Patent Application
20140259882
The subject matter disclosed herein relates to the gasification of slurry. More specifically, disclosed embodiments of the invention relate to the preparation and delivery of feed slurries to a gasifier.
Fossil fuels, such as coal or petroleum, may be gasified for use in the production of electricity, chemicals, synthetic fuels, or for a variety of other applications. Gasification involves reacting a carbonaceous fuel with sub stoichiometric amounts of oxygen at high temperature and pressure to produce syngas, a gaseous fuel containing primarily carbon monoxide and hydrogen, which burns much more efficiently and cleaner than the fuel in its original state.
Known gasification processes overcome the challenge of feeding solid carbonaceous fuels, such as coal, into a high pressure gasifier by grinding the coal to a fine powder and mixing the powder with water to form an aqueous coal slurry that can be transported into the gasifier using conventional process equipment such as positive displacement pumps. Slurries which are typically fed to a gasifier using this method must satisfy at least two conditions. First, the fuel itself must have a relatively high gross heating value. Second, the solids content of the aqueous slurry must be such that the energy content of the solid fuel is not diluted, as it were, by the presence of excessive amounts of water.
The gross heating value is a measure of the amount of thermal energy a fuel will release when completely reacted with oxygen via the combustion reaction. It includes the energy released when all of the water in the products of combustion is condensed from vapor to liquid. Using a fuel with a relatively high gross heating value is important for an efficient and economical gasification process. When a carbonaceous fuel such as coal is reacted with oxygen in a gasifier, some of the coal is fully oxidized to carbon dioxide and water via the combustion reaction, and some of the coal is partially oxidized to carbon monoxide and hydrogen via several gasification reactions (e.g. reactions of H2O and CO2 with solid carbon). The combustion reaction is exothermic, which means it releases thermal energy. The gasification reactions are endothermic, which means they require thermal energy input in order to proceed. So, in an operating gasifier, carbonaceous fuel is simultaneously combusted and gasified. The portion of the fuel that is combusted supplies the thermal energy that drives the gasification reactions as well as the energy that heats the reactants—the fuel, the oxidant and the water—from ambient temperature to the high temperature at which the reactions proceed inside the gasifier. If the heating value of a fuel is too low, a greater portion of the fuel must react via the combustion reaction in order to supply sufficient thermal energy to heat the reactants and to drive the gasification reactions. But this degrades the product syngas composition by shifting more of the gas to carbon dioxide and water and away from the more desirable carbon monoxide and hydrogen products. Likewise, if a slurry contains too much water, more fuel must react via the combustion reaction in order to provide extra energy to heat the excess water.
Petroleum coke, anthracite and bituminous coal are examples of fuels with high gross heating values that can be efficiently gasified. They also can be made into aqueous slurries with relatively high solids content, so they represent high quality gasification fuels. In contrast, lignites and some sub bituminous coals have lower gross heating values. In addition, many of them may have high internal moisture content, so they produce aqueous slurries with lower solids content. Both of these factors make lignites and sub bituminous coals less desirable feeds for gasifiers. Like sub bituminous coals and lignites, most biomass has relatively low gross heating values and produces aqueous slurries with low solids content. However, biomass is a carbon neutral, renewable feedstock and low rank coals, such as lignite and sub bituminous coal, are less expensive than high rank coals, such as anthracite and bituminous coal. Accordingly, it may be desirable to develop mixtures, systems, and methods for the gasification of low gross heating value fuels and of high moisture content fuels, including biomass and low rank coals.
Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below.
In a first embodiment, a gasification feedstock includes a mixture of a solid having a gross heating value of less than approximately 20 Megajoules/kilogram (MJ/kg) and a non-aqueous liquid having a heating value of greater than approximately 14 MJ/kg expressed on a wet basis.
In a second embodiment, a system includes a solid feed supply configured to supply a solid having a low gross heating value and a liquid feed supply configured to supply a liquid having a high gross heating value. The system further includes a feedstock mixing tank coupled to the solid feed supply and the liquid feed supply, wherein the feedstock mixing tank is configured to mix the solid and the liquid to provide a gasification feedstock.
In a third embodiment, a system includes a feedstock mixing controller configured to adjust a ratio of a liquid feedstock and a solid feedstock to obtain a gasification feedstock, wherein the solid feedstock comprises a first gross heating value of at least less than approximately 20 Megajoules/kilogram (MJ/kg) expressed on a wet basis, and the gasification feedstock has a third gross heating value of at least greater than 16 MJ/kg expressed on a wet basis.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
The disclosed embodiments include mixtures, systems and methods for utilizing, for example, low gross heating value solids and high moisture content solids (e.g., biomass and high moisture coals) as gasifier feedstock by adding high gross heating value non-aqueous liquids (e.g., vegetable oil, pyrolysis oil, orimulsion, biodiesel, liquid fossil fuels, petroleum-derived liquids, and the like). The moisture content of the fuel refers to the presence of water in the fuel and is usually measured as percentage of water by weight. The gross heating value of a fuel is a measure of the energy content of the fuel and is often measured in Megajoules per kilogram (MJ/kg). Low rank coals, for example, include coals that have low gross heating values, e.g., less than approximately 20 MJ/kg, as expressed on a wet basis. Examples include coals such as lignite coals and some sub-bituminous coals. Some low rank coals may also contain high moisture content, in some cases of approximately 20% to 40% or more by weight. Table 1 shows typical composition and gross heating value data for delayed petroleum coke, a bituminous coal (Illinois #6), two sub bituminous coals (Wyodak and Montana Rosebud) and a lignite. The moisture content of the fuels increases from left to right in the table and, as expected, the gross heating value generally decreases with increasing moisture content. The bottom four rows in the table show the range of typical aqueous slurry concentrations that may be more easily transported into a gasifier by commercially available pumps as well as the gross heating values of the slurries at the minima and maxima of those ranges. For delayed coke and Illinois #6 coal, economic gasifier operation may be achieved using aqueous slurry concentrations in the mid to upper portions of the slurry concentration ranges. However, the gross heating values of the sub bituminous coal and lignite slurries may be too low for economical gasifier operation at any point within the range aqueous slurry concentrations, in other words, at aqueous slurry gross heating values of approximately 16 MJ/kg and below.
Biomass fuels may include a wide range of fuels such as corn husks, rice husks, switchgrass, miscanthus, sugar cane bagasse, sorghum bagasse, wood and wood products, algae, manure, other agricultural products, and municipal solid waste. As with low rank coals, some biomass may contain considerable amounts of water, ranging from approximately 5 to 30 weight percent or, even in some cases, as high as 90 weight percent. Table 2 shows typical composition and gross heating value data for five typical biomass fuels. As with the coals in Table 1, moisture content increases from left to right, with the energy content of the fuels decreasing with increasing moisture content. In the case of rice waste, the high ash content makes up for the lower moisture content in the sense that ash, like moisture, dilutes the energy content of the organic matter. Also as in Table 1, the bottom four rows in Table 2 show the range of typical aqueous slurry concentrations that may be more reliably produced and transported into a gasifier by commercially available pumps as well as the gross heating values of the slurries at the minima and maxima of those ranges. Because of the low slurry concentrations for all of the biomass fuels, the gross heating values of all of the biomass slurries may be too low for economical gasifier operations.
As shown in Tables 1 and 2, sub bituminous coal, lignite and biomass slurries have lower gross heating values and higher water contents than petroleum coke and bituminous coal slurries. As explained above, the gasification of feedstocks with low gross heating values and high water contents may result in inefficient and uneconomical operation of the gasifier. One way to remove moisture is to dry the high moisture, low energy materials before slurrying them with water. However, water removal by drying may require a high amount of energy, which may negatively impact the overall energy efficiency of the gasification plant. However, a more desirable gasification feed may be produced by changing the slurrying medium from water, which has approximately zero energy content, to a non-aqueous liquid which can contribute energy content to the non-aqueous slurry. (Note that the non-aqueous liquids discussed below are referred to as having high gross heating value even though, in some cases, they may have gross heating values that are lower than the solid fuel component of the slurry. The reason for this is that the gross heating values of the non-aqueous liquids are being compared with water and not with the solid fuels. Water has zero energy content and, thus, cannot contribute any energy content to an aqueous slurry. However, by virtue of having at least some measurable gross heating value, the non-aqueous liquids are, by comparison with water, high gross heating value liquids.)
By judiciously selecting a desirable non-aqueous liquid slurrying medium, the non-aqueous slurry may attain a sufficient energy and moisture combination that enables its use in regular gasification operations. Additionally, the non-aqueous slurry may also be more easily transported by a fluid pump instead of a dry feed system, which may not be as efficient as the fluid pump. Table 3 shows composition and gross heating value data for four example non-aqueous liquids which may be mixed with low gross heating value and/or high moisture content solids in order to produce non-aqueous slurries having sufficient energy content for economical operation of a gasifier. The first three non-aqueous liquids are oils derived from the pyrolysis of switchgrass, hardwood and pine. The fourth liquid is heavy fuel oil, a readily available oil refinery byproduct.
Tables 4A and 4B show some representative results that may be obtained by mixing two of the non-aqueous liquids from Table 3 with each of the biomass fuels from Table 2 and the sub bituminous coals and lignite from Table 1. Table 4B is a continuation of table 4A but with additional columns. Just the highest gross heating value liquid (heavy fuel oil, or HFO) and the lowest gross heating value liquid (switchgrass pyrolysis oil, or SPO) were chosen for inclusion in Tables 4A and 4B in order to limit the size of the tables. It is to be understood, that in other embodiments, other values may be chosen. The first row of numbers shows the gross heating value for each of the solid feedstocks expressed on a wet basis. The second, third and fourth rows of numbers show the gross heating value for heavy fuel oil, an example solids concentration of the solids-HFO slurry and the resulting solids-HFO slurry gross heating value for each one of the solid feedstocks. The fifth, sixth and seventh rows of numbers show a similar set of data for switchgrass pyrolysis oil and solids-SPO slurry.
For both sets of examples slurries, a 50 wt % solids slurry concentration was assumed based on preliminary laboratory results that suggest that slurry concentrations of at least 50 wt % solids may be achieved when mixing solid fuels with non-aqueous liquids to form a slurry. As a consequence of the relatively high slurry solids concentration, the non-aqueous slurries based on heavy fuel oil, which has a high gross heating value, all may surpass the minimum gross heating value target of approximately 16 MJ/kg. But even the switchgrass pyrolysis oil, which has only 45% of the gross heating value of the heavy fuel oil, is able to make non-aqueous slurries with 50 wt % solids that have slurry gross heating values above the approximately 16 MJ/kg target—with an exception, miscanthus. For miscanthus, the solids concentration may be slightly decreased to approximately 46 wt % solids in order to reach the approximately 16 MJ/kg target. Nevertheless, in most cases, the use of non-aqueous liquids to produce non-aqueous slurries results in gasifier feedstocks of sufficient energy content to allow low gross heating value and/or low moisture content solids such as biomass, sub bituminous coal and lignite to be more efficiently and economically gasified. Given the results of preliminary laboratory results that suggest that non-aqueous slurries may be produced with solids concentrations as high as 60 wt %, it may be possible to tailor non-aqueous slurries with solids concentrations ranging from 1 wt % to as much as 60 wt % solids.
In the production of such non-aqueous slurries, certain mixing and delivery embodiments may include a mixing tank, a run tank, a set of pumps, interconnecting piping and control valves, a series of sensors associated with the tanks, pumps, piping and control valves, and a controller. Controller embodiments may manage the supply of solids and liquids into a mixing tank to achieve an approximate ratio of solid to liquid, the mixing of the solids and the liquids into a non-aqueous slurry, the delivery of the non-aqueous slurry into the run tank, which provides storage capacity within the system, and the pressurization and delivery of the non-aqueous slurry into a gasifier. The non-aqueous slurry embodiments may be produced by mixing low gross heating value solids and high gross heating value liquids at a ratio that may be derived from the energy content of the solids, the energy content of the non-aqueous liquids, and the moisture content of the solids, as discussed in more detail below. After the slurry is mixed, it may be delivered into a gasifier for use, for example, in a power generation system.
With the foregoing in mind and turning now to
The non-aqueous slurry may be pumped to a gasifier 16 from the feedstock mixing system 12 by using pump 18. Pump 18 may include any class of pump designed for handling non-aqueous slurries including, but not limited to, positive displacement pumps and progressive cavity pumps. The gasifier 16 may convert the non-aqueous slurry into syngas, a combination consisting primarily of carbon monoxide and hydrogen, but also containing carbon dioxide and water with lesser amounts of methane, nitrogen, argon, hydrogen sulfide, carbonyl sulfide and ammonia, plus trace components that depend on the composition of the non-aqueous slurry feed. This conversion may be accomplished by reacting the non-aqueous slurry with a controlled amount of steam or liquid water and an oxidant, such as oxygen, air or oxygen-enriched air, at elevated pressures (e.g., from approximately 400 psi-1250 psi) and temperatures (e.g., approximately 2200° F.-2700° F.), depending on the type of gasifier 16 utilized. At the high temperatures prevailing inside the gasifier, essentially all of the non-aqueous slurry may be converted to syngas. However, depending upon gasifier operating conditions, such as the ratio of oxygen in the oxidant to carbon in the slurry, some unconverted slurry may remain in the form of soot, or char, which may contain some carbon along with some of the ash material present in the feedstock. The amount of carbon exiting the gasifier in the form of soot or char may vary from 0 to 10 weight percent of the carbon in the feed slurry depending upon the operation conditions of the gasifier. The remainder of the ash material present in the feedstock may exit the gasifier in the form of slag 20.
As a result of the reactions that occur inside the gasifier 16, a resultant gas may be manufactured by the gasifier 16. The resultant gas may include approximately 60 to 90 volume percent of carbon monoxide and hydrogen, as well as CO2, H2O, CH4, N2, Ar, H2S, COS, NH3, trace amounts of other compounds that depend on the composition of the feedstock and unconverted feedstock in the form of soot or char. This resultant gas may be termed “untreated syngas.” The gasifier 16 may also generate byproduct solids, such as slag 20, which may be a wet ash material. As described in greater detail below, a gas treatment unit 22 may be utilized to treat the untreated syngas. The gas treatment unit 22 may scrub the untreated syngas to remove the soot or char, the NH3 and the sulfur compounds from the untreated syngas, which may include separation of sulfur 28 in a sulfur processor 30 by, for example, an acid gas removal process in the sulfur processor 30. Furthermore, the gas treatment unit 22 may separate salts 24 from the untreated syngas via a water treatment unit 26, which may utilize water purification techniques to generate usable salts 24 from the untreated syngas. Subsequently, a treated syngas may be generated from the gas treatment unit 22.
The IGCC system 10 may further include an air separation unit (ASU) 40. The ASU 40 may separate air 19 into component gases using, for example, distillation techniques. The ASU 40 may separate oxygen from the air supplied to it from an air compressor 42 and may transfer the separated oxygen to the gasifier 16. Additionally, the ASU 40 may direct separated nitrogen to a diluent gaseous nitrogen (DGAN) compressor 44. The DGAN compressor 44 may compress the nitrogen received from the ASU 40 at least to pressure levels equal to those in the combustor 36, for use in enhancing combustion of the syngas. Thus, once the DGAN compressor 44 has adequately compressed the nitrogen to an adequate level, the DGAN compressor 44 may direct the compressed nitrogen to the combustor 36 of the gas turbine engine 38.
As described above, the compressed nitrogen may be transferred from the DGAN compressor 44 to the combustor 36 of the gas turbine engine 38. The gas turbine engine 38 may include a turbine 46, a drive shaft 48, and a compressor 50, as well as the combustor 36. The combustor 36 may receive fuel, such as the syngas, which may be injected under pressure via fuel nozzles. This fuel may be mixed with compressed air from a compressor 50 as well as compressed nitrogen from the DGAN compressor 44 and combusted within the combustor 36. This combustion may create hot pressurized exhaust gases.
The combustor 36 may direct the exhaust gases towards an exhaust outlet of the turbine 46. As the exhaust gases from the combustor 36 pass through the turbine 46, the exhaust gases may force turbine blades in the turbine 46 to rotate the drive shaft 48 along an axis of the gas turbine engine 38. As illustrated, the drive shaft 48 may be connected to various components of the gas turbine engine 38, including the compressor 50.
The compressor 50 may include blades coupled to the drive shaft 48. Thus, rotation of turbine blades in the turbine 46 may cause the drive shaft 48 connecting the turbine 46 to the compressor 50 to rotate blades within the compressor 50. The rotation of blades in the compressor 50 causes the compressor 50 to compress air received via an air intake in the compressor 50. The compressed air may then be fed to the combustor 36 and mixed with fuel and compressed nitrogen to allow for higher efficiency combustion. The drive shaft 48 may also be connected to a load 52, which may be a stationary load, such as an electrical generator, for producing electrical power in a power plant. Indeed, the load 52 may be any suitable device that is powered by the rotational output of the gas turbine engine 38.
The IGCC system 10 also may include a steam turbine engine 54 and a heat recovery steam generation (HRSG) system 56. The steam turbine engine 54 may drive a second load 58, such as an electrical generator for generating electrical power. However, both the first and second loads, 52 and 58, may be other types of loads capable of being driven by the gas turbine engine 38 and the steam turbine engine 54, respectively. In addition, although the gas turbine engine 38 and the steam turbine engine 54 may drive separate loads, 52 and 58, as shown in the illustrated embodiment, the gas turbine engine 38 and the steam turbine engine 54 may also be utilized in tandem to drive a single load via a single shaft. The specific configuration of the steam turbine engine 54, as well as the gas turbine engine 38, may be implementation-specific and may include any combination of sections.
Heated exhaust gas from the gas turbine engine 38 may be directed into the HRSG 56 and used to heat water and produce steam used to power the steam turbine engine 54. Exhaust from the steam turbine engine 54 may be directed into a condenser 60. The condenser 60 may utilize a cooling tower 62 to exchange heated water for chilled water. In particular, the cooling tower 62 may provide cool water to the condenser 60 to aid in condensing the steam directed into the condenser 60 from the steam turbine engine 54. Condensate from the condenser 60 may, in turn, be directed into the HRSG 56. Again, exhaust from the gas turbine engine 38 may also be directed into the HRSG 56 to heat the water from the condenser 60 and produce steam.
As such, in combined cycle systems such as the IGCC system 10, hot exhaust may flow from the gas turbine engine 38 to the HRSG 56, where it may be used to generate high-pressure, high-temperature steam. The steam produced by the HRSG 56 may then be passed through the steam turbine engine 54 for power generation. In addition, the produced steam may also be supplied to any other processes where steam may be used, such as to the gasifier 16. The gas turbine engine 38 generation cycle is often referred to as the “topping cycle,” whereas the steam turbine engine 54 generation cycle is often referred to as the “bottoming cycle.” By combining these two cycles as illustrated in
Turning to
The non-aqueous slurry may be produced by the feedstock mixing system 12 which may be controlled by the controller 68. The high gross heating value non-aqueous liquid feedstock may be added first to the mixing tank by a pump 70 in order to establish an initial level or weight of liquid to which the solids may subsequently be added. Controller 68 may employ one or more sensors 78 to determine when the initial level or weight target has been achieved. Controller 68 may then energize motor 74 to begin rotation of mixer 76. The design of mixer 76, may have one or more sets of mixing blades connected to the same shaft at different elevations along the length of the shaft. The number of blade sets, the number of blades in each set and the size and shape of each blade may chosen so that rotation of mixer 76 by motor 74 more thoroughly stirs the contents of mixing tank 64, establishes vertical recirculation patterns within the stirred contents which aid in the mixing of the solids and the non-aqueous liquid and minimizes the settling and/or accumulation of solids in the bottom or along the vertical sidewall of mixing tank 64. Mixing tank 64 may also be configured with one or more vertical baffles (not shown) extending radially inwards from the vertical sidewall of mixing tank 64. After the controller 68 has started mixer 76, the controller 68 may also close block valve 73, open block valve 71 and start pump 80 in order to establish a recirculating flow of non-aqueous liquid around tank 64 via the pathway that includes the discharge of pump 80 and valve 71. The recirculation flow may turn over the contents of tank 64 in approximately 1 hour, in 30 minutes, in 15 minutes or in 5 minutes in order to assist mixer 76 with the stirring of the tank contents and with the prevention of solids settling once solids are added to the tank. Once the initial liquid level or weight has been established and the mixer 76 and the pump 80 started, solid feedstock processed by the grinder 66 may then be conveyed into the mixing tank by using a solid feedstock conveyor 77. As the solids enter the stirred and recirculating pool of liquid within mixing tank 64, they may become thoroughly mixed with the liquid by action of the mixer 76 and the recirculation flow maintained by pump 80. In one embodiment, the controller 68 may employ the solids conveyor 77, pump 70 and one or more sensors 78 to control the volume and/or the weight of each of the solid feedstock and the non-aqueous liquid feedstock to add to the mixing tank 64 in order to achieve an initial approximate solids-to-liquid ratio. In this embodiment, the controller 68 may then employ one or more additional sensors 78 to measure one or more properties of the initial mixture such as moisture content, gross heating value, viscosity, density and so forth, and then use the results to derive an actual as-mixed solid-to-liquid ratio. The ratio may be derived by the controller 68 based on several variables, including the gross heating value of the solid feedstock, the moisture content of the solid feedstock, the viscosity of the mixture, the temperature of the mixture, the density of the mixture, the heating value of the non-aqueous liquid, and so forth. In another embodiment, the variables that may be used for the determination of the solid-to-liquid ratio may be entered into the controller manually, for example, from the results of lab tests done on grab samples obtained from the mixing tank or from the recirculation line. For example, results from off-line analyses for moisture content, gross heating value, viscosity, weight percent solids, density and so forth, may be entered into the controller 68 manually, for example, by typing the values into a keyboard. In yet another embodiment, the ratio of solid to liquid fuel may be entered directly in the controller 68, for example by typing the ratio into a keyboard.
Once the initial as-mixed solids-to-liquid ratio has been determined, the controller 68 may compare the actual, as-mixed solids-to-liquid ratio with a target solids-to-liquid ratio in order to determine the size of the deviation, if any, from the target ratio. Depending upon the magnitude of the deviation, for example 2%, 1%, 0.5% or 0.1%, the controller may calculate an additional amount of solids and/or an additional amount of liquid to be added to the slurry in order to reach the target ratio after which the controller may add the calculated amount of additional solids via conveyor 77 or additional non-aqueous liquid via pump 70. In one embodiment, this procedure of calculating a deviation from a target solids-to-liquid ratio followed by making an appropriate adjustment, either by adding more solids or more liquid, may occur one time. In another embodiment, this “calculate deviation-make adjustment procedure” may occur “N” number of times, where “N” is an integer entered into controller 68, for example, by typing into a keyboard. In yet another embodiment, the “calculate deviation-make adjustment procedure” may occur as many times as needed to reduce the magnitude of the deviation to a predetermined level, for example to 2%, 1%, 0.5% or 0.1%.
Once the target solid-to-liquid ratio has been achieved in mix tank 64, the controller 68 may open block valve 73 and close block valve 71 to transfer the completed batch of slurry from mixing tank 64 to run tank 65 via pump 80. Once the transfer is complete, the above procedure may be repeated in order to make another batch of slurry. Run tank 65 may be the same size as mixing tank 64, or it may be larger, and in limited cases, smaller. In either case, the rate at which batches of slurry can be mixed and adjusted to a target solids-to-liquid ratio may be greater than the maximum rate at which slurry can be withdrawn from run tank 65 and fed to the gasifier 16 via slurry charge pump 18. This ability of system 12 to produce a non-aqueous slurry mixture faster than the mixture is fed to the gasifier ensures that run tank 65 will rarely or ever run dry and the gasifier 16 will rarely or ever be deprived of slurry during operation. Run tank 65 is equipped with a motor 75 and a mixer 79 which are similar in design and function to the motor 74 and mixer 76 in mixing tank 64. Controller 68 may start motor 75 as soon as an initial batch of slurry has been transferred into run tank 65 in order to maintain the transferred slurry in a thoroughly mixed state. Thereafter, controller 68 may continue operation of mixer 79 as long as there is any slurry within run tank 65. One or more level or weight sensors 72 may provide controller 68 with the required information about the level or weight of slurry within run tank 65 at operational times. If run tank 65 is full, as determined, for example, by one of the level or weight sensors 72, the controller 68 may close block valve 73 and open block valve 71 to prevent any more slurry from being transferred from mixing tank 64 to run tank 65. In addition, controller 68 may also direct solids conveyor 77 and liquid pump 70 to at least temporarily cease delivering more solids and more liquid, respectively, into mixing tank 64 in order to at least temporarily halt production of more slurry until such time as additional slurry is needed to maintain sufficient level or weight in run tank 65.
Run tank 65 is also equipped with a slurry recirculation pump 81 which recirculates slurry around run tank 65 in order to assist mixer 79 in maintaining the slurry in a thoroughly mixed state. The configuration of the recirculation line, e.g. pipe diameter and control valves (not shown), from the discharge of pump 81 back to the top of run tank 65 as well as the configuration of the slurry charge pump 18 suction line, e.g. pipe diameter and control valves (not shown), may be such that the flow rate in the recirculation line may be approximately 5 to 10 times larger than the flow rate of slurry to charge pump 18. This may ensure that the suction of charge pump 18 is always or almost always filled with an adequate supply of slurry. Once run tank 65 has been filled with a supply of slurry sufficient to start the gasifier 16, charge pump 18 may be started in order to deliver the non-aqueous slurry for use in the gasifier 16 as described above with respect to
In the embodiment described above, solid or semi-solid material and at least one non-aqueous liquid are mixed in order to produce the desired non-aqueous slurry. However, in another embodiment, any number of solid and liquid fuels may be added for mixing, for example, switchgrass, cornhusks, pyrolysis oil, and fuel oil. In yet another embodiment, the non-aqueous liquid may be solid or highly viscous at room temperature. For such cases, mixing tank 64 may be fitted with a control valve and a heat transfer coil 67 through which flows a heat transfer fluid from a heat transfer fluid source (HTFS) to a heat transfer fluid return (HTFR). The heat transfer fluid may include, but not be limited to, hot water, low pressure steam, high pressure steam and hot heat transfer oil. Likewise, run tank 65 may be fitted with a control valve and heat transfer coil 69 having a similar design and function. In such cases, the non-aqueous liquid would be delivered to mixing tank 64 via pump 70 from a heated source of non-aqueous liquid. Both mixing tank 65 and run tank 65 would be insulated, and the heat transfer coils 67 and 69 would be designed to maintain a desired temperature in the mixing tank 64 and the run tank 65 sufficient to ensure that the viscosity of the non-aqueous liquid and the viscosity of the non-aqueous slurry were within a range of values that could be handled by pumps 80, 81 and 18. Using one or more temperature sensors 78 in mixing tank 64 and its recirculation line and one or more temperature sensors 72 in run tank 65 and its recirculation line, controller 68 may adjust the amount of heating provided by coil 67 and/or by coil 69 to maintain the temperature and, thus, the viscosity of the contents of each tank at the desired level.
Turning to
GHV=33.86C+144.4(H—O/8)+9.428S (Equation 1)
Other GHV equations may also be used, some having been derived especially for calculating the gross heating value of biomass or for other types of fuels. The gross heating value of a fuel expressed on a wet basis can be calculated from the gross heating value of the fuel expressed on a dry basis using Equation 2, in which M is the mass fraction of moisture (water) in the wet fuel.
GHVwet=GHVdry(1−M) (Equation 2)
In addition to the above input data and GHV equations, control logic 82 may also contain code which, using the principals of mass balance, allows control logic 82 to calculate initial estimates of the quantities of liquid fuel and solid fuel that must be added to mixing tank 64 in order to produce the desired quantity of final mixed slurry with a gross heating value that falls within the range of acceptable values defined by the minimum GHV 90 and the maximum GHV 92. Once estimates for the initial quantities of liquid fuel and solid fuel have been calculated by control logic 82, an initial mass of liquid fuel may be added (block 100) to, for example, a mixing tank. Once the liquid fuel has been added, a mixer and a recirculation pump may be started (block 102) to provide a well stirred liquid into which the initial mass of solid fuel may then be added (block 104). Once the initial masses of solid fuel and liquid fuel have been added, the process 82 may continue via the connector 105 labeled A at the bottom of
In an alternative embodiment, the gross heating value of the slurry may be measured online by a sensor located, for example, on the recirculation line or at one or more points in the mixing tank. If, after mixing the initial masses of liquid and solid fuels, the slurry has a measured gross heating value that falls within the range of values defined by the minimum and maximum desired values (90 and 92 respectively), the process 82 moves on to the check slurry viscosity decision point 116. However, if the slurry does not have a gross heating value that falls between the minimum 90 and maximum 92 acceptable values, then the process 82 may adjust the slurry, for example, by adding liquid fuel (block 110) or by adding solid fuel (block 112). Liquid fuel may be added (block 110) if the slurry gross heating value testing or sensing determines that the slurry has too low of a gross heating value. Solid fuel may be added (block 112) if the slurry gross heating value testing or sensing determines that the slurry has too high of a gross heating value. After the addition of the fuel, the process 82 may again mix and recirculate (block 106) the slurry for a preset period of time followed by a repeat determination of the slurry's gross heating value at decision point 108. Each time that the process 82 proceeds from the mix and recirculate step 106 to the check slurry GHV decision point 108, a loop counter is checked and incremented (decision point 114) within the control logic 82 in order to keep track of the number of times through the loop, i.e. the number of slurry GHV adjustment iterations. The control logic 82 allows the process to continue to cycle through the slurry GHV adjustment loop until either the measured gross heating value of the slurry falls between the minimum 90 and maximum 92 acceptable values or the number of cycles, i.e. the number of iterations, equals or exceeds the maximum allowable iterations, N, that was input at block 98.
The loop counter (decision point 114) limits the number of slurry adjustment iterations that may be made by the control logic 82 in order to prevent the system from becoming stuck in an endless loop. When the checked number of iterations at decision point 114 equals or exceeds the maximum allowable iterations, N, the control process 82 skips to decision point 116 to check the slurry viscosity. However, it should be appreciated that, because the control program 82 has an updated GHV measurement following each slurry GHV adjustment iteration, each iteration represents a refinement of the previous set of fuel addition and mixing steps. Thus, it may be expected that the number of iterations required to produce a fuel slurry with a GHV falling within the acceptable range will be limited to no more than one or two or three iterations. The value for N can be determined empirically in the plant for a given set of liquid and solid feedstocks and adjusted as needed (block 98). For example, the minimum gross heating value may be approximately 1%, 2%, 3%, 4%, 5% below the target GHV. Alternatively, the minimum GHV may be about 2% or 0.5% or 0.1% below the target GHV. Likewise, the maximum gross heating value may be approximately 1%, 2%, 3%, 4%, 5% above the target GHV. Alternatively, the maximum GHV may be about 2% or 0.5% or 0.1% above the target GHV. It should also be appreciated that, the description of a preset mixing and recirculation time (blocks 102 and 106) does not mean that mixing and/or recirculation are stopped at the end of the specified time period. Instead, mixing and circulation may continue at all times that liquid is present in the mixing tank 64. The preset mixing and recirculation time merely refers to time that must elapse before the control logic 82 proceeds from step 102 to step 104 or from step 106 to step 114. In fact, for both the mixing tank 64 and run tank 65 in
Once the process 82 has produced a mixed fuel slurry with a gross heating value within the acceptable range of values, or once the number of iterations has reached the maximum allowable number N, the process may then test the slurry at decision point 116 to determine the slurry's viscosity to ensure that the slurry can be handled by downstream processing equipment. As mentioned above, it is to be understood that slurry may be collected for testing of the viscosity at different physical locations within, for example, the mixing tank 64 (e.g., bottom of the tank, top of the tank, left wall of the tank, right wall of the tank) or along the recirculation line. Each location sample may be tested and the results of the various tests may be averaged in order to arrive at a viscosity representative of the slurry. In an alternative embodiment, the viscosity of the slurry may be measured on line by a sensor located on the recirculation line or at one or more points in the mixing tank. If the slurry's viscosity is below the maximum desired slurry viscosity (block 94), the control process 82 moves on to the check moisture content at decision point 120. However, if the slurry viscosity is above the maximum desired slurry viscosity, the control logic 82 calculates an appropriate amount of additional liquid fuel to add to the mix in order to reduce the slurry viscosity, loops back to add this additional amount of liquid fuel (block 110) and then once again mixes and recirculates for a preset period of time (block 106).
Once the preset mixing and recirculation time has been computed for the viscosity adjustment, the controller 82 automatically returns to decision point 116 to recheck the slurry viscosity. It does this because, when it checked the viscosity the first time, the controller 82 also set the number of iterations to the maximum number, N, in order to disable the GHV adjustment loop by bypassing it. Thus, at this point in the process, the control logic 82 will only check and adjust viscosity. This is done because, while it is beneficial to get the GHV of the slurry within the desired range, it may be more beneficial that the slurry viscosity not exceed the maximum allowable value in order to avoid problems (e.g. plugging or high pressure drop or low flow rates) with slurry that may be too thick (viscosity too high) for the downstream equipment (e.g. pumps, piping or gasifier feed injector) to handle. Thus, the control logic 82 may ensure that the maximum allowable viscosity is not exceeded at the expense of possibly adding too much liquid fuel, which may raise the slurry gross heating value above the desired target range by a small amount. The maximum allowable slurry viscosity may be approximately 2 Pascal-seconds (2000 Centipoise). Alternatively, the maximum allowable slurry viscosity may be approximately 1 Pascal-seconds (1000 Centipoise) or 0.7 Pascal-seconds (700 Centipoise).
It should also be noted that the system 82 may adjust the temperature (and, thus, the viscosity) of the contents of the mixing tank 64 (block 118), for example, by adjusting the flow rate of heat transfer fluid through the internal heating coil 67. Adjusting the temperature in mixing tank 64 may impact the viscosity of the liquid fuel and the viscosity of the slurry. Increasing the temperature in the tank may decrease the viscosity; decreasing the temperature in the tank may increase the viscosity. Using the data on liquid fuel viscosity as a function of temperature that was entered at block 86, the system 82 may adjust the temperature (and, thus, the viscosity) during the mixing and recirculation that occurs after the initial mass of liquid fuel has been added to tank 64 (block 100) and/or during the mixing and recirculation that occurs during the GHV and/or viscosity adjustment steps (block 106). By using the ability to increase the temperature of the tank 64 contents, the system 82 may decrease the viscosity to an acceptable level while minimizing or eliminating the need to add additional liquid fuel.
Once the system 82 has generated a slurry mixture with a gross heating value that falls within the desired range and has adjusted the viscosity, if needed, to ensure that the slurry is not too thick, the system may test the slurry for moisture content, M (block 120). As with the GHV and the viscosity, it is to be understood that slurry may be collected for testing of the moisture content at different physical locations within, for example, the mixing tank 64 (e.g., bottom of the tank, top of the tank, left wall of the tank, right wall of the tank) or along the recirculation line. Each location sample may be tested and the results of the various tests may be averaged in order to arrive at a viscosity representative of the slurry. In an alternative embodiment, the moisture content of the slurry may be measured on line by a sensor located on the recirculation line or at one or more points in the mixing tank. The moisture content of the slurry may be something that needs to be known for gasifier operation but not necessarily something that may need to be manipulated by the control logic 82 in the slurry mixing system 12. Typically, a certain amount of water may be fed to the gasifier during gasifier operations in order to help control gasifier operating temperature, e.g., to help keep it from going too high. Water also may provide some of the oxygen that partially oxidizes the fuel during gasification, thereby reducing the gaseous oxygen feedstock requirement. However, water may be fed to the gasifier independently of the slurry mixing and feed system via a separate conduit connected to the gasifier feed injector; so there may be an independent means of controlling the amount of water that is fed to a gasifier. Therefore, in practical operation of the present techniques, the slurry that is prepared by mixing system 12 may be prepared with a moisture content that exactly matches the needs of the gasifier during operation. In that case, no additional water may need to be fed to the gasifier. Alternatively, the slurry may be prepared with a moisture content that is lower than that which may be needed by the gasifier during operation. In that case, additional water may be added to the gasifier using the independent water feeding system connected to the gasifier feed injector. However, for control purposes, the gasifier control system always needs to know the moisture content of the slurry so that the independent water feed system may be controlled, if needed. Therefore, control logic 82 measures the slurry moisture content, M, at decision point 120 and outputs the value M to the gasifier control system.
Once the control logic 82 has completed the preparation of a mixed fuel slurry with desired gross heating value and viscosity and has checked the moisture content, the control logic 82 checks the level in the run tank 65 (decision point 124) to see if it may be low enough to receive a full batch of recently mixed slurry from the mixing tank 64 without overflowing. If the level in run tank 65 is less than full, which may be defined as having a level low enough to accept the entire volume of slurry in mixing tank 64 without overflowing, control logic 82 may transfer the entire batch of recently mixed slurry from the mixing tank 64 to the run tank 65 via pump 80 by closing valve 71 and opening valve 73. Once the transfer is complete, control logic may turn off pump 80, close valve 73 and reopen valve 71, returning the system to its original condition. The system may then be ready to being making another batch of mixed fuel slurry according to process 82. However, if at decision point 124 the control logic 82 determines that the run tank 65 is still full, which is defined as having a level high enough that the transfer of a full batch of recently mixed fuel slurry would cause run tank 65 to overflow, the control logic 82 just waits at step 124 and continues to check the run tank level until the condition changes and the level in the run tank drops to the point where a new batch can be transferred, as described above. As noted above, the slurry mixing system 12 is capable of continuously supplying mixed fuel slurry to a gasifier 16 because the mixing tank 64 and its associated equipment and the control logic 82 are able to produce batches of freshly mixed slurry at a rate that exceeds the rate at which slurry is withdrawn from run tank 65 to feed gasifier 16. Thus, once filled, run tank 65 always remains full for the duration of operation of gasifier 16.
Technical effects of the invention include the gasification of low heating value solids such as biomass and low rank coals, the gasification of high moisture content solids such as low rank coals and lignites, the production and use of a non-aqueous slurry that is transportable through pumps, and the capability to mix a non-aqueous slurry so as to control the slurry's heating value and viscosity. The disclosed embodiments also allow for the use in regular gasification operations of certain solid fuels, including certain biomass fuels and low rank coals, which were traditionally not used as gasification fuels.
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.
