Natural gas: physical properties and combustion features

Transcription

1 Natural gas: physcal propertes and combuston features 39 X Natural gas: physcal propertes and combuston features Le orre Olver and Loubar Khaled GEPEA, Ecole des Mnes de Nantes, NRS, UMR 61 Ecole des Mnes de Nantes, NATech, GEM, PRES UNAM La hantrere,, rue Alfred Kastler, B.P. 07, F-307, Nantes, ede 3, France 1. Introducton One calls combustble natural gas or smply natural gas, any combustble gas flud comng from the basement. The concept of a unque natural gas s ncorrect. It s more eact to speak about natural gases. In fact, the chemcal composton of avalable natural gas (at the fnal customer) depends on ts geographc orgn and varous mtures carred out by networks operators. The majorty of natural gases are mtures of saturated hydrocarbons where methane prevals; they come from underground accumulatons of gases alone or gases assocated wth ol. There are thus as many compostons of natural gases as eploted hydrocarbon layers. Apart from the methane whch s the prevalng element, the crude natural gas usually contans decreasng volumetrc percentages of ethane, propane, butane, pentane, etc. The ultmate analyss of a natural gas thus ncludes/understands the molar fracton of hydrocarbons n, 6, 3 8, 10 and the remander of heaver hydrocarbons s generally ndcated under the term 5+. Table 1 gves typcal compostons. Apart from these hydrocarbons, one often fnds one or more mnor elements, or mpurtes, quoted hereafter: ntrogen N : t has as a dsadvantage ts nert character whch decreases the commercal value of gas, carbon dode O : t s harmful by ts corrosve propertes, hydrogen sulfde S: t s harmful by ts corrosve propertes, helum e: t can be developed commercally, water O: the natural gas of a layer s generally saturated wth steam. To be eploted, t undergoes a partal dehydraton. In ths chapter, the characterstcs of natural gas n term of composton and physcal propertes and combuston features are presented. The physcal models for the calculaton of the physcal propertes are developed and a synthess of the models selected s carred out.

2 0 Natural Gas Fuel N O MN No No No No No No No No No No Table 1. Sample group of fuel gases (Sakaly et al., 008). Varous technques of determnaton of combuston features such as equvalence rato, the low heatng value and Wobbe nde are eposed. These technques are based on drect or ndrect methods. The secton Physcal Propertes s a toolbo to calculate transport propertes (dynamc vscosty and thermal conductvty) and other mportant propertes such as speed of sound, refractve nde and densty. Regards tme, the ultmate consumer burns a fuel whose chemcal composton vares, see Fgure 1. These varatons brng problems for plant operaton, whatever s the prme mover (Internal ombuston engne, gas turbne or boler). The secton ombuston features detals: Ar-fuel rato s the rato of ar to fuel n stochometrc condtons. Network operator sells natural gas volume but fnal customer needs heat. Low heatng value LV s the lnk and s very mportant. By contract, network operator takes oblgatons on the LV mnmum value. Wobbe nde (W) s an mportant crteron of nter-changeablty of gases n the ndustral applcatons (engnes, bolers, burners, etc). Gas composton varaton does not nvolve any notable change of the factor of ar and the velocty burnng when the nde of Wobbe remans almost constant. Methane number (MN) characterzes gaseous fuel tendency to auto-gnton. By conventon, ths nde has a value 100 for methane and 0 for hydrogen (Leker et al., 197). The gaseous fuels are thus compared wth a methane-hydrogen bnary mture. Two gases wth same value MN have the same resstance aganst the spontaneous combuston.. Physcal Propertes.1 Introducton Physcal models of transport propertes relatng to the gases (vscosty, conductvty) result from the knetc theory of gases, see (rschfelder et al., 195) and (hapman & owlng, 1970).

3 Natural gas: physcal propertes and combuston features 1 Fg. 1. Methane Number durng 5 consecutve months (Sakaly et al., 008) The assumptons wth regards to the knetc theory of gases are: 1. The average dstance between the molecules s suffcently mportant so that the molecular nteractons (other than shocks) are neglgble,. The number of molecules per unt volume s large and constant (gas homogenety on a macroscopc scale). The followng assumptons are relatng to knematcs: 1. Between two shocks, presumed elastc, the movement of each molecule s rectlnear and unform,. The drecton of the Speed Vectors of the varous molecules obeys a unform space dstrbuton, 3. The module of the Speed Vectors vares accordng to a law of dstrbuton whch does not depend on tme when the macroscopc varables of state are fed. Natural gases are a mture of components. Ther physcal propertes such as dynamc vscosty and thermal conductvty, evaluated on the bass of knetcs of gases, are obtaned startng from the propertes of pure gases and correctve factors (related on the mtures, the polar moments, etc).. Dynamc vscosty Natural gas vscosty s requred to carry out flow calculatons at the varous stages of the producton and n partcular to determne pressure network losses. Natural gas generally behaves as a Newtonan flud, see (Rojey et al., 000) and, n ths case, dynamc vscosty n unt [Pa.s] s defned by Equaton (1): du Wth the shear stress and the shear rate. dy du (1) dy

4 Natural Gas..1 Pure gases onsderng brownan moton of the molecules regards to the ntermolecular forces, hapman and Enskog theory can be appled. Ths approach consders n detal the nteractons between molecules whch enter n collson and s based on equaton of Mawell-Boltzmann functon dstrbuton, see (hapman & owlng, 1970). For mono-atomc gases, analytc soluton of ths equaton gves the vscosty dependng of a (,) two double ntegrals, correspondng to molecules bnary collsons, often called collson ntegral for vscosty. owever, ths theoretcal approach s only applcable to mono-atomc gases under low pressures and hgh temperatures. To apply ths model to polyatomc gases, a correcton for energy storage and transfer are requred, see (Le Nendre, 1998). In general terms, the soluton obtaned for the dynamc vscosty of the mono-atomc gases whch do not have degree of freedom of rotaton or vbraton s wrtten: M T * (,) () Wth M the molar mass n [g mol -1 ], T the absolute temperature n [K], a characterstc * (,) dameter of the molecules, often called the collson dameter n [1 A ], the * * collson ntegral dependng on the reduced temperature T defned as T kt /, where k s the Boltzmann constant and s the mamum energy of attracton. orrelatons est to appromate the collson ntegral. For nonpolar gases, Neufeld et al. (197) have proposed the epresson: * * * B D T F T e E e * (,) T A (3) Where A=1.1615, B=0.187, =0.587, D=0.7730, E= and F= Equaton (3) s valuable n the range 0.3 T r 100, where T r T / Tc, T c beng the crtcal temperature, wth a standard devaton of 0.06%. hung et al. (198) and (1988) have epermentally obtaned: k T c / Vc (5) To take nto account molecule shapes, hung et al. have ntroduced a correctve factor F : c () 6 M T F * (6) (,) c V /3 c

5 Natural gas: physcal propertes and combuston features 3 Wth F c 1 r ; s the acentrc factor, s a correcton for gases beng strongly polar; the dmensonless dpole moment beng gven by 1/ r Vc Tc Rechenberg (197) have chosen a lnear dependence: * (,) Dynamc vscosty s then epressed by: a T * n n 6 1/ 1/ n 1 Tc / M T a /3 Vc (8) r (7).. Gaseous blends At low pressure, dynamc vscosty of gases blend, noted m, can be estmated from the vscosty of pure gases. For a mture of components, gaseous blend vscosty s gven by the epresson: Where m 1 K 1 j K j k K j K k (9) 1 j1 j1, k 1, K k k 1, k 3 M k / M (10) Where s the dynamc vscosty of th pure gas, and coeffcents j are obtaned by M ts molar mass, ts molar fracton j 1/ 1/ 6 M M j Tr, j Tr, j 1 FR, j 3 j (11) 1/ 3M M j Tr, j Reduced temperature T r, j s based on crtcal temperature of pure gases and j: T Tr, j 1/ (1) Tc, Tc, j

6 Natural Gas orrecton coeffcents F R, j s gven by: F R, j Tr 7 / T 7 /, j r, j 10 r, r, j 1 10 r, r, j 1/ 7 1/ 7 (13) oeffcents s obtaned by: 1/ M (1) U 1/ Wth: U 1/ 6 R, Tr, ( Tr, 1) 1/ Tr, F (15) Wlke (1950) have ntroduced smplfcatons nto equaton (9) by neglectng the term of the second order. The epresson of dynamc vscosty obtaned makes easer the applcaton: 1/ 1 1/ / j M / M j Wth j 8 1 M / 1/ M j m 1 j 1 j j (16) In the lterature, specfc correlatons were establshed to calculate the vscosty of gas hydrocarbons. In partcular, to calculate the vscosty of methane, an equaton of the followng general form was proposed by anley et al (1975) and ncluded by Vogel et al. (000): T ) ( T ) (, ) (17) m 0 ( 1 T where 0( T ) represents dynamc vscosty n etreme cases of 0. The sum 1( T ) (, T ) s the resdual dynamc vscosty whch takes account of the ncrease n vscosty from 0( T ).

7 Natural gas: physcal propertes and combuston features 5 functon vscosty = func_vscosty(compo) % compo s a vector n volume fracton % [ n O N O S O] P = 10135; % current gas pressure n Pa T = 73.15; % current gas temperature n K M = [ ]; % molar mass n g mol -1 Tc = [ ];% rtcal temperature Vc = [ ];%rtcal Volume cm3/mol Dp = [ ];% Dpolar Moment omega = [ ]; T_et = 1.593*T/Tc; % omegav = *T_et^(-0.187)+0.587*(ep( *T_et)) *(ep(-.3787*T_et)); mu_r = 131.3*Dp./sqrt(Vc.*Tc); Fc = ones(1,1)-0.756*omega *mu_r.^; eta = 0.785*(Fc.*sqrt(T.*M))./(Vc.^(/3).*omegaV)/ ; for = 1:1 for j = 1:1 A(,j) = (1 + sqrt(eta()/eta(j))*(m()/m(j))^(1/))^/sqrt(8*(1+m()/m(j))); % end end p1 = compo.*eta; for = 1:1 p() = p1()/sum(compo.*a(,:)); % end vscosty = sum(p); %Pa s-1 Sanda Natonal Laboratory ( has developed EMKIN, a reference tool for chemcal. The Gas Research Group ( carred out by the Unversty of alforna at Berkeley, Stanford Unversty, The Unversty of Teas at Austn, and SRI Internatonal, has set up the descrpton of methane and ts coproducts. The hand-made Matlab functon s compared to ths reference code. Error s defned as: hm ( T ) EM ( T ) T [ ] ma( ) ma (19) ( T ) EM Fg.. Dynamc vscosty for man consttuents of natural gases Fg. 3. Relatve error between hand-made functon and EMKIN for dynamc vscosty

8 6 Natural Gas The varaton of the vscosty of the varous components of natural gas accordng to the temperature s presented on Fgure at atmospherc pressure. Good agreement s obtaned for the 5 major gases consttutng a natural gas, see Fgure Vscometer Varous methods est to measure the dynamc vscosty of a gas (Guérn, 1981): U-tubes of Fagelson (199) are an etenson of Rankne apparatus (1910) Double-elmholtz resonator s frst conceved (Greenspan and Wmentz, 1953). The precson have been etended (Wlhem et al, 000). Rotatonal vscometers are avalable products..3 Thermal conductvty Fourer law characterzes heat conducton: the heat conducton flu crossng surface S n a gven drecton s proportonal to the gradent of temperature proportonalty s called thermal conductvty. T y. Ths factor of T S (0) y.3.1 Pure gases Thermal conductvty of a mono-atomc gas, for whch only the energy of translaton acts, s gven by the tradtonal epresson (Red et al., 1987): T M, * (1) 1 1 K Where s n [ Wm ] Usng Equaton (), thermal conductvty s epressed from dynamc vscosty by: 15 R () M For polyatomc gases (consttuents of natural gases), Euken number Eu s ntroduced: Where Eu M v s the heat capacty at constant volume. (3) v

9 Natural gas: physcal propertes and combuston features 7 For mono-atomc gases, Euken Number s close to 5/. For polyatomc gases, Euken Number s modfed by separatng the contrbutons due to translaton energy from those due to nternal energy (Red et al., 1987): 1 1 mol K M Eu v f tr tr v f Wth tr n [ J ] the part of the heat capacty due to translaton modes tr 3/ R and n, related to nternal modes, s defned as: n v tr, see (Red et al., 1987). Where M Eu p s the heat capacty at constant pressure. v n 9 1 p 1 R A modfed Euken relaton was proposed for whch f n s related to a coeffcent of molecular dffuson too. Ths new relaton s wrtten as, see (Red et al., 1987): n v () (5) M Eu v p 1 R (6) Mason and Monchck (196) worked out a theory based on a dynamc formalsm to calculate the conductvty of polyatomc gases. They obtaned for non-polar gases, by supposng the contrbutons of the neglgble modes of vbratons, the followng epresson: M 1.77 Eu v p 1 R 1 1 Wth rot n [ J mol K ] the part of the heat capacty due to rotaton modes and Z rot the number of collsons necessary to change a quantum of rotaton energy nto translaton energy. Equaton (7) was appled to hydrogen, ntrogen and carbon dode, but the man problem for ther use remans the precse determnaton of the number of collsons of rotaton Z rot whch s functon of the temperature. hung and al. (198) used smlar method to Mason and Monchck (196) and obtaned the relaton of thermal conductvty. Indeed, Euken number s epressed n ths case accordng to a coeffcent of correcton v as follows: rot Z rot v (7) M Eu v 3.75 p v R 1 (8)

10 8 Natural Gas oeffcent s gven by the followng formula: v ; Wth R v 1 (9) and T. r Term s gven by an emprcal correlaton for the contrbuton of translaton energy of the molecules to thermal conductvty for polyatomc gases and apples for the non-polar molecules. As the two man components of the natural gas (methane and ethane) are nonpolar and that the other components have weak dpole moment, ths correlaton represents well the behavour of natural gases. In the case of the polar molecules, a default value of 0,758 should be used. Term corresponds to the heat-storage capacty due to the nternal degrees of freedom. Thus, term can be ncluded/understood as beng a shape factor pontng out the devatons of the polyatomc molecules wth respect to the model of the rgd sphere..3. Gaseous blends Thermal conductvty of blends s estmated n the same manner as for vscosty. The thermal conductvty of a gas mture m can be thus calculated startng from a standard formula n the same way than Equaton (16), see (Red et al., 1987): m 1 Mason and Saena (1958) proposed the followng epresson for coeffcent A j 1 j1 j A 1/ tr, / tr, j M / M j 8 1 M / M 1/ j j 1/ A j : (30) (31) Where tr represents thermal conductvty of monoatomc gas and s a constant close to 1.0; Mason and Saena (1958) proposed eat conductvtes rato due to the energy of translaton of the molecules can be obtaned n a purely emprcal way: 1/ 6 3, Wth 10.0 Tc M, Pc j e e 0.06 Tr, 0.1 Tr, e tr, / tr, j (3) 0.06 T 0.1 T r, j e r, j ; P c, s the crtcal pressure of the th component.

11 Natural gas: physcal propertes and combuston features 9 functon thermal_conductvty = func_conductvty(compo) P = 10135; % current gas pressure n Pa T = 73.15; % current gas temperature n K R = 8.31; %deal gas constant J/K/mol M = [ ]; % molar mass n g mol-1 Tc = [ ];% rtcal temperature Vc = [ ];%rtcal Volume cm3/mol Pc = [ ];% rtcal pressure Dp = [ ];% Dpolar Moment omega = [ ]; methane = *(T/100)^ *(T/100)^ *(T/100)^(-0.5); ethane = *(T/100)-0.60*(T/100)^ *(T/100)^3; propane = *(T/100)-1.571*(T/100)^ *(T/100)^3; butane = *(T/100)-1.833*(T/100)^ *(T/100)^3; nbutane = *(T/100)-1.833*(T/100)^ *(T/100)^3; pentane = R*( *(T/100)+0.153*(T/100)^-0.037*(T/100)^ *(T/100)^); docarbone = *(T/100)^ *(T/100) *(T/100)^; azote = *(T/100)^(-1.5)+107.7*(T/100)^(-)-80.*(T/100)^(-3); oygene = *(T/100)^ *(T/100)^(-1.5)+36.88*(T/100)^(-); hydrogene = *(T/100)^(-0.75)+1165*(T/100)^(-1)-560.7*(T/100)^(-1.5); hydrosulf = R*( *(T/100) *(T/100)^+0.010*(T/100)^ *(T/100)^); monocarbone = *(T/100)^ *(T/100)^(-0.5) *(T/100)^(-0.75); pmol = [methane ethane propane butane nbutane pentane docarbone azote oygene hydrogene hydrosulf monocarbone]; cp=pmol.*1000./m; alpha = (cp.*m*1e-3-r*ones(1,1))./(r*ones(1,1))-1.5*ones(1,1); beta = 0.786*ones(1,1) *omega *omega.^; zed = *ones(1,1)+10.5*(t./tc).^; ps = ones(1,1)+alpha.*(0.15*ones(1,1)+0.888*alpha-1.061*beta *zed)./... (0.6366*ones(1,1)+beta.*zed+1.061*alpha.*beta); %**************Dynamc Vscosty********************************** T_et = 1.593*T./Tc; omegav = *T_et.^(-0.187)+0.587*(ep( *T_et)) *(ep(-.3787*T_et)); mu_r = 131.3*Dp./sqrt(Vc.*Tc); Fc = ones(1,1)-0.756*omega *mu_r.^; eta = 0.785*(Fc.*sqrt(T.*M))./(Vc.^(/3).*omegaV)/ ; %***************************************************************** lambda = 3.75*R*eta.*ps./M*1000; %for mture temp = 10*(Tc.*M.^3./(Pc*10).^).^(1/6); lambda_tr = temp.*(ep(0.06.*(t./tc))-ep(-0.1.*(t./tc))); for = 1:1 for j = 1:1 A(,j) = (1 + sqrt(lambda_tr()/lambda_tr(j))*(m()/m(j))^(1/))^/... sqrt(8*(1+m()/m(j))); end end p1 = lambda.*compo; for = 1:1 p() = p1()/sum(compo.*a(,:)); end thermal_conductvty = sum(p);

12 50 Natural Gas Fg.. Thermal conductvty for man consttuents of natural gases Fg. 5. Relatve error between hand-made functon and EMKIN for thermal conductvty The varaton of the thermal conductvty of the varous components of natural gas accordng to the temperature s presented on Fgure at atmospherc pressure. Good agreement s obtaned for the 5 major gases consttutng a natural gas, see Fgure 5. hm( T ) EM ( T ) T [ ] ma( ) ma (33) ( T ) EM.3.3 Thermal conductvty measurement Dfferent technques can be used to measure the thermal conductvty: Katharometer: Thermal conductvty determnaton of a gas s commonly based on the method of hot wres (Guérn, 1981). A wre s tended n the as of a metal cylndrcal room whose walls are mantaned at constant temperature and traversed by a gas, consttutng a cell. If one apples a constant electromotve force at the ends of ths wre, ts temperature rses untl the energy spent by Joule effect s, at each tme, compensated by the energy dsspated by radaton, convecton and thermal conducton. By choosng condtons such as the losses other than the last are neglgble (temperature of the wre lower than 00, dameter mamum of the tube of 1 cm, rather slow gas flow: 6 to 1 l/h), the temperature of the wre depends prmarly on the nature of the gas whch surrounds t. If the wre has a resstvty whose temperature coeffcent s rased, resstance s functon of the thermal conductvty of ths gas. Guarded ot Plate Method: Guarded hot plate s a wdely used and versatle method for measurng the thermal conductvty. A flat, electrcally heated meterng secton surrounded on all lateral sdes by a guard heater secton controlled through dfferental thermocouples, supples the planar heat source ntroduced over the hot face of the specmens (gas). The most common measurement confguraton s the conventonal, symmetrcally arranged guarded hot plate where the heater assembly s sandwched between two specmens, see Fgure 6. It s an absolute method of measurement and ts applcablty requres: (a) the establshment of steady-state condtons, and (b) the measurement of the undrectonal heat flu n the metered

13 Natural gas: physcal propertes and combuston features 51 regon, the temperatures of the hot and cold surfaces, the thckness of the specmens and other parameters whch may affect the undrectonal heat flu through the metered area of the specmen. Top cold plate Top aulary heater Specmen Guard Metered area Guard Specmen Bottom aulary heater Bottom cold plate Secondary guard Fg. 6. Guarded hot plate method confguraton.. Speed of sound Speed of sound s connected to thermodynamc scale of the flud by the relaton: P c S (3) where P and represent the pressure and the densty respectvely, and S the entropy. The prevous relaton shows the drect lnk between the speed of sound and state equaton of gas...1 Speed of sound for deal gas For deal gas, speed of sound s: For a mture of deal gases, speed of sound s: c R T M (35) p, m R T 1 R T c m (36) M m v, M 1 1 Ideal gas law s a good appromaton for low pressure. owever, n order to take nto account the real behavor of gases, several state laws were proposed. Van Der Waals equaton thus ntroduces two correctve terms: R T a P (37) ( V b) V

14 5 Natural Gas Then, n ths case, speed of sound s: c R T a r (38) b V 1 V Thermodynamc propertes models based on state equaton provde value of compressblty factor. It s useful, n the feld of gas ndustry, to have specfc methods of calculaton for natural gas of commercal type. The equaton derved from vral equaton, establshed by Groups European of Gas Research - GEGR (Jaescheke et al., 003), gves calculaton for the compressblty factor of commercal gas wth an average error of about 0.06% for a pressure up to 1 MPa. owever, one of the methods most used n ths feld s based on the model AGA8-D9 developed by Amercan Gas Assocaton (Starlng & Savdge, 199). Ths model makes t possble to estmate the densty wth an average absolute devaton (AAD) of 0.0% and the speed of sound wth AAD of 0.08%. In addton, Estela-Urbe et al. (003, 005) used another formulaton for natural gas n the range 70 T[ K] 330 and P 1 MPa. Ths model presents compressblty factor accordng to the densty by: oeffcents B m and m Z 1 B m (39) m respectvely represents the second and the thrd coeffcent of the vral development of the gas mture. They are gven accordng to temperature and composton of natural gas by the relatons: m B m j j k j k j B j jk (0) (1) Where B j and jk are gven by: bj,1 bj, Bj bj,0 T T () cjk,1 cjk, j cjk,0 T T (3) Reader s referred to Estela-Urbe et al. (003, 005) for coeffcents b j and c jk.

15 Natural gas: physcal propertes and combuston features 53 Speed of sound s wrtten: Where R T Z R Z c m Z Z T () M m, T T v m v, m s heat capacty at constant volume of the mture calculated by: IGL res v, m v, m v, m (5) IGL v m, s heat capacty calculated by deal gas law, see (Jaeschke & Schley, 1995), and, s resdual correcton, calculated by: res dbm d Bm dm T d m v, m T T T (6) dt dt dt dt res v m functon speedofsound = func_speedofsound(compo) P = 10135; % current gas pressure n Pa T = 73.15; % current gas temperature n K R = 8.31; %deal gas constant J/K/mol M = [ ]; methane = *(T/100)^ *(T/100)^ *(T/100)^(-0.5); ethane = *(T/100)-0.60*(T/100)^ *(T/100)^3; propane = *(T/100)-1.571*(T/100)^ *(T/100)^3; butane = *(T/100)-1.833*(T/100)^ *(T/100)^3; nbutane = *(T/100)-1.833*(T/100)^ *(T/100)^3; pentane = R*( *(T/100)+0.153*(T/100)^-0.037*(T/100)^ *(T/100)^); docarbone = *(T/100)^ *(T/100) *(T/100)^; azote = *(T/100)^(-1.5)+107.7*(T/100)^(-)-80.*(T/100)^(-3); oygene = *(T/100)^ *(T/100)^(-1.5)+36.88*(T/100)^(-); hydrogene = *(T/100)^(-0.75)+1165*(T/100)^(-1)-560.7*(T/100)^(-1.5); hydrosulf = R*( *(T/100) *(T/100)^+0.010*(T/100)^ *(T/100)^); monocarbone = *(T/100)^ *(T/100)^(-0.5) *(T/100)^(-0.75); pmol = [methane ethane propane butane nbutane pentane docarbone azote oygene hydrogene hydrosulf monocarbone]; MassMol =1/100*sum(M.*compo); eatapacty = 1/100*sum(pmol.*compo)*1000./MassMol; speedofsound = sqrt(eatapacty /( eatapacty -1000*R/MassMol)*R*T/MassMol*1000 The varaton of the speed of sound of the varous components of natural gas accordng to the temperature s presented on Fgure 7 at atmospherc pressure. Good agreement s obtaned for the 5 major gases consttutng a natural gas, see fgure 8. chm ( T ) cem ( T ) T [ ] ma( c ) ma (7) c ( T ) EM

16 5 Natural Gas Fg. 7. Speed of sound for man consttuents of natural gases Fg. 8. Relatve error between hand-made functon and EMKIN for speed of sound.. Sound velocty sensor Acoustc wave propagaton s characterzed by the speed of sound c n the propagaton medum. Several technques allow the measurement of ths characterstc n gases. Three methods of measurement can be dstngushed such as: - the acoustc waves dephasng, - the acoustc resonator, - the tme of transt. The last method s largely used n ndustral applcatons such as level measurement, flow meterng, etc (auptmann et al., 00). It nvolves measurement of the transt tme of an ultrasonc pulse travellng over a known propagaton dstance n the gas. Ths technque typcally employs one or more pezoelectrc transducers to generate and detect sound waves n the frequency range of about 0 kz to 1 Mz and hgher. A partcular technque known as a pulse echo technque uses a sngle transducer as both the transmtter and the recever see Fgure 9. The generated sound wave s reflected back to the source transducer from a target located at a known dstance from the transducer, and s receved by the same transducer. If the dstance between the transducer and the reflectng target s D, and the measured two-way travel tme s t, then the speed of sound s represented by: D c (8) t Ths method s advantageous because t uses only one transducer. owever, n applcatons requrng hgh precson speed of sound measurements, the method has the dsadvantage of ntroducng tme delay errors assocated wth mperfectly defned and varable dstance, D, and an mperfect ablty to determne the eact tme delay wth respect to the tme of the transmtted pulse and the tme nstant when the reflected sound wave s receved at the transducer.

17 Natural gas: physcal propertes and combuston features 55 Gas nput Gas nput Temperature Temperature Ultrasonc transducer t Obstacle Ultrasonc transducer t t 1 Obstacle D Gas output D 1 D DD Gas output Fg. 9. Pulse echo technque Fg. 10. Modfed pulse echo technque To reduce the tme delay error, the pulse echo method may be modfed to measure a tme dfference between two receved sgnals (Kelner et al., 00). A transmtted wave s reflected from two dfferent targets rather than a sngle target, see Fgure 10. The dstance, DD, between the two targets s known. Usng ths method, the speed of sound s represented by: where c gas t s the tme dfference between the two receved sgnals. DD (9) t.5 Refractve nde Guérn (1981) epressed refractve nde n g of a gas, for radaton of wavelength, accordng to the densty: n 0 g 1 q g def IR (50) Where q s a constant. Notng RI 0 the value of RI correspondng to the normal condtons (73,15 K, 1 atm) and assumng that the gases follow deal gas law, the value of RI (called co-nde of refracton, but named mproperly refractve nde too) relates to temperature T (n Kelvn) and pressure P (n atmosphere) s gven by: o-nde of refracton has an addtve property: T0 P RI RI0 (51) T P 1 0 RI RI (5)

18 56 Natural Gas Equatons (51-5) are enough to calculate wth precson the co-nde of refracton of natural gases..6 Densty and specfc densty In the case of a gas mture, the epresson of the specfc densty d m s wrtten: wth IGL Z ar ( T, P) dm dm (53) Z ( T, P) m Z m 1 1 Z (5) Wth Z compressblty factor of component, molar fracton of hydrogen. Specfc densty the equaton: IGL d m s ndependent of any state of reference and s calculated startng from In the same way, the densty s obtaned by: IGL M d m (55) M 1 IGL m ar ( T, P) T, P (56) Z ( T, P) IGL m P R T m 1 M (57).7 Synthess Qualty of natural gas, manly composed of methane, vares accordng to the varous sources of supply (layers). onsequently, physcal propertes and energy content are subject to varatons. As a result, one of the mportant nformaton requred for natural gas eplotaton relates to ts physcal propertes. Besdes the propertes of transport (vscosty, thermal conductvty), varous models of determnaton speed of sound, nde of refracton and densty were presented.

19 Natural gas: physcal propertes and combuston features ombuston features ombuston features of a gas such as the low heatng value, Wobbe nde and ar-fuel equvalence rato are of a great ndustral nterest. These propertes nterest both engne manufacturers and busness actvtes of P nstallatons and bolers. The commercal transactons on natural gas are generally based on the energy content of gas, obtaned by multplyng the volumes measured by the hgher heatng value. 3.1 Ar Fuel Rato Ar Fuel rato s defned as the rato of ar volume (or mass) V a (at normal condtons of temperature and pressure) requred to the theoretcal complete combuston per fuel volume unt (or mass). omplete combuston of generc fuel y O z N u under stochometrc condtons gves equvalence rato [Nm3/Nm3]: O O N O N N O O O N stoch u z y 1% % 79 (58) N O O N N O O O N O O O N O O O u z y (59) 1% 1 z y (60) 1 (61) Industral combuston s never complete, dssocatons/recombnatons occurred.... 1% 79% NO NO O O O O O N O N N O NO NO O O O O O N u z y (6) Where s the relatve ar fuel rato.

20 58 Natural Gas 3. eatng value Low heatng value s the energy released durng fuel combuston (of unt of mass or volume) under stochometrc condton and thermodynamc condtons (P, T) gvng O and O products. Through the world, dfferent thermodynamc reference condtons are consdered as reference condtons. LV LV (63) 1 gh heatng value V s deduced from low heatng value LV and s defned as the heat that can be obtaned by condensng the water vapor produced by combuston. 3.3 Wobbe nde Wobbe nde (W) s an mportant crteron of nter-changeablty of gases n the ndustral applcatons (engnes, bolers, burners, etc). Gas composton varaton does not nvolve any notable change of ar factor and of flame speed when Wobbe nde remans almost constant. Wobbe nde can be calculated startng from the hgh heatng value (V) and specfc gas densty (d) by: V W (6) d Ths parameter s usually used to characterze gas qualty. Indeed, two gases wth the same Wobbe nde delver the same quantty of heat for the same supply pressure. Thus, for an ndustral burner for eample, one mantans heat flow wth a constant value by the output control of gas accordng to the nde of Wobbe. In DOE report (007), a modfed Wobbe nde s used n real applcatons: LV W r (65) d T Ths modfed Wobbe nde takes account for heatng of the fuel and the uncovered heat from water vapour formed durng combuston. 3. Methane number Methane number (MN) characterzes gaseous fuel tendency to auto-gnton. By conventon, ths nde has the value 100 for methane and 0 for hydrogen (Leker et al., 197). The gaseous fuels are thus compared wth a methane-hydrogen bnary mture. Two gases wth same value of MN have the same resstance aganst the spontaneous combuston

21 Natural gas: physcal propertes and combuston features 59. Measurng nstruments ombuston features can be determned accordng to two types of methods: drect or ndrect. Drect methods are based on calormetrc measures where the energy released by the combuston of a gas sample s measured. Indrect methods are ssued of ether calculaton from gas composton, or of measurements of gas physcal propertes..1 alormeter Ths drect method s based on calormetrc measures. Ulbg & oburg (00) syntheszed measurement of heat value by: combuston of a gas sample nsde a calormetrc bomb (sochorc combuston), combuston of a gas wth a gas-burner (sobar combuston), catalytc combuston (sobar combuston wthout flame) by odaton of a gas on a catalyst. ombuston technque wth a gas-burner s largely used. Varous types of calormeters, based on ths technque, are employed: Junkers, Renke, Thomas--ambrdge and ulter-- ammer. Operaton prncple, presented on Fgure 11, s dentcal. Specfc quantty of gas s measured then burned completely. In a heat echanger, energy released by combuston heats a coolant (water or ar). onsequently, coolant temperature ncrease makes t possble to measure gas heatng value. Apparatus calbraton s done usng reference gas whch ts specfc heatng value s known (n general pure methane). Ehaust temperature Outlet temperature Burner eat echanger Inlet temperature Fuel Mer Water storage at T [K] Ar Fg. 11. alormeter operaton prncple atalytc combuston s safe way (flameless) to measure hgh heatng value of gases (ornemann, 1995), (eyden & Berg, 1998). Ths batch method s based on the followng prncple: gas mture and ar are ntroduced on a noble metal (platnum). Ar quantty ntroduced s suffcent for gas mture odaton. ydrocarbons are odzed over noble metal beng a catalyst. The procedure s renewed thereafter wth an unknown gas mture. eat released can be measured ether startng from temperature changes related to the catalytc reacton, or startng from electrc output changes requred to keep catalyst at

22 60 Natural Gas constant temperature. Ths method can however be subject at two errors: ncomplete gas odaton or catalyst posonng.. Stochometrc combuston For saturated lnear hydrocarbons (alkanes), there ests a lnear relaton between ar fuel rato and low heatng value of gas mtures, see Fgure 1. Ths measurement prncple s thus ssued on ar volume knowledge per unt of gas volume under stochometrc combuston. onsequently, that makes t possble to reach the calorfc value of gas startng from the followng relaton see (Ingran, 1990): V a LV K (66) Vm n-pentane Low eatng Value [MJ/m3] Ethane Propane -Butane n-µbutane 0 Methane Stochometrc ar-to-gas rato Fg. 1. Lnear relaton between LV and Stochometrc Ar-to-gas rato.3 Gas composton Gas chromatography and mass spectroscopy are the most commonly employed laboratory technques. These two technques are based upon the separaton of gas speces followed detecton..3.1 Gas chromatography Gas chromatography s a partton method. It s based on components dstrbuton of a sample between moble phase (the gas) and statonary phase (lqud or sold), see Fgure 13 upon a column. The column provdes a pathway, whch ams to separate the speces based upon molecular sze, charge, polarzablty, and other physcal parameters whch lmt nteractons between the gas speces and the column materals. If the components of the sample have dfferent partton coeffcents between the two phases, they mgrate wth dfferent speeds. An nert carrer gas (e.g. ntrogen or helum) s used to transport the gas sample through the columns.

23 Natural gas: physcal propertes and combuston features 61 Pressure controller Flter ontrol valve Processng Unt Mer Pressure controller Fg. 13. Gas chromatography prncple olumn Oven Detecton cell Gas chromatography s consdered accurate and relable. owever, t s usually slow and the columns requre mantenance. It s not consdered practcal for a contnuous on-lne applcaton. alculaton of heatng value and Wobbe nde of gas are obtaned regards to the standard procedure ISO For a natural gas, the hgher heatng value s wrtten see (Ingran, 1990): V Where Z m s the compressblty factor, see Equaton (5). V 1 (67) Z m.3. Mass Spectrometer Mass spectrometer s based manly upon the mass-to-charge rato of onzed speces. The mass spectrum s generated by frst onzng natural gas and the acceleratng t wth an electrc feld. The ons are separated by ther momentum dstrbuton. Most mass spectrometers have software that determnes gas concentratons from peak ntenstes and allows real tme calculatons. Gas composton by Infrared spectroscopy Infrared spectroscopy method eplots the property that natural gas components absorb lght n a gven wavelengh of the nfra-red spectrum. Only the hydrogen, whch do not absorb the nfrared radaton, and the carbon monode, whch absorbs n another area of the spectrum, do not take part n ths phenomenon. The general dagram s represented on the fgure 1. From calbraton, usng the absorpton band of the methane and the band of hydrocarbons hgher than methane, the measurement of radaton absorpton lead to the determnaton of the heatng value of natural gas. owever, the components hgher than the, as well as hydrogen, do not absorb the nfra-red radaton. onsequently, those components are not taken nto account for the calculaton of the calorfc value

24 6 Natural Gas Focusng mrror Measurement cell Gas output Infrared probe Wave pulse sgnal Gas nput Slt and Flters IR generator Fg. 1. Infrared spectroscopy prncple Focusng mrror 5. Physcal propertes methods Besdes the combuston or analyss of fuel gases, t s also possble to correlate and, respectvely, calculate the calorfc value by measurng the physcal propertes of the gas mture. orrelatons have to be determned, allowng the calorfc value to be relably calculated as a functon of one or several dfferent physcal propertes. For gases dstrbuted n the Unted Kngdom, Brtsh Gas (Thurston et al., 00) establshed correlaton to estmate low heatng value LV regards to speed of sound measurement and thermal conductvty of natural gas. Low heatng value s thus gven by the epresson : LV a 1 T a T a3 c a Ta a5 Ta a6 (68) L Where T s the thermal conductvty of natural gas at temperature T, respectvely T L the thermal conductvty at temperature T L, c s the speed of sound, ambent temperature, a 1 to a 6 are the ftted coeffcents. T a the Thurston et al. (00) proposed : a , a , a , a , a and a under condtons T Ta 70 K and TL Ta 50 K. In partnershp wth the department of energy of the Unted States, Morrow and Behrng (1999), of Southwest Research Insttute, have developed a correlaton based on speed of sound, ntrogen and carbon dode contents. V s evaluated by:

25 Natural gas: physcal propertes and combuston features B M V (69) M B c (70) Where N s the volume fracton of N, respectvely N O O the volume fracton of O, and c s the speed of sound, M s the mass molar of the mture, evaluated by: 0 M a 1 a N a O c 0 (71) Bonne (1996) proposed a general epresson : V T T 9. T L (7) Where s the thermal conductvty (cal/s m ) and T L, T are the low and hgh temperatures respectvely ( ). s the gas vscosty (µpose). Ths V estmaton 3 has a mamal error of MJ. m wth a standard error of MJ. m V p M (73) Pnvdc et al. (000), of Gaz de France, developed a correlaton of hgh heatng value regards to lght beam optcal absorpton by gas components. Lght beam defnes three bands of wave length measure, located to the near nfrared, wth a wdth of 10 to m. V s deduced from the measurement of temperature T, pressure P and, 1,, 3 of natural gas n three bands. V s epressed transmsson coeffcents thus by the followng relaton: Where a 0,, a 1,, 0 mamum error of 1%. 3 T T V a0, a1, ln a0 a1 (7) P P 1 a and a 1 are obtaned from reference gases. Ths fttng has a Tacke and Kastner (003) developed measure devce of V, n partcular, for natural gas. The apparatus set up uses nfrared radaton dsperson regards to wavelengths gettng a spectrum. Ths spectrum s detected by a system to be analyzed thereafter. V s deduced through the correlaton:

26 6 Natural Gas Where V V (75) s the absorpton coeffcent for wavelength and heatng value of the component whch has ts wavelength. V s the hgh Florsson and Burre (1989) determned Wobbe nde based on densty estmaton and ntrogen and carbon dode contents. The correlaton s vald only for gases whose Wobbe nde s n the range 3. and. MJ m d O 6.57 N W (76) d Pckenäcker et al. (000) showed that Wobbe nde can also be gven by a technque based on measurements of dynamc vscosty. They obtaned the followng correlaton: Rahmoun et al. (003-00) T ref W 8.86 ln (77) T Fg. 15. Determnaton of the ternary composton by usng a mture dagram.

27 Natural gas: physcal propertes and combuston features 65 A non-correlatve method was proposed by Rahmoun et al. (00). ombuston propertes of natural gas were calculated from a ternary composton that has the same physcal propertes as the tested gas. As llustrated on Fgure 15, when measured physcal propertes of natural gas are represented n a ternary dagram mture, an equvalent ternary composton s determned. That pseudo-composton has no connecton to the composton of the real gas (natural gas of fve or more consttuents). The term equvalent means that the three compounds gas, has the same two physcal propertes than the real gas (same refracton nde and same thermal conductvty for nstance). Then, the combuston propertes are evaluated usng the equvalent ternary composton (or pseudo-composton) and match the real gas combuston propertes. The choce of these propertes was based on a statstcal analyss (numercal eperment plan, prncpal component analyss) of a natural gas base dstrbuted n Europe. The basc assumpton s that f the measurement of two physcal propertes corresponds to the descrpton of a ternary gas, then the propertes of combuston of ths ternary gas wll be also those of the real gas whch models the ternary gas, wth the help of a reasonable error. The statstcal analyss concluded that methane and ethane, then comes propane and ntrogen, are the most nfluent component of natural gas. Thus, two ternary mtures can be consdered: or - 6 -N. Three combnatons Ternary dagram / physcal propertes were used: - The measurement of thermal conductvty and co-nde of refracton for T = 93 K and usng dagram, - ombnaton of the precedent measurements wth the - 6 -N dagram, - The measurement of thermal conductvty for two temperature levels 93 K and 33 K. The relatons below descrbe the ntersecton of two physcal propertes n a ternary dagram whch gve the ternary pseudo-composton: 1 1 b1 1 b 3 a a1 1 1 b b1 1 a a a b (78) 3 a1 b (79) (80) Where : Physcal property, 1, and 3 : Ternary gas composton, 10 : Lower lmt of 1 as, 0 Lower lmt of as, a1, b1, a and b : oeffcents dependng on physcal propertes and temperature. Once the composton of the ternary mture s determned, the combuston propertes can be calculated usng Equatons 63, 6 and 65.

28 66 Natural Gas 0, MAD = 0.01% 0,1 Relatve error [%] 0,0-0,1-0, V [MJ/m 3 ] Fg. 16. Relatve error between pseudo-composton and drect G analyss for hgh heatng value Based on the work of Rahmoun et al. (003-a and 003-b), Loubar et al. (007) have used a qunternary pseudo-composton n order to mprove the accuracy of combuston propertes determnaton. owever, wth a pseudo-composton of fve components, t s mpossble to use the prevous graphcal method nvolvng mture dagrams. So, they determned the compostons by solvng a nonlnear system of equatons epressng, for qunternary gases, the thermal conductvty (Equaton 30), at two fed temperatures (T 1 = 333 K and T = 383 K), the speed of sound determned at T 3 = 303 K (Equaton 36) and the carbon dode content. Fgure 16 shows the percentage of the relatve error of hgh heatng value, calculated from pseudo-composton and the G analyss. The mean absolute devaton (MAD) s about 0.01%. 6. Summary The actual composton of natural gas depends prmarly on the producton feld from whch t s etracted and lmted varatons n composton must therefore be accepted. Moreover, at a local dstrbuton level, seasonal adjustments by the local gas dstrbutor may cause sgnfcant varatons n the gas composton. onsequently, physcal propertes and energy content are subject to varatons and ther calculaton / estmaton s of great mportance for techncal and economcal aspects. In ths chapter, physcal models for the calculaton of physcal propertes of natural gas are presented. Physcal models of transport propertes (vscosty, conductvty) result from the knetc theory of gases. Vscosty and thermal conductvty of gases mture (lke natural

29 Natural gas: physcal propertes and combuston features 67 gas) can be estmated from the propertes of pure gases and requrng some correcton factors. These physcals models offer a good tool for ndustral calculatons and applcatons. Speed of sound can be easly determned from the composton of natural gas. Besdes, dfferent methods and devces are developed to measure ths property wth a good accuracy. A partcular technque known as a pulse echo was presented. Ths technque uses a sngle ultrasonc transducer as both the transmtter and the recever and two obstacles separated by a known dstance. The speed of sound of natural gas s then determned from the transt tme and the dstance between the targets. In addton, propertes such as refractve nde and the specfc densty were presented. These propertes can be used n correlatons n order to estmate other propertes that are dffcult to measure or need epensve nstruments. Varous technques of determnaton of combuston features such as ar-fuel rato, the low heatng value and Wobbe nde are eposed. These technques are based on drect or ndrect methods. Besdes the combuston or analyss of fuel gases, t s possble to correlate and, respectvely, calculate the combuston features by measurng the physcal propertes of the gas mture. Some correlatons can be lmted to a regon, a qualty of natural gas or a specfed range of combuston propertes varaton. So, t s mportant to consder those correlatons accordng to ther specfed condtons. Researchers and ndustrals contnue ther efforts n developng methods and devces to reach rapd and accurate measurement / determnaton of natural gas propertes regardng the stakes. 7. References Bonne U. (1996). Sensng fuel propertes wth thermal mcrosensors. Proc. SPIE Vol. 7, p , Smart Structures and Materals 1996: Smart Electroncs and MEMS, Vjay K. Varadan; Paul J. McWhorter; Eds. hapman. & owlng T.G. (1970). The mathematcal theory of non-unform gases, Press Syndcate of the Unversty of ambrdge, ISBN hung T.., Lee L.L. & Starlng K.E. (198). Applcatons of Knetc Gas Theores and Multparameter orrelaton for Predcton of Dlute Gas Vscosty and Thermal onductvty, Industral & engneerng chemstry fundamentals, Vol. 3, No. 1, p 8-13, ISSN hung T.., Ajlan M., Lee L.L. & Starlng K.E. (1988). Generalzed Multparameter orrelaton for Nonpolar and Polar Flud Transport Propertes, Industral & engneerng chemstry research, Vol. 7, No., p , ISSN DOE (007) LNG Interchangeablty/Gas Qualty: Results of the Natonal Energy Technology Laboratory s Research for the FER on Natural Gas Qualty and Interchangeablty, DOE/NETL-07/190, prepared by U.S. Department of Energy, Natonal Energy Technology Laboratory Estela-Urbe J.F., Jaramllo J., Salazar M.A. & Trusler J.P.M. (003). Vrel equaton of state for natural gas systems. Flud Phase Equlbra, Vol. 0, No., pp ISSN Estela-Urbe J.F. and Jaramllo J. (005). Generalsed vrel equaton of state for natural gas systems. Flud Phase Equlbra Vol. 31, No.1, pp , ISSN

30 68 Natural Gas Florsson O. & Burre P.. (1989). Rapd determnaton of the Wobbe nde of natural gas. Journal of physcs. Part E. Scentfc nstruments ISSN , Vol., No., p Guérn. (1981) Traté de manpulaton et d'analyse des gaz. Ed. Masson nd édton. Greenspan M. and Wmentz F.N. (1953). An acoustc vscometer for gases- Part I. Natonal Bureau of Standard Report No pages 5. OSTI ID: anley.j., Mcarty R.D. & aynes W.N. (1975). Equaton for the vscosty and thermal conductvty coeffcents of methane, ryogencs, Vol. 15, No. 7, p13-17 auptmann P., oppe N. and Püttmer A. (00). Applcaton of ultrasonc sensors n the process ndustry. Meas. Sc. Technol. 13 R73 R83, ISSN eyden W..V. & Berg R.A. (1998) Measurng heatng value usng catalytc combuston. Unted States Patent, US A, pp. 1--1, rschfelder J.O., urtss.f. & Brd R.B. (195). Molecular theory of gases and lquds, Ed. John Wley, ISBN ornemann J.A.T. (1995) Method for determnng the calorfc value of a gas and/or the Wobbe nde of natural gas. European Patent, EP B1, pp Ingran D. (1990). alorc value of gases (n french). Technque de l'ngéneur, ISSN Vol. R3, No. R 980, pp Jaescheke M., Bento A., Fredhem A., enault J. M., Vglett B., van Wesenbeeck P., Klmeck R., Kunz O. & Wagner W. (003), GERG Project : Wde-range reference equaton of state for natural gases. Gas- und Wasserfach. Gas Erdgas, Vol. 1, No. 7, p , ISSN Jaeschke M. and Schley P. (1995). Ideal gas thermodynamc propertes for natural gas applcatons. Internatonal Journal of Thermophyscs, Vol. 16, No. 6, p , ISSN X, DOI /BF Kelner E., Mnach A., Owen T. E, Burzynsk JR. M. and Petullo S. P. (00). Devce for precson measurement of speed of sound n a gas. Unted States Patent, US 00/ A1. Leker M., hrstoph K., Rankl M., arteller W. & Plefer U. (197). Evaluaton of antknockng property of gaseous fuels by means of methane number and ts practcal applcaton to gas engnes, ASME paperno. 7-DGP-, Vol. 9, Issue 7, p55 Le Nendre B. (1998). eat conductvty of gases and lquds (n french), Ed. Technque de l ngéneur, K 7, p1-3. Loubar K., Rahmoun., Le orre O. and Tazerout M. (007). A combustonless determnaton method for combuston propertes of natural gases, Fuel, Vol. 86, No. 16, p 535-5, ISSN , DOI: /j.fuel Morrow T.B. and Behrng K.A. (1999). Energy flow measurement technology, and the promse of reduced operatng costs. th Internatonal Symposum on Flud Flow Measurement, Denver, O. June 8--30, pp. 1--1, Mason E.A. and Monchck I. (196). eat onductvty of Polyatomc and Polar Gases, The journal of hemcal Physcs, Vol. 36, p ISSN do: / Mason E.A. and Saena S.. Appromate formula for the thermal conductvty of gas mtures, The Physcs of Fluds, Vol.1, No. 5, p , ISSN do: /

31 Natural gas: physcal propertes and combuston features 69 Neufeld P.D., Jansen A.R. & Azz R.A. (197). Emprcal equatons to calculate 16 of the * ( l, s) transport collson ntegrals for the (1-6) potental, Journal of hemcal Physcs, Vol. 57, p , ISSN Pckenäcker K., Trms D. & Wawrznek K. (000). Ecess ar controlled operaton of bolers and furnaces by means of Wobbe number measurement. 5 th European onference on Industral Furnaces and Bolers (INFUB5), ISBN , Espnho-Porto, Portugal (11/0/000) Pnvdc J.J., Juen G.L. & Pelous G.P. (000). Method and apparatus for determnng the calorfc value of a natural gas optcally and n real tme. Unted States Patent, US S615755A. Sakaly K., Rousseau S., Rahmoun., Le orre O. & Truffet L. (008). Safe operatng condtons determnaton for statonary SI gas engnes, Fuel Processng Technology, Vol. 89, No. 11, 1p , ISSN Rahmoun., Le orre O. and Tazerout M. (003-a). Onlne determnaton of natural gas propertes, omptes Rendus Mecanque, Vol. 331, No. 8, Pages , ISSN , DOI: /S (03) Rahmoun., Tazerout M. and O. Le orre (003-b)., Determnaton of the combuston propertes of natural gases by pseudo-consttuents*, Fuel, Vol. 8, No. 11, Pages , ISSN , DOI: /S (03) Rahmoun., Tazerout M. and Le orre O. (00). Method for determnng at least one energetc property of a gas fuel mture by measurng physcal propertes of the gas mture, Patent US , , Also publshed as US , FR87961, RU , WO030135, EP117 Red R.., Prausntz J.M. and Polng B.E. (1987). The propertes of gases & lquds. Ed. McGraw ll Book ompany, Fourth Edton, ISBN Rojey A., Durand B., Jaffret., Jullan S. & Valas M. (000). Natural gas : Producton processng transport (translate by N. Marshall), Eds Technp, ISBN Starlng K. E. and Savdge J. L. (199), ompressblty factors of natural gas and other related hydrocarbon gases. AGA Transmsson Measurement ommttee. Report 8, Amercan Gas Assocaton. Tacke M. & Kastner J. (003). Photometrc devce and photometrc method for determnng the gross calorfc value of a test gas. Unted States Patent, US B1. Thurston R.R., ammond P.S. and Prce B.L. (00). Method and apparatus for measurng the calorfc value of a gas. Unted States Patent, US 6996B1. Ulbg P. & oburg D. (00). Determnaton of the calorfc value of natural gas by dfferent methods. Thermochmca acta. Vol. 38, No.1-, pp. 7 35, ISSN Vogel E., Wlhelm J., Küchenmester. & Jaeschke M. (000). gh-precson vscosty measurements on methane, gh temperatures-hgh pressures, Vol. 3, No. 1, p , do: /htwu359. Wlhem J., Glls K.A., Mehl J.B. and Moldover M.R. (000). An mproved Greenspan acoustc vscometer. Internatonal Journal of Thermophyscs, Vol.1, No. 5, p , ISSN X, DOI: /A: Wlke.R. (1950). A vscosty for gas mtures, Journal of hemcal Physcs, Vol. 18, p , ISSN

32 70 Natural Gas

33 Natural Gas Edted by Prmoà ¾ Potoà  nk ISBN ard cover, 606 pages Publsher Scyo Publshed onlne 18, August, 010 Publshed n prnt edton August, 010 The contrbutons n ths book present an overvew of cuttng edge research on natural gas whch s a vtal component of world's supply of energy. Natural gas s a combustble mture of hydrocarbon gases, prmarly methane but also heaver gaseous hydrocarbons such as ethane, propane and butane. Unlke other fossl fuels, natural gas s clean burnng and emts lower levels of potentally harmful by-products nto the ar. Therefore, t s consdered as one of the cleanest, safest, and most useful of all energy sources appled n varety of resdental, commercal and ndustral felds. The book s organzed n 5 chapters that cover varous aspects of natural gas research: technology, applcatons, forecastng, numercal smulatons, transport and rsk assessment. ow to reference In order to correctly reference ths scholarly work, feel free to copy and paste the followng: Olver Le orre and Khaled Loubar (010). Natural Gas : Physcal Propertes and ombuston Features, Natural Gas, Prmoà ¾ Potoà  nk (Ed.), ISBN: , InTech, Avalable from: InTech Europe Unversty ampus STeP R Slavka Krautzeka 83/A Rjeka, roata Phone: +385 (51) Fa: +385 (51) InTech hna Unt 05, Offce Block, otel Equatoral Shangha No.65, Yan An Road (West), Shangha, 0000, hna Phone: Fa: