Physical And Chemical Characteristics Of Dispersion Combustion With Low Oxygen-content And Perturbation Analysis On Thin-walled Regeneration

High temperature air combustion (with low oxygen-content) has many advantages such as lower flame peak temperature, uniform temperature distribution, weaker oxidation loss and lower NO_x emission, and provides a new way to process the low-grade or low-density fuel such as low-heat-value gas, low-heat-value coal, biomass and municipal live waste. Deeper researches on its physical and chemical characteristics and thin-walled regeneration characteristics can improve the control level on NO_x and CO_x emission and energy saving in the combustion process and promote the sustained development of society and economy.Main contents and specialties of this work are summarized as following:(1) A new combustion system named "dispersion combustion with low oxygen-content" had been constructed. Dispersion combustion with low oxygen-content is a kind of combustion that the reaction spot is dispersed and the combustion process is delayed and controlled with the self-ignition of fuel mixture as the stable combustion condition. Its thermodynamic characteristics are the uniform heat release and the lower heat release per volume of reaction region. Its kinetic characteristic is the flame dispersion. Its dispersion performance can be evaluated by the dispersivity and the furnace-temperature unevenness and intensified by the methods that the air injects into the furnace at a high speed, the gas organization in the furnace is alternatively changed at a high frequency, the fuel gas and the air inject into the furnace from different positions or at different time and the dilution of flue gas partially or fully on the outside of furnace.(2) A new numerical simulation method for dispersion combustion with low oxygen-content had been proposed. The introduction of an equivalent specific heat capacity at constant pressure can decrease the simulation error because of the heat loss from combustion chamber and the decomposition heat of polyatomic gas. The combustion simulation based on eddy dissipation model and equivalent specific heat capacity can achieve temperature profiles that basically agree with the experimental ones and dispersion behaviors such as flat flame, unclear flame boundary, uniform temperature distribution, uniform heat release and lower heat release per volume of reaction region. The suitable equivalent specific heat capacity can obtain through the trial calculation on the actual combustion heat-balance test.(3) Kinetic conditions of dispersion combustion with low oxygen-content had been proved by the digital simulations as well as the hot tests. The combustion flame with low oxygen concentration can be converted from tradition to dispersion. The boundary between two flames is affected by the velocity threshold of eddy dissipation model. Eddy dissipation model can not exactly predict the boundary. The higher the fuel flow rate is, the larger the excess air coefficient is or the lower the oxygen concentration is, the lower the maximal furnace-temperature for dispersion combustion is.(4) A new perturbation analytic and numerical method for the thin-walled regenerative process had been presented. Based on a thin-walled assumption and a single parameter perturbation method, the first order asymptotic solution to the regenerator temperature distribution under weak solid heat conduction along the gas flow direction have been obtained. Analytic and numerical solutions agree well with tests and finite-difference numerical solutions. A new kind of digital simulation software of thin-walled regenerative process has been developed. The inlet temperature of air and flue gas and some operation conditions are taken as the information sources, the software can calculate regenerator temperature rapidly and meet the design and operation-control optimization on regenerative process. The optimization of switch time has been designed according to the demands of "steady low-oxygen-content combustion, no moisture condensation on the low-temperature end, economical operation, any large thermal stress and low oxygen concentration mainly from the dilution of remained furnace gas".(5) The temperature efficiency and the switch time of thin-walled regenerator had been optimized by the perturbation. There exists the maximal temperature efficiency and the corresponding optimal switch time. The solid heat conduction along the flow direction and the regenerator heat storage capacity for the unit volume has no impact on temperature efficiency peak and optimal switch time. The decrease of oxygen concentration in the air or circumferential length in the passage leads to the decrease of temperature efficiency peak. Maximal temperature efficiency is directly proportional to the passage length and optimal switch time is directly proportional to the matrix thickness. Temperature efficiency tendency from semi-analysis is the same as that in dispersion combustion industrial tests, and optimal switch time is basically in agreement with that in high-temperature gasification intermediate tests. As to thin-walled regenerators preheating the air and the fuel, the shorter switch time can result in the lower heat efficiency obviously and its later peak of heat efficiency. The longer time the flue gas passing through transition pipes has or the shorter switch time is, the higher heat efficiency error because of the fuel gas loss has.