Combustible coal/water mixtures for fuels and methods of preparing the same

There is disclosed a novel fuel slurry comprising a mixture of finely ground coal constituting 65 to 70 percent of the fuel, water, a wetting/dispersing agent for the coal constituting 0.5 to 5 percent of the fuel and a suspending agent for stabilizing the fuel slurry which agent constitutes a clay such as attapulgite clay in the dry, pregelled or predispersed form. Other clays can be employed as a stabilizer, as a predispersed non-gelling montmorillonate clay. If the coal slurry is to be used immediately the clay or other gelling agent is not necessary to maintain suspension stability.

BACKGROUND OF INVENTION 
This invention relates to a unique fuel and more particularly to a fuel 
comprising a mixture of finely ground coal, water, a wetting/dispersing 
agent and a suspending agent. 
It has long been known that the present supply of oil is rapidly being 
depleted. Because of the possible future world shortage of oil and the 
current price increases in petroleum products, the use of oil as a source 
of energy has become very expensive, while as indicated above, the supply 
has an uncertain future. The prior art cognizant of such problems 
attempted to alleviate the demand for fuel oils by substituting other type 
of fossile fuels. The abundance of coal and its ready accessibility 
suggested a direct substitution of coal for oil in many applications. 
Hence considerable effort has been expended in formulating mixtures of oil 
and finely ground coal in the ratio of 50 percent oil/50 percent coal to 
70 percent oil/30 percent coal. 
Examples of typical formulations and techniques are depicted in U.S. Pat. 
No. 4,147,519 entitled COAL SUSPENSIONS IN ORGANIC LIQUIDS issued on Apr. 
3, 1979 to Edgar W. Sawyer, Jr. the inventor herein and assigned to this 
assignee. 
This Patent describes coal dust powder suspended in fuel oil and stabilized 
against sedimentation by an additive that makes the mixtures viscous under 
conditions of low shear while exhibiting low viscosity under high shear 
conditions to facilitate pumping through long range pipelines by the 
combination of a suitable surfactant with a gelling grade clay as a 
suspending agent. 
U.S. Pat. No. 4,251,230 entitled COAL SUSPENSIONS IN ORGANIC LIQUIDS issued 
on Feb. 17, 1981 to E. W. Sawyer, Jr. describes a suspension of coal dust 
powder in fuel oil, which suspensions are also stabilized using a gelling 
grade clay suspending agent. 
Other Patents are U.S. Pat. No. 4,062,694 entitled STABLE TICULATE 
SUSPENSIONS which issued on Dec. 13, 1977 to E. W. Sawyer, Jr. describe 
the use of attapulgite clay as a suspending agent for coal dust in 
flammable liquid hydrocarbons to formulate fuels which facilitate the 
removal of fly ash from the fuel assembly after burning. As indicated such 
prior art as well as other Patents as U.S. Pat. No. 2,397,859 entitled 
LIQUID FUEL AND METHOD OF PRODUCING the same issued on Apr. 2, 1946 to A. 
B. Hersberger, et al. attempt to substitute coal for a portion of the oil 
and provide a composite fuel. 
U.S. Pat. No. 4,062,694 also describes coal dust-water slurries that 
exhibit good rheological characteristics and are useful for transporting 
and storing coal at a 60% concentration. The slurry is formed using coal, 
a wetting/dispersing agent for the coal and 1% to 3% gelling grade clay. 
The 60% coal concentration did not provide sufficient BTU's for the slurry 
to be considered a satisfactory fuel. The high water content had to be 
evaporated and that consumed too much of the heat content. The slurry also 
contributed to another problem, that being ash content. The 1 to 3% clay 
level contributed significantly to an unacceptable ash level. 
Certain of the mixtures depicted in the prior art have been burnt 
successfully in certain types of furnaces. 
The problem with the oil-coal mixtures is that they still contain fuel oil 
and do so in considerable percentages as between 50 to 70 percent oil. 
It is of course understood that the ideal situation is to completely 
eliminate oil from the composite fuel. Many present installations are 
designed for powdered coal combustion and have, due to environmental 
considerations, been retrofitted to burn oil. However, many of the 
previously retrofitted installations no longer have the auxiliary room and 
facilities to receive, store and grind coal and therefore changing back to 
coal or powdered coal combustion is not possible. 
It is therefore an object of the present invention to provide a fuel 
composition which will allow the use of powdered coal as a fuel in the 
above-mentioned isolated facilities, thus resulting in substantial savings 
in cost. 
Another object is to provide an improved fuel composition employing 
powdered coal without any fuel oil to provide an efficient stable and 
economical alternate fuel composition as well as economical processes for 
making the composition. 
SUMMARY OF THE INVENTION 
This invention consists of a composition that can be utilized as a fuel 
which composition comprises a mixture of coal, water, wetting/dispersing 
agent for the coal and a suspending agent for stabilizing the slurry. Coal 
is present in the mixture at the 65 to 70 percent level, with the 
wetting/dispensing agent for the coal employed in minor, functional 
percentages, the suspending agents employed are natural or synthetic 
hydrocolloid polymers, gelling type synthetic or natural minerals, or 
other agents that exhibit gel properties in the water which constitutes 
the remainder of the composition formula. 
DETAILED DESCRIPTION OF THE INVENTION 
As indicated the invention consists of a composition for a novel fuel 
comprising a mixture of finely ground coal, water, a wetting/dispersing 
agent for the coal and a suspending agent for stabilizing the slurry. As 
will be ascertained the suspending agent may not be required if a suitable 
wetting/dispersing agent for the coal is used, if the coal is finely 
ground, and if the slurry will be used in a relatively short period of 
time after it has been prepared; for example, within twenty-four hours. If 
a longer storage time is contemplated a suspending agent is necessary. 
Coal as anthracite, semi-anthracite, bituminous as well as semi-bituminous 
and other coal and semi-coal compositions is first crushed and ground by 
conventional techniques to a fairly fine powder. Examples of successful 
grinds which can be employed for the coal powder are 
1. 95%-100 mesh, 80%-200 mesh, 65%-325 mesh. 
2. 90%-100 mesh, 75%-200 mesh, 52%-325 mesh. 
3. 100%-325 mesh (44 microns), 55%-26 microns. 
The upper limit on the coal particle sizes is dictated by what will burn in 
the flame (.about.80 mesh). The amount of coal that can be incorporated 
into coal/water mixture slurries is a function of the particle size 
distribution, the particle morphology and the dispersing agents employed. 
Particle size distribution and particle shape can be established during 
dry processing or by post processing the slurry with various types of 
conventional wet-grinding equipment to achieve a change in particle size 
distribution and a rounding off of particle shapes. 
It is desirable to incorporate a maximum amount of coal in the slurry while 
maintaining rheological characteristics that insure good stability and 
sprayability in the burner nozzle for combustion. 
The maximum solids with coal appears to be about 70 to 75 percent and a 
satisfactory slurry can only be attained with relatively round particles, 
a high percentage of fine particles and a dispersing/wetting agent. For 
any medium to long range stability, a gelling agent that imparts gel 
properties to the continuous water phase is also required. The gelling 
agent may be omitted if the coal/water mixture is to be combusted soon 
after preparation and hence will not be stored. 
Dispersant/wetting agents effective in this invention are low to 
non-foaming in water, have a structure that consists of an organic moiety 
that is substantive to (absorbs on) the coal surface in preference to 
water and a charged hydrophilic portion that is lyophilic to the 
continuous water phase. These agents are employed at low concentrations 
and collect on the coal-water interface. They displace air and water, 
allow the coal agglomerates to be deflocculated with mild stirring and 
charge up the coal particles to give minimum viscosities. 
Many types of coal will only make thick pastes when 70 percent coal is 
blended with 30 percent water but when the dispersant/wetting agent is 
added, they will thin down to very low viscosity fluids when subjected to 
mild stirring. The dispersant/wetting agent must not interfere with any 
gellants that are incorporated to improve stability. Chemical materials 
that function well as dispersant/wetting agents for coal powders are Lomar 
D and PW (Diamond Shamrock Co.), sodium salts of condensed naphthalene 
formaldehyde sulfonates, Tamol N and SN (Rohm & Haas Co.), sodium salts of 
condensed naphthalene formaldehyde, sulfonates; Darvan 1 and Darvan 2 (R. 
T. Vanderbilt Co.), polymerized sodium salts of alkyl naphthalene sulfonic 
acid and sodium salts of polymerized substituted benzoid alkyl sulfonic 
acids respectively, and the series of Daxad wetting dispersing agents (W. 
R. Grace Co.) which are similar in composition to the Lomars & Darvans. 
Surfynol 104 (Air Products & Chemical Co.) can be used alone or in 
conjunction with the above-noted sulfonates as a wetting agent. Sodium 
lignosulfonates such as the Rayligs (ITT-Rayonier Co.), the Maracarbs & 
Marasperses (American Can Co.), Norligs (American Can Co.) and other 
lignosulfonates can be used. 
Stabilizing agents that maintain the coal powder in suspension in the 
continuous water phase act through gelling the water phase. The gelled 
water phase plus the suspended coal should exhibit a gel strength which 
reversibly breaks under shear so the coal/water mixture (C/WM) can be 
forced through a burner nozzle to make a burnable spray. Gelling agents 
that are employed are collodial grades of Wyoming bentonite, attapulgite, 
sepiolite, montmorillonites and synthetic smectite clays and gums and 
other hydrocolloids as carboxyl polymethylene (Carbopols, B. F. Goodrich 
Co.), carboxymethyl celluloses, alginates, xanthan gums, starches, guar 
gums, ethoxylated celluloses, and derivatives of these materials. 
When clays are employed as suspending agents, they can be predispersed with 
a chemical dispersant such as tspp (tetrasodium pyrophosphate) prior to 
use, for example, with attapulgite clay a predispersion would be: 
Water: 74.25% 
TSPP: 0.75% 
Attapulgite: 25.00% 
The resultant predispersion is thin and pourable and can be added to the 
water of the coal water mixture (C/WM) prior to adding the coal dispersant 
and coal solids. Montmorillonite can be made up at 30 percent clay solids 
with 3 percent TSPP base on the clay weight. 
Sepiolite can be made up in a mixture similar to the predispersed 
attapulgite. Wyoming bentonite is predispersed at the 10 percent clay 
level with 0.5 percent TSPP. Coal water mixtures containing reflocculated 
predispersed clays are very stable, have higher gel strengths than those 
containing gels made from dry clays and are pseudoplastic. 
When hydrocolloids are used as stabilizers, their makedown would be by 
conventional means as known by those skilled in the art. 
In order to gain a still clearer understanding of the composition and the 
problems attendant with the formulations as well as important data to 
enable those skilled in the art to provide the composition with a minimum 
of effort, examples as well as technical considerations will be discussed. 
If clays such as attapulgite, sepiolite and Wyoming bentonite are used to 
stabilize the coal water mixture (C/WM), they can be added to the mix (1) 
in the final concentration as a dry clay, (2) prehydrated by pregelling in 
water at a higher concentration and then adding to the slurry at the 
desired level or (3) predispersed at a high concentration in water using a 
chemical dispersant and then added to the slurry as a predispersion. If 
option #3 is used it may be necessary to add a flocculant or dispersant 
neutralizer to allow the clay particles to interact and form a stabilizing 
gel structure. 
Typical pregel compositions are: 
______________________________________ 
Formulations in Parts by Weight 
1 2 2 
______________________________________ 
Water 85-90 85-90 90 
Clay 
Attapulgite 10-15 -- -- 
Sepiolite -- 10-15 -- 
Wyoming Bentonite 
-- -- 10 
Total 100 100 100 
______________________________________ 
They are prepared by adding the water to a mixing tank, adding the clay 
while stirring and continuing agitation until a gel is formed and no clay 
lumps are present. Pregels are added to a coal/water slurry by addition of 
the desired aliquot to the water or to the final mix. 
Typical predispersion formulations are given below: 
______________________________________ 
Formulations in Parts by Weight 
1 2 3 
______________________________________ 
Water 74.25 74.25 89.50 
Chemical Dispersant 
0.75 0.75 0.50 
(i.e. TSPP) 
Clay 
Attapulgite 25.00 -- -- 
Sepiolite -- 25.00 -- 
Wyoming bentonite 10.00 
100.00 100.00 100.00 
______________________________________ 
They are prepared by dissolving the TSPP (tetrasodium pyrophosphate) in 
water in a mixing tank, adding the clay while agitating and continuing to 
agitate until the clay is dispersed. Predispersions of clay can be added 
at an early or late stage in the C/WM processing but are generally added 
prior to the coal. Usable chemical dispersants are TSPP, STP (sodium 
tripolyphosphate), Calgon, other condensed phosphate dispersants and other 
polyanionic organic dispersants that function as clay dispersants. 
Aluminum, magnesium and calcium montmorillonites of the non-swelling 
varieties (non-gelling grades) can be used as stabilizers in this 
invention. They can not be added dry or pregelled because they develop 
little viscosity but they will develop gel structures and viscosity if 
predispersed first and added as a predispersion. A typical predispersion 
formulation is shown below: 
______________________________________ 
Parts by Weight 
______________________________________ 
Water 69.0 
TSPP 1.0 
Ochlocknee Clay 30.0 
100.0 
______________________________________ 
It is prepared as described above and added in the same manner as the other 
predispersions. 
All predispersed clay-stabilized C/WM's should be examined to determine if 
a dispersant neutralizer is necessary. If a very clean coal is used and 
the viscosity and gel strength of the final C/WM is low, neutralizers such 
as hydrated lime, ammonium nitrate, aluminum sulfate, etc., can be added 
to the final mix to thicken it by floccing the clay component. With 
dirtier coals there is enough clay present to absorb some of the 
dispersant; thus, a gel is produced without the addition of a neutralizer. 
Normally the efficiency of clay utilization in suspension stabilization is 
predispersed&gt;pregelled&gt;&gt;dry addition. The same efficiency of usage has 
been noted in C/WM's. 
Examples of the utilization of this invention are shown as follows using a 
finely ground (80% minus 200 mesh) Kentucky bituminous coal and 
medium-shear (Sterling Multimixer) processing.