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Slurrying property and mechanism of coal–coal gasification wastewater–slurry Jinqian Wang, Jianzhong Liu, Shuangni Wang, and Jun Cheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00107 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018
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Slurrying property and mechanism of coal–coal gasification wastewater–slurry Jinqian Wang, Jianzhong Liu*, Shuangni Wang, Jun Cheng
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, China
Abstract: The co-slurrying technology of coal and organic wastewater, which achieves the simultaneous disposal and utilization of wastewater, has attracted extensive attention in recent years. The emission of coal gasification wastewater causes serious environmental hazards due to its large amount, high organic content, and high toxicity. In the present study, two types of wastewater, namely, gasifier wastewater (GW) and wastewater from a secondary sedimentation tank (SW), were used to prepare coal water slurry (CWS), which was labeled as coal–GW–slurry (CGS) and coal–SW–slurry (CSS), respectively, with coal–deionized water– slurry (CDS) as reference. The surface property of coal and the dispersing mechanism were characterized using zeta potential and contact angle analyses. Results showed that the fixed viscosity loading decreased with the addition of the two types of wastewater, and the decrease was more considerable with GW. All the slurries exhibited pseudo-plastic behavior and thixotropy, with the following order: CGS > CSS > CDS. Wastewater, particularly GW, strengthened the stability of CWS. The influences of ammonium and volatile phenols on slurry viscosity were also investigated. The effect of ammonium was related to the pH value of the solution. The effect of volatile phenols on the range of wastewater organic content was limited. The stability of CWS was obtained by combining the result of ammonium with organic matter. Ammonium affected the surface charge of coal particles and changed the electrostatic repulsion between coal particles. The amphiphilic organic molecules enhanced the hydrophilicity of coal, which weakened the hydrophobic interaction between particles
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and thickened the hydration layer. Consequently, the agglomeration of coal particles was weakened.
Keywords: coal gasification wastewater; coal water slurry; slurrying property; ammonium; interparticle interaction
Introduction Coal gasification is a key technology in the clean and efficient transformation of coal. It is the basis for the synthesis of various chemicals (e.g., ammonia, olefin, and methyl ether) and plays an important role in integrated gasification combined cycle poly-generation technology.1 However, coal gasification, particularly Lurgi pressurized gasification, is frequently accompanied by a large amount of highly concentrated and toxic wastewater. Statistics indicate that the conversion of 1 ton of coal emits approximately 0.8–1.1 tons of wastewater.2 Such wastewater generally contains high amounts of toxic substances, such as phenols, ammonia, cyanide, thiocyanide, and heterocyclic and polycyclic aromatic compounds.3-5 The components of different wastewater sources vary significantly depending on coal type and gasification. Consequently, the treatment of coal gasification wastewater has become one of the challenging problems in industrial wastewater disposal. Biochemical treatment, which is widely adopted at present, is limited due to its poor process stability, complicated process, and high investment cost.6 Therefore, a new and efficient disposal method for coal gasification wastewater should be developed.7 Coal water slurry (CWS) is an oil substitute technology developed in the 1970s. CWS exhibits superiority given its convenient transportation, high carbon conversion rate, and low pollution discharge.8-9 At present, it is widely used as a feeding technology for gasification.10 CWS is a highly concentrated suspension with pulverized coal dispersed in water. The slurrying property plays a vital role in the industrial application of CWS. For easy pumping, storage, and gasification, an ideal CWS should exhibit low viscosity, good fluidity, and stability.11 The adaptability of raw materials is gradually enhanced with the development of CWS gasification technology and the evolution of industrial operating devices. In recent years, researchers have used CWS technology to dispose of solid and liquid wastes (e.g., sewage sludge12, bioferment residue13, wastewater14, and
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engine oil15). In this manner, the calorific value of wastes is fully utilized and disposal cost is reduced, which are economically and environmentally beneficial. Zhou et al.16 used treated black liquor as CWS dispersant and found that such liquor demonstrated similar or better dispersing capability than a naphthalene dispersant. Xiang et al.17 used oilfield wastewater to prepare CWS and found that such wastewater promoted the slurrying property and improved the rheological properties of coal slurries. Wang et al.18 found that coal chemical wastewater improved the slurrying performance of CWS and petroleum coke water slurry by increasing the adsorption amount of the additive. Liu et al.19 obtained higher fixed viscosity concentration and better stability when using coal sludge slurry prepared from hydrothermal treatment waste liquid instead of from water due to the presence of dissolved organic compounds and ions. The aforementioned studies demonstrate the feasibility of coal–organic wastewater–slurry. However, studies on the preparation of coal–coal gasification wastewater–slurry are rare,20 and information regarding the effects of various components of coal gasification wastewater on the slurrying property is insufficient, i.e., the co-slurrying mechanisms of coal gasification wastewater and coal remain unclear. The present study focused on the viscosity, rheological property, and stability of coal–coal gasification wastewater–slurry. The effects of ammonium and volatile phenols on CWS viscosity were investigated. Coal surface property in coal wastewater slurry was analyzed via contact angle and zeta potential measurements. The slurrying micromechanism of coal–coal gasification wastewater–slurry was proposed by adopting the extended Derjaguin–Landau–Verwey– Overbeek (DLVO) theory.
Experiment 2.1 Materials Mixed coals from the Shenhua Group were selected. Raw coals were dried, ground in a ball mill, and passed through a 100-mesh sieve. The proximate and ultimate analyses of Shenhua coals are provided in Table 1. The particle size distribution of the coals was analyzed using a laser particle size analyzer (BT-9300ST, Baite, China), and the result is presented in Figure 1. The median diameter (D50) is c.a. 25.82 µm.
Table 1. Proximate and ultimate analyses of Shenhua coals
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Sample
Proximate analysis (%) Mar Ad Vd FCd Shenhua coals 8.26 14.68 29.38 55.94 M, A, V, FC refer to moisture, ash, volatile, refers to a dry basis.
Ultimate analysis (%) Qd (MJ/kg) Cd Hd Nd Sd Od 70.62 4.11 1.60 0.44 8.55 25.08 and fixed carbon contents; ar refers to an as-received basis;
4.5
d
100
3.5
80
3.0 60
2.5 2.0
40
1.5 1.0
20
0.5 0.0 0.1
1
10
100
Accumulative Volume (%)
4.0
Volume (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0 1000
Particle Size (µm)
Figure 1. Particle size distribution of Shenhua coals
Wastewater samples were collected from the Yima Coal Gasification Plant in Henan, China. The samples were classified into gasifier wastewater (GW) and wastewater from a secondary sedimentation tank (SW). GW is a puce troubled liquor with a certain viscosity and multi foams. It emanates a strong odor from phenols and ammonia. SW is a yellow liquid with precipitate. The supernatant liquid was used for the experiment. The main characteristics of GW and SW are listed in Table 2. GW has high chemical oxygen demand (COD) and NH3−N content due to the low gasification temperature in a Lurgi gasifier. With sludgelike floccules suspended in, GW showed higher kinematic viscosity than SW. Naphthalenesulphonic acid, sodium salt, polymer with formaldehyde (MF, CAS No.: 9084-06-4), which is one of the most common additives for preparing CWS, was adopted as the chemical additive. The chemical structure of the additive is shown in Figure 2. According to our former research, additive MF showed good suitability for the preparation of Shenhua coal water slurry. Additive dosage was fixed at 0.8 wt.% on a dry coal basis.
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Table 2. Main characteristics of wastewater samples Sample pH Kinematic viscosity (mm2/s) COD BOD5
GW (mg/L) 8.73 0.970 14400 5100
SW (mg/L) 7.68 0.926 578 244
Volatile phenol
5600
0.33
NH3−N
5260
228
NO3−N
35.5
18.0
TN
7830
315
Sodium Potassium Chloride Sulfate Sulfide Cyanide Oil
10.7 0.5 253 2510 0.017 0.359 37.7
379 10.9 232 6 CSS > CDS. The addition of the two types of wastewater decreased WSR and enhanced stability. CGS achieved the best stability. These findings indicate that wastewater is favorable for the storage, transportation, and atomization of CWS. (2) The effect of NH3−N content on slurry viscosity varied with the type of ammonium solution. The viscosity and zeta potential of CWS increased with an increase in the neutral or acidic ammonium salt content. For the alkaline ammonium solution, slurry viscosity initially decreased and then increased with an increase in NH3−N content. The turning point occurred when NH3−N content was c.a. 2700 mg/L and pH value was c.a. 11.2. The addition of volatile phenols exerted minimal effect on viscosity within the range of wastewater organic content but improved the wettability of coal. (3) The extended DLVO theory was applied to analyze the dispersion and stabilization mechanism of CWS. The amphiphilic molecules in wastewater, which were adsorbed onto coal surface, reduced the contact angle of coal and improved the hydrophilicity and polarity of coal surface. Thus, the hydration layer thickened, and hydrophobic attraction decreased. NH4+, K+, and other cations in SW compressed the electric double layer and decreased the electrostatic repulsion between particles. The weak alkaline GW enhanced the electronegativity of coal and strengthened electrostatic repulsion. CGS was more stable than CSS because of the higher organic content and floccule amount in GW.
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Acknowledgment The authors would like to acknowledge the financial support provided by the National Key Research and Development Program of China (Grant No. 2016YFB0600505).
ABBREVATIONS CWS = coal water slurry GW = gasifier wastewater SW = wastewater from a secondary sedimentation tank CDS = coal–deionized water–slurry CGS = coal–gasifier wastewater–slurry CSS = coal–wastewater from a secondary sedimentation tank–slurry COD = chemical oxygen demand η = apparent viscosity ηc = characteristic viscosity (the average η value presented at 100 s−1) SL = solid loading SL1000 = fixed viscosity loading (SL of the slurry sample when ηc = 1000 mPa·s) γ = shear rate τ = shear stress WSR = water separation ratio
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