Method for enhancing energy recovery from a high temperature, high pressure synthesis gas stream

A process for the partial combustion of finely divided coal at high temperature and pressures to make synthesis gas having entrained particles of fly ash which are separated from the gas at high pressure. The fly ash and a minor amount of entrained gas are handled in a batchwise manner to isolate a batch in a lock hopper, depressurize the batch, strip the synthesis gas from the fly ash and cool the fly ash prior to disposal. During the depressurizing steps, the kinetic energy represented by fluids flowing from the pressurized lock hopper is utilized to provide power to a pressure recovery means and do additional work. The additional work is used to aid in the pressurization of a second lock hopper. In an alternate embodiment the additional work may be used to aid in fluidizing the particulate coal flow to the burners of the gasifier.

BACKGROUND OF THE INVENTION 
The invention relates to a process for the partial combustion of a finely 
divided solid fuel, such as pulverized coal, in which the latter is 
introduced together with oxygen-containing gas via a burner into a reactor 
or gasifier from which a stream of high-temperature raw synthesis gas is 
discharged together with a minor amount of contaminating material, most of 
which is in the form of particles of fly ash. 
Partial combustion is the reaction of all of the fuel particles with a 
substoichiometrical amount of oxygen, either introduced in pure form or 
admixed with other gases, such as a transport stream of nitrogen, whereby 
the fuel is partially oxidized to predominantly hydrogen and carbon 
monoxide. This partial combustion differs from complete combustion wherein 
the fuel would be completely oxidized to carbon dioxide and water. 
During the process of partial combustion of pulverized coal in a gasifier, 
the mineral matter in the coal splits into two streams when the coal is 
gasified. Molten slag which is formed falls to the bottom of the gasifier 
where it is discharged. Lightweight particles of fly ash or fly slag which 
also are formed are carried out through the top of the gasifier by the 
stream of synthesis gas which is piped through a quench section and thence 
to a gas cooler, heat exchanger or waste heat boiler where steam may be 
generated. 
The product gas and fly ash pass through equipment at high pressures, say 
300 to 350 psig for example. The fly ash must then be separated from the 
product gas, collected, depressurized, purged of product and/or toxic 
gases, cooled, and converted to a form for easy disposal. 
An essential component of this process is a means for repeatedly isolating 
the particulate solids container, i.e., a lock hopper, for filling and 
emptying. Thus the lock hopper is (a) raised to an elevated pressure, 
e.g., 350 psig, and filled; (b) depressurized to transfer pressure; (c) 
emptied of particulate contents, then (d) isolated (purged) for 
repressurization, refilling and a repeat of the process. The 
depressurization and purging is generally performed at or near ambient 
pressure, i.e., the tank is vented to atmosphere thereby wasting this 
energy and material. 
To conserve some of the stored energy and improve efficiency, a dual lock 
hopper arrangement is sometimes used, as in Assignee's pilot plant, 
wherein one lock hopper (filled and at high pressure) is purged into the 
second (empty, low pressure) lock hopper thereby equalizing the pressures. 
The first lock hopper is then isolated, depressurized to transfer 
pressures and the particulate contents emptied to a receiver. After 
emptying, the first lock hopper is purged to atmosphere for a repeat of 
the refilling process. After being equalized, the second hopper is further 
pressurized, filled, purged into the first hopper (thereby equalizing 
pressures), isolated, depressurized and emptied to the receiver. This 
process repeats itself when the second lock hopper is empty and thus 
utilizes some of the stored energy which otherwise would be wasted (as in 
a single lock hopper). The dual lock hopper system, while reducing the 
wasted energy, still must purge to atmosphere the residual 
(pressure-equalized) pressure thereby wasting the stored energy during and 
after pressure equalization. This process is referred to as 
"cross-pressurization." The present invention improves upon the prior art 
by utilizing at least a portion of this otherwise wasted energy. 
SUMMARY OF THE INVENTION 
The present invention is directed to a process for the partial combustion 
of finely divided carbonaceous fuel containing at least 1% by weight ash 
in a reactor or gasifier to produce a product gas (mainly carbon monoxide 
and hydrogen) which carries along with it, as it leaves the reactor, 
sticKy particles of fly ash or fly slag, or ash-forming constituents which 
may consist of alkali metal chlorides, silicon and/or aluminum oxides. At 
the temperature prevailing in the reactor, the ash is usually sticky. In 
particular, when the partial combustion takes place by entrained 
gasification in the burner flame, the residence time in the gasifier or 
reactor is very short compared with gasification in a fluidized or moving 
bed process, and the temperature and pressure is very high. 
The fly ash that is formed during the gasification process is at least 
partly in liquid form at the conditions that prevail in the reactor, 
usually at temperatures from 2000.degree. F. to 4000.degree. F. If the ash 
particles are not fully in liquid form, they will generally consist at 
least partly of a molten slag or be a combustion product or residue having 
a partly molten consistency. The high temperature of a reactor is 
sufficient to vaporize certain organometallic by-products which may assume 
a sticky or solid form when cooled in the process equipment. 
In the present invention, a long straight quench section of pipe is 
provided which forms the first section of the discharge duct from the 
reactor. The temperature of the product gas at this point may be, say, 
2600.degree. F. for example. A stream of product gas, which has been 
cooled several hundred degrees, is recycled from a selected point in the 
process and injected as a quench gas into the upstream end of the quench 
section of the reactor discharge duct. By mixing the cool quench gas with 
the hot reactor effluent as it enters the quench section, and flowing the 
mixture through a preferably straight quench section of sufficient length, 
the hot synthesis product gas and the sticky particles carried thereby are 
thoroughly mixed with the cooler quench gas, allowing the molten or sticky 
particles of fly ash to "freeze" to the extent that they do not stick to 
the walls of any downstream equipment or piping. 
The "frozen" fly ash must be periodically discharged for disposal and 
requires pressure transitions from the very high operating pressures (350 
psig) to atmospheric. To conserve energy and improve efficiency, lock 
hoppers are utilized for isolating the various operating pressures of the 
system. Cross-pressurization in a dual lock hopper arrangement conserves 
some of the energy by using the isolated, stored energy pressure) of one 
lock hopper to partially pressurize (equalize) the pressure in the second 
which is then isolated and pressurized to operating pressure for 
refilling. The pressure remaining in the first lock hopper is then vented 
to atmosphere. The instant invention utilizes this otherwise wasted energy 
by directing it to other devices such as venturi eductors, turbines, 
compressors, blowers, etc. which can utilize this energy. 
An object of the present invention is to provide a coal gasification 
process in which the unwanted fly ash in the high-pressure, 
high-temperature system can be readily and efficiently separated from the 
product gas on the high pressure side of the system, then depressurized, 
purged of any toxic gases and cooled prior to disposal in a dual lock 
hopper process which enhances energy recovery. In a similar manner, the 
invention may be applied to a coal feed system utilizing dual lock hoppers 
which supply coal to the burners of the gasifier. The recovered energy may 
be used to power a compressor, for example, which supplies additional 
gaseous fluids for fluidizing the particulate coal which feeds the 
burners. 
Accordingly, the invention is designed for use in a synthesis gas 
generation complex comprising: 
(a) a coal gasification plant, including at least one gasifier or reactor 
for the gasification of coal to produce synthesis gas at a temperature of 
2000.degree. F. to 3000.degree. F., the gasifier having heat exchange 
surfaces adapted for indirect heat exchange with steam and water and 
preferably comprising a burner section having at least one burner adapted 
to utilize dry particulate coal which is mixed with oxygen; 
(b) A long straight cooling or quench section or conduit mounted at the gas 
discharge port of the gasifier and in flow communication therewith whereby 
a quenching gas of lower temperature may be injected into and mixed with 
the hot effluent synthesis gas and the fly ash carried thereby; 
(c) a heat exchange section comprising at least one heat exchanger in gas 
flow communication with said gasifier, said heat exchanger being adapted 
to further cool the gas and the fly ash carried thereby; 
(d) a gas cleanup section in flow communication with said heat exchanger 
including a gas/fly ash separator for removing substantially all of the 
fly ash from said synthesis gas; 
(e) a source of quenching gas at reduced temperature and reduced particle 
content for recycling back to the quench section; 
(f) means for accumulating a batch of fly ash under high pressure 
conditions; 
(g) means for depressurizing the batch of fly ash; 
(h) means for producing useful work during, and as a result of, said 
depressurizing; 
(i) means for sweeping or purging all toxic gases from the low pressure fly 
ash; and 
(j) means for cooling and disposing of the fly ash. 
The invention relates to a process for the production of synthesis gas 
comprising: 
(a) partially oxidizing coal at an elevated temperature by feeding dry 
particulate coal and oxygen to a gasification zone, the gasification zone 
preferably comprising at least one burner for oxidizing the coal, the 
ratio of coal to oxygen being such as to maintain a reducing atmosphere, 
and producing raw synthesis gas having a temperature of from about 
2000.degree. F. to about 3000.degree. F., and removing heat from said 
synthesis gas in said gasification zone by indirect heat exchange with 
steam and water; 
(b) passing raw synthesis gas and the fly ash particles carried thereby 
through a long straight quench chamber formed at the upstream end of the 
discharge duct from said gasification zone; 
(c) injecting a cooling quenching gas into said quench chamber and mixing 
the cooling quenching gas with the hot synthesis gas to cool the gas and 
the particles; 
(d) passing raw synthesis gas from step (c) to a heat exchange zone of any 
suitable cooler well known to the art and removing heat from said 
synthesis gas and the fly ash carried thereby; 
(e) removing fly ash from the raw synthesis gas in a high pressure 
environment to produce a synthesis gas substantially free of fly ash, a 
portion of the gas being adapted to be re-cycled back to and injected into 
the quench chamber; 
(f) depressurizing the separated fly ash in a batchwise manner to 
substantially atmospheric pressure; 
(g) converting the depressurizing energy to useful work; 
(h) purging each batch of fly ash of any residual synthesis gas or toxic 
gas in a continuous manner, and 
(i) cooling the fly ash to between 100.degree. F. to 200.degree. F. prior 
to disposal of the fly ash.

DETAILED DESCRIPTION OF THE INVENTION 
The gasification may be carried out utilizing techniques suitable for 
producing a synthesis gas having gasifier outlet temperature of from about 
2000.degree. F. to about 3000.degree. F., preferably 2350.degree. F. to 
about 2550.degree. F. Although some fluidized bed oxidizers are capable of 
producing such gas temperatures under the conditions mentioned herein, the 
process is preferably carried out with a gasifier comprising at least one 
burner. Such a process will preferably include combustion, with oxygen, of 
dry particulate coal, i.e., coal having less than about 10 percent water 
content. Steam may be added in some instances to assist in the combustion. 
The type of coal utilized is not critical, but it is an advantage of the 
invention that lower grade coals, such as lignite or brown coal, may be 
used. If the water content of the coal is too high to meet the 
requirements mentioned, supra, the coal should be dried before use. The 
atmosphere will be maintained reducing by regulation of the weight ratio 
of the oxygen to moisture and ash-free coal in the range of about 0.6 to 
1.0, preferably 0.8 to 0.9. The specific details of the equipment and 
procedures employed for gasification form no part of the invention, but 
those described in U.S. Pat. Nos. 4,350,103, 4,458,607, and 4,799,356, all 
of which are incorporated herein by reference, may be employed. In view of 
the high temperatures required, however, suitable structural materials, 
such as the Inconels and Incoloy 800, i.e., high chrome-molybdenum steels, 
should be employed for superheating duty for long exchanger life. By 
carrying out the preferred procedure described herein, or utilizing the 
preferred structural aspects mentioned, as described, a synthesis gas 
stream is produced free of fly ash particles. 
The essence of the present invention is to provide a novel method of 
removing and disposing of the tons of hot fly ash produced at a high 
temperature and pressure during the above-described synthesis gas process 
while maximizing efficiency. More particularly, the invention is directed 
to separating fly ash from synthesis gas at pressures of, say, 300 psig or 
more, reducing pressure to substantially atmospheric while utilizing the 
stored (and otherwise wasted) energy to produce useful work, detoxifying 
the fly ash and cooling it for disposal. A preferred alternate embodiment 
utilizes this recovered energy in the coal feed system itself. 
In order to disclose the invention more fully, reference is made to the 
accompanying drawing. The drawing is a schematic representation of the 
process flow type, and illustrates the efficient integration of the 
specialized gasifier with equipment for substantially eliminating the 
particles of fly ash that are produced in a gasifier and the subsequent 
treatment of the fly ash. All values specified in the description relating 
thereto hereinafter are calculated, or merely illustrative. 
Accordingly, FIG. 1 discloses a prior art process and apparatus as 
described in Assignee's U.S. Pat. No. 4,838,898, in which dry particulate 
coal (average particle size about 30 to 50 microns and moisture content of 
less than about 10 percent by weight) is fed via line 1 to burners 2 of 
gasifier 3. Gasifier 3 may be a vertical oblong vessel, preferably 
cylindrical in the burner area, with substantially conical or convex upper 
and lower ends, and is defined by a surrounding membrane wall structure 
(not shown) for circulation of cooling fluid. Preferably, the generally 
cylindrical reactor wall will comprise a plurality of heat exchange tubes, 
spaced apart from each other by "membranes" or curved plates, the tubes 
being connected at their extremities for continuous flow of a heat 
exchange fluid, such as water, and also having multiple inlets/outlets for 
the fluid, in a manner well known to the art. Concomitantly, oxygen is 
introduced to the burners 2 via line 5, the weight ratio of oxygen to 
moisture and ash-free coal being about 0.9, for example. The combustion 
produces a flame temperature of about 4000.degree. F., with a gas 
temperature at the outlet of the gasifier being about 2300.degree. F. to 
about 2600.degree. F. Regulation of gasifier and outlet temperature is 
assisted by coolant in the membrane wall structure. 
Hot raw synthesis gas leaves gasifier 3 through a straight elongated quench 
line 8 of selected length the interior of which forms a quench chamber in 
which the raw synthesis gas and the fly ash and impurities carried thereby 
are quenched, preferably by cooler synthesis gas through line 6 from any 
suitable point in the process. The quench gas may be from 300.degree. F. 
to about 1000.degree. F. The quenched gas then passes to a cooler or heat 
exchanger 7. Heat exchanger 7 is preferably a multiple section exchanger, 
the quenched synthesis gas being cooled by fluid in the tubes, and 
operates at substantially the same pressure as the gasifier. 
The raw synthesis gas, now cooled in the low temperature section of heat 
exchanger 7 to a temperature of from about 600.degree. F. to about 
300.degree. F., passes via line 14 to a cleanup section 15 which may be in 
the form of a cyclone separator for removing fly ash particles. The 
details of the gas cleanup after fly ash has been removed form no part of 
the invention. 
The dry solid fly ash separated from the synthesis gas in the cyclone or 
fly ash separator 15 is discharged to a high pressure fly ash accumulator 
or supply vessel 18 via line 19. The accumulator 18 may be a separate 
vessel displaced a distance from the cyclone separator 15, as illustrated. 
Alternatively, the bottom of the cyclone 15 may be designed as an 
accumulator in which case the fly ash would be discharged from the bottom 
of the cyclone 15 through line 19 and by-pass line 20, through open valve 
21 into a pressure-isolatable lock hopper 22. 
In the system illustrated in the drawing, the accumulator 18 is connected 
via line 23 and valve 24 to lock hopper 22. The lock hopper 22 is employed 
as a depressurizing chamber between the high pressure side of the fly ash 
handling system and the low pressure side which is downstream of the lock 
hopper 22. In normal operation, the fly ash in accumulator 18 may be at a 
pressure of 300 psig or more when the valve 24 in the discharge line 23 is 
opened so that a preselected amount of fly ash can drop or be conveyed 
into the top of the lock hopper 22 which is charged with a gas, such as 
nitrogen, to substantially the same pressure as the accumulator 18. If the 
fly ash cannot be dropped by gravity into the lock hopper 22, a transport 
gas such as nitrogen is injected, as through line 25 and valve 26. 
Injecting gas into the bottom of the accumulator 18, as well as the rest 
of the vessels in the system, helps to fluff up the fly ash in the vessel 
and break it loose from the cone-shaped bottom of the vessel. 
The lock hopper 22 is provided with a discharge or transfer line 27 with a 
discharge valve 28 through which a charge of fly ash from the lock hopper 
22 is transported to the top of a fly ash receiver and stripper vessel 30 
through valve 31. The discharge line is preferably elongated, say, from 
100 to 300 feet long, and is provided with heat-dissipating fins 29 to aid 
in cooling the fly ash before it gets to the stripper vessel 30. The 
temperature of flowing fly ash in a nitrogen carrier fluid can be reduced 
100.degree. F. to 150.degree. F. with 5 seconds of residence time in a 
200-foot transfer line 27. The lock hopper 22 is also provided with a vent 
line 32 and valve 33 whereby the lock hopper can be depressurized from its 
high pressure mode to its low pressure mode at substantially atmospheric 
pressure. The lock hopper 22 is also provided with a nitrogen supply line 
34 having a valve 35 therein and being connected to a nitrogen supply 
source. 
In the operation of the lock hopper 22, with the hopper empty, valves 21, 
24, 28 and 33 are closed prior to opening valve 35 in the nitrogen supply 
line 34. Valve 35 is opened and the empty lock hopper 22 is charged to a 
pressure substantially equal to that of the accumulator 18, say, 340 psig. 
Valve 35 is then closed and fly ash supply valve 24 is opened and a 
predetermined amount of fly ash is dropped into the lock hopper. If there 
is not sufficient fly ash in the lock hopper at that time, valves 36 and 
37 in the fly ash supply line 19 would be opened until sufficient fly ash 
had been received in the lock hopper 22. 
In order to change the lock hopper 22 from its high pressure mode, say, 340 
psig, to its low pressure mode, supply line valve 24 would be closed and 
vent valve 33 would be opened to bleed the gas through line 32 until the 
lock hopper is substantially at atmospheric pressure, say, 5 psig. The gas 
or gases from line 32 are preferably sent to a flare (not shown). At this 
point, the fly ash discharge valve 28 is opened together with valve 38 in 
the nitrogen supply line 39 whereby nitrogen under reduced pressure, say, 
30 psig, is used as a transfer fluid to convey fly ash to the stripper 30 
through the pneumatic conveyor line 27. With the entire charge of fly ash 
transferred from the lock hopper 22 to the stripper 30, valves 33 and 28 
are closed and valve 35 in the nitrogen supply line is opened to a high 
pressure nitrogen source to pressure up the lock hopper to its high 
pressure mode. With the pressures within the lock hopper 22 and the 
accumulator 18 substantially equal, the operation of the lock hopper is 
repeated with a second charge of fly ash. 
It is to be realized that as a batch of fly ash moves from the accumulator 
18 to the lock hopper 22 and thence on to the stripper 30, a minor amount 
of synthesis gas is carried by, entrained with, or adsorbed on the body of 
fly ash. To remedy this undesirable situation and to detoxify the body of 
fly ash, a continuous flow of low pressure nitrogen flows through line 40 
and open valve 41, into the bottom of the stripper vessel 30 and up 
through the body of fly ash in the vessel 30. At this time the inlet valve 
31 is closed and a fly ash discharge valve 42 in discharge line 43 is 
closed. 
The flow of nitrogen up through the body of fly ash in the stripper 30 
strips the synthesis gas from the fly ash with the gases being discharged 
through an open valve 44 in a vent line 45 from the top of the stripper. 
The carbon monoxide content of the gases vented through line 45 is 
preferably measured and monitored by a carbon monoxide analyzer and 
recorder 46 of any type well known to the art. when the carbon monoxide in 
the gas being vented to a flare drops below a predetermined value, say 10 
ppmv, the valve 41 in the stripping nitrogen line 40 is closed. Weigh 
cells 47 and its recorder 48 are provided on the stripper vessel for 
measuring and recording the gross weight after it has stabilized. 
The stripper vessel 30 is then isolated from the flare line by closing 
valve 44. The fly ash discharge valve 42 is then opened, allowing the fly 
ash to drop into a storage silo 50. The silo 50 is provided with a 
discharge line 51 having a valve 52 therein. A nitrogen supply line 53, 
having a valve 54 therein, is provided for introducing nitrogen into the 
bottom of the silo 50 to aid in discharging the fly ash. At this point, 
the temperature of the fly ash may be 200.degree. F. 
Any disposal or desired use of the fly ash may be made and such use is not 
part of this invention. The drawing illustrates one possible method of 
handling where the fly ash is dropped from the silo 50 into a pug mill 55 
with water being added through a line 56 to wet it down to prevent dust 
emissions during further handling. The wet paste of fly ash and water from 
the pug mill may be emptied into a transit mixer or cement truck 57. 
Cement is added to this mixture to densify the fly ash and make it more 
suitable for utilization or disposal. 
An automated control system is used in carrying out the fly ash collection 
and stripping sequences of the present invention, due to the complexity of 
the operation and the large number of steps which must be performed, some 
simultaneously and some in rapid succession. A programmable logic 
controller confirms when the vessel 22 has been emptied and isolated from 
the stripper 30. If desired, some stripping operations may take place in 
the lock hopper using nitrogen flow after the lock hopper has been 
depressurized. 
The prior art apparatus and process described above requires a waste of 
energy when the high pressure lock hopper is vented to the atmosphere. The 
improvement of FIG. 2 utilizes a large part of this energy to perform 
further useful work in the process. The portion of the equipment 100 shown 
by the dotted lines of prior art FIG. 1 is replaced by that of FIG. 2 
designated generally at 100 which shows dual lock hoppers "A" and "B" 
designated by numerals 22 and 22'. (Corresponding elements associated with 
lock hopper "B" are designated by a "prime," e.g., 22'. ) The process is 
the same as in the prior art process of FIG. 1 up to the venting to 
atmosphere of a lock hopper. In the instant improvement vent valves 33 and 
33' remain open throughout the entire operation. When one lock hopper 22 
is filled (high pressure, ready for emptying) and the other 22' is empty 
(low pressure, ready for repressurizing and filling), all valves to both 
lock hoppers are closed. Valves 60 and 62 are then opened by HIGH SELECT 
computer interlock circuit 63 and LOW SELECT interlock circuit 64, 
respectively, allowing the high pressure in lock hopper 22 to equalize 
with, and partially pressurize, the empty lock hopper 22' through conduit 
61. After equalization, valves 60 and 62 are closed and valve 28 is opened 
to permit transfer of fly ash to receiver/stripper 30 as before. In 
traversing conduit 61 through valves 60 and 62, the high pressure fluid 
from lock hopper 22 is forced through a pressure recovery device such as a 
turboexpander, a combination turboexpander/blower set or a venturi eductor 
70. The energy recovered thereby is used, preferably to compress 
additional gas in compressor 75 to assist in the simultaneous 
pressurization of lock hopper 22'. Compressor 75 is supplied with gas from 
a low pressure nitrogen source, or other exhaust nitrogen, and its 
compressed output is fed through conduit 71 and back into conduit 61. The 
combination of 70, 75 is thus used to convert kinetic energy from high 
pressure conduit 61 to pressure energy in conduit 71 which is used to 
pressurize lock hopper 22' at a slightly higher pressure than otherwise 
would be attainable. Upon partial pressurization of lock hopper 22' in 
this manner, and emptying of lock hopper 22 through line 27, the system is 
ready for reversing the process with the other lock hopper 22' after its 
complete pressurization and filling. Thus with lock hopper 22 filled and 
at high pressure and with all valves closed (except vent valves 33 and 
33'), valves 60' and 82' are opened by HIGH SELECT interlock circuit 63' 
and LOW SELECT interlock circuit 64' respectively and high pressure fluid 
flows from lock hopper 22' through conduit 61' and through the pressure 
recovery device 70 and the process is repeated as explained above. In this 
manner, up to 40% of the kinetic energy is recoverable during pressure let 
down. The process is then continued per the prior art process of FIG. 1 
for further treatment and disposal of the fly ash. 
While the invention has been described with particular reference to a fly 
ash disposal configuration, the disclosed enhanced pressurization system 
may be used for various purposes where dual lock hoppers are used. In an 
alternate preferred embodiment, for example, the enhanced pressure lines 
61 and 61' may instead be routed to the feed hopper 90, as shown in FIG. 
3, to provide additional pressure for fluidizing and controlling the flow 
of the particulate coal to the feed hopper 90 and thence to burners 2 of 
the gasifier 3. In this embodiment, a low pressure coal-receiving hopper 
80 in each coal-receiving train supplies coal to a single, multiple 
outlet, high pressure feed hopper 90 through parallel trains of lock 
hoppers 81,82. Each coal feed train has two lock hoppers, 81,82 which are 
alternately filled at low pressure by receiving hopper 80. High pressure 
nitrogen is introduced into the lock hoppers sequentially in order to 
pressurize the hopper and the coal therein is then discharged to the high 
pressure feed hopper 90. To conserve the high pressure nitrogen, i.e., to 
conserve energy, lock hoppers 81,82 are operated alternately so that when 
one lock hopper depressurizes, the gas from that lock hopper is used to 
partially pressurize the other lock hopper. So initially, for example, 
lock hopper 81 might be fully pressurized and loaded with coal while lock 
hopper 82 is empty and completely depressurized i.e., at atmospheric 
pressure. Then lock hopper 81 will vent through valve 33 in line 61 and 
the gas is introduced into the bottom of lock hopper 82 through line 61 in 
order to partially pressurize lock hopper 82. In the operating mode, 
assume lock hopper 81 starts at high pressure, say 455-500 psig. After 
depressurizing (equalizing), lock hoppers 81 and 82 each would end up at 
half that, say 225 psig. The total gas contained in the system does not 
change during the equalization process, it just goes from a high level of 
energy to a lower level of energy; in other words it is evenly divided 
between the two lock hoppers. The difference in what is called energy, or 
available energy from the initial difference in pressure of the two lock 
hoppers, is utilized, by discharging lock hopper 81 and expanding it 
through a turbo compressor 70, 75 into lock hopper 82. The "equalizing" 
gas is forced through the expander turbine or turbo compressor 70, 75 and 
the energy recovered thereby is used to pull in additional nitrogen from a 
relatively low pressure nitrogen source and pump it into the combined lock 
hopper system. Thus, by using the energy, i.e. the flow energy or energy 
from the high pressure nitrogen flowing from lock hopper 81 to lock hopper 
82 additional gas is drawn into the system. When the system is fully 
equilibrated in this manner, instead of having 225 psig each in lock 
hoppers 81 and as above 82, it is possible to pressurize to 250, 260 or 
even 300 psig in the lock hoppers and therefor save that amount of high 
pressure nitrogen (.DELTA.N.sub.2) required to pressure lock hopper 82 up 
from 225 to 300 psig. The cost in order to do that is very small because 
the turbo compressor is a small piece of mechanical equipment. A number of 
interlock valves are used to control the flow of nitrogen in this cycle as 
is known to those skilled in the art. After equalization, the high 
pressure nitrogen source may be used to bring lock hopper 82 to high 
pressure. Lock hopper 81 then is still at 225 or 300 psig, (it is at the 
higher pressure if the turbo compressor is used to boost the pressure in 
the whole system). By reconnecting the discharge from the turbo compressor 
to a low pressure destination (such as a flare) additional energy can be 
recovered for pumping in nitrogen from the low pressure nitrogen source 
and it continues to recover the energy from this lock hopper. 
Once lock hoppers 81 and 82 are at equilibrium, the interlock valve 72 
connecting the vent side(the expansion side of the turbo compressor,) is 
blocked off from the pressurization connection 62, 62' and is reconnected 
to a low pressure destination such as a flare system through valve 73. 
Venting of lock hopper 81 is continued through the turbo expander to the 
low pressure destination instead of to the lock hopper 82. We continue to 
use that expansion energy to pump the low pressure nitrogen up to the high 
pressure nitrogen level to continue to pressurize lock hopper 82 and then, 
if necessary, augment that with the high pressure nitrogen source. The 
vent from either of the lock hoppers, although it is filtered, may contain 
some volatiles from the coal or it also may contain some very fine coal 
particles so it is filtered or sent through the flare system in order to 
prevent any environmental impact. It is advantageous to vent one lock 
hopper as much as possible through the other lock hopper because that also 
recovers some of the coal material that is in it. 
A similar application or way to recover energy from the flow of gas from 
one lock hopper to another which would not use rotating machinery is to 
use a venturi eductor in which one lock hopper is vented through the 
venturi eductor to the other lock hopper and the venturi eductor is used 
to suck in some additional nitrogen. However, this will work only on the 
high pressure part of the cycle where you are equilibrating gas between 
the two lock hoppers. It will not work when venting to low pressure 
through valve 73. With the turbo compressor, once equilibrium is obtained, 
the depressurization cycle of lock hopper 81, i.e. the originally high 
pressure lock hopper, is continued through valve 73 to the flare. If a 
venturi is used, then you quickly run out of the capability to pump up the 
pressure as much as needed to get into the lock hopper 82 which is 
pressuring up. This is true because venturies are limited in the pressure 
ratio that they can achieve. The turbo compressor has the advantage that 
the expansion side and the compression side can be isolated from each 
other.