- Research article
- Open Access
Thermal decomposition of selected chlorinated hydrocarbons during gas combustion in fluidized bed
© Olek et al.; licensee Chemistry Central Ltd. 2013
Received: 6 September 2012
Accepted: 2 January 2013
Published: 6 January 2013
The process of thermal decomposition of dichloromethane (DCM) and chlorobenzene (MCB) during the combustion in an inert, bubbling fluidized bed, supported by LPG as auxiliary fuel, have been studied. The concentration profiles of C6H5CI, CH2Cl2, CO2, CO, NOx, COCl2, CHCl3, CH3Cl, C2H2, C6H6, CH4 in the flue gases were specified versus mean bed temperature.
The role of preheating of gaseous mixture in fluidized bed prior to its ignition inside bubbles was identified as important factor for increase the degree of conversion of DCM and MCB in low bed temperature, in comparison to similar process in the tubular reactor.
Taking into account possible combustion mechanisms, it was identified that autoignition in bubbles rather than flame propagation between bubbles is needed to achieve complete destruction of DCM and MCB. These condition occurs above 900°C causing the degree of conversion of chlorine compounds of 92-100%.
Chlorine derivatives of aliphatic and aromatic hydrocarbons are a group of compounds produced on a large scale. These substances are persistent environmental pollutants. Their degradation in nature is slow due to the presence of covalent bonds in their molecules and also they are often xenobiotics. Combustion is one of possible methods for chlorinated hydrocarbons (CHCs) destruction. Taking into account the average bond enthalpy  of C-Cl (338 kJ/mol) to C-C (348 kJ/mol) and C-H (412 kJ/mol)), at the elevated temperature chlorine should be easily dissociated. The main products of the process will be stable H2O, CO2 and HCl easily separated in scrubbers.
Fluidized bed incineration is one of technology which can be used in gaseous and liquid waste disposal. Usually wastes have low calorific value and auxiliary fuel (e.g. gaseous fuel) is needed to achieve stable combustion process. It is well known that in bubbling fluidized bed, gaseous fuel can be efficiently burnt [2–8]. The process of fuel conversion in inert bed material (eg. sand) is result of radical processes occurring in the bubbles [9, 10]. Combustion inside the bubbles has periodic character and are accompanied by pressure pulsations and visual effects [11–14]. Gaseous mixture fed into the fluidized bed quickly reach its temperature, and then the temperature inside the bubbles reach values of several hundred degrees higher than the temperature of the fluidized bed . The effect of overheating the gas mixture inside the bubbles can be used in the processes of waste disposal when the heteroatoms are present in them. Especially when heteroatoms such as Cl or Br participating in radical reactions they may cause the inhibition of the combustion process.
The combustion of solid and gaseous fuels in a fluidized bed in the presence of halogen derivatives have been studied before, but only concentration of CO, NO, NO2 have been analyzed. The general consensus was that the halogens have an inhibitory effect on the combustion process [15, 16]. The inhibition is due to catalytic effect of Cl radicals on the process of recombination of O, H, OH, and HO2 radicals. It was found that the presence of HCl inhibits the oxidation of CO to CO2 and decreases the formation of NO in the flue gas. This effect of halogen on CO and NO concentrations in flue gases was confirmed in other works [17–21].
As studies  and  show the combustion process of gaseous fuels in a bubbling fluidized bed in the temperature range of 300–1000°C is carried out at a certain height above the distributor. It has also been shown that one can designate two characteristic bed temperatures (Tcr.1 and Tcr.2), which enable distinguishing of three temperature ranges (combustion regime A, B and C). These regimes are associated with different physical mechanisms of combustion. According to  in regime A a mixture of gaseous fuel and air burn in flame above fluidized bed. During regime B and C it burn inside the bed in bubbles. Between Tcr.1 and Tcr.2, in regime B, combustion process in the bubbles is possible as a result of flame propagation between bubbles. During regime C - when the temperature of the bed is higher than the critical temperature Tcr.2 - the burning occurs as a result of self-ignition of the fuel mixture in bubbles [13, 14, 22]. The composition of the flue gases strongly dependences on type of combustion regime. It have been described earlier (for methane and ethane) [13, 14] and will be briefly discussed later in this work (for LPG).
The aim of present work was to determine how the addition of selected chlorinated hydrocarbons influences the fluidized bed combustion over the range 300-1000°C. This was done to recognize possibility of using the reactor with an inert fluidized bed to decompose selected chlorinated hydrocarbons. Measurements of concentration of chlorine byproduct in the flue gases, under different combustion regimes were also performed. The concentration profiles of initial, final and byproduct compounds were determined as a function of mean bed temperature. The profile of degree of DCM and MCB decomposition were compared with results obtained in tubular reactors, based on data from literature.
Materials, apparatus and analytical methods
Flue gases were monitored by a set of analyzers. The most important element of the analytic set was a GasmetTM DX-4000 IR spectrum analyzer (Figure 1, part C). It utilizes Michelson interferometer to obtain spectrum of analyzed gaseous sample. After Fourier transformation of interferogram concentration of selected compounds were calculated. The gas sample was periodically analyzed to determine concentration of HCl, CHCl3, CH3Cl, CH2Cl2, COCl2, C6H5Cl, CH4, C2H2 and C6H6. Concentrations of CO2, CO, NO and O2 were continuously monitored using a Horiba PG-250 analyzer (Figure 1, part A). A J.U.M. 3-200 analyzer (Figure 1, part B) was used to monitor VOCs (volatile organic compounds). Concentrations of CO2 and CO were measured using non-dispersive infrared detector (NDIR), NOx and O2 concentrations were determined using chemiluminescence (CLA) and electrochemical (EC) method respectively. The total concentration of VOCs was determined by using a flame-ionization method (FID). The accuracy of the measurement equipments for different species was as follow: CO and NO - 1 ppm, CO2, O2 - 0.1% vol., VOC - 0.1 ppm, HCl, CHCl3, CH3Cl, CH2Cl2, COCl2, C6H5Cl, CH4, C2H2 and C6H6 – 2 ppm.
At the beginning of experiment run the bed of sand was fluidized by air (1.66 dm3/s in normal conditions), which temperature was 80°C. Then a constant stream of chloroorganic additive was injected into the air. After dosing of hydrocarbon fuel the combustion process was initiated. The bed was gradually heated from c.a. 80°C to 1000°C at rate ~1.5 K/s and for 180 s the process was carried out isothermally. Afterwards the bed was cooled down to c.a. 800°C. Then it was heated again to 1000°C. After next 180 s the process was ended by turning off the supply of chlorine additives and next by cutting off the supply of hydrocarbon fuel. Initial concentrations of DCM introduced into the reactor were as follows: 1100 ppm (DCM1), 2700 ppm (DCM2) and 5350 ppm (DCM3). Similar experiments were carried out using MCB to give initial concentration of 900 ppm (MCB1), 1800 ppm (MCB2) and 3650 pmm (MCB3), respectively. To be able to find the influence of chloroorganic addition on combustion process, a reference experiment (w/o chloroorganic compounds addition) were also carried out. In the following discussion the results for the middle concentration of DCM and MCB have been consistently ignored because the results were located between values obtained for lower and higher concentrations of additives.
The results and discussion
The effect of additives on the combustion process
Adding of DCM (concentration 900 ppm – DCM1; Figure 2B) or MCB (concentration 1100 ppm, Figure 2C) to the reacting mixture did not change the values of critical temperatures, T’cr.1 and T’cr.2 were 535°C and 830°C respectively. Increasing the DCM concentration in the mixture at 5350 ppm and MCB to 3650 ppm increased the first critical temperature to T”cr.1 = 560°C (Figure 2B) and T”cr.1 = 585°C (Figure 2C). The second critical temperature was also changed. T”cr.2 = 840°C for DCM. After introducing MCB into the reactor T”cr.2 decreased to 800°C (Figure 2C).
As a result, the combustion of LPG in the presence of DCM is inhibited.
Different effects on the combustion process were observed when 3650 ppm of MCB was injected. Lowering Tcr.2 means that not every chlorinated hydrocarbon has negative impact on the combustion process. This follows from the fact that the molar ratio C:H:Cl in the MCB (6:5:1) is different than in DCM (1:2:2).
Products of LPG combustion in the presence of chlorinated hydrocarbons
Under regime A, below 200°C, the fluidization did not cause a significant turbulence on bed’s surface. LPG burns in the flame over the bed’s surface. Some of the solid particles were transported to the rare zone (freeboard) of the reactor. In that case combustion was almost completed and CO2 was the main carbon product. While the temperature increased, the velocity of gases and the turbulence of the fluidization increased and more solid particle were transported into the rare zone. The recombination of the radicals occurred on the surface of them. As the result a decrease of [CO2] and an increase of [CO] and [VOC] in the flue gases were observed (Figure 4B and C). Under regime B the combustion occurred in the large bubbles close to the bed's surface. CO2 concentration fell down until 630°C then increased till Tcr.2. The concentration of VOC and CO changed, but character of the changes was different. The peak concentration of VCO reached 2000 ppm at 470 - 570°C and decays to below detection at 800°C. The inhibitive effect of the particles in fluidized bed on the processes involving radicals, which are responsible for increase of the CO concentration is clearly observed in B regime. While bed temperature increase, instead of the expected decrease [CO], it rises to maximum concentration of 5000 ppm at 780°C. Then, at temperature near Tcr.2 [CO] fell rapidly. This was caused by the change of the mechanism controlling the combustion at the transition between B and C regimes. Above Tcr.2 spontaneous combustion inside the bubbles takes place, in a homogenous phase without solid particles present. There was no quenching effect from recombining radical adsorbed on the sand surface. Combustion in regime C is characterized by low CO and VCO concentration. Almost entire carbon was converted to CO2 (Figure 4A).
When chlorine compound is added to the combustible mixture, Cl and HCl, compete with CO for H and OH radicals (1-2 vs. 4-5). It leads to inhibition of CO oxidation. Analyzing the changes of CO concentration (CO1, CO3 lines, Figure 4B and E) it can be determined that this inhibition mechanism is significant under regime A and B. Under regime C, at higher temperature the concentration of OH is increased and catalytic processes involving chlorine is negligible.
Under regime A when DCM or MCB was added, VOCs concentration was lower, in comparison to LPG combustion. This change indicates that below Tcr.1, there is possibility of reaction between chlorinated byproducts and fuel which accelerates decomposition of the latter. Maximum of [VOC] was observed between regime A and B when LPG alone was burnt. Addition of CHCs shifts VOC peak slightly to higher temperature. Near Tcr.2 VOC was completely destroyed.
Chloroorganic compounds of the thermal decomposition of DCM
The temperature of the fluidized bed is a factor influencing the concentrations of toxic by-products resulting from incomplete combustion of DCM.
Under fuel-lean condition rather CHCl2 radicals will be created than CH2Cl .
The decrease in methyl chloride concentration along with the increase in DCM concentration may indicate that reaction (18) (limited by CH2Cl radical concentration) has a dominating role in the fluidized bed. That radical takes part in recreating DCM during regime B according to reactions (15) (Figure 5). In temperature above 900°C both CHCl3 and CH3Cl react completely.
where X = Cl, H, O, OH.
Above 900°C phosgene concentration was too small to be measured by the apparatus.
Chloroorganic compounds of the thermal decomposition of MCB
The results support that as with the decomposition of DCM, the degree of decomposition of MCB strongly dependent on temperature of the bed.
Below 825°C the following reactions initiating the MCB decomposition are possible :
Introduction MCB to the system not caused a significant increase in the concentration of benzene, but shifted its maximum concentration to the higher temperature.
During the combustion of LPG with MCB the main gas product containing chlorine was hydrogen chloride. Over 900°C the conversion factor of chlorine in MCB to hydrogen chloride was 92-100%.
Influence of presence of chlorine compounds on NOxformation
During combustion of fuels in the air nitrogen oxides are formed by oxidation of N2 or nitrogen fuel. The first process runs according to the Zeldovich mechanism , which is important when the temperature of the reagents exceeds 1400°C, or prompt mechanism, at lower temperature [16, 33]. In the latter mechanism crucial role play CHx forming HCN, CN, NH and hence NOx.
Thermal decomposition of chlorinated hydrocarbons during its combustion in mixture with supporting gaseous fuel, in the condition created by fluidized bed of sand, takes place in bubbles floating inside a bed’s dense phase. In relation to combustion in the tubular reactor, process of their decomposition occurs in a relatively lower temperature. This is due to specific role of the emulsion phase of fluidized bed. By contact with this phase mixture of utilized compound with air and supporting fuel are preheated to the temperature of the fluidized bed. After this preheating, the gaseous mixture (in the bubbles) ignited. When heat is released, temperature of the mixture in the bubbles increased. Due to the limited heat transfer between gases in the bubbles and the bed material, gases can reach maximum value of 1300°C or more while temperature of bed material is e.g. 850°C [14, 22]. It means that the effective temperature of the degradation of the pollutant is few hundred degrees higher than the temperature measured in the reactor. What is more, intensive mixing in the bed causes the alignment of the fluidized bed temperature. This ensures appropriate conditions for complete conversion of the reactants. Combustion of hydrocarbon (supporting) fuel in such an environment ensures stable conditions and is a source of H and OH radicals, which are necessary for disintegration of chloroorganic substances. On the other hand, adding to the fuel chlorine compounds influences the combustion process which leads to some changes of the values of critical temperatures Tcr.1 and Tcr.2. These small shifts do not make impossible to carry on thermal degradation of given additives. It was confirmed that phosgene can be formed only if, the oxidized substance is able to create a structure of two chlorine atoms bound to carbon atom. Therefore, in the case of chlorobenzene, the concentration of phosgene in the whole temperature range, was below detection limit, while during the oxidation of DCM reached a value 11 times higher, but only if the bed temperature is lower than 900°C .
If the main efficiency criterion is the minimization of CO and NOx concentration and preventing the formation of intermediate species from DCM or MCB oxidation, then it is preferable to carry out the process during regime C of the combustion in the fluidized bed. The optimum range for the process is limited by the minimum temperature of 900°C and the maximum temperature of the bed’s material. When the material used in the bed is SiO2 the temperature should not be higher than 1100°C. In a laboratory-scale reactor it has been found that even if the reactor has no automatic temperature regulation it is possible to have the temperature variation range at ±10 K. In a larger scale installation, which includes an automatic temperature regulation system, it is achievable to get the designated temperature range.
Notice: Some of the theses presented in this work, were previously published in Polish in: Przemysl Chemiczny 91 (5), 2012, pp. 912-919
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