Slagging Characteristics of Zhundong Coal during Circulating Fluidized Bed Gasification

Abstract

Three types of Zhundong coal with different sodium contentsShenhuazhundong coal (SHZD), Shaerhu coal (SEH), and Tianchimulei coal (TCML)were investigated to understand their slagging behaviors during circulating fluidized bed gasification (CFBG). Generally, the operational stability of Zhundong coal decreases with the rise of sodium content. Zhundong coal with low sodium content is suitable as a gasification fuel of a circulating fluidized bed. For high-sodium-content coal, agglomeration, and even defluidization, should be the focus. Agglomeration or defluidization could be alleviated or avoided by controlling the bed temperature. There is a one-to-one correspondence between temperature fluctuation and sodium content in bottom slags. Slagging is affected by sodium occurrence modes and some ash components (e.g., SiO2 and Fe2O3). Due to different contents of chemical components, especially chlorine, silicon, and sodium, the three types of Zhundong coal cause different problems. Chlorine and sodium in SEH, mainly existing as NaCl, play an important role in corroding metal walls. Microanalysis of agglomeration particles in the tests with TCML shows that the SiO2-to-Na2O ratio gradually increases from surfaces to the inner part, which confirms the effect of the Na2O–SiO2 reaction on the agglomeration. The SiO2 content in high-alkali coal decisively impacts on the slagging morphology by the interaction between Na2O and SiO2 in coal itself for high-SiO2 coals or additional SiO2 from quartz sands for low-SiO2 coals.

Effects of wall temperature on slagging and ash deposition of Zhundong coal during circulating fluidized bed gasification

“…Thus, CFB is highly potential for utilization of high-sodium Zhundong coal. We previously investigated the Na transformation during gasification/combustion [20] and compared the slagging behaviors among three types of Zhundong coal containing different sodium contents in a 0.25 t/d CFB test rig [21]. It was found Na behaved differently under these two reaction atmospheres: Na migrated from Zhundong coal to deposit mainly in the form of Na 2 SO 4 during combustion, but in the form of NaCl during gasification.…”

Effect of bed materials on slagging and fouling during Zhundong coal gasification

“…Na is the most important element for slagging and fouling (Song et al, 2016a(Song et al, , 2016b(Song et al, , 2016c(Song et al, , 2016d. Therefore, the capture capacity of Na by bed materials should be focused.…”

“…The Cl content in SEHc is up to 1.279% shown in Table 1. This indicates that Na and Cl in SEHc were mainly existed in the form of NaCl (Song et al, 2016a(Song et al, , 2016b(Song et al, , 2016c(Song et al, , 2016d. Under the action of SiO 2 and other minerals, Na silicates and aluminosilicates were formed by the following reactions (22) …”

“…This indicates fouling will be alleviated during Zhundong coal gasification, compared with Zhundong coal combustion. In our previous studies (Qi et al, 2016;Song et al, 2016aSong et al, , 2016bSong et al, , 2016cSong et al, , 2016d, slagging and fouling characteristics of Zhundong coal during circulating fluidized bed (CFB) gasification with quartz as bed material were initially proved, thus some measures still need to be taken to solve the ash-related problems ensuring smooth operation.…”

Steam gasification reactivity of a high-sodium coal fly ash obtained from a pilot scale CFB gasifier

“…The high-sodium content in coal ash may cause fouling and slagging problems, influencing the operation safety of units Wang et al 2015;Fan et al 2016;Song et al 2016a;Zhang et al 2017a). During the thermal utilization process, sodium transforms with the conversion of coal, and the properties of the derived fly ash may be different with other coal fly ashes.…”