Production of hydrogen-rich gas from methane by thermal plasma reform.

ABSTRACT

This study investigated the reforming characteristics and optimum operating condition of the high-temperature plasma torch (so called plasmatron) for hydrogen-rich gas (syngas) production. At the optimum condition, the composition of produced syngas was 45.4% hydrogen ([H.sub.2]), 6.9% carbon monoxide (CO), 1.5% carbon dioxide (C[O.sub.2]), and 1.1% acetylene ([C.sub.2][H.sub.2]). The [H.sub.2]/CO ratio was 6.6, hydrogen yield was 78.8%, and the energy conversion rate was 63.6%. To obtain the optimum operating condition, parametric studies were carried out examining the effects of [O.sub.2]/C[H.sub.4] ratio, steam/C[H.sub.4] ratio, and Ni catalyst addition in reactor. When the steam/C[H.sub.4] ratio was 1.23, the production of hydrogen was maximized and the methane conversion rate was 99.7%. The syngas composition was determined to be 50.4% [H.sub.2], 5.7% CO, 13.8% C[O.sub.2], and 1.1% [C.sub.2][H.sub.2]. The [H.sub.2]/CO ratio was 9.7, hydrogen yield was 93.7%, and the energy conversion rate was 78.8%. Hydrogen production with catalyst was effective, compared with no catalyst.

INTRODUCTION

Hydrogen ([H.sub.2]) is used for synthesis in the chemical engineering and oil refinery industries. [H.sub.2] is also expected to be used in the future as a clean replacement energy fuel and fuel cell. Therefore, because of this increasing importance of [H.sub.2], various studies have investigated [H.sub.2] production technology.

[H.sub.2] production processes through the fuel reforming reaction are steam reforming, (1) partial oxidation reforming, (2) and autothermal reforming. (3) However, the existing reforming processes have various technical limitations such as slow operation characteristics, requirement of high outside thermal sources due to strong thermal absorption reaction, and catalyst poison by a small amount of sulfur in fuel.

Therefore, to overcome such limitations, a plasma reforming (4) method that uses electric discharge is receiving attention. The plasma reforming method uses electrical energy and is effective in transforming various fuels because there is no generation of additional pollution impurities. In particular, electric discharge has characteristics that create electrons with high energy, easily transform compound materials with lower reaction, and because the plasma reaction takes place in any volume capacity held by plasma electric discharge, a reactor is compact and effective.

High-temperature plasma in plasma electric discharge can produce [H.sub.2]-rich gas because it has a very high reaction energy by maintaining a high density ion status through the generation of thermal plasma, which is ionized gas by direct current arc electric discharge in thermal dynamic equilibrium. In addition, by using plasma's own heat at the time of reforming and internal reaction heat according to partial oxidation, it is possible to apply various gas forms to the wide range of flow amounts because of fast starting within response times within a few seconds.

In this research, the optimum operation condition was found under the maximum [H.sub.2] production and methane (C[H.sub.4]) conversion rate by producing syngas, including [H.sub.2]-rich gas through reforming C[H.sub.4] by using plasmatron that has applied thermal plasma. In addition, reforming experiments to increase [H.sub.2] production were carried out, including changes of impact factors on the reforming reaction such as C[H.sub.4] flow ratio, steam flow ratio, and C[H.sub.4] flow ratio with the addition of catalyst.

THEORY AND TEST METHOD

Reforming Reactions

As the expected reactions are very diverse when we carry out the plasma reforming reaction by inputting steam into C[H.sub.4], in this research reforming characteristics were investigated by selecting eqs 1-8, (5,6) and similar decomposition mechanisms were described elsewhere. (7-9) Partial oxidation reforming reaction;

C[H.sub.4] + [1/2] [O.sub.2] [right arrow] CO + 2[H.sub.2] (1)

Steam reforming reaction;

C[H.sub.4] + [H.sub.2]O [right arrow] CO + 3[H.sub.2] (2)

C + [H.sub.2]O [right arrow] CO + [H.sub.2] (3)

Plasma (cracking) reforming reaction;

C[H.sub.4] [right arrow] C(s) + 2[H.sub.2] (4)

2C[H.sub.4] [right arrow] [C.sub.2][H.sub.2] + 3[H.sub.2] (5)

2CO [right arrow] C(s) + C[O.sub.2] (6)

Water-gas shift reaction;

CO + [H.sub.2]O [left and right arrow] C[O.sub.2] + [H.sub.2] (7)

CO oxidation;

CO + [1/2] [O.sub.2] [right arrow] C[O.sub.2] (8)

Reactant Gas Conversion Rate

Methane conversion rate was calculated in eq 9 by influx concentration and efflux concentration measured at the exit.

[eta] = [[[V.sub.X.sub.in] - [V.sub.X.sub.out]]/[V.sub.X.sub.in]] x 100 (9)

where [eta] is the conversion rate of the component X (%), [V.sub.X.sub.in] is the influx concentration of the component X (vol%), and [V.sub.X.sub.out] is the efflux concentration of the component X (vol%).

[H.sub.2] Yield and Energy Conversion Rate

[H.sub.2] yield and energy conversion rate (ECR) are calculated respectively by each eqs 10 and 11.

[H.sub.2.sub.yield](%) = [[[[H.sub.2]][.sub.Syngas]]/[[[H.sub.2]][.sub.Reaction gas]]] x 100 (10)

ECR(%) = [[([[H.sub.2]] + [CO])[.sub.LHV]]/[[C[H.sub.4]][.sub.LHV]]] x 100 (11)

In here, [H.sub.2yield] is hydrogen yield (%), [[H.sub.2yield]][.sub.Syngas] is the [H.sub.2] quantity within syngas (g/mole), and [[H.sub.2yield]][.sub.Reaction gas] is the [H.sub.2] quantity within reactant gas (g/mole). ECR (%) is the energy conversion rate, [C[H.sub.4]][.sub.LHV] is the low heating value of input fuel of C[H.sub.4] (kcal/N[m.sup.3]), and ([[H.sub.2]] + [CO]) is the low heating value (kcal/N[m.sup.3]) of syngas ([H.sub.2] + CO).

EXPERIMENTAL SETUP

The experimental apparatus setup that was used to construct an experiment for plasma reforming is shown in Figure 1. This consisted of a reformer, a power supply device, a gas/steam feeding line, and a measurement/analysis line. A reformer consists of a plasmtron and reactor. The reactor was charged, and the catalyst contained 6 wt% nickel in alumina carrier with a diameter of 4 mm.

To relieve the erosion phenomenon of the electrode by high temperature, water was supplied to the two electrodes of the plasmtron. The reactor used a castible for internal adiabatic and insulation.

[FIGURE 1 OMITTED]

The power supply device consisted of a power generator, ignitor, and trigger system. A power supplier can supply 10 kW of input power (maximum current 50 A, maximum voltage 200 V). The igniter supplied a high voltage up to 30 kV for initial starting, and the trigger system maintained plasma continuously.

Gas/steam lines supplied C[H.sub.4], air, and steam. Methane and air were supplied by precise flow calculation at each mass flow controller (MFC, Bronkhorst F201AC-FAC-22-V). The water flow rate was adjusted by the flow meter (B-175-X052) and metering valve, and then steam was generated by the heater.

The measurement/analysis lines were electricity characteristics, temperature, and gas analysis. Electricity characteristics measurements were made with a digital oscilloscope (Tektronix TDS 3052). Temperature measurements consisted of a K-type thermocouple and data analysis device (Fluke Hydra Data Logger). Gas analysis measurements were made with a sampling line and gas chromatographs (Varian CP-4900, Simazu 14B).

EXPERIMENTAL METHOD

The experiment started after the plasma reactor was stabilized at 1100 [degrees]C by partial oxidation reaction.

The flow quantity of input gases was precisely adjusted at each MFC. The steam was produced in a steam generator, which was fed water by a metering valve. Input voltage and electric current were adjusted with the knob of the power generator. The voltage and the current were measured by voltage probe and high current probe, respectively.

Gas and reactor temperature were measured through the port on the reactor. Syngas was sampled at the sampling port installed at the exit of the reactor. The gases were analyzed by gas chromatograph in dry basis gas. A TCD detector was used for analysis in production gases. Helium was used for a carrier gas. A Molecular Sieve 5A column was used for the analysis of hydrogen, nitrogen, and oxygen. A Molecular Sieve 13X column was used for the analysis of CO, and a Poropak Q column was used for the analysis of hydrocarbons (i.e., C[H.sub.4], acetylene [[C.sub.2][H.sub.2]]). The temperature of each of the detectors was 60, 150, and 80 [degrees]C.

Reforming experiments were carried out according to the C[H.sub.4] flow rate (C[H.sub.4]/C[H.sub.4] + air) and steam flow rate ([H.sub.2]O/C[H.sub.4] + air + [H.sub.2]O) without catalyst. The C[H.sub.4] flow rate and steam flow rate were approximately 23.7-47.1 and approximately 6.1-47%, respectively. To find out the characteristics of reforming by the addition of catalyst, the experiments were carried out with the same condition of C[H.sub.4] flow rate (i.e., ~23.7-47.1). The optimum [H.sub.2] production was termed the reference condition, found to be an input airflow rate of 5.1 L/min and an input power of 6.4 kW.

RESULTS AND DISCUSSIONS