PREDICTION OF INDOOR NITROGEN OXIDES CONCENTRATION

Transcription

1 PREDICTION OF INDOOR NITROGEN OXIDES CONCENTRATION I Senitkova *, S Vilcekova Civil Environmental Engineering Institute, Technical University Kosice, Slovakia ABSTRACT Indoor air is mostly contaminated from the use of gas for heating and cooking appliances. It is well known that the oxide of nitrogen production in the combustion products is the process that not been exactly described so far on all variables which are connected with its production. The comprehensive analysis of indoor air nitrogen oxides pollution show that extent of indoor air level exposure varies with many factor mainly with different distribution conditions and burning activity. It seems to be necessary study and formulate mathematical model for indoor nitrogen oxides production. A mathematical model has been developed to describe the physical processes determining indoor nitrogen oxides concentration as a function of outdoor concentration, indoor emission rates and building characteristic. The model has been parameterized for typical Slovak residences. The base of this activities is presented in the paper with belief in the next continue future work. INDEX TERMS Indoor environment, Nitrogen oxides concentration, Mathematical modeling INTRODUCTION Nitrogen monoxide and nitrogen dioxide are mainly produced by combustion at high temperatures and are formed by reactions between nitrogen and oxygen. Both nitrogen in the fuel and nitrogen in the air participate in reaction, NO is further oxidized and transferred into NO 2 in the atmosphere. Nitrogen dioxide is from the toxicology point of view the most important. During several last years the emissions of nitrogen oxides have increased. The mean daily outdoor nitrogen oxides concentrations achieve value 107,1 µg.m -3 in the winter and 35,2 µg.m -3 in the summer. The mean daily indoor nitrogen oxides concentrations achieve value 125,3 µg.m -3 in the winter and 48,5 µg.m -3 in the summer (Senitkova, 1999). The most important indoor source of nitrogen oxides is gas burning in the kitchen. Finally concerning to indoor environment, indoor air can be contaminated by using gas for heating and cooking and this probably involves a certain health hazard. Practical experiences show, that range of indoor nitrogen oxides concentration depends on energy conservation tendency interpret with ventilate level decrease. The highest nitrogen oxides concentrations are achieved during gas burning in the kitchen in dependence on intensity of gas burning, amount of gas burned and on room disposition (Senitkova, 2000). The indoor NO x concentrations were measured in order to understand the contribution of indoor sources to human exposure. The models and modeling substantially contribute to problem solution and to obtaining new cognition concerning system behavior respectively to air component dynamic changes. The mathematical description of the processes can generalize the achieved results and their interpretation on other various processes physically similar to introduced model. * Contact author Ingrid.Senitkova@tuke.sk 286

2 Although the modeling doesn t achieve all rules of similarity, but overall provides information concerning to systems behavior, which is only little different from real situation. Approximate model is simplifying description of the surveyed reality. The difficulty NO x emissions amount of prediction during combustion of fossil fuels is caused mostly by the complexity of the mechanism in oxide of nitrogen origin, which depends on many factors. Some partial mechanisms are not quite clarified so far and it is not possible to describe them exactly. Also the empirical correlation among the fuel properties, combustion regime and NO x emissions are not sufficiently known. For informative calculations is to be used mathematical model of indoor NO x production. It has to be assembled from the results obtained from statistical suitable collection of measurements as well as laboratory measurements (Senitkova, 2001a). INPUT PARAMETERS The research task of nitrogen oxides production in combustion process was described the measure of NO x influencing in such parameters which the combustion process directly characterize, i.e. intensity of gas burning I [m 3.s -1 ], exposure period t [s], amount of indoor air [kg] and air change n [s -1 ]. All the given variables are presented in basic dimensions, what is at the same time and conditions to utilize the dimensional analysis for creation the NO x production mathematical model. All these variables are simply measurable directly in operation, what allow us subsequently compare the NO x production during particular working conditions on the chosen combustion device on the basis of direct measurements and by means of the created mathematical model described in the following chapter (Senitkova, 2001b). DIMENZIONAL ANALYSIS APPLICATION Mathematical model describing NO x formation is based on formation of dimensionless arguments π i from the stated variables influencing the oxides of nitrogen formation. Their valuable property is, that in all existing systems of units they have the same numerical size and they have no dimension. Formation of a mathematical model rests on derivation of functional dependence from the expressed dimensionless variables, which in general has always exponential character. Transformation of this function into logarithmic coordinates corresponds to linear character that makes the work with model easier and enables simply to determine the parameters of linear function. The model presented in the article has a universal validity for all combustion devices that are noted for at least approximate geometric characteristics. For every combustion device the parameters of linear function i.e. regression coefficients has to be determined separately. The general relation among the selected variables, which effect the NO x production, can be put down in the form: φ (I, t, m, NO x, n) = 0. (1) The created dimensional matrix-relation (2) has the rank of matrix r = 3 and its lines are dimensionally independent on themselves. From n = 5 independent variables at rank of the matrix r, can be set up n-r of dimensionless arguments. 287

3 t I m NO x n s m kg For the general form of the argumentπ is valid: (2) x x x x 2 x x 5 π = t.i.m.no.n (3) From the condition that the left side identically equals to one (ordinary number π), the resulting exponent of every basic dimension equals to zero. By applying this condition for every from r basic dimensions we obtain a system of three linear equations with five unknowns. To solve it, we have to select two unknowns (and this always twice) and to calculate the rest. For the selected x 4 = 1, x 5 = 0 the exponents will be calculated from the linear equations system x 1 = 1, x 2 = 1, x 3 = -1, For the selected x 4 = 0, x 5 = 1 the exponents will be calculated from the linear equations system x 1 = 1, x 2 = 0, x 3 = 0. For dimensionless arguments is then valid: x 1 x 2 x 3 x 4 x 5 π π The searched dimensional homogeneous function in dimensionless form is (4) Ψ (π 1, π 2 ) = 0 (5) or after adjustment and backward transformation of dimensions for the particular variables, the function will have the form: t.i.no Ψ x, t.n = 0 m (6) The real course of the dependence of dimensionless arguments π 1 to π 2 calculated according to the relation (6) from the measured values. B π 1 = A. π 2 (7) Regression straight line for the calculation of the regression coefficients can be calculated by the method of least squares. After completing of relation (7) by relation (6) and its modification we obtain the relation characterizing the NO x production in the form: 288

4 NO xi = A. t B-1. n B. m. I 1 [µg.m -3 ] (8) NO xi NO xo NO xg = NO xo + NO xi (9) nitrogen oxides concentration from indoor sources background nitrogen oxides concentration (e.g. outdoor concentration) Relation (9) presents mathematical model of NO x production by using gas appliances in the kitchen. Presented mathematical model was verified for various gas consumption. In this paper verification for 0,1 m 3.h -1 is presented. The amount of indoor air was 28,28 kg and air change 3, s -1. Air change rate was calculated on the base of knowledge concentration differences. The main contribution of NO, NO 2 and NO x comes from the use of gas ovens. The experimental measurements were performed mostly in the kitchens of residential buildings.. The investigation included online gas measurement for NO, NO 2 and NO x by thermoenvironmental instrument Chemiluminiscence NO-NO 2 -NO x Analyzer - model 42. This analyzer was situated in the middle of the kitchen, approximately 1m above the floor. As the source is mostly located in the corner of the room the minimum 1m distance from the gas stove was also observed. Our results showed short concentration peaks during the cooking and large differences using various intensity of gas burning. Gas consumption per hour was estimated by gas-meter. In the figure 1 is described dependence of dimensionless arguments and regression line. The regression coefficients A equals , B equals 1,6493. π1 1 0,1 0,01 0,001 0,0001 1E-05 1E-06 1E-07 1E-08 1E-09 1E-10 1E-11 y = 5E-09x 1,6493 R 2 = 0,9928 0, π 2 Figure 1. Dimensionless arguments and regression line in logarithmic coordinates 289

5 On the base of measured input parameters and obtained located constant and regression coefficient was determinate nitrogen oxides concentrations according to relation (8) and (9). In the table 1 the value of measured and calculated nitrogen oxides concentrations are compared. The differences can be considered acceptable. Between measured and calculated NO x concentrations can be explained, that in the selection of relevant parameters were not involved all effects, which NO x production depends from. Table 1. Measured and calculated nitrogen oxides concentration t NO x measured. (µg.m -3 ) NO x calculated. Deviation (min) Min Max mean (µg.m -3 ) (%) 5 209,5 495,8 339,14 431,99-27, ,1 946,7 765,40 654,52 14, ,2 999,3 907,26 839,44 7, ,6 1315,5 1127, ,53 10, ,3 1586,5 1379, ,67 16, ,7 1698,5 1430, ,57 9, ,0 1836,1 1550, ,48 8, ,0 1891,3 1615, ,92 3, ,6 1982,0 1745, ,96 4, ,7 2058,8 1821, ,41 1, ,7 2177,6 1870, ,87-1, ,4 2209,4 1868, ,82-7, NOx (µg.m -3 ) NO x mesured values NO x calculated values time (min) Figure 2. Comparison of measured and calculated nitrogen oxides concentration 290

6 CONCLUSION The study of IAQ especially indoor air chemicals became a serious problem, concerning to estimation of mathematical methods using for concentration level prediction. Presented paper gives picture of possible mathematical approach in indoor nitrogen oxides concentration estimation. On the beginning of the mathematical model construction the selection of all relevant parameters is basically important. On the base of our experimental measurements it can be established, that indoor NO x production mainly effect intensity of gas burning, exposure period, amount of indoor air and air change level. Mathematical model was verified by gas consumption 0,1 m 3.h -1 in this paper. The presented selected relevant variables for construction of the mathematical model do not give a true picture of the combustion process consider to the NO x production wholly and exactly. Therefore were ascertained differences among measured and calculated indoor NO x concentrations. Presented mathematical model can be used for other values of gas burning intensity, room size, and air change rate. The mathematical model approach is introduced with belief in next continue work in order to precise input parameters as well as to achieve the complex approach. Consequently any exchange of experiences in this area would really be of mutual help. ACKNOWLEDGEMENTS The authors are grateful to the Grant Agency of Slovak Republic to support of project No. 1/7311/20, on the base of which the results are presented. REFERENCES Senitkova I Indoor Pollutants with Negative Health Impact (Buildings for Livings), Ministry of Building Industry of Slovakia Republic, Bratislava. Senitkova I, Zimermannova S, and Carnogurska M Indoor Air Toxicology - Nitrogen Oxides Study, Proceedings of the 5 th Conference on Environment and Mineral Processing, pp Ostrava. Senitkova I, Carnogurska M Mathematical model of NO x production, Proceedings of the VI International Scientific Conference Current Issues of Civil and Environmental Engineering, pp Lvov. Senitkova I, Vilcekova S Introduction to Mathematical Modeling of NO x Exposure, Proceedings of the 4 th International Conference Indoor Climate of Building 2001 Health, Comfort and Productivity Versus Cost Effective operation of HVACR, pp Strbske Pleso. 291