An investigation on pollutant emissions from co-firing of RDF and coal

Pollutant emissions from co-firing of refuse derived fuel (RDF) and coal were investigated in a vortexing fluidized bed combustor (VFBC). RDF-5 was made of common municipal solid waste (MSW). CaCO₃ was injected in the combustor to absorb HCl at 850 °C. The results show that NO_x and HCl emissions increase with RDF-5 co-firing ratio. The NO_x concentration in flue gas at the bottom of the combustor is higher than that at the top. However, the trend of HCl released is reverse compared with NO_x emissions. It was found that the HCl concentration decreases with increasing the molar ratio of Ca/Cl. However, the effect of CaCO₃ addition on HCl retention is not significant when the molar ratio of Ca/Cl is higher than 5. The chlorine content in fly ash increases obviously with the molar ratio of Ca/Cl. PCDD/Fs emissions decrease slightly with an addition of CaCO₃. In this study incomplete combustion is regarded as the main cause for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation.     

A Thermochemical Conversion Study on the Combustion of Residue-Derived Fuels

Two different waste-derived by-products were examined and compared. Based on the thermogravimetric tests performed, it was proved that their decomposition occurs in two weight loss steps represented by two shoulders in the derivative thermogravimetric curves. The first shoulder is attributed to the devolatilisation of hemicellulose, cellulose and lignin and the second one to the plastic fraction of the waste. Similarities in the degradation behaviour were observed for both wastes, despite of their different origin. Increased plastic fractions resulted in slightly higher conversions and lower pyrolysis rates. Enhanced lignocellulosic fractions led to higher rates during combustion. The lignocellulosic fraction was increased proportionally to the inorganic residue that remained after combustion. A wide variation of weight losses was attained even in refuse-derived fuel (RDF) samples of the same origin, whilst stronger deviations were observed in the decomposition of the plastic fraction. The independent parallel, first-order, reactions model was elaborated for the kinetic analysis of the pyrolysis results. The thermal degradation of the RDF samples was modelled assuming four parallel reactions corresponding to the devolatilisation of cellulose, hemicellulose, lignin and plastics. Increased activation energies were calculated for the plastics fraction, whilst lignin presented the lowest contribution in the pyrolysis of the samples. Generally, both RDF samples presented similar kinetic constants despite their heterogeneity.     

Absorption behavior of chlorine and sulfur by Ca(OH)<Subscript>2</Subscript> under simulated conditions with a fluidized bed combustor

This study was carried out to investigate the absorption reaction of chlorine and sulfur with Ca(OH)₂ under simulated conditions related to the co-combustion of refuse-derived fuel and sewage sludge in the fluidized bed combustor, such as high temperature, moisture and a mixing of samples. The combustion experiments were carried out in an electrical furnace with PVC and sulfur powders as the waste samples. High-purity slaked lime powder was used to capture the chlorine and sulfur. The experimental results showed that the removal efficiencies of both chlorine and sulfur were over 90% when the mole ratios of Ca on Cl and S were over 2.5. Furthermore, simultaneous absorption was helpful in improving the removal efficiency of chlorine and sulfur. The presence of chlorine markedly increased the removal efficiency of sulfur. The removal efficiency of chlorine was the highest at 700°C in both independent and simultaneous absorption. The removal efficiency of sulfur increased as the temperature increased, but the removal efficiency of chlorine sharply decreased from 95 to 83% when the moisture content in gases increased from 0 to 20 vol%. These results were confirmed by the combustion test of granular CaCl₂.     

Pyrolysis of mixtures of sewage sludge and manure: a comparison of the results obtained in the laboratory (semi-pilot) and in a pilot plant

A pilot-scale pyrolysis process was carried out for the treatment of a mixture of two types of waste, sewage sludge and cattle manure, comparing the results with others obtained under laboratory conditions (semi-pilot scale). The aim of this study was to obtain the energetic valorization of the products. Owing to the specific characteristics of the plant, two products were obtained from the process: gas and carbonized solid. As no liquid fraction was obtained, the gas fraction is a greater percentage made up of both condensable and non-condensable compounds, which were obtained separately at the laboratory scale. The pilot plant was designed so that the gases produced by thermolysis were burnt continuously in a combustion chamber, while the carbonized fraction was fed in batches for co-combustion. To determine composition and combustion ability, the gas and solid products from the pilot process were characterized by chromatographic analysis of the gaseous fraction and chemical analysis and programmed-temperature combustion of the carbonized solid. The composition of the combustion gases, rich in light hydrocarbons, and the carbon present in the carbonized fraction enable the energetic valorization of these products. The combustion gases were subjected to a cleaning process and their composition analysed twice: before and after the gas cleaning treatment. The study led to a positive assessment of the possible use of the process products as fuel, provided that the combustion gases are treated. As most of the sulphur and chlorine from the original waste are mainly concentrated in the solid fraction, the use of char as a fuel will depend on the effectiveness of clean-up techniques for combustion gases. During gas cleansing, neutralizing with sodium bicarbonate proved effective, especially for the acidic compounds HCl, HF and SO(2).     

化学循环燃烧载氧体的研究进展

化学循环燃烧是集分离捕集CO2和去除NOx为一体的新型燃烧技术。介绍了化学循环燃烧的发展背景、工艺原理及特点;综述了化学循环燃烧载氧体的选材、载氧体的性能要求,对各系载氧体（如NiO系、Fe2O3系、CuO系、CoO系和Mn2O3系载氧体）的性能进行了对比分析;提出了Fe2O3、Fe2O3／Al2O3（质量分数为60％的Fe2O3＋质量分数为40％的Al2O3）、NiO／NiAl2O3（质量分数为50％的活性NiO＋质量分数为50％的NiAl2O3）是当前的优势载氧体,并对载氧体的发展做了展望。     

Combustion kinetics of sewage sludge and combustible wastes

This study estimated the kinetics of the mono- and co-combustion of sewage sludge pellets and combustible wastes such as municipal solid waste (MSW) and refuse-derived fuel (RDF). Sewage sludge was manufactured into pellets with a diameter of 8, 12, or 16 mm and a length of 30 mm. The RDF was composed of paper and plastics and was formed into pellets with a diameter of 8 mm and a length of 30 mm. MSW samples were synthesized using combustible wastes such as garbage, paper, plastics, and wood. The MSW was adjusted to have a moisture content of around 40% after shredding to under 10 mm. A laboratory-scale batch type stoker incinerator was used for the combustion and the gas composition of the flue gas was measured. The activation energy was calculated using the experimental results, and then the relation of the decomposition rate and reaction time was evaluated using the shrinking core model. The decomposition rate of the sludge pellets decreased as their diameter and moisture content increased, and the co-combustion of sludge pellets and combustible waste was affected by the amount of combustible waste. The individual combustion rates of the cylindrical sludge pellets or RDF were mainly controlled by the chemical reaction, but in the case of shredded MSW it was mostly influenced by gas diffusion. The rate for the co-combustion of sludge pellets and combustible wastes was mainly determined by the combustion rate of the combustible waste. The activation energy of the 8-mm-diameter sludge pellets was between 6.70 and 10.0 kcal/mol, according to the moisture content, but it was lower for MSW and RDF. In the case of MSW co-combustion, the reaction rate accelerated as the moisture content of the sludge pellets decreased, but it was markedly increased by the addition of RDF, regardless of the sludge moisture content.     

Simulation of the flue gas cleaning system of an RDF incineration power plant

Because of the stringent pollutant emission standards introduced with the European Union guidelines for waste incineration, it is very important to optimize the flue gas cleaning systems which are able to result in a low environmental impact according to the emission limits. In this paper a thermochemical model has been proposed for the simulation of the flue gas cleaning system of an RDF incineration plant. The model simulates the operation of the flue-gas treatment section and the combustion section by using a simplified approach. The combustion includes the grate incinerator and the post-combustion chamber, while the cleaning section includes the NO(x) reduction process (urea injection) and the scrubbing of SO(2) and HCl (Ca(OH)(2) as sorbent). The modelling has been conducted by means of ASPEN PLUS code. The simulation results have been validated with the operating data. The model proposed by the authors can be a useful tool in both evaluating the efficiency of the gas cleaning system by verifying the environmental pollution of an incinerator power plant in nominal operating conditions and in forecasting the efficiency of the cleaning system in off-design operating conditions.     

Characterization of municipal solid waste combustion in a grate furnace

The objective of this paper is to evaluate the combustion process of municipal solid waste combustion in a grate furnace both experimentally and numerically by using data of a reference experiment with over-stoichiometric primary air supply. Measurements were carried out inside the combustion chamber of a pilot plant by monitoring temperatures and sampling gaseous combustion products along the bed surface. The data were assessed using elemental and energy balances. Experimental data of the axial temperature profiles of the flue gas, the fuel bed and the grate bars, as well as local gas flows and the flue gas composition measured above the fuel bed along the grate were used to describe the conversion process, including drying and carbon burnout. These data served as input to model the thermo- and fluid dynamic processes of the gas phase above the bed inside the combustion chamber. For this purpose the commercial code FLUENT was employed to carry out the simulations. Thus, the turbulent temperature, flow and species distributions in the combustion chamber of the pilot waste incinerator TAMARA were predicted. The results of the FLUENT modeling showed that under the prevailing conditions the flue gas burnout is almost completed before entering the first flue due to high temperatures, effective mixing and sufficient residence times of the flue gas inside the combustion chamber. This agrees well with the experimental results inside the first flue. On the basis of the above mentioned results, design and parametric studies can be carried out in a more efficient way by saving cost and time.     

A novel additive for the reduction of acid gases and NO(x) in municipal waste incinerator flue gas.

The reduction of SO2, HCl, and NO(x) concentrations using calcium magnesium acetate (CMA) as a novel sorbent in a simulated municipal waste incinerator flue gas was investigated. The reduction of individual SO2, HCl, and NO(x) concentrations was tested at 850 degrees C and it was found that CMA could reduce the SO2 concentration by 74%, HCl concentration by 64%, or NO(x) concentration by 94%. It was observed that individual SO2 or HCl capture increased with increasing initial oxygen concentration in the reacting gas or increasing sorbent input. NO(x) reduction decreased with increasing initial oxygen concentration in the reacting gas. The simultaneous reduction of SO2, HCl, and NO(x) concentrations by CMA was also investigated. It was found that CMA could simultaneously capture 60% SO2 and 61% HCl and reduce NO(x) concentrations by 26%, when the initial oxygen concentration in the reacting gas was 4%. During the simultaneous reduction of SO2, HCl, and NO(x), it was noted that as the initial oxygen concentration in the reacting gas increased, the efficiency of SO2 capture increased too, but the efficiency of HCl capture and the efficiency of NO(x) destruction decreased.     

Comparison of coal/solid recovered fuel (SRF) with coal/refuse derived fuel (RDF) in a fluidized bed reactor

An experimental study was undertaken to compare the differences between municipal solid waste (MSW) derived solid recovered fuel (SRF) (complying with CEN standards) and refuse derived fuel (RDF). Both fuels were co-combusted with coal in a 50 kW fluidised bed combustor and the metal emissions were compared. Synthetic SRF was prepared in the laboratory by grinding major constituents of MSW such as paper, plastic, textile and wood. RDF was obtained from a local mechanical treatment plant. Heavy metal emissions in flue gas and ash samples from the (coal + 10% SRF) fuel mixture were found to be within the acceptable range and were generally lower than that obtained for coal + 10% RDF fuel mixture. The relative distribution of heavy metals in ash components and the flue gas stream shows the presence of a large fraction (up to 98%) of most of the metals in the ash (except Hg and As). Thermo-gravimetric (TG) analysis of SRF constituents was performed to understand the behaviour of fuel mixtures in the absence and presence of air. The results obtained from the experimental study will enhance the confidence of fuel users towards using MSW-derived SRF as an alternative fuel.     

Clay-sewage sludge co-pyrolysis. A TG-MS and Py-GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Wastewater sewage sludge was co-pyrolyzed with a well characterized clay sample, in order to evaluate possible advantages in the thermal disposal process of solid waste. Characterization of the co-pyrolysis process was carried out both by thermogravimetric-mass spectrometric (TG-MS) analysis, and by reactor tests, using a lab-scale batch reactor equipped with a gas chromatograph for analysis of the evolved gas phase (Py-GC).Due to the presence of clay, two main effects were observed in the instrumental characterization of the process. Firstly, the clay surface catalyzed the pyrolysis reaction of the sludge, and secondly, the release of water from the clay, at temperatures of approx. 450-500 °C, enhanced gasification of part of carbon residue of the organic component of sludge following pyrolysis.Moreover, the solid residue remaining after pyrolysis process, composed of the inorganic component of sludge blended with clay, is characterized by good features for possible disposal by vitrification, yielding a vitreous matrix that immobilizes the hazardous heavy metals present in the sludge.     

Experimental and kinetic modeling of oxygen-enriched air combustion of municipal solid waste

The characteristics of oxygen-enriched air combustion of raw municipal solid waste (MSW) were studied by thermogravimetric analysis. Experiments on oxidative pyrolysis of MSW were carried out under different atmospheres (N(2), N(2):O(2)=7:3, N(2):O(2)=5:5, N(2):O(2)=4:6, and N(2):O(2)=2:8) at 30 degrees C/min. Two distinct peaks of weight loss were obtained according to the derivative thermogravimetric curves; one of them is centered on 305 degrees C with about 40% weight loss, and the second is centered on 420 degrees C with about 20% weight loss. Effects of oxygen concentration on the decomposition process and char combustion were analyzed, and then the process of oxygen-enriched air combustion of MSW was divided into four steps. Kinetic parameters were observed by direct non-linear regressions. According to the obtained data, the apparent activation energy and reaction order decreases along with the combustion process, while that of char combustion increases as oxygen concentration increases.     

Gasification of refuse derived fuel in a fixed bed reactor for syngas production

Steam gasification of two different refuse derived fuels (RDFs), differing slightly in composition as well as thermal stability, was carried out in a fixed-bed reactor at atmospheric pressure. The proximate and ultimate analyses reveal that carbon and hydrogen are the major components in RDFs. The thermal analysis indicates the presence of cellulose and plastic based materials in RDFs. H2 and CO are found to be the major products, along with CO2 and hydrocarbons resulting from gasification of RDFs. The effect of gasification temperature on H2 and CO selectivities was studied, and the optimum temperature for better H2 and CO selectivity was determined to be 725 degrees C. The calorific value of product gas produced at lower gasification temperature is significantly higher than that of gas produced at higher process temperature. Also, the composition of RDF plays an important role in distribution of products gas. The RDF with more C and H content is found to produce more amounts of CO and H2 under similar experimental conditions. The steam/waste ratio showed a notable effect on the selectivity of syngas as well as calorific value of the resulting product gas. The flow rate of carrier gas did not show any significant effect on products yield or their distribution.     

Treatment of tar pond sludge in a circulating fluidized bed combustor

A series of tests to burn mixtures of tar pond sludge and coal was carried out using a mini‐circulating fluidized bed combustor (mini‐CFBC). During the tests, carbon dioxide, oxygen, carbon monoxide, sulfur dioxide, and nitrogen oxides in the flue gas were monitored continuously. Stack gas sampling was carried out for hydrochloric acid, metals, particulate matter, volatile organic compounds (VOCs), total hydrocarbons, semivolatile organic compounds (SVOCs), dioxins and furans (PCDD/Fs), and polychlorinated biphenyls (PCBs). Results showed that hydrochloric acid, mercury, particulate matter, PCDD/F, and metal concentrations were all below both the current limits and the gas‐release limits to be implemented in 2008 in Canada. The new 2008 emissions limits will reduce the maximum allowable concentrations of most pollutants by half. Thus, the maximum concentration for particulate matter will be 5 mg/m³ (from the current maximum concentration of 10 mg/m³);the maximum concentration for hydrochloric acid will be 5 mg/m³ (from 10 mg/m³); and the‐maximum concentration for dioxins and furans will be 0.032 ng/m toxic equivalent (from 0.08 ng/mcurrently). Sulfur capture efficiency was 89–91 percent. The percentage of fuel nitrogen converted to nitrogen oxides was of the order of 4.7 to 6.1, which is significantly lower than that of conventional pulverized coal‐fired boilers and well within the normal range for fluidized bed combustors (FBCs). PCB and polycyclic aromatic hydrocarbon (PAH) emissions levels were comparable or lower than levels reported in the literature for industrial‐scale FBCs. VOC concentrations were low except for benzene, for which the concentration was higher than that reported for pulverized coal‐fired utility boilers. In addition, carbon monoxide concentration was high at 1,200 to 2,200 parts per million. However, these carbon monoxide concentrations are typical of the mini‐CFBC firing coal. The trials showed that for 10 percent by weight tar pond sludge mixed with 90 percent by weight coal, the combustion was both stable and efficient. The tests demonstrated that CFBC technology is an environmentally sound option for eliminating tar pond waste sludge. © 2005 Wiley Periodicals, Inc.     

Gaseous emissions from the combustion of a waste mixture containing a high concentration of N2O

This paper is focused on reducing the emissions from the combustion of a waste mixture containing a high concentration of N2O. A rate model and an equilibrium model were used to predict gaseous emissions from the combustion of the mixture. The influences of temperature and methane were considered, and the experimental research was carried out in a tabular reactor and a pilot combustion furnace. The results showed that for the waste mixture, the combustion temperature should be in the range of 950-1100 degrees C and the gas residence time should be 2s or higher to reduce emissions.     

钡渣用于三相流化床烟气脱硫

提出了在三相流化床内用钡渣进行烟气脱硫的方法。分析了钡渣湿法脱硫原理，并进行了工艺改进实验，对两种不同溶渣方式工艺进行了静态和动态模拟实验，考察了两种方式的离子浓度和吸收容量等因素。实验表明，将渣先和水混合再用脱硫废水调整水质、脱硫塔进水pH在7．5左右时有利于系统的稳定运行。当液气比为2．88、气速为9．5m／s时，脱硫效率可以达到85％以上。这种方法可大大降低脱硫成本，实现以废治废。     

Thermal behavior characteristics of Adhesive residue

Solid wastes from organic Adhesive production are identified as toxicant hazardous wastes in China’s National Catalogue of Hazardous Wastes. The aim of this study is analyzing the thermal behavior of Adhesive residue. Its pyrolysis and combustion characteristics were investigated using thermogravimetric analysis (TGA). Experiments were carried out in the temperature range of 50–950 °C in both nitrogen and air. The results indicate that combustion under these experimental conditions largely occurs between 210 and 410 °C, whereas pyrolysis proceeds between 260 and 430 °C. Almost all weight lost takes place before 430 °C during both pyrolysis and combustion of the residue. Fourier transform infrared spectroscopy (FTIR) was used to characterize the emission characteristics during pyrolysis. When the sample is heated in an inert atmosphere, the evolution of volatiles starts around 260 °C, and reaches a peak rate at 394 °C. Most organic products evolve in a narrow temperature range during pyrolysis. The evolving gaseous products were identified as Butyraldehyde, Ether, Acetonitrile and CO₂, accompanied with some CO.     

Simultaneous capture of metal, sulfur and chlorine by sorbents during fluidized bed incineration

Metal capture experiments were carried out in an atmospheric fluidized bed incinerator to investigate the effect of sulfur and chlorine on metal capture efficiency and the potential for simultaneous capture of metal, sulfur and chlorine by sorbents. In addition to experimental investigation, the effect of sulfur and chlorine on the metal capture process was also theoretically investigated through performing equilibrium calculations based on the minimization of system free energy. The observed results have indicated that, in general, the existence of sulfur and chlorine enhances the efficiency of metal capture especially at low to medium combustion temperatures. The capture mechanisms appear to include particulate scrubbing and chemisorption depending on the type of sorbents. Among the three sorbents tested, calcined limestone is capable of capturing all the three air pollutants simultaneously. The results also indicate that a mixture of the three sorbents, in general, captures more metals than a single sorbent during the process. In addition, the existence of sulfur and chlorine apparently enhances the metal capture process.     

次氯酸钙溶液的脱硫脱硝特性

采用Ca(ClO)2溶液在高效反应瓶中与模拟烟气进行脱硫脱硝实验,考察了模拟烟气中SO2和NOx的去除效果,以动力学为理论基础研究了Ca(ClO)2的脱硫脱硝性能。实验结果表明:在模拟烟气中SO2,NO,NO2的质量浓度分别为3700,1300,820 mg/m^3及Ca(ClO)2溶液浓度为0.35 mol/L的条件下,SO2去除率达到100%,NOx去除率达到67%。脱硫过程中吸收液中的ClO-是100%脱硫率的保障。Ca^2+吸收SO2的反应速率呈现一级反应的特征。得到的脱硫产物CaSO4·2H2O达到了工业石膏的品质要求。同时脱硫脱硝时NOx和SO2具有协同作用。     

高Ca~(2+)浓度CO_2沉淀法由电石渣制备纳米CaCO_3

以NH4Cl溶液为浸取剂、CO2为碳化剂、多聚磷酸钠(STP)为添加剂,由电石渣制备纳米Ca CO3。实验结果表明:电石渣浸取液Ca2+浓度为1.0 mol/L、STP加入量为3.00%时,可制备出粒径为30~60 nm的纳米Ca CO3;STP可有效控制纳米Ca CO3的粒度和形貌;在最佳工艺条件下,由电石渣制备纳米Ca CO3的产率为80%,处理1 t电石渣产生的经济效益约为2 670元。