Elmar R. Altwicker

Acetylene in the atmosphere: Sources, representative ambient concentrations and ratios to other hydrocarbons

Abstract Acetylene is a hydrocarbon of interest from a variety of viewpoints in atmospheric and pollution studies. This paper reviews representative data on absolute acetylene concentration in ambient air and on certain ratios of other hydrocarbons to acetylene. The possibility of non-automotive acetylene emissions to the atmosphere and the interference which this poses to the use of acetylene as an unbiased urban emission tracer is considered.

Relative rates of formation of polychlorinated dioxins and furans from precursor and de novo reactions

Abstract Though it is generally agreed that there are two major pathways for the formation of polychlorinated dibenzo-p-dioxins and furans (PCDD/F), namely the formation from chemically similar precursors (chlorobenzenes and chlorophenols) and the so-called de novo synthesis, some difficulties remain. Extrapolation from laboratory based studies to incinerators has not been straightforward. The question 0f relative rates is explored in this paper. It is concluded that the precursor-based reactions tend t0 be faster, when computed on a per carbon basis. Some observed incinerator PCDD/PCDF-ratios can be reconciled with this observation, if PCDD and PCDF formation occur at different rates as a function 0f temperature.

Formation of dioxins: Competing rates between chemically similar precursors and de novo reactions

Abstract The formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) on fly ash was investigated using polychlorinated phenols (principally 2,3,4,6-tetrachlorophenol) in competition with the de novo synthesis reaction (e.g., synthesis from native, nonextractable carbon in the fly ash). At the relatively high gas-phase concentrations employed, 38–370 ng ml −1 , the chlorophenol pathway was up to 100 times faster than the de novo pathway. For short reaction times (t ≤5 min) chlorophenols do not yield PCDF, suggesting that the formation of PCDF in incinerators is not controlled by this reaction over short time scales.

Some laboratory experimental designs for obtaining dynamic property data on dioxins

The presence of polychlorinated p-dibenzodioxins and polychlorinated dibenzofurans (hereafter PCDD/F) in municipal waste incinerator effluents has sparked a variety of laboratory investigations in the last few years in the hope of learning more about the formation and destruction mechanisms of these compounds. In the course of the investigations a variety of approaches have been employed. It is the purpose of this paper to review these designs and to comment on them in general as well as in specifics. Since under the conditions of interest the quantities of PCDD/Fs are miniscule compared with other products and reactants and because there are nearly 200 congeners, quantitative information is difficult and expensive to obtain. A second aspect concerns the reaction rates involved. No intrinsic kinetic studies have been conducted and no intrinsic values for PCDD/F rates of formation exist. Such information will be necessary, however. As will be shown below, there seems to exist a basic conflict between reaction rates as may be inferred from some laboratory experiments and rates seemingly required by the time/temperature histories of gases and particles in incinerators. The paper is divided into four major parts: homogeneous formation and destruction, heterogeneous formation and destruction, comments on discrepancies, and suggested designs.

Design and scale-up of a spouted-bed combustor

This paper discusses the effect of the second (heterogeneous) phase on the extent of heat recycle, lowering of the lean flammability limit, stability and performance of a spouted-bed combustor. Experiments suggest that the extent of heat recycle, and therefore the lowering in the lean flammability limit, is controlled by both particle circulation rate and particle physical properties. This concept of heat recycle can be utilized in the burning of low heating value chemical and toxic wastes. The stability of a spouted-bed combustor depends upon the geometric design ratios di/Dc,di/dp, and hd/H, based on studies with columns of two different sizes burning propane. The introduction of a draft tube improves the flexibility of the combustor in terms of residence time control and bed stability. A well-designed draft tube spouted bed combustor can yield a destruction and removal efficiency, i.e. conversion, of ⩾99.99.

Comparison of 2,4,6-trichlorophenol conversion to PCDD/PCDF on a MSWI-fly ash and a model fly ash

We performed experiments on two different matrices with 2,4,6-trichlorophenol as precursor to Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD)/F. A municipal solid waste incinerators (MSWI) and a model fly ash were spiked in two different ways. The experiments demonstrated a three times higher formation potential of the trichlorophenol to PCDD on MSWI fly ash compared with the model fly ash used. For both fly ashes the PCDD yield was higher when gaseous trichlorophenol was fed continuously compared to mixing the fly ashes prior to the experiments with the total amount of the precursor. Despite dilution of the fly ashes tenfold with an inactive matrix the conversion of the chlorophenol was very high.

Formation of PCDDF in municipal solid waste incinerators: laboratory and modeling studies

Recent laboratory results on the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDF) are reviewed, with emphasis on heterogeneous, fly ash-mediated reactions. The well-studied de novo synthesis yields PCDDF ratios less than one, a value frequently found in municipal solid waste incinerators. Its apparent rate cannot, however, explain PCDDF-formation in incinerators during flue gas residence times. Reactions of chlorophenols yield PCDD, either in the solid or gas phase, and are much faster, but generally lead to PCDDF ⪢ 1 at comparable temperatures. PCDF-yields from chlorobenzenes are much lower under comparable conditions, but increase at higher temperatures. Different experimental combustion systems have proven that PCDDF can form rapidly at rates comparable to apparent incinerator rates, i.e., formation during flue gas passage from the furnace through the air pollution control device. The properties of fly ash play an important role in determining rates and yields, but there is at present hardly a quantitative link between fly ash physical/chemical properties and chemical reactivity. Some success has been achieved in applying a modified four-step model of PCDD-formation to incinerator data. The concept of active and superactive sites on fly ash is introduced, it may be useful in future experimental designs. For future control approaches that are not add-on oriented a better understanding of the time/temperature history of fly ash formation and the intrinsic kinetics of precursor reactions is required.

Rapid formation of polychlorinated dioxins/furans during the heterogeneous combustion of 1,2-dichlorobenzene and 2,4-dichlorophenol

Abstract The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in a heterogeneous spouted bed combustor occurred within seconds from the oxidation of two different precursors; 1,2-dichlorobenzene and 2,4-dichlorophenol. Levels of PCDD/Fs produced from 2,4-dichlorophenol were two orders of magnitude higher than those produced from 1,2-dichlorobenzene, indicating that PCDD/Fs formation rate is much faster from chlorophenol precursors. In agreement with typical incinerator observations, the ratio of dioxins to furans was less than one from both precursors. Formation of chlorobenzenes and chlorophenols (other than the starting fuels) also occurred in the combustor; their concentration levels did not exhibit a strong dependence on the type of the precursors.

The role of precursors in formation of polychloro-dibenzo-p-dioxins and polychloro-dibenzofurans during heterogeneous combustion

Abstract Although by now literature reports suggest that compounds containing almost any combination of C, H, O, and Cl can form PCDD/PCDFs under suitable conditions, questions of the relevancy of such findings to the formation of these compounds in MSW-incinerators remain. To address these questions the formation and destruction of plausible precursors (intermediates) over time/temperature scales found in incinerators must be better understood. In our laboratory we have attempted to simulate MSW-incineration using a systems approach, i.e., a spouted bed combustor coupled to two post-combustion stages. In this system, formation of PCDD/PCDFs and potential precursors has been investigated in the bed (pre-flame), the post flame regions, and over fly ash (post flame combustion gases). From the lean combustion of chlorobenzenes (mono, 1,2-di-, and 1,2,4-tri-) hexachlorobenzene (HCB) and pentachlorophenol (PCP) are major PICs (products of incomplete combustion) formed in characteristic times 1, recent results suggest that this ratio may be very sensitive to local conditions. These two compounds are certainly possible precursors on the relevant time/temperature scales, but it has not been proven that they are required intermediates. In these combustions OCDD is the major dioxin congener, lower chlorinated congeners are only formed at longer residence times or over fly ash (at ∼300°C). A mechanism accommodating these observations will be proposed and the relevancy of the findings to recent measurements from incinerators will be discussed.

Mechanistic aspects of the de novo synthesis of polychlorinated dibenzo-p-dioxins and furans in fly ash from experiments using isotopically labeled reagents

13 C-activated carbon mixed with MSW incinerator fly ash and reacted with gas-phase oxygen at 300-350 o C resulted in the formation of 13 C-labeled chlorobenzene and PCDDJF products. Unlabeled chlorobenzene and PCDD/F derived from native carbon were also detected, but no scrambling of the added 13 C or native 12 C in the resultant products was evident. The [ 13 C]PCDD/F and [ 12 C]PCDD/F yields were both observed to have an optimum formation temperature around 325 o C, suggesting that the added 13 C-activated carbon was subject to the same fly ash-catalyzed formation and destruction mechanisms as the native carbon. 13 C-activated carbon added to otherfly ashes had PCDD/F formation rates reflective of their respective rates from the native carbon already present. Experiments with 13 C-labeled CO and CD, reacted with fly ash at 300 o C in the presence of gas-phase oxygen showed that neither is an effective precursor to PCDD/F