Preparation and Characterization of Coal-Water-Alcohol Slurry for Efficient Entrained-Flow Gasification

For efficient coal water slurry (CWS) gasification, it is very important to prepare CWS with higher carbon content and lower viscosity, which improve CWS heating values and atomization of CWS. However, the trade-off relationship between carbon content and viscosity gives rise to the difficulty in preparing CWS with high carbon content and low viscosity. Here we report the preparation of CWS with higher heating values and lower viscosity by employing various alcohol additives such as methanol, ethanol, 2-propanol, 1-pentanol, 1-hexanol, and 1-octanol with an alcohol content range of 1-10 wt %. In the case of methanol, ethanol, and 2-propanol with hydrophilic nature, as the added alcohol content was increased to 10 wt %, the slurry viscosity decreased from 2100 to 1089 cP, and heating values increased from 3613 to 4412 kcal/kg. However, in the case of 1-pentanol, 1-hexanol, and 1-octanol with lipophilic nature, the slurry viscosity significantly increased to the measurement limit (10 000 cP) of our viscometer with an increase in the added alcohol content, and the alcohol with a longer alkyl chain gave a much larger increase in viscosity. From the variation behavior of slurry viscosity by addition of various alcohol additives, it was considered that the viscosity variation behavior by addition of alcohols might be significantly associated with the hydrophilic nature of Shenhua coal. The swelling tests, energy dispersive X-ray spectroscopy (EDS), X-ray mapping, Fourier transform infrared (FTIR), and ash analyses of Shenhua coal were conducted to estimate the hydrophilicity of Shenhua coal. As a result, it was confirmed that Shenhua coal includes substantial hydrophilic groups such as hydroxyl groups, which are derived from silica and alumina well-distributed on the coal surface. In addition, water is considerably embedded in coal matrices after preparation of CINS. The water-swollen coal in CINS gives much higher hydrophilicity in comparison with coal before the preparation of CWS. Accordingly, when methanol, ethanol, and 2-propanol with hydrophilic nature are added into the as-prepared CWS, the coal content in CWS is diluted due to hydrophilic alcohols homogeneously mixed with water-swollen coal particles, which results in a falloff in slurry viscosity. By contrast, if 1-pentanol, 1-hexanol, and 1-octanol with relatively hydrophobic nature are added into CWS, water-swollen coal particles with high hydrophilicity agglomerate together to form larger clusters and the coal agglomerates are separated from an oil phase. The agglomeration and phase separation of coal particles are attributed to a drastic increase in slurry viscosity. Out of hydrophilic alcohols used in the present study, ethanol is expected to be the most promising candidate as an alcohol additive, because CO2 emission as a main drawback to entrained flow gasification of CWS can be considerably reduced by using crude bioethanol, which is produced from CO2-neutral biomass.