US2865344A - Apparatus and method for heating steam - Google Patents

Description

Dec. 23, 1958 A. c. FIRL 2,865,344

APPARATUS AND METHOD FOR HEATING STEAM Filed June 21, 1955 f has Q 22 4 Q X g t 7 K28? 32 5 0 t I2 6 I 32 l 5 l I i ,la

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I r I INVENTOR: 2 ARTHUR C. FIRL Unite tates APPARATUS AND METHOD FOR HEATING STEAM Application June 21, 1955, Serial No. 517,039

4 Claims. (Cl. 122-478) This invention relates to improvements in the operation of steam generating furnaces, steam boilers and steam heaters. t is more specifically concerned with ways and means for achieving highly eflicient combustion of fuel. Another aspect of the invention deals with an improved method of controlling the temperature of the superheated steam and the temperature of the reheated steam independently in a steam generating apparatus operating under the reheat cycle.

in the operation of furnaces in general and of furnaces for steam generation in particular, certain heat losses are encountered which determine the thermal efiiciency of the apparatus. These losses include among others carbon loss, moisture in fuel loss, moisture in air loss, radiation loss and dry gas losses. The dry gas losses are of rela tively high magnitude and are principally caused by the large amount of nitrogen contained in the air which supports combustion. Atmospheric air is a mechanical mixture of oxygen, nitrogen and slight amounts of carbon dioxide, water vapor, argon and other inert gases. In combustion calculations the carbon dioxide and the inert gases are ordinarily included with the nitrogen; and the generally accepted proportions of oxygen and nitrogen are 20.91% oxygen and 79.09% nitrogen by volume, or 23.15% oxygen and 76.85% nitrogen by weight. The oxygen fulfills its function in the promotion of combustion by separating itself from its mechanical union with nitrogen and by entering into chemical combination with the available combustible. On the other hand, nitrogen acting merely as a delutant serves no purpose in combustion and is instead, as pointed out hereinabove, a source of direct loss by absorbing heat while passing through the furnace and carrying 01f a portion of such heat in leaving the boiler. Attempts have been made to supply oxygen alone to the fuel to reduce the dry gas losses and gain other advantages such as reduction of furnace volume and sizeof boiler passages and fines. However the extremely high furnace temperatures produced when oxygen alone, or air greatly enriched with oxygen, is supplied for combustion, rendered such operation impracticable in the majority of cases.

My invention as herein described overcomes the above temperature dilhculties while still retaining most of the advantages by an improved method of combustion and operation of an apparatus in which the nitrogen in the air, .ordinarily supplied for combustion, is replaced with a smaller or larger amount of recirculated combustion gases. In this manner most, if not all of the nitrogen is excluded, permitting the elimination of up to about 75 percent of the dry gas losses, thereby achieving a substantial increase in the thermal efiiciency of the fuel burning apparatus. And at the same time by substituting r'ecirculated gas for the nitrogen in the air furnace temperatures can be retained at the conventional safe level; or the dilution effect of the recirculated gases can be lowered at will to obtain higher furnace temperatures to meet special operating conditions or to take. advantage of new advances in the development of heat resisting metals and refractory materials.

Furthermore in the evaporating and heating of steam for power generation in reheat boilers it is of great economic importance to maintain the temperature of the superheated steam as well as the temperature of the reheated steam constant over a wide range of steam generating load. Various means have been employed to accomplish such control. Notably among these is control of the reheated steam temperature by tilting burners and a supplemental control of the superheat temperature by means of gas by-passing. Or the reheat temperature may be controlled by gas recirculation and the superheat temperature by by-passing of gas. Another way of accomplishing such control would be by regulating the temperature of the reheated steam by one of the aforesaid means and additionally controlling the superheated steam temperature by desuperheating.

My invention contemplates individual control of the superheated steam temperature and the reheated steam temperature by means which permit a highly efficient combustion of the fuel and at the same time efiicient and independent control of these steam temperatures.

It is accordingly a primary object of the invention to reduce the stack gas heat losses in a vapor generating and steam heating apparatus.

Another important object of the invention is to improve combustion efiiciency and control the combustion temperatures in the furnace for the purpose of thereby controlling the temperature of the superheated and reheated steam independently.

Other and further objects of the invention will become apparent to those skilled in the art from the detailed description thereof when taken in conjunction with the accompanying drawing wherein:

Fig. 1 represents a diagrammatic illustration of a steam generating and steam heating unit operating under the reheat cycle and being equipped with the inventive improvements. t

Fig. 2 is a cross sectional plan view taken on line 22 of Fig. 1.

In Fig. 1 is shown a steam generating unit equipped with a furnace 4 having walls lined with closely spaced water cooled tubes 5. Feed water entering drum 6 by way of pipe 7 flows downwardly through pipe 8 into ring header 10 to supply the Water cooled walls of furnace 4 with water for the generation of steam. A mixture of steam and Water is discharged into drum 6 through tubes 12. A steam separator (not shown) located within drum 6 separates the saturated steam from the water in a well known manner. The saturated steam passes fro-m drum 6 via pipe 14 to a low temperature superheater section 16. After absorbing a predetermined amount of heat the steam flows by way of pipe 18 into high temperature superheater section 20 from whence it passes through pipe 22 to a high pressure turbine 24. After having given up some of the heat by expansion in the turbine the steam leaves the turbine and is returned to the steam generator to be reheated at lower pressure. For this purpose the steam flows consecutively to low temperature reheater section 26 by way of pipe 28 and to high temperature reheater section 30 by way of pipe 32. The reheated steam is then returned to the low pressure: turbine stage 34 by way of steam pipe 36 for further extraction of heat and generation of power.

The furnace 4 comprises a first combustion chamber 4A. This chamber is constructed in the form of a cyclone chamber having anash discharge opening v38 in the bottom thereof and a co-axial gas outlet 40 at the upper end of the chamber. The pulverized fuel suspended in a mixture of oxygen and recirculated combustion gas substantially free of nitrogen and primarily composed of CO gas is conveyed into the combustion chamber by way of burners 42. These burners are located adjacent the four corners of combustion chamber 4A and are arranged to project the fuel streams into the chamber in directions tangential to an imaginary firing circle 43 v(see Fig. 2). According to the invention the amount of oxygen supplied for combustion through burners'42 is proportioned so as to cause incomplete combustion of the mixture and to result in producing gaseous products of combustion, a large proportion thereof consisting of CO gas. Additional fuel is introduced at a lower level by way of fuel nozzles 44 also in directions tangential to an imaginary firing circle as illustrated in Fig. 2. This fuel is suspended in streams of cooled recirculated combustion gases which are substantially free of nitrogen and composed mostly of CO A lowering of the temperature and a reduction of CO to CO takes place in the lower portion of chamber 4A in an atmosphere charged with incandescent fuel particles which are violently agitated by the whirl of the gases which is caused by discharging the fuel streams in tangential directions. In order to create greater turbulence it may be desirable to direct the fuel streams issuing through burners 44 in a rotational direction which is opposite to that of burners 42.

The CO gases thus generated in the lower part of chamber 4A, are separated from the ash by cyclonic action and pass upwardly through axial gas offtake 46. The ash runs down the walls of chamber 4A passes out through ash opening 38 to be disposed of in a well known manner. The temperature of the gases having passed through outlet 40 is reduced by giving off heat to high temperature reheater 30 which is located directly thereabove. It is desirable that the temperature of the CO gas be thus lowered to a point somewhat above the ignition point of CO which lies in the neighborhood of 1250 F. Directly above reheater section 30 there is provided a second' combustion chamber 413. A mixture of oxygen and recirculated combustion gases substantially free of nitrogen is discharged into combustion chamber 4B by way of nozzles 45 and into the body of cooled CO gases causing these gases to burn to C The heat which is thus liberated is carried by these gases into high temperature superheater section 20, a large portion of heat being absorbed thereby and by the low temperature section 16 and low temperature reheat section 26 through which the gases pass. Further reduction of the temperature of the gases takes place by passing them over economizer heating surface 48 and possibly over other heat absorbing surfaces not shown.

A recirculating duct 50 with branches 62, 64 and fan 52 are provided-to recirculate portions of the cooled combustion gases to burners 42, 44 and oxygen nozzles 45 as earlier mentioned herein. Since the use of atmospheric air as a carrier of oxygen for combustion is avoided as far as possible the combustion gases are substantially free of nitrogen. A large portion of the stack losses is thereby eliminated.

The fuel burned in furnace 4 may be pulverized coal fed from a coal bin 54 by way of fuel pipes 56 and 58 into chamber 4A. In transporting the coal recirculated nitrogen-free combustion gas is used as carrier gas. This gas is supplied by way of pipe 60 from a point on the pressure side of recirculating fan 52. This fan also supplies recirculated nitrogen-free combustion gases to burners 42 and nozzles 45 by way of conduits 62 and 64 respectively as aforesaid.

Oxygen stored under pressure in tank 66 is conducte to burners 42 by way of conduit 68 and to nozzles 45 by way of conduit 70. Valves are provided as required in the oxygen supply lines 68, 70, in the fuel supply lines 56, 58 and in the recirculated gas line 60. There are also provided damper means 72, 74 and 76 in gas ducts 62, 64 and 50 respectively to control the flow of recirculated gas to the various points of the furnace.

In addition to the advantages resulting from replacing the nitrogen-free combustion gases my invention discloses a method of burning the fuel in two stages whereby the total heat released in furnace 4 for a given load is subdivided into a first part as a result of combustion of the fuel to CO gas in chamber 4A, and a second part as a result of combustion of CO gas to CO in chamber 4B. The maximum value of the furnace temperature is thereby greatly lowered, permitting a reduction of the amount of diluetent introduced with oxygen for combustion. This dilutent may consist of nitrogen as in a furnace which is supplied with oxygen enriched air. Or the dilutent may consist of a recirculated gas mixed with oxygen as disclosed in application Serial No. 488,682 filed February 16, 1955. Oxygen-enriched air on the one hand still contains a large amount of nitrogen and it is desirable to reduce this quantity in order to reduce the stack losses. A mixture of recirculated gas and oxygen on the other hand contains large amount of moisture and it is desirable to reduce this amount for the same reason, namely to lower the stack losses, as well as for other reasons such as reducing corrosion of the heating surfaces in the low temperature gas zone.

In an apparatus equipped with my improvements combustion of fuel and liberation of heat takes place in two distinct and controlled stages with cooling of the gases between these stages. Accordingly it is possible to reduce the quantity of dilutent (such as nitrogen in air, or recirculated gas) without encountering furnace temperatures which are excessive with respect to available heat resisting material.

Furthermore in addition tothe advantages set forth above my invention permits independent control of the superheated and reheated steam temperatures in a novel and superior manner.

In a steam generator and steam heater, as illustrated in Fig. 1 and operating under the reheat cycle, the superheated steam temperature as well as the reheated steam temperature must be kept constant over a wide range of steam generator load for maximum overall efiiciency. Since the temperature-load characteristic of the superheated steam does not coincide with the temperature-load characteristics of the reheated steam, desuperheating of the steam must be resorted to in the majority of cases. This lowers the overall eificiency of the power plant.

My invention permits individual and independent control of the superheated steam temperature and of the reheated steam temperature by increasing or decreasing the proportional amount of recirculated gas admixed with oxygen which enters the combustion chamber 4A and the combustion chamber 4B.

In this manner the furnace temperatures in chambers 4A and 43 can be varied within wide limits. Thus a change in furnace temperature in chamber 4A will alter the heat absorbed by reheater 30 and a change in furnace temperature in chamber 4B will alter the heat absorbed by superheater 20.

As shown in the illustrative embodiment of the invention the reheat temperature is controlled by opening or closing damper 74 and oxygen valve 78 in response to temperature variations recorded at the steam outlet of reheater 30 by a temperature recording device 80. Or, the superheat temperature is controlled by adjusting damper 72 and oxygen valve 82 in response to temperature variations recorded at the steam outlet of the superheater 20 by a temperature sensitive device 84. The temperature impulses received by devices. and 84 are translated into mechanical motion operative in adjusting valves 78, 82 or dampers 72, 74 by electric, pneumatic or hydraulic apparatus well known in the art. Devices of this kind are indicated at 86, 88, and 92.

Other control devices (not shown) of a conventional well known nature are provided to adjust the amount of fuel and oxygen supplied to the furnace in response to load requirements indicated by variations in steam pressure at the turbine inlet, or by other means. Furthermore, instead of taking the coal from a coal bunker as shown, pulverized coal can be supplied by one or more pulverizers directly to burners 42 and 44.

What I claim is:

1. In apparatus for generating and heating steam including means for burning pulverized ccal su pended in a mixture of oxygen and relatively cool recirculated combustion gas from which nitrogen is substantially excluded; the combination comprising a first water cooled cyclone furnace chamber having means to diszharge the combustion gas at one end thereof and means for discharg'ng the ash from the opposite end thereof; first burner means for feeding a mixture of coal and nitrogen-free oxygen into said first chamber in streams tangential to an imaginary firing circle, said burner means being circumferentially spaced and remote from said ash d'scharge end to cause combustion of said coal within a first combustion zone; second burner means for discharging into said first chamber a mixture of nitrogen-free flue gas composed mostly of CO and pulverized coal in streams tangential to an imaginary circle said second burner means being circumferentially spaced and located intermediate said first burner means and said ash discharge means to cause reduction of said CO gas to CO gas; a gas outlet duct axially arranged Within said first chamber and extending from the gas outlet end thereof to a point intermediate said second burners and said ash discharge means; a second combustion chamber adjoining said first chamber at the gas discharge end thereof for receiving said CO gas, said second chamber having a gas ofitake at the end thereof which is remote from said first chamber; a first heat exchanger arranged within said second chamber at a point remote from said gas otftake for absorbing heat from said CO gas; nozzle means for discharging a mixture of substantially nitrogen-free oxygen and substantially nitrogen-free recirculated flue gas into said second chamber for combustion of said CO to CO in a second combustion zone at a point beyond the downstream side of said first heat exchanger with respect to gas flow; second heat exchange means for absorbing heat from said second combustion gas; and means for recirculating a portion of said cooled combustion gas free of nitrogen to said first burner means and to said second burner means.

2. In apparatus for superheating and reheating steam by absorption of heat from products of combustion and having a gas outlet for discharging combustion gases substantially free of nitrogen; at first combustion chamber having a gas outlet; first burner means for discharging fuel, oxygen and recirculated combustion gases substantially free of nitrogen into said chamber for combustion r of said fuel to CO gas; a reheater disposed adjacent said gas outlet and in contacting relation with said CO gas for heating reheat steam by absorption of heat from said CO gas; means for increasing or decreasing the ratio of the amount of oxygen to the quantity of recirculated gases discharged through said first burner means in response to a temperature increase or decrease, respectively, of said reheated steam; a second combustion chamber adjoining said first chamber for receiving said cooled CO gas; second burner means for discharging oxygen and recirculated combustion gases substantially free of nitrogen into said CO gas containing second chamber for combustion of said CO gas to CO a superheater disposed within said second chamber and in contacting relation with said CO gases for heating primary steam by absorption of heat from said CO gases; means for increasing or decreasing the ratio of the amount of oxygen to the quantity of recirculated gases discharged through said second burner means in response to a temperature increase or decrease, respectively, of said superheated steam, and means for recirculating combustion gases from a point downstream of said superheater with respect to gas flow to said first burner means and to said second burner means as aforesaid.

3. in apparatus for heating steam by absorption of heat from products of combustion and having a gas outlet for discharging combustion gases substantially free of nitrogen; a first combustion chamber having a gas outlet; first burner means for discharging fuel, oxygen and recirculated combustion gases substantially free of nitrogen into said chamber for combustion of said fuel to CO gas; a first heat exchanger disposed adjacent said gas outlet in contacting relation with said CO gas for heating steam by absorption of heat from said CO gas; means for increasing or decreasing the ratio of the amount of oxygen to the quantity of recirculated gases discharged through said first burner means in response to a temperature increase or decrease, respectively, of said steam heat-ed in said first heat exchanger; a second combustion chamber adjoining said first chamber for receiving said cooled CO gas; second burner means for discharging oxygen and recirculated combustion gases substantially free of nitrogen into said CO gas-containing second chamber for combustion of said CO gas to CO a second heat exchanger disposed within said second chamber and in contacting relation with said CO gases for heating steam by absorption of heat from said CO gases; means for increasing or decreasing the ratio of the amount of oxygen to the quantity of recirculated gases discharged through said second burner means in response to a temperature increase or decrease, respectively, of said steam heated in said second heat exchanger; and means for recirculating combustion gases from a point downstream of said second heat exchanger with respect to gas flow to said first burner means and to said second burner means as aforesaid.

4. In apparatus for generating and heating steam including means for burning comminuted fuel. suspended in a mixture of oxygen and relatively cool recirculated combustion gas from which nitrogen is substantially excluded, the combination comprising a first water cooled furnace chamber having gas outlet means at one end thereof; first burner means for feeding a mixture of fuel and substantially nitrogen-free oxygen into said first chamber, said burner means being remotely disposed with respect to said gas outlet means to cause combustion of said fuel Within a first combustion zone; second burner means for feeding into said first chamber a mixture of nitrogen-free cooled combustion gas composed mostly of CO and comminuted fuel, said second burner means being located at a point beyond the donwstream side of said first burner means with respect to gas flow to cause reduction of said CO gas to CO gas; a second combustion chamber adjoining said first chamber at the gas outlet end thereof for receiving said CO gas and having a gas offtake at the end thereof which is remote from said first chamber; first heating means for absorbing heat from said CO gas said heating means being disposed within said second chamber at a point remote from said gas otftake; third burner means for discharging a mixture of substantially nitrogenfree oxygen and substantially nitrogen-free cooled recirculated combustion gas into said second chamber for combustion of said CO gas to CO gas in a second combustion zone and at a point beyond the downstream side of said first heat absorbing means with respect to gas flow; second heating means for absorbing heat from said second combustion zone gas; and means for recirculating a portion of said cooled second combustion zone gas to said first burner means and to said second burner means.

References Cited in the file of this patent UNITED STATES PATENTS;

1,950,454 Lucke Mar. 13, 1934 2,614,541 Armacost et al Oct. 21, 1952 2,697,482 Blizard -e Dec. 21, 1954

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