Two-stage coal gasification process

Coal is processed first through a moving bed reactor and then through a fixed bed reactor. Hot carbonized coal char is fed from the first stage reactor to the second stage reactor via a lock hopper and gas is taken off from the reactors either in separate streams or in a common stream.

FIELD OF INVENTION 
DISTILLATION PROCESSES THERMOLYTIC in class 201 
PRIOR ART 
U.S. patents; Alther U.S. Pat. No. 2,140,276, and Mansfield U.S. Pat. No. 
3,146,175; British Pat. No. 581,692--Bailey 
BACKGROUND AND OBJECTS 
Gasification of coal in fixed bed reactors is well-known. The British 
patent to Bailey (supra) discloses coal gasification first in a moving bed 
reactor and then in a fixed bed reactor. Although a fixed bed reactor 
provides for virtually complete gasification of coal, it is subject to 
certain limitations, one being that agglomerating or caking coals that 
have Free Swelling Indexes in excess of three cannot be tolerated, and 
another being that agglomerating coal sized with more than 20% by weight 
of less than 1/4 inch cannot be used. The object of this invention is to 
provide a process wherein coals having Free Swelling Indexes in excess of 
three and up to nine are first processed through a moving bed reactor so 
as to produce hot non-agglomerating char of substantially larger size than 
it was as raw feed stock for the fixed bed reactor. 
If the agglomerating coal containing more than 20% minus 1/4 inch size is 
fed to a conventional fixed bed reactor, the air and/or oxygen and steam 
fed to the reactor and the gases produced by the reaction cannot uniformly 
permeate the bed, and gas production rate and heating value thereby 
diminish to unacceptable levels. Channelization of the gases occurs and 
the resultant non-uniform bed causes further operating difficulties to the 
point of making the operation unfeasible. According to the subject 
process, however, agglomerating coals containing up to 70% of minus 1/4 
inch size can be charged into the first stage reactor and, with proper 
placement and distribution of air and/or oxygen and steam through the bed, 
char lumps are produced so as to provide properly sized feed stock for the 
second stage reactor. 
While Bailey (supra) discloses gas take-off from a fixed bed gasifier which 
is fed with hot coke falling from a moving bed reactor, certain inherent 
problems limit the transportation of the gas output to a very short 
distance without cooling the gas and forcing the cooled gas with a fan. 
The moving bed reactor is normally operated at about atmospheric pressure 
and hence it was impossible to pressurize the fixed bed reactor without 
pressurizing the moving bed reactor. The object now is to provide for 
feeding hot char from a moving bed reactor through a lock hopper which 
permits the moving bed reactor to operate at about atmosphere pressure and 
the fixed bed reactor to be pressurized to as to force the gas produced 
therein to a remote device. For versatility is is intended to provide for 
deriving the gas outputs from the first and second stage reactors either 
in a single common stream or in separate mutually exclusive streams.

Referring now to the drawing, the essential elements of a two-stage 
gasifier plant are shown. They are all of conventional construction and 
the operations of the individual elements are well-known. The invention is 
concerned with the method which makes it possible to use small size 
agglomerating coals with comparatively high Free Swelling Indexes as feed 
stock. The individual components and their general operation will first be 
described. 
Raw coal, which may contain more than 20% minus 1/4 inch sizes and which 
are of the agglomerating or caking type having a Free Swelling Index in 
excess of 3 is fed via elevator 4, bunker 6, and scales 8 to a layer 
loader 10 and there onto a horizontal chain grate 12 in the chamber of a 
moving bed reactor. The coal is spread onto the chain grate by means of a 
coal gate 15 to produce a bed on the grate ranging from 41/2 inches to 30 
inches. The chain grate is driven by a drive 16 at a controlled rate. As 
an example, Sewanee coal found in Tennessee having a Free Swelling Index 
of 71/2 and containing 60% minus 1/4 inch sizes was charged into the first 
stage reactor i.e., the moving bed furnace 14. On dry basis, this starting 
material consisted of about 8% ash, 28% volatile matter, and about 64% 
fixed carbon. In the fixed bed reactor the starting material reacted at a 
temperature of about 1800.degree. F. to produce carbon containing about 
10% minus 1/4 inch size and 4% volatile matter for charging into the 
second stage reactor described below. Air or oxygen and steam is provided 
to the air box 17 by means of an air fan 18 or other suitable air and/or 
gas feed device. Low BTU gas may be derived from the furnace chamber by 
means of a stack 20 controlled by damper 22. Alternatively, gas from the 
first stage reactor 14 may be taken off via a gas outlet 24 leading to the 
gas output pipe 26. Flow of gas through gas outlet 24 is controlled by a 
damper 28 so that, during one mode of operation of the plant, gas outlet 
24 may be closed by damper 28 and stack 20 opened by damper 22 so that the 
gas output from the first reactor may be fed to a utilization device 
independently of the gas output from the fixed bed gasifier 32 described 
below. Alternatively, if the gas from the fixed bed reactor is not to be 
fed to a remote utilization device, and if the two gases may be mixed 
damper 22 may be closed and damper 28 opened so that the gas output from 
the first stage reactor 14 is combined in a common stream with the gas 
output of the fixed bed gasifier 32. 
Char dropping off the end of chain grate 12 flows through a lock hopper 30 
from which it enters as feed stock to the fixed bed gasifier 32. Lock 
hopper 30, when damper 28 is closed, isolates the atmosphere of the fixed 
bed gasifier 32, from the atmosphere of the moving bed first stage reactor 
14 by operation of valves 33, 35. The gas output from fixed bed gasifier 
32 flows through an outlet 34 to a cyclone separator 36, where it is 
stripped from ash or other solid particulate matter and thence fed the 
output pipe 26 to a utilization device. 
Preferably the fixed bed gasifier 32 has a rotary grate 38 driven by a 
grate drive 40. Air and/or oxygen and steam is fed into the lower portion 
of fixed bed gasifier 32. The cyclone separator and fixed bed gasifier are 
provided with a conventional ash valve 44 and an ash lock 46, the ashes 
from which are discharged via an ash conveyor 50. Other sealing devices 
may be used. Under certain conditions, for example for start-up, gas may 
be vented from the cyclone 36 via the stack 20 whose lower end is 
controlled by damper 52. For normal operation it will be assumed that the 
fuel gas from the fixed bed gasifier 32 is to be piped to a utilization 
device which is remote from the subject plant. Because the first stage 
reactor, 14 is normally operated at atmospheric pressure (enough to 
prevent air from leaking into the furnace chamber) at atmospheric pressure 
and because in order to feed the gas output from the gasifier to a remote 
utilization device is necessary to operate the fixed bed gasifier 32 under 
pressure, the atmospheres of the first stage reactor 14 i.e., the chain 
grate furnace must be isolated from the atmosphere of the fixed bed 
gasifier 32. To do this, damper, 28, and 52 are closed and the char output 
of the first stage reactor is charged into the fixed bed gasifier 32 via 
lock hopper 30. 
Fixed bed gasifiers can tolerate very little material below 1/2 inch in 
size. Therefore, for successful gasification in the second stage, the coke 
produced in the first stage must be nominally larger than 1/2 inch. Coke 
of this size will not form a positive seal where, as in Bailey (supra) the 
coke from the moving bed reactor is charged directly into the fixed bed 
reactor. For applications where the fixed bed gasifier is to be operated 
at a higher pressure, positive sealing between the two stages provided by 
the lock hopper is necessary. 
The need for separating the gases produced by the first stages of the 
gasification process for some applications has been previously recognized. 
This is primarily because the gases produced in the two stages can have 
appreciably different qualities with some coals under certain operating 
conditions. In some instances the gas produced in the first stage reactor 
may not have the heating value high enough to be suitable for widespread 
application other than direct firing into an adjacent combustion chamber. 
The second stage of this process can be operated at significantly higher 
pressures than can the first stage. No known commercial installations have 
been successful in operating traveling grates at pressures significantly 
higher than atmospheric. However, fixed bed gasifiers like the second 
stage of this process, are commonly operated at pressures from 6 psi to 8 
psi; higher pressures are possible with special designs. Operating the 
second stage at higher pressure gives the capability to deliver the hot 
second stage gas to a remote location without having to cool the gas and 
pass it through a pressure boosting fan. Higher pressures can also 
increase the heating value of the gas produced. 
The bed depth in the second stage or fixed bed gasifier 32 is controlled 
primarily by the lock hopper control valves, the speed of the first stage 
grate and the flow rate of the air-steam mixture added to the second 
stage. This flow rate controls the rate which the fuel in the second stage 
gasifier is gasified. Secondary control of the level in the second stage 
can be accomplished by the rate of ash removal from the second stage. 
However, ash removal must be used primarily to control the depth of the 
ash layer in the second stage and the location of the fire zone in the 
fixed bed gasifier. 
Fixed bed reactors that are charged with coals having volatile content in 
excess of 12% will generate tars and oils as a result of distillation. In 
many cases the tars are not desirable and become an environmental 
liability. The tars can be removed from the product only by extensive gas 
cleaning steps. However, with the subject two stage gasification process, 
the proper control of air and/or oxygen and steam to the first stage 
reactor will result in temperatures that will crack the tars and oils to 
carbon monoxide, hydrogen and carbon thereby avoiding the production of 
undesirable materials. This is accomplished because the gasification 
reactions take place from the top of the bed downward toward the grate so 
that the tars and oils evolved by distillation must pass upward through 
incandescent carbon and are thereby reacted gases formed. This is opposed 
to conventional fixed bed reactors, wherein the gases evolved pass 
upwardly through the cold incoming material. The preferred method will 
result in a carbon feed to the second stage that contains less than 12% 
volatile matter; and the feeding of the hot incandescent carbon into the 
top of the fixed bed gasifier avoids condensation of the upwardly evolving 
gases onto the feed stock of the second stage reactor, which heretofore 
has been a problem where the feed stock is cold. 
For most coals the addition of steam to the undergrate zone of the first 
stage is desirable. Steam will control reaction temperatures and can 
increase the heating value of the first stage gas. Steam, gas, or water 
can also be added to protect the upper and lower gates of the lock hopper, 
although it is preferred that feed stock for the fixed bed gasifier should 
be hot enough to prevent condensation on the upwardly evolving gases onto 
it. 
In operating a fixed bed reactor into which cool coal is fed, there is a 
start-up problem, usually requiring 12 to 36 hours to reach stable 
operating conditions and the desired gasification rate. With the subject 
process, it is only necessary to put into the fixed bed reactor a layer of 
ash to protect the grate, after which hot coke from the first stage is 
dropped into the fixed bed reactor and the process starts immediately.