System for Cooling and Cleaning Synthesized Gas Using Ahot Gravel Bed

This invention relates to a method and apparatus for cleaning and cooling high temperature corrosive synthesized gas and, more particularly, to such a method and apparatus utilizing a static regenerative hot gravel bed, and a fluidized bed of fine particles which is recirculated through a heat exchanger.

Several well-known techniques, such as the gasification of coal, are available for producing synthesized gas (hereinafter referred to as "syngas") which may be passed to a topping combustor and a gas turbine, or the like, for an integrated gas combined cycle (IGCC) system and second generation pressurized circulating fluidized bed (PCSB), or, alternatively, for use in chemical processes wherein synthetic hydrocarbons are manufactured. However, the syngas so produced often contains a relatively high quantity of corrosive gaseous species and acid pollutants, such as CO, HCl, NH.sub.3, H.sub.2 S and sulphur constituents generally, and, to a lesser extent, other corrosive gaseous species, alkali particles, and hydrocarbons (hereinafter referred to as "pollutants") which must be removed to comply with federal and state emissions standards.

Such pollutants are typically removed using a metal regenerative process in which pollutants in the syngas react with metal and are then regenerated producing elemental sulphur and chlorine. To enhance this pollutant removal process, the syngas is often initially cooled from temperatures which may exceed 2600.degree. F. This cooling may be effected using a conventional tube and shell heat exchanger arranged to be either convective or radiant. In a convective (or "fired tube") arrangement, syngas is passed through the tubes and a cooling medium (such as water) is passed through the shell around the tubes. Alternatively, in a radiant arrangement, syngas is passed through the shell and a cooling medium is passed through the tubes. A disadvantage with either method though is that each is prone to corrosion and pluggage caused by the presence of corrosive gases and a low melt eutectic of inorganic compounds.

Alternatively, syngas may be cooled and pollutants removed therefrom using a system incorporating a circulating fluidized bed with a spouting bed. This method calls for mixing and reacting syngas with fresh sorbent and a cooled recycle stream of sorbent. A disadvantage of this method is that it utilizes a grid, which is typically water-cooled, resulting in the condensation of acid gases which may then plug nozzles in the grid.

It is therefore an object of the present invention to provide a syngas treatment method and apparatus which, utilizes a static hot gravel bed through which syngas is uniformly distributed with a minimum of pluggage.

It is a further object of the present invention to provide a method and apparatus of the above type in which the gravel bed may be continuously regenerated.

It is a further object of the present invention to provide a method and apparatus of the above type which utilizes a sorbent to absorb, and thereby remove, sulfur and/or other corrosive constituents from the syngas.

It is a further object of the present invention to provide a method and apparatus of the above type in which the temperature of the syngas is reduced to a level which is compatible for introduction into a topping combustor and a gas turbine and which is consistent with the absorption by the sorbent of the pollutants in the syngas.

It is a further object of the present invention to provide a method and apparatus of the above type which utilizes a fluidized bed of entrained particulate material formed in a vessel.

It is a further object of the present invention to provide a method and apparatus of the above type in which the syngas discharges from the vessel and is separated from the entrained particles, and in which the latter particles are cooled in a heat exchanger and returned to the vessel.

It is a further object of the present invention to provide a method and apparatus of the above type in which the separated syngas is introduced to a topping combustor and a gas turbine.

Towards the fulfillment of these and other objects, the method and apparatus of the present invention features the cooling of relatively high temperature syngas and the removal of pollutants therefrom. According to the present invention, the syngas is introduced into the lower portion of a vessel. The syngas then flows in the lower portion through a static regenerative hot gravel bed and into an upper portion of the vessel containing pollutant-absorbing particulate material at a temperature less than that of the syngas in a manner so that the syngas entrains at least a portion of the material and so that the material reduces the temperature of, and absorbs, thereby removing, pollutants from, the syngas. The entrained material is then separated from the syngas, cooled in a heat exchanger, and returned to the vessel, while the separated, and cleaned, syngas is passed to downstream facilities.

The above brief description as well as further objects, features and advantages of the method of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawing which is a schematic representation of the apparatus of the present invention.

Referring to the drawing, the reference numeral 10 refers, in general, to a vessel having an upper cylindrical section 10a, an intermediate frustoconical section 10b, and a lower cylindrical section 10c, of a lesser diameter than the upper section 10a. The wall of the frustoconical section 10b forms an angle A of between 75.degree. and 90.degree. from a horizontal plane. A horizontally-extending inlet duct 12 registers with a corresponding opening formed in the wall of the lower cylindrical section 10c, which opening includes a retaining screen or cross-bars (not shown) to retain gravel material (to be described).

A duct 14 registers with an opening formed through the upper cylindrical vessel section 10a and connects the vessel 10 with a cyclone separator 16 disposed adjacent the vessel. A dipleg 18 connects the lower portion of the separator 16 with a pressure seal, in the form of a J-valve 20, which, in turn, is connected, via a duct 22, to a heat exchanger 24. A duct 26 connects an outlet opening formed in the lower portion of the heat exchanger 24 to the vessel 10 via an opening formed in the wall of the lower portion of the upper vessel section 10a. A syngas outlet duct 28 extends from the interior of the separator 16 to downstream equipment such as a topping combustor and a gas turbine (not shown). A sorbent storage/transfer bin 30 is located adjacent the separator 16 and has an outlet connected, via a duct 32, to the top of the J-valve 20.

A gravel regenerator 34 is located adjacent the vessel 10 for regenerating gravel material. A duct 38 connects an inlet of the regenerator 34 to a valve 40 which in turn connects to an outlet opening extending vertically through the bottom end of the lower cylindrical vessel section 10c. A duct 42 connects an outlet of the regenerator 34 to an inlet opening extending through the wall of the lower portion of the upper cylindrical vessel section 10a. A gravel storage/transfer bin 44 is located adjacent the vessel 10 and has an outlet connected, via a duct 46, to the duct 42.

A conventional fluidized bed oxidizer/cooler 48 is located adjacent the vessel 10 for oxidizing and cooling spent fluidized bed materials. A duct 50 connects an outlet opening extending through the wall of the lower portion of the upper vessel section 10a to an inlet of the oxidizer/cooler 48. A duct 52 connects an outlet of the oxidizer/cooler 48 to an inlet opening extending through the wall of the upper vessel section 10a above the duct 50. The oxidizer/cooler 48 is also provided with a outlet drain 54 extending through the bottom of the oxidizer/cooler.

In operation, gravel material comprising fused silica or an equivalent which is resistant to temperatures as high as 2400.degree. F., is loaded into the bin 44. The gravel material is transferred from the bin 44 through the ducts 46 and 42 into the vessel 10 where it accumulates and forms, in the lower and intermediate vessel sections 10c, 10b, a gravel bed. Quenched (i.e., nonstoichiometric) air is then heated to a temperature of up to 2000.degree. F. by an induct burner (not shown) and passed through the inlet duct 12 into the lower vessel section 10c to heat the gravel bed to a temperature of between 1600.degree. and 2000.degree. F.

Relatively fine sorbent material, such as limestone or dolomite, stored in the sorbent bin 30 is then transferred via the duct 32, the J-valve 20, the duct 22, the heat exchanger 24, and the duct 26, into the vessel section 10a atop the gravel bed.

After the gravel bed is formed and heated and sorbent is transferred to the vessel 10, syngas, under reducing conditions at an elevated temperature typically in excess of 2000.degree. F. and containing sulfur and other pollutants, is transferred from an upstream production facility and introduced into the vessel 10 via the inlet duct 12.

The velocity of the syngas introduced into the vessel 10 is controlled so that the "solids" (i.e., all nongaseous material, including sorbent and excluding gravel, in the vessel 10) behave as a fluidized bed. It is understood that the gravel is sized (e.g., at 0.5 to 1 inch diameter) so that it will not fluidize at the velocity of the syngas flowing through the gravel bed. The gravel bed helps to evenly distribute the syngas across the cross section of the vessel, so that mixing and absorption of pollutants by the sorbent in the fluidized bed are optimized. As a result of the fluidization, the syngas and a portion of the entrained solids pass upwardly through the vessel 10 and exit through the opening in the upper vessel section 10a and into the duct 14. The separator 16 receives the syngas, with the entrained solids, from the duct 14 and operates to separate the former from the latter by cyclone separation in a conventional manner. The separated syngas in the separator 16 passes through the duct 28 to further processes, such as a fine particle removal filter and/or a topping combustor and a gas turbine (none of which are shown).

The separated solids from the separator 16 pass, via the dipleg 18, to the J-valve 22 which operates in a conventional manner to establish a pressure seal between the separator 16 and the heat exchanger 24 to prevent the material contained in the vessel 10 from backflowing through the heat exchanger to the separator.

A fresh supply of sorbent particles from the bin 30 passes into the J-valve 20 via the duct 32 and combines with the separated solids in the J-valve before the mixture of solids passes into the heat exchanger 24. The heat exchanger 24 is of a conventional design and, as such, includes heat exchange surfaces through which a cooling fluid is passed to remove heat from the mixture of solids thereby cooling the mixture down to approximately 1000.degree. F. before the mixture passes into the vessel 10.

The fluidized bed oxidizer/cooler 48 receives, via the duct 50, spent solids materials from the fluidized bed atop the gravel bed. The oxidizer/cooler 48 operates in a conventional manner to cool the solids and oxidize sulfur captured by the sorbent. For example, given a limestone-based sorbent, the oxidizer/cooler 48 will convert toxic non-disposable calcium sulfide (CaS) into non-toxic disposable calcium sulfate (CaSO.sub.4). The cooled and oxidized solids materials are then either returned to the vessel 10 via the duct 52 or are passed out of the system via the drain 54.

Solids may thus be added to the upper section 10a from the bin 30 and/or drained via the drain 54 and hence the height of the fluidized bed may thereby be controlled. By controlling the height of the fluidized bed, and also the freeboard differential pressure (i.e., the difference in the pressure between the top and the bottom of the freeboard, that is, the space in the vessel above the fluidized bed), the solids inventory and, hence, the solids residence time may also be controlled. Thus, for example, by maximizing the height of the fluidized bed, the solids residence time is increased and the sorbent has greater time to absorb more pollutants from the syngas.

As a result of the foregoing, the syngas is cooled in the vessel 10, by the cooled sorbent materials returned from the heat exchanger 24, to a temperature required for reaction of the sorbent with the corrosive gaseous species and other pollutants in the syngas which, for a limestone sorbent, is approximately 1600.degree. F.

In addition to the foregoing, the gravel may be regenerated by transferring it from the vessel 10 to the regenerator 34 via the duct 38 at either a constant or an intermittent rate selectively controlled by the valve 40. The regenerator 34 is operative in a conventional manner for first agitating the gravel in a mill to dislodge deposits that adhere to the gravel and for then sorting and screening the gravel to remove undesirable gravel. Useable gravel is then recycled back to the vessel 10 via the duct 42. Fresh gravel may be transferred to the vessel 10 from the bin 44 via the ducts 46, 42 to replace removed gravel.

It is understood that if the syngas transferred from the upstream production facility has a temperature in excess of 2400.degree. F., then the syngas should be cooled to less than 2400.degree. F. before it enters the gravel bed. This may be achieved in a conventional manner, for example, by replacing a portion of the inlet duct 12 with a cooled enclosure duct.

Several advantages result from the foregoing. For example, because the gravel material in the gravel bed is run hot, there is little or no heat transfer (loss) from the syngas to the gravel, and hence, there is minimal opportunity for molten solids to condense on the surface of the gravel. Moreover, if the molten solids do deposit on the gravel, they will, for the most part, remain molten and trickle down to the bottom end of the vessel 10 where they may be discharged using an appropriate removal system. In contrast, a cooled grid in a typical circulating fluidized bed system bed is prone to condensation and deposition of solids which may block nozzles in the grid, thereby reducing the effectiveness of the grid.

Further advantages result from using a static gravel bed instead of a grid because, by passing syngas through a gravel bed, the syngas is more uniformly distributed and mixed with the sorbent material thereby resulting in greater acid gas abatement.

Further advantages result from using a fluidized bed because the gravel particle size distribution according to the present invention is narrower than the sorbent particle size distribution in a spouting bed design, thereby facilitating an even and vigorous fluidization and mixing of the solids and syngas, and further reducing the agglomeration of the solids.

Further advantages result from using a gravel bed because the gravel may be continuously regenerated, rather than just during a unit shut-down as required when regenerating a grid system. Therefore, maintenance is simplified and unit shut-down time is reduced.

Further advantages result from using the static gravel bed because the free fluid flow area through the gravel bed between the gravel particles is small, relative to the duct 12 and the freeboard space in the upper vessel section 10a. As a consequence, the inter-particulate velocity of the syngas, and the solids entrained therein, flowing between the gravel particles is relatively high, e.g., greater than 50 ft/sec, thereby keeping the solids entrained in the syngas and further alleviating the deposition of solids on the gravel.

Further advantages result from using a circulating fluidized bed design and passing the recycled solids through the heat exchanger 24 to cool the recycled solids which in turn cools the syngas, thereby circumventing the need for conventional tube and shell heat exchangers which are prone to corrosion and pluggage caused by the presence of corrosive gases in the syngas.

Further advantages result from using the oxidizer/cooler 48 because it not only converts toxic, nondisposable CaS to non-toxic, disposable CaSO.sub.4, but, together with the bin 30, also allows for flexible control of the solids residence time in the vessel 10.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.