Patent Application
20160222306
Embodiments disclosed herein relate generally to a gasification system and processes for converting generally solid feedstocks, such as carbonaceous materials, into desirable gaseous products, such as synthesis gas.
Gasification processes are widely used to convert solid or liquid feedstocks such as coal, petroleum coke and petroleum residue into synthesis gas (syngas). Syngas is an important intermediate feedstock for producing chemicals such as hydrogen, methanol, ammonia, synthetic natural gas or synthetic transportation oil, or as a fuel gas for power generation.
A common practice for gasification processes is to recycle unreacted char back to the gasification reactor using a complex system of lock-hoppers, which generally includes multiple vessels connected in series, where each vessel can be individually pressurized and de-pressurized. These systems are typically used for transferring solids from a low pressure to a higher pressure environment. However, because of the frequent cycling and batch operations, lock-hoppers are very maintenance intensive, contributing to the high cost of operating such a system. In addition, there is a higher capital cost associated with the use of multiple vessels, valves, and instrumentation. Gas consumption, recycling, and management for pressurization and de-pressurization of the lock-hoppers is an additional factor for consideration.
As an alternative to lock hoppers, rotary valves have also been used for transferring solids from a low pressure environment to a higher pressure environment. However, high erosion wear in the rotor, especially for applications involving fine abrasive solids like char, is a serious problem.
Embodiments disclosed herein relate to a lower maintenance system that can be operated continuously, via which the flow rate of recycled char can be precisely metered and controlled, and which will be able to efficiently and effectively transport the solids from a lower pressure environment to a higher pressure environment.
In one aspect, embodiments herein are directed toward a system for gasification of a carbonaceous material. The system may include a gasification reactor for the gasification of a carbonaceous material, producing an overhead product stream containing char and syngas. The system may also include a separator for separating the overhead product stream into a solids stream including the char and a gas stream including the syngas. The system may also include a subsystem for recycling the solids stream to the gasification reactor. The recycling system may include a standpipe that receives the solids stream from the separator for generating a pressure differential across a bed of accumulated char, thereby producing a bottoms stream comprising char having a greater pressure than the solids stream. The recycling system may also include a holding vessel that receives the bottoms stream and a fluidized-bed distribution vessel that receives char from the holding vessel and is configured to provide a continuous and precise flow of recycled char to the gasification reactor.
In another aspect, embodiments herein are directed toward a system for gasification of a carbonaceous material. The system may include: a gasification reactor, for the gasification of a carbonaceous material producing an overhead product stream containing char and syngas, and a separator for separating the overhead product stream into a solids stream including the char and a gas stream including the syngas. The system may also include a subsystem for recycling the solids stream to the gasification reactor. The recycling system may include a standpipe that receives the solids stream from the separator and is configured for generating a pressure differential across a bed of accumulated char and for partially fluidizing a bottom portion of the bed of accumulated char to provide a continuous flow of recycled char to the gasification reactor.
In yet another aspect, embodiments herein are directed toward a process for recycling char to a gasification reactor. The process may include: separating a gasification reactor effluent including char and syngas to produce a solids stream including the char and a vapor stream including the syngas. The char in the solids stream may be fed to a standpipe, and an amount of char may be accumulated within the standpipe to generate a pressure differential, such that the char may be recycled to the gasification reactor.
Other aspects and advantages will be apparent from the following description and the appended claims.
In one aspect, embodiments herein relate to a process for the conversion of carbonaceous material to synthesis gas (syngas). In solid fuel gasification processes, a large quantity of dried and partially reacted particles, called char, may be entrained in a syngas produced in the gasification reactor. This char, which may include ash and unconverted carbon, needs to be separated, transported, and recycled back to the gasifier for final consumption, producing additional syngas and slag. For example, the char may be injected back into the gasifier with an oxidant such as air or oxygen through a burner or burners. The char/oxidant ratio for each burner needs to be controlled so that the gasifier does not operate at too low or too high a temperature. Too low a temperature results in incomplete conversion of the char, while too high a temperature may damage the refractory lining on the gasifier. Therefore, it is desired to maintain a steady char flow rate so that a precise amount of oxidant may be added to the burners. This may be accomplished with a standpipe-fluid bed hybrid recycle systems as described herein. Unlike lockhoppers, which consist of multiple vessels with frequent cycling, standpipe-fluid bed hybrid recycle systems described herein can increase the pressure of a solids recycle stream while advantageously maintaining a continuous, metered flow. The ability to provide a continuous, measurable, and controllable flow provides several advantages to the system, described further below. As noted briefly above, as used herein the term “char” refers to unconverted or partially converted carbonaceous particles and ash particles that may remain entrained within a gasification reactor effluent.
Systems and processes for gasification of a carbonaceous material according to embodiments herein may include a gasification reactor, or gasifier, for gasifying a carbonaceous material to produce a product stream comprising syngas and entrained char. Gasifiers useful in embodiments herein may include single stage or multi-stage gasifiers, such as a two-stage described below, where a fresh carbonaceous feed is introduced to a gasifier upper section, and recycled char is introduced to a gasifier lower section. The carbonaceous feed can be in the form of pulverized fine solids or fine particles suspended in a water slurry.
A separator, such as a cyclone separator, may be used to separate entrained char from the syngas. The entrained char recovered from the separator, which may include unconverted carbonaceous material, may then be recycled to the gasifier for production of additional syngas. The syngas recovered from the separator may also include a small amount of char, and a second separator, such as a cyclone separator or a filter system, may be used to remove additional char from the syngas, where the additional char may also be recycled to the gasifier.
Process dynamics result in a pressure drop between the gasification reactor and the solids outlet of the separators. As a result, recycle of the char requires a method to increase pressure to flow the char back to the gasification reactor. The abrasive properties of the char, however, affects reliability of systems that operate via pressurization and depressurization, and it is generally undesirable to use a liquid slurry system to recycle the char, as the amounts of liquid may adversely impact gasification reactor operations and conversion efficiency.
Recycle systems as described herein, including a standpipe, have been found to provide adequate pressurization of the recovered char to facilitate recycle to the gasification reactor. Standpipes, as used herein, may include relatively tall vessels, such that accumulation of char within the standpipe may produce a differential pressure, where the weight of accumulating particles causes the pressure at the bottom of the standpipe to be greater than the pressure at the top of the standpipe, facilitating transfer of the char back to the gasifier. For example, standpipes according to embodiments herein may have a height of 30 feet, 50 feet, 70 feet, 100 feet or greater, providing for a pressure build of 3 psi, 5 psi, 7 psi, 10 psi, 12 psi, 14 psi, or greater, as may be necessary for the transport of char through the recycle system. In some embodiments, standpipes according to embodiments herein may have a height sufficient to allow a pressure build in the range from about 3 psi to about 15 psi, such as in the range from about 5 psi to about 9 psi.
The required pressure build may depend upon the gasification system being used and the differential pressure needed to facilitate the solids transport and injection into the gasifier. Additionally, the realized pressure build may depend upon the properties of the char, which may in turn depend upon the type of carbonaceous feedstock being processed, operating conditions (e.g., temperature and pressure) within the gasification reactor, and the size, packing density, and porosity of the resulting char particulates, among other factors.
The overall system design may be configured for a consistent carbonaceous feedstock, or may be configured to operate with multiple carbonaceous feedstocks. For example, as compared to a high grade coal, a lower grade coal may be fed to an upper stage of a two-stage gasification reactor using a water slurry with a relatively high content of water. This, in turn, may result in a lower outlet temperature at the top of the upper reaction zone and a significantly greater amount of entrained char to be separated and recycled, and depending upon the differences in coal grades, could result in as much as ten times the amount of char recycle. Systems according to embodiments herein, utilizing a standpipe, may provide for efficient, continuous, measurable, recycle of char to a gasification reactor. For example, the char may be introduced to a lower section of a two-stage gasification reactor, the lower section processing char only or a mixture of char and carbonaceous material. Embodiments herein provide for feed of the recycle char as a dense phase, with limited amounts of fluidization medium, such as syngas, nitrogen, carbon dioxide, or other suitable fluidization gases. Carbon dioxide, a recoverable byproduct from the gasification process, may be used in particular embodiments.
Dense phase transport is preferred over dilute phase transport because of the amount of gas required to entrain the solids. For a dilute phase transport system, it may require 2 pounds of fluidization gas per pound of solid while a solid dense phase system may require only 0.02 pounds of fluidization gas per pound of the same solid, for a one hundred fold difference in the amount of gas required to entrain the solids. Also, the velocity of transport in a dilute phase transport system is in excess of 40 feet per second while it may be less than 20 feet per second in a dense phase system. The high transport velocity in the dilute phase system coupled with the entrained abrasive solids causes severe erosion problems in the piping system. If the recycle char is the primary feed to the gasifier reaction chamber, the huge volume of entrainment gas associated with a dilute phase transport system that will be fed with the recycle char to the gasifier makes the dilute phase transport system impractical to use. The ability to continuously recycle char to the gasification reactor as a dense phase afforded by the standpipe may advantageously provide for ease in reactor control and flexibility in feedstock.
Referring now to
According to the embodiment depicted in
The mixture product, including entrained solid particulates and a gaseous product, exits the reactor upper section 40 and is sent to a cyclone separator 50. The cyclone separator 50 splits the mixture product into a solid product stream, including the unreacted solid particulates, and a gaseous product stream, leaving only a small fraction of residual solid fines in the gaseous product stream. The solid product stream exits the cyclone separator 50 via an outlet 70.
The solid product recovered from the bottom of cyclone separator 50 is then fed to the top of standpipe 120. The solids accumulate and concentrate within standpipe 120. The height of accumulated solids in the standpipe results in the pressure build at the bottom of the standpipe. The accumulated solids are then transported from the bottom of standpipe 120 to a holding vessel 130 via flow line 125. The accumulated solids may be transported continuously or semi-continuously in various embodiments, and may be transported by gravity or via dense phase transport with a minimal amount of syngas, carbon dioxide, or nitrogen, for example, which may be introduced via flow line 126.
Holding vessel 130 may be disposed above a fluidized-bed distribution vessel 140, and may be used to facilitate transport of the char back to the gasifier via flow lines 142 as well as to facilitate measurement of the flow rate of char to the gasifier. For example, holding vessel 130 may be periodically opened to feed the solids into fluidized-bed distribution vessel 140 for recycle back to the reactor lower section 30, where a flow rate of solids may be determined by a drawdown in volume of particles within fluidized-bed distribution vessel 140, or a differential weight of fluidized-bed distribution vessel 140. Alternatively, commercially available solids flow meters used on lines 142 may be used to measure the flow rate of recycled char, where holding vessel 130 may facilitate periodic calibration of the flow meters via drawdown of particles within fluidized-bed distribution vessel 140. Holding vessel 130, while disposed above fluidized-bed distribution vessel 140, is independently supported, such that solids accumulating in holding vessel 130 do not affect the weight determination during drawdown of fluidized-bed distribution vessel 140 where a differential weight is required.
The standpipe 120, which is a length of pipe through which the solid product flows by gravity, may be used to transfer solids from a low pressure area, such as cyclone 50, to a higher pressure area, such as gasification reactor 10. The pressure available at the bottom outlet of standpipe 120 is dependent on the height of the standpipe, the height of the solids level in the standpipe, the characteristic of the solid (i.e., density, porosity, particle size distribution, packing efficiency, etc.), and how much gas is entrained in the solids, among other factors. Typically, with coal type carbonaceous materials, one can expect a pressure build-up of approximately 1-2 psi for every 10 feet of height of the standpipe. Therefore, with a 70-foot tall standpipe, the pressure of the solids exiting the bottom of the standpipe would be higher by approximately 7-14 psi relative to the top of the standpipe. For a two-stage gasification reactor, such as depicted in
Fluidized-bed distribution vessel 140 is used to transport and recycle char into the bottom of gasification reactor 10 through one or more transport lines 142 to one or more dispersion devices 60 and/or 60a on the reactor lower section 30. A fluidization medium, such as nitrogen or syngas fed via flow line 127, may be introduced to fluidized-bed distribution vessel 140 to fluidize and transport the solids. Typically, the lengths and configuration of the transport lines 142 between the fluidized-bed distribution vessel 140 and the dispersion devices 60 and/or 60a are adjusted so that the differential pressure drop for each line are the same, to ensure similar flow rates in each line. The pressure drop in the transport lines may be, for example, about 1-2 psi per 10 feet of piping. The pressure drop through the transport line may be used as a built-in restricting orifice to regulate the flow rate. Therefore, by varying the bed density in the fluidized-bed distribution vessel 140 by adjusting the amount of fluidization medium, the solids flow rate through the lines can be regulated, thereby eliminating the need for a flow control valve which typically needs a much higher differential pressure (e.g., 10-15 psi) to operate. The pressure drop in such a fluidized-bed distribution vessel 140 may be kept very low. By combining the standpipe 120, holding vessel 130, and the fluidized-bed distribution vessel 140, solids may be transferred from a lower pressure to a higher pressure region without the use of lock-hoppers.
Measurement of solids flow rate by flow meters can be challenging. There are flow meters used in the field that employ a capacitive principle to measure the density of the solids medium flowing through the pipe and its traveling velocity to calculate the mass flow rate. Such a flow meter does not work well for solids that are not very conductive, such as carbonaceous material and char that has a very low ash or mineral content such as petroleum coke. In contrast, systems according to embodiments herein may include solids flow measurements by gravimetric measurements, such as by weight loss or volume loss. For example, fluidized-bed distribution vessel 140 may be mounted on weight cells to monitor the rate of weight loss, or fitted with externally-mounted radiation-based sensors to monitor the bed level and therefore, the volume change. With the fluidized-bed distribution vessel 140 feed system, the solids material may be batched into the fluidized-bed distribution vessel 140 via holding vessel 130 so that weight loss (and therefore the flow rate of char to the burner) can be monitored. Similarly, for a char of known properties (density, packing density, etc.), volume loss may provide a sufficiently accurate measurement of recycled solids flow rate. Systems herein may additionally include one or more sample ports for withdrawing samples of char to determine the properties of the char.
In order to combine the standpipe 120 with the fluidized-bed distribution vessel 140, the holding vessel 130 is used to connect and to act as the interface between the two systems. This holding vessel 130 may be located directly on top of the fluidized-bed distribution vessel 140 and may be separated from the holding vessel 130 by an automated full-port quick opening valve, for example. Pressure in the holding vessel 130 will be the same or slightly higher than in the fluidized-bed distribution vessel 140. During operation, solid flows from the standpipe 120 into the holding vessel 130, with a valve located at the outlet of holding vessel 130 initially closed. When the holding vessel 130 is full, the valve will open and solids in the holding vessel 130 empty into the fluidized-bed distribution vessel 140. The valve will then close and the cycle will be repeated. No pressurization or de-pressurization of the holding vessel 130 is necessary. The solids flow from the fluidized-bed distribution vessel 140 through each transport line 142 and respective burners (or dispersion devices) will be uninterrupted, even during the solids transfer from the holding vessel 130.
The flow rate may be monitored gravimetrically by weight cells or volumetrically by radiation-based sensors fitted on the fluidized-bed distribution vessel 140. The weight or volume is reset after each solids transfer from the holding vessel 130, after which a differential weight or volume loss over time may be used to determine the rate of flow of solids from fluidized-bed distribution vessel 140 to the gasifier 10. Alternatively, as noted above, a solids flow meter can be installed at the outlet of the fluidized-bed distribution vessel 140 or on each individual transport line 142 from the vessel to the burners. If the solids flow meter is used to monitor the solids flow rate independently, the bottom valve on the holding vessel 130 can be left open at all times, and solids can flow directly from the standpipe 120, through the holding vessel 130, and into the fluidized-bed distribution vessel 140. The holding vessel 130 and the bottom valve will be used only when calibration of the solids flow meter is desired, such as once or twice a day or as frequently as desired.
The solid product stream is then recycled back to the reactor lower section 30 of the gasifier 10 through dispersion devices 60 and/or 60a. These devices mix the recycled solids with gaseous oxidant, such as air or oxygen, during addition of the solids and oxidant to the first stage of the reactor. The flow rate of oxygen or air, and thus the temperature of the gasifier, may be based at least in part on the flow rate of solids from fluidized-bed distribution vessel 140 to gasifier 10.
The solid product stream (primarily including char) reacts with oxygen in the presence of superheated steam in the reactor lower section 30 (or first stage reaction zone) of the gasification reactor 10. These exothermic reactions raise the temperature of the gas in the first stage to between 1500° F. and 3500° F., for example. The hot syngas produced in the reactor lower section 30 flows upward to the reactor upper section 40 where it comes into contact with the carbonaceous solid or slurry feedstock. The water content is evaporated and the feedstock particles are dried and heated to an elevated temperature by the hot syngas, then the dry particles react with steam to generate CO and hydrogen.
Again referring to the embodiment as shown in
Further referring to
In certain embodiments, such as illustrated in
Again referring to the embodiments depicted in
Further referring to the embodiments depicted in
The materials used to construct the gasification reactor 10 may vary. For example, the reactor walls may be steel and lined with an insulating castable or ceramic fiber or refractory brick, such as a high chrome-containing brick in the reactor lower section 30 and a dense medium, such as used in blast furnaces and non-slagging applications in the reactor upper section 40, in order to reduce heat loss and to protect the vessel from high temperature and corrosive molten slag as well as to provide for better temperature control. Use of this type of system may provide the high recovery of heat values from the carbonaceous solids used in the process. Optionally and alternatively, the walls may be unlined by providing a “cold wall” system for fired reactor lower section 30 and, optionally, unfired upper section 40. The term “cold wall”, as used herein, means that the walls are cooled by a cooling jacket with a cooling medium, which may be water or steam. In such a system, the slag freezes on the cooled interior wall and thereby protects the metal walls of the cooling jacket against heat degradation.
The physical conditions of the reaction in the first stage of the process in the slagging gasifier reactor lower section 30 are controlled and maintained to assure rapid gasification of the char at temperatures exceeding the melting point of ash to produce a molten slag from the melted ash having a viscosity not greater than approximately 250 poises. This slag drains from the reactor through the taphole 20, and may be further processed.
The physical conditions of the reaction in the second stage of the gasification process in the reactor upper section 40 are controlled to assure rapid gasification and heating of the carbonaceous feedstock, and in some embodiments may include heating of the coal above its range of plasticity. Some two stage gasification reactors may, however, control the temperatures in the reactor upper section 40 to be below the range of plasticity of the coal. The temperature of the reactor lower section 30 is maintained in a range between 1500° F. and 3500° F., or may be maintained in a range between 2000° F. and 3000° F. Pressures inside both the reactor upper section 40 and lower section 30 of the gasification reactor 10 are maintained at atmospheric pressure to 1000 psig or higher. The conditions in the upper reaction zone may impact not only the extent of reaction, but the favored reactions as well, and thus care should be used when selecting the operating conditions, so as to provide a desired product mixture from a particular carbonaceous feedstock.
As used herein, the term “oxygen-containing gas” that is fed to the reactor lower section 30 is defined as any gas containing at least 20 percent oxygen. Oxygen-containing gases may include oxygen, air, and oxygen-enriched air, for example.
Any carbonaceous material can be utilized as feedstock for the embodiments described herein. In some embodiments, the carbonaceous material is coal, which without limitation includes lignite, bituminous coal, sub-bituminous coal, and any combinations thereof. Additional carbonaceous materials may include coke derived from coal, coal char, coal liquefaction residue, particulate carbon, petroleum coke, carbonaceous solids derived from oil shale, tar sands, pitch, biomass, concentrated sewer sludge, bits of garbage, rubber and mixtures thereof. The foregoing exemplified materials can be in the form of comminuted solids.
When coal or petroleum coke is the feedstock, it can be pulverized and fed as a dry solid or ground and slurried in water before addition to the reactor upper section. In general, any finely-divided carbonaceous material may be used, and any of the known methods of reducing the particle size of particulate solids may be employed. Examples of such methods include the use of ball, rod and hammer mills. While particle size is not critical, the particles should be small enough to allow entrainment of the particles in the gas stream. Finely divided carbon particles are preferred for improved reactivity. Powdered coal used as fuel in coal-fed power plants is typical. Such coal has a particle size distribution such that 90% (by weight) of the coal passes through a 200 mesh sieve. A coarser size of 100 mesh average particle size can also be used for more reactive materials, provided that a stable and non-settling slurry can be prepared.
The embodiment described above with respect to
Referring now to
The holding vessel 200 may be a conical-shaped vessel with a capacity of approximately 15-30 minutes solids storage, for example. Holding vessel 200 may be separated from the partially fluidized standpipe 210 by a quick-opening block valve 212, for example, that may be remotely controlled. The partially fluidized standpipe 210 may be a vertical cylindrical vessel in which the solids are held and fluidized with a gaseous medium, such as nitrogen or syngas, introduced at the bottom of the partially fluidized standpipe 210 via flow line 215. The height of the standpipe should be tall enough to accumulate a solids level that generates sufficient static head pressure at the bottom of the standpipe to transport the solids to the higher pressure environment (e.g., the gasifier 10, such as to lower reaction section 30 of gasifier 10). The diameter of the partially fluidized standpipe should be large enough that the movement of the solids in the partially fluidized standpipe 210 is not hindered.
The bottom portion 218 of the partially fluidized standpipe 210 may be fitted with a porous medium or distribution nozzles (not shown) through which the fluidizing gas is introduced. The amount of fluidizing gas introduced via flow line 215 should be sufficient to fluidize the solids medium, but minimized so as to generate the maximum static head pressure at the bottom of the partially fluidized standpipe from the weight of the solids column (accumulated char). For example, depending upon the properties of the char, a partially fluidized standpipe could generate 1-2 psi of head pressure for every 10 feet of solids in the standpipe. As a particular example, a partially fluidized standpipe of 24 inches in diameter and 70 feet tall designed to handle a flow rate of 5,000 lb/hr of a pulverized coal may generate a pressure differential of 14.5 psi between the top and bottom of the partially fluidized standpipe.
Multiple conveying pipelines 143 may be disposed toward the bottom of the fluidized solids bed, just above the level where the fluidizing gas is introduced, to transport the solids to separate locations, such as the different burners (or dispersing devices) 60, 60a in a gasifier. Solids will flow in a dense phase mode through conduits 143, and a flow rate in each conveying line can be independently varied and controlled by adjusting an amount of transport gas introduced directly into the solids flow along the length of each conveying pipeline, such as via transport gas feed lines 144. Solids flow rate in each conveying line may be measured by a solids mass flow meter.
During normal operation, a remotely-controlled pneumatic ball valve 230 between the holding vessel 200 and the partially fluidized standpipe 210 may be left open. Solids from the cyclone separator flow through the holding vessel 200 into the partially fluidized standpipe 210. The solids level in the partially fluidized standpipe 210 is held constant in the upper part of the standpipe by balancing the outflow from the bottom of the standpipe with the incoming flow from the cyclone separator and holding vessel 200. Calibration of solids flow meters may be performed similar to that described above, by temporarily closing valve 230, where a differential volume or a differential weight may be used. For example, both the holding vessel 200 and the partially fluidized standpipe 210 may be equipped with a radiometric (radiation-based) sensor 240, 242, respectively, each having a radiation source 243, to measure a level of solids in the vessels, sensors 240 and 242 providing for a volume drawdown flow rate calibration, among other functions, and sensor 240 additionally providing a level indication so as to timely conclude the calibration tests. The solids flow rate in the conveying lines coming out of the bottom of the partially fluidized standpipe 210 can be adjusted by varying the amount of fluidization gas introduced to the partially fluidized standpipe 210 via flow line 215, by varying the transport gas directly added in the conveying pipelines 143 via flow lines 144, or by varying the level of solids in the partially fluidized standpipe 210.
The embodiment described above with respect to
Advantageously, the systems described in one or more embodiments above is capable of operating continuously, will be able to transport the solids from a lower pressure to a higher pressure environment with no cyclical pressurization and depressurization operations as required by the maintenance prone and high cost lock hopper system. The solids flow rate may also be more precisely monitored and controlled, providing enhanced reactor control as compared to slug feed resulting from pressurization and depressurization systems. The char transport systems disclosed herein may additionally provide flexibility in the gasification process, allowing a wider variety of feeds to be processed as compared to other char handling and gasification reactor systems.
While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.
This application, pursuant to 35 U.S.C. §119(e), claims benefit to U.S. Provisional Application Ser. No. 62/109,843 filed Jan. 30, 2015. This application is incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62109843 | Jan 2015 | US |
