CEM 2014 Istanbul - Turkey True trace emission monitoring system
Flue Gas - Can be dirty, wet, sour, hot etc. - Some species are reactive, some are soluble, some are unstable in mixtures of others - However, most flue gases are easily handled, and can be conditioned, without loosing its integrity - Others require much attention when being conditioned, prior to analysis
In-situ analysis - The analysis is conducted in the media conditions, without sample conditioning. - But some analysers uses a sintered filter, to protect the optical path, and this must be considered as sample conditioning - For optical principles, the majority is near-infrared, or visible - Dominating background "noise" will be water vapor - The more water vapor present, the lower the transmission will become, and hence the noise level increases - Trace level measurements becomes more predictive, than accurate, and the need for longer integration times, increases
Extractive analysis - A system where the actual sample is taken away from the process itself, at process conditions, and transported to an adjacent unit, defined as a "Sample Conditioning System" - The "SCS" must be specific for the analytical application, to avoid losing sample information and integrity - True emission analysis, is a system that does not alter or modify the species, during the sample conditioning phase, prior to analysis
Sample Conditioning - The combustion process, generates more or less water vapor. The process might also include a wet scrubber, or contactor, which again will rise the water content. - A traditional method of reducing the water influence in a sample, is to chill it, using a condensing cooler unit. - However, the condensing unit will also absorb soluble species such as SO2 and NO2, HCl etc. - For trace level measurements, ranges 0-50ppm or less, the soluble content is a major source of error
Soluble species - For emission monitoring, focus are on SO2 and NOx (NO+NO2) 70 Solubility/Water 60 50 40 ppm 30 20 ppm SO2 ppm NO (1000x) 10 0 0 10 20 30 40 50 60 70 80 90 100 temp C
Water vapor removal - For true emission monitoring, water vapor should be removed in the hot state, while the water is still in vapor phase, and will not react with the soluble species. Hot & Wet sample Hot Oven Membrane Cold & Dry sample Dryer gas (low pp H2O)
Drying technologies - Condensers function by cooling a gas stream until water coalesce. They are very nonspecific; not only do they remove whatever gases condense at lower temperatures, but also at least a portion of whatever gases dissolve in the condensate. Condenser systems are designed to minimize the contact of the gas stream with the condensate to limit this deficiency, but water-soluble gases are always lost to varying degrees, depending upon the solubility of the gas in question. Large amounts of gases such as sulfur dioxide are lost by condensers, and condensers are entirely inappropriate to dry gas streams containing hydrogen chloride or chlorine (unless its removal is desired). - Desiccant dryers function by binding water to an absorbant. The absorbant may be a solid (such as silica gel) or a liquid (such as sulfuric acid) that binds water to its chemical structure as water-ofhydration. Unfortunately, like condensers, they are very nonspecific, and remove many compounds other than water of desiccant while a second chamber is used, and the chambers alternate operation and regeneration.
Drying technologies - Permeation dryers function on a principle of selection on the basis of molecular size. Permeation dryers are a microporous material. When forced under pressure across the surface of the microporous material, large molecules tend to remain in the gas stream while small molecules tend to move through the microporous material and are removed. Permeation dryers are very simple to operate, but are primarily suitable as air dryers. Nitrogen and oxygen are larger molecules than water, and so air can be dried by this method. Permeation dryers too non-specific to dry complex gas sample streams. - Nafion dryers function on a principle of selection on the basis of affinity for the sulfonic acid group. Although water passage through Nafion is described as permeation, Nafion dryers do not operate on the same principles as permeation dryers. Nafion is not a microporous material, separating compounds on the basis of their molecular size. For example, Nafion dryers can remove water from a hydrogen stream, even though the hydrogen molecule is much smaller than water. Pressure is not required to drive the process; the driving force for the reaction is the partial pressure of water vapor. Unlike the competing methods, Nafion dryers are highly selective in the compounds they remove.
Membrane technology - Differential partial pressure of H2O is the driving force of this membrane, not molecular size. - Inlet dryer gas must have a lower pph2o than inlet sample.
Water vapor removal - For trace level measurements of sulphurs and nitric oxides, we recycle the sample, reducing the overall dewpoint of the sample, and hence, the dryer oven specifications are considerably reduced. Hot & Wet sample Hot Oven Membrane Cold & Dry sample Dryer gas (low pp H2O)
Capacity Water vapor removal is massive. With a reasonable low oven temperature, we can remove as much as 54 mol% H20, or a dewpoint of +84 degc. The sample is dried to a level of 0,1 mol% H2O. 0.60 Measurement Variance vs. Moisture Content in Scrubber Flue Gas ppm Gas Reading 0.50 0.40 0.30 0.20 0.10 Moisture Content (x100) ppm SO2 variance reading 0.00 55 56 56 60 64 68 71 75 79 83 84 Water DewPoint (degc)
Typical trace level emission monitoring system For ppm trace level measurements (< 10 ppm) of SO2, H2S, COS, CS2, mercaptans, NO and NO2. On-line field operations since 1989 on wet scrubbed Kraft Mill (sulphate) flue gases (Paper & Pulp). Lloyds Register approval for Ship Exhaust monitoring, using HFO
Typical trace level emission monitoring system For the shipping industries, with for the use of Heavy Fuel, and other dark liquids from the refinery site, containing high sulphur content.
Norsk Analyse Group of Companies, serving the Process Industries, Shipping, and Oil & Gas Exploration: Norway Sweden Denmark Finland Turkey Trinidad, West Indies
by Ivar S. Olsen previous Technical Director at Norsk Analyse AS, Norway > 35 years of process analyser system integration experience If any further request for information, please contact me: ivar.olsen@norskanalyse.no or our CEMS group, cems@norskanalyse.no