Numerical Simulation And Experimental On The Combustion Character In LCFB For Industrial Sludge And Coal Residue

There are large numbers of pathogens, heavy metals and other toxic substances in Industrial Sludge (IS) from different industries. Meanwhile, Coal Residue (CR) has become one of the largest emissions of industrial solid waste at present. CR is characterized by low volatility and low heat value, high ash, and difficult combustion. Stacking CR causes serious resources waste and environment pollution, which brings ecological damage. It is paid more attention and further study so that sludge is dealt with reduction, non-toxic, recovery. Coal refuse should be efficiently utilized in order to turn waste into wealth and improve the environment. In recent years, it has become a research hotspot to use the IS-CR energy and to decrease the emissions with the technology of fluidized bed combustion all over the word. It is used to treat industrial sludge (IS) and coal residue (CR), which could prove to solve both environmental and energy issues. The Low Circulating Ratio CFB (LCFB) combustion technology can improve the combustion stability of industrial sludge and coal refuse, reduce secondary pollution, effectively utilize waste and have an important practical value. However, there are few references to systematically research of the pyrolysis and combustion characteristics, and the influence of gas-solid multiphase flow. The combustion and products characteristics in LCFB is not enough clear. It will hinder the development and application of IS-CR-LCFB combustion technology.Therefore, based on the actual pyrolysis, flow and combustion characteristics of IS-CR, the gas-solid multiphase flow and combustion characteristics in LCFB furnace are numerically simulated and experimentally researched, to explore and make clear the gas-solid flow and combustion characteristics, to research and develop the IS-CR-LCFB clean combustion technology, to realize the utilization of waste energy and coal residue. It will enrich and promote the research of LCFB combustion technology. At the same time, it can promote the innovation and applications of co-combustion technology with IS-CR in LCFB. It can achieve comprehensive utilization of resources, promote energy saving and environmental protection, which is of great academic significance and wide application prospects.In this paper, based on in-depth analysis of IS-CR treatment, technology status and research of CFB combustion, it is the first time to systematically study pyrolysis and combustion characteristics of the different IS and CR under single and mixtures conditions by the thermogravimetric analysis method. The kinetic parameters and model are analyzed. Considering the combustion and structure characteristics of 10t/h LCFB boiler, and the IS-CR complex movement characteristics of gas-solid multiphase flow in the LCFB furnace, the multiphase flow mathematical models which describe the basic equations of gas-solid in turbulent are established and the cold flow characteristics are also simulated. Meanwhile, the overall mathematical models of multiphase turbulence which describes the mixed fuel combustion in LCFB are established. The effect of fuel ratio, different fuel characteristics and air distribution ratio on LCFB combustion characteristics of IS-CR are researched for the first time. The optimum IS-CR-LCFB boiler and system have been designed and manufactured. Besides, it is the first time to investigate the cold-state and combustion experiment in 10t/h industrial LCFB boiler, the high efficiency and stable combustion parameters(fuel ratio of CR and IS is 7:3 , the primary and second air distribution ratio is 8:2, rectangle dense segment ) are also gotten, and the test result is well agree with the numerical analysis.Results showed that different species, pyrolysis temperature, heating rate and pressure have an important influence on pyrolysis process, ignition, devolatilization and burnout characteristics, integrated combustion and heat emission performance for IS-CR. The pyrolysis characteristic of a single material in the mixed samples does not significantly change, and each material maintains the ignition and burnout characteristics itself for IS. So combustion characteristic can be expressed by the characteristic superposition of each single component. Moreover, the CR weight loss and preliminary pyrolysis temperature, characteristics are mainly determined by heating rate, pressure etc. For IS, the higher the volatility is, the combustion have gone easier, the igniting and burning out temperature is lower, the better flammability and integrated combustion performance will be. For CR, the activation energy is relatively higher in the initial reaction of pyrolysis, and activation energy decreases in the second stage, the integrated combustion performance is worse, and the reaction progression is respectively 1.5 and 2.0 during the low and high temperature segment. In view of the igniting and burning out characteristics, the integrated combustion performance can improve with mixing a little IS in CR, which is used for improve the combustion efficiency of CR.The gas-solid flow field results showed that smaller particle size which is easy to distribute well which can be beneficial to cold ignition, running load adjust and combustion optimization. And Air distribution ratio of 8:2 for the two densities and different particle sizes of the solid phase can cause reasonable, uniform and stable distribution and is the best operating conditions, which shows a continuous and stable flow along the LCFB height. Each particle concentration distribution is changed with adding the ratio of IS in fuel, the flow field and concentration distribution are useful for combustion with the ratio 7:3 of CR and IS. Furthermore, the stronger welt wall flow and particle back flow zone occurred for the shape of the rectangle, round and squareness dense region of LCFB. The fluid velocity gradually decreases for the rectangle dense region, as a result, the particle uniformity fluid state occurred for each LCFB section, at the same time, the flow of particles are continuous and stable along the height. The hearth shape is rectangle, the fuel ratio is 7:3 and the air distribution ratio is 8:2 which are the best operation conditions and are beneficial to combustion.Combustion simulation results show: With the increasing of the proportion of IS in the fuel of IS-CR, the combustion region is more intense, the temperature becomes higher, the volatility is greater and the ring-core flow character is more obvious, at the same time, the temperature, particle and O₂ concentration difference become smaller and smaller along the height of LCFB. The combustion intensity is the strongest, and the uniformity distribution of temperature, particle and O₂ concentration occurred for the fuel ratio of CR and IS. The IS character have a major impact on improving the combustion characteristics of IS-CR, the higher the volatility and heat value of IS is, the combustion intensity is stronger, the temperature is higher and its change is more obvious, the integrated combustion performance is better. With the adding of the primary air volume, the dense segment combustion temperature and its change increase, the particle concentration decrease in the dense segment but increase in the sparse segment, O₂ concentration decrease range sharply widen. At the same time, the effect to enhance combustion with the second air is weaken, the exit temperature decreases and O₂ concentration increases. The temperature, particle and O₂ concentration distribution are uniformity with the ratio of 8:2 of one and secondary air. It is the first time to obtain the best structure and combustion characteristics of IS-CR in LCFB, the parameters are: the dense segment is rectangle, the fuel ratio of CR and IS is 7:3, one and secondary air ratio is 8:2. The stable combustion is more enhanced, and its efficiency is 97%.Based on the numerical results, the 10t/h LCFB boiler is optimum designed (including boiler style, furnace structure, air chamber, air distributing board, wind cap, inertly separator and stuff back-feeder, air blower etc.) The industrial test results show: when material layer's thickness increases, material layer's resistance also increases. The resistance is related with the ratio of fuel CR and IS. With the increase of IS, the distribution uniformity and continuity of particle concentration become better. But when the ratio of IS is too bigger, the uniformity in the bottom and above of LCFB is easily formed in dense phase segment, which is adverse to combustion. The flue's combustion in each section mainly centralizes on dense phase and heat insulation section and the combustion of dilute section is heavily weak. When the ratio of fuel is 7:3, the temperature of combustion region is high and uniformity in dense phase, which is useful for combustion. The combustion intensity in the bottom region which the ratio of fuel is 7:3 greater than the ratio of fuel is 8:2 and CR, and the oxygen concentration is low in furnace exit. When the ratio of air distribution is 8:2, it not only keeps very relatively high combustion temperature in dense phase, but also oxygen amount and higher temperature which dilute phase's combustion needs are formed. The temperature is high in furnace exit and the combustion efficiency is relatively high. The industrial test study shows that the best combustion effect is gained when the ratio between primary and second wind is 8:2, and the ratio between CR and IS is 7:3. The test results agree with the numerical results.