Smells, scents, odours and aromas

In the holiday season, you might have found the time to relax and the opportunity to fill in a crossword puzzle. I noticed that puzzle composers often ask for a synonym for fragrance and it appears that there are quite a number of words that express a smell. A smell appears to be a sensation that remains very long in your memory. The smell of sulphur reminds me of romantic holidays in England of almost 40 years ago. In those days, coal was still frequently used as a fuel for cooking ranges and to heat homes. Smoke curling from the chimney of a bed-and-breakfast place often meant that a nice meal was being prepared for my wife and me. You can experience identical pleasant sensations when you enter a machine factory where turbines and engines are produced. The smell of evaporating cutting oil always reminds me of many interesting visits to the famous manufacturers of prime movers for decentralized equipment in the world. This smell is the same in Japan, Korea, the US or Europe, wherever you go. Scents, or whatever you call them, are important stimuli for your memory and can provide associations with pleasant events.

Recently, I had to write a chapter on the exhaust emissions of prime movers for a book on cogeneration, and also an article about the undesired species emanating from a chimney. During that writing, I realized that smells are increasingly banned from our society. It is true that some species in exhaust gases are harmful for human health and have negative effects for nature if their concentration is too high. The medical sector is gathering more and more knowledge about the relationship between species inhalation and health, especially in terms of long-term effects. Sensors and monitors check the local air quality and legislators try to find the best available emission abatement technologies for tightening the legislation. This is a good thing: if it is possible to avoid health problems and premature deaths, a rapid implementation of cleaning technologies is highly recommended. To be honest, I guess that any prime mover manufacturer would love to produce machines with zero emissions. That would make great advertising: use our equipment – zero emissions.

Yet I wonder if zero emissions is really what you should try to achieve. Civil servants are continuously searching for options to tighten legislation, sometimes only because there is a technique to further lower the emissions. An example is carbon monoxide. The toxicity of this species is about a factor of 150 lower than that of NO2, but most environmental standards have identical maximum values for CO and NOx. The concentration of CO in ambient air is very low nowadays because of the application of three-way catalysts and oxidation catalysts. This is in contrast with the past in most inner cities, where petrol-fuelled cars were notorious CO producers. The CO content of flue gases is often so low that one can easily stick one’s head into a chimney without getting carbon monoxide poisoning.

My recommendation is therefore to further reduce the exhaust emissions of prime movers only when it is necessary for health- and environment-related issues. Implementation of cleaning technologies just for the sake of getting lower and lower numbers is unnecessary and is only costing money.

A while ago, we moored our boat close to a bakery. When we woke up after a good night’s sleep, we smelled the wonderful aroma of freshly baked bread. Some policymakers now want to force bakeries to emit no smells at all. When I had almost finished this letter, my wife announced that lunch was ready. The smell of the good food was great and I would not do without it.

Dr Jacob Klimstra, Managing Editor

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