US20060196776A1

US20060196776A1 - Apparatus and method for producing sulfur from hydrogen sulfide

Info

Publication number: US20060196776A1
Application number: US11/073,231
Authority: US
Inventors: Burnard Bockris
Original assignee: World Hydrogen Inc
Current assignee: World Hydrogen Inc
Priority date: 2005-03-04
Filing date: 2005-03-04
Publication date: 2006-09-07
Also published as: WO2006096692A2; WO2006096692A3

Abstract

An apparatus is provided for producing sulfur from hydrogen sulfide. The apparatus includes a container containing a solution that includes hydrogen sulfide. Within the container, and at least partially submerged in the solution, is a photo-anode, formed of an N-type semiconductor, and a photo-cathode, formed of a P-type semiconductor. Each electrode is at least partially covered with a catalyst material. The photo-anode and photo-cathode are electrically coupled to each other and develop charges of opposite polarity when exposed to a light source, which causes the sulfur to separate from the hydrogen in the solution. Also provided is a method for producing sulfur from hydrogen sulfide.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is related to the inventor's application “APPARATUS AND METHOD FOR PRODUCING HYDROGEN FROM HYDROGEN SULFIDE,” Ser. No. ______, now ______, which was filed on the same day as the present application and commonly assigned herewith to World Hydrogen, Inc. This related application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates generally to the production of solid sulfur, and more particularly relates to the synthesis of sulfur atoms from a solution of hydrogen sulfide.
2. Description of the Related Art
Elemental sulfur (S) is used for, among other things, the making of landfills and roads. However, elemental sulfur is a relatively rare chemical, with the largest deposits in the United States being in the State of Louisiana. Because elemental sulfur is a commodity, the price varies from day to day. The present price for a ton of sulfur is estimated to be between $60-$70 USD, with the price overseas being much higher. Therefore, there is a constant need for reasonably-priced sulfur.
Sulfur is obtained from ore bodies, basically by melting the solid found in deposits. Some sulfur is found on the surface of the earth and, in this case, is set alight to melt the remainder of the underground deposit. Disadvantageously, about a quarter of the total sulfur is burned with this method. An even bigger disadvantage is that the environment is significantly damaged by the resulting sulfur dioxide (a gas), which makes the surrounding countryside barren.
Once the sulfur is melted, i.e., becomes a liquid. The liquid can then be collected, poured off, and allowed to solidify. However, a disadvantage to this method is that there is only a small temperature zone (˜5° C.) above the melting point, in which the sulfur can be poured. At higher temperatures, the polysulfide (rings of 8 sulfur atoms (S₈)), which are present from the solid, break open. The liquid, which results, is extremely viscous and it is no longer practical to pour.
An additional disadvantage to prior-art methods of extracting sulfur is that a great part of the sulfur stored in the earth is underground at depths up to 1,000 ft. Extraction is achieved by the insertion into the ore body of a large pipe containing three concentric pipes. Super-heated steam is passed down the first pipe and melts the sulfur. Through the second pipe is passed a strong stream of air, which produces from the molten sulfur a foamy liquid. Pressure forces the foamy liquid up the third pipe of the three concentric inner pipes. At the surface, the liquid is directed into vats where it cools and solidifies, ready for shipment.
Extraction from deep deposits is thus difficult and extraction of that on the surface environmentally damaging. Therefore a need exists to overcome the problems with the prior art as discussed above.

SUMMARY OF THE INVENTION

Briefly, in accordance with one embodiment of the present invention, disclosed is an apparatus for generating solid sulfur in an environmentally, non-impactful, and energy efficient manner. The apparatus includes a container containing a solution that includes hydrogen sulfide (H₂S). At least partially submerged in the solution is a photo-anode formed of an N-type semiconductor material and a photo-cathode formed of a P-type semiconductor material. The photo-anode and photo-cathode are electrically coupled to each other and develop charges of opposite polarity when exposed to a light source. The charges cause the hydrogen to separate from the sulfur in the solution, thus producing pure sulfur.
In one preferred embodiment of the present invention, the photo-anode comprises cadmium selenide over a stainless steel support and further covered by a plurality of ruthenium deposits on a surface of the cadmium selenide.
In some embodiments of the present invention, the photo-cathode comprises gallium phosphide with a plurality of platinum deposits on a surface of the gallium phosphide.
Another embodiment of the present invention provides a method for producing hydrogen from hydrogen sulfide. The method includes providing a solution including hydrogen sulfide within a container. An N-type photo-anode and a P-type photo-anode are provided and at least partially submerged into the solution. The photo-anode and the photo-cathode are exposed to at least one light source. A hydrogen gas byproduct is collected from the container.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
FIG. 1 is a diagram illustrating one embodiment of a two-chambered electrolysis container in accordance with the present invention.
FIGS. 2A & 2B are hardware block diagrams illustrating one embodiment of a cathode and an anode in accordance with the present invention.
FIG. 3 is a diagram illustrating one embodiment of a one-chambered electrolysis container in accordance with the present invention.
FIG. 4 is a flow diagram of the hydrogen evolution and sulfur deposition process in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. It is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention.
The present invention, according to one embodiment, overcomes problems with the prior art by efficiently producing elemental sulfur without the need for an outside source of electricity and without producing environmentally hazardous gasses. The present invention realizes an efficiency of from 10-100 times great than that of the prior-art devices and methods. Additionally, the present invention produces sulfur with negative costs.
Described now is an exemplary apparatus according to one embodiment of the present invention. Referring to FIG. 1, a container 102 is shown having a first chamber 104 and a second chamber 106. Each chamber 104 and 106 contains an electrode 108 and 110, respectively. The electrodes 108 and 110 are at least partially coated with a catalyst and connected to one another via conductive paths 112 and 113, which are coupled via a resistive element 114.
Each chamber 104 and 106 is an at least partially closed vessel and contains a quantity of hydrogen sulfide (H₂S), into which one of the electrodes 108 or 110 is at least partially submerged. The energy needed to split water at a reasonable rate is about 1.7 V producing a current of 10-100 mA/cm². However, the energy needed to split H₂S into hydrogen and sulfur is only about 1.1V (excluding IR drop) at about 10 mA/cm². By utilizing two catalyst-containing electrodes, as described in detail below, each having its own energy gap, enough power is available to increase the efficiency of the conversion to a reasonable value of between 10-20%, with only the input of illuminating light, to separate the hydrogen from the sulfur in each H₂S molecule.
In the illustrated embodiment, the chambers are bulbous, but in further embodiments can be any shape that will hold the solution. In other embodiments, the container 102 is a single chamber, which allows the electrodes to be moved closer together thus reducing the IR drop. The single chamber includes the elements shown in FIG. 1 to be within the two chambers 104 and 106. Other embodiments of the chamber are possible without departing from the spirit and scope of the present invention.
A light source 126 strikes both electrodes and develops charges on the electrodes, with each electrode developing a polarity opposite that of the other. The electrode composition and charge generating mechanics will be described in detail below.
The potential difference between the charges on the two electrodes 108 and 110 is significant enough to disassociate the H₂S molecules within the solution 124. As a result, hydrogen gas (H₂) 116 is produced at the first electrode 108 in the first chamber 104 and sulfur (S) 118 is produced at the second electrode 110 in the second chamber 106.
The hydrogen gas 116 is collected from an exit port 128 located at an upper portion of the first chamber 104 and the sulfur 118 is removed from a bottom portion of the second chamber 106, where the sulfur 118 collects.
One preferred embodiment of the present invention will now be explained in detail. Referring now to FIGS. 2A & 2B, the electrodes 108 and 110 of this preferred embodiment are shown. As previously stated, the electrodes 108 and 110 are each made of a doped semiconductor material. The first electrode 108 is made to evolve hydrogen and is made of P-type semiconductor material, while the second electrode 110 is made to deposit sulfur, and is made of an N-type semiconductor material.
Doping is a process well known in the semiconductor art that adds impurities to a normally balanced crystalline structure. In N-type doping, the “impurity” atoms have one extra electron in their valance shell than do the atoms making the crystalline structure. The extra electrons are free to move around because there is nothing with which they can bond. Only a small amount of the impurity is needed to create enough free electrons to allow an electric current to flow. N-type doping gets its name because the added electrons have a negative charge.
In P-type doping, the “impurity” atoms have one less electron in their valance shell than do the atoms making the crystalline structure. When mixed into the crystalline lattice, “holes” are formed, where electrons are missing. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current by accepting an electron from a neighbor, as to move the hole over a space.
In one embodiment, the cathode 108 includes a thin layer of gallium phosphide 202, P-doped, over an underlying support of stainless steel 206. In other embodiments, the layer is gallium arsenide (GaAs). In one embodiment, the anode 110 is made of a thin layer of cadmium selenide 208, N-doped, over an underlying support of stainless steel 210. It should be noted that the anode and cathode materials described here are just a single example and that other anode and cathode materials may also be used without departing from the spirit and scope of the present invention. The anode 110 and cathode 108 are coupled to each other through conductive paths 113 and 112.
The maximum energy available in a photo-electrolytic process is the “energy gap,” or difference in energy between the valence band and the conduction band, in the semiconductor material. For an electrode made of cadmium selenide (CdSe), the energy gap is 2.5 eV; cadmium sulfide (CdS) the energy gap is 1.1 eV; and cadmium telluride (CdTe) the energy gap is 1.5 eV. An eV (electron volt) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. This is a very small amount of energy: 1 eV=1.60217653×10⁻¹⁹J. Typical energy gaps needed for photo-electrolysis are between 1.0 and 2.0 eV, depending on the system being used.
Due to the stability of the CdSe in polysulfide solution, various forms of CdSe can be used—single crystal, polycrystal, and thin films, among others—to form the photo-anode 110. In the illustrated embodiment of the present invention, n-CdSe thin films deposited on stainless steel by chemical deposition techniques are used as the photo-anode 110. Similarly, p-gallium phosphide thin films deposited on stainless steel by chemical deposition techniques are used as the photo-cathode 108.
In the illustrated embodiment, deposited on top of the gallium phosphide 202, by pulse electrolysis, is platinum 204, which acts as a catalyst. The platinum 204 is arranged so that “spots” of platinum 204 cover between 10 and 20 percent of the gallium phosphide 202 in this embodiment. The “spots” are actually small mounds, or groups of atoms, deposited on the electrode. The spot diameter is about 0.2 mm.
In the illustrated embodiment, deposited on top of the cadmium selenide 208, by pulse electrolysis, is ruthenium 212, which acts as a catalyst. The ruthenium 212, similar to the platinum 204, is arranged so that “spots” of ruthenium 212 cover between 10 and 20 percent of the cadmium selenide 208 in this embodiment. The “spots” are actually small mounds, or groups of atoms, deposited on the electrode. The spot diameter is about 0.2 mm. In other embodiments of the present invention, other catalyst materials can be used, such as ferrous sulfide (FeS). For example, the ferrous sulfide can be applied by controlled vapor deposition in bursts until 10 to 20 percent coverage has been reached.
As shown in FIG. 1, the electrodes 108 and 110 are at least partially submerged in a solution 124. In one embodiment, the solution 124 is a 1M solution of NaOH, prepared by dissolving analar grade NaOH in triply distilled water. Hydrogen sulfide (H₂S) is bubbled through the NaOH and triply distilled water for about 1 hour. In the beginning, very small bubbles can be observed. The bubble size increases as the solution becomes saturated with H₂S.
H₂S, when dissolved in NaOH solution, partially dissociates to H⁺, HS⁻, and S⁻⁻. The latter two ions then deposit on the anode in the chamber. The initial state of the sulfur, after delivery into the anode, and charge transfer (S⁻→S+2e), is atomic sulfur, S. Fractions of a second later, the S leaves the anode to form chains of S atoms, mostly S₈, i.e., S-S-S-S-S-S-S-S (polysulfide). The lifespan of the polysulfide chains is in the order of minutes and then it gradually decomposes to form crystalline sulfur.
Accordingly, the starting solution is 1M with respect to NaOH. The pH level of the solution is calculated as log (1/H⁺), i.e., depends on the H⁺ (or OH⁻) in the solution 124. If the H is removed, (i.e., in the photo-stimulated evolution of H₂), the pH level will change. As the photo-stimulated electrolysis process proceeds (i.e., evolution of H₂), pH changes have to be compensated for. Therefore, in one embodiment of the present invention, appropriate amounts of acid, e.g., H₂SO₄, are added to the solution 124 to increase the H⁺ level to maintain the pH constant.
In one preferred embodiment of the present invention, cadmium acetate and Na₂SeSO₃are provided in the solution. Both substances are found to stabilize the CdSe used as the anode 104 and prevent it from dissolving. In another embodiment, the solution is heated to a temperature of about 80-90° C. The use of higher temperatures in the solution accelerates the synthesis.
Connecting the cathode chamber 104 and anode chamber 106 is a channel 120. The channel 120 allows the solution 124 to flow between the cathode chamber 104 to the anode chamber 106. Because the cathodic reactions is:
H₂O+→e½H₂↑+OH⁻
the channel 120 allows hydrogen ions (H⁺) to flow towards the cathode 102 as the cathode uses up H in the solution.
Although the channel 120 can be any shape and any size that allows fluid to flow between the chambers, in a preferred embodiment, the distance between the electrodes 108 and 110 is made as small as possible to reduce IR losses between the electrodes.
Between the cathode and anode chambers is a membrane 122. The membrane 122 prevents sulfur from drifting over to the cathode 108 and “poisoning” the catalyst on the cathode 108 during operation of the device. In one embodiment, the membrane 122 is a proton passing, cation selective Nafion membrane (such as Dupont #1100).
In the illustrated embodiment, the chambers are bulbous, but in further embodiments of the container can be any shape that will hold the solution. Referring to FIG. 3, a container 302 does not have the connecting tube 120 of FIG. 1 and allows the electrodes to be moved closer together, thus reducing the IR drop between them. The container 302 of FIG. 3 holds a solution 304 and includes a cathode 306 and an anode 308. The cathode 306 and anode 308 are separated by a membrane 310. Above and below the membrane 310 is a non-permeable separator 312 that prevents the solution 304 from passing between the cathode and anode sides of the chamber 302. The membrane 310 and the non-permeable separator 312 divide the container 302 into two chambers 318 and 320. Other embodiments of the container are possible without departing from the spirit and scope of the present invention.
Referring again to FIG. 1, it can be seen that the electrodes 108 and 110 are then exposed to a light source 126. It has been proven that light reaching both electrodes does not simply double the energy available from use of a single electrode exposed to light, but instead cause the reaction rate to react to the formula e^βΔE/RT, where ΔE is the change in potential, R is a gas constant, T is the temperature, and 0<β<1. Thus, if ΔE is 2 ev, e^βΔE/RThas a value of at least 10 ¹⁰.
When significant photocurrent flows through the container 102, H₂bubbles leave the P-type cathode 108, float to the surface of the solution 124, and exit the chamber 104 through a tube 128 for collection. Under these conditions, photo-generated holes on the surface of the photo-anode 106 oxidize S²⁻, formed as a result of reaction of H₂S with OH⁻,
H₂S+2OH⁻→S²⁻+2H₂O
to sulfide according to the reactions:
S²⁻+2h ⁺→S⁰
S²⁻+S⁰→S₂ ²⁻
while the electrons flowing from the anode 110 and then through the external resistor 114 to the cathode 108 reduce H₂S to H₂, i.e.,
2H₂S+2e ⁻→H₂+2HS⁻
The resistor 114 represents normal resistive characteristics of a conductive path or can be an actual load placed between the electrodes. As the distance between the electrodes increases, so do the IR losses in the circuit. As stated above, to reduce IR losses between the anode 110 and cathode 108, the two electrodes should be physically located in as close proximity as possible, while also preventing S⁻⁻from reaching the cathode, i.e., separating with a membrane 122.
The current flowing through the external resistor 114 produces a voltage that, although small (about 0.5V), can be harnessed and utilized. An array of such devices can be added together to produce more significant quantities of power.
In one embodiment of the present invention, the light source 126 is a large array of solar collectors or mirrors reflecting the sunlight. However, other light sources can also be used without departing from the spirit and scope of the present invention. For instance, solar simulators are available in many laboratory-type environments. Solar simulators are lamps which have an internal inert atmosphere. The gases which constitute this atmosphere are chosen so that their spectrum is about 95% the same as that of the sun.
In an embodiment of the present invention, the light source used is passed through a beam splitter that separates a single light beam into at least two. One or more mirrors can be used to direct the beams exiting the splitter onto each electrode. In addition, the strength of the light source reaching the electrodes can be controlled to regulate the rate of hydrogen production.
In one embodiment of the present invention, the light source is passed through a filter that removes light in the infrared region. Infrared light, if not filtered, works to heat the electrodes. Infrared light is a relatively low frequency light wave. Conversely, it is the portion of the light spectrum having higher frequency that is desirable for use with the present invention.
It has been shown that in both the cathode 108 and anode 110, the application of the “spots” 204 and 212, which act as catalysts, improves the efficiency of the electrodes by as much as 10-50 times. In the absence of the catalyst spots, H⁺ ions in the solution are discharged onto the gallium phosphide, a semiconductor. The kinetics of such a reaction are poor and need an electric potential to cause the deposition of H⁺ and evolution of H₂to function at a practical rate. The presence of a metallic catalyst changes the situation greatly. The H atoms are deposited on the “spots” and no longer deposits on the gallium phosphide, which remains clean and acts as a collector of light and supplier of the photo-generated electrons necessary for transfer to H₂O in solution. It should be noted that in an alkaline solution, the photo-electrons emit to H₂O according the following equation.
H₂O+e→H+OH⁻
The hydrogen photoevolved during the cell operation is collected. The sulfide formed according to S²⁻+2h ⁺→S⁰dissolves in the anolyte until the solubility limit of the polysulfide is reached. After this point, the excess sulfur precipitates out of the solution and the sulfur can be collected from the bottom of the chamber, where it has formed as a crystalline yellow mass. The net reaction, as shown in the formulas above, is the decomposition of H₂S into H₂and S.
Referring to container 302 of FIG. 3, it can be seen that the cathodic electrode 306 evolves hydrogen (H₂) 316 that rises to the top of the solution 304 and is collected. Correspondingly, the anodic electrode 308 deposits sulfur (S) 314 on the bottom of the chamber 320. As described above, with regard to FIG. 1, before the process begins, both electrodes are properly illuminated by a light source (not shown in FIG. 4). With the configuration of FIG. 3, it is difficult for a single light source alone to properly illuminate both electrodes simultaneously. Therefore, as is known by those of skill in the art, light waves from a single light source can be divided and directed onto both electrodes simultaneously by utilizing an appropriately placed prism and reflectors or mirrors.
Referring now to FIG. 4, a flow chart of the process for evolving hydrogen and sulfur from hydrogen sulfide according to a preferred embodiment of the present invention is shown. The flow begins at step 400 and moves directly to step 402, where hydrogen sulfide is bubbled through a solution of NaOH. The solution is placed into a two chamber container in step 404. One chamber contains an anode and the second chamber contains a cathode. The anode and cathode are exposed to a light source in step 406. Hydrogen gas is then devolved and collected from the cathode-containing chamber and sulfur is deposited and collected from the anode-containing chamber, in step 408. The flow stops at step 410.
Although H₂S is not as readily available as H₂O, it is known that H₂S is available in abundant quantities throughout the world. For instance, H₂S can be found in locations such as under the Gulf of Mexico, in many gas wells, and other places. In fact, H₂S is considered a nuisance or pollutant in over 10,000 wells in North America, effectively prohibiting the extraction of natural gas and oil.
The price of hydrogen is often stated in dollars per gigajoule (a joule is 4.18×10⁷ergs.) The cost of producing hydrogen with prior-art methods is between $10 and $20 per gigajoule. Interestingly, with the present invention, the sulfur produced with a gigajoule of hydrogen would be more than 10 tons. Sulfur, used for many practical applications, such as road making, landfills, and others, is a commercially viable substance, with a value of around $60-$70 per ton. Therefore, each gigajoule of hydrogen produced directly leads to the production of approximately $400-600 worth of sulfur. As a result, the production of hydrogen with the present inventive apparatus and method becomes a cost-free byproduct of the production of sulfur!
Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments, and it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.
The terms “a” or “an”, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used. herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

Claims

1. An apparatus for producing sulfur, the apparatus comprising:

a container;

a solution including hydrogen sulfide, the solution provided within the container;

a photo-anode formed of an N-type semiconductor, being at least partially coated with a first catalyst material, and at least partially submerged in the solution; and

a photo-cathode formed of a P-type semiconductor, being at least partially coated with a second catalyst material, and at least partially submerged in the solution, the photo-cathode being electrically coupled to the photo-anode, the photo-cathode and photo-anode developing charges of opposite polarity when exposed to a light source so as to separate sulfur from hydrogen in the hydrogen sulfide of the solution and deposit sulfur within the container.

2. The apparatus according to claim 1, wherein the photo-anode comprises one of cadmium selenide, cadmium sulfide, and cadmium telluride.

3. The apparatus according to claim 1, wherein the photo-anode further comprises a stainless steel support.

4. The apparatus according to claim 1, wherein the first catalyst material comprises a plurality of ruthenium deposits on a surface of the photo-anode.

5. The apparatus according to claim 4, wherein the ruthenium deposits cover approximately 10 to 20 percent of the surface of the photo-anode.

6. The apparatus according to claim 4, wherein the ruthenium deposits are each approximately 0.2 mm in diameter.

7. The apparatus according to claim 1, wherein the photo-cathode comprises gallium phosphide.

8. The apparatus according to claim 1, wherein the second catalyst material comprises a plurality of platinum deposits on a surface of the photo-cathode.

9. The apparatus according to claim 8, wherein the platinum deposits cover approximately 10 to 20 percent of the surface of the photo-cathode.

10. The apparatus according to claim 8, wherein the platinum deposits are each approximately 0.2 mm in diameter.

11. The apparatus according to claim 1, further comprising a membrane separating the container into at least two chambers, the photo-anode being disposed in one of the chambers and the photo-cathode being disposed in another of the chambers.

12. The apparatus according to claim 1, wherein the solution further includes NaOH.

13. A method for producing hydrogen and sulfur, the method comprising the steps of:

providing a solution including at least hydrogen sulfide within a container;

providing an N-type photo-anode that is at least partially covered with a first catalyst material and at least partially submerged in the solution;

providing a P-type photo-cathode that is at least partially covered with a second catalyst material and at least partially submerged in the solution;

exposing the photo-anode and the photo-cathode to at least one light source; and

collecting sulfur from the container, the sulfur being the byproduct of a reaction between the solution and the photo-anode.

14. The method according to claim 13, further comprising the step of:

bubbling hydrogen sulfide through the solution before the exposing step.

15. The method according to claim 13, wherein the solution further includes NaOH.

16. The method according to claim 13, further comprising the step of providing a membrane that separates the container into two chambers, one of the chambers including the cathode and one of the chambers including the anode.

17. The method according to claim 13, further comprising the step of:

adding cadmium acetate and Na₂SeSO₃to the solution.

18. The method according to claim 13, further comprising the step of:

heating the solution to a temperature of between about 80°-90° C.