| 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™-II 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-II conditioner utilizes a membrane gas dryer to remove water. This
dryer consists of tubing made of Nafion®, 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-II 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-II system offers an adaptable, easy-to-use
solution to gas sample conditioning applications. |
