Liquid purification or separation – Processes – Separating
Reexamination Certificate
2001-02-13
2004-03-16
Popovics, Robert J. (Department: 1724)
Liquid purification or separation
Processes
Separating
C210S415000, C048SDIG002
Reexamination Certificate
active
06706199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for withdrawing and dewatering slag from a gasification system. In particular, the present invention relates to a conveying lockhopper and method of using the conveying lockhopper with a gasification system.
2. Related Art
Gasification is among the cleanest and most efficient technologies for the production of power, chemicals and industrial gases from hydrocarbon feedstocks, such as coal, heavy oil, and petroleum coke. Gasification converts hydrocarbon feedstocks into clean synthesis gas, or syngas, composed primarily of hydrogen (H
2
) and carbon monoxide (CO). Gasification allows refineries to self-generate power and produce additional products. Thus, gasification offers greater efficiencies, energy savings, and a cleaner environment. For example, a gasification plant at a refinery in El Dorado, Kans. converts petroleum coke and refinery wastes into electricity and steam, making the refinery entirely self-sufficient for its energy needs and significantly reducing waste and coke handling costs. For these reasons, gasification has increasingly become popular among refineries worldwide. Currently, there are several hundred gasification plants in operation worldwide.
In a gasification plant, the feedstock is mixed with oxygen (O
2
) and injected into a gasifier. Inside the gasifier, the feedstock and the O
2
are subjected to high temperatures and pressures that cause the feedstock and O
2
to react and form syngas. Non-gasifiable ash material forms molten slag as a byproduct. Hot syngas exiting the gasifier is cooled either by direct contact with water in a quench chamber, or indirectly in a syngas cooler to recover excess heat/energy. In the direct quench mode, the hot syngas exiting the gasifier contacts water in the quench chamber located in the bottom of the gasifier vessel. The gas is cooled and saturated with steam. The molten ash is rapidly cooled and solidified into irregularly-shaped particles of varying size. Therefore, the quench chamber serves not only to cool and saturate the syngas, but also to disengage slag particles from the syngas, capturing the slag particles in the quench water.
In the syngas cooler mode, hot gas is indirectly cooled in a specifically designed heat exchanger where high pressure export steam is generated. At the exit of the syngas cooler, the cooled syngas is forced to make a rapid direction change as it passes over a pool of water called the syngas cooler sump. Molten and partially solidified slag entrained in the cooled syngas is thus disengaged from the syngas by the combined effects of gravity and momentum. The slag particles which drop down into the syngas cooler sump water then rapidly solidify into irregularly-shaped particles of various sizes.
In addition to H
2
and CO, the syngas contains other gases in small quantities, such as carbon dioxide (CO
2
), water, ammonia, methane, hydrogen sulfide (H
2
S), carbonyl sulfide (COS), nitrogen, and argon. As much as 99 percent or more of the H
2
S and COS present in the syngas can be recovered and converted to elemental sulfur for use in the fertilizer or chemical industries. The clean syngas is then used for generating electricity and producing industrial chemicals and gases.
Most of the ash present in the solid feedstock is removed from the gasifier as solid, glass-like slag particles through a water-sealed, depressurizing lockhopper system. The lockhopper is conventionally a cylindrical vessel vertically oriented with top and bottom valves. This vessel is located directly beneath the gasifier quench chamber or syngas cooler sump.
The conventional lockhopper cycles through collection and dump modes. In the collection mode the top valve—the lockhopper inlet—is open to the gasifier, and the bottom valve—the lockhopper outlet—is closed. The entire lockhopper is filled with water forming a continuous column of water with the quench water, in the quench chamber mode, or with the syngas cooler sump water, in the syngas cooler mode. Thus, during the collection mode, slag entering either the quench chamber or the syngas cooler sump is able to drift unhindered downwards through the lockhopper inlet valve and into the lockhopper vessel.
Slag collection from the gasifier usually lasts for a period of 15-30 minutes. In the dump mode, the inlet is closed and the lockhopper is depressurized. After being depressurized, the bottom valve—the lockhopper outlet—is opened for a short period of time, typically only a few seconds. During this period, the slag is flushed out with a deluge of water from a large overhead flush water tank. The large volume of water flushes the slag out of a conventional lockhopper either into a slag sump with a drag conveyor, or onto a slag pad. Conventional lockhoppers rely on slag sumps, drag conveyors, and slag pads to dewater the slag and prepare it for transportation.
If a slag sump is used, the drag conveyor slowly drags the water-soaked slag up a long incline by means of a series of heavy-duty scrapers connected by parallel chains in an arrangement that looks like a ladder. The drag conveyor circulates through the slag sump much like an escalator. The slag is dewatered by gravity-driven drainage as it get dragged up the incline over the top into a slag bin. In the case where the slag is flushed out of a conventional lockhopper onto a slag pad, the slag is drained of water by gravity as it sits atop the inclined concrete pad. Periodically, a front end loader is used to scoop-up dewatered slag and to transfer it to a slag bin for handling off site. Regardless of whether a slag sump/drag conveyor or a slag pad is used, the water which drains by gravity from the slag contains fine particles of slag which do not remain with the coarser material. This slag water is collected in a slag water sump from which it is pumped and processed as a dilute slurry for additional separation or filtration. Slag sump/drag conveyors, slag pads, and similar equipment usually require that large foundations be excavated in the earth beneath the gasifier and lockhopper so that the equipment can be positioned at the bottom of the lockhopper.
Since the lockhopper is a large elongated cylindrical vessel with top and bottom valves located directly beneath the gasifier, the gasifier structure must be supported high in the air. Because of the considerable weight and size of industrial gasification equipment, the need for structure to support that equipment at an increased elevation adds substantial expense to the gasification process. Because of its height, the elevated gasifier is more difficult to service.
In addition to the inefficiencies associated with supporting an elevated gasifier, the amount of water needed to flush the slag out of a conventional lockhopper during each dump cycle is typically 2-4 times the total volume of the lockhopper. This requires a considerable amount of processing equipment such as a slag drag conveyor or sump system to dispose of the slag and the water. Also, equipment is required to store intake water and to process and dispose of the resulting wastewater. Wastewater must be treated to meet environmental quality standards. The process of treating wastewater and adding new intake water is extremely costly. Additionally, to remove ammonia from the water within the system, an expensive reflux stripper is typically required. In some systems, even after the removal of the ammonia and a large percentage of contaminants, the purged water still does not meet strict environmental regulations.
For these reasons, a need has been recognized by the inventors for an improved lockhopper that can withdraw and process slag from a gasifier without the normal associated equipment and expense involved in supporting large gasification equipment above a large vertical lockhopper, and without expensive and wasteful water treatment processes and equipment.
SUMMARY OF THE INVENTION
The present invention relates to an apparatus and method for withdrawing and dewatering slag from a gasific
Kamarthi Rajasekar
Lee Katherine S.
Leininger Thomas F.
Tsang Chih-Hao M.
Vassantachart Pravit
Jones Josetta I.
Popovics Robert J.
Texaco Inc.
Turner Frank C.
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