Method of processing low rank coal

Method of processing low rank coals which comprises steps of classifying crushed low rank coals into middle sized lumps and minute particles, subjecting the middle sized lumps to a non-evaporating dehydration treatment to produce low moisture content coals, separating ash content from the minute particles and then at least partly liquefying the minute particles to produce liquefied oil, and mixing the liquefied oil with the low moisture content coals to thereby produce low moisture coal-oil slurry. The method makes it possible to utilize even small sized particles of coal in a practical manner.

This invention relates to the method of processing low rank coals such as 
brown coal, lignite, and subbituminous coal (hereinafter called as "brown 
coal"), which highly contain moisture and volatile matters, for producing 
low moisture content, oil coated coal, or oil slurry of low moisture 
content coal. 
Among low rank coals, brown coal has not been well used except in the 
vicinity of coal-mines because transportation is impractical since it has 
a danger of spontaneous combustion due to its high content of volatile 
matter and since it contains a high percentages of moisture. 
Due to unstable oil supply in recent years, development of techniques is 
being expedited in order to effectively utilize low rank coals, 
particularly brown coal. In effective utilization of brown coal, the 
important point to solve transportation and energy efficiency problems is 
to reduce its weight by dewatering or dehydrating its highly contained 
moisture. Brown coal can generate 6,000-7,000 k cal/kg at completely 
dehydrated basis, but its 20-70% moisture causes lowering of calory per 
unit weight, as well as necessity of huge amount of power for its 
transportation. To solve these defects dewatering or dehydration of brown 
coal is considered, however, this is uneconomical as this requires a huge 
heat energy for evaporating the moisture, and also causes risk of 
producing minute powder of dehydrated brown coal which leads to production 
of coal dust and to spontaneous combustion. It is also considered to 
dehydrate the coal by non-evaporation method, which is to separate 
moisture, without evaporation, by heating brown coal with steam or highly 
heated water under high pressure. This method has a problem that treatment 
of minute powder of brown coal is not easy and the efficiency of 
utilization of the coal is therefore decreased. 
On the other hand, since brown coal highly contains volatile matter, it 
retains high risk of explosion after being dehydrated even by 
non-evaporating dehydration method although the method decreases the 
volatile matter during the process. Therefore, in order to secure 
stability of the dehydrated coal in storage and transportation, it is 
necessary to cover the coal by an atmosphere of inert gas such as nitrogen 
or combustion gas, or to coat it with crude oil so as not to injure its 
ability as a fuel. However, the use of an inert gas is not economical 
because an additional power is required for its production. The method of 
coating brown coal with crude oil have been proved as quite effective, by 
the study of the inventors of this invention, for the prevention of not 
only the spontaneous combustion but also creation of coal dust during 
transportation. Thus, coal coating method is quite convenient, however, in 
case there is no crude oil in coal mining area, crude oil must be 
transported from other areas and this causes a further problem from the 
economical point of view.

It is therefore an object of the present invention to provide a method for 
processing low rank coals in which the aforementioned problems can be 
solved. 
Another object of the present invention is to provide an economical method 
for processing low rank coals, which can obtain a low moisture coal-oil 
slurry which has no risk of spontaneous combustion and is suitable for 
storage and transportation. 
According to the present invention, the above and other objects can be 
accomplished by a method of processing low rank coals which comprises 
steps of classifying crushed low rank coals into middle sized lumps and 
minute particles, subjecting the middle sized lumps to a non-evaporating 
dehydration treatment to produce low moisture content coals, separating 
ash content from the minute particles and then at least partly liquefying 
the minute particles to produce liquefied oil, and mixing the liquefied 
oil with the low moisture content coals to thereby produce low moisture 
coal-oil slurry or low moisture oil coated coal. 
The above and other objects and features of the present invention will 
become apparent from the following descriptions of a preferred embodiment 
taking reference to the accompanying drawing which shows a flow chart of 
an apparatus for carrying out the method of the present invention. 
By way of an example, the following descriptions will be made with 
reference to the processing of brown coal containing 20 to 60% of moisture 
and 10 to 20% of ash. Referring to the drawing, the reference numeral 1 
designates a crushing and classifying device which consists of a first 
screen 2, a second screen 3, and a crusher 4. The mined coal is thrown in 
the first screen 2 by a conveyor or other suitable means, and classified 
into large lumps of coal, for example, of larger than 150 mm in diameter 
and middle sized and small sized particles. The large lumps are further 
crushed in the crusher 4 and then thrown in the second screen 3 together 
with the smaller particles in order to be classified into middle sized 
lumps of 6-150 mm in diameter and minute particles of which diameter is 
less than 6 mm. As is the general case, ash content is substantially 
concentrated in the minute particles. Then, the middle sized lumps are led 
to a non-evaporating dehydrator 5 to be processed by a non-evaporating 
dehydration method by being heated with steam or highly heated water under 
high pressure, then to be separated to dehydrated brown coal (low moisture 
coal) and water under normal pressure. Brown coal contains oxygen in the 
form of functional groups such as --COOH group, --OH group, and --CO 
group, and due to the existence of these hydrophilic groups, brown coal 
contains much moisture in its capilary, and because of this, if it is 
heated in a non-evaporating atmosphere in which moisture cannot evaporate, 
for example, saturated steam atmosphere, the functional groups of --COOH 
and other groups are dissolved to produce dissolved gases, mainly carbon 
dioxide, with the result that moisture is separated in the form of liquid 
water. Thus in the dehydration under the non-evaporation method, it is not 
necessary to supply a huge amount of heat (approximately 550 k cal/kg) 
which is usually required in evaporating water. More specifically, plural 
autoclaves are combined, and raw coal is thrown in one of these autoclaves 
to be preliminary heated by being supplied with high temperature waste 
water from another autoclave which is in another processing stage, and 
then this preheated raw coal is further supplied with high temperature 
waste water or steam from a further autoclave which is in a highly heated 
condition, so that the coal be heated to a high temperature, and finally 
supplied with a fresh saturated steam of high temperature and high 
pressure or highly heated water of high pressure from a boiler. With this 
process, the liquid form dehydration of coal is completed at the final 
pressure and temperature. 
In this way, the heated waste water from one autoclave is led to the next 
autoclave, and the heat in the steam, or the hot water as well as that in 
the dehydrated water is utilized for preliminary heating processing. Thus, 
the temperature of the waste water is as low as 100.degree. C. or less so 
that a highly economical result can be established. In case of dehydration 
of raw coal by a conventional fluidized bed dryer, it is required for 
evaporating 1 kg of moisture a heat equivalent to 1.2 to 1.5 kg of steam, 
however, the heat required in non-evaporating dehydration method is only 
0.5 to 0.8 kg which is nearly half of the heat required in the 
conventional process. It has been known that non-evaporating dehydration 
cannot effectively applied to small particles of coal having particle size 
of smaller than 6 mm. And there is also the fact that such small particles 
contain relatively large amount of ash. 
The small particles of coal having diameter less than 6 mm is transferred 
from the device 1 to a coal washing apparatus 6 so as to remove the ash 
content therefrom. The ash content thus separated from the coal content is 
taken out through an ash discharge conduit 7. A flotation type coal 
washing apparatus may be used for the washing apparatus 6. In such a type 
of coal washing apparatus 6, the coal particles are charged in a vessel 
containing water and additives, and bubbles of air are introduced into the 
vessel at the bottom portion thereof. The brown grain coals are moved to 
the water surface since they are apt to be adhered to the bubbles. The ash 
content is therefore deposited in the bottom of the vessel. The separated 
coal particles are then transferred to a drying device 8 such as a 
fluidized bed type drying device to be dried therein. The dried coal 
particles are then applied in part to a gasifying device 9 and in the 
remaining part to a liquefying device 10. The gasifying device 9 is 
supplied with air or oxygen through a supply conduit 11 and with steam 
through a supply conduit 12 and the hydrogen and carbon monoxide produced 
in the device 9 are supplied through a conduit 13 to the liquefying device 
10. 
The liquefied oil as produced in the device 10 is in part circulated 
through a circulating conduit 14 and mixed with small particles of coal to 
form a slurry which is pressurized by a suitable pump (not shown) and 
heated to a predetermined temperature. Thereafter, the slurry is 
introduced into the liquefying device 10. In liquefying the coal particles 
anyone of known types of coal liquefying device may be used. In the 
liquefying device 10, it is not necessary to perform a complete 
liquefaction because the purpose of the liquefaction is to add oil to the 
coal lumps or to make a oil slurry from the coal lumps. Thus, even a 
mixture of brown coal and oil can be used provided that the liquid content 
is sufficiently high. 
The liquefied oil as produced in the device 10 is transferred together with 
the low moisture coal from the dehydrating device 5 to an agitating device 
15 such as a rotary drum type agitator or a slurry forming tank having a 
mixer. In the device 15, the low moisture coal is added or mixed with the 
liquefied oil to be coated with the oil and form a low moisture coal-oil 
slurry. The slurry thus produced is taken out through a conduit 16. 
A part of the liquefied oil, a part of the dehydrated middle sized coal 
particles and/or a part of the small sized coal particles having a 
particle size less than 6 mm are supplied to a boiler 17 to be burnt 
therein. The boiler 17 produces steam which is supplied to the dehydrating 
device 5 and the gasifying device 9. According to the process described 
above, since only the group having a particle size less than 6 mm is 
washed and liquefied, the types and the number of the crusher and the 
screen for coal washing can be minimized (Generally, the coal needs to be 
pulverized, before being washed), and also the quantity to be liquefied 
can be minimized (If all of the coal having the particle size of 0 to 150 
mm is to be liquefied, the energy requirement will be huge). Even small 
sized particles can be effectively and economically used. 
The invention has thus been shown and described with reference to a 
specific example, however, it should be noted that the invention is in no 
way limited to the details of the described processes but changes and 
modifications may be made without departing from the scope of the appended 
claims.