Note: Descriptions are shown in the official language in which they were submitted.
1~5~01~
It is well-~nown that vario-~s simple carbides can be used to produce
fuel gases such as acetylene, metha-le, allylene, etc., and that calcium
carbide manufacture and conversion to acetylene has provided the basis for a
major industry for many years. Nevertheless9 carbide-acetylene has never had
a measurable impace on the production of general purpose fuels. Rather, its
applications have been restricted to providing high energy fuel for the
welding gas industry plus major non-fuel or chemical applications, for example,
as a chemical intermediate. Several factors have precluded more widespread
use of calcium carbide derived acetylene for general fuel purposes; among
these are: ~1) High input power requirements resulting from the necessity of
employing electric arc furnaces to reach the temperatures needed to manufacture
calcium carbide (approximately 2000~C); (2) The hazards of handling the
derived acetylene, either under pressure or in mixtures with air, due to its
endothermic nature; (3) The magnitude of the disposal problem for the spent
lime that would arise if large amounts of carbide acetylene were used for
general fuel purposes; (4) Transportation costs of raw materials and calcium
carbide occasioned by high total tonnages per unit fuel value; and (5) Total
system costs in comparison with alternative fuel systems.
While the principal volatile products of conversion of carbide "fuel
precursors" are hydrocarbons, they may be accompanied by various lesser
amounts of hydrogen, carbon monoxide, carbon dioxide and compounds containing
carbon, hydrogen and oxygen (and/or additional elements). In the succeeding
discussion, the term "fuel" shall refer to all such volatiles, and
"fuel precursor" shall designate a ccmpound capable of generating all such
volatiles where used for fuel or non-fuel purposes.
It is therefore a primary o~ject of the present invention to provide
a process for the economical production of fuel precursors and for the conver-
sion of the fuel precursors to fuel gases or liquids for various fuel
or non-fuel uses.
--1--
ll~8~1~
:rn one a~pect of tile invcnt:ion Lhe above and
other ob jects are ac~lieve~ ~y a process ~or prod~lc:lng fuel
precursor~ comprlsing mi~ecl metal c~rbi~e material, the
process comprising the st~ps: (a) carboni~ing at a pyro-
lyzing temperature a raw carbonaceoug material to remove
volatile matter therefrom and form carbon as coke or char;
tb) mixing with the carbon a starting material selected
from the group consisting of metal oxides, metal hydro-
xides, and mixtures thereof, the carbon being present in
an amount sufficient to reduce the starting material to
free metal and saturate the metal with carbon, such re-
duction proceeding by application of heat and removal of
carbon oxides generated therein at an operating tempera-
ture sufficient to form from the metal and carbon a carbon-
saturated molten metal; (c) mixing the carbon-saturated
molten metal with excess of the coke or char in a finely
divided form while main~aining such mi.xture at a tempera-
ture above the freezing polnt of the molten metal for a
time sufficient to convert a major fraction of the mixture
to a metal carbide material and leave a minor fraction
of unconverted molten metal-rich solution and unconverted
carbon; (d) removing and lowering the temperature of the
carbide material, molten metal-rich solution and unconver-
ted carbon in fine particulate form to give a product at
which lower temperature the re.maining molten material has
become so viscous that migration of carbon or carbide
particles is inhibited and at least a portion of the
metal carbide material is capable of being formed into a
- metal carbide of higher carbon content, holding the pro-
duct at the lower temperature for further conversion of
the unconverted molten rich solution and unconverted car-
bon to a metal carbide, and for further conversion of the
~ .
- 2 -
mab/ ~
.
ll~BOl~
portion of metal carbide material formed in step (c) to
a metal carbide comprising carbides of higher carbon con-
tent than the metal carbide material formed in step (c);
(e) dividing and consolidating the product after the
further conversion of step (d) into agglomerates for
storage or handling; the agglomerates consisting of metal
carbides and a substance selected from the group consis-
ting of interstitial metal-rich phases, free carbon, and
mixtures thereof; and (f) cooling the agglomerates suf-
ficiently to permit storage or shipment as fuel precur-
sors.
The invention will be better understood with
reference to the following detailed discussion including
the accompanying drawings in which:
- 2a -
mab/('~
O 1 6
Figu~e 1 is a schemat;c representat;on of a process for producing
fuel precursors (svlid).
Figure 2 is a schematic representation of a process for converting
the fuel precursors into useful fuel gases or liquids.
~ recent review of metal carbides by Frad reports that of the 75
metallic and semi-metallic elements of atomic number 92 or lower, at least 48
form binary compounds with carbon (i.e. carbides) with another 7 elements
havir.g been reported as having formed carbides but whose existence requires
further confirmation. Hundreds of carbides have been investigated in which
two or more metallic elements are combined with carbon, both as true compounds
with more or less fixed ratios of metallic elements, and as solid solutions
between simple binary carbides. A problem in characterizing such complex
systems, as well as binary carbides, is the tendency toward defect structures
in which significant fractions of the lattice sites normally occupied by metal
or carbon atoms may be vacant.
It is desirable to classify carbides in regard to chemical and
physical properties or thermodynamic stabilities to identify those compounds
useful as fuel precursors, refractories, abrasives or for cutting tool applica-
tions, ~nd to indicate possible methods of preparation.
The classification of carbides into "salt-like" compounds and "metal-
like" compounds is most useful in describing their general properties. Salt-
like or ionic carbides are electrical insulators with thermophysical properties
similar to oxides and usually showing little tendency toward defect structures.
They are generally reactive toward water or dilute acids, are reducing agents
at elevated temperatures and tend to dissolve in fused salt systems. These
carbides are usually formed from the more basic metals such as alkali, alkaline
earth, and aluminum family elements. On the other hand, metal-like carbides
are electrical conductors with thermophysical properties similar to metals and
usually showing an appreciable tendency toward defect structures. Most are
L :1 5 ~0 1 6
substantially unreactive toward water or dilute acids at normal temperatures,
do not behave as reducing agents except at extremely high temperatures and are
generally chemically inert. Many are also extremely hard materials with high
melting points. These metal-like carbides are usually formed from less basic
elements such as transition metals or semi-metals; however, they also include
lanthanide or actinide element carbides. Some exceptions to the above general
trends may be noted in, for example, a number of carbides of the rare earth
elements, thorium and uranium. Such carbides behave principally like ionic
carbides but are electrical conductors, apparently due to lower than normal
chemical valence and close metal-metal atomic distances in the crystal struc-
ture. In addition, some metallic carbides of first transition series elements
such as vanadium, chromium, manganese, iron, cobalt and nickel behave in most
characteristics like metallic or interstitial carbides, but ~re more reactive
or corrodible by water or dilute acids, apparently resulting from atomic size
defects in the crystal structure.
A classification of carbides on the basis of thermodynamic stability
is useful in indicating possible methods of preparation, precautions in
storage and handling and reactivity at various temperatures. It is convenient
to use five categories which may be designated as highly stable, stable,
metastable, marginally unstable, and highly unstable.
The highly stable carbides are limited to interstitial carbides of
the transition elements which have negative heats of formation in excess of
-(25)Kcal/moleC and which are extremely hard, refractory, and chemically
inert. Examples include TiC, ZrC, TaC, NbC, and the like. Tungsten carbides,
boron carbide and silicon carbide have lower heats of formation but have
similar properties and should probably be included in this group.
The stable carbides include those carbides with negative free
energies of formation at all temperatures from room temperature to 1200C or
more except those carbides included in the first group. In this category are
the carbides of the alkaline earth metals (except magnesium), aluminum,
--4--
l l~B0~6
certain higher carbides of c~lrom;um, manganese and the rare earth metals.
Metastable carbîdes are those compounds which are thermodynamically
stable with respect to free metal and carbon at some elevated temperature but,
even though unstable at ambient temperatures, can be quenched and preserved
indefinitely at low or ambient temperatures. Examples include Mn3C, Mnl5C4,
UC2, MoC, etc.
The msrginally unstable carbides include carbides of iron, cobalt
and nickel which are slightly less stable than the corresponding metal and
graphite but which are also storable for indefinite periods at room temperature.
The last class, highly unstable carbides, consists of the carbides
that cannot be formed from the elements, but may only be prepared by lower
temperature indirect processes from carbon compounds. These include the
carbides of magnesium, zinc, copper, silver, etc. If their heats of formation
are too positive, they may be subject to explosive decomposition.
For the purpose of evaluating carbides of potential utility in the
generation of fuel gases or liquids, it is necessary to understand the chemical
reactions of the various carbides, especially toward water or steam. Unfor-
tunately, a large number of carbides which have previously been reported have
e;ther not been studied for hydrolyt;c behavior or such studies as have been
made are unreliable due to poorly characterized starting materials or imprecise
methods of analysis. This is evident from conflicting data from several
investigators.
Based on the first classification system (properties) discussed, one
may summarize hydrolysis studies as follows:
1. The salt-like or ionic carbides normally hydrolyze to yield a
single hydrocarbon characteristic of the carbide and on that basis may be
classified as acetylides, methanides or allylides corresponding to acetylene,
methane or allylene as the hydrolytic reaction product.
2. The metal-like carbides are for the most part substantially
chemically inert at low temperatures, but some may be hydrolyzed (corroded),
~ ~5~0 16
yielding a mixture of hydrocarbons and various amounts of hydrogen~ oxides of
carbon, etc., depending on the conditions of hydrolysis.
3. In the metal-like carbides which are hydroly~.able, the excess
metallic atoms are capable of reducing the water to form neutral hydrogen
atoms which can then directly reduce carbon atoms or react with unsaturated
hydrocarbons or chemical intermediates such as methylene groups (CH2). The
hydrolysis reactions of these compounds are consistent with treating them as
solid solutions of ionic carbides in excess free metal or alloys of metal and
hydrolyzable carbide.
Most studies of hydrolytic behavior have been carried out in aqueous
solution at moderate temperatures, but various studies with steam at elevated
temperatures have been made. It has long been known in the prior art that
evcn very inert carbides such as SiC are attacked slowly by prolonged exposure
to steam at about 2000~F. It appears that carbides, especially the metal-like
compounds, behave in many regards like the parent metal forming a superficial
layer of oxide or hydroxide in the presence of water vapor and that further
attack is inhibited unless some mechanism is present to accelerate diffusion
or migration of additional water molecules through the surface layers.
Composition of Fuel Products
Of the ionic carbides, it is well-known from previuus work that the
hydrolysis product of carbides formed from metallic elements of small ionic
radii such as aluminum and beryllium consists primarily of methane and the
product formed from the lower carbide of magnesium, with Mg2C3 forming
allylene (methylacetylene). Most of the remaining ionic carbides yield
acetylene upon hydrolysis.
For the metallic carbides, we may treat the mixture of carbide,
excess metal and/or free carbon as a "pseudocompound" of composition MzC,
where the metal is represented by the symbol M. A fraction x of the carbon
~toms yields hydrocarbons on hydrolysis, with a fraction l-x appearing as free
carbon. A nominal negative valence, v, is assigned to the x fraction of
~5~016
I
carbon ~corresponding to V =-v hydrogen atorns combined per carbon atom). Then
-V represents the valence of the carbon atom. Thus V = 4 for pure methane
formers, V = 3 for ethane formers, V = 2 for methylene or ethylene formers and
V = 1 for acetylene formers. We have found that the experimental results for
transition metal carbides in the Mn3C, Fe3C, Ni3C series can be accom-
modated with a carbon valence of about v= -2.8 to -3.4. The rare earth
dicarbides and sesguicarbides can be considered to have a carbon valence of
about v= -1. The carbon content then contains Vx equivalents which are
balanced by an equal number of metal ionic equivalents. The balance of the
metal atoms can be considered as neutral metal.
The total number of metallic equivalents after hydrolysis is given
by Z V where V is the average valence of lowest metallic states of con-
stituent elements stable in the presence of water. The difference ZV-Vx
represents the excess reducing power which appears as hydrogen in reaction
products. We may thus write the over-all reaction as:
MzC ZV2H20 ( ) V ~ 2 2 V/
The approximate value of fuel products heat of combustion per mole C
is given by:
~HcoM = x[136+19(V)] + (ZV-Vx) 68.3 Kcal/mole C
115BQ16
Using the above formula ~e may determine the fuel gas heat of
combustion for the following carbides:
For x=l,
AH (Kcal/mole C) ~H
Compound COM _ COM
M3C~V = 2, V = 3) x(193) + (6-3x~ 68.3 295.5
7 3 x(193) + (14-9x 68.3 249.9
M3C2 " " x(193) + (3-3x) 68.3 193
M C2 (V = 3, V = 1) x(155) + (3-2x) 68,3 172.1
M2C3 " " x(155) + (2-x) 68.3 189.2
(V = 3, V = 2) x(174) ~ x) 68.3 174
M~C2 (V = 2.5, V = 1) x(155) + (5-4x) 68.3 163.5
M2C3 " " x(155) + (5-3x) 68.3 177.8
The specific heating value of the fuel gas depends on the distribu-
tion of carbon atoms between methane and higher hydrocarbons with 2, 3 or
more carbon atoms per molecule.
--8--
L ~ S ~0 1 ~
Thus for V - 3, CH3 may be derived from C113 = 2CH4 + 4C211
(m = 1.33 ) or CH3 = 2C2H6 (m - 2), where m is the average number
of carbon atoms per hydrocarbon molecule. The heat of combustion per mole
of carbon is nearly the same for both systems, but the heat of combustion per
unit volume (gas mole) is 50% greater in the second case. As an examp]e, for
Mn3C the average number of carbon atoms per hydrocarbon molecule obtained on
hydrolysis equals 1.45.
Energy Balance for Synthetic Fuels
All of the existing or proposed processes for producing hydrocarbons
and/or carbon monoxide and hydrogen from carbonaceous sources may be
considered as a sum of individual chemical reactions in which all reactants
other than carbon and oxygen enter in a cyclic manner, emerging in the same
compounds as they enter. For a given quantity of carbon, the potential
heating value upon complete combustion is equal to 94.05 Kcal/gram mole
(169.29 KBTU/lb mole) which may be taken as the theoretical input energy
unless other forms of energy are also consumed. For every mole of carbon
consumed a fraction f will be recovered as useful fuel gases and the fraction
1 - f will be burned or otherwise lost to supply process heat. The fraction f
may be divided into two parts f = f + f where f is the fraction
' hc o hc
appearing in hydrocarbons and fO is the fraction appearing as carbon monoxide.
The useful output heat is then given by:
AH = f QHh + f-QH + f QH
out hc c o co H2 H2
where QH is the average heat of combustion of ~ per mole (with hydrocarbons
C H rewritten as CH ) and f is the fractional number of moles of hydro-
m n n/m H2
gen produced per mole of carbon consumed.
In the formation of fuel gas directly from carbon, the water gas (or
synthesis gas) reaction is of importance:
l~5~16
C + H 0 = C0 ~ H ~H = +41.9 Kcal/mole C.
Since this reaction is endothermic, the heat necessary is usually
supplied by burning extra carbon. By noting that the heat of combustion of
carbon to carbon dioxide is 94.05 Kcal/mole, iL may be readily calculated that
.445 additional moles of carbon must be burned to provide the theoretical
amount of heat necessary to produce one mole of carbon monoxide by the above
equation. Restated in terms of the previous discussion, the fraction f = .692
of the original carbon is converted to C0 while the fraction 1 - f = .308 is
burned to C02 to supply process heat. The output heat ~H t = .692(67.65) +
.692(68.3) = 94.05 Rcal which is just equal to the theoretical input energy.
All real processes will operate at less than 100~ thermal efficiency due to
thermal, frictional and other losses.
For processes that use metallic carbides as chemical intermediates,
two other thermal figures of merit are useful, namely, net and gross heat
ratios, where the net heat ratio is the ratio of heat of combustion of fuel
gases produced to the heat of combustion of the carbide, and the gross heat
ratio is the ratio of the sum of the heat of fuel gas combustion plus heat of
conversion (hydrolysis) to the heat of combustion of the carbide. In these
calculations, the oxidation of the metallic component is considered to be
carried to the valence state normally found after hydrolysis. Table I shows
heats of combustion and net heat ratios for various carbides.
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l:~$801~
TABLL I
Thermochemistry of Carbides
Net
Heat of Heat of Fuel Gas Heat
Compound Formation Combustion Heat of Combustion Ratio
CaC2 -15 325 310.61 .956
A14C3 -48.6 1031.55 638 .618
Mn3C 3 367.05 333 .907
eC2 5 316.5 310.6 .981
Be2C -22.2 363.85 212.8 .585
MgC2 ~ +21 352.9 310.6 .88
g2c3 ~l9 588.75 463.1 .787
Fe3C +5 290.15 269 .927
2 435.1
UC2 -42 416.1 360 .865
LaC2 (-30 est) (372 est) (323-355 est)
La2C3 (-50 est) (660 est) (505-568 est)
CeC2 (-30 est) (375.5 est) 355
Ce2C3 (-50 est) (667 est) (505-568 est)
In Table I heats of formation or combustion are expressed in kilo-
calories per mole.
The net heat ratios are generally above .8-.9, except for the ionic
carbides which yield methane on hydrolysis. Thus, A14C3 has a net heat ratio
of .618 while Be2C has a value of .585, which are both too low for efficient
synthetic fuel processes.
If we wish to formulate a carbide fuel process which is economical
and produces a fuel with lower handling hazards than acetylene, we may elimi-
nate from consideration all the simple ionic carbide The remaining carbides
are members of the alloy or metal-like carbide class. There are two principal
types of metal-like carbides which may be considered as candidates for a
synthetic fuel process: (1) Interstitial alloy carbides such as ~n3C,
--11--
1 158~16
Fe3C and related carbides of higher carbon content; (2) Rare earth or acti-
nide carbides such as cerium, thorium, or uranium carbides.
The first class (interstitial alloy carbides) contains metals whose
oxides are easily reducible, but their heating values per unit weight are low
and their reactivity toward water or steam is low in some cases. In addition,
they are difficult to prepare free of excess metal, which usually requires an
acidic medium to effect hydrolysis.
The second class (rare earth or actinide carbides) is more reactive,
but the metallic elements are difficult to reduce.
We have discovered that by modifying the composition of each class
we can achieve practical advantages in realizing a practical synthetic fuel
process.
It has been previously established that Fe3C and Mn3C form a
continuous series of solid solutions. As the iron conten~ increases5 the
energy efficiency of the fuel process increases, but the reactivity toward
water or steam is lowered. To hydrolyze pure iron carbide requires either
dilute acids or very high temperature steam. We have found that by incorporat-
ing small amounts of a reactive or corrodible metal such as calcium, magnesium,
zinc, and/or aluminum to the alloy which is reacted with carbon to form the
carbide, a limited amount of the reactive metal is incorporated in the carbide
solid solution and an additional amount remains in the alloy phase. Alterna-
tively, ternary phases such as Al Fe3C may be formed with the various mixed
carbides.
In the usual methods of forming the transition metal carbides from
molten metal and carbon, it is often difficult to prepare the carbides com-
pletely free from an excess metal-rich phase. This phase can be more resistant
to action of water or steam. By incorporating sufficient reactive metal (Ca,
Mg, Al, Zn) to reach a level of 2 to 30 atom percent of metal in the alloy
phase, the corrodibility is enhanced allowing easier hydrolysis. It also
tends to lower the melting point of the alloy, permitting synthesis at a lower
-12-
l 15~0~
temperature.
Slightly higher energy values per pound of fuel precursor can be
achieved by hydrolyzing carbides of higher carbon content, such as Mn7C3,
Cr7C3, or Cr3C2. These carbides, especially the chromium compounds,are
quite resistant to hydrolysis. We have found that by alloying again with
reactive metals such as Mg, Al, Zn, or Ca, we can produce a more easily
corrodible carbide.
While chromium or vanadium do not form trichromium or trivanadium
carbides, compounds may be formed by substituting one aluminum atom, as
Cr2AlC or V2AlC which have a high potential energy value per pound. These
compounds can also form solid solutions with the (Fe, Mn)3C system.
Among the Rare Earth carbides, cerium or lanthanum dicarbides or
sesquicarbides REC2 or RE2C3 have the best potential as a fuel precursor of
previously reported carbides, where RE represents a rare earth metal. Similar
compounds formed from Misch-metal or unseparated Rare Earth metals and carbon
have a lower system cost for fuel generation.
We have discovered that by alloying the Rare Earth metal with bi- or
trivalent metals of more easily reducible metals, especially of large ionic
radius, the energy of formation may be lowered. On size grounds, calcium,
stront;um, barium, bismuth, lead, and tin are candidates, but the alkaline
earth metals do not offer appreciable savings in energy. We have found that
the solubility of smaller ions such as zinc or iron or manganese can be
enhanced by co-dissolving a larger than normal ion such as barium.
While the substitution of divalent metal ions for Rare Earth ions
can lower the molecular weight and reduction energy of the fuel precursor,
it also lowers the fuel value of gases since the reducing capacity is lower
for divalent than trivalent metals.
l 6
Preparation t
It is well-known in the art that the following carbides May be
prepared by one or more of the following procedures:
1. Direct combination of metallic element(s) and carbon:
A. Solid state diffusion
B. Melting + congruent solidification
C. Peritectic freezing
D. Eutectic freezing
E. Eutectoid or peritectoid decomposition.
10 2. Reduction of oxide with excess carbon:
A. Solid state
B. From melt.
3. Reduction of sulfides with added carbon and optionally hydrogen.
4. Reduction of chlorides with added carbon and optionally hydrogen.
5. ~fetathesis with other carbides.
Of thesé methods only the first and second are applicable to a
cyclic process for fuel generation. Calcium carbide is made commercial-
ly by reduction of an oxide with excess carbon from a melt (method 2B), but
this requires exceptionally high temperatures and a massive input of heat,
~0 usually furnished by electric sources.
In contrast, the processes in the first group are exothermic, supply-
ing their own reaction heat, but require prior reduction of metal oxides to
metals. The most applicable method within the first group is governed by the
phase diagram of the system involved; however, methods based on solid state
transformations (lA and lE) are generally too slow to be useful for large
scale synthesis.
Most systems of interest cannot be readily produced by method lB,
congruent solidification, so we are generally left with methods lC or lD.
~lethod lC could yield pure carbide in principle, but for practical
30 configurations, the desired product will be mixed with a metal rich phase. Il
-14-
l ~5~016
This condition will also be found for method lD.
We have found that for most of the modified compositions discussed
earlier, a peritectic freezing process is the most favorable method of synthe-
sizing the desired carbide.
For the modified (Fe, Mn)C system, the following constraints are
present:
1. Pure Mn3C can only be made by solid state reaction since it is
unstable above 1050C.
2 Pure Fe3C is marginally unstable but may be produced by peritectic
freezing at temperatures above 1050C.
3. Fe-Mn alloys up to about 80% Mn may form carbides by peritectic
freezing at temperatures which dropjas the Mn contert increases.
I have discovered that a molten (Fe-Mn) alloy with dissolved carbon
may be modified by additions of metallic calcium, aluminum, magnesium or zinc
with the following results:
1. Melting point is lowered.
2. Peritectic freezing yields a trimetal carbide M3C whose metallic
atoms consist predominantly of Fe and Mn, but wlth lesser amounts
of low melting point metals.
3. In partial freezing, a slushy mixture of M3C and liquid metal
results which upon further cooling yields a multiphase solid mixture
which may be readily hydrolyzed with water or steam to yield a mix-
ture of hydrogen and hydrocarbons.
It will be understood by those familiar with the art that some
departure from specified metal/carbon ratios may commonly occur in these alloy
carbide systems without changing the general behavior or advantageous proper-
ties of such systems.
One may form higher carbides within this system by solid state
reaction of trimetal carbides with excess carbon to yield carbides of nominal
composition M7C3 (also M5C2). These may conveniently be formed by
partial peritectic freezing of M3C with equilibrium metal rich phases, then
adding additional powered carbon and completing solidification followed by
aging or curing at temperatures of 400-700C.
-15-
I ~ ~80 16
A similar procedure may be used to form M3C2 carbides with high
chromium content.
For the modified RE2C3 system (where R~ represents a rare earth
element), the following constraints are present:
1. Pure La2C3 or Ce2C3 may be produced by peritectic freezing at
temperatures above 800C.
2. Misch-metal alloy with dissolved carbon can yield RE2C3 by peri-
tectic freezing above 750~C.
I have discovered that molten lanthanum, cerium or misch-metal with
dissolved carbon ~odified by additions of barium, zinc, lead, and/or bismuth
give an alloy with the following results:
1. Melting point is lowered.
2 Peritectic freezing yields a sesquicarbide containing appreciable
quantities of non-lanthanide metal atoms.
3. The carbides formed upon partial or total freezing plus the inter-
stitial metal rich phase retain the reactivity toward water or
steam shown by the pure lanthanide carbides or metals.
4. The energy of reduction of the metallic alloy is lower than for
the pure lanthanide system equivalent to a given amount of carbon.
The higher rare earth carbides REC2 may be formed congruently from
the melt at extremely high temperatures, peritectically at intermediate
temperature or by solid state transformation.
I have found that the dicarbides REC2 may be conveniently prepared by
first forming the modified sesquicarbides, RE2C3 as previously described.
By mixing excess (powdered) carbon with a partially frozen peritectic mixture
consisting of a liquid metal-rich phase in equilibrium with crystalline
carbide, and then cooling, a 3 phase solid mass is obtained which will slowly
convert to a structure containing predominantly dicarbide if maintained at
elevated temperatures (400-700C.).
As a practical matter in forming carbides by peritectic freezing, by
mixing a controlled amount of powdered carbon with a partially frozen peritec-
tic mixture of carbide and metal-rich liquid, removing additional heat to
-16-
l ~5~01~
complete solidification and agin~ or curing to promote more complete transforma-
tion to carbide, one may produce carbides more easily than by employing separa-
tion techniques o~ partially frozen mixtures.
Hydrolysis
The results of hydrolysis experiments on metal~like carbides have
been reported on numerous occasions, often with conflicting results. An
e~amination of previous work along with my own studies has led to several
generalizations in this field:
1. The metal-like carbides can be considered to be an "alloy" between
excess metal and an ionic carbide where the ionic carbide would
contain metal ions in a normal valence state and carbon has a
nominal valence of -4 for most carbides whose structure leads to
large C-C bond distances and a valence of -1 for acetylides where
C-C distances below 1.3 A are found between isolated pairs of carbon
atoms.
2~ Upon hydrolysis, the excess metal reduces the water to hydrogen
while the ionic carbides yield methane or acetylene.
3. Some of the hydrogen tends to react with acetylene or unsaturated
carbon or hydrocarbon fragments which may evolve by other processes.
4. Metastable carbides such as Fe3C or Mn3C tend to partially re-
vert to metal and carbon which gives a higher than expected H2
content on hydrolysis. From 10 to 15% or more of the total carbon
content may revert to free carbon.
5. The rare earth carbides on hydrolysis behave as acetylides with
excess reducing agents; they may be viewed as alloys between excess
rare earth and hypothetical REC3. The hydrogen evolved by the ex-
cess metal partially reduces the acetylene and partially appears as H2.
6. The metal-like carbides generally produce lesser amounts of various
other hydrocarbon molecules as a result of secondary reactions of
intermediate or primary hydrolysis products.
Of prime importance in thermal efficiency of cyclic processes is the
final state of the metal atoms following hydrolysis. By reacting the carbides
with steam or water and allowing the reactants to increase in temperature, the
metallic constituents may be largely recovered as oxides rather than hydroxides.
As is well-known in the chemical arts, the alkali metal hydroxides cannot be
dehydrated at atmospheric pressure below their boiling points, but alkaline
earth metal hydroxides can be dehydrated at temperatures in the 200-600 C.
1~S8QlB
range and other metal hydroxides are more easily dehydrated. As a general
rule, the hydroxides with very low water solubility are easily dehydrated.
By recovering metals in oxide form, the energy of dehydration is
saved in processing the material for reduction to metal.
Description of Complete Process
The following describes the complete process for forming fuel
gases or liquids from carbonaceous sources using metallic carbide synthesis
and hydrolysis as intermediate steps. The process will be described in terms
of preferred embodiments, it being understood that certain variations in
operating conditions may be useful as operating experience is gained on larger
scale installations.
A. Forming the Fuel Precursors
l. Carbonizing or pyrolyzing raw carbonaceous material to remove
volatile matter and forming coke or char.
2. Mixing spent metal oxide-hydroxide residue with sufficient carbo-
nized coke or char to reduce said metal compounds to free metal and
saturate said metal with carbon and applying heat to cause such
reduction.
3. Mixing said carbon-saturated molten metal with excess finely divided
coke or char and maintaining such mixture at temperatures above the
freezing point of the molten metal until a major fraction of the
mixture has been converted to carbide compounds.
4. Removing the carbide material formed in step 3, along with various
amounts of molten metal-rich solution and optionally some uncon-
verted carbon in fine particulate form and holding said mixture at
some lower temperature at which the remaining molten material
solidifies, or alternatively becomes so viscous that migration of
carbon or carbide particles is inhibited, until further conversion
processes to form carbides are essentially complete.
5. Dividing and consolidating carbide product into agglomerates of
useful size for storage or handling and cooling sufficiently to
permit storage or shipment.
B. Conversion of Precursors to Fuel Products
.
6. Introducing said carbide agglomerates (fuel precursors) along with
steam or water into a gas tight reaction chamber to effect hydrolysis
to form metal oxides and/or hydroxides plus volatile fuel products
consisting of fuels and optionally hydrogen plus small amounts
of impurities.
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7. Removing spent metal oxides/hydroxides which are rcturned to step 2
and recovering the heat of hydrolytic reaction.
8. Purifying generated gases and molten metals by filtering, scrubbing
or slagging operations.
9. Removal and recovery of volatile fuel products.
Formation or ManuEacture of Fuel Precursors
Referring now to the drawings for a more detailed explanation of the
process of the invention, it can be seen that Fig. l is a schematic representa-
tion or flow diagram of a process for producing solid metallic carbides which
can be converted, by chemical reaction, to fuel gases or liquids. Raw
materials for the process are raw carbonaceous materials, and one or more
metal oxides or hydroxides which may be largely supplied as recycled material
from the gas generation stage of the process.
The raw carbonaceous material is reduced in particle size in crusher
16, to dimensions which will at least pass a lO mesh screen. All of the
entering carbonaceous material is pyrolyzed to remove a large portion of its
constituent volatiles, although conditions in pyrolyzer 18 need not be
controlled to achieve complete coking or devolatization. Thus, pyrolysis is
advantageously accomplished below normal coking temperatures of about 2000 F.
Rather, the raw, crushed carbonaceous materials are subjected only to a
pyrolyzing temperature of about 750-1250F., preferably 900-1100F. for a time
sufficient to remove from 80 to 95% by welght of the volatiles therein. The
time required will be limited by heat transfer considerations and will vary
from flash heating systems to 12 hours or more depending on process apparatus
employed. ~artial vacuum (for example, at pressures in the range pf 2 to 8
psia) may be used if desired. It is important that the pyrolyzing temperature
be sufficiently high that the partial pressure of water be kept sufficiently
low to convert to oxides the major proportion of dehydratable starting material
hydroxides in subsequent steps of the process. Water driven off in pyrolyzer
18 can be reintroduced into the process as steam. Volatile gases with fuel
value may be used to supply over-all process heat by combustion in the plant,
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mixed with hydrolytic process gas to augment output, or marketed as a separate
fuel gas. A portion of the pyrolyzed carbonaceous material continues on to
the mixer 20 for admixture with the metallic oxide/hydroxide materials. The
balance of the pyrolyæed carbonaceous material is directed to syn~hesizer 26
where it is used to form the solid mixed metallic carbide hydrocarbon gas
precursor elements.
The metallic oxide/hydroxide raw materials are passed directly to
one or more crushers 12, 14 where they are reduced in particle size for ease
of handling and to enhance subsequent processing, and then to mixer 20.
Although the particle size of the crushed metal compounds is by no means
critical, it is preferred that they pass a 20 mesh screen. In mixer 20, the
ambient temperature metal oxide/hydroxide materials are thoroughly mixed, as
by tumbling, with the relatively hot (800~900F.) pyrolyzed carbonaceous
material. If spent fuel precursors from which the fuel gas has already been
generated are recycled, they would be crushed, if necessary, and then metered
from supply 15 in appropriate proportions directly into mixer 20.
The thoroughly admixed constituents of mixer 20 are conveyed through
metering device 21 into reactor 22 where they are reduced to liquid metal at
the reactor temperature of about 1400-2400F. A considerable quantity of heat
must be supplied either by radiant energy or chamber walls heated by indirect
combustion or by concurrent combustion of excess carbon inside the chamber as
in a blast furnace. The time of reaction will depend on heat transfer limita-
tions but may be expected to require several hours for commercial size reduction
chambers. In the reactor, the metal oxides/hydroxides are reduced to free
metal according to the reactions:
M O + C--->xM + CO
M (OH) + 2C---->xM + 2CO + H2
M (OH) + C----->xM + CO + H2O
where M represents the metal element.
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Any by-product carbon monoxide gas generated in reactor 22 from
the reduction of the metallic compounds or from the residual volatiles in the
carbonaceous material is directed to a scrubber (not shown) and then marketed
as a fuel gas or used to supply process heat. The composition exiting from
reactor 22 is molten and consists primarily of liquid metal with dissolved
carbon and an insoluble slag or impurity layer which may be separately drained
and disposed. The metallic layer then enters a heated blender/reservoir 24
which has the capability to store the molten composition and via meter 25 to
control the quantity of composition which passes directly to synthesizer 26,
The balance of the molten composition is directed to a converter 28 which is
maintained at approximately 1000F. minimum temperature. In the converter the
composition is modified as necessary, with additional quantities of carbon
from supply 29 or liquid metal from supply 27. By virtue of meters 21 and 25
and the transformation capability in converter 28, the material flow to
synthesizer 26 can be controlled and stabilized in order that the composition
of the mixed metallic carbides produced in the process can be held at selected
levels.
Three separate materials flow streams enter synthesizer 26 --
the pyrolyzed carbonaceous material from the pyrolyzer 18, the molten composi-
tion exiting the blender/reservoir 24, and the residual molten material from
converter 28. Temperatures within synthesizer 26 are maintained in the range
of 1600-2400 F. so that a portion of the materials therein readily react to
form mixtures of ternary metallic carbides. Dwell time in the synthesizer is
about 4 to 7 hours and, nominally, about 5 hours to obtain the peritectic
mixture initially required to allow further processing to yicld hydrocarbon
precursor compositions. By controlling the input to the synthesizer as
hereinbefore described and by continuously monitoring and sampling the product
composition from the synthesizer, the desired mixture of modified metallic
carbides can be controlled.
. The partially converted material exiting from the synthesizer is
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conveyed through line 31 to converter 28 where additional conversion occurs.
In converter 28 additional carbon ;s added from supply 29 to react with molten
metal-rich phase or alternatively to react with intermediate carbides to form
carbides of higher carbon con~ent either through solid state diffusion pro-
cesses or by transmittal through a liquid metal film acting as a solvent.
The bulk of the output from converter 28 consisting of the desired
mixed metallic carbides is drained into a compactor 30 where the product is
compressed or otherwise formed into pellets, bricks or briquettes, preferably
spherical or at least non-angular in configuration, of a size which can
readily be handled and transported. Typical briquette sizes are in the range
1 to 9 inches in major dimension, although the size of the compacted fuel
precursor is not critical. The shaped fuel precursors cool {apidly to ambient
temperature and may be stored in bin 32, packed for shipment or immediately
used to generate fuels.
Conversion of Precursors to Fuel Products
The mixed metall;c carbide fuel precursors can be convert`ed
to fuels by the process which is schematically depicted in Fig. 2. The
input supply 38 to the conversion process is the fuel precursors
hereinbefore produced in the process of Fig. 1, which precursors are metered
20 from compactor 30 or bin 32 via hopper/feeder device 40 at a predetermined
rate into conversion chamber 42. In chamber 42, the mixed metallic carbide
fuel precursors are sprayed or otherwise contacted with water or steam to form
the volatile fuel gases and the spent mixed metallic oxides or hydroxides
exiting the converter are recycled to the fuel precursor production
process. If desired, additives, such as odorizers, may be added to the
fuel gas in the conversion chamber 42. The conversion chamber is maintained
at a temperature in the range 250-600F., preferably 300-450F. In this
temperature range, temperatures are high enough that most metal hydroxides
will be dehydrated and low enough that unwanted vapors, such as sulfur
dioxide, hydrogen sulfide, etc., can be readily removed. The reaction
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which forms the volatile fuels is exothermic, generating substantial
quantities of reaction heat and necessitating a heat exchange system 44
in the chamber 42 to capture this heat of reaction. Prefera~ly, coolant
water is caused to flow througll heat exchanger 44 to absorb and withdraw
the excess reaction heat and to control the converter temperature to the
desired range. The coolant water may be converted to steam which can be
transported through line 47 for use elsewhere.
The fuel precursors are moved through chamber 42 by a screw
conveyor 45 and are sprayed from above with the water by sprayers 46.
The water is advantageously distilled water condensed from the steam line
to avoid introducing dissolved minerals into the reduction stage during
recycling.
The gas exiting chamber 42 through line 54 is a mixture of fuel
gases or volatile fuels and water vapor which can be separated by
conventional techniques in scrubbers 48. For ease and continuity of
operation, a series of scrubber towers is preferably provided with
appropriate conventional valving means (not shown) to permit gas flow
through selected scrubber towers. In this manner, the towers can be
taken off line and repacked with fresh adsorbants when necessary.
Inasmuch as tllere is considerable vapor production in the
conversion chamber 42 creating a pressure in the range 2 to 8 psig
therein, fuel precursor feed into and spent fuel precursor removal
therefrom must pass through gas sealing mechanisms. The spent fuel
precursor, consisting now predominantly of mixed metallic oxides, is
passed into a storage pit 52 where any additionally generated volatiles
can be collected prior to recycling the spent carbides to mixer 20 (see
Fig. 1) as fuel precursor production input. The fuels pass from the chamber
42 and from the spent fuel storage pit 52 through scrubbers 48 into a
compressor stage 50 wherein the gas pressure is raised to a level suitable
for distribution, for example, in gas mains. Liquid fuels mey be removed
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at this stage if desired. The compressor 50 also serves as a mixer unit
wherein the fuel gas can be admixed with an inert ~e.g., nitrogen, carbon
dioxide) or active (e.g., carbon monoxide) diluent gas prior to distribution
through mains or otherwise.
Impurities present in the raw feed material may be removed at
special points in the process by one or more of the following methods:
1. Silica, phosphorus, sulfur or other acidic impurities arising pri-
marily from the carbonaceous input may be removed as a dross or
slag in the reactor 22 being generally immiscible and floating
on liquid metal layer. Additions of lime or magnesia in amount
sufficient to combine with the acidic impurities may aid in
separation of said impurities. Basic impurities such as alumina
would usually combine with acidic impurities normally present in
excess, but if necessary controlled additions of silica can be
made to effect removal.
2. Impurities introduced into the synthesizer 26 would normally be
carried over into the fuel precursor product. With the excep-
tion of sulfur, they would pass through the conversion chamber 42
largely unchanged and would be removed in the reactor 22 upon re-
cycling. Sulfur which may be released as H2S or SO2 in hydrolysis
can be removed by mild oxidation or absorptlon by basic materials
(lime, soda, ash or alkaline liquids) in scrubbers 48.
EX~IPLE 1
As an example of preferred compositions in the modified (Fe, Mn)C
system we list the following data.
The pyroly~er output yields approximately 70 lb coke or char per
100 lb raw coal for medium volatile coking coals and proportionately more or
less for coals of greater or lesser volatiles content.
Mixer 20 is charged with approxi~ately 497 lb of mixed metal oxides
(FeO and MnO) containing 10 - 65% FeO by weight, preferably about 50% with
approximately 84 lb of carbon (about 90 lb coke) plus sufficient excess carbon
to saturate the reduced liquid metals (about 4-5%). If the furnace is to be
internally fired, additional carbon must be added and burned to CO to provide
process heat. The synthesi~er is charged with approximately 385 lb of liquid
metal from the reactor 22 with about 36 lb of carbon (total of dissolved and
added C) to permit ultimate conversion to M7C3, where M represents the metal,
and sufficient soft metal to provide an interstitial liquid metal film to retain
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fluidity to the reacting mixture and promote diffusion of the metallic atoms.
The soft metal is preferably magnesium or magnesium zinc alloy and is present
in the synthesizer chamber 26 in an amount equal to 5 to 20% by weight of the
Fe, Mn content. Most of the soft metal content is retained in the synthesizer
and converter chambers. In the hydrolytic conversion chamber 42, approximately
421 lb of mixed metallic carbides are reacted with about 126 lb of water or
steam to provide approximately 45 lb of fuels of average composition
CnH3n and about 5 lb of hydrogen.
Example 2
For the modified rare earth carbide system we have the following pre-
ferred compositions.
Operation of the pyrolyzer would be essentially the same as for the
Fe, Mn carbide system described in Example 1.
Mixer 20 is charged with approximately 328 lb of mixed rare earth
oxides or La/Ce oxides with about 36 lb of carbon plus sufficient excess
carbon to saturate the reduced liquid metals (about 2-3%by weight). Reduction
of the rare earth oxides requires more drastic conditions than for Fe or Mn
such as higher temperatures or reduction of partial pressures of CO as by
vacuum or inert gas purge. Synthesiæer 26 is charged with approximately 280
lb of liquid metal from the reactor 22 with about 48 lb of carbon (total
dissolved plus added carbon~ to permit ultimate conversion to REC2 and
sufficient soft metal to provide an interstitial liquid metal film to retain
fluidity to the reacting mixture and pronote diffusion of the metallic atoms.
The soft metal is preferably magnesium or magnesium-lead~barium and/or zinc
alloy and is present in the synthesizer chamber 26 in an amount equal to 5 to
40% by weight of the rare earth metal content. A majority of the soft metal
is retained in synthesizer 26 and converter 28. In the hydrolytic conversion
chamber 42, approximately 328 lb of mixed metallic carbides are reacted with
about 54 lb of water or steam to provide approximately 54 lb of combined
fuels plus hydrogen.
i
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Throughout the specification and claims, unless otherwise specified,
parts and proportions are expres$ed in weight percent, pressures in pounds per
square inch absolute, temperatures in degrees Centigrade or Celsius, and heats
of Eormation, combustion, or the l~ke, in kiloc~lories per mole.
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