Inhibition and promotion: The effect of earth alkali metals and operating temperature on particle agglomeration/defluidization during incineration in fluidized bed
Graphical abstract
Different ratios of alkali metal (Na) to earth alkali metals (Mg and Ca) were employed to simulate the agglomeration/defluidization. Low Na/Mg and Na/Ca ratios inhibit the generation of agglomeration/defluidzation, but inhibition declines slowly as the temperature increases. When the ratios Na/Mg and Na/Ca exceeded two, the inhibition was weak. This result indicated that the inhibition by Mg and Ca was almost eliminated with the ratio increases.
Introduction
During fluidization, the accumulation of agglomerates changes the fluidization characteristics such as bubble size, bubble frequency, pollutant emission and minimum fluidization velocity [1]. This phenomenon influences the operation of fluidization even to the extent of shutting down the fluidized bed. Nonetheless, agglomeration requires many factors for it to take place. According to Langston and Stephens [2], Moseley and O'Brien [3] and Wank et al. [4], the agglomeration of particles is associated with particle size, surface, density, reaction mechanism, gas velocity, temperature, physical and chemical characteristics, area of contact and momentum of the particles. Therefore, it is clear that the agglomeration mechanism is fairly complex.
Fluidized bed reactors provide various advantages such as good solid mixing, large contact surface area and strong heat transfer [5], which have been exploited in incineration. However, most waste contains many materials such as plastics [6], heavy metals, alkalis and earth alkali metals [7]. Skrifvars et al. [8], [9] have reported that visco-materials cause the sintering of glassy materials, where liquid phase materials are produced by melting and chemical reactions which lead to agglomeration/defluidization. During incineration, the generation of agglomeration in a sand bed changes the fluidized behaviours and results in the production of secondary pollutants. Lin et al. [10] have pointed out that the concentration of emitted polycyclic aromatic hydrocarbons (PAHs) increases with agglomeration. Furthermore, defluidization significantly increases the emitted concentration of heavy metals (Pb, Cr and Cd). These results show that agglomeration/defluidization influences the emission of pollutants during incineration.
Some literatures reported that various elements markedly affect the agglomeration/defluidization during fluidization [7], [11], [12]; these elements include vanadium, nickel, ferrum, sulphur, chlorine, alkali metals and alkali earth metals. When alkali metals (Na and K) are present in coal or waste, they increase the risk of agglomeration during combustion [12], [13], [14]. However, the alkali earth metals (Mg and Ca) may also cause inhibition and promotion of agglomeration/defluidization. Atakül et al. [15] have reported that Mg and Ca accelerate agglomeration to form CaO and MgO in the sand bed. Fernández et al. [16] further stated that the amount of Ca in the agglomerates exceeds that in the non-agglomerated particles. This result reveals that Ca promotes the generation of agglomerates. Additionally, Mg and Ca may react with other elements to form the compounds magnesium sulphate and calcium carbonate, which agglomerate in the sand bed and ash [7], [15], [16]. On the other hand, Lin et al. [13] have reported that the addition of Mg and Ca to the waste has two effects (inhibition and promotion) on agglomeration. However, the results are inconclusive. Most researches have focused on analyzing the relationship between the components in coal and agglomerates during the combustion of coal. The components of coal are complex and contain various metal elements. Therefore, the relationships between various metal constituents and agglomeration have not been clarified.
However, municipal waste generally contains some alkali and alkali earth metals. Marco et al. [17] have pointed out that these metals may melt to form agglomerates at temperatures between 700 °C to 800 °C. The generation of agglomerates must be prevented during municipal incineration in order to reduce the secondary pollutants. However, few studies have addressed this issue. The effect of Mg and Ca on agglomeration/defluidization must be considered distinctly. Accordingly, this study considers the effects of alkali earth metals (Mg and Ca) and operating temperature on the agglomeration/defluidization of Na during incineration. Different ratios of alkali metal (Na) to alkali earth metals (Mg and Ca) were added to artificial waste. The signal addition and mixture addition of different metals were used to simulate the effects of Mg and Ca on agglomeration during incineration.
Section snippets
Experimental
The experimental apparatus was a bubbling fluidized bed incinerator as shown in Fig. 1. The reactor contains a preheated chamber (50 cm long), a main chamber (110 cm high) with an internal diameter of 10 cm, and a secondary combustion chamber (100 cm high) with an inner diameter of 25 cm. The chamber was composed of 3 mm thick stainless steel (AISI 310). The apparatus was surrounded by an electrically resistant material and packed with ceramic fibres for thermal insulation. The reactor was
Effect of Na concentration
Fig. 3 shows the effect of the Na concentration on the defluidization time. The defluidization time decreased gradually as the concentration of Na increased. When the Na concentration exceeded 1.4%, the variation of defluidization time was small at high concentrations. This result indicated that the effect of Na on agglomeration/defluidization is a negative correlation between time and concentration, in which the time decreases significantly as concentration increases. According to Atakül et
Conclusions
This study has examined the effect of Mg, Ca and operating temperature on the agglomeration/defluidization of sodium during incineration. Various compositions of artificial waste were employed to simulate agglomeration/defluidization during incineration. Therefore, the relationship between alkali earth metals and agglomeration/defluidization was considered. The experimental results reveal that the agglomeration/defluidization time decreases gradually as the concentration of Na increases,
Acknowledgement
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC 95-2218-E-040-001-.
References (29)
- et al.
Chemical reaction induced agglomeration and defluidization of fluidized beds
Powder Technol.
(1995) - et al.
A model for agglomeration in a fluidized bed
Chem. Eng. Sci.
(1993) - et al.
Vibro-fluidization of fine boron nitride powder at low pressure
Powder Technol.
(2001) - et al.
Photocatalytic oxidation of methyl orange in a three-phase fluidized bed reactor
Chemosphere
(2002) - et al.
The phenomenology of bed defluidization during the pyrolysis of a food-packaging plastic waste
Powder Technol.
(2001) - et al.
Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatments
Fuel
(2003) - et al.
Sintering mechanisms of FBC ashes
Fuel
(1994) - et al.
Emission characteristics of organic and heavy metal pollutants in fluidized bed incineration during the agglomeration/defluidization process
Combust. Flame
(2005) Laboratory techniques for evaluating ash agglomeration potential in petroleum coke fired circulating fluidized bed combustors
Fuel Process Technol.
(1995)- et al.
Formation of bed agglomeration in a fluidized mulit-waste incinerator
Fuel
(2003)
The effect of mineral compositions of waste and operating conditions on particle agglomeration/defluidization during incineration
Fuel
The ash chemistry in fluidized bed gasification of biomass fuels. Part Ⅰ: predicting the chemistry of melting ashes and ash-bed material interaction
Fuel
The relationship between the tendency of lignites to agglomerate and their fusion characteristics in a fluidized bed combustor
Fuel Process Technol.
Fluidization of agglomerating particles:influence of the gas temperature and composition on the fluidization of a Li/MgO catalyst
Powder Technol.
Cited by (51)
Transesterification of used cooking sunflower oil catalyzed by hazelnut shell ash
2022, Renewable EnergyBiomass-based systems
2021, Polygeneration Systems: Design, Processes and TechnologiesCarbon dioxide as a main source of air pollution: Prospective and current trends to control
2021, Sorbents Materials for Controlling Environmental Pollution: Current State and Trends