NON THERMAL PLASMA CONVERSION OF PYROGAS INTO SYNTHESIS GAS

Transcription

1 NON THERMAL PLASMA CONVERSION OF PYROGAS INTO SYNTHESIS GAS Fela Odeyemi, Alexander Rabinovich, and Alexander Fridman Mechanical Engineering and Mechanics Department, Drexel University, Philadelphia PA Abstract This paper discusses plasma assisted conversion of pyrolysis gas (pyrogas) fuel to synthesis gas (Syngas). Pyrogas is a product of biomass, municipal wastes or coal - gasification process. Pyrogas usually contains hydrogen (H 2 ), carbon monoxide (CO) as well as unreacted light and heavy hydrocarbons (especially methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) and tar. These hydrocarbons diminish the fuel value of pyrogas thereby necessitating the need for the conversion of the hydrocarbons. Various conditions and reforming reactions were considered for the conversion of pyrogas into syngas (a combination of H 2 and CO). Non thermal plasma is an effective homogenous process which makes the use of catalysts unnecessary for fuel reforming. The effectiveness of gliding arc plasma has a nonthermal plasma discharge is demonstrated in the fuel reforming reaction processes with the aid of a specially designed low current device called glid arc plasma reformer. Gliding arc plasma is a nonequilibrium discharge with multiple advantages over other reforming techniques which will be further discussed in this paper. Results obtained from thermodynamic simulations were compared with experimental results with emphases on yield, molar concentration, and enthalpy at different reaction temperatures. Keywords: Pyrogas, Dry CO2 reforming, Glid arc plasma 1. Introduction Biomass, municipal wastes, hydrocarbon fuels or coal can be reformed via one or a combination of pyrolysis, combustion and gasification processes. A series of chemical reactions during the course of these processes usually result in the formation of a complex mixture of combustible gases such as CH 4, CO, H 2, unreacted heavy hydrocarbons; tar and a noncombustible gas - CO 2. A combination of all these gases constitutes what is known as pyrolysis gas or pyrogas. The presence of heavy hydrocarbons and tar diminishes the quality of pyrogas from the perspective of its use as an intermediate for synthetic fuel production. This draw back therefore necessitates the removal of the unreacted hydrocarbons. A reasonable approach is the use of non-equilibrium gliding arc plasma for the chemical reformation of pyrogas into synthesis gas or syngas. Catalytic partial oxidation and plasma assisted fuel reforming are two different fuel reforming techniques. Non equilibrium gliding arc plasma reforming is a homogenous process of fuel reforming which eliminates the need for catalysts[1]. Catalytic partial oxidation has been known to have extensive drawbacks such as high cost, poisoning problem, large size and significant carbon footprint. Gliding arc plasma on the other hand has smaller reactors, fast start-up time, higher efficiency and low electrical energy cost to produce plasma; about 2 % 5 % of total power produced by the system [3]. The reforming reactions considered for pyrogas reforming are steam reforming reaction and dry CO2 reforming reaction, both of which are endothermic[2]. These two main reforming reactions produce hydrogen rich synthesis gas which can be used for power generation, utilized for fuel cells to produce electricity[3] and as a building block for production of synthetic fuels via the fischer tropsch process[4]. Syngas is a gas comprising a varying quantity of hydrogen (H 2 ) and carbon monoxide (CO). Partial oxidation reaction will be inappropriate in this situation due to the presence of hydrogen in the pyrogas mixture. Oxygen reacts with hydrogen to form water; this oxidation process diminishes the concentration of the hydrogen in the final mixture. Gliding arc plasma serves as a resource for active species and radicals such as O and OH which are necessary to stimulate the desired chemical reactions and reduce the initial temperature required to jump start fuel reforming chemical reactions[5-6]. The basis for this paper is the detailed description of the plasma catalytic reforming

2 of a complex mixture of hydrocarbons - Pyrolysis gas; in the presence of nonequilibrium gliding arc plasma with the aid of a gliding arc reactor which is essentially a plasma reforming device. Experimental results obtained from the plasma assisted reforming of pyrogas process were compared with thermodynamic predictions. Conversion rates were also quantified based on hydrogen yield, carbon monoxide yield, hydrocarbon conversion. Energy costs of the dry CO2 reforming and steam reforming reactions considered were also compared. 2. Experimental Apparatus The set up for the pyrogas reforming experiments essentially comprises a gliding arc plasma reformer, mass flow controllers, thermocouples, dc power supply, gas chromatograph, data acquisition unit with labview program, steam generator, furnace and heaters. 2.1 Gliding arc plasma reformer Gliding arc plasma discharge was selected for pyrogas reforming experiments due its relative high local temperature, low power and non-equilibrium plasma catalysis which are essential for stimulation of hydrocarbon reforming reactions. The gliding arc plasma reformer discussed in this paper is specially designed to function at temperatures above 850 degree celsius and atmospheric pressure. The gliding arc reformer also has the capability to work under auto thermal conditions. The gliding arc reactor shown in fig. 1 consists of a high voltage electrode made out of stainless steel material, a stainless steel ground electrode designed with multiple tangential jets to provide a vortex shaped discharge with gas flow; this system helps prevent heat losses to the walls of the electrode by propelling the gliding discharge over the electrodes. Other reactor parts include a fuel atomization nozzle, a glass filled teflon dielectric material which separates the high voltage electrode from the ground electrode. The spark gap (discharge breakdown gap) between the high voltage electrode and ground electrode is 3 mm. Fig. 1: The Gliding Arc Plasma reformer which consists of 2 stainless steel electrodes separated by glass filled Teflon which serves as a dielectric material. The ground electrode consists of multiple tangential gas jets which helps provide a vortex effect. The pyrogas composition that the experiments and results presented in this paper are based on is provided in table 1. The pyrolysis gas composition was determined after several laboratory tests were carried out on various pyrogas samples; it should be noted that the percentage composition provided in table 1 is the mean molar concentration (%) of the different constituent gases of pyrogas. The pyrolysis gas used in the experiments is a mixture of the gases listed in table 1 with corresponding percentage molar concentrations. Definite molar concentrations of individual gases were used during the experiments to ensure consistency during the course of the reforming experiments. The molar concentrations of the gases making up the pyrogas composition stated in table 1 are within the range of concentrations obtained from coal gasification processes. The mean value of the molar concentrations of the respective gases stated in table 1 was used for the steam reforming and dry CO2 reforming experiments of pyrogas. The total flow rate of the pyrolysis gas fuel mixture used for the experiments is 30 SLPM. The experiments were carried out at atmospheric pressure conditions. It should be noted that the parameters and conditions under which the reforming experiments were conducted were constant throughout to ensure a stable experimental set up. Water vapor was not included in the pyrogas composition used in

3 the reforming experiments due to the limitation of the gas chromatography equipment in detecting and measuring water vapor molar concentrations. Table 1 Pyrogas Composition Gas Mole % H CO CH CO C2H C3H8 1-2 C3H6 < 1 C4H10 < 1 The main hydrocarbon reforming reactions usually considered include partial oxidation reaction, steam reforming and dry CO2 reforming. Partial oxidation will be inappropriate for syngas production due to the presence of hydrogen and CO in the pyrogas mixture. Oxygen will easily oxidize hydrogen and CO, thereby reducing the concentrations of hydrogen and CO which is counterproductive to synthesis gas (syngas) formation. Hence, experiments were carried out with steam reforming and dry CO2 reforming reactions. Since CO2 is already contained in the pyrogas mixture in significant quantities; an external source of CO2 will not be required for the dry CO2 reforming reaction. A summary of some experiment parameters and working specifications of the plasma reformer is provided in Table 2. Table 2 Operating conditions of the plasma reformer Fuel Pyrolysis Gas Maximum pressure 1 atm Max Temperature 850 C Oxidants Water or CO2 Pyrogas Flow 30 SLPM Steam : Carbon Ratio Max Power 3 KW Spark Gap 3 mm 3. Results and Discussion 3.1 Thermodynamic Model The pyrolysis gas plasma steam reforming process was simulated with all the constituent gases that make up pyrogas. Chemical reactions calculations were conducted based on the supposition that the all the participating reactants completely mix in the reactor. The thermodynamic simulation was carried out using the reaction design software - CHEMKIN 4.1 package. The physical input parameters included in the steam reforming model conditions include temperature (degree celsius), reactor area and volume, pressure (atm), and mass flow rate. The reaction temperature was varied from 0 to 950 C. The output parameters are the gas composition and molar fractions of the resulting gases. 3.2 Plasma - Dry CO2 reforming Plasma Dry CO2 reforming is a reforming reaction which involves the reaction of hydrocarbons with CO2 in the presence of plasma discharge to form Syngas. With the already available CO2 present in the pyrolysis gas composition, plasma - dry CO2 reforming of pyrogas experiments were conducted at conditions stated in Table 2. The experimental setup for dry carbon dioxide CO2 reforming of pyrogas is identical to that described in the plasma steam reforming experiments without the steam generator. One of the advantages of this chemical reaction process is that the need for a supplementary source for carbon dioxide (CO2) is unnecessary due to the existing CO2 already present in the original pyrogas

4 concentration. This reduces the cost and increases of efficiency of the chemical reaction process. The non equilibrium plasma catalysis dry CO2 reforming process of pyrogas experiments were conducted at a temperature range of 800 C C. Exhaust gases from the plasma reforming process were analyzed with a gas chromatograph. Results show increases in the concentrations of hydrogen and carbon monoxide (CO) after the plasma chemical catalysis process. Also, the concentrations of methane, ethane, propane and carbon dioxide (CO2) reduced when compared to their initial individual concentrations in the pyrogas mix. Fig 2: Thermodynamic simulation of Steam reforming of Pyrolysis gas Fig 3: Graph shows the variations in gas concentrations compared to initial concentrations of individual gases that constitute Pyrogas. Changes in the concentration of individual gases with increase in enthalpy (KWhr/M3) can be observed. Conclusion The main purpose of this work is to demonstrate and compare the effectiveness of the gliding arc plasma assisted reformer in removing light and heavy hydrocarbons contained in pyrogas by converting the hydrocarbons and carbon dioxide to syngas using both steam reforming reactions and dry CO2 reactions at atmospheric pressure conditions. The data collected and results analyzed during the non equilibrium plasma reforming experiments indicate the plasma reforming of pyrogas into synthesis gas (syngas) using plasma - dry CO2 reforming reaction. The analyzed experimental results from plasma - dry CO2 reforming were also similar to thermodynamic predictions. Note that the pyrogas from an industrial gasifier is usually at a relatively high temperature ( C); hence plasma energy is mainly required to stimulate chemical reactions and not as a source of heat energy in the reforming processes discussed in this paper. This therefore makes energy consumption for pyrogas very low. Conversion rates of the different gases were higher with plasma -dry CO2 reforming reaction when compared to plasma - steam reforming reaction. Lower conversion rates observed with plasma steam reforming reaction may be attributed to the inability to maintain steam temperature required for homogenous reaction. This ultimately demands an increase in energy required to maintain steam at the same temperature of other reactants in the system. As a result, steam reforming occurs at a much lower temperature. The data presented in this work further suggests that pyrogas conversion with gliding arc plasma is stimulated by plasma catalytic effect. The results also show the ability to produce hydrogen rich syngas with the gliding arc plasma technology effectively. Gliding arc plasma does not just provide energy but stimulates attainment of thermodynamic equilibrium. The experimental results presented here further support some of the advantages of gliding arc plasma which include fast start-up time, compactness, robustness, relatively low power consumption and adaptability for various fuel types. Further work is required to optimize the gliding arc plasma reformer system for steam reforming conditions. Future optimization will also include minimizing heat losses at both pre-plasma and post plasma zones of the reformer; decreasing the enthalpy of the system while maintaining Syngas yield and conversion rates; and increasing residence time.

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