System and method for gasification of solid carbonaceous fuels

A system is disclosed for the conversion of solid carbonaceous fuels to combustible gases having a high energy content. The system utilizes a two-stage reaction system where a particulate fuel is entrained in a high velocity, hot gas stream emanating from a fixed-bed char reactor, the particulate fuel delivered to a gasification reactor by the hot gas stream while being rapidly heated (fast pyrolized). The gases produced are drawn a fixed distance through a bed of char at the bottom of the gasification reactor, after which they are withdrawn from the reactor and cooled. To promote methanation, the generated gases, after passing through the fixed bed of char in the gasification reactor, exit the reactor through a dip leg within the reactor filled with a catalyst which promotes methanation. Char from the gasification reactor is continuously removed and delivered to the fixed-bed char reactor where an oxygen-containing gas (air or oxygen) and steam are injected into the bed of char to produce the hot gas stream used to entrain the incoming particulate solid fuel.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a gasification system for carbonaceous solid 
fuels and to a method of gasifying such. 
2. Prior Art Relating to the Disclosure 
Gasification of solid fuels, such as biomass and coal, has become of 
increasing interest and importance because of rapidly rising petroleum 
prices, dwindling domestic petroleum and natural gas resources, and the 
increased dependence by the United States on foreign petroleum imports. 
Gasification of coal and biomass has been widely practiced for over 100 
years, and there are many varieties and types of gasifiers and routes to 
gasification. Generally, however, they all fall into the following 
categories: 
1. Pyrolysis 
2. Air gasification 
3. Oxygen gasification 
4. Anaerobic digestion 
Pyrolysis is the breakdown of biomass by heat at temperatures of 
200.degree.-600.degree. C. to yield a medium energy gas, a complex 
pyrolysis oil and char. All biomass gasification and combustion processes 
involve pyrolysis as a necessary first step. There are two kinds of 
pyrolysis--slow and fast. At slow heating rates or with large pieces of 
biomass, pyrolysis leads to a high proportion of charcoal which must then 
be gasified. At rapid heating rates, cellulose, for example, is converted 
to a gas containing a high proportion of olefins that are valuable as a 
chemical feed stock. Char production is minimal. Fast pyrolysis of finely 
divided biomass results in maximum gas yields. 
Air gasification of solid carbonaceous fuels, while requiring an initial 
pyrolytic step, uses a minimal quantity of air and steam to convert the 
char remaining from pyrolysis to gas in a single unit. The gas produced is 
a low energy gas (LEG) because it is diluted by the nitrogen of the air. 
Oxygen gasification of solid carbonaceous fuels produces a medium energy 
gas composed primarily of carbon monoxide and hydrogen. It can be used for 
chemical synthesis or to make methanol, ammonia, hydrogen, methane or 
gasoline, and is called "syngas." 
Hydro-gasification of solid carbonaceous fuels is where hydrogen gas is 
added under high pressure for direct and high yields of methane. 
Anaerobic digestion produces methane and carbon dioxide biologically from 
manure or sewage. It is not generally though of as a gasification process. 
The difficult problem in gasification is the conversion of all of the 
elements comprising the solid biomass into gases containing the highest 
amount of energy. Gasification at lower temperatures produces a high 
proportion of oil in addition to char. Conversion of this char and oils to 
gases can be done by the four basic types of gasification referred to 
earlier, i.e., air gasification, oxygen gasification, hydro gasification 
and pyrolytic processes. Gasifiers of numerous configurations are known 
and generally fall into four categories, namely: 
1. Entrained flow 
2. Fluidized bed 
3. Fixed bed 
4. Molten media Gasifiers which are either commercially available or are 
considered to have potential of becoming commercially available are 
described extensively in Handbook of Gasifiers and Gas Treating Systems, 
Final Report, Task Assignment No. 4, Engineering Support Services, by 
Dravo Corporation, Pittsburgh, Pa., prepared for the United States Energy 
Research and Development Administration under Contract No. 4(49-18)1772 
(February 1976). 
Gasifiers may also be classified as either updraft gasifiers or downdraft 
gasifiers. In downdraft gasifiers, combustion occurs first and the gases 
are then drawn through the hot char. In fluidized bed reactors a number of 
variations of temperature can be used to produce specific intermediate 
equilibrium states giving better control over gas composition. In updraft 
gasifiers, air or oxygen is drawn up through a fixed bed of biomass 
resting on a grate. At the lowest and hottest level on the grate, 
combustion and char gasification occur. As the gases rise, they reach 
successively lower temperature pyrolysis and drying zones and exit the 
gasifier at a low temperature saturated with pyrolysis oils and water. 
Such oil production is largely eliminated in downdraft gasifiers where air 
is introduced between the char zone and the pyrolysis zone with heat from 
the char zone pyrolyzing the biomass above. In this case, the tars and 
oils pass through a bed of hot charcoal where they are cracked and reduced 
mostly to hydrogen and carbon monoxide. 
Oxygen and air gasifiers consume char directly by increasing the oxygen 
content of the biomass to permit gas formation. In pyrolysis, gas, oil and 
char are all formed simultaneously, with the char and oil subsequently 
converted in a separate reactor to heat additional gas with recirculation 
of the hot gases as a heat exchange medium for additional conversion of 
the char and oil to gas. 
The four types of gasifiers mentioned and the type of gas produced depend 
on the number of parameters, including fuel type (biomass, solid municipal 
waste, peat, coal, etc.); fuel size (chunks, shreds, pellets, powders); 
fuel gas contact (updraft, i.e., counterflow; downdraft, i.e., coflow, 
fluidized bed or suspended particles); ash form (dry ash for temperatures 
below 1100.degree. C. or slagging for temperatures above 1300.degree. C. 
depending on feed); pressure; and catalyst use. 
An extensive survey of biomass gasification is given in "A Survey of 
Biomass Gasification"--Volume 1, "Synopsis and Executive Summary," 
SERI/TR-33239, July 1979, Solar Energy Research Institute. Additionally, 
process and equipment information on conversion of biomass to fuels and 
chemicals is given in a report by SRI International for the U.S. 
Department of Energy entitled "Mission Analysis for the Federal Fuels from 
Biomass Program," Vol. 4, Thermochemical Conversion of Biomass to Fuels 
and Chemicals (January 1979). 
SUMMARY OF THE INVENTION 
One of the primary objects of this invention is to provide a gasification 
system and method for the gasification of solid carbonaceous fuels to 
produce combustible gases having a high energy content. 
Another object of this invention is to provide a gasification system for 
gasification of solid carbonaceous fuels employing a fixed-bed char 
reactor in combination with a gasification reactor also having a fixed bed 
of char therein, means for entraining the particulate fuel to be gasified 
in the gasification reactor in a hot gas stream generated by the char 
reactor and means for withdrawing the gases generated in the gasification 
reactor through the bed of char therein. 
A further object of this invention is to provide a method and system for 
gasification of solid carbonaceous fuels wherein particulate solid fuels 
are metered into a high velocity, hot gas stream coming from a fixed-bed 
char reactor, the gas stream having a velocity sufficient to entrain the 
particulate solid fuel in the hot gas stream. The hot gas stream, 
containing little or no oxygen, rapidly pyrolyzes the particulate 
materials while delivering them to a gasification reactor containing a 
fixed bed of char maintained at a temperature of from 
1200.degree.-1800.degree. F. The gases generated in the gasification 
reactor are drawn a fixed distance through the bed of char at the bottom 
of the reactor and may then be contacted with a catalyst which promotes 
methanation, such as iron particles, iron wool or other such catalyst. The 
exiting gases may then be run through a heat exchanger to cool them and 
extract the heat content thereof for other uses. The char in the 
gasification reactor is continuously removed and fed to the fixed-bed char 
reactor where steam and oxygen or an oxygen-containing gas are injected 
thereinto to gasify the char and generate a hot gas stream having a 
temperature of from 2000.degree.-4000.degree. F. which is used to entrain 
the solid particulate fuel fed to the gasification reactor. 
It is a further object of this invention to provide a gasification system 
and method wherein the thermal efficiency of the unit is about 85 percent 
and the gas generated from the unit has a heat content of from 800-1000 
BTU/ft.sup.3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The schematic diagram illustrates a two-stage reactor system utilizing two 
separate reactors. It should be understood, however, that an integral 
reactor having two separate reaction zones may be used in place of the two 
separate reactor vessels. As illustrated, the system includes a char bed 
reactor 10 and a gasification reactor 40, the two reactors connected by a 
refractory-lined conduit 30. 
The char bed reactor 10 is lined with an insulative refractory lining 11. 
The reactor vessel is supported on legs 12 and has an opening in the 
bottom wall 13 through which char exits from the reactor. The vessel also 
includes an opening in the top wall 14 through which gases generated in 
the unit exit the vessel and an opening in the side wall 15 through which 
char from the gasification reactor is fed by a char conveyor 60. The 
reactor has a char bed 16 therein which is continuously stirred during 
operation by a rotary grate. The grate includes a central shaft 17 to 
which a set of lower arms 18 and upper arms 19 are attached. The arms 
extend at right angles to one another from their connection to the shaft. 
Each of the lower arms also includes a plurality of downwardly depending 
fingers 20 which serve to stir and keep the char bed from compacting. The 
grate is protected by a layer of insulative refractory lining. The shaft 
is driven by a motor 21 connected thereto by suitable reduction gearing. 
The char reactor also includes means for injecting water or steam and air 
or oxygen into the interior of the reactor. One or more steam inlets 22 
are provided for injecting steam into the reactor at a level beneath the 
surface of the char bed. One or more inlets 23 are also provided for 
injecting oxygen or air into the bed or beneath the bed of the reactor as 
illustrated. It is also desirable to include one or more inlets 24 for 
injecting air or oxygen into the reactor above the level of the char bed 
for purposes to be described. The reactor may also include an auxiliary 
heater 25 for initially heating the bed of char in the reactor to 
operating temperature. 
Gases generated in the char bed reactor exit the reactor through conduit 30 
which is lined with refractory 31. A venturi 32 is positioned in the 
conduit. The venturi is designed to increase the velocity of the hot gases 
exiting from the char bed reactor through the conduit sufficiently to 
entrain the particulate fuel injected into the hot gas stream through an 
infeed inlet 33 located downstream from the venturi 32. The venturi is 
preferably a "dumping type" venturi wherein the velocity of the gases is 
increased by decrease of the area of the conduit on the infeed side of the 
venturi while allowing the pressure to abruptly drop due to an abrupt 
increase in the area of the venturi on the downstream side of the venturi 
to aid in entraining the particulate fuel in the gas stream, due to 
turbulent mixing. The particulate solid fuel is fed into the conduit by 
any suitable feed means. Preferred is a conventional and commercially 
available rotary air lock feeder having multiple pockets containing the 
particulate fuel to be fed into the gasification reactor. 
The gasification reactor 40 is also supported on legs 41. The interior 
walls thereof are lined with an insulative refractory lining 42. A char 
bed 43 occupies the lower portion of the reactor, the char bed agitated 
and kept in motion by a rotary grate 55 of similar construction and 
operation as that of the char bed reactor. The reactor includes an opening 
in the bottom wall 44 for dumping of char from the bed 44 into a char 
conveyor 60 which conveys it to the char bed reactor. An opening 45 in the 
side wall of the reactor receives the hot gases from the char bed reactor 
containing the entrained particulate solid fuel. A further opening in the 
top wall of the gasification reactor receives a dip leg 46 extending 
beneath the surface of the char bed. The gasification reactor also 
contains means for withdrawing gases generated in the upper zone of the 
gasification vessel and feeding them through the char bed in the lower 
zone thereof before exit of the gases from the reactor through the dip 
leg. The lower end of the dip leg extending beneath the surface of the 
char bed is perforated to receive the gases exiting from the downcomer 
tubes 49 through the char bed. The gases generated in the gasification 
reactor exit through an opening 47 in a baffle 48 surrounding the dip leg 
and flow through a series of downcomer tubes 49 spaced circumferentially 
around the dip leg. The lower ends of each of the tubes 49 extend beneath 
the surface of the char bed. The gasification reactor also includes 
openings 50 beneath the surface of the char bed in the reactor for 
injection of air and/or oxygen thereinto. Means 51 may also be provided 
for injecting air and/or oxygen into the upper reaction zone above the 
char bed as illustrated. 
It is important that the gases generated in the gasification reactor be 
withdrawn an equal distance through the char bed before exit from the 
reactor. As illustrated, the gases generated in the gasification zone, as 
well as those generated by the char bed reactor, exit through the openings 
47 and are pulled beneath the char bed through a plurality of downcomer 
tubes extending around the circumference of the reactor and through the 
char bed. The gases flow a fixed distance "d" through the char bed before 
exiting through perforations 52 in the dip leg. 
The hot gases exiting the gasification reactor through conduit 53 may be 
run through a heat exchanger 54 for generation of steam for use for 
carrying out the reaction or for other purposes. The cooled gases exiting 
from the heat exchanger may be used directly for generation of heat, as a 
chemical feed stock or for other purposes. 
The respective speeds of char conveyor 60 and ash conveyor 70 are 
coordinated to maintain the levels of the respective beds in the reactors 
substantially constant. 
Method of Operation 
Referring to the schematic, the char bed reactor 10 is loaded with charcoal 
briquettes or other solid carbonaceous fuel to a level above the air or 
oxygen inlets 23. After loading, the heater 25, which may be a propane, 
oil or gas heater, is activated to heat the reactor and the contents to a 
temperature of between 1200.degree.-1300.degree. F. This results in 
charring if a carbonaceous fuel, such as wood wastes, is employed. When 
the bed has been sufficiently heated, a small amount of air or oxygen and 
steam is injected into the char bed through inlets 22 and 23 beneath the 
top surface thereof. The oxygen and steam react with the carbon in the 
char bed to generate gases containing principally carbon monoxide. The 
reaction is exothermic and therefore increases the temperature of the bed 
within the reactor to approximately 2000.degree. F. or more. Air and/or 
oxygen may be injected through inlets 24 to further increase the 
temperature of the gas stream by the exothermic conversion of carbon 
monoxide to carbon dioxide. The gases generated in the char bed exit the 
reactor through conduit 30, preferably at a temperature of about 
3000.degree. F. and a pressure of about 1-2 psig. The gases flow through a 
dumping venturi 32 whose area is about one-half to one-fourth the area of 
the incoming conduit. This results in an increase in velocity of the gases 
such that particulate solid fuels fed into the hot gas stream on the 
downstream side of the venturi are entrained in the very hot gas stream. 
The exposure of the solid particulate fuels to the hot gas stream results 
in fast pyrolysis of the solid particulate fuels simultaneously with entry 
of the entrained solid fuels into the upper zone of the gasification 
reactor. 
At the same time the char bed reactor is loaded with charcoal briquettes or 
other solid carbonaceous fuel, the gasification reactor is also loaded 
with charcoal briquettes and heated with an auxiliary heater 47 to 
1200.degree.-1300.degree. F. Oxygen or air is injected into the char bed 
to increase the temperature of the char bed to 1200.degree.-1300.degree. 
F. 
Once the exiting gases from the char bed reactor are sufficiently hot to 
instantly pyrolyze the incoming particulate solid fuel, the auxiliary 
heater 25 is turned off and the system is balanced by injection of air or 
oxygen and steam into the char bed of the char bed reactor and air or 
oxygen into the char bed of the gasification reactor to maintain the 
temperature substantially constant. At the same time, the speeds of the 
char bed conveyor, the ash conveyor and the amount of fuel fed into the 
gasification reactor are coordinated to maintain the level of the beds in 
each reactor substantially constant. 
The size of the particulate solid fuel fed into the system generally ranges 
from one-fourth inch to one-sixteenth inch in thickness or diameter. 
Larger or smaller particle sizes may be used, depending on the velocity of 
the gases into which the particulate fuel is fed. 
If it is desired to generate a combustible gas mixture coming from the 
gasification reactor containing a major amount of methane, the dip leg 
tube 46 may be filled with a catalyst, such as iron wool or chrome/nickel 
stainless mesh or other suitable catalyst which promotes methanation of 
the gas stream. 
The gasification reactor is, in effect, both an entrained reactor and a 
fixed-bed reactor. When the particulate fuel injected into the hot gas 
stream coming from the char bed reactor is pyrolyzed in the upper zone of 
the gasification reactor, the condensible pyrolysis oils generated are 
cycled through the bed of char in the lower zone of the gasification 
reactor where they are broken down at high temperature so that the gases 
leaving the gasification reactor contain a minimum amount of condensibles 
and a maximum amount of other useful gases, such as hydrogen, etc. The 
temperature of the char bed in the gasification reactor is preferably 
maintained between about 1600.degree.-2200.degree. F. Temperatures less 
than 1600.degree. F. result in slow reaction. The temperature of the bed 
in the char bed reactor is preferably maintained at a temperature of about 
1800.degree.-2500.degree. F. This is done by injection of steam which 
reacts with the carbon in the bed at the high temperature conditions to 
generate, principally, carbon monoxide. Air or oxygen injected into the 
reactor above the char bed results in increased temperature of the gas 
stream to 3000.degree. or more by the exothermic conversion of the carbon 
monoxide to carbon dioxide. The generation of the hot gas stream in the 
char bed reactor by injection of air and/or oxygen and steam into the char 
bed is an exothermic reaction, whereas the pyrolysis reaction occurring in 
the upper zone of the gasification reactor is principally an endothermic 
reaction. The amount of air and/or oxygen injected into the gasification 
reactor and the amount of air and/or oxygen and steam injected into the 
char bed reactor is controlled to maintain the endothermic/exothermic 
balance of the system. It is important to note that the gases formed by 
pyrolysis of the incoming particulate solid fuel in the gasification 
reactor are further heated by withdrawal through the char bed in the lower 
zone of the gasification reactor. The char withdrawn from the gasification 
reactor and delivered to the char bed reactor is also further heated in 
the char bed reactor before discharge. 
The system may be operated at atmospheric pressure, above atmospheric 
pressure, or below atmospheric pressure. A pump 80 may be provided to pull 
the gases from the gasification reactor through the downcomer tubes, char 
bed and dip leg and may contain a catalyst to promote methanation. The gas 
stream exiting the gasification reactor at a temperature of about 
1500.degree. F. may be run through trays of lime for promotion of 
methanation. 
Referring to the schematic, the distance "d," through which gases withdrawn 
from the upper zone of the gasification reactor must pass to enter the dip 
tube, is constant. This constant path is important in obtaining a uniform 
constituency of the gases exiting from the gasification reactor. 
The system disclosed accomplishes almost complete gasification of solid 
particulate fuels without generation of volatiles or condensibles in large 
quantities. In many prior art systems, the incoming fuel to be pyrolized 
falls directly on a carbon bed, with no entrainment of the incoming gas 
stream. This results in a high amount of condensibles, which require more 
oxygen if they are to be converted to non-condensible gases. In constrast, 
the incoming particulate fuels in the system described are entrained in a 
very hot, substantially oxygen-free gas stream which rapidly pyrolyzes 
them. Further, any condensibles generated in the upper zone of the 
gasification reactor of the system described are broken down as those 
generated gases are fed through the bed of char in the lower zone of the 
gasification reactor. 
The principal uses for a unit of this type are for generation of 
combustible gases containing a medium or high energy content for heating 
uses, such as heating of lime kilns or other high energy uses. 
EXAMPLE 
A char bed reactor 6 ft. in diameter and 12 ft. high containing a 6 in. 
insulative refractory lining (2 in. of high temperature fiber insulation 
and 4 in. of low iron content refractory) was filled with charcoal 
briquettes to a level above the point where the oxygen inlets 23 in the 
side walls of the vessel were located. An auxiliary propane heater having 
a heat output of about 200,000 BTU'S per hour was used to heat the bed of 
charcoal to 1200.degree.-1300.degree. F. At that temperature, a small 
amount of oxygen was injected into the bed to raise the temperature of the 
bed as high as possible, generally up to about 2000.degree. F. At that 
temperature, a small amount of steam was injected into the bed to increase 
the temperature of the bed and the temperature of the gas stream 
generated. A small amount of oxygen was also injected into the reactor 
above the fixed bed. The resulting gas stream excited the char reactor at 
a temperature of approximately 3000.degree. F. containing principally 
carbon dioxide. This hot gas stream, at a pressure of 1-2 psig, passed 
through the venturi which increases the velocity thereof. On the 
downstream side of the venturi, a sudden pressure drop aided in pulling in 
the particulate solid fuel being in-fed by a rotary airlock feeder 
containing eight pockets. The solid particulate fuel entrained in the very 
hot gas stream was rapidly pyrolyzed in the upper zone of the gasification 
reactor at approximately ambient pressure. The generated gases were drawn 
off through downcomer tubes in the gasification reactor and were withdrawn 
from the gasification reactor through the incandescent, fixed bed of 
carbon in the lower zone of the reactor through a dip leg in the reactor 
containing an iron wool catalyst. Oxygen was injected into the bed of 
incandescent carbon in the gasification reactor, as necessary, to maintain 
the temperature of the bed of carbon around 1600.degree. F. The gas was 
pulled through the bed of incandescent carbon and from the reactor by a 
vacuum pump. The hot gases exited the gasification reactor at a 
temperature of about 1500.degree. F. and were fed through a heat exchanger 
used to generate steam. The cooled gases exited the heat exchanger at a 
temperature of about 250.degree. F. and a pressure of about minus 5 psig.