March 11, 1992



MEMORANDUM

SUBJECT:   Modeling Credits for Stack Height Increases and       

           Merging Flue Gases at Taunton Municipal Light Plant

FROM:      Dean A. Wilson, Meteorologist
           Techniques Evaluation Section, SRAB (MD-14)

           Eric Ginsburg, Chief
           SO2/Particulate Matter Programs Branch, AQMD (MD-15)

TO:        Ian Cohen, Regional Meteorologist
           Technical Assistance Section, Region I

           Brian Hennessey, Regional Modeler
           Technical Assistance Section, Region I


     In response to your request, the Model Clearinghouse has
reviewed your position concerning Taunton Energy Center's request
to seek dispersion credits for a physical stack height increase
and merged flue gases for Taunton Municipal Light Plant (TMLP).  

     Your interpretations of the regulations as they apply to the
TMLP are consistent with EPA policy.  The request for dispersion
credit for the stack height increase must be justified by fluid
modeling.  The current stack height is considered to be Good
Engineering Practice (GEP) for the source unless demonstrated
otherwise; any additional stack height is therefore an increase
in excess of GEP.  Given the fact that TMLP emits less than 5000
tons of SO2/yr, the source may be eligible for a merger of gas
flues and could then model SO2 emissions using a single output
stream.  Yet, this exemption clause does not pertain to other
criteria pollutants.  In order to justify modeling TMLP's NO2 and
PM-10 emissions with merged gas streams, the source must install
pollution controls which result in a net reduction of allowable
emissions.   

     Regarding the issue of increment consumption, the New Source
Review Section (NSRS) has informed us that the Regional position
is consistent with guidance provided in the 1990 draft New Source
Review Workshop Manual (see page C.11, concerning stack height
changes occurring after the minor source baseline date).

     If you need additional information, please contact Dennis
Atkinson at FTS 629-0518 for modeling concerns or Gwen Jacobs at
FTS 629-5295 for questions relating to stack height policy.




cc:  D. Atkinson
     G. Blais 
     D. de Roeck
     J. Dicke 
     G. Jacobs