Liquid Effluents
Whether a gasification complex is a net water consumer or producer will depend on the feedstock and the downstream operation. In general, when coal or petroleum residues are used as feedstock, there is always a net consumption of water, whereas with natural gas-based plants, there can be a net production of water. Similarly, the application, chemicals or power, will have an influence over the overall water balance of the plant.
Most gasification plants have a water wash at some part of the syngas treatment. For coal gasifiers, its main purpose is to remove ammonia and chlorides, though many other constituents of the gas are also captured. Whether this process water is acidic or basic will depend on the amounts of nitrogen and chlorine in the coal. Whatever the pH of this water, it will almost certainly require to be adjusted during the flocculation step of the overall water treatment. In oil gasification plants, the main objective is removal of soot, but the water also contains ammonia, hydrogen cyanide, and H2S.
In practice, with appropriate design, an IGCC can be made with zero liquid discharge, whether using a dry feed technology as in Buggenum (Coste, Rovel, and George 1993) or a slurry feed as in Polk (U. S. Department of Energy 2000). In both these plants the final water treatment stage is a brine concentration and evaporation unit producing a solid waste salt.
Oil gasification units generally do not recycle the excess process water. In those applications, where the plant is located in a refinery, the water treatment is generally integrated into that of the overall refinery after ammonia, HCN, and H2S have been removed in a sour water stripper. Typical emission limits and performance for a stand-alone plant are given in Table 9-6
Specific aspects of water treatment are addressed in the following sections.
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Table 9-6 Emission Limits and Oil-Based IGCC Performance
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Fluorine. Fluorine in the gas will dissolve in the wastewater from the water wash, from which it can be removed by adding calcium ions that will precipitate the fluorine as CaF2. This salt will eventually end up in the same settler/filter cake as the heavy metal-containing precipitate from the flocculation unit.
Cyanide and Cyanometallates. A significant portion of the HCN produced in the gasifier is contained in process condensate. In any plant gasifying heavy oil, the excess water from the gasification section is usually stripped to remove free ammonia, H2S, and HCN. Typically, residual HCN values of 10-20 mg/1 can be achieved in a single-stage stripper. The residual HCN can be reduced to below 1 ppm in a biological treatment unit.
An alternative approach to cyanide and cyanometallates is to oxidize them. This is particularly appropriate when recycling all the process condensate for a zero- discharge system. Although aeration in a closed vessel is possible and has been employed, the required contact times are long and require large volumes. A more economic approach is the use of ozone as an oxidant, since this reacts rapidly with the cyanides (Coste, Rovel, and George 1993).
Heavy Metals Precipitation. Whether gasifying coal or heavy residues, there are heavy metals contained in the process condensate. Typically, these are subjected to treatment by flocculation and precipitation. The choice of flocking agent is determined by the metals to be removed, but is typically ferric chloride. The metals sludge from the precipitation stage is then thickened prior to filtration.
The excess water so treated would be “free of fluorides, cyanides and heavy metals and contain only dissolved soluble salts” (Coste, Rovel and George 1993). Sand filtration may be required to remove traces of metal hydroxides from the flocking step.