HIGH TEMPERATURE CO2 STEAM AND H2 REACTIONS FOR ENVIRONMENTAL BENEFITS.

Abstract

Presented are processes for the beneficial conversion of CO.sub.2 and other environmentally destructive compounds to their constituent parts by the application of thermal plasma containing activated species whereby the interaction of the plasma with the compounds and reactions of CO.sub.2 and H.sub.2 generate more environmentally friendly compounds comprising in part oxygen and hydrogen. The thermal plasma may be vibro-shear plasma generated by the superheating of either steam, gas or a combination of both.

Claims

1. A method for the reduction of CO.sub.2 compounds to environmentally friendly non-CO.sub.2 compounds comprising immersing the CO.sub.2 compounds in a thermal plasma plume.

2. The method of claim 1 wherein the reduction of the CO.sub.2 compounds occurs at a temperature below 1600° C.

3. The method of claim 1 wherein the thermal plasma plume is a vibro-shear plasma comprising activated species.

4. The method of claim 1 wherein the thermal plasma plume is comprised of a first activated species and the method is further comprised of a secondary thermal plasma comprised of a secondary activated species.

5. The method of claim 1 wherein the environmentally friendly non-CO.sub.2 compounds comprise O.sub.2 or H.sub.2O.

6. A method for the plasma valorization of a CO.sub.2 compound comprising immersing the CO.sub.2 compound in a thermal plasma plume and introducing a reducing reagent resulting in the generation of non-CO.sub.2 compounds.

7. The method of claim 6 wherein the thermal plasma plume is a vibro-shear plasma comprising activated species.

8. The method of claim 6 wherein the thermal plasma plume is comprised of a first activated species and the method is further comprised of a secondary thermal plasma comprised of a secondary activated species.

9. The method of claim 6 wherein the reducing agent is H.sub.2.

10. The method of claim 6 wherein the reducing agent is CH.sub.4.

11. The method of claim 9 wherein the generated non-CO.sub.2 compound comprises syngas.

12. The method of claim 11 wherein the generated non-CO.sub.2 compounds consist of compounds selected from the group of compounds consisting of methanol, ethanol, formic acid, acetic acid, propanoic acid and syngas.

13. The method of claim 1 wherein the thermal plasma plume is produced by a superheated steam generation device.

14. The method of claim 6 wherein the thermal plasma plume is produced by a superheated steam generation device.

15. The method of claim 1 wherein the thermal plasma plume is produced by a high temperature gas heating device.

16. The method of claim 6 wherein the thermal plasma plume is produced by a high temperature gas heating device.

Description

DETAILED DESCRIPTION

CO.SUB.2 .Conversion Employing Activated Species or Radicals Embodiment

[0037] Plasma technologies have many advantages over traditional CO.sub.2 conversion pathways and may provide a unique and economical process for the utilization of anthropogenic CO.sub.2. Plasma technologies provide gas activation by energetic electrons instead of heat, allowing thermodynamically difficult reactions, such as CO.sub.2 splitting, to occur with reasonable energy costs. Plasma technologies can also be easily switched on and off which is compatible with intermittent renewable energy and load-following applications. The most common types of plasma reported in the literature are dielectric barrier discharges (DBDs), microwave (MW), and gliding arc (GA); however, other types, such as radiofrequency, corona, glow, spark, and pulsed electron beam (PEB), have also been studied. Depending on the type of plasma, different CO.sub.2 conversions and energy efficacies have been reported. In terms of energy efficiency, a target of at least 60 percent energy efficiency has been suggested for plasma CO.sub.2 conversion to be competitive with other technologies. Plasma tends to be very reactive and not selective in the production of targeted compounds. Therefore, plasma CO.sub.2 conversion technologies may need a catalyst to increase selectivity and produce targeted compounds. Low conversion of CO.sub.2 could also require postreaction separation of the products from the reactants and may be cost prohibitive. methods are sought to convert CO.sub.2 to high value chemicals or fuels using plasma technologies in an economically viable process, overcoming challenges associated with energy efficiency, CO.sub.2 conversion and selectivity.

Key Reactions for this Embodiment

[0038] When it is shown that this process offers acceptable cost for CO.sub.2 conversion, then a wide range of potential industrial applications of the decomposition, such as treatment of waste, power plant exhausts that can lead to the synthesis of new materials including transportation fuels, will become feasible. CO.sub.2 conversion is an endothermic process. The main endothermic chemical processes of interest for carbon dioxide or assisted carbon dioxide decomposition (dry reforming of methane DMR reaction) can be presented by the reactions:

CO.sub.2.fwdarw.CO+1/2O.sub.2, ΔH=2.9 eV/molecule.  (R1)

CH.sub.4(g)+CO.sub.2(g).fwdarw.2CO(g)+2H.sub.2(g)ΔH°=2.55 eV/molecule  (R2)

2CH.sub.4(g)+CO.sub.2(g)+0.5O2(g).fwdarw.3CO(g)+4H.sub.2(g)+ΔG=0@280° C.  (R3)

[0039] Note that reactions R1 to R3 show an increase in entropy when converted, thus overcoming the endothermic barrier which should be enough to enable the reactions. Several of these reactions are possible at very high temperatures, however the possibilities of carrying them out at a lower temperature with either solid or plasma catalysts offers a cost reduction possibility. In this application, task one is directed towards plasma decomposition of reaction (R1) and task two towards a combination of plasma and solid catalysts for reactions (R2), but with an additional reaction (R4) that may be included in the method, namely:

2CH.sub.4(g)+CO.sub.2(g)+O*(g).fwdarw.3CO(g)+4H.sub.2(g)ΔG=0 even at room temperature  (R4)

[0040] With O* (activated species or radical) from plasma reaction R1 the 3CO(g)+4H.sub.2(g) ratioed syngas is immediately feasible. This implies no further energy input is required downstream. ΔG=0, even at room temperature, and remains negative as the temperature increases. Therefore, if R1 is proven with a high efficiency adjustments can be made to get 100% yield of the syngas by manipulating the sequence of sub reactions with positioning the inlet or temperatures.

Energy Efficiency and the Yield Landscape with Known Plasma Methods

[0041] Plasma processes accelerate dissociation with vibrational and ionization excitations. The plasma to be employed for the anticipated processes is an electro-shear-thermal plasma which is expected to primarily enhance vibrational excitations with phonon-boson interactions. The generating unit produces strong plasma plumes (a feature of vibro-ionizable plasmas) for Air, CO.sub.2, N.sub.2, N.sub.2—H.sub.2 and Steam. This type of plasma has also shown potential for use in plasma-nitriding applications. A reasonably high energy efficiency for reaction (R1) has been observed by plasma dissociation. The process efficiency η, of the process for reaction (R1), is the ratio of the dissociation enthalpy ΔH=2.9 eV/molecule to the actual specific energy requirement (called SER or total energy cost) to produce one molecule of CO in the plasma system i.e., η=ΔH/SER. The best η results, to date, appear to have been achieved in experiments with RF and microwave discharges at low pressure. A value of 60% energy efficiency has been achieved in non-equilibrium RF discharge at reduced pressure and with the GAP (gliding arc plasma). Notably, the cost of processing is not well reported. An average transferred arc plasma operates at about 20% efficiency which is low compared to simple heating to get a high η. Regardless, performing the process in subsonic flow has led to reports of energy efficiencies at 80%. In very costly supersonic flows, the energy efficiencies have reportedly reached 90%.

[0042] The reaction sequences for thermally excited but not fully ionized plasmas are known only in a speculative manner. The specific energy per molecule for the best conversion efficiency for reaction (R1) and the energy efficiencies at any conversion efficiency, is over several tests. When the temperature reaches the level that is high enough to support chemical reactions in a system (one can think of this as a sustainable ignition temperature), chemical reactions produce high concentrations of excited molecules, that could form a basis for stepwise ionization. This results in a significant drop in the energy necessary to support electric discharge in the system for two reasons. First, stepwise ionization that requires relatively low electron energy overcomes the requirement for complete direct ionization. Ionization is typical for low-temperature non-equilibrium plasmas requiring much higher ionization energy. Second, the high temperature of surrounding gas reduces heat losses from the discharge channel, whereas a significant portion of the discharge energy in semi-warm plasma systems should be spent to compensate these losses. Thus, an intensive chemical reaction, e.g. combustion, supports the existence of a warm electric discharge. However, another possibility to reduce SER can be realized in a process, in which a charged particle before recombination transfers virtually all the energy obtained from the electric field to a very efficient chemical channel. A unique example of such an efficient chemical channel is a class of reactions stimulated by a vibrational excitation of molecules.

[0043] Thus, this mechanism can be thought to possibly provide the highest energy efficiency of endothermic plasma-chemical reactions under non-equilibrium conditions because of the following four factors when discussing drawbacks to conventional and GAP plasmas. (1) The major fraction (70-95%) of discharge power in many molecular gases (including N.sub.2, H.sub.2, CO, CO.sub.2, etc.) that happens at the electron temperature Te≈1 eV can be transferred from plasma electrons to the vibrational excitation of molecules. (2) The rate of vibrational-translational (VT) relaxation is usually low, at low gas temperatures, thus spending most of the vibrational energy on stimulation of chemical reactions. (3) The vibrational energy of molecules is the most effective for stimulation of endothermic chemical processes. (4) The vibrational energy necessary for an endothermic reaction is usually equal to the activation barrier of the reaction and is significantly smaller than the energy threshold of the corresponding electronic excitation processes.

[0044] A good example of reduction to hydrogen is, therefore, now recognized, but it is not yet clear for CO.sub.2 dissociation. As an example, note that the dissociation of H.sub.2 through ionic processes requires 4.4 eV. On the other hand, dissociation of H.sub.2 with plasma can be done at ˜1-2 eV. Regardless, previous investigations have indicated that stimulation of vibrational excitation of CO.sub.2 molecules is the most effective route for CO.sub.2 dissociation in plasma. It is also well known that vibrational energy losses through vibrational-translational (VT) relaxation are relatively slow for CO.sub.2 molecules thus it is possible that the CO conversion is selectively enabled. Thus, there is a fair expectation that the vibro-shear plasma should be studied particularly as it has shown potential for large scale plasma-nitriding and aluminum dross reduction.

Importance of Selectivity

[0045] The combined conversion of CO.sub.2 and CH.sub.4, known as the dry reforming of methane (DMR), is analogous to the steam reforming of methane (SMR: Steam Methane Reforming and Methanol Synthesis: CH.sub.4+H.sub.2O.fwdarw.CH.sub.3OH+H.sub.2+122.0 kJ/mole), indicating the replacement of water by carbon dioxide. The DMR process is, however, not as straightforward as the steam reforming of methane, because CO.sub.2 is a highly oxidized, thermodynamically stable molecule, while its reaction partner, CH.sub.4, is chemically inert. Hence, the process needs to be carried out at high temperatures (900-1200° K) in the presence of a catalyst, typically containing Ni, Nickel Ferrite, Co, precious metals or Mo.sub.2C. as the active phase. At 1500° K, complete conversion is achieved, with an energy efficiency of 60%. However, the maximum energy efficiency of 70% is obtained before this at 1000° K, reaching a conversion of maximum of 83%, but this then decreases with increasing the temperature.

[0046] The DMR reaction was first studied by Fischer and Tropsch in 1928, and since has been a challenge for chemical engineering ever since. With the beginning of a new millennium and the increasing concern regarding climate change, DRM was a way to convert the major greenhouse gas CO.sub.2 into useful products with the aid of natural gas. To date, a true mixture of environmental and economical motivations are seen that include (i) the conversion of the greenhouse gas CO.sub.2, (ii) the capability of using biogas as a feedstock, and (iii) the search for a convenient way to liquefy CH.sub.4 for easier transport, and the availability of cheap CH.sub.4 through shale gas. There is one major pitfall, namely, the inherent susceptibility for soot deposition and the detrimental effect this has on the process through deactivation of the usable catalyst. Due to this drawback, DRM is to date not yet (widely) used on an industrial scale. Nevertheless, the inability to transform the alluring promises of DRM into reality through the traditional thermal methods, among other reasons, has sparked and fueled the growing interest for alternative reforming technologies and may prove to be more applicable for CO.sub.2 utilizations technologies. Potentially large benefits and volarization could arise from a continuous but two-step linear process where:

CO.sub.2(g).fwdarw.CO(g)+0.5O.sub.2(g) leading to

2CH.sub.4(g)+CO.sub.2(g)+0.5O.sub.2(g).fwdarw.3CO(g)+4H.sub.2(g)ΔG=0 at 280° C.

[0047] It is well known that various combinations if CO/H.sub.2 are suitable for different end products thus removing the need for additional separation sequences. Based on the literature it is now thought that this versatile plasma could be exactly what could assist CO.sub.2 conversion technologies.

More Product Reactions

[0048]
CO.sub.2+C==>>2CO

[0049] Carbon monoxide acts as a reducing agent and reacts with iron ore to give molten iron, which trickles to the bottom of the furnace where it is collected.

Fe.sub.2O.sub.3+3CO==>>2Fe+3CO.sub.2

[0050] The limestone in the furnace decomposes, forming calcium oxide. This is a fluxing agent and combines with impurities to make slag, which floats on top of the molten iron and can be removed.

CaO+SiO.sub.2==>>CaSiO.sub.3

1.5H.sub.2(g)+1.5CO(g)+Fe.sub.2O.sub.3=2Fe+1.5H.sub.2O(g)+1.5CO.sub.2(g): Feasible above 300° C. Good at 600° . Now recycle the CO.sub.2.
2H.sub.2(g)+2CO(g)+Fe.sub.3O4=3Fe+2H.sub.2O(g)+2CO.sub.2(g): Not feasible.
H.sub.2(g)+CO(g)+2FeO=2Fe+H.sub.2O(g)+CO.sub.2(g): Not feasible.
FeO+CO(g)=Fe+CO.sub.2(g): Negative free energy up to 500° C. Not reduced by H.sub.2
Fe.sub.2O.sub.3+3CO(g)=2Fe+3CO.sub.2(g): Negative free energy reduced by H.sub.2 above 600° C.
Fe.sub.3O.sub.4+4CO(g)=3Fe+4CO.sub.2(g): Negative free energy up to 500° C. Not reduced by H.sub.2.
Fe.sub.2O.sub.3+2CO(g)+H.sub.2(g)=2Fe+3CO.sub.2(g)+H.sub.2O(g) at 600° C. DG=−26.46 KJ/mol=−0.2 eV/molecule: (reaction is mildly exothermic).
CO(g)+2H.sub.2(g)=CH.sub.3OH(g): Negative free energy to 100° C.

Plasma Valorization of CO.SUB.2 .and CH.SUB.4

[0051] Plasma coupling of CO.sub.2 using CH.sub.4 as a reducing co-reactant has been reported for a variety of plasma systems. Many value-added products, such as methanol, ethanol, and various carboxylic acids like formic, acetic, and propanoic acids, have been identified as feasible liquid products. Among those gaseous products, syngas (primarily CO and H.sub.2) was reported as the prominent products. In addition, catalysts have been frequently reported showing beneficial effects in plasma catalytic transformation of CH.sub.4 and CO.sub.2 mixture and a common strategy is to use catalysts that are compatible for conventional thermal catalysis.

[0052] Depending on their respective dissociation rate, CO.sub.2 and CH.sub.4 may exhibit distinctive capability in producing reactive species.

[0053] Since Cu, Ni, MoSi.sub.2, Co, Nickel Ferrite and Mo-based catalysts have been widely used in thermal catalytic conversion of CO.sub.2 and/or CH.sub.4 into valuable products like CH.sub.3OH. Copper is anticipated to adsorb CO.sub.2 as COO species on its surface, which will then reduce to a crucial intermediate HCOO. This is an important advantage of Cu in the plasma catalytic conversion CO.sub.2 into alcohols.

Plasma Valorization of CO.SUB.2 .in the Presence of H.SUB.2

[0054] Different from CH.sub.4, H.sub.2 is frequently employed as a reducing reagent in chemical industry. It has been reported that plasma conversion of CO.sub.2 to syngas could be achieved with the co-introduction of H.sub.2.

[0055] A broad spectrum of materials have already been investigated for the plasma-catalytic DRM, of which Ni is by far the most commonly used active phase, such as in Ni/γ-Al.sub.2O.sub.3, Ni/SiO, Ni—Fe/γ Al.sub.2O.sub.3, Ni—Fe/SiO.sub.2, Ni—Cu/γ-Al.sub.2O.sub.3, Ni.sup.0/La.sub.2O.sub.3; Ni/MgO, Ni/TiO.sub.2, NiFe.sub.2O.sub.4, NiFe.sub.2O.sub.4#SiO.sub.2, LaNiO.sub.3/SiO.sub.2, LaNiO.sub.3 and LaNiO.sub.3@SiO.sub.2. Furthermore, alumina is the most commonly used support, i.e. in Ni/γ-Al.sub.2O.sub.3, Ni—Fe/γ-Al.sub.2O.sub.3, Mn/γ-Al.sub.2O.sub.3, Cu/γ-Al.sub.2O.sub.3, Co/γ-Al.sub.2O.sub.3, La.sub.2O.sub.3/γ-Al.sub.2O.sub.3, Ag/γ-Al.sub.2O.sub.3, Pd/γ-Al.sub.2O.sub.3, Fe/γ-Al.sub.2O.sub.3 and Cu—Ni/γ-Al.sub.2O.sub.3, or even in its pure form.

[0056] Many other catalytic systems are based on zeolites, e.g. 3A, A4, NaX, NaY and Na-ZSM-5. Besides, studies have also been conducted using BaTiO.sub.3, a mixture of BaTiO.sub.3 and NiSiO.sub.2, ceramic foams (92% Al.sub.2O.sub.3, 8% SiO.sub.2) coated with Rh, Ni or NiCa, quartz wool, glass beads, a stainless steel mesh, starch, BZT (BaZr.sub.0.75T.sub.0.25O.sub.3) and BFN (BaFe.sub.0.5Nb.sub.0.5O.sub.3). For a regular AC-packed DBD, the best result was obtained for the Zeolite Na-ZSM-5, with a total conversion of 37% and an energy cost of 24 eV per converted molecule. As for pure CO.sub.2 splitting, the addition of a catalyst does not seem to make the process more energy efficient, but it does yield higher conversions at the same energy cost. The best overall results in a packed-bed DBD were obtained for a quasi-pulsed DBD packed with BFN and BZN, with total conversions in the range of 45-60% and an energy cost in the range of 13-16 eV per converted molecule, which is lower than that for a DBD without packing, but this might also be due to the pulsed operation. Syngas is considered a renewable fuel since its origins mainly come from biological materials such as organic waste. Putting a carbonic waste stream through syngas synthesis converts waste to power through combustion. Benefits include renewable power, reduction of carbon emissions, problematic wastes to usable fuel, and onsite power production.

[0057] Besides the experimental work, major insights have been obtained in recent years based on modelling of the DRM process for a DBD. Different kinds of models and computational techniques have been successfully developed, including semi-empirical kinetic models, zero-dimensional chemical kinetic models with both simplified and extensive chemistry sets, a one-dimensional fluid model, a so-called 3D Incompressible Navier-Stokes model combined with a convection-diffusion model, a hybrid artificial neural network-genetic algorithm, a model focusing on a more accurate description of the electron kinetics and density functional theory (DFT) studies, to investigate the reaction mechanisms. Due to the complex chemistry taking place in a DRM, the development of accurate multidimensional models with extensive chemistry is currently restricted by computational limits.