Blog Post #1 - Technology Notes
INTRODUCTION - This is the first posting on the AirTech Innovations blog. In writing this blog, we are striving inform our readers on a wide variety of topics about the treatment of air and other gases. We hope that the published information that will be of practical value to engineers and operators concerned with meeting gas cleaning performance or handling objectives. This includes air emission control and process applications. While the thrust of the blog will be technical, we will try to keep the discussions informal and relevant to "real world" applications. We hope that the material published in the newsletter will stimulate discussion among our readers. Please feel free to contact us with your comments, questions and criticisms.
SULFUR SUBLIMATION - In working on the development of a process to control emissions from a mineral wool cupola we learned of the interesting phenomenon of sulfur sublimation. Certain forms of elemental sulfur will sublime (vaporize directly from the solid phase) at temperatures above 200°F. This can present a perplexing problem in processes where elemental sulfur is being collected by a fabric filtration device. At elevated temperatures sublimed sulfur vapor can pass through the filter medium and condense in the air to form a noticeable, white plume. This will have operators looking for bag leaks that don't exist. If this occurs in your baghouse, look for the presence of elemental sulfur, a yellow dust, in the baghouse catch.
SAMPLING ACID GASES - Current EPA-sanctioned test ,methods for particulate concentration in stack gases can yield inaccurate results when HCl is present and the gas stream is at or near the saturation point. Even low concentrations of HCl, e.g. 50 ppm, can attack the metal nozzle on the sampling probe and produce probe-wash weights that are significantly greater than the true particulate captured on the probe. In a recent experience we were faced with just such a problem. The probe rinse residue (glass probe, 316 ss nozzle) was an order of magnitude higher than the filter catch. Changing to an Inconel nozzle was not much help; significant quantities of metal chlorides were still present in the probe rinse residue. Finally, the equipment was modified with a glass nozzle and the problem was solved.
LIMITS OF THE DEUTSCH EQUATION - Virtually everyone involved in the design of electrostatic precipitators is familiar with the Deutsch Equation and its variations:
The Deutsch Equation:
Efficiency = 1 - exp(-AW/Q)
where,
A = collecting area
W = particle migration velocity
Q = gas flow rate.
The equation, by nature, predicts that no precipitator can be 100% efficient. This is true in real world application even if rapping and re-entrainment losses are eliminated. What many people do not realize is that the Deutsch Equation is based on the assumption of turbulent flow, which, in turn, precludes the possibility of 100% capture. However, if a precipitator operates on a laminar gas stream, then 100% efficiency is both theoretically and practically possible. A good example of this are household air cleaning precipitators which, with very narrow gas passages, approach a laminar flow situation. These devices typically demonstrate performance, in terms of efficiency per unit of collecting area, that is significantly greater than the performance of even the best turbulent flow devices.
HOW CLEAN IS CLEAN - We have frequently heard the comment that "the stack is cleaner than the ambient air". To see how true this is we have prepared the following table regarding particulate emissions.
| Source | Concentration |
|---|---|
| Uncontrolled pulverized coal boiler: | 5.0 gr/scf (11,350 mg/Nm3) |
| Typical performance standard: | 0.02 gr/scf (45 mg/Nm3) |
| Pulverized coal boiler w/multi-clones: | 0.2 gr/scf (454 mg/Nm3) |
| Typical performance standard: | 0.02 gr/scf (45 mg/Nm3) |
| Best expectable outlet w/baghouse: | 0.002 gr/scf (45 mg/Nm3) |
| Unattainable with traditional technology: | 0.0002 gr/scf (4.5 mg/Nm3) |
| NAAQS secondary air standard: | 0.0000066 gr/scf (15 µg/m3) |
| NAAQS primary air standard: | 0.0000052 gr/scf (12 µg /Nm3) |
| "Good" room air: | 0.0000022 gr/scf (5 µg /Nm3) |
Obviously, “clean” room air is much cleaner than any stack emission.
