Low-rank coals, including sub-bituminous coals, brown coals, lignites, peats and other humic solids, represent one of the largest fossil fuel resources in the world. Most of the low-rank coal deposits are located near the earth's surface, and can be mined at significantly lower cost than typical bituminous coals. In addition, many low-rank coals have a very low sulfur content, relative to bituminous coals. As a result, low-rank coals are emerging as preferred feedstocks for coal liquefaction.
However, low-rank coals present special problems in coal liquefaction. Low-rank coals are richer in oxygen than bituminous coals. Most of the additional oxygen is present as carboxylic acids and their salts. These carboxyl groups contribute to several important problems in liquefaction of coal. First, they bind water strongly, making drying low-rank coals difficult and costly. The bound water not removed by drying is liberated during coal liquefaction, thereby lowering hydrogen partial pressures and accelerating catalyst deactivation. Second, metals bound as the salts of these carboxylic acids are not effectively removed by conventional coal cleaning methods, and therefore can be liberated during liquefaction. This can result in the need for a de-asher to reduce high ash load. Moreover, once liberated, these metals can attack and deactivate the supported catalysts typically used to promote liquefaction of the coal. Third, carboxylic acids and their salts can undergo retrograde reactions, for example ketone formation, that make coal harder to liquefy. These retrograde reactions are especially troublesome when the coal contains appreciable amounts of calcium and magnesium. Finally, carboxylic acids and their salts can decarboxylate during coal liquefaction liberating carbon dioxide. In hydroliquefaction, this liberated carbon dioxide lowers the hydrogen partial pressure and requires scrubbing to maintain the desired purity of the recycle hydrogen stream.
Pretreatment of low-rank coals prior to liquefaction is well known in the industry. Most of these pretreatment processes are designed to address the problem of how to handle alkaline earth metals, particularly calcium, which are contained in the coal. These metals can react with available anions during liquefaction to form solid scale particles which deposit in the liquefaction reactor, thereby reducing reactor volume, liquefaction time, and total throughput. Moreover, a portion of the scale can remain in the liquid product and result in downstream plugging.
It has been discovered that alkaline earth metal deposits, which form during liquefaction of low-rank coal, can be avoided by converting these metals to a salt which will remain stable during liquefaction. For example, U.S. Pat. No. 4,332,668 discloses pretreating low-rank coal prior to liquefaction by contacting the coal with phthalic acid, phthalic anhydride, pyromellitic acid, or pyromellitic anhydride to convert the scale-forming components to the corresponding phthalate or pyromellitate prior to liquefaction. It is believed that the majority of the alkaline earth metals in the coal is converted into an insoluble, thermally stable alkaline earth metal phthalate which remains within the coal and is released during liquefaction as particulate solids which are recovered with the liquefaction bottoms.
Another coal pretreatment process for handling scale formation that occurs during liquefaction of low-rank coal is contacting the coal with a sulfur-containing compound prior to liquefaction. When the sulfur-containing compound is an oxide of sulfur, the addition of the sulfur dioxide or sulfur trioxide is believed to form an anion which combines with the alkaline earth metal to form a molecular species which precipitates within the pore as an insoluble molecular sulfate of the alkaline earth metal. An example of such a process is U.S. Pat. No. 4,304,655 which discloses contacting low-rank coal with a combination of pretreating agents comprising sulfur dioxide and an oxidizing agent. Another example is U.S. Pat. No. 4,161,440 which contacts a low-rank coal with an oxide of sulfur, in liquid phase. U.S. Pat. Nos. 4,149,959 and 4,094,765 contact coal with a H.sub.2 S gas prior to coal liquefaction.
Another way of addressing the scale formation problems associated with low-rank coals is to pretreat the coal by contacting it with a carbon dioxide-containing gas. It is believed that this process converts the alkaline earth metal to its corresponding carbonate which remains with the coal during liquefaction and, therefore, does not form scale. An example of such a process is U.S. Pat. No. 4,206,033 wherein a low-rank coal is contacted with carbon dioxide at a partial pressure above one atmosphere prior to coal liquefaction. Another example of a carbon dioxide pretreatment process can be found in U.S. Pat. No. 4,714,543.
U.S. Pat. No., 4,450,066 handles the problem of scale formation during liquefaction by hydrothermal pretreatment prior to liquefaction. It is believed that carbon dioxide is released during the hydrothermal treatment. The liberated carbon dioxide is then absorbed by the water, which is effectively retained in the coal pores by a hydrocarbon solvent and elevated pressure, and reacts with liberated alkaline earth metal to form the corresponding alkaline earth metal carbonate. These metal carbonates are not separated from the coal. Rather, they remain with the coal during liquefaction, where they can adversely affect coal conversion and product quality and can deactivate the catalysts used to promote liquefaction and product upgrading.
The above-described methods of pretreating low-rank coal can ameliorate scale problems in liquefaction. However, these methods do not remove the alkali and alkali earth metals from the coal during liquefaction. These metals can deactivate catalysts, and can adversely affect coal conversion and product quality.
U.S. Pat. No. 4,579,562 discloses decarboxylating low-rank coal by contacting the coal with water at a temperature of about 400.degree.-650.degree. F. and at a pressure sufficient to prevent boiling of the water prior to combusting the coal. This process makes the coal easier to dry, increases heating value, and makes it more economical to transport. Nowhere in this patent is there disclosed or suggested liquefying the decarboxylated coal.
European Patent No. 264,743 discloses contacting an aqueous suspension of coal with carbon monoxide in the presence of a hydroxide or an alkali metal carbonate at a temperature of about 662.degree.-809.degree. F. for about 5-60 minutes. Although this process can be effective at removing undesirable carboxylic acids, the carbon monoxide can react with the water to form hydrogen and carbon dioxide. The presence of carbon dioxide during liquefaction can lower hydrogen partial pressure, thereby decreasing coal liquefaction. Moreover, the presence of hydrogen can undesirably retard decarboxylation.