Operators of coal-fired power plants and gas turbine combined cycle power plants can utilize municipal wastewater for cooling and other needs. When they also eliminate discharges through zero liquid discharge (ZLD) technology they become positive rather than negative contributors to water quality. Water lost through evaporation can be greatly reduced by heat extraction from the flue gas before it reaches the scrubber. This also reduces total CO2/MW and makes the plant greener in every sense.
The markets and technology for this approach will be discussed in a free webinar on February 25th at 10 a.m. CST.
The various geographical markets will be analyzed. China is embracing ZLD in its power plants as well as its coal-to-chemical facilities. Gas turbine combined cycle power plant developers in the U.S. are selecting ZLD not only in arid areas but generally to avoid the need to obtain water discharge permits. ZLD is the preferred option in Saudi Arabia and surrounding countries.
The availability and cost of treated municipal wastewater will be reviewed. In general, power plants are near enough to municipal treatment plants to make the use practical.
Technologies to minimize evaporation losses will also be discussed. The potential is being increased by new developments such as catalytic filtration at 850oF.
China is making the largest investment in ZLD. Huaneng Group, China’s largest power producer, has constructed a 1.3 GW state-of-the-art ultra-supercritical coal-fired power plant at the Changxing Power Station in Zhejiang Province.
- The plant uses Oasys’ technology to treat the wastewater from flue gas desulfurization (FGD).
- The system treats up to 650 cubic meters of wastewater a day. Startup was early 2015.
- The system uses forward osmosis separation technology.
Aquatech has supplied ZLD systems for a number of power plants and coal-to-chemicals plants. Enel has purchased several systems from Aquatech for its Italian coal-fired power plants. Aquatech has a Zero Liquid Discharge (ZLD) plant in a coal-to-chemicals facility in Inner Mongolia. The ZLD plant incorporates HERO™ (High Efficiency Reverse Osmosis) and a Thermal Brine Concentrator. The plant will treat 2400 m3/day of wastewater and reuse 92.5 percent of it, producing 2000 m3/day of process water and 220 m3/day of distilled water for various end user applications.
China has some ZLD systems using evaporators but is leaning toward the cheaper route of using flue gas to evaporate liquid in the sludge. The spray drier approach is being offered by Mitsubishi, URS and other international companies.
The various approaches for evaporation of water from slurries will be analyzed. One option is a falling film evaporator (inside the brine concentrator vessel) that is seeded with calcium sulfate to minimize scale formation.
Mechanical vapor recompression (MVR) is the method by which a blower, compressor or jet ejector is used to compress, and thus, increase the temperature of the vapor produced. In this way, the vapor can serve as the heating medium for the ZLD solution being concentrated. The efficiency and feasibility of this process lies in the efficiency of the blower or compressor and the heat transfer co-efficient attained in the heat exchanger contacting the condensing vapor and the boiling ZLD liquid. Several different blower and compressor options will be reviewed.
The discussion will also include the co-combustion of sewage sludge and the use of gasified sludges as reburn fuels in coal-fired boilers.
A full review of the options for coal-fired power plants is provided in 44I Power Plant Air Quality Decisions. A full review of options for gas turbines is part of Gas Turbine and Combined Cycle Decisions. Both of these services are free of charge to power plants and by subscription for others.