Because of the variability of compounds and low concentrations in each emission stream, it is not reasonable to size a control device to minimize emissions of all compounds equally well during all inlet conditions. To optimize the minimizing of emissions, a target compound is used.
The target compound is the compound within an emission stream grouping that has a high usage rate and low effects screening level (ESL). This compound is selected using a two-step approach as follows:
- The three compounds with the highest monthly usage rates are identified.
- From these three compounds, the compound with the lowest ESL is selected as the target compound.
By sizing the control device around a target compound, the control device is more efficient at minimizing emissions of compounds with high usage rates and high potential for potential adverse health effects. This method differs from the idea of designing a control device around the compound with the lowest ESL.
This column also gives companies two ways to approach the design of the control device. If the highest expected inlet concentration for all nonhalogenated organic compounds is 50 ppmv or less, then it would not be beneficial to burn large quantities of natural gas just to achieve 95 percent destruction and removal efficiency (DRE). Instead, the company has the option of designing the control device to meet the outlet concentration limit.
The use of a target compound for design purposes implies that not all compounds will be controlled as efficiently as the target compound.
The DRE for thermal oxidation and Removal Efficiency (R.E.) for adsorption were determined by an internal TCEQ tier I review of semiconductor permits issued since 1994. The R.E. for absorption was taken from the TCEQ Technical Guidance Package for Chemical Sources - Absorption Units dated May 1995.
The design limit of 20 ppmv is taken from the Control Requirements in TCEQ Rule 115.122 (Vent Gas Control). This rule requires VOC streams from polypropylene and polystyrene plants to be controlled to 20 ppmv. Therefore, it is assumed that if 20 ppmv is an acceptable means of minimizing air emissions, then 20 ppmv is a reasonable outlet concentration limit for a similar VOC stream from a semiconductor facility.
Due to the requirement for ultra-clean manufacturing rooms, a 100 percent capture efficiency is assumed for all add-on controls. Therefore, a capture efficiency design criterion is not addressed.
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