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Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity
Fuel 174 (2016) 172–179
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Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity Dedy Eka Priyanto a,⇑, Shunichiro Ueno a, Naoki Sato a, Hidekazu Kasai b, Tatsurou Tanoue b, Hitoshi Fukushima b a b
Chemical Engineering Department, IHI Corporation, Yokohama 235-8501, Japan Engineering Centre, IHI Corporation, Tokyo 135-8710, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Co-firing 4 types of woody biomasses
with coal were conducted in a DTF. CCSEM and ternary phase diagram are
used to elucidate the ash formation mechanism. Co-firing using high-ash biomass produces significant changes in ash properties. The interaction between biomass ash and coal ash was discussed. Co-firing using high-ash biomass results in a rapid increase of slagging tendency.
a r t i c l e
i n f o
Article history: Received 1 September 2015 Received in revised form 10 December 2015 Accepted 22 January 2016 Available online 2 February 2016 Keywords: Woody biomass Co-firing Ash transformation Slagging Deposition
a b s t r a c t The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300 °C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4 wt%) biomasses (sugi and sakura). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0 wt%) biomasses (nara and bark of sugi). The significant transformation of ash with regard to its morphology and chemical composition,
⇑ Corresponding author at: Chemical Engineering Department, IHI Corporation, 1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan. Tel.: +81 45 759 2871; fax: +81 45 759 2208. E-mail address: [email protected] (D.E. Priyanto). http://dx.doi.org/10.1016/j.fuel.2016.01.072 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
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together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Co-firing technology, which involves the addition of some biomass to the coal used in coal-fired power plants, is a promising means of reducing greenhouse emissions, especially considering its lower additional investment requirement compared with CO2 capture technology [1]. Co-firing has been widely introduced particularly in the EU, with the ratio of the added biomass reaching more than 30% (energy basis) [2]. It has also been implemented in several coal-fired power plants in Japan, although the biomass ratio in the country is still low, being <3% [3]. However, Japan has large woody biomass resources, particularly coppice forests, which remain unutilized. The country has the ambitious target of reducing its 2030 total greenhouse gas emission to 26% of 2013 level [4]. The heavy application of biomass co-firing in coal-fired power plants in the country will not only contribute substantially to the achievement of this goal, but will boost the utilization of the national forest resources. Despite that woody biomasses generally have lower ash content compared to coal, they contain larger amounts of alkaline and alkali earth metals (AAEMs) such as potassium and calcium [5]. This high AAEM content may have an undesirable effect on the boiler operation during combustion. The interaction of AAEMs with the discrete coal minerals may reduce the melting point of the mineral particles [6], leading to severe deposition (slagging and fouling), which reduces heat transfer and causes corrosion and erosion problems under the high-temperature boiler conditions. Majority of coal-fired power plants in Japan and elsewhere, use a pulverized coal (PC) boiler as the steam generator. Compared to a circulating fluidized bed (CFB)-type boiler, which is mainly used for biomass combustion, a PC boiler operates at higher temperatures, with the furnace wall temperature reaching 1100–1400 °C. Thus, the effect of the inorganic component of the biomass on ash deposition in a PC boiler, particularly the slagging propensity, is a potential cause of concern regarding the use of high co-firing ratios. Several studies have been conducted on the use of woodderived fuels in low-ratio co-firing [7–9]. It was generally found that such conditions had a modest effect on the ash deposition. In contrast, there have been limited studies on co-firing using high woody biomass ratios, and on the effect of such on ash deposition especially in PC boiler applications. Moreover, there are different types of woody biomasses with varying ash contents, and few studies have investigated the effect of the type of woody biomass on the ash transformation during co-firing. In the present study, the effects of high-ratio co-firing and the woody biomass types on the ash transformation and slagging propensity in a drop tube furnace (DTF) were investigated. 2. Experimental 2.1. Solid fuels Pulverized bituminous coal was mixed with four different types of Japanese woody biomasses (30%, 50%, 70% on energy basis), namely, sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi, respectively. Sakura wood and sugi were used as examples of low ash biomasses, while nara
and bark of sugi were used as examples of high-ash biomasses. The biomass samples were dried at 105 °C for 1 day and then pulverized and shredded to less than 200 lm before use. The properties of the samples are presented in Table 1. 2.2. Combustion procedure The combustion tests were conducted in a drop tube furnace (DTF) (see Fig. 1). The DTF used in this study consisted of a ceramic tube of length 1000 mm and ID 50 mm. It comprised three zones, all which were heated electrically by siliconit heating elements. The temperature in all the zones could be raised up to 1400 °C. The samples were continuously supplied at a rate of 173 W (ex. coal: 0.35 g/min) through a table feeder and burnt together with air in the ceramic tube, which had been pre-heated to 1350 °C. The flow rate of the air was 3 L/min and the excess air ratio was 1.2. 2.3. Ash collection The combustion ashes (ashes) were collected during the combustion by a water-cooled suction probe inserted 270 mm beyond the furnace outlet. The ashes and flue gas passed through the inner tube of the suction probe and were then rapidly cooled to halt further combustion. The cooled ashes, including the char, were then trapped by a filter. The ashes were also collected in 400 mm beyond the furnace outlet where the slagging tests were conducted. The ashes were then subjected to thermogravimetric (TG)-differential thermal (DT) analysis to determine the combustion efficiency, which was always found to be higher than 95%. It was thus appropriate to conduct the slagging test at this location.
Table 1 Properties of tested samples. Property
Coal
Sugi
Sakura
Nara
Bark of sugi
High heating value (MJ/kg)
29.5
20.8
21.2
18.7
18.2
Proximate analysis (wt% db) Ash Volatiles Mixed carbon
12.7 30.9 56.4
0.4 84.6 15.0
0.4 82.2 17.4
1.1 81.9 17.0
5.3 71.1 17.4
Ultimate analysis (wt% daf) C H N O S
81.51 5.61 2.06 10.19 0.63
51.01 5.83 0.15 43.00 0.01
50.81 5.86 0.15 43.18 0.01
48.21 5.94 0.14 45.70 0.01
52.23 5.90 0.51 41.32 0.04
Ash analysis (wt% ash) SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 SO3
64.2 21.4 1.13 4.45 1.02 0.88 0.58 1.52 0.19 0.81
13.6 5.5 0.3 6.3 44.2 14.5 1.6 8.5 1.7 2.9
5.6 1.4 0.0 2.2 51.7 17.2 0.6 4.4 2.9 9.3
0.8 1.5 0.1 0.8 79.1 5.3 0.0 4.5 1.6 1.5
15.1 3.89 NA 2.51 55.2 2.99 0.74 1.96 1.46 NA
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backscattered image, and the constituent elements of each particle were determined by computer-controlled scanning electron microscopy analysis (CCSEM) analysis of the EDX spectra (SEM: JSM-5600, EDX: EDAX Genesis). XRD analysis was also conducted to analyze the crystallinity of the ash during the co-firing. Prior to the CCSEM analysis, all the samples were mounted on epoxy resin, cross-sectioned, and polished. They were then coated with a platinum layer to avoid the charging of the particles during the analysis. To obtain the backscattered electron image (BSE image) of samples, three magnifications were used, namely, 50, 250, and 500 for particles measuring 20–100, 4–20, and 1–4 lm, respectively. The compositions of the 10 constituent elements (Na, K, Mg, Ca, Al, Si, Fe, P, S, and Ti) in the thousands of particles were determined and expressed as the oxide. Each ash particle was then classified into one of several chemical/mineral classes based on its elemental composition, although some of the particles contained amorphous/glass phases. The mineral classification for coal-derived ashes proposed by the Energy and Environmental Research Center (EERC), University of North Dakota [10] was adapted for this purpose. Because calcium predominates in the inorganic compounds contained in woody biomass ash, a calcium-based chemical classification adapted from the classifications proposed by Zhang et al. [11] and Chen et al. [12] was also employed. A thermomechanical analysis (TMA) performed using a Hitachi TMA 7100 system, was also performed to examine the melting behavior of the ash deposits. This involved heating of 5 mg ash samples from 700 to 1400 °C at a rate of 5 °C/min in a N2 atmosphere under 49-mN load, and measuring the penetration of a ram into the sample.
3. Results and discussion Fig. 1. Schematic of drop tube furnace used for co-firing and slagging tests.
2.4. Slagging test procedure The slagging tests were conducted using a stainless steel (SUS304) water-cooled tube known, as ash deposition probe. The tip of the probe was tilted at 45° (Fig. 1) so that only the sticky ash particles were retained on the probe surface. The outer diameter of the ash deposition probe was 34 mm, and a thermocouple was installed on the probe surface. The probe was inserted 400 mm into the outlet, to a point where the inner furnace temperature was about 1300 °C, which is representative of the condition of the wall of the furnace boiler. The initial tube surface temperature was maintained at 500–600 °C by adjustment of the cooling water flow rate, which was fixed during the tests. The test for each sample was conducted two or three times, and the average mass of ash deposit was determined. The slagging propensity was evaluated by calculating the ash deposition ratio using the following equations:
Ash deposition ratio ¼
M dep Stube Mfuel t C ash Sfurnace
ð1Þ
where Mdep is the mass of the ash (g) deposited over a certain time (min); Mfuel and Cash are respectively the flow rate of fuel (g/min) and ash content (wt%) of the fuel; and Stube and Sfurnace are respectively the cross-sectional areas of the probe and furnace. 2.5. Ash characterization The morphological data of ash particles, such as the average diameter, perimeter, and shape index, were obtained from the
3.1. Formation of ash 3.1.1. Morphology of ash Fig. 2 shows BSE images of ash particles obtained during the combustion process. The bright particles in the figures are the ash particles. The co-firing of the low-ash biomasses, such as sugi and sakura, using ratios of up to 50% produced no significant morphological changes in the ash particles. However, there were changes when the biomass ratio reached 70%, as evidenced by the existence of large numbers of spherical ash particles. In contrast, the same significant morphological changes occurred during the co-firing of the high-ash woody biomasses, such as nara and bark of sugi using mixing ratios as low as 30%. Furthermore, the shape index of each particle obtained by CCSEM analysis was used to quantify the number of spherical particles. A common shape index is the circularity (fcirc), which is a function of the cross-sectional area (A) and perimeter (P) of the particle, as expressed by the following equation [13]:
f circ ¼
4p A P
Based on the circularity of a circle of 1, the numbers of particles with shape indexes ranging between 1.0 and 1.19 were compared, as shown in Fig. 3. As can be clearly observed, the number of spherical ash particles significantly increased with increasing ratio of the high-ash biomasses, especially bark of sugi. Compared to pure coalfiring, the numbers of spherical particles for nara 50% and bark of sugi 30% are more than twofold. The sphericity of an ash particle indicates that the entire particle or its surface became molten during the combustion. This strongly suggests that co-firing, especially when using high-ash biomasses, affects the ash melting behavior.
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Fig. 2. SEM–BSE images of ash particles produced by pure coal firing and co-firing of coal with woody biomass.
3.1.2. Chemical and mineralogical composition Because calcium is the most abundant element of the AAEMs in woody biomass ash, the behavior of the Ca-containing ash was investigated in detail. The compositions of the ash particles that contained more than 80% Ca, Al, and Si are plotted in the ternary diagrams shown in Fig. 4. The standard phase diagram of CaO– SiO2–Al2O3 [14] is also included in each of the ternary diagram to enable prediction of the mineralogy and the melting point of mineral in the ash samples. In the case of coal firing, most of the particles are distributed within the Si–Al or in the region containing mullite (Al6Si2O13), which has a melting point of about 1880 °C. Aluminosilicates, particularly mullite, are well known as the main constituent of the fly ash particles produced by pure coal firing. By comparison, the distribution of the ash particles produced by the
co-firing of coal with a low-ash biomass of at least 50% was not significantly different. In the case of sakura, when the biomass content exceeded 70%, more particles were distributed between the regions containing anorthite (CaAl2Si2O8) and mullite, suggesting the formation of calcium aluminosilicates (anorthite). Co-firing using a high-ash biomass produces significant change in ash particle distribution, even for low mixing ratios. In the case of co-firing with 30% nara, the particles were predominantly located between the regions containing mullite and anorthite. The number of particles in the region containing anorthite increased when the ratio was increased to 50%, indicating the possible formation of anorthite. Although the melting point of pure anorthite is 1555 °C, it can be significantly decreased to a value much lower than those of mullite and anorthite by the formation
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gaseous phase of Ca(OH)2/CaCO3 in the woody biomass ash [15] or CaSO4 produced by the reaction of Ca(OH)2/CaCO3 with flue gas (SOx), and the aluminosilicate minerals (mainly mullite) in the coal ash. The reaction is as follows:
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ xAl2 O3 ySiO2ðsÞ ! CaO xAl2 O3 ySiO2ðsÞ þ H2 O=CO2
ðR1Þ
CaSO4ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! CaO xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
Fig. 3. Comparison of numbers of spherical particles with shape index of 1.0–1.19, produced by pure coal firing and co-firing with woody biomass.
of an eutectic mixture of mullite and anorthite (eutectic melting point = 1347 °C [14]). It is possible for the eutectic mixture to be formed in deposit layer during the co-firing process. The most significant change in the ash particles was observed for co-firing using bark of sugi. The chemical composition of the particles was transformed from that of calcium aluminosilicate, (particularly a type rich in anorthite) for 30% bark of sugi, to the coexistence of calcium aluminosilicates (mainly anorthite and gehlenite (Ca2Al2SiO7, 1549 °C)) and calcium silicates (mainly pseudowollastonite (CaSiO3, 1540 °C)) for 50% of the biomass, and finally to rich calcium silicates (mainly a-calcium silicate (Ca2SiO4, 1540 °C) and rankinite (Ca3Si2O7, 1475 °C)) for 70% of the biomass. Because the ash obtained from co-firing using bark of sugi is dominated by calcium-containing minerals, it melts at a much lower temperature compared to ash obtained from pure coal-firing. The formation of eutectic mixtures of calcium minerals that further lower the melting point of the minerals is also possible. For instance, significant amounts of eutectic mixtures of pseudowollastonite, anorthite, and gehlenite (eutectic melting point = 1267 °C [14]), and rankinite, gehlenite, and a-Ca2SiO4 (eutectic melting point = 1317 °C [14]) are respectively formed during co-firing using 30–50% and 70% bark of sugi. These eutectic mixtures may melt at about 1200–1300 °C, which is much lower than the melting point of the pure mineral (about 1500 °C). From the above results, it can be concluded that the ash content of biomass as well as the ash composition affects the ash transformation by co-firing. The higher the ash content of biomass, more significant is the change in the ash properties. The presence of calcium-containing minerals and their eutectic mixtures during co-firing significantly contributes to the formation of low-melting-point-ash and the possibly of increased ash deposition.
3.2. Interaction between biomass ash and coal ash The fate of the alkali (K) and alkaline earth (Ca) species is the key to understanding the interaction between coal and biomass. Fig. 5 shows the constitution of the calcium/potassiumcontaining inorganic components of the ashes during combustion. For comparison, the results for co-firing using sugi, nara, and bark of sugi are shown as representative examples of low- and high-ash biomass co-firing. The interaction of the calcium species during the co-firing will be discussed in detail. There was a slight increase in the calcium minerals during the co-firing using the low-ash biomasses. In contrast, the increase was significant for co-firing using high-ash biomasses, especially bark of sugi. The calcium minerals in the ash predominantly comprised a calcium aluminosilicate (Ca–Al–Si) mineral, probably anorthite or gehlenite, produced by the interaction between the
ðR2Þ
where g denotes gas, s denotes solid, and c denotes condensed (liquid or solid). Generally, the concentration of Ca–Al–Si increases with the increasing biomass ratio. However, in the case of co-firing using 50% bark of sugi, the concentration of Ca–Al–Si decreased while that of calcium silicate minerals (Ca–Si) increased relative to using 30%. Ca–Si may occur in the form of pseudowollastonite, rankinite, or a-calcium silicate depending on the calcium content of the fuel ash. Ca–Si may also be formed by the reaction of Ca(OH)2/CaCO3/ CaSO4 in the biomass with SiO2-rich minerals in the coal ash, as shown in the reactions below. However, calcium silicate minerals have also been detected in biomass ash [16]. An XRD analysis conducted in the present study revealed a strong Ca2SiO4 peak particularly for the ash using 70% bark. Ca–Si was also present in the unreacted biomass ash (Ca2SiO4).
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ SiO2ðsÞ ! CaO SiO2ðsÞ þ H2 O=CO2
ðR3Þ
CaSO4ðgÞ þ SiO2ðsÞ ! CaO SiO2ðsÞ þ SO2 þ 1=2O2
ðR4Þ
Beside Ca–Al–Si and Ca–Si minerals, Ca–Fe–Al–Si and Ca–K–Al– Si were also observed. Ca–Fe–Al–Si was probably formed by the interaction of Ca(OH)2/CaCO3/CaSO4 from biomass ash with iron– aluminosilicates from coal ash through the following reactions.
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ Fe2 O3 xAl2 O3 ySiO2ðsÞ ! CaO Fe2 O3 xAl2 O3 ySiO2ðsÞ þ H2 O=CO2 CaSO4ðg or cÞ þ Fe2 O3 xAl2 O3 ySiO2ðsÞ ! CaO Fe2 O3 xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
ðR5Þ ðR6Þ
Conversely, Ca–K–Al–Si is produced from the reaction between K2Ca(SO4)2 or K2Ca2(SO4)3 from biomass ash [16] and aluminosilicates from coal ash. The interaction between potassium species such as K2SO4 and calcium aluminosilicates may also produce a Ca–K–Al–Si mineral.
K2 CaðSO4 Þ2ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! K2 O CaO xAl2 O3 ySiO2ðsÞ þ 2SO2ðgÞ þ O2ðgÞ
ðR7Þ
K2 Ca2 ðSO4 Þ3ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! K2 O 2CaO xAl2 O3 ySiO2ðsÞ þ 3SO2ðgÞ þ 3=2O2ðgÞ
ðR8Þ
K2 SO4 þ CaO xAl2 O3 ySiO2ðsÞ ! K2 O CaO xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
ðR9Þ
It should be noted that iron/potassium-containing calcium aluminosilicates have lower melting point s compared to calcium aluminosilicates [17,18], and this may increase the ash deposition tendencies of the former. Sulfur-containing calcium minerals such as anhydrite (CaSO4) and anhydrite-Al–Si were still observed in the products of cofiring, particularly when using bark of sugi. A moderate amount of anhydrate-Al–Si was also observed. As mentioned above, anhydrite is usually formed by the reaction of Ca(OH)2/CaCO3 from biomass ash with SO2 flue gas. A significant amount of anhydrite is usually produced by the combustion of woody biomass [19]. In co-firing, most of this mineral reacts with aluminosilicate from coal ash to produce SO2 as expressed by reaction (R2). When a big particle of anhydrite reacts with a small particle of
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Fig. 4. Distribution of chemical components of ash particles containing >80% of Ca + Si + Al, produced by pure coal firing and co-firing with woody biomass.
aluminosilicate mineral, anhydrite-Al–Si is formed. Ca-rich aluminosilicate minerals may also react with SO2 to form anhydrite-Al–Si. 3.3. Slagging propensity during co-firing One of the factors that determine the profile of the ash deposition ratio is the exposure time [20]. To determine the appropriate
exposure time, we conducted a slagging test using 50% bark of sugi. The determined time dependence of the ash deposition ratio and the mass of the deposited ash are shown in Fig. 6. While the mass of the ash deposited on the probe surface increased linearly, the ash deposition ratio was almost constant. However, after the saturation of the ash deposition, the deposition ratio tended to decrease. These results indicate that the exposure time of a
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Fig. 5. Fractions of calcium and potassium minerals in ashes produced by pure coal firing and co-firing with woody biomass.
the slagging propensity of the fuel. The higher the ash deposition ratio, the higher the slagging propensity. The test results showed that co-firing using a low-ash biomass did not increase the ash deposition ratio; rather, the deposition ratio tended to decrease with increasing biomass ratio at least up to 50%. In contrast, the ash deposition ratio increased with increasing biomass ratio for co-firing using a high-ash biomass. Co-firing using bark of sugi produced the highest ash deposition ratio among all the considered woody biomasses. Generally, the ash deposition ratio (slagging propensity) during the co-firing increased in the following order;
Bark of sugi > Nara > Sugi Sakura
Fig. 6. Time variations of ash deposition ratio and ash deposit for co-firing using 50% bark of sugi.
slagging test should be restricted to the duration when the deposition ratio is independent of time, which in the present case was determined to be 45 min. The total amount of fuel ash supplied during this period for all test cases was 1.2 g, and this made the exposure times of the slagging tests to be within 30–60 min depending on the ash content of the fuel. The profiles of the ash deposition ratio for co-firing and coal firing are shown in Fig. 7. The ash deposition ratio is an indication of
Because there was no significant difference between the chemical compositions of the combustion ashes and the ash deposits in this test, it is worthwhile to discuss the relationship between the combustion ash properties and the slagging propensity. The results of the slagging propensity agree with the transformation tendency of the ash properties during co-firing; specifically, co-firing using a high-ash biomass significantly affects the properties of the ash, whereas the use of a low-ash biomass does not. As noted earlier, co-firing increases the concentration of molten calcium minerals in the ash, with high ash biomasses (nara and bark of sugi) producing more significant changes. As is well known, the ash particles tend to adhere to the probe tube with increasing amount of molten ash particles [21]. In addition, the formation of the eutectic mixture of calcium minerals in the deposited layer may also further lower the melting point, resulting in the tendency of the co-firing deposits to be more receptive of oncoming particles, leading to rapid growth of the deposit. To investigate the eutectic formation in the ash deposits, we conducted a TMA analysis, which is widely used to investigate
35
Penetration (%)
30 25
50% Bark
20 15 10
Coal
50% Sugi
5 0 700
30% Bark 800
900
1000
1100
1200
Temperature (°C) Fig. 7. Ash deposition ratios (slagging propensities) for pure coal firing and co-firing with woody biomass.
Fig. 8. TMA profiles of ash deposits.
1300
1400
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the fusion behavior of mainly coal ashes [22,23]. The TMA profiles of the ash deposits obtained from co-firing and pure coal firing are compared in Fig. 8. The trend of the TMA profile of the ash deposit obtained using 50% sugi is similar to that of the ash deposit obtained from pure coal firing. In contrast, the TMA profile of the ash deposit obtained from co-firing with bark is significantly different, being characterized by a rapid increase in penetration within a narrow temperature range. The rapid increase was detected within 1210–1350 °C and 1170–1250 °C for 30% and 50% bark of sugi, respectively, and was due to the formation of eutectics in the deposit layer. These TMA results agree with our prediction of a high possibility of the formation of eutectic mixtures of pseudowollastonite, anorthite, and gehlenite, which melt at about 1200–1300 °C for 30% and 50% bark of sugi. The presence of iron (from coal ash) and alkali (potassium) in these mixtures may further decrease the melting point. The fact that no increase was observed in the slagging propensity for co-firing using the low-ash biomasses is also noteworthy. The ash deposition is not only affected by the chemical composition of the fuel ash, but also by the physical properties such as the particle and heat fluxes, and the particle velocity and impaction [24]. Sugi and sakura woods have an ash content of 0.4 wt% (Table 1), which is much lower than those of the other considered biomasses (>1.0 wt%) and coal (>12 wt%). Thus, the particle flux for co-firing using sugi/sakura should be much lower than those for high-ash biomasses. The lower the particle flux, the lower the probability of the ash sticking or colliding with the probe surface. In addition, co-firing was observed to produce no significant change in the ash properties. This together with the reduced particle flux probably contributed to the reduced slagging propensity observed for co-firing using a low-ash biomass. Hence, from the viewpoint of preventing severe slagging problems in a PC boiler during co-firing using a high ratio of woody biomass, a low-ash biomass is preferable. 4. Conclusions Co-firing of coal in a drop-tube furnace using high ratios of four different types of woody biomasses was investigated in this study. The properties of the combustion ashes and ash deposits were examined in detail by CCSEM analysis, CaO–Al2O3–SiO2 ternary phase diagrams and TMA analysis. Following is a summary of the findings: 1. Compared to pure coal firing, co-firing produces no significant changes in the morphology and chemical composition of the ash products when using a low-ash biomass (60.4 wt%) of up to 50 cal%. In contrast, the use of even a small ratio (30%) of a high-ash biomass (>1.0 wt%) such as nara or bark of sugi produces a large number of molten ash particles with a higher calcium mineral content. 2. The chemical composition of the co-firing products are mainly changed from those of the aluminosilicate (mullite)-rich ash particles produced by coal firing to those of calcium aluminosilicate (anorthite/gehlenite)-rich particles. With increasing biomass ratio, especially when using bark of sugi, there is a subsequent change in the chemical properties of the products to those of calcium silicate (pseudowollastonite, rankinite, aCa2SiO4)-rich ash. These calcium minerals have lower melting points than aluminosilicate minerals, which are predominant in the products of pure coal firing. In deposited layer, these calcium minerals easily form eutectics that decrease the melting point, accelerating growth of the ash deposit.
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3. An increase in slagging propensity was only observed for cofiring using high ash biomasses. This was because of the significant change in the ash properties (number of molten particles, and chemical composition) and the formation of the eutectic mixtures of calcium minerals in the deposited layer. To prevent severe slagging in a PC boiler during co-firing using a high woody biomass ratio, the use of a low-ash biomass is preferable.
Acknowledgement The financial support of the Ministry of the Environment (MOE) – Japan is gratefully acknowledged. References [1] Tillman DA. Biomass cofiring: the technology, the experience, the combustion consequences. Biomass Bioenergy 2000;19:365–84. [2] Almansour F, Zuwala J. An evaluation of biomass co-firing in Europe. Biomass Bioenergy 2010;34:620–9. [3] Suzuki A. Brief overview of woody biomass co-firing in Saijyo coal-fired power plant. Electr Rev 2005;90–12:74–7. [4]. [5] Vassilev SV, Bexter D, Andersen LK, Vassileva CG. An overview of the chemical composition of biomass. Fuel 2010;89:913–33. [6] Miller SF, Schobert HH. Effect of the occurrence and modes of incorporation of alkalis, alkaline earth elements, and sulfur on ash formation in pilot-scale combustion of beulah pulverized coal and coal-water slurry fuel. Energy Fuels 1994;8(6):1208–16. [7] Savolainen K. Co-firing of biomass in coal-fired utility boiler. Appl Energy 2003;71:369–81. [8] Robinson AL, Junker H, Baxter LL. Pilot-scale investigation of the influence of coal–biomass cofiring on ash deposition. Energy Fuels 2002;16:343–55. [9] Molcan P, Lu G, Le Bris T, Yan Y, Taupin B, Caillat S. Characterisation of biomass and coal co-firing on a 3 MWth combustion test facility using flame imaging and gas/ash sampling techniques. Fuel 2009;88:2328–34. [10] Zygarlicke CJ, Steadman EN. Advanced SEM techniques to characterize coal minerals. Scanning Microscopy 1990;4(3):579–90. [11] Zhang L, Sato A, Ninomiya Y. CCSEM analysis of ash from combustion of coal added with limestone. Fuel 2002;81:1499–508. [12] Chen Y, Shah N, Huggins FE, Huffman GP, Linak WP, Miller CA. Investigation of primary fine particulate matter from coal combustion by computer-controlled scanning electron microscopy. Fuel Process Technol 2004;85:746–61. [13] Ulusoy U, Yekeler M, Hicyılmaz C. Determination of the shape, morphological and wettability properties of quartz and their correlations. Miner Eng 2003;16:951–64. [14] Mao H, Hillert M, Selleby M, Sundman B. Thermodynamic assessment of the CaO–Al2O3–SiO2 system. J Am Ceram Soc 2006;89:298–308. [15] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the behaviour of biomass during combustion: Part II. Ash fusion and ash formation mechanisms of biomass types. Fuel 2014;117:152–83. [16] Vassilev SV, Baxter D, Andersen LK, Vassileva CG. An overview of the composition and application of biomass ash. Part 1. Phase–mineral and chemical composition and classification. Fuel 2013;105:40–76. [17] Higo T, Sukenaga S, Kanehashi K, Shibata H, Osugi T, Saito N, et al. Effect of potassium oxide addition on viscosity of calcium aluminosilicate melts at 1673–1873 K. ISIJ Int 2014;54:2039–44. [18] Matsuoka K, Yamashita T, Kuramoto K, Suzuki Y, Takaya A, Tomita A. Transformation of alkali and alkaline earth metals in low rank coal during gasification. Fuel 2008;87:885–93. [19] Wiinikka H, Gebart R, Boman C, Bostro D, Ohman M. Influence of fuel ash composition on high temperature aerosol formation in fixed bed combustion of woody biomass pellets. Fuel 2007;86:181–93. [20] Akiyama K, Pak H, Tada T, Ueki Y, Yoshiie R, Naruse I. Ash deposition behavior of upgraded brown coal and bituminous coal. Energy Fuels 2010;24:4138–43. [21] Raask E. Mineral impurities in coal combustion. Washington: Hemisphere Publishing Corporation; 1985. p. 137–60. [22] Liu Y, Gupta R, Elliotta L, Wall T, Fujimori T. Thermomechanical analysis of laboratory ash, combustion ash and deposits from coal combustion. Fuel Process Technol 2007;88:1099–107. [23] Wall TF, Creelman RA, Gupta RP, Gupta SK, Coin C, Lowe A. Coal ash fusion temperatures – new characterization techniques, and implications for slagging and fouling. Prog Energy Combust Sci 1998;24:345–53. [24] Stam AF, Livingston WR, Cremers MFG, Brem G. Review of models and tools for slagging and fouling prediction for biomass co-combustion. .
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Fuel journal homepage: www.elsevier.com/locate/fuel
Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity Dedy Eka Priyanto a,⇑, Shunichiro Ueno a, Naoki Sato a, Hidekazu Kasai b, Tatsurou Tanoue b, Hitoshi Fukushima b a b
Chemical Engineering Department, IHI Corporation, Yokohama 235-8501, Japan Engineering Centre, IHI Corporation, Tokyo 135-8710, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Co-firing 4 types of woody biomasses
with coal were conducted in a DTF. CCSEM and ternary phase diagram are
used to elucidate the ash formation mechanism. Co-firing using high-ash biomass produces significant changes in ash properties. The interaction between biomass ash and coal ash was discussed. Co-firing using high-ash biomass results in a rapid increase of slagging tendency.
a r t i c l e
i n f o
Article history: Received 1 September 2015 Received in revised form 10 December 2015 Accepted 22 January 2016 Available online 2 February 2016 Keywords: Woody biomass Co-firing Ash transformation Slagging Deposition
a b s t r a c t The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300 °C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4 wt%) biomasses (sugi and sakura). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0 wt%) biomasses (nara and bark of sugi). The significant transformation of ash with regard to its morphology and chemical composition,
⇑ Corresponding author at: Chemical Engineering Department, IHI Corporation, 1 Shin-Nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan. Tel.: +81 45 759 2871; fax: +81 45 759 2208. E-mail address: [email protected] (D.E. Priyanto). http://dx.doi.org/10.1016/j.fuel.2016.01.072 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
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together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Co-firing technology, which involves the addition of some biomass to the coal used in coal-fired power plants, is a promising means of reducing greenhouse emissions, especially considering its lower additional investment requirement compared with CO2 capture technology [1]. Co-firing has been widely introduced particularly in the EU, with the ratio of the added biomass reaching more than 30% (energy basis) [2]. It has also been implemented in several coal-fired power plants in Japan, although the biomass ratio in the country is still low, being <3% [3]. However, Japan has large woody biomass resources, particularly coppice forests, which remain unutilized. The country has the ambitious target of reducing its 2030 total greenhouse gas emission to 26% of 2013 level [4]. The heavy application of biomass co-firing in coal-fired power plants in the country will not only contribute substantially to the achievement of this goal, but will boost the utilization of the national forest resources. Despite that woody biomasses generally have lower ash content compared to coal, they contain larger amounts of alkaline and alkali earth metals (AAEMs) such as potassium and calcium [5]. This high AAEM content may have an undesirable effect on the boiler operation during combustion. The interaction of AAEMs with the discrete coal minerals may reduce the melting point of the mineral particles [6], leading to severe deposition (slagging and fouling), which reduces heat transfer and causes corrosion and erosion problems under the high-temperature boiler conditions. Majority of coal-fired power plants in Japan and elsewhere, use a pulverized coal (PC) boiler as the steam generator. Compared to a circulating fluidized bed (CFB)-type boiler, which is mainly used for biomass combustion, a PC boiler operates at higher temperatures, with the furnace wall temperature reaching 1100–1400 °C. Thus, the effect of the inorganic component of the biomass on ash deposition in a PC boiler, particularly the slagging propensity, is a potential cause of concern regarding the use of high co-firing ratios. Several studies have been conducted on the use of woodderived fuels in low-ratio co-firing [7–9]. It was generally found that such conditions had a modest effect on the ash deposition. In contrast, there have been limited studies on co-firing using high woody biomass ratios, and on the effect of such on ash deposition especially in PC boiler applications. Moreover, there are different types of woody biomasses with varying ash contents, and few studies have investigated the effect of the type of woody biomass on the ash transformation during co-firing. In the present study, the effects of high-ratio co-firing and the woody biomass types on the ash transformation and slagging propensity in a drop tube furnace (DTF) were investigated. 2. Experimental 2.1. Solid fuels Pulverized bituminous coal was mixed with four different types of Japanese woody biomasses (30%, 50%, 70% on energy basis), namely, sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi, respectively. Sakura wood and sugi were used as examples of low ash biomasses, while nara
and bark of sugi were used as examples of high-ash biomasses. The biomass samples were dried at 105 °C for 1 day and then pulverized and shredded to less than 200 lm before use. The properties of the samples are presented in Table 1. 2.2. Combustion procedure The combustion tests were conducted in a drop tube furnace (DTF) (see Fig. 1). The DTF used in this study consisted of a ceramic tube of length 1000 mm and ID 50 mm. It comprised three zones, all which were heated electrically by siliconit heating elements. The temperature in all the zones could be raised up to 1400 °C. The samples were continuously supplied at a rate of 173 W (ex. coal: 0.35 g/min) through a table feeder and burnt together with air in the ceramic tube, which had been pre-heated to 1350 °C. The flow rate of the air was 3 L/min and the excess air ratio was 1.2. 2.3. Ash collection The combustion ashes (ashes) were collected during the combustion by a water-cooled suction probe inserted 270 mm beyond the furnace outlet. The ashes and flue gas passed through the inner tube of the suction probe and were then rapidly cooled to halt further combustion. The cooled ashes, including the char, were then trapped by a filter. The ashes were also collected in 400 mm beyond the furnace outlet where the slagging tests were conducted. The ashes were then subjected to thermogravimetric (TG)-differential thermal (DT) analysis to determine the combustion efficiency, which was always found to be higher than 95%. It was thus appropriate to conduct the slagging test at this location.
Table 1 Properties of tested samples. Property
Coal
Sugi
Sakura
Nara
Bark of sugi
High heating value (MJ/kg)
29.5
20.8
21.2
18.7
18.2
Proximate analysis (wt% db) Ash Volatiles Mixed carbon
12.7 30.9 56.4
0.4 84.6 15.0
0.4 82.2 17.4
1.1 81.9 17.0
5.3 71.1 17.4
Ultimate analysis (wt% daf) C H N O S
81.51 5.61 2.06 10.19 0.63
51.01 5.83 0.15 43.00 0.01
50.81 5.86 0.15 43.18 0.01
48.21 5.94 0.14 45.70 0.01
52.23 5.90 0.51 41.32 0.04
Ash analysis (wt% ash) SiO2 Al2O3 TiO2 Fe2O3 CaO MgO Na2O K2O P2O5 SO3
64.2 21.4 1.13 4.45 1.02 0.88 0.58 1.52 0.19 0.81
13.6 5.5 0.3 6.3 44.2 14.5 1.6 8.5 1.7 2.9
5.6 1.4 0.0 2.2 51.7 17.2 0.6 4.4 2.9 9.3
0.8 1.5 0.1 0.8 79.1 5.3 0.0 4.5 1.6 1.5
15.1 3.89 NA 2.51 55.2 2.99 0.74 1.96 1.46 NA
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backscattered image, and the constituent elements of each particle were determined by computer-controlled scanning electron microscopy analysis (CCSEM) analysis of the EDX spectra (SEM: JSM-5600, EDX: EDAX Genesis). XRD analysis was also conducted to analyze the crystallinity of the ash during the co-firing. Prior to the CCSEM analysis, all the samples were mounted on epoxy resin, cross-sectioned, and polished. They were then coated with a platinum layer to avoid the charging of the particles during the analysis. To obtain the backscattered electron image (BSE image) of samples, three magnifications were used, namely, 50, 250, and 500 for particles measuring 20–100, 4–20, and 1–4 lm, respectively. The compositions of the 10 constituent elements (Na, K, Mg, Ca, Al, Si, Fe, P, S, and Ti) in the thousands of particles were determined and expressed as the oxide. Each ash particle was then classified into one of several chemical/mineral classes based on its elemental composition, although some of the particles contained amorphous/glass phases. The mineral classification for coal-derived ashes proposed by the Energy and Environmental Research Center (EERC), University of North Dakota [10] was adapted for this purpose. Because calcium predominates in the inorganic compounds contained in woody biomass ash, a calcium-based chemical classification adapted from the classifications proposed by Zhang et al. [11] and Chen et al. [12] was also employed. A thermomechanical analysis (TMA) performed using a Hitachi TMA 7100 system, was also performed to examine the melting behavior of the ash deposits. This involved heating of 5 mg ash samples from 700 to 1400 °C at a rate of 5 °C/min in a N2 atmosphere under 49-mN load, and measuring the penetration of a ram into the sample.
3. Results and discussion Fig. 1. Schematic of drop tube furnace used for co-firing and slagging tests.
2.4. Slagging test procedure The slagging tests were conducted using a stainless steel (SUS304) water-cooled tube known, as ash deposition probe. The tip of the probe was tilted at 45° (Fig. 1) so that only the sticky ash particles were retained on the probe surface. The outer diameter of the ash deposition probe was 34 mm, and a thermocouple was installed on the probe surface. The probe was inserted 400 mm into the outlet, to a point where the inner furnace temperature was about 1300 °C, which is representative of the condition of the wall of the furnace boiler. The initial tube surface temperature was maintained at 500–600 °C by adjustment of the cooling water flow rate, which was fixed during the tests. The test for each sample was conducted two or three times, and the average mass of ash deposit was determined. The slagging propensity was evaluated by calculating the ash deposition ratio using the following equations:
Ash deposition ratio ¼
M dep Stube Mfuel t C ash Sfurnace
ð1Þ
where Mdep is the mass of the ash (g) deposited over a certain time (min); Mfuel and Cash are respectively the flow rate of fuel (g/min) and ash content (wt%) of the fuel; and Stube and Sfurnace are respectively the cross-sectional areas of the probe and furnace. 2.5. Ash characterization The morphological data of ash particles, such as the average diameter, perimeter, and shape index, were obtained from the
3.1. Formation of ash 3.1.1. Morphology of ash Fig. 2 shows BSE images of ash particles obtained during the combustion process. The bright particles in the figures are the ash particles. The co-firing of the low-ash biomasses, such as sugi and sakura, using ratios of up to 50% produced no significant morphological changes in the ash particles. However, there were changes when the biomass ratio reached 70%, as evidenced by the existence of large numbers of spherical ash particles. In contrast, the same significant morphological changes occurred during the co-firing of the high-ash woody biomasses, such as nara and bark of sugi using mixing ratios as low as 30%. Furthermore, the shape index of each particle obtained by CCSEM analysis was used to quantify the number of spherical particles. A common shape index is the circularity (fcirc), which is a function of the cross-sectional area (A) and perimeter (P) of the particle, as expressed by the following equation [13]:
f circ ¼
4p A P
Based on the circularity of a circle of 1, the numbers of particles with shape indexes ranging between 1.0 and 1.19 were compared, as shown in Fig. 3. As can be clearly observed, the number of spherical ash particles significantly increased with increasing ratio of the high-ash biomasses, especially bark of sugi. Compared to pure coalfiring, the numbers of spherical particles for nara 50% and bark of sugi 30% are more than twofold. The sphericity of an ash particle indicates that the entire particle or its surface became molten during the combustion. This strongly suggests that co-firing, especially when using high-ash biomasses, affects the ash melting behavior.
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Fig. 2. SEM–BSE images of ash particles produced by pure coal firing and co-firing of coal with woody biomass.
3.1.2. Chemical and mineralogical composition Because calcium is the most abundant element of the AAEMs in woody biomass ash, the behavior of the Ca-containing ash was investigated in detail. The compositions of the ash particles that contained more than 80% Ca, Al, and Si are plotted in the ternary diagrams shown in Fig. 4. The standard phase diagram of CaO– SiO2–Al2O3 [14] is also included in each of the ternary diagram to enable prediction of the mineralogy and the melting point of mineral in the ash samples. In the case of coal firing, most of the particles are distributed within the Si–Al or in the region containing mullite (Al6Si2O13), which has a melting point of about 1880 °C. Aluminosilicates, particularly mullite, are well known as the main constituent of the fly ash particles produced by pure coal firing. By comparison, the distribution of the ash particles produced by the
co-firing of coal with a low-ash biomass of at least 50% was not significantly different. In the case of sakura, when the biomass content exceeded 70%, more particles were distributed between the regions containing anorthite (CaAl2Si2O8) and mullite, suggesting the formation of calcium aluminosilicates (anorthite). Co-firing using a high-ash biomass produces significant change in ash particle distribution, even for low mixing ratios. In the case of co-firing with 30% nara, the particles were predominantly located between the regions containing mullite and anorthite. The number of particles in the region containing anorthite increased when the ratio was increased to 50%, indicating the possible formation of anorthite. Although the melting point of pure anorthite is 1555 °C, it can be significantly decreased to a value much lower than those of mullite and anorthite by the formation
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gaseous phase of Ca(OH)2/CaCO3 in the woody biomass ash [15] or CaSO4 produced by the reaction of Ca(OH)2/CaCO3 with flue gas (SOx), and the aluminosilicate minerals (mainly mullite) in the coal ash. The reaction is as follows:
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ xAl2 O3 ySiO2ðsÞ ! CaO xAl2 O3 ySiO2ðsÞ þ H2 O=CO2
ðR1Þ
CaSO4ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! CaO xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
Fig. 3. Comparison of numbers of spherical particles with shape index of 1.0–1.19, produced by pure coal firing and co-firing with woody biomass.
of an eutectic mixture of mullite and anorthite (eutectic melting point = 1347 °C [14]). It is possible for the eutectic mixture to be formed in deposit layer during the co-firing process. The most significant change in the ash particles was observed for co-firing using bark of sugi. The chemical composition of the particles was transformed from that of calcium aluminosilicate, (particularly a type rich in anorthite) for 30% bark of sugi, to the coexistence of calcium aluminosilicates (mainly anorthite and gehlenite (Ca2Al2SiO7, 1549 °C)) and calcium silicates (mainly pseudowollastonite (CaSiO3, 1540 °C)) for 50% of the biomass, and finally to rich calcium silicates (mainly a-calcium silicate (Ca2SiO4, 1540 °C) and rankinite (Ca3Si2O7, 1475 °C)) for 70% of the biomass. Because the ash obtained from co-firing using bark of sugi is dominated by calcium-containing minerals, it melts at a much lower temperature compared to ash obtained from pure coal-firing. The formation of eutectic mixtures of calcium minerals that further lower the melting point of the minerals is also possible. For instance, significant amounts of eutectic mixtures of pseudowollastonite, anorthite, and gehlenite (eutectic melting point = 1267 °C [14]), and rankinite, gehlenite, and a-Ca2SiO4 (eutectic melting point = 1317 °C [14]) are respectively formed during co-firing using 30–50% and 70% bark of sugi. These eutectic mixtures may melt at about 1200–1300 °C, which is much lower than the melting point of the pure mineral (about 1500 °C). From the above results, it can be concluded that the ash content of biomass as well as the ash composition affects the ash transformation by co-firing. The higher the ash content of biomass, more significant is the change in the ash properties. The presence of calcium-containing minerals and their eutectic mixtures during co-firing significantly contributes to the formation of low-melting-point-ash and the possibly of increased ash deposition.
3.2. Interaction between biomass ash and coal ash The fate of the alkali (K) and alkaline earth (Ca) species is the key to understanding the interaction between coal and biomass. Fig. 5 shows the constitution of the calcium/potassiumcontaining inorganic components of the ashes during combustion. For comparison, the results for co-firing using sugi, nara, and bark of sugi are shown as representative examples of low- and high-ash biomass co-firing. The interaction of the calcium species during the co-firing will be discussed in detail. There was a slight increase in the calcium minerals during the co-firing using the low-ash biomasses. In contrast, the increase was significant for co-firing using high-ash biomasses, especially bark of sugi. The calcium minerals in the ash predominantly comprised a calcium aluminosilicate (Ca–Al–Si) mineral, probably anorthite or gehlenite, produced by the interaction between the
ðR2Þ
where g denotes gas, s denotes solid, and c denotes condensed (liquid or solid). Generally, the concentration of Ca–Al–Si increases with the increasing biomass ratio. However, in the case of co-firing using 50% bark of sugi, the concentration of Ca–Al–Si decreased while that of calcium silicate minerals (Ca–Si) increased relative to using 30%. Ca–Si may occur in the form of pseudowollastonite, rankinite, or a-calcium silicate depending on the calcium content of the fuel ash. Ca–Si may also be formed by the reaction of Ca(OH)2/CaCO3/ CaSO4 in the biomass with SiO2-rich minerals in the coal ash, as shown in the reactions below. However, calcium silicate minerals have also been detected in biomass ash [16]. An XRD analysis conducted in the present study revealed a strong Ca2SiO4 peak particularly for the ash using 70% bark. Ca–Si was also present in the unreacted biomass ash (Ca2SiO4).
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ SiO2ðsÞ ! CaO SiO2ðsÞ þ H2 O=CO2
ðR3Þ
CaSO4ðgÞ þ SiO2ðsÞ ! CaO SiO2ðsÞ þ SO2 þ 1=2O2
ðR4Þ
Beside Ca–Al–Si and Ca–Si minerals, Ca–Fe–Al–Si and Ca–K–Al– Si were also observed. Ca–Fe–Al–Si was probably formed by the interaction of Ca(OH)2/CaCO3/CaSO4 from biomass ash with iron– aluminosilicates from coal ash through the following reactions.
CaðOHÞ2ðgÞ =CaCO3ðgÞ þ Fe2 O3 xAl2 O3 ySiO2ðsÞ ! CaO Fe2 O3 xAl2 O3 ySiO2ðsÞ þ H2 O=CO2 CaSO4ðg or cÞ þ Fe2 O3 xAl2 O3 ySiO2ðsÞ ! CaO Fe2 O3 xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
ðR5Þ ðR6Þ
Conversely, Ca–K–Al–Si is produced from the reaction between K2Ca(SO4)2 or K2Ca2(SO4)3 from biomass ash [16] and aluminosilicates from coal ash. The interaction between potassium species such as K2SO4 and calcium aluminosilicates may also produce a Ca–K–Al–Si mineral.
K2 CaðSO4 Þ2ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! K2 O CaO xAl2 O3 ySiO2ðsÞ þ 2SO2ðgÞ þ O2ðgÞ
ðR7Þ
K2 Ca2 ðSO4 Þ3ðg or cÞ þ xAl2 O3 ySiO2ðsÞ ! K2 O 2CaO xAl2 O3 ySiO2ðsÞ þ 3SO2ðgÞ þ 3=2O2ðgÞ
ðR8Þ
K2 SO4 þ CaO xAl2 O3 ySiO2ðsÞ ! K2 O CaO xAl2 O3 ySiO2ðsÞ þ SO2ðgÞ þ 1=2O2ðgÞ
ðR9Þ
It should be noted that iron/potassium-containing calcium aluminosilicates have lower melting point s compared to calcium aluminosilicates [17,18], and this may increase the ash deposition tendencies of the former. Sulfur-containing calcium minerals such as anhydrite (CaSO4) and anhydrite-Al–Si were still observed in the products of cofiring, particularly when using bark of sugi. A moderate amount of anhydrate-Al–Si was also observed. As mentioned above, anhydrite is usually formed by the reaction of Ca(OH)2/CaCO3 from biomass ash with SO2 flue gas. A significant amount of anhydrite is usually produced by the combustion of woody biomass [19]. In co-firing, most of this mineral reacts with aluminosilicate from coal ash to produce SO2 as expressed by reaction (R2). When a big particle of anhydrite reacts with a small particle of
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Fig. 4. Distribution of chemical components of ash particles containing >80% of Ca + Si + Al, produced by pure coal firing and co-firing with woody biomass.
aluminosilicate mineral, anhydrite-Al–Si is formed. Ca-rich aluminosilicate minerals may also react with SO2 to form anhydrite-Al–Si. 3.3. Slagging propensity during co-firing One of the factors that determine the profile of the ash deposition ratio is the exposure time [20]. To determine the appropriate
exposure time, we conducted a slagging test using 50% bark of sugi. The determined time dependence of the ash deposition ratio and the mass of the deposited ash are shown in Fig. 6. While the mass of the ash deposited on the probe surface increased linearly, the ash deposition ratio was almost constant. However, after the saturation of the ash deposition, the deposition ratio tended to decrease. These results indicate that the exposure time of a
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Fig. 5. Fractions of calcium and potassium minerals in ashes produced by pure coal firing and co-firing with woody biomass.
the slagging propensity of the fuel. The higher the ash deposition ratio, the higher the slagging propensity. The test results showed that co-firing using a low-ash biomass did not increase the ash deposition ratio; rather, the deposition ratio tended to decrease with increasing biomass ratio at least up to 50%. In contrast, the ash deposition ratio increased with increasing biomass ratio for co-firing using a high-ash biomass. Co-firing using bark of sugi produced the highest ash deposition ratio among all the considered woody biomasses. Generally, the ash deposition ratio (slagging propensity) during the co-firing increased in the following order;
Bark of sugi > Nara > Sugi Sakura
Fig. 6. Time variations of ash deposition ratio and ash deposit for co-firing using 50% bark of sugi.
slagging test should be restricted to the duration when the deposition ratio is independent of time, which in the present case was determined to be 45 min. The total amount of fuel ash supplied during this period for all test cases was 1.2 g, and this made the exposure times of the slagging tests to be within 30–60 min depending on the ash content of the fuel. The profiles of the ash deposition ratio for co-firing and coal firing are shown in Fig. 7. The ash deposition ratio is an indication of
Because there was no significant difference between the chemical compositions of the combustion ashes and the ash deposits in this test, it is worthwhile to discuss the relationship between the combustion ash properties and the slagging propensity. The results of the slagging propensity agree with the transformation tendency of the ash properties during co-firing; specifically, co-firing using a high-ash biomass significantly affects the properties of the ash, whereas the use of a low-ash biomass does not. As noted earlier, co-firing increases the concentration of molten calcium minerals in the ash, with high ash biomasses (nara and bark of sugi) producing more significant changes. As is well known, the ash particles tend to adhere to the probe tube with increasing amount of molten ash particles [21]. In addition, the formation of the eutectic mixture of calcium minerals in the deposited layer may also further lower the melting point, resulting in the tendency of the co-firing deposits to be more receptive of oncoming particles, leading to rapid growth of the deposit. To investigate the eutectic formation in the ash deposits, we conducted a TMA analysis, which is widely used to investigate
35
Penetration (%)
30 25
50% Bark
20 15 10
Coal
50% Sugi
5 0 700
30% Bark 800
900
1000
1100
1200
Temperature (°C) Fig. 7. Ash deposition ratios (slagging propensities) for pure coal firing and co-firing with woody biomass.
Fig. 8. TMA profiles of ash deposits.
1300
1400
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the fusion behavior of mainly coal ashes [22,23]. The TMA profiles of the ash deposits obtained from co-firing and pure coal firing are compared in Fig. 8. The trend of the TMA profile of the ash deposit obtained using 50% sugi is similar to that of the ash deposit obtained from pure coal firing. In contrast, the TMA profile of the ash deposit obtained from co-firing with bark is significantly different, being characterized by a rapid increase in penetration within a narrow temperature range. The rapid increase was detected within 1210–1350 °C and 1170–1250 °C for 30% and 50% bark of sugi, respectively, and was due to the formation of eutectics in the deposit layer. These TMA results agree with our prediction of a high possibility of the formation of eutectic mixtures of pseudowollastonite, anorthite, and gehlenite, which melt at about 1200–1300 °C for 30% and 50% bark of sugi. The presence of iron (from coal ash) and alkali (potassium) in these mixtures may further decrease the melting point. The fact that no increase was observed in the slagging propensity for co-firing using the low-ash biomasses is also noteworthy. The ash deposition is not only affected by the chemical composition of the fuel ash, but also by the physical properties such as the particle and heat fluxes, and the particle velocity and impaction [24]. Sugi and sakura woods have an ash content of 0.4 wt% (Table 1), which is much lower than those of the other considered biomasses (>1.0 wt%) and coal (>12 wt%). Thus, the particle flux for co-firing using sugi/sakura should be much lower than those for high-ash biomasses. The lower the particle flux, the lower the probability of the ash sticking or colliding with the probe surface. In addition, co-firing was observed to produce no significant change in the ash properties. This together with the reduced particle flux probably contributed to the reduced slagging propensity observed for co-firing using a low-ash biomass. Hence, from the viewpoint of preventing severe slagging problems in a PC boiler during co-firing using a high ratio of woody biomass, a low-ash biomass is preferable. 4. Conclusions Co-firing of coal in a drop-tube furnace using high ratios of four different types of woody biomasses was investigated in this study. The properties of the combustion ashes and ash deposits were examined in detail by CCSEM analysis, CaO–Al2O3–SiO2 ternary phase diagrams and TMA analysis. Following is a summary of the findings: 1. Compared to pure coal firing, co-firing produces no significant changes in the morphology and chemical composition of the ash products when using a low-ash biomass (60.4 wt%) of up to 50 cal%. In contrast, the use of even a small ratio (30%) of a high-ash biomass (>1.0 wt%) such as nara or bark of sugi produces a large number of molten ash particles with a higher calcium mineral content. 2. The chemical composition of the co-firing products are mainly changed from those of the aluminosilicate (mullite)-rich ash particles produced by coal firing to those of calcium aluminosilicate (anorthite/gehlenite)-rich particles. With increasing biomass ratio, especially when using bark of sugi, there is a subsequent change in the chemical properties of the products to those of calcium silicate (pseudowollastonite, rankinite, aCa2SiO4)-rich ash. These calcium minerals have lower melting points than aluminosilicate minerals, which are predominant in the products of pure coal firing. In deposited layer, these calcium minerals easily form eutectics that decrease the melting point, accelerating growth of the ash deposit.
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3. An increase in slagging propensity was only observed for cofiring using high ash biomasses. This was because of the significant change in the ash properties (number of molten particles, and chemical composition) and the formation of the eutectic mixtures of calcium minerals in the deposited layer. To prevent severe slagging in a PC boiler during co-firing using a high woody biomass ratio, the use of a low-ash biomass is preferable.
Acknowledgement The financial support of the Ministry of the Environment (MOE) – Japan is gratefully acknowledged. References [1] Tillman DA. Biomass cofiring: the technology, the experience, the combustion consequences. Biomass Bioenergy 2000;19:365–84. [2] Almansour F, Zuwala J. An evaluation of biomass co-firing in Europe. Biomass Bioenergy 2010;34:620–9. [3] Suzuki A. Brief overview of woody biomass co-firing in Saijyo coal-fired power plant. Electr Rev 2005;90–12:74–7. [4]