Carbon dioxide collecting apparatus and method using independent power generation means

Abstract

A carbon dioxide capturing apparatus and process uses a self-generating power means that uses carbon dioxide in combustion exhaust gas through the convergence of a carbon dioxide absorption tower. The capturing apparatus and process also relies on ionic generator associated technology using a concentration difference between seawater and freshwater. The capturing apparatus and process result in increased production efficiency for electric energy and reduced costs for a carbon dioxide capturing process by increasing a concentration difference using an absorbent liquid for absorbing carbon dioxide and, at the same time, electricity is obtained through carbon dioxide which is a greenhouse gas.

Claims

1. A carbon dioxide capturing apparatus using a self-generating power means, comprising: an absorption tower (1) configured to absorb a gas including carbon dioxide through contact with an absorbent; an electricity-generating device (7) configured to generate electricity by a potential difference caused due to a difference in concentration between a fluid solution and an absorption solution having absorbed carbon dioxide, and comprising a space formed between a cation electrode (7c) and an anion electrode (7d), the space being divided by a cation exchange membrane (7a) and an anion exchange membrane (7b), a first flow path (7f) between the cation exchange membrane (7a) and the cation electrode (7c) through which the fluid solution moves, a second flow path (7g) between the anion exchange membrane (7b) and the anion electrode (7d) through which the fluid solution moves, and an absorption solution flow path (7e) between the first flow path and the second flow path through which the absorption solution having absorbed carbon dioxide moves; and a regeneration tower (20) configured to separate an regenerated absorption solution in which the absorption solution having absorbed carbon dioxide is introduced into an upper portion of the regeneration tower (20) and flows down to a lower portion of the regeneration tower (20), and comprising a reheater (22) configured to supply a heat source for separation of carbon dioxide and operated by electricity generated at the electricity-generating device; and a condenser (21) configured to discharge evaporated steam and carbon dioxide after the steam is condensed and the carbon dioxide is cooled, wherein the regenerated absorption solution which has passed through the regeneration tower is supplied to the absorption tower by a second liquid transfer pump (6b), and wherein, when the absorption solution having a relatively higher concentration than the fluid solution or the absorption solution having a relatively lower concentration than the fluid solution is supplied to absorption solution flow path (7e), a cation passes through the cation exchange membrane and an anion passes through the anion exchange membrane due to the difference in concentration between the fluid solution and the absorption solution having absorbed carbon dioxide.

2. The carbon dioxide capturing apparatus of claim 1, wherein the cation electrode (7c) is disposed to face the cation exchange membrane, and the anion electrode (7d) is disposed to face the anion exchange membrane.

3. The carbon dioxide capturing apparatus of claim 1, wherein the absorbent comprises, as a solute, one or more selected from an aqueous electrolyte solution group consisting of amines, alkali metal bicarbonates, alkali carbonates, carbonates, hydroxides, borates, phosphates, nitrates, acids, and sodium chloride, and an organic electrolyte solution group consisting of propylene carbonate (PC), diethyl carbonate (DEC), and tetrahydrofuran (THF).

4. The carbon dioxide capturing apparatus of claim 1, wherein the electricity-generating device is positioned at a place in which the absorption solution moves between the absorption tower and the regeneration tower.

5. The carbon dioxide capturing apparatus of claim 1, further comprising a preheater (25) configured to heat the absorption solution having absorbed carbon dioxide before the absorption solution flows into the regeneration tower and operated by electricity generated at the electricity-generating device.

6. The carbon dioxide capturing apparatus of claim 1, wherein the absorbent further comprises, as an additive, an anticorrosive agent, a coagulant aid, an antioxidant, an oxygen (O.sub.2) scavenger, an antifoaming agent, or a combination thereof.

7. The carbon dioxide capturing apparatus of claim 6, wherein the additive is further comprised at 1 wt % or less.

8. The carbon dioxide capturing apparatus of claim 1, further comprising a heat exchanger (5) in which heat is exchanged while the absorption solution having absorbed carbon dioxide and the regenerated absorption solution are passing through the heat exchanger (5).

9. The carbon dioxide capturing apparatus of claim 8, further comprising a regenerated absorption solution condenser (23) configured to cool the regenerated absorption solution which has passed through the heat exchanger, and an absorption solution replenishing unit (24).

10. The carbon dioxide capturing apparatus of claim 1, wherein the absorbent comprises, as a solvent, one or more selected from an aqueous solvent group or an organic solvent group, wherein the aqueous solvent group consists of pure water, freshwater, brackish water, saline water, all of which are in the form in which water is present, and a mixed solvent of an alcohol and water, wherein the organic solvent group consists of an aliphatic hydrocarbon group consisting of hexane; an aromatic hydrocarbon group consisting of benzene, toluene, xylene, and methylnaphthalene; a heterocyclic compound group consisting of quinoline and pyridine; a ketone group consisting of acetone, methyl ethyl ketone, and cyclohexanone; an ester group consisting of methyl acetate and methyl acrylate; an amine group consisting of diethylenetriamine and N,N-dimethylaminopropylamine; an ether group consisting of diethyl ether, propylene oxide, and tetrahydrofuran (THF); an amide group consisting of N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; and a polar aprotic solvent group consisting of hexamethylphosphoramide and dimethyl sulfoxide.

11. The carbon dioxide capturing apparatus of claim 10, wherein the absorbent comprises the solvent at 40 to 95 wt % and the solute at 5 to 60 wt %.

12. The carbon dioxide capturing apparatus of claim 11, wherein a mixed solution of the solvent, the solute and the additive has a pH value ranging from pH 2 to 12.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a configuration diagram of a carbon dioxide capturing device using a self-generating power means according to the present invention.

(2) FIG. 2 is a conceptual diagram of the carbon dioxide capturing device using a self-generating power means according to the present invention.

(3) FIG. 3 is a configuration diagram of a carbon dioxide capturing apparatus using a self-generating power means, which includes a heat exchanger, according to the present invention.

(4) FIG. 4 shows results of power density and closed circuit voltage according to Example 1.

(5) FIG. 5 shows results of changes in pH of an absorption solution and a fluid solution according to Example 1.

(6) FIG. 6 shows results of changes in maximum energy, power density, closed circuit voltage, and pH according to Example 2.

BEST MODE

(7) Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. It should be understood that it is possible for like elements to have like reference numerals even if they are shown in different drawings. When known functions and configurations are deemed to unnecessarily obscure the gist of the invention, detailed descriptions thereof will be omitted.

(8) The present invention is largely composed of three parts, that is, an absorption tower configured to selectively absorb carbon dioxide, an electricity-generating device using ion configured to generate electricity by a difference in concentration between an absorption solution and a fluid solution, and a regeneration tower configured to separate a regenerated absorption solution from the absorption solution having absorbed carbon dioxide using energy produced at the electricity-generating device using ion as a heat source, and a configuration diagram of the present invention is shown in FIG. 1.

(9) Also, the present invention is largely composed of four parts, that is, an absorption tower configured to selectively absorb carbon dioxide, a heat exchanger in which heat is exchanged between an absorption solution having absorbed carbon dioxide and an absorption solution that is regenerated by passing through an electricity-generating device using ion, an electricity-generating device using ion configured to generate electricity by a difference in concentration between an absorption solution and a fluid solution, and a regeneration tower configured to separate a regenerated absorption solution from the absorption solution having absorbed carbon dioxide using energy produced at the electricity-generating device using ion as a heat source, and a configuration diagram of the present invention is shown in FIG. 3. The components of the invention will be described.

(10) First, an absorption tower 1 will be described. The absorption tower according to the present invention is a device configured to absorb carbon dioxide from a mixed gas 2 containing carbon dioxide through contact, and is configured so that an absorption solution is supplied to an upper portion of the absorption tower, and the absorption solution 4 having absorbed carbon dioxide is transferred from a lower portion of the absorption tower to a heat exchanger 5. The absorption tower may include a filler, and the absorption solution having absorbed carbon dioxide may be transferred to the heat exchanger by a liquid transfer pump.

(11) In the heat exchanger 5, heat is exchanged between the absorption solution having absorbed carbon dioxide and the absorption solution that is regenerated at the regeneration tower by passing through the electricity-generating device using ion. Fluids flowing into the heat exchanger may be supplied in counter-flow or parallel-flow directions.

(12) In the electricity-generating device using ion, as shown in FIG. 1, a space formed between a cation electrode 7c and an anion electrode 7d is divided by a cation exchange membrane 7a and an anion exchange membrane 7b. That is, the electricity-generating device using ion 7 includes a first flow path 7f formed between the cation exchange membrane 7a and the cation electrode 7c, a second flow path 7g formed between the anion exchange membrane 7b and the anion electrode 7d, and an absorption solution flow path 7e formed between the cation exchange membrane 7a and the anion exchange membrane 7b.

(13) A fluid solution flows through the first flow path 7f and the second flow path 7g, and the absorption solution having absorbed carbon dioxide flows through the absorption solution flow path 7e.

(14) The cation exchange membrane 7a is a dense membrane that blocks a stream of the absorption solution and through which only cations selectively pass, and the anion exchange membrane 7b is a dense membrane that blocks a stream of the absorption solution and through which only anions selectively pass.

(15) The fluid solution may include an aqueous electrolyte such as NaCl, H.sub.2SO.sub.4, HCl, NaOH, KOH, Na.sub.2NO.sub.3, etc., and an organic electrolyte such as propylene carbonate (PC), diethyl carbonate (DEC), tetrahydrofuran (THF), etc.

(16) In particular, one or more solvents selected from the group consisting of an aqueous solvent such as pure water, freshwater, brackish water, saline water, or a mixed solvent of an alcohol and water, and an organic solvent including an aliphatic hydrocarbon such as hexane, etc.; an aromatic hydrocarbon such as benzene, toluene, xylene, methylnaphthalene, etc.; a heterocyclic compound such as quinoline, pyridine, etc.; a ketone such as acetone, methyl ethyl ketone, cyclohexanone, etc.; an ester such as methyl acetate, methyl acrylate, etc.; an amine such as diethylenetriamine, N,N-dimethylaminopropylamine, etc.; an ether such as diethyl ether, propylene oxide, tetrahydrofuran (THF), etc.; an amide such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, etc.; and a polar aprotic solvent such as hexamethylphosphoramide, dimethyl sulfoxide, etc. may be used as the fluid solution.

(17) Also, the absorption solution may move along the absorption solution flow path 7e, and the absorbent may include one or more selected from the group consisting of amines, alkali metal bicarbonates, alkali carbonates, carbonates, hydroxides, borates, and phosphates.

(18) The amines may include primary amines, secondary amines, and ammonia. The amines may be in a liquid or solid phase at room temperature and atmospheric pressure, or may include gases with vapor pressure or in a mist phase. The primary amines may include saturated aliphatic primary amines such as methylamine, ethylamine, isopropylamine, propylamine, butylamine, 2-aminoethanol, etc., unsaturated aliphatic primary amines such as allylamine, etc., alicyclic primary amines such as cyclopropylamine, etc., and aromatic primary amines such as aniline, etc., and the secondary amine may include saturated aliphatic secondary amines such as dimethylamine, diethylamine, diisopropylamine, etc., unsaturated aliphatic secondary amines such as diallylamine, etc., and aromatic secondary amines such as methylaniline, etc.

(19) Also, the amines may include ethyleneamine, ethanolamine (MEA), NN-butylethanolamine (BEA), ethylenediamine (EDA), diethanolamine (DEA), diisopropanolamine (DIPA), N-methyldiethanolamine (MDEA), diglycolamine (DGA), triethanolamine (TEA), o-methylhydroxylamine, ethanimidamine, N-(2-hydroxyethyl)ethylenediamine (AEEA), diethanoltriamine (DETA), N,N-dimethylethlethanolamine (DMMEA), 2-4n diisopropanolamine or 2n methyldiethanolamine (ADIP), piperidine, piperazine, morpholine, pyrrolidine, 2,2,6,6, tetramethyl-4-piperidinol (TMP), 2-amino-2-methylpropanol (AMP), 1-amino-2-propanol (MIPA), 2-methylaminoethanol (MMEA), 3-amino-1-propanol (MPA), diethylaminoethanol (DEMEA), 2-diisopropylaminoethanol (DIPMEA), 2-diethylaminoethanol (DEAE), 2-(diisopropylamino)ethanol (DIPAE), 2-(dimethylamino)-2-methyl-propanol (DMAMP), N-ethyldiethanolamine (EDEA), N-isopropyldiethanolamine (IPDEA), N-tert-butyldiethanolamine (tBDEA), 1-(2-hydroxyethyl)pyrrolidine (HEP), 1-(2-hydroxyethyl)piperidine (HEPD), 1-methyl-2-piperidineethanol (1M-2PPE), 1-ethyl-3-hydroxypiperidine (1E-3HPP), 2-{[2-(dimethylamino)ethyl]methylamino}ethanol (DMAEMAE), N,N,N,N-tetrakis(2-hydroxyethyl)ethylenediamine (TKHEEDA), 2-[2-(dimethylamino)ethoxy]ethanol (DMAEE), bis[2-(N,N-dimethylamino)ethyl]ether (DAEE), 1,4-dimethylpiperazine (DMPZ), N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA), N-methyl-N,N-bis[3-(dimethylamino)propyl]amine (PMDPTA), n-butyldiethanol amine (BDEA), triisopropanolamine (TIPA), 4-(2-hydroxyethyl)morpholine (HEM), hydroxyisopropylmorpholine (N-(2-hydroxypropyl)morpholine) (HIPM), 2-(dibutylamino)ethanol (2-DBAE), 2,2-bis(hydroxymethyl)-2,2,2-nitrilotriethanol (HMNTE), N-methyl-4-piperidinol (MP), hexamethylenetetramine (HMTA), N,N-dicyclohexylmethylamine (DCHMA), etc.

(20) In addition, the amines may be sterically hindered amines KS-1, KS-2, and KS-3. Also, sterically hindered cyclic amines may include 1-amino-4-methyl piperazine, 1-(2-aminoethyl)-4-methyl piperazine, 1-(2-hydroxyethyl)-4-methyl piperazine, 1-(2-aminoethyl)-piperazine, 1-(2-hydroxyethyl)-piperazine, 2-aminoethyl-piperazine, 1-ethyl-piperazine, 2,5-dimethyl-piperazine, cis-2,6-dimethyl-piperazine, 1,4-dimethyl-piperazine, trans-2,5-dimethyl-piperazine, 1-methyl piperazine, 2-methyl piperazine, 1-ethyl piperazine, 2-piperidineethanol, 3-piperidineethanol, 4-piperidineethanol, 2-aminoethyl-1-piperidine, and homopiperazine, etc.

(21) The alkali carbonates may include potassium carbonate (K.sub.2CO.sub.3), sodium carbonate (Na.sub.2CO.sub.3), sodium hydroxide (NaOH), potassium hydroxide (KOH), potassium bicarbonate (KHCO.sub.3), sodium bicarbonate (NaHCO.sub.3), etc. Also, the alkali carbonates may include compounds from a Benfield process developed by the Union Carbide Corporation, a HIPure process known as the improved Benfield process, a Catacarb process developed by A. G. Eickmeyer, FLEXSORB HP developed by the Exxon Mobil Corporation, etc.

(22) In particular, the absorption solution may also include an aqueous electrolyte such as NaCl, H.sub.2SO.sub.4, HCl, NaOH, KOH, Na.sub.2NO.sub.3, etc., and an organic electrolyte such as propylene carbonate (PC), diethyl carbonate (DEC), and tetrahydrofuran (THF), similar to the fluid solution.

(23) Particularly, one or more solvents selected from the group consisting of an aqueous solvent such as pure water, freshwater, brackish water, saline water, or a mixed solvent of an alcohol and water, and an organic solvent including an aliphatic hydrocarbon such as hexane, etc.; an aromatic hydrocarbon such as benzene, toluene, xylene, methylnaphthalene, etc.; a heterocyclic compound such as quinoline, pyridine, etc.; a ketone such as acetone, methyl ethyl ketone, cyclohexanone, etc.; an ester such as methyl acetate, methyl acrylate, etc.; an amine such as diethylenetriamine, N,N-dimethylaminopropylamine, etc.; an ether such as diethyl ether, propylene oxide, tetrahydrofuran (THF), etc.; an amide such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, etc.; and a polar aprotic solvent such as hexamethylphosphoramide, dimethyl sulfoxide, etc. may be used as the absorption solution.

(24) The absorption solution and the fluid solution may be supplied in counter-flow or parallel-flow directions.

(25) The cation electrode 7c, the anion electrode 7d, the cation exchange membrane 7a and the anion exchange membrane 7b may be used without limitation as long as they are used for conventional fluidized-bed electrode systems (cells, batteries, etc.), and may be properly selected and used by those skilled in the related art, depending on the purpose of use and conditions.

(26) The regeneration tower uses electric energy produced at the electricity-generating device using ion to drive a reheater in order to separate an absorption solution regenerated while the absorption solution having absorbed carbon dioxide is being allowed to flow in from an upper portion of the regeneration tower and flow down to a lower portion of the regeneration tower. In this case, the regeneration tower is configured to discharge evaporated steam and carbon dioxide after the steam is condensed and the carbon dioxide is cooled.

(27) Next, embodiments of the present invention in which the carbon dioxide capturing apparatus using an self-generating power means is used will be described.

(28) In FIG. 3, reference numbers 1, 5, 7, and 20 represent an absorption tower 1, a heat exchanger 5, an electricity-generating device using ion 7, and a regeneration tower 20, respectively.

(29) The absorption tower 1 may include a flow path 2 for an exhaust gas containing carbon dioxide, a flow path 3 for an exhaust gas which has passed through the absorption tower, a flow path 10 for a regenerated absorption solution supplied to the absorption tower through the heat exchanger 5, a flow path 4 for an absorption solution having absorbed carbon dioxide, which is discharged from a lower portion of the absorption tower, a filler 1a that comes in contact with a gas in the absorption tower, and a first liquid transfer pump 6a configured to transfer the absorption solution having absorbed carbon dioxide to the heat exchanger.

(30) In the heat exchanger 5, heat is exchanged when the absorption solution having absorbed carbon dioxide and the regenerated absorption solution which has passed through the electricity-generating device using ion pass through the heat exchanger.

(31) In the electricity-generating device using ion 7, a space formed between the cation electrode 7c and the anion electrode 7d is divided by the cation exchange membrane 7a and the anion exchange membrane 7b. That is, the electricity-generating device using ion 7 includes a first flow path 7f formed between the cation exchange membrane 7a and the cation electrode 7c, a second flow path 7g formed between the anion exchange membrane 7b and the anion electrode 7d, and an absorption solution flow path 7e formed between the cation exchange membrane 7a and the anion exchange membrane 7b.

(32) The regeneration tower 20 includes a preheater 25 configured to preheat and supply a carbon dioxide absorption solution, a reheater 22 in which electricity generated at the electricity-generating device using ion is used as a heat source, and a condenser 21 configured to discharge evaporated steam and after the steam is condensed and the carbon dioxide is cooled.

(33) Also, the regenerated absorption solution that has passed through the heat exchanger may pass through a regenerated absorption solution condenser 23 prior to flowing into the absorption tower, and a regenerated absorption solution replenishing unit 24 configured to replenish shortages of the absorption solution may be further configured.

(34) In the absorption tower 1, an absorption solution having a higher concentration than a fluid solution flowing in the first flow path 7f and the second flow path 7g is supplied to the absorption solution flow path 7e of the electricity-generating device using ion, or an absorption solution having a lower concentration than a fluid solution flowing in the first flow path 7f and the second flow path 7g is supplied to the absorption solution flow path 7e. Therefore, an ion concentration of the absorption solution passing through the absorption solution flow path 7e may decrease or increase.

(35) Then, an ammeter 8 configured to measure electric energy is connected with the electricity-generating device using ion, and thus may measure a potential difference generated by the concentration difference in an ion generating cell.

(36) Therefore, when the absorption solution having a relatively high concentration and the fluid solution having a relatively low concentration are supplied to the electricity-generating device using ion, cations and anions move toward the absorption solution from the first flow path 7f and the second flow path 7g having a low concentration to the cation electrode 7c and the anion electrode 7d through the catio exchange membrane 7a and the anion exchange membrane 7b, so that a potential difference is generated by the moving cations and anions.

(37) On the other hand, when the absorption solution having a relatively low concentration and the fluid solution having a relatively high concentration are supplied to the electricity-generating device using ion, cations and anions move toward the absorption solution from the first flow path 7f and the second flow path 7g having a high concentration to the cation electrode 7c and the anion electrode 7d through the cation exchange membrane 7a and the anion exchange membrane 7b, so that a potential difference is generated.

(38) A conceptual diagram of the present invention is shown in FIG. 2. An absorption process of the present invention operates in a temperature range of 5 to 80 C., a pressure range of atmospheric pressure to 20 atm and a range of a molar flux ratio (liquid/gas) of 2.0 to 10. Also, an ion exchange membrane process of the electricity-generating device using ion of the present invention operates in a temperature range of 5 to 80 C. and a pressure range of atmospheric pressure to 20 atm, and also operates in a range of a volumetric flow ratio (saline water:freshwater:electrolyte) of 2:1.0 to 2.0:2. It is possible to obtain effects of reducing greenhouse gases by removing carbon dioxide from exhaust gases and of producing electricity by inputting an absorbent which absorbs carbon dioxide to the electricity-generating device using ion which uses a concentration difference to generate a potential difference and thus operating a system without supplying external energy using the generated electricity as renewable energy, depending on the concept that a gas including carbon dioxide is absorbed by an absorption solution in the absorption tower, the absorption solution which has a high concentration and has absorbed carbon dioxide and freshwater which has a low concentration are supplied to the electricity-generating device using ion which uses the concentration difference, electricity is generated by generating the potential difference in the electricity-generating device using ion in which only ions selectively permeate, and then energy required to regenerate an absorbent at the regeneration tower is applied using the electricity generated at the electricity-generating device using ion as a heat source.

MODE FOR INVENTION

Example 1

(39) In Example 1 of the present invention, a carbon dioxide absorbing device that is capable of producing electricity and is configured of five each of a cation exchange membrane (commercially available from Fumatech and Astom Corporation) and an anion exchange membrane (commercially available from Fumatech) between rectangular positive and negative electrodes (graphite electrodes) having a micro-flow path, a spacer, an absorption solution (KIERSOL: a mixture including 15% by weight of K.sub.2CO.sub.3, 10% by weight of 2-methylpiperazine, and water), a vessel with a stirrer through which carbon dioxide is reacted (10 mL/min, 200 rpm, 40 C.), a vessel for inputting freshwater (10 mL/min) and a vessel for circulating an electrolyte (a mixed solution of ferrocyanide and NaCl:Fe(CN).sub.6.sup.3/450 mM, 20 mL/min), was manufactured.

(40) A total generated energy, the voltage, and the maximum power density per unit area of an exchange membrane were measured to be 0.05 W, 0.5 to 0.54 V, and 0.8 W/m.sup.2, respectively. FIG. 4 illustrates values of power density and closed circuit voltage according to reaction time. It can be seen that a power density value was 0.7 to 0.8 W/m.sup.2 within a measurement time. It was revealed that the generated energy per unit cell when KIERSOL was used as an absorbent was 300 kJ/(ton of KIERSOL).

(41) As shown in FIG. 5, it can be seen that a pH value of freshwater passing through an ion reactor decreased to pH 7.2 from pH 8.0 before passing through the ion reactor, and a pH value of the absorbent KIERSOL increased to pH 8.4 from pH 7.9 before passing through the ion reactor. Therefore, it was revealed that HCO.sub.3.sup. and H.sup.+ ions were transferred through an ion exchange membrane, and as a result, an absorption solution became basic as the absorption solution was regenerated, and a fluid solution became acidic as the fluid solution was regenerated.

Example 2

(42) Referring to FIG. 6, the experimental results of the carbon dioxide capturing apparatus capable of producing electricity were able to be confirmed. An experiment was performed using a selective ion exchange membrane (0.0071 m.sup.2) commercially available from Fumatech, a graphite electrode, five stacks of exchange membranes, a spacer (0.2 mm), saline water (10 mL/min), freshwater (5 mL/min), and an electrolyte (10 mL/min) were used.

(43) It was revealed that a KIERSOL solution was measured to have a voltage of 0.3 to 0.4 V and a power density value of 0.3 to 0.4 W/m.sup.2. Also, it was revealed that a KIERSOL solution having absorbed carbon dioxide was measured to have a voltage of 0.5 V and a power density value of 0.7 to 0.8 W/m.sup.2.

(44) Although the present invention has been described above in detail with reference to preferred examples thereof, it will be understood by those skilled in the art that various changes and modifications can be made to the detailed description and specific examples of the present invention without departing from the spirit and scope of the invention as defined by the appended claims.