Gas: heating and illuminating – Methane -containing product – or treatment or recovery process – Process including chemical reaction
Reexamination Certificate
2002-02-28
2003-12-16
Langel, Wayne A. (Department: 1754)
Gas: heating and illuminating
Methane -containing product, or treatment or recovery process
Process including chemical reaction
C048S210000, C423S359000, C423S657000, C423S658000
Reexamination Certificate
active
06663681
ABSTRACT:
STATEMENT REGARDING FEDERALLY FUNDED RESEARCH
Not applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the production of hydrogen gas. More particularly, the present invention is directed to a method for the production of hydrogen gas by steam reduction wherein steam is contacted with molten metal to form a metal oxide and a hydrogen-containing gas stream. The metal oxide can then be reduced back to the metal for further production of hydrogen gas. The hydrogen gas can be used for the generation of energy and in various chemical processes, such as the treatment of coal and the production of ammonia.
2. Description of Related Art
Hydrogen (H
2
) is a valuable commodity and specialty chemical that is critical to a number of industrial processes including the production of ammonia and the refining of oil. In addition, hydrogen can be converted directly to electricity in fuel cells at efficiencies approaching 80 percent. Water is the sole by-product of hydrogen conversion in fuel cells, and toxic emissions are eliminated. For these reasons, hydrogen is widely considered the fuel of the future.
It is known that hydrogen gas can be produced from many different feedstocks such as natural gas, biomass or water using different techniques such as reformation, gasification or electrolysis. The most common methods are steam methane reformation (SMR), coal gasification, steam reduction, biomass gasification and pyrolysis, and electrolysis.
Steam methane reformation is believed to be the most economical and commercially viable process that is presently available. In the SMR process, methane (CH
4
) is reacted with steam (H
2
O) to form a gas stream that includes hydrogen and carbon monoxide (CO). The feedstock is typically natural gas and the cost of the natural gas represents a significant portion of the total production cost.
At least two major difficulties are associated with the SMR method. One difficulty is the dependency of the hydrogen production cost on the price of natural gas. The price of natural gas is highly volatile due to supply/demand issues, which are projected to persist into the future. Secondly, hydrogen produced by SMR is co-mingled with a significant quantity of carbon oxides that can only be partially removed by scrubbing or pressure swing adsorption, both of which are costly. The carbon oxides remaining in SMR hydrogen are detrimental to the catalysts employed in fuel cells and in the production of ammonia (NH
3
) from hydrogen.
Hydrogen production by coal gasification is another established commercial technology, but is only economically competitive when natural gas is prohibitively expensive. In the coal gasification process, steam and oxygen (O
2
) are utilized in the coal gasifier to produce a hydrogen-containing gas. High purity hydrogen can then be extracted from the synthesis gas by a water-gas shift reaction followed by removal of the carbon dioxide (CO
2
) by pressure swing adsorption or scrubbing. Impurities such as acid gases must also be separated from the hydrogen. Hydrogen can also be formed by the gasification of other hydrocarbons such as residual oil.
The steam reduction method utilizes the oxidation of a metal to strip oxygen from steam (i.e., steam reduction), thereby forming hydrogen gas. This reaction is illustrated by Equation 1.
x
Me+
y
H
2
O→Me
x
O
y
+y
H
2
(1)
To complete the cycle in a two-step steam reduction process, the metal oxide must be reduced back to the metal using a reductant. For example, carbon monoxide (CO) has an oxygen affinity that is similar to the oxygen affinity of hydrogen and they are equal at about 812° C. At temperatures above about 812° C., CO has a greater affinity for oxygen than does hydrogen. Thus, if the CO has an oxygen affinity greater than the oxygen affinity of the metal at equilibrium, the CO will reduce the oxide of Equation 1 back to the metal.
Me
x
O
y
+y
CO→
x
Me+
y
CO
2
(2)
Generally stated, the function of the metal/metal oxide couple is to transfer oxygen from the steam to the reducing gas (CO) without allowing the steam/hydrogen of the hydrogen production step to contact the carbon monoxide/carbon dioxide of the metal oxide reduction step. The metal and metal oxide are not consumed by the overall process.
Oxygen partial pressure (pO
2
) relates to the facility with which the metal may be oxidized (e.g., by steam) and the oxide may be reduced (e.g., by CO). A related mathematical expression is pH
2
O/pH
2
, which is proportional to the oxygen partial pressure. Also, an equivalent and inversely related quantity is the hydrogen fraction, expressed as:
pH
2
(
pH
2
+
pH
2
O
)
(
3
)
Certain metals react strongly with water, releasing hydrogen. Examples of such metals include: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), silicon (Si), phosphorus, (P), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), yttrium (Y), zirconium (Zr), niobium (Nb), lanthanum (La), hafnium (Hf), tantalum (Ta) and gallium (Ga). The oxygen partial pressure in equilibrium with these metals and their oxides together is extremely low. Once the oxides are formed, they cannot be effectively reduced back to the metal by carbon monoxide. Conversely, there is another group of metals that produce insignificant quantities of hydrogen when reacted with water. Examples of these metals include: nickel (Ni), copper (Cu), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au), mercury, (Hg), lead (Pb), bismuth (Bi), selenium (Se) and tellurium (Te). The oxygen partial pressure in equilibrium with these metals and their oxides together is quite high. The oxides, therefore, can be easily reduced by carbon monoxide.
Between the two foregoing groups of metals are other metals characterized by an oxygen affinity that is roughly the same as the oxygen affinity of hydrogen. Included in this intermediate group of metals are: germanium (Ge), iron (Fe), zinc (Zn), tungsten (W), molybdenum (Mo), indium (In), tin (Sn), cobalt (Co) and antimony (Sb). These are elements that readily produce hydrogen from steam wherein the resulting oxide can be reduced by carbon monoxide. That is, these metals have an oxygen affinity such that their equilibrium pH
2
O/pH
2
is low enough to be practical for the production of hydrogen, yet the metal oxide is readily reduced by carbon at normal pyrometallurgical temperatures (e.g., about 1200° C.). These metals are referred to herein as reactive metals, meaning that both the metal can be oxidized by steam and the metal oxide can be reduced by carbon monoxide.
The steam reduction/iron oxidation process was the primary industrial method for manufacturing hydrogen during the 19th and early 20th centuries. At elevated temperatures, iron strips oxygen from water, leaving pure hydrogen.
3Fe+4H
2
O→Fe
3
O
4
+4H
2
(4)
Excess water is required to maximize hydrogen production from a given amount of iron. After the hydrogen is produced, excess water is condensed leaving an uncontaminated hydrogen gas steam. The reaction products of the steam reduction/iron oxidation process are pure hydrogen and wustite (FeO) and/or magnetite (Fe
3
O
4
). To regenerate the metal, carbon monoxide or carbon captures the oxygen from the iron oxide, forming iron metal and carbon monoxide or carbon dioxide (CO
2
).
In the early years of hydrogen manufacturing, these two steps were carried out at different locations. The primary cost components for producing hydrogen by this method included the cost of the iron used minus the value received for the iron oxide produced and the cost of producing the excess steam required to drive the reaction (a function of temperature). This cost was reduced by the benefits derived from recovering excess energy from the steam. There are numerous exam
Davis Boyd R.
Kindig James Kelly
Odle Robert R.
Weyand Thomas E.
Alchemix Corporation
Langel Wayne A.
Marsh & Fischmann & Breyfogle LLP
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