United States Patent: 3957459

As we've pretty thoroughly documented, the companies that ultimately coalesced into that beloved giant of the petroleum industry, ExxonMobil, had worked, and still are working, to refine the various processes by which Coal, and other Carbon sources, can be transformed into liquid hydrocarbon fuels.

One of the ways in which that can be accomplished is via a process of gasification, wherein the Coal, and/or additional Carbon sources, are transformed through partial oxidation into a synthesis gas blend of Hydrogen and Carbon Monoxide, which gases can then be catalytically condensed into the desired hydrocarbons.

The gasification process itself has grown increasingly sophisticated, since, as seen in:

Exxon 1973 CO2 + Coal = Hydrocarbon Syngas | Research & Development; concerning: "United States Patent 3,758,673 - Production of a Carbon Monoxide Containing Gas; 1973; Assignee: Esso Research and Engineering Company, NJ; Abstract: A process for producing carbon monoxide and hydrogen which comprises contacting a carbon dioxide containing gas with a carbonaceous material";

they long ago learned that Carbon Dioxide, recovered from whatever source, could be recycled through use as one of the agents of Coal gasification; and, as well, as seen much later in:

Exxon Co-Gasifies Coal and Carbon-Recycling Biomass | Research & Development; concerning: "United States Patent Application 20100083575 - Co-gasification Process for Hydrocarbon Solids and Biomass;  2010; Assignee: ExxonMobil Research and Engineering Company, NJ; Abstract: A process for the co-gasification of carbonaceous solids (coal) and biomass. A process for forming a slurry feed for the gasification to synthesis gas of solid carbonaceous particles and solid biomass ... wherein the solid carbonaceous particles comprise coal (and) wherein the biomass comprises biological matter selected from wood, plant matter, municipal waste, green waste, byproducts of farming or food processing waste, sewage sludge, black liquor from wood pulp, and algae ... followed by the conversion of the produced synthesis gas to form liquids by processes such as Fischer-Tropsch and its variants";

they determined that Carbon-recycling, renewable biomass could be so converted right along with the Coal.

Along with those developments, Exxon refined their gasification processes in other ways, as well, to make certain that as much of the original Carbon content of the basic solid raw material, Coal and/or biomass, as possible was converted into the Carbon Monoxide component of such liquid hydrocarbon synthesis gas.

For instance, in our report of:

Exxon Improves Coal & Char Hydrogasification | Research & Development; concerning: "United States Patent 3,874,739 - Method and Apparatus for the Transfer of Entrained Solids; 1975; Assignee: Exxon Research and Engineering, NJ; A method for the transfer of solid particles from a first vessel in which a stream of said particles moves downwardly in dense phase flow to a second vessel in which said particles are carried upwardly in dilute phase flow ... . (And) wherein said particles comprise carbonaceous solids. (And) wherein said additional gaseous fluid comprises steam. (And)  wherein said second vessel is a gasifier... .The solid feed material employed in the process (is) preferably a bituminous or lower rank coal (which is) reacted ... in a hydrogasification zone (and) the char produced is contacted with steam in a steam gasification zone to produce the synthesis gas";

two gasification zones can be employed to make certain that any Carbon not reacted in the initial  gasification can be separated and further reacted with Steam to produce more hydrocarbon synthesis gas.

As with processes entailing the complete combustion of Coal, to power electrical generators for instance, the inorganic mineral content of the Coal, the "Ash", will remain behind in such gasification and conversion processes as an un-reacted solid residue, much of which will be in a fine particulate form very similar in structure and composition to the more familiar Coal power plant "Fly Ash".

To avoid clogging of the catalysts used by, as noted above, "processes such as Fischer-Tropsch and its variants", which convert the synthesis gas into liquid hydrocarbons; and, to avoid or reduce the subsequent need for filtering the liquid hydrocarbons produced from that Coal-derived synthesis gas, Exxon also devoted effort to developing technology, whereby the mineral Ash could be removed from the synthesis gas, coincident with and preliminary to recovering any still-carbonaceous Char for further, as in our above citation of "United States Patent 3,874,739 - Method and Apparatus for the Transfer of Entrained Solids", gasification of that residual carbonaceous Char.

As seen in excerpts, with comment appended, from the initial link in this dispatch to:

"United States Patent 3,957,459 - Coal Gasification Ash Removal System

Date: May, 1976

Inventor: Willard Mitchell, et. al., TX

Assignee: Exxon Research & Engineering Company, NJ

Abstract: In a fluidized bed process for the gasification of coal or similar carbonaceous solids wherein char particles are withdrawn from a fluidized bed reaction vessel, transported to a second vessel, and later returned to the initial vessel, char particles of high ash content are separated from particles to be returned to the fluidized bed reaction vessel by injecting a dense phase stream of char particles including particles of both high and low ash content into a vertically moving gas stream having a velocity sufficient to transport relatively light particles of low ash content upwardly into the fluidized bed reaction vessel but insufficient to suspend relatively dense particles of high ash content, collecting the high ash content particles which are not entrained by the gas stream, and periodically withdrawing the collected particles from the system.

Claims:  A method for the removal of ash from a fluidized bed system for the gasification of coal or similar carbonaceous solids containing ash-forming constituents wherein a stream of char particles is continuously withdrawn from a fluidized bed reaction vessel having a substantially vertical bottom inlet line, circulated through another vessel, and returned to said reaction vessel through said bottom inlet line which comprises introducing a dense phase stream of said char particles withdrawn from said other vessel into said bottom inlet line in a substantially horizontal direction, introducing a gas stream upwardly into said bottom inlet line at a point below the point at which said dense phase stream is introduced into said inlet line, maintaining the velocity of said gas stream at a level sufficient to entrain from said dense phase stream char particles having an ash content less than about 65 weight percent but insufficient to suspend char particles contained in said dense phase stream which have an ash content greater than about 85 weight percent; passing said gas stream and char particles entrained therein upwardly through said inlet line into said fluidized bed reaction vessel; introducing upwardly into said reaction vessel independently of said bottom inlet line sufficient additional gaseous fluid to maintain the char particles contained therein in a fluidized state; and withdrawing char particles which are not entrained in said gas stream from said inlet line at a point below that at which said dense phase stream of char particles is introduced into said line. 

A method ... wherein said particles are coal char particles and said other vessel is a combustion vessel.

(Alternatively, as in the Claim immediately above, the separated Char can, rather than being returned to the initial, or a secondary, gasification process and be therein converted into more synthesis gas, instead be simply combusted to generate heat, and/or power one supposes, to help drive other needed processes and reactions within the total system.)

A method ... wherein the velocity of said gas stream is maintained between about 0.05 and about 3.0 feet per second.

(We won't reproduce all of the specifications, but, as above, Exxon does go into some detail about the gas velocities that are needed to effect the separation of the Ash and the Char, and, to maintain the Char particles in suspension.)

A method ... wherein said other vessel is a transfer line burner, said gas stream passed upwardly through the bottom inlet line of said other vessel contains insufficient oxygen to initiate combustion of char particles entrained therein, and said additional gaseous fluid introduced into said other vessel independently of said bottom inlet line comprises air. 

A method ... wherein said gas stream passed upwardly into said other vessel through the bottom inlet line thereof comprises recycle flue gas.

(Without specifying, or recording, the dirty words "Carbon Dioxide", by specifying "recycle flue gas", as above, as one of the agents which can be utilized in the gasification process; and, which "flue gas" one would presume to contain some Carbon Dioxide, they are hinting at the potentials for CO2's productive recycling, as more openly expressed in our above citation of their "US Patent 3,758,673 - Production of a Carbon Monoxide Containing Gas".)  

In a process wherein carbonaceous solids including ash-forming constituents are withdrawn from a first vessel having a substantially vertical bottom inlet line, introduced into a second vessel having a substantially vertical bottom inlet line through said bottom inlet line in said second vessel, withdrawn from said second vessel, and returned to said first vessel through said bottom inlet line in said first vessel and wherein carbon contained in said solids is partially consumed in at least one of said vessels and ash tends to accumulate in the system, the improvement which comprises introducing a dense phase stream of said carbonaceous solids withdrawn from said second vessel into said bottom inlet line of said first vessel in a substantially horizontal direction, introducing a stream of carrier gas into said bottom inlet line of said first vessel at a point below that at which said dense phase stream of solids is introduced into said inlet line of said first vessel, passing said stream of carrier gas upwardly in said inlet line of said first vessel at a velocity sufficient to entrain particles from said dense phase stream having an ash content less than about 65 weight percent but insufficient to entrain particles from said dense phase stream having an ash content greater than about 85 weight percent; passing said stream of carrier gas and solid particles entrained therein from said bottom inlet line of said first vessel into said first vessel; introducing additional gaseous fluid into said first vessel near the lower end thereof independently of said bottom inlet line therein to maintain the carbonaceous solids contained within said first vessel in a fluidized state; collecting solid particles which are not entrained in said stream of carrier gas in said bottom inlet line of said first vessel; and withdrawing the collected particles from the system.

Background and Field: This invention relates to the gasification of coal and similar carbonaceous solids and is particularly concerned with a method for the removal of ash during fluidized bed coal gasification and related operations. 

A number of different processes for the gasification of coal and similar carbonaceous solids have been developed in recent years. Among the most promising of these are fluidized bed processes in which feed coal particles are devolatilized to produce hydrocarbon gases and char and char is reacted with steam to form synthesis gas.

(The immediately above is as, again, in our above-cited earlier report concerning "United States Patent 3,874,739 - Method and Apparatus for the Transfer of Entrained Solids".)

The reactions involved, which may be carried out in a single vessel or in two or more reactors, are highly endothermic and require that large amounts of heat be supplied. This is generally done by burning a portion of the char, either by injecting oxygen into the fluidized bed with the steam or by withdrawing char from the bed, passing it to a separate combustion zone, and then returning hot char particles to the fluidized bed reaction vessel. The gasification and combustion reactions which thus take place result in the production of significant quantities of ash. The ash not carried overhead with the product gases tends to accumulate in the system and must be removed if the process is to operate continuously.

Summary: This invention provides an improved method for the removal of ash during coal gasification and similar operations requiring the circulation of char particles between a fluidized bed reaction vessel and a second vessel ... . In accordance with the invention, it has now been found that char particles having high ash contents can be effectively separated from particles to be returned to the fluidized bed reaction vessel during such an operation by introducing a dense phase stream of the solids into a vertically moving gas stream having a velocity sufficient to entrain low ash content particles and transport them upwardly in the fluidized bed reaction vessel but insufficient to suspend high ash content particles, collecting the particles of high ash content which are not entrained by the upwardly moving gas, and thereafter withdrawing the collected particles from the system."

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Don't be too confused by the complicated description; we caution that the full Disclosure is even more dense than our excerpts would indicate.

The overall concept, though, is a lot simpler than it might appear at first glance.

The process is only a modern-day, and much more "involved", version of threshing and winnowing, as it was once applied to wheat and other grains, wherein the crop residues, after an initial processing to harvest most of the wheat grains, were tossed up into the wind, and, the heavier remaining wheat grains would fall to the ground while the lighter "chaff" was blown away.

Herein, however, the lighter "chaff", or lower-density carbonaceous Char particles, are what they're really after; and, after the Char particles have been separated and entrained, in the wind, so to speak, they're blown back into the gasification process.

But, the heavier inorganic mineral Ash, i.e., "the particles of high ash content which are not entrained by the upwardly moving gas", which then falls out, and can be "collected" and "withdrawn .. from the system", shouldn't be thought of as just unwanted and useless "chaff".

As seen in our report of:

Exxon Converts Coal Conversion Residues to Cement | Research & Development; concerning: "United States Patent 4,260,421 - Cement Production from Coal Conversion Residues; 1981; Assignee: Exxon Research and Engineering Company, New Jersey; Abstract: Cement is produced by feeding residue solids containing carbonaceous material and ash constituents obtained from converting a carbonaceous feed material into ... gases into a cement-making zone and burning the carbon in the residue solids to supply at least a portion of the energy required to convert the solids into cement";

Exxon does have some thoughts about how to constructively utilize the mineral "chaff" separated by the process of our subject herein, "United States Patent 3,957,459 - Coal Gasification Ash Removal System", although there shouldn't be all that much remaining "carbon in the residue solids". However, the mineral content is still suitable, as in the above "United States Patent 4,260,421", for conversion into cement, and, there's no reason it just couldn't be dumped, along with the standard raw materials, into a conventional and typical cement kiln.

The basic process of our subject is an ancient agricultural art, literally thousands of years old. But, as it can be applied, as herein, to our most abundant natural resource, Coal, to help fulfill one of our most pressing current needs, liquid hydrocarbon fuels, it's so futuristic and advanced that news of it hasn't even yet reached us, here, in the very heart of United States Coal Country.

Why, we are compelled to wonder, is that


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