Method for monitoring and controlling the chemistry of a ZLD process in power plants

摘要

A method for monitoring and controlling the chemistry of a Zero Liquid Discharge (ZLD) process in power plants is described. The method identifies the principal phenomena of precipitation of calcium sulphate and calcium carbonate which can occur in such system and the principal critical sections affected by such precipitation phenomena. The interventions aimed at keeping or restoring the sustainable chemical conditions with respect to the precipitation of calcium sulphate and calcium carbonate in all the critical sections of the ZLD system are then indicated, ensuring saturation ratios of calcium sulphate and calcium carbonate lower than or equal to 1 through time.

主权项

1. A method for monitoring and controlling chemistry of a ZLD (Zero Liquid Discharge) system in a power plant, the power plant comprising a treatment plant for desulfurizing flue gas coming from said power plant, a treatment storage disposal unit (TSD) for treatment wastes coming from said flue gas treatment plant, a first fraction of a liquid effluent coming from said TSD unit being sent to an evaporation plant and a second fraction being recycled to a storage tank providing for a liquid make-up to said flue gas treatment plant, said evaporation plant comprising a softening unit and a crystallization-evaporation unit, said method for monitoring and controlling the chemistry of the ZLD system in the power plant comprising:
periodically sampling liquid streams circulating in predetermined, critical sections of said ZLD system, comprising an outlet section from the treatment storage disposal unit of the flue gas treatment plant, an outlet section from the softening unit, a feed section to the crystallization-evaporation unit, and a liquid make-up section from said storage tank to said flue gas treatment plant to determine chemical-physical parameters relating to calcium sulphate and calcium carbonate concentrations at said predetermined, critical sections; calculating saturation ratios relevant to calcium sulphate and calcium carbonate for each of said predetermined, critical sections; identifying the, critical sections which are subjected to a precipitation of calcium sulphate or calcium carbonate, wherein liquid streams passing through the critical sections have calculated saturation ratios for calcium sulphate and/or calcium carbonate higher than a prefixed attention threshold; and carrying out actions comprising variations of reactants/additive dosage to said crystallization-evaporation unit and/or to said treatment storage disposal unit or variation of a ratio between flow rates of said fractions of the liquid effluent coming from said TSD unit, said variations being such to maintain or restore suitable chemical conditions with respect to precipitation of calcium sulphate and calcium carbonate, ensuring saturation ratios for calcium sulphate and calcium carbonate is kept lower than, or equal to 1 through time.