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Influence of additives on potassium retention behaviors during straw combustion: A mechanism study
Bioresource Technology 299 (2020) 122515
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Influence of additives on potassium retention behaviors during straw combustion: A mechanism study
T
⁎
Fenghai Lia,b,c,d, Xiaochuan Wangb, , Chaoyue Zhaoc, Yang Lic, Mingxi Guoa, Hongli Fana, Qianqian Guoa, Yitian Fangd a
School of Chemistry and Chemical Engineering, Heze University, Heze 274015, China School of Power and Mechanical Engineering, Wuhan University, Wuhan, Hubei 430072, China c School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Potassium retention ratio Wheat straw ashes Oxidizing atmosphere Additive Variation mechanism
The potassium retention behaviors of wheat stalk and its modification mechanisms in an oxidizing atmosphere by three additives [kaolinite, fly ash from Jincheng coal fluidized-bed gasification (JFA), and ammonium dihydrogen phosphate (ADP)] were investigated. The potassium retention ratios (PRRs) increased with increasing additive mass ratio. The ADP was the optimal selection for wheat stalks combustion when the three additives at the same mass ratio because of its highest PRR and a slow decrease in PRR with an increase in temperature. With the increasing three additive mass ratios, the PRRs of their mixed ashes showed a similar trend [rapid increase (< 12%) and then a slow increase (12%–15%)]. The mass ratios of three additives were all < 1.0% and their mixed ash reached an appropriate PRR. For kaolinite or JFA, the formations of K–Al/Fe silicates prompted an increase in the PRR. For ADP, K–Ca/Mg phosphate generations increased the PRR.
1. Introduction To mitigate increasing environmental pollution and rapid reduction in fossil fuel reserves, it is necessary to apply renewable energy (Fang and Jia, 2012; Xu et al., 2014; Zhou et al., 2019a,b). Recently, increased
⁎
interest has been given to biomass worldwide because it is carbon neutral (Xiong et al., 2018), has a widespread distribution, is low cost (Nishiguchi and Tabata, 2016; Sikarwar et al., 2016; Wang et al., 2017), and is considered as potential energy to provide combined heat and power on a small or medium scale. In China, the installed capacity of
Corresponding author at: School of Power and Mechanical Engineering, Wuhan University, Wuhan, Hubei 430072, China. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.biortech.2019.122515 Received 23 September 2019; Received in revised form 26 November 2019; Accepted 27 November 2019 Available online 29 November 2019 0960-8524/ © 2019 Published by Elsevier Ltd.
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Limited systematic investigations have been conducted into potassium retention behaviors and the modification mechanism by different additives. Differences in straw ash composition and complexities in mineral interaction, the regulation behaviors, and the effecting mechanism of the additives on the straw-fixed potassium have not been clarified and need to be investigated further. The objectives of this paper were to investigate the influence of additives on the potassium retention behaviors and to explore the variation mechanism during WS combustion. Basic data and theoretical support is expected to mitigate the occurrence of ash-related issues during straw use.
biomass power will reach 30 GW by 2020 (Niu et al., 2013). Combustion or co-firing is regarded as an attractive and feasible method to use biomass as energy (Varol and Atimtay, 2015). Biomass tends to have a high alkali content (e.g., potassium and sodium), which makes its ashfusion temperature (AFT) low. These properties may lead to the occurrence of ash-related issues (e.g., fouling, agglomeration, slagging, and corrosion) (Link et al., 2018; Regueiro et al., 2017) during combustion, which results in a decrease in combustion efficiency, instability, and even unscheduled system shutdowns (Li et al., 2019). Among major forming ash elements in biomass, the potassium content indicates a potential ash fusion and deposition in conversion applications through vaporization and condensation (Wang et al., 2014). Thus, the mobility of potassium in combustion systems is a key factor that dictates ash behavior (Clery et al., 2018; Regueiro et al., 2017). During combustion, some elemental potassium converts to the gaseous phase (such as KOH, and KCl), and then reacts with chlorine or sulfide in the flue-gas to form hydrochloride or sulfate with a low melting point (MP). These low MP salts accelerate pipeline fouling and corrosion, and decreases boiler availability (Dirbeba et al. 2017; Niu et al., 2016; Zhang et al. 2018a). Alkali elements in the bottom ash and fly ash are mostly in the form of low MP silicates (e.g., K2SiO3 and K2Si4O9) and their eutectics, which may result in slag formation (Wang et al., 2015; Jobansen et al., 2017). With an increasing requirement for woody biomass to produce biomass fuels, extensive attention has been given to straw combustion or gasification to produce electricity, synthetic natural gas, and chemicals (Zhang et al., 2018b), because straw accounts for ~50%–60% of the total biomass in China from an energy perspective, and straw does not compete with land for food production requirements (Jiang et al., 2012; Mäkelä and Yoshikawa, 2016). However, straw tends to contain a higher alkali content (especially for potassium (K)), silicon, and ash content compared with woody biomass) (Li et al., 2020; Steer et al., 2013). The high potassium content decreases the AFT, which may lead to operational difficulties (Sarker et al., 2015). Thus, the investigation of potassium transformation behavior is important for straw thermal conversion. Potassium migration is dependent mostly on feedstock type and treatment conditions. The potassium emission profile is related closely to the K/Cl or K/(Si + Al) ratio in ash composition: high chlorine and/ or low (Si + Al) content promotes release as potassium chloride (KCl) or potassium hydroxide (KOH); whereas high (Si + Al) helps to retain K in the solid phase (fly or bottom ash) (Clery et al., 2018). Potassiumcontaining salt formations (e.g., KCl and K2SO4) are the main substances that influence ash-related issues during straw conversion (Nunes et al., 2016). Additive is a promising method in industrial practice to mitigate ash-related issues because it captures problematic ash species (e.g., gaseous phase KCl, KOH) by adsorption, and improves ash-fusion characteristics (Wang et al., 2015). Thus, it is very important in the selection of suitable additives with characteristics of a decrease in K release and an improvement in AFT for straw conversion on a large scale. Most investigations have focused on the effects of additives on ashfusion characteristics (Li et al., 2018; Liu et al., 2017; Wang et al., 2016). The influence of additives on potassium retention is low. Potassium fixation of maize straws and cotton stalks with two phosphorusbased additives was investigated and the influence of NH4H2PO4 on the potassium fixation ability was better than that of Ca(H2PO4)2 (Wang et al., 2017). Coal ash is a potential additive for easy-to-slagging biomass because it fixes potassium, and increases the AFTs of mixtures (Zhang et al., 2018b). However, these investigations are also related to the variation of AFT. For example, the performance of Ca(H2PO4)2 on the improvement of AFT was better than NH4H2PO4 because of the formations of high-MP potassium-calcium phosphates, i.e., K2CaP2O7, K2CaP4O12, and K4Ca(PO4)2 (Wang et al., 2017). The AFTs of the blended ash increased through coal ash addition because of the generations of high MP potassium aluminosilicates (Zhang et al., 2018b).
2. Materials and methods 2.1. Selections of samples Wheat is a type of gramineae plant that is planted worldwide, and is the second output grain crop in the world. Chinese wheat production is the largest in the world, and accounts for 17% of global production, which leads to a large amount of wheat stalk (WS) (wheat byproduct). Thermal conversion is the main method to use WS (Li et al., 2019). Airdried WS was selected from the countryside in Dongming, Shangdong, China. Samples were milled to 0.180–0.250 mm and stored in a cabinet dryer.
2.2. Preparation of ash samples 2.2.1. Preparation of WS ash The preparations of WS ash sample were as follows: the WS samples were placed in a muffle furnace (SX2-8-16ASP, Kewei Co., Beijing, China), and the temperature was increased from room temperature to 450 °C within 30 min, and then the muffle furnace door was closed and the temperature was maintained for 16 h. The WS ashes were removed, cooled to room temperature, and transferred into a drying cabinet for storage. The 450 °C ashing temperature was selected to prevent K volatilization in WS because elemental K in biomass is almost nonvolatile below 500 °C (Ma et al., 2017; Zhang et al., 2018b). The residual carbon contents of ash samples that were prepared at 450 °C for different ashing times were determined by using SC-444 equipment (Leco Corp., USA)) at 4-h intervals. The decrease in residual carbon content is obvious for ashing times from 4 h to 16 h (4 h: 9.36%; 8 h: 5.12%; 12 h: 2.29; 16 h: 0.75%); the variation of residual content is small for ashing times from 16 h to 20 h (20 h: 0.64%), and residual carbon content was less than 1% when the ashing time was 16 h. Thus, 16 h was selected as the WS ashing time.
2.2.2. Preparation of WS and its mixtures at different temperatures Three additives (kaolinite (KL), fly ash from Jincheng coal fluidizedbed gasification (JFA), and ammonium dihydrogen phosphate (ADP)) were crushed to a particle size < 0.198 mm and placed into the 450 °C WS ashes at mass ratios of 0%, 3%, 6%, 9%, 12%, and 15%, mixed manually and reached uniformity, respectively. The mixed WS ash samples at different temperature were prepared in the fixed-bed tube furnace (Fig. 1). Preparations were conducted according to the following procedure. Samples (450 °C WS ashes and their mixtures) were placed into a ceramic crucible and inserted into a low temperature zone of fixed-bed tube furnace. The oxidizing atmosphere (O2/CO2, 3:7, volume ratio) was introduced into the furnace at 2 L/min to simulate biomass combustion atmosphere (Li et al., 2019), and the furnace was heated at 20 °C/min. After the presetting temperature had been reached, the ceramic crucible was kept at this temperature for 1 h to simulate combustion. The samples were taken out, transferred into liquid-nitrogen cooling equipment, and cooled to room temperature to analyze the variation in potassium content and its mineral composition. 2
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hydrocarbons, carbon oxides, hydrogen, and tar vapors) in air- dried WS was high (72.30%), the content of fixed carbon, ash, and moisture was 15.60%, 7.60%, 3.36%, respectively. The ultimate analysis was performed on a PE 2400 analyzer (PerkinElmer, USA) (GB/T476-2001). The elemental content of carbon, hydrogen, oxygen, nitrogen, and total sulfur based on air-dried WS samples was 52.78%, 7.54%, 38.95%, 0.73%, and 0.85%, respectively. The ash compositions (prepared at 450 °C) measured by an ICP-AES (icap6300, Thermo Fisher Scientific, USA), in the form of oxides, are shown as (Na2O: 1.24%; K2O:34.56%; MgO: 9.22%; CaO: 8.64%; Fe2O3: 2.48%; SO3: 3.83%; Al2O3: 1.93%; SiO2:29.13%; Cl2O:7.85%; P2O5: 1.12%). The alumina content was low, whereas the contents of K and chlorine (Cl) were relatively high. Cl in biomass ash was volatilized mostly as sylvite (KCl), which promoted K volatilizes during straw thermal conversion (Ma et al., 2018). As shown above, compared with other straws, the WS had a moderate potassium content (generally from 2.29% to 63.90% (Stanislav et al., 2013)), a relatively high Cl content, and a low aluminum content. This ash composition reflected the characteristics of straw ashes. The WS had a wide distribution and was abundant globally. Thus, the WS can be regarded as a typical straw. Among the four category additives to improve the AFTs during biomass conversion (aluminum silicate-, sulfur-, calcium-, and phosphorus-based additives), the product of the sulfur-based additives during biomass combustion was mainly K2SO4. Its MP remained low and SO2 emission could not be ignored (Wang et al., 2017); the calciumbased additive tended to promote the release of potassium in biomass ashes at a high temperature because its ionic potential (20 nm−1) was higher than that of K+ (7.5 nm−1), and results in gradual replacement of K+ in silicates by Ca2+ (Li et al., 2019). Thus, aluminum silicate- and phosphorus-based additives were selected. Analytical pure kaolinite (Al2Si2O5(OH)4, KL) and ammonium dihydrogen phosphate (NH4H2PO4, ADP) were products of Kemeo Chemical Regent Co. Ltd (Tianjin, China). However, it might be expensive to buy these pure additives for biomass power plants, and residual materials with highadditive-composition content were economically attractive. Ash-agglomerate fluidized-bed (AFB) is considered one of the most promising technologies for three high coal [(high AFT, high ash content, and high sulfur content), is distributed widely in north and south-west China with reserves of ~62 billion tons, and accounts for ~25% total Chinese reserves] because of its wide fuel flexibility, uniform bed temperature, and environmental friendliness (Wang et al., 2012). The fly ash from AFB gasification of high AFT coal tended to contain high aluminum silicon content. Coal ash could absorb potassium (Liu et al., 2017). Thus, fly ash from Jincheng coal (Shanxi, China) AFB gasification (JFA, prepared based on the ASTM E1755-2001 standard) was also selected as an additive. The measured JFA compositions based on WS analytical methods are listed as (SiO2:44.25%; Al2O3:33.46%; CaO: 9.46%; Fe2O3:8.37%; SO3:1.68%; MgO: 1.24%; P2O5: 1.03%; K2O: 0.27%; Na2O: 0.12%; TiO2: 0.12%). As could be seen clearly that the contents of silicon and aluminum exist in the JFA were abundant.
Fig. 1. Schematic diagram of fixed-bed tube furnace. Legend: 1 oxygen gas cylinder; 2 carbon dioxide cylinder; 3 gas valve; 4 mass flow meter; 5 moving sample injector; 6 sealing element; 7 stainless steel tube; 8 electric heater; 9 temperature controller; 10 thermoelectric couple.
2.3. Experimental measurement 2.3.1. Potassium content in the ash samples The elemental potassium contents in ash samples were tested by using inductively coupled plasma atomic emission spectrometry (ICPAES, icap 6300, Thermo Fisher Scientific, USA). The ICP-AES with a high precision and high sensitivity at a high frequency power (1150 W) and 0.65 L/min gas velocity, could detect the concentrations from per million to percent levels. 2.3.2. X-ray powder diffractometer (XRD) measurements An XRD (D/max-rB, Rigaku Co., Japan) with Cu Kα radiation (40 kV, 100 mA) with software package MDI Jade 6.5 was used to test the mineral compositions of the ash samples. The samples were scanned at 5° 2θ/min from 10° 2θ to 75° 2θ with a step size of 0.01°. The semiquantitative normalized reference intensity ratio (RIR) method with a precision of approximately ± 10% (strong diffraction phase) or ± 25% (weak diffraction phases) was used to determine their mineral content (Huang et al. 2018). Amorphous matter content resulted from the difference between total crystalline phase content and ash bulk chemical composition (Li et al., 2020). 2.4. Potassium retention ratio The ash samples in the corundum boat that had been prepared in the fixed bed tube were removed and dissolved based on Chinese standard (MT/T1014-2006) to obtain an acid solution. The acid solution was diluted in a 100-mL volumetric flask with deionized water. ICP-AES was used to measure the potassium content [(Ci, Cc, and C0 (blank test)] in acid solution. To ensure the experimental data accuracy, each sample was measured three times and its RSD was within 2.5% in all cases. Because the K2O content in JFA was < 0.5% and the additives in the total ash were less than 15%. The ratios of potassium from JFA in the samples were < 0.1% and could be neglected. Therefore, the PRR can be calculated based on (Guo et al., 2017):
PRR% = (Ci − Co)/(Cc × (1 − W) i × 100%
3.2. Effects of temperature and additive mass ratio on PRR variation
(1)
3.2.1. WS PRR variation with increasing temperatures To explore the influence of additive on PRR and the effects of temperature on potassium release, in consideration of the WS mixture PRR variation with increasing three additive mass ratio (see Section 4.2.2), the ash samples of original WS ashes and mixtures with 6.0% three additives (mixed until uniform) were selected. These ash samples were prepared at different temperatures (750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) in the fixed-tube furnace. The temperatures were selected based on the operating temperature range of the biomass boiler. The potassium contents of the samples were determined by ICP-AES and are presented in Fig. 2. The WS PRR decreased obviously (from 750 °C to 900 °C) and then slowly (900 °C to 1100 °C) with an increase in temperature. During WS
where ΦK is the PRR of the sample, %; Ci is the potassium content in the ash sample, mg/L; Co is the potassium content in the blank test, mg/L; Cc is the potassium the content in original WS, mg/L; and Wi is the mass fraction of additives, %. 3. Results and discussions 3.1. Characteristics of CS and additives The proximate analysis of CS was conducted on a SDLA 718 proximate analyzer (SUNDY Co. Ltd., Changsha, China) (GB/T28731-2012) (Li et al., 2018). The volatile matter content (mostly composed of 3
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85
100
80
90
75
80 PRR/%
PRR/%
70 65 60
WS WS+KL WS+ADP WS+JFA
55 50
70 60 WS+KL WS+ADP WS+JFA
50
45 750
800
850
900
950
1000
0
o
Temperature/ C
2
4
6
8
10
12
14
16
Additive mass ratio/%
Fig. 2. Variation in PRR of WS and its mixtures with 6.0% three additives at different temperatures.
Fig. 3. Potassium retention variation with increasing three additive mass ratios at 950 °C.
ash heating, a large quantity of potassium was released in the form of a sylvite gaseous phase at a relatively low temperature (700–900 °C) in an O2/CO2 mixture atmosphere (Knudsen et al., 2014; Johansen et al., 2011). Some pores were covered by melting glasslike products with a further temperature increase (900–1100 °C). The covering blocked the release of potassium and resulted in a slow decrease in PRR (Wang et al., 2017). The PRR of mixed WS ashes with three additives increases for all temperatures investigated compared with WS. Among the three additives, ADP was optimal for potassium retention because of its highest PRR among the three additives and the PRR decreased slowly with an increase in temperature. ADP addition could inhibit the release of alkali metals even at a relatively high temperature, and it could fix potassium in the bottom ashes because of the formations of potassium metaphosphate (e.g., KPO3) and potassium phosphate (K2CaP2O7) (Wang et al., 2015). KL and JFA both had good fixed potassium ability at a relatively low temperature (800–900 °C). However, with an increase in temperature, its fixed potassium ability decreased slowly because the porosity was covered by a low-MP compound (Reinmöller et al, 2018). A reduction in the active surface areas of KL particles occurred and metakaolin converted to amorphous silica and alumina–silica spinel above 950 °C, which might convert to pseudo-mullite at 1000 °C with a low potential to react with KCl (Li et al., 2018). The potassium fixing ability of KL and JFA is almost the same from 750 °C to 1100 °C. However, the PRR of the WS with KL addition decreased more rapidly with an increasing temperature than that of JFA for two additives at the same ratio.
that of WS + KL for additives less than 9% and increased for additives above 9%. This behavior resulted from that silicon and aluminum were the keys to capture potassium in the solid/slag phase. For KL, the total contents of silicon (SiO2) and aluminum (Al2O3) accounted for 83.93%, whereas in JFA, the total contents of silicon and aluminum reached 77.71%. This difference resulted in a lower PRR of WS + JFA than that of WS + KL initially (< 9%). As the additive mass ratio increased further (> 9%), although the total mass ratio of silicon and aluminum in their additives did not change, the iron (Fe2O3) content in the WS + JFA increased (JFA contained 8.37% Fe2O3), and gradually fixed potassium (indicated by the formation of leucite (ferric) [K(FeSi2O6)]. Potassium may react with P2O5, CaO or MgO (from JFA) and generate K2CaP2O7 or KMgPO4, which caused the PRR of the WS + JFA to increase and exceed that of the WS + KL. Even when the mixture PRR reached 90%, as shown in Fig. 3, the mass ratio for the three additives was ~9.2% (ADP), 10.2% (JFA), and 11.3% (KL), respectively. Based on the WS ash content, the ratio of the three additives was 1:130, 1:116, and 1:103, correspondingly. The ratios of the three additives were < 1.0%, and they provided promising for use in straw combustion. As shown in Fig. 3, the fixed potassium efficiency of the ADP was the highest in the three additives. However, JFA might be a better selection for an industrial application, because JFA was cheap (it belongs to gasification residuals, is abundant, and is readily available) and environmentally friendly (residual recycling use) (Zhang et al., 2018a).
3.3. Mineral transformation of WS with increasing temperature
3.2.2. PRR variation with increasing additive mass ratio The operating temperature range of the biomass boiler varies from 800 °C to 850 °C, and the temperature range of the boiler was 100–200 °C higher than its operating temperature (Li et al., 2019). To explain the influence of additive mass ratio on the potassium retention of biomass during combustion, the ash samples of WS and its mixture were prepared at 950 °C in a fixed-bed tube furnace in an oxidizing atmosphere (Fig. 1), and their PRRs were calculated based on Eq. (1) with an increase in the three additives, as presented in Fig. 3. At 950 °C the PRR of the WS mixture increased with an increase in mass ratio of additives and showed a similar trend for three additives (increases rapidly [ < 12%) and then slowly (12%–15%)] (Fig. 3). The PRR of the ADP additive was highest in the three additives when they had the same mass ratio because the reactivity of Si or Al with K is lower than that of P (their ionic potentials decrease as P5+(147 nm−1) > Si4+ (95 nm−1) > Al3+(59 nm−1)) (Boström et al., 2012; Zhu et al., 2018). The PRR of WS + JFA was lower than
To investigate the PRR variation mechanism with an increase in additive mass ratio, the mineral evolution in WS ashes with an increase in temperature must be investigated. The WS ash samples at different temperatures (575, 750, 850, 950, and 1050 °C) in an oxidizing atmosphere were prepared in the fixed bed furnace, and their mineral phase compositions were measured by XRD. At 575 °C the minerals in the WS ashes were made of sylvite, potassium carbonate, quartz, goothite (α-FeO(OH)), and potassium sulfate (K2SO4). As the temperature increased, the sylvite contents transferred into the gaseous phase (Li et al. 2018), and potassium carbonate and quartz decreased; whereas the content of amorphous matter increased gradually. The potassium silicate (K2SiO3), monticellite (CaMgSiO4), kalsilite (KAlSiO4), calcium iron oxides (CaFe2O4), and forsenite (Mg2SiO4) were generated at 800 °C, and anorthite was formed at 950 °C. The reaction during WS ashes heating could be deduced and is listed as follows (Li et al., 2018; 4
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Phosphorus has priority over silicon of reactivity because the ionic potential of Si4+ (95 nm−1) was lower than that of P5+ (147 nm−1), thus K–Mg/Ca phosphates were formed before the corresponding K–Mg/Ca silicates (Bostrom et al., 2009). With ADP addition, potassium tended to react with ADP to produce potassium polymetaphosphate (KPO3) (700 °C (Wang et al., 2015)), and then might change to calcium or magnesium potassium phosphates with an increase in temperature (Wang et al., 2017). The generation of potassium magnesium phosphate (KMgPO4) resulted from the interaction of magnesium oxide and ADP. The derived reactions are listed as follows (Wang et al., 2015; Wang et al., 2017).
Wang et al., 2018; Zhou et al., 2019a,b): K2CO3 (s) + SiO2(s) → K2SiO3(s) + CO2 (g)
(2)
CaO(s) + MgO(s) + SiO2(s) → CaMgSiO4 (s)
(3)
KCl (g) + SiO2(s) + Al2O3(s) + H2O (g) → KAlSiO4 (s) + HCl (g) (4) MgO(s) + SiO2(s) → Mg2SiO4(s)
(5)
CaO(s) + α-FeO(OH)(s) → CaFeO4(s) + H2O(g)
(6)
CaO(s) + SiO2(s) + Al2O3(s) → CaAl2Si2O8(s)
(7)
3.4. Investigation of the influence mechanism of additive on WS PRR 3.4.1. Exploration of the influence mechanism of additive kind on PRR To explore the influencing mechanism of the three additives on the PRR during WS combustion, WS ash mixtures with three 6% additives at 950 °C were prepared in the fixed-bed furnace. The mineral phase composition of these mixed samples in an oxidizing atmosphere (O2/ CO2, 3:7, volume ratio) was determined by XRD. The minerals in the WS ashes at 950 °C were mostly in the form of sylvite, potassium carbonate, anorthite, potassium sulfate, forsterite, and leucite. In the WS ash samples with 6% KL, kalsilite formed (quartz and alumina derived from KL decomposition) and the leucite content increased. The mineral variation in mixed WS ashes by JFA addition was almost the same as KL addition because of the similarity in their main composition (Al2O3 and SiO2) content. Because of the high contents of silicon and aluminum in KL or JFA (Table 1), which are key compositions to capture potassium in the solid/slag phase (Clery et al., 2018; Liu et al., 2017), the contents of silicon and aluminum increased with an increase in KL or JFA mass ratio. The silicon and aluminum reacted with gaseous sylvite and resulted in high MP kalsiliate through Eq. (4). This reduced the sylvite gaseous concentration, retarded the release of potassium, and increased the PRR.
KAlSiO4(s) + SiO2(s) → KAlSi2O6(s)
(8)
NH4H2PO4(s) → NH4PO3(s) + H2O(g)
(9)
NH4PO3(s) + KCl(g) → NH4Cl(g) + KPO3(s)
(10)
KPO3(s) + CaO(s) → KCaPO4
(11)
KPO3(s) + MgO(s) → KMgPO4
(12)
NH4PO3(s) + KCl(g) + CaO(s) → K2CaP2O7(s) + NH4Cl(g)
(13)
NH4PO3(s) + KCl(g) + MgO(s) → KMgPO4(s) + NH4Cl(g) 2+
(14) 2+
Because of the ionic potential difference of Ca and Mg (Ca2+: 2.0; Mg2+: 3.0), Eq. (12) took precedence over Eq. (11). Moreover, the magnesium-oxide content was higher than that of calcium oxide (CaO: 8.64%; MgO: 9.62%) in WS ash, which resulted in KMgPO4 formation. 3.4.2. Effecting mechanism of additive mass ratio on PRR The migration and transformation of elements depends on a variation of the existing state of the elements or mineral compositions at certain conditions, which can be detected by XRD. In consideration of the ash composition similarity in KL and JFA, JFA was used as an example. The mineral phase composition of WS and its mixtures with different mass ratios of JFA and ADP at 950 °C was determined by XRD,
Table 1 Mineralogical composition contents of WS ashes and its mixed with 6% additives at 950 °C. Ash samples
WS WS WS WS WS WS WS WS WS WS WS
+ + + + + + + + + +
3%JFA 3%ADP 6%JFA 6%ADP 9%JFA 9%ADP 12%JFA 12%ADP 15%JFA 15%ADP
Ash samples
WS WS WS WS WS WS WS WS WS WS WS
+ + + + + + + + + +
3%JFA 3%ADP 6%JFA 6%ADP 9%JFA 9%ADP 12%JFA 12%ADP 15%JFA 15%ADP
Mineral (wt%) sylvite
potassium carbonate
anorthite
potassium sulfate
forsterite
leucite
kalsilite
14.37 12.15 11.87 10.42 9.95 8.37 7.80 6.24 5.28 3.15 3.08
4.63 2.31 – – – – – – – – –
18.19 16.71 16.24 14.53 13.29 12.38 12.39 10.72 10.04 7.26 7.98
6.27 6.19 6.15 6.12 3.20 5.79 2.58 4.16 2.10 3.92 1.98
7.82 7.63 7.73 7.54 4.32 5.81 4.01 5.06 3.78 4.97 2.99
5.87 7.24 7.08 8.26 9.62 7.34 8.35 6.97 7.75 9.24 6.09
– 8.38 – 16.89 – 22.37 – 24.94 – 28.36 –
monticellite
potassium feldspar
leucite, ferric
calcium potassium pyrophosphate
potassium magnesium phosphate
calcium potassium meta-phosphate
amorphrous matter
– – – – – 3.24 – 4.36 – 5.97 –
– – – – – – – 2.17 – 4.08 –
– – – – – – – 5.24 – 7.82 –
– – 7.24 – 13.37 – 18.32 – 20.15 – 24.58
– – 5.18 – 9.63 – 12.38 – 14.85 – 18.37
– – – – – – – – 5.28 – 8.24
42.85 39.49 38.51 37.31 36.62 34.70 34.17 31.14 30.77 27.34 26.69
Mineral (wt%)
5
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and their calculated mineral composition based on the RIR method is presented in Table 1. For JFA addition, when the JFA mass ratio was 3%, kalsilite formed through Eq. (4). Monticellite was generated when the JFA was 6% by Eq. (3) because of its high calcium content (CaO: 9.62%). With an increase in JFA mass ratio, the silicon and iron contents increased, which led to the generation of potassium feldspar (KAlSi3O8) and leucite (ferric) ([K(FeSi2O6)]) (Clery et al., 2018). These results might explain the trend of increasing WS PRR with increasing JFA mass ratio. Phosphorous mineral reacts preferentially with potassium before K-carbonate, K-sulphate, and K-chloride formation (Wang et al., 2017). Thus, with ADP addition, the potassium carbonate disappeared rapidly (ADP mass ratio < 3%), and the contents of K-sulfate and K-chloride decreased gradually, accompanied by the formation of high MP potassium phosphates (such as calcium potassium pyrophosphate (Eq. (13)), potassium magnesium phosphate (Eq. (14)), and calcium potassium metaphosphate (KCaP4O12)), and their content increased correspondingly. The reactions with increasing mass ratio of JFA or ADP are presented as follows. KAlSi2O6(s) + SiO2(s) → KAlSi3O8(s)
2012. Ash transformation chemistry during combustion of biomass. Energy Fuels 26, 85–93. Clery, S.D., Masona, P.E., Raynerb, M.C., Jones, M.J., 2018. The effects of an additive on the release of potassium in biomass combustion. Fuel 214, 647–655. Dirbeba, M.J., Brink, A., DeMartini, N., Zevenhoven, M., Hupa, M., 2017. Potential for thermochemical conversion of biomass residues from the integrated sugar-ethanol process-Fate of ash and ash-forming elements. Bioresour. Technol. 234, 188–197. Fang, X., Jia, L., 2012. Experimental study on ash fusion characteristics of biomass. Bioresour. Technol. 104, 769–774. Guo, S., Jiang, Y., Yu, Z., Zhao, J., Fang, Y., 2017. Correlating the sodium release with coal compositions during combustion of sodium-rich coals. Fuel 196, 252–260. Huang, Y., Liu, H., Yuan, H., Zhuang, X., Yuan, S., Yin, X., Wu, C., 2018. Release and transformation pathways of various K species during thermal conversion of agricultural straw. Part 1: devolatilization stage. 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Exploration of potassium migration behavior in straw ashes under reducing atmosphere and its modification by additives. Renew. Energy 145, 2286–2295. Link, S., Yrjas, P., Hupa, L., 2018. Ash melting behaviour of wheat straw blends with wood and reed. Renew. Energy 124, 11–20. Liu, Y., Duan, X., Cao, X., Che, D., Liu, K., 2017. Experimental study on adsorption of potassium vapor in flue gas by coal ash. Powder Technol. 318, 170–176. Ma, X., Li, F., Ma, M., Fang, Y., 2017. Investigation on blended ash fusibility characteristics of biomass and coal with high silica−alumina. Energy Fuels 131, 7941–7951. Ma, X., Li, F., Ma, M., Huang, S., Ji, S., Fang, Y., 2018. Regulation of ash fusibility characteristics for high-ash-fusion-temperature coal by bean straw addition. Energy Fuels 32, 6678–6688. Mäkelä, M., Yoshikawa, K., 2016. Ash behavior during hydrothermal treatment for solid fuel applications. Part 2: effects of treatment conditions on industrial waste biomass. Energy Convers. Manage. 121, 409–414. Nishiguchi, S., Tabata, T., 2016. Assessment of social, economic, and environmental aspects of woody biomass energy utilization: direct burning and wood pellets. Renew. Sustain. Energy Rev. 57, 1279–1286. Niu, Y., Du, W., Tan, H., Xu, W., Liu, Y., Xiong, Y., Hui, S., 2013. Further study on biomass ash characteristics at elevated ashing temperatures: the evolution of K, Cl, S and the ash fusion characteristics. Bioresour. Technol. 129, 642–645. Niu, Y., Tan, H., Hui, S., 2016. Ash-related issues during biomass combustion: Alkaliinduced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures. Prog. Energy Combust Sci. 52, 1–61. Nunes, L.J.R., Matias, J.C.O., Catalão, J.P.S., 2016. Biomass combustion systems: A review on the physical and chemical properties of the ashes. Renew. Sustain. Energy Rev. 53, 235–242. Regueiro, A., Patiño, D., Granada, E., Porteiro, J., 2017. Experimental study on the fouling behaviour of an underfeed fixedbed biomass combustor. Appl. Therm. Eng. 112, 523–533. Reinmöller, M., Schreiner, M., Guhl, S., Neuroth, M., Meyer, B., 2018. Formation and transformation of mineral phases in various fuels studied by different ashing methods. Fuel 202, 641–649. Sarker, S., Arauzo, J., Nielsen, H.K., 2015. Semi-continuous feeding and gasification of alfalfa and wheat straw pellets in a lab-scale fluidized bed reactor. Energy Convers. Manage. 99, 50–61. Sikarwar, V.S., Zhao, M., Clough, P., Yao, J., Zhong, X., Memon, M.Z., Shah, N., Anthony, E.J., Fennel, P.S., 2016. An overview of advances in biomass gasification. Energy Environ. Sci. 9, 2939–2977. Stanislav, V.V., David, B., Lars, K.A., Christina, G.V., 2013. A overview of the composition and application of biomass. Part 1. phase-mineral and chemical composition and classification. Fuel 105, 40–76. Steer, J., Marsh, R., Griffifiths, A., Malmgren, A., Riley, G., 2013. Biomass co-fifiring trials on a down-fifired utility boiler. Energy Convers. Manage. 66, 285–294. Varol, M., Atimtay, A.T., 2015. Effect of biomass-sulfur interaction on ash composition and agglomeration for the co-combustion of high-sulfur lignite coals and olive cake in a circulating fluidized bed combustor. Bioresour. Technol. 198, 325–331. Wang, Q., Han, K., Li, H., Qi, J., Lu, C., 2015. Influence of ammonium dihydrogen phosphates additive on potassium fixation capacity and ash fusibility for rice straw combustion in an O2/CO2 atmosphere. J. Fuel Chem. Technol. 43, 955–960. Wang, Q., Han, K., Wang, J., Gao, J., Lu, C., 2017. Influence of phosphorous based additives on ash melting characteristics during combustion of biomass briquette fuel. Renew. Energy 113, 642–645. Wang, Q., Han, K., Qi, J., Zhang, J., Li, H., Lu, C., 2018. Investigation of potassium transformation characteristics and the influence of additives during biochar briquette combustion. Fuel 222, 407–415. Wang, L., Hustad, J.E., Skreiberg, Ø., Skjevrak, G., Grønli, M., 2012. A critical review on
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Fe2O3(s) + SiO2(s) + KCl (g) + H2O(g) → [K(FeSi2O6)](s) + HCl (g) (16) NH4PO3(s) + CaO (s) + K2O(s) → KCaP4O12(s) + NH3 (g) + H2O (g) (17) 4. Conclusions The PRRs of mixed WS ashes increased with increasing three additive mass ratios in an oxidizing atmosphere (O2/CO2, 3:7, volume ratio). The mass ratios of three additives were < 1.0% when the WS ash reached an appropriate PRR. The PRR increase of WS mixture showed a similar trend (rapid increase (< 12%) and then a slow increase (12%–15%)). For KL or JFA addition, the formations of K–Al/Fe silicates through the interactions of alumina, magnesia, and sylvite prompted an increase in PRR WS mixture. For ADP, the generation of K–Ca/Mg phosphate increased their PRR. CRediT authorship contribution statement Fenghai Li: Writing - original draft, Methodology, Software. Xiaochuan Wang: Conceptualization, Investigation. Chaoyue Zhao: Visualization, Investigation. Yang Li: Validation, Data curation. Mingxi Guo: Writing - review & editing, Visualization. Hongli Fan: Software, Investigation. Qianqian Guo: Resources, Formal analysis. Yitian Fang: Supervision. Acknowledgements This work was financially supported by the Natural Science Foundation of China (21875059, 51504166), the Natural Science Foundations of Shandong Province, China (ZR2018MB037, ZR2017BB063), and the Fundamental Research Funds for the Central Universities (2042017KF0227). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122515. References Bostrom, D., Eriksson, G., Boman, C., Ohman, M., 2009. Ash transformations in fluidizedbed combustion of rapeseed meal. Energy Fuels 23, 2700–2706. Boström, D., Skoglund, N., Grimm, A., Boman, C., Öhman, M., Broström, M., Backman, R.,
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stalk coke. Bioresour. Technol. 270, 416–421. Zhang, H., Li, J., Guo, S., Wang, Z., Zhang, Y., Fang, Y., 2018a. Influence of coal ash on potassium retention and ash fusibility during gasification of corn stalk coke. J. Fuel Chem. Technol. 46, 1055–1062. Zhou, H., Luo, Z., Liu, D., Ma, W., 2019b. Effect of biomass ashes on sintering characteristics of high/low melting bituminous coal ash. Fuel Process. Technol. 189, 62–73. Zhou, A., Xu, H., Tu, Y., Zhao, F., Zheng, Z., Yang, W., 2019a. Numerical investigation of the effect of air supply and oxygen enrichment on the biomass combustion in the grate boiler. Appl. Therm. Eng. 156, 550–561. Zhu, Y., Hu, J., Yang, W., Zhang, W., Zeng, K., Yang, H., Du, S., 2018. Ash fusion characteristics and transformation behaviors during bamboo combustion in comparison with straw and poplar. Energy Fuels 32, 5244–5251.
additives to reduce ash related operation problems in biomass combustion applications. Energy Procedia 20, 20–29. Wang, L., Skreiberg, Ø., Becidan, M., 2014. Investigation of additives for preventing ash fouling and sintering during barley straw combustion. Appl. Therm. Eng. 70, 1262–1269. Wang, L., Skreiberg, Ø., Becidan, M., Li, H., 2016. Investigation of rye straw ash sintering characteristics and the effect of additives. Appl. Energy 162, 1195–1204. Xiong, Q., Li, J., Guo, S., Li, G., Zhao, J., Fang, Y., 2018. Ash fusion characteristics during co-gasification of biomass and petroleum coke. Bioresour. Technol. 257, 1–6. Xu, J., Yu, G., Liu, X., Zhao, F., Chen, X., Wang, F., 2014. Investigation on the hightemperature flow behavior of biomass and coal blended ash. Bioresour. Technol. 166, 494–499. Zhang, H., Li, J., Yang, X., Guo, S., Zhan, H., Zhang, Y., Fang, Y., 2018b. Influence of coal ash on potassium retention and ash melting characteristics during gasification of corn
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Influence of additives on potassium retention behaviors during straw combustion: A mechanism study
T
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Fenghai Lia,b,c,d, Xiaochuan Wangb, , Chaoyue Zhaoc, Yang Lic, Mingxi Guoa, Hongli Fana, Qianqian Guoa, Yitian Fangd a
School of Chemistry and Chemical Engineering, Heze University, Heze 274015, China School of Power and Mechanical Engineering, Wuhan University, Wuhan, Hubei 430072, China c School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454003, China d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China b
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Potassium retention ratio Wheat straw ashes Oxidizing atmosphere Additive Variation mechanism
The potassium retention behaviors of wheat stalk and its modification mechanisms in an oxidizing atmosphere by three additives [kaolinite, fly ash from Jincheng coal fluidized-bed gasification (JFA), and ammonium dihydrogen phosphate (ADP)] were investigated. The potassium retention ratios (PRRs) increased with increasing additive mass ratio. The ADP was the optimal selection for wheat stalks combustion when the three additives at the same mass ratio because of its highest PRR and a slow decrease in PRR with an increase in temperature. With the increasing three additive mass ratios, the PRRs of their mixed ashes showed a similar trend [rapid increase (< 12%) and then a slow increase (12%–15%)]. The mass ratios of three additives were all < 1.0% and their mixed ash reached an appropriate PRR. For kaolinite or JFA, the formations of K–Al/Fe silicates prompted an increase in the PRR. For ADP, K–Ca/Mg phosphate generations increased the PRR.
1. Introduction To mitigate increasing environmental pollution and rapid reduction in fossil fuel reserves, it is necessary to apply renewable energy (Fang and Jia, 2012; Xu et al., 2014; Zhou et al., 2019a,b). Recently, increased
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interest has been given to biomass worldwide because it is carbon neutral (Xiong et al., 2018), has a widespread distribution, is low cost (Nishiguchi and Tabata, 2016; Sikarwar et al., 2016; Wang et al., 2017), and is considered as potential energy to provide combined heat and power on a small or medium scale. In China, the installed capacity of
Corresponding author at: School of Power and Mechanical Engineering, Wuhan University, Wuhan, Hubei 430072, China. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.biortech.2019.122515 Received 23 September 2019; Received in revised form 26 November 2019; Accepted 27 November 2019 Available online 29 November 2019 0960-8524/ © 2019 Published by Elsevier Ltd.
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Limited systematic investigations have been conducted into potassium retention behaviors and the modification mechanism by different additives. Differences in straw ash composition and complexities in mineral interaction, the regulation behaviors, and the effecting mechanism of the additives on the straw-fixed potassium have not been clarified and need to be investigated further. The objectives of this paper were to investigate the influence of additives on the potassium retention behaviors and to explore the variation mechanism during WS combustion. Basic data and theoretical support is expected to mitigate the occurrence of ash-related issues during straw use.
biomass power will reach 30 GW by 2020 (Niu et al., 2013). Combustion or co-firing is regarded as an attractive and feasible method to use biomass as energy (Varol and Atimtay, 2015). Biomass tends to have a high alkali content (e.g., potassium and sodium), which makes its ashfusion temperature (AFT) low. These properties may lead to the occurrence of ash-related issues (e.g., fouling, agglomeration, slagging, and corrosion) (Link et al., 2018; Regueiro et al., 2017) during combustion, which results in a decrease in combustion efficiency, instability, and even unscheduled system shutdowns (Li et al., 2019). Among major forming ash elements in biomass, the potassium content indicates a potential ash fusion and deposition in conversion applications through vaporization and condensation (Wang et al., 2014). Thus, the mobility of potassium in combustion systems is a key factor that dictates ash behavior (Clery et al., 2018; Regueiro et al., 2017). During combustion, some elemental potassium converts to the gaseous phase (such as KOH, and KCl), and then reacts with chlorine or sulfide in the flue-gas to form hydrochloride or sulfate with a low melting point (MP). These low MP salts accelerate pipeline fouling and corrosion, and decreases boiler availability (Dirbeba et al. 2017; Niu et al., 2016; Zhang et al. 2018a). Alkali elements in the bottom ash and fly ash are mostly in the form of low MP silicates (e.g., K2SiO3 and K2Si4O9) and their eutectics, which may result in slag formation (Wang et al., 2015; Jobansen et al., 2017). With an increasing requirement for woody biomass to produce biomass fuels, extensive attention has been given to straw combustion or gasification to produce electricity, synthetic natural gas, and chemicals (Zhang et al., 2018b), because straw accounts for ~50%–60% of the total biomass in China from an energy perspective, and straw does not compete with land for food production requirements (Jiang et al., 2012; Mäkelä and Yoshikawa, 2016). However, straw tends to contain a higher alkali content (especially for potassium (K)), silicon, and ash content compared with woody biomass) (Li et al., 2020; Steer et al., 2013). The high potassium content decreases the AFT, which may lead to operational difficulties (Sarker et al., 2015). Thus, the investigation of potassium transformation behavior is important for straw thermal conversion. Potassium migration is dependent mostly on feedstock type and treatment conditions. The potassium emission profile is related closely to the K/Cl or K/(Si + Al) ratio in ash composition: high chlorine and/ or low (Si + Al) content promotes release as potassium chloride (KCl) or potassium hydroxide (KOH); whereas high (Si + Al) helps to retain K in the solid phase (fly or bottom ash) (Clery et al., 2018). Potassiumcontaining salt formations (e.g., KCl and K2SO4) are the main substances that influence ash-related issues during straw conversion (Nunes et al., 2016). Additive is a promising method in industrial practice to mitigate ash-related issues because it captures problematic ash species (e.g., gaseous phase KCl, KOH) by adsorption, and improves ash-fusion characteristics (Wang et al., 2015). Thus, it is very important in the selection of suitable additives with characteristics of a decrease in K release and an improvement in AFT for straw conversion on a large scale. Most investigations have focused on the effects of additives on ashfusion characteristics (Li et al., 2018; Liu et al., 2017; Wang et al., 2016). The influence of additives on potassium retention is low. Potassium fixation of maize straws and cotton stalks with two phosphorusbased additives was investigated and the influence of NH4H2PO4 on the potassium fixation ability was better than that of Ca(H2PO4)2 (Wang et al., 2017). Coal ash is a potential additive for easy-to-slagging biomass because it fixes potassium, and increases the AFTs of mixtures (Zhang et al., 2018b). However, these investigations are also related to the variation of AFT. For example, the performance of Ca(H2PO4)2 on the improvement of AFT was better than NH4H2PO4 because of the formations of high-MP potassium-calcium phosphates, i.e., K2CaP2O7, K2CaP4O12, and K4Ca(PO4)2 (Wang et al., 2017). The AFTs of the blended ash increased through coal ash addition because of the generations of high MP potassium aluminosilicates (Zhang et al., 2018b).
2. Materials and methods 2.1. Selections of samples Wheat is a type of gramineae plant that is planted worldwide, and is the second output grain crop in the world. Chinese wheat production is the largest in the world, and accounts for 17% of global production, which leads to a large amount of wheat stalk (WS) (wheat byproduct). Thermal conversion is the main method to use WS (Li et al., 2019). Airdried WS was selected from the countryside in Dongming, Shangdong, China. Samples were milled to 0.180–0.250 mm and stored in a cabinet dryer.
2.2. Preparation of ash samples 2.2.1. Preparation of WS ash The preparations of WS ash sample were as follows: the WS samples were placed in a muffle furnace (SX2-8-16ASP, Kewei Co., Beijing, China), and the temperature was increased from room temperature to 450 °C within 30 min, and then the muffle furnace door was closed and the temperature was maintained for 16 h. The WS ashes were removed, cooled to room temperature, and transferred into a drying cabinet for storage. The 450 °C ashing temperature was selected to prevent K volatilization in WS because elemental K in biomass is almost nonvolatile below 500 °C (Ma et al., 2017; Zhang et al., 2018b). The residual carbon contents of ash samples that were prepared at 450 °C for different ashing times were determined by using SC-444 equipment (Leco Corp., USA)) at 4-h intervals. The decrease in residual carbon content is obvious for ashing times from 4 h to 16 h (4 h: 9.36%; 8 h: 5.12%; 12 h: 2.29; 16 h: 0.75%); the variation of residual content is small for ashing times from 16 h to 20 h (20 h: 0.64%), and residual carbon content was less than 1% when the ashing time was 16 h. Thus, 16 h was selected as the WS ashing time.
2.2.2. Preparation of WS and its mixtures at different temperatures Three additives (kaolinite (KL), fly ash from Jincheng coal fluidizedbed gasification (JFA), and ammonium dihydrogen phosphate (ADP)) were crushed to a particle size < 0.198 mm and placed into the 450 °C WS ashes at mass ratios of 0%, 3%, 6%, 9%, 12%, and 15%, mixed manually and reached uniformity, respectively. The mixed WS ash samples at different temperature were prepared in the fixed-bed tube furnace (Fig. 1). Preparations were conducted according to the following procedure. Samples (450 °C WS ashes and their mixtures) were placed into a ceramic crucible and inserted into a low temperature zone of fixed-bed tube furnace. The oxidizing atmosphere (O2/CO2, 3:7, volume ratio) was introduced into the furnace at 2 L/min to simulate biomass combustion atmosphere (Li et al., 2019), and the furnace was heated at 20 °C/min. After the presetting temperature had been reached, the ceramic crucible was kept at this temperature for 1 h to simulate combustion. The samples were taken out, transferred into liquid-nitrogen cooling equipment, and cooled to room temperature to analyze the variation in potassium content and its mineral composition. 2
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hydrocarbons, carbon oxides, hydrogen, and tar vapors) in air- dried WS was high (72.30%), the content of fixed carbon, ash, and moisture was 15.60%, 7.60%, 3.36%, respectively. The ultimate analysis was performed on a PE 2400 analyzer (PerkinElmer, USA) (GB/T476-2001). The elemental content of carbon, hydrogen, oxygen, nitrogen, and total sulfur based on air-dried WS samples was 52.78%, 7.54%, 38.95%, 0.73%, and 0.85%, respectively. The ash compositions (prepared at 450 °C) measured by an ICP-AES (icap6300, Thermo Fisher Scientific, USA), in the form of oxides, are shown as (Na2O: 1.24%; K2O:34.56%; MgO: 9.22%; CaO: 8.64%; Fe2O3: 2.48%; SO3: 3.83%; Al2O3: 1.93%; SiO2:29.13%; Cl2O:7.85%; P2O5: 1.12%). The alumina content was low, whereas the contents of K and chlorine (Cl) were relatively high. Cl in biomass ash was volatilized mostly as sylvite (KCl), which promoted K volatilizes during straw thermal conversion (Ma et al., 2018). As shown above, compared with other straws, the WS had a moderate potassium content (generally from 2.29% to 63.90% (Stanislav et al., 2013)), a relatively high Cl content, and a low aluminum content. This ash composition reflected the characteristics of straw ashes. The WS had a wide distribution and was abundant globally. Thus, the WS can be regarded as a typical straw. Among the four category additives to improve the AFTs during biomass conversion (aluminum silicate-, sulfur-, calcium-, and phosphorus-based additives), the product of the sulfur-based additives during biomass combustion was mainly K2SO4. Its MP remained low and SO2 emission could not be ignored (Wang et al., 2017); the calciumbased additive tended to promote the release of potassium in biomass ashes at a high temperature because its ionic potential (20 nm−1) was higher than that of K+ (7.5 nm−1), and results in gradual replacement of K+ in silicates by Ca2+ (Li et al., 2019). Thus, aluminum silicate- and phosphorus-based additives were selected. Analytical pure kaolinite (Al2Si2O5(OH)4, KL) and ammonium dihydrogen phosphate (NH4H2PO4, ADP) were products of Kemeo Chemical Regent Co. Ltd (Tianjin, China). However, it might be expensive to buy these pure additives for biomass power plants, and residual materials with highadditive-composition content were economically attractive. Ash-agglomerate fluidized-bed (AFB) is considered one of the most promising technologies for three high coal [(high AFT, high ash content, and high sulfur content), is distributed widely in north and south-west China with reserves of ~62 billion tons, and accounts for ~25% total Chinese reserves] because of its wide fuel flexibility, uniform bed temperature, and environmental friendliness (Wang et al., 2012). The fly ash from AFB gasification of high AFT coal tended to contain high aluminum silicon content. Coal ash could absorb potassium (Liu et al., 2017). Thus, fly ash from Jincheng coal (Shanxi, China) AFB gasification (JFA, prepared based on the ASTM E1755-2001 standard) was also selected as an additive. The measured JFA compositions based on WS analytical methods are listed as (SiO2:44.25%; Al2O3:33.46%; CaO: 9.46%; Fe2O3:8.37%; SO3:1.68%; MgO: 1.24%; P2O5: 1.03%; K2O: 0.27%; Na2O: 0.12%; TiO2: 0.12%). As could be seen clearly that the contents of silicon and aluminum exist in the JFA were abundant.
Fig. 1. Schematic diagram of fixed-bed tube furnace. Legend: 1 oxygen gas cylinder; 2 carbon dioxide cylinder; 3 gas valve; 4 mass flow meter; 5 moving sample injector; 6 sealing element; 7 stainless steel tube; 8 electric heater; 9 temperature controller; 10 thermoelectric couple.
2.3. Experimental measurement 2.3.1. Potassium content in the ash samples The elemental potassium contents in ash samples were tested by using inductively coupled plasma atomic emission spectrometry (ICPAES, icap 6300, Thermo Fisher Scientific, USA). The ICP-AES with a high precision and high sensitivity at a high frequency power (1150 W) and 0.65 L/min gas velocity, could detect the concentrations from per million to percent levels. 2.3.2. X-ray powder diffractometer (XRD) measurements An XRD (D/max-rB, Rigaku Co., Japan) with Cu Kα radiation (40 kV, 100 mA) with software package MDI Jade 6.5 was used to test the mineral compositions of the ash samples. The samples were scanned at 5° 2θ/min from 10° 2θ to 75° 2θ with a step size of 0.01°. The semiquantitative normalized reference intensity ratio (RIR) method with a precision of approximately ± 10% (strong diffraction phase) or ± 25% (weak diffraction phases) was used to determine their mineral content (Huang et al. 2018). Amorphous matter content resulted from the difference between total crystalline phase content and ash bulk chemical composition (Li et al., 2020). 2.4. Potassium retention ratio The ash samples in the corundum boat that had been prepared in the fixed bed tube were removed and dissolved based on Chinese standard (MT/T1014-2006) to obtain an acid solution. The acid solution was diluted in a 100-mL volumetric flask with deionized water. ICP-AES was used to measure the potassium content [(Ci, Cc, and C0 (blank test)] in acid solution. To ensure the experimental data accuracy, each sample was measured three times and its RSD was within 2.5% in all cases. Because the K2O content in JFA was < 0.5% and the additives in the total ash were less than 15%. The ratios of potassium from JFA in the samples were < 0.1% and could be neglected. Therefore, the PRR can be calculated based on (Guo et al., 2017):
PRR% = (Ci − Co)/(Cc × (1 − W) i × 100%
3.2. Effects of temperature and additive mass ratio on PRR variation
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3.2.1. WS PRR variation with increasing temperatures To explore the influence of additive on PRR and the effects of temperature on potassium release, in consideration of the WS mixture PRR variation with increasing three additive mass ratio (see Section 4.2.2), the ash samples of original WS ashes and mixtures with 6.0% three additives (mixed until uniform) were selected. These ash samples were prepared at different temperatures (750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) in the fixed-tube furnace. The temperatures were selected based on the operating temperature range of the biomass boiler. The potassium contents of the samples were determined by ICP-AES and are presented in Fig. 2. The WS PRR decreased obviously (from 750 °C to 900 °C) and then slowly (900 °C to 1100 °C) with an increase in temperature. During WS
where ΦK is the PRR of the sample, %; Ci is the potassium content in the ash sample, mg/L; Co is the potassium content in the blank test, mg/L; Cc is the potassium the content in original WS, mg/L; and Wi is the mass fraction of additives, %. 3. Results and discussions 3.1. Characteristics of CS and additives The proximate analysis of CS was conducted on a SDLA 718 proximate analyzer (SUNDY Co. Ltd., Changsha, China) (GB/T28731-2012) (Li et al., 2018). The volatile matter content (mostly composed of 3
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Fig. 2. Variation in PRR of WS and its mixtures with 6.0% three additives at different temperatures.
Fig. 3. Potassium retention variation with increasing three additive mass ratios at 950 °C.
ash heating, a large quantity of potassium was released in the form of a sylvite gaseous phase at a relatively low temperature (700–900 °C) in an O2/CO2 mixture atmosphere (Knudsen et al., 2014; Johansen et al., 2011). Some pores were covered by melting glasslike products with a further temperature increase (900–1100 °C). The covering blocked the release of potassium and resulted in a slow decrease in PRR (Wang et al., 2017). The PRR of mixed WS ashes with three additives increases for all temperatures investigated compared with WS. Among the three additives, ADP was optimal for potassium retention because of its highest PRR among the three additives and the PRR decreased slowly with an increase in temperature. ADP addition could inhibit the release of alkali metals even at a relatively high temperature, and it could fix potassium in the bottom ashes because of the formations of potassium metaphosphate (e.g., KPO3) and potassium phosphate (K2CaP2O7) (Wang et al., 2015). KL and JFA both had good fixed potassium ability at a relatively low temperature (800–900 °C). However, with an increase in temperature, its fixed potassium ability decreased slowly because the porosity was covered by a low-MP compound (Reinmöller et al, 2018). A reduction in the active surface areas of KL particles occurred and metakaolin converted to amorphous silica and alumina–silica spinel above 950 °C, which might convert to pseudo-mullite at 1000 °C with a low potential to react with KCl (Li et al., 2018). The potassium fixing ability of KL and JFA is almost the same from 750 °C to 1100 °C. However, the PRR of the WS with KL addition decreased more rapidly with an increasing temperature than that of JFA for two additives at the same ratio.
that of WS + KL for additives less than 9% and increased for additives above 9%. This behavior resulted from that silicon and aluminum were the keys to capture potassium in the solid/slag phase. For KL, the total contents of silicon (SiO2) and aluminum (Al2O3) accounted for 83.93%, whereas in JFA, the total contents of silicon and aluminum reached 77.71%. This difference resulted in a lower PRR of WS + JFA than that of WS + KL initially (< 9%). As the additive mass ratio increased further (> 9%), although the total mass ratio of silicon and aluminum in their additives did not change, the iron (Fe2O3) content in the WS + JFA increased (JFA contained 8.37% Fe2O3), and gradually fixed potassium (indicated by the formation of leucite (ferric) [K(FeSi2O6)]. Potassium may react with P2O5, CaO or MgO (from JFA) and generate K2CaP2O7 or KMgPO4, which caused the PRR of the WS + JFA to increase and exceed that of the WS + KL. Even when the mixture PRR reached 90%, as shown in Fig. 3, the mass ratio for the three additives was ~9.2% (ADP), 10.2% (JFA), and 11.3% (KL), respectively. Based on the WS ash content, the ratio of the three additives was 1:130, 1:116, and 1:103, correspondingly. The ratios of the three additives were < 1.0%, and they provided promising for use in straw combustion. As shown in Fig. 3, the fixed potassium efficiency of the ADP was the highest in the three additives. However, JFA might be a better selection for an industrial application, because JFA was cheap (it belongs to gasification residuals, is abundant, and is readily available) and environmentally friendly (residual recycling use) (Zhang et al., 2018a).
3.3. Mineral transformation of WS with increasing temperature
3.2.2. PRR variation with increasing additive mass ratio The operating temperature range of the biomass boiler varies from 800 °C to 850 °C, and the temperature range of the boiler was 100–200 °C higher than its operating temperature (Li et al., 2019). To explain the influence of additive mass ratio on the potassium retention of biomass during combustion, the ash samples of WS and its mixture were prepared at 950 °C in a fixed-bed tube furnace in an oxidizing atmosphere (Fig. 1), and their PRRs were calculated based on Eq. (1) with an increase in the three additives, as presented in Fig. 3. At 950 °C the PRR of the WS mixture increased with an increase in mass ratio of additives and showed a similar trend for three additives (increases rapidly [ < 12%) and then slowly (12%–15%)] (Fig. 3). The PRR of the ADP additive was highest in the three additives when they had the same mass ratio because the reactivity of Si or Al with K is lower than that of P (their ionic potentials decrease as P5+(147 nm−1) > Si4+ (95 nm−1) > Al3+(59 nm−1)) (Boström et al., 2012; Zhu et al., 2018). The PRR of WS + JFA was lower than
To investigate the PRR variation mechanism with an increase in additive mass ratio, the mineral evolution in WS ashes with an increase in temperature must be investigated. The WS ash samples at different temperatures (575, 750, 850, 950, and 1050 °C) in an oxidizing atmosphere were prepared in the fixed bed furnace, and their mineral phase compositions were measured by XRD. At 575 °C the minerals in the WS ashes were made of sylvite, potassium carbonate, quartz, goothite (α-FeO(OH)), and potassium sulfate (K2SO4). As the temperature increased, the sylvite contents transferred into the gaseous phase (Li et al. 2018), and potassium carbonate and quartz decreased; whereas the content of amorphous matter increased gradually. The potassium silicate (K2SiO3), monticellite (CaMgSiO4), kalsilite (KAlSiO4), calcium iron oxides (CaFe2O4), and forsenite (Mg2SiO4) were generated at 800 °C, and anorthite was formed at 950 °C. The reaction during WS ashes heating could be deduced and is listed as follows (Li et al., 2018; 4
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Phosphorus has priority over silicon of reactivity because the ionic potential of Si4+ (95 nm−1) was lower than that of P5+ (147 nm−1), thus K–Mg/Ca phosphates were formed before the corresponding K–Mg/Ca silicates (Bostrom et al., 2009). With ADP addition, potassium tended to react with ADP to produce potassium polymetaphosphate (KPO3) (700 °C (Wang et al., 2015)), and then might change to calcium or magnesium potassium phosphates with an increase in temperature (Wang et al., 2017). The generation of potassium magnesium phosphate (KMgPO4) resulted from the interaction of magnesium oxide and ADP. The derived reactions are listed as follows (Wang et al., 2015; Wang et al., 2017).
Wang et al., 2018; Zhou et al., 2019a,b): K2CO3 (s) + SiO2(s) → K2SiO3(s) + CO2 (g)
(2)
CaO(s) + MgO(s) + SiO2(s) → CaMgSiO4 (s)
(3)
KCl (g) + SiO2(s) + Al2O3(s) + H2O (g) → KAlSiO4 (s) + HCl (g) (4) MgO(s) + SiO2(s) → Mg2SiO4(s)
(5)
CaO(s) + α-FeO(OH)(s) → CaFeO4(s) + H2O(g)
(6)
CaO(s) + SiO2(s) + Al2O3(s) → CaAl2Si2O8(s)
(7)
3.4. Investigation of the influence mechanism of additive on WS PRR 3.4.1. Exploration of the influence mechanism of additive kind on PRR To explore the influencing mechanism of the three additives on the PRR during WS combustion, WS ash mixtures with three 6% additives at 950 °C were prepared in the fixed-bed furnace. The mineral phase composition of these mixed samples in an oxidizing atmosphere (O2/ CO2, 3:7, volume ratio) was determined by XRD. The minerals in the WS ashes at 950 °C were mostly in the form of sylvite, potassium carbonate, anorthite, potassium sulfate, forsterite, and leucite. In the WS ash samples with 6% KL, kalsilite formed (quartz and alumina derived from KL decomposition) and the leucite content increased. The mineral variation in mixed WS ashes by JFA addition was almost the same as KL addition because of the similarity in their main composition (Al2O3 and SiO2) content. Because of the high contents of silicon and aluminum in KL or JFA (Table 1), which are key compositions to capture potassium in the solid/slag phase (Clery et al., 2018; Liu et al., 2017), the contents of silicon and aluminum increased with an increase in KL or JFA mass ratio. The silicon and aluminum reacted with gaseous sylvite and resulted in high MP kalsiliate through Eq. (4). This reduced the sylvite gaseous concentration, retarded the release of potassium, and increased the PRR.
KAlSiO4(s) + SiO2(s) → KAlSi2O6(s)
(8)
NH4H2PO4(s) → NH4PO3(s) + H2O(g)
(9)
NH4PO3(s) + KCl(g) → NH4Cl(g) + KPO3(s)
(10)
KPO3(s) + CaO(s) → KCaPO4
(11)
KPO3(s) + MgO(s) → KMgPO4
(12)
NH4PO3(s) + KCl(g) + CaO(s) → K2CaP2O7(s) + NH4Cl(g)
(13)
NH4PO3(s) + KCl(g) + MgO(s) → KMgPO4(s) + NH4Cl(g) 2+
(14) 2+
Because of the ionic potential difference of Ca and Mg (Ca2+: 2.0; Mg2+: 3.0), Eq. (12) took precedence over Eq. (11). Moreover, the magnesium-oxide content was higher than that of calcium oxide (CaO: 8.64%; MgO: 9.62%) in WS ash, which resulted in KMgPO4 formation. 3.4.2. Effecting mechanism of additive mass ratio on PRR The migration and transformation of elements depends on a variation of the existing state of the elements or mineral compositions at certain conditions, which can be detected by XRD. In consideration of the ash composition similarity in KL and JFA, JFA was used as an example. The mineral phase composition of WS and its mixtures with different mass ratios of JFA and ADP at 950 °C was determined by XRD,
Table 1 Mineralogical composition contents of WS ashes and its mixed with 6% additives at 950 °C. Ash samples
WS WS WS WS WS WS WS WS WS WS WS
+ + + + + + + + + +
3%JFA 3%ADP 6%JFA 6%ADP 9%JFA 9%ADP 12%JFA 12%ADP 15%JFA 15%ADP
Ash samples
WS WS WS WS WS WS WS WS WS WS WS
+ + + + + + + + + +
3%JFA 3%ADP 6%JFA 6%ADP 9%JFA 9%ADP 12%JFA 12%ADP 15%JFA 15%ADP
Mineral (wt%) sylvite
potassium carbonate
anorthite
potassium sulfate
forsterite
leucite
kalsilite
14.37 12.15 11.87 10.42 9.95 8.37 7.80 6.24 5.28 3.15 3.08
4.63 2.31 – – – – – – – – –
18.19 16.71 16.24 14.53 13.29 12.38 12.39 10.72 10.04 7.26 7.98
6.27 6.19 6.15 6.12 3.20 5.79 2.58 4.16 2.10 3.92 1.98
7.82 7.63 7.73 7.54 4.32 5.81 4.01 5.06 3.78 4.97 2.99
5.87 7.24 7.08 8.26 9.62 7.34 8.35 6.97 7.75 9.24 6.09
– 8.38 – 16.89 – 22.37 – 24.94 – 28.36 –
monticellite
potassium feldspar
leucite, ferric
calcium potassium pyrophosphate
potassium magnesium phosphate
calcium potassium meta-phosphate
amorphrous matter
– – – – – 3.24 – 4.36 – 5.97 –
– – – – – – – 2.17 – 4.08 –
– – – – – – – 5.24 – 7.82 –
– – 7.24 – 13.37 – 18.32 – 20.15 – 24.58
– – 5.18 – 9.63 – 12.38 – 14.85 – 18.37
– – – – – – – – 5.28 – 8.24
42.85 39.49 38.51 37.31 36.62 34.70 34.17 31.14 30.77 27.34 26.69
Mineral (wt%)
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and their calculated mineral composition based on the RIR method is presented in Table 1. For JFA addition, when the JFA mass ratio was 3%, kalsilite formed through Eq. (4). Monticellite was generated when the JFA was 6% by Eq. (3) because of its high calcium content (CaO: 9.62%). With an increase in JFA mass ratio, the silicon and iron contents increased, which led to the generation of potassium feldspar (KAlSi3O8) and leucite (ferric) ([K(FeSi2O6)]) (Clery et al., 2018). These results might explain the trend of increasing WS PRR with increasing JFA mass ratio. Phosphorous mineral reacts preferentially with potassium before K-carbonate, K-sulphate, and K-chloride formation (Wang et al., 2017). Thus, with ADP addition, the potassium carbonate disappeared rapidly (ADP mass ratio < 3%), and the contents of K-sulfate and K-chloride decreased gradually, accompanied by the formation of high MP potassium phosphates (such as calcium potassium pyrophosphate (Eq. (13)), potassium magnesium phosphate (Eq. (14)), and calcium potassium metaphosphate (KCaP4O12)), and their content increased correspondingly. The reactions with increasing mass ratio of JFA or ADP are presented as follows. KAlSi2O6(s) + SiO2(s) → KAlSi3O8(s)
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Fe2O3(s) + SiO2(s) + KCl (g) + H2O(g) → [K(FeSi2O6)](s) + HCl (g) (16) NH4PO3(s) + CaO (s) + K2O(s) → KCaP4O12(s) + NH3 (g) + H2O (g) (17) 4. Conclusions The PRRs of mixed WS ashes increased with increasing three additive mass ratios in an oxidizing atmosphere (O2/CO2, 3:7, volume ratio). The mass ratios of three additives were < 1.0% when the WS ash reached an appropriate PRR. The PRR increase of WS mixture showed a similar trend (rapid increase (< 12%) and then a slow increase (12%–15%)). For KL or JFA addition, the formations of K–Al/Fe silicates through the interactions of alumina, magnesia, and sylvite prompted an increase in PRR WS mixture. For ADP, the generation of K–Ca/Mg phosphate increased their PRR. CRediT authorship contribution statement Fenghai Li: Writing - original draft, Methodology, Software. Xiaochuan Wang: Conceptualization, Investigation. Chaoyue Zhao: Visualization, Investigation. Yang Li: Validation, Data curation. Mingxi Guo: Writing - review & editing, Visualization. Hongli Fan: Software, Investigation. Qianqian Guo: Resources, Formal analysis. Yitian Fang: Supervision. Acknowledgements This work was financially supported by the Natural Science Foundation of China (21875059, 51504166), the Natural Science Foundations of Shandong Province, China (ZR2018MB037, ZR2017BB063), and the Fundamental Research Funds for the Central Universities (2042017KF0227). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122515. References Bostrom, D., Eriksson, G., Boman, C., Ohman, M., 2009. Ash transformations in fluidizedbed combustion of rapeseed meal. Energy Fuels 23, 2700–2706. Boström, D., Skoglund, N., Grimm, A., Boman, C., Öhman, M., Broström, M., Backman, R.,
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