Combustion Characteristics of Spent Catalyst and Paper Sludge in an Internally Circulating Fluidized-Bed Combustor

Abstract

Combustion of spent vacuum residue hydrodesulfurization catalyst and incineration of paper sludge were carried out in thermo-gravimetric analyzer and an internally circulating fluidized-bed (ICFB) reactor. From the thermogravimetric analyzer-differential thermo-gravimetric curves, the pre-exponential factors and activation energies are determined at the divided temperature regions, and the thermo-gravimetric analysis patterns can be predicted by the kinetic equations. The effects of bed temperature, gas velocity in the draft tube and annulus, solid circulation rate, and waste feed rate on combustion efficiency of the wastes have been determined in an ICFB from the experiments and the model studies. The ICFB combustor exhibits uniform temperature distribution along the bed height with high combustion efficiency (>90%). The combustion efficiency increases with increasing reaction temperature, gas velocity in the annulus region, and solid circulation rate and decreases with increasing waste feed rate and gas velocity in the draft tube. The simulated data from the kinetic equation and the hydrodynamic models predict the experimental data reasonably well.     

垃圾炉一次风量对炉床燃烧影响的仿真研究

针对垃圾焚烧炉床层内垃圾燃烧过程复杂性及影响垃圾燃烧效率因素多的问题,运用数值仿真方法对某垃圾焚烧炉床层垃圾燃烧进行了模拟,获得了垃圾质量流失速率、水分蒸发速率、挥发分释放速率、焦炭燃烧速率和烟气的温度;并在50~1 000 Nm3/min范围内改变一次风流量,获得了床层质量流失比值、过剩空气系数和烟气中各组分的质量百分比含量。仿真结果显示,一次风流量取值517.47~6`32.09 Nm3/min时燃烧效率较高;炉排两端区域的一次风流量在50~75 Nm3/min时,可以使垃圾床表面平均温度明显提高,而且还可以提高燃烧效率,同时减少供风总量,节约能源与动力资源。本研究对优化设计垃圾焚烧炉,提高床层内垃圾的燃烧效率有一定参考价值。     

Combined simulation of combustion and gas flow in a grate-type incinerator

Computational fluid dynamic (CFD) analysis of the thermal flow in the combustion chamber of a solid waste incinerator provides crucial insight into the incinerator's performance. However, the interrelation of the gas flow with the burning waste has not been adequately treated in many CFD models. A strategy for a combined simulation of the waste combustion and the gas flow in the furnace is introduced here. When coupled with CFD, a model of the waste combustion in the bed provides the inlet conditions for the gas flow field and receives the radiative heat flux onto the bed from the furnace wall and gaseous species. An unsteady one-dimensional bed model was used for the test simulation, in which the moving bed was treated as a packed bed of homogeneous fuel particles. The simulation results show the physical processes of the waste combustion and its interaction with the gas flow for various operational parameters.     

Air Pollution Acronyms

Abstract

Computational fluid dynamic (CFD) analysis of the thermal flow in the combustion chamber of a solid waste incinerator provides crucial insight into the incinerator’s performance. However, the interrelation of the gas flow with the burning waste has not been adequately treated in many CFD models. A strategy for a combined simulation of the waste combustion and the gas flow in the furnace is introduced here. When coupled with CFD, a model of the waste combustion in the bed provides the inlet conditions for the gas flow field and receives the radiative heat flux onto the bed from the furnace wall and gaseous species. An unsteady one-dimensional bed model was used for the test simulation, in which the moving bed was treated as a packed bed of homogeneous fuel particles. The simulation results show the physical processes of the waste combustion and its interaction with the gas flow for various operational parameters.     

Trichloroethylene degradation by photocatalysis in annular flow and annulus fluidized bed photoreactors

The effects of trichloroethylene (TCE) gas flow rate, relative humidity, TiO(2) film thickness, and UV light intensity on photodegradation of TCE have been determined in an annular flow type photoreactor. Phosgene and dichloroacetyl chloride formation could be controlled as a function of TCE gas flow rate and photodegradation of TCE decreased with increasing relative humidity. The optimum thickness of TiO(2) film was found to be approximately 5 mum and the photocatalytic reaction rate of TCE increased with square root of UV light intensity. In addition, the effects of the initial TCE concentration, phase holdup ratio of gas and solid phases (epsilon(g)/epsilon(s)), CuO loading on the photodegradation of TCE have been determined in an annulus fluidized bed photoreactor. The TCE photodegradation decreased with increasing the initial TCE concentration. The optimum conditions of the phase holdup ratio (epsilon(g)/epsilon(s)) and CuO wt.% for the maximum photodegradation of TCE was found to be 2.1 and 1.1 wt.%, respectively. Therefore, an annulus fluidized bed photoreactor is an effective tool for TCE degradation over TiO(2)/silica gel with efficient utilization of photon energy.     

Nitrogen oxides emissions from the MILD combustion with the conditions of recirculation gas

The nitrogen oxides (NO_x) reduction technology by combustion modification which has economic benefits as a method of controlling NO_x emitted in the combustion process, has recently been receiving a lot of attention. Especially, the moderate or intense low oxygen dilution (MILD) combustion which applied high temperature flue gas recirculation has been confirmed for its effectiveness with regard to solid fuel as well. MILD combustion is affected by the flue gas recirculation ratio and the composition of recirculation gas, so its NO_x reduction efficiency is determined by them. In order to investigate the influence of factors which determine the reduction efficiency of NO_x in MILD coal combustion, this study changed the flow rate and concentration of nitrogen (N₂), carbon dioxide (CO₂) and steam (H₂O) which simulate the recirculation gas during the MILD coal combustion using our lab-scale drop tube furnace and performed the combustion experiment. As a result, its influence by the composition of recirculation gas was insignificant and it was shown that flue gas recirculation ratio influences the change of NO_x concentration greatly. Implications: We investigated the influence of factors determining the nitrogen oxides (NO_x) reduction efficiency in MILD coal combustion, which applied high-temperature flue gas recirculation. Using a lab-scale drop tube furnace and simulated recirculation gas, we conducted combustion testing changing the recirculation gas conditions. We found that the flue gas recirculation ratio influences the reduction of NO_x emissions the most.     

城市生活垃圾在机械炉排炉内焚烧过程研究及数值模拟

研究城市生活垃圾在机械炉排炉内焚烧过程，建立了垃圾在炉排炉移动床内燃烧过程中垃圾体积变化、水分蒸发、挥发分析出及燃烧、垃圾颗粒的移动、气-固两相热传递、焦炭燃烧等各个反应阶段的热化学模型，采用一维非稳态模型分别建立了床层内气固两相介质控制方程。通过对炉排上垃圾焚烧过程的数值模拟研究，获得料层中温度分布、料层高度、炉床机械负荷、气相中反应物组分和污染物浓度分布。利用建立的模型对某大型垃圾焚烧炉的燃烧过程进行数值仿真，对燃烧过程、烟气成分进行预测，为垃圾焚烧炉排炉的设计和燃烧控制提供理论依据。     

Kinetics of catalytic oxidation of benzene, n-hexane, and emission gas from a refinery oil/water separator over a chromium oxide catalyst

With the advances made in the past decade, catalytic incineration of volatile organic compounds (VOCs) has become the technology of choice in a wide range of pollution abatement strategies. In this study, a test was undertaken for the catalytic incineration, over a chromium oxide (Cr2O3) catalyst, of n-hexane, benzene, and an emission air/vapor mixture collected from an oil/water separator of a refinery. Reactions were carried out by controlling the feed stream to constant VOC concentrations and temperatures, in the ranges of 1300-14,700 mg/m3 and 240-400 degrees C, respectively. The destruction efficiency for each of the three VOCs as a function of influent gas temperature and empty bed gas residence time was obtained. Results indicate that n-hexane and the oil vapor with a composition of straight- and branch-chain aliphatic hydrocarbons exhibited similar catalytic incineration effects, while benzene required a higher incineration temperature or longer gas retention time to achieve comparable results. In the range of the VOC concentrations studied, at a given gas residence time, increasing the operating temperature of the catalyst bed increased the destruction efficiency. However, the much higher temperatures required for a destruction efficiency of over 99% may be not cost-effective and are not suggested. A first-order kinetics with respect to VOC concentration and an Arrhenius temperature dependence of the kinetic constant appeared to be an adequate representation for the catalytic oxidation of these volatile organics. Activation energy and kinetic constants were estimated for each of the VOCs. Low-temperature destruction of the target volatile organics could be achieved by using the Cr2O3 catalyst.     

Waste burning and heat recovery characteristics of a mass burn incineration system

An experimental investigation on waste combustion characteristics of a mass burn incinerator is conducted in this study. Three different charging modes, including operator manipulation, periodic feeding, and temperature control, are taken into consideration. The results indicate that the burning characteristics in the combustion chambers are closely related to the operating modes. For the operator manipulation where the wastes are sent into the incinerator in two short periods, the entire temperature distribution of the primary combustion chamber can be partitioned into two parts, thereby yielding waste group combustion. Temperature oscillations in both the primary and secondary combustion chambers are characterized for the periodic feeding. However, because of the shorter charging period and smaller amount of waste, the burning interaction between the two chambers is initially weak and becomes notable in the final stage. When temperature control is performed, the burning oscillation of the primary combustion chamber is further amplified so the combustion interaction is drastic. These exhibitions are mainly caused by the competition between endothermic and exothermic reactions. The instantaneous heat exchange efficiency of the cyclone heat recovery system (CHRS) installed in the incineration system is also evaluated to obtain details of energy recovery behaviors. As a result, the efficiency tends to decrease linearly with increasing temperature of hot flue gas. This arises from the fact that heat loss from the gas to the environment is increased when the temperature of the former is higher, even though the temperature gradient across the cyclone is enlarged.     

双喷嘴矩形喷动床流体力学及脱硫性能的实验研究

针对半干法烟气脱硫方法,提出了一种新型的双喷嘴矩形喷动床半干法脱硫技术.在相同的静床层高度下,研究了最小喷动速度、最大喷动压降和喷动高度;在相同的Ca/S比、进气SO2浓度、进口温度、绝热饱和温差条件下,以消石灰为脱硫剂,研究了脱硫性能,并与传统柱锥形喷动床进行了比较.结果表明,矩形喷动床拥有更好的流动性能和脱硫率,得出了实验条件下矩形喷动床的最佳操作范围.     

Thermodynamic analysis and kinetic modelling of dioxin formation and emissions from power boilers firing salt-laden hog fuel

Both organic chlorine (e.g. PVC) and inorganic chlorides (e.g. NaCl) can be significant chlorine sources for dioxin and furan (PCDD/F) formation in combustion processes. This paper presents a thermodynamic analysis of high temperature salt chemistry. Its influence on PCDD/F formation in power boilers burning salt-laden wood waste is examined through the relationships between Cl2, HCl, NaCl(g) and NaCl(c). These analyses show that while HCl is a product of combustion of PVC-laden municipal solid waste, NaCl can be converted to HCl in hog fuel boilers by reactions with SO2 or alumino-silicate materials. Cl2 is a strong chlorinating agent for PCDD/F formation. HCl can be oxidized to Cl2 by O2, and Cl2 can be reduced back to HCl by SO2. The presence of sulphur at low concentrations thus enhances PCDD/F formation by increasing HCl concentrations. At high concentrations, sulphur inhibits de novo formation of PCDD/Fs through Cl2 reduction by excess SO2. The effect of NH3, CO and NOx on PCDD/F formation is also discussed. A semi-empirical kinetic model is proposed. This model considers both precursor and de novo formation mechanisms. A simplified version is used as a stack emission model. The kinetic model indicates that stack dioxin emissions will increase linearly with decreasing electrostatic precipitator (ESP) efficiency and exponentially with increasing ESP temperature.     

湿污泥在流化床中干燥特性

在鼓泡流化床干燥器内,床料采用0～1 mm的石英砂,以热空气作为干燥介质,对城市污水污泥的流态化干燥特性进行实验研究。通过控制送风温度、流化风量、加料量、污泥粒径等参数,研究在不同工况条件下床温以及干污泥含水率的变化情况。结果表明,随着送风温度的升高,床温升高,干污泥含水率下降,最终趋向于一个稳定值8%。床温随流化风量的增加先是减小而后升高,干污泥的含水率随流化风量的增加先是降低而后增大。当加料量从11 kg/h增加到13kg/h时,干污泥的含水率基本没有变化,当加料量超过13 kg/h时,干污泥的含水率从8.1%开始呈递增趋势。随污泥粒径的增大,床温升高,干污泥含水率也随之升高。     

Formation and transport of PCDD/Fs in the packed bed of soil containing organic chloride during a thermal remediation process

The authors previously proposed the concept of a new thermal remediation process for particulate/powder materials contaminated by polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and experimentally verified its validity on the basis of process efficiency. However, contaminees such as soils and fly ashes from waste incinerators often contain a considerable amount of other chlorides, which may act as a main source of chlorine in the formation of PCDD/Fs via thermal processes. The present study aims to examine the formation and transport of PCDD/Fs in the packed bed of soil containing a chloride during the process. Polyvinyl chloride (PVC) polymer was mixed with soil sample as an organic chloride model. A laboratory-scale apparatus was employed as a process simulator. Further, a technique to quench the process was applied to observe the concentration distribution of PCDD/Fs in the solid bed in the vertical direction. The result shows that the PCDFs tend to form dominantly in the high temperature (calcination and/or combustion) zone and are successively trapped in the low temperature (wet) zone. Especially, TeCDF is the most dominant homologue contained in the wet zone and outlet gas. Although PCDD/Fs are once trapped at the wet zone, the concentration of the remediated materials gives fairly low value (1.9 pg/g-dry, 0.04 pg-TEQ/g-dry). It signifies that organic chlorides mingled in the solid contaminee not affect the removal efficiency of PCDD/Fs in the process. Nevertheless, attention should be paid to the potential emission of PCDD/Fs in the outlet gas due to the presence of organic chloride in the soil.     

基于正交实验的颗粒移动床烟气脱硫工艺参数优化

石灰石颗粒移动床脱硫工艺参数是影响脱硫效率和操作压降变化的重要因素,考虑到工艺参数对脱硫过程的影响,基于正交实验方法,通过直观分析和方差分析得到喷淋密度、空床气速、SO2浓度、烟气温度和床层下移速度对脱硫效率影响程度的主次顺序和脱硫效率与各因素之间的关系,并且分析了各因素对脱硫效率的影响规律。研究发现,各因素对脱硫效率的影响程度依次是空床气速>喷淋密度>SO2浓度>烟气温度>床层下移速度。喷淋密度对脱硫效率的影响高度显著,呈正相关;空床气速对脱硫效率的影响高度显著,呈负相关;SO2浓度和烟气温度对脱硫效率有影响但不显著,呈负相关。床层下移速度与脱硫效率呈正相关。     

采用循环流化床技术焚烧城市生活垃圾

根据目前我国城市生活垃圾热值偏低的实际情况 ,提出以煤为辅助燃料 ,采用循环流化床焚烧技术处理生活垃圾 ,可大幅度降低运行成本 ,符合国情。阐述了循环流化床焚烧技术在降低NOx、SO2 和二噁等方面的优越性。尾气处理建议采用喷活性碳 +布袋除尘组合工艺 ,飞灰处理建议采用高温熔融处理     

生活垃圾高温空气气化处理的低污染特性分析

本文介绍了高温空气气化城市生活垃圾的工作原理及过程 ,分析了城市生活垃圾高温气化反应机理。通过对高温空气 /蒸汽发生器、高温气化炉及湿式除尘器等关键部件的分析发现 ,采用高温空气气化系统可实现城市生活垃圾低污染处理     

Kinetics of Catalytic Oxidation of Benzene,n-Hexane,and Emission Gas from a Refinery Oil/Water Separator over a Chromium Oxide Catalyst

ABSTRACT

With the advances made in the past decade, catalytic incineration of volatile organic compounds (VOCs) has become the technology of choice in a wide range of pollution abatement strategies. In this study, a test was undertaken for the catalytic incineration, over a chromium oxide (Cr₂O₃) catalyst, of n-hexane, benzene, and an emission air/vapor mixture collected from an oil/water separator of a refinery. Reactions were carried out by controlling the feed stream to constant VOC concentrations and temperatures, in the ranges of 1300–14,700 mg/m³ and 240–400 ° C, respectively. The destruction efficiency for each of the three VOCs as a function of influent gas temperature and empty bed gas residence time was obtained.

Results indicate that n-hexane and the oil vapor with a composition of straight- and branch-chain aliphatic hydrocarbons exhibited similar catalytic incineration effects, while benzene required a higher incineration temperature or longer gas retention time to achieve comparable results.

In the range of the VOC concentrations studied, at a given gas residence time, increasing the operating temperature of the catalyst bed increased the destruction efficiency. However, the much higher temperatures required for a destruction efficiency of over 99% may be not cost-effective and are not suggested. A first-order kinetics with respect to VOC concentration and an Arrhenius temperature dependence of the kinetic constant appeared to be an adequate representation for the catalytic oxidation of these volatile organics. Activation energy and kinetic constants were estimated for each of the VOCs. Low-temperature destruction of the target volatile organics could be achieved by using the Cr₂O₃ catalyst.     

Prediction of Flue Gas Composition of an Incinerator Based on a Nonequilibrium-Reaction Approach

A nonequilibrium-reaetion model is developed to predict the flue gas composition of an incinerator for solid waste combustion and gasification. The model may also be effectively utilized to predict the composition of exit gas from the primary chamber of an incinerator where invariably substoichiometrie combustion conditions exist The model assumes that the drying and pyrolysis of waste occurs in sequence followed by combustion of devolatilized gas and residue separately. The model is applied to the combustion of pine wood, and the flue gas compositions are computed as a function of operating parameters such as temperature (1100 to 1500 K), initial moisture content (0 to 40 percent), and percentage excess air (up to 100 percent). The model is extended to include substoichiometric combustion (to simulate the gasification process without steam injection) to compare its predictions with the available experimental data. The general agreement of theory and experiment is good.     

A Comparison of Dioxin,Furan and Combustion Gas Data from Test Programs at Three MSW Incinerators

Dioxin, furan and combustion gas data from test programs conducted at the Pitts field, Prince Edward Island, and Peekskill municipal solid waste incinerators were compared to identify similar and discrepant results. Dioxin and furan concentrations sampled upstream of air pollution control devices are related to a variety of combustion gas variables monitored in the high temperature zones of the incinerators. The effects of temperature, excess oxygen, carbon monoxide, hydrochloric acid gas, flue gas moisture, particulate loading and refuse moisture and PVC content are discussed. Conclusions are made and future research needs are discussed.     

Combustion modeling and performance evaluation in a full-scale rotary kiln incinerator.

This work summarizes the results of numerical investigations and in situ measurements for turbulent combustion in a full-scale rotary kiln incinerator (RKI). The three-dimensional (3D) governing equations for mass, momentum, energy, and species, together with the kappa - epsilon turbulence model, are formulated and solved using a finite volume method. Volatile gases from solid waste were simulated by gaseous CH4 distributed nonuniformly along the kiln bed. The combustion process was considered to be a two-step stoichiometric reaction for primary air mixed with CH4 gas in the combustion chamber. The mixing-controlled eddy-dissipation model (EDM) was employed to predict the conversion rates of CH4, O2, CO2, and CO. The results of the prediction show that reverse flows occur near the entrance of the first combustion chamber (FCC) and the turning point at the entrance to the second combustion chamber (SCC). Temperature and species are nonuniform and are vertically stratified. Meanwhile, additional mixing in the SCC enhances postflame oxidation. A combustion efficiency of up to 99.96% can be achieved at approximately 150% excess air and 20-30% secondary air. Reasonable agreement is achieved between numerical predictions and in situ measurements.