Article pubs.acs.org/EF
Cite This: Energy Fuels XXXX, XXX, XXX−XXX
Experiment Study on Ash Fusion Characteristics of Cofiring Straw and Sawdust Yiming Zhu,† Hao Zhang,‡ Yanqing Niu,*,†,‡ Haitao Xu,† Xiao Zhang,† Shi’en Hui,‡ and Houzhang Tan† †
Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, and ‡State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China ABSTRACT: Considering the remedy of cofiring on biomass slagging and less concerns on different biomass cofiring, the effects of blending ratio of straw (a representation of agricultural biomass) and sawdust (a representation of woody biomass) during cofiring, as well as CaO, MgO, SiO2, and K2O additives, on ash fusion characteristics are studied using sintering tester, XRF, XRD, and SEM. The ash fusion temperatures (AFTs) of straw are significantly lower than those of sawdust due to higher K2O, SiO2, and Cl, and lower CaO and MgO contents in the straw ash, consequently, the AFTs decrease with increased straw/sawdust blending ratio because of the formation of low-melting K2SO4, NaCl, CaSi2, K2MgSi3O8, Na2Si2O5, and K2Si4O9. However, attributed to the dominating straw ash content and components in mixtures, the AFTs keep unchanged with increasing blending ratio above 3:2. SiO2 and K2O improve the generation of K2O·nSiO2, KCl, and K2SO4 with low-melting temperature, thus lowering AFTs and worsening slagging; Whereas CaO and MgO improve the formation of high-melting silicates such as Ca2SiO4 and CaMgSiO4, thus rising AFTs and weakening slagging. There exists a transformation from chemical reactions to physical addition with increased dosages of CaO and MgO. To ease fusion slagging of agricultural residues during combustion, the cofired woody biomass should account for at least 40 wt %. These general results are useful for biomass-fired power plants arranging the cofiring of woody biomass and agricultural residues.
1. INTRODUCTION Biomass has attracted worldwide attention due to the worsening energy crisis and environmental issues. In China, the biomass power installed capacity will increase to 30 GW by 2020, and accounts for 3% of the national installed capacity.1 Although its well-known advantages such as high regeneration potential and carbon dioxide neutrality encourage the rapid development of biomass-fired power plants, biomass combustion attributed to high concentrations of Cl and alkali metals (K and Na, especially K), whether in fluidized beds or grate furnaces, remains a challenge for slagging, corrosion, ash utilization, etc.2−4 Those result in a decrease in boiler efficiency and operation safety. High concentrations of Cl and K in biomass result in unmanageable deposits on the fired surfaces, particularly alkaliinduced slagging on the superheater2,5−7 and high-temperature silicate melt-induced slagging on water cooler,7 which inhibit heat transfer and reduce boiler efficiency. Furthermore, the deposits with a high Cl concentration on tube surfaces lead to corrosion.3,8 Up to now, some sporadic results regarding slagging have reached consensus, such as the functions of Cl and K,2,9 the formation and growth mechanisms of alkaliinduced slagging,2,9 agglomeration mechanisms,2,4 and some evaluation criteria (for example, (K+Na)/(Ca+Mg),10 (Cl +K 2 O+Na 2 O)/(SiO 2 +Al 2 O 3 ), 9 (S v o l a t i l e +K 2 O+Na 2 O)/ (SiO2+Al2O3),9 and S/Cl11). However, being different from coal, biomass has various sources including woody, agricultural, waste, and excrement. Woody biomass is low in Si and K, yet high in Ca; agricultural residues are high in Si and K, yet low in Ca; animal residues are high in both P and Ca.12,13 Even for a © XXXX American Chemical Society
specific biomass, different planting environments and harvest seasons can produce distinct ash contents and compositions.2,9 Some additives (such as metal oxides, 7,14 alkali earth metals,15,16 kaolin,7,15−18 lime or calcite,16,19 zeolites,20 and coal21−23) are adopted for K-capture or elevation of the ash fusion temperatures (AFTs) to avoid or ease slagging; however, several additives with high Si/Al ratio may pull down the AFTs and result in low-temperature silicate melt-induced slagging.2,20,24 The author found that high-temperature silicate melt-induced slagging showed ‘V’ shaped distribution with increased SiO2, Al2O3, and K2O content in ash.7 Meanwhile, the silicate melt-induced slagging is closely related to the initial deformation temperature (IDT)18,25 and fluidized temperature (FT).7 In comparison with additives that bring huge extra costs on additive purchase, biomass cofiring in coal-fired boilers is a promising biomass utilization approach.2 The ash-related problems with biomass in general can be reduced by cofiring with coal because of positive interactions between clay minerals and/or sulfur in coal and alkali in biomass (especially for agricultural residues).2,22,26 Thus, biomass cofiring in existing pulverized coal fired boilers has been widely implemented in Europe and the USA.27 Fang and Jia21 reported that the AFTs, changing as “V” shape, first decrease and then increase with increasing content of the corn straw blended into bituminous coal; however, a majority of studies agreed that the fouling Received: October 12, 2017 Revised: November 10, 2017
A
DOI: 10.1021/acs.energyfuels.7b03104 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Ultimate Analysis and Proximate Analysis of Sawdust and Straw, wt %a proximate analysis
a
ultimate analysis
biomass
Mar
Aar
Var
FCar
Car
Har
Oar
Nar
Sar
Clar
straw sawdust
8.11 8.10
7.03 0.94
67.45 74.94
17.41 16.02
41.78 46.03
4.63 5.05
37.19 39.31
1.14 0.53
0.12 0.03
0.395 0.038
Subscript ar denotes as-received basis. milled and sized below 200 μm. To avoid K and Cl losses, the ashing is conducted at 600 °C in a muffle furnace for 2.5 h.34 Six blending ratios of straw/sawdust (0:5, 1:4, 2:3, 3:2, 4:1, and 5:0) are designed. Meanwhile, to further elucidate the mechanisms of biomass cofirng on AFTs, varied dosages of additives, including CaO, MgO, SiO2, and K2O, are added into the biomass fuel by mechanical mixing before ashing. According to the ash composition analysis of 30 biomass in the literature,18 the contents of CaO, MgO, SiO2, and K2O in both straw and sawdust ash are formulated in range of 15−46 wt %, 10−25 wt %, 4.5−30 wt %, and 22−42 wt % by addition of Ca(OH)2, MgO, SiO2, and KCl-based element conservation, respectively. 2.2. Experiment Apparatus. The ash fusion tests are conducted in a sintering instrument (HR-8000B, China) according to China standard GBT/219−2008. The ash is modeled into a standard triangle cone with a height of 20 mm and bottom side of 7 mm. During ash fusion tests, the heating rate is designed at 45 °C min−1 (below 900 °C) and 5 °C min−1 (above 900 °C), respectively. X-ray fluorescence (XRF, S4-Pioneer, Bruker Co., Germany) is used for elemental determinations in the ash. The main crystalline compounds in the ash samples are identified by X Ray Diffraction (XRD, D/max2400X, Japan) with the characteristic Cu Ka radiation, which is operated under 40 kV and 100A. The morphology analysis is performed by scanning electron microscopy (SEM, JSM-6390A, Japan). An electronic scale (JA2003, China) with a precision of 1× 10−3 g is used for sample weighing.
(deposition or slagging) of biomass/coal cofiring is receded compared to unblended biomass, and becomes more serious than unblended coal.22,23 Consequently, the biomass cofiring ratio is recommended to be kept to less than 20 cal % in pulverized coal-fired boilers after giving consideration to the boiler efficiency and slagging potential,2,23,28−30 and the biomass is cofired as an additional fuel rather than primary fuel. Moreover, although biomass/coal cofiring has been studied intensively, research on the AFTs that influence slagging during the cofiring of different biomass, such as woody biomass and agricultural residues, both of which are the dominating fuel in biomass-fired boilers, is scarce. Overall, the number and biomass consumption of biomass-fired power plants are far beyond those of biomass/coal cofiring power plants, especially in China, where although the biomass power installed capacity will increase to 30 GW by 2020,1 only one 140 MW demonstration biomass/coal cofiring power plant has been running for a long time and only one 300 MW coal-fired power plant has tried the biomass/coal cofiring twice,29,30 and most biomass fired power plants are constructed for dedicated biomass firing. In the biomass-fired power plants, woody biomass is primary fuel in general because of its stable source and fewer ash-related issues compared to agricultural residues; whereas in harvest seasons, agricultural residues may be the sole or blending fuel because of their low prices. Although attributing to the dilution of the woody biomass on the ash forming elements of the agricultural residues and a change in the ash composition mainly due to direct deposition of Ca-enriched particles from woody biomass in the agricultural residues based K-silicate, thus providing for the formation of more high temperature melting silicates and oxides, and the slagging propensities of cofiring agricultural residues and woody biomass were reduced compared to that of agricultural residues,31 the blending ratio of agricultural residues into woody biomass becomes a key point in order to avoid slagging effectively.32,33 Therefore, to provide guidelines on cofiring of agricultural residues and woody biomass, the effect of blending ratio of corn straw (a representation of agricultural biomass) and sawdust (a representation of woody biomass) on AFTs is studied in detail. Meanwhile, to elucidate the influence mechanisms, a further study on the effects of SiO2, K2O, CaO, and MgO as the major ash components in biomass on AFTs is conducted by means of additives.
3. RESULTS AND DISCUSSION 3.1. Effect of Straw/Sawdust Blending Ratio. Figure 1 shows the effect of straw/sawdust blending ratio on the AFTs
Figure 1. Effect of straw/sawdust blending ratio on AFTs.
2. EXPERIMENTAL METHODS AND APPARATUS
including initial deformation temperature (IDT), soften temperature (ST), and fluidized temperature (FT). Compared to sawdust, which shows temperatures of 1225, 1416, and 1428 °C corresponding to IDT, ST, and FT, the AFTs of straw (corresponding to 860, 1033, and 1132 °C in turn) are remarkably lower. Meanwhile, with an increase of straw/ sawdust cofiring ratio (i.e., increasing fraction of straw in the straw/sawdust mixtures), the AFTs decrease and access to the
2.1. Experimental Methods. In the experiment, two types of biomass, corn straw (a representation of agricultural biomass, cultivated in Shaanxi province and harvested in autumn, 2016) and sawdust (a representation of woody biomass, mixture of rosewood), are selected. The ultimate analysis and proximate analysis, tested according to GB/T 31391−2015 and GB/T 212−2008 (Chinese norms), are listed in Table 1. In comparison with sawdust, the straw contains high contents of ash, S, and Cl. Before ashing, the biomass are B
DOI: 10.1021/acs.energyfuels.7b03104 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 2. XRF Analysis Results of the Ashes for Varied Straw/Sawdust Ratios ratio
Na2O
K2O
SiO2
Al2O3
CaO
MgO
Fe2O3
TiO2
Cl
SO3
P2O5
others
0:5 1:4 3:2 5:0 0:5 1:4 3:2 5:0
2.04 0.52 0.41 0.28 1.69 1.10 0.68 0.48
22.60 23.90 26.90 28.50 8.04 23.90 26.80 28.50
4.54 16.30 25.80 29.30 11.10 28.80 36.30 37.30
1.45 1.62 1.48 1.65 3.23 2.14 1.99 1.65
33.10 24.20 18.10 15.40 40.90 24.30 15.60 13.70
11.70 14.10 11.60 9.98 18.70 7.39 6.48 6.98
1.50 0.90 0.67 0.81 2.34 1.38 1.72 0.87
4.63 2.35 0.61 0.12 6.39 2.19 0.76 0.12
0.80 1.12 1.68 3.04 0.02 0.02 0.09 1.47
4.35 4.87 4.73 4.38 4.73 5.35 6.08 4.97
2.27 4.14 4.91 4.72 2.20 3.16 3.24 3.67
2.04 0.52 0.41 0.28 1.69 1.10 0.68 0.48
ashes at 600 °C
ashes at IDT
AFTs of straw. Whereas, when the straw/sawdust cofiring ratio is higher than 3:2, the AFTs keep unchanged with increasing blending ratio. Table 2 summarizes the XRF analysis results of the cofiring biomass ashes at 600 °C and IDT. It can be seen that the changing trends of various components in the ashes at 600 °C and IDT are similar. In comparison with straw ash, sawdust ash possesses higher contents of Na2O, alkali earth metals (CaO and MgO), Fe2O3, and TiO2, whereas lower contents of K2O, SiO2, Cl, and P2O5, and comparable S and A12O3 contents. As a result, with increasing straw/sawdust blending ratio, the contents of K2O, SiO2, and Cl increase, whereas the contents of Na2O, CaO, MgO, and TiO2 decrease. In biomass, the content of Na2O is significantly lower than that of K2O, and can be ignorable in general.2 Generally, the coexistence of K2O, SiO2, and Cl generates low-melting substances such as K2O· nSiO2(the melting points are lower than 800 °C when n = 1, 2, 4) and KCl (melting point 772 °C), but high alkali earth metals improve the generation of high-melting silicates.2,31 Thus, straw shows lower AFTs, and with increasing straw/sawdust blending ratio, the AFTs decrease. On the basis of the XRF analysis results shown in Table 2, the losses of volatile K, Cl, and S in ash between ashing temperature (600 °C) and IDT are calculated by SiO2 trace method and illustrated in Figure 2. Assuming that the loss of SiO2 is zero in the experiment temperature range, and the losses of volatile K, Cl, and S are calculated by the differences of K2O/SiO2, Cl/SiO2, and SO3/SiO2 in the ash at 600 °C and in the triangle ash cone at IDT, respectively. With increasing straw/sawdust blending ratio, the absolute losses of K, Cl, and S decrease (Figure 2a). That means that by addition of straw into sawdust during cofiring, more K, Cl, and S are retained in the ash and improve the generation of low-melting K2O·nSiO2, KCl, and K2SO4,5,9 thus, resulting in decreasing AFTs. Furthermore, it can be seen from Figure 2b that when the straw/sawdust blending ratio is less than 3:2, the relative losses of K and S keep similar decreasing pace with increasing straw/ sawdust blending ratio, and the change of the relative loss of Cl is slight; However, when the straw/sawdust blending ratio is above 3:2, the relative loss of K and Cl decrease with increasing straw/sawdust blending ratio, and the change of the relative loss of S is ignorable. Thus, it can be deduced that K is mainly released as K2SO4 when the straw/sawdust blending ratio is less than 3:2, whereas with increased straw/sawdust blending ratio above 3:2, K mainly released in the form of KCl. Figure 3 shows the XRD analysis results of the ashes at IDT of various straw/sawdust blending ratios. It can be seen that in the ash of straw/sawdust blending ratio of 5:0 (i.e., pure straw ash), besides of Ca3(PO4)2, all identifiable major components including K2SO4 (melting point 1069 °C), CaSi2 (1020 °C), K2MgSi3O8 (927 °C), Na2Si2O5 (874 °C), and K2Si4O9 (
