Grantee Research Project Results

Final Report: Investigation of the Elementary Reaction Mechanisms of Fly-Ash Mediated Formation of PCDD/F

EPA Grant Number: R826166
Title: Investigation of the Elementary Reaction Mechanisms of Fly-Ash Mediated Formation of PCDD/F
Investigators: Dellinger, Barry , Khachatryan, Lavrent , Alderman, Steven
Institution: Louisiana State University - Baton Rouge
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1997 through September 30, 2000
Project Amount: $202,976
RFA: Exploratory Research - Environmental Engineering (1997) RFA Text |  Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management

Objective:

The goal of this project was to prevent the formation of polychlorinated dibenzodioxin/polychlorinated dibenzofuran (PCDD/PCDF) through modification of conditions in the source. Since it is now well established that PCDD/PCDF are formed in the postcombustion region, our strategies were focused on techniques that apply to "cool-zone" chemistry. Our approach is to examine the pathways of formation from a prevention perspective, identify the rate controlling reactions and conditions, and develop methods for reducing their rate and ultimately the yield of PCDD/PCDF. The main thrusts and results of this project were the following:

1. Control through prevention of de novo formation of small-molecule, PCDD/PCDF precursors that are formed from gas—solid reactions of combustion-generated radicals with combustion generated soot and char.
2. Control of chemisorption of large-molecule precursors (formed from gas-phase molecular growth involving the de novo precursors) on fly-ash surfaces.
3. Control of surface catalyzed chlorination by transition metal chlorides.

Summary/Accomplishments (Outputs/Outcomes):

We demonstrated the formation of acetylene and ethylene at low temperatures (200-500°C) from the reaction of hydroxyl radicals (photolytically generated using ArF laser photolysis of N₂O in the presence of water) with the activated carbon (as a surrogate for combustion-generated soot and char). This represents a partial confirmation of the initial step in our proposed, extended precursor pathways of PCDD/PCDF formation. The formation of these unsaturated hydrocarbons are significant, as we also have shown that these species can be chlorinated by a transition metal and undergo further reaction to form chlorinated aromatic species and PCDD/PCDF.

Our research has shown that formation of these species by the de novo formation pathway can be minimized if the cool zone temperature is maintained below 380°C, where a low rate of formation of these compounds is observed. An inhibition effect on the gasification of carbon also is observed if gasification components like N₂O(H₂O) are flowing over carbon for an extended period. These gases are apparently absorbed by the carbon reduced (or block active sites) by the effective area of surface available for reaction and consequence, the overall rate of oxidation.

Carbon tetrachloride is the dominant halocarbon released from chlorination of carbon pellets by Cl₂ gas molecules or Cl atoms (generated by N₂ laser). Catalytic conversion of carbon tetrachloride can yield olefinic CHCs, which are precursors in the formation of chlorinated aromatic species and PCDD/PCDF. A low temperature zone (200-350°C) and a deactivation of the catalyst is needed to prevent the formation of chlorinated hydrocarbons and hence, PCDD/ PCDF.

Increasing the water concentration and maintaining the particulate temperature outside of specific temperature regimes can block the chemisorption of precursors (phenol, halogenated phenols, and benzenes) and chlorinating agents on fly-ash surfaces by formation of annealed siloxanes. Other combustion-generated species also may block reactive sites. Chlorinated phenols are more sensitive to the changing of experimental conditions, as they are more readily chemisorbed than chlorobenzenes in the temperature region 100-500°C.

Using EPR spectroscopy, we have discovered what appear to be surface-stabilized radicals of hydroxyphenols. The discovery of these stable radicals places an entirely new perspective on the mechanism of formation of PCDD/PCDF and strategies for control. Because our investigation show these radicals might be the principal intermediates in the formation of PCDD/PCDF on surfaces, methods for scavenging or oxidizing these radicals may be effective in preventing formation of PCDD/PCDF.

Journal Articles on this Report : 2 Displayed | Download in RIS Format

Publications Views

Other project views:	All 11 publications	3 publications in selected types	All 2 journal articles

Publications

Type	Citation	Project	Document Sources
Journal Article	Asatryan RS, Mailyan NS, Khachatryan L, Dellinger B. Electronic elasticity-toxicity relationships for polychlorinated dibenzo-p-dioxin congeners. Chemosphere 2002;48(2):227-236.	R826166 (Final)	not available
Journal Article	Khachatryan L, Dellinger B. The formation of olefins and alkynes from the reaction of hydroxyl radical and carbonaceous material. Journal of the Chemical Society - Perkin Transactions 2 2002;(4):779-783.	R826166 (2000) R826166 (Final) R827719 (2000)	not available

Supplemental Keywords:

air, atmosphere, adsorption, absorption, exposure, carcinogen, mutagen, chemicals, toxics, particulates, PAHs, Dioxin, nitrogen oxides, pollution prevention, waste reduction, waste minimization, treatment cleanup, incineration, oxidation, environmental chemistry, engineering, ecology, modeling, monitoring, measurement methods, south central, Gulf Coast, Louisiana, LA, industry, incineration., RFA, Scientific Discipline, Waste, Toxics, pesticides, Environmental Chemistry, Incineration/Combustion, Environmental Engineering, combustion contaminants, metal chlorides, dioxins, emission controls, kinetc models, combustion generated radicals, furans, fly ash, sustainable development, reaction mechanisms

Progress and Final Reports:

Original Abstract

1998

1999