Comparison between gray melt-polymerized fluidized bed gasifiers and circulating fluidized bed boilers: Issues regarding coal moisture content and particle size
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I recently came across some information stating that the coal requirements for a gray fused bed fluidized bed are: a moisture content of less than 5%, and a particle size of less than 6 mm； The moisture content in circulating fluidized bed boilers can exceed 30%, with particle sizes of less than 12 mm. Since both are fluidized bed systems, what is the reason for such a large difference? I would appreciate advice from experts!
The reason, as I understand it, for the requirement of low moisture content in the raw coal for ash fusion: 1. It helps to ensure smooth operation of the coal feeding system； 2. The influence of moisture in the gasifier on the steam-coal ratio during coal gasification.
I cannot agree with this answer: Circulating fluidized bed boilers also face the issue of coal feeding – aren’t they at risk of clogging? It’s true; water is also steam. If there’s less steam, then the ratio of gas to coal will be maintained, right? Moreover, if the particle size and moisture content can be kept so high, it would be a boon for the gasification furnace, as crushing, screening, and drying the coal before feeding it into the furnace are very time-consuming processes.
I agree with your argument about steam later on, but regarding the previous sentence about circulating fluidized bed boilers, since they operate at atmospheric pressure, blockages are not a major issue
Reply to 3# xxxs: Assuming your theory regarding steam is correct, let me share some practical information with you. The coal intake is 8 tons per hour, and the normal amount of steam generated is 4500 kg/h. The moisture content in the coal is 8000*0.3=2400 kg/h; therefore, the actual amount of steam required is 4500-2400=2100 kg/h； While ensuring the oxygen concentration of the gasifying agent, the amount of gasification agent is not sufficient to fluidize 8 tons of coal. I am ignorant and lack knowledge; I earnestly ask the experts for their criticism and guidance.
How does a circulating fluidized bed boiler achieve fluidization?
The analysis is quite reasonable; a higher moisture content in coal can indeed reduce the amount of steam that enters the furnace, but it’s not a simple matter of subtraction. The issue of fluidization still needs to be taken into consideration; by ensuring proper fluidization, it is possible to reduce the amount of steam used to some extent. Otherwise, due to the large amount of steam (including the water present in coal), more oxygen is required, which results in a higher concentration of H2 and CO2 in the output gas, while the amount of CO decreases.
I’m not sure whether the poster wants to burn a boiler or carry out coal gasification – can you control the level of moisture in coal with precision? With 30% moisture, what will be the product after vaporization? Does the gasifier have one like this? Please, the original poster, give me some advice! The amount of steam is relatively easy to control; it can be adjusted well depending on the composition of the coal!
Hehe, Rhinocao, of course I know that coal needs to be dried before gasification, because if the coal isn’t dried, it’s impossible to control the water content when it enters the furnace; factors such as ash content and moisture levels change, making it very difficult to maintain control!
The coal feed requirements for the ash fusion fluidized bed are: 15% moisture content