This invention is in the technical field of coal burners for furnaces and gas turbine engines.
Prior efforts to burn coal in gas turbine engines, such as by use of pulverized coal, or coal in water slurries, have been unsatisfactory due to turbine blade maintenance problems, caused by coal ash particles being carried into the turbine blades, with the hot gases flowing therethrough.
As a result, gas turbine engines, such as are used for electric power generation in combined cycle plants, today burn natural gas, or petroleum distillate fuels, and these fuels are increasingly in short supply, and thus expensive.
As of October 2004, coal cost is about one-fifth of natural gas cost, per unit of energy. Known coal reserves are much greater than known petroleum and natural gas reserves, both nationally, and internationally.
In a mixed fuel coal burner of this invention, coal chunks pass through two separate reaction chambers in series. In the first, ODD, reaction chamber the coal is heated by a throughflow of hot gas, containing some oxygen, in order to carry out oxidative destructive distillation, ODD, of the coal volatile matter. The oxygen content of these hot gases is less than stoichiometric, relative to the coal volatile matter, so that partial oxidation of only the volatile matter occurs in the ODD reactor. Two fuel products thus emerge from the ODD reactor, a devolatilized coke product, which is passed into the second coke reactor, and a partially oxidized coal volatile matter gas. The partially oxidized coal volatile matter gas is mixed with an overfire air, and fully burned, in an ODD overfire burner, and the resulting burned gases pass into the turbine. Partially oxidized volatile matter can thus be burned cleanly in the ODD overfire burner, with greatly reduced creation of undesirable soot or tar, and this is one of the beneficial objects of this invention.
The devolatilized coal chunks are delivered, by overfeed, into the top, or gas exit end of a bed of hot burning coke, in the second coke reactor. Primary air flows upward through the coke reactor and countercurrent to the coal chunk flow direction. The coke reacts rapidly with oxygen in the counterflowing primary air, and the resulting very hot burned gases quickly heat up entering coke chunks by connective heat transfer. The coke is thus burned up rapidly, and completely, to carbon monoxide and carbon dioxide while passing through coke reactor. This coke burn rate is proportional to the rate of supply of primary air into the coke reactor. Thus the power output of a gas burning engine, using this coal burner, can be controlled by control of the primary air flow rate into the coke reactor over a very wide range of engine power output, and corresponding coal burn rates.
The exit gases from the second coke reactor are rich in carbon monoxide fuel, and these are mixed with additional overfire air, and burned fully to carbon dioxide, in a carbon monoxide overfire burner, and these fully burned gases flow into the turbine inlet nozzles.
Prior art underfeed coal burners used a single reaction chamber to achieve similar clean burning of high volatile matter bituminous coals. With these underfeed burners, the coal and primary air moved in the same direction through the reactor. As a result, the fresh coal volatile matter evaporates into the oxygen rich incoming primary air. In this way the evaporating volatile matter receives the partial oxidation, needed for clean burning thereof, without soot and tar formation. But the entering coal chunks are heated up to rapid burning temperatures, by slow radiation heat transfer between chunks, and not by rapid convective heat transfer from hot burned gases. This radiation heat transfer rate, and hence the coal burn rate, is not only slow, but cannot be controlled by control of the primary air flow rate, as is needed for control of the power output of a gas turbine engine.
This is another beneficial object of this invention, over the prior art, that high volatile matter bituminous coals can be burned cleanly, at a high burn rate, and that this burn rate can be controlled, over a wide range, by control of the rate of flow of primary air into the second coke reactor.
With overfeed supply of coal chunks into the coke reactor final coke burnup to ashes occurs at the bottom of the second coke reactor, and the ash particles, which are smaller than the coal chunks, are restrained from being blown out of the coke reactor, and into the gas turbine engine, by the overlying coke bed. With prior art, underfeed coal burners, the small ash particles are formed at the top of the fuel bed, and can thus be blown out of the fuel bed, and into the gas turbine engine, resulting in turbine blade damage.
It has been this carryover of ash particles, into the gas turbine engine, and resulting turbine blade damage, which has previously prevented the use of low cost, and readily available, coal fuels in gas turbine engines. At present, gas turbine engines, such as are widely used in combined cycle electric power generating plants, operate only on expensive natural gas, or petroleum distillate fuels. This is a principal beneficial objet of this invention, that low cost, ash containing, coals can be cleanly burned, in a gas turbine engine, without ash carryover into the turbine blades.
A mixed fuel coal burner of this invention can additionally comprise a supplementary fuel air mixture overfire burner, which burns a gas or liquid fuel, such as natural gas, in the overfire space, above the coke fuel bed. This supplementary fuel burner can be used to control gas turbine engine speed very closely, as such burners can respond quickly to speed changes. The coal burner, while readily governable, responds slowly to speed changes, whereas in most applications, such as electric power generation, very close speed control is needed.
This supplementary fuel air mixture overfire burner also provides a method for adjusting the relative fuel quantities being used by the gas turbine engine, and these quantities can then be changed, in response to changes in fuel prices and availability.
A schematic diagram of an example form of mixed fuel coal burner, of this invention, is shown in FIG. 1.
In FIG. 2 an example controller, for controlling a mixed fuel burner of this invention, is shown schematically for use on a gas turbine engine.
Additional related controls are shown schematically on FIG. 3.
The preferred operating regions, for the ODD reactor portion, of a mixed fuel coal burner of this invention, are shown graphically in FIG. 4.
The preferred operation regions for the coke reactor portion, of a mixed fuel coal burner of this invention, are shown graphically in FIG. 5.
The operating characteristics of an example generator of hot oxygen containing gases is shown graphically on FIG. 6.
An example refuel mechanism for use on a mixed fuel coal burner is shown schematically in FIGS. 7 and 8. FIG. 8 is the cross section, A-A, of FIG. 7. FIG. 7 is the cross section, B-B, of FIG. 8.
An example gas manifold and inlet ports system for admitting hot oxygen containing gases into an ODD reaction chamber is shown in cross section in FIG. 9.
The effects of coal chunk size, and fragmentation, on chunk lift off limited air mass velocities, through the ODD reactor, and coke reactor, are shown graphically on FIG. 10.
An example air manifold and inlet ports system for admitting primary air into the coke reactor is shown in cross section in FIG. 11.
None of the apparatus drawings are to scale.
One example form of a mixed fuel coal burner of this invention is shown schematically in FIG. 1, as adapted for use with a gas turbine engine, and comprises the following elements:
The hot oxygen containing mixer exit gases are ideally admitted uniformly over the ODD reactor cross sectional area. But this uniform admission would require a flow of these hot gases through the refuel mechanism, 3, with resulting maintenance problems on the refuel mechanism. A compromise plan is to admit the hot mixer exit gases peripherally into the ODD reaction chamber, 2, via several ports, 101, distributed peripherally around the chamber, from a supply manifold, 102, as illustrated in FIG. 9. These gas admission ports, 101, are preferably somewhat above the refuel mechanism, 3, by at least one refuel package, 103, in order to insulate the refuel mechanism, 3, from the hot mixer exit gases flowing through the ODD reaction chamber.
With peripheral admission of hot mixer exit gas, into the ODD reactor, as illustrated in FIG. 9, coal chunks, in the reactor center, will receive mixer exit gas partially oxygen depleted by flow through outer coal chunks. This could result in a possible increased yield of soot and tar from these central coal chunks. This effect becomes aggravated as ODD reactor cross section area is increased. By using two or more ODD reaction chambers, 2, for each coke reactor, 7, the area of each ODD reactor can be reduced, and the formation of soot and tar reduced.
For the purposes of this invention, the flow rate of oxygen, into the ODD reactor, 2, via the hot mixer exit gas, is to be less than stoichiometric, relative to the flow rate of coal volatile matter, into the ODD reactor, 2, via the refuel mechanism, 3. Details for achieving this result are described hereinbelow in the ODD reactor sizing section.
But volatile matter molecules, not thusly partially preoxidized, are known to create soot and tar when burned. This is one of the beneficial objects of this invention, that coals, containing appreciable volatile matter, such as bituminous coals, can be burned cleanly, with little or no formation of soot or tars, by applying this oxidative destructive distillation, ODD, process to the coal chunks within the ODD reactor chamber.
The primary air is preferably admitted peripherally into the coke reaction chamber, 7, at several points, 104, distributed peripherally around the chamber from a supply manifold, 105, as shown in FIG. 11. These primary air admission points, 104, are preferably somewhat above the ash removal mechanism, 8, so that the high temperature oxygen burn zone, 11, is separated from the ash removal mechanism, by an insulating layer of ash, 9, located just below the admission points, 104.
If coals which tend to form klinkers are to be used, the coke reaction chamber can be tapered, as shown in FIG. 11, with the cross section area increasing in the direction of coke motion. In this way, downward motion of the coal chunks may not be impeded by klinkers.
With peripheral admission of primary air into the coke reactor as illustrated in FIG. 11, centrally located coal chunks will receive air partially depleted of oxygen by flow through outer coal chunks. These central coal chunks may thus react more slowly than the outer coal chunks and this effect is aggravated as coke reactor cross sectional area is increased. This uneven reaction of coke can be minimized by using two or more coke reaction chambers.
A mixed fuel coal burner of this invention can alternatively be used in applications other than gas turbine engines, such as boiler furnaces, or process furnaces, where the furnace functions as the receiver of hot burned gases created by the mixed fuel coal burner.
As primary air flow rate is increased, the stagnant gas film on the hot coke surface is reduced in thickness, with a consequent increase of both the heat transfer rate from the hot gases to the surface, and the rate of diffusion of oxygen and carbon dioxide molecules, and hence reaction with, the hot carbon surface. In this way the rate of burning of coke in the coke burner, 7, can be controlled by control of the rate of flow of primary air into the reactor.
With overfeed delivery of coke chunks, into the coke reactor, 7, the coke is gradually gasified, as it descends through the reactor, and final coke burnup to ashes occurs at the bottom of the coke fuel bed. The resulting ash particles are much smaller than the coke chunks, but are nevertheless prevented from being blown through the coke reactor, and into the gas turbine engine nozzles and blades, by the overlying bed of coke chunks. With prior art underfeed burners, this final burnup to ashes occurs at the top, gas exit, end of the fuel bed, and the smaller ash particles can be blown over, through the overfire burner, and into the gas turbine engine, resulting in turbine blade damage.
Prior art efforts to burn coal, in gas turbine engines, using underfeed burners, or pulverized coal burners, have been commercially unsuccessful, due to this ash carryover causing turbine blade damage. It is a principal beneficial object of this invention, that high volatile matter, low cost, bituminous coals, can be cleanly burned, in a gas turbine engine, without ash carryover into the turbine blades.
Primary air is delivered into the gas inlet near the bottom of the coke burner, 7, and flows upward countercurrent to the coke flow direction. The ashes in the ash removal mechanism, 8, and in the protective ash layer, 10, can thus be cooled by the relatively cold incoming primary air. Such cooling, as the ash is emerging from the coke, can function to limit ash particle fusion and agglomeration into klinkers.
When using free burning, non caking, coals, at essentially steady coal burn rates, adjustable flow dampers could alternatively be used to distribute the compressor air flow portions, into the several burners, mixers and reactors. But where caking coals were being used, with consequent variations in coal bed flow resistance, or where coal burn rates varied over a wide range, such flow dampers would require frequent adjustment, and a meter, or other sensor means, to check on flow rates.
An example of a suitable disc or plate type force feed refuel mechanism is described in U.S. Pat. No. 5,485,812, Firey, 23 Jan. 1996, in FIG. 2, and column 4, lines 16 through 45. For purposes of the present invention, a mask element can be added, between the refuel driver piston, 35, and the reaction chamber, 7, to prevent fuel bed slumping when the driver piston, 35, is retracted at the time of movement of the transfer plate, 33. Also the driver piston, 35, stroke length would be limited to the depth of the refuel cavity, 34. This material from U.S. Pat. No. 5,485,812 is incorporated herein by reference thereto.
Another example of a suitable rotary force feed refuel mechanism is described in U.S. Pat. No. 4,653,436, Firey, 31 Mar. 1987, and this material is also incorporated herein by reference thereto. This rotary force feed mechanism can also be adapted for use as an ash removal mechanism, for purposes of the present invention, as is described in U.S. Pat. No. 4,653,436.
The refuel mechanism driver controller, for the present invention, may differ from the driver controllers described in these referenced US patents.
Another example refuel mechanism is illustrated schematically in cross section in FIGS. 7, and 8, and comprises the following components:
At the lower refuel end, 4, of the ODD reaction chamber, 2, the refuel mechanism, 3, comprises:
An example of a suitable sliding plate type of ash removal mechanism is described in U.S. Pat. No. 5,613,626, Firey, 25 Mar. 1997, and this material is incorporated herein by reference thereto. The ash removal mechanism driver controller for the present invention may differ from the driver controller described in referenced U.S. Pat. No. 5,613,626. The mechanism, described in U.S. Pat. No. 5,613,626, can also be modified to function as a positive, force feed, refuel mechanism, as described therein on column 7, lines 6 through 44.
The ash removal form of the mechanism, described in U.S. Pat. No. 5,613,626, provides for positive removal of the ash from the ash cavity, 5, by the transfer driver piston, 6, and subsequently also by the dump driver piston, 25. Such positive final dumping of the ashes may be preferred when ash klinkering is a problem, as with those coals having a low ash fusion temperature.
The refuel mechanism, 3, when driven through a refuel cycle by the driver, delivers an essentially constant volume of coal into the ODD reaction chamber, 2, on each cycle. The average rate of coal supply into the ODD reaction chamber, can be controlled by adjusting the length of the refuel time interval, (tRF), between refuel cycles, coal supply rate increasing when refuel time interval is shortened. The refuel time interval can be thusly adjusted by the controller, in terms of clock time, or, for constant speed gas turbine engines, in terms of engine shaft revolutions between refuel cycles.
The ash removal mechanism, 8, when driven through an ash removal cycle by the driver, removes an essentially constant volume of ashes from the coke reaction chamber, 7, on each cycle. An example controller, of this ash removal process, senses the ash depth in the chamber 7, as by a thermocouple or infrared temperature sensor, and, when ashes accumulate to the sensor depth, and thus reduce the temperature there, initiates an ash removal cycle. The sensor depth is set high enough above the ash removal mechanism, that, after completion of an ash removal cycle, an adequate depth of ash remains to protect the ash removal mechanism from the very hot oxygen burn zone, 11.
The ODD reactor burner, 14, and ODD mixer, 19, can be combined, as is common practice with aircraft gas turbine engine burners and cooling air mixers.
In both the ODD reactor burner, 14, and the supplementary fuel air mixer, 30, the ratio of fuel to air is to remain constant, at somewhat fuel lean from stoichiometric, and the fuel supply meters, 17, 33, and corresponding air supply meters, 15, 29, are to function in this manner. For example, where natural gas fuel is used, with positive displacement air and gas supply meters, a common drive can be used to drive both the fuel meter, and the air meter, at the same speed, with the ratio of meter displacement volumes per revolution being equal to the intended fuel to air ratio.
When gas turbine engine load increases, a sensor of turbine power output, such as an electric generator wattmeter, 36, or a turbine power output shaft torque meter, could act via a coal controller, 37, to carry out the following control functions:
This coke bed exit gas composition sensor has the advantage that coke fuel bed depth is automatically compensated for changes in coke fuel chunk size (dCH), and primary air flow rate, (G0). However, drawing a suitable sample of coke fuel bed exit gas, for this composition sensor to analyze, is a difficult problem.
Automatic sensor and control apparatus, as described hereinabove, will usually be preferred, but hand sensor and control methods could also be used, for example in small plants.
The gas turbine engine type, size, speed, and allowable turbine inlet temperature, will determine the proportion of compressor discharge air available as air flow to the coal burner, at rated gas turbine engine power output. Also determined are the burner operating pressure (PO), the burner air inlet temperature (TA3°R), and the engine thermal efficiency relative to the lower heating value (LHV), of the coal.
The coal properties needed are the coal lower heating value (LHV), the weight fraction volatile matter, (VM), the weight fraction fixed carbon (FC), the weight fraction oxygen, and the weight fraction ash (ASH), by proximate analysis, and the coal density.
The equivalent coal chunk size (dCH), can be preselected as a design variable, or can be measured approximately by a technique described hereinbelow.
For the approximate reactor sizing methods described herein, the rough approximation is made that all coals have an “equivalent” molecular weight of 12, as carbon. This approximation ignores the different burning stoichiometry of the hydrogen and sulfur portion of a coal. However, the resulting design errors are small, since coal hydrogen contents are small.
The coal energy rate per megawatt of gas turbine rated power output (MW), can be calculated:
And the coal feed rate into the ODD reactor, 2, by the refuel mechanism, 3, can be calculated:
The fuel for the ODD Burner, 14, and the supplementary overfire burner, 31, is, in this example calculation, assumed to be natural gas, composed largely of methane (CH4), each mol of which requires at least a stoichiometric air flow of 9.52 mols.
A. ODD Reactor Sizing
The ODD burner, 14, and mixer, 19, are to supply a hot gas, containing some oxygen, into the ODD reactor, 2 in order to increase the temperature of the coal therein, from the coal refuel temperature, (To°R), up to the coal rapid devolatilization temperature (Tcx°R). An energy balance on this overall process yields the following approximate relation for the molal ratio of ODD burner methane to coal:
Wherein:
This relation between mixer exit gas temperature, (Tmx°R) and molal ratio of mixer air to ODD burner CH4, is shown graphically on FIG. 6, together with the variation of ODD mixer exit gas oxygen concentration.
The experimental data on rate of coal devolatilization, as a function of coal temperature, and heating gas temperature, presented in reference C, indicate that coal devolatilization is rapid at temperatures at or above 1500°R, and is essentially complete, for very small coal particles, in less than one minute's time. For larger coal chunks, the heat transfer process, from the hot through flow gas, into the coal chunks, is unsteady, the chunk centers being the last to reach a rapid devolatilization temperature, and hence the last to undergo devolatilization. Additionally, for coal chunks, the rate of heat transfer from the hot gas, into the coal, increases as throughflow gas mass velocity (Gf), increases due to thinning of the stagnant gas film on the coal surface. An approximate analysis of this unsteady heat transfer process, using the methods of Gurney and Lurie, as presented in reference D, together with the film heat transfer coefficient relations, for gas to a bed of solid chunks, presented in reference E, indicates that the coal chunk centers reach within 100 degrees Rankine of the adjacent hot gas temperature, in less than about two minutes' time, for hot gas throughflow mass velocities (Gf), above about 350 lbsmass, per hour, per square foot, of ODD reactor cross sectional area.
As described hereinbelow, the coal refuel process is intermittent, a refuel time interval, (tRF) intervening between refuel steps. Hence each refuel package of coal chunks remains inside the ODD reactor for at least one refuel time interval Since refuel time intervals will preferably exceed two minutes, it follows that essentially complete devolatilization of the coal fuel chunks can occur in the ODD reactor during the first refuel time interval.
Devolatilization of the central portions of a coal chunk may take place in the absence of the oxygen needed for partial oxidation to prevent tar and soot formation. Hence the amount of tar and soot may increase as coal chunk size is increased.
The molal air flow rate to the ODD burner, via air supply meter, 15, can be estimated as about 9.52 times the molal CH4 flow rate into the burner, via gas supply meter, 17.
Volatile matter, emerging from the coal chunks in the ODD reactor, 2, during devolatilization, mixes into the hot oxygen containing gases from the mixer, 19, flowing through the ODD reactor. Thus the emerging volatile matter has first call on the oxygen available in these throughflowing gases. To avoid any oxidation of the coke, formed by devolatilization, to ashes while within the ODD reactor, the oxygen available for the coal volatile matter is preferably less than the stoichiometric oxygen, for full burnup of the volatile matter. In this way, only partial oxidation of the volatile matter takes place, and essentially all of the oxygen, in the throughflowing gases, is reacted only with emerging volatile matter. Ash formation is to be avoided, within the ODD reactor since the small ash particles, emerging at the ODD reactor upper gas exit, can easily be carried into the turbine blades by the throughflowing gas. This preferred operation, of the ODD reactor can be achieved by controlling the flow of mixer air, into the ODD mixer, 19, to be less than stoichiometric for the flow of coal volatile matter, into the ODD reactor by refueling.
An approximate energy balance on the ODD reaction chamber, 2, yields the following relations for the coal temperature (Tcx°R), in the chamber:
For this ODD reaction chamber energy balance, the approximation is made, that the endothermic heat of reaction of the destructive distillation of the volatile matter, is offset by the exothermic heat of reaction of the partial oxidation of the volatile matter.
The fraction of stoichiometric oxygen flowing through the ODD reactor relative to the flow rate of coal volatile matter can be estimated by the following relations:
Wherein:
An example ODD reactor chamber energy balance result is shown graphically on FIG. 4, for a coal with a volatile matter weight fraction of 0.35, and for a range of values of coal devolatilized temperatures, (Tcx°R), and also a range of value of (r), the stoichiometric oxygen fraction.
On this coal, the ODD reaction chamber could be operated at the following example conditions, as shown on FIG. 4:
Preferably, the ODD reaction chamber is to be operated, with the stoichiometric oxygen traction less than 1.0, to assure no ash formation within the ODD reactor, as described hereinabove.
At very low values of (r), the desired partial oxidation of the volatile matter may be incomplete, resulting in increased formation of soot and tar. For values of (r) close to stoichiometric, burnup of the volatile matter may be almost completed within the ODD reactor. The subsequent overfire burning, of the small residual unburned volatile matter, may be incomplete, due to excess dilution of the reactants. Hence intermediate values of (r) are preferable, to be selected experimentally, at full overfire burnup of coal volatile matter, with minimum soot and tar.
Whatever soot and tar are formed in the ODD reactor can be largely removed from the gases flowing through the reactor, by filtering these gases through a deep bed of the coke chunks, produced by the devolatilization process. For this reason, each refuel package of coal chunks preferably remains inside the ODD reactor for several refuel time intervals. The depth of the coal bed, inside the ODD reactor preferably sufficient to hold at least two refuel packages, and preferably more. With three or more refuel packages, within the ODD reactor the most recent package can function to insulate the refuel mechanism, by admitting the hot oxygen containing gas well above the refuel mechanism, as shown in FIG. 9.
The longer the residence time inside the ODD reactor the greater the extent of capture of soot and tar, and the more completely these captured products of devolatization are transformed into coke, and subsequently transferred into the coke reaction for complete burnup to CO2.
For preferred filtering of soot and tar, within the ODD reactor the depth of the ODD reactor (LODDR), is to be essentially a constant, and independent of the refuel package volume, (RCV), or the refuel time interval (tRF). This depth is another design variable, wherein greater depth yields more complete capture of soot and tar, together with a larger pressure drop through the ODD reactor.
The volume of a cylindrical ODD reactor, (VODDR), can be estimated as follows:
Wherein:
The ODD reactor cross sectional area, (AODDR) can be estimated in terms of the throughflow gas mass velocity (Gf), which is a design variable.
(Gf)(AODDR)=(292+138w)(H)(J)(MW Power Output)
Wherein:
(Gf)=pounds mass flow, per hour, per square foot of ODD reactor cross section area, when empty, of gases flowing through the ODD reactor excluding the volatile matter flow;
Design values of the throughflow gas mass velocity (Gf), and hence of the ODD reactor class sectional area (AODDR), can be based on the following considerations:
Tentatively, design values for gas mass velocity (Gf), appear to lie preferably between about 300 lbsmass, per square foot, per hour, and about 1500 lbsmass, per square foot, per hour;
Design values for the refuel package volume (RCV), as well as the refuel time interval (tRF), are more readily determined from the coke reaction sizing, as described hereinbelow.
B. Coke Reactor Sizing
As primary air flows through the reacting coke fuel bed, the oxygen reacts rapidly with the carbon fuel, by diffusing through a stagnant gas film, to the hot carbon surface. This reaction is rapid, and oxygen is largely depleted within the early portions of the fuel bed, the reaction products being mostly carbon dioxide, with some carbon monoxide. The carbon dioxide subsequently reacts further with the carbon surface, by diffusing back thereto, and forms additional carbon monoxide further along in the fuel bed. The carbon dioxide reaction, with the carbon surface, is appreciably slower than the oxygen reaction therewith. Nevertheless, the early formed carbon dioxide is, in turn, depleted as the gases flow further through the fuel bed. The gases, leaving the fuel bed, may thus contain carbon monoxide, and some carbon dioxide, plus nitrogen, with only trace amounts of surviving oxygen.
In an equilibrium coke fuel bed, all of the fuel, supplied to the top of the coke reactor, from the exit of the ODD reactor is gasified, and only ashes remain at the bottom of the coke reactor. Thus coke feed rate, into the coke reactor is to equal carbon gasification rate therein. This carbon gasification rate is proportional to the rate at which oxygen is supplied into the coke reactor by the primary air, supplied via the positive displacement air supply meter, 12. As primary air flow rate, G0, is increased, the stagnant gas film on the carbon surface becomes thinner, and both oxygen and carbon dioxide react more rapidly with the carbon.
Two differing reaction zones are thus created within the reacting coke fuel bed: an oxygen burn zone, 11, where oxygen in the primary air reacts rapidly with carbon, to form largely carbon dioxide, with some carbon monoxide; and a carbon dioxide reaction zone, 25, where the carbon dioxide, from the oxygen burn zone, reacts further with carbon, to form additional carbon monoxide. These two reaction zones overlap in part. The carbon dioxide reaction, while slower than the rapid oxygen reaction, is nevertheless rather fast, the endothermic heat of the carbon dioxide reaction being supplied by heat transfer to the coal chunks from the very hot gases, flowing out of the oxygen burn zone.
For efficient combustion, all of the carbon monoxide, formed inside the coke reactor, 7, is to be burned further to carbon dioxide, in an overfire burner, 26, supplied with overfire air via the air supply meter, 27.
As the coke reactor exit gas carbon monoxide concentration is decreased, a weaker carbon monoxide plus air overfire flame results. With room temperature reactants, a carbon monoxide in air flame becomes non-burnable at molal diluent ratios (DRCO), greater than about 0.55, as described in reference B. This dimensionless molal diluent ratio can be described as follows:
The nitrogen and carbon dioxide, in the coke reactor exit gases, are the principal diluents, and the carbon monoxide overfire air, supplied via air supply meter, 27, is the air for CO burnup. At elevated gas temperature, the carbon monoxide flame, being less chilled, becomes burnable at diluent ratios greater than 0.55, but the relation of usable diluent ratio to gas temperature is not well established.
Herein, the conservative sizing assumption is illustrated, that a molal diluent ratio no greater than about 0.55 is used, for the carbon monoxide overfire burner, 26, and that the coke fuel bed, in the coke reactor, is to be sufficiently deep to react most of the diluent carbon dioxide into carbon monoxide, in order to achieve this diluent ratio.
Some bituminous coals break up, during devolatization, into smaller coal chunks. As a result the coal chunk equivalent diameter (dCHE), within the coke reactor, may be one half, or one fourth, or less, of the coal chunk diameter (dCH), as refueled into the ODD reactor. These smaller coal chunks offer a greater surface area for the oxygen and carbon dioxide reactions, which thus occur more rapidly, and a shallower coke fuel bed will be adequate to meet the diluent ratio requirement.
Herein, the additional conservative sizing assumption is illustrated, that the coke chunk diameter (dCHE) is the same as the coal chunk diameter (dCH), for sizing the coke fuel bed depth, within coke reactor. The resulting design coke fuel bed depth will be adequate for coals which do not break up on devolatization, and will be more than adequate for those coals which break up into smaller chunks.
The relations of, fuel bed exit gas composition, mols carbon gasified per mol of primary air, and diluent ratio of the coke fuel bed exit gas at overfire, obtained by approximate analysis of this model of the fuel bed reactions, are plotted graphically in FIG. 5, against fuel bed depth factor (bm)(z);
Wherein:
These analytical results, shown in FIG. 5, agree reasonably well with experimental results, such as are presented in reference A, even though the mol fractions were not corrected for the changes in total number of mols, due to reaction.
The preferred operating range, for the coke reactor, shown in FIG. 5, lies between a fuel bed depth factor (bm)(z), of about 1.6, at the diluent ratio limit for carbon monoxide burnability at overfire, and a fuel bed depth factor about 3 to 4, at the carbon dioxide depletion limit, hence:
1.6<(bm)(z)<4.0
Within this preferred coke reactor operating range, the carbon gasification rate is roughly constant, at about 0.40, as shown in FIG. 5:
The reacting fuel bed depth (Z), in feet, can be calculated in terms of the fuel bed depth factor (bm)(z):
The total coke fuel bed depth, (ZT), is to exceed these reacting bed depths, by an additional ash bed depth, (ABD4), as described hereinbelow.
The maximum refuel quantity volume, (RCV) should be less than the product of coke reactor grate area, (GAR), times the maximum change of reacting fuel bed depth:
(RCV)<[(Zmax)−(Zmin)](GAR), in ft.3
The required total coke reactor grate area, (GAR) is related to the primary air mass velocity (G0), the carbon gasification rate, and the gas turbine power output (MW), as follows:
Wherein:
The refuel quantity (RCV), and refuel time interval (tRF), are related to the gas turbine engine power output as follows:
Wherein:
(RCV)=Volume of total refuel quantity in cubic feet;
(tRF)=Refuel time interval between refuel processes, mins.;
(df)=Coal fuel density, lbsmass per cubic foot;
For reactor sizing purposes, the refuel quantity, and refuel time interval, are selected for maximum gas turbine engine power output. At reduced power output, longer refuel time intervals are used, with a constant refuel quantity.
The internal volume of the coke reactor, 7, is to at least equal the product of grate area (GAR), and the total maximum coke fuel bed depth, (ZTmax)=[(Zmax)+ABD4)];
The net ash removal rate is necessarily related to the coal refuel rate, and the coal ash content, as follows:
Wherein:
(ARV)=Ash removal cavity, 8, volume;
(da)=Ash density;
(tAR)=Ash removal time interval;
The packing factor, (PF), is assumed equal for both coal particles and ash particles;
For the common case, where the ash removal cavity, 8, is fully aligned with the burner, the cross sectional area of the cavity, will equal the cross sectional area of the grate, (GAR).
To protect the ash removal mechanism, from the high temperature of the oxygen burn zone, 11, in the fuel bed, the minimum ash bed depth, (ABD3), is to exceed the ash removal cavity depth,
by a protective ash bed layer, 10, of thickness, (PABD). Current experience with coal stokers indicates that a protective ash bed layer of as little as one or two inches is adequate.
During each ash removal interval, ash accumulates above the minimum ash bed depth (ABD3), up to the maximum ash bed depth (ABD4), at which depth the ash bed depth sensor initiates an ash removal process, which returns the ash bed depth to its minimum value (ABD3), by removing an ash volume of (ARV). Hence:
The primary air mass velocity, (G0), through the coke reactor grate area, is limited to less than the lift off mass velocity (GLO), at which coke chunks start to lift off the fuel bed. Design limiting values for (GLO) can be estimated from the following approximately relation:
(GLO)=√{square root over ((dfx)(PF)(dCHE)(3.2×106))}
Wherein:
(GLO)=Air mass velocity, in lbsmass, per hour, per square foot of grate area, at incipient lift off;
(dfx)=Coke chunk density at fuel bed gas exit, in lbsmass per cubic foot;
(PF)=Coke chunk packing factor=0.74;
(dCHE)=Coke chunk equivalent diameter, in feet, at fuel bed gas exit;
The coke density at fuel bed gas exit (dfx), can be estimated as the coal density, reduced by the volatile matter removed in the ODD reactor; (dfx)=(df)(1−VMP)
(VMP)=coal volatile matter weight fraction;
The coal chunk equivalent diameter, (dCHE), can be estimated as the coal chunk equivalent diameter, (dCH), divided by the number of fragments, (FRAG), into which the coal chunk breaks up during devolatilized:
An example calculation, of limiting air mass velocity (GLO), for a typical bituminous coal, is shown on FIG. 10 for several values of fragmentation factor (FRAG), and coal chunk starting size (dCH). For this particular coal, design values of coal chunk size of one inch or greater, can be selected, with air mass velocities up to about 1000 lbsmass per hour per square foot of grate area, or more.
Where several separate coke reactors are used, on a single gas turbine engine, and each of these separate coke reactors receive coke from several separate ODD reactors the total refuel quantities, and air and gas flow quantities, will usually be equally divided among these several reactors. Multiple coke reactors could have separate multiple overfire burners, or, alternatively, a single combined overfire burner could receive exit gases, from these several coke reactors, and all of their connected ODD reactors.
C. Overfire Burners Sizing
A mixed fuel coal burner of this invention, will use at least two overfire burners. In the ODD overfire burner, the partially oxidized volatile matter, emerging from the ODD reactor exit, is mixed with ODD overfire air, and ignited and burned, largely to CO2 and H2O. In the carbon monoxide overfire burner the coke reactor exit gases, containing carbon monoxide are mixed with carbon monoxide overfire air and ignited and burned largely to CO2 and H2O. In this way the coal supplied to the reactors, is finally completely bummed, as desired for high engine efficiency, and these burned gases flow into the turbine inlet nozzles.
In many applications of mixed fuel coal burners, an additional supplementary overfire burner may be preferred, in order to assure close speed and load governing of the gas turbine engine. In this supplementary overfire burner, a supplementary fuel, such as natural gas, or distillate petroleum fuel, is mixed with supplementary overfire air, and ignited and burned in the supplementary overfire burner.
1. ODD Overfire Burner Sizings
Wherein:
2. Carbon Monoxide Overfire Burner Sizing
Wherein:
3. Supplementary Overfire Burner Sizing
The following calculated results were obtained for a 25 MW gas turbine engine:
Analysis of coke reactor exit gas composition, at one or more values of coke fuel bed depth, with overfire air briefly stopped, could provide experimental values for the reaction rate factors, (a), and (bm). With these data, obtained over a range of values of primary air flow rate, G0, the coke bed reaction properties could be experimentally determined.
Coal chunk equivalent diameter, (dCH), can be approximated, if screened, as the average of the screen sizes. For unscreened, coal, or where chunk size varies widely, a light oil holdup versus time test, can be used to compare a coal against spherical chunks of known diameter.
A strong economic incentive exists to use low cost coal, in place of high cost natural gas, in gas turbine engine driven electric generators, As of October 2004, coal cost, per unit of energy, is about one fifth of the natural gas cost per unit of energy.
The mixed fuel coal burner, of this invention, describes apparatus, and a process, for clean burning of coal, in gas turbine engines, without ash particle carryover into the turbine blades.
At current fuel prices, as of October 2004, a fuel cost savings of about three cents per kilowatt hour can be realized by substituting coal for natural gas, in a gas turbine electric generator, such as a combined cycle plant.
A modified mixed fuel coal reactor of this invention could be used in cupola furnaces for melting cast iron or iron blast furnaces. The mixed fuel coal reactor creates two different fuel products from the coal, a clean burning fuel, derived from the coal volatile matter, and a coke fuel, derived from the coal fixed carbon. The coke fuel would be used in the cupola furnace, and the volatile matter derived fuel could be used as the energy source for a gas turbine, or piston engine, driving the air blower for the cupola furnace.
By adding limestone into the original coal, the sulfur oxides, formed from the coal sulfur content, and the nitrogen oxides, formed from the coal nitrogen content, can be captured, in the deep coke fuel bed of the coke reactor. In this way undesirable emissions of sulfur and nitrogen oxides can be reduced.
A gas turbine engine, using a mixed fuel coal burner of this invention, could be the electric power generator for a total energy system, to heat homes and factories cleanly, with low cost coal, instead of the high cost natural gas, or petroleum distillate, fuels currently used.