No matter how we convert our abundant Coal into more versatile hydrocarbons, no matter how completely we utilize it's carbon content, there will, just as when we combust Coal for power generation, remain behind an inorganic mineral residue.
That residue is most often referred to and known most commonly as "fly ash", which most think of as settling out of exhaust gases, but larger and heavier agglomerations, "slag" and "clinker", are also formed.
Processes for the indirect conversion of Coal, wherein it is first partially oxidized to form a blend of Carbon Monoxide and Hydrogen, a "synthesis gas", which can then be catalytically condensed into liquid and gaseous hydrocarbons, are similar in many ways to the more straightforward combustion of Coal for the purposes of power generation, and residues from the two processes will be quite similar in composition.
The same is true to a certain extent of direct Coal conversion technologies, such as the WVU's "West Virginia Process", wherein the carbon content in the Coal is more directly hydrogenated and liquefied through contact with a liquid Hydrogen donor solvent.
The inorganic mineral content originally contained in the Coal will remain as an unconverted residue.
We've previously documented a number of uses for Coal ash, and will address those, and others, a little further on in this dispatch; but, first, we wanted to demonstrate that the petroleum industry, who, as should now be apparent from our literally thousands of reports attesting to the fact, know all about the art and science of converting Coal into hydrocarbons, has addressed the issue of Coal ash, especially as that issue applies to processes for the indirect conversion of Coal, wherein it is first transformed into synthesis gas, and, how to, through re-processing of that ash, make certain all of the available Carbon has been extracted and utilized in the production of the desired hydrocarbon synthesis gas.
Additional comment and links follow excerpts from the initial link in this dispatch to:
"United States Patent 4,969,931 - Process for the Preparation of Synthesis Gas
Date: November, 1990
Inventor: H.L. Wu, et. al., Amsterdam
Assignee: Shell Oil Company, Texas
Abstract: A process for the preparation of synthesis gas by the partial combustion of an ash-containing fuel with an oxygen-containing gas is described, the synthesis gas formed being removed from the top of the reactor through a gas discharge pipe, and slag formed through a slag discharge at the bottom of the reactor, the process being characterized by the counter-current contact of the synthesis gas in the reactor with cold fly-slag agglomerates.
Claims: A process for the preparation of synthesis gas comprising: partially combusting an ash-containing fuel with an oxygen-containing gas in a reactor, and producing synthesis gas, fly slag, and slag;
(And) removing synthesis gas and fly slag formed through a gas discharge pipe at the top of the reactor and removing slag formed through a slag discharge at the bottom of the reactor;
(And) introducing cold fly-slag agglomerates produced from said fly slag particles into the reactor and counter-currently contacting the synthesis gas in the reactor with said cold fly-slag agglomerates.
(And) wherein the agglomerates of the fly slag is effectuated with an agglutinant.
(And) wherein the agglutinant is a water-glass, bitumen, tar or pitch (or) cement.
(And) wherein the fuel is coal or lignite.
Background and Field: The invention relates to a process for the preparation of synthesis gas by the partial combustion of an ash-containing fuel with an oxygen-containing gas in a reactor, synthesis gas formed being removed from the reactor through a gas discharge pipe at the top and slag formed through a slag discharge in the reactor bottom.
In the gasification of an ash-containing fuel, synthesis gas is prepared by partially combusting the fuel with an oxygen-containing gas. The fuel used for this purpose can be coal, but lignite, peat, (and) wood ... are also suitable.
(Note yet again, as in many of our previous reports attesting to the fact, that a note of sustainability and Carbon recycling can be introduced by using "wood", presumably any cellulose, along with Coal.)
Besides the fuel and the oxygen-containing gas, a moderator is conveniently passed into the reactor as well. Said moderator exercises a moderating effect on the temperature of the gasification reaction by entering into an endothermic reaction with the reactants and/or the products.
(Be sure to make note of the following.)
Suitable moderators are steam and carbon-dioxide.
(As we've many times documented, Steam can be added to Coal synthesis gas generation processes to supply more Hydrogen to the mix of products. And, again, Carbon Dioxide, recovered from whatever source, can be recycled through utilization as one of the agents of Coal gasification, in a process intended to generate a synthesis gas suitable for catalytic condensation into hydrocarbon fuels.)
In the gasification reaction, slag is formed in addition to synthesis gas. A large proportion of the slag falls down and disappears from the reactor through the slag discharge. It has been found, however, that a proportion of the slag is entrained with the product gases to the discharge pipe. The entrained slag is in the form of small droplets or porous particles. It is called fly slag and can create severe disturbance by causing contamination in the equipment.
In order to prevent contamination as far as possible, the discharged synthesis gas with the fly slag is quenched, so that the fly slag rapidly solidifies. Said quenching is preferably effectuated by injecting a cold gas and/or water into the gas discharge pipe. After the gas has cooled down the fly slag is removed from the gas, for example by means of one or more cyclones.
When the fly slag has been separated from the synthesis gas, all the fly slag is in the form of fine, porous particles.
A proportion of the fuel in the fly slag is not converted into synthesis gas. The solidified fly slag therefore contains a considerable percentage of carbon.
It has been proposed to recycle the fly-slag particles via the burners to the reactor together with the fuel to be gasified, so that said particles are again contacted with oxygen. In this way practically all the carbon in the fly slag is partially combusted. Even more importantly, the fly slag then melts again and at least a proportion thereof falls down to the slag discharge. However, this proposal has the drawback that a proportion of the recycled fly-slag particles are again entrained with the synthesis gas.
That means that more fly slag has to be separated in the cyclones, so that the latter have to be larger and therefore more expensive. Moreover, the pneumatic transport of fly slag to the reactor requires a considerable quantity of carrier gas. These quantities may become such as to have an adverse effect on the thermal efficiency of the combustion and therefore the carbon monoxide and hydrogen yield.
Summary: The object of the present invention is to convert the fly slag to slag such as that which is discharged through the slag discharge, without the above-mentioned drawbacks being encountered. To that end the fly slag is recycled to the reactor in such a form that there is no risk of its being reentrained with the synthesis gas, during which process it is remelted and the remaining carbon which it still contains is converted into synthesis gas.
(This process, thus, embodies a way in which all, or nearly all, of the Carbon content of the original Coal can be extracted and utilized in the synthesis of hydrocarbons, or at least of hydrocarbon synthesis gas.
It represents a more efficient way to extract all of the Carbon, similar to other, related, thorough Coal gasification and conversion processes we've documented for you, such as in:
Consol Hydrogasifies CoalTL Residues | Research & Development; concerning: "United States Patent 4,248,605 - Gasification of Coal Liquefaction Residues; 1981; Conoco, Inc.; Abstract: A method for gasifying the bottoms fraction from a coal liquefaction process by ... reacting the agglomerate particles with steam in a fluidized bed; and:
Texaco Recycles Coal Conversion Residues | Research & Development; concerning: "United States Patent 2,980,521 - Carbon Separation Process; 1961; Texaco, Inc.; Abstract: This invention relates to a process for the production of synthesis gas from carbonaceous fuels. More especially, the invention is concerned with improvements in a method for the separation and recovery of free carbon particles from a carbon-water slurry formed in a synthesis gas generation process".
It has, thus, long been known by the petroleum industry that processes of Coal conversion, whether direct or indirect, do not initially extract all of the Carbon in the Coal feed; and, the still-carbonaceous residues resulting from an initial Coal extraction can, as in our subject Shell Oil process of "US Patent 4,969,931", be subjected to further treatment for the full hydro-conversion of nearly all the available Carbon. All of which Shell Oil goes on to further explain.)
The invention therefore relates to a process for the preparation of synthesis gas by the partial combustion of an ash-containing fuel with an oxygen-containing gas in a reactor, synthesis gas formed being removed from the top of the reactor through a gas discharge pipe and slag formed through a slag discharge at the bottom of the reactor, characterized in that the synthesis gas is countercurrently contacted in the reactor with cold fly-slag agglomerates.
According to the invention, therefore, agglomerates of fly-slag particles are produced and introduced into the reactor. It is preferable to inject the agglomerates into the reactor at the top thereof. In this way the duration of their fall to the slag discharge is comparatively large. During their fall, they come into contact with the hot synthesis gas. This heats them up. Moreover, the carbon in the agglomerates undergoes partial combustion with the oxygen and/or steam in the reactor. The reaction with oxygen generates a great deal of heat, thereby promoting the melting of the agglomerates. This yields slag from which, once it has solidified, heavy metals are not readily lixiviated and which has a low carbon content.
Agglomeration of the separated fly slag can be effected with mechanical or electrostatic aids. For example, it is possible to compact fly slag into larger particles. Preferably, however, agglomeration is effectuated with an agglutinant, so that agglomerates are obtained which consist of fly slag with an agglutinant. Water forms fairly good agglomerates. If agglomerates of fly slag with water come into contact with high-temperature gases, the agglomerates explode as a result of the sudden evaporation of the water. The resultant steam can participate in the gasification. Water is only a suitable agglutinant if the fly-slag particles remaining after the sudden evaporation of the water are not so small that they are all re-entrained with the synthesis gas. Preferably, the agglutinant is water-glass. As is known, water-glass consists of water and sodium silicate.
Other suitable agglutinants are bitumen, tar or pitch. These enable good agglomerates to be obtained. Moreover, when the agglomerates return into the reactor the agglutinant is gasified as well. As a result of the gasification reaction of this agglutinant with oxygen, heat is generated in the agglomerates, thereby promoting their melting.
In addition, the yield of synthesis gas also becomes higher.
(Thus, if we elect to use, for instance, Coke Oven "bitumen, tar or pitch" to extract the fly ash from the Coal synthesis gas, and recycle the resulting "agglomerates" into the Coal "gasification reaction", then "the yield of synthesis gas" will be even "higher".)
Cement is also suitable as an agglutinant. Cement yields firm agglomerates.
(And) a side-effect of cement is caused by its calcium oxide content: hydrogen sulfide present in the synthesis gas is bonded by the calcium oxide. Accordingly, if cement is used as an agglutinant, the synthesis gas is also partly stripped of H2S.
(Thus) the largest quantity of useful gas, (i.e.,) carbon monoxide and hydrogen, is obtained with the process according to the invention."
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And, through generating the "largest (possible) quantity of ... carbon monoxide and hydrogen" from a given amount of Coal and, as Shell Oil notes, CO2-recycling "wood", the process thus also utilizes nearly all of the original Carbon contained in the raw materials.
Not only that, but, since Shell Oil specifies herein that both "steam and carbon-dioxide" can be utilized as agents of gasification, the potentials for Carbon recycling, in addition to full Carbon utilization, are even further expanded.
Resulting from this process, in addition to "the largest quantity of useful gas", will be a relatively-inert and very nearly carbon-free ash, or slag, similar in many respects of composition to Coal power plant fly ash.
We've documented that such ash does have a number of potentials for use, as seen, for instance, in:
Scientists Convert Coal Ash to Cement | Research & Development; concerning: "New Use for Coal Ash: Material Provides Strong and Lightweight Alternative to Concrete – without Cement; An assistant professor in Georgia Tech’s School of Civil and Environmental Engineering, Doyoyo, has developed a new structural material based on these leftovers from coal burning. Known as Cenocell (TM), the material offers attributes that include high strength and light weight – without the use of cement, an essential ingredient of conventional concrete"; and:
Carbon Dioxide + Coal Fly Ash = Synthetic Lumber | Research & Development; concerning: "US Patent Application 20080029925 - Filled Polymer Composite and Synthetic Building Material; 2008; Abstract: The invention relates to composite compositions having a matrix of polymer networks and dispersed phases of particulate or fibrous materials. The matrix is filled with a particulate phase, which can be selected from one or more of a variety of components, such as fly ash particles ... . A method of continuously forming a molded material ... (including) a building material (such as synthetic) lumber (or) roofing (or) siding (consisting of) a polyurethane .... filled with a particulate ... such as fly ash."
However, before such ash is used simply as an additive and reinforcement for cement and plastic, it still has other values which could, and should, be realized, as seen in:
Coal Waste Uses - Aluminum Ore | Research & Development; wherein we're told that a: "Chinese power company has succeeded in producing alumina from coal ash, a step that could help ease China's chronic raw materials shortage ... . Datang International Power Generation Co has completed construction of a plant capable of producing 3,000 tonnes of alumina a year from coal ash ... . Ash remaining after coal is burned typically contains metals, including alumina, the raw material for aluminium, and recapturing and using them could reduce demand for natural resources."
Such "alumina", and other "metals", would be present in the Coal ash resulting from our subject Shell Oil Company technology of "United States Patent 4,969,931 - Process for the Preparation of Synthesis Gas", as well, just as it would be in Coal-fired power plant ash, after, through Shell Oil's process, all of the original Carbon content of the Coal had been extracted and converted into the raw materials, Hydrogen and Carbon Monoxide, needed for hydrocarbon synthesis.
And, in reports to follow, we will address even further the potentials for such "mining" of Coal ash, whether that ash results from the conventional conversion of Coal into electric power or, as herein, into the components of a "synthesis gas" suitable for catalytic condensation, as through the Fischer-Tropsch and related processes, into liquid and gaseous hydrocarbon fuels.