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July 17, 2007
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Home / Issue Archive / 2007 / February #2 / Plasmochemical Processing of Methane Using Microwave Discharge

№ 2 (February 2007)

Plasmochemical Processing of Methane Using Microwave Discharge

Plasmochemistry technologies are a strong alternative to standard thermal and catalytic thermal conversion of hydrocarbon material.

By P. Guschin, E. Ivanov, V.Vinokurov

Plasma enables the elimination of many processing and kinetic constraints typical of high-temperature reactions, including:

• pyrolysis of saturated hydrocarbons, natural gas, and other gaseous and liquid hydrocarbon mixtures, coal with acetylene and hydrogen formation; acetylene, ethylene and hydrogen; aromatic hydrocarbons (thermal gasoline fraction) and hydrogen; polyaromatic hydrocarbon and hydrogen; and carbon black and hydrogen;
• methane oxidizing conversion with synthetic gas formation.
Nonequilibrium plasma hydrocarbon conversion is of particular interest since it has apparent advantages compared to quasi-stationary plasma.

Nonequilibrium plasma enables chemical reactions even at low temperatures due to reactive species generated by high-velocity electrons. Thus, high _editor_2_fig1.jpgspecific output peculiar to plasma methods is combined with low specific energy consumption typical of standard catalytic methods. Under plasma conversion conditions, feed methane conversion may reach 95-97 percent and power consumption will be 1.0-1.2 kWh/cu. m. It is also possible to vary the proportion of H2/CO (ranging from 3/1 to 1/1) in products, subject to a medium applied (oxygen, water steam, and CO2), similar to catalytic thermal conversion.

The non-application of catalysts results in competitive advantages to plasma hydrocarbon conversion technologies:

• clean environment;
• high specific capacity;
• quick start and the potential for an unrestricted number of start-stop cycles;
• less severe requirements to feedstock purity (presence of sulfur or nitrogen);
• the potential for conversion under atmospheric pressure.

Carbon dioxide plasma conversion is of great interest since it enables entrainment of technogenic СО2 generated during combustion of hydrocarbon energy carriers. There are two principal reactions observed under plasma conditions:

CH4(g) + СО2(g) = 2 CO(g) + 2 H2(g) (1)
CH4(g) = 3/2 H2(g) + 1/2 C2H2(g) (2) 

Under microwave plasma conditions, conversion of СН4 and СО2 may reach 95 percent. There is a weak relationship between the synthetic gas generation output performance and microwave discharge exposure time that is determined by the gas mixture flow rate. It is explained by the fact that the volume of the discharge channel peripheral area with a lower temperature (where methane conversion and pyrolysis are also on) increases with decreasing time of the mixture exposure to the high energy plasma zone.

The least power is consumed during plasma chemical methane conversion in oxygen or air media due to heat released in the oxidizing reactions. For instance, when acetylene is synthesized from natural gas in oxygen plasma, specific power consumption decreases from 9.5 kWh/kg to
5 kWh/kg; the characteristic time of acetylene synthesis is (2-5)10-4 s; and acetylene output drops to 60 percent due to carbon oxide formation.

However, the output doubles compared to the acetylene output during natural gas oxidizing pyrolysis and the reaction time decreases.

Under plasma processing conditions, it is much easier to eliminate problems connected with high endothermicity of methane conversion reactions in steam and carbon dioxide media due to reaction in a combined mixture of CH4 + CO2 (or H2O) + O2. Such thermally neutral conversion can be carried out by combining carbon dioxide (steam) and oxygen media of methane conversion. Thermodynamic calculation of methane conversion in combined (oxygen and carbon dioxide) media proves that in a mixture of 50 percent  СН4 + (50-х)percent СО2 + х percent О2 at 800 С, thermoneutrality is achieved at х = 23 percent. During reaction in a mixture of 50 percent СН4+ 27 percent СО2+23 percent О2 at 800 С and 1 atm, equilibrium outputs are 49.3 percent Н2 and 36.5 percent СО, (i.e. a proportion of CO/H2 significantly differ from 1).

During conversion of a similar composition mixture in microwave discharge plasma, the results obtained prove that methane conversion achieves 92-95 percent; plasma exposure time is 1.88•10-4 s; power consumption is 1.67 kWh/cu. m of synthetic gas (proportion of Н2/СО = 3/2); and the selective output is rather high. Basic by-products of these processes are carbon white with a perspective to be used in the future, and acetylene.

The prospects of combined plasma and catalysts utilization are also obvious. Combined employment of plasma and catalytic hydrocarbon processing may become a major source of liquid hydrocarbon fuel and feedstock for hydrogen power engineering. The mechanism of the latter is described on Fig. 2.

Using such a combination makes it possible to integrate multistage reactions in a single process flow, enabling GTL processes in one reactor; and to create plasma and catalytic packaged units for installation directly in oil and gas production areas.

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