Thermochemical generation of hydrogen and carbon dioxide

Mixing of carbon in the form of high sulfur coal with sulfuric acid reduces the temperature of sulfuric acid decomposition from 830.degree. C. to between 300.degree. C. and 400.degree. C. The low temperature sulfuric acid decomposition is particularly useful in thermal chemical cycles for splitting water to produce hydrogen. Carbon dioxide is produced as a commercially desirable byproduct. Lowering of the temperature for the sulfuric acid decomposition or oxygen release step simplifies equipment requirements, lowers thermal energy input and reduces corrosion problems presented by sulfuric acid at conventional cracking temperatures. Use of high sulfur coal as the source of carbon for the sulfuric acid decomposition provides an environmentally safe and energy efficient utilization of this normally polluting fuel.

2. Technical Field 
The present invention relates to the thermochemical production of hydrogen 
and other useful gases such as carbon dioxide. More particularly, the 
present invention relates to water splitting thermochemical cycles based 
upon thermal cracking of sulfuric acid. 
3. Background Art 
Hydrogen has been identified as a flexible fuel form that: 
1. permits storage of energy over a wide range of time periods (from hours 
to years); 
2. enables efficient energy transport over long distances within available 
distribution networks; 
3. offers chemical and physical characteristics which propose several 
distinct applications: 
as a chemical feedstock, 
as a fuel for electrochemical fuel cell systems in decentralized power 
generating stations, and 
as a fuel supplement to natural gas. 
Most of the hydrogen is currently produced through steam reforming of 
natural gas. Natural gas is a depleting, finite resource as are all of the 
other fossil fuel sources and is becoming increasingly expensive. New 
hydrogen production technologies are in the process of development such as 
coal gasification, water electrolysis or thermochemical water splitting. 
While coal gasification appears to be the most likely alternative for 
large scale production of hydrogen, the other technologies appear more 
competitive for supplying small scale uses (i.e. less than 1 million cubic 
feet a day). 
Thermochemical processes are being explored since thermal energy available 
from thermonuclear reactors and/or from solar collectors can be fixed as 
hydrogen, a storable fuel. The decomposition of water by thermochemical 
means proceeds according to the reaction: 
EQU H.sub.2 O(1).fwdarw.H.sub.2 (g)+1/2O.sub.2 (g) (1) 
An analysis of the thermodynamics of the cycle requires that energy and 
entropy be supplied in the cycle. The main feature of this reaction is its 
highly endothermic nature requiring an input of 286,000 kj/kg-mol. 
Therefore, the reaction must be practiced as close as possible to ideal 
conditions in order to be practical. 
In a number of thermochemical cycles under active investigation, the oxygen 
release step is the thermal decomposition of sulfuric acid: 
EQU 2H.sub.2 SO.sub.4 (g).fwdarw.2SO.sub.2 (g)+O.sub.2 (g)+2 H.sub.2 O(g) (2) 
This reaction is highly endothermic requiring a temperature of between 
800.degree. C. and 900.degree. C. to thermally crack the sulfuric acid. 
The high temperature required to thermally crack sulfuric acid causes 
severe equipment problems in providing suitable materials to withstand the 
corrosive action of the superheated sulfuric acid and additionally, severe 
problems have been experienced in attempts to adequately heat the sulfuric 
acid to the high temperatures necessary for cracking. 
The thermochemical system disclosed in pending patent application entitled 
THERMOCHEMICAL GENERATION OF HYDROGEN AND CARBON DIOXIDE, (D. D. Lawson, 
et al--inventors, U.S. Ser. No. 145,207 filed on Apr. 30, 1980 now U.S. 
Pat. No. 4,314,984, issued Feb. 9, 1982) is an example of recent attempts 
to improve thermal efficiency in heating the sulfuric acid to cracking 
temperatures. The contents of this prior application are hereby 
incorporated by reference. This prior art thermochemical system includes a 
thermal source, such as a very high temperature nuclear reactor or solar 
concentrator, an oxygen splitting decomposition reactor, an 
electrochemical hydrogen reactor, a sulfuric acid preheater, and a 
decomposition gas heat exchanger and separator. The thermal source heats a 
heat exchange fluid such as helium to the high temperatures necessary for 
thermal cracking of sulfuric acid. The hot heat exchange fluid is passed 
through a coil to provide sufficient heat to decompose the sulfuric acid 
to sulfur dioxide, oxygen and water. The hot decomposition gases 
(SO.sub.2, O.sub.2, H.sub.2 O) are passed to the heat exchanger where heat 
is transferred to a special perfluorocarbon heat exchange liquid. The 
perfluorocarbon heat exchange liquid is then passed to a sulfuric acid 
preheater for directly heating the sulfuric acid to temperatures of 
300.degree. C. to 400.degree. C. prior to entry into the thermal 
decomposition reactor. 
After partial cooling in the heat exchanger, the oxygen is removed from the 
decomposition gases with the remaining sulfur dioxide and water being 
passed to an electrochemical hydrogen reactor where hydrogen and sulfuric 
acid are generated. 
The above-described prior art system provides an improved method for 
heating sulfuric acid to cracking temperatures and is well suited for its 
intended purpose; however, it is desirable to provide a thermal chemical 
cycle based on sulfuric acid decomposition which does not require high 
cracking temperatures and the resultant corrosion and heating problems. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a thermal chemical method and 
system is provided where decomposition of sulfuric acid is carried out at 
a much lower temperature than the thermal cracking temperatures of 
conventional processes. Specifically, the decomposition temperature is 
reduced from about 800.degree. C. to 900.degree. C. to about 300.degree. 
C. to 400.degree. C. The low temperature sulfuric acid decomposition of 
the present invention is particularly useful in thermo chemical cycles for 
splitting water to produce hydrogen. By lowering the temperature at which 
sulfuric acid decomposition takes place, equipment is simplified, energy 
input is lowered and corrosion problems are reduced. 
Further, the low temperature decomposition of sulfuric acid in accordance 
with the present invention produces high purity carbon dioxide along with 
sulfur dioxide and water. This highly pure carbon dioxide is a 
commercially useful gas which can be used in secondary oil recovery, 
carbonation of soft drinks, beer and charging of fire extinguishers. 
Additionally, the low temperature sulfuric acid decomposition of the 
present invention may be utilized in heat transfer systems based on 
catalytic reaction of sulfur dioxide, oxygen and water to form useful heat 
and sulfuric acid. 
The present invention is based on the discovery that carbon, in the form of 
carbonaceous material, when added to sulfuric acid, which has been heated 
to a temperature of between 300.degree. C. and 400.degree. C., reacts with 
the hot acid to produce sulfur dioxide, carbon dioxide and water as 
decomposition gases. Virtually any carbonaceous material may be used in 
accordance with the present invention. However, carbon containing 
materials which do not include hydrogen, such as graphite, will not react 
with the acid. Coals, peat, shale oil, rubber, biomass and many other 
carbonaceous materials may be used. The use of high sulfur coal, oil or 
residuals as a suitable carbonaceous material is particularly attractive 
since it provides a new use for high sulfur carbonaceous materials which 
are presently an environmental and economic liability. 
The decomposition gases produced in accordance with the present invention 
may be used in a number of different processes. For example, in addition 
to use of the SO.sub.2 and H.sub.2 O generated during decomposition for 
water splitting processes as cited above, the SO.sub.2 and H.sub.2 O may 
be transported to a catalytic reactor where oxygen is added in the 
presence of a suitable catalyst to produce heat and sulfuric acid. In 
addition, the carbon dioxide in the decomposition gases may be separated 
from the sulfur dioxide and water prior to or after transportation and 
utilized commercially as previously discussed. 
As contemplated by the present invention, the use of high sulfur coal is 
particularly desirable in the new thermochemical system. At present, high 
sulfur coal is economically and environmentally undesirable due to high 
sulfur dioxide emissions during burning. Since in the present invention, 
sulfur dioxide is a desirable decomposition gas, high sulfur content coal 
is an advantageous reactant. As will be realized, the present invention 
not only significantly reduces heating and corrosion problems inherent in 
high temperature sulfuric acid cracking, but at the same time provides a 
desirable, environmentally safe use for high sulfur coal. 
These and many other features and attendant advantages of the present 
invention well become apparent as the invention becomes better understood 
by reference to the following detailed description when considered in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The conventional thermochemical cycle for producing hydrogen is based on a 
series of reactions of at least two and generally three reactions to split 
water into hydrogen and oxygen. The generalized three step reaction based 
on sulfuric acid is as follows: 
EQU 2HX.fwdarw.H.sub.2 +X.sub.2 (3) 
EQU X.sub.2 +SO.sub.2 +2H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 +2HX (4) 
EQU H.sub.2 SO.sub.4 .fwdarw.H.sub.2 O+SO.sub.2 +1/2O.sub.2 (5) 
Examples of suitable cycles are the sulfur-iodine cycle where X is I, and 
the sulfur-bromine cycle where X is Br. Usually step 3 is driven 
electrochemically. The conventional hybrid electrochemical sulfur cycle is 
based on only two reactions: 
EQU 2H.sub.2 O+SO.sub.2 .fwdarw.H.sub.2 +H.sub.2 SO.sub.4 (6) 
EQU H.sub.2 SO.sub.4 830.degree. C. H.sub.2 O+SO.sub.2 +1/2O.sub.2 (7) 
Reaction (6) is driven electrochemically. 
In accordance with the present invention, carbonaceous material is added to 
the sulfuric acid in the cracking or oxygen release step (reactions 5 and 
7) according to the following equation 
EQU 2H.sub.2 SO.sub.4 +C 300.degree. C. 2SO.sub.2 +2H.sub.2 O+CO.sub.2 (8) 
The particular carbonaceous material which is added to the sulfuric acid is 
not important. Suitable carbonaceous materials include rubber, peat, shale 
oil, biomass and coal. The most important criteria is that the 
carbonaceous material include hydrogen bound in one form or another to the 
carbon. For example, carbon in the form of graphite has been found 
unsuitable for use in the present invention. The lack of reaction between 
graphite and hot sulfuric acid is believed due to the absence of hydrogen. 
The presence of other materials found in carbonaceous materials, such as 
oxygen, sulfur, and nitrogen, do not appear to promote or inhibit the 
reaction except for the dilution effects caused by their presence. The 
exact amount of hydrogen required to carry out the reaction is not known. 
It appears only that some hydrogen must be present in the carbonaceous 
material to carry out the reaction. 
Coals are preferred carbonaceous materials. A wide variety of coals ranging 
from sub-anthracite and anthracite through bituminous and sub bituminous 
to lignite may be used. 
The preferred coals are high sulfur coal. High sulfur coal is preferred not 
so much because it cracks the sulfuric acid better, but is preferred 
because use of high sulfur coal in this process provides a valuable use 
for this environmentally burdensome fuel. The high sulfur coal typically 
will have a sulfur content above 2 percent by weight. The sulfur in the 
coal or any other carbonaceous material is believed to be converted to 
sulfur dioxide which is a desired product of the reaction. 
The coal may be added to the sulfuric acid in any convenient manner. The 
size of the coal particles and method of mixing are not critical and only 
affect the speed of reaction. It is preferred that the coal be added to 
the hot sulfuric acid (330.degree. C.) as a slurry in room temperature or 
slightly warmed concentrated sulfuric acid. The particles should be 
between about 50 to 250 mesh. The amount of acid necessary to form the 
slurry may be varied depending on desired viscosity. 
The coal is added in sufficient amounts to hot sulfuric acid to decompose 
the entire amount of sulfuric acid with 1 gram of coal being sufficient to 
decompose about 15 grams of concentrated sulfuric acid. This ratio may 
vary depending upon contaminants, such as oxygen and other impurities in 
the coal. It is preferred that the sulfuric acid be concentrated, i.e. 98% 
of H.sub.2 SO.sub.4 by weight; however, less concentrated acid may be used 
in accordance with the present invention, but this results in slower 
reaction times and less efficient production of decomposition gases. 
Referring to FIG. 1, a suitable acid resistant thermal reactor 110 is 
seated in a heating mantle 112. The heating mantle generates sufficient 
heat to raise the temperature of the sulfuric acid 114 within the thermal 
reactor 110 to between 300.degree. C. and 400.degree. C. The concentrated 
sulfuric acid is introduced into the thermal reactor 110 through conduit 
116. Once the sulfuric acid has reached the desired temperature, high 
sulfur coal is introduced into the thermal reactor through line 118 in the 
form of the above identified slurry. Upon mixing, the sulfuric acid is 
decomposed while at the same time the coal is digested and gasified. The 
reaction which takes place as et forth in reaction (8) produces 
decomposition gases comprising sulfur dioxide, water and carbon dioxide. 
For tests on gram quantities of coal, the reaction proceeds rapidly with 
the bulk of the reaction being complete within a few minutes. The 
decomposition gases are passed out through conduit 120 for further use in 
a thermochemical system for generating hydrogen or other suitable use. 
Alternatively, heat for reaction can be generated by directly injecting 
oxygen or air into the reactor 110, producing heat by direct reaction of 
oxygen with coal. This procedure has the advantage of eliminating the need 
for indirect heat transfer, and increases the ratio of carbon dioxide to 
sulfur dioxide in the product gases. More coal is used, however, than when 
using nuclear, solar or other indirect heat source. A suitable oxygen 
inlet and valve 113 are provided for allowing controlled introduction of 
oxygen into the reaction 110 when this type of heating is desired. 
The decomposition gases produced in accordance with the present invention 
may be used in a number of different processes. For example, carbon 
dioxide may be separated out from the sulfur dioxide and water by well 
known separation techniques to provide a process for producing pure carbon 
dioxide. Also, the produced sulfur dioxide and water may be passed under 
pressure through pipes, as in a chemical heat pipe system, to a catalytic 
bed where the sulfur dioxide is reacted with oxygen to generate heat. 
However, the preferred use of the present invention is in water splitting 
thermochemical cycles where the oxygen release step involves cracking or 
decomposition of sulfuric acid. 
FIG. 2 is a schematic view of a system for a water splitting thermochemical 
cycle in accordance with the present invention. The system generally 
includes a thermal reactor 130, a heat exchanger 132, a separator 134 and 
an electrochemical hydrogen reactor 136. The sulfuric acid which is 
produced in electrochemical hydrogen generator 136 is passed through 
conduit 138 and into the heat exchanger 132. Heat exchange fluid is heated 
in heater 140 by heating mantle 142 or other suitable heating device. 
Preferably, the heat exchange fluid is a polyperfluoropropylene oxide as 
disclosed in copending patent application as set forth in the Background. 
The heat exchange liquid is heated to a temperature of between 300.degree. 
C. and 400.degree. C. and passed through transfer line 144 and into the 
heat exchanger 132 where it is directly mixed with the sulfuric acid. 
Since the heat exchange liquid is not miscible with sulfuric acid, it will 
rise to the top of the heat exchanger 132 and be removed through removal 
conduit 146. The heat exchange liquid removed in removal conduit 146 is 
then passed back to the heater 140 for reheating. 
The sulfuric acid which has been heated by direct mixing with the heat 
exchange fluid forms an immiscible phase on the bottom of heat exchanger 
132 and is removed through acid removal line 148. The heated sulfuric acid 
(300.degree. C. to 400.degree. C.) is then passed to thermal reactor 130. 
Heating of the sulfuric acid to reaction temperature as described above is 
preferred since it provides a particularly effective means for efficiently 
providing heat to the sulfuric acid. However, any other suitable means for 
heating the sulfuric acid to the desired temperature may be employed. For 
example, as shown in FIG. 2, a simple heating mantle placed about the 
thermal reactor 130 would provide a suitable means for heating the 
sulfuric acid to reaction temperature. 
High sulfur coal is fed through feed line 150 into the thermal reactor 130. 
The high sulfur coal reacts with the sulfuric acid to produce copious 
amounts of sulfur dioxide, water and carbon dioxide. These decomposition 
gases are removed from the thermal reactor 130 by way of gas removal line 
152. These relatively high temperature decomposition gases are passed to 
an annular chamber 154 surrounding the electrochemical reactor cell 156. 
Heat present in the decomposition gases is transferred to the 
electrochemical reactor cell 156 after which the decomposition gases are 
removed through line 158 and transferred to separator 134. In separator 
134, the carbon dioxide is separated by well known fractionation 
techniques from the sulfur dioxide and water and removed through carbon 
dioxide removal line 160. The carbon dioxide may then be used for any 
number of useful purposes such as carbonating soft drinks or in oil well 
operations. 
The decomposition gases remaining after carbon dioxide separation (sulfur 
dioxide and water) are then passed through feed conduits 162 and 164 
respectively to the electrochemical reactor cell 156. The electrochemical 
reactor cell 156 contains electrodes 166 and a semipermeable membrane 
separator 168. The sulfur dioxide and water are electro-chemically 
converted into hydrogen gas and sulfuric acid. The two products are 
separated with the hydrogen being removed through product removal line 170 
and the sulfuric acid being removed through sulfuric acid conduit 138 for 
recycling back to heat exchanger 132. 
The present invention therefore, provides a suitable thermochemical cycle 
in which equipment requirements are simplified since high acid cracking 
temperatures are not necessary, and problems of high temperature acid 
corrosion are minimized and in the process a relatively undesirable 
material, i.e. high sulfur coal is utilized in a productive and 
environmentally safe manner. 
Having thus described exemplary embodiments of the present invention, it 
should be noted by those skilled in the art that the within disclosures 
are exemplary only and that various other alternatives, adaptations and 
modifications may be made within the scope of the present invention. For 
example, at secondary oil recovery sites, a portion of the recovered heavy 
oil can be utilized in accordance with the present invention to produce 
carbon dioxide which is pumped down the well during recovery operations. 
Accordingly, the present invention is not limited to the specific 
embodiments as illustrated herein.