Slurry concentration for coal slurry gasification
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The generally accepted view at present (including what GE Texaco itself promotes) is that if the concentration of water-coal slurry is below 60%, it cannot be used in Texaco’s gasifiers. If the concentration of water-coal slurry decreases, the specific oxygen consumption and specific coal consumption in the gasifier will increase; however, I believe that specific circumstances need to be taken into account in such cases. Compared to coal consumption, it is of course a matter of coal usage; in my opinion, aside from equipment investment, oxygen consumption can ultimately also be attributed to coal consumption. Since the compressors in air separation plants are usually driven by steam turbines, and the steam comes from coal-fired boilers, oxygen is ultimately also derived from coal. In areas where coal is very cheap, even if its slurry-forming properties are poor and the oxygen consumption per unit of coal used is higher, the cost of the final chemical products is not necessarily lower than that of using high-quality coal with better slurry-forming properties, due to the low cost of the raw material.
The original poster makes a valid point. This is also why various gasification technologies can have a certain market share among each other. Because, as a business, the outcome ultimately depends on profitability; although there are differences in the pulping properties of the raw materials, price is the decisive factor. Good raw materials are expensive, and currently they are mainly found in regions such as Inner Mongolia, Gansu, and Shaanxi in China; transportation costs also represent a significant constraint. Also, the claim by the original poster that gasifiers with a concentration below 60% are unusable is a bit extreme; a more reasonable threshold is that the concentration should not be lower than 58%. The problem arises when there is too much water, as this leads to an imbalance in the heat generated during the reaction, resulting in insufficient heat. Because manufacturers are already operating at a 59% concentration.
In my opinion, Texaco gasification uses coal water slurry technology; water itself is one of the raw materials in the gasification reaction. Additionally, Texaco gasification can utilize a wide range of coal types, and most of the high-quality coals are used as fuel. Therefore, a coal water slurry concentration of 58% to 62% is sufficient for Texaco gasification.
It would be worthwhile to do a careful cost analysis to determine which option is more suitable. For example, comparing lignite, which has poor slurry-forming properties, with bituminous coal, which has better slurry-forming properties: roughly speaking, if the concentration of the coal water slurry is reduced from 60% to 50%, the oxygen consumption increases from 400 to 500. Taking an output of 50,000 Nm3 of (CO+H2) per hour as an example, this is equivalent to approximately 23 tons of ammonia being produced per hour. In such a case, the power required for the air separation compressors would need to increase from 14,000 kW to 18,000 kW, and this electricity naturally comes from burning coal. If in Inner Mongolia, at 100 yuan per ton for lignite, then compared with using Shenmu bituminous coal at 500 yuan per ton, the cost per ton of ammonia increases by only about 10 yuan due to the increased oxygen consumption (which is related to the boiler efficiency). Everyone can also estimate it based on coal consumption.
Normal ash should have a uniform particle size (around Ф5mm), a smooth surface, and an appropriate amount of ash; it is advisable that the ash content make up more than 50% of the total ash volume. (1) There is a considerable amount of fine slag; the slag volume is high, but no stringing phenomenon occurs, which indicates that the furnace temperature is too high. However, since the slag has good fluidity, it is possible to lower the temperature slightly in order to reduce the O/C ratio ； (2) There is an excess of fine slag, which has a yellow-green color; analysis shows a high Cr2O3 content. At this time, the furnace temperature is high, so it is necessary to slowly reduce the O/C ratio and gradually lower the operating temperature ； (3) If there is stringing phenomenon and the amount of slag is moderate, it indicates that the fluidity of the slag has decreased and the slag outlet is blocked, resulting in an irregular shape; in such cases, the temperature should be increased appropriately to raise the O/C ratio ； (4) If there are fibers similar to rock wool, with no formed slag lumps or particles and a relatively large amount of such fibers, it indicates that the slag outlet or downcomer is severely blocked; in this case, it is necessary to reduce the load or shut down the operation ； (5) The presence of coarse slag particles, along with a small number of small fragments of irregular shape, and an appropriate amount of slag indicates that the furnace temperature is too low and the fluidity has decreased; it is necessary to increase the temperature accordingly ； (6) The coarse particles have increased significantly; larger slag lumps and adhering slag are present, and their shape is very irregular. This indicates that the furnace temperature is low, and the furnace temperature should be raised to improve slag management ； (7) The slag contains both lumps and filaments; its amount is moderate. Analysis shows a high carbon content in the slag, indicating poor atomization effect of the burner or uneven spraying, which leads to incomplete gasification and results in large temperature variations in the slag holding plate ； (8) An excessive amount of slag indicates a high ash content in the coal; it is necessary to raise the furnace temperature, reduce the load, or improve operational practices ； If the amount is low, scum or slag may form; in such cases, the slag breaker can be operated in reverse mode or the load can be reduced ； In practical operation, decisions regarding treatment should be made based on a comprehensive assessment of the slag, DCS operation instructions, CS concentration, and coal quality analysis