Бурение грунтовых зондов, установка энергетических колодцев

Solid effluents from gasification plants are essentially related to the ash in the feed­stock, the quantities of which can vary from as much as 40wt% in some coals to under 1 wt% for petroleum feeds. In many cases gasification plants are able to make the solid residue available in a form that can be used as a raw material in other industries.

In contrast to a PC boiler, gasification has no FGD sludge or gypsum disposal problem because the sulfur is all captured as elemental sulfur in the Claus plant. At present this is a saleable product, although one should be aware that, were a large part of the power industry to switch to gasification-based processes, this together with increasing sulfur production from oil refineries would oversaturate the market. Nonetheless, even in this case the amounts of material would be substantially smaller than with gypsum.

Dry Coal Ash. Dry ash from nonslagging gasifiers is essentially the same material as the product of a PC boiler processing the same feed. This ash can often be utilized in the cement or building industry. If this option is not available due to the nature of the ash or due to transport problems, the ash can become a major environmental liability that can be aggravated if the ash is leachable or caustic in nature. The ash is then often stored in large ponds, which have become more and more unacceptable. This problem can be largely avoided by selecting a slagging gasifier from which the quantity of dry ash waste is one to two orders of magnitude lower than from a dry ash gasifier.

Slag. Slagging gasifiers have in general an advantage over dry ash processes in that most of the ash components leave the gasifier as molten slag, which on being quenched turns into a fine inert gritty material that can be used as a replacement for sand and aggregate in concrete, for example. Van Liere, Bakker, and Bolt (1993) give the particle size of water-quenched slag as being about 78-80% in the range 0.5-4 mm in a paper in which they describe usage trials in Dutch road construction. Data from Geertsema, Groppo, and Price (2002) generally supports this size data for the actual slag component, although the material analyzed contained consider­able quantities of unreacted carbon, which after separation are recycled to the gasi­fier or used as fuel in a PC boiler. They report the slag as being used for blasting grit and roofing granules. Amik and Dowd (2001) provide an analysis for slag produced at the Wabash River plant (Table 9-7), which essentially mirrors the ash content of the coal used. The slag contains trace metals such as lead, arsenic, selenium, chrome, antimony, zinc, vanadium, and nickel, which is captured in the nonleachable glassy matrix. The results of some leachability tests are included in the paper. They report marketing for asphalt, construction backfill, and landfill cover applications.

Heavy Metals. The nature of any heavy metals leaving the plant as solid effluent will depend heavily on the feedstock. With a coal feed, most heavy metals will end up in the slag or fly ash of a gasifier. Heavy metals that are not removed together with the fly ash and slag will eventually end up in the wastewater from the water wash. As described on page 345 the heavy metals are removed from the water as a filter cake after treatment by flocculation and precipitation. With coal feeds this filter cake may contain such elements as arsenic, antimony or selenium. Unlike the slag, this concentrate is not inert and has to be considered as chemical waste.

In the case of oil feeds, the main heavy metals are vanadium, nickel, and iron. There are a number of processes available for recovering the ash for use in the metallurgical industry (see Section 5.4). The economics of such processes depends very much on the vanadium content in the ash.

Salt. All coal gasifiers will have dissolved salts in the treated wastewater. When this water cannot be disposed of, the water has to be evaporated and a salt mix is obtained that consists of over 98% NaCl. This product is often not suitable as road salt and may have to be considered as chemical waste.

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