Solving the Problem of Low Level NOx measurements in a Hot Wet Sample Stream


An automotive catalyst manufacturer in Southern California had major difficulties with their NOx measurement and calibration due to moisture effects until they installed a Perma Pure GASS-II into their emissions monitoring system.

The South Coast Air Quality Management District (SCAQMD), which encompasses the greater Los Angles area, has the toughest air quality standards in the nation. In 1994, they promulgated the Reclaim Act, an innovative, market-based set of air emission regulations that allows trading of emission ‘credits’. A significant key to the success of this program has been an increased focus on the monitoring equipment used in collection and reporting of emissions data, as opposed to simply concentrating on the pollution control systems. SCAQMD has taken an active role in evaluation the ‘best available technology’ for control and analysis and will make recommendations to local sources as to how they can best meet these standards.

TABC is a wholly owned subsidiary of Toyota Motor Manufacturing USA. They operate a manufacturing plant in Long Beach, Ca that produces truck beds and other sheet metal components, as well as catalytic converters and coated catalytic substrates. This proprietary coating process generates NOx as a by-product. The levels were high enough (in excess of 1,000ppmv) to require the addition of a high efficiency pollution control system in order to satisfy RECLAIM requirements. A multi-stage NOx control system, the Tri-NOx (Tri-Mer Corp.), was installed based on SCAQMD’s recommendation. This new equipment reduced these emissions to less than 50 ppmv.

Once the Tri-NOx system was operational, the plant was confronted with a new problem: unreliable effluent gas analysis. NOx measurements were a constant headache. They were frequently failing their daily calibrations; the analyzers couldn’t achieve 90% of the span gas concentration within the specified time period. Secondly, NOx measurements fluctuated wildly, even when other process parameters indicated steady state operations. Formal re-certification of the system had to be performed every six months, with the company barely able to meet the relative accuracy limits and narrowly avoiding fines for their inability to prove compliance. Lack of confidence in the measurements prevented the analyzers from being fully integrated into control of the Tri-NOx process.

Measuring NOx at low concentrations (less than 100 ppmv) is not trivial, but becomes very difficult in wet sample gas streams. Nitrogen oxides (NOx) are a combination of Nitrogen oxide (NO) and nitrogen dioxide (NO2). Nitrogen dioxide is extremely water soluble and has the potential to be lost in a wet gas stream. When water is removed from the gas stream via condensation, this created the ideal environment for NO2 losses. The main by-product of the TABC catalyst manufacturing process is nitrogen dioxide. The first stage of the Tri-NOx system is a water quench, and the second is an oxidation stage whereby NO is converted to NO2: the result is a very wet (15% by volume) stream containing a low concentration of a very soluble pollutant.

The original sample conditioning system TABC used was a thermoelectric cooler that removes water by condensation.

This equipment will bring the sample dew point down to about 40F, which corresponds to about 1% water by volume. This has been the commonly accepted method of drying sample gases in most environmental monitoring systems. However, TABC’;s difficulties with their NOx measurement were finally attributed to excess moisture in the sample. SCAQMD recommended that they install a Perma Pure GASS system utilizing Nafion™ membrane dryers. The GASS-II had been successfully utilized at other industrial plants in the Los Angeles basin. Previous studies conducted at Brookhaven National Laboratories and the Gas Research Institute confirmed that Perma Pure dryers allowed more accurate analysis of nitrogen oxides than condenser based systems.
The GASS-II is complete sample conditioning system that filters, dries, and cools hot, we44t gases prior to analysis. Unlike coolers, GASS drying technology selectively removes water in the vapor phase. This eliminates losses of soluble compounds like NO2. The drying material is a corrosion-resistant membrane that can continuously operate at elevated temperature (up to 250F). This system can also remove more water vapor than a cooler, further reducing analytical problems associated with moisture. The system is mechanically simple and has problem to be quite reliable in numerous installations worldwide.

Once TABC replaced their cooler with a GASS-II system, dramatic improvements were seen. NOx measurements stabilized due to improved sample integrity. The GASS-II system also dried the sample down to a dew point of 0F, or about 0.1% water, so there was not 90% less residual moisture than in the previous system. The sample lines dried out, and consequently TABC began to routinely pass their daily calibrations. Confidence in their analysis allowed them to use their data in process control, resulting in more accurate caustic additions in the Tri-NOx. Because of these improvements, TABC can now re-certify their analyzer system annually, as opposed to every six months. Cost savings were realized in materials, labor and testing. The system was installed in August 1996 and has been working flawlessly since then.

The GASS family of systems have been used in many challenging sample conditioning applications It is the best available technology to filter and dry a gas sample. It is the only choice when accurate measurements of low concentrations of soluble compounds are required, such as the case with TABC.

Perma Pure wishes to thank Tim Scott of PE Systems for his efforts in getting the GASS system installed at TABC.

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