Patent Application
20140224636
The field of the invention relates generally to gasification systems and, more specifically, to gasification systems for high-ash content fuels.
Feedstock, such as coal, petroleum coke, biomass, wood-based materials, agricultural wastes, tars, coke oven gas and asphalt, and other carbon containing items may be gasified for use in the production of electricity, chemicals, synthetic fuels, and for a variety of other applications. Gasification involves reacting a carbonaceous fuel and oxygen at a very high temperature to produce syngas, a fuel containing carbon monoxide and hydrogen. Syngas typically burns more efficiently and cleaner than the fuel in its original state before gasification. The syngas may be used for power generation, chemical production, and any other suitable application. Some carbon containing feedstocks have an ash content, however, that renders such feedstocks unsuitable for use in connection with some known gasification systems.
In one aspect, a coal separation unit for a gasification system having a grinding unit and a gasification unit is provided. The separation unit includes a vessel having a top portion and a bottom portion, a first inlet configured to receive a feed stream including at least coal and ash from the grinding unit, and a second inlet configured to receive a wash fluid stream. The separation unit further includes a first outlet configured to convey a low-ash feed stream, and a second outlet configured to convey an ash-rich stream. In operation, the feed stream is washed with the wash fluid stream in the separation unit to separate at least a portion of the ash from the coal. The separated ash settles to the bottom portion of the vessel and is conveyed via the second outlet as the ash-rich stream. The separated coal floats to the top portion of the vessel and is conveyed via the first outlet to the gasification unit as the low-ash feed stream.
In another aspect, a gasification system for high-ash content coal is provided. The system includes a grinding unit configured to reduce the particle size of a high-ash feed stream comprising at least coal and ash, a gasification unit configured to receive a low-ash feed stream and produce a syngas from the low-ash feed stream, and a coal separation unit coupled between the grinding unit and the gasification unit. The separation unit includes a vessel having a top portion and a bottom portion, a first inlet in flow communication with the grinding unit and configured to receive the high-ash feed stream having reduced particle size, and a second inlet configured to receive a washing fluid stream. The separation unit further includes a first outlet coupled in flow communication with the gasification unit and configured to convey the low-ash feed stream thereto, and a second outlet configured to convey an ash-rich stream. In operation, the high-ash feed stream is washed with the wash fluid stream in the separation unit to separate at least a portion of the ash from the coal. The separated ash settles to the bottom portion of the vessel and is conveyed via the second outlet as the ash-rich stream, and the separated coal floats to the top portion of the vessel and is conveyed to the gasification unit via the first outlet as the low-ash feed stream.
In yet another aspect, a method of reducing ash content of a high-ash feed stream in a gasification system is provided. The method includes providing a coal separation unit, the separation unit including a vessel having a top portion, a bottom portion, a first and second inlet, and a first and second outlet. The method further includes providing the high-ash feed stream to the first inlet, the high-ash feed stream including at least coal and ash, providing a wash fluid stream to the second inlet, contacting the high-ash feed stream with the wash fluid stream in the vessel, and separating at least a portion of the ash from the coal. The separated ash settles to the bottom portion of the vessel and the coal floats to the top portion of the vessel.
As used herein, the terms “high-ash” and “low-ash” are used in a relative sense, and for high-ash, means an ash content greater than about 20%, and for low-ash means an ash content less than about 20%.
In the exemplary embodiment, grinding unit 12 receives a feedstock of high-ash content coal (i.e., greater than 20% ash content) (not shown), reduces the coal to a predetermined particle size, and conveys a coal feed stream 24 to separation unit 14. Coal and ash are separated in separation unit 14, as described further herein, to produce an ash-rich stream 26 and a low-ash feed stream 28. Gasification unit 16 receives a fluid stream 30 from ASU 22 and low-ash feed stream 28 and produces a syngas stream 32. At least a portion of syngas stream 32 is conveyed to black water handling unit 18 and grey water handling unit 20 for removal of impurities from the syngas stream.
In the exemplary embodiment, a feedstock 102 (e.g., high-ash content coal) and a fluxant 104 are conveyed to a feedstock handling and storage unit 106. Fluxant 104 may be added to feedstock 102 in feedstock handling and storage unit 106 to adjust the melting behavior of feedstock 102. Fluxant 104 may include, for example, sand, rock, limestone, bauxite, and iron ore. Feedstock handling and storage unit 106 directs the mixture of feedstock 102 and fluxant 104 to a feedstock grinding and slurry preparation unit 108 that resizes or reshapes the mixture of feedstock 102 and fluxant 104 by chopping, milling, shredding, pulverizing, briquetting, or palletizing feedstock 102 and fluxant 104 to generate gasification fuel. Additionally, water 110 and/or a reagent feed 244, or other suitable fluids may be added to the mixture of feedstock 102 and fluxant 104 in the feedstock grinding and slurry preparation unit 108 to create a slurry fuel 112.
In the exemplary embodiment, slurry fuel 112 is conveyed to separation unit 14. The feed to separation unit 14 may include a solid feed, a liquid feed, and/or a gas feed. Separation unit 14 includes a flotation separation-type column 200 (shown in
In the exemplary embodiment, gasifier 118 converts feed stream 116 into a syngas, e.g., a combination of carbon monoxide and hydrogen. This conversion may be accomplished by subjecting the fuel to a controlled amount of any moderator and limited oxygen at elevated pressure (e.g., between approximately 40 bar and 90 bar) and elevated temperatures (e.g., between approximately 1200° C. and 1500° C.), depending on the type of fuel used. The heating of fuel during a pyrolysis process may generate a slag 120 and residue gases (e.g., carbon monoxide, hydrogen, and nitrogen).
In the exemplary embodiment, a partial oxidation process (i.e., gasification) may then occur in gasifier 118. To aid with the gasification process, an oxygen stream 122 is supplied to gasifier 118 from ASU 22, which operates to separate air 124 into component gases. ASU 22 separates air 124 into oxygen 122 and nitrogen 126, and oxygen 122 is directed to gasifier 118 and nitrogen 126 may be conveyed to other components or processes in system 10.
In the exemplary embodiment, char and residue gases may react with oxygen 122 to form carbon dioxide and carbon monoxide, which provides heat for subsequent gasification reactions. The temperatures during the gasification process may range from approximately 1200° C. to approximately 1500° C. In addition, steam (not shown) may be introduced into gasifier 118 to enable some of the feedstock to be burned to produce carbon monoxide and energy, which may drive a second reaction that converts further feedstock to hydrogen and additional carbon dioxide.
In the exemplary embodiment, a resultant syngas stream 128 and slag 120 are produced by gasifier 118. Slag 120 is conveyed to a coarse slag handling system 130 that produces coarse slag 132 to be sold as product, and syngas stream 128 is conveyed to a scrubber 134 for removing certain particulate matter and other pollutants. In the exemplary embodiment, any suitable scrubbing technique may be used. For example, scrubber 134 may utilize a water spray supplied with water from a condensate return 136 and a grey water return 138. Syngas stream 128 may contact the water spray, which condenses certain condensables such as tar and oil. Syngas stream 128 may also be quenched for removal of particulate matter.
In the exemplary embodiment, scrubber 134 produces a water 144 containing particulate matter removed from syngas stream 128 during the scrubbing process. Water 144 is directed to a water flash system 146 to flash water 144 into a high pressure flash gas 148 and a black water 150, which is directed to a fine slag and grey water handling system 152 for reuse. For example, fine slag and grey water handling system 152 may process black water 150 to recover solids and water 154 for reuse by feedstock grinding and slurry preparation system 108. Fine slag and grey water handling system 152 may also produce grey water 138 suitable for reuse in the scrubbing process of scrubber 134. An excess water stream 156 is directed to a grey water pretreatment system 158 for further processing. For example, excess water stream 156 is filtered, cleaned and directed as a cleaned water stream 160 to a bio-pond (not shown) for further filtration and reuse. A filter cake 162 may also be produced by fine slag and grey water handling system 152, which may include non-reusable fine particulates, such as powdered ash.
In the exemplary embodiment, a scrubbed syngas stream 140 is conveyed from scrubber 130 to an expander 142 suitable for energy recovery through the expansion of syngas stream 140. Syngas stream 140 may be cooled in a cooler 164 and further purified and/or cleaned (not shown) and directed into a power system or power island 166 for use in the generation of power, and/or for the production of chemicals. For example, power island 166 may include a gas turbine (not shown) suitable for using syngas stream 140 as fuel and converting the fuel to rotational energy, which can be converted by a generator into electrical power.
In the exemplary embodiment, separation of ash and coal contained in high-ash content coal feed 212 is further facilitated by air unit 204 and reagent unit 206. Air unit 204 provides an air feed 238 to inlet 240 located in column bottom 224. As such, air feed 238 generates air bubbles 242 that create turbulent flow within wash fluid 220 to facilitate separation high ash content coal feed 212 and to enhance upward flow of separated coal 226. In the exemplary embodiment, reagent unit 206 provides a reagent feed 244 to coal feed 212. As such, reagent feed 244 increases the hydrophobicity of the coal in coal feed 212 to facilitate upward flow of separated coal 226. In the exemplary embodiment, reagent feed 244 is diesel and/or a glycol containing composition. Alternatively, reagent feed 244 is any composition that enables system 14 to function as described herein.
In operation, grinding unit 12 supplies high-ash content coal feed 212 through inlet 214 into separation column 200. Wash fluid unit 202 supplies down-flowing wash fluid 220 through column inlet(s) 218 to contact feed 212 and to subject feed 212 to a froth floatation process to separate the ash and coal of feed 212. As such, separated ash 222 settles to column bottom 224, and separated coal 226 floats to column top 228. Feed 212 is further separated by injecting air feed 238 through column inlet 240 to generate upward flowing air bubbles 242 and by injecting reagent feed 244 through column inlet 246 to enhance coal hydrophobicity. Separated ash 222 collected at column bottom 224 is removed from column 200 via outlet 230, and separated coal 226 is removed from column 200 via outlet 234 as low-ash stream 236. In the exemplary embodiment, the froth flotation process of separation unit 14 produces low-ash stream 236 with an ash content of less than 20%, and stream 236 may be subsequently treated in a slurry thickening/thinning system 248 to produce a low-ash coal slurry 250 with a solid content of at least 40% (by weight), which is then conveyed to gasification unit 16 for production of syngas. In an alternative embodiment, separation unit 14 produces low-ash stream 236 with an ash content of less than 10%.
As described herein, systems and methods are provided for reducing the ash content of high-ash content fuels such as high-ash content coal. The high-ash content fuel is subjected to froth flotation to separate ash from the fuel to produce a low-ash slurry. Typically, entrained flow gasification is not an efficient method to gasify high-ash containing fuel. The systems and methods described herein enable entrained flow gasification systems to utilize high-ash fuels, thus enabling access to otherwise unusable coal or fuel sources. In addition, such systems and methods reduce or eliminate the cost associated with coal beneficiation, which is a process to reduce non-combustible content of fuel and to improve fuel quality. As such, the systems and methods described herein enable gasification systems to utilize fuels from markets such as India and South America, which have vast reserves of low-cost, high-ash content coals.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
