Coal gasification process with inhibition of quench zone plugging

A process for the gasification of coal to produce synthesis gas is disclosed, the process being characterized by passage of product gas stream containing sticky or molten particles upward from the gasification zone and quenching of the product gas stream and particles in a quench zone coated or lined internally with boron nitride.

BACKGROUND OF THE INVENTION 
Partial combustion or gasification of coal involves reaction of the coal at 
elevated temperatures, and possibly elevated pressures, with a limited 
volume of oxygen, the reaction preferably being carried out in a reactor 
or reaction chamber or vessel by means of "burners" in the presence of 
additional agents such as steam, carbon dioxide, or various other 
materials. Gasification of coal produces a gas, known as synthesis gas, 
that contains mostly carbon monoxide and hydrogen. Also produced are 
varying minor quantities of other gases, such as carbon dioxide and 
methane, and, at least with some coals, various heavier materials, such as 
small sticky or molten particles. In some processes, the design of the 
gasifier or reactor is such that the sticky or molten particles are 
carried downward principally by the synthesis gas through a water quench 
area or zone, and thence to a slag recovery area. Remaining fine 
particles, now solidified, pass with the synthesis gas from the bottom of 
the quench zone or cyclones, where the particles are separated. 
In at least one other coal gasification process undergoing development, the 
design of the gasifier is such that a rough separation of the molten 
particles takes place in the gasifier vessel or chamber. That is, the 
heavy particles drop to the bottom of the gasifier vessel to a slag 
recovery area or bath, and lighter and molten particles are carried by the 
synthesis gas upward and out of the reactor chamber into a quench zone 
which is mounted generally above the gasifier, and wherein a cool quench 
gas is employed to quench the gas and particles. The particles carried 
upward, in the aggregate, tend to be of somewhat different chemical 
composition than the "slag" which falls to the bottom of the vessel, and 
are designated collectively herein as "flyslag." The solidified material, 
because it is derived from a "reducing" atmosphere, may be different in 
composition and properties from flyash normally associated with combustion 
boilers, wherein a fully oxidizing atmosphere is utilized. For example, 
the flyslag from processes for partial combustion of coal may contain 
elemental iron and sulphides, components not normally associated with 
boiler flyash. 
A significant concern in processes where the molten or sticky particles are 
transported up into the quench zone is the possibility that the flyslag 
particles will stick to the walls of the quench zone. Unlike the 
down-fired processes, where water may be present or injected to quench and 
help wash down the particles, a quench gas, such as a cool recycle gas, 
may be employed, along with indirect heat exchange, for quenching and 
cooling the synthesis gas and the sticky or molten particles. Sticking of 
the flyslag particles will cause loss of heat transfer, and, of greatest 
concern, possibly result in plugging of the quench zone. The invention 
addresses this problem. 
SUMMARY OF THE INVENTION 
Accordingly, the invention relates to a process for the gasification of 
coal in which particulate coal is partially oxidized in a gasification 
zone or gasifier, producing a hot synthesis gas containing sticky or 
molten flyslag particles. The hot synthesis gas containing the sticky or 
molten flyslag particles is then passed upward from the gasification zone 
to a quench zone where the gas is quenched, and the particles are 
solidified, the quench zone comprising an indirect heat exchange zone, the 
heat transfer surfaces of which indirect heat exchange zone in contact 
with the hot synthesis gas and through which heat is extracted from the 
hot gas to a coolant at least partly being composed of boron nitride. 
As used herein, the terms "surface" or "surfaces", in referring to the 
material in contact with the hot synthesis gas and the sticky or molten 
flyslag particles, refer either to a coating of the boron nitride on the 
quench zone heat exchanger wall or walls, or to a liner of boron nitride 
positioned between the synthesis gas and the flyslag and heat exchanger 
wall or walls (or tubes). The coating or liner may be fabricated according 
to techniques known to those skilled in the art. Coatings have the 
advantage that they may be readily applied to the walls of the exchanger, 
such as by spraying, vacuum application, or brushing, e.g., in the form of 
a dispersion, and may be applied in layers, while a liner will generally 
require fabrication offsite, hanging mechanism(s), and insertion in place. 
On the other hand, a liner may be fabricated with grooves or channels, for 
heat exchange fluids, such as steam, and may tend to last longer because 
of its greater thickness. In general, coatings will normally be applied in 
a thickness of up to 30 mils or so, preferably from 10 to 12 mils, while 
liners may range up to 1/4 to 1/2 inch, or even more, in thickness. In 
both cases, consideration must be taken of differences in the coefficients 
of expansion of the boron nitride coating or liner and the quench zone 
heat exchanger wall or walls. In the case of a coating, a pre-coating of 
the wall or walls with another material which absorbs some of the 
expansion differences or which enhances bonding may be employed, or the 
dispersion of the boron nitride may be formulated to provide some "flex", 
as understood by those skilled in the art. In the case of a liner, 
provision may be made in the mounting of the liner, and/or a coolant may 
be circulated in channels of the liner to regulate expansion. It is not 
necessary that all of the quench zone be coated or lined with boron 
nitride; preferably, however, at least the lower half of the vertically 
disposed zone is coated or lined. 
Preferably, the temperature of the surface of the boron nitride in contact 
with the hot synthesis gas and the flyslag should be below a certain 
temperature or temperature range. If the surface of the boron nitride is 
above a certain temperature or temperature range, a tendency for some 
particles to stick and accumulate may arise. The invention has the 
advantage that even if the temperature of the liner or coating is such 
that there is a tendency to stick, the "non-stick" character of the boron 
nitride coating or liner inhibits sticking. As used herein, the term 
"temperature at which particles tend to stick" is a range of temperatures, 
and will vary from coal to coal, depending on the composition of the 
matter which forms the particles. Accordingly, a precise number or range 
cannot be given, but simple testing will determine this temperature or 
range. For example, a testing procedure analogous to that described in the 
New York State Energy Research and Development Report 82-36 (12-1982) may 
be employed. In the case of most coals, particles may tend to stick at a 
boron nitride surface temperature (in contact with the gas and particles) 
which is above about 500.degree. C. 
After the starting materials have been converted, the reaction product, 
which has a temperature of between about 1050.degree. C. and about 
1800.degree. C., and which comprises hydrogen, carbon monoxide, carbon 
dioxide, and water, as well as the aforementioned impurities, is passed 
upward from the reactor. As will be evident, passing the hot synthesis-gas 
containing sticky particles upward from the reactor provides a separation 
of the synthesis gas and the particles, so that, in some instances, this 
separation, along with rapid quench and/or cooling, is sufficient to 
prevent deposition of the particles. In other cases, however, the sticky 
particles represent the problem mentioned, and the particles must be taken 
into account. By use of a coating or liner of boron nitride, as specified, 
with an appropriate liner temperature, the particles will proceed upward 
without sticking, or will fall back into the gasifier. The upward moving 
particles will then be solidified by the quench gas and indirect heat 
exchange, and the synthesis gas stream with solidified particles then 
passes on for further cooling and treatment. As indicated, a variety of 
elaborate techniques have been developed for quenching and cooling the 
gaseous stream, the techniques in the quench zone and primary heat 
exchange zone in general being characterized by the use of a quench gas 
and a boiler in which steam is generated with the aid of the waste heat. 
The walls of the quench zone, i.e., the external or wall surfaces not in 
contact with the synthesis gas, and those of the primary heat exchange 
zone, are cooled with boiling water or steam. 
DETAILED DESCRIPTION OF THE INVENTION 
The partial combustion of coal to produce synthesis gas, which is 
substantially carbon monoxide and hydrogen, and particulate flyslag, is 
well known, and a survey of known processes is given in "Ullmanns 
Enzyklopadie Der Technischen Chemie", vol. 10 (1958), pp. 360-458. Several 
such processes for the preparation of hydrogen and carbon monoxide, 
flyslag gases are currently being developed. Accordingly, details of the 
gasification process are related only insofar as is necessary for 
understanding of the present invention. 
In general, the gasification is carried out by partially combusting the 
coal with a limited volume of oxygen at a temperature normally between 
about 1050.degree. C. and about 2000.degree. C. If a temperature of 
between 1050.degree. C. and 2000.degree. C. is employed, the product gas 
may contain very small amounts of side products such as tars, phenols and 
condensable hydrocarbons, as well as the molten or sticky particles 
mentioned. Suitable coals include lignite, bituminous coal, subbituminous 
coal, anthracite coal, and brown coal. In order to achieve a more rapid 
and complete gasification, initial pulverization of the coal is preferred. 
Particle size is preferably selected so that 70% of the solid coal feed 
can pass a 200 mesh sieve. The gasification is preferably carried out in 
the presence of oxygen and steam, the purity of the oxygen preferably 
being at least 90% by volume, nitrogen, carbon dioxide and argon being 
permissible as impurities. If the water content of the coal is too high, 
the coal should be dried before use. The atmosphere will be maintained 
reducing by the regulation of the weight ratio of the oxygen to moisture 
and ash free coal in the range of 0.6 to 1.0, preferably 0.8 to 0.9. The 
specific details of the equipment and procedures employed form no part of 
the invention, but those described in U.S. Pat. No. 4,350,103, and U.S. 
Pat. No. 4,458,607, both incorporated herein by reference, may be 
employed. Although, in general, it is preferred that the ratio between 
oxygen and steam be selected so that from 0.1 to 1.0 parts by volume of 
steam is present per part by volume of oxygen, the invention is applicable 
to processes having substantially different ratios of oxygen to steam. The 
oxygen used is preferably heated before being contacted with the coal, 
preferably to a temperature of from about 200.degree. C. to 500.degree. C. 
The details of the gasification reactor system form no part of the present 
invention, and suitable reactors are described in British Pat. No. 1501284 
and U.S. Pat. No. 4,022,591. The high temperature at which the 
gasification is carried out is obtained by reacting the coal with oxygen 
and steam in a reactor at high velocity. A preferred linear velocity is 
from 10 to 100 meters per second, although higher or lower velocities may 
be employed. The pressure at which the gasification can be effected may 
vary between wide limits, preferably being from 1 to 200 bar. Residence 
times may vary widely; common residence times of from 0.2 to 20 seconds 
are described, with residence times of from 0.5 to 15 seconds being 
preferred.

Accordingly, pulverulent coal is passed via line (1) into a coal dryer (2) 
where the coal is dried, suitably at a temperature of about 200.degree. C. 
The dry coal is subsequently discharged through a line (3) and passed into 
a gasification reactor (4) where it is gasified at a temperature of about 
1500.degree. C. to about 2000.degree. C., a pressure of about 35 
atmospheres absolute, with oxygen, which is supplied through a line (5). 
The gasification produces a product or synthesis gas containing sticky 
molten particles which is removed from the upper portion (6) of the 
reactor, and a slag which is removed from the lower portion of the reactor 
via line (7). The gasification product is passed upward via conduit or 
quench zone (8) where it is quenched by cooled synthesis gas supplied via 
line (9) and indirect heat exchange with steam, and is then passed via 
duct (8a) through a boiler or heat exchange zone (10) where it is cooled 
to a temperature of about 200.degree. C. The walls and tubes of quench 
zone (8) in contact with the synthesis gas are coated with boron nitride. 
In the heat exchange zone (10), water, which is supplied through line 
(11), is converted by indirect heat exchange to high pressure steam, the 
steam being discharged through a line (12). The cooled gasification 
product is passed through a line (13) to a series of cyclones (14) where 
the bulk of the the particulates (flyslag) is removed and sent via line 
(15) to storage. The synthesis gas then passes via line (16) to a series 
of purification steps designated as (17) where a final, cooled product 
synthesis gas is removed via line (18). A portion of the cooled gas is 
recycled via line (19) to quench zone (8) for quenching the hot gas from 
reactor (4). A partially cooled, impure gas is removed and utilized (not 
shown). 
While the invention has been illustrated with particular apparatus, those 
skilled in the art will appreciate that, except where specified, other 
equivalent or analogous units may be employed. The term "zone", as 
employed in the specification and claims, includes, where suitable, the 
use of segmented equipment operated in series, or the division of one unit 
into multiple units to improve efficiency or overcome size constraints, 
etc. For example, a series of scrubbers might be employed, with different 
aqueous solutions, at least the bulk of the "loaded" solutions being sent 
to one or more strippers. Parallel operation of units is, of course, well 
within the scope of the invention.