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Properties of Artificial Gaseous Mixtures for their Safe Use and Support the Natural Gas Supply Networks / Własności Sztucznych Mieszanin Gazowych do Bezpiecznego ich Użytkowania i Wspomagania Zasilania Sieci Gazu Ziemnego

Properties of Artificial Gaseous Mixtures for their Safe Use and Support the Natural Gas Supply... Arch. Min. Sci., Vol. 57 (2012), No 2, p. 351­362 Electronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.2478/v10267-012-0022-5 MARIUSZ LACIAK* PROPERTIES OF ARTIFICIAL GASEOUS MIXTURES FOR THEIR SAFE USE AND SUPPORT THE NATURAL GAS SUPPLY NETWORKS WLASNOCI SZTUCZNYCH MIESZANIN GAZOWYCH DO BEZPIECZNEGO ICH UYTKOWANIA I WSPOMAGANIA ZASILANIA SIECI GAZU ZIEMNEGO The increase in natural gas consumption by the general public and industry development, in particular the petrochemical and chemical industries, has made increasing the world interest in using gas replacement for natural gas, both as mixtures of flammable gases and gas mixtures as LPG with air (SNG ­ Synthetic Natural Gas). Economic analysis in many cases prove that to ensure interchangeability of gas would cost less than the increase in pipeline capacity to deliver the same quantity of natural gas. In addition, SNG systems and installations, could be considered as investments to improve security and flexibility of gas supply. Known existing methods for determining the interchangeability of gases in gas gear based on Wobbe index, which determines the heat input and the burning rate tide, which in turn is related to flame stability. Exceeding the Wobbe index of a value increases the amount of carbon monoxide in the exhaust than the permissible concentration. Methods of determining the interchangeability of gases is characterized by a gas in relation to the above-described phenomena by means of quantitative indicators, or using diagrams interchangeability, where the gas is characterized by the position of a point in a coordinate system. The best known method for determining the interchangeability of gases is Delbourg method, in which the gas is characterized by the revised (expanded) Wobbe Index (Wr), the combustion potential, rate of soot formation (Ich ) and the ratio of the formation of yellow ends (Ij ). Universal way to determine the interchangeability of gas is also Weaver accounting method. It does not require determination of the reference gas. It is designed for utensils for household gas and gas pressure p = 1.25 kPa. The criteria and definition of gas interchangeability volatility in practice to the combustion in a gas gear. In the case of gas exchange in industrial furnaces, interchangeability criteria are usually not very useful because of other conditions of combustion and heat exchange. In industrial reheating furnace gas is combusted in a sealed combustion chambers. Air supply is regulated. The exhaust gases are discharged into canals and the chimney to the atmosphere. The temperature difference between load (fuel gas) and the flame is much less than in the case of gas household appliances. In the furnace heat exchange takes place mainly by radiation in 85% to 95%. The value of heat flux flowing from the gas to a heated charge is not proportional to the heat load burners. Interchangeability of gas is linked by adding to natural gas, a certain amount of gas that is a substitute for natural gas in meeting the criteria for substitution in order to ensure certainty of supply of natural gas to customers. Gases that can be used in the processes of blending and used as replacement gases are mainly a mixture of propane and propane ­ butane (LPG ­ Liquid Petroleum Gas), landfill gas or biogas (LFG ­ Landfill Gas) and dimethyl ether (DME). One * AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF DRILLING, OIL AND GAS, AL. MICKIEWICZA 30, 30-059 CRACOW, POLAND of the more well-known gas mixtures used in many countries around the world to compensate for peak demands is a mixture containing about 75% of natural gas and approximately 25% propane / air (LPG / air). Also in Poland is prepared to amend the provisions in this regard (at this moment ­ oxygen in the gas network can not exceed 0.2%). In this paper, the calculations of interchangeability of gas mixtures LFG ­ LPG and LPG ­ air (SNG) for natural gas was made. It was determined whether the analyzed mixtures have similar stable flame zones regardless of the quality of LFG fuel and whether they may in whole or in part replace CH4, without any modification of equipment suction air for combustion. The obtained results will determine whether the fuel can be used as a replacement for natural gas used in such household appliances and, possibly, industrial burners. In connection with the possibility of changes in the quality of LFG, depending on such factors as storage time, as pre-treatment, will be determined the degree of interchangeability of LFG as a fuel mixed with regard to its quality. Keywords: Natural gas, interchangeability of gases, burning velocity, peak shaving, Liquid Petroleum Gas, propane, Landfill Gas, Wobbe Index Wzrost zuycia gazu ziemnego przez odbiorców komunalnych oraz rozwój przemyslu w szczególnoci petrochemicznego i chemicznego sprawil, e na calym wiecie wzroslo zainteresowanie zastosowaniem gazów zamiennych za gaz ziemny, zarówno jako mieszanin gazów palnych jak i jako mieszanin gazów plynnych z powietrzem (SNG ­ syntetyczny gaz ziemny). Przeprowadzane analizy ekonomiczne w wielu przypadkach dowodz, e zapewnienie wymiennoci paliwa gazowego kosztowalo by mniej ni zwikszenie przepustowoci gazocigów dla dostarczenia tej samej iloci gazu ziemnego. Ponadto systemy i instalacje SNG, mona by uzna za inwestycje poprawiajce bezpieczestwo i elastyczno dostaw gazu. Znane dotychczasowe metody okrelania zamiennoci gazów w przyborach gazowych oparte s na liczbie Wobbego, która decyduje o obcieniu cieplnym przyboru i szybkoci spalania, z któr z kolei zwizana jest stabilno plomienia. Przekroczenie liczby Wobbego o pewn warto powoduje wzrost iloci tlenku wgla w spalinach ponad dopuszczalne stenie. Sposoby okrelajce wymienno gazów charakteryzuj dany gaz w odniesieniu do opisanych wyej zjawisk przy pomocy wskaników liczbowych lub za pomoc diagramów wymiennoci, na których gaz jest scharakteryzowany przez poloenie punktu w ukladzie wspólrzdnych. Najbardziej znan metod okrelenia zamiennoci gazów jest metoda Delbourga, w której gaz scharakteryzowany jest przez skorygowan (rozszerzon) liczb Wobbego (Wr ), potencjal spalania, wspólczynnik tworzenia si sadzy (Ich) oraz wspólczynnik powstawania óltych koców (Ij ). Uniwersalnym sposobem okrelenia zamiennoci gazu jest równie metoda rachunkowa Weavera. Nie wymaga ona okrelenia gazu odniesienia. Przeznaczona jest dla przyborów gazowych uytku domowego i cinienia gazu p = 1,25 kPa. Kryteria zmiennoci gazów i definicja zamiennoci w praktyce dotyczy spalania gazów w przyborach gazowych. W przypadku wymiany gazu w piecach przemyslowych kryteria zamiennoci s zazwyczaj malo przydatne z powodu innych warunków spalania i wymiany ciepla. W przemyslowych piecach grzewczych gaz spala si w zamknitych komorach spalania. Doplyw powietrza jest regulowany. Spaliny odprowadzane s kanalami i kominem do atmosfery. Rónica temperatur nagrzewanego wsadu (paliwa gazowego) i plomienia jest duo mniejsza ni w przypadku przyborów gazowych domowego uytku. W piecach wymiana ciepla odbywa si glównie przez promieniowanie w 85% do 95%. Warto strumienia cieplnego plyncego od gazu do ogrzewanego wsadu nie jest proporcjonalne do obcienia cieplnego palników. Zamienno gazów zwizana jest dodawaniem do gazu ziemnego pewnej iloci gazu bdcego substytutem naturalnego gazu ziemnego przy spelnieniu kryteriów zamiennoci w celu zagwarantowania pewnoci dostaw gazu ziemnego do odbiorców. Gazy mogce by uyte w procesach mieszania i wykorzystane jako gazy zamienne to przede wszystkim propan lub mieszaniny propan ­ butan (LPG ­ z j.ang. Liquid Petroleum Gas), gazy wysypiskowe lub biogazy (LFG ­ z j.ang. Landfilll Gas) oraz eter dimetylowy (DME). Jedn z bardziej znanych mieszanek gazowych stosowanych w wielu krajach wiata do wyrównywania szczytowych zapotrzebowa jest mieszanka zawierajca ok. 75% gazu ziemnego i ok. 25% mieszanki propan / powietrze, (LPG / air). Równie w Polsce przygotowywana jest zmiana przepisów w tym wzgldzie (obecnie zawarto tlenu w sieci gazowej nie moe przekracza 0,2 %). W artykule przeprowadzono obliczenia zamiennoci mieszanin paliw gazowych LFG ­ LPG i LPG ­ powietrze (SNG) za gaz ziemny. Okrelono czy analizowane mieszaniny maj podobne stabilne strefy plomienia niezalenie od jakoci LFG i czy paliwa te mog w pelni lub w czci zastpi CH4 , bez adnych modyfikacji urzdze zasysajcych powietrze do spalania. Uzyskane wyniki, pozwol stwierdzi, czy paliwa te mog by wykorzystane jako zamienne za gaz ziemny uytkowany we wspomnianych urzdzeniach gospodarstwa domowego i ewentualnie palnikach przemyslowych. W zwizku z moliwoci zmian jakoci LFG w zalenoci od takich czynników jak czas skladowania, sposób obróbki wstpnej, zostanie okrelony równie stopie wymiennoci LFG jako paliwa mieszanego w odniesieniu do jego jakoci. Slowa kluczowe: Gaz ziemny, zamienno gazów, szybko spalania, szczytowe zapotrzebowanie, gaz plynny, propan, biogaz, liczba Wobbe'go Index Hs Hi Ws d SL KG Pmax (dP/dt)max Pi, Pk td ts ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ gross calorific value (GCV). net calorific value (NCV). Wobbe index. relative density. laminar burning velocity. mixture concentration ratio. explosion index. maximum pressure of explosion. maximum increase of explosion pressure. initial and final pressure, respectively. ignition delay. total combustion time. With the increasing gas consumption, gas industry stays responsible for gas production, transport and storing but not always and not everywhere can fully satisfy the end consumer's needs for this fuel. When the gaseous fuel is supplied to the gas networks, peak demands, especially local and short-term ones, can be made by the construction and operation of the so-called satellite installations re-gasifying liquid natural gas (LNG) or through gaseous mixtures. This however necessitates re-dimensioning of gas network at the stage of designing or later reconstruction works have to be done to increase its capacity (Nagy, Olajossy, Siemek, 2004; Laciak 2009). The unused increased capacity of pipelines beyond the peak demand obviously increases the cost of exploitation of the network. The economic case analyses prove that providing interchangeability of gaseous fuel would bring greater economic effect than increasing the capacity of gas pipelines providing the same amount of natural gas. Moreover, the synthetic natural gas (SNG) systems and installations can be considered to be investments which can improve the safety and flexibility of gas deliveries. As is known the gross calorific value (GCV) and net calorific value (NCV) characterize the amount of heat energy, which can be obtained from a unit of natural gas in the process of combustion. Those parameters can be used for calculating energy provided to the end customer with the received portion of gas. Another important parameter depending on gross calorific value (Hs) and relative density (d ) of gas is the Wobbe index, Ws = Hs / . The upper and the lower Wobbe figures decide about whether or not gas can be efficiently used in the gas receiver with nominal heat load. The concentration of the main hydrocarbon components (aliphatic hydrocarbons from C1 to C6+) is the parameter on the basis of which Hs, Hi, compressibility coefficient, divisor and dividend of Wobbe index can be calculated. Those calculations also require knowing the composition of hydrocarbon fraction C6 as well as composition of hydrogen and non-combustible components of natural gas, i.e. nitrogen, carbon dioxide, helium, argon, oxygen, etc. Knowing the aromatics, mercury vapours and sulphur compounds contents one can evaluate whether or not the using of gas by individual and industrial users is safe for human health and the environment. The interchangeability of gases is related with admixing a certain amount of gas, which is a substitute of natural gas, meeting the interchangeability criteria, in order to guarantee the safety of gas deliveries to the customer. Gases which can be used in the process of mixing and used as interchangeable gases are mainly propane or propane-butane mixtures (Liquid Petroleum Gas ­ LPG), landfill gas and biogas (Landfill Gas ­ LFG) and in certain conditions di-methyl ether (DME), (Marchionna, 2008). One of the best known gaseous mixtures applied in numerous countries all over the world for balancing peak demands is a mixture containing ca. 75% natural gas and ca. 25% propanebutane (LPG/air) mixture. Already known methods of determining gas interchangeability for specific gas appliances are mainly based on the Wobbe index, which determines the heat load of the appliance, combustion rate, and which is related with the stability of combustion. The issue of threatened natural gas deliveries necessitates adding gaseous mixtures to the gaseous networks, however neither causing any qualitative changes in the distributed gas nor having any negative impact on the security of the users, (Eaton, 2005; Laciak, 2011). In a number of countries the LGP-air mixtures are applied. Polish regulations are about to be changed; presently, (PN-C-04752: 2002) the oxygen content in the gas network cannot exceed 0.2%. However, introduction of those changes will require investigating the process of mixing and analyzing safety of use of gaseous mixtures. A number of industrial processes employing gaseous fuels are run uninterruptedly. Any break in fuel deliveries may result in stopping the production and consecutively, considerable economic losses. Such companies are frequently required to have an alternative source of energy in case the energy deficiency situation appears. This can assume the form of systems for mixing propane and air, i.e. systems producing synthetic liquid gas (SNG) as a counterpart of natural gas. Opposite to diesel fuel, furnace oil or propane, SNG does not require any additional gas appliances, pipelines, regulators, or special fuel reception systems. Another cheap renewable energy source is the landfill gas (LFG) generated spontaneously or produced (biogas) from organic matter. After cleaning and treating, the gas could be interchanged with natural gas in the combustion processes for domestic and industrial purposes (Lee, Hwang, Lee 2008). Gases acquired from new sources do not burn properly in the presently existing gas installations, although their properties meet the standards, or reversely, the can be well combusted even though they do not meet the standards. Therefore, new criteria of gas evaluation and interchangeability have been worked out. It has been assumed that gases interchange among themselves if they well burn in the same appliances and in the same conditions, at the same pressure and when the following conditions are satisfied: · Heat load of gas appliance remains unchanged. Gas composition and also amount of sucked primary air have influence on the above conditions. This condition can be also fulfilled if the Wobbe index for both gases has the same value, with admissible allowance ± 5.0%. · The flame is stable, does not have any tendency to sever or counter-flow into the burner. The stability of flame depends on the maximum rate of gas burning and air suction coefficient. With its growth, the flame is less stable. · Combustion should be complete. The carbon dioxide content in waste gases should not exceed admissible values. Partial combustion manifests itself in the presence of carbon oxide in waste gases and yellow-ended flames. Admissible carbon oxide content in undiluted waste gases is 0.1÷0.05 vol.%. The limit value for the gas is also established with the use of carbon monoxide to carbon dioxide ratio, which should not exceed 0.01. The ways of determining interchangeability of gases are characteristic of a given gas in reference to above described phenomena with the use of figure data or interchangeability diagrams, for which the gas has been characterized (position of a point in the coordinates system). The Delbourg method is the best known method for determining gas interchangeability, where gas has been characterized by: - corrected (extended) Wobbe index, - combustion potential, D, - soot formation coefficient, Ich, - coefficient of yellow-ended flames formation, Ij. The two last coefficients depend on the hydrocarbons content in gas. The gas interchangeability can be also defined in a universal way by the Weaver calculation method. It does not require defining any reference gas. This method is fit for domestic gas appliances and gas pressure p = 1.25 kPa. The interchangeability is determined by calculating six indices for two gases, where one of them is the reference gas. If the listed interchangeability indices, two of which: - refer to heat load of gas appliance, - have stabilization of flame, - the quality of combustion, i.e. soot and carbon oxide formation and their values stay within values given by Weaver, then gases are mutually interchangeable. Weaver indices: - interchangeability index for heat load ­ IH - interchangeability index for sucked primary air ­ IA - interchangeability index for counter-flow flame IF - interchangeability index for flame severing ­ IL - interchangeability index for carbon monoxide formation ­ II - interchangeability index for yellow-ended flame formation ­ IY In practice the gas interchangeability criteria and definition of interchangeability refers to gas combustion in gas appliances. When gas is exchanged in industrial furnaces, the interchangeability criteria are of little use because of different combustion conditions and heat exchange. In industrial furnaces gas is combusted in closed combustion chambers. The airflow is regulated. The waste gases are discharged through canals and chimney to the atmosphere. The temperature differences of heated charge (gaseous fuel) and flames are much lower than in the case of do- mestic gaseous appliances. In 85% to 95% of cases the heat exchange in furnaces mainly takes place through radiation. The heat stream flowing from gas to the heated charge is not proportional to the heat load of the burners. Heat from radiation increases nearly proportionally to the 4th power of temperature of the source of radiation. The calorimetric temperature of combustion of gas supplied to the furnace has an influence on the result of the heating process, (Grzybczyk, Laciak, Grela, 2011). Landfill Gas (LFG) A number of papers on the use of LFG, both in the form of biogases and landfill gases can be found in literature. In the case of landfill gases, it is generated by anaerobic decomposition of municipal solid waste (MSW) in landfills and mainly consists of methane (CH4) and carbon dioxide (CO2) as well as minor quantities of nitrogen and oxygen, and also traces of volatile organic compounds (VOCs). Recently, the use of LFG has raised considerable interest as an alternative energy source for heat, electric energy and fuels production. The LFG production usually begins immediately after opening the MSW landfill, to reach its top production after about 10 years, and lasting for about 40 years or longer. Another source of LFG can be biogases generated in biogas plants. The quality of LFG as a fuel increases with the growing CH4 content and its composition may considerably differ, depending on the type of landfill, time elapsed, preprocessing method, etc. The net calorific value of LFG usually ranges from 12.56 to 25.12 MJ/mn3, which is enough for it to be used as fuel in various heating systems, though the mixing methods may differ, depending on the quality of LFG. The investigations, which have been described in literature focused mainly on the influence of CO2 content in LFG on the combustion rate, contraction of combustibility and explosiveness limits as well as the emission of contaminations. Qin et al. (2001) performed complex fundamental and environmental investigations of LFG use for energy generation. In their studies Lee et al. (2008) proposed new correlations of determining combustion rate of LFG and LFG ­ LPG as a function of equivalence ratio. They revealed that the CO2 content in LFG causes that heat losses increase through radiation in low flames with simultaneous lowering of NOx emission. Lee and Hwang (2007) investigated the stability of flames when combusting LFG/LPG mixtures in domestic appliances and industrial turbulent-jet burners without pre-mixing of gas and air. Various gaseous mixtures were analyzed; they were so selected as to reach the Wobbe index (Ws) and net calorific value (Hi) equivalent to the respective values typical of natural gas (group E). The required values of LFG, being a component of the analyzed gaseous mixtures, were obtained by selecting contents of CH4 from 55 vol.% to 30 vol.% in the mixture making up LFG. The basic properties of fuels were obtained by comparing the temperature of the flame, combustion rate and stability of combustion. The purpose of the study was to check out whether the mixtures had similar flame stability zones regardless the LFG quality or if the fuels could fully substitute CH4, without necessary modifications of the air suction elements used for the combustion process. The temperature profile of flames of mixed fuels turned out to be similar to that of pure CH4. Although LFG contains about 45% of neutral gases, i.e. CO2 and N2, relatively high flame temperatures were obtained, i.e. about 1900 K. The thermodynamic calculations and the obtained calculation results reveal that both the combustion rate and stability of flames confirmed that the LFG/LPG mixtures could be used interchangeably with natural gas without any changes in the gas burning appliances. In experiments, presented in literature, special attention was paid to defining basic properties of combustible mixtures, i.e. laminar rate of burning, minimum ignition energy, delayed ignitions, combustibility boundaries and extinction diameters. Synthetic Natural Gas (SNG) The propane-air mixtures (known as LPG-Air or SNG), are the synthetic natural gas which was created by mixing vapor of LPG with air, most frequently about 45% of air and about 55% of gas (LPG or propane, mixture concentration ratio = 1.22). After mixing up, we had a homogeneous mixture which could be used as a substitute of natural gas (to 25% of gas stream) in the combustion processes, e.g. to regulate peak demands. Defining burning velocity as one of the basic tasks related with the possibility of interchanging gaseous fuels, propane/air and LPG/air. The usability of propane and LPG as components of mixtures used as interchangeables was analyzed by, e.g. Huzayyin et al. (2007). The author addressed the problem of changes in the laminar burning velocity (SL) and determining the so-called "explosion index" (KG) of LPG/air and propane/air mixtures in a wide range of mixture concentration ratio ( = 0.7-2.2), initial temperature (Ti = 295-400 K) and pressure (Pi = 50-400 kPa). For this reason a cylinder pressure tank ("bomb") was construed for further experiments. The combustion rate was defined with the use of a number of models, depending on the changes of pressure (P-t) of the combustion process at minimum ignition energy. The explosion index KG was constant for a given mixture or for specific components of the mixture. Dahoea et al. (2003) give a simple dependence for KG: KG = (dP/dt)maxV = (36 p ) (Pk ­ Pi) ( Pk Pi )1/T SL (1) where (dP/dt)max is maximum rate of growth of explosion rate, Pi and Pk are initial and final pressure, respectively. On the basis of their investigation, Huzayyin at al. (2007) give a new equation for combustion rate (SL ) of LPG/air mixtures in the form: SL = S LO (T/T n) (P/Pn) where: SLO = 1209.1 ­ 935.5 2 + 708.1 3 ­ 229.0 4 ­ 492.8 = 275.0 ­ 2.13 2 + 2.0 = 0.029 ­ 0.026 2 ­ 0.137 Pn and Tn are pressure and temperature in normal conditions. (3) (4) (5) (2) Exemplary changes of combustion rate SL in a function of pressure and temperature in reference to eqs. (2-5) for various values of have been presented in figs. 1 and 2. F = 0,9 450 400 350 300 P [kPa] 250 200 150 100 50 0 0 100 200 300 400 S L [mm/s] 500 600 F = 1,0 450 400 350 300 P [kPa] 250 200 150 100 50 400 500 S L [mm/s] Fig. 1. Burning velocity SL depending on pressure and temperature ( = 0.9 and 1.0) F = 1,2 450 400 350 300 P [kPa] F = 1,4 450 400 350 300 P [kPa] 250 200 150 100 50 S L [mm/s] S L [mm/s] Fig. 2. Burning velocity SL depending on pressure and temperature ( = 1.2 i 1.4) In another model construed on the basis of Dahoea et al. (2003) the following formulae were proposed for calculating the burning velocity in a function of pressure: X= P - Pi P - Pi e 1 Tu ; rb = r 1 ­ PTu Pe - Pi i PTi Pe - Pi 1 Tu (6) SL = ( ) 3 P r Pi [1­ ( P ) ( P - P )] (P - P ) Pi P -P e e i e i dP dt (7) 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 140 Metghalchi Rallis Huzayyin 190 240 290 S L [mm/s] 340 390 440 Fig. 3. Comparison of burning velocity SL vs. coefficient for propane/air mixtures in normal conditions according to models by: Metghalchi & Keck. Rallis & Huzayyin et al. The analysis of experiments for various models, e.g. Huzayyin et al. (2007) and earlier Metghalchi & Keck (1980) reveals that the maximum laminar burning velocity SL for propane equals to 455 mm/s at = 1.1, whereas for LPG it is 432 mm/s ( 1.5). In nearly all researches the value of SL for propane/air mixture at = 1.0 equals to about 415 mm/s. The explosion index (KG) can be olso calculated on the basis of laminar burning velocity (SL) which is 93 bar m/s for propane and almost 88 bar m/s for LPG, fig. (2-5). The maximum pressure of explosion Pmax, maximum rate with which explosion pressure grows (dP/dt)max, ignition delay (td) and total combustion time (ts) greatly depend on the type of the gaseous fuel and coefficient . The course of changes remains similar, therefore: ­ For a given value within the combustion limits, in the case of propane/air mixtures the flame is spreading faster than in LPG/air mixtures, which can be indicated by relatively higher Pmax, (dP/dt)max values at lower ignition delay td and combustion time ts values. The value, corresponding to the maximum rate with which flame moves, is lower ( = 1.2) for the propane-air mixture than for LPG-air mixture ( = 1.5). The differences are related with the structure of hydrocarbon molecules (length of chains and chain branchings) Glassman (1996). ­ In both types of mixtures the maximum value of burning velocity is reached for mixtures richer in fuel. This is related with the necessity to compensate for the effect of hydrocarbons dissociation at higher temperatures. ­ The closer to the explosion limits, the lower are the combustion rates. At the ,,poor" side ( < 1), part of the liberated energy is lost because of the excessive air; this leads to lowering of Pmax and (dP/dt)max values, and increase of td and ts values. At the opposite side ( > 1) the oxygen deficiency causes incomplete combustion, and so reduced the reaction of oxidation. ­ With the increase of initial pressure of mixture Pi the combustion rate SL decreases for both fuels, and grows with the increase of the initial temperature Ti. ­ The maximum NOx emission is nearly the same for both mixtures, though appears at various concentrations: for propane/air mixtures at the level of a poor mixture ( 0.9) and for LPG/air at the level of a rich mixture ( = 1.2). Determining the explosion parameters of mixtures it can be said that for low initial values of pressure Pi the maximum explosion pressure Pmax for propane mixtures reaches higher values than for LPG mixtures. With the increase of pressure Pi the situation changes and Pmax grows for LPG mixtures. The maximum value of explosion pressure increase (dP/dt)max for propane is about 20% higher than for LPG (depending on composition). The importance of the growth of explosion pressure KG is definitely much bigger than the influence of the growth of initial temperature Ti. Those and other results would indicate propane as more fit for interchangeability processes than LPG. Undoubtedly, one of the reasons is the lack of butane (a component of LPG) which has a considerably higher specific weight, hindering proper mixing of gases. Conclusions (a) The propane (LPG)-air mixtures (known as LPG-Air or SNG) can be used as a direct substitute of natural gas in the combustion processes, e.g. for regulating peak demands in proper proportions and concentration. (b) One of the cheapest renewable energy sources is LFG which is generated spontaneously (landfill gas) or produced (biogas) from organic matter. The net calorific value of LFG usually ranges between 12.56 and 25.12 MJ/mn3. (c) Adiabatic temperatures of flames for all types LFG/LPG mixtures are very similar to temperature profiles of CH4, though the temperatures drop down with the decreasing percentage of CH4 in the LFG mixture. (d) Maximum burning velocity of mixed fuels decreases linearly with the drop of CH4 content in LFG. However, even in the case of a mixture with LFG containing about 70% of neutral gases, the measurements of burning velocities reveal that it can be used in any combustion system. (e) Most of LFG/LPG mixtures have very similar stable flame zones, and regardless the quality of LFG can completely substitute CH4 without having to modify the gaseous appliances. (f) The stability of flame zone of mixtures decreases with the drop of CH4 content in LFG. The LFG/LPG mixture can substitute CH4 entirely or with some limitations covering, e.g. regulation of burner sucking the air. (g) The maximum rate of laminar burning velocity (SL) for propane-air mixtures is 455 mm/s at = 1.1 and 432 mm/s for LPG/air mixtures ( 1.5). (h) The maximum increase of explosion rate (dP/dt)max for propane is about 20% higher than for LPG, whereas the maximum explosion pressure Pmax at higher initial pressures Pi is higher for LPG. (i) Better results in all gas interchangeability experiments have been obtained when low- or zero-butane LPG mixtures were used (technical propane as LPG). (j) The difference of density (specific weight) of components of gaseous mixtures used for gas interchangeability processes may constitute a significant problem in certain conditions, sometimes neglected at the stage of designing. (k) Experimental admixing di-methyl ether (DME) directly to the low-pressure gas networks gave positive results. DME as gaseous fuel has numerous advantages: considerably lowers ignition temperature of DME/natural gas mixture (lower activation energy), improves combustion efficiency, lowers the temperature of flame ­ lower NOx emission. Among other advantages, besides methane interchangeability, are easy transport and safety of deliveries. (l) The first and basic criterion of gas interchangeability remains the constant Wobbe index (±5.0%). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Mining Sciences de Gruyter

Properties of Artificial Gaseous Mixtures for their Safe Use and Support the Natural Gas Supply Networks / Własności Sztucznych Mieszanin Gazowych do Bezpiecznego ich Użytkowania i Wspomagania Zasilania Sieci Gazu Ziemnego

Archives of Mining Sciences , Volume 57 (2) – Nov 12, 2012

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Abstract

Arch. Min. Sci., Vol. 57 (2012), No 2, p. 351­362 Electronic version (in color) of this paper is available: http://mining.archives.pl DOI 10.2478/v10267-012-0022-5 MARIUSZ LACIAK* PROPERTIES OF ARTIFICIAL GASEOUS MIXTURES FOR THEIR SAFE USE AND SUPPORT THE NATURAL GAS SUPPLY NETWORKS WLASNOCI SZTUCZNYCH MIESZANIN GAZOWYCH DO BEZPIECZNEGO ICH UYTKOWANIA I WSPOMAGANIA ZASILANIA SIECI GAZU ZIEMNEGO The increase in natural gas consumption by the general public and industry development, in particular the petrochemical and chemical industries, has made increasing the world interest in using gas replacement for natural gas, both as mixtures of flammable gases and gas mixtures as LPG with air (SNG ­ Synthetic Natural Gas). Economic analysis in many cases prove that to ensure interchangeability of gas would cost less than the increase in pipeline capacity to deliver the same quantity of natural gas. In addition, SNG systems and installations, could be considered as investments to improve security and flexibility of gas supply. Known existing methods for determining the interchangeability of gases in gas gear based on Wobbe index, which determines the heat input and the burning rate tide, which in turn is related to flame stability. Exceeding the Wobbe index of a value increases the amount of carbon monoxide in the exhaust than the permissible concentration. Methods of determining the interchangeability of gases is characterized by a gas in relation to the above-described phenomena by means of quantitative indicators, or using diagrams interchangeability, where the gas is characterized by the position of a point in a coordinate system. The best known method for determining the interchangeability of gases is Delbourg method, in which the gas is characterized by the revised (expanded) Wobbe Index (Wr), the combustion potential, rate of soot formation (Ich ) and the ratio of the formation of yellow ends (Ij ). Universal way to determine the interchangeability of gas is also Weaver accounting method. It does not require determination of the reference gas. It is designed for utensils for household gas and gas pressure p = 1.25 kPa. The criteria and definition of gas interchangeability volatility in practice to the combustion in a gas gear. In the case of gas exchange in industrial furnaces, interchangeability criteria are usually not very useful because of other conditions of combustion and heat exchange. In industrial reheating furnace gas is combusted in a sealed combustion chambers. Air supply is regulated. The exhaust gases are discharged into canals and the chimney to the atmosphere. The temperature difference between load (fuel gas) and the flame is much less than in the case of gas household appliances. In the furnace heat exchange takes place mainly by radiation in 85% to 95%. The value of heat flux flowing from the gas to a heated charge is not proportional to the heat load burners. Interchangeability of gas is linked by adding to natural gas, a certain amount of gas that is a substitute for natural gas in meeting the criteria for substitution in order to ensure certainty of supply of natural gas to customers. Gases that can be used in the processes of blending and used as replacement gases are mainly a mixture of propane and propane ­ butane (LPG ­ Liquid Petroleum Gas), landfill gas or biogas (LFG ­ Landfill Gas) and dimethyl ether (DME). One * AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF DRILLING, OIL AND GAS, AL. MICKIEWICZA 30, 30-059 CRACOW, POLAND of the more well-known gas mixtures used in many countries around the world to compensate for peak demands is a mixture containing about 75% of natural gas and approximately 25% propane / air (LPG / air). Also in Poland is prepared to amend the provisions in this regard (at this moment ­ oxygen in the gas network can not exceed 0.2%). In this paper, the calculations of interchangeability of gas mixtures LFG ­ LPG and LPG ­ air (SNG) for natural gas was made. It was determined whether the analyzed mixtures have similar stable flame zones regardless of the quality of LFG fuel and whether they may in whole or in part replace CH4, without any modification of equipment suction air for combustion. The obtained results will determine whether the fuel can be used as a replacement for natural gas used in such household appliances and, possibly, industrial burners. In connection with the possibility of changes in the quality of LFG, depending on such factors as storage time, as pre-treatment, will be determined the degree of interchangeability of LFG as a fuel mixed with regard to its quality. Keywords: Natural gas, interchangeability of gases, burning velocity, peak shaving, Liquid Petroleum Gas, propane, Landfill Gas, Wobbe Index Wzrost zuycia gazu ziemnego przez odbiorców komunalnych oraz rozwój przemyslu w szczególnoci petrochemicznego i chemicznego sprawil, e na calym wiecie wzroslo zainteresowanie zastosowaniem gazów zamiennych za gaz ziemny, zarówno jako mieszanin gazów palnych jak i jako mieszanin gazów plynnych z powietrzem (SNG ­ syntetyczny gaz ziemny). Przeprowadzane analizy ekonomiczne w wielu przypadkach dowodz, e zapewnienie wymiennoci paliwa gazowego kosztowalo by mniej ni zwikszenie przepustowoci gazocigów dla dostarczenia tej samej iloci gazu ziemnego. Ponadto systemy i instalacje SNG, mona by uzna za inwestycje poprawiajce bezpieczestwo i elastyczno dostaw gazu. Znane dotychczasowe metody okrelania zamiennoci gazów w przyborach gazowych oparte s na liczbie Wobbego, która decyduje o obcieniu cieplnym przyboru i szybkoci spalania, z któr z kolei zwizana jest stabilno plomienia. Przekroczenie liczby Wobbego o pewn warto powoduje wzrost iloci tlenku wgla w spalinach ponad dopuszczalne stenie. Sposoby okrelajce wymienno gazów charakteryzuj dany gaz w odniesieniu do opisanych wyej zjawisk przy pomocy wskaników liczbowych lub za pomoc diagramów wymiennoci, na których gaz jest scharakteryzowany przez poloenie punktu w ukladzie wspólrzdnych. Najbardziej znan metod okrelenia zamiennoci gazów jest metoda Delbourga, w której gaz scharakteryzowany jest przez skorygowan (rozszerzon) liczb Wobbego (Wr ), potencjal spalania, wspólczynnik tworzenia si sadzy (Ich) oraz wspólczynnik powstawania óltych koców (Ij ). Uniwersalnym sposobem okrelenia zamiennoci gazu jest równie metoda rachunkowa Weavera. Nie wymaga ona okrelenia gazu odniesienia. Przeznaczona jest dla przyborów gazowych uytku domowego i cinienia gazu p = 1,25 kPa. Kryteria zmiennoci gazów i definicja zamiennoci w praktyce dotyczy spalania gazów w przyborach gazowych. W przypadku wymiany gazu w piecach przemyslowych kryteria zamiennoci s zazwyczaj malo przydatne z powodu innych warunków spalania i wymiany ciepla. W przemyslowych piecach grzewczych gaz spala si w zamknitych komorach spalania. Doplyw powietrza jest regulowany. Spaliny odprowadzane s kanalami i kominem do atmosfery. Rónica temperatur nagrzewanego wsadu (paliwa gazowego) i plomienia jest duo mniejsza ni w przypadku przyborów gazowych domowego uytku. W piecach wymiana ciepla odbywa si glównie przez promieniowanie w 85% do 95%. Warto strumienia cieplnego plyncego od gazu do ogrzewanego wsadu nie jest proporcjonalne do obcienia cieplnego palników. Zamienno gazów zwizana jest dodawaniem do gazu ziemnego pewnej iloci gazu bdcego substytutem naturalnego gazu ziemnego przy spelnieniu kryteriów zamiennoci w celu zagwarantowania pewnoci dostaw gazu ziemnego do odbiorców. Gazy mogce by uyte w procesach mieszania i wykorzystane jako gazy zamienne to przede wszystkim propan lub mieszaniny propan ­ butan (LPG ­ z j.ang. Liquid Petroleum Gas), gazy wysypiskowe lub biogazy (LFG ­ z j.ang. Landfilll Gas) oraz eter dimetylowy (DME). Jedn z bardziej znanych mieszanek gazowych stosowanych w wielu krajach wiata do wyrównywania szczytowych zapotrzebowa jest mieszanka zawierajca ok. 75% gazu ziemnego i ok. 25% mieszanki propan / powietrze, (LPG / air). Równie w Polsce przygotowywana jest zmiana przepisów w tym wzgldzie (obecnie zawarto tlenu w sieci gazowej nie moe przekracza 0,2 %). W artykule przeprowadzono obliczenia zamiennoci mieszanin paliw gazowych LFG ­ LPG i LPG ­ powietrze (SNG) za gaz ziemny. Okrelono czy analizowane mieszaniny maj podobne stabilne strefy plomienia niezalenie od jakoci LFG i czy paliwa te mog w pelni lub w czci zastpi CH4 , bez adnych modyfikacji urzdze zasysajcych powietrze do spalania. Uzyskane wyniki, pozwol stwierdzi, czy paliwa te mog by wykorzystane jako zamienne za gaz ziemny uytkowany we wspomnianych urzdzeniach gospodarstwa domowego i ewentualnie palnikach przemyslowych. W zwizku z moliwoci zmian jakoci LFG w zalenoci od takich czynników jak czas skladowania, sposób obróbki wstpnej, zostanie okrelony równie stopie wymiennoci LFG jako paliwa mieszanego w odniesieniu do jego jakoci. Slowa kluczowe: Gaz ziemny, zamienno gazów, szybko spalania, szczytowe zapotrzebowanie, gaz plynny, propan, biogaz, liczba Wobbe'go Index Hs Hi Ws d SL KG Pmax (dP/dt)max Pi, Pk td ts ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ ­ gross calorific value (GCV). net calorific value (NCV). Wobbe index. relative density. laminar burning velocity. mixture concentration ratio. explosion index. maximum pressure of explosion. maximum increase of explosion pressure. initial and final pressure, respectively. ignition delay. total combustion time. With the increasing gas consumption, gas industry stays responsible for gas production, transport and storing but not always and not everywhere can fully satisfy the end consumer's needs for this fuel. When the gaseous fuel is supplied to the gas networks, peak demands, especially local and short-term ones, can be made by the construction and operation of the so-called satellite installations re-gasifying liquid natural gas (LNG) or through gaseous mixtures. This however necessitates re-dimensioning of gas network at the stage of designing or later reconstruction works have to be done to increase its capacity (Nagy, Olajossy, Siemek, 2004; Laciak 2009). The unused increased capacity of pipelines beyond the peak demand obviously increases the cost of exploitation of the network. The economic case analyses prove that providing interchangeability of gaseous fuel would bring greater economic effect than increasing the capacity of gas pipelines providing the same amount of natural gas. Moreover, the synthetic natural gas (SNG) systems and installations can be considered to be investments which can improve the safety and flexibility of gas deliveries. As is known the gross calorific value (GCV) and net calorific value (NCV) characterize the amount of heat energy, which can be obtained from a unit of natural gas in the process of combustion. Those parameters can be used for calculating energy provided to the end customer with the received portion of gas. Another important parameter depending on gross calorific value (Hs) and relative density (d ) of gas is the Wobbe index, Ws = Hs / . The upper and the lower Wobbe figures decide about whether or not gas can be efficiently used in the gas receiver with nominal heat load. The concentration of the main hydrocarbon components (aliphatic hydrocarbons from C1 to C6+) is the parameter on the basis of which Hs, Hi, compressibility coefficient, divisor and dividend of Wobbe index can be calculated. Those calculations also require knowing the composition of hydrocarbon fraction C6 as well as composition of hydrogen and non-combustible components of natural gas, i.e. nitrogen, carbon dioxide, helium, argon, oxygen, etc. Knowing the aromatics, mercury vapours and sulphur compounds contents one can evaluate whether or not the using of gas by individual and industrial users is safe for human health and the environment. The interchangeability of gases is related with admixing a certain amount of gas, which is a substitute of natural gas, meeting the interchangeability criteria, in order to guarantee the safety of gas deliveries to the customer. Gases which can be used in the process of mixing and used as interchangeable gases are mainly propane or propane-butane mixtures (Liquid Petroleum Gas ­ LPG), landfill gas and biogas (Landfill Gas ­ LFG) and in certain conditions di-methyl ether (DME), (Marchionna, 2008). One of the best known gaseous mixtures applied in numerous countries all over the world for balancing peak demands is a mixture containing ca. 75% natural gas and ca. 25% propanebutane (LPG/air) mixture. Already known methods of determining gas interchangeability for specific gas appliances are mainly based on the Wobbe index, which determines the heat load of the appliance, combustion rate, and which is related with the stability of combustion. The issue of threatened natural gas deliveries necessitates adding gaseous mixtures to the gaseous networks, however neither causing any qualitative changes in the distributed gas nor having any negative impact on the security of the users, (Eaton, 2005; Laciak, 2011). In a number of countries the LGP-air mixtures are applied. Polish regulations are about to be changed; presently, (PN-C-04752: 2002) the oxygen content in the gas network cannot exceed 0.2%. However, introduction of those changes will require investigating the process of mixing and analyzing safety of use of gaseous mixtures. A number of industrial processes employing gaseous fuels are run uninterruptedly. Any break in fuel deliveries may result in stopping the production and consecutively, considerable economic losses. Such companies are frequently required to have an alternative source of energy in case the energy deficiency situation appears. This can assume the form of systems for mixing propane and air, i.e. systems producing synthetic liquid gas (SNG) as a counterpart of natural gas. Opposite to diesel fuel, furnace oil or propane, SNG does not require any additional gas appliances, pipelines, regulators, or special fuel reception systems. Another cheap renewable energy source is the landfill gas (LFG) generated spontaneously or produced (biogas) from organic matter. After cleaning and treating, the gas could be interchanged with natural gas in the combustion processes for domestic and industrial purposes (Lee, Hwang, Lee 2008). Gases acquired from new sources do not burn properly in the presently existing gas installations, although their properties meet the standards, or reversely, the can be well combusted even though they do not meet the standards. Therefore, new criteria of gas evaluation and interchangeability have been worked out. It has been assumed that gases interchange among themselves if they well burn in the same appliances and in the same conditions, at the same pressure and when the following conditions are satisfied: · Heat load of gas appliance remains unchanged. Gas composition and also amount of sucked primary air have influence on the above conditions. This condition can be also fulfilled if the Wobbe index for both gases has the same value, with admissible allowance ± 5.0%. · The flame is stable, does not have any tendency to sever or counter-flow into the burner. The stability of flame depends on the maximum rate of gas burning and air suction coefficient. With its growth, the flame is less stable. · Combustion should be complete. The carbon dioxide content in waste gases should not exceed admissible values. Partial combustion manifests itself in the presence of carbon oxide in waste gases and yellow-ended flames. Admissible carbon oxide content in undiluted waste gases is 0.1÷0.05 vol.%. The limit value for the gas is also established with the use of carbon monoxide to carbon dioxide ratio, which should not exceed 0.01. The ways of determining interchangeability of gases are characteristic of a given gas in reference to above described phenomena with the use of figure data or interchangeability diagrams, for which the gas has been characterized (position of a point in the coordinates system). The Delbourg method is the best known method for determining gas interchangeability, where gas has been characterized by: - corrected (extended) Wobbe index, - combustion potential, D, - soot formation coefficient, Ich, - coefficient of yellow-ended flames formation, Ij. The two last coefficients depend on the hydrocarbons content in gas. The gas interchangeability can be also defined in a universal way by the Weaver calculation method. It does not require defining any reference gas. This method is fit for domestic gas appliances and gas pressure p = 1.25 kPa. The interchangeability is determined by calculating six indices for two gases, where one of them is the reference gas. If the listed interchangeability indices, two of which: - refer to heat load of gas appliance, - have stabilization of flame, - the quality of combustion, i.e. soot and carbon oxide formation and their values stay within values given by Weaver, then gases are mutually interchangeable. Weaver indices: - interchangeability index for heat load ­ IH - interchangeability index for sucked primary air ­ IA - interchangeability index for counter-flow flame IF - interchangeability index for flame severing ­ IL - interchangeability index for carbon monoxide formation ­ II - interchangeability index for yellow-ended flame formation ­ IY In practice the gas interchangeability criteria and definition of interchangeability refers to gas combustion in gas appliances. When gas is exchanged in industrial furnaces, the interchangeability criteria are of little use because of different combustion conditions and heat exchange. In industrial furnaces gas is combusted in closed combustion chambers. The airflow is regulated. The waste gases are discharged through canals and chimney to the atmosphere. The temperature differences of heated charge (gaseous fuel) and flames are much lower than in the case of do- mestic gaseous appliances. In 85% to 95% of cases the heat exchange in furnaces mainly takes place through radiation. The heat stream flowing from gas to the heated charge is not proportional to the heat load of the burners. Heat from radiation increases nearly proportionally to the 4th power of temperature of the source of radiation. The calorimetric temperature of combustion of gas supplied to the furnace has an influence on the result of the heating process, (Grzybczyk, Laciak, Grela, 2011). Landfill Gas (LFG) A number of papers on the use of LFG, both in the form of biogases and landfill gases can be found in literature. In the case of landfill gases, it is generated by anaerobic decomposition of municipal solid waste (MSW) in landfills and mainly consists of methane (CH4) and carbon dioxide (CO2) as well as minor quantities of nitrogen and oxygen, and also traces of volatile organic compounds (VOCs). Recently, the use of LFG has raised considerable interest as an alternative energy source for heat, electric energy and fuels production. The LFG production usually begins immediately after opening the MSW landfill, to reach its top production after about 10 years, and lasting for about 40 years or longer. Another source of LFG can be biogases generated in biogas plants. The quality of LFG as a fuel increases with the growing CH4 content and its composition may considerably differ, depending on the type of landfill, time elapsed, preprocessing method, etc. The net calorific value of LFG usually ranges from 12.56 to 25.12 MJ/mn3, which is enough for it to be used as fuel in various heating systems, though the mixing methods may differ, depending on the quality of LFG. The investigations, which have been described in literature focused mainly on the influence of CO2 content in LFG on the combustion rate, contraction of combustibility and explosiveness limits as well as the emission of contaminations. Qin et al. (2001) performed complex fundamental and environmental investigations of LFG use for energy generation. In their studies Lee et al. (2008) proposed new correlations of determining combustion rate of LFG and LFG ­ LPG as a function of equivalence ratio. They revealed that the CO2 content in LFG causes that heat losses increase through radiation in low flames with simultaneous lowering of NOx emission. Lee and Hwang (2007) investigated the stability of flames when combusting LFG/LPG mixtures in domestic appliances and industrial turbulent-jet burners without pre-mixing of gas and air. Various gaseous mixtures were analyzed; they were so selected as to reach the Wobbe index (Ws) and net calorific value (Hi) equivalent to the respective values typical of natural gas (group E). The required values of LFG, being a component of the analyzed gaseous mixtures, were obtained by selecting contents of CH4 from 55 vol.% to 30 vol.% in the mixture making up LFG. The basic properties of fuels were obtained by comparing the temperature of the flame, combustion rate and stability of combustion. The purpose of the study was to check out whether the mixtures had similar flame stability zones regardless the LFG quality or if the fuels could fully substitute CH4, without necessary modifications of the air suction elements used for the combustion process. The temperature profile of flames of mixed fuels turned out to be similar to that of pure CH4. Although LFG contains about 45% of neutral gases, i.e. CO2 and N2, relatively high flame temperatures were obtained, i.e. about 1900 K. The thermodynamic calculations and the obtained calculation results reveal that both the combustion rate and stability of flames confirmed that the LFG/LPG mixtures could be used interchangeably with natural gas without any changes in the gas burning appliances. In experiments, presented in literature, special attention was paid to defining basic properties of combustible mixtures, i.e. laminar rate of burning, minimum ignition energy, delayed ignitions, combustibility boundaries and extinction diameters. Synthetic Natural Gas (SNG) The propane-air mixtures (known as LPG-Air or SNG), are the synthetic natural gas which was created by mixing vapor of LPG with air, most frequently about 45% of air and about 55% of gas (LPG or propane, mixture concentration ratio = 1.22). After mixing up, we had a homogeneous mixture which could be used as a substitute of natural gas (to 25% of gas stream) in the combustion processes, e.g. to regulate peak demands. Defining burning velocity as one of the basic tasks related with the possibility of interchanging gaseous fuels, propane/air and LPG/air. The usability of propane and LPG as components of mixtures used as interchangeables was analyzed by, e.g. Huzayyin et al. (2007). The author addressed the problem of changes in the laminar burning velocity (SL) and determining the so-called "explosion index" (KG) of LPG/air and propane/air mixtures in a wide range of mixture concentration ratio ( = 0.7-2.2), initial temperature (Ti = 295-400 K) and pressure (Pi = 50-400 kPa). For this reason a cylinder pressure tank ("bomb") was construed for further experiments. The combustion rate was defined with the use of a number of models, depending on the changes of pressure (P-t) of the combustion process at minimum ignition energy. The explosion index KG was constant for a given mixture or for specific components of the mixture. Dahoea et al. (2003) give a simple dependence for KG: KG = (dP/dt)maxV = (36 p ) (Pk ­ Pi) ( Pk Pi )1/T SL (1) where (dP/dt)max is maximum rate of growth of explosion rate, Pi and Pk are initial and final pressure, respectively. On the basis of their investigation, Huzayyin at al. (2007) give a new equation for combustion rate (SL ) of LPG/air mixtures in the form: SL = S LO (T/T n) (P/Pn) where: SLO = 1209.1 ­ 935.5 2 + 708.1 3 ­ 229.0 4 ­ 492.8 = 275.0 ­ 2.13 2 + 2.0 = 0.029 ­ 0.026 2 ­ 0.137 Pn and Tn are pressure and temperature in normal conditions. (3) (4) (5) (2) Exemplary changes of combustion rate SL in a function of pressure and temperature in reference to eqs. (2-5) for various values of have been presented in figs. 1 and 2. F = 0,9 450 400 350 300 P [kPa] 250 200 150 100 50 0 0 100 200 300 400 S L [mm/s] 500 600 F = 1,0 450 400 350 300 P [kPa] 250 200 150 100 50 400 500 S L [mm/s] Fig. 1. Burning velocity SL depending on pressure and temperature ( = 0.9 and 1.0) F = 1,2 450 400 350 300 P [kPa] F = 1,4 450 400 350 300 P [kPa] 250 200 150 100 50 S L [mm/s] S L [mm/s] Fig. 2. Burning velocity SL depending on pressure and temperature ( = 1.2 i 1.4) In another model construed on the basis of Dahoea et al. (2003) the following formulae were proposed for calculating the burning velocity in a function of pressure: X= P - Pi P - Pi e 1 Tu ; rb = r 1 ­ PTu Pe - Pi i PTi Pe - Pi 1 Tu (6) SL = ( ) 3 P r Pi [1­ ( P ) ( P - P )] (P - P ) Pi P -P e e i e i dP dt (7) 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 140 Metghalchi Rallis Huzayyin 190 240 290 S L [mm/s] 340 390 440 Fig. 3. Comparison of burning velocity SL vs. coefficient for propane/air mixtures in normal conditions according to models by: Metghalchi & Keck. Rallis & Huzayyin et al. The analysis of experiments for various models, e.g. Huzayyin et al. (2007) and earlier Metghalchi & Keck (1980) reveals that the maximum laminar burning velocity SL for propane equals to 455 mm/s at = 1.1, whereas for LPG it is 432 mm/s ( 1.5). In nearly all researches the value of SL for propane/air mixture at = 1.0 equals to about 415 mm/s. The explosion index (KG) can be olso calculated on the basis of laminar burning velocity (SL) which is 93 bar m/s for propane and almost 88 bar m/s for LPG, fig. (2-5). The maximum pressure of explosion Pmax, maximum rate with which explosion pressure grows (dP/dt)max, ignition delay (td) and total combustion time (ts) greatly depend on the type of the gaseous fuel and coefficient . The course of changes remains similar, therefore: ­ For a given value within the combustion limits, in the case of propane/air mixtures the flame is spreading faster than in LPG/air mixtures, which can be indicated by relatively higher Pmax, (dP/dt)max values at lower ignition delay td and combustion time ts values. The value, corresponding to the maximum rate with which flame moves, is lower ( = 1.2) for the propane-air mixture than for LPG-air mixture ( = 1.5). The differences are related with the structure of hydrocarbon molecules (length of chains and chain branchings) Glassman (1996). ­ In both types of mixtures the maximum value of burning velocity is reached for mixtures richer in fuel. This is related with the necessity to compensate for the effect of hydrocarbons dissociation at higher temperatures. ­ The closer to the explosion limits, the lower are the combustion rates. At the ,,poor" side ( < 1), part of the liberated energy is lost because of the excessive air; this leads to lowering of Pmax and (dP/dt)max values, and increase of td and ts values. At the opposite side ( > 1) the oxygen deficiency causes incomplete combustion, and so reduced the reaction of oxidation. ­ With the increase of initial pressure of mixture Pi the combustion rate SL decreases for both fuels, and grows with the increase of the initial temperature Ti. ­ The maximum NOx emission is nearly the same for both mixtures, though appears at various concentrations: for propane/air mixtures at the level of a poor mixture ( 0.9) and for LPG/air at the level of a rich mixture ( = 1.2). Determining the explosion parameters of mixtures it can be said that for low initial values of pressure Pi the maximum explosion pressure Pmax for propane mixtures reaches higher values than for LPG mixtures. With the increase of pressure Pi the situation changes and Pmax grows for LPG mixtures. The maximum value of explosion pressure increase (dP/dt)max for propane is about 20% higher than for LPG (depending on composition). The importance of the growth of explosion pressure KG is definitely much bigger than the influence of the growth of initial temperature Ti. Those and other results would indicate propane as more fit for interchangeability processes than LPG. Undoubtedly, one of the reasons is the lack of butane (a component of LPG) which has a considerably higher specific weight, hindering proper mixing of gases. Conclusions (a) The propane (LPG)-air mixtures (known as LPG-Air or SNG) can be used as a direct substitute of natural gas in the combustion processes, e.g. for regulating peak demands in proper proportions and concentration. (b) One of the cheapest renewable energy sources is LFG which is generated spontaneously (landfill gas) or produced (biogas) from organic matter. The net calorific value of LFG usually ranges between 12.56 and 25.12 MJ/mn3. (c) Adiabatic temperatures of flames for all types LFG/LPG mixtures are very similar to temperature profiles of CH4, though the temperatures drop down with the decreasing percentage of CH4 in the LFG mixture. (d) Maximum burning velocity of mixed fuels decreases linearly with the drop of CH4 content in LFG. However, even in the case of a mixture with LFG containing about 70% of neutral gases, the measurements of burning velocities reveal that it can be used in any combustion system. (e) Most of LFG/LPG mixtures have very similar stable flame zones, and regardless the quality of LFG can completely substitute CH4 without having to modify the gaseous appliances. (f) The stability of flame zone of mixtures decreases with the drop of CH4 content in LFG. The LFG/LPG mixture can substitute CH4 entirely or with some limitations covering, e.g. regulation of burner sucking the air. (g) The maximum rate of laminar burning velocity (SL) for propane-air mixtures is 455 mm/s at = 1.1 and 432 mm/s for LPG/air mixtures ( 1.5). (h) The maximum increase of explosion rate (dP/dt)max for propane is about 20% higher than for LPG, whereas the maximum explosion pressure Pmax at higher initial pressures Pi is higher for LPG. (i) Better results in all gas interchangeability experiments have been obtained when low- or zero-butane LPG mixtures were used (technical propane as LPG). (j) The difference of density (specific weight) of components of gaseous mixtures used for gas interchangeability processes may constitute a significant problem in certain conditions, sometimes neglected at the stage of designing. (k) Experimental admixing di-methyl ether (DME) directly to the low-pressure gas networks gave positive results. DME as gaseous fuel has numerous advantages: considerably lowers ignition temperature of DME/natural gas mixture (lower activation energy), improves combustion efficiency, lowers the temperature of flame ­ lower NOx emission. Among other advantages, besides methane interchangeability, are easy transport and safety of deliveries. (l) The first and basic criterion of gas interchangeability remains the constant Wobbe index (±5.0%).

Journal

Archives of Mining Sciencesde Gruyter

Published: Nov 12, 2012

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