Method Of Operating A Combined Gasification-steam Generating Plant

Abstract

A steam generating plant is provided which includes an entrainment, slagging, air blown atmospheric pressure coal gasifier, a heat recovery train, atmospheric pressure desulfurization and a steam generator designed to burn the low Btu fuel gas produced in the gasifier. Heat recovered from the gas produced in the gasifier is used to raise the temperature of a portion of the steam generator feedwater, generate steam for use in the gasifier, and to reheat the fuel gas following desulfurization. Heat recovered from boiler flue gas is utilized in drying and preheating the reactants to the gasifier.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to steam generating plants and more particularly to a system wherein a steam generator is integrated with an on-site coal gasification plant which provides a low Btu gaseous fuel for the steam generator.

2. Description of the Prior Art

The electric utility industry in the United States is currently the target of many regulatory agencies and environmental groups whose goal is the reduction or elimination of objectionable emissions such as sulfur oxide, nitrogen oxide, and particulate matter from the stacks of power generating stations. Of the many solutions to this problem now being studied one of the most promising for near term relief is the on-site gasification and desulfurization of a high sulfur content coal to produce a clean burning low Btu fuel gas which may be burned in a steam generator. Various systems making use of this idea have been proposed; however, none have appeared particularly attractive from an economic standpoint, primarily because of the gasification losses where as much as 20 percent of the Btu content of the coal may be lost.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention there is provided a steam generating plant fully integrated with a coal gasification plant which produces a gaseous fuel to be burned in the steam generating plant. The interrelationship of the two plants is such as to take maximum advantage of the available Btu content of the coal.

The hot gas formed in the gasifier is passed through a series of three heat exchangers to effect cooling thereof before it is sent to a desulfurization complex where the hydrogen sulfide present in the gas is removed. Heat is extracted in the first of the series of heat exchangers using water as the heat exchange fluid. The hot water from this heat exchanger is then circulated through water cooling passages in the walls of the gasifier itself where additional heat is extracted. This hot water is then passed directly to the boiler of the steam generator. Heat extracted in the second heat exchanger is used to generate steam which is used as a reactant in the gasifier. The third heat exchanger is one-half of a fluid coupled heat exchanger and its function is to further cool the gas prior to desulfurization. The desulfurized gas passing from the desulfurization complex is then reheated in the second half of the fluid coupled heat exchanger, prior to being burned in the steam generator.

A gas storage capacity is included in the system in order to permit the gas supply to more promptly respond to increases in demand from the steam generator.

Other important features of the invention include; the use of the steam generator air preheater to provide hot air, which is used to dry a portion of the coal to be gasified and as a source of oxygen in the gasification reaction, and the use of spent flue gas from the steam generator to dry another portion of the coal.

Other objects and advantages of the invention will become apparent upon reading the following detailed description of an illustrative embodiment and upon reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a steam generating plant embodying the principles of the invention.

FIG. 2 is a schematic showing a representative desulfurization complex which may be used with the system and the gas storage system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 there is shown a gasifier 10 for producing a low Btu gaseous fuel, by the reaction at high temperatures of oxygen and steam with a solid carbonaceous fuel, and a steam generator 12 having a boiler 14 adapted to burn the low Btu gas produced by the gasifier 10.

With the exception that the burners (not shown) in the boiler furnace 16 are adapted to burn a low Btu gas the steam generator 12 is of a conventional design. Air for combustion is supplied to the boiler furnace 16 by the forced-draft fan 18, which takes atmospheric air and forces it through the air preheater 20 and into the furnace 16 to be burned with the low Btu gas which enters the furnace along with the air as shown at 22. The product of combustion gases from the furnace 16 are removed by an induced draft fan 24 and are drawn successively through the superheater 26 where the steam temperature is raised above that existing in the boiler, the reheater 28 where superheat is added to the steam between power turbine stages, the economizer 30 where feedwater to the boiler 14 is heated, and the air preheater 20, and discharged to the stack 32.

Turning to the gasifier 10, its operation and association with the steam generator 12 in such a way as to make maximum use of the heat generated in both systems will now be described. The gasifier 10 is of the entrainment, slagging, air blown atmospheric pressure type, and makes use of the so-called two stage reaction process.

The first reaction stage, which takes place in the lower end of the reactor involves the near complete combustion of a quantity of pulverized coal with preheated air in stoichiometric proportions. These reactants are injected into the gasifier through a plurality of burner nozzles, one of which is shown schematically at 34. These burners 34, in a typical gasifier of this type having a circular cross section, are equally spaced about the outer circumference of the gasifier and operate in the same manner as the burners conventionally utilized in pulverized coal fired steam generating boilers. Primary air and the pulverized coal are delivered to the burner 34 through a coal pipe 36 while the balance of the air, the secondary air is delivered through a separate conduit 38 and mixed with the primary air and coal as it leaves the burner nozzles.

The mixture of primary air and pulverized coal is prepared in the combustor mills 40, which may be of any well known design such as a bowl mill or a ball mill. Coal is supplied to the combustor mills 40 from a suitable source, as represented at 42, while the primary air is supplied through conduit 44. The air entering the mill through conduit 44 is preferably maintained at a temperature between about 550.degree.-600.degree. F. This temperature is maintained by mixing, as shown at 50, a stream of hot air 46, which has passed through the air preheater 20 of the steam generator 12, with a stream of cooler atmospheric air 48. Both streams of air are provided by a fan 51, the cool air being bled off from the main stream 52 before it passes through the air preheater 20. The stream of hot air 46 to the combustor mills is taken from the main stream of hot air 52 as required while the balance of this air passes to the burner 34 through the secondary air conduit 38. Such an arrangement is typical of pulverized coal burning equipment and is designed to reduce the moisture content of the coal to an acceptable level and to preheat it to a temperature generally not exceeding 180.degree. F.

The complete combustion of these reactants which occurs upon their injection into the gasifier 10 results in the production of a very high temperature product of combustion gas. This high temperature, preferably 3100.degree.F or above is required for two reasons: it is above the fusion temperature of the ash in the coal, thus resulting in the formation of molten slag which is removed from the lower end of the reactor by a suitable slag disposal system 54; and it provides, in the form of the hot rising gases, a high temperature heat source which is necessary to sustain the primarily endothermic reduction reaction, the second stage of gasification, which occurs in the region overlying the combustion zone.

The second reaction stage involves the reaction of a second stream of reactants, containing pulverized coal and steam, with the hot gases rising from the oxidation or combustion zone. As indicated the reactions occurring here are primarily endothermic and result in the formation of a low Btu fuel gas containing mostly carbon monoxide and hydrogen with some methane. Hydrogen sulfide in quantities proportionate to the initial sulfur content of the coal is also present in the gas.

The stream of steam and coal for the second reaction stage is injected into the gasifier at 56 and is supplied thereto by means of a suitable conduit 58 which communicates with a mixing chamber 60. The mixing chamber 60 is in turn supplied with separate streams of dried pulverized coal 62 and steam 64. The coal is pulverized and dried in the reductor mills 66 which are, as were the combustor mills 40, of conventional design. The drying of the coal in the reductor mills 66, which is supplied from the same coal source 42 as the combustor mills, is accomplished by passing through the mills a stream of hot flue gas 68, passing from the steam generator economizer 30. This gas is drawn from the passage 70, through duct 72, by a suitable fan 74 and delivered to the mills 66 through duct 76. Hot flue gas, instead of hot air, is used to dry this coal in order to insure a minimum oxygen level in the coal when it enters the gasifier.

The dry coal/flue gas mixture from the reductor mills 66 is then passed through a cyclone separator 78, where the coal and flue gas are separated, and the coal is passed from the bottom of the separator as at 62 to the mixing chamber 60. The flue gas from the separator 78 is drawn out through conduit 80 by means of a fan 82. This gas is at a substantially reduced temperature and a portion of it is recirculated through duct 84 to the duct 76 which carries flue gas to the reductor mills 66 thereby maintaining the temperature of this gas in the range needed to accomplish sufficient drying of the coal in the mills. The balance of the flue gas passing from fan 82 is drawn by a second fan 86 and directed to a wet scrubber 88 which removes any particles of pulverized coal which may have passed with the gas through the cyclone separator 78 before directing it to the stack 32 and thence to the atmosphere.

The gaseous fuel exiting from the gasifier is preferably at a temperature ranging up to about 1,700.degree. F. This is considerably cooler than the 3,100.degree. F existing in the combustion zone, due to the endothermic nature of the reactions in the reduction zone, however, the gas still contains a large quantity of sensible heat which should be utilized in order to increase the overall efficiency of the entire system. Accordingly, the hot fuel from the gasifier 10 is passed directly, through conduit 90, to a high pressure heat exchanger 92 where it gives up heat to water bled through pipe 94 from the boiler feedwater supply line 96. In a typical installation the water passing through the high pressure heat exchanger 92 would enter at a pressure of 3,050 psia and a temperature of 487.degree. F and leave at 3,000 psia and 630.degree. F. The hot water from this heat exchanger 92 is then passed via conduit 98 to the inlet manifold (not shown) of a water wall cooling system in the gasifier 10. The water is circulated through the wall cooling system where it is further heated so that the water exiting from the system, at 99, is at a temperature and pressure of 690.degree. F and 2,900 psia respectively. This water is then directed through conduit 100 to the steam generator 12 where it is admitted directly to the boiler steam drum 102.

The gaseous fuel passes from the high pressure heat exchanger at a reduced temperature of about 1,040.degree. F, and is conducted directly through conduit 104 to a low pressure heat exchanger 106. The heat exchange fluid in the low pressure heat exchanger is a suitable supply of process water 108 at atmospheric pressure which is converted to steam by the passage therethrough. The steam generated therein typically will be at 700.degree. F and 165 psia, and passes from the heat exchanger 106 through a suitable steam line 110 to the mixing chamber 60, thus providing the steam supply required to carry out the gasification reaction.

The gas passing from the low pressure heat exchanger 106 is reduced in temperature to about 640.degree. F. It is then further cooled to about 300.degree. F before entering the desulfurization complex 114 by passing through the first section 112 of a fluid coupled heat exchanger 115. The fluid coupled heat exchanger 115 comprises two heat exchange sections 112, 113 remotely located from one another and interconnected by a flow of heat exchange fluid through closed circuit flow path 116. Fluid flow through this circuit is maintained by a suitable pump 117. The second section 113 of the fluid coupled heat exchanger 115 returns to the gas the heat removed from it in the first section 112, after the gas has been desulfurized as will be described hereinbelow. Each of the three heat exchangers 92, 106, 112 through which the gas passes prior to desulfurization is provided at the bottom of the gas passage with a hopper to collect char which may accumulate. This char is periodically recycled back to the gasifier through recycling lines 109.

As indicated above the gas passes from the first section 112 of the fluid coupled heat exchanger 115 to the desulfurization complex 114 where the hydrogen sulfide present in the gas is removed. Desulfurization may be carried out by one of several known processes such as for example solvent extraction, or dry techniques. A typical desulfurization system is shown in FIG. 2 and will be described in connection with a fuel storage arrangement also illustrated therein.

Referring to FIG. 2 the gas from the first section of the fluid coupled heat exchanger 112 passes first to a raw gas scrubber 118 where any remaining particulates are removed from the gas and it is further cooled by adiabatic humidification. The water spray 119 to the gas scrubber 118 saturates the gas and cools it to the dew point. In order to preclude formation of hydrogen sulfide and the resulting acid corrosion which would occur, a heater 120 positioned at the top of the scrubber 118 raises the temperature of the exiting gas to about 10.degree. F above the dew point. The gas then flows via conduit 122, through a booster fan 124 to the absorber 126 where the H.sub.2 S is removed as a result of chemical reactions occurring upon exposure of the H.sub.2 S in the gas with a suitable absorption solution, which is admitted at 127.

The used solution from the absorber is pumped, by pump 128, to the regenerator 130 where air is blown through the solution as at 132. The air regenerates the absorbant to an oxidizing from so that it may be returned through sulfur melt tank 134, and cooler 136 to the absorber 126 where it is reused. Elemental sulfur is separated from the absorbant upon passage through the sulfur melt tank 134 and the sulfur is heated therein so that it may be conveniently removed in a liquid form for disposal.

The desulfurized gas passing from the absorber is then conducted, as needed, through conduits 138 and 140 (FIGS. 1 and 2) to the second section 142 of the fluid coupled heat exchanger 116 where it is reheated, and from there through conduit 141 to the steam generator 12 to be burned.

In order to provide for occasional imbalance which may exist between the gasifier output and steam generator demand a gas compression and storage capability is provided in the system. Referring to FIG. 2, during periods when steam generator demand is less than gasifier output, quantities of clean gas passing from the absorber 126 are compressed by compressor 144, further reduced in volume by cooler 146, and then passed to storage tank 148 where it is stored at about 150 psia. When steam generator demand temporarily exceeds gasifier output the quantity of gas necessary to supplement the gasifier output in order to meet the demand is supplied from the storage tank 148. The flow of gas from the tank 148 is controlled by a throttling valve 150 which is located in the tank storage supply conduit 152 which ties into the conduit 140 which feeds the gas to the steam generator. The throttling valve 150 also serves to let down the pressure of the gas coming from the storage tank 148 to atmospheric pressure.

The diversion of gas to the storage tank 148 of course can only be carried out when the tank is at less than full capacity. Accordingly, when the tank is full and the steam generator demand is less than maximum, the output of the gasifier must be reduced to match the steam generator demand.

While this preferred embodiment of the invention has been shown and described, it will be understood that it is merely illustrative and that changes may be made without departing from the scope of the invention as claimed.

Claims

What is claimed is:

1. A method of operating a steam generating plant in conjunction with a coal gasification plant which produces gaseous fuel to be burned in the steam generating plant, comprising the steps of:

producing a low Btu gaseous fuel in said gasifier by the reaction at high temperatures of oxygen and steam with a solid carbonaceous fuel;

flowing the gaseous fuel from the gasifier in heat exchange relation with a flow of water;

passing said flow of water through the walls of said gasifier to effect the cooling thereof;

burning said gaseous fuel in said steam generating plant to produce heat for the generation of steam therein; and

conducting the hot water exiting from said gasifier walls to the boiler of said steam generating plant.

2. A method of operating a steam generating plant in conjunction with a coal gasification plant which produces gaseous fuel to be burned in the steam generating plant, comprising the steps of:

burning a first reactant stream of pulverized coal and preheated stoichiometric air in said gasifier to produce hot product of combustion gases;

reacting a second reactant stream of pulverized coal and steam with said hot product of combustion gases in said gasifier to produce a low Btu gaseous fuel therein;

flowing water through the walls of said gasifier to effect the cooling thereof;

burning said gaseous fuel in said steam generating plant to produce heat for the generation of steam therein; and

conducting the hot water exiting from said gasifier walls to the boiler of said steam generating plant.

3. The method of claim 2 including the steps of flowing the hot gaseous fuel from the gasifier in heat exchange relation with process water to form steam; and

mixing the steam formed in the preceeding step with pulverized coal to form said second reactant stream.

4. The method of claim 2 including the steps of: flowing the hot flue gases from the steam generating plant in heat exchange relation with atmospheric air to raise the temperature of said air;

mixing a first portion of said air with pulverized coal to dry the coal; and

mixing a second portion of said air along with said previously mixed air and pulverized coal, to form said first reactant stream.

5. The method of claim 2 including the steps of:

mixing a quantity of hot flue gases from the steam generating plant with pulverized coal to lower the moisture content of the coal;

separating the dry pulverized coal from the hot flue gases; and;

mixing the dry puvlerized coal with steam to form said second reactant stream.

6. A method of operating a steam generating plant in conjunction with a coal gasification plant which produces gaseous fuel to be burned in the steam generating plant, comprising the steps of:

producing a low Btu gaseous fuel in said gasifier by the reaction at high temperatures of oxygen and steam with a solid carbonaceous fuel;

flowing water through the walls of said gasifier to effect the cooling thereof;

cooling the gaseous fuel from the gasifier by passing it in heat exchange relation with the fluid in one half of a fluid coupled heat exchanger;

removing the H.sub.2 S from the gaseous fuel;

reheating the cleaned gas by passing it in heat exchange relation with the fluid in the second half of said fluid coupled heat exchanger;

burning said gaseous fuel in said steam generating plant to produce heat for the generation of steam therein; and

conducting the hot water exiting from said gasifier walls to the boiler of said steam generating plant.

7. The method of claim 1 wherein the step of producing a low Btu gaseous fuel comprises the steps of:

burning a first reactant stream of pulverized coal and preheated stoichiometric air in said gasifier to produce hot product of combustion gases; and

reacting a second stream of pulverized coal and steam with said hot product of combustion gases.

8. The method of claim 7 including the steps of:

flowing the hot gaseous fuel from the gasifier in heat exchange relation with process water to form steam; and

mixing the steam formed in the preceeding step with pulverized coal to form said second reactant stream.

9. The method of claim 8 including the steps of:

flowing the hot flue gas from the steam generating plant in heat exchange relation with atmospheric air to raise the temperature of the air;

mixing a first portion of said heated air with pulverized coal to dry the coal;

mixing a second portion of said air along with said previously mixed air and pulverized coal, to form said first reactant stream;

mixing a quantity of hot flue gases from the steam generating plant with pulverized coal to dry the coal;

separating the dry pulverized coal from the hot flue gases; and

using said dry pulverized coal as the coal which is mixed with steam to form said second reactant stream.