U.S. patent application number 11/880690 was filed with the patent office on 2008-01-24 for conversion of carbonaceous materials to synthetic natural gas by pyrolysis, reforming, and methanation.
This patent application is currently assigned to Clean Energy, L.L.C.. Invention is credited to Stanley R. Pearson.
Application Number | 20080016769 11/880690 |
Document ID | / |
Family ID | 38982018 |
Filed Date | 2008-01-24 |
United States Patent
Application |
20080016769 |
Kind Code |
A1 |
Pearson; Stanley R. |
January 24, 2008 |
Conversion of carbonaceous materials to synthetic natural gas by
pyrolysis, reforming, and methanation
Abstract
The production of synthetic natural gas from a carbonaceous
material, preferably a biomass material, such as wood. The
carbonaceous material is first pyrolyzed, then subjected to steam
reforming to produce a syngas, which is then passed to several
clean-up steps then to a methanation zone to produce synthetic
natural gas.
Inventors: |
Pearson; Stanley R.; (Baton
Rouge, LA) |
Correspondence
Address: |
KEAN, MILLER, HAWTHORNE, D'ARMOND,;MCCOWAN & JARMAN, L.L.P.
ONE AMERICAN PLACE, 22ND FLOOR, P.O. BOX 3513
BATON ROUGE
LA
70821
US
|
Assignee: |
Clean Energy, L.L.C.
|
Family ID: |
38982018 |
Appl. No.: |
11/880690 |
Filed: |
July 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60832803 |
Jul 24, 2006 |
|
|
|
Current U.S.
Class: |
48/197R ;
201/13 |
Current CPC
Class: |
C10K 1/004 20130101;
C10J 2300/1662 20130101; C10J 2300/0916 20130101; C10K 1/18
20130101; C10K 1/002 20130101; C10J 3/466 20130101; Y02P 20/145
20151101; Y02P 20/146 20151101; C10J 3/62 20130101; C10J 2300/093
20130101; C10K 1/143 20130101; C10K 1/005 20130101; C10K 3/00
20130101; C10K 1/024 20130101; C10J 3/66 20130101; C10K 1/101
20130101; C10K 1/026 20130101; Y02P 20/00 20151101; C10K 1/02
20130101; C10K 1/003 20130101; C10K 3/026 20130101; C10K 1/165
20130101; C10L 3/08 20130101; C10L 3/102 20130101; C10K 1/12
20130101; C10K 1/20 20130101 |
Class at
Publication: |
48/197.R ;
201/13 |
International
Class: |
C10J 3/46 20060101
C10J003/46 |
Claims
1. A process for converting carbonaceous material to synthetic
natural gas, which process comprising: a) feeding said carbonaceous
material and an effective amount of superheated steam through a
plurality of vertically oriented tubes in a pyrolysis furnace,
which tubes are at a temperature of about 400.degree. C. to about
650.degree. C. for an effective amount of time to produce a
reaction product stream; b) quenching the reaction product stream
thereby resulting in a gaseous fraction, a liquid fraction and a
solids fraction; c) collecting at least a portion of the solids
fraction; d) passing the gaseous and liquid fractions of the
reaction product stream to a separation zone wherein the gaseous
fraction is separated from the liquid fraction; e) collecting the
gaseous fraction for further use; f) passing at least a portion of
the liquid fraction and an effective amount of superheated steam to
a reforming zone operated at a temperature of about 850.degree. C.
to about 1000.degree. C. and a pressure form about 3 psig to about
500 psig wherein said liquid fraction is reformed to produce a
synthetic gaseous product comprised of hydrogen, carbon monoxide,
carbon dioxide, and methane, which synthetic gaseous product stream
is at an elevated temperature; g) passing said synthetic gaseous
product stream at an elevated temperature to a heat recovery zone
wherein its temperature is substantially lowered; h) passing said
lowered temperature synthetic gaseous product stream to a solids
recovery zone wherein substantially all remaining solids are
removed; i) passing said synthetic gaseous product stream having a
reduced amount of solids to an organics removal zone wherein
substantially any remaining organic material is removed by contact
with an organic liquid in which the organic material is at least
partially soluble; j) passing said synthetic gaseous product stream
from said organics removal zone to an acid gas removal zone wherein
substantially all acid gases are removed; k) passing said synthetic
gaseous product stream from said acid gas removal zone to a
methanation process unit containing at least one methanation
catalyst and operated at methanation process conditions thereby
resulting in a product stream comprised predominantly of
methane.
2. The process of claim 1 wherein the carbonaceous material is a
source of fossil fuels selected from the group consisting of coal,
peat, lignite, tar sands, and bitumen from oil shale.
3. The process of claim 1 wherein the carbonaceous material is a
biomass material.
4. The process of claim 3 wherein the biomass material is a
cellulosic material.
5. The process of claim 4 wherein the cellulosic material is
selected from the group consisting of wood, bagasse, rice hulls,
rice straw, kennaf, old railroad ties, dried distiller grains, corn
stalks and cobs and straw.
6. The process of claim 5 wherein the cellulosic material is
selected from wood and dried distiller grains.
7. The process of claim 1 wherein the carbonaceous material is
dried to a moisture content of less than or equal to about 15% by
weight before pyrolysis.
8. The process of claim 1 wherein the carbonaceous material is with
the size range of about 1/16 inch to about 1/2 inch.
9. The process of claim 1 wherein the gaseous product collected
from the separation zone is a fuel gas a portion of which is used
to fuel the pyrolysis unit, the reforming zone, or both.
10. The process of claim 1 wherein the heat recovery zone uses
water to recover heat and wherein at least a portion of the heated
water is used as preheated steam to the pyrolysis unit, the
reforming zone, or both.
11. The process of claim 1 wherein the scrubbing agent used in the
acid gas removal zone is selected from the group consisting of
alcohols and amines.
12. The process of claim 11 wherein the scrubbing agent is an
alcohol.
13. The process of claim 12 wherein the alcohol is methanol.
14. The process of claim 11 wherein the amine is selected from the
group consisting of diethanol amine and mono-ethanol amine.
15. The process of claim 14 wherein the amine is diethanol
amine.
16. The process of claim 1 wherein the methanation zone contains
three reactors in series and wherein heat is removed from the
stream passing from the first reactor and the second reactor.
17. The process of claim 1 wherein at least a portion of the
methane produced in the methanation unit is introduced into a
natural gas pipeline.
18. The process of claim 17 wherein methane is removed from a
pipeline and converted to a syngas.
19. The process of claim 1 wherein prior to organic removal step
(j) the synthetic gaseous product stream is subjected to a water
wash wherein is flowed countercurrent to a stream of water to
remove any remaining solids material.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is based on Provisional Application 60/832803 filed
Jul. 24, 2006.
FIELD OF THE INVENTION
[0002] The present invention relates to the production of synthetic
natural gas from a carbonaceous material, preferably a biomass
material, such as wood. The carbonaceous material is first
pyrolyzed, then subjected to steam reforming to produce a syngas,
which is then passed to several clean-up steps then to a
methanation zone to produce synthetic natural gas.
BACKGROUND OF THE INVENTION
[0003] The world's energy supplies, particularly liquid and gaseous
fuel from fossil fuels, are being depleted faster than they are
replaced. Consequently, the development of techniques for producing
energy are urgently needed for avoiding the depletion of limited
fossil fuel resources as well as for alleviating the global warming
problem. Among various types of natural energy, biomass energy is
regarded as one of the most promising natural energy from the
viewpoint of its abundance, renewability and storability.
Cellulosic materials, such as wood, have great potential for
providing large amounts of energy. Direct combustion of woody
biomass suffers from a limited amount of resource and low
efficiency, and, further, only electric power can effectively be
supplied from the direct combustion of woody biomass. The
development of techniques that can utilize the entire biomass,
including cellulose and hemicellulose, to produce energy,
particularly in the form of liquid and gaseous fuels is of great
interest. At the present time, however, such techniques are not in
a practical stage for technical as well as economical reasons.
[0004] There is increasing interest in the production of synthetic
natural gas as an alternative to natural gas. Synthetic natural
gas, A large portion of synthetic natural gas is often referred to
as "green gas" because it is a renewable gas typically obtained
from biomass and having natural gas specifications. Thus, it can be
transported through the existing natural gas infrastructure,
substituting for natural gas in all existing applications. Also,
the use of biomass as the feedstock will not generally result in a
net CO.sub.2 emission as long as the source material can be
replanted to replace those used as fuel. It may even be possible to
reduce atmospheric CO.sub.2 by sequestering the CO.sub.2 that is
released during the conversion of biomass (negative CO.sub.2
emission).
[0005] Various problems exist in the art for pyrolyzing or
gasifying carbonaceous materials, such as cellulosic materials. For
example, vessels that have traditionally been used for gasifying
biomass, such as wood chips and similar cellulosic material have
been cylindrical, or often wider or narrower at the grate level
than at the surface of the fuel bed, relative to the flow of feed
and the forced air (or other gases) draft. Concerns with the
settling of the fuel bed so that combustion takes place without the
need to poke or otherwise stir the fuel bed have provoked a variety
of vessel construction. None of these lends themselves well to a
high volume, precisely controlled, continuous process wherein the
biomass fuel is efficiently converted to the target gas for supply
to and likely, additional energy or waste in the process. Exposing
the base fuel during the pyrolysis to air, water vapor or other
components has a direct impact on the products of pyrolysis, as
does the temperature of the process and the duration thereof. By
using any of the processes of the prior art, such as a fluidized
bed, which is, at least initially exposed to air and can be
additionally exposed to oxygen, or other input gasses, some portion
of the fuel for gasification is consumed, as by oxidation (burning)
affecting the output of the process by producing ash or other
undesirable residue.
[0006] Although several prior processes have met with varying
degrees of both commercial and technical success, there is still a
need in the art for improved and more efficient processes for
converting biomass to synthetic natural gas.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention there is provided a
process for converting carbonaceous material to synthetic natural
gas, which process comprising:
[0008] a) feeding said carbonaceous material and an effective
amount of superheated steam in a plurality of vertically oriented
straight tubes in a pyrolysis furnace, which tubes are at a
temperature of about 400.degree. C. to about 650.degree. C. for an
effective amount of time to produce a reaction product stream;
[0009] b) quenching the reaction product stream thereby resulting
in a gaseous fraction, a liquid fraction and a solids fraction;
[0010] c) collecting at least a portion of the solids fraction;
[0011] d) passing the gaseous and liquid fractions to a separation
zone wherein the gaseous fraction is separated from the liquid
fraction;
[0012] e) collecting the gaseous fraction for further use;
[0013] f) passing at least a portion of the liquid fraction and an
effective amount of superheated steam to a reforming zone operated
at a temperature of about 850.degree. C. to about 1200.degree. C.
and a pressure form about 3 psig to about 500 psig wherein said
liquid fraction is reformed to produce a synthetic gaseous product
comprised of hydrogen, carbon monoxide, carbon dioxide, and
methane, which synthetic gaseous product stream is at an elevated
temperature;
[0014] g) passing said synthetic gaseous product stream at an
elevated temperature to a heat recovery zone wherein its
temperature is substantially lowered;
[0015] h) passing said lowered temperature synthetic gaseous
product stream to a solids recovery zone wherein substantially all
remaining solids are removed;
[0016] i) passing said synthetic gaseous product stream having a
reduced amount of solids to an organics removal zone wherein
substantially any remaining organic material is removed by contact
with an organic liquid in which the organic material is at least
partially soluble;
[0017] j) passing said synthetic gaseous product stream from said
organics removal zone to an acid gas removal zone wherein
substantially all acid gases are removed;
[0018] k) passing said synthetic gaseous product stream from said
acid gas removal zone to a methanation process unit containing at
least one methanation catalyst and operated at methanation process
conditions thereby resulting in a product stream comprised
predominantly of methane.
[0019] In a preferred embodiment there is a water wash step between
before the organic removal step wherein the synthetic gaseous
product stream is passed countercurrent to a stream of water to
remove any remaining solids.
[0020] In another preferred embodiment the carbonaceous material is
selected from the group consisting of wood and dried distillers
grains.
BRIEF DESCRIPTION OF THE FIGURES
[0021] FIG. 1 hereof is a generalized flow scheme of a preferred
embodiment of the present invention wherein a carbonaceous
material, such as wood chips, are pryolyzed to produce a pyrolysis
oil, which is then reformed to produce a syngas, which is then sent
through various clean-up steps then to a methanation unit to
produce synthetic natural gas.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present invention is directed to the production of
synthetic natural gas (predominantly methane) from carbonaceous
materials, preferably biomass materials. Synthetic natural gas,
also sometimes called "green gas" is a renewable gas from biomass
with natural gas specifications. Therefore, it can be transported
through the existing gas infrastructure, substituting for natural
gas in all existing applications. Another advantage of green gas is
that is carbon neutral. That is, using biomass as an energy supply
will typically not result in a net CO.sub.2 emission since its
source can be replanted and uses CO.sub.2 from the atmosphere
during its growth period.
[0023] While this invention is applicable to a broad range of
carbonaceous feedstocks including the traditional naturally
occurring solid fossil fuels such as coal, peat, lignite, tar
sands, and bitumen from oil shale, the preferred feedstocks for use
in the present invention are biomass feedstocks Non-limiting
examples of biomass feedstocks suitable for being converted in
accordance with the present invention include trees such as red
cedar, southern pine, hardwoods such as oak, cedar, maple and ash,
as well as bagasse, rice hulls, rice straw, kennaf, old railroad
ties, dried distiller grains, corn stalks and cobs and straw.
Cellulosic materials are the more preferred biomass feedstocks,
with wood and dried distillers grains being the most preferred.
Biomass is typically comprised of three major components:
cellulose, hemicellulose and lignin. Cellulose is a straight and
relatively stiff molecule with a polymerization degree of
approximately 10,000 glucose units (C.sub.6 sugar). Hemicellulose
are polymers built of C.sub.5 and C.sub.6 sugars with a
polymerization degree of about 200 glucose units. Both cellulose
and hemicellulose can be vaporized with negligible char formation
at temperatures above about 500.degree. C. On the other hand,
lignin is a three dimensional branched polymer composed of phenolic
units. Due to the aromatic content of lignin, it degrades slowly on
heating and contributes to a major fraction of undesirable char
formation. In addition to the major cell wall composition of
cellulose, hemicellulose and lignin, biomass often contains varying
amounts of species called "extractives". These extractives, which
are soluble in polar or non-polar solvents, are comprised of
terpenes, fatty acids, aromatic compounds and volatile oil.
[0024] In most instances the carbonaceous mateials used in the
practice of the present invention will be found in a form in which
the particles are too large for conducting through the tubes of the
pyrolysis unit. Thus, it will usually be necessary to grind the
carbonaceous material to an effective size. In this case, the
carbonaceous material is ground, otherwise reduced in size, to a
suitable size of about 1/32 inch to about 1 inch, preferably from
about 1/16 inch to about 1/2 inch, and more preferably from about
1/8 inch to about 1/4 inch. Grinding techniques are well know and
varied, thus any suitable grinding technique and equipment can be
used for the particular carbonaceous material being converted.
[0025] The type of pyrolysis preferred for use in the practice of
the present invention is known as "fast pyrolysis" which is a
thermal decomposition process that occurs at moderate temperatures
with a high heat transfer rate to the carbonaceous particles and a
short hot vapor residence time in the reaction zone. Several
conventional reactor configurations have been used in the art, such
as bubbling fluid beds, circulating and transported beds, vortex or
cyclonic reactors, and ablative reactors. While all of these
reactors have their advantages they are all faced with limitations,
such as the tendency of fluid bed reactors to produce more gas and
coke then the desired pyrolysis oil, the preferred pyrolysis
product of the present invention. The pyrolysis reactor of the
present invention contains a plurality of vertically oriented
straight tubes within the enclosed reactor vessel which is heated
by use of a suitable heating device, such as one or more
burners.
[0026] The pyrolysis of biomass as practiced by the present
invention produces a liquid product, pyrolysis oil or bio-oil that
can be readily stored and transported. Pyrolysis oil is a renewable
liquid fuel can be used for production of chemicals and liquid
fuels, or as herein for the production of synthetic natural gas. As
previously mentioned, synthetic natural gas is a very desirable
product because it is derived from a renewable source and it can be
used as a substitute for natural gas for all natural gas
applications.
[0027] Generally, pyrolysis requires that a feedstock have less
than about 15% moisture content, but there is an optimization
between moisture content and conversion process efficiency. The
actual moisture content will vary somewhat depending on the
commercial process equipment used. Since some of the biomass
received for processing can have a moisture content from about 40
to 60% it will have to be dried before pyrolysis. Any conventional
drying technique can be used as long as the moisture content is
lowered to less than about 15% when mixed with the superheated
steam. For example, passive drying during summer storage can reduce
the moisture content to about 30% or less. Active silo drying can
reduce the moisture content down to about 12%. Drying can be
accomplished either by very simple means, such as near ambient,
solar drying or by waste heat flows or by specifically designed
dryers operated on location. Also, commercial dryers are available
in many forms and most common are rotary kilns and shallow
fluidized bed dryers.
[0028] This invention can be better understood with reference to
the sole figure hereof. The carbonaceous feedstock is conducted via
line 10 and superheated steam is conducted via line 12 to mixing
zone Mix wherein the two are sufficiently mixed before being
conducted via line 14 into pyrolysis process unit P. The
superheated steam, which will be at a temperature from about
315.degree. C. to about 700.degree. C. acts as both a source of
hydrogen as well as a transport medium. The amount of superheated
steam to feedstock will be an effective amount. By effective amount
we mean at least that amount needed to provide sufficient transport
of the feedstock. That ratio of superheated steam to feedstock, on
a volume to volume basis, will typically be from about 0.2 to 2.5,
preferably from about 0.3 to 1.0. The temperature conditions for
the pyrolysis reaction will be described later in detail. The steam
is preferably introduced so that the feedstock is diluted to the
point where it can easily be transported through the reactor tubes.
Fluidization will typically result and can realize fluid pyrolysis
by virtue of good contact among steam, feed polymers and heat
decomposition products of carbonaceous material liberated in the
gas phase.
[0029] The mixture of steam and feedstock, which will be at a
temperature of above its dew point of greater than about
230.degree. C., is fed to the pyrolysis reactor P via line 14 into
a flow divider FD where it is distributed into the plurality of
vertically oriented straight reactor tubes of effective internal
diameter and length within a metal cylindrical vessel of suitable
size. Flow divider FD can be any suitable design that will divide
the feedstock substantially equally among the plurality of reactor
tubes. The reactor tubes for the pyrolysis reactor are straight
instead of coiled because the residence time needs to be very short
in order to produce the maximum amount of oil without the
production of an undesirable amount of gas. The temperature of the
mixture entering the pyrolysis unit will be at least about
230.degree. C. Typical internal diameters for the pyrolysis reactor
tubes will be from about 2 to about 4 inches, preferably from about
2.5 to about 3.5 inches, and more preferably about 3 inches.
[0030] The feedstock passing though the pyrolysis reactor tubes is
subjected to fast pyrolysis at temperatures from about 400.degree.
C. to about 650.degree. C. and pressures from about 3 to 35 psig,
preferably from about 5 psig to about 35 psig. The residence time
of the feedstock in the pyrolysis reactor will be an effective
residence time. By "effective residence time" we mean that amount
of time that will result in the maximum yield of oil without excess
gas make. Typically this effective amount of time for purposes of
this invention will be from about 0.2 to about 7 seconds,
preferably from about 0.3 seconds to about 5 seconds. The heating
rate will be a relatively high heating rate of about 1,000.degree.
C. per second to about 10,000.degree. C. per second. The high
heating rate in the pyrolysis reactor of the present invention, at
temperatures below about 650.degree. C. and with rapid quenching,
causes the liquid intermediate products of pyrolysis to condense
before further reaction breaks down higher molecular weight species
into gaseous products. The high reaction rates also minimize char
formation, and under preferred conditions substantially no char is
formed. At high maximum temperature, the major products is gas,
thus the need for the present process to operate at low enough
temperatures to maximize the production of pyrolysis oils.
[0031] Although the source of heat for the pyrolysis unit, as well
as the reformer of the present invention, can be any suitable
source, it is preferred that the source of heat be one or more
burners B located at bottom of the pyrolysis and reforming process
unit. Fuel for the burners B can be any suitable fuel. It is
preferred that at least a portion of the fuel to the burners be
obtained from the present process itself, such as the syngas
produced in either the pyrolysis reactor or in the reformer. For
example at least a portion of syngas stream 20 can be diverted via
line 21 and used as a fuel to burners B. A portion of the syngas
stream 20 can also be combined with syngas stream of line 30.
[0032] Flue gas, which will typically be comprised of CO.sub.2 and
N.sub.2 is exhausted from the pyrolysis reactor via line 15 and the
reaction products from the pyrolysis reactor are sent via line 16
to quench zone Q resulting in a mixture of liquid, gaseous and
solid products. Most of the solids, which will typically be in the
form of ash, will be collected from quench zone Q via line 17. The
liquid product will be in the form of a pyrolysis oil and the
gaseous product will be a syngas. The resulting liquid and gaseous
products are conducted via line 18 to first separation zone Si
wherein a syngas stream is separated from the pyrolysis oil and
collected overhead via line 20 or a portion being diverted via line
21 to either or both of burners B. This syngas stream is comprised
primarily of hydrogen, carbon dioxide, carbon monoxide, and
methane. The pyrolysis oil stream, which may contain some remnants
of char and ash formed during pyrolysis, is conducted via line 22
to reformer R along with an effective amount of superheated steam
via line 23. It is preferred that reformer R contains a plurality
of coiled reactor tubes within an enclosed reactor vessel heated by
a suitable heating means, such as one or more burners.
[0033] At least a portion of the pyrolysis oil is converted to
syngas in reformer R, which syngas is also composed primarily of
hydrogen, carbon dioxide, carbon monoxide and methane. The inlet
temperature of the feedstock and superheated steam entering
reformer R will preferably be about 200.degree. C. The exit
temperature of the product syngas leaving reformer R via line 24
will typically be from about 850.degree. C. and 1200.degree. C.,
preferably between about 900.degree. C. and about 1000.degree. C.
At a temperature of about 1100.degree. C. and above and with a
contact time of about 5 seconds, one obtains less than about 5 mole
percent of methane and about 15 mole percent of CO.sub.2, which is
an undesirable result. Pressure in the reformer is not critical,
but it will typically be at about 3 to 500 psig. Also, it is
preferred that the residence time in the reformer be from about 0.4
to about 1.5 seconds.
[0034] For any given feedstock, one can vary the proportions of
hydrogen, carbon dioxide, carbon monoxide and methane that comprise
the resulting syngas product stream as a function of the contact
time of the pyrolysis oil feedstock in the reformer, the exit
temperature, the amount of steam introduced, and to a lesser
extent, pressure. Certain proportions of syngas components are
better than others for producing synthetic natural gas, thus
conditions should be such as to maximize the production of carbon
monoxide and methane at the expense of hydrogen.
[0035] Returning now to the Figure hereof flue gas is exhausted
from the reformer via line 23 and the product syngas stream from
reformer R is conducted via line 24 to heat recovery zone HR1 where
it is preferred that water be the heat exchange medium and that the
water be passed as preheated steam to one or both of the pyrolysis
reactor P or reformer R via lines 25 where it is further heated to
produce at least a portion of the superheated steam used for both
units. Heat Recovery zone HR1 can be any suitable heat exchange
device, such as the shell-and-tube type wherein water is used to
remove heat from product stream 24. From heat recovery zone HR1 the
product syngas is passed via line 26 through second separation zone
S2 which contains a gas filtering means and preferably a cyclone
(not shown) and optionally a bag house (not shown) to remove at
least a portion, preferably substantially all, of the remaining ash
and other solid fines from the syngas. The filtered solids are
collected via line 28 for disposal.
[0036] The filtered syngas stream is then passed via line 30 to
water wash zone WW wherein it is conducted upward and
countercurrent to down-flowing water via line 31. The water wash
zone preferably comprises a column packed with conventional packing
material, such as copper tubing, pall rings, metal mesh or other
such materials. The syngas passes upward countercurrent to
down-flowing water which serves to further cool the syngas stream
to about ambient temperature, and to remove any remaining ash that
may not have been removed in second separation zone S2. The water
washed syngas stream is then passed via line 32 to oil wash zone OW
where it is passed countercurrent to a down-flowing organic liquid
stream to remove any organics present, such as benzene, toluene,
xylene, or heavier hydrocarbon components via line 35 that may have
been produced in the reformer. The down-flowing organic stream will
be any organic stream in which the organic material being removed
is substantially soluble. It is preferred that the down-flowing
organic stream be a hydrocarbon stream, more preferably a petroleum
fraction. The preferred petroleum fractions are those boiling in
naphtha to distillate boiling range, more preferably a C.sub.16 to
C.sub.20 hydrocarbon stream, most preferably a C.sub.18 hydrocarbon
stream.
[0037] The resulting syngas stream is conducted via line 34 to acid
gas scrubbing zone AGS wherein acidic gases, preferably CO.sub.2
and H.sub.2S are removed. Any suitable acid gas treating technology
can be used in the practice of the present invention. Also, any
suitable scrubbing agent, preferably a basic solution can be used
in the acid gas scrubbing zone AGS that will adsorb the desired
level of acid gases from the vapor stream. It will be understood
that it may be desirable to leave a certain amount of CO.sub.2 in
the scrubbed stream depending on the intended use of resulting
methane product stream from the methanation unit. For example, if
the methane product stream is to be introduced into a natural gas
pipeline, no more than about 4 vol. % of CO.sub.2 should be remain.
If the methane product stream is to be used for the production of
methanol, then at least that stoichiometric amount of CO.sub.2
needed to result in the production of methanol should remaing. One
suitable acid gas scrubbing technology is the use of an amine
scrubber. Non-limiting examples of such basic solutions are the
amines, preferably diethanol amine, mono-ethanol amine, and the
like. More preferred is diethanol amine. Another preferred acid gas
scrubbing technology is the so-called "Rectisol Wash" which uses an
organic solvent, typically methanol, at subzero temperatures. The
scrubbed stream can also be passed through one or more guard beds
(not shown) to remove catalyst poisoning impurities such as sulfur,
halides etc. The treated stream is passed via line 36 from acid gas
scrubbing zone AGS to methanation zone M. Methanation of syngas
involves a reaction between carbon oxides, i.e. carbon monoxide and
carbon dioxide, and hydrogen in the syngas to produce methane and
water, as follows:
CO+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O (1)
CO.sub.2+4H.sub.2.fwdarw.CH.sub.4+2H.sub.2O (2)
[0038] Methanation reactions (1) and (2) take place at temperatures
of about 300.degree. C. to about 900.degree. C. in methanation zone
M which is preferably comprised of two or more, more preferably
three, reactors each containing a suitable methanation catalyst.
The methanation reaction is strongly exothermic. Generally, the
temperature increase in a typical methanator gas composition is
about 74.degree. C. for each 1% of carbon monoxide converted and
60.degree. C. for each 1% carbon dioxide converted. Because of the
exothermic nature of methanation reactions (1) and (2), the
temperature in the methanation reactor during methanation of syngas
has to be controlled to prevent overheating of the reactor
catalyst. Also high temperatures are undesirable from an
equilibrium standpoint and reduce the amount of conversion of
syngas to methane since methane formation is favored at lower
temperatures. Formation of soot on the catalyst is also a concern
and may require the addition of water to the syngas feedstock.
[0039] A preferred way to control heat during the methanation
reaction is use a plurality of reactors with heat removed between
each reactor. Thus, methanation zone M preferably comprises a
series of three adiabatic methanation reactors R1, R2 and R3. Each
of these reactors is configured to react carbon oxide and hydrogen
contained in the syngas in the presence of a suitable catalyst to
produce methane and water, in accordance with the reactions (1) and
(2) set forth hereinabove. Each of the methanation reactors
includes a catalyst capable of promoting methanation reactions
between carbon oxides and hydrogen in the syngas feedstock. Any
conventional methanation catalyst is suitable for use in the
practice of the present invention, although nickel catalysts are
most commonly used and the more preferred for this invention. Such
catalysts are, especially those containing greater than 50% nickel,
are generally stable against thermal and chemical sintering during
methanation of undiluted syngas streams. Alternatively, other
stable catalysts that are active and selective towards methane may
be used in the methanation reactors.
[0040] As previously mentioned because the methanation reaction is
strongly exothermic, heat needs to be removed between reactors.
Thus, heat recover zones HR2 and HR3 are used to remove heat from
the stream as it passed from reactor R1 to reactor R2 and reactor
R2 to reactor R3 respectively. Any suitable exchange device can be
used, preferably a shell-and-tube type wherein water can be used to
remove heat from the product stream. The water can then be recycled
to one or both of 12 and 23 where it can be further heated to
produce superheated steam. As can be appreciated from the above and
as shown in the examples discussed below, the inlet and outlet
temperatures of the streams entering and exiting methanation
reactors R1-R3 can be controlled by varying the percentage of
syngas being delivered to each of the reactors as well as how much
heat is exchanged by heat exchangers HR2 and HR3. Typically, the
inlet temperature of reactors R1 and R2 will be from about
400.degree. F. to about 450.degree. F. with an outlet temperature
of about 500.degree. F. to about 800.degree. F. The third reactor,
which will operate at a lower temperature than that of reactors R1
and R2 will have an inlet temperature of about 400.degree. F. and
an outlet temperature of about 500.degree. F.
[0041] In a preferred embodiment of the present invention, the step
of recovering at least a part of generated heat and/or at least a
part of waste heat in the regeneration zone and effectively
utilizing the recovered heat is further provided. The recovered
heat can be effectively utilized, for example, for drying and
heating of the biomass feedstock and the generation of steam as the
gasifying agent.
[0042] The product stream from the methanation unit will be
comprised predominantly of methane. That is, it will contain at
least about 75 vol. %, preferably at least about 85 vol. %, and
more preferably at least about 95 vol. % methane. If the methane
product stream is to be introduced into a natural gas pipeline,
then it must meet the specification requirements for the pipeline.
Such a specification for most pipelines, with respect to CO.sub.2
content will be less than about 4 volume percent. If the methane
product stream is to be used for the production of methanol, then
higher amounts of CO.sub.2 will be required. The product methane
stream is preferably introduced into a natural gas pipeline and
utilized at any downstream facility. One such facility if
preferably a plant that converts the methane to syngas then to
other products, such as alcohols, transportation fuels, or
lubricant base stocks. If it is desired to produce syngas from the
methane produced in the methanation unit M, then any suitable
process can be used that convert methane or natural gas to syngas.
Preferred methods include steam reforming and partial oxidation.
More preferred is steam reforming. Steam reforming of methane is a
highly endothermic process and involves following reactions:
[0043] Main reaction
CH.sub.4+H.sub.2O.fwdarw.CO+3H.sub.2 -54.2 Kcal per mole of
CH.sub.4 at about 800.degree. C. to about 900.degree. C.
[0044] Side reaction
CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2 +8.0 kcal per mole of CO at
about 800.degree. C. to about 900.degree. C.
[0045] CO.sub.2 reforming of methane: It is also a highly
endothermic process and involves the following reactions:
[0046] Main reaction
CH.sub.4+CO.fwdarw.2CO+2H.sub.2 -62.2 kcal per mole of CH.sub.4 at
about 800.degree. C. to about 900.degree. C.
[0047] Side reaction: Reverse water gas shift reaction
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O -8.0 kcal per mole of CO.sub.2
at about 800.degree. C. to about 900.degree. C.
[0048] The steam reformer will preferably be one similar to
reformer R hereof, which is a coiled tubular reactor. Preferred
steam reforming catalysts are nickel containing catalysts,
particularly nickel (with or without other elements) supported on
alumina or other refractory materials, in the above catalytic
processes for conversion of methane (or natural gas) to syngas is
also well known in the prior art. Kirk and Othmer, Encyclopedia of
Chemical Technology, 3rd Ed., 1990, vol. 12, p. 951; Ullmann's
Encyclopedia of Industrial Chemistry, 5th Ed., 1989, vol. A12, pp.
186 and 202; U.S. Pat. No. 2,942,958 (1960); U.S. Pat. No.
4,877,550 (1989); U.S. Pat. No. 4,888,131 (1989); EP 0 084 273 A2
(1983); EP 0 303 438 A2 (1989); and Dissanayske et al., Journal of
Catalysis, vol. 132, p. 117 (1991).
[0049] The catalytic steam reforming of methane, or natural gas, to
syngas is a well established technology practiced for commercial
production of hydrogen, carbon monoxide and syngas (i.e., a mixture
of hydrogen and carbon monoxide). In this process, hydrocarbon feed
is converted to a mixture of H.sub.2, CO and CO.sub.2 by reacting
hydrocarbons with steam over a supported nickel catalyst such as
NiO supported on alumina at elevated temperature (850.degree. C. to
1000.degree. C.) and pressure (10-40 atm) and at steam to carbon
mole ratio of 2-5 and gas hourly space velocity of about 5000-8000
per hour.
[0050] This process is highly endothermic and hence it is carried
out in a number of parallel tubes packed with a catalyst and
externally heated by flue gas to a temperature of 980.degree. C. to
about 1040.degree. C. (Kirk and Othmer, Encyclopedia of chemical
Technology, 3rd, Ed., 1990, vol. 12, p. 951, Ullmann's Encyclopedia
of Industrial Chemistry, 5th Ed., 1989, vol. A12, p. 186).
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