Article pubs.acs.org/IECR
Effective Utilization of Water Hyacinth Resource by Co-Gasification with Coal: Rheological Properties and Ash Fusion Temperatures of Hyacinth-Coal Slurry Haifeng Liu,*,†,‡ Menghan Xu,†,‡ Qiang Zhang,†,‡ Hui Zhao,†,‡ and Weifeng Li†,‡ †
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, No. 130 Meilong Road, Shanghai 200237, People’s Republic of China ‡ Shanghai Engineering Research Center of Coal Gasification, No. 130 Meilong Road, Shanghai 200237, People’s Republic of China ABSTRACT: Water hyacinth has attracted extensive attention due to its capability to capture carbon dioxide and remove excess nutrients and toxic metal ions; however, its exuberant growth also leads to environmental problems. In this study, water hyacinth was introduced to prepare bioslurry fuels with coal in the entrained-flow gasification process. Water hyacinth was modified with the addition of Fe2(SO4)3. Rheological properties and ash fusion temperatures of the modified-hyacinth-coal slurry (MHCS) were investigated. MHCS with solids loading of 60.0 wt % was prepared by adding 19.2 g of modified water hyacinth to 100 g of coal that showed more stability and shear-thinning behavior (thixotropy) than those of coal-water slurry. The ash fusion temperatures of most water hyacinth-coal blends are lower than those of coal and water hyacinth. The modified water hyacinth could further reduce the ash fusion temperature of coal because low-melting eutectic mixtures were formed.
1. INTRODUCTION A huge amount of water hyacinth is harvested every year to control its exuberant growth in India, South Africa, USA, and China.1−3 Water hyacinth produces about 140 tons of dry mass per hectare per year and is an ideal plant for carbon dioxide capture and biomass production.4−6 Moreover, water hyacinth is capable of removing excess nutrients and toxic metal ions from environment.7−10 Common methods for water hyacinth utilization include anaerobic digestion, composting, and its usage as fodder, silage, or green manure; however, these are not enough to deal with their excessive harvest from the seasonal exuberant growth in large water bodies.1,5,11 Besides, water hyacinth suffers from its low energy density and high moisture content, so it is not economically viable to dry water hyacinth for various uses.12,13 A few proper treatment methods limit water hyacinth to play a greater role in environmental protection. Therefore, it is urgent to develop a suitable water hyacinth treatment technology to reduce environmental problems and treatment costs. Preparing bioslurry fuels with water hyacinth and coal followed by processing it in an entrained-flow gasifier could be an effective strategy to utilize biomass as direct fuel.14 The preparation of hyacinth-coal slurry (HCS) does not require predried water hyacinth because a certain quantity of water is needed during the slurry gasification process. Therefore, both the water and calorific value in water hyacinth can be adequately utilized. Since the main solids present in HCS are coal, it would help in effective gasification, thereby meeting the industrial criterion for its use. The primary factors responsible for the use of HCS depend on the influence of water hyacinth on (i) the rheological properties of slurry15 and (ii) the ash fusion temperatures of coal as it is an important factor in the entrained-flow gasifier operation.16−18 High solids content and low viscosity of HCS are important criteria for its storage, transportation through pipelines, subsequent atomization, and gasification. It is difficult © 2013 American Chemical Society
to convert water hyacinth into slurry because it contains many polar oxygen-containing functional groups. During the slurry formation, water can easily interact with the hydrophilic functional groups, and, therefore, the content of free water would be reduced, resulting in an increase in the viscosity of the slurry.19−22 To improve the maximum solids loading of HCS, easy and affordable pretreatment of water hyacinth is required. The entrained-flow gasification technology requires that the coal ash fluid temperature should be lower than 1400 οC because of the thermal properties of refractory materials of gasifier. If it is off-limits, it would bring many operational problems like reducing the life of refractory materials. Because of the liquid-phase epitaxy slag, the reduction of ash fusion temperature can favor the smooth operation of entrained-flow and reduce the oxygen consumption. Li and co-workers have applied algae and sewage sludge to make bioslurry with coal for entrained-flow gasifier.20,23 However, few data are available where higher plants have been used to prepare bioslurry fuel with coal. In this study, HCS was prepared by mixing water hyacinth to coal as a substitute for coal-water slurry (CWS) to use in hydrogen production. The effect of different ratios of water hyacinth on the slurryability of HCS was investigated. A method for modification of water hyacinth is introduced to improve the solids loading and flowability of HCS. The rheological properties (such as viscosity, yield stress, and thixotropy), stability, and ash fusion temperature of the HCS were studied and compared to CWS. Received: Revised: Accepted: Published: 16436
July 8, 2013 October 9, 2013 October 31, 2013 November 7, 2013 dx.doi.org/10.1021/ie402163c | Ind. Eng. Chem. Res. 2013, 52, 16436−16443
Industrial & Engineering Chemistry Research
Article
Table 1. Proximate Analysis and Ultimate Analysis of Shenfu Coal and Water Hyacinth ultimate analysis (wt %)b
proximate analysis (wt %) a
sample
Mar
coal water hyacinth
7.17 94.02
Ad
a
6.58 30.95
Vda
a
FCd
39.7 55.32
53.72 13.73
Cd
Hd
Nd
St,d
Qdc (MJ·kg−1)
69.23 25.86
4.72 2.19
0.86 3.11
0.49 0.85
28.36 12.70
Mar, Ad, Vd, and FCd refer to moisture, ash, volatile, and fixed carbon on a dried basis. bUltimate analysis is also on a dried basis. cQd refers to the calorific value on a dried basis. a
2. EXPERIMENTAL SECTION 2.1. Materials. Shenfu coal from Inner Mongolia and water hyacinth collected from Huangpu River, Shanghai, were chosen for this study. As shown in Table 1, the moisture content of fresh water hyacinth is more than 94%, and its ash, volatile matter, and nitrogen contents are significantly higher than those of coal. Mixing water hyacinth with coal is hopeful to overcome these problems. Sodium naphthalene sulfonate formaldehyde condensate (A1) and modified sodium lingosulfonate (A2) were used as the dispersing agent. 2.2. Experimental Procedure. Raw coal was comminuted in a ball-milling machine and passed through 40−200 and 200 mesh screens to obtain particles of two particle size distributions. The mean volume diameters of coarse particles and fine particles are 36 and 140 μm, respectively. First, water hyacinth was washed to remove sand and soil and then milled in a planetary ball mill for 20 min. Modified water hyacinth was prepared by adding 1.2 wt % of Fe2(SO4)3 (as-received water hyacinth weight basis) during milling of water hyacinth. To prepare HCS, coarse and fine coal particles were mixed by a mass ratio of 6:4. The resulting coal particles were mixed with water hyacinth and 1.0 wt % dispersing agent (as-received basis of the weight of dry solids) in a vessel containing a certain quantity of deionized water. Then, the mixture was stirred by a mechanical agitator at 1,000 rpm for approximately 20 min to ensure homogenization. The amount of water hyacinth added to coal is expressed by the ratio of as-received basis of water hyacinth to coal. The maximum solids loading is defined as solids content of slurry with viscosity (1,000 ± 100) mPa·s at a shear rate of 100 s−1.20 The moisture in water hyacinth is reckoned into the total water of slurry. For ash preparation, a series of water hyacinth-coal blends were dried and converted to ashes according to the Chinese GB/T212-2001 standard. The dried sample was placed on a cupel and heated in a muffle furnace up to 500 οC for 30 min. After keeping it at 500 οC for 30 min, the temperature was raised to 815 οC at a rate of 25 οC min−1 and kept there for 1 h. At last, the temperature was reduced to room temperature, and the sample ash was prepared. 2.3. Analytical Procedure. The rheological property measurements were performed using Malvern Bohlin CVO rheometer. The temperature was controlled at 25 ± 0.1 οC. The viscosity of slurry was measured as follows: shear rate was smoothly increased from zero to 100 s−1 and then kept constant at 100 s−1 for 30 s for further viscosity measurements. The yield stress of slurry was determined as follows: the critical stress at which the suspension begins to flow was measured, such as the point at which slope of the strain (as a function of shear stress) changes from a very low value to a high value, or a rapid reduction in the measured viscosity occurs.24 The stability of slurry was measured according to “glass rod penetration test” described by Qiu et al.25 Prepared slurry was poured into a glass cylinder (3 cm in diameter) to 15 cm in
height at room temperature. A glass rod (5 mm diameter, 20 g) was spontaneously dropped from the slurry surface to the bottom of cylinder at a certain time interval, and it stopped when the tip got in contact with the hard sediment. The time taken by slurry to hold without hard sediment is defined as the storage time. DSC 2910 (Thermo Analysis company) differential scanning calorimeter equipped with a cooling device was used to measure the heat absorption (Q) of freezable water phase transition in water hyacinth. Sample temperature was raised from −30 to 30 °C at a rate of 10 °C/min. According to Z. H. Ping et al.,26 the mass of freezable water in water hyacinth is obtained as Wc = Q /ΔH(g )
(1)
where ΔH is the melting enthalpy of bulk water (333.5 J/g). Infrared spectrum of the sample was analyzed using a MagnaIR 550 Fourier transform infrared (FT-IR) spectrometer of American Thermo Nicolet Corporation. The chemical composition of ash was determined using a X-ray fluorescence spectrometer (XRF-1800) produced by Shimadzu Corporation in Japan. Ash fusion temperatures of the samples were determined using a HR-A5 AFT autoanalyzer (Kaiyuan, China) under a reducing atmosphere according to the Chinese Standard GB/T219-2008. The reducing atmosphere was created by the incomplete combustion of black lead and charcoal in a corundum tube during the heating of ash cones. On the basis of the “Seger Cone” method, the measurements were carried out by heating the ash cone at a rate of 15 οC/min from room temperature to 900 οC and then to the maximum temperature (1,600 οC) at a rate of 5 οC/min. During this process, the deformation of cone with respect to the temperature was photographed. According to the specific shapes of the ash cones, the initial deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT), and fusion temperature (FT) were recorded. X-ray diffraction (XRD, Rigaku D/max-2550VB/PC diffractometer produced by Japan neo-Confucianism Company) was used to identify the mineral composition in the ashes at different temperatures. Each of the ash samples was heated in a reducing atmosphere from 800 οC to FT with an interval of 100 ο C and then dampened in water. The mineral composition and type of mineral were identified using XRD. The diffraction intensities were recorded in the 0−80ο 2θ range.20
3. RESULTS AND DISCUSSION 3.1. Effect of Water Hyacinth on the Viscosity of Slurry. Obviously, the viscosity of slurry is very sensitive to the amount of water hyacinth. Figure 1 shows the dependence of slurry viscosity on the water hyacinth ratio, evaluated at a single shear rate of 100 s−1 when A1 was used as the dispersing agent and the solids loading is 60.0 wt %. The viscosity of HCS increases rapidly with an increasing ratio of water hyacinth. 16437
dx.doi.org/10.1021/ie402163c | Ind. Eng. Chem. Res. 2013, 52, 16436−16443
Industrial & Engineering Chemistry Research
Article
is 19.2 g/100 g coal. MHCSs with four different ratios of water hyacinth were prepared and compared to CWS. The results are shown in Figure 2. It can be seen that the MHCSs and CWS
Figure 1. The influence of the water hyacinth ratio on the viscosity of slurry.
When 25 g of water hyacinth is added to 100 g of coal, the slurry viscosity increases from 578 to 1,598 mPa·s. Therefore, in order to prepare a HCS with a low viscosity, pretreatment of water hyacinth is required. Two types of pretreatment methods are proposed as follows: (i) by prolonging the milling time and (ii) by adding various additives during milling. Water hyacinth was added to the slurry at a constant ratio of 19.2 g/100 g coal, and the solids loading of slurry was 60.0 wt %. The results obtained are shown in Table 2. Prolonging milling time helps to improve the degree of fragmentation of water hyacinth resulting in the release of intracellular water. Therefore, the amount of free water is increased resulting in a decrease in the viscosity of the HCS. Although the viscosity of HCS decreased with increasing milling time, the effect was not significant. Sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], and ferric sulfate [Fe2(SO4)3] were chosen for chemical modification of water hyacinth (1.2 wt %, as-received basis of water hyacinth weight). As shown in Table 2, Fe2(SO4)3 is more effective than Ca(OH)2, while NaOH leads to a slight increase in viscosity. The pH of slurry has no effect on the slurry viscosity, which is different from the corresponding experimental results in case of algae.20 However, the viscosity of HCS decreases significantly by increasing the metal ion valency from Na+ and Ca2+ to Fe3+ ions. Thus, Fe3+ ion is most effective for reducing the viscosity of HCS. This may be because the high valence metal ions have a good ability to compress the electrical double layer and thereby neutralize the charge. Thus, the composite structure of water hyacinth and coal particles is destroyed and more free water is released. Two types of ferric salts [Fe(NO3)3 and FeCl3] were added with the same concentration of Fe3+ ions. Similar effects observed by the use of different ferric salts prove that Fe3+ ions could reduce the viscosity of slurry. Considering the negative effects of nitrogen and chlorine on entrained-flow gasifier, Fe2(SO4)3 was finally chosen for further investigation. The viscosity of modified-hyacinth-coal slurry (MHCS) versus the water hyacinth ratio is shown in Figure 1. The difference in viscosity between HCS and MHCS increases with the ratio of water hyacinth. For MHCS with acceptable viscosity (
