Characteristics of the Blockage from Air Nozzle Guide Duct in Circulating the Fluidized-Bed Coal Gasifier and Its Formation Mechanism

Blockage is often generated in the air nozzle guide duct in a circulating fluidized-bed coal gasifier (CFBG), especially with Zhundong sub-bituminous coal (ZSBC) as the raw material. A typical example is found in one CFBG sample from Xinjiang Yihua Chemical Industry Co, Ltd. The serious blockage can be observed obviously. As so far, it is not clear for the characteristics and generation mechanism of the blockage. For analysis, the blockage can be classified into two parts, wall-layer blockage (WLB) and center-layer blockage (CLB). To inhibit its formation, it is of significance to analyze the composition, surface morphology, and formation mechanism of the two blockages. In our experiments, WLB and CLB were tested by XRF, XRD, FTIR, SEM-EDS, and SEM-mapping methods. Results showed that WLB presents high content of Fe, Cr, and Ni, and Fe mainly existed in the form of metal oxides. CLB is dominated by Si (43.04%), derived from silica and alkali and alkaline-earth metals silicates, and the migration of Fe, Cr, and Ni elements from the duct material was observed. Compared with WLB, from FTIR analysis, CLB contains more inorganic minerals, and the absorption peak of inorganic minerals is mainly attributed to asymmetric Si–O–Si. Many fine particles are attached to the surface of the WLB, while the surface of the CLB is smooth, and there is noticeable raised texture, which is presumed to be the result of particle melting and agglomerating as the bottom ash enters the duct in the gasification process. For the formation of the blockage, this paper speculates that it is mainly due to the difference in flow resistance near the air nozzle outlet, resulting in the formation of a flow dead zone at the bottom of the gasifier, which leads to large amounts of ash overcoming the outlet resistance and leaking into the air nozzle, and next, the ash corrodes in the tube, resulting in wall deposition and ultimately blocking the air guide duct. Two methods can be tried to avoid or inhibit the formation of blockage in the duct, including optimizing air nozzle with more wear-resistant and heat-resistant materials and adjusting the distance between air nozzles to avoid mutual interference from ash particles.