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The experimental study on the incineration of the acrylonitrile effluent in the CFB boiler burning oil shale.

1. Introduction

Acrylonitrile is an essential raw material mainly for the synthesis of polyacrylonitrile fiber, ABS/SAN resin, adiponitrile, acrylamide and carbon fiber [1]. In China, the demand for and production of acrylonitrile are huge [2, 3], so investigation of the possibilities of disposal of the wastewater generated is of great practical significance. The by-products in the process of propylene ammoxidation are mainly hydrogen cyanide, acetonitrile, acrolein, carbon dioxide and carbon monoxide. Provided by Chinese Guangxi Keyuan Corporation, the effluent sample was from the 200,000 t/y acrylonitrile project, whose components and design flow values are given in Table 1.

The acrylonitrile effluent has a very complex composition, consisting not only of acrylonitrile, acetonitrile and hydrocyanic acid, but also of acrylamide, acrylic acid, and acrolein [4]. The acrylonitrile, acetonitrile and hydrocyanic acid in the effluent are extremely harmful to human health. In direct contact, acrylonitrile is able to damage the skin and mucosa of the human body, inflict harm on the organs, and has proved to be carcinogenic [5, 6]. After entering the human body, cyanides generate the [CN.sup.-] group, which can bind enzymes relevant for respiration, making cells unable to utilize oxygen normally [7].

The Chinese Integrated Wastewater Discharge Standard GB8978-1996 [8] sets the emission standards of cyanides, requiring that the first and secondary level emission should be below 0.5 mg/L and the third level emission should not exceed 1.0 mg/L. Therefore, the acrylonitrile effluent has to be treated to make it harmless.

Hitherto, there have been used various approaches for the harmless treatment of the acrylonitrile wastewater. These include enrichment by evaporation, hydrolysis, ozonation, ion exchange and activated carbon adsorption, as well as biochemical, ferrous sulfate, peroxide and liquid membrane methods. The incineration of the acrylonitrile effluent is able to oxidize the complex organic components, realizing the harmless treatment. The circulating fluidized bed (CFB) technology has many advantages, such as high combustion efficiency, greater flexibility in fuels use, high heat transfer efficiency, and low emissions [9, 10]. Besides, the huge amount of inert material present in the furnace at high temperature creates an ideal environment for the evaporation, pyrolysis and combustion of the wastewater, which is especially suitable for the incineration of the hazardous effluent.

In China, along with the economic development and social advancement, the consumption of traditional fossil energies, mainly coal and oil, is increasing rapidly. Therefore, seeking substitutes for them is becoming more and more vital. Oil shale is an important alternative energy. China has abundant oil shale resources and rich reserves [11]. The conventional methods of oil shale utilization in China include retorting and burning in the boiler [12]. Besides, oil shale is an ideal co-firing fuel, the mixture of which with high sulfur coal demonstrates good combustion characteristics [13]. However, there have been no investigations as yet on the co-combustion of oil shale and hazardous material in the CFB boiler for harmless treatment.

The originality of the trial described in this paper consists in that the simultaneous incineration of oil shale and the acrylonitrile effluent in CFB renders the latter harmless. The following two main objectives were set:

1) to verify whether the process of incineration can transform acrylonitrile, acetonitrile and hydrogen cyanide into harmless compounds;

2) to investigate the mechanism of transformation of the hazardous material by measuring the component concentrations in the flue gas.

At first, the current paper presents the characteristics of the effluent used, reports the results of proximate and ultimate analyses of oil shale, and gives the particle size distribution of the quartz bed material. Then, the experimental methods and measurement equipment are described. The third section discusses the measurement results and analyzes the regularities of transformation of hazardous substances.

2. Materials and methods

2.1. Material characteristics

For experiments, the Agilent 7890A Gas Chromatograph was used according to the U.S. Environmental Protection Agency method USEPA8031-1994, to measure the concentration of acrylonitrile and acetonitrile in the wastewater, the results are given in Table 2.

The oil shale sample used in the experiment was from Jilin Huadian City, the results of proximate and ultimate analyses of the sample are presented in Table 3.

The bed material was quartz. Its particle size distribution in weight is given in Table 4. The motionless height of the bed material was kept at 300 mm, which needed 2.95 L quartz sand.

2.2. Experimental methods

The test facility used in the experiment resembled a large CFB boiler, as shown in Figure 1. The height and inner diameter of the furnace were 5 m and 210 mm, respectively. Oil shale was added from the top of the furnace to prevent blocking. The effluent was injected from the bottom of the furnace. The distance between the two points was 3.8 m. The effluent injection point was 1.2 m above the grid. Silicon carbide bars, which were used to heat the furnace, were placed outside it. The tail flue was collected into a bag-type dust collector.

During the trial, the furnace was heated to 300 [degrees]C by silicon carbide bars. After oil shale was added, the temperature was controlled by the air volume and feed rate to maintain the combustion. Depending on whether the wastewater was injected, as well as on whether the temperature of the dense-phase zone was 750 or 850 [degrees]C, the experiment was carried out with four material groups: two blank groups and two experimental groups. During the measurement of concentrations, the combustion of each group was maintained for 15 minutes. In the two experimental groups, the acrylonitrile effluent was injected into the furnace as a flow of 35 ml/min by a peristaltic pump.

The component concentrations of the flue gas were measured. The measuring points were all situated at the exit of the separator. Acrylonitrile and acetonitrile in the flue gas were detected by the PerkinElmer Clarus 500 gas chromatograph according to the solvent desorption and gas chromatography method recommended by the Chinese national standard Determination of nitriles in the air of workplace GBZ/T160.68-2007 [14]. The concentration of hydrogen cyanide (HCN) was measured by a UV/Vis spectrophotometer according to the Iso-nicotinic-acid-3-methyl-1-phenyl-5-pyrazolone spectrophotometric method as described in the Chinese national standard Determination of hydrogen cyanide of the Stationary source emission HJ/T28-1999 [15]. The concentrations of conventional gas were measured by a flue gas analyzer Gasmet FTIR DX4000. All the measured concentrations were converted into concentrations in dry flue gas and the 1.4 excessive air coefficient based on Chinese Emission standard of air pollutants for coal-burning, oil-burning, gas-burning boiler GB13271-2001 [16].

3. Results and discussion

3.1. The decomposition rate of acrylonitrile and acetonitrile

The main hazardous components of the effluent were hydrogen cyanide, acrylonitrile and acetonitrile, whose measured and converted concentration values are given in Table 5.

According to the results of analysis of the wastewater and the measured concentrations of hazardous components in the flue gas, the conversion rates of acrylonitrile and acetonitrile after combustion were calculated, the results are presented in Table 6.

From Table 5 and Table 6 it can be seen that the conversion rates of the two compounds are over 95% at both 850 and 950 [degrees]C, however, being higher (97.5%) at 750 [degrees]C. The average concentration of oxygen in the exit of the separator at 850 [degrees]C is only 3.76%, remaining far below the 8.06% at 750 [degrees]C. The low oxygen concentration causes a relatively low oxygenolysis, which leads to a low conversion rate of acrylonitrile and acetonitrile. In the process of the acrylonitrile effluent incineration, the excessive air coefficient can be increased to enhance the decomposition rate of both compounds. The emission limits of the three contaminants as set in Chinese standards are listed in Table 7.

The Chinese standard GB16297-1996 [17] stipulates that the emission limit of acrylonitrile is 22 mg/[m.sub.3], the results obtained at the two different temperatures prove that the incineration process can make the concentration of acrylonitrile meet the Chinese standard. However, GB16297-1996 [17] does not set the specific emission concentration limit of acetonitrile. Therefore, the limit value of 30 mg/[m.sub.3] in the Chinese national standard Occupational exposure limits for hazardous agents GBZ2.1-2007 [18] was employed as the emission limit. Similarly, after the incineration the acetonitrile concentration in the flue gas met the standard.

From Table 5 it is clear that the concentration of HCN in the flue gas at both the temperatures was almost 40 mg/[m.sub.3]. According to the Chinese Integrated emission standard of air pollutants GB16297-1996 [17], the hydrogen cyanide emission limit of a polluting unit built after January 1, 1997 must be lower than 1.9 mg/[m.sub.3]. In this work, the corresponding measured figure was far beyond this value, which means that the incineration process is unable to meet the national standard. The trial demonstrated that the direct incineration of the flue gas with low HCN concentration gave no desired disposal effect. So, the flue gas needs further treatment. Besides, the combustion of the fuel containing elemental nitrogen generates hydrogen cyanide [19]. Hence the most effective way to remove HCN is to subject the flue gas to purification treatment.

3.2. The transformation of hazardous materials

The analysis of different experimental compound groups showed that the main oxygenolysis products of acrylonitrile, acetonitrile and hydrogen cyanide were NO and [N.sub.2]O. The four experimental conditions were as follows: 750 [degrees]C without effluent, 750 [degrees]C effluent added, 850 [degrees]C without effluent and 850 [degrees]C effluent added. The emission concentrations of NO and [N.sub.2]O were converted into concentrations in dry flue gas and the 1.4 excessive air coefficient and are given in Figure 2.

The main decomposition products of the three hazardous materials were NO and [N.sub.2]O generated from two sources: oxygenolysis of the oil shale organism containing elemental nitrogen, and conversion of acrylonitrile, acetonitrile and hydrogen cyanide in the wastewater. The measured concentration values of the flue gas demonstrated that after the injection of the effluent, the concentrations of NO and [N.sub.2]O increased significantly, which proves that the hazardous components in the wastewater underwent oxidative decomposition. The overall incineration process proceeded through the following reaction sequence:

C[H.sub.2] = CH - CN + [O.sub.2] [right arrow] C[O.sub.2] + [H.sub.2]O + HCN (1)

C[H.sub.3] - CN + [O.sub.2] [right arrow] C[O.sub.2] + [H.sub.2]O + HCN (2)

HCN + [O.sub.2] [right arrow] [H.sub.2]O + C[O.sub.2] + NO + [N.sub.2]O (3)

After the addition of the wastewater, the concentration of NO increased much more than that of [N.sub.2]O. Due to the injection of the acrylonitrile effluent into the oil shale CFB, the concentration of NO and [N.sub.2]O in the flue gas no longer met the limit standard for nitrogen oxide [16]. The flow rate of the wastewater has to be dealt with more thoroughly in order to make the concentrations of emitted NO and [N.sub.2]O acceptable for practical applications. The water in the effluent evaporated rapidly after the injection. Thus acrylonitrile, acetonitrile and hydrogen cyanide went through rapid gasification. Due to the high triple bond energy, the cyano in the cyanides was unable to react with other chemicals. Therefore, the gaseous acrylonitrile and acetonitrile were converted into hydrogen cyanide [20], a major intermediate from the reaction of the cyano with hazardous NO and [N.sub.2]O [19, 21]. In the reaction process, the oxidation product of HCN was NCO. Then, NCO was converted into NO after oxidation. The reaction of NCO with NO gave [N.sub.2]O [22, 23]. A large amount of NCO was oxidized to generate NO in a competing way. This is why the NO concentration rose significantly after the addition of the wastewater, whereas the [N.sub.2]O concentration increased negligibly.

The increase of the [N.sub.2]O emission concentration was more noticeable at 750 [degrees]C, being smaller at 850 [degrees]C. This might be due to that the decomposition of [N.sub.2]O occurred more easily at a higher temperature [24] and the conversion of [N.sub.2]O took place in a lower oxygen concentration atmosphere [25]. Also, with rising temperature the concentration of NO increased and that of [N.sub.2]O decreased. This may be explained by that the concentration of oxygen in the furnace was lower at 850 [degrees]C than at 750 [degrees]C. The production of NO increased with increasing temperature, while the generation of [N.sub.2]O decreased with decreasing excessive air coefficient and increasing temperature [26, 27].

4. Conclusions

In this work, the oil shale sample from Jilin Huadian City was used to incinerate the acrylonitrile effluent. The effluent was provided by the Guangxi Keyuan Corporation, China. In the experiments, the bed temperature of the dense-phase zone was maintained at 750 and 850 [degrees]C, respectively. Based on the results of analysis of the acrylonitrile effluent composition and the measured values of the flue gas, as well as comparisons with Chinese national standards, the following conclusions can be drawn.

Under the experimental conditions, the conversion rates of the acrylonitrile and acetonitrile in the wastewater were both slightly over 95%. Most of the hazardous material underwent oxygenolysis, whose major product was NO and minor, [N.sub.2]O. The incineration of the effluent caused an evident increase of the NO concentration and only a slight rise in the [N.sub.2]O concentration. The harmless treatment carried out during the experiment showed that the concentrations of acrylonitrile and acetonitrile in the flue gas were lower than the emission limits set in Chinese national standards. However, the hydrogen cyanide concentration in the flue gas did not meet the requirements, which indicates that the direct incineration of this compound is not a suitable method for harmless treatment and the flue gas needs further disposal.

doi: 10.3176/oil. 2016.1.05


The authors would like to thank all the staff involved in carrying out the experiment. The Guangxi Keyuan Corporation is acknowledged for providing the materials and relevant information.


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Presented by J. Qian

Received October 9, 2014


Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China

* Corresponding author: e-mail

Table 1. The components of the effluent and design flows

Component                    Design flow, kg/h

Acrylonitrile and            4.5
hydrogen cyanide
[H.sub.2]O                   3098.6
CO                           948.1
C[O.sub.2]                   2926.1
[C.sub.3][H.sub.6]           254.4
[C.sub.3][H.sub.8]           77.8
[O.sub.2]                    1910.8
[N.sub.2]                    84950.1
N[O.sub.x], mg/N[m.sup.3]    [less than or equal to] 500
Total flow, kg/h             94165.9
Operating temperature,       36
Operating pressure, MPa      0.015
Average molecular weight     28

Table 2. The concentration of acrylonitrile and acetonitrile

Compound        Concentration, mg/L

Acrylonitrile          1720
Acetonitrile           1180

Table 3. The proximate and ultimate analysis of oil shale

Proximate analysis, %

[]   []   []   []    [Q.sub.

11.54          36.21        48.24          4.02        11076.07

Ultimate analysis, %

[]   []   []    []    []

27.33           3.59         7.89          0.57          0.84

Table 4. The particle size distribution of quartz sand

Particle    > 4.0   4.0-3.0   3.0-2.5   2.5-2.0
size, mm

wt%           0      18.02     25.37     29.21

Particle    2.0-1.38   1.38-1.1   1.1-0.8   < 0.8
size, mm

wt%          16.85       7.13      2.08     1.34

Table 5. The measured and converted concentrations
of hazardous components in the flue gas

Temperature,             750    850

Average                 8.06   3.76
concentration of
oxygen at the exit
of the separator

Gas                           Concentration, mg/[m.sup.3]

                      Measured    Converted   Measured   Converted
                        value       value      value       value

Hydrogen cyanide        36.6        42.43       45.1       39.24
Acrylonitrile           2.39        2.77        3.86       3.36
Acetonitrile            1.78        2.06        3.89       3.38

Table 6. The conversion rate of hazardous components

Parameter                    Unit

Fuel feed rate               kg/h
Effluent flow rate          ml/min
Air flow rate             [m.sup.3]/h
Flue gas flow rate        [m.sup.3]/h

Feed rate of hazardous      mg/min      Acrylonitrile
components                              Acetonitrile

Hazardous components        mg/min      Acrylonitrile
residue                                 Acetonitrile

Conversion rate of             %        Acrylonitrile
hazardous gas                           Acetonitrile

Parameter                  Temperature      Temperature
                          750 [degrees]C   850 [degrees]C

Fuel feed rate                 6.3              10.3
Effluent flow rate              35               35
Air flow rate                  21.6             29.2
Flue gas flow rate            34.29            44.46

Feed rate of hazardous         60.2             60.2
components                     41.3             41.3

Hazardous components           1.37             2.05
residue                        1.02             2.07

Conversion rate of            97.72            96.59
hazardous gas                 97.53            94.99

Table 7. The emission limits of three contaminants

Contaminant     Chinese standard                Concentration limit,

Acrylonitrile   Integrated emission standard             22
                of air pollutants
                GB16297-1996 [17]

Acetonitrile    Occupational exposure limits             30
                for hazardous agents
                GBZ2.1-2007 [18]

Hydrogen        Integrated emission standard            1.9
cyanide         of air pollutants
                GB16297-1996 [17]
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Author:Ma, Yang; Liu, Jianguo; Fan, Chao; Wang, Sha; Jiang, Xiumin
Publication:Oil Shale
Article Type:Report
Date:Mar 1, 2016
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