Article pubs.acs.org/EF
Cite This: Energy Fuels 2018, 32, 416−424
Ash Formation and Fouling during Combustion of Rice Husk and Its Blends with a High Alkali Xinjiang Coal Jianqun Wu, Dunxi Yu,* Xianpeng Zeng, Xin Yu, Jingkun Han, Chang Wen, and Ge Yu State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, China ABSTRACT: Limited data from fluidized bed combustion tests have shown that rice husk, a silicon-rich residual biomass, has the potential to be cofired with coal while not inducing unacceptable ash-related problems. However, there is great concern regarding the behavior of rice husk ash under pulverized fuel combustion conditions, where the temperatures are much higher and expected to facilitate fuel interactions. This work, to the authors’ knowledge, is the first to investigate both ash formation and fouling behavior in rice husk firing and its cofiring with coal at a high temperature relevant to pulverized fuel combustion. A Chinese rice husk and a high alkali Xinjiang coal were selected. Combustion tests of individual fuels and their blends (with the share of rice husk being 10% and 20%, respectively) were performed at 1573 K on a laboratory drop tube furnace. Both bulk ash and fouling deposit samples were collected in each test. Techniques including X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), and ash fusion temperature testing were used for sample characterization. The results show that the rice husk ash generated at 1573 K is dominated by amorphous silica with only a trace amount of quartz. Partial melting of ash particles is observed and attributed to enhanced formation of potassium silicates, which were not found in fluidized bed combustion. Nevertheless, such differences do not seem to change the nonfouling nature of rice husk ash. The ash from cofiring rice husk and coal possesses similar crystalline structures to the coal ash, but extensive fuel interactions are observed. The high fouling tendency of the coal ash is greatly reduced by cofiring with rice husk. The effects of rice husk are both physical and chemical in nature. The presence of high-fusion-temperature, nonsticky rice husk ash in the deposits and the capture of fouling-inducing species from coal by rich husk ash are considered to play important roles in reducing ash fouling. The nonlinear dependence of ash fouling tendency on the rice husk ratio, as found in fluidized bed combustion, is also observed. This work suggests that rice husk may also be cofired with coal in pulverized fuel boilers while not causing significant ash-related problems, though further investigations are still necessary.
1. INTRODUCTION Concerns over the health and environmental effects of coal utilization1−5 have stimulated the use of biomass, the third largest and renewable energy resource in the world.6,7 Cofiring biomass (a carbon-neutral renewable energy source) and coal has been identified as a promising technology to use bioenergy at large scales, which also has great potential in reducing SO2, NOx, CO2, and trace element emissions associated with coal utilization.8 However, despite its environmental benefits, cofiring biomass and coal is still facing a number of technical challenges such as fuel supply, handling, ash corrosion and deposition issues.9 Among them, ash deposition (including ash formation, fouling, and slagging), mainly resulting from high alkali/alkaline earth metals (AAEMs) in biomass and their interactions with coal ash components, can significantly affect energy conversion, boiler performance, and safe operation. Therefore, it has long been a primary concern and a subject of extensive research. Rice husk is an important residual biomass generated from the rice milling industry. The world rice production in 2014 was approximately 740 million tons, while in China this value reached about 200 million tons.10 If assuming that rice husk accounts for about 20 wt % of the rice grain on average,11 its production can be tremendous. Rice husk contains energy comparable to lignite11 and has been recognized as a key renewable energy source especially in large rice producing countries like China. However, although the utilization of rice © 2017 American Chemical Society
husk in small-scale boilers has a long history, it was not until the late 20th century that rice husk became an attractive fuel for large-scale combustors.12 While there have been a number of studies on combustion characteristics and pollutant emissions in firing or cofiring rice husk,13−15 very little work is available on ash-related issues especially at high temperatures (e.g., pulverized fuel combustion). Quite different from other herbaceous biomass fuels, rice husk has a high ash content (13−29%) and a very high silica content (87−97%) of the ash.16 In contrast, the contents of AAEMs can be much lower.17 These unique features are expected to clearly distinguish rice husk from other biomass fuels in ash behavior. However, this was not purposely investigated until 2005, when Skrifvars et al. published a series of two impressive papers18,19 on the fouling behavior of rice husk in fluidized-bed combustion. The study of fuel characteristics18 showed that, when ashed at 500 and 900 °C in air, rice husk produced large ash particles with almost identical structures to the fuel particles. The ash consisted almost purely of silica and no potassium silicates were observed, suggesting little interactions between potassium and silica during combustion. The initial deformation temperature of the rice Received: August 4, 2017 Revised: December 16, 2017 Published: December 17, 2017 416
DOI: 10.1021/acs.energyfuels.7b02298 Energy Fuels 2018, 32, 416−424
Article
Energy & Fuels Table 1. Fuel Properties proximate analysis (wt %, air-dry basis) RH XJ
RH XJ a
fixed carbon
volatile matter
12.7 50.4
69.0 33.6
ash
ultimate analysis (wt %, air-dry basis) moisture
C
14.8 3.5 40.5 10.0 5.9 59.1 Low temperature ash analysis (wt %)
H
S
N
Oa
5.6 4.3
0.1 0.4
0.9 0.7
34.9 19.5
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
Fe2O3
0.1 6.2
0.4 8.3
0.7 17.4
93.5 28.0
0.7 0.8
0.3 7.9
2.7 1
1.2 21.6
0.4 8.8
By difference.
husk ash was as high as 1480 °C. Consequently, no ash melting was expected under fluidized-bed combustion conditions (∼850 °C). The Multi Fuel Fouling index suggested that rice husk was a nonfouling fuel. These were consistent with the subsequent work performed on an entrained-flow type of pilot furnace and a large-scale (157 MWth) bubbling fluidized-bed boiler.19 The data collected from both facilities showed that no melting or deformation of rice husk ash could be observed, and the combustion of rice husk alone did not lead to any detectable fouling. When eucalyptus bark (a fouling biomass) was cofired with rice husk, there were no clear chemical interactions between ashes from two fuels. However, a physical effect (“sand blasting”) caused by rice husk particles was observed and seemed to be able to alleviate fouling of eucalyptus bark ash. The nonfouling nature of rice husk ash was also observed in tests performed at a commercial 370 MWth CFB boiler.20 These studies suggest that rice husk may be a good candidate for cofiring with fouling biomass fuels or low-rank coals. It can be noted that the above-mentioned work was primarily concentrated on the behavior of rice husk ash under fluidizedbed combustion conditions (1723 >1723 >1723
Figure 6. Ash deposition propensities for different fuel combustion cases.
DP of about 26 mg/g ash, suggesting the most severe ash fouling problems. This is consistent with AFT test results (Table 2) and field observations of a very speedy buildup of fouling deposits in the convection pass of a boiler firing the XJ coal. Figure 7a shows that XJ ash deposits have a dense
among all the fuels investigated, the XJ coal has the lowest AFTs. This is expected as it is enriched with AAEMs (Table 1), which can readily react with silicates to form low melting point eutectic compounds. The IDT, HT, and FT of the XJ coal ash are far lower than the combustion temperature (1573 K) adopted in the experiments. It suggests that the coal ash is prone to extensive melting during combustion. This is consistent with Figure 4b, where combustion-generated ash particles are seen to be nearly entirely spherical in shape. Quite differently, the AFTs of the rice husk are astonishingly high, even beyond the limits of the CAF (1723 K) that is much higher than the combustion temperature used in this work. This is attributed to the extremely high SiO2 content and low contents of AAEMs in rice husk (Table 1). Due to its high AFTs, the rice husk ash is not expected to undergo extensive melting in the DTF (Figure 4a) and is nonsticky in nature. For the blends, because of the introduction of high melting point rice husk ash, the AFTs of the blends are higher than those of the XJ coal and they are seen to be increase with the share of rice husk (Table 2). The data presented in Table 2 show that the AFTs of the fuels increase in the order of XJ < XJ-10-RH < XJ-20-RH < RH. Therefore, the tendency of ash fouling is estimated to decrease in the order of XJ > XJ-10-RH > XJ-20RH > RH. The results suggest that the XJ coal would have the highest ash fouling tendency while the rice husk would not have fouling problems. The addition of rice husk will alleviate ash fouling when compared with the XJ coal. 3.3.2. Fouling Tests in the DTF. Although the AFT test has become the most accepted approach for assessing ash fouling propensity, it has shortcomings in the uncertainties as a predictive tool for plant performance and the reproducibility of
Figure 7. Photographs of deposits on coupons for different fuel combustion cases.
structure with fine ash particles adhering tightly to the coupon. For the RH, an extremely low DP of approximate 3 mg/g ash is observed. The fouling deposits (Figure 7d) mainly consist of large particles that could easily fall off when the coupon was slightly shaken manually. It suggests that, even at high combustion temperatures as used in this work, the RH ash would not be sticky enough to cause appreciable fouling problems though partial surface melting may occur (Figure 4a). This nonfouling nature of rice husk ash is in accordance with its 421
DOI: 10.1021/acs.energyfuels.7b02298 Energy Fuels 2018, 32, 416−424
Article
Energy & Fuels high AFTs (Table 2) and similar to that found in fluidized-bed combustion.19 Of particular interest in this work is the ash behavior during combustion of coal-rice husk blends. Figure 6 shows that, compared with the XJ coal, the DPs of the blends (XJ-10-RH and XJ-20-RH) decrease significantly. It demonstrates that the addition of rice husk results in an apparent mitigation of ash fouling caused by firing the XJ coal. The deposits formed on the coupon become sparse due to the presence of large rice husk ash particles (Figure 7b and c), in contrast to the dense structure of the deposits from sole combustion of the XJ coal. It is somewhat surprising that, if the error bars are taken into consideration, the DP of the XJ-10-RH is similar to that of the XJ-20-RH. This result shows that increasing the share of rice husk from 10% to 20% has insignificant influence on the ash deposition propensity though differences exist in the AFTs (Table 2). This is understandable, because AFT tests only focus on ash particles themselves but do not consider their aerodynamic behavior in real combustion processes.36 In summary, the data presented in Figure 6 show the important role of rice husk in mitigating ash fouling of the high fouling XJ coal, and the nonlinear dependence of ash fouling propensity on the share of rice husk in the blends. This nonlinear dependence is most likely related to changed interactions between different ash components. When the share of rice husk in blends is low, the fouling-inducing species from coal may be far more than needed for interactions with rice husk ash. In this case, ash fouling behavior in cofiring would possess similar features to that in sole coal combustion. However, when the share of rice husk increases greatly, there may be excessive rice husk ash particles for interactions with fouling-inducing species. As a result, ash fouling behavior in cofiring would be similar to that in sole rice husk combustion. Such changes are expected to lead to the nonlinear dependence of ash fouling propensity on the rice husk share. These findings are similar to those from the study by Skrifvars et al.,19 who investigated the ash fouling behavior of rice husk-eucalyptus bark blends under fluidizedbed combustion conditions. This work, together with that by Skrifvars et al.,19 suggests that rice husk may be a good nonfouling fuel for cofiring with fouling coals or other biomass whatever under pulverized fuel or fluidized-bed combustion conditions. Nevertheless, this conclusion is yet to be further validated by additional studies and tests. 3.4. Chemical Composition of Fouling Deposits. The information on the deposit composition is critical to a fundamental understanding of ash fouling behavior. The data of the chemical composition of fouling deposits from different combustion cases are compared in Figure 8. As expected, the RH deposit is dominated by SiO2 (88.6 wt %). The content of K2O in the deposit is about 4.4 wt %, apparently higher than that (2.7 wt %) in the low temperature ash of the RH (Table 1). The enrichment of K2O in the deposit is consistent with its fouling characteristics.34 It may deposit on the coupon by condensation, chemical interaction in the form of potassium silicates. It is likely that only some ash particles are sticky enough to adhere to the coupon surface. This is consistent with the nonfouling nature of the RH ash, as shown in Figure 6. In contrast to the RH deposit, the XJ deposit is rich in foulinginducing components such Na2O, SO3, CaO, and Fe2O3. This feature is closely related to the coal properties. The XJ coal is characterized by high contents of AAEMs and iron but low contents of silicon and aluminum (Table 1), which are similar to some low-rank coals found in the US and Australia.38,39 It
Figure 8. Deposit chemical composition for different fuel combustion cases.
has been well established that the fouling-inducing components in low-rank coals generally find their way into the deposit in the forms of sulfates and/or low melting point silicates. This can account for the high fouling tendency of the XJ coal ash as observed in Figure 6. When the coal-rice husk blends (XJ-10-RH and XJ-20-RH) are combusted, the generated deposits show apparently different composition from that of the XJ coal (Figure 8). The SiO2 content is greatly elevated, while those of Al2O3 and fouling-inducing components are seen to decrease accordingly. The K2O content of the deposits is relatively low and only increases slightly. When all the composition data (i.e., XJ, XJ10-RH, XJ-20-RH, and RH) are compared, it can be seen that the contents of SiO2 and K2O in the deposits generally increase with increasing the share of rice husk from 0% (XJ) to 100% (RH). These observations clearly demonstrate the dilute effect of the RH ash on deposit composition. Based on the data presented in this work, the effects of rice husk addition on ash fouling propensity may lie in two folds. First, the rice husk ash acts as a diluting agent in the deposits. During cofiring, the incoming large RH ash particles maybe captured by the sticky surface. With the occupation of the surface by RH ash particles, further buildup of the deposits would be suppressed because the RH ash possesses very high AFTs (Table 2) and is expected to be far less sticky. Due to the presence of RH ash in the deposits, the contents of SiO2 and K2O increase while those of other oxides from the XJ coal decrease (Figure 8). It can be seen that such effects of the RH on ash fouling is physical in nature. Second, chemical reactions between fuels (fuel interactions) tend to reduce vaporized AAEMs and fine ash particles that are critical contributors to ash fouling. The results presented in Figure 4c and d have shown enhanced interactions between XJ coal ash and RH ash in cofiring. These processes transform vaporized AAEMs into high-temperature eutectic compounds. These materials are usually silicates, whose volatility is much lower than that of other AAEM-containing compounds such as chlorides or hydroxides. Also fuel interactions may facilitate the transformation of fine ash particles into coarse ones. As a consequence, fouling-inducing components can be greatly reduced. Although the rice husk ash particles show partial melting of the surfaces (Figure 4a), they are large in size and may not be sticky enough to enhance ash fouling during cofiring. In this regard, fuel interactions also play an important role in mitigating ash fouling in cofiring. Based on the above 422
DOI: 10.1021/acs.energyfuels.7b02298 Energy Fuels 2018, 32, 416−424
Article
Energy & Fuels
2016YFB0600601) and National Natural Science Foundation of China (Grant Nos. 51520105008 and 51676075). The support from the Analytical and Testing Center at Huazhong University of Science & Technology is also acknowledged.
analyses, the observed reduction in ash fouling propensities of the blends compared with the XJ coal (Figure 6) is reasonably attributed to a combination of physical and chemical effects of the RH. Nevertheless, which one is the dominant effect is currently unknown and need further investigation. Skrifvars et al.19 reported a “sand blasting” phenomenon caused by rice husk particles. It was considered an important mechanism for the reduction of ash fouling in rice husk-eucalyptus bark cofiring under fluidized combustion conditions. In this work, the effect of “sand blasting” may be insignificant, because the particle size of the RH used (
