Production of hot, saturated fuel gas

A clean, cool stream of low BTU gas is heated and saturated with water vapor by means of a hot liquid stream containing water. Contamination of the gas stream by droplets, particulate matter and salt content of the liquid stream is obviated by maintaining separation between the gas and liquid phases by means of a microporous barrier. The barrier is made of a material selected from the group consisting of hydrophobic polymer material and hydrophilic polymer material having a gel structure.

BACKGROUND OF THE INVENTION 
The instant invention relates to the production of low BTU fuel for use in 
a combined cycle power plant. In order to maximize the efficiency of the 
gas turbine component in such an arrangement, it is preferred that the 
fuel gas supplied to the combustor thereof be hot and be saturated with 
water vapor. Therefore, it has been proposed to subject a gasifier product 
gas stream, which has been cooled to accommodate the cleanup sequence, to 
a reheat and resaturation step prior to entry thereof into the combustor. 
The reheat and resaturation step so proposed would be accomplished in a 
packed tower or tray tower by direct contact between the cool, clean gas 
and a hot, water-containing liquid stream, generated in preceding process 
steps. 
Unfortunately, the liquid stream contains particulates and dissolved salts. 
Direct contact between the liquid and gas streams as proposed would result 
in the entrainment of droplets in the product gas stream in normal 
operation and, in the event that foaming occurs in the tower, such 
carryover would be inevitable. The entrainment of droplets in the fuel gas 
will result in the transfer of such droplets to the combustor and this in 
turn will cause undesirable flame conditions (e.g., a luminous flame which 
results in overheating of the combustor lining) and such droplets may 
contain alkali metal salts, which pose problems of corrosion of the hot 
gas path including the transition piece and turbine section. 
The term "microporous" as used in describing the barrier employed herein 
refers to conditions of porosity such that the cross-sections of the 
individual pores have areas equivalent to circles having diameters of less 
than about 1 micrometer and flow communication through the barrier occurs 
via more than about 5% of the pores (e.g., through interconnection of the 
pores). 
Percentages used herein to describe water vapor content of a gas stream 
refer to percent by volume. 
DESCRIPTION OF THE INVENTION 
A clean, cool stream of low BTU gas is heated and saturated with water 
vapor by means of a hot liquid stream containing water. Contamination of 
the gas stream by liquid droplets, particulate matter and salt content 
from the liquid stream is obviated by maintaining separation between the 
gas and the liquid phases by means of a microporous barrier. The barrier 
is made of a material selected from the group consisting of hydrophobic 
polymer material and hydrophilic polymer material having a gel structure. 
The heated liquid stream is brought into contact with one side of the 
microporous barrier and the cool product gas stream is brought into 
contact with the opposite side of the same microporous barrier. The high 
vapor pressure of the liquid promotes rapid transfer of water vapor 
through the barrier and into the gas stream. Preferably the flow is 
countercurrent in order to more effectively transfer heat from the hot 
liquid stream to the gas stream via the hot water vapor being transferred 
and via heat exchange through the microporous barrier.

MANNER AND PROCESS OF MAKING AND USING THE INVENTION 
In the process outlined in FIG. 1, raw product gas leaves the fixed bed 
gasifier 10 at temperatures ranging from about 900.degree. to 1200.degree. 
F and contains dust and vaporized condensible hydrocarbons including tar. 
In order to drop the temperature of this product gas and provide an 
initial cleanup, the gas stream enters the direct contact cooler 11 
wherein water is sprayed into the gas stream. The product gas so quenched 
leaves cooler 11 at about 330.degree. F saturated with water. 
Next, the temperature of the product gas stream is reduced still further 
(to about 180.degree.-200.degree. F) by passage through a series of heat 
exchangers 12. In the process of cooling the product gas stream to this 
extent, a condensate is produced comprising water, oils, tar and other 
hydrocarbon liquids. It is desirable to reintroduce the water and oil 
content of this condensate stream into the product gas. To accomplish this 
it has been proposed to raise the temperature of the condensate stream and 
then conduct this hot liquid stream to the reheat/resaturator 13, which is 
normally a packed tower or a tray tower. 
As shown in FIG. 1 the condensate stream leaving heat exchanger units 12 is 
made to re-enter one (indicated as 12a) of the multiple heat exchangers 12 
where it is heated to about 320.degree. F. From heat exchanger 12a the hot 
condensate stream passes to pump 14 whereby it is introduced at the top of 
tower 13. Alternatively, the heating of the condensate stream can be 
carried out in a steam heat exchanger. 
After the cooled product gas stream leaves heat exchangers 12, it enters 
scrubber 16 for the removal of hydrogen sulfide and ammonia therefrom. The 
clean, cooled (about 180.degree. F) dry (about 3% water vapor) product gas 
stream would then enter tower 14 wherein by direct contact with the hot 
liquid gas stream it would be converted to a temperature of about 
300.degree. F and be saturated with water vapor and some hydrocarbon 
vapor. Now the product gas stream would be ready for use as a fuel in a 
low BTU combustor 17. 
The disadvantages of utilizing a direct contact device as the 
reheat/resaturator 13 have been set forth hereinabove. In accordance with 
the objective of the instant invention, tower construction 13 is replaced 
with device 20 (FIG. 2) schematically illustrating the principle of 
maintaining the gas and liquid streams separate from each other while 
still achieving the requisite functions of reheating and resaturating the 
gas and, moreover, obviating the introduction of particulates, liquid 
droplets, or dissolved salts therein. 
As is schematically represented in FIG. 2, the hot condensate stream is 
maintained separate from the product gas flow by microporous barrier 21 
through which heat and water vapor are able to transfer from the 
condensate stream to the gas stream raising the temperature thereof from 
about 180.degree. F to about 300.degree. F and increasing the water 
content from about 3% to a water content in the 20 - 30% range 
(saturation). Microporous barrier 21 may be made from a material selected 
from the group consisting of hydrophobic polymer material and hydrophilic 
polymer material having a gel structure. 
FIG. 3 is a schematic representation of the interrelationship occurring 
between liquid (condensate) A, pore B, and gas C when the microporous 
polymer barrier 21 is a hydrophobic material. The pores of interest will 
directly or indirectly (i.e., by interconnection with each other) provide 
a continuous path from one face to the opposite face of membrane D and, 
for convenience, these pores are shown as holes B passing directly through 
membrane D. The small cross-section of each pore B and the hydrophobic 
nature of the walls thereof prevent liquid A from entering and passing 
through membrane D via these pores. However, pores B do provide flow 
communication through membrane D for water vapor to leave liquid A (at a 
high vapor pressure) and enter gas C. At the same time heat is transferred 
from liquid A to gas C via the hot vapor and by heat exchange through 
membrane D. 
Suitable microporous membranes made of hydrophobic polymer material are 
commercially available. One is made of fluorinated hydrocarbon polymer 
(Gore-Tex, a trademark of W.L. Gore and Associates, Inc.) and another is 
made of polypropylene (Celgard.RTM. made by the Celanese Corporation of 
America). 
In the case of microporous barrier of hydrophilic polymer membrane material 
having a gel structure, as is shown in FIG. 4 the aqueous component of the 
liquid stream will enter and fill pores F, but will not enter the gas 
stream in the liquid state. Once again, the high vapor pressure of the 
liquid stream provides ready transport of water vapor into the gas stream. 
Heat is transferred as described above with respect to the hydrophobic 
barrier. Thus, in this manner, membrane G of hydrophilic gel-like 
structure material also functions to provide the necessary transfer of 
heat and water vapor from the liquid stream to the gas stream. 
Representative of such hydrophilic materials are sulfonated polyxylylene 
oxide, cellulose and cellulose acetate. 
The preparation of suitable membranes from these latter materials is 
described in one or more of the following: "Synthetic Polymer Membranes" 
by R. E. Kesting (McGraw Hill, New York, 1971, pp 116-157); "Reverse 
Osmosis Performance of Poly (2.6-dimethylphenylene ether) Ion Exchange 
Membranes" by S. G. Kimura (Ind. Eng. Chem. Prod. Res. Develop., 10 No. 3, 
1971), and U.S. Pat. No. 3,259,592 -- Fox et al. 
The terms "gas" or "gases" as used herein are interchangeable with "vapor" 
or "vapors". 
Microporous hydrophilic materials that are not polymers having a gel-like 
structure (i.e., which have an open pore structure) are not useful for the 
manufacture of barriers for the practice of this invention, because such 
barriers will result in "seepage" of liquid into the gas stream possibly 
resulting in liquid entrainment, if the seepage is excessive. 
BEST MODE CONTEMPLATED 
The arrangement shown in FIG. 2 is illustrative of the principle of 
separation, but the preferred structure will be a shell and tube 
arrangement as is shown in FIG. 5. Shell 30 is divided into three 
compartments by headers 31 and 32. As shown, the liquid flow is confined 
to compartments 33, 34 and the interior of tubes 36, which are 
schematically illustrated extending between headers 31 and 32 and 
providing flow communication between chambers 33 and 34. The cool dry gas 
enters chamber 37 via conduit 38, passes between and around tubes 36, is 
reheated and becomes resaturated and then leaves chamber 37 via conduit 39 
as a clean, hot saturated gas ready for use in a combined cycle power 
plant. 
One of tubes 36 having a microporous hydrophobic wall is shown in FIG. 6. 
The inner diameter of tubes 36 may range from about 10 mils to about 1/2 
inch, and the tubes would be made of Gore-Tex, or similar fluorinated 
hydrocarbon polymer porous membrane. 
It is preferred to pass the liquid flow through the tubes with the gas 
passing external to the tubes in order to minimize fouling, because the 
fouling of the tubes is less apt to occur in this configuration.