Article pubs.acs.org/IECR
Elemental Distribution and Mineralogical Composition of Ash Deposits in a Large-Scale Wastewater Incineration Plant: A Case Study Lin Mu, Liang Zhao, Liang Liu, and Hongchao Yin* School of Energy and Power Engineering, Dalian University of Technology, 116023, Dalian, Liaoning, China ABSTRACT: Release and transformation of incombustible inorganic substances during petrochemical industrial wastewater incineration form a large number of ash-forming species, which deposit and accumulate on the surfaces of heat transfer tubes, and can pose some severe operational problems, such as fouling, slagging, and even low heat transfer efficiency. Ash deposits characterization has been performed to investigate ash transformation and deposition behavior in a large-scale petrochemical industrial wastewater incineration plant using analytical techniques, including XRF, SEM−EDS, and XRD for elemental composition, morphology, and mineralogy. The results show that this volatile element has a dominated contribution to form varied ash deposits in a wide temperature range from 1000 to about 260 °C. Nickel and iron mainly in the oxide forms are transported by ash particles, have strong deposition propensity at high temperatures, and can be captured by the sticky initial deposition layer. Meanwhile, sulfur is enriched at low temperatures due to condensation, nucleation of alkali metal sulfates, and sorption of SO2/SO3. In the temperature range of 550−900 °C, eutectic mixtures which are temperature-dependent are formed in the molten phase and an “evolving” branched structure detected from SEM indicates the formation of a sintered deposition layer. Acidic salt sodium sesquisulfate is generated at low temperatures and should be responsible for the initiation of low temperature corrosion.
1. INTRODUCTION Industries, such as petroleum refineries and petrochemical plants, as well as pulp and paper mills are a major anthropogenic source of hazardous industrial wastewater, which can pose serious and wide-ranging effects to the environment and human health. There has been a great deal of interest in promoting effective and responsible wastewater treatment and reducing potential secondary pollutant emissions by engineers and researchers around the world. The incineration method is a well established technology which is often used to dispose industrial wastewater with highly toxicity, poor biodegradability, and complex composition.1 Furthermore, significant emphasis is now placed on recovering and utilizing the energy, i.e., waste heat from the hot flue gas to create the steam.2,3 Among various hazardous wastewater incineration treatment facilities for environmental protection in industries, liquid−injection incinerators,4 rotary kiln incinerators,5 and circulating fluidized bed boilers6,7 combined with heat recovery steam generators are the common applications for thermal degradation of hazardous substances contained in industrial wastewater, since they are easy to control and simple in structure. Generally, two basic classes of materials are principally involved in industrial wastewater: organic carbonaceous materials and inorganic species. During the incineration process, the hazardous organic substances are destroyed by intense but controllable chemical reactions, forming innocuous substances, such as CO2 and H2O, and the quantity of hazardous wastewater can be effectively reduced to a small amount of stabilized ash. The following procedures are included in an overall wastewater incineration treatment process: (1) preparation; (2) concentration/evaporation; (3) incineration; (4) heat recovery; and (5) exhaust treatment and emission, as illustrated in Figure 1. Issues © 2012 American Chemical Society
Figure 1. Typical process flow diagram of wastewater incineration.
limiting the wastewater incineration and heat transfer process include: wastewater chemistry, wastewater preparation, wastewater supply, and auxiliary fuel chemistry and supply, as well as control of pollutant emissions.7 In addition, release and transformation of incombustible inorganic substances during wastewater incineration form a large number of ash-forming species which consists mainly of crystalline phases, derivatives of thermally decomposed minerals, and some nonreacted minerals.8 Received: Revised: Accepted: Published: 8684
February 2, 2012 June 3, 2012 June 5, 2012 June 5, 2012 dx.doi.org/10.1021/ie301074m | Ind. Eng. Chem. Res. 2012, 51, 8684−8694
Industrial & Engineering Chemistry Research
Article
Ash-forming species transported by hot flue gas into the heat recovery section can cause some severe operation-related problems, such as fouling, slagging, and corrosion, which lead to low heat transfer between the hot flue gas and the water/steam system inside the tubes and can, in serious cases, even damage the incineration equipment, resulting in unscheduled shutdown and increasing maintenance costs.8,9 According to a general definition of particles and a typical bimodal distribution obtained from the experiments, ash forming species can be divided into two categories based on their varied sizes.9 The first category is vapors derived from vaporization of volatile inorganic components (e.g., Na and K) during incineration. When supersaturation occurs, vapors further grow due to the mechanisms such as homogeneous nucleation, adsorption, condensation, or chemical reaction, forming the submicrometer ash particles.10 The second category, i.e. coarse ash particles, is generated from in-flight combustion and entrainment of nonvolatile ash elements which are originally from the feedstock and process conditions, as well as re-entrained ash deposit fractions from the furnace walls or surfaces of heat transfer tubes.2,11 Some transient or heavy metal oxides act as condensation seeds for the condensable vapors; furthermore some volatile materials which are partially or fully vaporized can also be enriched on the surfaces of large ash particles by heterogeneous condensation as the flue gas cools during heat recovery process, enlarging the ash particles' size.12 Ash transport and deposition is a sophisticated combination of physical and chemical processes, which plays a crucial role in the operating efficiency and security.9,13 Many prominent researchers have pointed out that the formation of ash deposits depends on many parameters, such as temperature distribution in the incineration system, air/waste ratio, gas velocity, trace elements in the wastewater, and use of additives.2,6,7,14−17 It is generally acknowledged that the ash-related operational problems in the incineration plants are primarily due to either inorganic substances containing in the wastewater or the impact of these substances on the resultant gases, ash particles, and deposits.2 Furthermore, it should be noted that ash particles produced from wastewater incineration are quite different from those generated from combustion of conventional fossil fuel or biomass in terms of combustion behavior and physical properties, such as lower melting temperatures, higher contents of Na/S, and lower contents of Ca/Cl.18,19 On the basis of laboratory tests, some researches have indicated that the release of high alkali contents markedly reduce the sintering and melting temperatures and further exacerbate the influence of the ash deposits on heat transfer process.20−23 However, limited investigations have been published on industrial-scale hazardous wastewater incinerators, especially in petrochemical plants. It is thus of great interest to investigate ash deposition behavior as well as mineral transformation in a petrochemical industrial wastewater incineration plant. The main objective of the present work is to characterize various ash deposits sampled from different sections of an industrial-scale hazardous wastewater incineration plant by using some analytical techniques, and then based on the analytical results, some useful references and conclusions can be obtained in order to reduce ash-related problems and minimize ash deposits in the similar wastewater incineration plants.
SINOPEC, China. Because of technical restrictions and the complexity of chemical compositions, it is really impossible to separate each composition in the studied industrial wastewater and characterize them separately. However, a rough approximation of chemical compositions of the industrial wastewater was still obtained and presented in Table 1. The wastewater Table 1. Main Chemical Component of Petrochemical Industrial Wastewater pH chemical component (wt %) acid byproduct (mainly of hydroxyl acid, benzoic acid, their derivatives, etc.) organic sulfonates organic ammonium salt ammonium sulfate caprolactam (CPL) sodium sulfate water
6.30 11.92 36.81 3.42 6.41 1.88 0.49 39.07
was weakly acidic with a pH of 6.3 and contained a high content of acid byproduct (mainly of hydroxyl acid, benzoic acid, their derivatives, etc.) and organic sulfonates. The ultimate analysis, proximate analysis, and ash compositions of the solid content of the studied wastewater are shown in Table 2. Elemental compositions were determined by an elementary analyzer (vario MACRO CHNS) while the oxygen content was obtained by mass difference. Proximate analysis of the solid content was measured using a muffle furnace. The higher heating value (HHV) was measured by a bomb calorimeter (IKA C2000), and the lower heating value (LHV) was determined based on HHV. The ash compositions (as percent of oxide) of the wastewater were determined by XRF.7 On the basis of ultimate analysis, the solid content had a higher level of oxygen and a lower level of carbon in comparison with the traditional fossil fuel24 and biomass.10,25 In addition, high levels of nitrogen and sulfur in the solid content implied that potential secondary pollutant emissions in the atmosphere might be significant (if under favorable conditions for formation of SOx and NOx).26 Water in industrial wastewater resulted from the original moisture in the feedstock and varied over a wide range depending on the process conditions. The presence of high level water lowers the heating value of wastewater; whereas, proper water content can improve the flow characteristic and reduce the wastewater viscosity, which is beneficial for atomization and incineration. Figure 2 is the relations between the wastewater viscosity, temperature, and concentration. The wastewater viscosity is strongly dependent on the temperature and solid content concentration in the wastewater. As the solid content increases, the wastewater viscosity rises significantly, while the wastewater viscosity decreases with the increased temperature. For example, the wastewater viscosity with solid content of 24.55% at 60 °C is 1.6 × 10−3 Pa·s, which rises up to 1 505 × 10−3 Pa·s when the solid content in the wastewater increases to 74.68%. Furthermore, the wastewater viscosity with the highest level of solid content is only 115 × 10−3 Pa·s at 105 °C which is less than one-tenth of that at 60 °C. 2.2. Experimental Facility. The incineration experiment was conducted on an industrial-scale petrochemical wastewater treatment plant, which included a liquid injection incinerator (LII), heat recovery steam generator (HRSG), and flue gas desulphurization (FGD) apparatus, at Shijiazhuang Refining & Chemical Company,
2. MATERIALS AND METHODS 2.1. Studied Wastewater. In this study, the incinerated industrial wastewater was the byproduct mainly produced from the unit processes at Shijiazhuang Refining & Chemical Company, 8685
dx.doi.org/10.1021/ie301074m | Ind. Eng. Chem. Res. 2012, 51, 8684−8694
Industrial & Engineering Chemistry Research
Article
Table 2. Chemical Composition, Physical Properties of Petrochemical Industrial Wastewater, and Main Composition of Auxiliary Fuel Gas ultimate analysis on a dry basis (wt %)
a
carbon (C)
hydrogen (H)
42.80
8.12
volatiles
ash
62.23
17.71
oxygen (O)
nitrogen (N)
sulfur (S)
14.04
3.78
31.26 proximate analysis on a dry basis (wt %) fixed C
ρa (kg/m3)
20.06 1126.32 ash composition (as percent of oxide) of wastewater by XRF
Na2O
Al2O3
SiO2
28.59
22.89
1.30
SO3
CaO
SVb (mL/g)
HHV (MJ/kg) 21.31
1.47
Fe2O3
NiO
ZnO
Cl
27.27 1.27 8.67 main composition of auxiliary fuel gas (vol %)
9.34
0.38
0.30
H2
C5
C3H8
C3H6
C4H10
CO2
C2H4
C2H6
O2
N2
CH4
CO
37.80
2.77
0.19
0.21
0.19
1.60
10.78
10.10
0.28
14.59
19.86
1.63
Density. bSwelling volume.
and for wastewater ignition. Then, the wastewater was injected into the combustion chamber A, while the injection amount of auxiliary fuel gas decreased gradually to 0. On the basis of formed furnace temperature and heating value of wastewater itself, a steady combustion flame can be obtained in order to degrade hazardous materials in the wastewater. During the experiment, the total amount of incinerated wastewater was kept at 8 ± 0.2 t/h and distributed equally through 12 burners, i.e. the injection amount of each burner was about 0.180− 0.190 kg/s. The main composition of auxiliary fuel gas is also listed in Table 2. The HRSG manufactured by Dalian boiler company, Dalian, Liaoning, China, was a horizontal corner tube boiler. The furnace dimension was 17.4, 2.6, and 3.8 m in length, width, and height, respectively, with two circular smoke boxes at the inlet and outlet. Tube banks in the furnace were in a staggered arrangement with hot flue gas traversing across them. On the basis of different tube pitches, two patterns of tube arrangements were adopted. In the first half section, 12 rows of tubes with the transverse pitch 170 mm and the longitudinal pitch 125 mm were introduced and divided into 3 groups, each consisting of 4 rows. In the second half section, there were 76 rows of tubes with the transverse pitch 130 mm and the longitudinal pitch 125 mm, respectively, each 4, 5, or 6 rows was combined into one group. There were several working passages between neighboring two groups used for slag cleaning manually during overhaul. In addition, there are altogether 24 detonation-wave soot blowers which were manufactured by Harbin modern soot-blowing company (Harbin, China) installed along the flue gas journal in the furnace of the HRSG as well as in the economizer. During the operation, operators could activate the soot blowers manually based on the operating data and actual slagging conditions, as well as their experience. Although the detonation-wave soot blowers were effective enough to remove most of ash deposits, some of strongly linked deposits can not be removed completely, leaving a thin coverage of deposits on which subsequent deposition took place.13 Therefore, the incineration plant was usually shut down every 30−60 days, and then workers entered into the furnace to clean the ash deposits that were strongly attached to the tube surfaces. 2.3. Operating Conditions and Test Procedure. In the present work, our focus is on the formation of ash deposits as well as further characterization. Over a period operation of 15 days without activating soot-blowers, the whole incineration
Figure 2. Relations between the wastewater viscosity, temperature, and concentration.
SINOPEC, China, as shown in Figure 3. Concentrated industrial wastewater, divided by wastewater distributor, was injected into the combustion chamber A of the incinerator through 12 mechanical atomization burners which were installed uniformly on the upper combustion chamber A. The central axis of each burner was 30° from the central axis of combustion chamber A. Primary air (PA) as well as auxiliary fuel gas was introduced through the PA inlet and an auxiliary fuel gas burner, both of which were installed vertically at the top of combustion chamber A, while secondary air (SA) was supplied into the furnace together with the incinerated industrial wastewater through each burner in order to enhance turbulent mixing for sufficient atomization and incineration. At the tail of the incinerator, the DeNOx device (SNCR, selective non-catalytic reduction) was installed in order to convert potential flue gas NOx to the elemental nitrogen through both high-temperature and use of NH 3/steam mixture for the purpose of reducing NOx emissions. The DeNOx device did not work under the normal operating condition as well as during the experiment period because the NOx emissions of wastewater incineration can completely meet pollutants emission standard for hazardous wastes incineration (NOx < 500 mg/m3) according to Chinese standard GB18484-2001. For starting the wastewater incineration plant, auxiliary fuel gas was injected into the combustion chamber A from the auxiliary fuel gas burner, and only used for preheating of the incinerator up to 600−800 °C 8686
dx.doi.org/10.1021/ie301074m | Ind. Eng. Chem. Res. 2012, 51, 8684−8694
Industrial & Engineering Chemistry Research
Article
Figure 3. Schematic overview of the combined wastewater incineration facility, marking five sampling locations of ash deposits as numbers 1−5.
using a step size of 0.04° 2θ, and the scanning speed was 4°/min.
unit was shut down and cooled to the room temperature. According to field observations, severe ash-related problems mostly took place in the HRSG; therefore, all the characterized ash deposits were collected from the furnace of the HRSG. The mature ash deposits formed on the heat transfer tubes which were exposed at different furnace temperatures were removed carefully without destroying initial structures. They were numbered as (1) the 1st row; (2) the 13th row; (3) the 37th row; (4) the 61st row, and (5) the 81st row (see Figure 3). Upon collection, all the ash samples were stored in a sealed bag separately to prevent them from polluting each other. After visual inspection, ash samples were comminuted by the sample preparation comminuter (GJ-I, Xin Ke Instrument Co., Ltd., Henan) and stored in clean, sealed bags for further characterization. For measuring the furnace temperature, a modified flue gas-pumped thermocouple with a heat insulation cover was adopted in order to reduce the measurement errors. The cover was used to insulate the thermocouple detector from the surroundings in order to reduce the radiate heat transfer error, and the scouring velocity of the flue gas to the thermocouple as well as the cover was increased by air pumping, leading to an increased coefficient of convection heat transfer. 2.4. Analytical Determinations. In order to ascertain a general view of ash deposits, the collected ash samples were characterized using analytical techniques, including a X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Japan), scanning electron microscope equipped with energy dispersive X-ray spectrometer (SEM, JEOL JSM-5600LV, Japan; EDS, Oxford Inca X-Sight, UK), and X-ray powder diffractometer (Shimadzu XRD-6000, Japan). The detailed elemental compositions and distribution characteristics along the flue gas journal in the HRSG were analyzed by XRF. SEM−EDS was adopted to determine the morphology of the ash deposits and elemental compositions of typical positions in SEM photos. XRD was used to detect the mineralogical compositions of ash deposits. For X-ray data collection, the diffractometer was operated with a copper X-ray tube at operation voltage 40 kV and current 30 mA. Measurements were realized by diffraction angle 2θ over a range from 20° to 100° (or 10° to 90° for some samples)
3. RESULTS AND DISCUSSION 3.1. Deposit Formation and Classification. Figure 4 is the visual inspection of ash deposits sample 1 which was
Figure 4. Visual inspection of ash sample 1.
collected from the surface of heat transfer tube in first row. The mountain-like-shaped deposits represent a typical layered structure along the development direction of ash deposits thickness. A thin but dense deposition layer whose thickness is only 1−5 mm is formed first next to the surface of heat transfer tube. The formation of this initial deposition layer can be explained by the fact that owing to the cooling effect of the heat transfer tube surface, alkali vapors which were released during wastewater incineration diffuse toward the tube surface, and then condense on it. While the main body layer is basically formed by the ash particles which are transported by flue gas and mainly in solid state. The main body deposition layer shows an apparent characteristic of overlying in the form of sheets, which implies that slagging or even flowing deposits trend to be formed due to significant increase in surface temperature and may flow down along the tube surface. The texture of main body deposition layer is relatively dense, and moderate physical force is 8687
dx.doi.org/10.1021/ie301074m | Ind. Eng. Chem. Res. 2012, 51, 8684−8694
Industrial & Engineering Chemistry Research
Article
Figure 5. Visual inspection of (a) ash sample 2, (b) ash sample 3, and (c) ash sample 4.
Figure 6. Visual inspection of ash sample 5.
of samples shown in Figure 5a, b, and c are 50−70, 40−60, and 30−40 mm, respectively. Figure 6 is ash samples collected from the heat transfer tubes in the 81st row near to the outlet of the HRSG. In this section, the amount of deposited ash is much smaller compared to those in the front section of the HRSG and the thickness is only 5−10 mm. Ash deposits at low temperatures are fine grained but quite friable with a color of gray−white. Ranges of flue gas temperature around the sampling locations and general description of each deposit sample are listed in Table 3. 3.2. Elemental Analysis. Some studies have indicated that deposition behavior of ash particles, as well as subsequent transformation mechanisms on the tube surfaces, depends heavily on the inorganic mineral substances which are closed related not only to the chemical composition but also to the physical states, while, elemental analysis of ash deposits is an established method to identify element migration and deposition behavior along the flue gas journal.27,28 Table 4 lists the element compositions of ash samples in the oxide form. The dominant mineral constituents in all the ash samples are sodium oxide (Na2O), nickel oxide (NiO), ferric oxide (Fe2O3), and sulfur trioxide (SO3), whereas the concentrations of chromium oxide (Cr2O3), aluminum oxide (Al2O3), calcium oxide (CaO), silicon dioxide (SiO2), and others are insignificant. For major elements, Ni, Fe, and most of minor elements, they have high melting points and thus do not volatilize under the relatively low furnace temperature during wastewater incineration.29 The comparison of elemental compositions between the ash samples is contingent on two distinct behaviors of mineral elements, i.e. their volatilities.3 Sodium is a highly reactive alkali metal which can easily vaporize and stay in the gas phase during the wastewater incineration. Overall, the contents of sodium oxide in the different ash samples are similar. For ash sample 1, the sodium oxide content
required to break it. The ash deposits collected from sampling points 2, 3, and 4 are shown in Figure 5a−5c and have a similar macroscopic view of irregular and heterogeneous shapes which can be distinguished by the different colors of formed deposition layers. The color of ash deposits which are adjacent to the tube surface is gray−green; whereas, the ash deposits on the outside have a color of ochre or chocolate-like dark brown. Ash deposits on the inner layer show an unconsolidated looseshaped structure which appears to be porous (although minor sintered structure was detected); however, the relatively obvious sintered structures are identified on the outside due to direct exposure to the hot flue gas and a striking increase in surface temperature. Because of ash particle−to−particle bonding and agglomeration at high temperatures, the pores appearing in the sintered deposition sections are larger than those in the unsintered deposition layer. In addition, the initial deposition layer which has been observed in ash sample 1 is not detected in ash samples 2, 3, and 4. According to the operating data, the boiler feedwater temperature was near 130−150 °C, and it was used to generate steam with pressure of 2.0 MPa and temperature of 216 °C. Therefore, the temperature of tube surface was only about 220−250 °C. We, thus, believe that strong heat transfer process between the hot flue gas and working fluid inside the tubes makes the temperature of flue gas decreases remarkably, and then, alkali vapors still containing in the flue gas undergo significant homogeneous nucleation, forming the fine ash particles in submicrometer size, or heterogeneous condensation on the surface of high melting point ash particles which play the role of the “seed”. Therefore, rapid condensation of alkali vapors on tube surfaces becomes weaker and less perceptible. On the whole, the amount of ash deposits declines gradually from sample 2 to sample 4, and the average thicknesses 8688
dx.doi.org/10.1021/ie301074m | Ind. Eng. Chem. Res. 2012, 51, 8684−8694
Industrial & Engineering Chemistry Research
Article
Table 3. General Description of Ash Deposits Collected from Different Positions in the HRSG no.
position
flue 1 2 3 4 5 flue
temperature range (°C)
thickness (mm)
color
description
gas temperature at the inlet: 1027−1043 °C first row 1025−1040 70−90 dark brown 13th row 903−925 50−70 gray−green next to the tube surface; red−brown outside 37th row 683−708 40−60 61st row 488−525 30−40 81st row 260−280
