Article pubs.acs.org/EF
Migration and Transformation of Ash-Forming Elements during Steam Gasification of Biomass and Polyethylene Tianhua Yang, Fei Liu, Rundong Li,* Xingping Kai, Kai Jia, and Yang Sun College of Energy and Environment, Shenyang Aerospace University, Key Laboratory of Clean Energy, Liaoning Province, Shenyang 110136, PR China ABSTRACT: Corn straw and polyethylene were subjected to steam gasification in a tube furnace at three different temperatures to investigate the migration and transformation of ash-forming elements. Overall, 84% of the alkali metals, 52% of the alkaline earth metals, and 66% of P were water-soluble and were completely volatilized at 750 °C in the steam gasification process. Of the alkaline earth metals, 26% were organic and were completely volatilized at 850 °C. A portion of these volatile elements remained in the ash as (SiO2·K2O)(l) slag, (SiO2·Na2O)(l) slag, Ca9MgK(PO4)7, Mg2P4O12, Mg2SiO4, and CaMgSi2O6. When the steam gasification temperature reached 750−950 °C, 54−88% of alkali metals, 45−63% of alkaline earth metals, and 50−68% of P were released into the gas phase. Water-soluble Cl (91%) and S (55%) as well as organic Cl (9%), S (7%), and P (10%) were completely released at 750 °C, whereas 29−32% of the insoluble S was released as gaseous sulfur. Fe and Al were relatively stable throughout the gasification process. Over 87% of Si was in the eutectic composition and was present as cristobalite. When the temperature increased, crystalline Si transformed into amorphous Si. form of KOH via a reaction between K2SO4 and H2O. Jiang12 studied the release characteristics of AAEMs during biomass pyrolysis and steam gasification; the results indicated that carboxyl and other functional groups decomposed and released 53−76% of K/Na and 27−40% of Ca and Mg during pyrolysis, while releasing 12−34% of K/Na and 12−16% of Ca/Mg during gasification. However, there was no explanation for the release mechanism of AAEMs during gasification. Studies on the ash characteristics of biomass-derived fuel during steam gasification are scarce. In this study, scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), and chemical fractionation analysis (CFA) were performed at 750, 850, and 950 °C to reveal the migration and transformation of ash material, particularly of AAEMs, which are closely linked to ash-related problems in steam gasification.
1. INTRODUCTION Currently, biomass energy contributes approximately 10% of the total global energy requirement. By 2050, biomass will be able to satisfy a quarter, or even a third, of the global energy demand. Hence, there is considerable opportunity for development in the biomass industry in the coming years. Thermochemical processes, such as gasification, liquification, pyrolysis, and combustion, are the most common methods for converting biomass into fuels with high heating values. Steam gasification is one of the most effective and efficient techniques for generating a high stoichiometric yield of H from biomass.1,2 Plastic contains high amounts of volatiles and occupies a large proportion of waste. Therefore, the addition of plastic to biomass can improve its gasification quality and can aid in waste disposal.3,4 However, biomass, particularly annual biomass, contains various forms of alkali and alkaline earth metals (AAEMs) along with considerable amounts of silica; therefore, biomass ash melts easily in the low-temperature eutectic system generated. S and Cl can promote the volatilization of alkali metals, which are then deposited on the surface of the heat exchanger in the form of sulfates and halides; these components further evolve into the slag by capturing the coarse ash particles in the flue gas. These phenomenona affect the safety, reliability, and economy of the gasification device and increase the probability of shutting down for maintenance.5−11 For a fluidized bed system, the accumulation of agglomerates resulting from the lowtemperature eutectic system may cause fluidization loss. To solve this problem, the migration characteristics of ash at high temperatures should be investigated. Niu7 reported that Cl disappeared at 815 °C in the form of HCl because of the aluminosilicate of sylvite. Above 1000 °C, inorganic S was released in the form of SO2 by the silicate of K2SO4. K can be reduced by organic decomposition and releases metal K and KOH. Du8 reported that KCl and (KCl)2 were released between 750 and 950 °C; above 1000 °C, K was released in the © XXXX American Chemical Society
2. MATERIALS AND METHODS 2.1. Materials. Corn straw was obtained from Shenyang, Liaoning Province, China. Corn-straw-derived fuel (CSDF) was produced by mixing 50% polyethylene and 50% corn straw. The size of the raw material for gasification was less than 5 mm. 2.2. Research Gasifier. The schematic of the experimental apparatus is shown in Figure 1. In this experiment, 1 g of raw material was prepared for gasification. The steam flow output from the steam generator was maintained at 2 mL/min by a constant-flow pump. The temperature was set at 750/850/950 °C. The N2 input was closed after blowing out the tube before each experiment. When the temperature reached the predetermined value, a constant steam flow was fed into the reactor by the controlling steam generator and the constant-flow pump. After the system was stable, a crucible filled with 1 g of raw material was pushed into the reaction zone. After 30 min of reaction, the gasification ash was collected in the crucible. Received: November 4, 2014 Revised: May 19, 2015
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DOI: 10.1021/acs.energyfuels.5b00335 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
3. RESULTS AND ANALYSIS 3.1. Distribution of Ash-Forming Elements in CSDF. The concentrations of the extracted elements according to CFA and their original concentrations in the fuel are given in Table 1. 3.1.1. Water Fraction. Water leached 83% of K and 88% of Na from the fuel. S (55%), P (66%), and Cl (91%) were also discovered in the water fraction. Given that alkali metal chlorides, phosphates, carbonates, and sulfates were dissolved in water, the alkali metals leached by water were present as alkali metal chlorides, sulfates, phosphates, and carbonates. The molar ratios of Cl/(K+Na), S/(K+Na), and P/(K+Na) in the water fraction were 0.66, 0.09, and 0.09, respectively. Therefore, alkali metals were most likely associated with chloride (as well as phosphate or sulfate) in the raw material. Approximately 42% of Ca and 71% of Mg were present as water-soluble salts in the straw. Water-soluble Mg was likely bound to Cl and S, which were its only readily water-soluble forms, and Ca was likely to exist as CaCl2, which was its only readily water-soluble form.15 3.1.2. NH4Ac Fraction. The primary leached elements in the NH4Ac fraction were Ca (32%), Mg (18%) and Mn (44%). The second extraction with NH4Ac dissolved ion-exchangeable elements, such as K, Ca, and Mg, which are generally in the form of oxalates and carboxylate salts in raw materials.13,14,16,17 Mn coprecipitates with Ca because it occurs as solid inclusions of MnC2O4 in CaC2O4.14,18 In addition, 8% of K was extracted in this fraction, indicating the existence of potassium oxalates. For nonmetals, Cl (9%), S (7%), and P (10%) were extracted in this fraction, revealing that these elements were organically associated in the biomass. 3.1.3. HCl and the Insoluble Fraction. The elements in the last two fractions are generally considered inert in the thermochemical process. Therefore, they remained in the ash during the chemical reaction. Si was the primary element in these fractions. Quartz (SiO2) is a common mineral in biomass. According to Lier,19 the solubility of quartz [SiO2(s)] in pure water at 25 °C is 6 mg of Si per liter (ppm). As shown in Table 1, the Si concentration in the water solution was 192 ppm. Therefore, corn straw may contain other types of Si, such as opal, an organic silica mineral in biomass14 that has a higher solubility than crystalline Si. In addition, clay (i.e., Al silicates), which generally consists of K, Na, and Ca aluminosilicates, was inevitably collected during the corn straw harvesting. Nearly all Fe and Al were present as acid-soluble and insoluble species. Al was most likely present in the clay. P
Figure 1. Fixed-bed gasifier system. 2.3. Methods. 2.3.1. Chemical Fractionation Analyses. The CSDF and laboratory-prepared ash samples were crushed, ground, and sieved to a particle size of less than 5 mm. Each sample was extracted by three successive solutions of deionized water, 1 mol/L NH4Ac, and 1 mol/L HCl. The various types of ash-forming materials were identified according to their solubility in these solvents. Water easily washes out soluble salts, such as alkali metal chlorides, sulfates, and carbonates. The ion-exchangeable part of the fuel was extracted into the NH4Ac solution. Hydrochloric acid primarily leached acid-soluble salts and minerals, such as alkaline earth metal carbonates and phosphates. The insoluble fraction primarily consisted of insoluble silicates and oxides. Generally, the ash-forming elements leached out by water and NH4Ac are the volatile species, whereas the elements leached out by HCl and those present in the remaining fraction are the nonvolatile ash-forming elements in the raw materials. After each leaching process, the samples were centrifuged for 9 min at 20 °C and 4000 rpm and then filtered through 0.45 μm membrane filters. The solid residue was washed twice with deionized water. Further details of the CFA procedure can be found in the study by Pettersson.13 2.3.2. Content of Ash-Forming Elements. Acid digestion of the CSDF and ash samples after gasification was conducted in a microwave acid digester according to the European Committee for Standardization technical specification 15290:2006.14 The leachates and solid samples were analyzed for Si, K, Mg, Ca, Al, Fe, Mn, Na, P, and S using inductively coupled plasma atomic emission spectroscopy. It was assumed that all Cl was extracted in the first two solutions, and the Cl concentrations were determined using an ion chromatograph (Metrohm AG, Switzerland). The eluent flow (0.7 mL/min) contained 1.0 mM NaHCO3(aq) and 3.2 mM Na2CO3 (aq) to buffer to pH 10. Prior to detection, the Na ions in the eluent were removed via chemical suppression in a cation-exchange column that had been regenerated with 50 mM H2SO4 (aq). 2.3.3. Species of the Ash-Forming Elements. The solid ash phase was identified via XRD using a PW3040/60 X’pert PRO X-ray diffractometer (PANalytical, The Netherlands). The scanning angle was set from 10 to 70°, and the scanning rate was set to 2° per minute. SEM/EDS analysis of the gasification ash was conducted using an S3400N scanning electron microscope (Hitachi, Japan) and an Oxford INCA spectrum analyzer (Oxford Instruments, United Kingdom).
Table 1. Concentration of Elements Extracted from CSDF by Chemical Fractionation Analysis element
fuel (kg/mg)
H2O (mg/kg)
NH4Ac (mg/kg)
HCl (mg/kg)
residue (mg/kg)
total leachable element (mg/kg)
recovery
S P Cl Al Fe Mn K Mg Ca Na Si
819 725 3600 553 247 117 4834 2118 3469 1153 10060
466 467 3276
