P. O. Box 1520, Thessaloniki, Greece Tel : , Fax gstaurop@cheng.auth.gr Abstract Exergy is a relatively new concept and its primary use is to analyze processes for energy inefficiencies. Further, by definition, exergy is a measure of the deviation of a system from the environmental state. By choosing an appropriate environmental state, exergy can be used to measure the pollution to the environment. This intuitive notion guided many researchers to propose quantitative methods that could express pollution in exergy terms for comparing pollution effects of processes. In this paper a method based on the environmental negative effect, which is an index that accounts environmental effects based on the exergies of the wastes is discussed. For the calculation of the exergy, the exergy of mixing substitutes the physical and chemical exergy conventionally used. Exergy of mixing is that portion of the chemical exergy, which is due only to material transfers or changes in composition. The method is applied in a coal (anthracite) fired steam boiler and environmental pollution is evaluated either with the use of total exergy and exergy of mixing. Results for anthracite are compared with methane as an alternative fuel and differences in pollution potentials are discussed. 1. Introduction

2 Resource utilization and environmental protection are issues of major concern for the sustainable development of the modern society. Raw material and energy consumption is directly linked with environmental problems now and in the future. Except of the economic benefit of the industrial development, pollution control strategies must be adopted in the present social model. Cleaner production processes are necessary in order to make effective use of the resources and preserve the environment. Qualitative and quantitative tools must be invented that will permit to measure the degree of resource use and pollution. 2. Discussion Exergy is a relatively new concept and is currently used as basic theory in analyzing energy intensive processes for inefficiencies. In the traditional exergy analysis the main work consists in calculating the exergy losses and improving the overall system efficiency. This is achieved by conducting the exergy balance that locates exergy losses, since output exergy is not equal to the input exergy in a system. Further, by definition, exergy is a measure of the degree of deviation of a system from the environmental state. By choosing the environmental state as its dead state, it can be applied to account the difference between a system and the environment, which is a measure of the pollution to the environment. Exergy losses are divided into exergy dissipation and the exergy discharge loss. It is the last that affects the environment because accounts for the waste going directly to the environment. A system is polluting when its composition, concentration and state are different from those of the environment. The difference can be measured by the exergy discharge losses. Serious global environmental problems are created by waste emissions such as ozone depletion, greenhouse effect and acid rain. Another aspect of the waste influence on the environment is that wastes of different systems do different harm to the environment and the exergy losses alone are not sufficient to measure their impact. A solution to this problem could be the use of a harm coefficient to show the different harm to the environment.

3 Based on the above considerations an index called environment negative effect (ENE) was introduced [1] defined as: ENE = B E (1) i i x, i Where E x,i is the exergy discharge losses of the component i in the system s waste and includes the physical and chemical exergy. B i is the harm coefficient of i and is used to exemplify the different harm to the environment due to the different chemical nature of components. The determination of B i is not easy for the various pollutants because many harm effects to the environment (health, climate etc) should be included. Here a simple method based on the experience is adopted, and the B i of the four main components of the flue gases are estimated, Table 1. The use of waste exergy in expressing the environmental impact has been discussed in literature. Some authors argue that the waste exergy approach can provide only a first approximation of the environmental impact. This is primary because waste exergy can have different forms that may have different quantitative and qualitative environmental effects. For this reason, another approach has been proposed and focuses on the portion of the chemical exergy, which is due only to material transfers or changes in composition [2]. This part of chemical exergy is referred as exergy of mixing and is a measure of the chemical change attributable to the introduction of any pollutant into the environment. It can be computed for the i-th chemical species of any system as: y Bi = ni RTln (2) y i 0 i Where B i is the exergy of mixing in joules, n i the total number of moles of species i, y i the activity in the thermodynamic system under consideration and 0 y i the reference activity in the appropriate environmental sink. Although exergy of mixing is small compared to total exergy, from an environmental impact standpoint is an objective

4 measure of chemical change produced in the environment by release of wastes. Therefore exergy of mixing can be used to replace total exergy when assessing environmental effects. Especially suitable seems to be its application in the calculation of the ENE mentioned earlier. In this paper the ENE index is calculated both with the total exergy and exergy of mixing and comparison is made to explore any differences. Calculations are performed in a 20 MPa steam boiler fueled with anthracite described in literature [3]. Basis for the calculations is taken: 100 kg of anthracite with a composition shown in Table 2. The combustion reaction is: ( 6510C+ 1200H2+ 444H2O() + 46O2+ 31S+ 32N2) O l 2( g) 6510CO H O + 32NO + 31SO ( g) coal The physical exergy of the flue gas components i. e. the exergy due to heat content is calculated from the equation: ph i p ( ) E = n c T T 0 Where T=473K, the flue gas temperature and T 0 =298K. Chemical Exergy, E 0, is calculated based on the standard chemical exergy tables, [3]. Both Exergies are shown in Table 1. Total flue gas exergy amounts to kj. The exergy of mixing is calculated with the application of equation 2 using y i =1. In table 4 the values of 0 y i employed are reported [2]. The flue gas exergy of mixing is and is 15 % lower than total exergy because does not include physical exergy and also because of lower contributions of CO2 and SO 2, (compare Tables 3 and 5). The calculation of the ENE is easily accomplished both with total exergy and exergy of mixing and the results are shown in Table 6. ENE is given in kj and quantifies the harm to the environment in exergy terms. A comparison of the ENE calculated either by E tot and E mix shows substantial difference

5 in values. The former is 38 % grater and this is caused from the use, in the calculation of ENE, of the harm coefficients. As can be deduced from Table 1, ENE value is greatly affected from the CO 2 and H 2 O harm coefficients, 0.1 and 0 respectively. Therefore from a quantitative point of view, the ENE from E mix anticipates a considerably lower damage to the environment than that resulting from the E tot. In order to explore, in exergy terms, the environmental benefits from the use of a cleaner fuel, the same analysis is performed with CH 4. The combustion reaction of CH 4 is: 3672CH O 3672CO H O mol CH 4 are required to provide the same exergy of anthracite. The exergy values are reported in Table 7 and are lower than those of anthracite mainly due to the absence of N 2 and S in CH 4 fuel and the consequent absence of NO 2 and SO 2 in the flue gases. Another interesting future is the different ratio H/C in the two fuels, which is greater for CH 4 resulting in lower CO 2 and greater H 2 O amounts in the flue gases. This affects exergy by diminishing the contribution of the CO 2 and increasing that of H 2 O, although the last does not contributes to the ENE having harm factor equal to zero, Table 1. Based on these considerations, it is clear the reason why ENE of CH 4 results to be lower than anthracite, Table Conclusions A method of quantifying environmental effects from energy consuming processes is to calculate the total exergy of waste streams. The use of exergy of mixing seems to be a more rational approach because focus on the differences in concentration of pollutants. Further, the introduction of the Environmental Negative Effect, which takes in to account the harm potential of the substances, is also an improvement in estimating the environmental impacts. Calculations reported in this paper show that environmental effects are overestimated when expressed in terms of total exergy. Moreover, the ENE based on exergy of mixing estimates an even lower

6 environmental harm. The method exemplifies very well the different environmental impacts of two fuels one clean (CH 4 ) and one less clean (anthracite). References [1] Y. Wang and X. Feng. Computers and Chemical engineering, 24 (2000) [2] T. P. Seager and T. L. Theis, J. of Cleaner Production, 10 (2002) [3] T. J. Kotas, The Exergy method of Thermal Plant Analysis, 1 st edition, Butterworths, Melbourne (1985) Table 1. Harm coefficients CO 2 SO 2 NO 2 H 2 O Bi

7 Table 2. Anthracite composition, wt % C H 2 H 2 O O 2 S N 2 Ash

8 Table 3. Exergy of anthracite flue gases CO 2 SO 2 NO 2 H 2 O E ph E Total

9 Table 4. Activities of flue gases, 0 y i CO SO NO 2 1 ppb H 2 O 35 ppb

10 Table 5. Exergy of mixing of anthracite flue gases CO 2 SO 2 NO 2 H 2 O

11 Table 6. ENE of flue gases, (kj) Anthracite CH 4 Based on E total Based on E mixing

12 Table 7. Energy of methane flue gases, kj CO 2 H 2 O E ph E E total E mixing