Synthesis of Ammonia Using CH₄/N₂ Plasmas Based on Micro-Gap Discharge under Environmentally Friendly Condition

Abstract

The synthesis of ammonia has been studied in methane-nitrogen plasmas using a micro-gap discharge under an environmentally friendly condition. The effects of some parameters such as the specific input energy, the discharge gap, the volume ratio of CH₄/N₂, the residence time, and the gas temperature on the yield of NH₃ and conversion rate of CH₄ are discussed in the paper. The results show that the highest yield of NH₃ is 8000 ppm for a residence time of 1.6 s. In addition, the yield and generating rate of H₂ are 9.1% (v/v) and 1879.8 μmol/min, respectively. Therefore, the micro-gap discharge is an efficient method for NH₃ synthesis and H₂ generation from CH_4.

Introduction

Ammonia (NH₃), as an important chemical product, can be used to produce ammonium fertilizer, such as carbamide ammonium nitrate and ammonium bicarbonate. Hydrogen (H₂) is definitely a green energy source, and hydrogen based fuel-cell technology has been investigated extensively to develop fuel cell vehicles and cogeneration system. Methane is the major constituent of natural gas. Activation of saturated hydrocarbons and especially methane is difficult due to the high dissociation energy of C–H bonds. The direct synthesis of NH₃ using CH₄ rather than H₂ by non-equilibrium plasma at atmospheric pressure is of great importance.

A number of studies on plasma synthesis of ammonia have been carried out by radio frequency (=13.56 MHz) or microwave (=2540 MHz) discharge at low pressure. In 1994, Oumghar, Legrand et al. [1–3] investigated the conversion of CH₄ by N₂ or air microwave plasma, as well as kinetic mechanism of CH₄ conversion. C₂ hydrocarbons (C₂H₂, C₂H₄, C₂H₆) and trace amount of HCN, NH₃ were obtained, and the methane conversion rate was 80% within the power range of 350~650 W. Some studies [4, 5] reported that ammonia molecules would be formed by the reaction between NH_x radicals and hydrogen atoms. In some cases, catalysts were placed in the discharge or afterglow region, or on the cold trap wall in order to enhance the ammonia yield [5–8]. Tanaka et al. [7, 8] studied the synthesis of ammonia in N₂-H₂ plasmas by microwave or radio frequency discharge at a pressure of 650 Pa (5 Torr) with catalysts of iron and molybdenum wires of 100 pieces. The yields of NH₃ and N₂H₄ were only 1.5 mmol/g and 2.5 μmol/g, respectively, with a residence time of 2 h and power input of 180 W. The main drawback of low pressure plasmas is low energy efficiency and low yields of NH₃.

H₂ generation from water, methane, and methanol was investigated in a ferroelectric pellet packed-bed reactor (FPR) and a silent-discharge-type reactor (SDR) by Kabashima et al. [9] in 2003. The H₂-generation rate with FPR was 54 μmol/min in ambient Ar. Suid et al. [10] have reported water splitting by a dielectric barrier discharge (DBD) plasma reactor, of which the inner electrode was an Au-coated copper rod. The highest H₂-generating rate was 3.4 μmol/min in ambient Ar.

In previous studies performed by the author, the NH₃ synthesis in N₂–H₂ plasmas was carried out using the combined surface discharge and DC (direct current) discharge [11]. NH₃ yield was 5000 ppm with MgO catalyst. Subsequently, the concentration of NH₃ reached 12500 ppm by the micro-gap DBD without any catalyst [12, 13]. In this paper, the synthesis of NH₃ by N₂-CH₄ plasmas was performed. The yield of NH₃ was 8000 ppm for a residence time of 1.6 s. The whole process was carried out under atmospheric pressure without any catalyst, which is an environmentally friendly condition in line with Green Chemistry Principle [14, 15].

Plasma Processes of Micro-Gap Discharge

Comparison of Main Parameters between Several Kinds of Gas Discharges

The comparison of main parameters between several kinds of gas discharges is shown in Fig. 1. At high pressure (≥10⁵ Pa), the main gas discharges are the high-voltage corona, pulse corona, general DBD, and our micro-gap discharge. Strong and weak ionization zones are separated by the line which is defined by the following parameters: the electric field intensity E_g, 100 kV/cm; the electron density in discharge path N_e, 10¹⁴/cm³; the average electron energy T_e, 10 eV; and the fractional active volume (δ, %), 1%. The main parameters of our micro-gap discharge, which falls in the strong ionization zone, are much higher than that of other gas discharges; in particular the fractional active volume is about 2%. The fractional active volume refers to the volume ratio of the volume of all ionization discharge paths to that of micro-gap. Plasma chemistry reactions take place in the ionization discharge paths. The increase of the fractional active volume leads to more discharge paths, resulting in greater whole electron density in the micro gap. Therefore, our micro-gap discharge is an efficient method for the conversion of CH₄ into NH₃.

Fig. 1

Comparison of main parameter for several kinds of gas discharge

Excited Dissociation, Direct Ionization of CH₄ and Formation of H₂

The dissociative potential of C–H bond of CH₄ is less than 4.6 eV. The excited methane S₁ (9.6 and 10.4 eV) and S₂ (11.7 eV),which are formed by the electron-methane collisions, are so unstable that rapidly dissociate into radicals such as CH₃, CH₂, CH, C. The plasma reactions are as follows [1–3]:

$$ {\text{e}}\left( {\varepsilon > 10{\text{eV}}} \right) + {\text{CH}}_{{ 4 {\text{ }}}} \to {\text{CH}}_{{ 4 {\text{ }}}} \left( {{\text{S}}_{ 1} ,{\text{ S}}_{ 2} } \right) + {\text{e}} $$

(1)

$$ {\text{CH}}_{ 4} \left( {{\text{S}}_{ 1} , {\text{ S}}_{ 2} } \right) \to \left\{ \begin{gathered} {\text{CH}}_{ 3} + {\text{H}}\quad \quad \quad \; ( 2 )\hfill \\ {\text{CH}}_{ 2} + {\text{H}} + {\text{H}}\quad \;\;(3) \hfill \\ {\text{CH}}_{ 2} + {\text{H}}_{ 2} \quad \quad \;\;\;(4) \hfill \\ {\text{CH}} + {\text{H}}_{ 2} + {\text{H}}\quad \;\; ( 5 )\hfill \\ {\text{C}} + {\text{H}}_{ 2} + {\text{H}}_{ 2} \quad \quad (6) \hfill \\ \end{gathered} \right. $$

Direct Ionizations of CH₄ are as follows [12]:

$$ {\text{CH}}_{ 4} + {\text{e}}*\left( { 1 2. 7 5 {\text{eV}}} \right) \to {\text{CH}}_{ 4}^{ + } + 2 {\text{e}} $$

(7)

$$ {\text{CH}}_{ 4} + {\text{e}}*( 1 4. 3 {\text{eV}}) \to {\text{CH}}_{ 3}^{ + } + {\text{H}} + 2 {\text{e}} $$

(8)

$$ {\text{CH}}_{ 4} + {\text{e}}*( 1 5. 1 {\text{eV}}) \to {\text{CH}}_{ 2}^{ + } + {\text{H}}_{ 2} + 2 {\text{e}} $$

(9)

$$ {\text{CH}}_{ 4} + {\text{e}}* \to {\text{H}}^{ + } + {\text{CH}}_{{ 3 {\text{ }}}} + 2 {\text{e}} $$

(10)

$$ {\text{CH}}_{ 4} + {\text{e}}*( 2 2. 2 {\text{eV}}) \to {\text{CH}}^{ + } + {\text{H}} + {\text{H}}_{ 2} + 2 {\text{e}} $$

(11)

$$ {\text{CH}}_{ 4} + {\text{e}}* \to {\text{CH}}_{ 2} + {\text{H}}_{ 2}^{ + } + 2 {\text{e}} $$

(12)

$$ {\text{CH}}_{ 4} + {\text{e}}*( 2 5 {\text{eV}}) \to {\text{C}}^{ + } + 2 {\text{H}}_{ 2} + 2 {\text{e}} $$

(13)

T_e in our micro-gap discharge is ≥10 eV, which means a lot of electrons have the energy ≥9.6 eV. In this study, the above plasma reactions might be dominant reactions for H₂ generation as well as the formations of CH₄, CH₃, CH₂, CH radicals and CH₄ ⁺, CH₃ ⁺, CH₂ ⁺ ions.

Dissociation and Ionization of N₂ Molecule [16]

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 1 5. 6 3 {\text{eV}}} \right) \to {\text{N}}_{ 2}^{ + } \left( {{\text{X}}^{ 2} \sum {\text{g}}^{ + } } \right) + 2 {\text{e}} $$

(14)

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 1 6. 8 4 {\text{eV}}} \right) \to {\text{N}}_{ 2}^{ + } ({\text{A}}^{ 2} \Uppi {\text{u}}) + 2 {\text{e}} $$

(15)

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 1 8. 7 6 {\text{eV}}} \right) \to {\text{N}}_{ 2}^{ + } \left( {{\text{B}}^{ 2} \sum {\text{u}}^{ + } } \right) + 2 {\text{e}} $$

(16)

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 2 3. 5 3 {\text{eV}}} \right) \to {\text{N}}_{ 2}^{ + } \left( {{\text{C}}^{ 3} \sum {\text{u}}^{ + } } \right) + 2 {\text{e}} $$

(17)

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 2 4. 3 2 {\text{eV}}} \right) \to {\text{N}}^{ + } \left( {^{ 3} {\text{P}}} \right) + {\text{N}}\left( {^{ 4} {\text{S}}} \right) + 2 {\text{e}} $$

(18)

$$ {\text{N}}_{ 2} \left( {{\text{X}}^{ 1} \sum {\text{g}}^{ + } } \right) + {\text{e}}*\left( { 2 6. 6 6 {\text{eV}}} \right) \to {\text{N}}^{ + } \left( {^{ 3} {\text{P}}} \right) + {\text{N}}\left( {^{ 2} {\text{D}}} \right) + 2 {\text{e}} $$

(19)

Plasma Processes of NH₃ Synthesis

The CH₄, CH₃ etc. react with N₂ ⁺, N⁺, N(⁴S) to form the NH radicals as follows [1, 2]. Ion reactions (20) with high rate constant are the major reactions.

$$ {\text{CH}}_{{ 4 {\text{ }}}} + {\text{N}}_{ 2}^{ + } \to {\text{N}}_{ 2} {\text{H}}^{ + } + {\text{CH}}_{ 3} $$

(20)

$$ {\text{CH}}_{{ 3 {\text{ }}}} + {\text{N}}_{ 2}^{ + } \to {\text{N}}_{ 2} {\text{H}}^{ + } + {\text{CH}}_{ 2} $$

(21)

$$ {\text{CH}}_{{ 4 {\text{ }}}} + {\text{N}}^{ + } \to {\text{CH}}_{ 3}^{ + } + {\text{NH}} $$

(22)

$$ {\text{CH}}_{{ 4 {\text{ }}}} + {\text{N}}\left( {^{ 4} {\text{S}}} \right) \to {\text{CH}}_{ 3} + {\text{NH}} $$

(23)

NH radicals, which are considered as the ammonia precursors, are formed by the reactions of N₂ ⁺ with H₂, H. Also N atoms adhered to the wall react with hydrogen to form NH radicals [1, 2, 12].

$$ {\text{N}}_{ 2}^{ + } + {\text{H}}_{ 2} \to {\text{N}}_{ 2} {\text{H}}^{ + } + {\text{H}} $$

(24)

$$ {\text{N}}_{ 2}^{ + } + {\text{H}} \to {\text{N}}_{ 2} {\text{H}}^{ + } $$

(25)

$$ {\text{N}}_{ 2} {\text{H}}^{ + } + {\text{e}} \to {\text{NH}} + {\text{N}} $$

(26)

$$ {\text{N}}\left( {{\text{ads}}} \right) + {\text{H}} \to {\text{NH}}\left( {{\text{ads}}} \right) $$

(27)

Synthesis of NH₃ molecule [12]

$$ {\text{NH}} + {\text{H}}_{ 2} \to {\text{NH}}_{ 3} $$

(28)

$$ {\text{NH}} + {\text{H}} \to {\text{NH}}_{ 2} $$

(29)

$$ {\text{NH}}_{ 2} + {\text{H}} \to {\text{NH}}_{ 3} $$

(30)

The N₂ ⁺ ion is the major activated particle to form the NH radical and leads to NH₃. As for the micro-gap discharge, a lot of electrons have the energy ≥15.6 eV (ionization potential of N₂). Therefore, the micro-gap discharge is an efficient plasma method for the synthesis of NH₃ at atmospheric pressure.

Experiment

Experimental Setup

The experimental setup for synthesis of NH₃ using the micro-gap discharge is shown in Fig. 2. The plasma reactor 3 is rectangular, of which the dimension is 260 mm (length) × 130 mm (width) × 35 mm (thickness). The thin dielectric layers of α-AL₂O₃ are attached to both sides of discharge electrodes, of which the dimension is 175 mm (length) × 80 mm (width) × 0.3 mm (thickness). The discharge gap is 0.47 mm or 0.64 mm. The dielectric constant and the electric insulation intensity are 10 and 350 kV/cm, respectively. With this thin dielectric layer, strong streamer discharge can be obtained in the micro-gap.

Fig. 2

Schematic diagram for the synthesis of NH₃ by CH₄–N₂ plasmas Note: 1. Gas flow meter; 2. Gas mixing system; 3. Plasma reactor; 4. Earthing electrode 5. Discharge electrode; 6. Dielectric layers; 7. NH₃ analyzer; 8. Glass for Liquid fuel; 9. Gas chromatograph; 10. Discharge gap; 11. Power supply; 12. Insulator 13. Oscillograph; 14. Electrostatic voltage meter

Measurement System

The CH₄–N₂ gas mixture with a purity of 99.98% is fed into the reactor. The gas flow rate and mixing ratio are adjusted by the mass-flow controller 4000 Series Gas Mixing System (Environics INC in USA). The yield of ammonia is monitored on-line by GXH-105 Type Infrared Analyzer. The feed and product of gaseous hydrocarbons are analyzed with a gas chromatograph of GC-102 or HP4890 equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The relative standard deviation (RSD) is <0.5% based on the five times measurement of one sample.

The self-made power supply is applied to the discharge electrodes, leading to a continuous strong streamer discharge, of which the main parameters are as follows: the discharge power, 240 W; the applied frequency, 10 kHz; the pulse width, 40 μs. The electric parameters are measured using HV-60 High Voltage Probe (Iwatsu, Japan), SS-240 Pulse Current Probe (Iwatsu, Japan), TDS-3032B Oscilloscope (Tektronix, USA), Model HC-F1000L Frequency Meter (Hong Chong Electronic Co. Ltd) and Q3-V Electrostatic Voltage Meter (Beijing Electric Meter Factory, China).

Evaluation of System Performance

In order to evaluate the energy efficiencies for methane conversion, the specific input energy (SIE, electrical input energy to unit volume of gas), defined in Eq. 1, is used. The discharge power is measured using the method of charge-voltage figure [17, 18], which can efficiently eliminate the measure error caused by discharge gap that was equal to a capacitor. Equations 2–3 indicate the calculation methods for generation rate and conversion rate of CH₄. The volume per mol in our experiments is 24.2 L/mol with a temperature of 22°C and an atmospheric pressure.

$$ 1.\;{\text{Specific}}\;{\text{input energy}} = \frac{{{\text{Discharge power}}}}{{{\text{Flow rate feed gas}}}}\quad \left( {{\text{J}}/{\text{L}}} \right) $$

(1)

$$ 2.{\text{ Generation or conversion}}\;{\text{rate}} = \frac{{{\text{Yield }}\ \left( {{\text{v}}/{\text{v}}} \right) \times {\text{Flow rate}}}}{{{\text{Volume per mol}}}}\; \left( {\mu {\text{mol}}/{ \min }} \right) $$

(2)

$$ 3.{\text{ Methane conversion}} = \frac{{{\text{Consumed}}\;{\text{CH}}_{4} }}{{{\text{Initial}}\;{\text{CH}}_{4} }}\; \times 100 \left( {{\text{mol}}\% } \right) $$

(3)

Experimental Results

Analysis of Gaseous Products for the Conversion of CH₄

The experiments were carried out at 22°C, 10⁵ Pa, without any catalyst. The mixed gases of CH₄/N₂=3:1 (v/v) were fed into the reactor at a flow rate of 0.5 L/min. The specific input energy (SIE) is 29.2 kJ/L with a discharge power of 243.6 W. The discharge gap width of the plasma reactor is 0.47 mm. The yields of gaseous products in Table 1 were analyzed with a GC-102 gas chromatograph by Institute of Guang Ming Chemical Engineering. The standard gases of hydrocarbons, NH₃ and H₂ were injected into the Chromatogram to calibrate their peak values.

Table 1 Data for the conversion of CH₄ into hydrocarbons (≤C₄), NH₃ and H₂

As shown in Table 1, hydrocarbons (≤C₄) such as ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), propane (C₃H₈), propylene (C₃H₆), butane (i-C₄H₁₀, n-C₄H₁₀) are produced by electron collision reaction and recombination reaction. The yield and generation rate of hydrocarbons are of the following relationship C₂H₆ > C₃H₈ > C₂H₂ > C₂H₄ > i-C₄H₁₀, n-C₄H₁₀ > C₃H₆. 10.13% of the residual is not identified, which is possibly to be gaseous hydrocarbon (≥C₅). The conversion rate of CH₄ is 4752 μmol/min.

With the electron-methane collisions, large numbers of H₂ are produced by exited dissociation and direct ionization of CH₄. The yield and generation rate of H₂ are 9.1% (v/v) and 1879.8 μmol/min, respectively. These data show that the micro-gap discharge is a promising method to obtain high H₂-generating rate in the absence of catalyst.

The NH₃ is synthesized by atmospheric plasma in ambient N₂ and CH₄. The yield of synthesized NH₃ is 8000 ppm equal to 0.42 mmol/g for a residence time of 1.6 s. The yield of NH₃ would be1.89 × 10³ mmol/g if the gas residence time was 2 h. However, the yield of NH₃ was only 1.5 mmol/g for a residence time of 2 h by Tanaka et al. [7, 8].

Effect of the Specific Input Energy on the Concentration of Synthesized NH₃ and Conversion Rate of CH₄

As shown in Fig. 3, the SIE has a vital effect on the concentration of synthesized NH₃ and the conversion rate of CH₄. The greater the SIE is, the more molecules of CH₄, N₂ are ionized into activated particles, thus enhancing synthesis of NH₃. When the SIE changes in the range of 5.38–29.2 kJ/L, the NH₃ concentration increases from 3600 ppm to 7000 ppm, meanwhile CH₄ conversion rate increases from 47.3% to 59.0%. The trend slows after the point where SIE is 29.2 kJ/L. When the SIE is 35 kJ/L, the highest CH₄ conversion rate of 60.0% is obtained, accordingly the highest NH₃ concentration is 7400 ppm. However SIE is not further increased due to the possibility of dielectric layer breakdown, in addition, high SIE is also unsuitable for synthesis of NH₃ from energy consumption perspective.

Fig. 3

SIE vs. NH₃ concentration and CH₄ conversion. Q = 0.5 L/min; CH₄/N₂ = 3:1; Gap = 0.64 mm

Effect of the Volume Ratio of CH₄ to N₂ on the Concentration of Synthesized NH₃ and Conversion Rate of CH₄

As shown in Fig. 4, the volume ratio of CH₄/N₂ has a strong effect on the concentration of NH₃, and the optimum volume ratio is 3:1, where the concentration of NH₃ and the conversion rate of CH₄ reach 7000 ppm and 59%, respectively. When the volume ratio of CH₄/N₂ is in the range of 0.25~3, the two values increase with the volume ratio. When the ratio of CH₄/N₂ exceeds 3, the concentration of NH₃ remains constant at 7000 ppm, and the conversion rate of CH₄ decreases with the increase of the volume ratio. The optimum ratio of CH₄/N₂ 3:1 is the same as that of conventional NH₃ synthesis process.

Fig. 4

Volume ratio of CH₄/N₂ vs. NH₃ concentration and CH₄ conversion rate. SIE = 30 kJ/L; Q = 0.5 L/min; Gap = 0.64 mm

Effects of the Residence Time and Discharge Gap on the Concentration of Synthesized NH₃

As shown in Fig. 5, curve 1 (0.47 mm) and curve 2 (0.64 mm) for the different discharge gaps have almost the same trend. The concentration of NH₃ increases with the residence time in the range of 0.5–2.25 s. The residence time is much shorter than that of NH₃ synthesis by Tanaka et al. [4, 5], because more CH₄ and N₂ are ionized and dissociated in our micro-gap discharge.

Fig. 5

Residence time vs. NH₃ concentration. Discharge power = 240 W; CH₄/N₂ = 3:1

The discharge gap has a great effect on the concentration of NH₃. When the residence time is 1.65 s, the concentration of NH₃ is 6930 ppm for a discharge gap of 0.64 mm, 8000 ppm for a discharge gap of 0.47 mm with a 1070 ppm increase. With the same residence time, the SIE for the discharge gap of 0.47 mm is higher than that of 0.64 mm, so that higher concentration of NH₃ is obtained with the gap of 0.47 mm. In addition, when the discharge gap becomes narrow, the fractional active volume is correspondingly increased, resulting in the increase of high-energy electron density. As a result, more molecules of CH₄ and N₂ are ionized and dissociated to produce N₂ ⁺, NH, CH₄, CH₃·radicals for the synthesis of NH₃.

Effect of the Gas Temperature on the Concentration of Synthesized NH₃ and Conversion Rate of CH₄

Even though the feed gas isn’t heated, its temperature can go up to about 70–80 °C owing to the micro-gap discharge. As shown in Fig. 6, the concentration of NH₃ increases with the gas temperature in the range of 14.9~137.3 °C. When the gas temperature exceeds 137.3 °C, the concentration of NH₃ is almost constant at 7000 ppm. The gas temperature has light effect on the conversion rate of CH₄, which is almost constant at 60%. The activation energy of gas molecules increase with the gas temperature so that more CH₄ and N₂ are ionized and dissociated for the synthesis of NH₃. However, high temperature isn’t necessary for synthesis of NH₃, and high temperature leads to electrodes breakdown and reactor destruction.

Fig. 6

Gas temperature vs. NH₃ concentration and CH₄ conversion rate. SIE = 29.2 kJ/L; CH₄/N₂ = 3:1; Q = 0.5 L/min; Gap = 0.64 mm

Conclusions

The conversion of CH₄ into NH₃ is investigated using the micro-gap discharge plasma. The whole reaction processes is carried out under the conditions of normal temperature and pressure without any catalyst, which is an environmentally friendly condition in accordance with Green Chemistry Principle. The results show that:

• 1) Analysis of gas ingredient shows that the main products of CH₄/N₂ discharge plasma are NH₃, H₂ and hydrocarbons (≤C₄) such as C₂H₆, C₂H₄, C₂H₂, C₃H₈, C₃H₆, i-C₄H₁₀, n-C₄H₁₀. The yield and generation rates of hydrocarbons (≤C₄) are of the following relationship C₂H₆ > C₃H₈ > C₂H₂ > C₂H₄ > i-C₄H₁₀, n-C₄H₁₀ > C₃H₆.

• 2) The synthesis of NH₃ is realized using CH₄/N₂ micro-gap discharge at an atmospheric pressure. The yield of NH₃ is 8000 ppm for a residence time of 1.6 s, and 1.89 × 10³ mmol/g if the residence time was 2 h.

• 3) The micro-gap gas discharge is a promising method for high H₂ generation rate in the absence of catalyst. The yield and generation rate of H₂ are 9.1% (v/v) and 1879.8 μmol/min, respectively.