Patent Application
20110011719
The present invention relates to the field of coal processing, and more specifically to a process for treating various types of bituminous coal for the production of coal derived liquids (CDLs) and gaseous fuel, and other higher value coal derived products, suitable for metallurgical or power production uses.
Coal in its virgin state is sometimes treated to improve its usefulness and thermal energy content. The treatment can include drying the coal and subjecting the coal to a pyrolysis process to drive off low boiling point organic compounds and heavier organic compounds. Thermal treatment of coal causes the release of certain volatile hydrocarbon compounds having value for further refinement into transportation liquid fuels and other coal derived chemicals. Subsequently, the volatile components can be removed from the sweep gases exiting the pyrolysis process. Thermal treatment of coal causes it to be transformed into coal char by virtue of the evolution of the coal volatiles and products of organic sulfur decomposition, and the magnetic susceptibilities of inorganic sulfur in the resultant char are initiated for subsequent removal of coal ash, sulfur and mercury from the coal char.
The effective removal of such volatile components as coal ash, inorganic sulfur and organic sulfur, and mercury, from coal char is problematic. It would be advantageous if bituminous coal could be treated in such a manner that would enable volatile components to be effectively removed from the coal at more desirable concentrations thereby creating a coal char product having reduced ash and sulfur. It would be further advantageous if bituminous coal could be refined in such a manner to create a second revenue stream (i.e., condensable coal liquids), which could be collected to produce syncrude. A process for beneficiating bituminous coal, including reducing sulfur and ash, evolving valuable coal liquids and fuel gas, increasing calorific value, and improving other properties of the resultant coal char product, is desirable.
In a broad aspect, there is provided herein a process for treating bituminous coal. The process includes providing dried, pulverized coal, and treating the pulverized coal in a vessel with a gas stream having an oxygen content sufficient to form oxides on a surface of the coal particles. The treated coal is transferred into a pyrolyzing chamber and an oxygen deficient sweep gas is passed into contact with the coal, the sweep gas being at a higher temperature than the temperature of the coal so that heat is supplied to the coal. The process further includes providing additional heat to the coal indirectly by heating the chamber, wherein the heating of the coal by the sweep gas and by the indirect heating from the chamber causes condensable volatile components to be released into the sweep gas, and wherein some of the oxides are converted into paramagnetic mineral components. The coal, including the paramagnetic mineral components, are removed from the pyrolyzing chamber as coal char. The paramagnetic mineral components are removed from the coal char, thereby creating a coal char having reduced ash and sulfur.
In certain embodiments, the coal is pulverized to a size within a range of from about minus 60 mesh to about minus 200 mesh.
In certain embodiments, the oxygen content of the gas stream is sufficient to cause the coal to gain weight in an amount within a range of from about 0.5% to about 2.0% of the weight of the coal when the coal is treated for a time of about 30 minutes at a temperature within a range of from about 400° F. to about 600° F.
In certain embodiments, the treating of the pulverized coal is effected by heating the coal to a temperature within a range of from about 400° F. to about 650° F. in an oxidizing rotary retort or an oxidizing fluidized bed vessel.
In certain embodiments, the treated coal is pre-heated to a temperature within a range of from about 550° F. to about 900° F. in a pre-heat rotary retort or a pre-heat fluidized bed vessel.
In certain embodiments, the temperature of the pre-heat rotary retort or pre-heat fluidized bed vessel is controlled to about 850° F. so as to remove about 2% to about 10% by weight of coal volatile components from the treated coal while allowing desirable volatiles to remain with the coal particles.
In certain embodiments, the pre-heating step removes volatiles from the treated coal and includes withdrawing off gases from a pre-heat rotary retort or a pre-heat fluidized bed vessel, and then combusting the volatiles in the off gases and transferring thermal energy from the combustion to the pre-heating step.
In certain embodiments, the chamber is a rotary retort, and the coal is heated in the retort to a temperature within a range of from about 900° F. to about 1200° F. so that the sweep gas exiting the retort has a condensable hydrocarbon content of at least about 25%.
In certain embodiments, the pyrolyzing step creates sulfur in the form of at least one of H2S, CS2, and COS, with the H2S, CS2, and COS being removed from the chamber with the sweep gas, and further includes removing sulfur from the sweep gas.
In certain embodiments, the treated coal entering the chamber includes pyrite (FeS2) and hematite (Fe2O3), and wherein the pyrolyzing of the coal in the chamber causes the conversion of pyrite to pyrrhotite (Fe7S8) and the conversion of hematite to magnetite (Fe3O4).
In certain embodiments, the process further includes the step of removing pyrrhotite and magnetite from the coal by magnetic separation.
In certain embodiments, the coal is cooled to a temperature below 350° F. prior to removing pyrrhotite and magnetite from the coal.
In certain embodiments, at least 80% of the sweep gas entering the chamber is comprised of CO2 and H2O.
In certain embodiments, the sweep gas removed from the chamber includes at least one of C3H8, CH4, and CO, and further includes at least one of H2S, CS2, and COS.
In another broad aspect, there is provided herein a process for treating bituminous coal. The process includes providing dried, pulverized coal, and treating the pulverized coal in a vessel with a gas stream having an oxygen content sufficient to cause the coal to gain weight in an amount within a range of from about 0.5% to about 2.0% of the weight of the coal, and to form oxides on a surface of the coal particles. The treated coal is transferred into a pyrolyzing chamber and an oxygen deficient sweep gas is passed into contact with the coal, the sweep gas being at a higher temperature than the temperature of the coal so that heat is supplied to the coal. The process further includes providing additional heat to the coal indirectly by heating the chamber, wherein the heating of the coal by the sweep gas and by the indirect heating from the chamber causes condensable volatile components to be released into the sweep gas, and wherein some of the oxides are converted into paramagnetic mineral components. The coal, including the paramagnetic mineral components, are removed from the pyrolyzing chamber as coal char. The paramagnetic mineral components are removed from the coal char, thereby creating a coal char having reduced ash and sulfur.
Various advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.
The process of the present invention pertains to treating various types of bituminous coal for the production of coal derived liquids (CDLs) and other higher value coal derived products, such as a high calorific value, low volatile, low ash, low sulfur coal (char), suitable for a variety of uses in industry, including metallurgical and power production and the like. Desired amounts of volatile components are removed from the resultant coal char through the use of low temperature carbonization (i.e., less than about 1300° F.) so as to refine the solid product and to create a second revenue stream, the condensable coal liquids, which can be collected to produce syncrude. Further, desirable condensable hydrocarbon liquids are removed from the coal at more desirable concentrations than capable with conventional coal treating processes. In particular, the process combines the advantages of pyrolytic heating with an attemperated, high sensible heat oxygen deficient gas stream (sweep gas) coupled with indirect heating by passing a portion of the required heat through a rotating metal shell of a rotary pyrolyzer retort as described below. Pyrolytic heating is a desirable step in the process as coal feedstock is separated into a coal char and a vapor, which when passed through downstream condensers, such compounds can be separated into coal tar, water, and a fuel gas.
The process further combines the advantages of a pretreatment or chemisorption step (apparatus 32) in order to destroy or reduce the caking properties of the bituminous coal in refining the coal to a coal char product having reduced ash and sulfur. The process is a dual zone pyrolysis process. During the first step, the bituminous coal is heated to a certain temperature, and during the second step, the coal is heated to a higher temperature than the first step. By using the dual zone pyrolysis process, the indirect/direct pyrolytic heating step of the second pyrolysis step is optimized. A primary reason for indirect heating is that it maximizes the vapor pressure of the condensable hydrocarbon components and minimizes the carryover or lofting of fine coal or coal char particles. A further advantage of dual pyrolysis is to reduce the thermal requirement for the second pyrolysis step. The operating temperature in the second pyrolysis step is controlled to maintain a target or desirable volatile content in the coal char as some volatile in the coal char is desirable for both metallurgical and steam coal char product requirements.
It is to be understood that the process disclosed herein is suited for various types of bituminous coal, particularly caking, coking coal having a free swelling index (FSI) of greater than 1.0.
In consideration of the figures, it is to be understood that for purposes of clarity certain details of construction are not provided in view of such details being conventional and well within the skill of the art once the present invention is disclosed and described herein.
Reduction of volatiles, including moisture, involves several thermal processing steps. Typically, bituminous coals from surface mining operations are washed to remove mineral matter normally associated with these coals. Washing is dependent on large density differences between the organic coal substance and the mineral matter included therein with the as mined coal. After washing, a typical Western Kentucky bituminous coal will have a moisture content of nearly 12% by weight, even though the equilibrium moisture content is within a range of from about 7% to about 9%. Therefore, the as-received coal must be dried as the first step in the series of thermal steps described below.
Referring now to
A heat exchange manifold 20, which functions as a heat transfer fluid conduit, is configured within a bottom portion of the fluidized bed dryer 14, from which a heat transfer fluid return flows downstream through conduit 22 into a heat exchanger 24 for heating the heat transfer fluid. Heat exchanger 24 is configured within a waste fuel gas combustor 26 for the combustion of gaseous CH4, CO, H2S, and other compounds. A heat transfer fluid conduit 28 exits from the heat exchanger 24 and flows upstream to a heat exchange manifold 30, which functions as a heat transfer fluid conduit, and is configured within a vessel such as a fluidized bed chemisorption apparatus 32. While a preferred apparatus 32 for the chemisorption process is a fluidized bed heater, an indirectly heated retort (not shown) having a retention time of at least 30 minutes can be used in the alternative. The fluidized bed chemisorption apparatus 32 includes a heat exchange embedded tubular surface 34 configured therein. An air blower 36 configured outside the fluidized bed chemisorption apparatus 32 supplies air to the coal 12 during the chemisorption treatment process. A vent 38 extends upstream from the fluidized bed chemisorption apparatus 32 and directs waste to the waste fuel gas combustor 26 for the combustion of gaseous carbon-oxygen compounds, which compounds may be formed during the chemisorption treatment process.
Over a temperature range that coincides relatively closely with that of the intended active thermal decomposition, bituminous coals pass through a transient plastic state in which they soften, swell and finally resolidify into a more or less distended cellular cake mass. These coals are referred to as caking coals, as opposed to those that do not become plastic on heating, which are referred to as non-caking coals. The caking or swelling nature of coals is evaluated using the empirical free-swelling test. The free-swelling index (FSI) is commonly used to rank various coals, the index having a range of from 1 to 10. Non-caking coals normally exhibit an FSI of 1 or less. Western Kentucky bituminous coals typically have an FSI of 4, or within a range of from about 1 to about 6. The plastic or coking nature of these bituminous coals leads to agglomeration of the coal particles when heated to the intended decomposition temperature range of from about 350° F. to about 1050° F. Agglomeration leads to sticking, which phenomenon causes plugging in the various heating devices. These caking properties are impediments to the intended thermal process and should be destroyed or counteracted, or at least greatly reduced.
Pilot plant experiments in accordance with the process disclosed herein have shown that pulverized bituminous coal sized to minus 60 mesh can be treated with chemisorption of oxygen and slow heating so as to convert the particulate dried coal to non-caking coal.
Exposure of freshly mined coal to air at ambient temperature conditions for as little as a few days will cause a marked deterioration of any caking properties. While not being bound by any theory, this deterioration of the caking properties is believed to be caused by two substantially concurrent processes—(1) progressive oxidative destruction of non-aromatic configurations, such as CH3, OCH3, or (CH2)n, in the coal molecules, and (2) simultaneous chemisorption of oxygen at aromatic carbon sites.
In one embodiment, the coal is treated in a vessel with a gas stream having an oxygen content sufficient to form oxides on a surface of the coal particles. The oxygen content of the gas stream is sufficient to cause the coal to gain weight in an amount within a range of from about 0.5% to about 2.0% of the weight of the coal 12 when the coal is treated for a time of about 30 minutes at a temperature within a range of from about 400° F. to about 650° F. It should be understood that the vessel used for treatment can be either an oxidizing fluidized bed vessel 32 or an oxidizing rotary retort (calciner) of the type described below.
Following treatment of the coal by chemisorption, the chemisorbed or treated coal 40 can be transferred to either a fluidized bed, or, preferably, a dual zone pyrolysis, for pre-heating in accordance with the process of the present disclosure. It is advantageous to separate the two stages of the dual zone pyrolysis process for several reasons, including: (1) to reduce the coal mass flow heating requirement for the indirect heating required for the second stage; (2) to reduce the sensible heat required for the indirect second stage as coal will enter at about 900° F.; (3) to increase the partial pressure of the condensables released in the second stage, i.e., C5+ and the like; (4) to burn combustible components released in the first zone in a slipstream combustor; and (5) to separately treat effluent for removal of mercury using activated carbon.
In one embodiment, the first zone pre-heats the coal to a temperature within a range of from about 550° F. to about 900° F. in either a pre-heat rotary retort 42 or a pre-heat fluidized bed vessel (not shown). It is contemplated that the first zone will raise the coal temperature to a temperature within a range of from about 550° F. to about 900° F. so as to both preheat and produce CO2, CO, and CH4, by partial pyrolysis. The CO2 is used as a recycle fluidizing gas (i.e., off gas) 44, partially slipstream passing through a combustor 46 and prior to venting so as to combust any hydrocarbons or CO that may be involved in the partial pyrolysis process. Combustion of any fuel gases other than CO2, including CO, CH4 and the like, will provide all or a portion of the thermal energy required for pre-heating and partial pyrolysis of the coal in the pyrolyzer 42. It is further contemplated that the temperature in the first zone is controlled to about 850° F. so as to remove about 2% to about 10% by weight of coal volatile components from the treated coal 40 while allowing desirable volatiles to remain with the coal particles.
In certain embodiments, the temperature of the first zone is no greater than 850° F., which is the temperature incipient for release of condensable coal volatile vapors.
In a further embodiment, the pre-heating step removes volatiles from the treated coal and includes withdrawing off gases (i.e., CO2, CO, CH4 and the like) 44 from a pre-heat rotary retort 42 or a pre-heat fluidized bed vessel (not shown), and then combusting the volatiles in the off gases in combustor 46 and transferring thermal energy from the combustion to the pre-heating step 42. The off gases 44 pass through a recirculation fan 48 before flowing either through a slipstream combustor air supply fan 50 prior to combustion or through a heat exchanger 52 to provide on gas 54 to the first pyrolysis retort 42. The on gas 54 and first stage coal char 56 from the pyrolyzer 42 can be vented at 55 as shown in
Referring to
Various reactions in the second pyrolysis step occur at a temperature within a range of from about 900° F. to about 1200° F. in accordance with the process of
The rotary retort 58 used for the combined direct/indirect pyrolytic heating process may be selected from a type of heat transfer device for the indirect thermal processing of bulk solid materials commonly referred to as a rotary calciner. The rotary calciner consists principally of an alloy rotary shell 62, enclosed in and indirectly heated on its exterior in a stationary furnace. The process material (i.e., coal) 56 moves through the interior of the rotary shell 62, where it is heated through a combined radiative and convective/conductive mode of heat transfer through the rotary shell wall 64. Operating temperatures of up to 2200° F. can be achieved. Rotary calciners can be small pilot-scale units, or full-scale productions units as large as 10-12 feet in diameter with a heated length of up to 100 feet. Units can be heated by a variety of fuels, such as gas (
It is contemplated that the rotary retort 58 is of sufficient length and capacity so as to provide pulverized coal particle residence time within a range of from about 15 minutes to about 25 minutes, which time is desirable for conversion of the non-magnetic pyrite (FeS2) to paramagnetic pyrrhotite (Fe7S8) and for reduction of the non-magnetic iron oxides to paramagnetic magnetite. In some embodiments, the residence time is no greater than 22 minutes, which residence time will not cause reduction of the newly formed magnetic iron oxides, forming therefore undesirable, non-magnetic wustite (FeO).
Materials of construction of the rotary shell 62 are selected for high-temperature service, corrosion resistance, and compatibility with process materials. The rotary shell 62 may be fabricated from a wrought heat and corrosion-resistant alloy steel. For example, Type 309 alloy is the nominal material for indirectly heated rotary calciners operating in the 1300° F. metal temperature range. A variety of features and auxiliary equipment is available to accommodate many process requirements.
Rotary calciners are ideal for specialized processing due to the indirect heating mechanism. As the heat source is physically separated from the process environment, specific process atmospheres can be maintained. Processes requiring inert, reducing, oxidizing, or dehumidified atmospheres, or those with a solids/gas phase reaction can be accommodated. Depending on the process requirements, rotary calciners can operate under positive or negative pressure, and a variety of seal arrangements are available. Internal appurtenances affixed to the rotary shell interior 62 can be employed to promote uniform heat transfer and exposure of the material to a process gas (i.e., sweep gas) 66. The indirect heating also allows for temperature profiling of the process, which provides the capability of maintaining the material temperature at a constant level for specific time periods. Multiple temperature plateaus can be achieved in a single calciner unit in this manner.
Indirectly heated rotary calciners are well known to those knowledgeable with thermal heating of bulk free flowing solids. A typical rotary retort suitable for heating coal to 1200° F. is manufactured by The A. J. Sackett & Sons Co. (Baltimore, Md.) and it is rated for transfer of 6,240,000 BTU/hour having a surface area of 602.88 ft2 of indirect rotary calciner surface and a heat flux in the range of about 10,350 BTU/hr/ft2.
For a heating retort having a combination of indirect and direct heating, when indirect heating is in the range of about two thirds of the total, the one third balance of heat must be supplied by a flow of gases (sweep gases 66) passing into contact with the coal 12. One method of providing sweep gases 66 is to pass a stream of oxygen deficient gases containing both inert and combustible components through an indirect heat exchanger in which the temperature of the gas stream may be heated and/or cooled so as to provide the optimum temperature and composition. Another method of providing sweep gases 66 is to admit the oxygen deficient gas stream containing both inert and combustible components into a combustion chamber with oxygen or combustion air to release sensible heat. The gas stream serves a second purpose, other than partial heat input, serving as a sweep gas to cause the outflow of gases released in the pyrolytic treatment of the continuously flowing dried and preheated coal entering the system.
An advantage of the combined direct/indirect pyrolytic heating process is the co-current flow configuration. The temperatures of the heated coal char 56 and the sweep gases containing the gaseous volatiles having been pyrolytically released from the solid coal char can be brought essentially to equilibrium at the discharge end 68 of the rotating retort 58 via a steam quench 69. Steam quench 69 at the exhaust of retort 58 reduces the gaseous exhaust temperature. The heated coal char 56 can be controllably released at the discharge end 68 of the retort 58 via a product char outlet rotary valve (not shown). The temperature differential between the coal char 56 and the sweep gases 66 at the point of desired pyrolysis process completion is in the range of from about 100° F. to about 200° F. In one embodiment, the temperature differential is about 150° F. Other ranges can be used.
Although in the embodiment shown in the drawings the flow of coal char 56 and sweep gases 66 is co-current, it is to be understood that the flow could be counter-current.
Another advantage of the combined direct/indirect pyrolytic heating process is the relatively substantial permissible thermal temperature differential at the charge end 70 of the retort 58. Differential temperatures between the coal char 56 and the sweep gases 66 at the charge end may be in the range of about 650-750° F., or higher, resulting with an overall retort log mean differential temperature of about 300-400° F.
A further advantage of the combined direct/indirect pyrolytic heating process is found in the fact that the concentration of condensable volatiles is increased when compared to a direct heating process employing attemperated high sensible heat oxygen deficient gas for 100% of the heating. For a conventional 100% direct gas heated system, processing a dried and pre-heated coal, the condensable hydrocarbon concentration is typically about 6.2% of the gaseous stream 72 exiting from the pyrolyzer 58. On the other hand, with 100% indirect heating, the condensable component is about 51.3% of the total gas, including water of pyrolysis released when pyrolytically processed at 1200° F. For a combined indirect/direct heated system with 50% direct gas and 50% indirect heating, the condensable hydrocarbon component is expected to be in the range of about 27.4% of the gas stream 72 leaving the retort 58.
Optional internal lifting flights 74 (
In some embodiments of the rotary pyrolytic retort 58, the coal bed 56 moves in a rolling mode according to Hencin's classification. In this mode, the bed of coal char particles 56 can be considered as those rolling on the surface as opposed as to those that are embedded. Those on the surface roll due to the effect of gravity. This surface layer is commonly referred to as the “active layer”. These particles 56 receive heat from the sweep gases 66 by convection. The oxygen deficient sweep gas 66, containing no greater than about 1% by volume oxygen, is at a higher temperature than the temperature of the coal char 56 so that heat is supplied to the coal. In other embodiments, it is contemplated that the oxygen deficient sweep gas 66 contains no greater than about 2% by volume oxygen. The active layer is enhanced by virtue of staged lifters 74 so as to promote additional internal convective heat transfer from the sweep gas 66 to the coal char particles 56. Beneath the active layer is the mass of the coal bed 56, which is in contact with the metal wall, receiving indirect heat by conduction, as shown in
As schematically illustrated in
In one embodiment, the proportion of heat supplied to the coal char 56 by the sweep gas 66 is less than 40% of the total heat supplied to the coal char 56. In further embodiments, at least 80% of the sweep gas 66 includes CO2 and H2O, and the mass ratio of sweep gas 66 to the coal char 56 supplied into the chamber 58 is less than about 0.50. In still further embodiments, at least 80% of the sweep gas 66 includes CO2 and H2O, and the mass ratio of sweep gas 66 to the coal char 56 supplied into the chamber 58 is less than about 0.25.
A further advantage of the high specific heat sweep gas 66 is the relatively high emissivity in accordance with the process of the present invention. Nitrogen (N2) is a symmetrical molecular gas, which does not contribute to the radiative component of the gas stream. Nitrogen (N2), Oxygen (O2), Hydrogen (H2) and dry air have symmetrical molecules and are practically transparent to thermal radiation—they neither emit nor absorb appreciable amounts of radiant energy at temperatures of practical interest, i.e., 1000-1500° F. On the other hand, radiation of heteropolar gases and vapors such as CO2, H2O, and hydrocarbons are of importance in heat transfer applications. In one embodiment, the intended sweep gas, 40,000 lb/hour of gas having a constituency of approximately 67.3% H2O, 2.9% N2 and 29.2% CO2, supplied into the chamber has an emissivity within a range of from about 0.5 to about 0.7, optimally with an emissivity of about 0.65. There may be H2S present also. When both CO2 and H2O are present in high concentrations, the emissivity can be estimated by adding the emissivities of the two components. The primary components of the composite emissivity with a beam length of 9.0 feet are about 0.45 from water vapor and about 0.20 from the carbon dioxide, with an internal retort pressure within a range of from about 0.85 to 1.3 atmospheres or, alternatively, a range of from about 1.05 to 1.20 atmospheres, and optimally at about 1.15 atmosphere. The optimal internal retort pressure enhances the downstream oil recovery process as the downstream oil collection apparatus (absorption apparatus 82) can be smaller in cross-section, i.e., absorption apparatus can be a lesser diameter, which contributes to a more effective absorption and a lower cost.
The heating of the coal char 56 by the sweep gas 66 and by the indirect heating from the chamber 58 causes condensable volatile components to be released from the coal into the sweep gas. The temperature of the retort 58 can be controlled so as to produce pulverized coal char 56 having a volatile component within a range of from about 10% to about 25% by weight. In one embodiment, the temperature of the coal char 56 within the chamber 58 is raised to a temperature within a range of from about 1200° F. to about 1500° F. in order to improve removal (e.g., volatilization) of organic sulfur.
Optional seals (not shown) can be provided to restrain gas and dust flow at the charge 70 and discharge end 68 of the pyrolytic retort 58. The seals are typically mechanical in nature with a riding/wear component, typically graphite or the like. The seal components are restrained with springs so as to maintain the seal between the static end housings and the rotating cylindrical metal shell 62. Other types of seals can be used.
For a typical pyrolytic coal heating process, the heat required to cause a continuously entering stream of 360,000 lbs/hour of bituminous coal previously dried and preheated in the range of about 850-900° F. to be pyrolyzed has been determined by heat balance and computation to be about 32,000,000 BTU/hour. The specific heat requirement is approximately 95 BTU/lb-dried coal entering at 900° F. For the typical pyrolytic coal heating process, having an indirect heating effective surface area of 2880 ft2, with a heat flux rate of 9,000 BTU/hr/ft2, the heat supplied is therefore about 25,500,000 BTU/hr. The indirect heating component would be in the order of 25,000,000 BTU/hr divided by the total requirement of 32,000,000 BTU/hr or 80% of the total. Other rotary calciners examined show heat flux rating of from about 4000 BTU/hr/ft2 to 12,000 BTU/hr/ft2 with 10,000 BTU/hr/ft2 being typical for the present embodiment.
It should be understood that a very short gaseous residence time in the retort is desirable to avoid thermal cracking of the high molecular weight hydrocarbon vapors at temperatures of about 950° F. and higher. For temperatures in the 950° F. to 1,300° F. range, gaseous residence times of five seconds or less are desirable to avoid measurable cracking of the desirable hydrocarbons. Conversely, with gaseous residence times of one to two seconds, hydrocarbon cracking requires temperatures in the 1,650 to 1,850° F. range. For a 10-foot diameter retort having a length of 100 feet, the gaseous interior volume is calculated to be 5,500 cubic feet (30% filled with coal/char). With a sweep gas flow of 75,000 actual cubic feet per minute (measured at the exit and including the make gas, i.e., gas evolved during pyrolysis), the residence time is in the range of about 0.25 seconds. In one embodiment, the average gaseous residence time within the retort 58 is within a range of from about 0.2 second to one second. In an alternative embodiment, the average gaseous residence time within the retort 58 is less than about one second.
Following the removal of the sweep gas 66 from the chamber 58, the sweep gas is appropriately treated to remove condensable components of the coal char 56, including hydrocarbons, water vapor, and other volatile compounds, in accordance with the process 10 schematically illustrated in
Optionally, the remaining gaseous compounds and water vapor can be passed through a final stage quench tower (not shown) to remove a portion of the contained water vapor.
Referring to
It is further contemplated that increased energy efficient volatilization and desorption cooling process stages can be realized by using less sweep gas, replacing the convective heat transfer of the sweep gas wholly or partially with indirect heating of the coal being treated in the pyrolytic retort 58. In one embodiment, the condensable hydrocarbon (C5+) components represent about 50% (25-75 wt %) of the volatiles evolved in the pyrolysis process. At this concentration, the condensation temperatures are more representative of the respective boiling points and the volatile hydrocarbons can be efficiently cooled, condensed and separated in a multi-stage downstream absorption system (shown as a single-stage absorption system 82 in
Referring to
It is contemplated that the process derived gaseous fuel 110 may be used as a sweep gas 66 for the second zone pyrolysis process. The process derived gaseous fuel 110 flows into the waste fuel gas combustor 26 in which the gaseous fuel 110 is heated by a heat exchanger 158. After appropriate heating in the combustor 26, the sweep gas 66 flows upstream into the second zone pyrolytic retort 58.
After the second zone pyrolysis process is completed and the pulverized coal has been transformed into coal char (containing paramagnetic components and other ash components) 118 by evolution of the coal volatiles, and products of organic sulfur decomposition and the magnetic susceptibilities of the inorganic sulfur in the resultant char have been enhanced, the coal char can be cooled via a coal char cooler 120 and transferred to a dry magnetic separation device 122. The coal char cooler 120 is configured to have a heat exchange embedded tubular surface 124. The coal char 118 enters the char cooler 120 at a temperature within a range of from about 950° F. to about 1150° F. and is cooled to a temperature within a range of from about 250° F. to about 350° F. Other temperatures are possible. The coal char cooler 120 can be a fluidized bed cooler having internal embedded coiling coils. The coolant used in conjunction with the coal char cooler 120 can be a commercial heat transfer fluid of the type manufactured by Solutia, Inc. (St. Louis, Mo.) called Therminol. Optionally, the coolant is circulated to an upstream heating/drying unit, where the heat is transferred to the incoming coal. The intended purpose for the cooling step is to remove sensible heat from the solid, and a secondary purpose is to quench the pyrolysis process, which process continues in the hot char as it enters the coal char cooler 120. Exhaust gases from the cooler 120 are treated in the waste fuel gas combustor 26.
The cooled coal char 126 can be passed through a dry magnetic separator 122 so as to remove at least a portion of the magnetic pyrrhotite and magnetite to produce a beneficiated coal char. Dry magnetic separation of coal ash, sulfur, and mercury from comminuted coal is known in the art. The cooled coal char 126 can be magnetically treated by using a conventional dry magnetic separator of the type manufactured by the EXPORTech Company, Inc. (Pittsburgh, Pa.). A preferred dry magnetic separator is an open gradient, free flow, Para Trap separator capable of separating very weakly magnetic materials, such as iron pyrites, which contribute to the sulfur and trace metals such as mercury and arsenic in some coals so treated. It has been shown with two passes of the coal through the Para Trap separator, reductions of 28% ash, 78% pyritic sulfur, 31% arsenic, and 72% mercury were achieved, when used in accordance with the process disclosed herein.
It should be understood that ash removal and carbon carryover results vary with the degree of comminution afforded the raw coal, the iron content, and the degree of magnetic conversion attained in the pyrolysis chamber. The actual retention time and temperature experienced by the coal in the thermal reduction process can affect the ash/sulfur removal results.
In one embodiment, a further reduction of other iron oxide materials occurs in the pyrolysis process, such that this mineral matter is transformed into magnetite, which mineral matter is subsequently removed by the same dry magnetic separation means used to remove the inorganic sulfur. Magnetic ash minerals (containing inorganic sulfur and iron oxides) 128 exit the dry magnetic separator 122 while coal char 130 flows downstream into a mixer 132 in which the coal char is combined with a centrifuge output (containing coal tar, char fines, and a suitable binder) 134 for briquetting. A desirable ingredient for briquetting pulverized coal char is a binder. In one embodiment, the binder is coal tar from the liquid recovery portion of a coal beneficiation plant.
It is contemplated that coal tar is condensed and collected prior to its use as a binder for briquetting of the coal char. Heat is recovered from hot coal tar using an external heat exchanger and is directed to the fluidized bed dryer 14 for drying the pulverized coal 12. Overhead gases from a coal tar collection apparatus (not shown) contain various fuel components, including C3H8, CH4, CO and the like, and gaseous sulfur compounds, including H2S, CS2, and COS. The overhead gases can be used for fuel for the drying, preheating, and pyrolysis functions. The effluent from the heating units contain SO2, which can be removed using conventional scrubber technology.
Condensed hydrocarbon vapors exiting the second pyrolysis zone contain solid coal char fines. The liquid recovery system includes a centrifuge for separation of highly viscous coal liquids and coal char fines. This stream of viscous coal tar containing coal char fines (centrifuged bottom portions) can be pumped into a mixer or blender where the coal tar, char fines and product coal char, are intermittently mixed and blended prior to briquetting. The nominal addition of coal tar may be equal to about 3% of the product coal char. The coal tar adds to the volatile content in the product briquettes. The addition of coal tar can be adjusted as might be necessary to correct for over or under removal of volatiles in the pyrolysis process. The beneficiated coal char and binder 136 can be briquetted using any suitable apparatus, such as a conventional roll briquetting machine 138 of the type manufactured by K.R. Komarek, Inc. (Wood Dale, Ill.). The product coal char briquettes 140 formed in accordance with the process disclosed herein are synthetic metallurgical grade, high calorific value, low sulfur coal. The product briquettes 140 can be transshipped using traditional coal transport means.
In the alternative, following the dry magnetic separation step, the beneficiated coal char is suitable for transfer to a pulverized coal power generation facility. Transfer may be accomplished by using an inert pneumatic transfer means. A further technique is to use inerted, enclosed gondola rail cars for long distance transshipment.
It is to be understood that when electric resistance heating is the indirect heating source of the outer shell 80 of the rotary retort 58, elements 106, 108, and 112 shown in
Referring further to
The present disclosure is further defined in the following Examples, in which all parts and percentages are by weight and degrees are Fahrenheit, unless otherwise stated. It should be understood that these Examples, while indicating preferred embodiments of the invention, is given by way of illustration only. From the discussion herein and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
The content of the resultant coal char product according to the process described herein is shown in Table 1 below. It is to be understood that the composition of the resultant coal char product is very much a function of the feed coal, and laboratory testing is needed to verify yields for each product for various types of bituminous coals.
A forensic analysis of Western Kentucky, Ohio County and Paradise Quadrant bituminous coal was conducted. It was determined that based on 25 samples 36.91% of the dry coal ash formed after combustion is non-oxide hematite (Fe2O3). It was further discovered that 34% of the resultant iron oxide resulted from pyrite. Therefore, the other 66% of the iron oxide preexisted in the coal ash. This iron oxide and mineral matter so associated with the iron oxide is reduced to the paramagnetic form, magnetite (Fe3O4). This leads to the further beneficiation of the coal as up to 50% of the coal ash may be removed in the downstream dry magnetic separation step of the process disclosed herein.
While the invention has been described with reference to various and preferred embodiments, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the essential scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the particular embodiment disclosed herein contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.
The present invention claims the benefit of U.S. Provisional Patent Application No. 61/225,406, filed Jul. 14, 2009, the disclosure of which is incorporated herein by reference in its entirety. This invention is related to co-pending applications entitled “Process For Treating Coal By Removing Volatile Components,” and “Process For Treating Agglomerating Coal By Removing Volatile Components,” filed concurrently herewith. This invention was made with no Government support and the Government has no rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61225406 | Jul 2009 | US |
