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

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

A large portion of the Department of Energy (DOE) radioactive waste inventory is composed of metal-bearing liquid wastes, which may or may not also contain organics. It is highly desirable to concentrate the metals in order to reduce the volume of these wastes and to render them into an environmentally benign form. One method for doing this is to exploit high-temperatures to reactively capture metals by sorbents, and thus to isolate them from the environment. The objective of this research is to provide the background information necessary to design a process that accomplishes this on a large scale, namely in hot turbulent flows, into which are injected the wastes to be treated and, also the sorbents that scavenge the metals. The current work focuses on cesium and strontium, which are present in the DOE inventory as radioactive isotopes. The project involves five investigators at three institutions, and is comprised of the following parts: (1) Experimental research at the University of Arizona focuses on the chemistry of cesium and strontium sorption on kaolinite and lime sorbents in a laminar flow environment. (2) Theoretical research pursued jointly by the University of Arizona and Sandia Laboratories, Livermore, focuses on prediction of droplet trajectories in a turbulent flow environment. (3) Experimental research at the Air Pollution Technology Branch of the US Environmental Protection Agency, to investigate the process in turbulent flows. (4) Experimental research at the University of Illinois focuses on design, construction, and application of a laser based LIBS system for measuring droplet size, metal concentration in the gas phase, and metal concentration in the vapor phase. This analysis procedure will be used both at the University Of Arizona laminar flow reactor and the EPA turbulent flow reactor. (5) Theoretical research at the University of Illinois to provide input into the drag model to be used to predict droplet trajectories in hot turbulent flows

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