Patent Application
20170343145
This invention relates to apparatus for directing the flow of gases and to gasification reactors that include such apparatus in associated ductwork.
In various gas handling applications, process and layout constraints often require bends or elbows in the gas handling ductwork to change the direction of gas flow. Some elbows may be simple bends in the ductwork with the same cross-sectional shape as the inlet and outlet ducts. However, certain gas flows, such as those with a high concentration of particulate matter (PM), may demand a different elbow design to avoid particulate build-up within the elbow. Build-up is undesirable for a number of reasons. For example, build-up increases the resistance to flow through the elbow by constricting the cross-sectional flow area. This may increase the power consumption of the fans employed to drive the gas flow, and/or may reduce the gas flow rate through the system. In addition, build-up must often be removed manually, which typically involves shutdown of the gas handling system in order to access the elbow.
There are two principal mechanisms for PM build-up in elbows: impaction of particulate along the walls of the elbows as the walls change direction, which promotes adhesion of PM to the elbow walls; and collection/settling of particles on horizontal or near-horizontal surfaces within the elbow.
Gases that include a high concentration of particulate matter can be produced in gasification reactors, such as those that process municipal solid waste to produce syngas. The ductwork immediately downstream of certain municipal solid waste (MSW) gasifier vessels employs an elbow to re-direct a vertically-upward syngas flow into a generally downward direction. Conventional self-cleaning elbows cannot withstand the high pressures associated with certain MSW gasification exhaust systems. It would desirable to have a self cleaning elbow that can be used in combination with MSW gasifier vessels.
In one embodiment, an exhaust elbow includes an inlet, an outlet, a curved gas guiding duct between the inlet and the outlet, and a plurality of thermally insulated stiffeners connected to an external surface of the curved gas guiding duct, each of the stiffeners including a metallic component and thermal insulation adjacent to at least a portion of a surface of the metallic element.
In one aspect, the present invention relates to self cleaning elbows for directing the flow of gases that may contain particulate matter. Such self cleaning elbows can be used in ductwork associated with gasification reactors.
Plasma gasification reactors (sometimes referred to as PGRs) are a type of pyrolytic reactor known and used for treatment of any of a wide range of materials including, for example, scrap metal, hazardous waste, other municipal or industrial waste and landfill material, and vegetative waste or biomass to derive useful material, e.g., metals, or a synthesis gas (syngas); or to vitrify undesirable waste for easier disposition.
As shown in the example of
A gasification process performed in gasification reactors can produce a syngas with a relatively high particulate loading (e.g., solids content potentially exceeding 1,000 kg/h), which must be conveyed to downstream gas cleaning equipment for particulate matter (PM) and contaminant removal.
Elbows in the exhaust gas ductwork should be designed to operate at the pressures and temperatures of gases exiting gasification reactors, and to handle the particulate loading in gas supplied from the reactor, while minimizing particulate material build-up within the elbows.
A schematic representation of a common elbow geometry employed to handle gas flows with high PM content is illustrated in
The elbow of
Elbows having geometries as illustrated in
Embodiments of the invention can have an overall geometry that is generally similar to the elbow of
The elbow of
Additional stiffeners 82 are positioned adjacent to generally flat sides 84 of the transition portions 70, 74. Sight glasses 86 are provided to allow visual inspection of the interior of the elbow. As more fully described below, the stiffeners can be constructed to provide the desired mechanical strength, and also to reduce the possibility of hot or cold spots occurring along the walls of the elbow. In some embodiments, the structural reinforcement of the self-cleaning elbow can allow the elbow to withstand internal design pressures ranging from about −34.5 kPag to about 50 kPag. In other embodiments, the structural reinforcement of the self-cleaning elbow can allow the elbow to operate in combination with a pressurized reactor that would operate at 300 kPag or more.
The bottom wall of the curved portion of the elbow has a radius in a crotch area 88 to reduce stress in this region. This differs from the sharp bend 54 in the elbow of
Stiffeners are used to minimize the required thickness of the duct. The stiffener arrangement in the crotch region is configured to support the elbow, but not over-stiffen it. If the walls of the elbow are too rigid high thermal stresses will result. The stiffener design can be more robust than what is required for shell strength due to pressure. This is because of the internal refractory which may fail if the shell deflects too much. Thickness of insulation around the duct stiffeners can be selected to minimize deflections and achieve the desired shell and stiffener temperatures.
The thickness and material of the insulation layer 94 and the refractory layer 96 can be selected to maintain the steel shell 92 temperature within design limits (e.g., between 120° C. and 350° C.) under all expected process conditions when the elbow is used in ductwork connected to gasification reactor (based on the thermal and corrosion protection requirements).
There are two features of the stiffener arrangement which are employed to limit temperature differentials in the elbow assembly, in order to mitigate any issues associated with thermal expansion and contraction. First, the insulation 104 on the steel stiffener 102 is employed to prevent excessive heat dissipation to the external environment through the stiffener, which could result in a localized cool spot where the steel stiffener 102 is fixed to the steel shell 92. Second, the thickness of the insulation 104 is tapered in the area 116 near the connection point between the steel stiffener 102 and the steel shell 92. This prevents over-insulation of the steel shell 92 near the area 118, which could result in a localized hot spot. Both of these measures reduce temperature gradients that could lead to unacceptable thermal expansion and/or contraction of the assembly. In the example of
The insulation on the stiffener is intended to keep the temperature of the stiffener uniform and as similar to the shell as possible. If the shell is hot and the stiffener's outer extremity is cold, the shell will tend to bow inwards which could damage the refractory layer.
In the embodiment of
The stiffener is preferably welded to the shell to add the appropriate stiffness to the shell. In other embodiments, it may be possible to bolt the stiffener to the shell. A bolted connection may require some form of rigid insulation between the stiffener and the shell and sliding joints for the thermal expansion differences, but this might potentially avoid having to insulate the stiffener.
The angle of taper of the insulation can be, for example, about 45°. Tapering the insulation to a point adjacent to the shell of the elbow avoids extra insulation adjacent to the shell and consequently avoids overheating of the shell in the region of the stiffeners. A balance needs to be struck between preventing the stiffener from acting like a fin and over insulating the region.
As shown in
Various design objectives have been established in order to accommodate the significant process variations (e.g. gas flow rate, gas temperature, and gas contaminant levels) that may occur during operation of elbows coupled to MSW gasifiers. For example, the self-cleaning elbows employed in the ductwork for gasifier systems can be designed to meet several design elements including gas flow geometry; thermal and corrosion protection; and structural reinforcement for pressure loadings.
For thermal and corrosion protection, in some embodiments such as where the elbow is used in an outlet duct of a gasification reactor, it is desirable to limit the maximum design temperature of steel elbow shell and steel stiffeners to 350° C., since temperatures above this limit may result in an unacceptable reduction in steel strength, as dictated by pressure vessel design codes. In addition, it may be desirable to limit the minimum design temperature of steel elbow shell to 120° C., since temperatures below this limit may result in condensation of chemical species such as H2S on the steel shell, with a resultant risk of corrosion. Corrosion by contaminants within the gas stream (e.g., primarily hydrogen sulfide (H2S)), may occur when the temperature of metallic surfaces (e.g. the steel shell of the elbow) drops below a specified temperature.
It may also be desirable to limit temperature differentials across the steel elbow shell and steel stiffeners in order to minimize differential thermal expansion and contraction, which may compromise the structural integrity of the assembly.
Various embodiments can also be designed to withstand a maximum internal gas temperature of 1,300° C., in conjunction with a minimum exterior (ambient) temperature of 32° C.
Embodiments of the elbow may be suitable for use in gas handling applications involving a significant pressure differential between the internal gas flow environment and the ambient environment, and gas handling applications with significant variability in gas temperature and gas contaminant levels (which present technical challenges related to corrosion protection and management of thermal expansion and contraction).
While particular aspects of the invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the disclosed embodiment may be made without departing from the invention as defined in the appended claims.
