Comparison of SO2 Losses in Continuous Emission Monitoring Systems

Sulfur dioxide solubility in water as a function of temperature
Temp (°C) SO2 solubility in H2OmL SO2/100mL H2O
01
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
35
40
90
100		 79.877.2
74.7
72.2
69.8
67.5
65.2
63.0
60.8
58.7
56.6
54.7
52.7
50.8
49.0
47.3
45.8
43.9
42.4
40.8
39.4
38.0
36.6
35.3
34.0
32.8
31.6
30.4
29.3
28.2
27.2
22.5
18.8
2.1
0.0

Continuous emissions monitoring systems (CEMS) to trace the release of acid rain gases are a necessity due to the mandates of Title IV of the Clean Air Act. Unfortunately, theses systems are often a disagreeable necessity due to reliability and performance problems related to poor gas sample conditioning. Modern gas analyzers work quite well when given a cool, clean, dry sample, but preparing the hot, dirty, wet sample from a stack for the analyzer without changing the concentrations of the gases of interest can be challenging. The GASS™-2040 gas analysis sampling system, figure 1, is designed to be a solution to this problem.
The main problem is typically removal of water. In the past, the preferred sample cleanup methods were to dilute with clean, dry air to lower the sample dew point or to condense the water with a chiller system then drain it away. Dilution is increasingly a disfavor because it can be difficult to detect low levels of acid rain gases after their concentration has been reduced 200 times. Chillers have difficulty processing sample that are highly corrosive or those that have a very high water content or flow rate. Furthermore, since chiller/condensation systems typically operate above freezing, at least 0.6% water remains in the sample under the best circumstances.
Chillers also drain away water-soluble acid gases such as sulfur dioxide (SO2) and hydrogen chloride (HCl) along with the water. HCl is so soluble in water that it is impossible to use chillers to prepare a sample for analysis of it; all of the HCl is removed along with the water. High-temperature analyzers that keep the gas sample hot until after it is analyzed have been used to circumvent this problem, but they are very expensive and are plagued with reliability problems. Significant amounts of SO2 are lost when water is condensed, since it is more soluble in cold water (see table).
The GASS-2040 conditioner utilizes a membrane gas dryer to remove water. This dryer consists of tubing made of Nafion™ polymer, a corrosion-resistant polymer that is selectively permeable to water. As the sample flows through the tubing, water is removed indirectly from the vapor phase, so no condensation forms to dissolve water-soluble compounds. In addition, the tubing permeability is so highly selective that acid rain gases and most other compounds monitored in stack gases remain the sample while water is removed. The GASS-2040 conditioner incorporates this dryer technology into a system that controls the sample temperature as it removes water and particulates, producing a cool, clean, dry sample for the analyzer,. It accepts flow rates up to 25 lpm and water concentrations up to 75%, yielding final dew points as low as -25°C( 600 ppm of water).
The system is suitable for CEMs applications in major industries such as refineries, pharmaceutical, incinerators and utilities. Lower sample dew points, less analyzer maintenance, and more accurate analysis for water-soluble gases such as sulfur dioxide and nitrogen oxides are the result. The system has no moving parts and requires little energy to operate; it can be mounted on the stack in close proximity to the sampling probe, eliminating heated sample lines and the attendant maintenance problems. Once the sample is dry and cool, it can be transported through unheated lines.
As monitoring requirements become more stringent, and dilution or condensation systems for CEMs prove more problematic, the GASS-2040 system offers an adaptable, easy-to-use solution to gas sample conditioning applications.