ncasi 

NATIONAL COUNCIL FOR AIR AND STREAM IMPROVEMENT, INC. 

P.O. Box 13318, Research Triangle Park, NC 27709-3318 

Phone (919) 941-6400 || Fax (919) 941-6401 

Vipin K. Varma, PhD 

Vice President – Air Quality 

vvarma@ncasi.org

January 8, 2013 

Mr. Michael S. Hannan 

Office of Atmospheric Programs, Climate Change Division 

US Environmental Protection Agency 

1310 L Street NW 

Washington, DC 20005 

Dear Mr. Hannan, 

This is in response to your email dated December 20, 2012 to Ms. Nancy Clark of AFPA, requesting 

written authorization from NCASI to upload Technical Bulletin No. 998, Methane and Nitrous Oxide 

Emissions from Biomass-Fired Boilers and Recovery Furnaces in the GHGRP docket. By way of this 

letter, NCASI grants EPA written permission to upload the above-mentioned Technical Bulletin to the 

docket, in connection with the petition for reconsideration of the CH4 and N2O emission factors in the 

GHGRR. 

Additionally, you requested clarification on (a) the statement in page 27 of the Technical Bulletin 

discussing improper sampling techniques for Nitrous Oxide (N2O) and (b) how this relates to the Fortum 

2011 report referenced in the Technical Bulletin. Our responses are provided below; 

• As discussed in p.27 of the Technical Bulletin, unconditioned grab samples, collected from 

combustion sources and stored prior to analysis, can generate N2O and yield positively biased 

emission numbers. Muzio et al studied this phenomenon and reported that combustion gases 

containing sulfur dioxide (SO2), nitric oxides (NOx), and moisture can form several hundred 

parts per million (ppm) of N2O in the sampling container, even within short durations of storage 

of up to 2 hours (Muzio 1989, referenced in the Technical Bulletin). 

 

• Muzio et.al reported that the mechanism for this artifact N2O formation involved gas-phase 

oxidation of NO to NO2, followed by liquid phase conversion of NO2 and SO2 to N2O and 

sulfuric acid (H2SO4). The mechanisms also indicate that a lower pH would favor the liquid 

phase reactions. The removal of moisture and SO2 from the gas stream, and storage at a higher 

aqueous-phase pH are viable options for conditioning these grab samples. EPA’s Air and 

Energy Research Laboratory (AEERL) subsequently developed a sampling protocol for 

collecting grab samples for the measurement of N2O in combustion sources in 1993. The 

sampling methodology included the use of reactant-specific SO2 and H2O sorbents followed by 

sample collection in a stainless steel container. 
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• The Fortum study (Fortum 2001) acknowledged the potential for positive bias due to 

inappropriate grab sampling, cited the above study by Muzio et al, and reiterated that many of 

the primary N2O measurement data available at that time could be regarded as inaccurate due to 

this artifact formation. Our reference to the Fortum study in connection with the statement in 

p.27 is primarily intended to communicate this technical group’s acknowledgement of the 

inadequacy as it relates to existing data.

• The Fortum study utilized a modified sampling / continuous measurement technique for 

obtaining N2O emission data. Moisture from the flue gas was removed using a cooler and the 

gases were immediately analyzed using an infrared analyzer using the Non-Dispersive Infrared 

(NDIR) technique. Measurements made using the NDIR analyzer also compared favorably with 

those made using a Fourier Transform Infrared (FTIR) analyzer at one of the sources tested 

during the study. It is NCASI’s assessment that this technique of cooling the sample gas (to 

remove entrained moisture) followed by immediate analysis (no sample storage) minimizes the 

potential for interferences or positive biases due to spontaneous N2O formation.

• As discussed in section 2.0 and Appendix C of Technical Bulletin 998, NCASI developed a 

sample conditioning system (drop-out impinger + silica gel impinger) to remove entrained 

moisture and SO2 from the flue gas in conjunction with this study to develop emission factors 

for CH4 and N2O. The removal of both moisture and SO2 allows for grab sample storage, 

especially in cases where instantaneous continuous measurement was not feasible. As discussed 

in Appendix C, samples collected using the above conditioning techniques were stable up to 72 

hours in storage. 

Note on Emission Factors Related to Wood Combustion Reported in the Fortum Study 

On the subject of the applicability of emission factors developed during the Fortum study, the 

following points are worth reiterating; 

• N2O emissions from Fluidized Bed (FB) Boilers – Six out of 13 boilers tested during the Fortum 

study were FB boilers (3 circulating fluidized bed (CFB) and 3 bubbling fluidized bed (BFB)). 

This study focused more on fluidized bed boilers given the expectation that the prevalent lower 

combustion zone temperatures (800oC - 900o C) would result in higher levels of N2O emissions. 

As discussed in section 5.2 of Technical Bulletin 998, combustion zone temperatures in 

industrial boilers are expected to be significantly higher than 950oC, the presumptive threshold 

above which N2O formation is expected to be negligible. As expected, N2O emissions measured 

during the Fortum study from pulverized coal boilers were significantly lower than the emissions 

from CFBs. Additionally, only two of the boilers (one CFB and one BFB) tested during the 

Fortum study burned predominantly wood/bark during the tests. Lastly, fluidized bed boilers 

account for <5% of all forest products industry boilers that burn biomass fuels. It is therefore 

inappropriate to use the N2O emission factors for fluidized bed boilers, developed during the 

Fortum study, for the broader category of industrial boilers burning biomass fuels. The N2O 

emission data provided in Technical Bulletin 998 are more comprehensive and reflect periods 

with predominantly wood/bark firing.

• CH4 and N2O emissions from Recovery Furnaces – CH4 emissions from the recovery boilers 

tested in the Fortum study were either at or below the confidence limit of 1 kg/TJ (<0.001 

kg/MMBtu). This is comparable to the average CH4 emission factors reported in the NCASI 
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study (Table 5.2 in Technical Bulletin 998). N2O emissions from recovery furnaces in the 

Fortum study were also below the confidence limit of 1 kg/TJ (<0.001 kg/MMBtu). The average 

N2O emission factor reported in the NCASI study (0.00044 kg/MMBtu in Table 5.3) is 

consistent with the conclusion of the Fortum study.

Please do not hesitate to contact me at 352- 331-1745 (ext. 260) or vvarma@ncasi.org, if you have 

additional questions on this topic or on the NCASI Technical Bulletin. 

Sincerely, 

 

Vipin K. Varma 

cc: 

Derek Sain, Senior Research Associate, NCASI 

Brad Upton, Principal Research Engineer, NCASI 

Reid Miner, Vice President, Sustainable Manufacturing, NCASI 

Nancy Clark, Director, Climate and Air, AFPA