Classification procedure of the explosion risk areas in presence of hydrogenrich syngas: Biomass gasifier and molten carbonate fuel cell integrated plant

Fuel 99 (2012) 245–253 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Classification procedure of the explosion risk areas in presence of hydrogen-rich syngas: Biomass gasifier and molten carbonate fuel cell integrated plant A Molino a,⇑, G Braccio a, G Fiorenza a, F.A Marraffa b, S Lamonaca b, G Giordano c, G Rotondo b, U Stecchi b, M La Scala b a Italian National Agency for New Technologies, Energy and Sustainable Development, Trisaia Research Centre, S.S 106 Jonica, km 419+500 – 75026 Rotondella, Matera, Italy Politecnico of Bari, Electrical and Electronics Dept., Via E Orabona – 70125 Bari, Italy c University of Calabria, Chemical and Materials Engineering Dept., Via Pietro Bucci – 87036 Arcavacata di Rende, Cosenza, Italy b a r t i c l e i n f o Article history: Received 24 January 2012 Received in revised form 20 April 2012 Accepted 23 April 2012 Available online 12 May 2012 Keywords: Explosion risk Biomass gasifier Molten carbonate fuel cell a b s t r a c t This paper deals with the safety aspects of a 500 kWth (thermal power) biomass gasification plant coupled with a 125 kWe (electric power) molten carbonate fuel cell In particular, it describes the procedure for assessing the explosion risk in presence of hydrogen-rich syngas and compares the results given by the application of technical standards with those obtained by the implementation of a fluid dynamic model for the potential emission scenarios Ó 2012 Elsevier Ltd All rights reserved Introduction Among several hypotheses of energy development, a now well established prospect look at the hydrogen as an energy carrier The reasons for this choice are essentially due to environmental factors, rather than to the hydrogen use flexibility and, not least, to the uncertainty on supply costs of the existing conventional primary energy sources This rationale has oriented research to develop technologies that allow to directly use hydrogen in energy conversion systems (fuel cells) with high efficiency and low environmental impact; unlike other devices normally used for energy production, this technology gives back only water vapor emissions Assuming to use hydrogen as the energy carrier of the future, a crucial aspect is its production The biomass gasification is of great interest because of its renewable nature In this respect, the ENEA Trisaia Research Centre is involved in developing a 500 kWth biomass gasifier and a 125 kWe molten carbonate fuel cell integrated plant The integration between fuel cell and gasification plant represents a potential path to the electric generation from biomass, increasing the efficiency and lowering the environmental impact (50% CO2 reduction) Based on a fluid-dynamic approach, the explosive atmosphere area has been assessed in order to satisfy a ⇑ Corresponding author Tel.: +39 (0)835 974736; fax: +39 (0)835 974210 E-mail address: antonio.molino@enea.it (A Molino) 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.fuel.2012.04.040 fire engineering performance-based approach In addition, the explosion risk area has been evaluated with the well-known ATEX risk assessment This paper is aimed at comparing both procedures in order to validate the ATEX-based technical standards for the pilot plant located at the ENEA – Trisaia Research Center At the time of this study, no Integrated Gasification Fuel Cell (IGFC) system is operating worldwide but ENEA Trisaia pilot plant As it can be observed from the literature survey, most of the available studies in this field deal with general aspects of the involved technologies and perspectives of their combination [1–3] The biomass gasification pilot plant operating at the ENEA Research Centre of Trisaia exploits a dual fluidised bed (DFB) reactor having 500 kWth capacity The reactor uses steam as gasification agent, so that a fuel gas nearly nitrogen free is produced characterized by a relatively high Lower Heating Value, approaching 13 MJ/ Nm3 on a dry basis The gasification concept is the well-known Fast Internally Circulating Fluidised Bed (FICFB) [4], which was developed since the mid-nineties by the Vienna University of Technology (TUV) and Austrian Energy & Environment (AE&E) on a laboratory test ring (100 kWth) [5] Subsequently, within the scope of the European project ‘‘Hydrogen-rich gas from biomass steam gasification’’ the ENEA Trisaia 500 kWth gasifier was designed, constructed and tested [6] In the framework of the following European project ‘‘Clean energy from biomass’’, an innovative hot gas cleaning section 246 A Molino et al / Fuel 99 (2012) 245–253 Nomenclature a A c cp cv Co hazardous distance in the emission direction (m) surface emission (m2) gas concentration specific heat–constant pressure (J/kg K) specific heat–constant volume (J/kg K) number of the air exchange, referred to the total volume (sÀ1) hazardous distance from a sorgent emission (m) dz DHc combustion heat (kcal/mol) E specific internal heat (J/kg) f efficiency factor fe external force for volume (N/m3) k security factor kdz safety factor applied to the LFL Kx, Ky, Kz gas diffusion coefficients LFL lower flammability limit (m3/m3Á100) UFL upper flammability limit (m3/m3Á100) LOC minimum oxygen concentration M molar mass of flammable substance (kg/kmol) P absolute pressure in the containment system at the point of emission (Pa) was added consisting of an adsorbing reactor for the removal of acid compounds, a hot gas cyclone and a hot gas filter for the removal of coarse and fine particles, respectively [7] The DFB steam gasification pilot plant has been coupled with a molten carbonate fuel cell (MCFC) Under the maximum load conditions characterized by a current of 1100 A, this MCFC can generate 125 kW The stack operating temperature is around 650 °C, in order to avoid the carbonate salt mixture solidification, and the operating pressure is 3.5 bar, in order to create the appropriate fluidodynamic conditions for the system The MCFC technology exploits also the carbon monoxide to produce electricity, this is an important fraction of the producer gas of around 25% of the volume on dry basis [8] The reaction quickly balances because of the high temperature of the stack and a greater amount of H2, flowing into the fuel cell, is available for the following anodic reaction Safety standards for explosive atmosphere risk analysis Areas in which there is a risk of explosion that may harm people or the environment are subject to legal or technical comparable rules in most countries of the world While these rules were initially issued at the national level, in Europe they have since been replaced over the last years by regional European Directives and Standards, and in the field of standardization they have partially been replaced by international regulations 1.1 European directives In 1976, the Council of the European Community established the prerequisite for the free trade of explosion-protected electrical equipment within the European Union by ratifying the ‘‘Directive on the harmonization of the laws of the member states concerning electrical equipment for use in potentially explosive atmospheres (76/117/EEC)’’ This directive has since been adapted to the state of the art by means of national laws and guidelines on electrical equipment ~ q Qamin Qg R S t T Ta ~ v Vz wa yj c u q ry, rz heat flux (W/m2) minimum ventilation mass flow rate (m3/s) maximum flow rate emission of gas/vapor (kg/s) universal gas constant = 8314 J/kmol K stress tensor (Pa) time (s) temperature (K) environment temperature vector of gas velocity whose components are respectively u, v, w (m/s) volume of potentially explosive atmosphere (m3) reference velocity of the air in the considered ambient (m/s) molar fraction specific heats ratio (cp/cv) critical ratio of flow rate density (kg/m3) standard deviation of the wind speed in the transversal and orthogonal direction Complete harmonization and extension to all types of equipment was achieved with the new Directive 94/9/EC in 1994 [9] The Directive 99/92/EC, which regulates operation in hazardous areas and defines safety measures for the concerned personnel, was issued in 1999 [10] In addition to the 94/9/EC Directive, which regulates how explosion-protected equipment and protective systems are placed on the market and the design, construction and quality requirements to be met by them The 99/92/EC Directive stating ‘‘Minimum requirements for improving the health and safety protection of works potentially at risk from explosive atmospheres’’ refers to the operation of potentially explosive installations, and is, therefore, intended for the employer This directive contains only minimum requirements When implementing it into national law, the single states can adopt further regulations Examples are the implementation of the directive into the British law by ‘‘The Dangerous Substances and Explosive Atmospheres Regulations’’ and into the German law by ‘‘The Betriebssicherheitsverordnung’’, the German regulation on Industrial Safety and Health Protection, which takes into account further European directives on safety on work Comparable regulations are found in other European countries According to the 99/92/EC Directive, it is duty of the employer to verify where there is a risk of explosion, classify the hazardous areas into zones accordingly, and document all measures taken to protect the personnel in the so-called explosion protection document 1.2 Assessment of explosion risks When assessing the risks of explosion, the following factors are to be taken into account: À the likelihood that explosive atmospheres will occur and their persistence; À the likelihood of ignition sources, including electrostatic discharges; À the installations which can give rise to an explosion, substances used, processes, and their possible interactions; À the scale of the anticipated effects 247 A Molino et al / Fuel 99 (2012) 245–253 The employer has to classify the areas in which explosive atmospheres may appear into risk-zones, and ensure that the minimum organisational and technical requirements of the Directive are observed Since our assessment refers to gases, we restrict our discussion about ATEX Directives applied to gas explosive atmosphere [10] The zone classification refers to gases only Zone 0: An area in which an explosive atmosphere consisting in a mixture of air and flammable substances in the form of gas, vapor or mist is present continuously or for long periods or frequently Zone 1: An area in which an explosive atmosphere consisting of a mixture with air or flammable substances in the form of gas, vapor or mist is likely to occur in normal operation occasionally Zone 2: An area in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapor or mist is unlikely to occur in normal operation but, if it does occur, will persist for a short period only An explosion protection document has to be generated, which contains at least the following information: À À À À assessment of the explosion risk protective measures taken zone classification observance of minimum requirements This piece of information can be divided into organisational measures (e.g instruction of workers) and technical measures (e.g explosion protection measures) Hazardous areas are classified into zones to facilitate the selection of the appropriate electrical equipment as well as the design of suitable electrical installations Information and specifications for the classification into zones are included in IEC 60079-10 [11] In Italy this assessment procedure is introduced through standard CEI (Comitato Elettrotecnico Italiano) 31-30 The methodology adopted in this paper is based on two guidelines adopted in Italy: (1) Guide CEI 31-35, ‘‘Electrical apparatus for explosive atmospheres Guide for the application of the Norm CEI EN 60079-10 (CEI 31-30)’’ and (2) Guide CEI 31-35/A, ‘‘Electrical apparatus for explosive atmospheres – Guide for the application of the Norm CEI EN 60079-10 (CEI 31-30) Classification of hazardous zones, Examples of application’’ [11,12] We refer to CEI 31-35 since this standard has no equivalent among the IEC standards Furthermore, we have perceived this methodology, as an important tool for the application of IEC 60079-10, since it includes a lot of specific issues not considered there These two guidelines give special features for determination of the type of the zone and for the evaluation of its extension The standard EN 60079-10 does not provide sufficient information about the decision process for classification The Italian Guide CEI 31-35 reports some detailed mathematical formulation on how to proceed to the application of standard CEI 31-30 When the type of the zone has been determined, the Italian methodology include a procedure for checking that the likelihood of the explosive atmosphere in one year and the total duration of the explosive atmosphere in one year (release duration plus time of persistence after the release has ends up) are below some critical values This verification introduces a probabilistic risk-based approach The method is a stepwise process that gives both the type and extension of the zone The guideline contains indications on: (1) the most suitable leakage hole dependent on the type of component (i.e pump/compressor, piping connections, valve, etc.); (2) flow rates for structural/continuous grade gas release as a function of the component type based, on statistical data; (3) flow rates for primary and secondary grade gas release calculated by specific reference formulas; (4) evaluation of the extension of the hazardous zone as a function of the release flow rate, ventilation and flammable substances Plant description The plant under study is installed at the ENEA Trisaia Research Centre It is an experimental pilot plant for the biomass-derivedhydrogen-rich syngas production and consists of a 500 kWth biomass gasifier coupled with a 125 kWe molten carbonate fuel cell [13] Two different areas for storing technical gases used in the process are utilized The first area relates to a plant of storage and vaporization of nitrogen, carbon dioxide and oxygen The second one is devoted to the implementation of a bunker for storing hydrogen in a cylinder, with the relative unit of decompression Fig 2.1 shows an advanced technological platform on biomass gasification is available at ENEA Research Centre of Trisaia, including: (1) two air-blown fixed bed downdraft gasifiers having a fuel capacity of 120 and 300 kW, respectively, with conventional gas cleaning, consisting in filtration units and water scrubber, and combined to an internal combustion engine (ICE); (2) a dual fluidised bed steam gasification pilot plant, having a fuel capacity of 500 kW, with both hot gas cleaning, via an adsorbing reactor and a filtration unit (cyclone plus ceramic filter), and conventional cold gas cleaning; (3) air/steam-blown fixed bed updraft gasifier, having a fuel capacity of 150 kW, with advanced gas cleaning: coalescent filters and bio-diesel scrubber; (4) interconnected fluidised bed steam/oxygen gasification pilot plant, having a fuel capacity of MWth, with catalytic ceramic candles located inside the gasifier The evaluation of the risk areas in presence of hydrogen-rich syngas was carried out with the dual fluidised bed steam gasifier (2) coupled with a molten carbonate fuel cell The dual fluidised bed steam gasifier is based upon the FICFB (Fast Internally Circulating Fluidised Bed) gasification process, which was formerly developed by the Vienna University of Technology [14] This process is considered as commercial, (1) fixed bed downdraft gasifiers (2) dual fluidised bed steam gasifier (3) fixed bed updraft gasifier (4) interconnected fluidised bed gasifier (5) molten carbonate fuel cell Fig 2.1 3D view of the plant 248 A Molino et al / Fuel 99 (2012) 245–253 provided that it has been tested since 2002 on the MW thermal capacity plant in Güssing and has by now reached a level of use of 80% Furthermore, being rich in hydrogen, the producer gas is especially suitable to be used as the fuel for a MCFC For these reasons, amongst the different gasification systems available at the Research Centre of Trisaia, the dual fluidised bed steam gasifier has been selected in order to be directly coupled to a MCFC The MCFC system is provided by Ansaldo Fuel Cells (AFCo), an Italian manufacturing company, which is part of the of the Ansaldo Group The electrical capacity is 125 kW, while operating temperature and pressure are 650 °C and 3.5 bar, respectively The high working temperature is required in order to keep the electrolyte, which is an alkali carbonate salt mixture, in the liquid state An horizontal ‘‘hot vessel’’ configuration is adopted, which requires the anodic stream at 200 °C, while the cathodic stream can be utilized at ambient temperature [15] In effect, the cathodic stream is heated up to the operating temperature via an internal catalytic burner, where exhaust gases from the anode are conveyed The high temperature exhaust gases leaving the burner are additionally used to heat up to the working temperature the incoming anodic stream, via an heat exchanger located inside the vessel also Finally an internal blower allows the recirculation of the cathode stream, thus minimizing the correlated energy requirements As it can be deduced, the operation of the MCFC requires several auxiliary devices Besides the equipments which are located inside the vessel and the anode heater, a cathode pre-heater is necessary in order to increase the cathodic stream temperature up to 300 °C during the plant heat up stage However, the most onerous auxiliary device is represented by the system for storage, vaporization and mixing of technical gases, such like hydrogen, nitrogen and carbon dioxide These gases are necessary during MCFC heat up and cool down, which must occur under controlled feed conditions Furthermore, they can be used to create apposite fuel gas mixtures simulating, during the tests, a gas from a given process (biomass gasification, anaerobic digestion, etc.) Flammable substances Flammable gases are in various sections of the plant The syngas produced by gasification contains H2, CO, CH4, C2H2, C2H4, N2 In some tests, the fractions of the combustible products in the mixture could reach values such as to achieve a high calorific value Therefore, in order to guarantee the absolute safety of the plant, the syngas has been characterized from a flammability point of view, so as to explore any possible flammable scenarios that could involve the reactor The flammability limits of each components of the typical mixture in the gasifier are reported hereafter: (see also Table 3.1) When the fuel is a mixture of several substances, the lower and the upper limits are computed on the basis of additivity criteria A Table 3.1 Lower flammability limit (LFL) and upper flammability limit (UFL) of each syngas component CO CO2 CH4 H2 N2 C2H6 C3H8 %Vol LFLair25°C UFLair25°C 25.00 19.00 10.00 33.00 10.00 0.50 2.50 12.50 0.00 5.30 4.00 0.00 3.00 2.37 74.00 0.00 15.00 75.00 0.00 12.40 9.50 widely used rule is the so-called Le Chatelier rule, also known as the law of mixtures, as follows: 100 LFL ¼ P yj j LFLj 100 UFL ¼ P yj j UFLj where LFLj and UFLj denote lower/upper flammability limit in air of the j-th substance, respectively Once knowing the flammability limits of each component of the mixture, it is possible to compute the lower and the upper limits in the case of a mixture with a greater amount of hydrogen [16,17] obtaining the following results: LFLmix25 C ¼ 7:11½m3 =m3 Á 100Š UFLmix25 C ¼ 63:45½m3 =m3 Á 100Š In order to evaluate the risk related to the explosive atmospheres, one have to consider that the mixture, for incidental causes, may be in contact with the combustive agent (air, oxygen and nitrogen) at the process temperature (around 900 °C); consequently, it is necessary to take into account these change of LFL, UFL with the temperature Usually this effect is taken into account through safety margins, in our approach we propose to actually evaluate these variables at the process temperature Safety analysis according the CEI 31-35 norm 4.1 Flammability limits The LFL, UFL have significance if computed at 25 °C Therefore, because the mixture may come into contact with the oxidizing at the process temperature (900 °C), they have to be assessed as a function of the temperature To address this need, the norm CEI 31-35 adopts wide margins using the safety coefficient k that is the safety factor applied to the LFL for the definition of the minimum ventilation mass flow rate Qamin and hypothetical volume of potentially explosive atmosphere Vz [12] So, applying the CEI 31-35 Italian standards, this correction coefficient is assumed equal to 0.5 4.2 Sources of emissions The Emission Source (ES) is a point or a part of a plant from which a flammable gas, vapor or liquid can be emitted to generate an explosive atmosphere The Norm CEI 31-35 provides some estimates, useful for making an assessment of the emission flow For that it regards the plant under study, the sources of emissions are essentially represented by flanges and valves The size of the emission hole of a flange is defined by taking into consideration the seal failure For a ring type joint (RTJ) flange (metal-to-metal), a serious failure may lead to a hole with a thickness of 0.05 mm and a length of 10 mm, or an area of 0.5 mm2 The size of the emission hole of a valve is instead defined by taking into account the emission of the stem In industrial practice, the area of the emission hole is assumed equal to 0.25 mm2 for general use valves on pipes having a diameter smaller or equal to 150 mm; 2.5 mm2 for general use valves on pipes having a diameter higher than 150 mm and for severe service valves on pipes of any diameter 249 A Molino et al / Fuel 99 (2012) 245–253 Safety valves that not discharge into a torch or blow down provide both second-grade and first-grade emissions depending on their behavior during the ordinary operation of the plant [12] In CEI 31-35 the Emission Sources are categorized according to level of hazard: Continuous Grade Emission Sources, when the flow is continuous or at least the case for a long time First Grade Emission Sources, when the emission is in regular form, but not prolonged, or occasional, but nevertheless expected in normal operation Second Grade Emission Sources, when the emission is not provided for short periods and in normal operation 4.3 Emission discharge For each SE, the emission discharge under cautionary conditions can be computed In case of continuous or first-grade emissions, it should be evaluated according to the features of the containment system and the effective size of the openings; in case of secondgrade emissions the above mentioned evaluation criteria should be applied In order to assess the emission discharge in case of a gas leakage from a containment system in which the pressure does not substantially drop for the effect of the considered emission, the following formula has to be applied: " Qg ¼ u Á c Á A Á c Á b #0:5 P ÁÀ Á0:5 cþ1 RÁ T  M where b ¼ ccþ1 , Qg is the maximum flow rate emission of gas/vapor, À1 u is critical ratio of flow rate, c is the concentration of gas, A is the surface emission, c is the specific heats ratio, P is the absolute pressure in the containment system at the point of emission, T is the reference temperature and M is the molar mass of flammable substance 4.4 Degree and availability of ventilation With regard to ventilation in areas with flammable gases or vapors, the CEI EN 60079-10 guide considers the ventilation in a quantitative manner (degree of ventilation) and according to the reliability which air is available with [11] The degree of ventilation represents the ratio between the amount of air which affects the emission source and the amount of flammable substances emitted in the environment The assessment of the degree of ventilation requires first information on the minimum mass flow rate of ventilation air (Qamin), defined as the mass flow rate of air (m3/s) needed for diluting the mass flow rate (Qg) of the dangerous substance associated to the emission, below the LFL, with a safety margin varying with the emission degree Either for indoors or outdoors, the minimum mass flow rate of ventilation air (Qamin) can be computed using the following formula: Q a ¼ Qg Ta Á k Á LFLm 293 where Ta is environment temperature and k is a safety factor applied to the LFL Then, it is necessary to determine the hypothetical volume of potentially explosive atmosphere (Vz) around the source of emission This can be done using the following formula: Vz ¼ f Á Q a Co For outdoors, ventilation per unit time Co is assumed equal to 0.03 sÀ1 The efficiency factor f, in the case of outdoors with the presence of some free air circulation impediments not able to reduce the effective dilution capacity of the air in the volume affected by the flammable emissions, was assumed to be two [12] Looking at the obtained results, for the plant under study the hypothetical volume of explosive atmosphere Vz can be considered negligible, as a result the degree of ventilation is high In order to define the effectiveness of ventilation, another important parameter should be considered: its availability The availability of ventilation has an influence on the presence or formation of an explosive atmosphere and expresses the availability level of the degree of ventilation It can be: good, adequate or poor The availability is good when ventilation (mass flow rate and factor of efficiency) is continuous Very brief interruptions can sometimes be admitted With natural outdoors ventilation, the availability is generally good if a wind velocity of 0.5 m/s is considered, conventionally representative of ‘‘calm wind’’ which is always present in practice For what concerns the plant under study, the wind measures have shown that a wind velocity of m/s can be assumed, therefore a good availability is present 4.5 Zone type Once known the degree of emission, the degree of ventilation and the availability of ventilation the zone can be classified Basically, there are three zone types: 0, and The type of emission is closely related to the degree of emission Generally, a continuous-grade emission produces a 0-type zone, a first-grade emission a 1-type zone and a second-grade emission a 2-type zone The element which can affect this biunique correspondence is the ventilation [12] In this case, since the sources of emission in the plant are second-grade, the degree of ventilation medium and the availability of ventilation good, the risk areas can be classified as 2-type areas 4.6 The risk distance Once determined the zone type, the risk distance have to be computed The risk distance (dz) is the distance from the source of emission (SE) starting from which the flammable gas or vapor concentration in the air is less than LFL Again, technical literature provides adequate formulas for computing this value Hereafter, some of them, taken from the CEI 31-35 guide, provide precautionary values for the classification of risk areas The formulas are applicable to outdoors problems [12] For emissions as a free jet of gas or vapor with high velocity the formula reads: dz ¼ 1650 Á ðP  10À5 Þ Á M À0:4 A0:5 kdz Á LFLV where dz is the dangerous distance from the emission source, M is the molar mass of flammable substance (kg/kmol), A is the area (section) of the hole of emission (m2) and P is absolute pressure in the containment system at the point of emission (Pa) For emissions as a free jet of gas or vapor with slow velocity, instead, the formula reads: dz ¼  42300 Á Q g Á f M Á kdz Á LFLV Á wa 0:55 where wa is the reference velocity of the air in the considered ambient (m/s), kdz is the safety coefficient applied to the LFL for the definition of the distance dz The risk distance, computed with the above mentioned formulas, can be used to approximately evaluate the extension of the risk zone but not its real size, which instead, have to be defined considering the specific situation by an expert technician, through experimental works and/or specific guides or recommendations Therefore, it is needed to evaluate the extension in the emission 250 A Molino et al / Fuel 99 (2012) 245–253 direction (quota ‘‘a’’) which has to be at least equal to the risk distance dz Usually, this distance is assumed for safety scopes In the absence of exact data, it is reasonable to assume a safety margin of 20% for defining the quota ‘‘a’’ Table 5.2 LFL, UFL, LOC at 900 °C LFLmix900°C UFLmix900°C LOCmix900°C 6.37 76.45 3.5 Explosion risk analysis with a fluid dynamic model 5.1 Flammability limits LOC ¼ LFL Á The flammability limits computed in Section are valid at the temperature of 25 °C Being the process temperature around 900 °C, in our approach we introduce their evaluation as a function of temperature At this aim, the following empiric relationships valid for alkanes are used [16]: LFLt ¼ LFL25 À 0:75 Á ðT À 25Þ DHC UFLt ¼ UFL25 À 0:75 Á ðT À 25Þ DHC where DHc is the combustion heat and t is the time The applicability of these empiric equations requires, because of the different species involved in the syngas, data on the flammability limits at the same temperature and pressure conditions These equations requires the following assumptions During the evolution of the reaction, the thermal capacity and the molar composition of the mixture are considered constant The kinetics of the combustion of the pure species is not significantly influenced by the presence of other fuels In Table 5.1 the values of the upper and lower flammability limits, LFL/UFL (expressed in volume percent of the substance, %vol.) are shown when the temperature varies for each species in the syngas: While these assumptions may be considered valid for calculating the lower flammability limit, but they introduce not negligible errors for upper flammability limits Fortunately most of the procedure illustrated before about the application of Guidelines CEI 31-35 is based on LFL which plays a major role in safety The low flammability limit influence directly the LOC number while the upper flammability limit has lower influence for the analysis because in this work is considered the diameter risk necessary to exit outside the flammability area started from syngas concentration composed by a fixed concentration of combustible gases Moreover, the flammability limits, as well as the reaction velocity and flame propagation velocity, are influenced by the pressure The effect of the pressure on the flammability limits is not always easily predictable, since it is specific for each mixture The flammability range of fuel-oxidizer mixtures is more easily computable by using the triangular diagram (mixture-oxidizerair) For the construction of this diagram, it is necessary to know the flammability limits in air, in pure oxygen and the lower oxygen concentration (LOC) under which the reaction does not provide the energy needed to heat the entire mixture The minimum oxygen concentration is defined by the following relationship: molO2 stoichiometric moltotal The table below (Table 5.2) shows the lower and the upper flammability limits (% of Volume, express in m3/m3Á100) in oxygen and the minimum oxygen concentration at the temperature of 900 °C: The flammability diagram (Fig 5.1) at the temperature of 900 °C represents the worst condition in which the syngas is utilized namely the temperature of the process Flammability diagrams show the regimes of flammability in mixtures of fuel, oxygen and an inert gas, typically nitrogen Mixtures of the three gasses are usually depicted in a triangular diagram, also known as a Ternary plot All the black lines represent the different composition of the syngas mixture The air line represents all the possible combinations of air/fuel The UFL and the LFL spots are defined just above this line The stoichiometric mixture line describes all the possible combinations between fuel and oxygen 5.2 Environmental conditions From a meteorological point of view, both the wind and the atmospheric stability widely affect the gas dispersion Wind is described and quantified by the following attributes: velocity, direction and turbulence In meteorology, atmospheric conditions can be: stable, unstable and neutral Dispersion is greatest for unstable conditions and lowest for stable conditions The gas dispersion mainly depends on meteorology (wind, atmospheric stability, humidity, solar radiation, ambient temperature, cloudiness) Other important aspects to take into consideration are: latitude, month of year, time of day, roughness, topography of the area In meteorology, the atmospheric turbulence seriously affect the dispersion of dangerous substances In order to study the atmospheric turbulence, the atmospheric boundary layer needs to be assessed focused For the proposed analysis the considered environmental conditions are: À wind speed: m/s; À wind direction: x; À solar radiation: 1520 kWh/mq/year These environmental conditions influence the standard deviation rx and ry contained in the Pasquill–Giffors law’s that will be used for the evaluation of the risk distances Table 5.1 LFL e UFL (%Vol.) for different temperatures and different species in the syngas [18] T (°C) LFL CO UFL CO LFL CH4 UFL CH4 LFL H2 UFL H2 LFL C2H6 UFL C2H6 LFL C3H8 UFL C3H8 100 200 300 400 500 600 700 800 900 12.3 12.0 11.7 11.5 11.2 10.9 10.7 10.4 10.1 73.8 73.5 73.2 73.0 72.7 72.4 72.2 71.9 71.6 5.23 5.15 5.06 4.98 4.89 4.81 4.73 4.64 4.56 14.9 14.8 14.7 14.6 14.5 14.5 14.4 14.3 14.2 3.87 3.87 3.87 3.87 3.87 3.87 3.87 3.87 3.87 74.8 74.8 74.8 74.8 74.8 74.8 74.8 74.8 74.8 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 12.4 12.4 12.4 12.4 12.4 12.4 12.4 12.4 12.4 2.35 2.35 2.35 2.35 2.35 2.35 2.35 2.35 2.35 9.48 9.48 9.48 9.48 9.48 9.48 9.48 9.48 9.48 251 A Molino et al / Fuel 99 (2012) 245–253 Fig 5.1 Triangular diagram for the fuel mixture leaving the Joule plant at 900 °C 5.3 Emission sources modeling Once defined the flammability range, the accidental release of dangerous chemical substances from different sources (pipes, flanges, valves) have to be modeled In this case, the more cumbersome (and less probable) situation has been modeled: the release of syngas from a pipe after a cumbersome breakdown The dispersion close to the gasifier area has been modeled The input data for doing this evaluation are: the mixture features, the source, the modality and the duration of the release The dispersion of syngas represents an emission of a gaseous substance in the environment followed by a dispersion of the gas (vapor) cloud The dispersion is therefore an effect of the emission where c is the gas concentration and Kx, Ky, Kz are gas diffusion coefficients (assuming anisotropy) Depending on simplifying hypothesis (isotropy of diffusion, absence or constancy of the velocity component/s), the convection/ diffusion equation may have an analytical solution, otherwise it needs to be integrated The dispersion of alight gas having a neutral buoyancy is defined a passive dispersion In general, the neutral buoyancy is given either by the high emitted gas dilution (low concentration) or by its molecular weight similar to that of the surrounding air (in this case the emitted gas temperature is similar to the atmospheric one) This representation has been longly adopted to describe the emission from chimney [19,20] These models also describe the dispersion of substances for instantaneous or continuous emissions from land Experimentally it was detected that either for instantaneous or continuous releases from a punctiform source posed on the ground, the concentration profiles are Gaussian (Pasquil–Gifford) [20,21] At the same time, for both kind of releases the variability of concentration increases with sampling time The plume generated from a continuous release tends to spread It follows that the dispersion due to the turbulence is increased The concentration downstream of the point of emission depends on the intensity of the source except in the case it is significantly responsible of the convective motion transferred to the emitted fluid For that it concerns the continuous and punctiform source, the concentration is inversely proportional to the average of wind velocity In order to assess the concentration of pollutants and the effect of the relative dilution, the model of Pasquill–Gifford has been chosen as a starting point [19,20] As an example, for a continuous punctual dispersion from land the following functional dependence is valid:  cðx; y; zÞ ¼ Qg pry rz u e À12 y2 z2 þ r2y r2z ! 5.4 Dispersion modeling The laws equations: modeling dispersions are the Navier–Stokes @q mÞ ¼ þ rðq~ @t @ðq~ vÞ þ rðq~ m~ v À ~SÞ ¼ ~f e @t where ry, rz are standard deviations of the wind velocity in the transverse and vertical directions, Qg is the mass flow rate of the dangerous substance associated to the emission, u is the wind speed, y is the transversal distance from the emission hole and z is the orthogonal distances from the emission plane The essential feature of the dispersion coefficients is that they depend on the downwind distance and the class of meteorological stability @ðqEÞ v À ~S Á ~ v þ ~qÞ ¼ ~f e Á ~ v þ rðqE~ @t where ~ v is vector of gas velocity whose components are respectively u, v, w (m/s), q is the density (kg/m3) and E is the specific internal heat (J/kg) The frequently modeled and described scenarios are the instantaneous release (snort) and the continuous release (plume) from a punctiform source [22] There are two approaches for modeling the turbulent dispersion: the Eulerian and the Lagrangian approach, respectively Focusing attention only on the Eulerian approach, a possible dispersion model is that based on the convection/diffusion equation (k model) [23] The convection/diffusion for a gas in rectangular coordinates is: @c @c @c @c @2c @2c @2c þu þv þ w ¼ Kx þ Ky þ Kz @t @x @y @z @x @y @z Fig 6.1 Concentration profiles in a fuel mixture versus transversal and horizontal distance from the emission hole (A = 0.5 mm2) 252 A Molino et al / Fuel 99 (2012) 245–253 Table 6.1 Results obtained by applying the CEI 31–35 guide and the fluid dynamic model Method LFL (%) A (mm2) Vz (m3) dz (cm) a (cm) Fig 6.2 Concentration profiles in a fuel mixture as a function of transversal and longitudinal distance from the emission hole (A = 0.5 mm2) for the syngas mixture percentage lower than the lower flammability limit Test results On the basis of the assumptions made for the two approaches, namely the CEI 31-35 standard method and the fluid dynamic simulation, it was possible to compare them in order to validate the standard approach when applied to a new technology such the use of syngas obtained from biomass The fluid dynamic simulation is characterized by a more detailed representation of the problem and introduced LFL evaluation at the process temperature (900 °C) instead of applying safety coefficients In this case, assuming an hypothetical failure of a flange with an emission hole of 0.5 mm2, a continuous spill of the overall gaseous mass flow rate and a wind velocity of m/s, the trends of the syngas concentration at 900 °C as a function of distance can be obtained Fig 6.1 shows the effect of the dilution as a function of the longitudinal and transversal distances from the emission hole In Flange Valve CEI 31-35 Fluid dynamic CEI 31-35 Fluid dynamic 7.11 0.5 0.02 10 12 6.37 0.5 // 7.11 0.25 0.01 6.37 0.25 // order to appreciate the lower flammability limit of the syngas mixture at the emission temperature (LFLmix900°C = 6.73%), it is necessary to show a zoom for mixture composition lower that LFL, as shown in Fig 6.2: Fig 6.2 shows the volumetric percentage of syngas as a function of the transversal and longitudinal distance from the emission hole for volumetric percentage of syngas mixture around the LFL For a distance about of cm from the emission hole, a syngas mixture is lower than 2%Vol This value vanishes almost completely for a distance of 2.5 cm from the emission hole (dz = cm) at an orthogonal distance from the emission plane of z = cm At the other hand, assuming an hypothetical failure of a valve with an emission hole of 0.25 mm2, a continuous spill of the overall gaseous mass flow rate and a wind velocity of m/s, a total dilution of the syngas mixture is observed at almost cm from the emission hole can be obtained Because the emission direction of the Emission Sources in the plant is unknown, a spherical shape of the dangerous area has been assumed The study showed that all the emission sources give rise to 2-type areas Figs 6.1 and 6.2 show that the dangerous distance dz is equal to 10 cm for the flanges and cm for valves Table 6.1 shows the results obtained applying the CEI 31-35 guide compared with the fluid dynamic model Where a is the effective extension of the dangerous area in the direction of emission (m), Vz is the hypothetical volume of potentially explosive atmosphere Table 6.1 shows that the approach proposed by CEI 31-35 guidelines results very conservative with regard to the fluid dynamic approach The results of the risk analysis are shown in the picture Fig 6.3: Fig 6.3 shows that the biggest volumes of risk are located both near the compression zone and in the exhaust gases zone in outlet of the molten carbonate fuel cells Conclusions This article discusses the safety aspects related to a 500 kWth biomass gasifier and a 125 kWe molten carbonate fuel cell integrated plant, actually under construction at the ENEA Trisaia Research Centre In particular, it describes the procedure to assess the explosion risk due to the electricity in presence of hydrogenrich syngas The results obtained by following the CEI 31-35 guide were compared with a fluid dynamic model The most interesting result is that, either by applying the CEI 31-35 guide or by performing a fluid dynamic analysis, the dangerous distance from the emission sources has the same order of magnitude however the guidelines provide conservative results Therefore, the validity of the Italian guide is confirmed for this specific plant although it results very conservative References Fig 6.3 3D plant view with risk volumes identification [1] Amos WA, Analysis of two biomass gasification/fuel cell scenarios for smallscale power generation National Renewable Energy Laboratory NREL/TP-57025886; 1998 A Molino et al / Fuel 99 (2012) 245–253 [2] Lobachyov KV, Richter HJ An advanced integrated biomass gasification and molten fuel cell power system Energy Convers Manage 1998;39:1931–43 [3] McIlveen-Wright DR, Williams BC, McMullan JT Wood gasification integrated with fuel cells Renew Energy 2000;19:223–8 [4] [5] Hofbauer H, Veronik G, Fleck T, Rauch R The FICFB gasification process Dev Thermochem Biomass Convers 1997;2:1016–25 [6] Hofbauer H, Rauch R Hydrogen-rich gas from biomass steam gasification Publishable final report of project JOR3-CT97-0196; 2001 [7] Hofbauer H, Rauch R, Loeffler G, Kaiser S, Fercher E, Tremmel H Six years experience with the FICFB-gasification process In: Proceedings of 12th European conference and technology exhibition on biomass for energy Industry and climate protection, Amsterdam; 2002 [8] Fiorenza G, Canonaco J, Blasi A, Braccio G Biomass steam gasification at the Trisaia dual fluidized bed pilot plant: experimental results and optimization analysis of process variables In: proceedings of zero emission power generation workshop, Gebze, Turkey; 2007 [9] Directive 94/9/EC of the European parliament and the council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres [10] Directive 1999/92/EC of the European Parliament and of the Council of 16 December 1999 on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres [11] Norma EN 60079–10 (CEI 31–30): Costruzioni elettriche per atmosfere esplosive per la presenza di gas Parte 10: Classificazione dei luoghi pericolosi 253 [12] Guida CEI 31–35 Costruzioni elettriche potenzialmente esplosive per la presenza di gas Guida all’applicazione della norma CEI EN 60079–10 (CEI 31–30) Classificazione dei luoghi pericolosi [13] Fiorenza G, Molino A Analysis of integration of a molten carbonate fuel cell in a biomass steam gasification plant; 2008 [14] Hofbauer H, Veronik G, Fleck T, Rauch R The FICFB gasification process In: Proceedings of the international conference developments in thermochemical biomass conversion, vol Banff, Alberta, Canada; 1996 p 1016–25 [15] Rossetti A, Scagliotti M, Valli C, Giannotti A, Bertone R Molten carbonate fuel cells research capabilities In: proceedings of international symposium on diagnostic tools for fuel cell technologies, Trondheim, Norway; 2009 [16] Lees FP Loss prevention in the process industries, vols 1–3, Butterworth; 1996 [17] Lees FP Loss prevention in the process industries, vol 1–3, Butterworth; 2004 [18] Zabetakis, MG Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, US Bureau of Mines, XMBUA, USA; 1965 [19] Gifford FA Atmospheric dispersion calculations using generalized Garrison plume model Nucl Safety 1961;2:56 [20] Gifford FA Use of routine meteorological observations for estimating atmospheric dispersion Nucl Safety 1961;2:56 [21] Pasquill F The estimation of the dispersion of windborne materials Metall Mag 1961;90:33 [22] Seinfeld JH, Pandis SN Atmospheric chemistry and physics New York: WileyInterscience; 1997 [23] Sharan M, Modani M Variable K-theory for the dispersion of air pollutant in low wind condition in the surface-based inversion Atmos Environ 2007;41(33):6951–63