Fluidized Bed Furnace Research Articles

Combustibility analysis of high-carbon fine slags from an entrained flow gasifier

The fine slag produced from the entrained flow gasifier in coal chemical industry contains a high amount of unburned carbon content, which can reach more than 40%. The coal gasification fine slag is dissipated just by land filling which occupies a lot of land. Consequently, it causes the pollution of soil, water and wastes the combustible carbon in coal gasification fine slag. It is crucial to develop an environmental friendly and economical scheme for the utilization of coal gasification fine slag. To achieve this aim, it is significant to investigate the combustibility of coal gasification fine slag and then propose a comprehensive utilization technology. In this study, the physical and chemical properties of the raw bituminous coal and the produced coal gasification fine slag, including proximate and ultimate analysis, particle size distribution, ash composition, morphology, and specific area were investigated. The combustion and co-combustion characteristics of coal gasification fine slag were analyzed by a thermo-gravimetric analyzer. A drop tube furnace and a fluidized bed reactor were employed to test the combustibility of coal gasification fine slag in a pulverized furnace and a fluidized bed furnace, respectively. Results show that the carbon content in dried coal gasification fine slag is >40% with a heating value > 16 MJ kg−1. Further, thermo-gravimetric analyzer test showed that the combustion property of coal gasification fine slag is worse than that of anthracite and close to that of high ash coal, and there is a non-negligible synergistic effect for raw bituminous coal and coal gasification fine slag co-firing. The combustibility test in drop tube furnace and fluidized bed reactor showed that coal gasification fine slag can be well burned in a pulverized furnace requiring combustion temperature >900 °C and oxygen concentration >10 vol%. However, the fluidized bed furnace was not appropriate for high efficiency coal gasification fine slag burning, because the unburned carbon content of fly ash after coal gasification fine slag combustion is still >14%, even at 900 °C, 21% oxygen concentration and a low fluidization number. It is suggested that coal gasification fine slag will be better to burned it in a pulverized furnace rather than fluidized furnace.

Shedding light on the governing mechanisms for insufficient CO and H2 burnout in the presence of potassium, chlorine and sulfur

Based on the experiences of insufficient burnout in industrial fluidized bed furnaces despite adequate mixing and availability of oxidizer, the influence of potassium on CO and H2 oxidation in combustion environments was investigated. The combustion environments were provided by a laminar flame burner in a range relevant to industrial furnaces, i.e. 845 °C to 1275 °C and excess air ratios ranging from 1.05 to 1.65. Potassium, in the form of KOH, was homogeneously introduced into the hot gas environments to investigate its effect on the radical pool. To quantitatively determine key species that are involved in the oxidation mechanism (CO, H2, KOH, OH radicals, K atoms), a combination of measurement systems was applied: micro-gas chromatography, broadband UV absorption spectroscopy and tunable diode laser absorption spectroscopy. The inhibition effect of potassium on CO and H2 oxidation in excess air was experimentally confirmed and attributed to the chain-terminating reaction between KOH, K atoms and OH radicals, which enhanced the OH radical consumption. The addition of chlorine or sulfur could reduce the concentrations of KOH and K atoms and consequently eliminated the inhibition on CO and H2 oxidation. Existing kinetic mechanisms underestimate the inhibiting effect of potassium and they fail to predict the effect of temperature on CO and H2 concentration when potassium and sulfur co-exist. This work advances the need to revise existing kinetic mechanisms to fully capture the interplay of K and S in the oxidation of CO and H2 in industrial fluidized bed furnaces.

Desulfurization using limestone during sludge incineration in a fluidized bed furnace: Increased risk of particulate matter and heavy metal emissions

Incineration of sludge can be an effective method to minimise waste whilst producing useful heat. However, incineration can cause secondary pollution issues due to the emission of SO2, therefore a set of experiments of sludge incineration in a bubble bed furnace were conducted with limestone addition to study desulfurization of sludge incineration flue gas. As expected, over 93% emission of SO2 was reduced with limestone addition, and that of CO and NOx were increased and decreased respectively when the fuel feeding rate raised. The distribution of fly ash was also increased by raising the fuel feeding rate due to increasing fragmentation of the ash. However, distributions of PM2.5 and heavy metals in submicron particles have dramatically increased with limestone desulfurization. The mechanism was revealed by SEM and EDS statistical analysis, indicating that the reaction between aluminosilicate and calcium made particles agglomerate and eutectic mixtures form, these larger ash particles were found to divide between collection as cyclone ash and fragmentation into finer particles that bypassed the cyclone. Those fine particles provided more surface area for heavy metal condensation. Furthermore, it was found that the reaction mechanism for semi-volatile metals involved them being released from the sludge and forming PM1 particles due to the vaporization-condensation mechanism, leading to higher emission of PM1 and distribution of heavy metals in PM1. Thus, it should be considered that there may actually be higher emission risks of PM and heavy metal emissions when aiming to desulfurize a flue gas using Ca-based minerals in certain circumstances.

Pollutant Emissions during Co-incineration of Landfill Material Refuse-Derived Fuel in a Lab-Scale Municipal Solid Waste Incineration Fluidized Bed Furnace

Landfill material refers to the waste that has been buried in landfill sites for a long time. The combustible components of landfill material can be made into refuse-derived fuel (RDF) for energy u...

The immobilisation of heavy metals from sewage sludge ash in geopolymer mortars

Sewage sludge is a semi-solid waste material created as a result of the sewage treatment of industrial or municipal wastewater. Because the laws and regulations of the European Union require not only a reduction in waste generation but also the preparation of waste for reuse and disposal, it is necessary to look for new methods of the application of sewage sludge as part of sustainable waste management. In this study, ash formed as a result of the combustion of sewage sludge from the sewage treatment plant in Płaszów, Krakow in a fluidised bed furnace at a temperature of around 800°C was used. Sewage sludge ash (SSA) contains over 30% SiO2 and approx. 10% Al2O3, which indicates potential applications in geopolymer materials. In this study, samples of geopolymer mortars with a binder containing sewage sludge ash as well as fly ash (FA) and ground granulated blast furnace slag (GGBFS) were prepared. The mechanical parameters were determined after 2, 7, 14, and 28 days. The results show that the sewage sludge ash-based geopolymer shows binding properties at ambient temperature and, depending on the presence of FA and/or GGBFS, the compressive strength varies from 5 to 45 MPa after 28 days. The aim of the research was also to determine the total content of heavy metals (Sb, As, Cr, Cd, Cu, Ni, Pb, Hg, Zn) in the raw materials used and their leachability from the structure of the hardened materials. Immobilisation of heavy metals is very promising. Based on the results of tests, it seems possible to use SSA in geopolymer materials, but not as the main component of the binder.

Research and Application on Bed Material Preparation System of CFB Boiler

This paper introduces a technology of preparing bed material with bottom slag of circulating fluidized bed boiler, and designs the system of filling the bed material into the furnace, which has been applied in engineering. This technology uses bottom slag to prepare bed material. Through screening treatment of developed ultra-fine roller screen, the bed material with particle size meeting combustion requirements is prepared and fed into fluidized bed furnace. It not only ensures the combustion effect of the boiler, but also saves a lot of bed material procurement and operation costs, at the same time, it reduces the discharge of boiler bottom slag, which is conducive to reducing pollutant emission. Moreover, some limestone which is not involved in the desulfurization reaction in the furnace is screened out and returned to the furnace again as bed material, so as to improve the utilization rate of limestone powder. The success of this system in engineering application provides a low-cost and reliable new bed material preparation and filling method for the industry, and provides a reliable guarantee for the combustion of CFB boilers. The system meets the requirements of energy conservation and environmental protection, enriches the auxiliary system of CFB boiler, improves the supporting level of auxiliary system of CFB boiler, and further promotes the application of clean combustion fluidized bed unit.

Particle temperature and flue gas emission of a burning single pellet in air and oxy-fuel combustion

Pelletization of biomass increases the bulk energy density and the uniformity in size and shape with minimized mechanical degradation from transport, storage, and handling. A pellet can be burned in industrial circulating fluidized-bed or fixed-bed furnaces. This experimental study examines the combustion behavior of single pine wood and empty fruit bunch pellets in a laboratory-scale entrained-flow reactor. Individual biomass pellets were oxidized under both N2/O2 (79%/21%) and CO2/O2 (79%/21%) to investigate the burning characteristics during oxy-fuel combustion. The gas temperature was set at 860 K, 970 K, and 1090 K to examine its effect on combustion behavior. The burning pellets were directly observed in a visualization window in the reactor throughout the sequential combustion process. Direct observation, optical pyrometry, and flue gas analysis are synchronized for each pellet to interpret the relation between phenomenological events, particle temperature, and flue gas emission. The experimental results shows an increased homogenous ignition delay and a conical, less sooty flame in the presence of CO2. The particle temperature under CO2/O2 atmosphere is slightly lower than that under N2/O2, especially for the volatile combustion. The CO emission under CO2/O2 is significantly higher and produced from incomplete combustion. These combustion characteristics are attributed to low oxygen diffusion and the high volumetric heat capacity of CO2. The pine wood displayed a fast, homogeneous ignition and a sooty flame with high NO and SO2 emissions compared to the empty fruit bunches.

Analysis of Technological Issues, Related to Processing of Alunite at Ganja Alumina Plant (GAP), and Ways of Their Solving

In 1965, the Ganja Alumina plant (GAP) started implementing an alkaline reduction technology for processing of alunite ore on an industrial scale. Technological deficiencies, together with design errors, led to unprofitable production. Since the plant was established, studies have been conducted to eliminate deficiencies in the reduction process, through alkaline technology and hardware design. A “reversed” scheme was developed for hydrochemical processing of alunite restored with the conversion of sodium sulphates using a KOH solution. Despite the elimination of several shortcomings in alkaline reduction technology, certain drawbacks remained, in particular: 1) significant emission of gas and dust from the kilns of fluidized bed furnace during roasting and recovery; 2) insufficient time for recovery of alunite powder, which complicates and worsens the technological and economic aspects of the process; 3) passivation of alumina in the roasting and reduction processes; 4) low yield alumina yield in the commercial product (≤ 75%); and 5) a significant amount of solid waste: 5 tonnes of red sludge per 1 tonne of AL2O3, and errors. As a result, the alunite ore processing line ceased production in 1992 and has not operated since.
 
 This article is devoted to the development of new technologies and the improvement of a new potash-alkaline method and new soda-alkaline technology for processing alunite ores. The replacement of potash with soda (sodium carbonate), using new soda-alkaline technology, is proposed. Processing of solution from the first leach with sodium sulphate by conversion with KCl leads to production of K2SO4 and NaCl. Use of the soda-alkaline technology allowed us to obtain the same products as with potash-alkaline technology, with an additional product – table salt. The fluidized bed furnace was replaced by a new type of kiln.

A fast-solving particle model for thermochemical conversion of biomass

Computational fluid dynamics (CFD) simulations of large-scale furnaces or reactors for thermal conversion of solid fuels remains challenging partially due to the high computational cost related to the particle sub-models. Owing to the thermally thick nature, it is particularly expensive to simulate the conversion of large fuel particles such as biomass particles. To address this issue, a fast-solving particle model was developed in this work with special attention to the computational efficiency. The model spatially discretizes a fuel particle in one homogenized dimension. The conversion process of the fuel particle is treated as a reactive variable-volume one-dimensional transient heat conduction problem. The model also utilizes several features that are typically found in sharp interphase-based models to reduce the computational cost. Validation of the model was carried out by comparing with experimental results under both pyrolysis and combustion conditions. The accuracy and computational efficiency of the model was thoroughly examined by varying the degrees of temporal and spatial discretization. It was found that the model well predicted pyrolysis and combustion of a single biomass particle within a broad range of temporal and spatial discretization. The time used to simulate the conversion of a biomass particle using the developed model can be more than one order of magnitude smaller than the conversion process itself. It was also revealed that a well-predicted conductive heat transfer inside the particle is essential for a precise simulation of the drying and devolatilization process. The char conversion process, however, is less sensitive to the external heat transfer as it is mainly controlled by the mass diffusion process. Further studies showed that a time step of 1×10−3 s and a spatial discretization of 20 cells were sufficient for simulating the conversion of typical fuel particles in grate-fired and fluidized-bed furnaces. We also demonstrated that when the particle model was implemented in a CFD solver, only 2.2% of computational overhead was introduced by the model. As the model can efficiently employ fixed time stepping, optimal load balancing during parallel computing of many simultaneous conversion processes becomes trivial. This performance opens up new possibilities for treating fuel polydispersity in Eulerian CFD simulations of biomass conversion.

Thallium isotopic fractionation in industrial process of pyrite smelting and environmental implications

Thallium (Tl) is typical rare element with severe toxicity comparable to Hg and Pb. To track Tl pollution, isotopic fractionation of Tl was evaluated during pyrite smelting for sulfuric acid production. Large variations in Tl isotope compositions were observed among the pyrite ore (PO) and its four different smelting wastes. The starting raw PO had an ε205Tl value of +1.28. The fluidized-bed furnace slag generated by high-temperature smelting had the heaviest ε205Tl (+16.24) in the system. Meanwhile, the boiler fly ash (ε205Tl = +8.34), cyclone fly ash (ε205Tl = +2.17), and electrostatic precipitation fly ash (ε205Tl = –1.10), with decreasing grain sizes during the treatment processes, were characterized by elevated levels of Tl contents and substantial enrichment in the light Tl isotopes relative to the furnace slag. Further calculation and high-resolution transmission electron microscopy indicated that Tl isotope fractionation could be governed by both Rayleigh-type fractionation and adsorption of volatilized Tl by particles of various grain sizes. According to the substantial differences in the PO from its smelting wastes and the measurement precision of isotopic fractionation, it is suggested that Tl isotopes can serve as a new tool for tracing pollution of Tl.

Comparison of the work of Reskvik, Smith, PI regulators in the circuit of maintaining the temperature in a fluidized-bed furnace

This paper dwells upon fluidized-bed furnaces. It shortly describes the theory behind the firing process. Special emphasis is put on calculating the air feed through the nozzles. These calculations serve as the basis for compiling the material and heat balance formulas. The process is modeled mathematically in MatLab. The main parameter of the firing process is the bed temperature. It has been found out that raising the concentrate firing temperature beyond 1050°C enlarges the cinder pieces due to flashing while lowering it below 800°C results in excessive size reduction and incomplete oxidation of the sulfides, i.e. in a worse performance of the furnace. The analysis of firing nickel concentrate in a fluidized-bed furnace shows that for the process to occur appropriately, it is advisable to design the following systems for the automatic control and adjustment of process parameters that determine the quality of firing nickel sulfide concentrate: automatic fluidized-bed temperature control systems, junction-box air feed control systems, and under-roof rarefaction controls. The paper compares three types of controllers as deployed on the furnace temperature maintenance loop.

Experimental fatigue life determination of thermo-diffusion surface boronized of AISI 1040 steel

In this study, the effect of boronizing on the fatigue life of AISI 1040 steel was investigated. The boronizing process was carried out in the fluid bed furnace at temperatures 850 °C and 950 °C for 2, 4 and 6 hours. Boron layer thickness, micro structure, micro hardness and fatigue strength of the treated samples were investigated. The boronized layer on the surfaces became thicker by the increment of the process time and in addition micro hardness increased with the increment in process temperature and time. As a result of the fatigue tests conducted for comparison; shorter fatigue strength values were determined in boronized samples compared to cold drown AISI 1040 sample experiments. Samples which boronized at 950 °C for 6 hours had the lowest fatigue live under all experimental conditions and the highest fatigue cycle numbers were determined in non-boronized samples in fatigue tests.

Effects of Austempering Process to Mechanical Properties of Thin Wall Ductile Iron Connecting Rod.

Austempering process was carried out on thin wall ductile iron (TWDI) connecting rod produced by the design built in previous research. Austempering process was applied to improve the mechanical properties of the component, so it will be equal to forged steel. The aim of this work is to study the effects of austempering holding time to microstructure and mechanical properties of TWDI connecting rod. The process was applied to all connecting rods resulting from the vertical casting. The austenitising temperature was 960°C with 35 minutes holding time and the austempering temperature was 350°C with varied austempering holding time, that were 15, 30, 45 and 60 minutes. The process was held in fluidized bed furnace. Metallographic examination, tensile and compression stress-strain test were conducted directly to the connecting rod. The results showed that austempering process improved the mechanical properties of TWDI connecting rod and the result of the compression stress-strain test showed that the austempered TWDI connecting rod met its requirements.

The Effect of Adding Slag, Achieved from Wastewater Sludge Incineration in Fluidized-Bed Furnace, on the Quality of Concrete

Abstract Due to a constant increase in generating the amount of sewage waste it is necessary to find an alternative method of its use or disposal. One of such methods can be utilization of sewage sludge in construction materials industry, particularly in concrete technology and other materials based on cement. It allows using waste materials as a passive additive (filler) or also as an active additive (replacement of part of bonding material). The article aims at presenting the analysis of the effect of adding slag, achieved from wastewater sludge incineration in sewage treatment plant, on properties and quality of concrete mix and hardened concrete. Using an experimental method, the researcher designed the composition of the control concrete mix, which was then modified by means of slag. For all concrete mixtures determined – air content with the use of pressure method and consistency measured by the use of concrete slump test. For all concrete series the following tests were conducted: compressive strength of concrete after 7, 28 and 56 days of maturing, frost resistance for 100 cycles of freezing and thawing, water absorption. The use of slag, ground once in the disintegrator, causes a decrease of in compressive strength of concrete samples in relation to the control concrete series as well as bigger decrease in compressive strength after frost resistance test.

Automatic control scheme development for process of sulfatizing roasting of iron-manganese concretions in fluidized bed

The article describes the problem of regulating the process of sulfatizing roasting in a fluidized bed (FB) furnace of an enlarged laboratory facility for processing iron-manganese concretions (IMC) together with pyrite concentrate in a continuous mode. The features of roasting in the fluidized bed and technological factors affecting the control process and, as a consequence, the quality indicators of the technology are considered. The results of large-scale laboratory studies on a continuous installation of a FB controlled in manual mode are given. There is a scheme of adaptive automatic control of the roasting process in the FB with regulation of feed charge components. It allows maintaining a stable hydrodynamic state of the fluidized bed and control of key parameters, such as granulometric composition of the cinder, dust removal from the FB, blast pressure, temperature, etc., affecting the sulfatization of IMC metals.

Biomass steam gasification in bubbling fluidized bed for higher-H2 syngas: CFD simulation with coarse grain model

A comprehensive coarse grain model (CGM) is applied to simulation of biomass steam gasification in bubbling fluidized bed reactor. The CGM was evaluated by comparing the hydrodynamic behavior and heat transfer prediction with the results predicted using the discrete element method (DEM) and experimental data in a lab-scale fluidized bed furnace. CGM shows good performance and the computational time is significantly shorter than the DEM approach. The CGM is used to study the effects of different operating temperature and steam/biomass (S/B) ratio on the gasification process and product gas composition. The results show that higher temperature enhances the production of CO, and higher S/B ratio improves the production of H2, while it suppresses the production of CO. For the main product H2, the minimum relative error of CGM in comparison with experiment is 1%, the maximum relative error is less than 4%. For the total gas yield and H2 gas yield, the maximum relative errors are less than 7%. The predicted concentration of different product gases is in good agreement with experimental data. CGM is shown to provide reliable prediction of the gasification process in fluidized bed furnace with considerably reduced computational time.

Emission characteristics for co-combustion of leather wastes, sewage sludge, and coal in a laboratory-scale entrained flow tube furnace.

Four different mixed fuels consisted of leather waste, coal, and sewage sludge were combusted in a lab-scale entrained flow fluidized bed furnace. The influence of blending ratio on emission characteristics of SO2, NOx, HCl, particulate matter (PM), heavy metals, and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) was studied. Results showed that the mixing of coal with sewage sludge had a complex effect on the emission characteristics. On the one hand, with more sewage sludge blending in the mixed fuel, the acid gas pollutant (SO2, NOx) decreased a lot, and the recovery of volatile heavy metals (Cd, Pb) increased at the same time. Furthermore, the leaching toxicity of Cr in the fly ash and bottom ash went down below the national standard with the adding of sewage sludge. On the other hand, the mixing of sewage sludge which consisted of more ash content resulted in the increase of the PM emission. Moreover, the high content of Cu and chlorine in the sewage sludge can promote the formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) when the fuel 3 and 4 were combusted. Most importantly, the concentration of toxic PCDD/Fs in the flue gas produced from fuel 3 and fuel 4 was successfully controlled down below 0.20ng I-TEQ/Nm3 by the active carbon.

Evaluation of Fluidized Bed Ashes from Animal Wastes, Properties, Environmental Impact and Valorization

Use of secondary fuels for power production, such as animal and agricultural wastes, is becoming important nowadays, in view of the declining reserves of fossil fuels. Fluidized bed furnaces offer many advantages, such as fuel flexibility, high efficiency and low pollutant emissions. However, bottom or fly ashes produced from these appliances may create technical and environmental problems. In this study, agricultural wastes from the island of Crete (S. Europe) were co-fired in a fluid bed unit and bottom and fly ashes were characterized by mineralogical and chemical analyses. The effect of ash materials on system’s performance, as well as their environmental impact and valorization were assessed. The results showed that all ashes were enriched in Ca, Si, Mg, P, K. These elements were principally associated with calcite, anhydrite, quartz, dolomite, hydroxyapatite, fairchildite and arcanite minerals. Animal waste ashes contained whitlockite, calcium sodium phosphate and epsonite. Combustion of animal manure above 1000°C could lead to deposition problems in furnaces. Heavy metals Cu, Zn, Mn and Sr dominated in fly ashes. Toxic elements leached through alkaline soil of phyllitic and quarzitic nature were below legislation limits for land disposal. All ashes could be used for soil amelioration, liming or possibly fertilization.

Influence of gas flow oscillation on the efficiency of particulate solids treatment in a circulating fluidized bed

Apparatuses with fluidized bed are widely used in the solid fuel energetic, chemical and other industries. In particular, fluidized bed furnaces play the increasing role for thermo-chemical treatment and burning the solid fuels including bio-fuels. The use of pulsating gas flows is one of the possible ways to intensify the process of gas-particle interaction. However, the efficiency of the way is not always obvious, and new approaches are needed for preliminary estimation of its advantages. The method of mathematical modeling is used to solve the above problem. The model is based on the set of differential equations of a single particle motion in the upstream gas flow with variable with time velocity, and with variable with time particle properties due to this or that thermo-physical or chemical process of gas-particle interaction. The study of influence of the process parameters on its efficiency is done by numerical methods. The model of the effect of oscillations in the gas flow on the efficiency of particle processing in a circulating fluidized bed has been developed. The necessary time for enough particle treatment is compared for different amplitude and frequency of gas flow oscillation. The most considerable gain in time is being reached for the strong dependence of the reaction rate constant on the velocity of flow around the particle. In this case the optimum frequency of oscillations exists that give the maximum gain in time. The growth of the amplitude leads to the monotonic increase of the gain in time, or gives the optimum amplitude. Transition to the pulsating gas flow in a circulating fluidized bed reactor leads to considerable gain in time of particle treatment only in the case of strong dependence of the reaction rate constant on the velocity of flow around the particle. In this case the optimum frequency and amplitude of oscillation exist that gives the maximum gain in time.

Model development of sustainability assessment from a life cycle perspective: A case study on waste management systems in China

Due to the problems of municipal solid waste (MSW) during the expanding urbanization, strategy to find an environmentally friendly, energy efficient, cost-effective, and socially acceptable MSW management system is essential for sustainable development. This study establishes a novel environment-energy-economy-society (3E + S) model from a life cycle perspective for sustainability assessment: life cycle assessment for evaluating environmental performance and energy consumption, life cycle costing for recording economic burden, and social life cycle assessment for reflecting social impacts; based on the individual 3E + S results, the final ranking of alternatives is obtained by multi-criteria decision making, which is integrated with analytic hierarchy process and entropy weight method. This model is implemented to identify a sustainable MSW management system among four typical treatment alternatives. Results show that incineration with fluidized bed furnace is the best choice in this study; incineration with moving grate furnace follows after with a slight gap; landfill with and without energy recovery rank the third and the last. The framework of SLCA on MSW field is built in this study and sensitivity analysis is provided for further discussion on social impacts. The calculation method of weight factors reduces man-made disturbances and the sensitivity analysis demonstrates strong robustness of the results and effectiveness of the modification for the model.