Flour particles average about 80 microns (P.M. 80) in size with the smallest about 10 microns (P.M. 10) and the largest 300 microns. A micron is one millionth of a meter, or about 0.000039 inch. A typical yeast cell is 3 to 4 microns; 100 microns is the average diameter of a strand of human hair.
The E.P.A. has regulations for P.M. 10 and P.M. 2.5. Some applications for an air permit require the estimation of the potential emissions of P.M. 10 or P.M. 2.5. Flour milled to less than P.M. 1 is no longer flour but has been broken down into starch granules and other molecules. About 6% of a load of flour may have particles smaller than 75 microns.
Flour usually is transferred pneumatically from trucks and rail cars to silos. Flour then usually is transferred pneumatically to scales and mixers in the facility. The transfers are controlled with filters for loading and for the scaling hoppers in the bakery. The filters will capture particle sizes of 0.5 microns and larger. Baghouses that are flour control fabric filter collectors are common silo controls and are 99.99% capture efficient. Inside the bakery, suction systems often keep flour from circulating in the air.
For bakeries the potential particulate emissions are insignificant because the particles are too big to fit through the filters. For regulatory purposes, the emission would be insignificant, in some areas at less than 0.01 lbs per hour or in some areas less than 100 tons per year. For example, a bakery uses 5,000 lbs of flour per hour. Worst case, if 99.99% is captured by the baghouse then 0.5 lbs per hour, or 2 tons per year (8,760 hours), is the potential release.
Note that flour that may escape from an oven stack would be heavy enough to fall on the roof and may be a sanitation problem and a storm water permit violation.
Some readers may be familiar with the estimation of volatile organic compound air emissions of ethanol from the baking of yeast raised products found by going to www.epa.gov, search for AP 42 Bakery Oven Emissions and then go to Chapter 9.9.6.
There is no methodology for estimating potential emissions for flour. For purposes of air permit compliance most companies use AP 42 Portland Cement, Chapter 11.6. Cement kiln dust ranges from 5 microns to greater than 50 microns. Use of this substance is a conservative choice but defensible and reasonable. The emission factor from Table 11.6-2 Dry process kiln with fabric filter is 0.2 lbs per tons handled.
As background, in the E.P.A. document, “Technical background document on control of fugitive dust at cement manufacturing facilities,” Chapter 3 is titled “Emissions estimation” and 3.1.1 is titled, “Description of cement kiln dust handling processes at the sample factory.”
In the E.P.A. document AIRS Source Classification Codes and P.M. 10 Emission Factor Listing by Technology Transfer, EPA-45014-89-022, Cement Manufacturing, Dry Process, Raw Material Transfer the emission factor of 0.15 lbs per tons handled is used.
Some companies use AP 42 Concrete Batching, Chapter 11.12 Cement Unloading to elevated storage silo, pneumatic, controlled. Note that the particle size of the components is about 62 to 2,000 microns. In Table 11.12-2 the emission factor is 0.00099 lbs per tons handled.
P.M. 2.5 particles are created by the burning of fossil fuels. To estimate emissions for P.M. 2.5 and other constituents from Natural Gas Combustion go to AP 42 Chapter 1.4. Small Boilers.
The National Fire Protection Association Standards 61 for Dust from Food Processing and 654 Combustible Particulate Solids set concentration levels low enough to avoid explosions and fire (www.nfpa.org). Inside the bakery, the Occupational Safety and Health Administration sets concentration levels low enough to protect human health and for adequate sanitation standards. Other federal and state regulations to control dust also apply.
Sosland Publishing has a database of articles that I have written about calculating air and water emissions and sustainability. Go to www.bakingbusiness.com and select either Milling & Baking News or Baking & Snack, and search for “Giesecke.” Many bakers have said that the information was very helpful.
This is my final column for Milling & Baking News as I am retiring from the food industry at the end of this year. It has been a pleasure to work with so many wonderful people. Thanks to Sosland for the writing opportunity. I will be spending time with family and will increase time in pursuit of land and underwater archeology.