Causes of blockage in the downcomer of the water-coal slurry pressurized gasification furnace

The main reason is the deterioration of coal quality, with an increase in the ash fusion point

Blockages in the downcomer tubes occur most often during shutdown periods. In my opinion: 1. It is necessary to raise the temperature in order to remove the slag adhering to the furnace walls, which causes the slag to accumulate at the slag outlet； 　　2. During pressure release, the control of the release speed is inadequate; if the speed is too fast, it causes the gas flow to increase, which in turn leads to more slag accumulating at the slag outlet. 　　3. If blockage occurs in the downcomer or if the pressure difference at the slag outlet increases during normal production, it is usually caused by the furnace temperature being lower than the ash fusion point of the coal. This may be due to poor production control, delayed analysis, or changes in coal quality. Generally, reducing the load and increasing the furnace temperature is used to melt the slag blockage at the downcomer outlet; if the pressure difference at the slag outlet does not decrease, it is necessary to shut down the plant for repairs. It is important to contact relevant departments such as those responsible for coal supply, coal grinding, and analysis promptly to determine the cause. (Actually, it takes seven or eight hours to analyze the ash fusion point once.) ）

We have encountered the problem of downcomer blockage many times, which can be mainly attributed to two aspects: 1) Changes in coal quality, an increase in the ash melting point, lower furnace temperature control, and an increase in the viscosity of the ash, all of which lead to slag accumulation at the slag outlet. An increase in the flow rate of the process gas or deviations in its flow direction can also break through the water film in the downcomer, resulting in slag accumulation there as well. 2) Excessively high load on the gasifier and low pressure in the gasifier result in too high velocity of the gases leaving the reactor; this causes the gas flow to break through the water film, leading to slag accumulation in the downcomer. If slag accumulates in the downcomer, the system should be shut down as soon as possible to prevent equipment damage.

As a supplementary point, damage to the slag outlet of the gasification furnace is also a cause of slag accumulation in the downcomer; therefore, slag outlet maintenance should be given priority in the maintenance of the gasification furnace

The main cause of blockages in the downcomer, for everyone, is the quality of the coal. However, when the coal quality changes, it is possible to adjust the temperature of the furnace. Moreover, if slag accumulates in the downcomer, it is possible that there are changes in the quench water flowing into it; for example, a brief interruption in the supply of quench water may not cause any blockage at that moment, but prolonged operation can lead to slag accumulation on the walls. It is also possible that uneven distribution of water within the downcomer could cause such problems.

As mentioned above, when there is a brief interruption in the supply of cold water, it is necessary to stop the plant; otherwise, an overestimated temperature of the process gas at the outlet can also trigger an interlock shutdown, and then there would be no issue with continuing to feed material or with slag accumulation. And when sudden cooling water causes a shutdown, pressure retention is definitely necessary; otherwise, heat transfer between the downcomer and the high-temperature process gas will lead to damage to the downcomer. The slag that reaches the downcomer simply flows downward naturally, and generally does not cause significant blockage. It is very dangerous to keep the furnace running with cold water suddenly cut off.

Stopping the cold water supply is only temporary; in fact, stopping it for a short time is done so that traffic can resume. Profit is what drives such actions, but in the end it’s not worth it, haha