 

MEMORANDUM - DRAFT

DATE: 	December 12, 2007

SUBJECT:	Conversion of NOx, CO, and VOC to Concentration-Based Limits 

FROM:	Lucy Adams, Alpha-Gamma Technologies, Inc.

		Tanya Parise, Alpha-Gamma Technologies, Inc.

TO:	Jamie Pagán and Christian Fellner, EPA Energy Strategies Group

This memorandum provides concentration-based emission standards for
nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic
compounds (VOC) for stationary spark ignition (SI) engines subject to
the final Standards for Performance for Stationary Spark Ignition
Internal Combustion Engines.  The proposed rule (71 FR 33803) provided
emission standards in units of grams per horsepower-hour (g/HP-hr). 
Comments on the proposed standards suggested that concentration-based
emission limits would, in some cases, be a more appropriate and easier
approach to measuring engine emissions.  The emission standards in units
of g/HP-hr and parts per million by volume, dry basis (ppmvd at 15
percent oxygen (O2)) are presented in Table 1.  

Table 1:  Final SI NSPS NOx, CO, and VOC Emission Standards 

Engine Type 

and Fuel	Maximum 

Engine 

Power	Stage	Emission Standards



	g/HP-hr	ppmvd @15% O2



	NOx	CO	VOCa	NOx	CO	VOC

Non-Emergency SI	HP>25	Stage 1	2.0	4.0	1.0	160	540	86



Stage 2	1.0	2.0	0.7	82	270	60

Landfill/

HP≥130	Stage 2	2.0	4.0	1.0	160	540	86

aEPA proposed emission standards in terms of non-methane hydrocarbons,
but is finalizing these standards as VOC.

bThe emission standards applicable to emergency engines between 25 HP
and 130 HP are in terms of NOx+HC.

 	The following equation shows how the concentration-based emission
standards were calculated for NOx:

8,710 dscf/MMBtu	=	Natural gas fuel factor (from 40 CFR part 60,
appendix A,

				Table 19-2)	

46 lb/lb-mol		=	Molecular weight of NOx

385.5 dscf/lb-mol	=	Volume occupied by 1 lb-mole at 68°F and 14.7 psia 

%O2			=	Exhaust stack oxygen level of 15 percent

35%			=	Engine thermal efficiency

60 min/hr		=	Conversion factor

450 g/lb		=	Conversion factor	

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owever, based on available information, 35 percent efficiency is
believed to be representative of most engines.  

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