Combustion control methods

Selection of pollution control methods is generally based on the need to control ambient air quaUty in order to achieve compliance with standards for critetia pollutants, or, in the case of nonregulated contaminants, to protect human health and vegetation. There are three elements to a pollution problem a source, a receptor affected by the pollutants, and the transport of pollutants from source to receptor. Modification or elimination of any one of these elements can change the nature of a pollution problem. For instance, tall stacks which disperse effluent modify the transport of pollutants and can thus reduce nearby SO2 deposition from sulfur-containing fossil fuel combustion. Although better dispersion aloft can solve a local problem, if done from numerous sources it can unfortunately cause a regional one, such as the acid rain now evident in the northeastern United States and Canada (see Atmospheric models). References 3—15 discuss atmospheric dilution as a control measure. The better approach, however, is to control emissions at the source. 

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

NOx emission controls in large engines and turbines are based on the same principles. However, special designs must be applied to accommodate differences in the combustion process. Methods to control NfJ include the following. 

The excellent detection ability for flames makes UV sensing a good method for remote fire alarm-monitoring. UV radiation after the outbreak of a fire reaches a sensor much faster than heat or smoke. Also, the distance between sensor and fire is less critical. Requirements for the sensor are high sensitivity and excellent selectivity. Radiation intensities at the sensor position may be even lower and the ambient light conditions less restricted than for combustion controlling. When used outside, solar-blindness is a must. These stringent requirements make UV fire alarm monitors expensive, and they are used in industrial environments such as production floors or warehouses rather than in private homes. 

The PC JVS PDF method [23, 24] requires remarkable computational power for attaining reasonable accuracy of results. Hence an alternate mathematical method is needed to determine interrelations between various governing parameters. This method should have acceptable accuracy to obtain necessary estimates relatively fast and to test the efficiency of various approaches to passive and active combustion control. The corresponding estimates obtained would be used for more detailed studies by the PC JVS PDF method. 

The Presumed Probability Density Function method is developed and implemented to study turbulent flame stabilization and combustion control in subsonic combustors with flame holders. The method considers turbulence-chemistry interaction, multiple thermo-chemical variables, variable pressure, near-wall effects, and provides the efficient research tool for studying flame stabilization and blow-off in practical ramjet burners. Nonreflecting multidimensional boundary conditions at open boundaries are derived, and implemented into the current research. The boundary conditions provide transparency to acoustic waves generated in bluff-body stabilized combustion zones, thus avoiding numerically induced oscillations and instabilities. It is shown that predicted flow patterns in a combustor are essentially affected by the boundary conditions. The derived nonreflecting boundary conditions provide the solutions corresponding to experimental findings. 

The use of feedback-control techniques to modulate combustion processes in propulsion systems has recently received extensive attention [1-3]. Most of the previous studies involved direct implementation of existing control methods designed for mechanical devices, with very limited effort devoted to the treatment of model and parametric uncertainties commonly associated with practical combustion problems. It is well established that the intrinsic coupling between flow oscillations and transient combustion responses prohibits detailed and precise modeling of the various phenomena in a combustion chamber, and, as such, the model may not accommodate all the essential processes involved due to the physical assumptions and mathematical approximations employed. The present effort attempts to develop a robust feedback controller for suppressing combustion instabilities in propulsion systems. Special attention is given to the treatment of model uncertainties. Various issues related to plant... 

In recent decades, catalytic combustion has been explored as a primary control method for the production ofheat and energy with two main goals (i) to achieve ultra-low emissions of NO, CO and unburned hydrocarbons (UHCs) and (ii) to obtain stable combustion under conditions not allowed by conventional methods. 

Precombustion control involves removal of sulfur compounds from fuel prior to combustion. Control during combustion employs techniques to minimize the formation and/or release of SO2 and N0X during the combustion process. Finally, SO2 and N0X can be removed from the combustion flue gas using various postcombustion control methods. This chapter discusses the potential of mitigating acid deposition through precombustion cleaning of coal to remove sulfur compounds. 

Emissions from biomass-fueled boilers can be controlled by a variety of methods. The control systems needed depend mainly on the composition of the feedstock. First, good combustion control is essential to maximize combustion and to minimize emissions of unburned hydrocarbons and carbon monoxide. Efficient removal of particulate matter in the flue gases can be achieved by various combinations of cyclonic separation, electrostatic precipitation, agglomeration, and filtration. Removal of acid gas emissions can be achieved by flue gas scrubbing and treatment with lime. There are several approaches to the control of NO, emissions (Clearwater and Hill, 1991). Combustion control techniques include use of staged combustion, low excess... 

Environmental concerns and strict pollution legislation prompted action in the construction of up to date coal-fired boilers and adaptation of existing plants. At the present time, flue gas desulfurization is the only conventional method employed on a commercial scale for reducing sulfur emissions after coal combustion. Over 90% reduction of sulfur dioxide in flue gases can be achieved by this process. Combustion control techniques of the flames will effectively reduce oxides of nitrogen emissions into the atmosphere. 

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