There is a general understanding that the size of ash particles produced during coal combus- tion decreases with decreasing coal particle size and with decreasing mineral content of the parent coal particles.
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There are, however, no fundamental models that allow the researchers to predict the change in the size of ash particles when coal is finely ground or beneficiated or how ash size is affected by combustion conditions. Bead T 0. The utilization of the calcium is currently as low as 20 percent, which results in a large volume of spent sorbent. The problems preventing better sorbent utili- zation are the sintering of pores at high tem- peratures near the name zone, the low diffusivity of sulfur dioxide through the layer of calcium sulfate that forms on grains of the calcium oxide sorbent, and pore plugging.
There is opportunity for major innovation in the design of sorbents for sulfur capture in combustors by tailoring their physical and chemical properties. The key characteristics of an ideal sorbent are large surface area, mechanical strength, and fast and complete utilization. Used sorbent should be regenerable or usable as a by-product. The hot stream of gases generated carries small particles up the combustor 2.
The particles of ash, gypsum, and limestone called "sand" in the gas stream are separated and collected by large cyclone separators 3. The hot gases 4 are ducted to preheat boiler water. The hot sand transfers heat to boiler tubes 5 , generating steam. Cooled sand particles 6 are recycled to the combustor.
Hazardous Substances (Chemicals)
Courtesy, E. Fires and Explosions Fires and explosions cause major property loss within the chemical process industry; more significantly, they account for an annual loss in this country of thousands of lives and the destruction of billions of dollars of property. The chemical engineer can contribute to the solution of the overall fire problem by providing means of estimating the flammability, flame propagation rates, and products of incomplete combustion for the increasing diversity of in- dustrial and manufacturing materials, including polymer and ceramic composites.
Examples of the pressing problems to which the chemical engineer can contribute follow. At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiDammable fire-retardant material. The principal elements of the problem include pyrolysis of solids; char- layer buildup; buoyant, convective, turbulent- boundary-layer heat transfer; soot formation in the flame; radiative emission from the sooty flame; and the transient nature of the process char buildup, fuel burnout, preheating of areas not yet ignitedJ.
Efforts are needed to develop computer models for these effects and to de- velop appropriate small-scale tests. Most fire deaths are caused by smoke inha- lation rather than by burns. Buildings now contain many synthetic polymeric materials that can burn to yield such toxic compounds as hydrogen cyanide and hydrogen chloride in addition to common combustion products such as carbon monoxide.
For this reason, consid- eration is being given to banning certain mate- rials, at least in public buildings. Realistic hazard analyses for materials would be facilitated by computer models that could interrelate such significant factors as the identity and amount of toxic products formed by combustion of these materials, the rate at which the materials burn, and the ease of ignition and smoke-forming tendency of the materials. Furthermore, as combustion products are transported away from the flame e. Any interdisciplinary effort to un- derstand the hazards of fires involving synthetic materials would benefit from chemical engi- neering research expertise in reaction modeling, chemical kinetics, and heat and mass transfer.
A small fire in a computer room, a telephone exchange, or an assembly plant for communi- cation satellites can cause enormous damage because of minute amounts of corrosion on circuit elements.
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Furthermore, if either water or a halogenated agent is used to control the. Energy and material :recovery processes: Management paths for hazardous waste include temporary storage, treatment, disposal, and dispersion. Courtesy, Office of Technology Assess- ment. As automated and robotic systems become more prevalent in manufacturing plants, vulnerability of plants to small fires will in- crease. Chemical engineering research relevant to this challenge includes the development of sensors for more sophisticated fire detection, the design and development of materials and techniques for encapsulation of sensitive device elements, research on surfaces and interfaces to facilitate more effective equipment salvage, and research to develop a better understanding of corrosion mechanisms so that optimal strat- egies for fighting small fires can be developed.
Considerable basic research has been conducted to understand this transition in the combustion of hydrogen and significant progress has been made.
Extrapolating this understanding to more complex compounds and to mix- tures of the chemicals found in chemical plants is a challenging problem. The problems of han- dling and disposing of radioactive waste are largely the concern of nuclear engineers, often working with chemical engineers to de- velop separation and encapsula- tion technologies for radioactive nuclides, and are not discussed here.
The most basic way to deal with the continuing generation of hazardous waste is to accumulate, encapsu- late, and store only as a temporary measure and to develop new approaches to reduce the volumes generated and to concentrate hazard- ous components or convert them into nonha- zardous materials. Abandoned waste sites are remediated by cleaning them up or containing them before they contaminate groundwater sup- plies. The establishment of priorities for site cleanup and the development of appropriate detoxification technologies require an under- standing of the processes by which the waste can migrate or be transformed in the natural environment.
The development of a fundamental under- standing of the behavior of toxic chemicals in atmospheric, soil, and aquatic environments Figure 7. Nevertheless, the ability of American manufacturing industries to remain internation- ally competitive depends on this. Engineers in. A major effort must be mounted to conduct advanced research and to educate engineers to solve the problems associated with the disposal and environmental behavior of toxic chemicals.
Detoxification of Currently Generated Waste Many technologies have been proposed for detoxifying waste by processes that destroy chemical bonds: pyrolytic; biological; and cat- alyzed and uncatalyzed reactions with oxygen, hydrogen, and ozone. Courtesy, Of lice of Technology Assessment.
Hazardous Substances (Chemicals) - OHS Reps
Heating methods include resistive electrical heating, the use of radio frequencies or microwaves, radiative heating, and the use of hot combustion products as a heat source. Heating can be done in the absence of oxygen, in which case the process is known as pyrolysis. Pyrolysis yields different products than does combustion of waste in the presence of oxygen.
The most important current technique for the thermal destruction of waste is incineration, where the energy required for destruction is provided by oxidation of the waste, sometimes supplemented with a fossil fuel. The major question about all thermal destruction tech- niques is whether products from the process- either traces of unreacted parent compound or compounds synthesized from the parent com- pound at high temperature will pose a health hazard.
Concerns have been expressed about incin- eration on land and in the water. EPA's Science Advisory Board, in a report entitled Incin- eration of Hazardous Liquid Waste, stated, "The concept of destruction efficiency used by the EPA was found to be incomplete and not useful for subsequent exposure assessments.
Mere destruction of the original hazardous material is not, however, an adequate measure of the performance of an incinerator. Products of incomplete combustion can be as toxic as, or even more toxic than, the materials from which they evolve. Indeed, highly mutagenic PAHs are readily generated along with soot in fuel-rich regions of most hydrocarbon flames. Formation of dioxins in the combustion of chlorinated hydrocarbons has also been re- ported. We need to understand the entire se- quence of reactions involved in incineration in order to assess the effectiveness and risks of hazardous waste incineration.
The routine monitoring of every hazardous constituent of the effluent gases of operating incinerators is not now possible. EPA has es- tablished procedures to characterize incinerator performance in terms of the destruction of selected components of the anticipated waste stream. These compounds, labeled principal organic hazardous components POHCs , are currently ranked on the basis of their difficulty of incineration and their concentration in the anticipated waste stream.
The destruction effi- ciency is expressed in terms of elimination of the test species, with greater than The effectiveness of incineration has most commonly been estimated from the heating value of the fuel, a parameter that has little to do with the rate or mechanism of destruction. Alternative ways to assess the effectiveness of incineration destruction of various constituents of a hazardous waste stream have been pro- posed, such as assessment methods based on the kinetics of thermal decomposition of the constituents or on the susceptibility of individual constituents to free-radical attack.
Laboratory studies of waste incineration have demonstrated that no single ranking procedure is appropriate for all incinerator conditions. For example, acceptably low levels of some test compounds, such as methylene chloride, have proved diffi- cult to achieve because these compounds are formed in the flame from other chemical species. Rather than focus on specific incineration technologies, one must address the fundamental physical and chemical processes common to many of the possible incineration systems through studies of 1 reaction kinetics of selected waste materials and 2 behavior of waste solutions, slurries, and solids in the incineration environ- ment.
The combustion chemistry of methane and C-2 hydrocarbons is reasonably well under- stood. Progress is being made in addressing the pyrosynthesis reactions that lead to the for- mation of toxic PAHs.
Much of the literature on combustion, though, is devoted to the flame zone, where heat release rates and free-radical concentrations are high. A key problem in incineration chemistry involves understanding the late stages of degradation of waste materials, where temperatures and free-radical concentra- tions are lower than in the flame zone. The effect of such reactions on the incineration of hazardous substances containing halogen atoms needs to be determined. We are concerned both with the destruction of the original compounds and with the production of trace quantities of other hazardous species dur- ing the reaction.
Thermal pyrolysis and reac- tions of the waste with common radicals such as OH, O. Both reducing and oxidizing atmospheres are encountered in turbulent diffusion flames; therefore, an under- standing is needed of the chemistry over the entire range of combustion stoichiometries. Studies of the incineration of liquid and solid wastes must determine the rates at which haz- ardous compounds are released into the vapor phase or are transformed in the condensed phase, particularly when the hazardous mate- rials make up a small fraction of the liquid burned.
We must be particularly concerned with understanding the effects of the major compo- sition and property variations that might be encountered in waste incinerator operations- for example, fluctuations in heating value and water content, as well as phase separations. Evidence of the importance of variations in waste properties on incinerator performance has been demonstrated by the observation of major surges in emissions from rotary-kiln in- cinerators as a consequence of the rapid release of volatiles during the feeding of unstable ma- terials into the incinerator.
For example, the insertion of foreign genes from. One microorganism into another has become relatively routine. Progress in achieving high expression of a foreign inserted gene has also been Impressive. A combination of molecular biology and chemical engineering could lead to the design of new processes for waste treatment. The controlled use of biological systems or their products to bring about chemical or phys- ical change is particularly attractive when deal- ing with dilute waste streams. Biological sys- tems thrive in dilute aqueous media, where they can effectively degrade organic pollutants, ab- sorb heavy metal ions, or change the valence state of heavy metal ions Table 7.
In addition, many microbial systems have high affinity for metal ions, and metal ions are often moved from an aqueous solution into the cell through active transport. Accordingly, such reactions as the biological reduction of a heavy metal ion can be carried out at relatively fast rates, although at millimolar concentrations.
Related research oppor- tunities for chemical engineers include the for- mulation of biocatalysts, the development of bioseparations, and the use of chemical engi- neering expertise in process control and opti- mization to better understand the behavior of large microbial populations. The promise of biological treatment of heavy metal ions has already been illustrated by strains of microorganisms that tolerate mercury, chro- mium, and nickel heavy metals that are gen- erally toxic to microorganisms.
Tolerant micro- organisms have been isolated through classical adaptation and strain-selection studies rather than by recombinant DNA techniques. Mer- cury-tolerant microorganisms have been shown to possess an enzyme that is not present in nonresistant strains. This enzyme, mercuric reductase, is able to catalyze the reduction of mercury II ions to metallic mercury.
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Since the mercury II ion is the toxic species and the insoluble metal is chemically inactive, the mi- croorganism is able to detoxify a solution that contains mercury II ions. Microorganisms hold tremendous promise for improvements in the treatment of hazardous waste, but genetically altered microorganisms present both regulatory bodies and industry- the complex task of identifying, managing, and controlling their use.