Air Mixtures at Elevated

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Lean flammability limits of syngas/air mixtures at elevated temperatures and pressures Daniel Jaimes, Vincent McDonell, and Gary Scott Samuelsen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02031 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Lean flammability limits of syngas/air mixtures at elevated temperatures and pressures Daniel Jaimes,* Vincent G. McDonell, and G. Scott Samuelsen Advanced Power and Energy Program, Department of Mechanical and Aerospace Engineering, University of California, Irvine, Irvine, California, United States of America

ABSTRACT: New experimental results for lean flammability limits (LFLs) of syngas/air (H2/CO/air) mixtures have been obtained at temperatures up to 200° C and pressures up to 9 bar. ASTM Standard E918 (1983) provided the framework for tests at these elevated conditions, using a one-liter pressure-rated test cylinder in which the fuel-air mixtures were prepared and then ignited. The purpose for characterizing the flammability limits for these gaseous mixtures is to facilitate development of appropriate procedures for the safe industrial use of syngas, which contains large quantities of hydrogen and carbon monoxide gas. The LFLs for each gas mixture are found to decrease linearly with increasing temperature at all test pressures. The LFL results at atmospheric pressure are consistent with previous flammability studies, while those at elevated pressures represent new flammability data. An increase in the initial test pressure results in an increase of the LFLs for each test mixture, which also serves to address the lack of syngas/air flammability data at elevated pressures. An empirical formula is derived which allows for the calculation of the LFLs of all syngas/air test mixtures in the temperature and pressure range of the current study in an effort to promote ease of use in practical applications. Predicted LFL values

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obtained using Le Chatelier’s mixing rule and an appropriate choice for the lower flammability limit of pure carbon monoxide are consistent with the experimentally determined values near ambient conditions of temperature and pressure.

1. INTRODUCTION Flammability limits are the concentration limits (lower and upper) of a combustible gas mixed homogeneously with an oxidizing gas within which a flame is able to propagate independently through the mixture. Gas mixtures are deemed flammable if and only if the concentration of the combustible gases lies between the lower flammability limit (LFL) and the upper flammability limit (UFL). Inherently, the primary method for preventing a gaseous explosion is to prevent the formation of a flammable atmosphere. Therefore, knowing the flammability limits of a substance relative to an oxidizing agent is necessary. Common fuel mixtures of interest include process gases such as natural gas, coke oven gas and IGCC syngas fuel.1 In addition, interest in renewably derived fuels is increasing. Such technologies include the recycling of carbon already circulating in the environment by producing gas from waste feedstocks, as well as Power-to-Gas (P2G), which uses excess electricity from renewable sources to produce renewable hydrogen.2,3 The gases produced by renewable methods are either used directly in industry or transportation, or reformed to match the pipeline quality before introduction into the natural gas network. An understanding of the flammable or “explosive” range of gaseous fuels at elevated conditions is critical for establishing appropriate safety standards and protocols. Extensive flammability limit data for many gaseous fuels have been published. However, inconsistency is observed and only limited data are available for 1) elevated temperature and pressure conditions and 2) mixtures of fuels. Some of the earliest reports on flammability data


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were conducted by the U.S. Bureau of Mines, with Coward & Jones (1952) first contributing on limits of flammability followed by Zabetakis (1965) reporting on flammability characteristics of combustible gases and vapors.4,5 Although these reports are thorough and widely referenced, the data are limited to the atmospheric conditions the tests were conducted under, which means they cannot be applied directly to applications under elevated pressure conditions. An effort to standardize test methods was developed by the American Society for Testing and Materials (ASTM) and resulted in the development of two standards, E681 for atmospheric conditions and E918 for elevated conditions.6,7 A European standard was also developed, namely EN 1839, and describes two different methods for determining the flammability limits (FLs).8,9 Although the data collected by using these standards can be consistent, a comparison of the conditions as shown in Table 1 displays the lack of agreement on what determines flammability. Standard ASTM E681

ASTM E918

EN 1839 - Tube

EN 1839 - Bomb

Test Apparatus

Ignition Criteria

Conditions of Use

Glass sphere

Visual flame propagation over 0.2 m

Atmospheric pressure,

V = 5 L; D = 222 mm Steel cylinder V ≥ 1 L; D ≥ 76 mm Vertical glass tube D = 80 mm; L = 0.3 m Pressure resistant vessel V ≥ 5 L; D/L = 1 - 1.5

Pressure rise of 7% of initial pressure Visual flame propagation over 0.1 m Pressure rise of 5% of initial pressure

Ambient temperature up to 150 °C Pressures less than 13.8 bar (200 psia) Ambient temperature up to 200 °C Atmospheric pressure, Ambient temperature up to 200 °C Atmospheric pressure, Ambient temperature up to 200 °C

Table 1. Comparison of current standards for the determination of flammability limits Numerous parameters influence flammability limits, but the three main factors include: the test apparatus, the ignition criterion, and the conditions of use (e.g. pressure, temperature). Many studies were conducted based on the suggested apparatus, criterion and conditions of the


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aforementioned standards. Karim et al. investigated the lean flammability limits (LFLs) of methane, hydrogen, carbon monoxide and mixtures of these gases, and primarily used a vertical tube apparatus as suggested by the U.S. Bureau of Mines.10,11 Smedt et al. also chose experimental setups based on standards, including a glass tube per EN 1839, and a 20-liter pressure resistant vessel per ASTM E918.12 The flammability study conducted by Cashdollar et al. used an assortment of test chambers including an 8-liter, 120-liter and 25,500-liter (25.5 m3) pressure resistant vessel.13 Other studies similar to this were performed to not only determine the flammability limit dependence in regards to the type of gas but also based on the size of the vessel. The studies of Smedt et al.12 and Cashdollar et al.13 are also significant in that they investigated the effect of different ignition criteria on the flammability data. Smedt et al. initially used a pressure rise criterion of 7% to indicate ignition, but when comparing the results of the tube and pressure resistant vessel, ultimately concluded that a 2% pressure rise resulted in improved agreement for the flammability data collected. Cashdollar et al. also compared different ignition criteria and noticed the LFLs for methane and propane were consistent, regardless of the criteria. However, they observed that results for hydrogen varied significantly. Miao et al.14 presented a comprehensive review of published flammability data for methane and hydrogen separately, making sure to include the details of the apparatus size, type and ignition criteria; the conclusions drawn from this study were similar to that of Cashdollar et al. The previously mentioned studies investigate the first two main factors; however, the resulting data are relevant for only ambient conditions. Only a few studies focus on elevated temperature and/or pressure conditions, and even fewer investigate the behavior of gas mixtures as well. Wierzba and Ale investigated the flammability limits of binary fuel mixtures of hydrogen with methane, ethylene and propane in air at temperatures up to 350 °C using a stainless steel tube and


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upward vertical flame propagation as the ignition criterion.15 Wierzba and Wang determined experimental values of the flammability limits of some fuel mixtures made up of H2, CO, and CH4 in air at different initial mixture temperatures up to 300 °C.16 Hustad and Sonju conducted experimental studies of lower flammability limits for methane, butane, hydrogen, carbon monoxide, and, in addition, for mixtures of these gases at temperatures up to 450 °C.17 With regard to the effect of pressure, Ding et al. studied the pressure dependence of the LFL of various flammable liquids at pressures from 35 to 101 kPa, and observed an increase in the LFL with decreasing pressure.18 Le at al. also studied the pressure dependence of LFLs of hydrogen and light hydrocarbons at pressures from 101 kPa down to 10 kPa.19 The LFL results for the tested hydrocarbons agreed with Ding et al. as the LFL increased as the pressure decreased, with the exception of the LFL for methane that did not change with pressure. Askari et al. experimentally investigated the laminar burning velocity and flammability limits of landfill gas (CH4/CO2) and biogas (H2/CO/N2) mixtures at pressure up to 5 bar and found similar results to the previous studies that demonstrated an increase in the flammable range (increase of UFL, decrease of LFL) with an increase in the initial test pressure.20,21 Studies that investigate the dependence of the flammability limits on both pressure and temperature include a comprehensive report published by the SAFEKINEX (SAFe and Efficient hydrocarbon oxidation processes by KINetics and Explosion eXpertise) program supported by the European Union which reported the explosion limits of methane, hydrogen and propylene at elevated pressures up to 20 bar and temperatures up to 200 °C.22 Tschirschwitz et al. studied the flammability limits of methane-air and hydrogen-air mixtures, for pressures up to 175 bar and temperatures up to 150 °C.23 Van den Schoor has actively researched the influence of pressure and temperature on flammability limits, including a study on the upper explosion limit of lower alkanes


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and alkenes at elevated pressure up to 30 bar and temperature up to 250 °C (2006), as well as the determination of flammability limits of hydrogen/carbon monoxide/nitrogen air mixtures at temperatures up to 200 °C (2009).24,25 As the current study seeks to characterize the lean flammability limits of various syngas/air mixtures at elevated temperature and pressure conditions, the following studies are of particular relevance. Wierzba and Kilchyk reported on the flammability limits of hydrogen-carbon monoxide mixtures at temperatures up to 300 °C, and comment on the accuracy of LFL estimates using Le Chatelier’s rule.26 Li et al. also investigated the effect of initial temperature on the lean flammability limit of syngas/air (hydrogen/carbon monoxide/air) mixtures and found that the LFL decreases linearly with preheating temperature.27 Li et al. also claim that Le Chatelier’s Rule can be used to predict the LFL of syngas with fair accuracy until moderately diluted conditions. For reference, another study that successfully used Le Chatelier’s formula to reproduce flammability limit values for binary fuel mixtures was conducted by Zhang et al. using the ASHRAE method at ambient conditions.28 Comparable techniques for the prediction of flammability limits include the calculated flame temperature method with a fixed critical temperature employed by Hu et al. for investigating the diluent effect of CO2 on the LFL of oxy-methane mixtures, as well as a constant flame temperature model developed by Liaw et al. for estimating the flammability limits of various fuel-air diluent mixtures in a constant pressure vessel at ambient conditions of temperature and pressure.29,30 A summary of key flammability literature, including the aforementioned studies, are shown in Table 2. Overall, the literature presented here affirms the limited amount of flammability studies at elevated temperature and pressure conditions for gaseous mixtures. However, the available experimental results at these conditions provide important data for validation of the current


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experimental methods. The objective of the present study is to address the lack of flammability data for syngas/air mixtures at elevated temperature and pressure conditions. Author(s) Van den Schoor

Year 2007

Type of Gases Studied Methane, ethane, methane/hydrogen

Vessel(s) Tube and bomb

Criterion Various

Conditions Pressures up to 30 bar, temperatures up to 250 C

Ale et al.

1981

Ammonia/Hydrogen/ Methane

8 L bomb, 6 L bomb

Not mentioned

Pressures up to 175 bar, temperature of 150 C

Tschirschwitz et al.

2015

Methane, Hydrogen

11 L cylinder, 9 L sphere (bombs)

Various

Pressures up to 20 bar, temperatures up to 300 C

Van den Schoor & Verplaetsen

2006

Lower alkanes/alkenes

4.2 L spherical bomb

1% pressure rise


ASTM E918

1983

Chemicals (vapors and liquids)

SS cylinder (bomb), V = 1 L

7% pressure rise

Pressures up to 13.8 bar, temperatures up to 200 C

Mitu et al.

2015

Ethane/air

Spherical SS bomb, V = 0.52 L

Unknown pressure rise

Pressures up to 1.3 bar, temperatures up to 150 C

Liu & Zhang

2014

Hydrogen

5 L cylinder bomb

7% pressure rise


Pekalski & Pasman

2009

n-butane-oxygen, C1C2-oxygen

V = 20 L spherical bomb

5% and 7% pressure rise

Holtappels

2007

Methane, Hydrogen

Various

5% pressure rise

Pressures up to 4 bar (nbutane) and up to 16 bar (C1-C2), Temp = 500 K (230 C) Pressures up to 30 bar, temperatures up to 250 C

Grune et al.

2015

8.2 L spherical bomb

5% pressure rise

Atmospheric pressure, temperatures up to 250 C

Van den Schoor et al.

2009

Hydrogen/Carbon Monoxide/H2O/CO2/ N2 Hydrogen/Carbon monoxide/Nitrogen

Tube, D = 80 mm, L = 300 mm

Visual flame propagation


Wierzba & Wang

2006

Hydrogen/Carbon monoxide/Methane

SS tube, D = 50.8 mm, L = 1 m



Wierzba & Kilchyk

2001

Hydrogen/Carbon Monoxide (Syngas)

Stainless steel tube



Wierzba & Ale

2000

Methane, Hydrogen, Methane/Hydrogen

Stainless steel tube



Hustad & Sonju

1988

Vertical tube, D = 100 mm, L = 3 m


Atmosphere pressure, temperatures up to 450 C

Miao et al.

2011

5.34 L cylinder bomb

5% and 7% pressure rise

Atmospheric pressure, ambient temperature

Pahl

1994

Methane, butane, hydrogen, carbon monoxide Methane, Hydrogen, Natural Gas, Methane/Hydrogen, Natural Gas/Hydrogen Methane/Hydrogen

2.7 L cylinder bomb

10% pressure rise



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Wang et al.

2014

Methane/Carbon Dioxide/Nitrogen

22 L spherical bomb

7% pressure rise


Van den Schoor et al.

2008

Methane/Hydrogen

Glass tube



Zhao

2008

Vertical tube, D = 4 inches



Liao et al.

2005

Methane/ethylene, methane/n-butane, methane/acetylene, etc. Natural Gas

1.57 L cylinder bomb

7% pressure rise


Karim et al.

1985

SS tube, D = 5 cm, L = 1 m


Atmosphere pressure, temperatures down to 130 C

Karim et al.

1984

Hydrogen/Carbon monoxide, Hydrogen/Methane, Hydrogen/Propane, Hydrogen/Ethane Methane/Hydrogen/Ca rbon monoxide

SS tube, D = 5 cm, L = 1 m


Atmosphere pressure, temperatures down to 130 C

Gant et al.

2011

Hydrocarbons and CO2 mixtures

V = 20 L spherical bomb

Not mentioned


Hu et al.

2014

Methane/CO2/Oxygen

Glass tube



Cashdollar et al.

2000

Methane, Propane, Hydrogen, Deuterium

8 L chamber, 20 L chamber, 120 L sphere, 25.5 cubic meter sphere (bombs)

3% and 7% pressure rise, visual flame propagation


De Smedt et al.

1999

Methane, ethane, propane, butane

Various


ASTM E681

1985

Chemicals (vapors and liquids)

Glass tube, V = 20 L spherical bomb Glass flask, V = 5 L



Zabetakis

1965

Methane, hydrogen, etc.




Coward & Jones

1952

Methane, hydrogen, etc.




Zlochower

2012

120 L steel sphere (bomb)

7% pressure rise


Kutzler

2008

2005


Atmospheric pressure, temperatures above 100 C Atmospheric pressure, ambient temperature

Liaw et al.

2016

SS cylinder, D = 152-168 mm, L = 1295 mm Glass cylinder, D = 60 mm, L = 300 mm V = 20 L spherical bomb


Schroder & Molnarne

Methane, ethylene, dimethyl ether, carbon monoxide Methane/Ethane/Hydr ogen/Oxygen and Steam (Premixed) Methane, ethane, hydrogen, etc.

7% pressure rise

Atmospheric pressure, T = 150 C

Acetone, methyl formate, methanol, isopropanol (FTIR)


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Giurcan et al.

2015

Methane, propane, ethane, etc.

Several bombs

5% pressure rise


Table 2. Summary of key flammability literature 2. METHODS AND APPROACH 2.1 Experimental Setup. The experimental setup was modeled after the suggested setup and conditions provided by ASTM E918.7 The test vessel chosen is required to have a volume of no less than 1 liter and a minimum inside diameter of 76 mm (3 in.). The apparatus also needs to be rated to a maximum pressure of 206.8 bar (3000 psi) in order to be used for mixtures at initial pressures up to as much as 13.8 bar (200 psia). The specific test vessel chosen was a HOKE sampling cylinder constructed of 304 stainless steel, with a 3.5 in. diameter and equipped with two ½” female NPT connections. Variations to the suggested experimental setup were considered in order to simplify the procedure for flammability testing while ensuring consistent results. The spark ignitor was chosen according to the ASTM standard as a Champion spark plug that would be fitted in the lower half of the cylinder and extending inside to reach the vertical centerline of the test apparatus. The thermocouple chosen for temperature measurements was an Omega TJ36CAIN type-K thermocouple with a diameter of 1/16” in order to provide a rapid response. Modifications were necessary to prepare the experimental setup for conditions of use up to 13.8 bar (200 psia) and 200 °C. In order to allow the complete system to tolerate the high pressure testing, an Omega high pressure (HP) PX41T0 0-300 psig pressure transmitter was used for recording pressure-time histories. A LabVIEW virtual instrument (VI) program was developed to display and collect data from both the Omega thermocouple and HP transmitter. Instead of the manual addition of the gas components, it was determined to use Schrader solenoid valves in combination with appropriately sized orifices to ensure remote yet accurate partial pressure


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addition. The desire to operate the valves remotely is to ensure safety during each test, especially for higher pressure procedures. The addition of remote ignition of the cylinder was also considered not only to ensure safety but to also be able to quantify the amount of ignition energy provided. Limit mixtures that depend on the ignition source strength may be defined as limits of ignitibility, while mixtures that are essentially independent of the ignition source strength and that give a measure of the ability of a flame to propagate away from the ignition source may be defined as limits of flammability.5 The standard for which the experimental setup of the current study is based on (ASTM E918) does not specify an amount of time to provide ignition energy, however the European Standard (EN 1839) mentions an ignition energy strength range of 10 – 20 Joules.7,8 This range is mentioned for the determination of flammability limits utilizing closed test vessels referred to as the bomb method, and is sufficient to establish flammability limits as opposed to ignitibility limits as can be seen in Figure 1. Although minimum ignition energy varies as fuel composition changes, ignition energy strength for determining limits of flammability are chosen to be considerably greater than the minimum ignition energy in order to be independent of fuel type. As shown in Figure 1, limits of ignitibility are defined for low ignition energies on the order of milli-Joules (mJ); however, the chosen ignition energy strength range is significantly higher. To consistently avoid approaching the region of ignitibility limits rather than flammability limits at the various pressures investigated in the current study, the suggested amount of ignition energy strength was chosen to more closely match with the European Standard EN 1839. Spark duration was kept consistently in the range of 0.04 and 0.08 seconds to ensure an ignition energy strength range of 10 – 20 Joules at the rated ignition transformer power of 250 VA.


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Figure 1. Ignitibility curve and limits of flammability for methane-air mixtures at atmospheric pressure and 26 °C5 It was necessary to add a check valve after the installation of the solenoid valve so as to prevent the backflow of gaseous components during partial pressure addition. Finally, BriskHeat BS0 silicone rubber heating tape was chosen as it was suitable for electrical conductive surfaces such as this application, and would allow for maximum exposure temperatures up to 230 °C. A full schematic of the experimental setup is shown in Figure 2.


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Figure 2. Experimental set up for elevated flammability limit testing (1) One-liter stainless steel cylinder; (2) Omega high pressure transmitter; (3) Powered solenoid valves; (4) Silicone rubber heating tape 2.2 Experimental Test Parameters. Defining the test parameters for the experimental approach required investigating typical compositions of syngas, such as those derived from renewable sources. Table 3 summarizes a representative composition of renewably-derived syngas, and shows that the majority of the fuel mixture is made up of two fuel components. Common syngas mixtures derived from processes such as methane reforming or oxygen-blown gasification of coal can contain up to 90% combined hydrogen and carbon monoxide. Focusing on these two primary components is reasonable for simplifying the experimental procedure and for providing applicable flammability data for use in the field. Hence, all simulated syngas/air fuel blends are treated as


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binary fuel mixtures. As shown in Table 3 simulated syngas for the purpose of this study is composed of varying amounts of hydrogen and carbon monoxide. These binary fuel blends varied by 20% per case and are chosen with the intention of covering the scope of the previously mentioned typical renewable syngas composition. In the following sections, syngas fuel blends will also be referred to as splits and notated as %/% (e.g. 60/40 to designate a 60% H2/40% CO syngas fuel blend (by volume)). Finally, with respect to the initial temperature and pressure test parameters, a test range of 25 C to 200 C for temperature and 1 bar to 9 bar for pressure was chosen in order to be able to compare the LFL results from the relevant literature. Syngas (Typical)

Syngas (Simulated)

CO

30-60%

H2

CO

H2

25-30%

60%

40%

CH4

0-5%

40%

60%

CO2

5-15%

20%

80%

Table 3. Comparison of typical renewable1 gas composition and simulated syngas fuel mixture composition used for LFL testing 2.3 Le Chatelier’s Law. In addition to comparing the experimental results of the current study with available literature data, lean flammability limit (LFL) results for the hydrogen/carbon monoxide (H2/CO) fuel blends will also be compared with LFL estimates found by using Le Chatelier’s (LC) Law:31 𝐿𝐹𝐿𝑚𝑖𝑥 =

1

1

𝑥𝑖 𝐿𝐹𝐿𝑖

∑

(1)

Source: www.netl.doe.gov


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With respect to the flammability testing of gaseous mixtures, pertinent literature typically suggests the use of Le Chatelier’s law as a predictive method for the determination of these limits. This rule has been used extensively in the literature and has been confirmed to be accurate based on studies conducted primarily at standard temperature and pressure conditions. The standard for the determination of flammability limits at elevated temperature and pressure conditions, ASTM E918, does not explicitly mention Le Chatelier’s law as a valid method for estimating the lean flammability limit; however, ASTM E681 suggests that lower flammability limits of known mixtures may be estimated from known lower flammable limits of the mixture components using Le Chatelier’s law.6,7 An LFL estimation using the above equation for a natural gas with the composition shown in Table 4 gives the following result:

𝐿𝐹𝐿𝑚𝑖𝑥 =

1 100 = 4.55% 𝑥𝑖 = 80 15 4 1 ∑ 𝐿𝐹𝐿𝑖 5.30 + 3.22 + 2.37 + 1.86

Fuel Component

Composition (%)

LFL (% vol2)

Methane

80

5.30

Ethane

15

3.22

Propane

4

2.37

Butane

1

1.86

Table 4. Sample natural gas composition for LC example.

2

% volume (% vol) of fuel in air; interchangeable with mol % (% mol)


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The benefits of a simple calculation to estimate the LFL of various types of gaseous mixtures for a range of conditions are evident. However, it is important to compare these estimates with experimental results in order to assess the validity of Le Chatelier’s law to estimate LFLs. Discussion of the LFL results for hydrogen/carbon monoxide (H2/CO) gas mixtures in comparison to estimates using Le Chatelier’s law can be found in the Results and Discussion section. 2.4 Pressure Rise Criterion. In order to determine the lower flammability limit (LFL) experimentally at elevated temperature and pressure conditions, it is necessary to decide on an ignition criterion appropriate for the given experimental setup. As previously mentioned, an ignition criterion based on the amount of pressure rise after the test mixture has been ignited is preferred for these conditions as visual flame propagation is not typically possible due to the design of the test vessel at elevated pressures. The criterion suggested by the ASTM E918 standard that this experimental setup was first modeled after is a 7 percent pressure rise, however, it is widely accepted that a lower percent pressure rise is more suitable as it would result in a more conservative flammable range.7 The bomb method discussed in the European Standard EN 1839 includes a pressure rise criterion of 5% which is widely used in the literature.8 A criterion of 2% pressure rise reportedly agrees consistently with visual flame propagation data according to De Smedt et al. although this may not necessarily extend to conditions of elevated pressures.12 In the present study, the noise associated with the pressure transducer and data acquisition system was determined to be 1.5%. Hence, to avoid erroneous data due to noise or small pressure rises due to local heating rather than complete ignition of the fuel mixture, a 5% pressure rise was selected as the ignition criterion for determining the LFL at all test conditions. 2.4 Fundamental Theory and Uncertainty of Measured Data. Regarding the effect of temperature on the limits of flammability, the rate of an elementary reaction relevant to ignition


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of a combustible mixture is generally proportional to the rate coefficient in Arrhenius form, dω/dt ~ exp(-Ea / RT), and therefore increases with an increase in temperature.32 The temperature dependence of the flame speed for a spherically expanding isobaric flame is reflected in the following derived expression, Sf ~ (Tf / Tu)×[exp(-Ea / RTf)]1/2, and shows that flame speed increases with increasing temperature due to the exponential term dominating over the fractional term. Regarding the effect of pressure on the limits of flammability, the reaction rate is proportional to the product of the concentrations of the reacting species, dω/dt = kCACB ~ pn, and therefore increases with an increase in pressure. For a spherically expanding isobaric flame, flame speed is proportional to burning velocity, Sf = Su×(ρu / ρb) ~ Su×(Tf / Tu), while burning velocity is proportional to pressure through the following derived expression: Su ~ p(n-2)/2. Therefore, given an overall reaction order of 2 or less, an increase in pressure would result in a decrease in the flame speed. The combination of these factors plus others related to the flammability of different types of fuel mixtures cause there to be uncertainty with predicting the influence of pressure on the LFL. The uncertainty for all test measurements of the current study, including experimentally derived LFL values, was established based on a 95% confidence interval. A 95% confidence level corresponds to a probability of 0.95 that the true value will be bounded by the mean value +/1.96×(σ / √n) in which σ is the standard deviation of the population and n is the number of tests for a given condition. At least ten data points were measured for a given LFL test point. Error bars are included in each of the following plots to denote the 95% confidence interval calculated for each LFL measurement. In some cases, the error bars are too small to be visible for some test mixtures.


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3. RESULTS AND DISCUSSION 3.1 Effect of Temperature and Pressure on Syngas LFL. The dependence of the lower flammability limit (LFL) for hydrogen/carbon monoxide (H2/CO) mixtures in air on pressure at T = 25 °C is shown in Figure 3.

Figure 3. Dependence of pressure and composition on the LFL of hydrogen/carbon monoxide/air mixtures at T = 25 °C The effect of fuel composition on the LFL of hydrogen/carbon monoxide mixtures is clear and distinct and this is due to the widely different combustion characteristics of hydrogen and carbon monoxide as individual gaseous fuels. At atmospheric conditions, the reported LFL value for hydrogen is approximately 4.0 vol % while the reported LFL value for carbon monoxide is about 12.5 vol %. The results clearly show that the addition of hydrogen results in a much lower LFL value, and consequently, a wider flammable range. This observation holds for all experimental values for a given initial test pressure. Regarding the effect of pressure on the LFL of H2/CO fuel mixtures in air, all three fuel splits (60/40, 40/60 and 20/80) exhibit a linear increase of the LFL with pressure. Each linear trend is confirmed because it does not deviate from the data points more than the respective experimental


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uncertainty of each data set. At T = 25 °C, the 95% confidence interval is approximately ±0.109 vol % for the 60/40 (% H2/% CO) blend, approximately ±0.129 vol % for the 40/60 blend, and approximately ±0.108 vol % for the 20/80 blend. In addition to the theoretical background discussed earlier, flame stretch is another physical mechanism that can clarify the pressure dependence on the limits of flammability. For weakly stretched flames, the burning velocity Su is found to be linearly dependent on the flame stretch rate K, with the unstretched burning velocity given by (Su)0, the flame thickness given by δ, and the Markstein number given by Ma: Su = (Su)0 – δ×Ma×K. For a flame kernel propagating upwards, it is found that its upward velocity v increases with the density ratio of unburned to burned gases, consistent with expected natural convective flow. Consequently, the upward velocity increases slightly with pressure. Since a higher upward velocity leads to a higher flame stretch rate K, the slight increase of v and, hence, of the flame stretch rate K with pressure causes a slight decay in the flame propagation 33. Another explanation for the increase of the lower flammability limit with an increase in pressure is due to heat loss due to convection, which is greater at high pressures than at low, as density differences become greater 4

. The specific amount of increase of the LFL per increase of pressure is determined from the slope

of the linear trend lines shown in Figure 3. At T = 25 °C, the increase of the LFL per 1 bar of pressure is approximately 0.115 vol % for the 60/40 blend, 0.152 vol % for the 40/60 blend, and 0.180 vol % for the 20/80 blend. It is evident that an increase in the amount of hydrogen in the fuel results in a decrease of the “slope” or unit change/increase of the LFL as a function of pressure. The dependence of the lower flammability limit for hydrogen/carbon monoxide mixtures in air on pressure at T = 200 °C is shown in Figure 4. The results again clearly show that the addition of hydrogen results in a much lower LFL value, similar to the results at T = 25 °C. Comparing Figure 3 and Figure 4, it is noted that an increase in the temperature from T = 25 °C to T = 200 °C results


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in a clear decrease in the LFL for all the fuel blends tested and at all test pressures. Regarding the effect of pressure on the LFL of H2/CO fuel mixtures in air, and similar to the results at T = 25 °C, the fuel 40/60 and 20/80 fuel splits exhibited a confirmed linear increase of the LFL with pressure. The trend line for the 60/40 blend at T = 200 °C, however, could not be confirmed as it deviates by approximately 0.02 vol% or 0.3% of the LFL value at P = 5 bar. For the linear trends at T = 200 °C, the interval of uncertainty is approximately ±0.104 vol % for the 60/40 (% H2/% CO) blend, approximately ±0.081 vol % for the 40/60 blend, and approximately ±0.093 vol % for the 20/80 blend. The specific amount of increase of the LFL per increase of pressure is determined from the slope of the linear trend lines shown in Figure 4. At T = 200 °C, the increase of the LFL per 1 bar of pressure is 0.120 vol % for the 60/40 blend, 0.122 vol % for the 40/60 blend, and 0.256 vol % for the 20/80 blend. Similar to the results at ambient temperature, an increase in the amount of hydrogen in the fuel results in a decrease of the unit change of the LFL as a function of pressure.

Figure 4. Dependence of pressure and composition on the LFL of hydrogen/carbon monoxide/air mixtures at T = 200 °C Literature data for the dependence of the lower flammability limit for hydrogen/carbon monoxide mixtures in air on pressure were not available for comparison, however, recent


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flammability studies that investigate the dependence of the LFL for syngas/air mixtures on temperature can be compared with the current results at 1 bar.26,27 Figure 5 shows a comparison of the experimentally determined LFL data of the current study with the literature data for similar composition syngas/air mixtures in the temperature range of T = 20 °C to T = 200 °C. Five syngas fuel blends of varying composition studied by Li et al. are included for comparison and had an average composition of 10% H2, 22% CO, 4% CH4, 49% N2 and 16% CO2. The remaining fuel blends, both of the current study and those investigated by Wierzba and Kilchyk, are labeled by their % H2/% CO (hydrogen/carbon monoxide) composition. Overall, all LFL data confirmed the expected linear decrease of the LFL with an increase in temperature, as well as a definite decrease in the LFL with an increase in the hydrogen concentration of the mixture. As previously mentioned, an increase in the initial temperature promotes ignition and flame propagation by leading to an increase in reaction rate and flame speed, respectively, and has an overall agreed upon inverse relationship with LFL in the temperature range studied.5 The linear dependence on temperature can be explained by the assumption that the flame temperature at LFL for a given fuel composition is constant, as the adiabatic flame temperature exhibits a linear dependence with equivalence ratio at extremely-lean conditions.34 Varying fuel composition may or may not affect the flame temperature at the LFL as it depends on the fuel components. If the fuel is composed of combustibles with a constant flame temperature at LFL (e.g. paraffin hydrocarbons), then the fuel mixture will have the same flame temperature at LFL regardless of composition. If a fuel component is added with a distinct flame temperature at LFL (e.g. hydrogen), varying the fuel composition will also vary the flame temperature at the LFL. For the syngas mixtures of the current study as well as those from the literature, the presence of different concentrations of hydrogen in the fuel corresponds to different resultant flame temperatures at LFL.


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Figure 5. Comparison of current and published LFL data26,27 for various syngas/air mixtures at P = 1 bar As the simulated syngas/air mixtures of the current study are most similar to the mixtures tested by Wierzba and Kilchyk, these two were the most notable to compare. Wierzba and Kilchyk tested a fuel mixture of 75% H2 and 25% CO which resulted in the lowest LFL data set at all test temperatures due to the small LFL associated with hydrogen. The largest LFL data set at all temperatures tested was for the 20% H2 and 80% CO of the current study. Although the most diluted fuel mixture tested by Li et al. was their SYN10 fuel mixture with about 70% diluent (N2 + CO2), the 20/80 syngas fuel blend of the current study was determined to have a narrower flammable range (higher LFL) despite having four times the amount of hydrogen in the fuel mixture. It should be noted that the LFLs for the 40/60 syngas fuel blend of the current study were comparable to the LFLs for the 25/75 syngas fuel blend provided by Wierzba and Kilchyk. This unexpected result is likely attributed to the different methods of determining flammability. Li et al. used a counterflow flame burner, Wierzba and Kilchyk determined flammability through visual


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flame propagation using a stainless steel tube, while the current study selected a pressure resistant vessel and a pressure rise ignition criterion to determine flammability. The effect of the choice of ignition criterion and experimental apparatus on the LFL results can be seen more clearly in Figure 6. The binary syngas LFLs from the Wierzba and Kilchyk study as well as the current study are plotted in a flammability ternary plot in order to better compare the results of the different mixture compositions. To clarify, the three components that compose the ternary plot are hydrogen, carbon monoxide and air. The hydrogen content can be read on the leftmost “axis”, following the lines horizontally ranging from 0-10 vol %. The air content can be read on the rightmost “axis”, following the lines diagonally ranging from 90-100 vol %. The carbon monoxide content can be read on the bottommost “axis”, following the lines diagonally ranging from 0-10 vol %. Every point within the ternary plot corresponds to components totaling 100% (e.g. 2% H2, 6% CO, 92% air). By definition, the LFL in air is (100% - % air) and as a result is directly related to the air content. Therefore, LFL data is naturally included in this ternary plot and the effect of fuel composition and experimental conditions such as temperature on the LFL can be clearly observed. For a given temperature, an increase (decrease) in the hydrogen (carbon monoxide) content results in a decrease (increase) of the LFL, across both data sets. An increase in temperature for a given syngas mixture results in a decrease of the LFL which is also consist with previous findings. According to Figure 6, the LFLs for the syngas mixtures from the Wierzba and Kilchyk study at T = 20 °C, and the syngas mixtures of the current study at T = 200 °C, are comparable. This result is similar to the aforementioned result of comparable LFLs for the 40/60 syngas fuel blend of the current study and the 25/75 syngas fuel blend shown in Figure 5, and is similarly attributed to the generally lower LFLs that result from the determination of flammability limits through visual flame propagation. As previously mentioned, the choice of ignition criterion


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of 5% pressure rise is consistent with established flammability standards and is appropriate for the determination of flammability limits at conditions of elevated pressure. De Smedt et al. claim that there is a close agreement between the visual indication of flammability and a 2% pressure rise criterion, and thus a 5% pressure rise criterion would produce narrower flammability limits as a result.12 Therefore, the primary physical explanation for lower LFL data determined using a visual criterion is the sensitivity of flame observation. As flammability tests at elevated pressures are performed in closed pressure vessels, practical problems arise with respect to observing the flame visually and thus result in higher LFL values. Nevertheless, important observations regarding the influence of initial pressure and temperature on the LFLs of hydrogen/carbon monoxide/air mixtures were made and additional investigations are necessary to corroborate the conclusions.

Figure 6. Flammability ternary plot of current and published LFL data26 for syngas/air mixtures at P = 1 bar 3.2 Empirical Formula for Syngas/Air LFLs at Elevated Conditions. Overall, the complete LFL data set for syngas (H2/CO) as the fuel for the current study can be condensed to an empirical formula provided in Table 5. The formula was derived from the specific trend line equations previously determined and as shown in Figure 3 and Figure 4, assuming also a linear relationship with respect to temperature to help determine the f (T) and g (T) functions.


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𝑳𝑭𝑳𝒔𝒚𝒏𝒈𝒂𝒔 = 𝒇(𝑻) ∙ 𝑷 + 𝒈(𝑻) = [𝒂 ∙ 𝑻 + 𝒃] ∙ 𝑷 + 𝒄 ∙ 𝑻 + 𝒅; 𝑻 = [𝑪]; 𝑷 = [𝒃𝒂𝒓] % H2 / % CO

𝒂

𝒃

𝒄

𝒅

20/80

4.3714 × 10-4

1.6857 × 10-1

-1.214 × 10-2

1.0282 × 10+1

40/60

-1.7143 × 10-4

1.5629 × 10-1

-6.7543 × 10-3

7.8609 × 100

60/40

2.8571 × 10-5

1.1429 × 10-1

-7.0457 × 10-3

6.4371 × 100

Table 5. Empirical relationship for the LFL of syngas/air mixtures as a function of temperature and pressure The derived formula can be used to calculate the experimentally determined LFL values of the current study but can also extend to intermediate temperatures and pressures within the scope of this study (25 °C ≤ T ≤ 200 °C, 1 bar ≤ P ≤ 9 bar). Although literature data of syngas LFLs are limited, lower flammability limits for hydrogen/carbon monoxide mixtures at similar elevated conditions as the current study are available to test the aforementioned derived formula. Van den Schoor

et

al.

experimentally

determined

flammability

limits

of

hydrogen/carbon

monoxide/nitrogen/air mixtures for hydrogen fuel molar fractions of 0.44, 0.62 and 0.71 at atmospheric pressure and initial temperatures up to 200 °C using a glass cylindrical tube.25 The LFL values shown in Table 6 were interpolated from the literature data reported by Van den Schoor et al. and are compared with the calculated values from the derived empirical formula. Although both sets of LFLs agree with respect to the trend of decreasing LFL with either an increase in initial temperature or an increase in hydrogen fraction, the LFL values themselves differ quite significantly. The main reason for the difference in LFL values is due to the literature data being determined using the “tube” method while the current study determined LFLs using the “bomb” method, as using a tube as the experimental apparatus results in a wider flammable range. Further tests are necessary in order to increase the amount of reliable flammability data using the “bomb”


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method and use this data to fully validate this formula for syngas/air mixtures at elevated initial conditions of temperature and pressure. (% / %)

Literature Data (vol %)

H2/CO

Current Study (vol %)

Percent Difference (%)

Literature Data (vol %)

25 °C

Current Study (vol %)

Percent Difference (%)

200 °C

40/60

6.21

7.84

21

4.53

6.63

32

60/40

5.01

6.38

21

4.11

5.15

20

Table 6. Comparison of lower flammability limits of syngas/air mixtures, at different initial temperatures, from the literature25 and calculated using the derived empirical formula 3.3 Validity of Le Chatelier’s Law. Hydrogen/carbon monoxide/air LFL data and Le Chatelier estimates at P = 1 bar and T = 25 °C are plotted versus hydrogen fraction in Figure 7. It should be noted that the LFL of pure carbon monoxide (CO) at various temperatures and pressures are needed to calculate estimated LFL values at elevated conditions using Le Chatelier’s law, however, these values are outside of the scope of the current study. The predicted values plotted in Figure 7 use the literature LFL value for carbon monoxide of 12.5 vol %, which is only applicable at ambient conditions of temperature and pressure.


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Figure 7. Comparison of hydrogen/carbon monoxide/air LFL data with Le Chatelier estimates; P = 1 bar, T = 25 °C, LFLCO = 12.5 vol %5 For the three fuel blends tested, the estimated LFLs calculated using Le Chatelier’s law closely follow the general trend of the actual, experimentally-derived values, which is a decrease of the LFL with increasing amounts of hydrogen in the fuel mixture. The predicted values are on average 0.62 vol % less than the actual LFL values. Some authors claim that the presence of hydrogen in a gaseous mixture is the cause of deviation in the effectiveness of Le Chatelier’s law.26,35,36. The deviation for the current study, however, is not attributed to the presence of hydrogen but rather to an inaccurate choice for the LFL of pure carbon monoxide. Assuming a value of 15.0 vol % for the LFL of pure carbon monoxide instead of the reported literature of 12.5 vol % provides a much better agreement between predicted LFL and experimentally determined LFL for all syngas/air mixtures at atmospheric conditions as shown in Figure 8. The higher LFL value is a result of the LFL data of the current study being determined with a pressure rise criterion rather than the visual flame propagation criterion used to determine the original literature value. Taking this modification into consideration, it is determined that the estimates found using Le Chatelier’s law are sufficiently accurate as a predictive tool. This result is appealing, as it indicates Le Chatelier’s law can be used to estimate LFLs if data are not available, and it provides a wider


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(safer) flammable range than actual. Wierzba and Kilchyk came to a similar conclusion with their assertion that LFLs could be predicted reasonably well using Le Chatelier’s rule for fuel mixtures containing more than 20% of hydrogen.26 Li et al. confirmed that Le Chatelier’s rule could be used with fair accuracy until moderately diluted conditions, as the limiting flame temperature increases sharply at high inert dilutions.27 In general, the above results for hydrogen/carbon monoxide/air mixtures are presented for atmospheric conditions only, and thus are only applicable at these conditions if a predictive method for determining the lean flammability limit is desired.

Figure 8. Comparison of hydrogen/carbon monoxide/air LFL data with Le Chatelier estimates; P = 1 bar, T = 25 °C, LFLCO = 15.0 vol %

4. CONCLUSIONS New data for lean flammability limits (LFLs) for syngas/air (H2/CO/air) mixtures at temperatures up to 200 °C and pressures up to 9 bar have been obtained. A full literature review provided context for choosing an appropriate test apparatus and ignition criterion. Although flammability data determined at ambient conditions is widely available, data at elevated pressures as well as for syngas/air mixtures is lacking. Conditions of elevated pressures require use of a


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closed test vessel, and as a result a pressure rise ignition criterion. In order to promote standardization for the determination of flammability limits, the test apparatus for the current study was chosen to match the ASTM E918 standard, while the ignition criterion was chosen to be a 5% pressure rise for consistency with the limited flammability data available. Based on analysis of the results obtained, the key conclusions regarding flammability limits of hydrogen/carbon monoxide (H2/CO) mixtures in air include: •

Experimentally derived LFLs for syngas/air mixtures were confirmed to decrease consistently with either an increase in initial mixture temperature or an increase in hydrogen fraction. The decrease of the LFL with increasing hydrogen concentration is due to the lower characteristic LFL of hydrogen relative to that of carbon monoxide at all test conditions, while the decrease of the LFL with increasing temperature is consistent with the assumption that the flame temperature at LFL for a given fuel composition is constant.

•

Experimentally derived LFLs for syngas/air mixtures exhibited an increase with an increase in initial pressure, and the influence of pressure on the LFL decreased with an increase in hydrogen fraction. The increase of the LFL with pressure can be attributed to the flame stretch rate with pressure which causes a slight decay in the flame propagation.

•

An empirical formula was derived which allows for the calculation of the LFLs of all syngas/air test mixtures at the specific test pressure and temperature conditions investigated in the current study. The derivation of the empirical formula directly from the experimental LFL data, taking into account uncertainty based on 95% confidence intervals, ensures accurate performance of the formula to calculate a desired LFL in an effort to promote ease of use in practical applications.


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•

Le Chatelier’s law was shown to provide relatively close estimates for the lower flammability limits of hydrogen/carbon monoxide/air mixtures distinctively at conditions of atmospheric temperature and pressure.

AUTHOR INFORMATION Corresponding Author *Tel: +1-9498245950 ext. 11-142. E-mail: [email protected]. ACKNOWLEDGMENTS The work was financially supported by the Advanced Power and Energy Program (APEP) at UC Irvine. ABBREVIATIONS LFL, lean flammability limit; ASTM, American Society for Testing and Materials; LC, Le Chatelier’s Law; ETP, elevated temperature and pressure. REFERENCES (1)

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Air Mixtures at Elevated

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