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

Classification procedure of the explosion risk areas in presenceof hydrogen-rich syngas: Biomass gasifier and molten carbonate fuel cell integrated plant A.. Nomenclature a hazardous dista

Classification procedure of the explosion risk areas in presence

of hydrogen-rich syngas: Biomass gasifier and molten carbonate fuel cell

integrated plant

A Molinoa,⇑, G Braccioa, G Fiorenzaa, F.A Marraffab, S Lamonacab, G Giordanoc, G Rotondob,

U Stecchib, M La Scalab

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

b

Politecnico of Bari, Electrical and Electronics Dept., Via E Orabona 4 – 70125 Bari, Italy

c

University of Calabria, Chemical and Materials Engineering Dept., Via Pietro Bucci – 87036 Arcavacata di Rende, Cosenza, Italy

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 cou-pled 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

0 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

de-velop technologies that allow to directly use hydrogen in energy

conversion systems (fuel cells) with high efficiency and low

envi-ronmental 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 kWemolten carbonate fuel cell

inte-grated plant

The integration between fuel cell and gasification plant

repre-sents 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

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 avail-able 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 Re-search Centre of Trisaia exploits a dual fluidised bed (DFB) reactor having 500 kWthcapacity The reactor uses steam as gasification agent, so that a fuel gas nearly nitrogen free is produced character-ized by a relatively high Lower Heating Value, approaching 13 MJ/

Nm3on a dry basis

The gasification concept is the well-known Fast Internally Cir-culating 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

0016-2361/$ - see front matter Ó 2012 Elsevier Ltd All rights reserved.

⇑ Corresponding author Tel.: +39 (0)835 974736; fax: +39 (0)835 974210.

E-mail address: antonio.molino@enea.it (A Molino).

Contents lists available atSciVerse ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / f u e l

was added consisting of an adsorbing reactor for the removal of

acid compounds, a hot gas cyclone and a hot gas filter for the

re-moval 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

or-der to create the appropriate fluidodynamic conditions for the

system

The MCFC technology exploits also the carbon monoxide to

pro-duce electricity, this is an important fraction of the propro-ducer 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

follow-ing anodic reaction

1 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

ini-tially 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

Complete harmonization and extension to all types of equip-ment 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 explo-sion-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 require-ments for improving the health and safety protection of works potentially at risk from explosive atmospheres’’ refers to the oper-ation of potentially explosive installoper-ations, and is, therefore, in-tended 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 implementa-tion of the directive into the British law by ‘‘The Dangerous Sub-stances 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:

 the likelihood that explosive atmospheres will occur and their persistence;

 the likelihood of ignition sources, including electrostatic discharges;

 the installations which can give rise to an explosion, substances used, processes, and their possible interactions;

 the scale of the anticipated effects

Nomenclature

a hazardous distance in the emission direction (m)

A surface emission (m2)

cp specific heat–constant pressure (J/kg K)

cv specific heat–constant volume (J/kg K)

Co number of the air exchange, referred to the total volume

(s1)

dz hazardous distance from a sorgent emission (m)

DHc combustion heat (kcal/mol)

E specific internal heat (J/kg)

fe external force for volume (N/m3)

kdz safety factor applied to the LFL

Kx, Ky, Kz gas diffusion coefficients

LFL lower flammability limit (m3/m3100)

UFL upper flammability limit (m3/m3

100) 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)

~q heat flux (W/m2)

Qamin minimum ventilation mass flow rate (m3/s)

Qg maximum flow rate emission of gas/vapor (kg/s)

R universal gas constant = 8314 J/kmol K

Ta environment temperature

~v vector of gas velocity whose components are

respec-tively u,v, w (m/s)

Vz volume of potentially explosive atmosphere (m3)

wa reference velocity of the air in the considered ambient

(m/s)

yj molar fraction

c specific heats ratio (cp/cv)

u critical ratio of flow rate

q density (kg/m3)

ry,rz standard deviation of the wind speed in the transversal

and orthogonal direction

The employer has to classify the areas in which explosive

atmo-spheres may appear into risk-zones, and ensure that the minimum

organisational and technical requirements of the Directive are

ob-served Since our assessment refers to gases, we restrict our

discus-sion 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:

 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

selec-tion 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

guide-lines 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

hazard-ous 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

impor-tant 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

methodol-ogy 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 cal-culated by specific reference formulas;

(4) evaluation of the extension of the hazardous zone as a func-tion of the release flow rate, ventilafunc-tion and flammable substances

2 Plant description The plant under study is installed at the ENEA Trisaia Research Centre It is an experimental pilot plant for the biomass-derived-hydrogen-rich syngas production and consists of a 500 kWth bio-mass gasifier coupled with a 125 kWemolten 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 sec-ond one is devoted to the implementation of a bunker for storing hydrogen in a cylinder, with the relative unit of decompression

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 scrub-ber, 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 1 MWth, with catalytic cera-mic 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

2

3 4 5

(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

provided that it has been tested since 2002 on the 8 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

espe-cially suitable to be used as the fuel for a MCFC For these reasons,

amongst the different gasification systems available at the

Re-search 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

re-quires 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

con-veyed 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

auxil-iary 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

mix-tures simulating, during the tests, a gas from a given process

(bio-mass gasification, anaerobic digestion, etc.)

3 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

mix-ture 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

widely used rule is the so-called Le Chatelier rule, also known as the law of mixtures, as follows:

LFL ¼P100

j

yj LFL j

UFL ¼P100

j

y j

UFLj

where LFLjand UFLjdenote 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  C¼ 7:11½m3=m3 100

UFLmix25  C¼ 63:45½m3=m3 100

In order to evaluate the risk related to the explosive atmo-spheres, 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); conse-quently, 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

4 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 mini-mum ventilation mass flow rate Qaminand 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 esti-mates, useful for making an assessment of the emission flow For that it regards the plant under study, the sources of emis-sions are essentially represented by flanges and valves The size

of the emission hole of a flange is defined by taking into consider-ation the seal failure

For a ring type joint (RTJ) flange (metal-to-metal), a serious fail-ure 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 mm2for general use valves on pipes having a diameter smaller or equal to

150 mm; 2.5 mm2for general use valves on pipes having a diame-ter higher than 150 mm and for severe service valves on pipes of any diameter

Table 3.1

Lower flammability limit (LFL) and upper flammability limit (UFL) of each syngas

component.

%Vol LFL air25°C UFL air25°C

Safety valves that do 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:

1 Continuous Grade Emission Sources, when the flow is

continu-ous or at least the case for a long time

2 First Grade Emission Sources, when the emission is in regular

form, but not prolonged, or occasional, but nevertheless

expected in normal operation

3 Second Grade Emission Sources, when the emission is not

pro-vided 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

second-grade 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:

Qg¼u c  A  c 2

cþ 1

 b

" #0:5

R T M

 0:5

where b ¼c þ1

c 1, Qgis the maximum flow rate emission of gas/vapor,

uis critical ratio of flow rate, c is the concentration of gas, A is the

surface emission,cis the specific heats ratio, P is the absolute

pres-sure in the containment system at the point of emission, T is the

ref-erence 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

va-pors, 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

re-quires first information on the minimum mass flow rate of

ventila-tion 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

com-puted using the following formula:

Qa min¼ Qg

k  LFLm

 Ta

293

where Tais environment temperature and k is a safety factor

ap-plied to the LFL

Then, it is necessary to determine the hypothetical volume of

potentially explosive atmosphere (Vz) around the source of

emis-sion This can be done using the following formula:

Vz¼f  Qa min

Co

For outdoors, ventilation per unit time Cois assumed equal to

0.03 s1 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 Vzcan be consid-ered 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 for-mation 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 consid-ered, conventionally representative of ‘‘calm wind’’ which is al-ways present in practice For what concerns the plant under study, the wind measures have shown that a wind velocity of

2 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 Basi-cally, there are three zone types: 0, 1 and 2 The type of emission

is closely related to the degree of emission Generally, a continu-ous-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 ven-tilation[12]

In this case, since the sources of emission in the plant are sec-ond-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 precaution-ary 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:

dz¼ 1650

kdz LFLV

 ðP  105Þ  M0:4A0:5 where dzis 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, in-stead, the formula reads:

dz¼ 42300  Qg f

M  kdz LFLV wa

where wais the reference velocity of the air in the considered ambi-ent (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 formu-las, can be used to approximately evaluate the extension of the risk zone but not its real size, which instead, have to be defined consid-ering the specific situation by an expert technician, through exper-imental works and/or specific guides or recommendations Therefore, it is needed to evaluate the extension in the emission

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

mar-gin of 20% for defining the quota ‘‘a’’

5 Explosion risk analysis with a fluid dynamic model

5.1 Flammability limits

The flammability limits computed in Section2are 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

va-lid for alkanes are used[16]:

LFLt¼ LFL250:75  ðT  25Þ

DHC

UFLt¼ UFL250:75  ðT  25Þ

DHC

where DHcis 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

flammabil-ity limits at the same temperature and pressure conditions These

equations requires the following assumptions During the

evolu-tion of the reacevolu-tion, the thermal capacity and the molar

composi-tion 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 InTable 5.1the values of the upper

and lower flammability limits, LFL/UFL (expressed in volume

per-cent of the substance, %vol.) are shown when the temperature

var-ies for each specvar-ies in the syngas:

While these assumptions may be considered valid for

calculat-ing the lower flammability limit, but they introduce not negligible

errors for upper flammability limits Fortunately most of the

proce-dure 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

be-cause 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

veloc-ity and flame propagation velocveloc-ity, 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-oxidizer-air) 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:

LOC ¼ LFL molO2 stoichiometric

moltotal

The table below (Table 5.2) shows the lower and the upper flammability limits (% of Volume, express in m3/m3100) 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 combina-tions 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 de-scribed and quantified by the following attributes: velocity, direc-tion and turbulence In meteorology, atmospheric condidirec-tions 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 meteorol-ogy, the atmospheric turbulence seriously affect the dispersion of dangerous substances In order to study the atmospheric turbu-lence, the atmospheric boundary layer needs to be assessed focused

For the proposed analysis the considered environmental condi-tions are:

 wind speed: 2 m/s;

 wind direction: x;

 solar radiation: 1520 kWh/mq/year These environmental conditions influence the standard devia-tionrxandrycontained 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 CH 4 UFL CH 4 LFL H 2 UFL H 2 LFL C 2 H 6 UFL C 2 H 6 LFL C 3 H 8 UFL C 3 H 8

Table 5.2 LFL, UFL, LOC at 900 °C.

LFL mix900°C UFL mix900°C LOC mix900°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

cumber-some (and less probable) situation has been modeled: the release

of syngas from a pipe after a cumbersome breakdown The

disper-sion 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

5.4 Dispersion modeling

The laws modeling dispersions are the Navier–Stokes

equations:

@q

@tþrðq~mÞ ¼ 0

@ðq~vÞ

@t þrðq~m ~v ~SÞ ¼ ~fe

@ðqEÞ

@t þrðqE~v ~S  ~vþ ~qÞ ¼ ~fe ~v

where ~v is vector of gas velocity whose components are

respec-tively u,v, w (m/s),qis the density (kg/m3) and E is the specific

internal heat (J/kg)

The frequently modeled and described scenarios are the

instan-taneous release (snort) and the continuous release (plume) from a

punctiform source[22]

There are two approaches for modeling the turbulent

disper-sion: the Eulerian and the Lagrangian approach, respectively

Focusing attention only on the Eulerian approach, a possible

dis-persion model is that based on the convection/diffusion equation

(k model)[23]

The convection/diffusion for a gas in rectangular coordinates is:

@c

@tþ u

@c

@xþv@c@yþ w@c

@z¼ Kx

@2c

@x2þ Ky

@2c

@y2þ Kz

@2c

@z2

where c is the gas concentration and Kx, Ky, Kzare gas diffusion coef-ficients (assuming anisotropy)

Depending on simplifying hypothesis (isotropy of diffusion, ab-sence 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 de-fined 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 emis-sions 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 con-centration increases with sampling time The plume generated from a continuous release tends to spread It follows that the dis-persion due to the turbulence is increased

The concentration downstream of the point of emission de-pends on the intensity of the source except in the case it is signif-icantly 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

pryrzue

 1 y2

r 2 y

þ z2

r 2 z

where ry, rzare standard deviations of the wind velocity in the transverse and vertical directions, Qgis 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

Fig 5.1 Triangular diagram for the fuel mixture leaving the Joule plant at 900 °C.

Fig 6.1 Concentration profiles in a fuel mixture versus transversal and horizontal

2

6 Test results

On the basis of the assumptions made for the two approaches,

namely the CEI 31-35 standard method and the fluid dynamic

sim-ulation, 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

de-tailed representation of the problem and introduced LFL evaluation

at the process temperature (900 °C) instead of applying safety

coef-ficients 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 2 m/s, the trends of

the syngas concentration at 900 °C as a function of distance can be

obtained

Fig 6.1shows the effect of the dilution as a function of the

lon-gitudinal and transversal distances from the emission hole In

order to appreciate the lower flammability limit of the syngas mix-ture at the emission temperamix-ture (LFLmix900°C= 6.73%), it is neces-sary to show a zoom for mixture composition lower that LFL, as shown inFig 6.2:

Fig 6.2shows 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 1 cm from the emission hole, a syngas mixture is lower than 2%Vol This value vanishes almost completely for a dis-tance of 2.5 cm from the emission hole (dz= 5 cm) at an orthogonal distance from the emission plane of z = 0 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 2 m/s, a total dilu-tion of the syngas mixture is observed at almost 3 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 6.2show that the dangerous distance dzis equal to

10 cm for the flanges and 5 cm for valves

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 poten-tially 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 pictureFig 6.3:

Fig 6.3shows 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

7 Conclusions This article discusses the safety aspects related to a 500 kWth biomass gasifier and a 125 kWemolten carbonate fuel cell inte-grated plant, actually under construction at the ENEA Trisaia Re-search Centre In particular, it describes the procedure to assess the explosion risk due to the electricity in presence of hydrogen-rich 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 danger-ous distance from the emission sources has the same order of mag-nitude however the guidelines provide conservative results Therefore, the validity of the Italian guide is confirmed for this spe-cific plant although it results very conservative

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