Acid Rain in a Pipe

One of my frustrations watching the so called march of technology is that the whole problem of smoke stack pollution is readily solvable. Yet we have stayed with old systems, if any are used at all, that only partially ameliorate the problem. We have even exported our smelters offshore and wink at the horrific and noxious pollution thrown into the atmosphere.

Our coal burners currently use a fluidized bed that is charged with limestone. The limestone reacts with the sulphur to produce gypsum while absorbing some heat. This is good for about 60% of the sulphur and little else. Most such gypsum ends as waste. Not a great solution.

In the late seventies I met a technologist who had the simple insight that since natural ozone produces acid rain in the first place, it may be possible to achieve the same result in the stack using the best and fastest oxidizer possible. That is chlorine gas. He patented the idea and became its champion.

He met me and I persuaded him to run proper bench tests under the auspicious of the University of British Columbia. This ensured that the results would be credible. After that he continued to champion the protocol with little additional progress, in part due to his own business perspective.

What we developed was a very promising protocol.

The flue gas, whether from a smelter or a coal burner is well over 600 degrees when it exits the combustion chamber. It is also traveling fast. At this point water is sprayed into the gas stream along with chlorine gas. This produces hydrochloric acid in the gas stream. This acid reacts preferentially with the SOx first and secondly the NOx converting them into first sulphuric acid and then nitric acid. And the surplus hydrochloric acid is sponged up by the CO2 to produce some carbonic acid. These acids continue to additionally react with any metals in the flue stream converting them into salts that are usually soluble.

Our bench tests confirmed the implied stoichemistry of the reactions and showed a complete reduction of the SOx and NOx in the flue gas.

The spent flue gas was then sent through a water quench to sponge up any excess chlorine and to strip the heat, acids and salts out of the stream. This also would collect most of the particulate. The end result is a clean stack gas that is primarily CO2 and nitrogen.

In the heyday of the Acid Rain scare, a literature search search isolated over 150 separate strategies been explored, all stuck with slower reaction speeds than we could easily achieve with chlorine gas.

What I have just described is an aggressive reaction protocol that can be tuned nicely to be fast and efficient. The capital cost to implement this procedure is minor for a new plant and likely very doable as a retrofit for older facilities. We are only engineering a reaction chamber for the flue gas.

The waste is in the form of a hot solution of acids and salts in addition to the particulate already handled. The solution mix would be run through a small acid plant that would recover the chlorine, and produce both sulphuric acid and nitric acids as salable products. The salts would also presumably be recovered at least as a blend for later processing off site.

The total consumables for a typical power plant would be around one carload of chlorine gas per year.

Of course, even the scientifically literate shy away from the word chlorine, making this protocol a hard sell. But it is the real solution to our second major source of atmospheric pollution.

As an aside. Ozone is likely as good. The problem is producing pure ozone. The plasma arc produces mostly nitrous oxide rather than ozone which is very counter productive. Other methods of producing ozone are costly compared to chlorine.

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