Flash hydropyrolysis of bituminous coal

A process is described for the flash pyrolysis of a high rank caking coal in a pyrolysis chamber in which the coal passes through a tacky state during flash pyrolysis. According to the novel feature, before entering the pyrolysis chamber, the particles of high rank caking coal are blended with a diluent comprising a finely ground non-caking coal, whereby agglomeration and caking of the high rank coal is prevented during flash pyrolysis.

BACKGROUND OF THE INVENTION 
This invention relates to the flash hydropyrolysis of high ranking caking 
(agglomerating) coals, such as bituminous coal, to produce gaseous, liquid 
and solid decomposition products. More particularly, it relates to a 
process in which high rank caking coals can be flash hydropyrolyzed in a 
tubular reactor or a fluidized bed reactor. 
Pyrolysis or carbonization of coal and other carbonaceous solids is a 
well-established technique. It comprises heating carbonaceous material to 
temperatures at which thermal decomposition occurs with the formation of 
condensible organic liquids, non-condensible gases and solid residue. The 
condensible organic liquids obtained are normally referred to as tars and 
light oils, while the solid residue is normally referred to as char. The 
tar as produced by the process can be further refined with hydrogen to 
produce a range of liquid fuel products. 
The total yields of tar and liquid hydrocarbons from pyrolysis of coal and 
other carbonaceous material are markedly influenced by pyrolysis 
conditions such as heating rate, temperature and residence time of the 
liberated volatiles and coal particles in the pyrolysis zone. When coal is 
subjected to rapid or flash pyrolysis followed by rapid quenching of the 
volatile products, the yields of liquids from the process are maximized 
and secondary decomposition of the tar product is minimized. This concept 
of flash pyrolysis has been widely accepted as a carbonization technique 
for the production of oil from coal. 
Flash hydropyrolysis must be carried out at very high heating rates of the 
coal particles and also with very low residence time of the volatiles in 
the pyrolysis zone. These conditions are readily met by processing finely 
divided coal particles in either a fluidized bed or entrained flow 
reactor. 
Problems are experienced when caking coals, e.g. bituminous coals, are used 
in flash pyrolyzers because it is necessary to take the coal particles 
through a temperature range at which they become plastic, and in which 
stage the coal particles tend to agglomerate or cake, before good yields 
of volatiles are obtained. With caking coals, severe build up of caked or 
agglomerated char can occur in the pyrolyzer or the product outlet lines, 
or in both. These caked or agglomerated char deposits can adversely affect 
the operating characteristics of the pyrolyzer and can ultimately render 
the process inoperable. 
Various techniques have been proposed for overcoming or reducing the 
problems experienced with agglomerating or caking coals. For instance, the 
caking carbonaceous material may be mixed with non-agglomerating materials 
such as hot char. One such process is described in Sass et al., U.S. Pat. 
No. 3,736,233. However, when the caking coal particles are diluted by 
mixing them with non-agglomerating solid material, such as char, the 
quantity of recycled char required is excessively large when this material 
is derived from an external source, and internal recycling of char or 
other inert material introduces an additional hot surface which enhances 
the cracking reactions with a resultant loss in char yield. Moreover, the 
use of a large amount of inert materialreduces the reactor efficiency by 
occupying a significant portion of the effective reactor volume. 
It is an object of the present invention to provide a new and simplified 
technique for overcoming the agglomeration or caking problems associated 
with the flash pyrolysis of caking coals. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a continuous process for the 
pyrolysis of high rank agglomerative or caking coals, such as bituminous 
coals, in which the coal passes through a tacky state during pyrolysis 
without forming deposits thereof on the reactor walls. In the present 
invention, the agglomeration problems associated with the tacky state are 
avoided by mixing the high rank caking coal with a finely ground low rank 
non-caking coal, such as partially oxidized coal, sub-bituminous coal and 
lignites, to prevent agglomeration and caking of the high rank coals 
during the flash pyrolysis. 
The non-caking coal should be ground to a very fine particle size of 
typically less than 40 microns and preferably less than 10 microns. The 
effective concentration of non-caking coal diluent depends on the swelling 
index, the wetability of the diluent by tarry substances that exude from 
the caking coal particles, and the relative particle diameter of the 
caking coal to that of the diluent. 
For use in the process of this invention, the particle size of the high 
rank caking coal can vary quite widely, e.g. from about -35 mesh to +150 
mesh (Canada standard sieve). 
The blend of caking coal and non-caking coal typically contains about 10 to 
50%, preferably 20 to 50%, by weight of the non-caking coal and it may 
also include a small amount, e.g. in the range of about 1 to 5% by weight, 
of a finely divided inert material, such as silica powder. This aids in 
the inhibition of caking. 
In the method of the present invention, the above mixture is fed into a 
tubular or fluidized bed flash pyrolyzer at a temperature in the range of 
about 500.degree. C. to 950.degree. C. and a pressure of at least 4.0 MPa. 
Preferably the temperature is maintained at a level above 600.degree. C. 
with a retention time of coal in the reaction zone of under 10 seconds. 
Many different materials were tried as diluent for caking coals for 
instance, lime and Prince Mine char were tested as diluents at particle 
sizes of less than 10 microns. Even though the lime and char were very 
finely divided, their surface properties were such that caking could not 
be reduced substantially during flash hydropyrolysis.