An examination of combustion byproducts from wood structural panels

Discarded wood products are sometimes burned due to environmental concerns associated with other disposal options such as landfills and recycling, and because there’s a desire to produce energy from wood and other biomass. Since wood structural panels such as softwood plywood and oriented strand board (OSB) contain small amounts of synthetic resin-based adhesives, questions have arisen about the byproducts of combustion. This article summarizes the information available concerning such byproducts.

 

Background

Combustion byproducts from any material depend greatly on a number of combustion-related variables, including: temperature, amount of oxygen, rate of heating and resistance times.¹ Any complex organic material (e.g., wood, plastics, leather, wool, foodstuffs) is capable of releasing large amounts of toxic chemicals if it is not burned under optimum conditions in an efficient combustion device, such as a boiler or specially designed incinerator (the presence of smoke indicates chemicals are being released). Potentially harmful chemicals released from incomplete combustion typically include carbon monoxide, formaldehyde, acetaldehyde, acrolein, hydrogen cyanide, benzene, phenol, polycyclic aromatic hydrocarbons and other organic compounds. The combustion of materials that contain chlorine will also produce hydro-chloric acid and certain chlorinated organic compounds. In addition to being acutely toxic, many of these chemicals are considered carcinogens. Carbon monoxide, an acute toxin of particular concern, has caused most fire-related deaths in residences.²

Most wood structural panels are bonded with adhesives that are based on phenol formaldehyde (phenolic) resins. In the case of plywood, these resins are normally mixed with fillers, such as wheat flour or ground walnut shells. Some OSB panels also use methylene diisocyanate (MDI) adhesives as well as phenolics.

 

Combustion Byproducts

The resins and fillers used in wood structural panels contain only carbon, hydrogen, oxygen, nitrogen and sodium—the same elements that comprise wood. From a theoretical standpoint, there is no reason to believe the combustion of wood structural panels will release chemicals that are different or more toxic than those which result from the combustion of solid wood.

Complete combustion of both solid wood and wood structural panels would only result in the emission of carbon dioxide, water and nitrogen oxides, none of which is normally considered toxic. Combustion in modern boilers and incinerators is nearly complete, and the release of toxic chemicals from either solid wood or wood structural panels in such combustion systems is very low or nonexistent.

Most studies of combustion byproducts have involved simulating fires in residences. Typically in such cases, the fire is smoldering, and combustion is suppressed and incomplete due to relatively low temperatures, inadequate oxygen supplies and other “worse-case” conditions for combustion. Under these circumstances, many toxic substances are produced that would not be present in systems designed to optimize combustion. The results of such studies cannot be applied to combustion that occurs in an efficient combustion system, such as a boiler or specially designed incinerator.

APA is not aware of any published information comparing chemical-by-chemical combustion byproducts from solid wood and wood structural panels in boilers and incinerators. However, the comparative information generated by studies of smoldering combustion is likely relevant when considering combustion that is more complete, provided it is recognized that the quantities of toxics produced by boilers and incinerators are minuscule when compared to those produced during “open” burning.

Although detailed comparative analyses of all the incomplete combustion byproducts from solid wood and wood structural panels are not available, a study was performed by Rubicon Chemical Co.³ to determine the amounts of carbon monoxide, carbon dioxide, hydrogen cyanide and nitric oxide released during the combustion of solid wood and wood bonded with various adhesives at 500°C. The study found no major differences in the amounts of combustion byproducts that resulted from solid wood versus wood chips bonded with either 6% phenolic or MDI resins. The product which contained the phenolic resin produced a little more carbon monoxide but less nitric oxide and hydrogen cyanide than the solid wood. Since no statistical parameters were reported, it is not possible to conclude whether these small differences are statistically significant; but even if one assumes they are, the conclusion must be that they were very minor.

A number of studies have investigated the byproducts of the incomplete combustion of phenolic plastics (without wood)⁴ and found the major decomposition byproducts to be carbon monoxide, carbon dioxide, water and methane. Smaller amounts of hydrogen, formaldehyde, phenol and other volatile organic compounds were also found. All of these compounds are also present in smoke emanating from solid wood.

As with solid wood, when pure phenolics are burned, the toxic chemical of upmost concern is carbon monoxide. Some studies indicate that certain types of phenolic compounds produce more carbon monoxide at certain temperatures than solid wood,⁴ but this is not relevant to the use of efficient combustion systems, as the formation of carbon monoxide is a result of the lack of adequate oxygen for complete combustion.

Even if larger quantities of carbon monoxide (or other toxics) were produced by the synthetic resins, the amount of these resins used in wood structural panels is so low (typically 1.5%-4%, by weight) that their overall effect would be negligible.

 

Toxicity

A considerable amount of information regarding the toxicity of combustion byproducts for various wood-based materials has been published. Studies based on the toxicity of smoke on rats indicate the byproducts that result from the incomplete combustion of wood structural panels are no more harmful than those that result from solid wood. Some of the most direct evidence comes from a smoke toxicity study commissioned by the National Forest Products Association in response to a New York State law that requires manufacturers to provide information on the toxicity of smoke from their products:⁵ the results clearly indicate smoke from wood structural panels (and a wide variety of other wood products) is no more toxic than smoke from solid wood.

APA has field representatives in many major U.S. cities and in Canada who can help answer questions involving APA trademarked products. For additional assistance in specifying engineered wood products, contact APA, 7011 S. 19th St., Tacoma, WA 98466; Tel: 253-565-6600; Fax: 253- 565-7265. APA Product Support Help Desk: 253-620-7400; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

Disclaimer

The information contained herein is based on APA—The Engineered Wood Association’s continuing programs of laboratory testing, product research and comprehensive field experience. Neither APA nor its members make any warranty, expressed or implied, or assume any legal liability or responsibility for the use, application of, and/or reference to opinions, findings, conclusions or recommendations included in this publication. Consult your local jurisdiction or design professional to assure compliance with code, construction and performance requirements. Because APA has no control over quality of workmanship or the conditions under which engineered wood products are used, it cannot accept responsibility of product performance or designs as actually constructed.

Other studies also indirectly indicate there is no significant difference between wood structural panels and solid wood relative to smoke toxicity. Despite common beliefs to the contrary, the research shows byproducts of incomplete combustion from the majority of synthetic plastics, including the adhesives used in wood structural panels, are no more hazardous than those from solid wood.^2,4,6 There are a few halogen-containing synthetics, such as polytetrafluoroethylene (e.g., Teflon), which can produce more hazardous fumes, but they are the exception.³ Surprisingly, some of the most toxic combustion byproducts are produced by common natural proteinaceous materials, such as wool and silk.⁶

 

Conclusions

There is no evidence to suggest that the combustion byproducts that result from wood structural panels should cause any more concern than those that result from solid wood.

 

References

¹ USDA Forest Products Laboratory. 2010. Wood Handbook – Wood as an Engineering Material, Chapter 18. Madison, WI.

² Hirschler, Marcelo M. and Gregory F. Smith. 1991. “Investigation of a Smoke Toxicity Fire Model for Use on Wood.” Paper presented at the Third International Symposium of Fire Safety Science, July 8-12, Edinburgh, Scotland.

³ Galbraith, C.J., Jr., S.C. Cohen and G.W. Ball. 1983. “Self-Releasing Emulsifiable MDI Isocyanates: An Easy Approach for All-Isocyanate Bonded Boards.” Proceedings of the Seventeenth Washington State University International Particleboard/Composite Materials Series, Washington State University, Pullman, Washington.

⁴ Johnston, P.K., E. Doyle and R.A. Orzel. 1988. “Phenolics: A Literature Review of Thermal Decomposition Products and Toxicity.” Journal of the American College of Toxicology 7(2):201.

⁵ Anon. 1989. “Request for Approval of Classes, Submitted to New York State Department of State Fire Gas Toxicity Data File in Compliance with Uniform Fire Prevention and Building Code Article 15, Part 1120 Combustion Toxicity Testing Section 11210.3(b).” National Forest Products Association, Washington, D.C.

⁶ Hilado, Carlos J. and Patricia A. Huttlinger. 1981. “Toxic Hazards from Common Materials.” Fire Technology 17(3):177.

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