Jost O.L. Wendt

Toxic metal emissions from incineration: Mechanisms and control

Abstract Toxic metals appear in the effluents of many combustion processes, and their release into the environment has come under regulatory scrutiny. This paper reviews the nature of the problems associated with toxic metals in combustion processes, and describes where these problems occur and how they are addressed through current and proposed regulations. Although emphasis in this paper is on problems associated with metals from incineration processes, conventional fossil fuel combustion is also considered, insofar as it pertains to mechanisms governing the fate of metals during combustion in general. This paper examines the release of metals into the vapor phase, with the particle dynamics of a nucleating, condensing, and coagulating aerosol that may be subsequently formed, and with the reactive scavenging of metals by sorbents. Metals can be introduced into combustion chambers in many physical and chemical forms. The subsequent transformations and vaporization of any volatile metal depend on the combustion environment, the presence of chlorine and other species (reducing or oxidizing), on the nature of the reactive metallic species formed within the furnace, and on the presence of other inorganic species such as alumino-silicates. Some insight into how these factors influence metal release can be gained by considering the release of organic sodium during coal char combustion. Once vaporized, a metal vapor cloud will normally pass through its dewpoint to form tiny nuclei, or condense around existing particles. These aerosols are then affected by other dynamic processes (including coagulation) as they evolve with time. This paper shows how current mathematical descriptions of aerosol dynamics are very useful in predicting metal aerosol size distributions in combustion systems. These models are applied to two prototype problems, namely: the prediction of the temporal evolution of a particle size distribution of a self-coagulating aerosol initially composed of nuclei; and the scavenging of nuclei by coagulation with larger sorbent particles. A metal vapor can also react with certain aluminosilicate sorbents. This process, which will occur at temperatures above the dewpoint, is described, and is important, since it allows the high temperatures in incineration processes to be exploited to allow the formation of water-unleachable metal-containing compounds that can be isolated from the environment. Future research problems are also identified.

Trace metal transformation mechanisms during coal combustion

Abstract Mechanisms governing the fate of trace metals during coal combustion are reviewed, and new theoretical results interpreting existing data are presented. Emphasis is on predicting the size-segregated speciation of trace metals in pulverized coal-fired power plant effluents. This facet, which determines how trace metals originally in coal impact the environment, is controlled by fuel composition and combustion conditions. Multicomponent equilibrium calculations are used to predict vaporization/condensation temperatures for antimony, arsenic, beryllium, cadmium, chromium, lead, mercury, nickel, and selenium compounds in coal combustion flue gases, for a representative Illinois No. 6 coal. Experimental data show that equilibrium provides a good guide on the effect of chlorine on the partitioning of pure nickel, cadmium, and lead salts, introduced separately into a gaseous turbulent diffusion flame within an 82 kW combustor. Metal nuclei coagulation mechanisms are examined using existing computer codes, and these predict that coagulation does not allow condensed metal nuclei to be scavenged by existing coal ash particles. Rather, literature data on trace metal enrichment on small particles are consistent with processes of reactive scavenging of metals by larger particles, and it is suggested that these processes might be exploited further to convert these metals into environmentally benign forms.

Comparison of Particle Size Distributions and Elemental Partitioning from the Combustion of Pulverized Coal and Residual Fuel Oil

ABSTRACT U.S. Environmental Protection Agency (EPA) research examining the characteristics of primary PM generated by the combustion of fossil fuels is being conducted in efforts to help determine mechanisms controlling associated adverse health effects. Transition metals are of particular interest, due to the results of studies that have shown cardiopulmonary damage associated with exposure to these elements and their presence in coal and residual fuel oils. Further, elemental speciation may influence this toxicity, as some species are significantly more water-soluble, and potentially more bio-available, than others. This paper presents results of experimental efforts in which three coals and a residual fuel oil were combusted in three different systems simulating process and utility boilers. Particle size distributions (PSDs) were determined using atmospheric and low-pressure impac-tion as well as electrical mobility, time-of-flight, and light-scattering techniques. Size-classified PM samples from this study are also being utilized by colleagues for animal instillation experiments. Experimental results on the mass and compositions of particles between 0.03 and >20 μm in aerodynamic diameter show that PM from the combustion of these fuels produces distinctive bimodal and trimodal PSDs, with a fine mode dominated by vaporization, nucleation, and growth processes. Depending on the fuel and combustion equipment, the coarse mode is composed primarily of unburned carbon char and associated inherent trace elements (fuel oil) and fragments of inorganic (largely calcium-alumino-silicate) fly ash including trace elements (coal). The three coals also produced a central mode between 0.8- and 2.0-μm aerodynamic diameter. However, the origins of these particles are less clear because vapor-to-particle growth processes are unlikely to produce particles this large. Possible mechanisms include the liberation of micron-scale mineral inclusions during char fragmentation and burnout and indicates that refractory transition metals can contribute to PM <2.5 μm without passing through a vapor phase. When burned most efficiently, the residual fuel oil produces a PSD composed almost exclusively of an ultrafine mode (~0.1 μm). The transition metals associated with these emissions are composed of water-soluble metal sulfates. In contrast, the transition metals associated with coal combustion are not significantly enriched in PM <2.5 μm and are significantly less soluble, likely because of their association with the mineral constituents. These results may have implications regarding health effects associated with exposure to these particles.

ON TRIMODAL PARTICLE SIZE DISTRIBUTIONS IN FLY ASH FROM PULVERIZED-COAL COMBUSTION

Combustion-generated fine particles, defined as those with aerodynamic diameters less than 2.5 μm, have come under increased regulatory scrutiny because of suspected links to adverse human health effects. Whereas classical theories regarding coal combustion suggest that mechanisms of ash vaporization and fragmentation lead to bimodal ash particle size distributions (PSDs), this paper presents experimental results supporting other existing hypotheses that three distinct ash modes may be more appropriate. This paper focuses on the existence and generality of a central mode, between approximately 0.7 and 3.0 μm diameter. This central mode is presumably caused by fragmentation mechanisms, but is still important from a health perspective, because a large portion is contained within the 2.5 μm particle size fraction. Presented here are experimental results from two different laboratory combustors and one industrial boiler, all burning pulverized coals. Use of a variety of particle-sampling and size classification methods, including electrical mobility, time-of-flight, and inertial (low-pressure impaction) methods, confirms that the central mode is not an artifact of the particle-sampling and -sizing methods used. Results from the combustion of 10 different coals consistently show that this central mode is significant for both high-and low-rank coals. Size-segregated elemental distributions of calcium, iron, and aluminum provide additional insight into mechanisms of formation of each mode. Field tests show that the central mode can be the major contributor to fine particle emissions leaving an electrostatic precipitator (ESP). The new experimental results presented here are interpreted in the light of complementary existing data and available theories from the literature.

Metal capture by sorbents in combustion processes

Abstract The use of sorbents to control trace metal emissions from combustion processes was investigated, and the underlying mechanisms governing the interactions between trace metals and sorbents, were explored. Emphasis was on mechanisms in which the metal vapor was reactively scavenged by simple commercial sorbents, to form water unleachable products, which are easy to collect and isolate from the environment. Results are presented from two different scales of experimentation, involving a bench scale thermo-gravimetric reactor and a 17 kW down-fired laboratory combustor, respectively. Results from the bench scale tests showed that lead and cadmium, vaporized from the chloride salt, could be reactively captured at temperatures above the dew point. Both kaolinite and bauxite were effective sorbents for lead, while bauxite but not kaolinite was effective for cadmium. The primary reaction products, as identified by X-ray diffraction analyses, consisted of lead and cadmium aluminosilicates. Laboratory combustor tests, completed in the absence of coal char or coal ash particles, showed that lead could be effectively reactively scavenged in situ, in a combustor, downstream of the primary flame. Here, the high temperatures of the combustion process were being exploited to promote the reactions between the metal vapor and kaolinite sorbent, that were identified in the bench scale tests.

Partitioning of arsenic, selenium, and cadmium during the combustion of Pittsburgh and Illinois #6 coals in a self-sustained combustor

Abstract An important environmental issue is the emission of semi-volatile toxic metals, such as arsenic, selenium, and cadmium, from the combustion of coal. These materials may vaporize in the hot portions of the combustor then return to the solid phase in cooler zones of the process downstream. Understanding the mechanisms by which toxic metals partition between the vapor and solid phases is an important step for predicting and mitigating the effect of these metals upon the environment. Particulate ash samples were withdrawn from a 17-kW pilot-scale downflow combustor in which pulverized coal was burned under self-sustaining conditions. The samples were size segregated in a Berner low pressure impactor, and then analyzed using neutron activation. This research approach has suggested mechanisms, which govern the partitioning of arsenic, selenium, and cadmium in practical pulverized coal combustion processes. The results suggest that volatilization and subsequent transfer of selenium to submicron particle surfaces appears to be an important post-combustion phase mechanism for Illinois #6 coal but not for Pittsburgh seam coal. Most of the selenium in the Pittsburgh submicron fly ash, cadmium in Illinois #6 submicron fly ash, and arsenic in both Pittsburgh and Illinois #6 submicron fly ash enters the post-combustion zone in the solid phase. The dominant heterogeneous partitioning mechanism for transformation to large, supermicron particles is the reaction of metal vapor on the surface or within the pores of an ash particle. The results also suggest that the rate of transformation is dominated either by an exterior surface reaction-controlled regime or a pore diffusion-controlled regime. A relationship between the concentration of solid phase arsenic, selenium, and cadmium to calcium in supermicron particles was also observed, suggesting the formation of As–Ca, Se–Ca, and Cd–Ca reaction products.

Sorbent capture of nickel, lead, and cadmium in a laboratory swirl flame incinerator

Abstract The in situ capture of toxic metals by sorbents was investigated in a small semi-industrial scale 82 kW research combustor. Metals considered were nickel, lead, and cadmium. These metals were introduced into the system as aqueous nitrate solutions, sprayed down the center of a natural gas flame, supported on a variable swirl burner. Kaolinite, bauxite, and hydrated lime were injected along the centerline in the postflame, near the peak system temperature. Measurements of both the submicron acrosol size distribution and the size segregated particulate composition in the exhaust allowed the effects of sorbent injection to be ascertained, both with and without the presence of chlorine. Lead and cadmium could be almost completely scavenged by kaolinite, which formed melted particles. Bauxite, which did not melt, was exceedingly effective in capturing cadmium. However, chlorine inhibited metal capture in these instances. Hydrated lime also captured cadmium to form a eutectic melt, and this process was slightly enhanced by chlorine. Nickel alone did not significantly vaporize and was not captured by kaolinite. However, in the presence of chlorine, nickel did vaporize and was effectively captured. These results are interpreted and compared to bench scale results in the literature. Two mechanisms, or scenarios, for toxic metal capture are presented.

High-temperature interactions between multiple-metals and kaolinite

High-temperature combustion flue gases provided the environments for this investigation of multiple-metals interactions with dispersed kaolinite. Whereas previous research was restricted to interactions of single-metals with kaolinite powders, this work focuses on understanding how kaolinite transformations induced by reaction with one metal species can affect the mechanisms of interaction with other metal species. Experiments were performed in an 18 kW downflow furnace, by doping combinations of semi-volatile metals through a natural gas flame, and injecting supermicron sized powder kaolinite sorbent into the post flame. An aerosol size fractionation method was used to quantify the fraction of metal captured. It was found that the melt-associated restructuring of the meta-kaolinite crystal, caused by eutectics formed with lead and sodium reaction products (self-enhancing for lead and sodium capture), enhances the capture of cadmium by kaolinite at 1160°C. Cadmium reaction products by themselves fail to initiate a self-enhancing eutectic-melt at this temperature, because cadmium forms a higher temperature eutectic with the meta-kaolinite crystal than does lead or sodium. In fact, at temperatures in the range 1000°C to 1300°C, cadmium enhances the capture of lead and sodium by slowing down the deactivating excessive-melt driven by the lead or sodium/sorbent eutectic at these temperatures. Hence, total metal capture is enhanced in this temperature range for the Cd/Pb and Cd/Na systems by the formation of an optimum eutectic-melt, whereby significant melt-enhancement is induced without sorbent deactivation. Lead and sodium reaction products form similar high-temperature eutectics with kaolinite. However, sodium capture dominates over lead capture with a slightly higher reaction rate than lead, thus achieving capture before the most significant sorbent deactivation occurs. Sodium also dominates by effectively displacing lead previously captured on sorbent sites. These observations are quantified using models that extend those developed for single-metal capture to multiple-metal situations.

Air staging and reburning mechanisms for NOx abatement in a laboratory coal combustor

Abstract Mechanisms that govern the formation and destruction of nitrogenous species in fuel rich zones caused by air staging and reburning were explored in a laboratory coal combustor. The objective was to determine whether the same simplified, but fundamentally based, gas phase mechanism would predict time resolved profiles of all nitrogenous species, for both NO x abatement procedures. Experimentation was conducted on a 17 kW down-fired pulverized coal combustor, burning a variety of coals, under various air staged and fuel staged configurations. In the fuel rich pulverized coal post-flame, HCN appearance in the bulk gas phase was not due to the slow release of nitrogen from the coal residue, as was previously hypothesized. HCN formation and destruction in fuel rich regimes were governed by homogeneous gas phase kinetics alone. The fixation of N 2 by hydrocarbons to form HCN, together with reactions of hydrocarbon radicals with either NO or N atoms, could explain HCN formation, during both air and fuel staging. The interconversion of nitrogenous species in the fuel rich zones of air or fuel staging configurations could be adequately described by a simplified mechanism based on known detailed gas phase reactions and partial equilibrium assumptions. Profiles of NO, HCN and NH 3 were adequately predicted for both air and fuel staged NO x abatement configurations, and the proposed mechanism should prove useful for incorporation into more complicated furnace models to predict NO emissions from practical coal combustion systems.

The partitioning of iron during the combustion of pulverized coal

Abstract The partitioning of iron during pulverized coal combustion was investigated both experimentally and theoretically. Emphasis was on determining how coal variables and combustion conditions influenced the formation of slagging precursors. Experimental work consisted of burning a suite of six well-characterized coals in an aerodynamically well-defined 17 kW downflow combustor. Speciation of iron in collected ash samples was determined by Mossbauer Spectroscopy. A model was developed to predict the partitioning of iron for the coals and experimental conditions examined. The model was based on a competition between pyrite oxidation and iron capture by silicates and required as input, CCSEM data on the initial distribution of mineral matter in the coal. It used literature-derived mechanisms and kinetics to calculate pyrite oxidation and char combustion rates, together with a new glass formation mechanism, coupled with current knowledge of sintering. Requiring only two unknown parameters to be fitted, the model agreed well with 26 sets of experimental data, which covered a wide range of iron partitioning in the fly ash. Both experimental and theoretical results suggest that removal from the parent coal, of extraneous iron alone, may not be very effective in eliminating slag formation in boilers.