Conversion Of Lignite And Coking Wastewater To Hydrogen In Supercritical Water

Hydrogen has become one of the primary green energy resources because of its zero pollution and sufficient energy. However, hydrogen energy is a kind of secondary energy which will consume a lot of energy in the hydrogen production process. Traditionally, hydrogen is produced in large scale by coal gasification because our country is richer in the coal resources. The main problems needed to be overcome are the conventional usage manner of the coal leads to lower efficiency and more seriously environmental pollution. So it has very important significance to explore a new high efficiency of coal conversion hydrogen technology.A supercritical fluid can be described as a fluid with a single phase where the distinction between a liquid and a gas can no longer be made. Water's critical point is at 374.3℃and 22.05 MPa. The properties of water change dramatically around the critical point. The use of SCW in conversion of low rank coal appears to be a promising process, which may not only easily control the solvent power and the diffusivity of the medium by changing the reaction temperature and pressure, but also easily separate gases and liquids from SCW after reaction. The current study is focused on the reaction characteristics of the low rank coal and coking wastewater in SCW. The coal-water slurry (CWS) prepared with coking wastewater and coal was firstly studied to provide basic date for continuous hydrogen generation. In the following experiments, the effect of raw materials, temperature, CWS concentration and additives on continuous hydrogen production process in supercritical water from lignite and coking wastewater was investigated. The main conclusions are summarized as follows:1. Influencing factors during coal-water slurry (CWS) prepared with lignite and coking wastewaterThe water quality, dispersant dosage and the CWS concentration is the main factor during coal-water slurry (CWS) prepared. In the same conditions, different coking wastewater has different effect of viscosity reduction for CWS. The highest phenols content of CWS has the best stability and rheological property. The best dispersants adding amount was varies with water quality.2. Conversion of lignite and coking wastewater to hydrogen in SCW1) There exists an obvious synergistic effect in the co-gasification of lignite and coking wastewater. In the conditions of 20wt%,600℃and 25 MPa, The hydrogen yield and carbon gasification efficiency increased 141.9 mL·g-1 and 6.1%, respectively.2) Temperature is a key parameter affecting the characteristics of supercritical water, and the hydrogen concentration and yield increased from 21.5% and 85.3 mL·g-1 to 42.3% and 371.8 mL·g-1 by increasing the temperature from 450℃to 600℃.3) Carbon gasification efficiency and hydrogen yield decreased by increasing the slurry concentration.50 wt% coal-water-slurry was continuously transported and stably gasified without plugging problems.3. The effects of additives on conversion of lignite and coking wastewater to hydrogen1) The addition of additives in high concentration coal-water slurry could effectively improve the Hydrogen yield and carbon conversion efficiency. The influence of adding Ca/C molar ratio of 0.15 much more greater than 2% of KOH2) The yield of hydrogen and carbon conversion efficiency were effective improved by adding Ca/C molar ratio of 0.15 and 2% of KOH compared to adding signgle additive, but the yield of CH4 decreases.