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Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant
Energy xxx (2015) 1e8
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Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant Makoto Kobayashi*, Hiroyuki Akiho, Yoshinobu Nakao Energy Engineering Research Institute, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka 240-0196, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2014 Received in revised form 10 April 2015 Accepted 11 April 2015 Available online xxx
Integrated coal gasification combined cycle power generation that adopts oxy-fuel concept has potential to achieve thermal efficiency of 44% at lower heating value besides its CO2 separation ability. Halide control as well as dry gas sulfur removal is inescapable process to the proper operation of the plant. This study investigates performance of dry halide sorbents to establish suitable halide control procedure for the power plant. The most appropriate position of the process is considered to be in the upper stream of the dry sulfur removal process, where the process is able to prevent both corrosion of gas turbine and halogenation of zinc in the desulfurization sorbent. Preproduction sorbents for the dry-gas halide removal process were prepared as pellets containing sodium aluminate as a main ingredient. The sorbents could absorb stoichiometric amount of hydrogen chloride at 450 C and 0.98 MPa absolute in a simulated coal derived gas environment. The fresh and spent sorbents were examined for dependence of kinetics of halide removal reaction on their pore structures. The results showed that the pore structure comprising of macropore peak at around 5.0 mm and smaller peaks at 1.0 mm or below was suitable to attain affordable kinetics on halide removal and sodium conversion of the sorbent. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Halide removal Sodium aluminate Pore structure IGCC (Integrated gasification combined cycle) Hydrogen chloride
1. Introduction Annual coal consumption of the world has been rapidly increasing since 2003, and its 70% was consumed by the countries of Asia Pacific region in 2013 [1]. Among fossil fuels, coal is far behind of natural gas in CO2 emission intensity due to the significant decrease in thermal efficiency as well as the intrinsic fuel composition. Because the CO2 emission from existing pulverized coal-firing plants is estimated to exceed 870 g-CO2/kWh [2], its reduction becomes burning issue in the countries with high dependence on coal-firing power plant. Application of the CO2 capture technologies to pulverized coal-firing plants and IGCC (Integrated Gasification Combined Cycles) are extensively investigated to find out the most realistic procedure for reducing CO2 emission. It is reviewed that the thermal efficiencies of coal firing power plants suffer decrease of 10% in absolute value due to
Abbreviations: EGR, exhaust gas recirculation; FID, flame ionization detector; FPD, flame photometric detector; HHV, higher heating value; ICP, inductively coupled plasma; IGCC, integrated gasification combined cycle; LHV, lower heating value; SAS, sodium aluminate sorbents; TCD, thermal conductivity detector; XRD, X-ray diffraction. * Corresponding author. E-mail address: [email protected] (M. Kobayashi).
introduction of post combustion type CO2 capture technologies [3]. Approximately two third of the efficiency decrease results from the power consumption of the CO2 capture process [3]. The large efficiency loss causes an increase in fuel consumption resulting in higher operating cost. Besides the operating cost for fuel, the investment cost for the CO2 capture and storage considerably increases the cost for electricity production, that is illustrated by a case study on USC (ultra supercritical) coal firing plant [4]. The other comparison between USC and IGCC revealed that the investment costs of both plants with CCS (CO2 capture and storage) equipment are comparable [5]. Thus, IGCC with CCS equipment exhibits better economic performance because of the lower efficiency penalty of 8.6 points, in contrast with the value being 10.5 points for USC with CCS [5]. An exergy analysis on IGCC and CFB (circulating fluidized bed) cogeneration plants also revealed that the plant efficiency suffers from the large efficiency loss due to the CO2 capture and compression [6]. It is urged from these results that an alternative process to the conventional CO2 capture equipment should be introduced to enhance economics and efficiency of coal fueled power generation with CO2 separation. Despite various attempts to reduce the efficiency penalty of CO2 capture with the conventional amine based process, considerable decrease in net thermal efficiency seem to be unavoidable so far as the CO2 capture process exhibits the large energy consumption. There are several
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Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055
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M. Kobayashi et al. / Energy xxx (2015) 1e8
assessments of thermal efficiencies on IGCC based power plant with CO2 capture facilities. An IGCC power plant with Shell gasifier was analyzed so that the efficiency decreased from 46% to 36% in LHV (lower heating value) base due to adoption of CO2 capture process [7]. An efficiency analysis of IGCC with and without CO2 capture was performed for three types of gasifier from General Electric Energy, ConocoPhillips, and Shell Global Solutions [8]. The analysis concluded that the relative efficiency penalty for CO2 capture installation to the IGCC power plants is 21.4% on average. Although the CO2 capture has immediate effect on reducing the emission to the atmosphere, considerable decrease in thermal efficiency of the power plant results in exhaustion of coal resource in longer term. It is important to avoid absurdity that environmental conservation on utilization of fossil fuels will accelerate the depletion of energy resource. Because the hot fuel gas desulfurization process has potential to improve thermal efficiency of the IGCC power plants [9], there is an attempt to apply the process to the IGCC system along with CO2 capture. Thermodynamic estimation showed that IGCC power plants with hot gas clean-up and post-combustion CO2 capture has potential to improve thermal efficiency as high as 41.5% at LHV [10], while the estimation is still on the system analysis base. Recent review on CO2 capture technology illuminated the potential technologies includes absorption, adsorption, cryogenic separation, and membrane separation [11], whereas the direction for technology improvement is remained uncertain. The other review revealed that the existing technology still requires improving their performance and reducing cost and energy consumption for CO2 capture [12]. Considering these backgrounds for CO2 capture in coal firing power plants, variety of concepts on new power generation system are proposed to reconcile the thermal efficiency with the efficient CO2 capture for coal-fueled power generation. Zecomix cycle is a conceptual system comprised mainly of a hydrogasification equipped with the dry CO2 absorbing process using calcium oxide sorbent, and a combined cycle power generator with semi-closed gaseturbine cycle on syngas oxy-firing. Zecomix cycle was initially proposed as a system for coproduction of electricity and hydrogen with CO2 capture [13]. The recent estimation on the plant performance of Zecomix cycle indicated that net thermal efficiency varies from 47% to 44% at LHV base along with decreasing utilization of CaO, which is dry sorbent for CO2 capture, from 67 to 20% [14]. Among various technological hurdles that the cycle has, the most crucial issue seems to be the dry CO2 capture process, where the CaO sorbent particulate should be circulated between absorbing reactor and desorbing reactor under high temperature and high pressure. Similar concept called Zecomag cycle is suggested as a modification of the power island of Zecomix cycle introducing open gas turbine cycle at which the syngas is burnt with air [15]. The cycle also has same difficulty in the dry capturing of CO2 with the CaO sorbent process. Other approach to reduce the efficiency penalty due to the CO2 capture is proposed for future IGCC adopting one of emerging technologies, such as membraneenhanced CO conversion, carbonate looping process for CO2 capture, and oxy-fuel concept by replacing cryogenic air separation with oxygen transfer membrane [16]. The other concept to lowering the energy consumption for CO2 capture is so called “oxyfuel IGCC” power generation system. The distinctive concept of IGCC power plant is expected to be as a candidate of power generation system that will reconcile effective CO2 separation with high efficiency of the coal firing power plant. The concept of oxyfuel IGCC power generation plant is basically comprised of a semi-closed cycle operation of a gas turbine with an oxy-fuel combustor that is fueled by an oxygen-CO2 blown coal gasification. The CO2 separation in the plant is efficiently established by steam condensation from circulating exhaust gas and compression.
While the exhaust circulation brings advantage in the plant efficiency to the oxy-fuel IGCC, contaminant issues arise from the semi-closed cycle operation [17]. As coal-derived syngas contains various contaminants derived from coal; sulfur compounds, halides, nitrogen compounds, alkaline and alkaline earth metals, and volatile heavy metal are suspected as target of consideration. Those contaminants should be properly treated in the plant to protect equipment in the plant and achieve the environmental tolerance for CO2 storage as well as the environmental regulations. The system requires a pre-combustion type sulfur removal process to attain sulfur tolerant level of the gas turbine. The removal process should be dry sulfur removal process so that the processed gas retains its H2O and CO2 concentrations. The retained amount of H2O and CO2 will not only act as working fluid of the gas turbine, but also decrease the potential of carbon deposition. The proper operation of the dry sulfur removal process was sought and suggested that the exhaust circulation to the upper stream of the process is effective to prevent carbon deposition [17]. The exhaust circulation is effective in inhibiting carbon deposition at all expected condition in the actual plant operation and the power consumption for the exhaust circulation is small enough to keep those advantages [17]. Besides the sulfur compounds in the syngas, halide impurities are also harmful to the health of gas turbine materials. It is expected that the tolerant concentration of chloride is lower than ppm level to protect gas turbine from corrosion. Hydrogen chloride is also detrimental to the desulfurization performance of the zinc ferrite sorbent because the halide causes zinc chlorination. This paper focuses the target for impurity removal on halides so that the system configuration of the oxy-fuel IGCC power generation is adequately established for the treatment of contaminants in the syngas. Alkaline compounds of sodium are candidates for use in halide-removal sorbent, due to the favorable thermodynamic affinity of sodium for chlorine [18]. Industrially available alkaline compounds of sodium, such as sodium hydroxide, sodium carbonate, and sodium aluminate, have poor reactivity with hydrogen chloride, because those compounds are prepared in bulk solid forms. It was revealed from former work that sodium aluminate is able to acquire considerable reactivity with hydrogen chloride if the material is carefully synthesized from sodium carbonate and sol of alumina [18]. Although the specially synthesized sodium aluminate is a candidate material for the halide removal from coal-derived fuel, the synthetic procedure requires expensive reagents and consumes significant amount of energy for condensation and drying of the raw materials. In order to resolve the restriction on economic manufacturing, a trial production of SAS (sodium aluminate sorbents) was performed by using industrially available sodium aluminate and additional plasticizer, as well as pore builder. Sodium aluminate is generally difficult to form into pellet shape, because of its deliquescent property. This trial production of halide removal sorbents is quite challenging and such sorbents were not investigated before. The prototype sorbents were investigated on hydrogen chloride removal performances in simulated coal-derived syngas, and their applicability to the halide removal process in the oxy-fuel IGCC power generation plant was discussed. 2. Halide removal process for oxy-fuel IGCC 2.1. Process position on the oxy-fuel IGCC plant Prior to the experimental investigation of the halide sorbents, we have to determine the operating condition for the halide removal process in the oxy-fuel IGCC power generation. The halide removal process has roles for protection of the zinc ferrite sorbent from chlorination, as well as the protection of gas turbine from corrosion. The sequence of the processes is then naturally
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determined for the halide removal being at the upstream of the sulfur removal process as indicated in Fig. 1. The EGR (exhaust gas recirculation) for inhibiting carbon deposition in the sulfur removal process is injected downstream of halide removal. This will reduce pressure drop through the sorbent bed of the halide removal reactor, and avoid the possibility of sodium carbonate formation in the halide sorbent. EGR to downstream of the halide removal process is also important to avoid deliquescence of sodium aluminate, because the exhaust contains steam at high concentration around 30 vol%. These requirements make the halide removal process be anchored to that position. Although the schematic flow diagram shown in Fig. 1 does not illustrate auxiliary equipment, each process requires equipment such as sorbent silos and hoppers for actual process operation. Because the halide sorbent is not regenerable, the sorbent in the reactor should be changed periodically during plant operation. The specific of the equipment configuration and the manufacturing design of the practical process should be considered along with the plant design in the future. The halide removal process well fits to the dry gas purification, because the process can be operated at the same temperature and pressure with the desulfurization process. Thus, the introduction of the halide removal process would give slight effect on the plant efficiency of the oxy-fuel IGCC power generation. Earlier thermal analysis revealed that the plant exhibits very high net thermal efficiency of 44.0% LHV (41.9% higher heating value) at 99% CO2 capture ratio [19]. 2.2. Consumption of chemicals for halide removal process The environmental impact for introducing the halide removal process should be assessed from the viewpoint of consumption of chemical products. This section explains the advantage of the dry halide removal process in the viewpoint by comparing with conventional wet scrubbing process. If wet gas purification is applied to the oxy-fuel IGCC power plant, water-scrubbing process will mostly capture hydrogen chloride and other halides in the syngas and produce a solution containing hydrogen chloride and other water-soluble contaminants. Then the solution should be neutralized by sodium hydroxide as shown in Eq. (1). Wastewater from the scrubber requires more chemicals for water treatment process. The dry halide sorbent, SAS also acts as a neutralizer for HCl as indicated in Eq. (2). The difference is that the SAS will leave aluminum oxide along with steam. If the residue of SAS utilization causes environmental impact, appropriate treatment is required for the residue. Both sodium chloride and aluminum oxide are well-understood chemical substances that can be treated by established procedure with small environmental impact. We also have to consider the consumption of sodium aluminate from the point of environmental impact during the production, utilization, and disposal of SAS for
3
Table 1 Preparation specification of the sodium aluminate sorbents. Sorbent name
SAS A
SAS B
SAS C, SAS D
Calcination cond. Pellet size (mm f mm L) Raw material comp. (wt%) Sodium aluminate Plasticizer,#1 Plasticizer,#2 Pore builder
700 C, 4 h 33
700 C, 4 h 3.5 3.5
700 C, 4 h 3 3.5
94.3 5.7 e e
66.7 6.7 e 26.6
75.2 e 9.8 15.0
the dry halide removal process. The halide removal sorbent, SAS may have small environmental impact because the utilization of SAS produces neither hazardous waste nor waste-water requiring chemical treatment.
HCl þ NaOH/NaCl þ H2 O HCl þ NaAlO2 /NaCl þ
1 1 Al O þ H O 2 2 3 2 2
(1) (2)
3. Experimental 3.1. Preparation of sodium aluminate sorbent The preproduction sorbents were prepared on an experimental basis, because the sorbent performance highly depends on the physical properties developed with the preparation procedure. At the initial stage of this study, SAS was prepared by trial-and-error production to find out appropriate production procedure for building suitable pore structure to the SAS. Sodium aluminate is available as an industrial raw material in the form of powder or solution. SAS should be prepared as small pellet to filling up a packed bed in the reactor. It is required to add some plasticizer to form pellets of sodium aluminate through an extruder. Then the raw extruded pellets were dried and calcined at 700 C for 4 h. In this preliminary production, three kinds of SAS were prepared with different compositions of raw material and additives as summarized in Table 1. The recipe for SAS A provides benchmark of the sorbent, which is simple combination of a sodium aluminate powder and a plasticizer. A pore builder, which is burned out during calcination to leave pore structure in the pellet, was introduced to the recipe for SAS B. The third recipe for SAS C and SAS D is in the same concept as SAS B, but it used different plasticizer giving better molding characteristics. SAS C and SAS D is essentially same recipe but the SAS D is produced in semi-production scale reactor to obtain larger amount of sorbent pellet. The calcined product of SAS was partly carbonated by carbon dioxide in the atmosphere, according to the reaction shown in Eq. (3), because the sorbent was
Fig. 1. Suitable position of halide removal process determined for the oxy-fuel IGCC power generation plant.
Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055
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Fig. 2. Test apparatus for the sorbent performance evaluation in a fixed bed reactor.
exposed ambient condition during the calcination. Thus, the product pellet mainly consisted of sodium aluminate, sodium carbonate, and aluminum oxide. Sodium carbonate is also possible to react with hydrogen chloride according to the reaction shown in Eq. (4). Although the proportion of sodium carbonate was rather minor, its contribution to the halide removal reaction is not clear. We also have to examine the effect of the carbonate content to the sorbent performance.
CO2 þ 2NaAlO2 /Na2 CO3 þ Al2 O3
(3)
Na2 CO3 þ 2HCl/2NaCl þ H2 O þ CO2
(4)
3.2. Fixed bed reactor test for SAS performance evaluation To determine halide removal performance of SAS, a fixed bed reactor shown in Fig. 2 was operated at the condition summarized in Table 2. The same weight of sample was packed in the reactor for each SAS pellet. Because the sizes of those SAS pellets were slightly different each other, the apparent volume of the packed bed varied from 30 to 32 cm3. Thus, the space velocity according to the same flow rate of 2200 cm3/min varied between 4100 and 4400 h1. Gas analysis was performed with combination of gas chromatograph with three TCD (thermal conductivity detector) detecting inorganic gas species (Round Science, AG-1 TTTH), and gas chromatograph with FID (flame ionization detector) and FPD (flame photometric detector) for hydrocarbons and sulfur compounds (Round Science, AG-1 FH-FPD). The instrument AG-1 TTTH is able to determine steam concentration as well as the other inorganic gas concentration in the sample. Prior to the chlorination of the sorbents, the
Table 2 Operating condition of the fixed bed reactor instrument.
Temp., C Press., MPa abs. Gas comp. Additional impurities, ppm Pellet size of sorbent, mm Sorbent amount, g Bed volume, cm3 Gas flow rate, cm3/min Space velocity, h1 Reaction time, h a
HCl
Reduction
HCl removal
450 0.98 a 1 0 3.0e3.5 15 30 32 2200 4100 4400 1
) ) a 1 900 ) ) ) ) ) 8
1: Air blown coal gas composition was selected for comparison.
sorbent bed was preheated to the desired temperature under flowing nitrogen through the bed. Then the chlorination of the sorbents was performed in the simulated gas, which was supplied as a mixture of each component gases from cylinder through mass flow controller as shown in Fig. 2. Steam was separately injected through vaporizer from micro pump to establish the desired concentration, which was confirmed by the gas chromatograph. The performance evaluation initially started with the reduction condition in Table 2 and lasted for 1 h. Then the reaction gas was switched to the HCl removal condition, and the evaluation test continued for 7e17 days with typically 8 h duration, while the halide removal period depended on the reaction condition applied. The gas composition was measured at the reactor outlet with the gas chromatographs at 5 min interval. The breakthrough curves of HCl was measured with ion electrode HCl analyzer (Toa DKK, GNC224), the data was used to evaluate halide capacity, prebreakthrough concentration, and halide removal rate. 3.3. Analyses of chemical composition and physical property of SAS SAS was prepared as mixed compounds of sodium aluminate, sodium carbonate, and aluminum oxide. It was difficult to determine the quantitative composition of those compounds directly by chemical analysis. The composition of those compounds was determined indirectly from elemental analysis of the sample. Required data was contents of chloride, aluminum, sodium, and carbon. The whole sample was dissolved to determine chloride content using an ion chromatography system (Dionex, ICS-1500). Aluminum and sodium was analyzed by an ICP optical emission spectroscopy (Shimadzu, ICPS-8100). Carbon content was determined with a carbon and sulfur content analyzer (Horiba manufacturing, EMIA-920V2). The obtained data was processed to estimate the component compounds. The existence of compounds in the SAS was confirmed by powder XRD (X-ray diffraction) analysis. Pore size distribution of SAS was measured with a mercury porosimeter (Micromeritics, Auto Pore IV 9520). 4. Results and discussions 4.1. Halide removal performance of SAS The condition for performance evaluation of SAS represented the expected operating condition of the halide removal process in the oxy-fuel IGCC, in which the process is placed in upstream of the sulfur removal process. Table 3 shows estimated gas composition of syngas produced by the O2eCO2 blown gasification, and its
Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055
M. Kobayashi et al. / Energy xxx (2015) 1e8 Table 3 Estimated gas composition produced by O2eCO2 blown gasification, its equilibrium condition, and the test conditions. Condition
Gasifier outleta Halide removala Prelim. tests Oxy-fuel test
Temp. ( C) Press. abs, (MPa) Gas comp. (vol%) H2O CO H2 CO2 CH4 N2 O2 HCl
895 2.8
450 2.75
450 0.98
450 0.98
3.2 66.4 18.8 8.6 1.5 1.5 0.0 100 ppm
0.4 63.6 21.6 11.4 1.5 1.5 0.0 100 ppm
5.0 20.0 12.0 5.0 1.5 56.4 0.0 900 ppm
3.0 66.0 19.0 9.0 1.5 1.41 0.0 900 ppm
a
Chemical equilibrium estimation at the condition of each process stream.
equilibrium composition for water gas shift reaction at 450 C, that is the operation temperature of the halide removal process. The first two test results were done in the preliminary test condition in Table 3. Because this performance evaluation in the preliminary condition has screening purpose, the gas composition was settled to simulate the syngas produced by the air-blown gasification. Fig. 3 displays a breakthrough curve of hydrogen chloride for the SAS A. The horizontal axis express the reaction time, tN, that is normalized based on the theoretical chloride capacity. Despite of sample amount shown in Table 2, this test was done with sample weight of 10 g. Thus, the space velocity value became large as 7200 h1. After the instant period of pre-breakthrough, the outlet concentration increased steadily. This means that the sorbent amount is too small
5
to exhibit breakthrough curve expressing the reaction kinetics between sorbent bed and HCl in the gas. The next test was performed with the exact amount of sorbent shown in Table 2 and the preliminary test condition shown in Table 3. Fig. 4 shows the results obtained for 15 g of SAS B sample that was installed in a packed bed reactor as shown in the photograph in the figure. Unexpectedly the sorbent could not keep low HCl concentration at the reactor outlet, despite of the lowered space velocity being 4500 h1. As outlet HCl concentration exceeded 100 ppm at the initial stage, the test was discontinued at the reaction time, tN being 0.2. Fig. 5 shows the results obtained for 15 g of SAS C sample that was packed in the reactor in the same manner. After the pre-breakthrough continued until tN exceeded 0.4, the outlet concentration gradually increased toward the inlet HCl concentration. SAS C solely exhibited the affordable performance of halide removal among the preliminary production sorbents. The moderate slope of breakthrough curves shown in Figs. 3 and 5 imply the relatively lower rate of halide removal. In both results, the breakthrough occurred far before tN comes closer to one, and bed conversion reached scarcely 70% at the complete breakthrough. SAS C, as well as SAS A, requires more acceleration of the halide removal kinetics, to exhibit sufficient reactor performance for the halide removal process. 4.2. Pore size distribution of SAS and possibility for enhancing reaction kinetics To check the possibility for enhancing reaction kinetics of SAS, pore size distributions of prepared sorbents were examined. Figs. 6e8 show the pore size distribution of SAS A, SAS B and SAS C respectively. The distribution of SAS A, which solely introduced plasticizer to form the pellet, exhibited a macropore peak at around 1 mm. The cumulative pore volume was relatively low as 0.4 ml/g.
Fig. 3. Preliminary halide removal test result for SAS A at relatively high space velocity (7182 h1).
Fig. 5. Preliminary halide removal test result of SAS C at space velocity of 4150 h1.
Fig. 4. Preliminary halide removal test result for SAS B at space velocity of 4150 h1.
Fig. 6. Pore size distribution of fresh SAS A.
Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055
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Fig. 7. Pore size distribution of fresh SAS B.
Fig. 8. Pore size distribution of fresh SAS C.
Superior sorbent prepared in former work showed the pore distribution peak at 0.1 mm and cumulative pore volume of 0.6 ml/g. These differences in the pore structures may affect to the earlier breakthrough observed for SAS A shown in Fig. 3. Dissimilarity in the pore structure of SAS B is more conspicuous as expressed in Fig. 7. Main pore distribution peak became as large as 10 mm, nevertheless the cumulative pore volume exceeded 0.7 ml/g. Introduction of pore builder to SAS C produced triplex distribution of macropore peaks at 5 mm, 1 mm and 0.1 mm as shown in Fig. 8 in addition to enlarging the cumulative pore volume to 0.8 ml/g. It seems that building pore structure below 1 mm is effective to enhance reaction kinetics of halide removal with SAS. The favorable pore structure may enhance the diffusion of reactant and product across the gas solid interface through the pore. Although building smaller pore structure with sodium aluminate is difficult due to deliquescent property of the material, it should be emphasized that enhanced kinetics will decrease the size and capacity of required reactor of the halide removal process, which will drastically decrease the amount of consumed sorbent and the energy consumption for the sorbent preparation. Although the poor performance of SAS B can be attributed to the absence of smaller pore, difference between SAS A and SAS C requires more detail description. Changes in the pore structures during the reaction with HCl were examined. Prior to the discussion on the pore structure changes, the specific volume changes during the reaction formulae in Eqs. (2)e(4) were estimated. Table 4 summarizes the formula volume change of solid reactants according to the chlorination or carbonation expressed in the formulae.
Chlorination of sodium carbonate solely increases its formula volume, while chlorination and carbonation of sodium aluminate both reduce its formula volume. It might be possible to discuss the pore structure change during the chlorination of SAS by taking the volume change into consideration. Fig. 9 displays the pore distributions of SAS A measured before and after halide removal. The major peak observed at 1 mm significantly reduced after chlorination. Peak at around 0.01 mm emerged instead after chlorination. To explain the change reasonably, the solid reactant of chlorination reaction was Na2CO3 at least around the inner surface of the pore structure. Because the peak at 1 mm and shoulder peak at 0.2 mm both shrunk and shifted to smaller diameter, volume expansion due to chlorination of the carbonate is reasonable to describe the behavior. Other possibility of the pore decrease is the shrinkage of bulk structure due to chlorination of sodium aluminate. The cause of the emerged peak at 0.01 mm is not clear at this moment. The pore of SAS A was gradually blocked during progress of chlorination of the sorbent. Then the gas reactant and products of chlorination reaction, viz. HCl, CO2, and H2O, should have diffused across the solid phase, which would have diminished the reaction rate considerably. Pore structure behavior during chlorination of SAS C is presented in Fig. 10. The peaks at 1 mm and 0.1 mm shrunk and peak at 0.01 mm emerged, which is quite similar behavior to that observed for SAS A. The difference is peak at 5 mm that remained largely after the chlorination. The relatively large pore probably took a role of gas channel between outer surface and bulk of the sorbent pellet. The gas channel might allow the gas reactant and products of chlorination to diffuse easily through the gas phase in the pore structure, which would keep larger reaction rate of SAS C compared to SAS A as shown in the Figs. 3 and 5. 4.3. Sorbent performance in the oxy-fuel IGCC gas composition Finally, SAS D sorbent, which was prepared by the essentially same recipe of SAS C, was subjected to the halide removal test in the
Fig. 9. Comparison of pore size distribution of fresh and spent SAS A.
Table 4 Volume change according to chlorination and carbonation of sodium aluminate. Formula volume of solid reactant
Formula volume of solid product
Expansion ratio
2NaAlO2 e 109.3 ml 2NaAlO2 e 109.3 ml Na2CO3, Al2O3 e 67.5 ml
2NaCl, Al2O3 e 79.7 ml Na2CO3, Al2O3 e 67.5 ml 2NaCl, Al2O3 e 79.7 ml
27% 38% þ18%
Fig. 10. Comparison of pore size distribution of fresh and spent SAS C.
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Fig. 13. Powder XRD chart obtained for fresh SAS B. Fig. 11. Halide removal test result for SAS D at oxy-fuel test condition.
oxy-fuel gas composition. Fig. 11 displays breakthrough curve of hydrogen chloride obtained for 15 g of SAS D. After the prebreakthrough continued until tN exceeded around 0.6, the outlet concentration gradually increased. The pre-breakthrough period extended compared with the preliminary test shown in Fig. 5, which indicates the gas composition of the oxy-fuel condition enhanced the reaction kinetics of SAS D. The most plausible explanation for the enhancement can be derived from equation (2). The lower steam concentration of oxy-fuel condition might accelerate the reaction. The possibility of the acceleration from the slight difference in the steam concentration for the two conditions, namely 3% and 5%, should be examined in further investigation. In order to evaluate the practical feasibility of the halide removal process with the sorbent, quantitative analysis for the heat and material balance of the process operation is required. It is also the future work to reveal the appropriate engineering design of the process and to extract the tasks for improvement.
4.4. Comparison of the compound composition of fresh and spent sorbents Compositions of compounds in the fresh and spent sorbents were estimated from the chemical analysis of constituent elements of those samples. The sample of the spent sorbent was collected from the sorbent bed at the most inlet part, where the sorbent was expected to be sufficiently saturated with chloride, to compare the maximum halide capacity of chlorinated SAS samples. Fig. 12 displays the estimated composition in bar graph. The summation of the composition does not reach 100% in some cases, which indicates the error in analysis and estimation. The worst case was SAS B fresh sorbent whose estimated summation
Fig. 12. Composition of compounds in fresh and spent SASs estimated from chemical analysis.
of the composition exhibited 84%. The importance of the figure is its qualitative tendency of the composition. All sorbents consisted of major portion of sodium aluminate and 13e38% of sodium carbonate. Free aluminum oxide was in minor portion in the fresh sorbent. Giving attention to the comparison between fresh and corresponding spent sorbent, significant characteristics of the sorbents were revealed. Both sodium aluminate and sodium carbonate were chlorinated as the composition change indicates that the most portions of both compounds were converted to sodium chloride. Sodium conversion of whole sorbent reached 80, 95 and 95% for SAS A, SAS C and SAS D respectively. Thus the halide capacity of the SAS is very efficient to utilize sodium in the sorbent. In order to confirm the estimation of the sorbent composition, powder X ray diffraction patterns were measured for the fresh and spent sorbents. Distinctive results were obtained for SAS B and SAS C. Fig. 13 illustrates the XRD pattern of fresh SAS B, the marker with chemical formulae is indicating the strongest peak of each compound. The other mark is labeled at the major peaks of each compound. The strong peak of Na2CO3 indicates that the main portion of the sorbent was sodium carbonate, whereas the sodium aluminate structure was recognized as a minor portion. This result is quite consistent with the composition estimated for SAS B in Fig. 12. The XRD pattern of spent SAS B shown in Fig. 14 indicates that at least the surface of the sorbent was completely converted to sodium chloride. The higher content of sodium carbonate might retard the chlorination kinetics due to the increasing formula volume during the reaction as described in the former section. On the contrary, the major portion of the fresh sorbent C was clearly sodium aluminate as shown in Fig. 15, which is well corresponds to the estimation shown in Fig. 12. The spent SAS C also exhibited the sodium chloride formation on the sorbent Fig. 16.
Fig. 14. Powder XRD chart of spent SAS B.
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to the normalized time being 0.6; the sorbent attained sufficient capacity and pre-breakthrough duration in the oxy-fuel IGCC operating condition.
Acknowledgment A part of this work was carried out as development program P08020 of New Energy and Industrial Technology Development Organization (NEDO), Japan.
References Fig. 15. Powder XRD chart obtained for fresh SAS C.
Fig. 16. Powder XRD chart of spent SAS C.
5. Conclusion Halide control process is essential for realizing the oxy-fuel IGCC power generation plant. It was recommended that the dry halide removal process should be placed at the upstream of the dry sulfur removal, where the process protects desulfurization sorbents as well as gas turbine. Sodium aluminate sorbent were prepared by the preliminary production procedure and subjected to the experimental examination into the relationship between halide removal performance and their pore structure. A sorbent prepared with plasticizer and pore builder exhibited the most promising performance under syngas composition expected for the oxy-fuel IGCC at the temperature being 450 C and absolute pressure being 0.98 MPa. Investigation of change in the pore structure and estimation of the compound composition of the sorbents prepared with different recipe revealed that the physical and chemical properties are closely rerated to the sorbent performance. Coexistence of macropore peak at around 5 mm and smaller peaks at 1 mm or below provided suitable kinetics on halide removal and higher conversion of sodium chlorination. The higher ratio of sodium aluminate to sodium carbonate was desirable to enhance both chlorination kinetics and sodium conversion. The best sorbent obtained in this work exhibited the sodium conversion being 95%, and the pre-breakthrough period extended
[1] The editor of BP statistical review of world energy, BP statistical review of world energy June 2014. 2014.. [2] CRIEPI Research Report: Y09027 Imamura E, Nagano K. Evaluation of life cycle CO2 emissions of power generation technologies e update for state-of-the-art plants e in Japanese. 2010. 978-4-7983-0339-0, . [3] Goto K, Yogo K, Higashii T. A review of efficiency penalty in a coal-fired power plant with post-combustion CO2 capture. Appl Energy 2013;111:710e20. [4] Viskovi c A, Franki V, Valenti c V. CCS (carbon capture and storage) investment possibility in South East Europe: a case study for Croatia. Energy 2014;70: 325e37. [5] Tola V, Pettinau A. Power generation plants with carbon capture and storage: a techno-economic comparison between coal combustion and gasification technologies. Appl Energy 2014;113:1461e74. [6] Liszka M, Malik T, Budnik M, Zie˛ bik A. Comparison of IGCC (integrated gasification combined cycle) and CFB (circulating fluidized bed) cogeneration plants equipped with CO2 removal. Energy 2013;58:86e96. [7] Martelli E, Kreutz T, Consonni S. Comparison of coal IGCC with and without CO2 capture and storage: Shell gasification with standard vs. partial water quench. Energy Proc 2009;1:607e14. [8] DOE/NETL. Cost and performance baseline for fossil energy plants. Bituminous coal and natural gas to electricity, vol. 1; 2007. Final Report, DOE/NETL-2007/ 1281. [9] Giuffrida A, Romano MC, Lozza G. Thermodynamic assessment of IGCC power plants with hot fuel gas desulfurization. Appl Energy 2010;87:3374e83. [10] Giuffrida A, Bonalumi D, Lozza G. Amine-based post-combustion CO2 capture in air-blown IGCC systems with cold and hot gas clean-up. Appl Energy 2013;110:44e54. [11] Olajire AA. CO2 capture and separation technologies for end-of-pipe applications e A review. Energy 2010;35:2610e28. [12] Mondal MK, Balsora HK, Varshney P. Progress and trends in CO2 capture/ separation technologies: a review. Energy 2012;46:431e41. A, Deiana P, Fiorini P, Girardi G, Stendardo S. Possible optimal con[13] Calabro figurations for the ZECOMIX high efficiency zero emission hydrogen and power plant. Energy 2008;33:952e62. [14] Romano MC, Lozza GG. Long-term coal gasification-based power plants with near-zero emissions. Part A: Zecomix cycle. Int J Greenh Gas Control 2010;4: 459e68. [15] Romano MC, Lozza GG. Long-term coal gasification-based power with nearzero emissions. Part B: Zecomag and oxy-fuel IGCC cycles. Int J Greenh Gas Control 2010;4:469e77. [16] Kunze C, Spliethoff H. Assessment of oxy-fuel, pre- and post-combustionbased carbon capture for future IGCC plants. Appl Energy 2012;94:109e16. [17] Kobayashi M, Nakao Y, Oki Y. Exhaust circulation into dry gas desulfurization process to prevent carbon deposition in an oxy-fuel IGCC power generation. Energy Convers Manage 2014;87:1315e23. [18] Ohtsuka Y, Tsubouchi N, Kikuchi T, Hashimoto H. Recent progress in Japan on hot gas cleanup of hydrogen chloride, hydrogen sulfide and ammonia in coalderived fuel gas. Powder Technol 2009;190:340e7. [19] CRIEPI Research Report: M08006 Nakao Y, Koda E, Takahashi T. Development of CO2 capture IGCC system e investigation of aiming at higher efficiency in CO2 capture IGCC system e in Japanese. 2008. 978-4-86216-864-1, .
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Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant Makoto Kobayashi*, Hiroyuki Akiho, Yoshinobu Nakao Energy Engineering Research Institute, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka 240-0196, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 December 2014 Received in revised form 10 April 2015 Accepted 11 April 2015 Available online xxx
Integrated coal gasification combined cycle power generation that adopts oxy-fuel concept has potential to achieve thermal efficiency of 44% at lower heating value besides its CO2 separation ability. Halide control as well as dry gas sulfur removal is inescapable process to the proper operation of the plant. This study investigates performance of dry halide sorbents to establish suitable halide control procedure for the power plant. The most appropriate position of the process is considered to be in the upper stream of the dry sulfur removal process, where the process is able to prevent both corrosion of gas turbine and halogenation of zinc in the desulfurization sorbent. Preproduction sorbents for the dry-gas halide removal process were prepared as pellets containing sodium aluminate as a main ingredient. The sorbents could absorb stoichiometric amount of hydrogen chloride at 450 C and 0.98 MPa absolute in a simulated coal derived gas environment. The fresh and spent sorbents were examined for dependence of kinetics of halide removal reaction on their pore structures. The results showed that the pore structure comprising of macropore peak at around 5.0 mm and smaller peaks at 1.0 mm or below was suitable to attain affordable kinetics on halide removal and sodium conversion of the sorbent. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Halide removal Sodium aluminate Pore structure IGCC (Integrated gasification combined cycle) Hydrogen chloride
1. Introduction Annual coal consumption of the world has been rapidly increasing since 2003, and its 70% was consumed by the countries of Asia Pacific region in 2013 [1]. Among fossil fuels, coal is far behind of natural gas in CO2 emission intensity due to the significant decrease in thermal efficiency as well as the intrinsic fuel composition. Because the CO2 emission from existing pulverized coal-firing plants is estimated to exceed 870 g-CO2/kWh [2], its reduction becomes burning issue in the countries with high dependence on coal-firing power plant. Application of the CO2 capture technologies to pulverized coal-firing plants and IGCC (Integrated Gasification Combined Cycles) are extensively investigated to find out the most realistic procedure for reducing CO2 emission. It is reviewed that the thermal efficiencies of coal firing power plants suffer decrease of 10% in absolute value due to
Abbreviations: EGR, exhaust gas recirculation; FID, flame ionization detector; FPD, flame photometric detector; HHV, higher heating value; ICP, inductively coupled plasma; IGCC, integrated gasification combined cycle; LHV, lower heating value; SAS, sodium aluminate sorbents; TCD, thermal conductivity detector; XRD, X-ray diffraction. * Corresponding author. E-mail address: [email protected] (M. Kobayashi).
introduction of post combustion type CO2 capture technologies [3]. Approximately two third of the efficiency decrease results from the power consumption of the CO2 capture process [3]. The large efficiency loss causes an increase in fuel consumption resulting in higher operating cost. Besides the operating cost for fuel, the investment cost for the CO2 capture and storage considerably increases the cost for electricity production, that is illustrated by a case study on USC (ultra supercritical) coal firing plant [4]. The other comparison between USC and IGCC revealed that the investment costs of both plants with CCS (CO2 capture and storage) equipment are comparable [5]. Thus, IGCC with CCS equipment exhibits better economic performance because of the lower efficiency penalty of 8.6 points, in contrast with the value being 10.5 points for USC with CCS [5]. An exergy analysis on IGCC and CFB (circulating fluidized bed) cogeneration plants also revealed that the plant efficiency suffers from the large efficiency loss due to the CO2 capture and compression [6]. It is urged from these results that an alternative process to the conventional CO2 capture equipment should be introduced to enhance economics and efficiency of coal fueled power generation with CO2 separation. Despite various attempts to reduce the efficiency penalty of CO2 capture with the conventional amine based process, considerable decrease in net thermal efficiency seem to be unavoidable so far as the CO2 capture process exhibits the large energy consumption. There are several
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assessments of thermal efficiencies on IGCC based power plant with CO2 capture facilities. An IGCC power plant with Shell gasifier was analyzed so that the efficiency decreased from 46% to 36% in LHV (lower heating value) base due to adoption of CO2 capture process [7]. An efficiency analysis of IGCC with and without CO2 capture was performed for three types of gasifier from General Electric Energy, ConocoPhillips, and Shell Global Solutions [8]. The analysis concluded that the relative efficiency penalty for CO2 capture installation to the IGCC power plants is 21.4% on average. Although the CO2 capture has immediate effect on reducing the emission to the atmosphere, considerable decrease in thermal efficiency of the power plant results in exhaustion of coal resource in longer term. It is important to avoid absurdity that environmental conservation on utilization of fossil fuels will accelerate the depletion of energy resource. Because the hot fuel gas desulfurization process has potential to improve thermal efficiency of the IGCC power plants [9], there is an attempt to apply the process to the IGCC system along with CO2 capture. Thermodynamic estimation showed that IGCC power plants with hot gas clean-up and post-combustion CO2 capture has potential to improve thermal efficiency as high as 41.5% at LHV [10], while the estimation is still on the system analysis base. Recent review on CO2 capture technology illuminated the potential technologies includes absorption, adsorption, cryogenic separation, and membrane separation [11], whereas the direction for technology improvement is remained uncertain. The other review revealed that the existing technology still requires improving their performance and reducing cost and energy consumption for CO2 capture [12]. Considering these backgrounds for CO2 capture in coal firing power plants, variety of concepts on new power generation system are proposed to reconcile the thermal efficiency with the efficient CO2 capture for coal-fueled power generation. Zecomix cycle is a conceptual system comprised mainly of a hydrogasification equipped with the dry CO2 absorbing process using calcium oxide sorbent, and a combined cycle power generator with semi-closed gaseturbine cycle on syngas oxy-firing. Zecomix cycle was initially proposed as a system for coproduction of electricity and hydrogen with CO2 capture [13]. The recent estimation on the plant performance of Zecomix cycle indicated that net thermal efficiency varies from 47% to 44% at LHV base along with decreasing utilization of CaO, which is dry sorbent for CO2 capture, from 67 to 20% [14]. Among various technological hurdles that the cycle has, the most crucial issue seems to be the dry CO2 capture process, where the CaO sorbent particulate should be circulated between absorbing reactor and desorbing reactor under high temperature and high pressure. Similar concept called Zecomag cycle is suggested as a modification of the power island of Zecomix cycle introducing open gas turbine cycle at which the syngas is burnt with air [15]. The cycle also has same difficulty in the dry capturing of CO2 with the CaO sorbent process. Other approach to reduce the efficiency penalty due to the CO2 capture is proposed for future IGCC adopting one of emerging technologies, such as membraneenhanced CO conversion, carbonate looping process for CO2 capture, and oxy-fuel concept by replacing cryogenic air separation with oxygen transfer membrane [16]. The other concept to lowering the energy consumption for CO2 capture is so called “oxyfuel IGCC” power generation system. The distinctive concept of IGCC power plant is expected to be as a candidate of power generation system that will reconcile effective CO2 separation with high efficiency of the coal firing power plant. The concept of oxyfuel IGCC power generation plant is basically comprised of a semi-closed cycle operation of a gas turbine with an oxy-fuel combustor that is fueled by an oxygen-CO2 blown coal gasification. The CO2 separation in the plant is efficiently established by steam condensation from circulating exhaust gas and compression.
While the exhaust circulation brings advantage in the plant efficiency to the oxy-fuel IGCC, contaminant issues arise from the semi-closed cycle operation [17]. As coal-derived syngas contains various contaminants derived from coal; sulfur compounds, halides, nitrogen compounds, alkaline and alkaline earth metals, and volatile heavy metal are suspected as target of consideration. Those contaminants should be properly treated in the plant to protect equipment in the plant and achieve the environmental tolerance for CO2 storage as well as the environmental regulations. The system requires a pre-combustion type sulfur removal process to attain sulfur tolerant level of the gas turbine. The removal process should be dry sulfur removal process so that the processed gas retains its H2O and CO2 concentrations. The retained amount of H2O and CO2 will not only act as working fluid of the gas turbine, but also decrease the potential of carbon deposition. The proper operation of the dry sulfur removal process was sought and suggested that the exhaust circulation to the upper stream of the process is effective to prevent carbon deposition [17]. The exhaust circulation is effective in inhibiting carbon deposition at all expected condition in the actual plant operation and the power consumption for the exhaust circulation is small enough to keep those advantages [17]. Besides the sulfur compounds in the syngas, halide impurities are also harmful to the health of gas turbine materials. It is expected that the tolerant concentration of chloride is lower than ppm level to protect gas turbine from corrosion. Hydrogen chloride is also detrimental to the desulfurization performance of the zinc ferrite sorbent because the halide causes zinc chlorination. This paper focuses the target for impurity removal on halides so that the system configuration of the oxy-fuel IGCC power generation is adequately established for the treatment of contaminants in the syngas. Alkaline compounds of sodium are candidates for use in halide-removal sorbent, due to the favorable thermodynamic affinity of sodium for chlorine [18]. Industrially available alkaline compounds of sodium, such as sodium hydroxide, sodium carbonate, and sodium aluminate, have poor reactivity with hydrogen chloride, because those compounds are prepared in bulk solid forms. It was revealed from former work that sodium aluminate is able to acquire considerable reactivity with hydrogen chloride if the material is carefully synthesized from sodium carbonate and sol of alumina [18]. Although the specially synthesized sodium aluminate is a candidate material for the halide removal from coal-derived fuel, the synthetic procedure requires expensive reagents and consumes significant amount of energy for condensation and drying of the raw materials. In order to resolve the restriction on economic manufacturing, a trial production of SAS (sodium aluminate sorbents) was performed by using industrially available sodium aluminate and additional plasticizer, as well as pore builder. Sodium aluminate is generally difficult to form into pellet shape, because of its deliquescent property. This trial production of halide removal sorbents is quite challenging and such sorbents were not investigated before. The prototype sorbents were investigated on hydrogen chloride removal performances in simulated coal-derived syngas, and their applicability to the halide removal process in the oxy-fuel IGCC power generation plant was discussed. 2. Halide removal process for oxy-fuel IGCC 2.1. Process position on the oxy-fuel IGCC plant Prior to the experimental investigation of the halide sorbents, we have to determine the operating condition for the halide removal process in the oxy-fuel IGCC power generation. The halide removal process has roles for protection of the zinc ferrite sorbent from chlorination, as well as the protection of gas turbine from corrosion. The sequence of the processes is then naturally
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determined for the halide removal being at the upstream of the sulfur removal process as indicated in Fig. 1. The EGR (exhaust gas recirculation) for inhibiting carbon deposition in the sulfur removal process is injected downstream of halide removal. This will reduce pressure drop through the sorbent bed of the halide removal reactor, and avoid the possibility of sodium carbonate formation in the halide sorbent. EGR to downstream of the halide removal process is also important to avoid deliquescence of sodium aluminate, because the exhaust contains steam at high concentration around 30 vol%. These requirements make the halide removal process be anchored to that position. Although the schematic flow diagram shown in Fig. 1 does not illustrate auxiliary equipment, each process requires equipment such as sorbent silos and hoppers for actual process operation. Because the halide sorbent is not regenerable, the sorbent in the reactor should be changed periodically during plant operation. The specific of the equipment configuration and the manufacturing design of the practical process should be considered along with the plant design in the future. The halide removal process well fits to the dry gas purification, because the process can be operated at the same temperature and pressure with the desulfurization process. Thus, the introduction of the halide removal process would give slight effect on the plant efficiency of the oxy-fuel IGCC power generation. Earlier thermal analysis revealed that the plant exhibits very high net thermal efficiency of 44.0% LHV (41.9% higher heating value) at 99% CO2 capture ratio [19]. 2.2. Consumption of chemicals for halide removal process The environmental impact for introducing the halide removal process should be assessed from the viewpoint of consumption of chemical products. This section explains the advantage of the dry halide removal process in the viewpoint by comparing with conventional wet scrubbing process. If wet gas purification is applied to the oxy-fuel IGCC power plant, water-scrubbing process will mostly capture hydrogen chloride and other halides in the syngas and produce a solution containing hydrogen chloride and other water-soluble contaminants. Then the solution should be neutralized by sodium hydroxide as shown in Eq. (1). Wastewater from the scrubber requires more chemicals for water treatment process. The dry halide sorbent, SAS also acts as a neutralizer for HCl as indicated in Eq. (2). The difference is that the SAS will leave aluminum oxide along with steam. If the residue of SAS utilization causes environmental impact, appropriate treatment is required for the residue. Both sodium chloride and aluminum oxide are well-understood chemical substances that can be treated by established procedure with small environmental impact. We also have to consider the consumption of sodium aluminate from the point of environmental impact during the production, utilization, and disposal of SAS for
3
Table 1 Preparation specification of the sodium aluminate sorbents. Sorbent name
SAS A
SAS B
SAS C, SAS D
Calcination cond. Pellet size (mm f mm L) Raw material comp. (wt%) Sodium aluminate Plasticizer,#1 Plasticizer,#2 Pore builder
700 C, 4 h 33
700 C, 4 h 3.5 3.5
700 C, 4 h 3 3.5
94.3 5.7 e e
66.7 6.7 e 26.6
75.2 e 9.8 15.0
the dry halide removal process. The halide removal sorbent, SAS may have small environmental impact because the utilization of SAS produces neither hazardous waste nor waste-water requiring chemical treatment.
HCl þ NaOH/NaCl þ H2 O HCl þ NaAlO2 /NaCl þ
1 1 Al O þ H O 2 2 3 2 2
(1) (2)
3. Experimental 3.1. Preparation of sodium aluminate sorbent The preproduction sorbents were prepared on an experimental basis, because the sorbent performance highly depends on the physical properties developed with the preparation procedure. At the initial stage of this study, SAS was prepared by trial-and-error production to find out appropriate production procedure for building suitable pore structure to the SAS. Sodium aluminate is available as an industrial raw material in the form of powder or solution. SAS should be prepared as small pellet to filling up a packed bed in the reactor. It is required to add some plasticizer to form pellets of sodium aluminate through an extruder. Then the raw extruded pellets were dried and calcined at 700 C for 4 h. In this preliminary production, three kinds of SAS were prepared with different compositions of raw material and additives as summarized in Table 1. The recipe for SAS A provides benchmark of the sorbent, which is simple combination of a sodium aluminate powder and a plasticizer. A pore builder, which is burned out during calcination to leave pore structure in the pellet, was introduced to the recipe for SAS B. The third recipe for SAS C and SAS D is in the same concept as SAS B, but it used different plasticizer giving better molding characteristics. SAS C and SAS D is essentially same recipe but the SAS D is produced in semi-production scale reactor to obtain larger amount of sorbent pellet. The calcined product of SAS was partly carbonated by carbon dioxide in the atmosphere, according to the reaction shown in Eq. (3), because the sorbent was
Fig. 1. Suitable position of halide removal process determined for the oxy-fuel IGCC power generation plant.
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Fig. 2. Test apparatus for the sorbent performance evaluation in a fixed bed reactor.
exposed ambient condition during the calcination. Thus, the product pellet mainly consisted of sodium aluminate, sodium carbonate, and aluminum oxide. Sodium carbonate is also possible to react with hydrogen chloride according to the reaction shown in Eq. (4). Although the proportion of sodium carbonate was rather minor, its contribution to the halide removal reaction is not clear. We also have to examine the effect of the carbonate content to the sorbent performance.
CO2 þ 2NaAlO2 /Na2 CO3 þ Al2 O3
(3)
Na2 CO3 þ 2HCl/2NaCl þ H2 O þ CO2
(4)
3.2. Fixed bed reactor test for SAS performance evaluation To determine halide removal performance of SAS, a fixed bed reactor shown in Fig. 2 was operated at the condition summarized in Table 2. The same weight of sample was packed in the reactor for each SAS pellet. Because the sizes of those SAS pellets were slightly different each other, the apparent volume of the packed bed varied from 30 to 32 cm3. Thus, the space velocity according to the same flow rate of 2200 cm3/min varied between 4100 and 4400 h1. Gas analysis was performed with combination of gas chromatograph with three TCD (thermal conductivity detector) detecting inorganic gas species (Round Science, AG-1 TTTH), and gas chromatograph with FID (flame ionization detector) and FPD (flame photometric detector) for hydrocarbons and sulfur compounds (Round Science, AG-1 FH-FPD). The instrument AG-1 TTTH is able to determine steam concentration as well as the other inorganic gas concentration in the sample. Prior to the chlorination of the sorbents, the
Table 2 Operating condition of the fixed bed reactor instrument.
Temp., C Press., MPa abs. Gas comp. Additional impurities, ppm Pellet size of sorbent, mm Sorbent amount, g Bed volume, cm3 Gas flow rate, cm3/min Space velocity, h1 Reaction time, h a
HCl
Reduction
HCl removal
450 0.98 a 1 0 3.0e3.5 15 30 32 2200 4100 4400 1
) ) a 1 900 ) ) ) ) ) 8
1: Air blown coal gas composition was selected for comparison.
sorbent bed was preheated to the desired temperature under flowing nitrogen through the bed. Then the chlorination of the sorbents was performed in the simulated gas, which was supplied as a mixture of each component gases from cylinder through mass flow controller as shown in Fig. 2. Steam was separately injected through vaporizer from micro pump to establish the desired concentration, which was confirmed by the gas chromatograph. The performance evaluation initially started with the reduction condition in Table 2 and lasted for 1 h. Then the reaction gas was switched to the HCl removal condition, and the evaluation test continued for 7e17 days with typically 8 h duration, while the halide removal period depended on the reaction condition applied. The gas composition was measured at the reactor outlet with the gas chromatographs at 5 min interval. The breakthrough curves of HCl was measured with ion electrode HCl analyzer (Toa DKK, GNC224), the data was used to evaluate halide capacity, prebreakthrough concentration, and halide removal rate. 3.3. Analyses of chemical composition and physical property of SAS SAS was prepared as mixed compounds of sodium aluminate, sodium carbonate, and aluminum oxide. It was difficult to determine the quantitative composition of those compounds directly by chemical analysis. The composition of those compounds was determined indirectly from elemental analysis of the sample. Required data was contents of chloride, aluminum, sodium, and carbon. The whole sample was dissolved to determine chloride content using an ion chromatography system (Dionex, ICS-1500). Aluminum and sodium was analyzed by an ICP optical emission spectroscopy (Shimadzu, ICPS-8100). Carbon content was determined with a carbon and sulfur content analyzer (Horiba manufacturing, EMIA-920V2). The obtained data was processed to estimate the component compounds. The existence of compounds in the SAS was confirmed by powder XRD (X-ray diffraction) analysis. Pore size distribution of SAS was measured with a mercury porosimeter (Micromeritics, Auto Pore IV 9520). 4. Results and discussions 4.1. Halide removal performance of SAS The condition for performance evaluation of SAS represented the expected operating condition of the halide removal process in the oxy-fuel IGCC, in which the process is placed in upstream of the sulfur removal process. Table 3 shows estimated gas composition of syngas produced by the O2eCO2 blown gasification, and its
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M. Kobayashi et al. / Energy xxx (2015) 1e8 Table 3 Estimated gas composition produced by O2eCO2 blown gasification, its equilibrium condition, and the test conditions. Condition
Gasifier outleta Halide removala Prelim. tests Oxy-fuel test
Temp. ( C) Press. abs, (MPa) Gas comp. (vol%) H2O CO H2 CO2 CH4 N2 O2 HCl
895 2.8
450 2.75
450 0.98
450 0.98
3.2 66.4 18.8 8.6 1.5 1.5 0.0 100 ppm
0.4 63.6 21.6 11.4 1.5 1.5 0.0 100 ppm
5.0 20.0 12.0 5.0 1.5 56.4 0.0 900 ppm
3.0 66.0 19.0 9.0 1.5 1.41 0.0 900 ppm
a
Chemical equilibrium estimation at the condition of each process stream.
equilibrium composition for water gas shift reaction at 450 C, that is the operation temperature of the halide removal process. The first two test results were done in the preliminary test condition in Table 3. Because this performance evaluation in the preliminary condition has screening purpose, the gas composition was settled to simulate the syngas produced by the air-blown gasification. Fig. 3 displays a breakthrough curve of hydrogen chloride for the SAS A. The horizontal axis express the reaction time, tN, that is normalized based on the theoretical chloride capacity. Despite of sample amount shown in Table 2, this test was done with sample weight of 10 g. Thus, the space velocity value became large as 7200 h1. After the instant period of pre-breakthrough, the outlet concentration increased steadily. This means that the sorbent amount is too small
5
to exhibit breakthrough curve expressing the reaction kinetics between sorbent bed and HCl in the gas. The next test was performed with the exact amount of sorbent shown in Table 2 and the preliminary test condition shown in Table 3. Fig. 4 shows the results obtained for 15 g of SAS B sample that was installed in a packed bed reactor as shown in the photograph in the figure. Unexpectedly the sorbent could not keep low HCl concentration at the reactor outlet, despite of the lowered space velocity being 4500 h1. As outlet HCl concentration exceeded 100 ppm at the initial stage, the test was discontinued at the reaction time, tN being 0.2. Fig. 5 shows the results obtained for 15 g of SAS C sample that was packed in the reactor in the same manner. After the pre-breakthrough continued until tN exceeded 0.4, the outlet concentration gradually increased toward the inlet HCl concentration. SAS C solely exhibited the affordable performance of halide removal among the preliminary production sorbents. The moderate slope of breakthrough curves shown in Figs. 3 and 5 imply the relatively lower rate of halide removal. In both results, the breakthrough occurred far before tN comes closer to one, and bed conversion reached scarcely 70% at the complete breakthrough. SAS C, as well as SAS A, requires more acceleration of the halide removal kinetics, to exhibit sufficient reactor performance for the halide removal process. 4.2. Pore size distribution of SAS and possibility for enhancing reaction kinetics To check the possibility for enhancing reaction kinetics of SAS, pore size distributions of prepared sorbents were examined. Figs. 6e8 show the pore size distribution of SAS A, SAS B and SAS C respectively. The distribution of SAS A, which solely introduced plasticizer to form the pellet, exhibited a macropore peak at around 1 mm. The cumulative pore volume was relatively low as 0.4 ml/g.
Fig. 3. Preliminary halide removal test result for SAS A at relatively high space velocity (7182 h1).
Fig. 5. Preliminary halide removal test result of SAS C at space velocity of 4150 h1.
Fig. 4. Preliminary halide removal test result for SAS B at space velocity of 4150 h1.
Fig. 6. Pore size distribution of fresh SAS A.
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Fig. 7. Pore size distribution of fresh SAS B.
Fig. 8. Pore size distribution of fresh SAS C.
Superior sorbent prepared in former work showed the pore distribution peak at 0.1 mm and cumulative pore volume of 0.6 ml/g. These differences in the pore structures may affect to the earlier breakthrough observed for SAS A shown in Fig. 3. Dissimilarity in the pore structure of SAS B is more conspicuous as expressed in Fig. 7. Main pore distribution peak became as large as 10 mm, nevertheless the cumulative pore volume exceeded 0.7 ml/g. Introduction of pore builder to SAS C produced triplex distribution of macropore peaks at 5 mm, 1 mm and 0.1 mm as shown in Fig. 8 in addition to enlarging the cumulative pore volume to 0.8 ml/g. It seems that building pore structure below 1 mm is effective to enhance reaction kinetics of halide removal with SAS. The favorable pore structure may enhance the diffusion of reactant and product across the gas solid interface through the pore. Although building smaller pore structure with sodium aluminate is difficult due to deliquescent property of the material, it should be emphasized that enhanced kinetics will decrease the size and capacity of required reactor of the halide removal process, which will drastically decrease the amount of consumed sorbent and the energy consumption for the sorbent preparation. Although the poor performance of SAS B can be attributed to the absence of smaller pore, difference between SAS A and SAS C requires more detail description. Changes in the pore structures during the reaction with HCl were examined. Prior to the discussion on the pore structure changes, the specific volume changes during the reaction formulae in Eqs. (2)e(4) were estimated. Table 4 summarizes the formula volume change of solid reactants according to the chlorination or carbonation expressed in the formulae.
Chlorination of sodium carbonate solely increases its formula volume, while chlorination and carbonation of sodium aluminate both reduce its formula volume. It might be possible to discuss the pore structure change during the chlorination of SAS by taking the volume change into consideration. Fig. 9 displays the pore distributions of SAS A measured before and after halide removal. The major peak observed at 1 mm significantly reduced after chlorination. Peak at around 0.01 mm emerged instead after chlorination. To explain the change reasonably, the solid reactant of chlorination reaction was Na2CO3 at least around the inner surface of the pore structure. Because the peak at 1 mm and shoulder peak at 0.2 mm both shrunk and shifted to smaller diameter, volume expansion due to chlorination of the carbonate is reasonable to describe the behavior. Other possibility of the pore decrease is the shrinkage of bulk structure due to chlorination of sodium aluminate. The cause of the emerged peak at 0.01 mm is not clear at this moment. The pore of SAS A was gradually blocked during progress of chlorination of the sorbent. Then the gas reactant and products of chlorination reaction, viz. HCl, CO2, and H2O, should have diffused across the solid phase, which would have diminished the reaction rate considerably. Pore structure behavior during chlorination of SAS C is presented in Fig. 10. The peaks at 1 mm and 0.1 mm shrunk and peak at 0.01 mm emerged, which is quite similar behavior to that observed for SAS A. The difference is peak at 5 mm that remained largely after the chlorination. The relatively large pore probably took a role of gas channel between outer surface and bulk of the sorbent pellet. The gas channel might allow the gas reactant and products of chlorination to diffuse easily through the gas phase in the pore structure, which would keep larger reaction rate of SAS C compared to SAS A as shown in the Figs. 3 and 5. 4.3. Sorbent performance in the oxy-fuel IGCC gas composition Finally, SAS D sorbent, which was prepared by the essentially same recipe of SAS C, was subjected to the halide removal test in the
Fig. 9. Comparison of pore size distribution of fresh and spent SAS A.
Table 4 Volume change according to chlorination and carbonation of sodium aluminate. Formula volume of solid reactant
Formula volume of solid product
Expansion ratio
2NaAlO2 e 109.3 ml 2NaAlO2 e 109.3 ml Na2CO3, Al2O3 e 67.5 ml
2NaCl, Al2O3 e 79.7 ml Na2CO3, Al2O3 e 67.5 ml 2NaCl, Al2O3 e 79.7 ml
27% 38% þ18%
Fig. 10. Comparison of pore size distribution of fresh and spent SAS C.
Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055
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Fig. 13. Powder XRD chart obtained for fresh SAS B. Fig. 11. Halide removal test result for SAS D at oxy-fuel test condition.
oxy-fuel gas composition. Fig. 11 displays breakthrough curve of hydrogen chloride obtained for 15 g of SAS D. After the prebreakthrough continued until tN exceeded around 0.6, the outlet concentration gradually increased. The pre-breakthrough period extended compared with the preliminary test shown in Fig. 5, which indicates the gas composition of the oxy-fuel condition enhanced the reaction kinetics of SAS D. The most plausible explanation for the enhancement can be derived from equation (2). The lower steam concentration of oxy-fuel condition might accelerate the reaction. The possibility of the acceleration from the slight difference in the steam concentration for the two conditions, namely 3% and 5%, should be examined in further investigation. In order to evaluate the practical feasibility of the halide removal process with the sorbent, quantitative analysis for the heat and material balance of the process operation is required. It is also the future work to reveal the appropriate engineering design of the process and to extract the tasks for improvement.
4.4. Comparison of the compound composition of fresh and spent sorbents Compositions of compounds in the fresh and spent sorbents were estimated from the chemical analysis of constituent elements of those samples. The sample of the spent sorbent was collected from the sorbent bed at the most inlet part, where the sorbent was expected to be sufficiently saturated with chloride, to compare the maximum halide capacity of chlorinated SAS samples. Fig. 12 displays the estimated composition in bar graph. The summation of the composition does not reach 100% in some cases, which indicates the error in analysis and estimation. The worst case was SAS B fresh sorbent whose estimated summation
Fig. 12. Composition of compounds in fresh and spent SASs estimated from chemical analysis.
of the composition exhibited 84%. The importance of the figure is its qualitative tendency of the composition. All sorbents consisted of major portion of sodium aluminate and 13e38% of sodium carbonate. Free aluminum oxide was in minor portion in the fresh sorbent. Giving attention to the comparison between fresh and corresponding spent sorbent, significant characteristics of the sorbents were revealed. Both sodium aluminate and sodium carbonate were chlorinated as the composition change indicates that the most portions of both compounds were converted to sodium chloride. Sodium conversion of whole sorbent reached 80, 95 and 95% for SAS A, SAS C and SAS D respectively. Thus the halide capacity of the SAS is very efficient to utilize sodium in the sorbent. In order to confirm the estimation of the sorbent composition, powder X ray diffraction patterns were measured for the fresh and spent sorbents. Distinctive results were obtained for SAS B and SAS C. Fig. 13 illustrates the XRD pattern of fresh SAS B, the marker with chemical formulae is indicating the strongest peak of each compound. The other mark is labeled at the major peaks of each compound. The strong peak of Na2CO3 indicates that the main portion of the sorbent was sodium carbonate, whereas the sodium aluminate structure was recognized as a minor portion. This result is quite consistent with the composition estimated for SAS B in Fig. 12. The XRD pattern of spent SAS B shown in Fig. 14 indicates that at least the surface of the sorbent was completely converted to sodium chloride. The higher content of sodium carbonate might retard the chlorination kinetics due to the increasing formula volume during the reaction as described in the former section. On the contrary, the major portion of the fresh sorbent C was clearly sodium aluminate as shown in Fig. 15, which is well corresponds to the estimation shown in Fig. 12. The spent SAS C also exhibited the sodium chloride formation on the sorbent Fig. 16.
Fig. 14. Powder XRD chart of spent SAS B.
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to the normalized time being 0.6; the sorbent attained sufficient capacity and pre-breakthrough duration in the oxy-fuel IGCC operating condition.
Acknowledgment A part of this work was carried out as development program P08020 of New Energy and Industrial Technology Development Organization (NEDO), Japan.
References Fig. 15. Powder XRD chart obtained for fresh SAS C.
Fig. 16. Powder XRD chart of spent SAS C.
5. Conclusion Halide control process is essential for realizing the oxy-fuel IGCC power generation plant. It was recommended that the dry halide removal process should be placed at the upstream of the dry sulfur removal, where the process protects desulfurization sorbents as well as gas turbine. Sodium aluminate sorbent were prepared by the preliminary production procedure and subjected to the experimental examination into the relationship between halide removal performance and their pore structure. A sorbent prepared with plasticizer and pore builder exhibited the most promising performance under syngas composition expected for the oxy-fuel IGCC at the temperature being 450 C and absolute pressure being 0.98 MPa. Investigation of change in the pore structure and estimation of the compound composition of the sorbents prepared with different recipe revealed that the physical and chemical properties are closely rerated to the sorbent performance. Coexistence of macropore peak at around 5 mm and smaller peaks at 1 mm or below provided suitable kinetics on halide removal and higher conversion of sodium chlorination. The higher ratio of sodium aluminate to sodium carbonate was desirable to enhance both chlorination kinetics and sodium conversion. The best sorbent obtained in this work exhibited the sodium conversion being 95%, and the pre-breakthrough period extended
[1] The editor of BP statistical review of world energy, BP statistical review of world energy June 2014. 2014.
Please cite this article in press as: Kobayashi M, et al., Performance evaluation of porous sodium aluminate sorbent for halide removal process in oxy-fuel IGCC power generation plant, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.04.055