Mechanism and Kinetics of Lead Capture by Kaolinite in a Downflow Combustor

An 18 kW, 6 m long, gas-fired downflow combustor was used to examine the postflame reaction between lead vapor and kaolinite. Since the focus of this work was to evaluate the reaction with lead vapor, samples were extracted at a temperature above the metal dew point. The sample was rapidly diluted with nitrogen such that lead vapor homogeneously nucleated to form small particles in the presence of a pre-existing particle population. These small lead particles were easily distinguished from lead reacted to large sorbent particles; hence, multicomponent particle size distributions were used to determine the extent of lead capture. A parametric study was conducted in which sorbent/metal molar ratio and sorbent injection temperature were controlled. Temperatures and residence times were varied by changing the sorbent injection point in the combustor, which had the approximate time and temperature profile of practicalscale units. The effect of chlorine was evaluated by doping chlorine gas into the flame. Results show that lead capture by kaolinite was reduced at higher temperatures and in the presence of chlorine. A two-reaction mechanism is proposed to model the apparent temperature inhibition. In the primary capture reaction, lead oxide reacts with activated kaolinite and forms a lead aluminosilicate product. Subsequently, a reaction between this product and activated kaolinite acts to inhibit further lead capture. First-order rate expressions are proposed for each reaction, and kinetic parameters are estimated from experimental results. The primary capture reaction appears to have an activation energy that is approximately zero. The inhibition reaction has an activation energy of ϳ10 2 kJ mol ‫1מ‬ . To model the effect of chlorine, the reaction scheme is modified to account for the partitioning of lead between lead oxide and lead chloride. Based on experimental results, the concentration of lead chloride vapor in the system is significantly higher than predicted by equilibrium calculations. Introduction The emission of toxic elements from stationary combustion sources, such as incinerators and coalfired boilers, is a major concern. Of particular concern are semivolatile toxic elements, for example, lead and cadmium, which vaporize and condense within the combustion system. At high temperatures, lead vaporizes and is liberated from ash particles. As the temperature of the combustor decreases, the vapor condenses to form, in part, submicron particles. These submicron particles can penetrate conventional air pollution control systems, like baghouses and electrostatic precipitators, and be emitted to the environment One potential method to control toxic metal emissions is to inject a high-temperature sorbent into the postflame region. The sorbent powder, which is easily collected, reacts with metal vapors and prevents the subsequent vapor-to-particle transformation processes that form submicron particles. Previous researchers have examined the reaction of lead and a clay-based sorbent, kaolinite. In a series of benchtop experiments where large kaolinite flakes were exposed to lead chloride vapor, Uberoi and co-author

Isolation of Metals from Liquid Wastes: Reactive Scavenging in Turbulent Thermal Reactors

The Overall project demonstrated that toxic metals (cesium Cs and strontium Sr) in aqueous and organic wastes can be isolated from the environment through reaction with kaolinite based sorbent substrates in high temperature reactor environments. In addition, a state-of-the art laser diagnostic tool to measure droplet characteristic in practical 'dirty' laboratory environments was developed, and was featured on the cover of a recent edition of the scientific journal ''applied Spectroscopy''. Furthermore, great strides have been made in developing a theoretical model that has the potential to allow prediction of the position and life history of every particle of waste in a high temperature, turbulent flow field, a very challenging problem involving as it does, the fundamentals of two phase turbulence and of particle drag physics

Isolation of Metals from Liquid Wastes: Reactive Scavenging by Sorbets in Turbulent Reactors

The objective of this work is to develop the fundamental knowledge base for the design of a broad class of high-temperature reactive capture processes to treat metals-bearing liquid waste in the DOE inventory. The major thrust is devoted to understanding phenomena that govern process performance and are critical to achieving emission specifications

NOx, FINE PARTICLE AND TOXIC METAL EMISSIONS FROM THE COMBUSTION OF SEWAGE SLUDGE/COAL MIXTURES: A SYSTEMATIC ASSESSMENT

This research project focuses on pollutants from the combustion of mixtures of dried municipal sewage sludge (MSS) and coal. The objective is to determine the relationship between (1) fraction sludge in the sludge/coal mixture, and (2) combustion conditions on (a) NOx concentrations in the exhaust, (b) the size segregated fine and ultra-fine particle composition in the exhaust, and (c) the partitioning of toxic metals between vapor and condenses phases, within the process. The proposed study will be conducted in concert with an existing ongoing research on toxic metal partitioning mechanisms for very well characterized pulverized coals alone. Both high NOx and low NOx combustion conditions will be investigated (unstaged and staged combustion). Tradeoffs between CO{sub 2} control, NO{sub x} control, and inorganic fine particle and toxic metal emissions will be determined. Previous research results have demonstrated that the inhalation of coal/MSS ash particles cause an increase in lung permeability than coal ash particles alone. Elemental analysis of the coal/MSS ash particles showed that Zn was more abundant in these ash particles than the ash particles of coal ash alone

Oxy-coal combustion for retrofit: challenges and opportunities

This paper is concerned with Oxy-Fuel Combustion, a process for the control of carbon dioxide from a range of solid fuel combustion processes. The focus is on the applicability of this technology for retrofit and hence on entrained flow combustors rather than on circulating fluidized bed systems. First, the importance of retrofit technology for CO2 control from coal fired units in the US is outlined. The current state of the technology (applied mainly to pulverized coal combustors), and critical research needs are identified. There are three over-arching issues that must be resolved in order for oxy-coal retrofit to be attractive: the first is related to the O2 supply energy penalty since current cryogenic technology can consume 15-20% of energy produced; the second is related to the purity of CO2 in the flue gas for sequestration; and the third is related to air ingress through leaks that are present in most existing systems. Resolution of these over-arching issues requires efforts by industrial gas engineers, politicians, and combustion engineers, respectively. One objective of current research in oxy-coal combustion for retrofit is to create enabling technology to be used for: 1) extrapolation of boiler performance from conventional air firing to oxygen firing with sufficiently large amounts of flue gas recycle to match heat transfer to existing heat exchange surfaces. 2) extrapolation to substantially modified existing units that "still look like boilers" but that optimize and reduce the amount of flue gas to be recycled and therefore may require some relocation of heat transfer surfaces. This enabling technology will be comprised of simulations employing validated heat transfer sub-models (radiant and convection zones), and validated chemistry sub-models (coal jet ignition, chemistry, char burnout, ash partitioning, trace metals, and combustion by-products - NOx, SOx, Hg), where validation must be under oxy-fuel combustion conditions, with varying amounts and compositions of flue gas recycle streams. Recent research results in each of these areas are presente

Oxy-coal combustion for retrofit: challenges and opportunities

reportThis paper is concerned with Oxy-Fuel Combustion, a process for the control of carbon dioxide from a range of solid fuel combustion processes. The focus is on the applicability of this technology for retrofit and hence on entrained flow combustors rather than on circulating fluidized bed systems. First, the importance of retrofit technology for CO2 control from coal fired units in the US is outlined. The current state of the technology (applied mainly to pulverized coal combustors), and critical research needs are identified. There are three over-arching issues that must be resolved in order for oxy-coal retrofit to be attractive: the first is related to the O2 supply energy penalty since current cryogenic technology can consume 15-20% of energy produced; the second is related to the purity of CO2 in the flue gas for sequestration; and the third is related to air ingress through leaks that are present in most existing systems. Resolution of these over-arching issues requires efforts by industrial gas engineers, politicians, and combustion engineers, respectively. One objective of current research in oxy-coal combustion for retrofit is to create enabling technology to be used for: 1) extrapolation of boiler performance from conventional air firing to oxygen firing with sufficiently large amounts of flue gas recycle to match heat transfer to existing heat exchange surfaces. 2) extrapolation to substantially modified existing units that "still look like boilers" but that optimize and reduce the amount of flue gas to be recycled and therefore may require some relocation of heat transfer surfaces. This enabling technology will be comprised of simulations employing validated heat transfer sub-models (radiant and convection zones), and validated chemistry sub-models (coal jet ignition, chemistry, char burnout, ash partitioning, trace metals, and combustion by-products - NOx, SOx, Hg), where validation must be under oxy-fuel combustion conditions, with varying amounts and compositions of flue gas recycle streams. Recent research results in each of these areas are presente

Particle Size Distributions of Fly Ash Arising from Vaporized Components of Coal Combustion: A Comparison of Theory and Experiment

A 100-kW-rated down-fired pilot-scale combustor was used to explore sub-micrometer coal ash aerosol formation for two coals under various air and oxy-combustion atmospheres. Particle size distribution (PSD) data were obtained through isokinetic sampling and then by electron mobility and light-scattering particle sizing. The sub-micrometer portion of the PSD exhibited an “accumulation” mode at ∼0.3 μm and, in some cases, an additional “nucleation” mode between 0.03 and 0.07 μm. Predictions of the temporal evolution of the sub-micrometer aerosol were made using a sectional coagulation model. A comparison to experimental measurements suggested that the “accumulation” mode was formed by coagulation of vaporized silicon-rich species, which occurred and was completed very close to the parent char particle and not in the mixed flue gas. This showed the importance of carefully characterizing microscale mixing phenomena around individual particles. For the sodium-rich species that had heretofore been thought to nucleate in the sampling probe, it now seems that they nucleate within the furnace, but coagulation without particle growth was insufficient to explain the location of the “nucleation” modes for all but one case explored. For that one coal, the “nucleation” mode was dominated by high concentrations of particles containing calcium, and there, its location was consistent with coagulation. Additional modeling involving both coagulation and particle growth is required