Carbon slurry fuels

A liquid composition suitable as a high performance fuel and having improved rheological and stability properties comprising a liquid hydrocarbon having carbon particles dispersed therein of at least two disparate particle sizes, one of said carbon particles having an average particle diameter of from about 300 to about 350 mu and the other particle having an average particle diameter of about 60 to about 80 mu.

It is known that the volumetric net heat of combustion of liquid fuels can 
be increased by adding to the liquid finely divided solids such as carbon 
which have densities substantially greater than that of the liquids 
themselves and are also capable of being consumed via an oxidation process 
involving the formation of gases. In making such slurry fuels it is 
desirable to obtain a liquid product which is pumpable, i.e., it should 
exhibit a low viscosity, at least under shear, and will have other 
rheological properties that will enable it to be atomized in a nozzle and 
burned, say in a ramjet or turbine engine. A particle of high density is 
important in order to increase the volumetric heat of combustion, or 
conversely, for a given volumetric heat of combustion a high density 
particle enables a lower particle concentration to be used resulting in 
better rheological properties (e.g. a more fluid dispersion). Also, the 
dispersion of particles in the fuel must also be stable and not settle out 
over a period of time. 
U.S. Pat. No. 2,754,267 discusses this problem to some extent and discloses 
suspensions of carbon in a fuel oil used for increasing flame radiation 
where the carbon-containing fuel oil is injected into steel mill furnaces. 
In the disclosure of this patent carbon blacks are used with a large 
exposed surface area having monomolecular layers of certain polymers on 
the surface, the carbon particles being preferably anisometric (i.e., not 
spherical) and of a particle size diameter of the shortest dimension of 
less than about 1 micron, preferably 0.02 to 0.8 micron. 
It is also known that spherically shaped carbon particles can be dispersed 
in jet fuels (Final Report; Ramjet Fuels Analysis Supplement, Contract No. 
Y6E140, Ashland Chemical Company, Research and Development Division, 
Process Development Section, L. J. Frainier and P. M. Colling). However, 
as pointed out in this report, a concentration of carbon particles above 
40% by weight could not be achieved and such concentrations are below that 
required to give an effective carbon-slurry fuel. 
What is required for an effective carbon-slurry fuel is a liquid slurry 
which is highly stable, has suitable viscosity characteristics (e.g. will 
flow properly during use), and which has a high carbon content so that the 
formulation can yield high thermal energy. Thus, the carbon slurry fuel 
composition must have properties that are unique with respect to its 
rheological properties and stability and, therefore, which make it a 
highly desirable fuel for use in high performance gas turbine engines such 
as those utilized by manned aircraft, military missiles, or various types 
of self propelled vehicles such as trucks, military tanks, etc. where 
there is a premium placed on the heat content of the fuel in terms of 
BTU/gallon, i.e., volume limited applications. These fuel compositions 
must be pumpable, which puts constraints on the viscosity, while at the 
same time exhibiting a high degree of stability with respect to the 
settling of the dispersed particles over a long time period. In addition 
the net volumetric heat of combustion expressed as BTU/gallon must be kept 
as high as possible which requires as high a dispersed carbon content as 
can be tolerated in the hydrocarbon fuel and still maintain a flowable 
dispersion under the conditions of shear encountered in fuel transport 
from the vehicle storage vessel to the combustion chamber. In general, the 
required heat of combustion for such fuels must be at least about 170,000 
BTU per gallon of slurry, preferably equal to or greater than 180,000 
BTU/gal. 
The addition of appropriate amounts of a medium thermal black to a fuel 
will give a composition of high energy, but such a composition is 
unstable; that is, the particulates will settle out. The use of 
deflocculants help reduce this instability, but to overcome it 
satisfactorily so much deflocculant is needed that the heat of combustion 
is significantly reduced. A semi-reinforcing furnace black may also be 
used in a hydrocarbon to produce a high energy fuel, but in this instance 
so much is required for about 180,000 BTU per gallon of slurry that the 
fuel composition becomes very viscous or even gelled and cannot be pumped 
to the engine. 
In accord with this invention a novel slurry fuel composition of high 
particulate concentration and of satisfactory stability and rheological 
properties is provided, comprising a liquid hydrocarbon having dispersed 
therein at least about 50% by weight of the hydrocarbon of carbon 
particles comprising two disparate particle sizes and wherein one carbon 
particle has an average particle diameter of from about 300 to about 350 
mu and the second carbon particle has an average particle diameter of 
about 60 mu to about 100 mu. A suitable carbon particle having an average 
particle diameter of from about 300 to about 350 mu is available 
commercially as medium thermal (MT) black. A semi-reinforcing furnace 
black has an average particle size of from about 60 mu to about 100 mu and 
is quite suitable as the smaller carbon particle for the invention. 
The liquid hydrocarbons employed to make the slurry fuels will be high 
density fuels having a density of at least about 0.9 and will be 
conventional jet fuel types such as methylcyclohexane, JP-4, JP-5, JP-9, 
JP-10, RJ-4, RJ-5, RJ-6 and the like or their mixtures. The technology for 
making slurry fuels from these materials is well known and will be 
employed in making the fuels of this invention. In one embodiment of the 
invention a blend of RJ-5 and JP-10 will be used preferably in a 
volumetric ratio of about 60:40. 
The carbon slurry formulation will contain appropriate additives in 
stabilizing amounts to aid in the dispersing and stabilizing of the 
suspension. Any one or more of a number of commercial surfactants can be 
utilized as, for example, succinnimide types, barium sulfonate, calcium 
sulfonate, imide type pigment dispersants, and the like. For long-term 
stability, it may be desirable to add an aluminum soap to the dispersion. 
A preferred agent to impart stability to the suspension is an oligomeric 
succinnimide containing about 2.3% nitrogen (sold by Edwin Cooper as Hitec 
E-645 Deflocculant) and is added at a level of 5% of the carbon content of 
the slurry. 
The two blacks of different particle sizes will be used in a weight ratio 
of from about 40:60 to about 60:40, preferably about 50:50. As indicated, 
both the medium thermal black (MTB) and the semi-reinforcing furnace black 
(SRB) are well known products readily available commercially and require 
no special treatment before use in the formulation. In making the carbon 
slurry fuel of the invention all that is required is to thoroughly mix the 
ingredients into the hydrocarbon liquid with a high speed mixer or other 
device to ensure that a well dispersed product is obtained. 
The concentration of the carbon in the fuel will be at least about 40% in a 
high density fuel such as RJ-6 (density=1.02) and up to about 65% in a 
high energy fuel having a density of about 0.93 (JP-10). It will be 
understood, that appropriate milling procedures, use of dispersants and 
other conventional techniques will be used in preparing the dispersions. A 
concentration much above 65% by weight, although achievable, will 
generally be too viscous to be used as a fluid fuel composition. 
The fuel composition of the invention will have a stability of at least 
about 0.9 which is the ratio representing the concentration of black in 
the upper one ml. in a standard 15 ml. centrifuge tube after centrifuging 
the composition for eight hours at 2285 RPM (about 1000 G's), divided by 
the black concentration before centrifugation. Such stability will be 
obtained by using semi-reinforcing furnace black in an amount of about 45 
to about 60% of the total black (MT black and SRF black) used. Above about 
60% SR black the composition is too viscous for use in jet engines and the 
like. Reference is made to FIG. 2 where these limiting values are evident. 
Also of significance from the figure is the sudden change in slope of the 
curve at about 45% SRF black. This clearly illustrates the unexpected 
improvement in stability that occurs at an SRF black content of about 45%. 
The fuel composition of the invention, when used as fuel for a missile 
system, will be optimally characterized by a pour point of -65.degree. F. 
and a viscosity at ambient conditions of less than 1000 cps. The slurry 
fuel is readily pumped and atomized, and will be stable over a temperature 
range of from about -65.degree. to about 250.degree. F.

In order to further illustrate the invention the following example is 
given: 
EXAMPLE 1 
To a mixture of RJ-5 and JP-10 in a volumetric ratio of 60:40 there was 
added various amounts of a medium thermal black (ASTM designation N-990) 
and a semi-reinforcing furnace black (N-754) and a deflocculant (Hitec 
E-645) at a concentration of 5% by weight of the carbon black. A high 
speed mixer (15,000 to 20,000 RPM) was used to mix the ingredients for one 
hour in a two liter stainless steel flask with cooling provided to 
maintain the temperature at ambient conditions. 
The finished slurry was subjected to measurements of viscosity and 
stability. 
Viscosity was measured on a Haake rotating cylinder viscometer at an rpm 
where the slurry viscosity was substantially constant with shear rate, 
i.e., Bingham or Newtonian in behavior. 
Stability was measured by centrifuging 15 ml. of the slurries for eight 
hours at 2285 rpm (ca 1000 G's) in a laboratory bench top centrifuge and 
sampling the top 1 ml. 
The variation of viscosity with carbon composition is shown in FIG. 1 and 
the stability in FIG. 2. Note in FIG. 1 that the blends of the MT and SR 
black deviate from linearity with the viscosity of the blends being less 
than would be expected from linear blending behavior while in FIG. 2 the 
stability of the mixtures shows a distinct increase beginning at about a 
45 volume percent SRF black. 
The carbon dispersion is fluid at -65.degree. F. and has a Brookfield 
viscosity of 20,000 to 30,000 cps whereas SRF black alone dispersed in a 
60:40 RJ-5/JP-10 mixture is solid by -40.degree. F. 
Table I is a summary comparison of various 180,000 BTU/Gal. fuel 
compositions prepared containing MT and SRF blacks from a 60:40 by volume 
mixture of RJ-5 and JP-10 and JP-10 alone and indicates the superiority of 
mixtures of MT and SRF blacks in viscosity and stability in accord with 
the invention. 
TABLE I 
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PROPERTIES OF SLURRIES WITH HEATS OF COMBUSTION 
OF 180,000 BTU/GAL FUEL COMPOSITION 
BLACK FUEL WT % % N.sub.60 CP 
STABILITY* 
MT SRF 
JP-10 
RJ-5 
CARBON 
GELLANT.sup.+ 
68.degree. F. 
-65.degree. F. 
C/C.sub.o 
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100 
-- 100 -- 64 -- 85 1,000 
Not Stable 
100 
-- 100 -- 63 0.5 600 15,000 
0.67 
50 
50 100 -- 64.5 -- 1000 
15,000 
0.90 
100 
-- 40 60 56 -- 105 8,000 
0.35 
-- 100 
40 60 56 -- 1600 
SOLID 
0.94 
50 
50 40 60 56 -- 360 20,000 
0.96 
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*C = Black concentration after centrifugation. 
C.sub.o = Black concentration before centrifugation. 
.sup.+ Gellant was a hydrocarbon polymer used to stabilize composition.