Use of emission factors for wood fired boilers in the DRAFT Supplement F to AP-42 Section 1.6 "Wood Waste Combustion in Boilers"

Use of emission factors for wood fired boilers in the DRAFT Supplement F to AP-42 Section 1.6 "Wood Waste Combustion in Boilers"
5-29-01 update by James H. Southerland QEP

NC DAQ continues to get inquiries regarding the use of emission factors for wood combusted in boilers. The confusion seems to relate primarily to what is the "best available information."

In February 1999, EPA published an update of AP-42 which expanded upon the available test data and information. One of the major clarifications and expansions related to the use of moisture to adjust the Btu content of the wood; this had direct consequences on the quantity of fuel being used with the emission factor and thus the resulting emissions. In the course of doing this work, added data were found on burning of dry wood, allowing better separation of wet and dry wood factors.

Almost immediately after the February 1999 Supplement, there was an additional effort by EPA which resulted in a September 1999 draft Section with additional enhancements. Though there were comments on that draft made by various parties, the section is yet to be made final. The statements below from AP-42 summarize the changes in these two supplements.

Supplement E, February 1999

  • In the footnotes of tables 1.6-1, 2, 3, 4, 5, 6, 7, some text was removed that described how to adjust the factors when burning wood with moisture and thermal content significantly different from 50% or 4500 Btu/lb, respectively. The EPA is revising Section 1.6 and, in the interim, consistent with EPA's recommendations regarding proper use of AP-42, the EPA encourages users of the wood combustion emission factors to account for the specific assumptions included in the factors and to convert the factors to a thermal content basis (i.e., lb/MMBtu) to estimate emissions when burning wood that differs significantly from 4500 Btu/lb or 50% moisture.

Supplement F, (date to be determined.) (Estimated out about end of June, 2001/jhs)

  • Original draft F - All emission factors were revised and new factors were added. Separate factors were developed for wet wood (greater than or equal to 20 percent moisture content) and dry wood (less than 20 percent moisture).
  • JHS - In discussions with EPA on 5-30-01, it was determined that some additional changes have been made in this supplement to be published on EPA's web page about mid to late June 2001. This page is: They will include further refinements in the factors due to availability of test data and its interpretation.

In the meanwhile, NC DAQ staff reviewed the dry wood factors in the September 1999 draft and determined that these would likely to provide the best estimates available and incorporated them into the DAQ spreadsheets.

DAQ has accepted the draft as "best available information" (but perhaps subject to further future revisions), and incorporated these into the DAQ spreadsheets that are on the DAQ web page (courtesy of former employee William Willets of Permits) and have in 1999 and 2000 advised facilities at workshops and by other means that we are accepting the new numbers. DAQ also met and talked with Andy Counts of AFMA in this regard in 1999 and he indicated at that time that he would be making this information available to members of the AFMA. No official notification was sent to facilities by DAQ or on the web page because the release of the new supplement was expected to be forthcoming shortly.

Furniture Companies have apparently been in agreement with the use of the September 1999 factors for burning dry wood. The following is a quote from one of their environmental officials:

" The factors for dry wood in Supplement F are probably more representative of furniture plant wood waste than the factors in the current version of AP-42 Section 1.6 which are based upon wood at 50% moisture and 4500 Btu/lb. A recent analysis of the wood fuel from { xx plant in xx} indicated the moisture in the wood to be only 4.89% and the heat content to be 7816 Btu/lb (wet basis), 8218 Btu/lb (dry basis). A stack test was conducted along with the fuel analysis: PM emissions (Method 5, adjusted for soot blowing) were 0.167 lb/MMBtu for a spreader stoker boiler (no flyash reinjection) controlled by a multicyclone. "

The spreadsheet now on DAQ's web was developed using Supplement F. However, all DAQ spreadsheets come with caveats that there may be errors and that the facility is responsible for the best emission estimates possible with the best available data, whether that be from AP-42, testing overseen by DAQ or other similar information. There have indeed been errors detected in the DAQ spreadsheets on the DAQ web page that have not yet been corrected. In addition, the new final revision for Supplement F is due out very shortly. Therefore, it is not prudent to update any such spreadsheet until that update is available.

NC DAQ has reiterated on many occasions that "We (DAQ) will continue to insist on plant-specific and equipment-specific calculations and assumptions for calculation of yield, control device efficiency and emission of specific species/compounds." Therefore, the "best information available" remains the standard which DAQ will use to judge individual situations and such applications, even when errors are detected in spreadsheets which DAQ has developed and made available to facilities for their information and use.

For CY 2000 emission inventories, the information in AP-42 draft supplement F (September 1999) is acceptable. For CY 2001 emission inventories, the Supplement F will be presumed to be final and will represent the "best available" information when there is no source specific information.

Sorry for any confusion!!!

PS: IF you wish to have a copy of the Draft Supplement F and can not open and unzip from EPA's web page, please send an e-mail to and I will forward you a copy in Microsoft Word.

Last Modified: Mon May 04 13:20:17 2015