Method and Apparatus for Gasification Wastewater Treatment

#As the coal-based power generation industry continues to evolve, there is a greater emphasis on strategies that limit or prevent the emission of harmful chemicals into the atmosphere. The coal-fired power plants that were once common are now being developed in limited quantity as future power needs will more preferably be met by clean coal technology, or through the use of other fuel sources such as natural gas and biomass, or through renewable energy forms such as solar and wind.

#Gasification technology is one such technology that has been developed and applied in recent decades. In the gasification process, a carbon-bearing feedstock is converted to a mixture of hydrogen, carbon monoxide and carbon dioxide. This gas mixture is called synthesis gas (or syngas) and can be combusted as fuel for power generation. Other beneficial uses of syngas include methanol, hydrogen, hydrocarbon production, among others.

#Gasifiers function by heating the carbon feedstock to very high temperatures which drives the various chemical reactions forward for the production of syngas. One of the undesirable byproducts of the gasification process is the production of “black water.” Black water contains residual carbon-based suspended solids, other colloidal particles, as well as dissolved impurities. The suspended solids present in this stream are removed and the resulting wastewater is blended with other wastewater streams from the facility and sent to a water treatment system. This stream is referred to as “grey water.”

#FIG. 1: Conventional Technology: Gasification with treated water for discharge.

#FIG. 2: Conventional Technology: Gasification with partial evaporation and concentrated brine discharge.

#FIG. 3: Technology Claimed: Gasification with Crystallizing Reactor for zero-liquid discharge.

#Embodiments as reported herein include a crystallizing reactor that modifies the primary dissolved components into a form which is conditioned and amenable to concentration and precipitation. Salt forms that would otherwise be very highly soluble and resistant to concentration are converted into a form that can readily be precipitated at low to moderate concentration and subsequently dewatered to achieve zero-liquid discharge.

#We expect that embodiments of this new process may, but are not required to yield one or more of the following benefits for the technology compared to the conventional processes: Zero-liquid discharge is achieved, which eliminates wastewater discharge and the corresponding danger of constituents of concern entering local bodies of water. The influent dissolved solids are ultimately converted to a suspended solid and dewatered for solids disposal in a landfill. Cleaning frequency of equipment is significantly reduced since the evaporation process is substantially non-scaling; that is, a crystallizing reactor is typically designed for chemical reaction and precipitation and does not experience scaling like a conventional evaporator. Conversion of dissolved solids to low solubility salts allows the process to operate reliably and with high availability. Heat transfer is done very efficiently by limiting the viscosity of the solution by eliminating the highly soluble salt species.

#Another design feature of the process is the ability to reduce the corrosion potential by modifying the operating chemistry. This can be done, in one embodiment, by adding chemicals such as Ca(OH)2 to maintain a pH greater than 11.0. In some embodiments the pH is between 10-12. In other embodiments the pH is between 10.5 and 11.5. In further embodiments a suitable chemical is sodium hydroxide or strontium hydroxide. In typical embodiments the added chemical is hydrated lime or caustic soda. This mode of operation would allow the evaporation process to be constructed with leaner metal alloys (lower cost) since the corrosion potential of the process can be substantially reduced.