Ind. Eng. Chem. Res. 2008, 47, 1331-1335
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Influence of Different Lignosulphonates on the Properties of Desulfurant Sorbents Prepared by Hydration of Ca (OH)2 and Fly Ash M. Josefina Renedo* and Josefa Ferna´ ndez Departamento de Ingenierı´a Quı´mica y Quı´mica Inorga´ nica, ETSIIyT, UniVersidad de Cantabria, AVenida de los Castros s/n, 39005-Santander, Spain
The influence of different lignosulphonates used as additives on the physicochemical properties and desulfurization behavior of mixtures of Ca(OH)2 and fly ash was investigated. Sorbents were prepared by the hydration of fly ash and Ca(OH)2 at 3:1 or 5:3 fly ash /Ca(OH)2 initial weight ratios for 7 or 15 h, at 90 °C and atmospheric pressure. The lignosulphonates tested are trade named: Borrebond 55-L, Borrebond 55-S, Borresperse CaI-50, and Borresperse NaI-50. The properties characterized in the sorbents include Ca(OH)2 conversion, BET surface area, and X-ray diffraction pattern. The sorbents were tested in a flue gas desulfurization reaction at low temperature. The effect of the additives on the desulfurant properties of the sorbents depends greatly on the amount of additive, with the type of lignosulphonate used being less relevant. A positive effect was found when the weight of the additive was 0.1 or 0.2% of the initial amount of solid raw materials. Higher percentages of additive produced the opposite effect, decreasing greatly the specific surface area and the SO2 capture of the sorbents. When additives are used in low amounts, a general low increase in the specific surface area of the sorbents is found; this structural property and the hydrophilic character of lignosulphonates present in the sorbents, that increase the retention of the water from the flue gas, can explain the higher amounts of SO2 retained per mol of calcium with these sorbents. When lignosulfonates are added at higher amounts to the slurry of fly ash and Ca(OH)2, their agglomerating properties prevail and the raw materials remain unreacted in the sorbents that exhibit low surface areas and are unable to capture SO2 on their surface. Introduction Sulfur dioxide is one of the main air pollutants emitted from thermal power plants firing fossil fuels. In Spain, 42% of the electrical power plants are coal-fired plants burning different national or imported coals.1 Hydrated lime is mainly used in flue gas desulfurization technologies to abate SO2 from these power plants, with the low sorbent utilization being its main disadvantage in dry processes. SO2 retention can be greatly improved by using modified calcium hydroxide. Zhao et al.2 used humic acid (H.A.), a surfactant agent to modify calcium hydroxide, getting an increase in the desulfurization activity depending on the weight percentage of H.A. in the sorbent. Kirchgessner and Jozewicz3 prepared modified calcium hydroxide by the hydration of CaO in water containing a desugared lignosulphonate. They found that the predominant effect of the additive was a reduction in the rate of sintering of the sorbents used in desulfurization in the post flame zone. Ghosh-Dastidar et al.4 found a great increase in the BET surface area and in the conversion value for sulfation, using a modified hydrated lime prepared from a commercial calcium hydroxide calcined and rehydrated in water containing dissolved calcium lignosulphonate at the optimal concentration. Testing different modifiers of calcium hydroxide, Ada´nez et al.5 found the best sulfur retention with the sorbent obtained by CaO hydration with ethanol-water and using a calcium lignosulphonate as additive. Tseng et al.6 studied the effect of calcium and sodium lignosulphonates on the preparation and the use of modified calcium hydroxide as a sorbent for acid gases. They found a great * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 34-42-201580.
increase in the acid gas removal by the use of these additives. In a previous work,7 CaCO3 was synthesized by the carbonation of a suspension of calcium hydroxide in the presence of different lignosulphonates founding that the CaCO3 obtained in the presence of lignosulphonate NaI-50, showed a specific surface area five times higher than that of the sorbent obtained without additive. The modification of calcium hydroxide, by making a slurry of this base with fly ash, the most voluminous byproduct of all of the coal power plants, renders sorbents with a higher specific surface area and calcium utilization than calcium hydroxide, due to the products of the pozzolanic reaction among raw materials. In previous works,8-12 the optimal conditions to prepare desulfurant sorbents from fly ash and hydrated lime and the influence of additives such as CaSO4 or seawater salts on the properties of these sorbents were studied. It was found that the best desulfurant solids were obtained by mixing raw materials at 5:3 or 3:1 fly ash/Ca(OH)2 ratios and maintaining hydration for 7 or 15 h. It was also found that the influence of sea salts12 on the increase in calcium utilization of the sorbents was principally due to the ability of the remaining salts to retain moisture from the flue gas; respect to the calcium sulfate, the positive influence was only found at long hydration times, principally due to the great increase in the specific surface area values of the sorbents prepared with sulfate respect to the sorbents prepared without additive. In both cases, any influence of the additives on the pozzolanic reaction rate was not found. Lignosulphonates are byproducts of wood pulp manufacturing. In our region of Cantabria (Spain), Lignotech Ibe´rica, a Company of the Sniace Group, produces these compounds by recovering the lignin resulting from the cellulose production process. Physical and chemical properties of these products
10.1021/ie071003o CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
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Table 1. Physical and Chemical Properties of the Raw Materials Physical properties
Hydrated lime
Fly ash (F.a.)
Adsorption-Desorption of N2 BET specific surface area (m2/g) 16.20
2.98
Intrusion-Extrusion of Hg pore volume (cm3/g) 1.395 macropore volume (>50 nm) (cm3/g) 1.319 mesopore volume (6.7-50 nm) (cm3/g) 0.076
0.680 0.658 0.022
Laser Diffraction mean volume diameter (D 4,3) 6.59
27.64
Chemical Composition (wt %) SiO2 Al2O3 Fe2O3 MgO CaO Ca(OH)2 CaCO3 Na2O K2O insoluble loss by heat impurities
Hydrated Lime
Fly Ash 49.26 30.04 6.92 0.85 1.91
84.32 11.98 0.64 2.44 0.17 6.31 3.70
differ, depending on the extraction technology and source of wood and are widely used as dispersant agents in an aqueous environment, as an agglomerant, as an emulsifier as a wetting agent, or as a chelating agent. In this work, following the study of the influence of additives on the properties of desulfurant sorbents prepared with fly ash and hydrated lime, the effect of different lignosulphonates added in different amounts on the physicochemical properties and desulfurization behavior of the sorbents is studied. The use of these compounds in the process of the preparation of desulfurant sorbents can contribute to diversify the use of these by products that are still burnt in 95% of the amount produced. Experimental Section Preparation of Sorbents. The sorbents were prepared using commercial calcium hydroxide supplied by Calcinor S.A. and ASTM Class F coal fly ash from a coal-fired power plant in Pasajes (Guipuzcoa, Spain). Ca(OH)2 and fly ash chemical composition and physical properties are shown in Table 1. Four different lignosulphonates supplied by Lignotech Ibe´rica were tested: Borrebond 55-L, an aqueous solution at 55% of calcium lignosulphonate with insoluble impurities of CaSO4; Borrebond 55-S, the same aqueous solution, without CaSO4 but containing sugars as impurities; Borresperse CaI-50, an aqueous solution of calcium lignosulphonate at 50% free of CaSO4 and sugars and Borresperse NaI-50, which is an aqueous solution at 50% of sodium lignosulphonate, also free of impurities. Solids were prepared by slurring fly ash and calcium hydroxide mixtures in a discontinuous stirred tank reactor at a constant temperature of 90 °C, with a reflux refrigerator to maintain a constant 10:1 water/solid ratio and at a stirring rate of 1400 rpm maintained for 7 or 15 h. The amount of liquid was 150 cm3 being 15 grams the total amount of fly ash and Ca(OH)2, with a fly ash/Ca(OH)2 weight ratio of 3:1 or 5:3. Lignosulphonates were added to the water in amounts varying from 0.05 to 10% with respect to the total initial amount of fly ash and Ca(OH)2. After hydration time, samples were filtered with a 0.45 µm mesh filter, dried in an oven at 105 °C, and ground. Sorbents obtained with high amounts of additives are granules, being the rest of the sorbents free-flowing powders.
Physical and Chemical Analysis of the Sorbents. The amount of unreacted Ca(OH)2 in the solid was measured by dissolving a small sample of the sorbent in a sugar solution at ambient temperature and titrating the dissolved Ca(OH)2 with HCl following the procedure given by Peterson and Rochelle.13 From the amount of unreacted Ca(OH)2, its conversion was calculated as follows, this value being a measure of the extent of the pozzolanic reaction between Ca(OH)2 and the fly ash. Ca(OH)2 used in the preparation of solid - Ca(OH)2 after hydration Ca(OH)2 used in the preparation of solid
× 100
The Ca(OH)2 used in the preparation of the solid was calculated by considering the composition of commercial Ca(OH)2 (Table 1). The specific surface area of the sorbents was measured using a Micromeritis ASAP-2000 apparatus. It was determined following the BET standard method. X-ray powder diffraction patterns of the samples (less than 0.150 mm) were collected in air at room temperature in a PW 1729 Phillips diffractometer, using Cu KR (λ ) 1.5418 Å) radiation. The evolution of the starting materials was followed in the diffractograms. Sulfation Test. The reaction between the solid sorbents and SO2 was performed in a glass-made jacketed fixed-bed reactor under isothermal conditions at 57 °C and at a relative humidity of 56%. Approximately 1 g of the sorbent was weighed and dispersed manually in 30 g of an inert silica sand bed, and the whole bed was supported on a 3.6 cm diameter fritted glass plate contained in the glass cylinder. The volume composition of the gas was 5000 ppm SO2 (measured with a mass flow meter), 12% CO2, 2% O2, and balanced N2 (measured with rotameters), at a rate of 1000 cm3/ min. The gas stream without SO2 passed through the humidification system, where it was in contact with water in an absorber flask. The absorber flask was submerged into a water bath at a constant temperature of 50 °C. After humidification, the gas mixture with SO2 flowed through the reactor. The water content in the gas stream was obtained by cooling the stream with cool water in a reflux refrigerator for 3 h and measuring the condensed water. From this data, the experimental vapor pressure at the reaction temperature (57 °C) was obtained. The quotient between the experimental pressure vapor and the saturation pressure vapor at the reaction temperature gave the relative humidity. When the reaction time (1 h, determined in a previous work8) was over, the reactor content was sieved to separate the sorbent from the sand, and the reacted sorbent was analyzed by the thermogravimetric technique. Determination of the Calcium Utilization of the Sorbent in the Desulfurization Reaction. For the thermogravimetric determination of the calcium utilization, a PerkinElmer TGA unit with a temperature furnace program between 50 and 1250 °C and a Pyris program (software for Thermal Analysis from Perkin-Elmer) were used. Synthetic air was used as carrier gas (30 cm3/min). TG curves of the reacted sorbent showed a weight loss between 850 and 1250 °C that was attributed to the SO3 released in the sulfate decomposition in CaO(s) and SO3(g). This analysis allowed the calculation of the calcium utilization. This value expressed as the number of moles of captured SO2 per mole of calcium in the sorbent was obtained as follows.
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1333
moles of SO2 captured grams of sorbent ) moles of calcium grams of sorbent % SO3 1 mole SO3 grams of reacted sorbent 100 80 grams SO3 grams of sorbent moles of calcium grams of sorbent
that the lignosulphonate Borresperse CaI-50, a purified lignosulphonate, added at 0.1%, slightly increases both parameters, Ca(OH)2 conversion, and specific surface area in all of the sorbents. Table 2 and Figure 2 show the influence of the additive on the calcium utilization of the sorbent in the desulfurization process, quantified as mol SO2/mol of calcium (%). In Figure 2, the SO2 capture for sorbents prepared at 5:3 fly ash/Ca(OH)2 ratio (1) and 3:1 ratio (2) prepared with 0.1% if additive or without additive (no add) and 7 h of hydration is shown. This figure shows that, at 0.1% of the additive, a general increase in the SO2 capture is found with respect to the SO2 uptake obtained with the sorbents prepared without additive. This effect is more important in 3:1 sorbents, being that the Borresperse CaI-50 is the most effective additive among sorbents prepared with 7 h of hydration. Table 2 also shows that the sorbent that exhibits the highest calcium utilization (75.1%) was prepared with 0.1% of NaI-50 additive for 15 h. Data of the calcium utilization (mol SO2/mol of Ca) in Table 2 show that, for sorbents prepared with 0.2% of additive, the influence of the additive is not clear, being that this influence is negative at higher amounts (1 or 10%). The XRD pattern of selected samples, prior to the sulfation reaction, is shown in Figure 3. XRD of sorbents was performed to study in depth the influence of lignosulphonates on the pozzolanic reaction between Ca(OH)2 and the fly ash components. The additives are not present in the figures as they are not crystalline. The diffraction figure of the raw material fly ash and Ca(OH)2 are omitted, but their more relevant peaks are marked in the figures of the sorbents. The fly ash presents a mild hump at 2θ ) 24° characterizing the amorphous SiO2, peaks of quartz at 26.64 and 20.8° (C in the figure), and typical peaks of mullite at 2θ ) 26.3, 26, and 40.8° (D in the figure); the peaks of the commercial Ca(OH)2 were found at 34, 18.1, 47.1, 50.4, 53.9, and 28.7° (A in the figure). The order of the angles of the peaks is written following their intensities. Comparing XRD pattern of sorbents prepared without additive (3:1, 7 h; and 5:3, 7 h) it can be seen that at a 5:3 initial ratio, peaks of Ca(OH)2 still remain at 7 h of reaction, and these peaks
To calculate the moles of calcium/grams of sorbent, a different fly ash/Ca(OH)2 initial ratio must be considered. From the weights of the sorbent before (grams of sorbent) and after (grams of reacted sorbent) the reaction with SO2, the ratio (grams of reacted sorbent)/(grams of sorbent) was determined. Results and Discussion The Ca(OH)2 conversion, the specific surface area values of the sorbents, and the calcium utilization values of these sorbents tested in the desulfurization process are shown in Table 2. Comparing 3:1 or 5:3 sorbents with or without additive each other, calcium hydroxide conversion evaluates the extent of the pozzolanic reaction. Results show that when the percentage of additive is 1 or 10% with respect to the amount of solid, a drastic reduction in the Ca(OH)2 conversion is found with any lignosulphonate. At lower percents of additive, 0.05, 0.1, or 0.2, a clear influence is not found, being in general the Ca(OH)2 conversion values similar in sorbents prepared with or without additive. To complete the influence of the additives on the specific surface area, Figure 1 shows values of this property in sorbents prepared with the four additives essayed, including solids that have not being used in a desulfuration process and do not appear in Table 2. Results show that when the additive was used at 0.05%, no influence was found on the surface area values; at 0.1 or 0.2% of additive, values of surface area remain unchanged or are in general slightly higher; a relevant decrease was detected at higher percentages (0.5, 1, 5 or 10%). It is also remarkable
Table 2. Ca(OH)2 Conversion, Specific Surface Area, and Calcium Utilization of Sorbents Prepared at Different Raw Material Ratios, Hydration Time and Percentage of Additive Additive
Hydration time (h) 7
no 15 lignosulphonate Borresperse NaI-50
7 15
F.a./Ca(OH)2 ratio
3:1 5:3 3:1 5:3
7 5:3 15
3:1 5:3
lignosulphonate Borrebond 55-L
3:1 7 5:3 15
lignosulphonate Borrebond 55-S
7 15
Ca(OH)2 conversion
Specific surf. area (m2/g)
Mol SO2/mol Ca (%)
0.1 10 0.1 0.1 1 0.05 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.1 0.2 0.1 0.2 1 0.1 0.1 0.2
89 65 98 86 92 31.6 54.6 97.2 1.6 77.8 92.3 90.1 68.6 62.1 98.1 97.6 94.4 92.9 88.4 67.7 48.5 25.6 93.9 49.1 85.7
21.6 29.8 57.3 33.1 28.8 6.9 25.5 55.9 9.5 22.5 28.3 28.1 32.4 27.8 71.5 55.8 40.4 28.4 27.3 32.4 25.4 21.1 29.9 28.2 33.3
50.1 54.1 55.9 60.2 55.5 7.9 66.3 75.1 20.8 59.7 70.0 56.7 69.6 48.0 61.8 61.8 67.8 64.6 59.5 51.1 48.6 38.7 65.3 45.1 59.1
3:1 5:3 3:1 5:3
3:1 lignosulphonate Borresperse CaI-50
% of additive
5:3 3:1 5:3 5:3
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Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008
Figure 1. Specific surface area of sorbents prepared at 3:1 or 5:3 ratios with different amounts of different additives.
Figure 2. Influence of different additives on the capture of SO2 for sorbents prepared at 5:3 (1) or 3:1 (2) fly ash/Ca(OH)2 ratio, 7 h of hydration, and 0.1% of additive. Samples prepared at 5:3 (1) or 3:1 (2) ratios without additive are marked as “no add”.
are not detected when the initial ratio is 3:1, as calcium hydroxide, present in a lower amount, has reacted at this hydration time with fly ash. In the diffraction figure of the 5:3 sorbent at 15 h of the reaction (no presented), these peaks have
Figure 3. XRD diffraction patterns for sorbents prepared without or with additives. A, peaks of Ca(OH)2; C and D, peaks of fly ash.
disappeared. The intensity of the peaks of the fly ash does nearly not change with hydration time, showing that the fly ash reacts with calcium hydroxide to produce silicates in a lower amount than the calcium base. The diffraction pattern of the sorbents prepared with the additives presented in Figure 3 has been selected, as they clearly
Ind. Eng. Chem. Res., Vol. 47, No. 4, 2008 1335
summarize the effect of these additives on the process of the preparation of the sorbents. The XRD of raw materials and other sorbents prepared with additives can be consulted in the Supporting Information. The influence of the additives has been principally related to the evolution of the starting materials because the peaks of the products are difficult to identify because they overlap with those of the raw materials. The XRD pattern of the 3:1 sorbents without and with the additive are similar, not founding any relevant difference on each other. As peaks of calcium hydroxide are still present at 7 h of reaction in 5:3 sorbents, the influence of additives is easier to detect in these solids. Comparing 5:3, 7 h sorbents, Figure 3 shows that the main peaks of calcium hydroxide have the same intensity (with lignosulphonate CaI-50) or higher (with lignosulphonate NaI50) than that of the sorbent prepared without additive, when additive was used at 0.1%. This means that the presence of additive either retards or does not have any influence on the pozzolanic reaction rate. When the amount of the additive is 1%, the diffraction pattern shows the peaks of both raw materials, meaning that the pozzolanic reaction did not take place in the hydration process. These findings are according to the results of the Ca(OH)2 conversion and of the specific surface area found in the sorbents prepared at high additive percentages. As Figure 2 shows, a general relevant increase in the SO2 capture per mol of calcium has been found for sorbents prepared with any additive at 0.1%. This increase cannot be explained taking into account the effect of the additives on the pozzolanic reaction. The improvement in the SO2 capture found in the sorbents prepared mainly with 0.1% of additive, considering the information obtained in this work, can be related to the dispersant effect of the lignosulphonates. These lignosulphonates added to the slurry of the raw materials produce, in general, sorbents with a slightly higher surface area values; moreover, the hydrophilicity of the additives present in the sorbents in low amounts could retain the humidity of the flue gas, increasing the SO2 capture. When lignosulphonates are present in high amounts in the slurry of raw materials, their agglomerant properties joins particles of reactives together, preventing the pozzolanic reaction. The obtained sorbents are an agglomerate of raw materials with low specific surface area that cannot react with the SO2. Conclusions The effect of lignosulphonates on the properties of desulfurant sorbents depends principally on the amount used. Lignosulphonates used as soluble additives in the process of the preparation of these sorbents increase the desulfurant ability of the sorbents when they are used in low amounts (0.1%). The opposite effect is found when they are used in higher amounts (from 1%). The increase in the SO2 capture when the additive is used at 0.1% can be related to the increase in the surface area of these sorbents and to the hydrophilicity of the lignosulphonates and not to their influence on the pozzolanic reaction; the decrease in the SO2 capture at higher amounts of additive is probably due to the agglomerant properties of these additives. No relevant differences are found when varying the type of additive, as there are not substantial chemical differences among
them that could have an influence on the process of the preparation of sorbents. But solids prepared with lignosulphonates CaI-50 and NaI-50, with additives free of impurities, exhibit the best desulfurization behavior. XRD is a good technique to study the process of the pozzolanic reaction if the evolution of the raw materials is followed. Acknowledgment We acknowledge M.C. y T. for financial support of this work under Project MAT 2006-03683. Supporting Information Available: XRD pattern of raw materials and of 3:1 sorbents prepared with and without lignosulfonate. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Relacio´n de Centrales Te´rmicas del Estado Espan˜ol. Available at http://es.geocities.com/ecored2000/termicas5.html.) (2) Zhao, R.; Liu, H.; Ye, S.; Xie, Y.; Chen, Y. Ca-Based Sorbents Modified with Humic Acid for Flue Gas Desulfurization. Ind. Eng. Chem. Res. 2006, 45, 7120-7125. (3) Kirchgessner, D. A.; Jozewicz, W. Enhancement of Reactivity in Surfactant-Modified Sorbents for Sulfur Dioxide Control. Ind. Eng. Chem. Res 1989, 28, 413-418. (4) Ghosh-Dastidar, A.; Mahuli, S. K.; Agnihotri, R.; Fan L-S. Investigation of High-Reactivity Calcium Carbonate Sorbent for Enhanced SO2 Capture. Ind. Eng. Chem. Res. 1996, 35, 598-606. (5) Ada´nez, J.; Fierro, V.; Garcı´a-Labiano, F.; Palacios, J. M. Study of Modified Calcium Hydroxides for Enhancing SO2 Removal during Sorbent Injection in Pulverized Coal Boilers. Fuel 1997, 76, 257-265. (6) Tseng, H. H.; Wey, M-Y.; Lu, C-Y. The Study of Modified Calcium Hydroxide with Surfactant for Acid Gas Removal during Incineration. EnViron. Technol. 2002, 23, 109-119. (7) Renedo, M. J.; Ferna´ndez, J. Use of Lignosulphonates as Modifiers of CaCO3 Specific Surface Area for Desulfurization. Materials of the 16th International Congress of Chemical and Process Engineering, Chisa Set; 2004, 3, 1207. (8) Renedo, M. J.; Ferna´ndez, J.; Garea, A.; Ayerbe, A.; Irabien, J. A. Microstructural Changes in the Desulfurization Reaction at Low Temperature. Ind. Eng. Chem. Res. 1999, 38, 1384. (9) Ferna´ndez, J.; Renedo, M. J.; Pesquera, A.; Irabien, J. A. Effect of CaSO4 on the Structure and Use of Ca(OH)2 /Fly Ash Sorbents for SO2 Removal. Powder Technol. 2001, 119, 201. (10) Renedo, M. J.; Ferna´ndez, J. Preparation, Characterization, and Calcium utilization of Fly-Ash / Ca(OH)2 Sorbents for Dry Desulfurization at low Temperature. Ind. Eng. Chem. Res. 2002, 41, 2412. (11) Ferna´ndez, J.; Renedo, M. J. Study of the Influence of Calcium Sulfate on Fly Ash / Ca(OH)2 Sorbents for Flue Gas Desulfurization. Energy & Fuels 2003, 17, 1330. (12) Ferna´ndez, J.; Rico, J. L.; Garcı´a, H.; Renedo, J. Development of Sorbents for SO2 Capture Prepared by Hydration of Fly Ash and Hydrated Lime in Sea Water. Ind. Eng. Chem. Res. 2006, 45, 856. (13) Peterson, J.; Rochelle, G. Aqueous Reaction of FlyAsh and Ca(OH)2 to Produce Calcium Silicate Absorbents for Flue Gas Desulfurization. EnViron. Sci. Technol. 1988, 22, 1299.
ReceiVed for reView July 24, 2007 ReVised manuscript receiVed October 25, 2007 Accepted October 26, 2007 IE071003O
