Modelling agricultural ammonia emissions: impact on particulate matter formation

. To improve the existent approach which is not fully satisfying in terms of model performances regarding PM10 in spring, a coupling process has been built between CHIMERE and a mechanistic VOLT'AIR model computing the NH 3

Methods

volatilization.

To calculate the ammonia emissions after N application on agricultural lands in France, a mechanistic model "VOLT'AIR" (Génermont and Cellier, 1997) is used offline with CHIMERE. It describes the action of environmental conditions and soil properties on ammonia volatilization. The calculation of ammonia fluxes is made over agricultural plot of land on an hourly basis depending on collocated input data. The NH 3 volatilization process is highly dependent on the meteorological conditions: air/soil temperature, air/soil humidity, wind speed and rainfall are provided to Volt'Air by the outputs of the meteorological mesoscale model WRF (Weather Research and Forecasting; http://www.wrfmodel.org). The physicochemical properties of the soil (texture and pH) (INRA infosol; http://www.orleans.inra.fr/les_unites/us_infosol) are available at local scale and have been interpolated on the chosen grid scale (0.15° X 0.10°). Data about crop and grassland areas are available at cantonal level from the national agricultural census. Using a Geographic Information System (GIS), data are intersected with the corresponding land cover types of the Corine Land Cover database (UE-SOeS, CLC 2006) and spatialized on a regular grid. Fertilizer types considered are ammonium nitrates, nitrogen solutions and urea. They constitute the bulk of the synthetic fertilizers used in France [START_REF] Unifa | Les livraisons de fertilisants minéraux en France : exercice 2006 -2007[END_REF]. Fertilization rates and dates of spreading are available at regional scales. The chemistry-transport model CHIMERE is used to simulate the observed PM concentrations for France. The meteorological input fields are computed using the WRF mesoscale model. Boundary conditions are provided by a large-scale simulation covering Western Europe with a 0.5° horizontal resolution. We carried out two simulations. The first is the reference simulation using emissions provided by EMEP inventory (http://www.emep.int/). The second simulation is the same as the first one but ammonia emissions are calculated as the sum of the NH 3 emissions due to fertilization (VOLT'AIR), animal husbandry and manure spreading. These two last NH 3 emissions are provided by the National Spatialized Inventory (NSI) for France. For the rest of the domain, EMEP NH 3 Results emissions are used. The CHIMERE-VOLT'AIR runs were carried out on the period February-April 2007 over France with a resolution of 0.15° X 0.10° (~ 10 km).

Ammonia emissions:

The emissions from fertilizers calculated by Volt'air model present a significant spatiotemporal variation. The spatial variation of the emissions depends especially on soil pH and the agricultural practices (type and amount of fertilizers). The temporal variation depends on the soil temperature (diurnal variations of the emissions) and on the dates of the fertilization varying with different regions for different crops. Ammonia concentrations: The ammonia concentrations are calculated with CHIMERE using the corresponding emissions. They vary between 0 and ~ 80 ppb when the Volt'air plus NSI emissions are used and between 0 and ~ 50 ppb when EMEP emissions are used. The results show significant differences in the spatial variations using the two methods (fig. 1.a). PM10 concentrations: The PM10 concentrations are calculated with CHIMERE CTM using ammonia emissions provided by Volt'air + NSI and using those provided by EMEP. The differences are significant in some regions. In the north, north-east, centre and south-west of France, the PM10 mean levels calculated using the new method are the highest. The maximum of this difference is about 4µg.m -3 (fig. 1.b) on the total spring period and the increase of the PM10 mean levels varying from 0 to 16 %. For the episode of PM10 pollution of April 17, the daily mean difference can reach locally 15µg.m -3 (fig. 1.c) using the new emission method and the difference can range from 0 to 30 % of PM10 mean concentrations. These results confirm the impact of high NH 3 emissions on PM formation at favourable meteorological conditions and thus the importance of ammonia modelling in CTMs. The comparison of PM concentrations with observations will be shown.

Fig. 1 .

 1 Fig.1. The difference of NH 3 [ppbv] (a) and PM10 [µg.m -3 ] mean concentrations on the period February-April (b), and on April 17 2007 (c) between Voltair_NSI method and EMEP method of NH 3 emissions implemented in CHIMERE CTM.


A Janes

F Malet

V Herrenschmidt-Munoz

Risks generated by the treatment of aerosol in the recycling of metals: How to secure the process?

Keywords: metal waste recycling, aerosols, fire hazard, explosion hazard

Aerosols consumed at home are supported in France in the recycling of aluminum or steel household waste stream. This process requires storage and crushing in a press, the latter operation leading as indirect consequence in random perforation of aerosols. As aerosols include flammable gaseous, liquid or liquefied chemical compounds, this process can induce fire and explosion hazards. Such incidents have been reported by many waste sorting plants in France. This work analyses potential of hazard associated to full or empty aerosols recycling. Three types of hazardous scenarios have been identified: 1) an ignition of an explosive atmosphere, 2) a BLEVE occurrence and 3) a liquid pool fire. In each case, the potential effects have been quantified. Then, technical and organizational solutions are proposed to prevent these fires and explosions and to protect waste sorting plants, regarding hazards associated to the treatment of aerosols.

OVERVIEW

Products packaged in aerosol propellant form include 1) a liquid base containing the active ingredient dissolved in a flammable solvent or in water and 2) a propelling gas, which is often liquefied petroleum gas (LPG) (iso-butane, butane and/or propane) or dimethyl ether. Some hazardous properties of these substances are listed in tables 1 and 2. Flammable propellant replaced the chloro-fluoro-carbons (CFCs) nonflammable, but that were progressively banned from the market due to their ozone depletion potential in recent past. .

Table 1. Available information on the hazardous properties of most common flammable propelling

gases [START_REF]Modélisation d'un incendie affectant un stockage d'aérosols, Rapport -4, Méthodes pour l'évaluation et la prévention des risques accidentels[END_REF] to [ Due to fire and explosion risks, aerosols have always been considered as hazardous by safety regulations. Indeed, aerosols are classified as "flammable aerosols" on the basis on the following criteria: Fraction of components classified as "flammable", as defined in the CLP regulation [START_REF]Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing[END_REF]: it can be "Flammable Liquids" (if the flash point is less than 93 °C), "Flammable Gases" or "Flammable solids". The composition threshold is 1 %.

Heat of combustion.

Tests results: foam flammability test for a foam producing aerosol can, or ignition distance test and enclosed space ignition test for a spray forming aerosol can. These test methods are described in the UN Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria [START_REF]UN Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria[END_REF], Part III, sub-sections 31.4, 31.5 and 31.6. Table 3. Label elements for flammable aerosols [START_REF]Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing[END_REF] As they are usually made of steel or aluminum, aerosols can consumed at home are considered in France as household packaging waste. The process requires several operations comprising handling, storage, crushing in a press. This latter operation leading indirectly to perforation of the aerosols cans.

Although they are mixed with other aluminum or steel waste, their quantity in the overall metal waste mix is not negligible. For example, the proportion of aerosols in the aluminum packaging waste is about 20 to 30% of the overall weight. In an aluminum waste ball of 1.5 m 3 compressed at 330 kg/m 3 , the estimated number of aerosols is 1600 [START_REF] Company | Rapports d'analyses de composition d'aluminium issu de collectes sélectives[END_REF]. Even if it is emptied by the user, an aerosol still contains residual gas or liquid fractions. The estimated residual weight filling rate is about 2% of initial contents of commercially available products [START_REF] Smith | The use of can flatteners in MRFs for processing material including post consumer aerosolsa risk assessment[END_REF] and [START_REF] Smith | Health and safety issues in port-consumer aerosol container recycling[END_REF]. Moreover, it is not uncommon to find in the waste not emptied aerosols. It is why the press operation, and more generally the storage and the treatment of waste containing aerosol, generates fire and explosion risks that need appropriate consideration.

HAZARD POTENTIAL

Numerous incidents and accidents have occurred on many waste sorting plants in France. Events identified are fires in press, at the output of waste balls, and explosions in press compression chamber. The impact of these events was sometimes significant: pressure effects, fires spreading and other domino effects have been observed, nevertheless no injury was reported. This work analyses potential of hazard associated to full or empty aerosols under recycling operations. Three potential hazardous situations have been identified: 1) an ignition of an explosive atmosphere, 2) a BLEVE phenomenon and 3) a liquid pool fire. In each case, the potential effects have been quantified.

Ignition of a vapour/air explosive atmosphere

The volume of an explosive atmosphere is related to the lower flammability limit and the amount of vaporized fuel mixed with air. Table 4 shows that in most cases, the volume of residual propellant gas content in an empty aerosol is enough to constitute a hazardous explosive atmosphere. The volume of gas required to obtain a significant effect in case of an ignition is very low. Indeed, the volume affected by thermal effects induced by the ignition of such an explosive atmosphere can be estimated to 10 times the initial explosive atmosphere volume. For example, the ignition of 10 ml of iso-pentane at 1.4 % vol. in the air may produce thermal effects in a more than 1 m radius sphere.

However, the probability that the gas expulsed is mixed with the air to reach the lower flammability limit is not very high. Actually, the flammable gas concentration is inhomogeneous and this is likely to reduce the actual explosive atmosphere volume obtained. When aerosols are not completely emptied, the explosive atmosphere volume is also considerably influenced by the flammable liquid or liquefied flammable gas flow due to the crushing operation, which vaporizes as a function of its volatility.

BLEVE

Distances of to which significant effects due to a BLEVE phenomenon may be observed are depending on the weight of product involved. These effects were evaluated for an aerosol propellant containing a few hundred ml of liquefied flammable gas and with a maximum internal pressure resistance of 21 Bar: the fireball generated is about 5 m in diameter and has an overall duration of 1 s. Side effects related to fragments projections can be much larger. This must however be tempered by the fact that, in principle, aerosols crushed in the process essentially contain a small amount of liquid. Indeed, if no liquefied gas is present in addition, the BLEVE phenomenon cannot occur.

Flammable pool fire

The distances of thermal effects related to a pool fire scenario can be estimated on the basis of the pool surface and some characteristics of the liquid, such as its specific combustion rate and the flame emissivity. These effects are more important in the case of propellants (e.g. butane, propane or iso-butane) than in the case of solvents (e. g. ethanol or methanol). Indeed, in the case of a 1 m diameter pool, significant burns are reached at about 1 m for solvents and 5 m for the propellants.

FIRE AND EXPLOSION RISKS MITIGATION

One of the aims of this study is to identify technical and organizational solutions to prevent these fires and explosions and to protect waste sorting plants, regarding hazards associated to the treatment of aerosols. Some of them have been listed in [START_REF] Smith | The use of can flatteners in MRFs for processing material including post consumer aerosolsa risk assessment[END_REF] and [START_REF] Smith | Health and safety issues in port-consumer aerosol container recycling[END_REF].

Prevention of a flammable pool and/or an explosive atmosphere formation

In order to reduce the probability of a fire or an explosion, one workable way consists in controlling the flammable substances overall emission release potential. By contrast, preventing the occurrence of ignition of a flammable atmosphere is not feasible in the crushing process. Indeed, several safety measures can be considered without modifying the sorting plant processes.

Prohibition of not completely emptied aerosols from entering the crush process Introduction of control procedures at the most appropriate steps for collecting, receiving and sorting wastes can help the verification that aerosol propellants does not contain residual liquid. Generators containing liquid should not be directed in the press. When aerosols contain residual liquid or if they are not empty, a safe drain step is necessary. This could be done manually in an inerted fume hood or eventually in a special automatic device. In both cases, liquid and gaseous effluents must be treated to prevent their release into the environment.

Prevention of the liquids accumulation on the bottom of the press: It is recommended to alternate the type of wastes treated in a press, so as to avoid sequences which lead to processing series of balls of metal containing aerosols within a row. A retention vat located at the bottom of the press would help to collect the liquid flow and reduce the potential surface of the pool. This vat must be cleaned at regular intervals.

Aspiration and dilution of flammable gases in the press: A vapour extraction system can be implemented into press compression chamber. The extraction flow must be adapted to the amount of aerosols contained in the wastes. This extraction system must be shutdown in case of fire detection.

Mitigation of the detrimental effects of a fire or an explosion

Effects of fires or explosions that could occur must be controlled to ensure the health and safety of workers. The spread of a fire can be avoided by using some safety measures: Any combustible material must be kept in a remote position from the press. This includes waste bales waiting for evacuation. The minimum safe distance is estimated to 8 m. Another solution can be the protection by a protection wall made of incombustible material (concrete…) between combustible materials and the press.

A water injection system is recommended into the press compression chamber and into the press supply column. This water injection could be combined with automatic fire detection and emergency procedure like powering off the crush operation.

Organizational safety measures

Technical solutions described above must be completed by organizational measures: Any intervention in or near the press must be prohibited during the crush operation of aluminum wastes. These hazardous locations around the press must be clearly identified, and marked by visual signs and not reachable (or rendered as normally prohibited areas).

Finally, operators must be informed about hazards and risks linked to the crushing of metals wastes containing aerosols. They must also be trained to have an appropriate response in case an incident occurs like a fire or an explosion into or near a press.

CONCLUSION AND PROSPECTS

Evaluation and mitigation of fire and explosion risks generated by the recycling of metal wastes containing aerosols is particularly important insofar as the amount of waste to be recycled is still increasing. Moreover, labor regulation entails general requirements from employers that the explosion and fire hazards are assessed and that appropriate measures are taken to ensure worker safety.

Table 2 .

 2 Available information on the hazardous properties of most common flammable solvents

		5]
	Butane	Iso-butane	Propane	Dimethylether

Table 4 .

 4 Calculation of maximal explosive atmosphere volumes according to the nature and amount of fuel

			Estimated volume (l) of the explosive atmosphere corresponding to:
	Fuel	LEL (%)	230 ml of vapour (average	1000 ml of vapour (maximal
			volume of an aerosol propellant)	volume of an aerosol propellant)
	Butane	1.8	12.78	55.56
	Iso-butane	1.8	12.78	55.56
	Propane	2.4	9.58	41.67
	Dimethylether	3.4	6.76	29.41
	Iso-pentane	1.4	16.43	71.43
	Ethanol	3.3	6.97	30.30
	Methanol	6.0	3.83	16.67
	Iso-propanol	2.3	10.00	43.48


Maxime Beauchamp

Laure Malherbe

Laurent Létinois

Chantal De Fouquet

Spatial representativeness of an air quality monitoring station. Application to NO 2 in urban areas

Keywords: geostatistics, kriging, spatial representativeness, nitrogen dioxide (NO 2 )

The present study aims at setting up a geostatistical methodology that could be implemented in an operational context to assess the spatial representativeness of a measurement station. In the proposed definition, a point is considered as belonging to the area of representativeness of a station if its concentration differs from the station measurement by less than a given threshold. Additional criteria related to distance or environmental characteristics may also be introduced.

Concentrations are first estimated at each point of the domain applying kriging techniques to passive sampling data obtained from measurement surveys. The standard deviation of the estimation error is then used, making a hypothesis on the error distribution, to select the points, at a fixed risk, where the difference of concentration with respect to the station is below the threshold.

The methodology is then applied to NO 2 experimental datasets for different French cities.

Introduction

Local agencies in charge of air quality monitoring are concerned with assessing the geographical areas in which concentrations may be assumed similar to those measured by monitoring stations. Spatial representativeness of a monitoring site is a recurrent notion that appears in European regulatory requirements on air quality but has not been precisely defined so far. A definition will be proposed and its practical implementation will lead to the production of maps to characterize areas represented by the stations.

Application of the method for the background pollution [START_REF]Classification et critères d'implantation des stations de surveillance de la qualité de l'air[END_REF] will be presented and some issues concerning the consideration of a traffic-related pollution model will be discussed.

Materials and Methods

First, an estimation of the NO 2 annual average of the background pollution is provided at each point of the domain applying kriging techniques to passive sampling surveys data. High resolution auxiliary variables, like the NOx emissions density in a 2km radius are also used as external drift.

A first approach to define the area of representativeness of a monitoring station located in is to consider all the sites where the concentrations are sufficiently close to the station measurement, which implies the introduction of a threshold notion [START_REF] Bobbia | Représentativité spatiale d'une station de mesure de la pollution atmosphérique[END_REF][3][4]:

Let's consider the estimation error of the pollution . We don't take the measurement error at the station into account.

(E.2)

A sufficient condition for (E.2) is:

(E.3)
We introduce the statistical risk that the concentration of a point considered in the area of representativeness of differs from the station measurement by more than the given threshold :

(E.4)
Then, making a Gaussian hypothesis on the error distribution, the standard deviation of the estimation error is used to select the points in the area of representativeness:

(E.5)
In this approach, a point can be considered as belonging to several areas of representativeness. So, additional criteria related to distance, minimal deviation of concentration, or environmental features are introduced to make a point belong to a unique station.

Local scale also enables to estimate concentrations taking traffic-related pollution into account: distance to the road, traffic-related NOx emissions, or road traffic informations can be considered to develop and improve a model.

Results

To illustrate the results of the methodology, passive sampling data provided by a survey carried out in the French city of Montpellier in 2007 are used. Figure 1 shows the application of the method on the background pollution for a threshold of 10µg/m 3 and a statistical risk fixed at 10%. Two areas of representativeness can be obtained: a first one for the downtown pollution and a second one for the suburb pollution.

Results can be helpful in providing some recommendations for setting up new fixed monitoring sites. In this case, sampling passive data can be used to find an appropriate site where the concentration of NO 2 is the most representative of the missing information.

Concluding remarks

Application of the method for background pollution using analyzed data of NO 2 annual concentrations produced on national scale shows its sensitivity to the criterion selected to remove intersections between representativeness areas. Stability in time of the areas is also related to variations of concentrations on the domain.

This study underlines the difficulty to set up a reliable traffic-related pollution model and the influence of the passive sampling data location on the quality of the model.

The way of taking account of the error of this traffic-related pollution model could also be discussed in future studies: the introduction of a Gaussian hypothesis as well as the results of Chilès and Delfiner under a continuous and unimodal distribution error [START_REF] Chilès | Geostatistics: Modeling Spatial Uncertainty[END_REF] are envisaged.

  Area of the monitoring site 8005Area of the monitoring site 8016 Area of the new monitoring site

Figure 1 :

 1 Figure 1: Areas of representativeness of the background monitoring sites for the French city of Montpellier in 2007, for a threshold of 10µg/m 3 and a risk fixed at 10%


Laure Malherbe
email: laure.malherbe@ineris.fr

Maxime Beauchamp

Laurent Létinois

Anthony Ung

Chantal De Fouquet

Estimation of the areas of air quality limit value exceedances on national and local scales. A geostatistical approach. 1

Keywords: threshold exceedance, geostatistics, kriging, NO 2, PM 10

Each year Member States have to report to the European Commission on the exceedances of air quality limit values which occurred on their territory. Quantitative information is required about the areas and population exposed to such exceedances. A probabilistic methodology for defining exceedance zones has been developed, based on preliminary air quality mapping. Atmospheric concentration fields estimated by kriging and the corresponding kriging variance are used to identify areas where the exceedance or non-exceedance can be considered as certain and areas where the situation with respect to the limit value is indeterminate. The methodology is applied on national and urban scales focusing on exceedances of PM 10 daily limit value and NO 2 annual limit value. Results are discussed from operational perspectives.

Introduction

In addition to reporting air quality measurement data above limit values, Member States have to provide estimates of the surface and population exposed to the observed exceedances. This study aims at developing a methodology that can be easily implemented both at national level for an overall evaluation of exceedance areas, and at local level for a more detailed assessment.

A two-stage methodology is proposed. It first involves estimating concentrations over the domain of interest and computing the estimation variance. A kriging based mapping approach can be used at that stage. Non-exceedance and exceedance zones are then determined from kriging results, considering the risk of misclassifying a point. The calculation steps are described in section 2. Section 3 provides application examples on national (PM 10 ) and urban (NO 2 ) scales. Improvement issues are discussed in the concluding part.

Materials and Methods

Let LV designate the considered limit value. LV = 40 µg/m 3 for NO 2 or PM 10 annual mean concentrations; LV = 50 µg/m 3 , not to be exceeded more than 35 times per year, for PM 10 daily mean concentrations (Directive 2008/50/EC).

Let Z(x) denote the concentration at location x that has to be compared to LV, Z * (x) its estimate from kriging and σ K (x) the kriging standard deviation. Let us take the estimation error ε(x) into account, conventionally assumed to be a Gaussian process with zero mean and a standard deviation equal to σ K (x):

with (1)
Evaluating whether Z(x) exceeds the limit value can be written as follows:

(2)

In the proposed method, non-exceedance and exceedance areas are delimited from inequality (2), considering a non-detection probability threshold α, which is the risk of x belonging to a non-exceedance zone whereas Z(x) is above the limit value, and a false detection probability threshold β, which is the risk of x belonging to an exceedance zone whereas Z(x) is below the limit value.

If the priority is to keep the number of exceedance points wrongly included in the nonexceedance area as small as possible, then α should be set to a low value whereas a higher value may be allowed for β. Cori (2005) suggests that α be given the classical value of 5% while β could empirically be set to 1/3 to have a moderate risk of false detection. This leads to the following definitions:

non-exceedance zone: {x} such as P[Z(x) > LV] < α for α=5% (3) -exceedance zone: {x} such as P[Z(x) ≤ LV] < β for β=34% (4) q 1-α and q β are the (1-α) and β-quantiles from the standard normal distribution.

The locations satisfying none of those conditions make the "uncertainty zone". In section 3 this formal approach is compared to a more empirical methodology previously developed for identifying exceedances of PM 10 daily limit value and rapidly answering to urgent regulatory requests [START_REF] Malherbe | Evaluation des zones touchées par les dépassements de valeurs limites[END_REF]; GT Zones sensible, 2010).

Results

National level. On French scale, daily PM 10 and annual NO 2 concentrations are estimated on a 1 km x 1km grid by combining surface observations from background monitoring stations with outputs from the chemistry-transport model CHIMERE (resolution : about 10 km). For NO 2 , which is mainly related to local emission sources, additional high resolution variables, precisely NO x emission density and population density within a 2-km radius, are introduced in the kriging as external drift. Figure 1 shows the example of one polluted day during a PM 10 event (April 2009). Results are provided both for the methodology described in section 2 and the more empirical methodology in which only two states are defined (hypothesis (1) being unchanged):

exceedance: {x} such as P[Z(x) ≤ LV] < ߟ with ߟ: false detection probability threshold

(5) qߟ is empirically adjusted by comparing the annual numbers of exceedances estimated by cross-validation at the monitoring sites with the actual observed numbers. In this application qߟ has been set to approximately 0.52, which amounts to defining a falsedetection probability threshold of 70% and taking a cut-off value lower than LV.

-non-exceedance: {x} making the complementary set, i.e. : {x} such as ( 6)

Local level

Exceedance areas defined for NO 2 on national scale are very small since NO 2 exceedances mostly occur at traffic-related sites. On local scale, detailed concentration maps accounting for both background and roadside pollution can be established from passive sampling surveys, using high resolution auxiliary variables and additional information about traffic emissions and distance to the roads [START_REF] Malherbe | Using different spatial scale measurements in a geostatistically based approach for mapping atmospheric nitrogen dioxide concentrations[END_REF].

Results obtained for the French city of Montpellier are displayed in Figure 2. During year 2007 an extensive sampling campaign was carried out in this city by the local agency Air Languedoc-Roussillon. The sampling dataset includes eight 14-day periods of measurement at background and traffic sites.

Concluding remarks

Annual reporting to the European Commission but also the working out of local air quality plans require the delimitation of areas where atmospheric concentrations do not comply with environmental objectives. A first approach was developed with a view to rapidly producing realistic exceedance maps for PM 10 . The identified areas are consistent with observed exceedances but might somewhat be overestimated, especially where or when kriging variance is high. The notion of exceedance and non-exceedance was then formalized making some conventional assumptions due to operational constraints. The advantage of this second approach is that it distinguishes the nondetection and false detection probability thresholds which can be adjusted according to the objectives of the study. However, a remaining issue is the way of addressing the uncertainty area. In the end authorities and decision makers will rather have a single figure (spatial extent of the exceedance) than an interval of values. Among envisaged solutions are the refining of the uncertainty area and its inclusion in the exceedance area.

Figure 1 :

 1 Figure 1: PM 10 . Exceedance of the daily threshold (50 µg/m 3 ) on a highly polluted day.Left: newly formalized methodology. Right: empirical methodology.

Figure 2 :

 2 Figure 2: NO 2 . Left: sampling points in Montpellier -year 2007 (source of the data: Air Languedoc Roussillon). Right: exceedance of the annual limit value (40 µg/m 3 ).

This work was funded by the French Ministry in charge of the Ecology and Sustainable Development.


J Hébrard

E Studer

S Jallais
email: simon.jallais@airliquide.com

V Blanchetière
email: vincent.blanchetiere@gdfsuez.com

O Gentilhomme
email: olivier.gentilhomme@ineris.fr

SAFETY OF HYDROGEN/NATURAL GAS MIXTURES BY PIPELINES: ANR FRENCH PROJECT HYDROMEL

In order to gain a better understanding of hazards linked with Hydrogen/Natural gas mixtures transport by pipeline, the National Institute of Industrial Environment and Risks (INERIS) alongside with the Atomic Energy Commission (CEA), the industrial companies Air Liquide and GDF SUEZ, and the French Research Institutes ICARE and PPRIME (CNRS) have been involved in a project called HYDROMEL. This project was partially funded by the French National Research Agency (ANR) in the framework of its PAN-H program aimed at promoting the R&D activities related to the hydrogen deployment. Firstly, the project partners investigated how a NG/H 2 mixture may influence the modeling of a hazard scenario, i.e. how the addition of a quantity of hydrogen in natural gas can increase the potential of danger. Therefore it was necessary to build an experimental database of physics properties for mixtures. Secondly, effect distances in accidental scenarios that could happen on pipelines have been calculated with existing models adapted to the mixtures. This part was preceded by a benchmark exercise between all partners' models and experimental results found in the literature. Finally the consortium wrote a "good practice guideline for modeling the effects related to the release of natural gas /hydrogen mixture". The selected models and their comparison with data collected in the literature as well as the experimental results of this project, and the main conclusions of the guidelines are presented in this paper.

INTRODUCTION

The development of hydrogen as energy carrier in our society raises issues (storage, production, transportation ...). Within the project HYDROMEL, the partners Air Liquide, GDF SUEZ, CEA, two CNRS laboratories ICARE and PPRIME and INERIS focused on assessing the risks associated with transport of hydrogen, pure or mixed with natural gas. Indeed, the most cost effective solution for large-scale distribution of hydrogen as energy seems now to be the pipeline transport. Either by adding a larger or smaller fraction of hydrogen in existing natural gas networks or specific networks dedicated to hydrogen. For each scenario, the safety and the finding of an economic optimum are key elements for the design of future hydrogen transport systems. It is therefore necessary to assess all risks during transportation and distribution of this gas, particularly related to accidental leakage. In the HYDROMEL project, two main objectives were:

• Study the constraints associated with the addition of hydrogen in natural gas pipelines and the impact on the safety of a high content of hydrogen • Realization of experiments in order to obtain data for the validation of computational models of the hazardous consequences while advancing these models. In the first step, the consortium has achieved a full state of the art in order to identify plausible scenarios. INERIS, GDF SUEZ, Air Liquide and CEA compared their models.

In the experimental part of the project, the Institute ICARE focused on the characterization of the inflammation and explosion for different blends of hydrogen and natural gas. LCD worked on a specific issue concerning the risk of self-ignition in case of high pressure leakage of hydrogen in air.

RISK ANALYSIS

The use of hydrogen as energy carrier requires its transport and distribution by pipelines.

Two sorts of transport solution by pipeline have been considered:

The use of the existing natural gas pipeline network with modifications, by which the hydrogen will be transported mixed with the natural gas or pure in some parts of the network A new network dedicated to pure hydrogen with some specific characteristics of diameters, pressure and materials. The production capacities of hydrogen will condition the possible mass flow into the natural gas network or dedicated hydrogen network, and the injections conditions. The production rate of hydrogen by high temperature electrolysis linked with a nuclear reactor would be in the order of 350 000 m 3 / h. Considering this hypothesis hydrogen would be injected at high pressure (100 bar) into the network (Diameter: 500 mm) dedicated to the transport of hydrogen.

.Hydrogen can be produce by: Steam Methane Reforming (SMR) with CCS1 Gasification (biomass or coal + CCS 1 ) Water electrolysis (coupled with wind or photovoltaic power).

Concerning the transport of natural gas/hydrogen mixtures by pipeline two configurations are envisaged: Injection of hydrogen in the small local NG networks. The hydrogen content in this case is lower than 20% .

Injection of hydrogen in the national NG network. The hydrogen content will be lower than 10% in this case.

Summary of scenarios used in the Safety Studies for transport of hydrogen and natural gas

In order to determine realistic leak scenarios we referenced our work to the methodology used in the French guide GESIP 96/082 . Three types of breach are taken into account:

• a small breach by corrosion,

• a medium size breach,

• and the full bore rupture.

The orientation of the leak flow in the case of full bore rupture has a vertical direction. For small breaches, the release can be inclined or horizontal in the case of overhead pipelines and jets can disturbed by an obstacle.

The studied hazardous phenomena in this work are dispersion, thermal radiations, and overpressures following the inflammation of the release.

In general, the impact of these hazardous phenomena on humans is, considering overpressure, worse for hydrogen and, considering thermal radiations, worse for natural gas. The methodologies used for the pipeline transport safety studies are relatively similar for hydrogen and natural gas. Differences concern the definition of the breach size and the consideration of overhead pipelines or equipments. Another difference is the consideration of the influence of a crater formation on the direction of the gas release. The models used for the estimation of effects distances are also, of course, different.

The geometry of the breach was chosen circular (overestimation of the mass flow rate).

The following transport equipments have been taken into account:

-Pipeline -Switching stations -Crossing rivers, roads ... -Compressor stations online.

The chosen "top events":

-the full bore rupture of the pipeline (aggression by a powerful vehicle or by ground movement including earthquakes),

-medium size breach of 70 mm diameter, corresponding mainly to the attack on the pipeline by a tool used for construction, -a small 12 mm diameter breach of , corresponding mainly to a crack or corrosion, From this analysis, and this choice of scenarios, the partners managed to reach an agreement that resulted in the writing of a "good practice guideline for modeling the effects of a gas natural/hydrogen mixtures release".

GUIDELINE

The main objective of this guideline is to provide some indications for users of the different modeling methods used in HYDROMEL project to calculate H 2 /CH 4 pipelines transport hazard scenario consequences.

We will discuss:

• The physical models developed in the different software used by the project partners (PHAST, PERSEE, CAST3M, ALDEA et EXPLOJET) • The input parameters (weather, pressure, temperature…) • The initial assumptions (homogeneous mixtures methane / hydrogen, perfect gas…). The models used by the partners (the list presented in the following table is not exhaustive) have been tested on few experimental configurations with data found in literature.

Each partner used its own internally developed model and sometimes also commercial software. GDF-SUEZ used its own Persee platform, Air Liquide used Phast 6.53 software and the internally developed ALDEA. INERIS used also Phast 6.53 and its own EXPLOJET model and CEA used the internal CAST3M platform. The detailed models [START_REF] Studer | Benchmark exercises to the safety analysis of hydrogen/Natural gas transmission in pipelines[END_REF] Some of the models have been used out of their validation domain given by their editor. This stresses the need of further developing and validating models for use in new configurations. All models have been developed for pure gases. For example Persee is for natural gas only, Explojet can accept pure hydrogen or pure methane. For commercial softwares (Flacs, Phast), mixing laws are already integrated.

Source term calculations: this aspect is relatively well controlled for high pressure gas releases (pure or mixtures). The two main difficulties are:

1. The choice of the state equation: it is advisable to choose a real gas equation (Abel Noble and Van der Waals for example) when the pressure reaches 70-80 bar. For lower pressures, the assumption of perfect gas gives satisfactory results, especially for methane.

2. Pressures losses evaluation in the network (choice of the discharge coefficient for example). In some cases, all the inputs are unavailable, so it is of course advisable to take a small loss in order to increase results (taking a Cd around 0.8 for the release scenarios of gas through an orifice for example).

For mixtures, properties are balanced by ideal laws or Le Chatelier laws with masses or molar fractions of each pure gas.

NB: the software Persee has not been used for the calculation of mixtures.

When the breach is small compared with the pipe diameter, the mass flow rate is considered as constant. All partners found very similar results because the calculation then depends principally on the equation of state and the choice of the discharge coefficient.

For the full bore rupture of a pipeline, the pipeline depressurization can be modeled (this is not necessary for the other breaches). Experiments [START_REF] Cliff | Mass flow rate measurements from ruptured high pressure gas pipelines[END_REF] have been run by Shell to measure the mass flowrate from a pressurized air pipe suddenly open at one end. The test conditions are summarized in Figure 1 (left), the mass flow rate evolution is plotted in Figure 1 (right). All models used by the partners gave results very close to experimental observations, particularly for the 20 first seconds. For later times, some models tend to underestimate mass flow rate. For consequences analysis mean values over one or two periods are usually considered. The mean mass flow rate for duration of 60 s is assumed to be very close for all models. These models can be used for pure gases and mixtures.

Figure 1: Comparison between all pipeline decompression models

The dispersion: The high pressure release of gas is quite well known in general. A comparison has been made in [START_REF] Studer | Benchmark exercises to the safety analysis of hydrogen/Natural gas transmission in pipelines[END_REF] between the experiments of [START_REF] Roberts | Dispersion of hydrogen from high pressure sources, Hazard XIX, process safety and environmental protection[END_REF] for hydrogen and [START_REF] Birch | Velocity decay of high pressure jets[END_REF] for methane, and the model predictions. For the hydrogen experiments [START_REF] Roberts | Dispersion of hydrogen from high pressure sources, Hazard XIX, process safety and environmental protection[END_REF], an upstream pressure of 31 bar was used associated with an exit diameter of 2.7 mm. For methane experiments [START_REF] Birch | Velocity decay of high pressure jets[END_REF], the pressure was 100 bar with a diameter of 3 mm. Both experiments have been run at room temperature. The resulting axial center line gas concentrations are plotted in Figure 2. The results are within an accuracy of 30 %, which is reasonable for dispersion modeling. Another safety information is the mass of fuel in the flammable cloud. The results presented in [START_REF] Studer | Benchmark exercises to the safety analysis of hydrogen/Natural gas transmission in pipelines[END_REF] shows for hydrogen jet an explosive mass varying by a factor five. However, the associated overpressures are proportional to the cubic root of this quantity and a factor less than two is expected for the overpressure at a certain distance from the ignition point.

Experiments

The pseudo-source models seem particularly adapted for the dispersion of high pressure releases for pure gas and even for mixtures. Even if sometimes this model allows to slightly increasing results. The balance laws allowed keeping coherence between results obtained from pure gas to mixtures.

To take into account the atmospheric conditions in integral models seems superfluous regarding the high rate of establishment of a cloud of light gas at high pressure. However, in the case of large jets oriented vertically, taking into account the effects of a strong side wind and buoyancy phenomena may allow a more accurate simulation.

It should be noted that the software PERSEE was developed specifically for natural gas, the results for hydrogen are then not relevant.

Thermal radiation calculations: natural gas safety is mainly driven by thermal radiations from large scale jet fires. Exclusion distances are usually determined for worst case scenario by explosion phenomenon in hydrogen related accidents. Safety properties of the mixtures should then include each as the above explained features and it is a challenging issue to determine the exact location of the transition between thermal effects is worst or explosion effect is worst. Regarding this important issue, experiments have been conducted within the HYDROMEL project to address properties of large scale H2/CH4 jet fires. A short description of the INERIS test facilities is presented on figure 3. Heat fluxes, temperature and flame lengths have been recorded using thermocouples, thermal and speed cameras. These experimental results with validations of numerical models are presented by [START_REF] Studer | Properties of largescale methane/hydrogen jet fires[END_REF] Practically, the Chamberlain model implemented in PHAST, ALDEA or PERSEE give good results. They are validated on a lot of cases and are considered dependable. PERSEE has been validated on natural gas. The model of [START_REF] Schefer | Spatial and radiative properties of an openflame hydrogen plume[END_REF] (CAST3M, ALDEA) is limited to small jet fires. Its use should be limited to flame length lower than 50 m. The mixture composition should be based on mass weighted properties. It was justified by comparing them with test data. The experiment results [START_REF] Studer | Properties of largescale methane/hydrogen jet fires[END_REF] show the non dimensional flame length in the momentum dominated regime is affected by the composition of the released gas. Pure hydrogen is very close to the results obtained by [START_REF] Schefer | Spatial and radiative properties of an openflame hydrogen plume[END_REF] for vertical jet fire. Visible flame length (figure 4) compares well with the already available results, and engineer correlations have been proposed. Blow-out characteristics are in agreement with the properties predicted by [START_REF] Kalghatgi | Lift-off heights and visible lengths of vertical turbulent jet diffusion flames in still air[END_REF] model. Radiative properties of H 2 /CH 4 jet fires scale with the parameter proposed by [START_REF] Molina | Radiative fraction and optical thickness in large-scale hydrogen-jet fires[END_REF] τ f T 4 f a p . Finally, the phenomenological model developed within the project [START_REF] Studer | Properties of largescale methane/hydrogen jet fires[END_REF] has shown great capabilities to predict the main characteristics of the jet fires: flame length, blow-out velocities and radiant fluxes.

Over pressure calculations: the modeling of the blast effects showed several differences between all partners' calculation methods. This can be explained by the initial assumptions concerning the flame propagation and not really by the dispersion calculations. The pseudo-source models used to calculate the inflammable mass are good and provide very comparable results. The differences come from the over pressure calculations. The three methods are presented below; the partner did not find any agreement to use one of them:

1) The first method consists in considering the entire flammable mass dispersed (i.e. the part of the cloud between LFL and UFL). Then a multi energy strength number of 3 for natural gas or 4 for hydrogen and mixtures are applied.

2) The second method seems to take into account the physical phenomena more accurately. A combustion (variable velocity) model is applied to a spherical volume (the sphere radius equal the jet radius at the ignition point). This modeling method has been validated on experimental data [START_REF] Chaineaux | EMERGE Extended modeling and experimental research into gas explosion[END_REF] for pressure up to 40 bar and breach diameters up to 100 mm for natural gas. Beyond this limit, the turbulence levels in the jet may be large enough to induce very high flame velocities, which would tend to make models out of there validation domain.

3) If the flame velocity model is out of there validation domain, a multi energy strength number of 6 could be used but on a reduced spherical volume able to accelerate the flame (i.e more than 11% for H2) [START_REF] Dorofeev | Flame acceleration and explosion safety applications[END_REF]. This method is generally conservative.

Figure 2 :

 2 Figure 2 : Gas concentration along the center line of the cloud: top -inputs, bottom left: hydrogen jet [12], right: methane jet [6]

Figure 3 :Figure 4 :Figure 5 :

 345 Figure 3: Facility design and example of measured and post-processed flame shape

Table 1 : Presentation of the software and models used by partners

 1 are presented on the following table:

	Partner	Code	Model	Description
	GDF	Persee	Dispersion	Integral model [2] or pseudo source
	SUEZ		Over-pressure	model (CATS)
			Jet fires	Deshaies [3]
				Chamberlain [4, 5]
		Flacs	Building explosion	CFD
		CALDEIRA	Mass flow rate	Real gas EOS for CH 4 and Perfect gas
		3.0		EOS for H 2
	Air	Phast (DNV)	Dispersion	Unified Dispersion model (Gaussian)
	Liquide		Jet fires	Chamberlain [4,5]
			Over-pressure	TNO multi-energy
		ALDEA	Dispersion	Birch [6]
			Jet fires	Schefer [7]
		Flacs (H 2	Building explosion	CFD
		version)
	INERIS	Phast (DNV)	Dispersion	Unified Dispersion model
			Jet fires	Chamberlain [4,5]
			Over-pressure	TNO multi-energy
		Explojet	Dispersion	Analytical formulas for gas
				concentration decay
			Over-pressure	In-house method (constant volume
				explosion and acoustic wave source)
	CEA	CAST3M	Dispersion	Integral model [8]
			Jet fires	Pseudo source model [6]
			Over-pressure	Schefer [7]
			Building explosion	Dorofeev [9]
				CFD

CCS = CO

Capture and Storage2 le Guide GESIP 96/08 « Méthodologie pour la réalisation d'une étude de sécurité concernant une installation de transport (hydrocarbures, gaz, produits chimiques) »

ACKNOWLEDGMENTS

The authors would like to thank Dr. C. Proust for helpful discussions during the preparation of the manuscript. This work has been supported by French Research National Agency (ANR) through Plan d'Action National sur l'Hydrogène et les piles à combustible program (project HYDROMEL n°ANR-06-PANH-001).

NB : From [START_REF] Sorin | Deflagration to Detonation Transition in Binary Fuel H2/CH4 with Air Mixtures[END_REF] it was found that the introduction of a small amount of CH4 (x ≥ 0.8, i.e. ratio of CH4 less than 20% in the fuel) desensitizes the mixture compared to H2 -Air mixture and increases the run-up distance L DDT to obtain transition to detonation.

The poor number of experimental data on the blast effects did not allow the partners to conclude. Therefore, the models cannot be validated and further experimental work is needed to conclude on the appropriate approach for modelling H 2 -CH 4 mixture explosion.

CONCLUSIONS

The two main objectives of the HYDROMEL project were to:

• Study the constraints associated with the addition of hydrogen in natural gas pipelines and the impact on the safety of a high content of hydrogen • Produce experimental data to validate computational models of the hazardous consequences while advancing these models. In this work a practical modeling guideline has been realized which identifies a few methods and models for calculating effects (thermal and over pressure) following a breach on pipelines using hydrogen/natural gas mixture. The different dispersion models showed a good accordance with experimental data found in the literature. The experimental set-up allowed partners to validate their radiation models on H 2 /CH 4 mixtures. Finally, all the partners agreed on the modeling methods, even for mixtures; except for blast effects. Indeed, no database is being available for blast effects that would have allowed partners to validate their models. It is necessary to undertake additional experimental work to reach an agreement on modeling these effects.


Jérome Daubech
email: jerome.daubech@ineris.fr

Christophe Proust
email: christophe.proust@ineris.fr

Didier Jamois
email: didier.jamois@ineris.fr

Emmanuel Leprette
email: emmanuel.leprette@ineris.fr

Dynamics of vented hydrogen-air deflagrations

The use of hydrogen as an energy carrier is a real perspective for Europe since a number of breakthroughs now enable to envision a deployment at the industrial scale. However some safety issues need to be further addressed but experimental data are still lacking especially about the explosion dynamics in realistic dimensions. A set of hydrogen-air vented explosions were thus performed in two medium scale chambers (1 m 3 and 10 m 3 ). Homogeneous mixtures were used (10% to 30% vol.). The explosion overpressure was measured inside the chamber and outside on the axis of the discharge from the vent. The incidence of the external explosion is clearly seen. All the results in this paper and the predictions from the standards differ greatly meaning that a significant effort is still required. It is the purpose of the French project DIMITRHY to help progressing.

Introduction

Especially along the last decade a growing interest in the potential uses of hydrogen as an energy carrier has been observed. A significant impulse was given by international organisations like IEA [1] overseas and by the European network HySAFE [2] by organizing networking, promoting research projects. Important progresses were made. Besides a number of technical achievements, the main outcome of this past effort may be that the safety issues could be mastered so that a manageable hydrogen economy could appear. Today, R&D activities are still going in close connection to practical applications via more industry targeted projects like HyPE [3], DIMITHRY [4] and H 2 E [5] in France. Safety issues still constitute the red line of these programmes but looking for practical solutions. DIMITRHY, in particular targets on the development of explosion mitigation techniques for stationary H 2 fuel cell systems. Explosion venting is clearly an option which needs to be efficient if all other defence lines fail. The subject of explosion venting has received a large attention during the second part of the last century. The flame theory was applied to this matter and equations, graphs were issued [START_REF] Simmonds | The design of explosion reliefs for industrial drying ovens, 1 st[END_REF][START_REF] Runes | Explosion venting[END_REF][START_REF] Rust | Explosion venting for low pressure equipment[END_REF][START_REF] Bradley | The venting of gaseous explosions in spherical vessels I-theory[END_REF][START_REF] Swift | Gaseous combustion venting -a simplified approach[END_REF][START_REF] Chippett | Modeling of vented deflagrations[END_REF][START_REF] Cates | A simple assessment methodology for vented explosions[END_REF][START_REF] Molkov | Dynamics of gas explosions in vented vessels : a critical review and progress[END_REF][START_REF] Wu | A mathematical model for vented explosions in a cylindrical chamber[END_REF][START_REF] Puttock | Explosion assessment in confined vented geometries. SOLVEX large scale explosion tests and SCOPE model development[END_REF]. Some appear in official documents [START_REF]Gas explosion venting protective systems[END_REF]18]. However, the models developed for this purpose are based on simplifying assumptions and even if a number of physical phenomena are tentatively introduced like the evolution of the flame shape as function of the geometry of the vessel [START_REF] Runes | Explosion venting[END_REF][START_REF] Bradley | The venting of gaseous explosions in spherical vessels I-theory[END_REF][START_REF] Wu | A mathematical model for vented explosions in a cylindrical chamber[END_REF], the hydrodynamic instabilities [START_REF] Chippett | Modeling of vented deflagrations[END_REF][START_REF] Puttock | Explosion assessment in confined vented geometries. SOLVEX large scale explosion tests and SCOPE model development[END_REF], the turbulence of the flow ahead of the flame [START_REF] Swift | Gaseous combustion venting -a simplified approach[END_REF][START_REF] Cates | A simple assessment methodology for vented explosions[END_REF][START_REF] Puttock | Explosion assessment in confined vented geometries. SOLVEX large scale explosion tests and SCOPE model development[END_REF], the characteristics of the vent cover (inertia, discharge coefficient) [START_REF] Molkov | Vented gaseous deflagrations: modeling of translating inertial vent covers[END_REF]... Despite these refinements, venting equations are numerous tend to produce conflicting results [START_REF] Razus | Comparison of empirical and semi-empirical calculation methods for venting of gas explosions[END_REF] suggesting several phenomena may not yet be well understood or correctly accounted for. A set of excellent papers [START_REF] Solberg | Observations of flame instabilities in large scale vented gas explosions[END_REF][START_REF] Van | Venting of gas explosions in large rooms[END_REF][START_REF] Cooper | On the mechanisms of pressure generation in vented explosions[END_REF][START_REF] Bimson | An experimental study of the physics of gaseous deflagration in a very large vented enclosure[END_REF][START_REF] Chow | An experimental study of vented explosions in a 3:1 aspect ratio cylindrical vessel[END_REF] suggest that flame instabilities or different nature (Taylor, hydrodynamic, acoustic...) play a great role and that in particular the external combustion of the cloud in front of the vent [START_REF] Palmer | External pressures caused by venting gas explosions in a large chamber[END_REF][START_REF] Harrison | External Explosions" as a result of explosion venting[END_REF][START_REF]Scale effects on the external combustion caused by venting of a confined explosion[END_REF] interacts. In fact, the degree of interaction is very strong especially at large scale with large "vent" ratios [START_REF] Proust | The dynamics of vented gas explosions[END_REF]. Given the inherent complexity of flame instabilities [START_REF] Bychkov | Dynamics and stability of premixed flames[END_REF], it is not so surprising that accurate prediction of "vented explosion" remains a difficult task even using complex CFD modelling [START_REF] Bauwens | Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures[END_REF]. Because hydrogen-air flames tend to be more unstable than many fuels, vent dimensioning for H 2 systems is a challenging question. The difficulty is amplified by a severe lack of experimental data. The purpose of the present paper is to provide additional experimental results about vented hydrogen-air explosions in vessels of industrial sizes.

Available data

To the knowledge of the present authors, only a few articles present experimental results about vented hydrogen-air explosions [START_REF] Bauwens | Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures[END_REF][START_REF] Pasman | Design of pressure relief vents[END_REF][START_REF] Kumar R | Vented explosions of hydrogen-air mixtures in large volumes[END_REF][START_REF] Tanaka | Experimental study on hydrogen explosions in a full scale hydrogen filling station model[END_REF] and even a more limited number are directly usable ( [START_REF] Bauwens | Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures[END_REF][START_REF] Pasman | Design of pressure relief vents[END_REF][START_REF] Kumar R | Vented explosions of hydrogen-air mixtures in large volumes[END_REF] As mentioned before there are a number of very diverse methods to calculate vent sizes (see for instance [START_REF] Razus | Comparison of empirical and semi-empirical calculation methods for venting of gas explosions[END_REF]). In two recognised guidelines [START_REF]Gas explosion venting protective systems[END_REF]18], the same equations are proposed :

( ) ( ) [ ] ( ) [ ] 750 0 ; 2 max 1 . 0 126 . 0 0567 . 0 log 127 . 0 2 3 2 572 . 0 582 . 0 10 - ⋅ + ⋅ - ⋅ ⋅ + ⋅ - ⋅ = - - D L K V P P P K A g stat red red g vent
• A vent : vent area in m 2 • P red , P stat , maximum (reduced) explosion overpressure and static opening overpressure of the vent panel in barg • Kg explosion severity index1 • L/D length over diameter ratio of the vented vessel • V volume of the vented vessel in m 3

Experimental details

Two experimental chambers were used (Figure 1 and2) of 1 and 10.5 m 3 with an L/D ratio of respectively 1.4 and 3.3. Only one vent area was arranged on the 1 m 3 vessel leaving the flange opposite to the ignition location fully open which corresponds to a vent area of 0.15 m 2 . Similarly, on the 10.5 m 3 chamber, only one vent area was arranged by leaving the flange opposite to the ignition location totally open which corresponds to a vent area of 2 m 2 .

The fuel-air mixture was prepared by injecting pneumatically the fuel from the content of pressurised bottles (1 of 5 l for the 1 m 3 vessel and 4 bottles of 8 l for the 10 m 3 chamber). The final composition was carefully checked by measuring precisely the oxygen content prior to ignition. A sheet of paper was lightly taped on the vent area in order to prevent the hydrogen mixture to escape before ignition. It was verified that the mixture was homogeneous by measuring the gas concentration at several locations into the chambers with accuracy around ± 0.5 % vol.

Ignition was performed with a pyrotechnical match 2 , releasing 60 J, located on the axis of the chamber, at the opposite end to the vent. The evolution of the cloud and flame outside was recorded by high-speed video (500 to 2000 f/s). In order to ease the observation of the cloud, the gaseous mixture inside the chamber was seeded with fine silica particles (order of 1 g/m 3 ).

Results

Experiments were done varying the initial pressure into the dispersion reservoir. The main results are given in table 3 but some further details are given hereafter.

Table 2 : main results of the experiments

The pressure traces and flame trajectories for the typical tests in the 1 m 3 vessel are presented in figure 5. The internal overpressure trace has a classical shape with a single dome. It is interesting to note that the overpressure does not drop when the flame rushes out but continues to rise. Note that as soon as the flame exits, the overpressure measured just on the axis of the vent at 1 m drops sharply. This is a local phenomenon since it does not appear further downstream. But after this initial pressure drop (at 1 m on the axis of the vent discharge) a strong pressure burst is superposed. This pulse appears also at 3 and 5 m although with a smaller amplitude (it can be verified that the time difference between these pulses correspond to the speed of the sound). The magnitude of this burst is larger the richer the mixture. It is not detectable when %H 2 =10. After this pressure burst the pressure inside the explosion chambers drops (taking into account the time delay corresponding for the trailing edge of the pulse to join the pressure transducer : about 5 ms at the speed of the sound).

The pressure traces and flame trajectories for the typical tests in the 10.5 m 3 vessel are presented in figure 6. Much more complex signals are produced : there is a first pressure bulge lasting two thirds of the propagation of the flame in the chamber (up to 0.35 s for test 13 and 0.12 s for test [START_REF] Molkov | Vented gaseous deflagrations: modeling of translating inertial vent covers[END_REF]. The pressure at the end of this period drop to zero and then rises sharply just before the flame rushes out. After this several peaks appear with a period of 20-40 ms corresponding to the first acoustic mode of the chamber ("organ pipe"). Peaks with a much smaller period (2 ms) are also visible on the traces of test 16. It seems to correspond to the natural vibration of the metallic envelope of the chamber rather than to any acoustic mode of the chamber [START_REF] Dorofeev | Transition to detonation in vented hydrogen-air mixtures[END_REF].

Discussion and conclusion

The main objective of this paper is to provide additional data about explosion venting of hydrogen air mixtures. Tests were performed in a 1 m 3 vessel and in a 10.5 m 3 totally vented at one extremity. In addition to the pressure trace presented herein, high speed films were performed which results would certainly add. For instance, the pressure bursts seen on the pressure trace measured on the axis of the vent for the 1 m 3 vessel can clearly be attributed to a significant external explosion. This burning outside of the vent prevents for a while the discharge of the burned gases from the vessel as suggested earlier by Cooper at al. [START_REF] Cooper | On the mechanisms of pressure generation in vented explosions[END_REF]. This combustion also occurs for the larger vessel some of the late oscillations.

Finally, it was attempted to compare all the results in this paper to the predictions from the standards (Table 3 and figure 7). As expected the actual data differ greatly from the prediction (NFPA 68) meaning that a significant effort is still required. It is the purpose of the French project DIMITRHY to help progressing. To this end another type of experimental device enabling the full visualization of the explosion is being prepared.

Test 10.5-16 (23%H2)

Reference

Volume (m3) Vent area (m2) Measured (bar) [START_REF] Bauwens | Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures[END_REF] 64 2.700 0.32 [START_REF] Bauwens | Vented explosion overpressures from combustion of hydrogen and hydrocarbon mixtures[END_REF] 64 5.400 0.13 [START_REF] Pasman | Design of pressure relief vents[END_REF] 0.95 0.200 1.25 [START_REF] Pasman | Design of pressure relief vents[END_REF] 0.95 0.300 0.4 [START_REF] Kumar R | Vented explosions of hydrogen-air mixtures in large volumes[END_REF] 6.85 0.018 4.8 [START_REF] Kumar R | Vented explosions of hydrogen-air mixtures in large volumes[END_REF] 6

2

  Pyrotechnical match is a good ignition source to detect the initial time and seems not to disturb flame development during the initial times

Figure 1 :Figure 2 Figure 3 :Figure 4 :

 1234 Figure 1 : sketch (dimensions in mm) and views of the 1 m 3 vessel (injection device of the fuel on the right picture)

Figure 5 :

 5 Figure 5 : Flame position, internal and external overpressures (1 m 3 vessel, rear ignition, 0.15 m 2 vent, % H 2 in air = 10; 15; 20 and 27)

Figure 6 :

 6 Figure 6 : Flame position, internal and external overpressures (10.5 m 3 vessel, rear ignition, 2 m 2 vent, % H 2 in air = 14; 23)

Figure 7 :

 7 Figure 7 : experimental data and prediction with the standards

Table 3 :

 3 Characteristics of experimental data

		10
		9
		8
		7
	Overpressure -bar-	4 5 6								Measured Predicted
		3
		2
		1
		0
		[30]	[30]	[31]	[31]	[32]	[32]	[32]	present	present

Kg = 550 bar.m/s for hydrogen-air explosions[18]


V Bert

Del Grata

R 1 Gaucher

Study of the potential valorization of metal contaminated Salix via phytoextraction by combustion

Keywords: trace elements, phytoextraction, Salix, combustion, dredged sediment landfill site

Phytoextraction is an emerging technology to remediate soils and sediments contaminated with trace elements. In the particular case of dredged sediment landfill sites contaminated with metals, phytoextraction may contribute to the management scheme of sediments and to the revaluation of certain land-disposed contaminated sediments. In this study, rapidly growing trees (willows) were cultivated in short rotation coppice (SRC) on a metal contaminated dredged sediment landfill site to produce biomass for heat and financial return. Three willow clones were planted on 4.5 hectares of this disposal site to assess phytoextraction efficiency and biomass production. Two combustion assays were performed in a biomass boiler of 30 KW, the first one with Salix Zn and Cd enriched wood harvested from the SRC and the second one with Salix wood bought at a wood producer (control). Combustibles, ashes and gases were analyzed to study trace element behaviour during the combustion process. For phytoextraction efficiency, best results are obtained when exportation of both stems and leaves are carried out. The most optimistic scenario led to a reduction of the total Cd content of the sediment from 3.16 to 2 mg kg -1 in 23 years. Due to boiling points, Cd and Zn were more found in fumes than in ashes. According to current regulation, Salix wood issued from phytoextraction should be used for non-domestic purpose, in industrial or collective boilers equipped with efficient filters.

Introduction

Phytoextraction is an emerging technology to remediate soils and sediments contaminated with trace elements [START_REF] Vangronsveld | Phytoremediation of contaminated soils and groundwater: lessons from the field[END_REF]. In the particular case of dredged sediment landfill sites contaminated with metals, phytoextraction may contribute to the management scheme of the sediments and to the revaluation of certain land-disposed contaminated sediments (Bert et al., 2009a). In this case, phytoextraction may be combined with bioenergy production to provide a benefit to the land owner and, at the same time, to guarantee pollutant impact control according to current regulation. In the last years, different energy-recoverytechniques (incineration, combustion, gasification, pyrolysis, anaerobic digestion and pure plant oil production) using metalaccumulating bioenergy plants were investigated for this purpose [START_REF] Keller | Thermal treatment of metalenriched biomass produced from heavy metal phytoextraction[END_REF][START_REF] Van Ginneken | Phytoremediation for heavy metalcontaminated soils combined with bioenergy production[END_REF][START_REF] Stals | Flash pyrolysis of heavy metal contaminated biomass from phytoremediation: Influence of temperature, entrained flow and wood/leaves blended pyrolysis on the behaviour of heavy metals[END_REF]. The aim of this study is to assess phytoextraction efficiency of Salix on a metal contaminated dredged sediment landfill site and perform a combustion experiment with a Salix SRC sample to see the viability of the overall process, i.e. phytoextraction combined with reuse of metal enriched biomass.

Materials and Methods

In 1998, a short rotation coppice (SRC) of three willow clones (Salix viminalis "Tora" Salix schwerinii x Salix viminalis "Jorr" and "Björn") was set up on 45,000 m² of a metal contaminated dredged sediment landfill site located in the North part of France. The SRC was harvested in 2001 to assess phytoextraction efficiency and biomass production. In 2006, trees were harvested and the crushed wood left on site. In April 2010, based on a sediment characterization of the SRC area, samples of sediments and above ground parts of willows were taken in the most contaminated zone. Five samples of willows and associated sediments were taken for metal analysis. Two combustion experiments were performed with two different Salix woods. One was harvested on the sediment disposal site (named "phytoextraction") whereas the second was bought at a wood producer (named "control"). Woods were dried at 40 °C in a forced air oven until a moisture content of 15 %, thinned out the foliage of, ground and riddled (35 mm). In June 2010, combustion experiments were carried out with a biomass boiler of 30 KW, built in a similar manner as higher boilers. Sixty three kilograms of "control" and 54 kg of "phytoextraction" were burnt. Combustibles, ashes and gases were analyzed to study trace element behaviour during the combustion process.

Results

Time needed to reduce Cd concentration in the upper 50 cm from 3.16 to 2 mg kg -1 sediment should be 23 years. This result is obtained supposing a linear decrease of Cd content and with exportation of leaves and stems. Cd and Zn concentrations in "control" and "phytoextraction" combustibles, ashes and gases are reported in Table 1. As expected, Cd and Zn concentrations in "phytoextraction" combustible are higher than in "control". For "phytoextraction" and "control" combustibles, Cd and Zn are more found in the particle fraction of gases than in ashes. Indeed, in ashes, Cd represents less than 1% of Cd combustible content and Zn around 5% of Zn combustible content. In gases, Zn concentrations exceeded 100% mainly due to analysis uncertainties. (2009b) on Arabidopsis halleri who performed incineration assays.

In the absence of metal emission limit values for boiler < 2 MW, trace elements in gas emissions were compared with emission limit values for 20 MW combustion plants. Cd and Zn exceeded these values for "phytoextraction". As a consequence, gases issued from the combustion of Salix wood should be treated (e.g. efficient filters) to avoid air pollutant impact. In addition, such wood should be used for non domestic purpose, in industrial or collective boilers. From the phytoextraction point of view, the best scenario will be to harvest Salix before leaves fall. Nevertheless, combustion of leaves and stems may increase metal concentrations in gases and modify combustion performances.

Table 1

 1 Cd and Zn concentrations in combustibles (= fuels), ashes and gases. Salix and Thlaspi sp. and Bert et al.

			Concentrations in
			mg.kg -1 of stems burnt
			Control	Phytoextraction
	Fuels	Cd	1.44	7.34
		Zn	38.01	284.6
	Ashes	Cd	0.02	0.06
		Zn	1.54	15.03
	Gases	Cd Zn	0.96 50.44	6.33 358.1
	Discussion
	For phytoextraction efficiency, best results are
	obtained when harvesting of both stems and
	leaves is supposed. This result is in accordance
	with those reported by Vangronsveld et al.
	(2009) on Salix.
	Inside the boiler, temperature was about
	1000°C. During the combustion assay, the
	boiling points of Cd (767 °C) and Zn (907 °C)
	were reached which explains that most Cd and
	Zn are found in fumes and not in ashes. This
	result confirms those found by Keller et al.
	(2005) on


S Denys

F Dor

C Dabin

Integration of the oral bioaccessibility of trace elements in the Human exposure and risk assessment of contaminated soils

Keywords: oral bioavailability, oral bioaccessibility, human health risk assessment, exposure assessment

Bioaccessibility and bioavailability are often mentioned as parameters that might greatly improve the accuracy of human health risk assessment of contaminated sites. The objective of this paper is to give some insights into the implemention of using the bioaccessibility concept, focusing on the French methodology. In this framework, the assessment of the exposure media and the more classical human health risk assessment method are the two methodologies defined by the regulatory guidelines. The advantages and limits of integrating the bioaccessibility concept are developed. Bottlenecks are also identified to promote the utilization of such parameters more extensively in the management of contaminated sites.

Introduction

Oral bioaccessibility is defined as the fraction of a soil contaminant that is solubilized in the lumen by the digestive fluids. Oral bioavailability is defined as the fraction that has been absorbed and metabolized. The link between bioavailability (BD) and bioaccessibility (BA) can be formalized as follows (equation 1; [START_REF] Denys | Biodisponibilité et bioaccessibilité pour l'Homme des métaux et metalloïdes des sols pollués pour la voie orale : définitions, protocoles de mesure et retour d'expérience international[END_REF] : BD = BA x AB x M = BA x AB equation where M is the metabolization of the pollutant (=1 in the case of trace metals) and AB the absorption of the pollutant through the gastro-intestinal membrane.

These two concepts are often mentioned as parameters that might greatly improve the accuracy of human health risk assessments at contaminated sites. As inclusion of this parameter would allow for a more accurate assessment of the exposure, via the oral route. Numerous in vitro protocols have been designed to assess the bioaccessibility of soil contaminants. Few in vivo protocols have been developed to assess the bioavailability of the contaminants and to validate the physiological accuracy of the bioaccessibility protocols.

Recently the Unified Barge 1 Method was validated for

As, Cd and Pb and the UBM-bioaccessibility was demonstrated to be linked to the soil-metal speciation [START_REF] Caboche | Validation d'un test de mesure de la bioaccessibilité[END_REF]. Due to the number of contaminated sites, the amount of land that is affected and the cost and ethic issues linked to in vivo protocols, bioaccessibility appears to be a more useful parameter to include in a site specific risk assessment. However, despite intensive research on the development of in vitro methods, no clear guidelines have ever been published related to the possible integration of bioaccessibility data in a quantitative human exposure or risk assessment. The objective of this work is to give some insights into the implementation of bioaccessibility data into the French human health risk 1 Bioaccessibility Research Group, Europe assessment methodology. Whereby two regulatory methods might be used: the "assessment of the exposure media" and the more classical quantitative risk assessment. The first method intends to assess the consequence of pollutant emission from a source to its external environment. It consists of a comparison of the concentrations in the different exposure media to either regulatory guidelines or to the local background concentrations.

Material and Methods

For the "assessment of the exposure media" method, the work was undertaken at the field scale on a former mining site located at Saint-Laurent-le-Minier (Gard). Accordingly to the regulatory guidelines, concentrations of As and Pb in the superficial soil layers (0-5 cm) were measured in the residential areas impacted by the contamination. These measures were compared to the concentrations of As and Pb in the background soils. Then the UBM-bioaccessibility measurements of As and Pb were included in the comparison. For the quantitative risk assessment, the work redefined the initial hypothesis that is implicit in the current exposure assessment calculations for the oral route when the bioavailability or the bioaccessibility of the soil contaminants are not considered.

Then, implementation of the bioaccessibility into the dose calculation will be developed. An illustration will be given for As.

Results

In the assessment framework for exposure media, the total concentrations of As and Pb were not statistically significant (P<0.05). In this context and according to the regulatory guidelines, it can be defined that there is no difference in terms of human exposure to As and Pb between the contaminated and the background environment. Following this result, no corrective actions to reduce the human exposure in the contaminated environment are needed. However, this result does not consider the potential differences either in speciation or in contaminant distribution within the soil between the impacted and the background environment. Implementing the method including the bioaccessibility of the contaminants allows for consideration of such differences. In this specific case, and similarly to the total concentrations, the bioaccessibility of As and Pb were not different between the contaminated and the background environment. Thus it confirmed that no corrective action were needed, thereby improving the robustness of this conclusion. In the framework of the quantitative risk assessment method, the human exposure following soil ingestion is calculated as follows (Equation 2 It is currently considered for the exposure assessment that the relative bioavailability of the pollutant between the soil and the reference matrix used to establish the toxicological reference (reference matrix) value is 1.

The relative bioavailability is defined as the following ratio (Equation 4)

BDrelative = BDsoil/BDi equation 4
where BDsoil: soil bioavailability of the contaminant; BDi: bioavailability of the contaminant from the reference matrix

According to equation 1, equation 4 is equivalent to (equation 5): BDrelative = (BA x AB)soil / (BA x AB)i equation 5

Thus in the framework of the quantitative exposure assessment, the improvement of equation 1 requires knowledge of

• either the bioavailiabilities of the pollutants from both the soil and the reference matrix;

• or the bioaccessibility and the absorption of the substances.

In the case of As, the reference matrix is water. It was recently calculated that bioaccessibility of As was close to 1 and that the absorption of As from contaminated soil is equal to the absorption of As from water [START_REF] Caboche | Validation d'un test de mesure de la bioaccessibilité[END_REF]. Thus, in this case, equation 5 can be simplified as:

BDrelative= BAsol equation 6

In this case, the measurement of the bioaccessibility of As in soil using the UBM is an easy way to improve the accuracy of the exposure calculation.

Discussion

Integrating the bioaccessibility measured by UBM in the management of contaminated sites allows for improvement of the exposure assessment step. It permits a better integration of the trace elements speciation or distribution within the soil, that are both known as sensitive parameters that influence their bioavailability. It also better reflects the actual human exposure of soil contaminants than the total concentrations, as the UBM was validated against in vivo experiments [START_REF] Caboche | Validation d'un test de mesure de la bioaccessibilité[END_REF]. However, several bottlenecks do exist before a total integration of the bioaccessibility parameter within the exposure and risk assessment methodologies can be achieved. Firstly, the analytical validation of the UBM should be completed to permit an extensive use of this test by routine laboratories. Second, it is necessary to have more transparency in the construction of the toxicological reference values as precise information on the composition of the reference matrix and data about the bioavailability and the absorption of the element from the reference matrix might be needed.

Conclusion

This paper provides two ways to implement the human exposure and risk assessment regarding the management of contaminated sites. It also allows for the identification of bottlenecks that could be solved by further research programs.

Aknowledgements

Authors acknowledge the French Agency of Environment (Ademe) and Total for funding these researches