Gas turbine slagging combustion system

There is provided a slagging combustion system for generating high purity working fluid suitable for driving gas turbines. The system consists of a precombustor for preheating oxidant tangentially fed to a primary slagging combustor where a solid carbonaceous material is combusted under substoichiometric slagging conditions. Slag is collected in the primary slagging combustor and products of combustion passed to a transition section where tertiary oxidant and sulfur-gettering agents are added, and then to a cyclonic secondary combustion chamber.

BACKGROUND OF THE INVENTION 
Over the past twenty years, the use of gas turbine systems has been on the 
increase among utilities, as well as in industry. Gas turbine systems, 
whether used in conjunction with steam turbines or alone, provide their 
users with an efficient source of energy, mechanical or electrical, at a 
modest capital cost. Typically, capital cost is one-half to one-third that 
of more conventional steam-powered plants. Other advantages associated 
with gas turbine systems stem from their availability at modest sizes when 
compared to steam turbines, the relatively fast delivery time to the user, 
and the flexibility that a modular approach can provide to satisfying 
increasing energy demands. 
The working fluid for gas turbine systems is directly derived from the 
products of combustion of carbonaceous fuels. The highly stressed working 
elements (blades) of the turbine need to preserve their shape and 
integrity for efficient operation over protracted periods of time. In 
consequence, exacting requirements are usually imposed upon the fuels 
which can be used to generate the working fluid for such systems. Up until 
now, non-solid fuels, such as natural gas or oil, have been required to 
provide a working fluid of a purity which minimizes deposition, erosion, 
and corrosion of the turbine elements. 
In recent years, oil prices have increased by about a factor of ten. Many 
electric utility and industrial plants are caught in a cost squeeze. 
Restriction to the use of gas or oil as the fuel for the operation of gas 
turbine systems, makes such systems less attractive to the user. Coal and 
related solid carbonaceous fuels are our most abundant reserve of fossil 
fuels. Accordingly, efforts are presently underway to develop systems 
which would make possible the use of coal in gas turbine systems. Such 
systems, if economical, would be very attractive to users in the utility, 
industrial, and transportation markets. 
Two approaches aimed at using coal with gas turbine systems have received 
attention. The first uses a process in which coal is first gasified, and 
the products of gasification, after being suitably cleaned of impurities 
at moderate or low temperatures, are made available for use in the gas 
turbine system. Drawbacks of this approach are reduced efficiency and a 
substantial increase in capital cost associated with the gasification and 
clean-up processes. 
The second approach relies on coal beneficiation. This approach involves 
removal of the major fraction of the coal impurities. Typically, a 
reduction of the mineral matter content to levels below one-half of one 
percent by weight of the coal, and comminution to sizes small enough so 
that, after combustion, residual ash particles will not exceed dimensions 
of the order of five microns, are required. Removal of a major fraction of 
the sulfur in the coal is also desirable to eliminate the need for 
back-end SOx removal equipment. This approach promises to preserve the low 
capital cost of existing gas turbine systems, but the high cost of 
chemical beneficiation of coal would result in fuel costs comparable to 
oil. 
The present invention is directed to the resolution of the problem by a 
process in which clean-up of the working fluid is carried out in 
conjunction with its generation. With this approach, the benefits of 
modest capital and fuel costs are retained. The present invention utilizes 
a slagging combustor in combination with other equipment and operating 
regimes to provide a working fluid of desired purity for direct use in gas 
turbine systems. 
Slagging combustors are described in U.S. Pat. No. 4,217,132 to Burge, et. 
al. and U.S. application Ser. No. 788,929 filed Oct. 18, 1985, now U.S. 
Pat. No. 4,685,404 which is a continuation of application Ser. No. 
670,417, each assigned to the assignee of record and each incorporated 
herein by reference. 
We have found that the aforementioned slagging combustion systems can 
provide the following advantages. High power density: about 1.0 million 
Btu/hr per cubic foot of volume and per atmosphere of pressure in the 
primary combustion chamber of the slagging combustor. High carbon 
conversion: conversion of substantially all carbon to oxides of carbon 
within the combustion system. Removal of non-combustibles: Capture and 
removal from the gaseous products of combustion of most, of the order of 
95 percent, of the non-combustible mineral content of the fuel before the 
fluid leaves the slagging combustion system. Low NOx: Low nitrogen oxide 
emissions achieved by fuel and air staging for fuels naturally containing 
substantial amounts of nitrogen. Low SOx: Control of sulfur oxide emission 
by the addition of suitable gettering agents into the slagging combustion 
system. Thermal efficiency: Delivery to the end-use equipment of a gaseous 
working fluid having about 85 to 95 percent of the chemical potential 
energy of the carbonaceous fuel. Durability: Protection of the walls of 
the high temperature primary combustion chamber by a layer of slag so that 
deleterious corrosion and/or erosion of the walls can be kept within 
commercially acceptable limits. 
The present invention is directed to improved apparatus utilizing a 
slagging combustor and a process which generates a high-purity working 
fluid from carbonaceous fuels, such as coal, for use in gas turbine 
systems. 
SUMMARY OF THE INVENTION 
According to the present invention, therefore, there is provided a compact 
apparatus and method for efficient combustion of particulate carbonaceous 
materials at high energy output per unit volume, removing noncombustibles 
to the highest levels possible, while minimizing the generation of 
nitrogen oxides and removing from the products of combustion a major 
portion of the fuel's sulfur content. All of this is accomplished prior to 
delivery of purified products of combustion to the end-use equipment at 
pressures and temperatures compatible with the operation of gas turbines. 
The apparatus comprises, in combination, a slagging combustor comprising a 
primary combustor including means for removal and disposal of slag formed 
in the combustion of fuel. The primary combustor, having a head end and an 
opposed apertured baffle at the exit end communicates with a secondary 
chamber by tertiary oxidant injection-transition means for adding 
supplementary oxidant to the fluid passing to the secondary combustion 
chamber to complete combustion and in sufficient quantity to reduce the 
fluid temperature to levels compatible with the end-use equipment. 
Preheated oxidant for the primary combustor is generated in a precombustor 
positioned normal to the axis of the primary combustor and tangentially 
introduced to the primary combustor, along with injection of particulate 
carbonaceous fuel such as coal to the primary combustor. This creates a 
split-flow operation with fuel being combusted in flight to form 
combustion products comprising carbon monoxide and hydrogen with 
generation of slag which is collected on the walls of the primary 
combustor. Means of injecting suitable gettering agents for the control of 
sulfur and, if necessary, alkali vapor are provided in the transition 
means. The secondary combustor includes a cyclonic solids separation 
system which ensures the system will provide a working fluid for a gas 
turbine which is tailored, in purity and temperature, to its needs. 
The apparatus is used in a process for generating a purified working fluid 
suitable for use in a gas turbine and the like. The process comprises 
forming a preheated oxidant in a precombustor zone by combusting first 
particulate carbonaceous material with oxidant to form first combustion 
products and combining in the precombustor zone the first combustion 
products with additional oxidant to yield preheated oxidant at a 
temperature of from about 1200.degree. to about 2500.degree. F. The 
preheated oxidant is tangentially introduced to a primary combustion zone 
hving a head end and an apertured baffle exit end simultaneously with 
injection of second particulate carbonaceous material into the primary 
combustor zone at a point between the head end and exit end in a manner to 
establish first and second high-velocity flows of a mixture oxidizer, 
particulate carbonaceous material, and combustion products with the first 
and second high-velocity flows proceeding respectively toward the head end 
and exit end of said primary combustor zone. 
The oxidizer and fuel input velocities and mass-flow rates are regulated to 
maintain a substoichiometric combustion regime within said primary 
combustion zone, most of the slag content of the fuel is driven to the 
walls of the primary combustion zone, and substantially all of the carbon 
content of the fuel is converted to oxides of carbon before the gaseous 
products of combustion leave the primary combustion zone. Slag formed in 
the combustion of said carbonaceous fuel is removed from the primary 
combustion zone, separately from the gaseous products. 
Tertiary oxidant is introduced into the combustion products in a transition 
zone between the exit of said primary combustion zone and a secondary 
combustion zone to form an oxygen-rich fluid at a temperature suited for 
use in end-use apparatus. Oxidizing of carbon monoxide and hydrogen and 
gettering sulfur oxides and any alkali metal vapor by solid getting agents 
occurs in said secondary combustor zone simultaneously with centrifuging 
and separating of solids, including gettering agents, from the oxygen-rich 
fluid to form a substantially solids-free working fluid which is conducted 
to end-use apparatus. 
In operation, only the primary combustion chamber is operated under wet 
wall (liquid slag-coated) conditions. The secondary combustion chamber, 
including the centrifuging means, is operated in a temperature regime such 
that substantially all of the non-combustible minerals, i.e. fly ash and 
slag, that may be carried over from the primary combustion zone, are 
centrifuged out of the gaseous products and are separately removable from 
the system.

DETAILED DESCRIPTION 
There is provided, in accordance with the present invention, apparatus and 
a method for efficiently combusting particulate carbonaceous materials and 
removing solid noncombustibles to the highest levels possible, while at 
the same time minimizing the generation of the oxides of nitrogen and 
providing an efficient means to remove sulfur compounds before the gaseous 
products are introduced into an associated system for utilizing the 
thermal and chemical energy of the products of combustion, or working 
fluid. The working fluid is provided at sufficient purity for use in a gas 
turbine. 
With reference to FIGS. 1, 2, and 3, the following mechanical units are 
coupled together: a precombustor 10, a primary combustor 12, a slag 
collection unit 14, a tertiary oxidant injection transition section 16, a 
secondary combustor chamber 18, and an exit conduit 20. The apparatus is 
compact and capable of operation at pressures typical of gas turbine 
systems and supplying a working fluid which satisfies stringent gas 
turbine specifications. 
By the term "particulate carbonaceous fuel" used herein, there is meant 
carbon-containing substances that include noncombustible minerals and 
which can be provided as a fuel in a dispersed state, either suspended in 
a gaseous carrier fluid as free particles, or as a slurry. Representative 
carbonaceous materials include, among others, coal, char, the organic 
residue of solid waste recovery operations, tarry oils that are 
dispersible in liquid, and the like. All that is required is that the 
carbonaceous material be oxidizable in the primary combustor chamber, and 
amenable to dispersion as discrete particles in the carrier fluid. 
Preferably, the fuel is powdered coal of a grind so that about 70 percent 
will pass through a 200-mesh screen. By the term "oxidant" there is meant 
air or oxygen-enriched air. By the term "carrier fluid" there is meant a 
gas or liquid which may be inert, or an oxidant. 
With reference to FIGS. 1 and 2, the precombustor 10 comprises, first, an 
oxidant addition chamber defined by converging end wall 24 and a first 
apertured baffle 26, spaced from end wall 24. This chamber includes means 
for imparting a swirling motion to the first oxidant stream. A first 
combustion zone 28 extends along the precombustor axis, from baffle 26 to 
a second oxidant introduction zone 30. Nozzle means 32 is adapted to 
inject the carbonaceous fuel into precombustor 10. The fuel is dispersed 
into the zone 28 by virtue of the initial momentum provided by the nozzle 
and/or by the centrifugal forces which result from the swirling motion of 
the oxidant flow near the baffle aperture. The second oxidant introduction 
zone 30 starts near the exit plane of the cylindrical pre-combustion 
chamber 36 and terminates in a duct extending to the primary combustor 12 
and is attached thereto at an opening 40, preferably rectangular, 
positioned to enable introduction of oxidant and products of combustion 
from the precombustor tangentially and adjacent to the walls of the 
primary combustor 12. The axis of precombustor 10 is preferably 
horizontally oriented. 
With specific reference to FIGS. 1, 2 and 3, cylindrical primary combustor 
chamber 12 is normally positioned with its main axis horizontally oriented 
and in a direction orthogonal to that of the precombustor axis. The 
primary combustor 12 includes a fuel injector 42 for introduction of 
particulate carbonaceous material, which extends into the primary 
combustion chamber from the end wall 44 thereof. 
With reference to FIGS. 5 and 6, primary combustion chamber 12 provides an 
inner wall surface 86, adapted to retain and maintain thereon a slag layer 
resulting from the combustion of the carbonaceous material. The oxidizer 
entering the primary combustor chamber effectively splits into two flows. 
One is directed towards the head-end 44 and fuel nozzle 42, the other is 
directed towards the apertured baffle exit end 52. Preferably, one or more 
dampers 48 are provided at the inlet 40 to the primary combustor to 
control the flow velocity at the inlet of the primary combustor chamber. 
The products of combustion leave the primary combustor chamber through the 
opening 50 of a second baffle 52 with a high velocity swirling motion. 
An opening 54 with a vertically oriented axis is located immediately 
upstream of the exit baffle 52 and collects the flow of slag flowing 
towards the exit of the primary combustor 12. Slag flow occurs primarily 
as a result of the shear forces imparted by the gas motion, and of gravity 
forces, thus effectively removing mineral matter from the system. The 
primary combustor exit baffle 52 may include a cylindrical reentrant 
member 56 extending into the primary combustor to further hinder the flow 
of molten slag to the downstream components. 
Products of combustion from the primary combustor 12 pass into a tertiary 
oxidant injection transition section 16. Tertiary oxidant is added in 
amounts sufficient to complete oxidation of any as-yet unburned 
constituents of the flow, mainly hydrogen and carbon monoxide, and to 
reduce the effluent temperature to a level compatible with the end-use 
equipment. Oxidant injection and transition section 16 is also preferably 
provided with means of injecting suitable gettering agents for the control 
of sulfur emission and/or alkali carry-over in vapor form. The gettering 
agents may be introduced with the tertiary oxidant flow or by separate 
injection nozzles. The tertiary oxidant injection and transition zone 16 
extends to secondary combustor 18 and is attached thereto by an opening, 
preferably rectangular, positioned to enable introduction of the product 
mixture tangentially and adjacent to the inner wall of the secondary 
combustor 18. 
The secondary combustor 18 is geometrically configured as a cyclone 
separator with a symmetry of revolution about a vertically oriented axis. 
It comprises a vertically oriented, cylindrical portion 58 and a conical 
bottom portion 60 for the removal of solid particulate matter from the 
system. The products of combustion exit vertically with a high swirling 
motion through a reentrant outlet conduit 20 having inlet 62 extending 
into the secondary combustion chamber to a point preferably below that of 
the tangential entry port 64. After exiting the secondary combustor 18, 
the products of combustion or working fluid are ducted by conduit 20 to 
the end-use equipment, typically a gas turbine. 
In the presently preferred embodiment, and with reference to FIGS. 1 and 2, 
a fraction of the oxidant flow to the precombustor 10 enters a swirl can 
66 through one or several openings 68 which impart a strong swirling 
motion to the fluid oxidant. This motion can also be imparted or is 
enhanced in the vicinity of the fuel nozzle 32 by a set of guide vanes 70 
generally inclined at an angle of 45.degree. to 60.degree. relative to the 
precombustor axis. This arrangement is effective in preventing coking of 
the fuel nozzle and aids in dispersing the fuel within the combustion 
chamber 36. The fuel nozzle tip is preferably located at or near the plane 
of the baffle 26 which forms the back wall of combustion chamber 36. The 
amount of oxidant entering the combustion chamber 36 is preferably that 
required for stoichiometric combustion of that portion of the fuel being 
fed to the precombustor subassembly. The rest of the oxidizer flows 
outside the combustion chamber 36 through an annular opening 72, whose 
dimension is such that the desired partition of the oxidant is achieved. 
The oxidant mixes in zone 30 with the products of combustion from the 
combustion chamber 36 to yield a heated oxidant with a temperature in the 
range of 1200.degree. to 2500.degree. F. At the same time, a transition in 
the flow cross section from circular to rectangular is effected. The 
oxidant mixture is then injected into the primary combustor through the 
rectangular inlet 40. Damper 48 is used to control the oxidant injection 
velocity as the load dictates. Heat losses and cooling requirements are 
minimized by providing thermal insulation 74 for the walls of precombustor 
10. This is achieved either through the use of refractories or other 
equivalent thermal insulation. The combustion chamber 36 is water-cooled. 
Thermal insulation or an uncooled annular shield 76 minimizes heat loss 
from the oxidant normally introduced at a temperature from about 
250.degree. to about 700.degree. F. to the water-cooled walls of 
combustion chamber 36. 
With reference now to FIG. 3, the primary combustor 12 extends from head 
end 44 to the exit end defined by apertured baffle 52. Particulate 
carbonaceous fuel, along with its carrier fluid, is introduced through 
fuel injector 42, which is preferably located on the axis of the primary 
combustor 12. Fuel injector 42 extends from end wall 44 to a position 
along primary combustor 12 such that the particulate fuel is injected in 
the primary combustion zone 78 at a location just upstream from the 
oxidizer inlet aperture 40. A slag-coated hot sleeve 80, such as described 
in U.S. application Ser. No. 670,417, surrounds the fuel injector 42 and 
promotes rapid ignition of the fuel upon entry into combustion zone 78. 
The oxidant and fuel flows are selected to yield a net oxygen-to-fuel 
mixture ratio from about 70 to about 90 percent of balanced stoichiometry. 
The oxidant velocity at the inlet 40 of the primary combustor 12 is from 
about 200 to about 400 feet per second. This, in combination with head end 
44, the apertured baffle 52, results in a strong confined vortex motion 
characterized by high turbulent sheer rates, complex recirculation zones, 
such as depicted as 82 and 84, and strong centrifugation of nongaseous 
products. This flow field enhances mixing of the reactants and stabilizes 
the combustion process. The centrifugal forces drive substantially all 
solid and liquid noncombustibles, and any noncombusted combustibles, to 
the wall surfaces 86 of primary combustor 12 in the form of molten slag. 
Slag flows towards the baffle 52, and is collected at aperture 54 where it 
is removed by gravity from the primary combustor 12 to a slag collection 
system (not shown). 
With reference to FIGS. 8A and 8B, reentrant portion 56 of baffle 52 serves 
to inhibit the flow of molten slag to the downstream components. 
Typically, the baffle aperture 50 is half the primary combustion chamber 
diameter, and the length-to-diameter ratio of the reentrant portion 56 of 
the baffle is approximately 1 to 1. This, in combination with the strong 
whirling motion, prevents substantially all but the smallest particles to 
be entrained in the gaseous fluid exiting the primary combustor 12. 
With additional reference to FIG. 4, upon leaving the primary combustor, 
the products of combustion enter tertiary oxidant injection and transition 
section 16, where tertiary oxidant is injected from conduit 88 to 
surrounding plenum 90 and through ports 92 and 94 in circular grid plates 
96 and 98. The pattern of oxidant flow, as illustrated in FIG. 4, 
maintains the surfaces of plates 96 and 98 sufficiently cool to prevent 
corrosion and helps insure against burnout of sulfur or other getters 
introduced with the oxidant. Mixing of the two streams results in final 
combustion, mainly of CO and H.sub.2 and oxidation of H.sub.2 S to 
SO.sub.x with dilution to temperatures which are compatible with operation 
of gas turbines, typically in the vicinity of about 1500.degree. to about 
2000.degree. F. As previously noted, selected gettering materials 
(sorbents) can be mixed with the tertiary oxidant prior to injection into 
the system. In the presently preferred embodiment, sulfur getters are 
introduced with the tertiary oxidant and alkali or by conduits 100 
positioned about converging duct 102 forming part of opening 64 leading to 
secondary combustor 18. Duct 102 preferably terminates in a vertically 
oriented rectangular tangential opening 64 at the inlet of the secondary 
combustor 18. The flow in the combustion zone of secondary combustor 18 
may be characterized by a helical motion with a descending outer zone 104 
and an ascending inner zone 106. Residence times in the order of 200 
milliseconds or more are provided for gettering of sulfur and alkali vapor 
species, as required. Ash, slag droplets and debris, and gettering agents 
are removed from the system through aperture 108. The purified products of 
combustion leave the system entering inlet 62 of duct 20. 
With reference to FIG. 7, refractory thermal insulation 110 is provided on 
the internal walls of the secondary combustion chamber so as to minimize 
heat losses and reduce cooling requirements. 
FIGS. 5, 6, and 7 illustrate preferred structures for providing thermal and 
corrosion protection of the walls of the primary and secondary combustors. 
FIG. 5 illustrates the preferred configuration for the slagging walls of 
the primary combustion chamber. Cooling is provided by the flow of coolant 
112 at a suitable velocity inside a passage enclosed by surfaces 114 and 
116. The passage may be a tube, a double-walled membrane construction, or 
the like. When first constructed, a suitable sacrificial refractory 118, 
such as Missouri Flint Clay, is placed on the hot-gas side of surface 116 
in a nominal thickness of about 0.5 inch. During operation, slag deposits 
on the clay. The combination of liquid slag 120, frozen slag 122, and the 
clay 118 are of a thickness such that, when subjected to the heat flux 
from the combustion gases, a freely flowing fluid layer of slag will 
result at steady state. As time progresses, the original refractory is 
partially or completely replaced by the solid and liquid slag layers. The 
flowing slag layer 120 provides a source for curing and replenishment for 
any loss of thermal protection of the wall surface 118. The coolant is 
kept in the temperature range from about 325.degree. to 600.degree. F. 
This minimizes acidic corrosion and also guards against hydrogen sulfide 
corrosion. Water is preferably utilized as the coolant for the primary 
combustion chamber. The water can be used in the overall cycle for 
combined cycle systems or flashed into steam and injected into the 
products of combustion, thus augmenting the turbine work. 
FIG. 6 illustrates the presently preferred wall construction arrangement 
for securely retaining refractory and/or slag. Here, the coolant passage 
surfaces 114 and 116 are the interior and exterior surfaces of a 
cylindrical metal tube. Attached by welding to surface 116 are studs 124, 
generally staggered along the coolant passage length. A tube-and-membrane 
construction is utilized for the containment walls. Each tube is joined to 
adjacent tubes by a full penetration weld at mid-diameter with membranes 
126. This wall construction can also be used for the secondary combustor 
18, studs being effective in retaining refractory of sufficient thickness 
so that heat losses and cooling requirements are minimized. For this 
application, high-porosity, alumina-based refractories, which are 
available as castable, concrete and ramming mixes, can be used. 
FIG. 7 illustrates an alternate wall configuration for the secondary 
combustor 18. Here, metallic anchors 128 are used to hold the refractory 
110 in place. A high-temperature metallic liner 130 can also be used to 
preserve the refractory integrity. The preferred cooling fluids are air 
and water. 
The system operates at the pressure level of the available oxidant, which 
for gas turbine systems is generally in the range of six to fifteen 
atmospheres. Nominal comprised oxidant temperature ranges typically from 
about 250.degree. to about 700.degree. F. A portion of the primary oxidant 
stream is mixed with from about 10% to about 30% of the total particulate 
carbonaceous material to be fed to the system for feed to the combustion 
chamber 36 and is normally sufficient for stoichiometric combustion of all 
the fuel fed to the precombustor 10. Products of combustion chamber 36 are 
diluted by the secondary oxidant flow to form an oxidant-rich effluent, 
e.g. from about 2 to about 4 times that stoichiometry required for the 
precombustor 10, and is suitable for injection into primary combustor 12 
and used therein as the sole, or substantially the sole, source of 
oxidizer for the carbonaceous fuel fed to primary combustor 12. Some 
oxidant may be used as the carrier fluid for particulate carbonaceous 
material introduced to primary combustor 10, but this is a relatively 
negligible amount when compared to the whole. The balance of the 
particulate carbonaceous material is fed by the fuel injector 42 to the 
primary combustor 12 and mixes with the oxidant-rich preheated effluent 
from precombustor 10, which is delivered at a temperature from about 
1200.degree. to about 2500.degree. F. Combustion in the primary combustor 
is substoichiometric with the total oxidizer fed to the primary combustor 
12 being in the range from about 0.7 to about 0.9 times the stoichiometric 
amount that would be required for combustion of the fuel. In the primary 
combustor 12, combustion occurs substantially in flight with conversion of 
a large fraction of the noncombustibles to molten slag which, by the 
whirling action of flow fields within the primary combustion chamber, is 
centrifugally driven to the walls 86 of the primary combustor and collects 
thereon as a slag layer whose surface is liquid or molten. Combustion 
temperatures in the primary combustion zone are maintained above the 
ash-fusion temperature of the non-combustible mineral constituents of the 
fuel. In steady-state operation, slag flows towards the exit baffle 52 and 
is removed from the system through a slag tap opening 54. 
The hot oxidant inflow from the precombustor 10 is beneficial in deterring 
the accumulation of frozen slag near the oxidant-inflow aperture 40. It 
also maintains a high temperature environment throughout the head-end 82 
portion of the primary combustor 12, thereby assuring prompt and stable 
fuel combustion closely adjacent the fuel injector 42. 
The gaseous products of combustion flow through the apertured baffle 52 
into the tertiary oxidant injection transition section 16, where tertiary 
oxidant at operating pressure is introduced to complete oxidation of any 
fuel species, namely, H.sub.2 and CO, and reduce by dilution the 
temperature of the products of combustion to levels in the vicinity of 
about 1500.degree. to about 2000.degree. F., depending on turbine 
specification. The addition of air can be staged so as to avoid quenching 
the kinetics for oxidation of carbon monoxide. Air-staging in the system 
also limits of the formation of the oxides of nitrogen. Sufficient 
tertiary oxidant is added to the products of combustion to form a working 
fluid and so that the temperatures in the secondary combustion zone are 
kept below the ash-fusion temperature of the non-combustible mineral 
constituents of the fuel and sufficiently low to avoid deactivation of the 
sulfur-gettering agents used for removing sulfur constituents of the fuel 
from the working fluid. 
Gettering materials, i.e. sorbents for capturing sulfur and/or alkali 
containing compounds, can be added with the tertiary air, but can also be 
added immediately thereafter, to getter gaseous sulfur and alkali vapor 
species. 
Calcium-based sorbents are preferably used for sulfur capture. Such 
sorbents include limestone, lime, dolomite, hydrated dolomitic lime, and 
the like. For alkali vapor capture, aluminosilicate-based getters are 
preferably used in a similar manner. 
In the particularly preferred embodiment of the invention, the precombustor 
10 has a length-to-diameter ratio of 1.5 to 1; the primary combustor 12 
has a length-to-diameter ratio of 2.4 to 1; the baffle aperture area is 
approximately one-quarter of the primary combustion chamber cross 
sectional area; the cylindrical portion of the secondary combustor 18 has 
a diameter 1.5 to 2 times the primary combustor diameter and a 
length-to-diameter ratio of 1.5 to 2 to 1. The conical bottom portion 60 
of the chamber forms a cone with half angle of the order of 30.degree.. 
Exit duct diameter 108 is one-third to one-half that of the secondary 
combustor diameter. 
As indicated, preconditioning of the oxidant is accomplished in the 
precombustor 10 to which all, or essentially all, the primary combustor 
oxidant is supplied. The primary combustor oxidant is used to combust from 
about 10% to about 30% of the total carbonaceous feed under dry wall, i.e. 
nonslagging condensing conditions, to form a first reaction product. The 
hot, oxidant-rich gas stream containing essentially all of the first 
reaction product is directed in a controlled fashion into the primary 
combustor 12. The oxidant-rich gas stream carries all the precombustor 
fuel and noncombustibles, including still-burning carbonaceous particles 
dispersed throughout its volume. The precombustor exit temperature may 
range from about 1200.degree. to about 2500.degree. F. Precombustor 
geometry provides self-sustaining combustion when air is used as the 
oxidant, and such air is introduced at temperatures of from about 
250.degree. to about 700.degree. F. 
The heated oxidant and reactants, generated in precombustor 10, move 
through a rectangular exit 40 to primary combustor 12 of cylindrical 
geometry. This precombustor-effluent stream is introduced essentially 
tangential to the interior wall 86 of the primary combustor 12. The 
rectangular exit 40 from precombustor 10 is sized such that the dimension 
parallel to the primary combustor axis is larger than the dimension 
perpendicular to the axis of the primary combustor. A length-to-height 
ratio of approximately 2.5 to 1 is preferred. Preferably, the centerline 
of the rectangular exit is aligned with the longitudinal axis of the 
precombustor and is positioned upstream from the mid-point of the primary 
combustor longitudinal axis, i.e. about 1/4 to 1/3 of the distance from 
the head-end 44 to apertured baffle 52. 
By locating the rectangular exit 40 of the precombustor 10 in the 
above-described manner, the precombustor effluent causes a whirling motion 
to be imparted to the flow within the primary combustor 12. We have found 
that, by controlling the precombustor velocity to the order of 200 to 400 
feet per second through the use of one or several damper plates 48 located 
within the rectangular exit 40 of the precombustor, satisfactory 
combustion can be achieved over a wide range of primary combustor fuel 
feed rates. The above location also causes a division of the effluent into 
two whirling flows of comparable magnitude, one whirling towards the 
head-end 44, while the other moves toward the baffle 52 at the exit of the 
primary combustor 12. 
The major portion of the carbonaceous fuel is introduced into the primary 
combustor 12 through a fuel injector 42. The assembly causes the 
carbonaceous material to be introduced in a conical flow pattern into the 
whirling flow field, preferably at a point slightly upstream of the 
precombustor exit rectangular opening 40. The bulk of the combustibles for 
the fuel are consumed in flight through the heated oxidizer flow field, 
giving up energy in the form of heat of reaction and further heating the 
resultant combustion products. The particles in free flight follow 
generally helical flow paths towards the lateral walls and exit baffle of 
the primary combustor 12. 
The fuel-rich gases generated in the head-end of the primary combustion 
chamber generally flow towards the exit baffle 52 while the whirling 
motion is maintained. That portion of the precombustor effluent which 
initially divided from the head-end flow proceeds towards the exit baffle 
plate 52, mixes and reacts with the fuel-rich gases to yield a stream of 
hot combustion products, rich in CO and H.sub.2. 
The particles of fuel while in flight are subjected to enhanced mass and 
heat transport from the turbulent whirling flow field. When most of the 
carbon has been consumed, the remaining particulate melts and, as it 
impacts the walls of the combustor, mixes with the molten slag to form a 
continuous self-replenishing protective layer. The slag flows in a 
generally helical path, under the influence of gaseous aerodynamic shear 
forces and of gravity forces, towards the primary combustor exit and the 
slag tap 54 located near the exit baffle 52. Only the very small 
particulates, generally smaller than 10 microns in diameter, escape the 
chamber with the effluent gas as they are weakly centrifuged by virtue of 
their increased drag in relationship to their mass. 
With reference to FIGS. 8A, B, and C, there are shown three functional 
constructions for the baffle exit. FIG. 8A utilizes lip 132 at the end of 
cylindrical section 56 preferably attached to baffle plate 52. This 
directs slag which flares off back to cylindrical inner wall 86 of primary 
combustor 12, instead of slag passing through port 50 into section 16. The 
construction of FIG. 8B may also use a lip, but has a divergent conical 
configuration for unit 56 to aid recapture of pressure through gas 
deceleration in passing through opening 50. FIG. 8C is the simplest and 
eliminates unit 56. Any lost slag showers into transition zone 16 and 
nominally hardens before striking and sticking to a surface. 
Slag also flows along the exterior surface 80 of the fuel injector 42, from 
the head-end 44 towards the point of injection of the fuel. This very hot 
(molten slag) exterior surface on the injector assembly functions as a 
flame holder to assure stable ignition of the fuel particles as they leave 
the injector. In operation, the flowing slag along the injector strips off 
short of the point of solid particle injection, and provides small-point 
centers of intense radiation and ignition of the head-end-generated 
fuel-rich gases. 
The slag tap 54 preferably is a circular opening at the bottom of the 
primary combustion chamber, located near the upstream side of baffle 52. 
This opening intercepts the flow of slag flowing along the combustor walls 
86 and is used to efficiently remove slag from the system. Baffle 52 also 
acts as a dam which curbs the flow of slag out from the primary chamber 12 
and, by its proximity to the slag tap, alone or in combination with 
reentrant member 56, increases slag collection effectiveness. 
When burning 200-mesh coal, up to about 90 percent or more of the 
noncombustible content of the coal can be captured as slag and removed 
from the system. The balance of the mineral matter leaves the primary 
combustor 12 with the combustion gases, either as fluid-borne particulates 
of very small sizes, less than 10 micron diameter typically, or as liquid 
slag sheared along the inner rim of the baffle assembly by aerodynamic 
forces. In order to prevent the potential problem associated with freezing 
of a wall slag layer in the tertiary oxidant injection and transition 
section 16 located immediately downstream of baffle 52, it is preferable 
to provide for a smooth, well-cooled, inner baffle surface, so that the 
liquid slag layer will become detached from the walls and will form small, 
frozen, air-borne slag droplets. 
The products of combustion leaving the primary combustion chamber enter 
oxidant injection and transition chamber 16. The flow cross section in 
that chamber changes from circular, with a diameter comparable in 
magnitude to that of the primary combustion chamber, to rectangular, with 
an exit aspect ratio of about 2.5 to 1. Preferably, the longer axis of 
said rectangular opening is oriented in a vertical direction. 
In the preferred embodiment, and with reference to FIG. 4, the tertiary air 
is injected in an axisymmetric fashion from low velocity plenum 90, 
radially into the effluent from the primary combustor, as discreet jets or 
sheets through ports 92 and 94 into the fluid-exiting opening 50. Rapid 
mixing, followed by oxidation of the primary combustor effluent and 
cooling down to temperatures suitable for use in gas turbines, takes place 
in the oxidant injection and transition section 16 and in the secondary 
combustor 18. Finely-divided getters for sulfur oxides are preferably 
mixed with the tertiary air prior to injection into the system. When 
exposed to the high-temperature environment of the combustion gases, 
calcium carbonate will calcine to yield calcium oxide, and a highly 
divided surface and porous structure will result which is beneficial to 
capturing sulfur oxides. In the case of lime, the calcination step is not 
necessary. Mixing of the sorbent with the relatively cold tertiary air 
avoids exposure to high temperatures and the ensuing dead-burning which, 
if occurs, greatly reduces the specific surface area available for 
sulfation. Dead-burning can be further avoided by delaying the injection 
of the sorbent in relationship to that of the tertiary air. Preheating of 
the tertiary air is not required. Rather, it may be fed to the transition 
section plenum 90, from the compressor 134 (FIG. 10) of a gas turbine, at 
a mass flow rate, relative to the stoichiometry of the first combustion 
zone, selected to provide optimum temperatures in the secondary chamber 
for sulfur capture without significant dead burn of the sulfur sorbent. 
Injection of the sorbent near the inlet to the secondary combustor 18 also 
avoids exposure to the high temperatures of the effluent of the primary 
combustor 12. 
With reference to FIG. 9, the oxidant injection and transition section 16 
and secondary combustor 18 provide a gas media of the region of SO.sub.2 
capture, where reaction of the sulfur oxides such as: 
EQU SO.sub.2 +CaO+1/2O.sub.2 .fwdarw.CaSO.sub.4 
are favored without the generation of oxides of nitrogen, still keeping the 
object of combusting to extinction of substantially all hydrogen and 
carbon monoxide. 
Gettering agents for the alkali metal (sodium and potassium) vapors can be 
introduced into the system in a similar way. Suitable sorbents include 
active alumina, bauxite, silicate compounds, and the like. 
The mixture consisting of the primary combustor exhaust gases, airborne 
particulates from the carbonaceous fuel, slag droplets sheared away by the 
gases at primary combustion chamber baffle rim, tertiary air, and 
particulate sorbent material is injected tangentially into the secondary 
chamber 18 at velocities of from about 200 to about 400 feet per second. 
The secondary chamber functions as a cyclone separator with flow at the 
periphery having a helically downward motion which effectively transports 
the particulate matter towards the removal point at the apex of the 
conical section. Flow near the axis of the chamber, by contrast, is 
upwards and leaves the chamber with a strong whirling motion through a 
centrally located reentrant pipe. The reentrant exhaust pipe 20, extending 
to a point preferably below the rectangular inlet 64 into the secondary 
combustor 18, also minimizes the amount of particulate matter which could 
directly escape the system. If desired, a conical exhaust pipe can be used 
here to reduce system pressure drop. The secondary combustor is also 
effective in capturing and removing any large debris and/or pieces of slag 
detached from the walls of the primary combustion chamber. 
The exit duct 20 diameter is one-third to one-half that of the cylindrical 
section of the secondary combustor 18. 
The products of combustion or working fluid leaving the system contain a 
very small amount of inert solid particles. 
As an example, initially selecting a coal with an ash content of 5 percent 
by weight, and in the case where only 3 percent of the solids escapes the 
system, the particulate loading in the exhaust is only approximately 50 
parts per million by weight at 15 percent excess O.sub.2. Furthermore, the 
median size for particles will be in the range of 1 to 2 microns, with a 
negligibly small amount larger than 10 microns in size. This is equivalent 
to the carryover that one would achieve using a micronized coal with an 
ash content of 0.15 percent. Furthermore, assuming that by washing, a 
sulfur removal from the coal in the range of 25 to 50 percent is obtained, 
then for a sulfur removal in the range of 60 to 80 percent in the 
combustion system, overall sulfur reduction levels in the range 70 to 90 
percent will be achieved without resorting to any back-end clean-up. 
Staging will also minimize the formation of nitrogen oxides with levels of 
the order of 100 parts per million at 15 percent excess O.sub.2 or lower 
being achievable. 
FIG. 10 illustrates the apparatus of this invention as part of a turbine 
operating system. With reference thereto, air is fed by line 132 to 
compressor 134 driven by turbine expander 136 through shaft 138. Turbine 
136 also does useful work such as powering generator 140 to generate 
electricity. The compressed oxidant is fed to precombustor 10 and tertiary 
oxidant-transition unit 16 and becomes part of a working fluid. Combustion 
is completed and sulfur oxides and/or alkali vapor gettered in cyclone 
secondary combustor 18 and the highly pure working fluid fed to expander 
136 where mechanical energy is generated and spent fluid exhausted by line 
142. 
While we have shown and described a specific embodiment of our invention, 
it is to be understood that various modifications may be made therein 
without departing from our invention; and it is, therefore, our intent to 
cover all changes and modifications as fall within the true spirit and 
scope of our invention.