CONTROL OF SUSPENSION DENSITY USING RADIATION SOURCE

Abstract: A method and apparatus for controlling the suspension density directly, and the mass flow rate indirectly, of a particulate solids and gas mixture transported to a reactor using a radiation source and detector.

Description: The present invention relates to a method for controlling the suspension density of a particulate solid coal and gas mixture having a suspension density ranging from about 50-800 kg/m³, advantageously 200-500 kg/m³, delivered to a gasifier where the residence time for the fuel within the reactor is approximately 0.2 seconds. Conventional coal feed systems using gravity flow of solids, such as coal feed to coal-fired boilers, allow major fluctuations in the coal suspension density and mass flow rate, which is a product of the density and velocity of the solids. However, fluctuations of coal suspension density and mass flow rate to burners within a coal gasification reactor, hereinafter referred to as a gasifier, are detrimental to gasifier performance. For example, such fluctuations cause inefficient combustion of fuel within the gasifier and damaging heat fluxes to the burner face within a reaction zone near the burner which result in thermal stresses on the burner face. Since the residence time of the coal within the reaction zone of the reactor is approximately 200 milliseconds or less, advantageously 80 milliseconds, the coal suspension density and mass flow rate should be constant over periods of this order and over shorter periods to maintain constant local conditions. U.S. Patent Specification No. 4,049,394 is related to maintaining a predetermined volumetric ratio between fuel and a gasification agent during feeding thereof into a gasification reactor. However, this patent is not directed to controlling the suspension density to yield a uniform, constant mass flow rate of the fuel and gasification agent to the reactor. The present invention, therefore, is directed to controlling the suspension density of the coal to provide a uniform, constant mass flow rate to the burners of a gasifier over time periods as short as 0.2 seconds or less. The invention therefore provides a method for controlling the suspension density of a particulate solid coal and carrier gas mixture having a suspension density ranging from about 50-800 kg/m³ conveyed from a feed hopper through a discharge port and a conduit having a diameter less than about 40 mm to at least one burner for injecting said mixture into a reactor for partial oxidation of the particulate solid coal where the residence time of the particulate solid coal within the said reactor is 0.2 seconds or less, comprising: transmitting radiation from a radiation source to a detector; receiving radiation by said detector, said detector including means for converting the transmitted radiation to yield a determination of the suspension density of said mixture; transmitting signals relative to the determined suspension density of said mixture to means for comparing the determined suspension density with a preselected value; comparing the determined suspension density with a preselected value; and adjusting the suspension density of said mixture to obtain substantially the preselected value; characterized by the steps of: injecting carrier gas into a lower portion of said feed hopper; positioning at least one radiation source to penetrate the cross-sectional area of said mixture being conveyed, wherein said source is located downstream of injecting said gas into the lower portion of said feed hopper and upstream of the discharge port of said feed hopper; positioning at least one radiation detector substantially diametrically opposite said radiation source to receive radiation from said source at least portion of which is transmitted through said mixture; and wherein said adjusting the density of said mixture includes adjusting the flow rate of the gas injected into the lower portion of said vessel by adjusting a flow control valve. It can be remarked that WO-A-82/03273 discloses the use of a radiation source means on a duct for controlling the mass flow of a coal/gas suspension. The bulk density of powdered coal in flow streams of coal entrained with air and/or other gases is mentioned. However, this document is completely silent on specific locations where the radiation measurement should take place and the specific features of the present invention can not be derived therefrom. In a gasifier generation of synthesis gas occurs by partially combusting organic or carbonaceous fuel, such as coal, at relatively high temperatures in the range of 800 to 2000 °C and at a pressure range of from about 1 to 200 bar in the presence of oxygen or oxygen-containing gases. Oxygen-containing gases include air, oxygen-enriched air, and oxygen optionally diluted with steam, carbon dioxide and/or nitrogen. In the present invention, the fuel and gas mixture is discharged from a feed vessel apparatus, advantageously having multiple outlets, each outlet being in communication with at least one burner associated with the gasifier. Typically, a gasifier will have burners in diametrically opposing positions. Generally, the burners have their discharge ends positioned to introduce the resulting flame and the agent of combustion into the gasifier. Of particular importance in the manufacture of synthesis gas is the uniform manner in which the particulate fuel is introduced to the burners within the gasifier so as to minimize fluctuations in the mass flow rate of the fuel being supplied to the burners. If the mass flow rate of the particulate fuel fluctuates, zones of underheating are generated next to zones of overheating in the gasifier. As a result, in the zones of underheating the fuel is not completely gasified and in zones of overheating the fuel is completely converted into less valuable products, viz. carbon dioxide and water vapor. Additionally, locally high temperatures in the gasifier could damage the refractory lining which is normally arranged at the inner surface of the gasifier wall. Various factors substantially affect the mass flow rate of the fuel being supplied to the burners. In particular, the discharging of the fuel from a feed vessel and the pneumatic transporting by conduit of the fuel from the feed vessel to the gasifier affect the mass flow rate of fuel. Specifically, fuel and gas mixtures having suspension densities ranging from about 50-800 kg/m³ transported through a conduit having a diameter less than about 40 mm experience significant pressure drop due to the summation of various contributions such as frictional losses, restrictions, curvatures, etc., within the conduit. The present invention advantageously utilizes local suspension density measurements of the mixture discharged from the feed vessel to the gasifier that are derived from density-indicating devices to independently control the suspension density and the mass flow rate of the mixture conveyed to the reactor. An advantage of the present invention is the controlling of the suspension density of the coal and gas mixture to provide a uniform, steady mass flow rate of the mixture to the reactor. Another advantage of the present invention is the rapid detection of changes in coal mass flow rate so that the oxygen or oxidizing agent may be adjusted in ratio to the coal mass flow rate to the gasifier which provides more efficient and safer operation of the gasification process. Advantageously, the size of solid carbonaceous fuel is such that 90 percent by weight of the fuel has a particle size smaller than 3.35 mm (No. 6 mesh (A.S.T.M.)). The invention will now be described by way of example in more detail by reference to the accompanying drawing, in which the figure represents schematically the principle of the invention. The drawing is of the process flow type in which auxiliary equipment, such as pumps, compressors, cleaning devices, etc. are not shown. Referring to the figure, an apparatus for controlling the suspension density of a particulate solids and gas mixture pneumatically conveyed via conduit 18 from a vessel, such as a feed hopper 10, to means for injecting the mixture into a reactor 16, such as a burner 17, includes a hopper 10 including injection of gas into the lower portion thereof. Gas, such as nitrogen, carbon dioxide, or synthesis gas, can be injected from a source 12 into the lower portion of the hopper through porous material 14, which serves to distribute the gas throughout the bed 15 formed in the lower portion of the hopper 10. Injecting gas into the lower portion of the hopper 10 provides for a uniform discharge flow rate of the mixture exiting the hopper 10 and allows for a smaller diameter conduit 18, say 40mm, so as to maintain the high velocities, say 10 m/s, within the conduit 18 which are required to achieve a uniform mass flow rate of the mixture to the burners 17 of the gasifier 16 for the suspension densities used in the present invention. At least one radiation source 20, for example a gammaray, source, is positioned downstream of injecting the gas from source 12 into the lower portion of the hopper 10 and upstream of the discharge port of the feed hopper to direct radiation to penetrate the cross-sectional area of the mixture. A collimator may be used to reduce the scatter of radiation received by the detector 22 to increase the sensitivity of the densitometer at low suspension densities. Advantageously, the source 20 is located in proximity to the discharge port 21 but downstream of injecting the gas from source 12 since injecting the gas into the lower portion of the hopper 10 causes the suspension density of the mixture to fluctuate. In addition to the source located in proximity to the discharge port 21 of the hopper 10, another source (not shown) can be located downstream of the discharge port 21 but upstream of the burner 17. However, if the source and detector are located in close proximity to injecting transport gas from source 30, then correction of the suspension density would be required for the reason stated above. At least one radiation detector/transmitter 22 is positioned substantially diametrically opposite the source 20 to receive radiation transmitted from the source 20 after it has penetrated at least a portion of the mixture. The detector/transmitter 22 includes means for converting the received radiation to yield a determination of the suspension density of the mixture. Signals relative to the determined suspension density of the mixture are transmitted from the detector/transmitter 22 a processor-controller 25. The processor-controller 25 compares the determined suspension density with a preselected value. If the determined suspension density is higher than the preselected value, then the suspension density of the mixture can be decreased to provide a uniform, constant mass flow rate to the gasifier by increasing the rate of injecting gas from source 12 by actuating flow control valve 27. In a like manner, if the determined suspension density is lower than a preselected value, then the suspension density of the mixture can be increased to provide a uniform, constant mass flow rate to the gasifier by decreasing the rate of injecting gas from source 12. The figure illustrates that the processor-controller 25 transmits signals, shown for ease of illustration as dashed line 26, to actuate valve 27 for increasing or decreasing the flow rate of the gas injected from source 12. Additionally, gas can be vented from the hopper 10 by actuating flow control valve 28 via transmission of a signal 29 from the processor-controller 25. Although the system for controlling the suspension density shown is shown in the figure in its distributed form as discrete components, it would be readily understood by those skilled in the art that these components could be combined into a single unit or otherwise implemented as may be most convenient for the particular application at hand. Furthermore, although the embodiment has been shown as using an electronic process control system for controlling the suspension density to provide a uniform, constant mass flow rate of the coal transported to the gasifier, it is also understood by those skilled in the art that the present invention could be effected using manual or pneumatic controls.

Claims: A method for controlling the suspension density of a particulate solid coal and carrier gas mixture having a suspension density ranging from about 50-800 kg/m³ conveyed from a feed hopper through a discharge port and a conduit having a diameter less than about 40 mm to at least one burner for injecting said mixture into a reactor for partial oxidation of the particulate solid coal where the residence time the particulate solid coal within the said reactor is 0.2 seconds or less, comprising: transmitting radiation from a radiation source to a detector; receiving radiation by said detector, said detector including means for converting the transmitted radiation to yield a determination of the suspension density of said mixture; transmitting signals relative to the determined suspension density of said mixture to means for comparing the determined suspension density with a preselected value; comparing the determined suspension density with a preselected value; and adjusting the suspension density of said mixture to obtain substantially the preselected value; characterized by the steps of: injecting carrier gas into a lower portion of said feed hopper; positioning at least one radiation source to penetrate the cross-sectional area of said mixture being conveyed, wherein said source is located downstream of injecting said gas into the lower portion of said feed hopper and upstream of the discharge port of said feed hopper; positioning at least one radiation detector substantially diametrically opposite said radiation source to receive radiation from said source at least portion of which is transmitted through said mixture; and wherein said adjusting the density of said mixture includes adjusting the flow rate of the gas injected into the lower portion of said vessel by adjusting a flow control valve. The method as claimed in claim 1, characterized by the additional step of venting gas from the feed hopper by actuating a second flow control valve.

Inventors: DEWITZ, THOMAS SHAUN
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1988
Main classification: C10J3
IPC codes: F23N5, F23K1, B01J8, G05D11, C10J3, G01F1