Calcium Silicate



Cement 3.3MMT/yr

Inc. for the US Environmental Protection Agency (Motley and Cosgrove 1978). The idea was motivated by the fact that flue gas desulfurization (FGD) technology was just being introduced by coal-burning electric power plants. The technology then being adopted was lime/limestone scrubbing, which captures sulfur dioxide quite effectively but generates large quantities of calcium sulfite/sulfate wastes. The TRW study evaluated a possible use for these wastes.

The scheme was based on a conceptual coal-burning power plant generating 1000MW, which generates 1MMT/yr of lime/limestone scrubber wastes. The core of the scheme would be a sinter plant in which the sulfate sludges react with carbon monoxide produced by burning coal (273kMT), clay (300kMT/yr) and soda ash (12kMT/yr), to yield soluble sodium aluminate, dicalcium silicate and hydrogen sulfide. These, in turn, are processed by standard means (indicated briefly in the description of the kombinat above), to yield calcined alumina (70kMT), elemental sulfur (156kMT) and dicalcium silicate (625kMT). The latter, in turn, is the major ingredient to produce 850kMT of Portland cement. At typical market prices, this scheme appeared to be viable, or nearly so. It would certainly be viable given a realistic credit for FGD waste disposal.

Another interesting proposal for an industrial ecosystem comes from Poland (Zebrowski and Rejewski 1987). It is actually a set of interrelated proposals utilizing two basic technologies that have been under development in Poland. The first is coal pyroly-sis in the gas stream (PYGAS), a patented technology,2 that has already been adopted at several Polish industrial sites. It is particularly suited to upgrading existing power plants at minimal capital cost. The basic idea is to feed powdered coal into a hot gas stream (about 800°C) where it pyrolyzes very rapidly (in the order of one second), and pyritic sulfur also decomposes at this temperature. The gas stream passes through a cyclone, where desulfurized carbon char dust is collected and removed. It is usable as a direct substitute for powdered coal in the boilers. Some of the gas is recycled. The pyrolysis gas can be desulfurized and burned or used as feedstock for chemical processing. The second building block is a technology derived from PYGAS for pyrolysis of recycled gas streams, PYREG, specialized to the case of lignite. It has been developed to the large-scale laboratory test stage at the Industrial Chemistry Research Institute (ICRI) in Warsaw.

The idea is not qualitatively different from numerous other proposals for coal gasification, but the authors have given careful consideration to the use of these technologies to integrate existing disconnected systems, especially with respect to sulfur recovery and fertilizer production. This concept is called the Energo-Chemical PYREG site, or simply ENECHEM. The base case for comparison would be a surface lignite mine (18MMT/yr), with 0.5 per cent sulfur content. This would feed a power station generating 2160MW of electricity. Lignite in Poland (and central Europe generally) contains 2 per cent -10 per cent xylites (5 per cent average). Xylites are potentially useful organic compounds related to xylene (C6H4(CH3)2), which are not recovered when lignite is simply burned.

In the base case, annual wastage of xylites would be 900kMT. By contrast, PYREG technology permits the direct recovery of xylites in the form of high-grade solid fuel (semicoke, 200kMT/yr), fatty acids and ketenes (65kMT/yr) and gaseous aromatics (benzene, toluene, xylene or BTX), which are normally derived from petroleum refineries. The proposed ENECHEM site would include a power station, but instead of burning lignite directly to generate 2160MW as in the base case, it would gasify the lignite, via PYREG, as shown in Figure 5.2, yielding semicoke powder plus volatile hydrocarbons, tar and phenolic water. The semicoke powder would then be burned in the power station (generating 1440MW, and emitting about 97kMT/yr SO2). Sulfur recovery in PYREG technology is only 40 per cent-60 per cent, but since Polish lignite has a very low sulfur content (0.5 per cent) this is not considered to be a major disadvantage.

Figure 5.2 Lignite-burning power plant modified via PYREG

The volatile hydrocarbon fraction of the PYREG output would be desulfurized - by conventional Claus technology - yielding about 67kMT/yr of elemental sulfur (S). The condensibles would be separated as liquid propane gas (LPG) for domestic use (150kMT/yr). The non-condensibles, consisting of methane and ethane or synthetic natural gas (SNG), would be available as a feedstock to any natural gas user, such as an ammonia synthesis plant (318kMT/yr). The tar from the PYREG unit could be refined much as petroleum is, yielding liquid fuels and some light fractions (C2-C4) that would go to the gas processing unit. The yield of gasoline and diesel oil would be 430kMT/yr and 58kMT/yr, respectively, plus 75kMT/yr of heavy fuel oil. (Obviously, the tars could be a supplementary feed to a co-located conventional petroleum refinery, but the incremental outputs would be much the same.) The phenolic water would be processed to recover phenols (13kMT/yr), cresols (27kMT/yr) and xylols (26kMT/yr).

Obviously, the details of ENECHEM could be varied considerably, but the scheme as outlined in the previous paragraph would reduce sulfur dioxide emissions by roughly half (from 180kMT to 97kMT). It would produce less electric power but, in exchange for a reduction of 720MW, it would yield 318kMT (400 million cubic meters) of SNG, 150kMT LPG, 430kMT gasoline, 480kMT diesel fuel, 75kMT heavy fuel oil (less than 1 per cent S), 66kMT of phenols, cresols and xylols, and 67kMT of sulfur (99.5 per cent). This is shown in Figure 5.3.

NaCl Crude Oil 4.5MMT

NaCl Crude Oil 4.5MMT

20 million Nm3/y

Figure 5.3 Systems integrated with ENECHEM with additional plant for xylite processing

20 million Nm3/y

Figure 5.3 Systems integrated with ENECHEM with additional plant for xylite processing

A recent proposal by Cornell University to the US Environmental Protection Agency differs sharply from the schemes outlined above. Instead of focusing on utilizing an existing natural resource more efficiently, it would attempt to assemble the elements of an industrial ecosystem around a municipal waste treatment facility. In other words, it is essentially a scheme to 'mine' wastes per se. The proposal points out that in the 1970s a number of facilities were built with the idea of reducing landfill volumes by recovering the combustible fraction, along with ferrous metals, and converting it to a refuse-derived fuel (RDF), to be sold to a local utility to help defray the costs of operating the facility. Many of these facilities operated only briefly or not at all.

The Cornell proposal would extend the earlier waste treatment concept in two ways. First, it would include not only municipal wastes but also a variety of other industrial wastes for a whole county. Second, it would employ advanced technologies to produce a number of salable by-products, one of which would be fuel gas. (Nevertheless, its success would still depend on one or more utilities that would undertake to accept the gaseous fuel generated.)

Also, in contrast with other schemes outlined in this chapter, it would involve no detailed prior planning of the site or the technology to be used, beyond the creation of an organizational structure to seek out potential participants. This approach is almost mandatory, at least in the USA, where central planning is virtually anathema today. Nevertheless, the proposal (if supported) would offer some useful insights as to how a cooperative entity might be created from essentially competitive, independent production units - or, indeed, whether this is possible.

A final example might be COALPLEX, first proposed by the author some years ago (Ayres 1982) and revised more recently (Ayres 1993c, 1994b). It too would be coal-based. Like the Polish scheme it would start with gasification of the coal, recovering sulfur for sale and using the coal ash as a source of alumina (and/or aluminum) and ferrosilicon (Figure 5.4). The most attractive version - albeit somewhat theoretical - would utilize the direct (hydrochloric) acid leaching process for aluminum chloride recovery and the ALCOA process for electrolysis of the chloride. The gasified coal would be (partly) burned on site to produce electric power for the aluminum smelter and electric furnaces. A variant would also produce carbon anodes for the aluminum smelter from coke made from gasified coal (instead of petroleum coke). There are, in fact, a number of possible variants, none of which have been adequately analyzed to date.

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