Theoretical analysis of explosion behavior for CO/O2/N2, CO/O2/N2/H2O and CO/O2/N2/CO2 mixtures

1. Introduction

Oxidation of solid, liquid and gaseous fuels either in oxygen atmosphere is surely attractive in the worldwide challenge for zero emission technology and in chemical process intensification. However, this opportunity raises several issues not only in terms of costs, but also in terms of safety. Indeed, in the case of loss of control, explosion consequences can be dramatic. Safety is a primary concern when dealing with flammable gases mixtures and the study of explosion pressures is an important consideration in addressing this issue (Lees, 2012). This work aims at studying the explosion behavior of CO/O₂/N₂ at different initial temperature and pressure and with one of two inerts H₂O (vapor) and CO₂ (gas). Nowadays, experimental tests are carried out in a closed cylindrical vessel. Experiments could be, however, expensive and time consuming, especially at elevated conditions of temperature and pressure, at which many industrial processes occur (Pekalski et al., 2005). Therefore, approximate calculation methods are being sought with which approximate values of explosion pressure can be calculated in reasonable time.

2. Analysis

Due to the complexity of the involved physical phenomena and to the lack of an adequate amount ofreliable experimental data, a number of different models and calculation procedures for estimating the physical consequences following the physical explosion of a gaseous state are presently reported in the literature. Due to the consistency of the input phys.-chem. data used in this study were chosen those from standard database (DIPPR, 2012) based mostly on experimental values with standard errors according to AIChE. The recommended values and temperature correlations are based on a critical, comprehensive evaluation by experts of the available data including inter-property relationships, inter-chemical relationships, and predicted/expected values.

Element balanced equation:

(1)

where a_j are moles of j-th element (atom); n_iare moles of i-th species; A_i,j number of j-th element in i-th species; l is the number of elements; j is the number of species.

The free energy (Helmholtz function) of the system F equation:

(2)

where F is the free energy (Helmholtz function) of the system; n_i are moles of i-th species; f_i is free energy of i-th species.

(3)

where f_i is free energy of i-th species; λi are the Lagrangian multipliers for each element (j=1 to J); A_i,j number of j-th element in i-th species.

(4)

where g_i is the Gibbs free energy for single i-th species; ΔH_fis the enthalpy of formation of the i-th species at standard conditions;

(5)

where g_iis the Gibbs free energy for single i-th species; R is the universal gas constant; T is temperature; n_i is the number of moles of every species; V is the volume.

(6)

where a_j are moles of j-th element (atom); n_iare moles of i-th species; A_i,j number of j-th element in i-th species; l is the number of elements; j is the number of species; g_i is the Gibbs free energy for single i-th species; λi are the Lagrangian multipliers for each element (j=1 to J); R is the universal gas constant; T is temperature; n_i is the number of moles of every species; V is the volume.

(7)

where g_i is the Gibbs free energy for single i-th species; A_i,j number of j-th element in i-th species; l is the number of elements; λi are the Lagrangian multipliers for each element (j=1 to J); R is the universal gas constant; T is temperature; n_i is the number of moles of every species; V is the volume.

It has been shown (Skřínský et al., 2015) that the model is able to predict, with a reasonable accuracy in different fuel-enriched conditions, for different types of gaseous mixtures explosions. The equations 1-7 were used to investigate and quantify the role of initial temperature and pressure in affecting the explosion behavior of CO/O₂/N₂, CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures.

3. Results and discussions

Table 1: Explosion characteristics of flammable gas component

G = GESTIS-Substance database (IFA); CO = Compendium of Environmental Standards (CES); D = Design Institute for Physical Properties (AiChE).

Table 1 compares the theoretically derived data for the maximum explosion pressure of the studied gas mixture components. The values were adopted from the databases as the IFA and the AiChE, and from the literature as the Compendium of Environmental Standards (CES). In Table 1, for example the values of the P_max adopted from IFA, AiChE and CEI (8.2 bar(a), 8.3 bar(a) and 8.0 bar(a), respectively) clearly appear to be quite different. All values were obtained by using standard testing conditions.

A) Adiabatic temperatures and explosion pressures for CO/O₂/N₂

Calculated adiabatic temperatures, T_ad, and explosion pressures, P_ex, for CO/O₂/N₂ were determined at various initial temperatures and ambient initial pressure. The results of calculations are given in Table 2 and Figures 1a-2a.

Table 2: Computed explosion pressures for CO/O₂/N₂ mixtures at p₀ = 1 bar(a)

From the numerical results presented in Table 2 it is possible to identify that the increase in the initial temperature lowers the maximum explosion pressure (37% for 30.0 vol. %). The value of the explosion pressure with varying CO/O₂/N₂ concentration is similar at all investigated initial temperatures. The maximum value of the explosion pressure is found close to 35 vol. % of CO for all fuel fraction vol. % (see Figure 1a). Further, in Table 3, we reported simulations on the explosion properties of CO/O₂/N₂ mixtures at various initial pressure and ambient initial temperature.

Table 3: Computed explosion pressures for CO/O₂/N₂ mixtures at T₀ = 298 K

From the data performed in Table 3 and Figure1b-2b, it is clear that the increase of initial pressure has significant effect on explosion pressure when only the air as oxidant.

a)
b)

Figure 1: Calculated explosion pressure vs fuel fraction for explosions of a) CO/O2/N2 mixture at 298 K (top), 358 K (upper middle), 418 K (lower middle), and 478 K (bottom); b) CO/O2/N2 mixture at 1 bar(a) (top), 5 bar(a) (upper middle), 10 bar(a) (lower middle), and 15 bar(a) (bottom)

a)
b)

Figure 2: Calculated explosion pressure vs initial temperature for explosions of a) CO/O₂/N₂ mixture at 10.00 vol. % (top), 35.00 vol. % (upper middle), 60.00 vol. % (lower middle), and 85.00 vol. %  (bottom); b) CO/O₂/N₂ mixture at 1 bar(a) (top), 5 bar(a) (upper middle), 10 bar(a) (lower middle), and 15 bar(a) (bottom)

B) CO₂ and H₂O inerting CO/O₂/N₂ mixtures

Further, maximum explosion pressure of CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ at 300 K for four different fractions of H₂O and CO₂ was investigated. The initial ambient temperature profile estimation with different O₂/N₂ atmospheres similar to industrial application conditions was also analyzed. The numerical results of calculation are summarized in Figure 3. The flammability data for CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures obtained in the present work provides adequate information for plant design purposes for a range of presented gas atmospheres, ambient temperature and pressures up to 15 bar(a).

a)
b)

Figure 3: Calculated explosion pressure vs fuel fraction for explosions of CO/O₂/N₂ mixture at a) different initial H₂O volume fractions 2.50 vol. % (top), 5.00 vol. % (upper middle), 10.00 vol. % (lower middle), and 15.00 vol. % (bottom); b) different initial CO₂ volume fractions 2.50 vol. % (top), 5.00 vol. % (upper middle), 10.00 vol. % (lower middle), and 15.00 vol. % (bottom)

Figure 3 shows the calculated explosion pressure behavior of stoichiometric CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures. The pressure trends for both mixtures are similar. Depending on the contents of H₂O / CO₂ and fuel fraction, they exhibit a single pressure peak at the maximum pressure. From Figure 3 it appears that the mixture reactivity, as quantified through P_max, increases on decreasing the initial H₂O / CO₂ vol. % fraction. Furthermore, it can be noted that the explosion pressure is higher for the mixture with H₂O for the same vol. % fraction values of fuel, in all cases. Therefore, based on equilibrium calculation presented in Figure 3 CO₂ seems to be a better inert for CO/O₂/N₂ mixture. The results will be experimentally proofed in 1 m³ explosion apparatus.

4. Conclusion

The adiabatic explosion pressures and constant volume adiabatic explosion temperatures of CO₂/O₂/N₂, CO/O₂/N₂/H₂O  and CO/O₂/N₂/CO₂ mixtures at various initial temperatures and pressures were calculated. The model predictions for the CO₂/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures are compared for four different initial H₂O / CO₂ vol. % fractions. Finally, the results of simulations are compared to clarify the role of CO₂ and H₂O. Although the results from the evaluation indicate that presented theoretical simulations can become a valuable tool for rough estimation, the modeling requires further improvements to be useful for consequence modeling and design of industrial facilities. Thus, at the first stage, the equilibrium calculations can be used as a rough calculation of a worst case scenario. At the same time, these values will be used as approximate initial values for explosion experiments carried out in heated 1 m³ explosion apparatus designed by OZM Research s.r.o. at Energy Research Centre, VŠB - Technical University of Ostrava. The experimental data on the flammability of CO₂/O₂/N₂, CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures can readily be applied to the hazards of handling solutions of carbon monoxide gas mixtures. These conclusions apply only to solutions in closed vessels, but similar calculations based on gas-gas and vapor-gas equilibrium data can be made for solutions in opened vessels. The results represents a continuation of numerous efforts by various research groups, where the key underlying problem has been the understanding of results obtained in laboratory tests for predicting the consequences of gas/vapor explosion scenarios in industry (Skřínský et al., 2015).

5. Acknowledgment

This work would not have been possible without the financial support of two grants: (1) Innovation for Efficiency and Environment, reg. no. ED0036/01/01 supported by Operation Program Research and development for Innovation and financed by the Ministry of Education, Youth and Sports; (2) Innovation for Efficiency and Environment - Growth, reg. no. LO1403 supported by National Programme for Sustainability and financed by the Ministry of Education, Youth and Sports. J.V. is thankful for the financial support of the project “Support research and development in the Moravian-Silesian Region 2014 DT 1-International research teams” 02722/2014/RRC, financed from the budget of the Moravian-Silesian Region.

6. Literature

[1] LEES, F. P. Loss Prevention in the Process Industries: Hazard Identification, Assessment, and Controls: volume 1-3. Oxford: Butterworth-Heinemann, 2012.

[2] PEKALSKI, A. …[et al.]. Determination of the Explosion Behaviour of Methane and Propene in Air or Oxygen at Standard and Elevated Conditions. Process Saf. Environ, 2005, vol. 83, s. 421-429.

[3] SKŘÍNSKÝ, Jan …[et al.]. Explosion chracteristics of methane at elevated initial temperatures. Časopis výzkumu a aplikací v profesionální bezpečnosti [online], 2015, roč. 8, č. 2-3. Dostupný z WWW: <http://www.bozpinfo.cz/josra/josra-02-03-2015/methane-explosion-characterist.html>.  ISSN 1803-3687.

[4] GESTIS-Substance database. Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA), 2015.

[5] Design Institute for Physical Properties. DIPPR Project 801: Full Version. Design Institute for Physical Property Research/AIChE, 2012.

Vzorová citace

SKŘÍNSKÝ, J. …[et al.]. Theoretical analysis of explosion behavior for CO/O₂/N₂, CO/O₂/N₂/H₂O and CO/O₂/N₂/CO₂ mixtures. Časopis výzkumu a aplikací v profesionální bezpečnosti [online]. 2015, roč. 8, č. 4. Dostupný z WWW: <http://www.bozpinfo.cz/josra/josra-04-2015/analysis-explosion-behavior.html>. ISSN 1803-3687.

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