using synthetic gas mixtures are considered to be bench scale.”

Spokesmen who affirm that industrial-scale reliability is now available, as well as spokesmen who deny it, often quote the NAE panel’s requirement of 1 year of operation at the 100-Mw scale. They argue whether or not this has been acheived in a particular unit, and whether or not the experience in this unit is generally applicable.

Not much attention has been paid to the other important ingredient by the panel as necessary to insure industrial process availability: the requirement that technical and economic data must be available to permit design of full-scale units to meet specific local and regional conditions.

The NAE panel did its work at a time when the chemistry of sulfur oxides scrubbing appeared far simpler than it does today. The panel considered 16 stack gas control procedures. A reflection of the subsequent advance in knowledge is the fact that 10 of the 16 were not represented by presentations at a meeting that EPA held in Atlanta in early November of 1974 to review the status of control technology. The Atlanta meeting considered 13 processes, of which 7 were not on the list of the 1970 NAE panel.

It should also be remembered that there have been expensive large-scale development failures in sulfur oxide emission control (see Table 11–1). One process, limestone injection into a boiler followed by a scrubber, that EPA urged upon utilities as late as early 1972 (Walsh 1972), is no longer being offered for sale.

The record would stand as an indictment of the engineering profession were it not for the fact, now evidient, that the engineer was compelled to press forward into design and construction of scrubbing equipment of unprecedented size in absence of adequate chemical knowledge. Never before had the chemical engineer been asked to treat such a large flow of gas even for a chemistry that was well understood. It is not surprising, therefore, that many of the early disappointments involved failure of large



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