2 Prozess- und Anlagensicherheit
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Eingeladener Vortrag
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The flammable hydrogen-blended methane–air and natural gas–air mixtures raise specific safety and environmental issues in the industry and transportation; therefore, their explosion characteristics such as the explosion limits, explosion pressures, and rates of pressure rise have significant importance from a safety point of view. At the same time, the laminar burning velocities are the most useful parameters for practical applications and in basic studies for the validation of reaction mechanisms and modeling turbulent combustion. In the present study, an experimental and numerical study of the effect of hydrogen addition on the laminar burning velocity (LBV) of methane–air and natural gas–air mixtures was conducted, using mixtures with equivalence ratios within 0.90 and 1.30 and various hydrogen fractions rH within 0.0 and 0.5. The experiments were performed in a 14 L spherical vessel with central ignition at ambient initial conditions. The LBVs were calculated from p(t) data, determined in accordance with EN 15967, by using only the early stage of flame propagation. The results show that hydrogen addition determines an increase in LBV for all examined binary flammable mixtures. The LBV variation versus the fraction of added hydrogen, rH, follows a linear trend only at moderate hydrogen fractions. The further increase in rH results in a stronger variation in LBV, as shown by both experimental and computed LBVs. Hydrogen addition significantly changes the thermal diffusivity of flammable CH4–air or NG–air mixtures, the rate of heat release, and the concentration of active radical species in the flame front and contribute, thus, to LBV variation.
Safety concerns on cable tray fires in urban utility tunnels, which may further trigger huge casualties, ceiling structure damages, power failures and other domino effects, attract increasing attention in recent years. Determining the maximum excess ceiling gas temperature (MECT) induced by cable tray fires in urban utility tunnels is crucial to evaluate the fire risks. A series of one-layer horizontal cable tray fire experiments to explore the MECT were carried out in a large-scale utility tunnel without mechanical ventilations. The number of cables on the tray was varied from 8 to 18 in the experiments. The experimental results showed that the cable tray fire burning could be divided into three distinct stages, including ignition, self-sustaining and decaying stages. In the self-sustaining combustion stage, the cable tray was found to burn relatively steady. The mean MECT was also investigated since it represents one of the main characteristics of the cable tray fire. By redefining two parameters (the heat release rate and the effective ceiling height) in three classical MECT models proposed originally based on pool-fire, these three models could be extended to be able to predict the mean MECT generated from the cable tray fire (solid combustible) within 20% deviations. Consequently, two novel models were respectively proposed to predict the mean MECT at the self-sustaining burning period and the instantaneous MECT of one-layer horizontal cable tray fire in utility tunnel, which would be useful in the field of fire protection engineering.
A series of experiments were conducted in a 10L closed and vented tube with L/D = 10.0, and effects of initial fuel volume concentration, inert gas dilutions (diluted by N2 and CO2), inert gas–water mist twin fluid medium dilutions (diluted by N2-H2O twin fluid medium, CO2-H2O twin fluid medium) and end boundary conditions on overpressure transients of hydrocarbon fuel–air mixtures explosion were revealed. Results show that the overpressure-time profiles consistent with the dynamic evolution law of ‘approximately zero-1st overpressure rising stage-2nd overpressure rising stage-descending stage’, and ‘rate of overpressure rise-time’ curves exhibit the characteristics of multi-stages and multi-peaks, such as (dp/dt)(1,max), (dp/dt)(1,min), (dp/dt)(2,max) and (dp/dt)(2,min). Specifically, as the fuel volume concentration increased, both the maximum overpressures (pmax), and the maximum rates of overpressure rise ((dp/dt)(1,max) and (dp/dt)(2,max)) show a variation trend of increasing firstly and then decreasing, while the corresponding times (tmax, θ(1,max) , θ(2,max)) show a total different variation trend. Moreover, when YCH is lower than 1.88%, the value of (dp/dt)(1,max) is greater than (dp/dt)(2,max), while the value of (dp/dt)(1,max) was less than (dp/dt)(2,max), and when YCH was higher than 1.88%. The addition of N2 and CO2 can obviously inhibit the explosion intensity of hydrocarbon fuel, and the inhibition effect of CO2 is better than that of N2. Due to the synergy inhibition effect of the inert gas and ultrafine water mist, all the values of pmax, (dp/dt)(1,max) and (dp/dt)(2,max) diluted by inert gas-ultrafine water mist twin fluid medium were smaller than those diluted by sole inert gases. In addition, there are significant differences in the overpressure-time and the rate of overpressure rise-time profiles between closed and end venting explosions. The values of maximum overpressure and the rates of overpressure rise of the closed explosion were higher than those of the venting explosion, but the minimum rate of overpressure rise is a smaller one.
Hydrogen leakage of vehicles in the tunnel is a great threat to the safety operation of the tunnel and longitudinal ventilation strategies have always been utilized to control the fire and smoke movement of rail transit, electric and fossil-fueled vehicles in the engineering field. It is in doubt whether the longitudinal ventilation strategy could still help to reduce the jet fire hazard of transportation with H2 power in the tunnel, considering the rapid development of the hydrogen energy. In present work, a numerical research on effects of longitudinal ventilation strategies on hydrogen jet flames in the tunnel is conducted. The results illustrate that longitudinal ventilation could affect the flame characteristics of jet flames greatly in the tunnel. The critical ventilation velocity increases firstly with the increase of hydrogen leakage rates and then changes little after a critical value. The predicted theoretical model of pool fires could well predict the critical ventilation velocity for hydrogen jet fires. With the increase of longitudinal ventilation velocity, maximum ceiling temperatures are decreased greatly. According to the heat releases, jet speeds and ventilation velocities, three kinds of flame bending characteristics of hydrogen jet fire could be observed due to different effects of the inertial force. At last, the stable thermal stratification could also be destroyed by large ventilation velocities but the corresponding ventilation velocity is far larger than the critical ventilation one. With the increase of longitudinal ventilation velocities, the height of thermal layer is reduced firstly and then maintained at a constant value.
Self-sustaining smoldering as a novel disposal approach for food waste with high moisture content
(2021)
This work aims to explore the self-sustaining smoldering treatment (SSST) for the destruction of food waste. The food waste used in this work mainly consists of cereal, vegetable, and meat. To examine the processing procedures and parameters, three experimental groups (raw food waste, food-waste paste and food-waste paste blended with anthracite) and five variables (moisture content, sand size, sand-to-food waste ratio, air flow and food waste-to-coal ratio) were extensively investigated. Results show that raw food waste with 40% moisture content can be disposed by SSST with the mass destruction ratio over 90%. However, the robustness of SSST for raw food waste is constrained by the heterogeneous issue due to a variety of components and sizes of food waste. This issue is addressed through stir pre-processing of raw food waste as food-waste paste. The robustness of SSST for food waste is significantly further enhanced only if a slight mass fraction of anthracite (<20% of raw food waste) is supplemented. This provides an alternative solution for employing SSST for raw food waste with very high moisture content.
The space heating system accounts for 20%~50% of building energy consumption, and may lead to energy waste due to unreasonable controls. In this study, an energy-efficient floor heating system with intelligent control was proposed to improve energy efficiency of the system. In order to validate the concept of the proposed intelligent control, an experimental system was designed and constructed in Chengdu, China. Temperature, control cycle and energy consumption were then studied under different control strategies. The result shows that a larger flow rate of supply water will result in a longer control cycle and a lower control frequency, i.e., the average control cycle at 7 L min−1 is 1.7 h during the test day, while it is 1.5 h at 5 L min−1. Moreover, adopting water with a higher temperature and flow rate could achieve a higher efficiency of the system. The energy consumptions in case 1 (5 L min−1, 50 °C), case 2 (5 L min−1, 55 °C), case 5 (7 L min−1, 55 °C) and case 6 (7 L min−1, 60 °C) are 4746 kJ, 3534 kJ, 3093 kJ and 3028 kJ, respectively. Based on the experimental data, the supply water temperature is suggested to set lower than 60 °C considering human comfort.
The Oxygen Pressure Surge Test (OPST) is considered to be best practice for evaluating the burn out safety of plant components and testing the compatibility of nonmetallic materials for oxygen service. Nevertheless, there are only a few laboratories worldwide performing the OPST. The Federal Institute for Materials Research and Testing (BAM) already operates oxygen pressure surge test facilities with maximum OPST pressures of 450 bar.
However, to ensure technical innovation and to enhance research and development, the decision was made to develop a new and large-scale oxygen pressure surge test facility. Due to the size and the high potential impact energies, the test facility is located outside of Berlin at BAM Test Site for Technical Safety (TTS), where large scale testing is carried out. In addition to an intended maximum OPST pressure of 750 bar, there is the possibility of testing industrial equipment with large inner volumes and diameters. The new quick opening valve constitutes the core of the system and was developed by BAM. It allows to perform tests with the standardized pressure rise times of 15 ms to 20 ms or with adjustable pressure rise times at higher or lower values. As a result, it is possible to provide customized test parameters for specific practical applications as well as research projects.
This presentation shows the status quo of this unique test facility, its performance, its technical specifications, and its new possibilities for research and development for a long term enhancement of burn out safety in oxygen application.
The general requirement, that nonmetallic materials for oxygen service must be tested and found suitable for their intended use in plants and components is the current state of technical safety. However, numerous requests of manufacturers, sales offices, and users show that there is still a strong need for information on how these materials are tested and evaluated. Based on more than 60 years of expertise, this paper provides background information on BAM’s test approach. It explains why BAM applies additional assessment criteria on nonmetallic materials for certain use conditions. Hence, the test schedule as well as the final evaluation shall reflect the practical application in a more customer-related way in combination with a higher safety level. That is the demand of BAM’s safety philosophy.